United States
Environmental Protection
Agencv
Offif e of Solid Waste and
Emerqency Response
Office of Emergency and
Remedial Response
Washington DC 20460
Office of Research and
Development
Superfund
EPA/540/2-85/004 Nov 1985
£EPA
Leachate Plume
Management
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EPA/540/2-85/004
LEACHATE PLUME MANAGEMENT
by
Edward Repa and Charles Kufs
ORB Associates
8400 Uestpark Drive
McLean, Virginia 22102
EPA Project Officer
Naomi Barkley
OFFICE OF SOLID WASTE AND EMERGENCY RESPONSE
OFFICE OF EMERGENCY AND REMEDIAL RESPONSE
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, DC 20460
HAZARDOUS WASTE ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
• rental Protection Agenc*
jS. Environmental nut
,«aS
Chicago, Illinois
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NOTICE
The information in this document has been funded, wholly or in part, by
the United States Environmental Protection Agency under Contract No.
68-03-3113 to Science Applications International Corporation/JRB
Associates (hereafter SAIC/JRB). This document has been subject to the
Agency's peer arid administrative review and has been approved for
publication as an EPA document.
This handbook is intended to present information on the application of
technologies for the control of specific problems caused by uncontrolled
waste sites. The handbook is not intended to address every conceivable
waste site problem or all possible applications of these technologies.
Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
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FOREWORD
This is one of a series of reports being published to implement CERCLA,
otherwise known as Superfund legislation. These are documents explaining the
hazardous response program and, in particular, the technical requirements for
compliance with the National Contingency Plan (NCR), the analytical and
engineering methods and procedures to be used for compliance, and the
background and documenting data related to these methods and procedures. The
series may include feasiblity studies, research reports, manuals, handbooks,
and other reference documents pertinent to Superfund.
This handbook provides an overview of the fundamental concepts,
procedures, and technologies used in leachate plume management. Plume
generation dynamics and delineation are discussed. Plume control technologies
are evaluated and selection criteria for site applications are defined.
Groundwater pumping, subsurface drains, low permeability barriers, and
innovative technologies are the aquifer restoration technologies addressed in
this handbook.
The handbook provides governmental and industrial technical personnel
with the means to successfully control leachate plumes from uncontrolled
hazardous waste sites. In conjunction with other publications in this series,
this handbook will assist in meeting the national goal of a cleaner, safer
environment.
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ABSTRACT
The problem of leachate plume management has been aggravated to some
extent by a lack of understanding of plume dynamics and the various remedial
options available. This handbook summarizes information in the areas of
leachate plume dynamics and plume management alternatives. The handbook
describes factors that affect leachate plume movement, key considerations in
delineating the current and future extent of the leachate plume, technologies
for controlling the migration of plumes, and criteria for evaluating and
selecting plume management alternatives. The handbook consists of eight
chapters:
0 Chapter 1 -- Introduction includes an overview disc'ussion of leachate
plume generation and a summary of the handbook's contents.
0 Chapter 2 --Plume Dynamics includes discussions on the effects of
groundwater flow patterns, leachate characteristics, and plume and
geologic media interactions.
• Chapter 3 -- Plume Delineation includes discussion on data sources and
procedures for estimating plume boundaries, characterizing plume
chemistry, and extrapolating future plume movement.
• Chapter 4 -- Plume Control Technologies includes an overview
discussion of plume control technologies and how they can be evaluated
and selected for site applications.
• Chapter 5 -- Groundwater Pumping includes discussions on well
hydraulics and well design, installation, operation, and maintenance.
• Chapter 6 — Subsurface Drains includes discussions on drain
hydraulics and drain design, installation, and maintenance.
• Chapter 7 --Low Permeability Barriers includes discussions on barrier
materials, placement, design, installation, and maintenance.
• Chapter 8 -- Innovative Technologies includes discussions on
bioreclamation, in situ chemical treatment, and emerging technologies.
This report was submitted in fulfillment of Contract No. 68-03-3113,
Task No. 38-1 by SAIC/JRB under the sponsorship of the U.S. Environmental
Protection Agency.
IV
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CONTENTS
Page
1.0 INTRODUCTION 1-1
2.0 PLUME DYNAMICS 2-1
2.1 Groundwater Flow Patterns 2-2
2.1.1 Basic Hydrogeologic Concepts 2-2
2.1.2 Effects of Subsurface Conditions 2-15
2.1.3 Effects of Human Activities 2-28
2.2 Effects of Leachate Characteristics 2-39
2.2.1 Effects of Physical Characteristics 2-40
2.2.2 Effects of Geochemical Interactions 2-50
3.0 PLUME DELINEATION 3-1
3.1 Plume Delineation Procedures 3-2
3.1.1 Calculate Possible Plume Boundaries 3-2
3.1.2 Modify and Verify Calculated Boundaries 3-8
3.1.3 Extrapolate Future Plume Movement 3-11
3.2 Indirect Data 3-16
3.2.1 Previously Collected Data 3-22
3.2.2 Aerial Imagery 3-28
3.2.3 Geophysical Methods 3-35
3.3 Direct Data 3-47
3.3.1 Hydrologic Testing 3-49
3.3.2 Sampling 3-62
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CONTENTS (Continued)
Page
4.0 PLUME CONTROL TECHNOLOGIES 4-1
4.1 Control Techniques 4-2
4.1.1 Groundwater Pumping 4-2
4.1.2 Subsurface Drains 4-5
4.1.3 Low Permeability Barriers 4-9
4.1.4 Innovative Technologies 4-14
4.2 Technology Evaluation and Selection 4-17
4.2.1 Development of Alternatives 4-17
4.2.2 Screening of Alternatives 4-20
4.2.3 Detailed Analysis of Alternatives 4-30
4.3 Design and Implementation of Alternatives 4-43
5.0 GROUNDWATER PUMPING 5-1
5.1 Introduction 5-1
5.2 Well Theory 5-2
5.2.1 Darcy's Law 5-2
5.2.2 Equilibrium Well Formula 5-7
5.2.3 Non-Equilibrium Well Formula 5-11
5.2.4 Semiconfined Aquifers 5-15
5.2.5 Partially Penetrating Wells 5-21
5.2.6 Cumulative Drawdown 5-24
5.2.7 Hydrogeologic Boundary Effects 5-28
5.2.8 Flow Between Discharge and Recharge Wells .... 5-31
5.2.9 Radius of Influence 5-31
5.3 Applications 5-38
5.3.1 Groundwater Level Adjustment 5-38
5.3.2 Plume Containment 5-39
5.3.3 Plume Removal 5-42
5.4 Design and Construction 5-44
5.4.1 Well Design 5-44
5.4.2 System Design 5-80
5.4.3 Installation and Maintenance of Wells 5-95
5.5 Costs of Well Systems 5-109
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CONTENTS (Continued)
Page
6.0 SUBSURFACE DRAINS 6-1
6.1 Introduction 6-1
6.2 Theory 6-8
6.2.1 Drainage System Terminology 6-12
6.2.2 Depth and Spacing 6-17
6.3 Design 6-38
6.3.1 Flow Capacity 6-39
6.3.2 Filters and Envelopes 6-46
6.3.3 Design and Selection of Pipes 6-53
6.3.4 Drainage Sump and Pumping Plant 6-57
6.4 Installation and Maintenance 6-60
6.4.1 Trench Excavation 6-60
6.4.2 Drain Installation 6-81
6.4.3 Inspection and Maintenance 6-93
7.0 LOW PERMEABILITY BARRIERS 7-1
7.1 Slurry Walls 7-2
7.1.1 Applications and Limitations 7-2
7.1.2 Theory 7-8
7.1.3 Design and Construction 7-37
7.1.4 Completed Wall Costs 7-61
7.2 Grouting 7-63
7.2.1 Theory 7-65
7.2.2 Design and Construction 7-81
7.2.3 Grouting Costs 7-104
8.0 INNOVATIVE TECHNOLOGIES 8-1
8.1 Introduction 8-1
8.2 Bioreclamation 8-9
8.2.1 Applications and Limitations 8-11
vn
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CONTENTS (Continued)
8.2.2 Theory
8.2.3 Design and Operation
8.3 Chemical Treatment
8.3.1 Soil Flushing/Solution Mining
8.3.2 Oxidation/Reduction
8.3.3 Precipitation/Polymerization
8.3.4 Neutralization/Hydrolysis
8.3.5 Permeable Treatment Beds
8.4 Block Displacement
8-15
8-23
8-49
8-49
8-53
8-68
8-70
8-80
8-92
Bibliography
COPYRIGHT NOTICE
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FIGURES
Number Page
2-1 Example Calculation of Horizontal Hydraulic
Gradient 2-8
2-2 Potentiometric Surfaces in Confined and
Unconfined Aquifers 2-10
2-3 Example Calculation of Vertical Gradient 2-12
2-4 Example of How Groundwater Velocity Can Vary with
Differing Hydrogeologic Conditions and with
Differing Scales of Observation for Key Parameters 2-13
2-5 Characteristic Plume of Miscible Contaminants in
a Homogeneous Aquifer .... 2-16
2-6 Effects of Variable Hydraulic Conductivity on
Leachate Plume Movement in Unconsolidated Material 2-18
2-7 Effects of Secondary Porosity on Leachate
Plume Movement 2-19
2-8 Effects of Groundwater Flow Direction and Geologic
Heterogeneities on Leachate Plume Movement 2-22
2-9 Effect of a High-Head, Semi-Confined Unit on
Leachate Plume Movement 2-24
2-10 Example of the Effects of Site Geology on
Leachate Plume Movement (Cross-Sectional View) 2-25
2-11 Example of the Effects of Site Geology on
Leachate Plume Movement (Map View) . 2-26
2-12 The Effect of Waste Site Location Relative to
Location of Recharge and Discharge Areas on
Plume Migration 2-29
2-13 Effects of Human Activities on Leachate Plume
Movement (Map View) 2-36
IX
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FIGURES (Continued)
Number Page
2-14 Effects of Human Activities on Leachate Plume
Movement (Cross-Sectional View) 2-37
2-15 Effects of a Heterogeneity on the Flow of a
High Density Plume 2-44
2-16 Effect of Density on Leachate Plume Movement 2-46
2-17 Configurations Based on Solubility and Density 2-47
2-18- Types of Breakthrough Curves Generated by the
Soil Column Technique 2-60
3-1 Generalized Approach to Delineating Leachate Plumes 3-3
3-2 Well Configurations Used for Groundwater Monitoring 3-67
3-3 Well Design for Preventing Interaquifer Contamination .... 3-74
5-1 Development of Flow Distribution about a Discharging
Well in a Free Aquifer - A Fully Penetrating and
33-Percent Open Hole 5-4
5-2 Formation of Cone of Depression Around a Pumping Well .... 5-5
5-3 Effect of Storage and Transmissivity on the Shape
of the Cone of Depression 5-6
5-4 Unconfined Aquifer Flow . „ 5-8
5-5 Confined Aquifer Flow 5-9
5-6 Plots of H-h Versus r for Unconfined and
Confined Aquifers 5-10
5-7 Semi confined Aquifer 5-19
5-8 Flow to Partially Penetrating Wells 5-23
5-9 Flow in Confined Aquifer with Partially
Penetrating Well 5-25
5-10 Composite Drawdown in a Confined Aquifer 5-26
5-11 Composite Drawdown in an Unconfined Aquifer 5-27
5-12 Effect of Recharge and Barrier Boundaries
on Drawdown 5-29
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FIGURES (Continued)
Number Page
5-13 Method of Images for Determining Resultant
Cone of Depression 5-30
5-14 Recharge and Discharge Wells in a Confined Aquifer 5-32
5-15 Distance Drawdown Diagrams for A) Varying Pumping
Rating and B) Varying Pumping Times 5-35
5-16 Plume Diversion Using Injection Wells 5-40
5-17 Containment Using Extraction Wells 5-41
5-18 Extraction and Injection Wells Patterns for
Plume Removal 5-43
5-19 Components of Typical Deep Well 5-48
5-20 Performance Curves 5-56
5-21 Range of Filter Selections 5-66
5-22 Grain-Size Analysis Curve 5-69
5-23 Componennts of One-Pipe and Two-Pipe Ejector Wells 5-72
5-24 Driven Uellpoint (a), Jetted Wellpoint (b), and
Drilled Wellpoint (c) 5-77
5-25 Basic Injection Well 5-79
5-26 Potentiometn'c Surface Map and Geologic Cross-Section
of Gasoline Pollution Site 5-81
5-27 Drawdown Versus Yield and Specific Capacity for
a Water Table Aquifer 5-91
5-28 Well Placement and Drawdown Superimposed onto
Groundwater Maps 5-93
5-29 Typical Well System Components 5-111
6-1 The Use of Subsurface Drainage to Contain a
Leachate Plume. 6-4
6-2 The Use of Subsurface Drainage to Lower Groundwater
Levels 6-5
i
6-3 The Use of a One Sided Subsurface Drain for Reducing
Flow from Uncontaminated Sources 6-6
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FIGURES (Continued)
Number Page
6-4 The Use of Subsurface Drainage in a Completely
Encapsulated Site 6-7
6-5 The Use of Subsurface Drainage to Prevent
Overflow and Ponding 6-7
6-6 Differences Between Groundwater Flow Toward
Drains Versus Wells 6-9
6-7 Relationship Between Drawdown (H-h) in a Drain
and a Well 6-11
6-8 The Affect of Relief and Intercepter Drains in
Altering the Configuration of the Water Table 6-13
6-9 The Use of Interceptor Drains to Collect Flow
Induced by Groundwater Mounding 6-14
6-10 Components of a Drainage System 6-15
6-11 The Relationship of Drain Depth and Spacing to
Water Table Drawdown 6-18
6-12 The Effect of Depth to a Low Permeability
Barrier on Drain Spacing 6-20
6-13 The Effect of Hydraulic Conductivity Drain Spacing 6-21
6-14 Flow to a Drain Resting on a Low Permeability
Barrier 6-23
6-15 Flow to Drain Not Resting on a Low Permeability
Barrier 6-24
6-16 Symbols for the Ernst Equation for Flow in a Two-Layered
Soil with (A) the Drain in the Lower Layer
and (B) the Drain in the Upper Layer 6-27
6-17 Nomograph for Determining the Geometry Factor,
in the Ernst Equation 6-30
6-18 Site Conditions Requiring Interceptor Drains for
Plume Management 6-33
6-19 Location of a Subsurface Drain with Respect to
Topography and the Direction of Groundwater Flow 6-34
xn
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FIGURES (Continued)
Number Page
6-20 Symbols for the Glover and Donnan Equation for
Calculating the Downgradient Influence of an
Interceptor Drain 6-37
6-21 Capacity Chart for N = 0.013 6-44
6-22 Capacity Chart for N = 0.015 6-45
6-23 Mechanical Analysis of a Gravel Filter Material 6-51
6-24 Joint Design for Rigid Drain Tiles 6-54
6-25 Typical Design of an Automatic Drainage
Pumping Plant 6-58
6-26 Correlation of Required Excavation Methods with
Seismic Testing Data 6-62
6-27 Costs of Shoring for Deep Trenches 6-72
6-28 Pipe Drain with Filter Fabric 6-88
6-29 Typical Manhole Design for a Closed Drain 6-91
6-30 Typical Design of Sediment Trap and Wetwell 6-93
7-1 Hanging Slurry Wall 7-5
7-2 Hanging Slurry Wall 7-6
7-3 Plan of Circumferential Wall Placement 7-7
7-4 Cut-Away Cross-Section of Circumferential
Wall Placement 7-7
7-5 Plan of Upgradient Placement with Drain 7-9
7-6 Cut-Away Cross-Section of Upgradient Placement
with Drain 7-9
7-7 Plan of Downgradient Placement 7-10
7-8 Cut-Away Cross-Section of Downgradient Placement 7-10
7-9 Fluid Loss During Filter Cake Formation 7-18
XII 1
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FIGURES (Continued)
Number Page
7-10 Relationship Between Permeability and Quantity
of Bentonite Added to SB Backfill 7-23
7-11 Effect of Plastic and Non-Plastic Fines Content
on Soil-Bentonite Backfill Permeability ..... 7-24
7-12 Typical Slurry Wall Construction Site 7-44
7-13 Cross-Section of Slurry Trench, Showing
Excavation and Backfilling Operations 7-53
7-14 Grouting Pipe Layout for Constructing a
Barrier Uall 7-98
7-15 Stage-up Grout-Port Injection Process 7-100
7-16 Diagram of a Double Packer Used in
Grout Port Injection 7-101
7-17 Vibrating Beam Grout Placement Process 7-102
8-1 Simplified View of Groundwater BiorecTarnation 8-10
8-2 Typical Groundwater Temperatures (°F) at 100 Foot
Depth in the Conterminous United States 8-14
8-3 Simplified Selective Adaption/Mutation Process 8-19
8-4 Configuration of Reinjection Trenches 8-29
8-5 Configuration of Static Mixer 8-31
8-6 Possible Configuration of In Situ Aeration
Well Bank 8-34
8-7 Configuration of In Situ Oxygenation Well System
and Dissolved Oxygen Concentrations as a
Function of Distance from Well 8-36
8-8 Installation of a Permeable Treatment Bed 8-81
8-9 Schematic Diagram of Block Displacement 8-94
8-10 Schematic Cross-Section of Block Displacement
Showing Separating to Induce Displacement 8-97
8-11 Slurry Jet in Air Notching Operation 8-99
xiv
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TABLES
Number Page
2-1 Range of Porosity Values for Various Geologic Materials. . . 2-4
2-2 Range of Hydraulic Conductivity Values for
Various Geologic Media 2-6
2-3 Effects of Several Remedial Action Measures on
Leachate Plume Migration 2-31
2-4 Distribution of Contaminant Types for Superfund
Sites 2-40
2-5 Factors Affecting Leachate Volume Generation 2-42
2-6 Examples of Plume Constituents Based on
Solubility and Density 2-48
2-7 Probable Effect of Various Processes on the
Mobility of Constituents in Subsurface
Waters Contaminated by Waste Disposal 2-55
2-8 Processes Which May Control Amounts of Certain
Constituents in Subsurface Waters Contaminated
by Waste Disposal 2-58
2-9 Behavior of Specific Chemical Wastes at Landfills 2-59
2-10 Pesticide Mobility Based on Distribution
Coefficients 2-62
3-1 Information Sources for Calculating Velocity
Using Darcy's Law (V = Kl/n) 3-6
3-2 Example Calculations of Maximum Plume Limits (D ) 3-9
3-3 Release Rate Models • 3-13
3-4 Analytical Solute Transport Models 3-17
3-5 Numerical Transport Models 3-19
3-6 Type of Information Generally Available in
U.S. EPA Site Investigation Reports 3-23
xv
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TABLES (Continued)
Number Page
3-7 Types and Sources of Maps, Reports, and
Related Information 3-24
3-8 Examples of Additional Sources of Information
for Remedial Investigations 3-29
3-9 Summary of Borehole Log Applications 3-48
3-10 Significance of Selected On-Site Observations
to Plume Delineation 3-50
3-11 Laboratory Methods for Determining Hydraulic
Conductivities 3-57
3-12 Effects of Various Types of Errors on Laboratory
Measured Values of Hydraulic Conductivity 3-58
3-13 Single Well Tests 3-60
3-14 Multiple-Well Pump Tests 3-61
3-15 Advantages and Disadvantages of Various Types
of Monitoring Well Configurations 3-68
3-16 Design Specifications for Monitoring Wells 3-70
3-17 Selected Groundwater Sample Withdrawal Methods 3-78
3-18 Containers, Preservation, and Holding Times 3-79
3-19 Potential Sources of Factors Affecting
Groundwater Sampling Validity 3-85
4-1 Criteria for Well Selection 4-4
4-2 The Influence of Site Geology on the Selection and
Performance of Leachate Migration Control Technologies. . . 4-21
4-3 The Influence of Site Hydrology on the Selection
and Performance of Leachate Migration
Control Technologies 4-22
4-4 The Influence of Plume Characteristics on the
Selection and Performance of Leachate Migration
Control Technologies 4-24
4-5 The Influence of Surface Conditions on the
Selection and Performance of Leachate Migration
Control Technologies 4-26
xvn
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TABLES (Continued)
Number Page
4-6 Factors Affecting the Relative Reliability of
Leachate Migration Control Technologies 4-32
4-7 Factors Affecting the Implementability of
Leachate Migration Control Technologies 4-34
4-8 Selected Regulatory Requirements for Aquifer
Restoration Activities 4-38
4-9 Relative Costs and Key Cost Hens of
Leachate Migration Control Technologies 4-42
4-10 Summary of the Four Basic Leachate Migration
Control Technologies 4-44
5-1 Simplifying Assumptions for Steady State Equations. ..... 5-12
5-2 Values of W (M) for Various Values of M 5-14
5-3 Values of the Function W (MA«T) f°r Water-Table
Aquifers 5-16
5-4 Values of the Function W (Mn,r ) for Water-Table
Aquifers 5-17
5-5 Values of Functions W (M, r/B) and W (//., r/B) for
Various Values of M or//. 5-20
5-6 Values of the Function H (M,0) 5-22
5-7 Radius of Influence for Various Unconsolidated
Materials 5-33
5-8 Methods for Calculating the Radius of Influence (R ) 5-37
5-9 Data Requirements for Well System Design 5-46
5-10 Criteria for Well Selection 5-47
5-11 Deep Well Components and Selection Criteria 5-49
5-12 Loss of Head Due to Friction in Smooth Pipe 5-54
5-13 Approximate Head Loss Equivalents for Pipe Fittings 5-55
5-14 Recommended Well Casing Diameters 5-57
5-15 Well Screen Types 5-59
xvn
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TABLES (Continued)
Number Page
5-16 Well Screen Materials and Applications 5-60
5-17 Criteria for Well Screen Length Selection 5-62
5-18 Recommended Entrance Velocities for Various Filters 5-63
5-19 Open Areas of Commercially Available Wellscreens 5-64
5-20 Filter Selection Criteria 5-68
5-21 Well Screen Selection for Stratified Soils 5-70
5-22 Recommended Casing and Riser Sizes for
Ejector Systems 5-73
5-23 Ejector System Performance Specifications 5-75
5-24 Advantages and Disadvantages of Pumping Methods 5-83
5-25 Radius of Influence Equations 5-85
5-26 Methods of Well Installation 5-97
5-27 Range of Costs for Selected Pumps and Accessories 5-113
5-28 Typical Range of Costs for Well Screens and
Wellpoints 5-114
5-29 Average Drilling Costs (1981) for Unconsolidated
Materials 5-115
6-1 Values for Equivalent Depth d(m) for r, = 4 inches
Calculated for Different Values of Drain Spacing (L)
and Saturated Thickness Below Drains (D) 6-26
6-2 Minimum Grades for Various Pipe Sizes 6-41
6-3 Critical Velocity of Various Soil Types 6-41
6-4 Drain Grades for Selected Critical Velocities 6-42
6-5 A Classification to Determine the Need for
Drain Filters or Envelopes 6-50
6-6 Approximate Hourly Production in Cubic Yards
for Ladder and Wheel Trenchers Operating at
100 Percent Efficiency 6-67
xv m
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TABLES (Continued)
Number Page
6-7 Theoretical Hourly Production of a Hydraulic
Backhoe 6-69
6-8 Production Costs for Trenching Using Backhoes 6-69
6-9 Theoretical Hourly Production Rate of a
Dragline Excavator 6-70
6-10 Typical Characteristics of Self-Priming Centrifugal
Pumps 6-76
6-11 Representative Specifications and Performances
of Centrifugal Submersible Pumps 6-77
6-12 Representative Specifications and Performances
of Diaphragm Pumps 6-78
6-13 Cost for Open Pumping 6-79
6-14 Specifications for Stabilizing Gravel . 6-82
6-15 Installation Costs for Drainage Pipe 6-85
6-16 Estimated Costs and Outputs for Backfilling
by Dozer 6-90
6-17 Installed Costs for Manholes 6-92
7-1 Summary of Slurry Wall Configurations . 7-11
7-2 Specified Properties of Bentonite and Cement
Bentonite Slurries 7-13
7-3 Comparison of Selected Properties of Clays 7-15
7-4 Typical Compositions of Cement-Bentonite Slurries 7-20
7-5 Types of Physical Constraints and Their Effects
on Slurry Wall Construction 7-46
7-6 Excavation Equipment Used for Slurry Trench
Construction 7-49
7-7 Materials Quality Control Program for Soil-
Bentonite Walls 7-54
7-8 Materials Quality Control Program for Cement-
Bentonite Walls 7-62
xix
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TABLES (Continued)
Number Page
7-9 Relation of Slurry Cut-Off Wall Costs per Square
Foot as a Function of Medium and Depth 7-64
7-10 Approximate Costs of Grouts 7-105
8-1 Hazardous Soil Contaminants at Superfund Sites 8-5
8-2 Potential Useful Soil In Situ Treatment Processes 8-7
8-3 BOD5/COD Ratios for Various Organic Compounds 8-12
8-4 Refractory Indices for Various Organic Compounds 8-13
8-5 Problem Concentrations of Selected Chemicals 8-24
8-6 Solubilities of Oxygen in Water at Selected
Temperatures 8-31
8-7 Oxygen Solubility in Water in Equilibrium with
Oxygen Gas at One Atmosphere 8-35
8-8 Capital Costs for Ozone Treatment 8-41
8-9 Power Costs for Ozone Generator System 8-42
8-10 Composition of Basal Salts Medium 8-45
8-11 Basal Salt Medium Used by GDS Inc 8-45
8-12 Surfactant Characteristics 8-54
8-13 Environmental Chemical Properties of Selected
Commercial Surfactants 8-56
8-14 Waste Chemical Classes Ability to React with
Hydrogen Peroxide .... 8-59
8-15 Waste Chemical Classes Ability to React with Ozone 8-62
8-16 Waste Chemical Classes Ability to React with
Hypochlorites 8-64
8-17 Costs for In Situ Detoxification of Cyanide 8-67
8-18 Hydrolysis of Alkyl Hal ides 8-72
8-19 Hydrolysis of Epoxides 8-73
xx
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TABLES (Continued)
Number Page
8-20 Hydrolysis of Esters 8-74
8-21 Hydrolysis of Amides 8-76
8-22 Hydrolysis of Carbamates 8-77
8-23 Hydrolysis of Phosphoric and Phosphonic Acid Esters 8-78
8-24 Hydrolysis of Miscellaneous Compounds Including
Pesticides 8-79
8-25 Results of Chemical Analyses of Greensand
Filtration of Pigeon Point Landfill Leachate 8-87
8-26 Chemical Composition of Glauconites from the
Delaware Coastal Plain 8-88
8-27 Estimated Costs for Installation of a Permeable
Treatment Bed 8-93
xxi
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SYMBOLS
2
A Area or cross-sectional area (length )
2
Ad Area affected by drain (length )
2
A Area normal to flow direction (length )
2
AQ Open area of well screen (length )
A Cross-sectional area of pipe
2
A Cross-sectional area of sample (length )
o
2
A Cross-sectional area of a well (length )
w
a Geometric factor for radial flow (dimensionless) or volume fraction
(dimension less)
B Width of area drained by a drainage pipe (length)
b Width of trench (length)
C Concentration of a substrate (wt/vol)
Cf Final concentration of substrate (wt/vol)
C.j Initial concentration (wt/vol)
C. Concentration of oxygen in a liquid (wt/vol)
C Microbe concentration (wt/vol)
C Concentration of oxygen in a liquid (wt/vol)
Concentration of substrate supporting half of the maximum growth
rate (wt/vol)
C Uniformity coefficient (dimensionless)
c Constant
D Height of the drain above an impervious layer (length)
D, Thickness of water bearing zone below drain (length)
xxii
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Dd Distance downgradient from the drain where the water table is
lowered to a desired depth (length)
De Difference in water elevation in a pumped sump (length)
D Distance from the water level in a drain to the next soil layer
(length)
D Distance a leachate plume migrates from source (length)
Dr Aquifer thickness where radial flow occurs (length)
DU Effective distance of drawdown upgradient (length)
DV Aquifer thickness where vertical flow occurs (length)
D Diameter (length)
A
D, Distance from the ground surface to water table at the drain
(length)
D? Distance from the ground surface to water table at distance D, from
the drain (length) °
d Equivalent depth of aquifer below drain (length)
dg Effective drain depth (length)
E Elevation (length)
Ej Groundwater elevation at start of aerated flow (length)
E£ Groundwater elevation at end of aerated flow (length)
E Activation energy
e Base of natural logarithms
G Geometric factor for radial flow
o
g Acceleration of gravity (length/time )
H Total hydraulic head (length) or
Maximum height of water table above the drain midway between drains
(length)
H^ Henry's law constant for oxygen
H-h Drawdown (length)
h Height of water table after drawdown (length) or depth to bottom
separation
xxm
-------
h. Desired depth of water table after draining (length)
h Final head (length)
tv Total frictional losses in well system expressed as head (length)
h. Initial head (length)
h. Total vertical lift in a well (length)
h Head difference across sample (length)
h. Total dynamic head in a well (length)
h Velocity head required to produce flow in a well (length)
h Height of water in the well (length)
h Optimum drawdown (length)
h,-h? Difference in water table levels at two points on a line parallel
to flow direction (length)
I Hydraulic gradient
i Inflow rate (vol/time)
K Hydraulic conductivity (length/time)
K. Horizontal hydraulic conductivity (length/time)
K. Hydraulic conductivity of lower aquifer (length/time)
K Hydraulic conductivity of aquifer with radial flow (length/time)
K Hydraulic conductivity of aquifer with vertical flow (length/time)
K Weighted average hydraulic conductivity (length/time)
K Vertical hydraulic conductivity (length/time)
K, Hydraulic conductivity of aquifer above drain (length/time)
Kp Hydraulic conductivity of aquifer below drain (length/time)
K1 Hydraulic conductivity of leaky confining layer (length/time)
p
k Intrinsic permeability (length )
k Acid catalyzed hydrolysis constant
a
k Apparent constant
xxiv
-------
k. Base catalyzed hydrolysis constant
k. Distribution coefficient
k Roughness factor
k.. Neutral hydrolysis constant
k Partition coefficient of substance
L Influence of relief drain or drain spacing (length)
L Length of aerated zone (length)
a
L Downslope influence of drains (length)
Lf Length of groundwater flow path between h, and h^ (length)
LS Length of sample (length)
1 Length of well screen as a fraction of aquifer thickness
(dimensionless)
m Aquifer thickness (length)
m Saturated thickness at aquifer (length)
m1 Thickness of leaky confining layer (length)
N Roughness coefficient
n Effective porosity (percent)
P Air pressure (wt/vol)
P Average earth mass density
AP Pressure in excess of the overburden
p Density (wt/vol)
Q Discharge (vol/time)
Q Gravity discharge capacity (vol/time)
Qj Recharge rate of injection well (vol/time)
Qm Pumping discharge rate (vol/time)
Q Pumped discharge capacity (vol/time)
O Discharge rate of pumping well (vol/time)
xxv
-------
Q Specific capacity of fully penetrating well (vol/time)
Q Specific capacity of partially penetrating well (vol/time)
Q Expected or desired yield of well (vol/time)
Q, Discharge of high pressure supply water (vol/time)
Q? Discharge of groundwater to supply line (vol/time)
q Rate of water flow, specific discharge, or drainage coefficient
(length/time)
R Hydraulic radius (length)
R Radius of influence of well (length)
R Gas content (vol)
R. Initial reaction rate
R Shear rate
Rt Temperature correct reaction rate
r Distance from well to measured drawdown (length)
r. Drain radius (length)
r Equivalent well radius (length)
r Well radius (length)
S Coefficient of storage
S Sump storage volume (vol)
S Specific yield (percent)
J
s Slope of the angle between the water table and horizontal plane
2
T Transmissivity (length /time)
T Absolute temperature (degrees)
a
T Initial gel strength
TQ Yield stress
T Shear stress
t Time (time)
xxvi
-------
t Cycle time of pump (time)
L*
t Pumping time (time)
t Residence time (time)
Half life (time)
U Uniformity coefficient
U Maximum growth rate of microbes
u Wetted perimeter and drain (length) or specific weight of fluid
V Velocity or average linear velocity (length/time)
V, Water velocity in permeable treatment bed (length/time)
V Groundwater velocity (length/time)
V Entrance velocity (length/time)
v Viscosity
v Apparent viscosity
a
v Plastic viscosity
W(u) Well function (dimensionless)
W, Thickness of permeable treatment bed
x Length of drain pipe (length)
Yd Yield coefficient
Volume fraction
/LU, fj.fr, T Well functions
xxvi i
-------
CONVERSION FACTORS
Length-
1 inch (in) = 25.4 millimeters (mm).
1 foot (ft) = 0.3048 meters (m).
1 yard (yd) = 0.9144 meters (m).
1 mile (mi) = 1.6093 kilometers (km)
Area-
2 2
1 square inch (in ) = 645.2 square millimeters (mm )
2 9
1 square foot (ft ) = 0.0929 square meters (m).
9 2
1 square yard (yd") = 0.8361 square meters (m ).
2 2
1 square mile (mi ) = 2.59 square kilometers (km ).
1 acre (ac) = 0.4047 hectares (ha).
Volume-
3 3
1 cubic inch (in ) = 16.39 cubic centimeters (cm )
1 cubic foot (ft3) = 0.02832 cubic meters (m3)
= 28.32 liters (1)
= 0.7646 cubic meti
= 764.6 liters (1)
ubic meti
= 3.785 liters (1)
= 1,233.5 (
= 1,233,000 liters (1)
= 2,451.3 cubic
= 2,450,000 liters (1).
1 cubic yard (yd3) = 0.7646 cubic meters (m3)
1 gallon = 0.0038 cubic meters (m3)
1 acre-foot (af) = 1,233.5 cubic meters (m )
2
I second foot day (sfd) = 2,451.3 cubic meters (m )
xxvi11
-------
Mass-
1 ounce (oz.) = 28.35 grams (g).
1 pound (Ib.) = 0.4536 kilograms (kg).
1 ton = 907.2 kilograms (kg)
= 0.907 tonne (t)
Velocity, linear-
1 foot per second (ft/sec) = 30.48 centimeters per second (cm/s)
1 mile per hour (mph) = 1.609 kilometers per hour (km/hr) =
4.47 x 10" meters per second (m/s).
Flow Rate (volumetric)-
1 gallon per minute (gprn) = Q.0631 liters per second (1/s) = 6.309 x 10
cubic meters per second (m /s) = 5.451 cubic meters
per day (m /d)
1 cubic foot per second (ft3/s) = 0.02832 cubic meters per second (nr/s)
= 28.32 liters per second (1/s).
Transmissivity-
7 2
1 square foot per day (ft /day) = 0.09289 square meters per day (m /d).
1 gallon per day per foot (gpd/ft) = 0.01242 square meters per day
(mVd).
Viscosity-
_c y
poise = 1.45 x 10~ pounds (weight) seconds per square inch (Ib.sec/in )
Density-
1 pound per cubic foot (pcf) = 16.02 kg/m
Pressure-
2
1 pound per square inch (psi) = 70.31 grams per square centimeter (g/cm )
= 68948 dynes per?square centimeters (dynes/cm ) = 6894.8 newtons per
square meter (N/m ).
xxix
-------
Force-
1 pound (Ib, weight) = 4.448 meters = 4.448 x 10 dynes
= 33.36 poundals.
Temperature-
Degrees Celsius (°C) = 5/9 (°F-32)
Degrees Kelvin (°K) = degrees °C + 273.16
Work, Energy, Power, Quantity of Heat-
1 kilowatt-hour = 3.6 x 106 joules = 3,409.52 British Thermal Units
(Btu).
1 horsepower = 0.746 kilowatt (kW).
xxx
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ACKNOWLEDGEMENTS
This document was prepared by SAIC/JRB for EPA's Office of Research and
Development in partial fulfillment of Contract No. 68-03-3113, Task 38. Ms.
Naomi Barkley, of the Hazardous Waste Engineering Research Laboratory, Land
Pollution Control Division, served as the EPA Project Officer. Dr. Edward
Repa and Mr. Charles Kufs were Task Managers and principal authors for
SAIC/JRB. Other major contributors from SAIC/JRB included Mr. Paul
Rogoshewski, Ms. Kathleen Wagner, Mr. Edward Tokarski, Ms. Marjorie Kaplan,
Mr. Philip Spooner, and Ms. Constance Spooner.
Preparation of this handbook was aided greatly by the constructive
contributions of the following reviewers:
Mr. Donald Sanning U.S. EPA HWERL
Mr. Richard Stanford U.S. EPA OERR
Mr. Joseph Keely U.S. EPA Robert S. Kerr Environmental
Research Laboratory
Dr. David D'Appolonia ECI
Dr. Wayne Pettyjohn Oklahoma State University
Dr. Rip Rice Rip Rice, Inc.
Mr. William Walker Walker Wells, Inc.
Appreciation is also extended to other numerous individuals from SAIC/JRB and
Federal, state, and industrial organizations who contributed or were contacted
on matters related to development of this handbook.
xxxi
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CHAPTER 1
INTRODUCTION
Cleaning up the thousands of hazardous waste sites identified across the
United States is a serious environmental challenge. Of particular importance
is protecting and cleaning up contaminated groundwaters that now serve or
could serve as drinking water supplies. The magnitude of this problem is
illustrated by the fact that nearly 70 percent of sites surveyed in 1983 and
now undergoing corrective action have contaminated groundwaters (JRB
Associates and Environmental Law Institute, 1983).
Contaminants can enter the groundwater system through a variety of
mechanisms. Some disposal sites contain fine solids or sludges that can be
transported via infiltrating solutions. The amount of solids transported
depends on the size of the particles, the size and interconnectedness of void
spaces, the speed of percolating solutions, and the surface area of waste in
contact with the solutions. The maximum size of a particle transported is a
function of pore space diameter and percolation rate. As the rate of solution
movement increases, the solution's capacity to carry solids in suspension
increases. When large areas of fine particle wastes are in contact with per-
colating solutions, a greater opportunity for transport of solids exists.
In areas where the geologic material between the waste site and the
groundwater table is permeable, movement of liquid waste to the groundwater
regime is possible. The amount of direct liquid waste seepage is highest
when:
• Highly permeable material exist within the waste site and between the
site and the groundwater table
1-1
-------
t The distance separating wastes and groundwater is minimal, or the
wastes are in direct contact with the groundwater
• The wastes consist of uncontainerized liquids that have a higher
density and lower viscosity than water.
Waste organic and inorganic solutions can dissolve other waste constit-
uents, thereby greatly increasing the mobility of some contaminants.
Solvents, soaps, and emulsifying agents can also dissolve or suspend waste
constituents in infiltrating solutions. Therefore, water insoluble wastes
should not necessarily be considered immobile.
By far the most predominant means of contaminant movement to the ground-
water system is via dissolution by infiltrating precipitation. Wastes dis-
solved into infiltrating solutions are carried through the waste site and
underlying soil, along solution channels or seepage paths, and to the
groundwater table.
Solutions resulting from the dissolution of waste constituents into
percolating water are called leachate. Leachate concentration depends in part
on waste constituent solubility, volume of precipitation, and length of time
the infiltrating solution is in direct contact with waste. Certain
constituents are more soluble in rainwater than distilled water because
rainwater is chemically different from distilled, water. For example, the pH
of distilled water is 7.0, while rainwater in nonurban, nonindustrialized
areas commonly has a pH between 5 and 6 (Freeze and Cherry, 1979). Part of
this acidity is caused by the presence of dissolved C(L which produces up to
one percent carbonic acid (Gilluly, et al., 1975). The pH of acid rain can be
as low as 3 to 4 because of high sulfur oxides or nitrogen oxides in air which
dissolve to produce sulfuric and nitric acids in the rainwater (Freeze and
Cherry, 1979). In addition to low pH, rainfall also contains dissolved solids
and can entrain microbes and other matter as it percolates through soils or
wastes. Thus, wastes that do not have a high solubility in distilled water
under laboratory conditions may become mobile in rainwater.
1-2
-------
The final means by which contaminants can be leached from a waste dispos-
al site is dissolution by groundwater. While this route is important only
when wastes are in direct contact with groundwater, this route can be signifi-
cant because of the large volumes of water that can become contaminated.
Once in groundwater, contaminants are not diluted and flushed from the
system to the extent they would be in surface water because flow rates-are
slower and flow paths more tortuous. As a result, contaminants tend not to
disperse but form slugs or plumes. Contaminant concentrations throughout a
leachate plume can vary considerably but are generally highest in the center
of the plume and closest to the contaminant source area. The boundaries of a
leachate plume are usually indistinct, which makes vertical and horizontal
plume delineation and control a very complicated. process.
Leachate plume management has been complicated to some extent by a lack
of understanding of plume dynamics and the various site remediation options
available. Generally, methods for controlling the migration of a leachate
plume can be placed into one of four categories--groundwater pumping,
subsurface drains, low permeability barriers, and innovative technologies.
Under the Comprehensive Environmental Response, Compensation, and Liability
Act of 1980 (CERCLA), also known as "Superfund," the Environmental Protection
Agency is implementing a program to assess and disseminate information on
these technologies and procedures related to plume management.
The purpose of this handbook is to serve as a summary reference on
factors that affect leachate plume movement and considerations necessary to
develop effective plume management plans. The handbook consists of eight
chapters as follows:
• Chapter 1 -- Introduction.
• Chapter 2 — Plume Dynamics. Includes discussions of the factors that
affect a plume's movement and shape in an aquifer.
• Chapter 3 -- Plume Delineation. Includes discussions of plume
delineation techniques such as the use of models, and indirect and
direct methods.
1-3
-------
t Chapter 4 -- Plume Control Technologies. Includes brief discussions
about various plume control technologies including groundwater
pumping, drains, barriers, and innovative technologies, and on the
selection of a plume control technology to best meet site conditions.
• Chapter 5 -- Groundwater Pumping. Includes discussions of well
hydraulics; well construction and installation; and system design,
operation, and maintenance.
• Chapter 6 -- Subsurface Drains. Includes discussions of drain hydrau-
lics; drain components and materials; and system design, installation,
and maintenance.
t Chapter 7 -- Low Permeability Barriers. Includes discussions of
barrier placement; types of barrier material; and barrier design and
installation.
• Chapter 8 -- Innovative Technologies. Includes discussions of
bioreclamation, in-situ chemical treatments, and emerging
technologies.
Many topics related to controlling the migration of leachate plumes are
not covered in this handbook. Treatment of leachate is discussed by EPA
(1980), De Renzo (1978), Hammer (1975), and Liptak (1975). Complementary
remedial action technologies are summarized by EPA (1982a). Procedures for
safeguarding worker health and safety are addressed by Heiss (1980), Lippitt,
et al. (1982), Allcott, et al. (1981), Turpin (1981), Weitzman and Cohen
(1981), and NEIC (1980). The role of community relations during site
remediation is discussed by Cohen, et al . (1981), Neuman and Drake (1982), and
ICF (1982). Finally, case studies of leachate plume control have been
described by SCS (1980), JRB and ELI (1983), and Nielson (1982).
1-4
-------
CHAPTER 2
PLUME DYNAMICS
Leachate plumes are subject to various processes which alter their size
and shape, and their direction and flow rate. These processes are related to
the nature of the contaminants; the groundwater flow patterns existing in the
area of concern; and the interactions between the contaminants, groundwater,
and the geologic media through which the plume moves. While all the effects
of these processes are not known precisely and are still being researched,
general relationships have been observed or can be approximated. The purpose
of this section is to discuss, in general terms, the processes affecting plume
movement.
When leachate seeps into groundwater, the leachate tends not to disperse
evenly. Instead, variations occur in contaminant concentration depending on
position within the groundwater system. The highest concentrations of
contaminants are generally found close to the source and near the center of
the plume where dilution is minimal. As plume size and degree of dilution
increase, the relative contaminant concentration decreases. However, the
volume of groundwater affected increases, and thus the cost for treating
affected water also increases.
Among other factors, plume size is determined by hydrologic factors, age
of the contaminant source, and rate of contaminant release. The width and
depth of a plume are directly influenced by the width and thickness of the
aquifer, the aquifer's flow characteristics, and the properties of the con-
taminants. In most instances, plumes move in the direction of groundwater
flow and are elongated in that direction. Some hydrogeologic conditions,
however, cause anomalous plume migration patterns to develop. Plume shape is
2-1
-------
a reflection of the flow patterns within the aquifer and the chemical charac-
teristics of the leachate. In general, groundwater flow patterns tend to
exert the greatest influence on plume shape and path.
Factors affecting plume migration can be grouped into three broad areas:
• Groundwater flow patterns--factors that alter groundwater flow
patterns
• Leachate characteristics—the properties of the hazardous materials
• PI time and geologic media interactions—the reactions between waste and
contaminant constituents and the geologic materials.
These topics are discussed in greater detail in the following sections.
2.1 Groundwater Flow Patterns
Groundwater flow patterns probably exert the greatest influence on
leachate migration of all the factors affecting plume movement. This factor
is probably the best understood of all the factors. Nevertheless, the site-
specific influence of groundwater flow on plume movement at individual hazard-
ous waste disposal sites is often difficult to assess because of variations in
site conditions. Variations can be a result of:
t Subsurface hydrogeologic conditions
• Surface hydrologic conditions
• Human activities.
In order to understand the effects of these conditions, certain hydro-
geologic concepts must be understood. These concepts are outlined in the
following section.
2.1.1 Basic Hydrogeologic Concepts
Precipitation falling on land can run off into surface water bodies,
directly evaporate, be taken up and used by vegetation (transpiration), or
2-2
-------
infiltrate into the subsurface. Once in the subsurface, water may be forced
back to the surface and evaporated through capillary action, may be taken up
by deeper root systems and transpired, or may be transported downward under
the influence of gravity. Water in the unsaturated material will move
downward until it reaches the saturated zone where all available pore space
(i.e., openings between particles making up unconsolidated materials, or
cracks, fissures, openings in consolidated rock) are filled with water; These
processes and their interactions are termed the hydrologic cycle.
Aquifers are subsurface water bearing zones capable of supplying water in
usable quantities (Gary, et al., 1974). Both unconsolidated and consolidated
(i.e., rock) materials can function as aquifers. Groundwater flows through
voids within the aquifer, and the ratio (usually expressed as a percentage) of
the volume of voids in a material to the total volume of material is termed
porosity (Gary, et al., 1974). Table 2-1 lists some typical ranges of values
for porosity for different geologic materials. Porosity and rate of flow are
not necessarily proportional; a sample with a high porosity may not allow the
passage of water unless pore spaces are interconnected. An example of this is
cork, which has a high porosity yet is relatively impermeable. In addition,
some highly porous, well interconnected samples (e.g., clays) still do not
allow for the passage of water for other reasons. The effective porosity (n)
refers to the ratio of the volume of interconnected pore spaces to total
volume, and is equal to the ratio of the volume of liquid that a sample will
yield under specific conditions after it is saturated (Freeze and Cherry,
1979). Groundwater flows through the interconnected pores, fractures, joints
or openings that provide a flowpath.
A measure of the rate of groundwater flow is the hydraulic conductivity
(K). Hydraulic conductivity is the volume of water that will move per unit
time under a unit hydraulic gradient through a unit area measured at right
2-3
-------
TABLE 2-1.
RANGE OF POROSITY VALUES FOR VARIOUS GEOLOGIC MATERIALS
ro
Geolog ic
Material
Gravel , mixed
Gravel , well sorted
Gravel and sand, mixed
Sand, mixed
Coarse sand
Mediun sand
Fine sand
Silt
Clay
Glacial till
Carbonate Rocks
(e.g., limestone)
Sandstones
Shales
Crystal 1 ine Rocks
(i .e., igneous
and met amor phic
rocks)
highly fractured
relatively unfractured
Freeze and
Cherry (1979)
25-40%
25-50%
35-50%
40-70%
0-50%
5-30%
0-10%
5-50%
0-10%
Fetter (1980) Pettyjohn (1975) Linsely,
et al. (1975)
25%
25-50%
20-35% 20%
35-40% 35%
39-41%
41-48%
44-49%
35-50%
33-60% 50% 50%
10-20%
1-30% 5%
3-30% 15-20% 15%
0-10% 5%
1%
Davis and
De Wiest (1966)
25-38%
33%
36-42%
45-52%
49-51%
48-67%
2-16%
12-29%
36-40%
14%
-------
angles to the direction of flow (Freeze and Cherry, 1979). K has the
dimensions of length divided by time (commonly expressed in feet/day or
centimeters/second). A related term is intrinsic permeability (k). Intrinsic
permeability is a property of the media through which groundwater flows, has
the dimensions of length squared, and is a measure of the resistance to fluid
flow through the media. Of the two, K is more useful for studying plume
dynamics and delineation. Table 2-2 lists ranges of values for K for various
geologic materials.
While intrinsic permeability (k) is a property of the media only,
hydraulic conductivity (K) depends on both the media and the fluid. For
example, a material with a hydraulic conductivity for pure water of 10 ft/day,
will have a lower hydraulic conductivity for a more viscous fluid. This is
one reason leachate and waste contaminants may move slower than groundwater.
For most groundwater situations the influence of changes in liquid properties
is usually slight, and can be ignored (Freeze and Cherry, 1979). However,
this may not be the case in highly concentrated or multiphase plumes.
When the density of fluid varies significantly from that of pure water,
hydraulic conductivities can be calculated to account for these density
changes. Hydraulic conductivity (K) can be redefined as follows (Jorgensen,
et al., 1982):
K = kpg/v = ku/v
where:
k = intrinsic permeability
p = density of fluid
g = acceleration of gravity
u = specific weight of fluid
v = viscosity of fluid
The new K value then can be used in subsequent calculations,
2-5
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TABLE 2-2
RANGE OF HYDRAULIC CONDUCTIVITY VALUES
FOR VARIOUS GEOLOGIC MEDIA (After Freeze and Cherry, 1979)
Geologic
Material
Gravel
Sand, well sorted
Silty sand
Silt
Clay, unweathered
Glacial till
Carbonate rocks
Sandstones
Shales
Crystalline rocks
Highly fractured
Relative unfractured
cm/sec
lo-1 -
io-4 -
io-5 -
io-7 -
io-10 -
io-10 -
io-7 -
10"8 -
io-11 -
io-6 -
in-12 -
Hydraulic
IO2
1
io-1
io-3
io-7
io-4
1
io-4
io-7
1
io-8
Conductivity
gal /day/
IO4
10
1
10-2
io-5
c
10 D
io-3
io-3
10'6
10"1
io-7
sq.ft.
-IO7
-IO5
- IO4
-IO2
-io-2
- 10
- IO5
- 10
-io-2
- IO5
-io-3
1 gal/day/sq.ft. = 1.74 x 10"6 ft/day
1 gal/day/sq.ft. = 4.72 x 10"5 cm/sec
2-6
-------
The average linear velocity (V ) with which groundwater flows in a. system
is related to aquifer properties through a set of empirical relationships
named after their discoverer, Henri Darcy. Darcy's law, in its most useful
form, states:
Vg = KIA/n.
where: V = average linear velocity of groundwater
K = hydraulic conductivity
I = hydraul ic -gradient
A = cross sectional area (usually 1)
n = effective porosity.
The velocity calculated using Darcy's law is not a measure of the
velocity of a water molecule as it moves within the void spaces of an aquifer;
rather, it is the average rate of movement between two distant points along
the same groundwater flowline in an aquifer. Darcy's law is important in
delineating plumes because if V and the time (t) from which contaminants
entered groundwater are known, then the distance that groundwater, and theo-
retically contaminants, will have moved over that time period (i.e., assuming
non-degradation of contaminants) can be estimated.
In order to use Darcy's law in a specific situation, the hydraulic gradi-
ent (I) must be known. Hydraulic gradient is the change in head per unit
distance in the direction of maximum rate of decrease in head (Gary, et al.,
1974). Hydraulic head (H) is the height above a datum (usually sea level) to
which a column of water can be supported by the pressure at that point. The
head represents the energy level of the water at that point. The hydraulic
gradient (I) is the difference in elevation of water levels in any two wells,
A and B, (assuming A and B lie along the path of maximum rate of change in
head), divided by the distance between A and B. The concept of head and
hydraulic gradient are illustrated in Figure 2-1.
Hydraulic gradient can also be expanded to account for the effects
of pressure on groundwater flow (Jorgensen, et a!., 1982). The relationship
2-7
-------
FIGURE 2-1.
EXAMPLE CALCULATION OF HORIZONTAL HYDRAULIC GRADIENT
Total
Head
Pressure
Head
Well A
U—
1000 Ft
Well B
—M
Elevation Head
= 80Ft
Sea Level ^
Horizontal Gradient = (Elevation of the water table in Well A minus the elevation of the water table in Well B), divided by- rhp
horizontal distance between the two wells
[ = 10 Ft./1000 Ft.
I = 0 01 %
2-8
-------
P = pgH can be used to convert from head change equations to pressure change
equations when significant density variations are present. This relationship
points out that one may have zero head gradient but a definite pressure
gradient because of density differences (e.g., Hj = H2, but P^ i- P? because
P! * P2).
Groundwater flows from higher energy levels to lower energy levels in the
direction of decreasing head. In unconfined aquifers (aquifers that do not
have a bed or layer of significantly lower K overlying the aquifer and causing
the water in the aquifer to exist under pressure), the top of the saturated
zone represents the energy level of the water in the aquifer and is called the
water table. The water table often tends to follow land surface topography,
such that the direction of flow is from areas of higher land elevation to
areas of lower elevation. This is not the case in all situations. For
example, in areas of flat terrain, the water table may follow steep bedrock
slopes.
In situations where aquifers are saturated and separated from the land
surface by zones of lower K, the energy level and direction of flow are more
complicated to determine. The low K zones, known as aquitards, increase the
hydrostatic pressure of groundwater in aquifers beneath them by limiting the
ability of this groundwater to move upward. In these confined aquifers, the
energy level of the groundwater will be greater than the saturated thickness
of the aquifer. Water levels in wells penetrating a confined aquifer will
rise above the level of the aquifer/aquitard interface.
An imaginary surface representing the energy level of the groundwater is
known as the potentiometric surface. The potentiometric surface in an
unconfined aquifer is simply the water table. Groundwater always flows from
higher to lower potentiometric elevations and may be totally independent of
surface topography. This concept is illustrated in Figure 2-2. Note that
water levels in wells screened in the unconfined aquifer stand at different
elevations than the water levels in the artesian wells.
2-9
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FIGURE 22.
POTENTIOMETRIC SURFACES IN CONFINED AND UNCONFINED AQUIFERS
ro
t—•
o
Potentiometnc
Surface of
Confined
Aquifer
-------
Groundwater can flow not only horizontally but also vertically. For
example, two wells located close together but screened at different depths may
indicate that there is a vertical hydraulic gradient, and thus a component of
groundwater flow in the vertical direction, by the fact that the elevation of
the potentiometric surface is different for each. This situation is illus-
trated in Figure 2-3. A vertical gradient may also exist between two wells
screened at the same depth and having equal potentiometric surface elevations
if there are sufficient differences in the fluid density in the two wells.
The fundamental problem with delineating and predicting the migration
pattern of contaminated groundwater plumes is that the subsurface environment
is rarely homogeneous. Zones of lower permeability and lower hydraulic con-
ductivity can exist within aquifers, and the shape and configuration of
aquifers may be highly variable. Groundwater and plume flow rates and
directions will vary depending upon the characteristics of the aquifer through
which they are passing. Figure 2-4 illustrates how groundwater velocity can
vary with changing hydrogeologic parameters. Transmissivity (T) in Figure 2-4
is defined as the hydraulic conductivity (K) of the aquifer multiplied by the
saturated thickness of the aquifer (m). Transmissivity is a measure of the
quantity of groundwater flowing through an aquifer.
Variations in aquifer characteristics, primarily with regard to K, are
discussed in terms of homogeneity and isotropy. If K is independent of posi-
tion within an aquifer, the aquifer is homogeneous. If K is independent of
the direction of measurement at a point in an aquifer (i.e., K in the vertical
and horizontal directions are equal), the aquifer is isotropic. Most aquifers
are heterogeneous and anisotropic, (i.e., K varies depending upon the point of
measurement and the direction of measurement).
If an anisotropic aquifer has a greater vertical K than horizontal K, and
all other properties (i.e., hydraulic gradient and porosity) are equal in both
directions, then groundwater would flow faster vertically than horizontally.
A plume in this case would tend to migrate preferentially in the vertical
direction and be limited in horizontal extent. If the aquifer were also
2-11
-------
FIGURE 2-3.
EXAMPLE CALCULATION OF VERTICAL GRADIENT
Aquifer Discharge Region
Aquifer Recharge Region
Well
A
Well
B
560 Ft
500 ft
450 Ft
Well
C
Well
D
550 Ft
500 Ft
450 Ft
Vertical Recharge
Gradient
Elevation of water table in
Well A minus the elevation of
the water table in Well B,
divided by the vertical distance
between the midpoints of the
screens in the two wells.
Aquifer Discharge Region
I = (HA-HBI/ (& -/B)
I = (540-5501/1515-445)
I = -10/70
I = 0.14 (upward flow)
Aquifer Recharge Region
' = '£-J&
I = (540-5301/I515-445)
I = 10/70
I = -0.14 (downward flow)
2-12
-------
FIGURE 24.
EXAMPLE OF HOW GROUNDWATER VELOCITY CAN VARY WITH DIFFERING HYDROGEOLOGIC
CONDITIONS AND WITH DIFFERING SCALES OF OBSERVATION FOR KEY PARAMETERS
PO
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-------
heterogeneous, then depending upon where the plume is located, the preferen-
tial flow paths would change. For example, the vertical K may become less
than the horizontal K or the magnitude of difference between vertical K and
horizontal K may change. The result of heterogeneity and anisotropy is that
groundwater flow paths vary within aquifers and are not perpendicular to water
level contours, thus affecting plume dynamics. For a detailed discussion of
the effect of anisotropy on flow lines in aquifers the reader should refer to
Fetter (1981).
In summary, groundwater flows in aquifers from higher energy levels to
lower energy levels along paths of least resistance. Energy levels are
represented by water levels in wells. For unconfined aquifers, groundwater
flow direction in many cases mimics surface topography while direct measure-
ments are required for confined systems. The rate of groundwater flow can be
calculated using Darcy's law, if hydraulic conductivity (a measure of
rate of flow in an aquifer, and indirectly a measure of resistance to flow),
hydraulic gradient (a measure of the energy level driving the system), and
porosity are known. Groundwater flow is complicated by the heterogeneous and
anisotropic nature of the subsurface environment. This causes changes in
preferential flowpath directions, and in rates of flow, depending upon
position in the aquifer and direction of movement. This is a partial cause
for the uneven distribution of contaminants in groundwater.
General groundwater flow theory is more complicated than explained here.
For example, as described previously, hydraulic conductivity (K) and pressure
(P) within the aquifer can be affected by fluid density and viscosity. These
factors can impact groundwater flow directions and cause plumes to move in
seemingly random directions at variable rates. However, this simplified
discussion should prove adequate to understand the following sections
discussing plume dynamics. Numerous introductory hydrogeology texts and
research papers are available if more information on groundwater flow is
desired.
2-14
-------
2.1.2 Effects of Subsurface Conditions
The size, shape, and flow direction of a leachate plume are strongly
affected by the geometry and uniformity of the aquifer through which it flows.
The presence of geologic discontinuities and hydrologic barriers modify a
plume's shape and flow direction. These factors are discussed below.
2.1.2.1 Aquifer Geometry
In large, homogeneous porous strata, the shapes of water soluble leachate
plumes are relatively easy to predict. Figure 2-5 illustrates the "character-
istic" shape of a water soluble contaminant migrating through a homogeneous
aquifer in the direction of regional groundwater flow. In large aquifers, a
low density (i.e., density less than water) contaminant can form a thin film
over the upper surface of the aquifer unless the spread of the low density
contaminant over the surface is inhibited for some reason. In this case, the
contaminants have been observed to form a mounded layer extending above the
water table (Yaniga, 1982). This situation is most likely to occur in
relatively thin, unconfined aquifers.
Many highly productive aquifers do not have the classical sheet or
tabular shape commonly associated with sedimentary deposits. One of the more
common of these aquifer shapes is the elongate or "shoestring" aquifer, which
typically consists of stream deposits. These aquifers can appear as isolated
pods or lenses; as long, sinuous, and sometimes braided ribbons; as complex
dendroids; or as highly variable belts (Pettyjohn, 1975). This type of
deposit is generally very difficult to trace in the subsurface and can exert a
major impact on plume migration.
Even more complex than shoestring deposits are aquifers having major
fracture sets or solution cavities. In these aquifers, groundwater flowpaths
are usually impossible to identify completely and are generally neither con-
tinuous nor uni-directional. Thus, leachate movement patterns can be highly
variable and often unpredictable.
2-15
-------
FIGURE 2-5.
"CHARACTERISTIC" PLUME OF MISCIBLE CONTAMINANTS IN A HOMOGENEOUS AQUIFER
-------
Aquifer geometry, particularly aquifer thickness, also affects plume
migration rates. In general, as groundwater velocity increases, plume
migration rates also increase. Thinner aquifers having the same hydraulic
gradient and discharge rate as thicker aquifers have faster flow rates because
the same volume of water is forced to move through a smaller volume of
aquifer. Because of this, aquifer size is not a reliable measure of plume
migration rates. In situations where an aquifer decreases in thickness or
"pinches out" in the direction of flow, flow rates can be expected to
increase, increasing the migration rate of plumes. The opposite effect can
occur in an aquifer which increases in thickness downflow.
2.1.2.2 Aquifer Hydraulic Conductivity
Most aquifers do not exhibit constant hydraulic conductivities throughout
their depth, width, and length. In fact, hydraulic conductivities can vary by
orders of magnitude from area to area and commonly diminish with depth into
the aquifer. Changes in hydraulic conductivity may be gradual, or abrupt as
in the case of multilayered aquifers. One example of the effect of nonuniform
hydraulic conductivity on plume movement patterns is shown in Figure 2-6. In
this example, contaminants have moved farthest from the source area in a
deeper, gravel layer having a larger hydraulic conductivity than in the
overlying strata. This is not an uncommon occurrence, and highlights the
importance of understanding site geology prior to designing and implementing
plume sampling programs.
Geologic strata which have been uplifted and tilted can alter or reverse
the direction of plume movement by channelling fluid flow along zones of
secondary porosity (i.e., bedding planes, fractures, faults, or solution
cavities). Possible effects of various types of hydraluic conductivities
caused by secondary porosity are illustrated in Figure 2-7.
Faults are discontinuities in geologic materials that exhibit vertical or
lateral displacement or both of one side of the break with respect to the
other. Faults can be highly permeable or nearly impervious. If the fault has
2-17
-------
FIGURE 26.
EFFECTS OF VARIABLE HYDRAULIC CONDUCTIVITY ON LEACHATE
PLUME MOVEMENT IN UNCONSOLIDATED MATERIAL
(\3
I—•
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-------
FIGURE 2 7.
EFFECTS OF SECONDARY POROSITY ON LEACHATE PLUME MOVEMENT
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a low hydraulic conductivity, the fault can be a subsurface barrier to ground-
water flow and deflect or redirect the regional flow pattern. If the fault
has a higher hydraulic conductivity than surrounding strata, the fault can
channel groundwater either upward or laterally at a higher rate than regional
flow. Highly permeable faults can permit very rapid groundwater flow which
promotes extensive plume migration or expansion within the groundwater system.
Some geologic units have been subjected to various types of stresses that
cause fracturing of the rock. Other units, particular carbonates, can contain
large, interconnected solution cavities. If a sufficient number of fractures
are present, even relatively non-porous rocks can function as aquifers. Frac-
tures and solution cavities can have pronounced effects on leachate plume flow
patterns and rates, as illustrated in Figure 2-7. When these features are
present within an aquifer, solution movement is controlled by the width,
length, frequency, interconnectedness, angle, and surface area of the frac-
tures or cavities. If the fractured rock dips at an angle opposite to the
direction of groundwater flow in the uppermost aquifer, the direction of plume
flow can be altered in the lower units. Fractures or solution cavities can
also channel contaminants from an upper aquifer to a lower one.
Fractures, faults and bedding planes may also act as barriers to flow.
For example, in channels that are "sealed" because of secondary deposition of
clays and silicates, groundwater flow rates can be minimal. These zones can
impede the spread of contaminants significantly.
Flow rates in open fractures or solution cavities can be many times
higher than flow rates in non-fractured media. In unfractured water table
aquifers, flow rates generally range between 3 and 330 feet/year. In
unfractured confined aquifers, groundwater generally flows much more slowly,
between 0.03 and 33 feet/year (Jackson, 1980). Measured rates of plume
migration in unfractured deposits can range from less than 6 feet/year to more
than 4410 feet/year in a very permeable sand and gravel aquifer (Apgar and
Langmuir, 1977; Walker, 1973). In contrast, groundwater tracing studies
conducted in solution cavities through a limestone formation in Jamaica found
groundwater movement through large cavities to be occurring at rates ranging
2-20
-------
from 18,350 to 1,468,000 feet/year (Wedderburn, 1977). From this data, plume
migration rates can be highly variable and certain hydrogeologic conditions at
the site can permit extremely rapid flow rates.
The migration of contaminants in fractured rock can be modified by
adsorption of contaminants onto materials lining fracture surfaces and by
diffusion into the rock matrix. These processes are most influential when
flow velocities are relatively slow and the surface area in contact with the
contaminants is large. Other factors affecting attenuation include the amount
of dispersed clay (for metals) and the amount of organic carbon (for organic
chemicals). Matrix diffusion exerts a greater influence within porous rocks
than relatively non-porous rocks. Aquifers having a high concentration of
adsorptive substances coating the fracture faces are more likely to modify the
characteristics of solutions flowing through the fractures than are aquifers
composed primarily of relatively non-reactive substances. Types of minerals
that can function as adsorptive substances include various oxides, chlorite,
illite, other clay minerals, and chalk, as well as carbonates (Jackson, 1980).
The preceeding discussion focuses on faults and fractures as conduits for
leachate flow to lower permeable units. Leakage to lower aquifers is not
always so rapid or dramatic. Some units classified as aquitards actually
contain hairline fractures, or relatively permeable areas which can allow
contaminants to migrate into lower water bearing zones. Although flow rates
may be extremely slow, a substantial amount of groundwater and leachate may
migrate between aquifers when large areas of an aquitard leak.
Figure 2-8 illustrates one example of how contaminants can penetrate a
leaky aquitard. In this example, contamination in the lower aquifer would
probably not be expected because of the presence of the clay zone. However, a
locally high proportion of sand or hairline fractures in the clay could permit
downward leakage. Once within the lower aquifer system, contaminants could
flow in entirely different patterns than expected within the upper aquifer.
This figure also illustrates the differences that can exist between ground-
water flow patterns within separate aquifer systems at a given locality.
2-21
-------
FIGURE 28.
EFFECTS OF GROUIMDWATER FLOW DIRECTION AND GEOLOGIC HETEROGENEITIES
ON LEACHATE PLUME MOVEMENT
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Leakage through an aquiclude does not necessarily have to be downward.
If the hydraulic pressure (i.e., head) in a serniconfined aquifer were higher
than the head in an overlying unconfined aquifer, groundwater would flow
upward as shown in Figure 2-9. This groundwater influx could cause greater
lateral spreading of the plume, faster groundwater and plume flow, and
accelerated plume dispersion and dilution.
In practice, the presence of leaky aquitards tends to complicate dramat-
ically the prediction of leachate plume migration rates, extent, directions
and flow patterns.
2.1.2.3 Site Geology
Site geology can often have a significant effect on plume migration. At
some sites the pattern of plume migration in the overburden (i.e., unconsoli-
dated materials overlying bedrock) may differ from the pattern in the rock
under the site. An example of this phenomena is shown in Figure 2-10. In
this example, water in the overburden aquifer flows through the wastes forming
leachate which is discharged at a spring or seepage area. The leachate then
flows over land and contaminates a nearby lake. Some of the contaminants in
the lake then enter the permeable sandstone unit underlying the lake, thus
further spreading the zone of pollution. In addition to the leachate that
seeps out of the hillside, a larger volume of leachate migrates downward and
spreads through the permeable sandstone unit beneath the waste site. Thus,
because of the geology at this site, three contaminated aquifers exist—the
overburden, and two separate sandstone units. Figure 2-11 is a map view of
this same example. Situations such as this would be difficult to interpret
correctly unless the effects of site geology on plume migration patterns were
taken into account.
This discussion of the general effects of subsurface geology and
hydrology illustrate some of the types of plume migration patterns that can
occur. These factors and their effects highlight the need for site-specific
2-23
-------
FIGURE 29.
EFFECT OF A HIGH HEAD, SEMI CONFINED UNIT ON LEACHATE PLUME MOVEMENT
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FIGURE 2-11.
EXAMPLE OF THE EFFECTS OF SITE GEOLOGY ON LEACHATE PLUME MOVEMENT
(MAP VIEW)
Sand
stone
rsi
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Sandstone
Sandstone
111
m
';. Waste $&//•/;
; Disposal :;.:i •:;•;••/'.•':.
.;....site..$%y%i
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hydrogeo"logic information when attempting to predict plume movement. In addi-
tion to subsurface conditions, recharge, discharge, and other surface con-
ditions can have a great influence on plume movement. These effects are
described in the following section.
2.1.2.4 Site Hydrology
The hydrology of unconfined aquifers is in many cases controlled by
surface topography. As was described in the section on basic hydrologic
concepts, the water table can be a subdued expression of land surface
elevations, or topography (assuming the conditions previously noted in Section
2.1.1). This generalization is true where the primary form of aquifer
recharge (i.e., the addition of water to an aquifer) is by precipitation
infiltration. The water table will, however, deviate from topographic
patterns at hydrologic boundaries represented by discharge or recharge areas.
The location of a waste site relative to the recharge area of the nearest
aquifer can affect leachate flow. Some waste sites are highly permeable and
allow a considerable amount of recharge (e.g., infiltration of precipitation)
to occur. This recharge can increase the volume of leachate produced, causing
groundwater mounding under the waste site (Freeze and Cherry, 1979). Leachate
entering groundwater in these areas tends to flow radially outward before
gradually conforming to regional groundwater flow (Figure 2-8). In areas
where the aquifer's vertical gradient is upward (i.e., discharge zones),
opposing the downward migration of percolating leachate, mounding will occur
more rapidly. In recharge zones the natural vertical gradient of the aquifer
is downward, causing extensive travel of contaminants into the aquifer, but
minimal mounding.
Floodwaters can also recharge groundwater. The rise in water tables as a
result of flooding can cause groundwater to contact waste not normally
saturated, increasing the rate of leachate generation. Also, rivers at flood
stage can cause redirection of groundwater contaminant plumes because of the
temporary creation of hydraulic divides by riverbank storage effects.
2-27
-------
A third effect of recharge area location on leachate plume migration
patterns is illustrated in Figure 2-12. Sites near recharge areas are likely
to produce plumes that flow for long distances before reaching a natural
discharge point. Conversely, plumes from sites located near discharge areas
pollute a smaller volume of the aquifer before contaminants are dispersed into
surface waters (Fenn, et al., 1980; Lindorff and Cartwright, 1977). At some
sites, only a portion of the leachate plume enters the surface water at
discharge areas, as was illustrated in Figures 2-10 and 2-11.
Recharge and discharge do not typically occur at constant rates over
time. As a result, the potentiometric surfaces of aquifers can and do change,
sometimes significantly. This is especially true of unconfined surface
aquifers which are subject to seasonal changes in precipitation, stream flow,
evaporation and transpiration, snow melt, and other conditions. These changes
can occasionally be of such a magnitude that water table gradients (and hence,
groundwater and pollutant flow directions) are reversed. Thus, these cyclic
hydrologic changes are an important consideration in an aquifer system when
assessing leachate migration patterns (refer to Pettyjohn, 1982, for more
details).
The migration of leachate plumes is controlled by a multitude of
naturally occurring situations and processes. The location of recharge and
discharge areas, for example, can have a large impact on plume dynamics.
Plume migration can also be either slowed or promoted by various human
activities. These human activities are discussed in the next section.
2.1.3 Effects of Human Activities
A number of human activities can modify the flow patterns of both
groundwater and leachate plumes. The types of effects that human activities
can cause include:
• Increasing or lowering the water table
• Modifying water table gradients
• Creating or obliterating recharge and discharge areas
2-28
-------
FIGURE 2-12.
THE EFFECT OF WASTE SITE LOCATION RELATIVE TO LOCATION OF RECHARGE AND DISCHARGE
AREAS ON PLUME MIGRATION
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Permeable
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• Diverting groundwater or plume flow
• Reducing or increasing leachate production
• Containing or removing the plume from the aquifer.
The ways in which human activities accomplish these effects include:
• The use of extraction wells which can cause localized lowering of the
water table, creating cones of depression surrounding the wells.
• The use of injection wells which can cause groundwater mounding,
increased water table elevations, and radial flow away from the wells.
• The use of construction dewatering techniques which can redirect
groundwater movement near the construction site by the installation of
subsurface low permeability barriers, the construction of drainage
ditches, or the use of extensive pumping from extraction wells
surrounding the site.
• The presence of surface impoundments, such as lakes and lagoons, which
can cause groundwater recharge.
t The presence of irrigation or drainage ditches which can cause either
groundwater recharge or discharge.
• The presence of numerous other subsurface man-made features including
mines, tunnels, foundations, and sewer lines.
• The use of various remedial action measures to contain or control
leachate plumes.
Planned human interventions in the groundwater regime of a waste disposal
site (i.e., remedial actions) can cause a significant reduction of hazards to
public health and the environment. The effects of several types of remedial
actions for leachate plume control are summarized in Table 2-3.
Groundwater flow patterns and leachate plume migration rates can also be
inadvertently altered by human activity. Figures 2-13 and 2-14 illustrate a
hypothetical example of several types of human activities and their unexpected
effects on plume migration.
2-30
-------
TABLE 2-3
EFFECTS OF SEVERAL REMEDIAL ACTION MEASURES ON LEACHATE PLUME MIGRATION (After EPA, 1982a)
Remedial Action Measure
General Effects
Effects on
Leachate Plume
Sealing of site surfaces,
grading, revegetating and
diverting surface water
from site
ro
Installing subsurface low
permeability barriers
upgradient from site
2.
1.
2.
Reduces the amount of water
(from rain, snow or surface
run-on) that infiltrates into
the site and flows through
the wastes.
Reduces groundwater recharge
and mounding beneath the site.
Initially diverts uncontaminated
groundwater away from the site
and the plume.
Initially reduces the water table
evaluation on the down-gradient
(waste site) side.
1.
4.
1.
2.
Minimizes the volume of
leachate generated.
May result in a short-term
increase in the concentra-
tion of contaminants in the
plume because less dilution
will occur.
Concentrations of water
soluble constituents from
above the water table
should be reduced in the
leachate because solub111-
zation is less likely.
Radial spread of the plume
beneath the landfill will
be reduced.
Initially reduces the like-
hood of horizontal ground-
water flow through the
waste site bottom.
Initially reduces the
volume of groundwater
exposed to the leachate.
-------
TABLE 2-3 (continued)
Remedial Action Measure
General Effects
Effects on
Leachate Plume
Installing of subsurface low
permeability barriers
upgradient from site (cont'd)
3. Initially reduces the flow rate
on the downgradient side
because the hydraulic gradient
is reduced.
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3. Temporarily lessens the
potential for leachate
dilution
4. May temporarily reduce the
severity of conditions
within the lower parts of
the waste site.
5. Should temporarily reduce
amount of dissolution
occurring (if waste was
previously in contact with
groundwater).
6. Reduces the flow rate of
the plume.
7. Slightly reduces the degree
of plume extent.
8. May result in a short-term
increase in the concentra-
tion of contaminants in the
plume because less dilution
will occur.
9. Slower flow rates can allow
additional attention and
degradation to occur.
-------
TABLE 2-3 (continued)
Remedial Action Measure
General Effects
Effects on
Leachate Plume
Installing subsurface,
low permeability barriers
downgradient from the site
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Installing subsurface, low
permeability barriers completely
surrounding the site
1. Minimizes groundwater movement
downgradient from barrier.
2. Causes rise in water table
elevation upgradient from wall
unless groundwater pumping or
subsurface drains are used.
1. Isolates groundwater in the
site area from regional ground-
water.
2. Reduces groundwater velocity.
1. Prevents longitudinal
migration of plume.
2. Causes lateral migration of
the plume along the sub-
surface barrier unless
groundwater pumping sub-
surface drains are used.
1. Prevents longitudinal
migration of plume.
2. Minimizes dilution of
leachate and leachate
volume.
3. Reduces cost of treating
contaminated groundwater
by minimizing the amount
of groundwater affected.
4. Allows dewatering of the
site via groundwater
pumping or subsurface
drains.
-------
TABLE 2-3 (continued)
Remedial Action Measure
General Effects
Effects on
Leachate Plume
Permeable Subsurface Treatment
Beds
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Groundwater Pumping
Installing Subsurface
Drainage System
1. Modify groundwater chemistry
by introducing sorptive or
chemically active substances
such as activated carbon or
crushed limestone.
2. May cause local increases in
flow velocity through the beds.
1. Upgradient pumping wells reduce
the amount of groundwater flowing
through the site and lower the
water table elevations locally
2. Downgradient pumping wells
reduce the amount of groundwater
flowing off the site and lower
the water table in localized
areas downgradient of the site.
1. Same as for groundwater pumping.
May reduce concentration of
certain contaminants via
sorption, precipitation
or neutralization.
2. May change pH and solids
content of leachate.
1. Upgradient wells can help
stabilize the plume if
operated carefully
2. Downgradient wells can be
used to extract the plume
if pumped or divert the
plume if used for injection
3. Wells may not affect all of
plume if not properly
sited.
1. Same as for groundwater
pumping but with less of a
change of allowing enhance-
ments to escape from the
system if properly
designed.
-------
TABLE 2-3 (continued)
Remedial Action Measure
General Effects
Effects on
Leachate Plume
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Bioreclamation
1. Allows microbial degradation
within contaminated aquifers.
1. Promotes microbial degrada-
tion of plume constituents
2. Can alter physical and
chemical properties of the
leachate.
-------
FIGURE 2-13.
EFFECTS OF HUMAN ACTIVITIES ON LEACHATE PLUME MOVEMENT
(MAP VIEW)
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O - Urn ontamin.ited Private Wells
-------
FIGURE 2 14.
EFFECTS OF HUMAN ACTIVITIES ON LEACHATE PLUME MOVEMENT
(CROSS SECTIONAL VIEW)
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The waste disposal site shown in map view in Figure 2-13 is apparently
releasing a leachate plume which has contaminated downgradient private wells
in a somewhat anomalous pattern. A nearby lake is inexplicably free of
contaminants as are wells adjacent to the lake and between the lake and the
site. A river system downgradient of the lake is slightly contaminated. Some
wells between the lake and the river are contaminated and some are not. Wells
across the river from the disposal site are uncontaminated with one exception,
a high-production industrial well. The geology of the area is fairly uniform
and does not appear to be the cause of the contamination pattern.
This pattern of surface and groundwater contamination might be confusing
unless changes in site hydrology from human activities are understood. To
assist in understanding this situation, Figure 2-14 illustrates the site in
cross section. In this figure, the lake is recharging groundwater and causing
the plume to deflect downward as it flows beneath the lake. This causes
shallow wells nearest the lake to remain unaffected by the site and wells
extending to greater depths within the aquifer to be contaminated. Contam-
inated groundwater flowing under the lake discharges into the river without
affecting most of the wells located on the opposite bank. Because a large
volume of water is extracted from the deep industrial well a steep gradient
toward this well is formed. This causes groundwater flow toward the well and
pulls the leachate plume beneath the river. Private wells nearby are not
contaminated because smaller volumes of water are withdrawn from these wells.
In addition to the effects of human activities illustrated in the hypo-
thetical example given above, many other activities can channel plumes in
seemingly anomalous patterns. These include constructing high permeability
zones, such as tile drainage systems on low-lying farmland, domestic septic
tank drainfield lines, and underground mine workings; and low permeability
areas, such as foundations, retaining walls, tunnels, and areas where
compacted fill materials have been placed. The effects of these features must
be kept in mind when attempting to delineate the areal extent, and estimate
flow rates, of leachate plumes.
2-38
-------
2.2 Effects of Leachate Characteristics
The types and concentrations of contaminants strongly influence the
physical and chemical properties of leachate. In turn, the flow patterns and
chemical interactions of leachate are determined to a great extent by the
distinctive set of chemical and physical properties of the constituents.
Properties that influence leachate migration patterns are described below.
These properties affect the types of interactions that can occur between
leachate, aquifer, and groundwater constituent. Knowledge of these inter-
actions assists in understanding the data obtained from site investigations
and from studies of leachate behavior within aquifers.
The characteristics of a plume emanating from a waste site are strongly
affected by the wastes present in the site. However, the waste and the
leachate it produces are rarely similar in their chemical and physical
properties because of variable contaminant release rates and the effects of
dilution. Furthermore, the extent of mixing of different types and quantities
of wastes in a disposal site will also influence leachate characteristics.
For this reason, generalizations about the types and concentrations of
specific contaminants found in leachates are difficult. Nevertheless,
leachates from waste organic chemicals probably pose the greatest environ-
mental concern. The approximate percentages of general contaminant types
found at Superfund sites (Table 2-4) indicate that over three-quarters of the
waste constituents in these sites are organics or organics mixed with metals.
The remainder consist of metals and radioactive materials. A survey of
leachate from solid waste disposal facilities, on the other hand, would result
in much different findings. Thus, great variability in plume dynamics can be
expected within any class of leachate-producing facility because of varying
physical and chemical characteristics of the leachates produced.
2-39
-------
TABLE 2-4
DISTRIBUTION OF CONTAMINANT TYPES FOR SUPERFUND SITES
(Based on EPA, 1982b)
Primary Type Percentage of Sites*
of Contaminant
at Site 358 of 418 sites 86 of 114 sites
on expanded on initial
priority listing priority listing
Organics
Solvents, oils, fuels
PCBs
Pesticides
Organics and Metals, mixed
Metals, acids and bases,
and radioactives
54%
(42%)
(7%)
(5%)
26%
20%
44%
(32%)
(6%)
(6%)
36%
20%
*Percentages are based on varying levels of data completeness
2.2.1 Effects of Physical Characteristics
The physical characteristics of leachate that can affect plume migration
include:
o Volume
o Viscosity
o Density
o Solubility
o Degree of dispersion
o Temperature.
These factors, which have varying degrees of influence on migration patterns
and rates, are described below. Other factors which can affect plume
migration, but not discussed herein, include chemical stability, reactivity,
and fluid phases present in the leachate.
2-40
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2.2.1.1 Leachate Volume
The amount of leachate produced at a site affects both the volume and
flow rate of the plume. Larger volumes of leachate can produce larger or more
concentrated plumes or both.
The factors that affect the amount of leachate produced include:
• Duration, frequency, type, and amount of precipitation
• Mean annual temperature, temperature fluctuation, and degree of
insolation
• Surface characteristics of the site including the steepness of the
slopes, type and permeability of the cover material used, presence and
types of vegetation present, length of the slopes, and other features
which impact percolation rates
• Site subsurface characteristics including the presence of water within
the wastes, volume of flow through the wastes, and rate of flow
through the waste site.
Table 2-5 indicates the effects of each of these parameters on leachate pro-
duction.
For some sites, estimations of leachate volume have been made using a
water balance approach, whereby the interactive effects of climatic conditions
such as precipitation, temperature, insolation, and evapotranspiration are
calculated to produce an estimate of the yearly volume of water available for
infiltration. From this value, the amount of runoff is subtracted and the
remaining water is assumed to seep through the waste site to generate
leachate. One of the disadvantages of the water balance method is the
assumption that the waste site is not in direct contact with groundwater.
Thus, the method is not directly applicable to sites where waste material has
been disposed of below the level of the seasonal high water table. Additional
information on using the water balance approach to predict leachate generation
is provided in Fenn, et al. (1975) and Perrier and Gibson (1980).
2-41
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TABLE 2-5
FACTORS AFFECTING LEACHATE VOLUME GENERATION
Factor
Effect
Precipitation
Amount Per Year
Highest 24-hour Rainfall
Mean Annual Temperature
Degree of Insolation
Surface Slope
Cover Permeability
o Greater the amount of precioation, the
greater-volume of leachate generated.
o No real effect, but the greater the number
of short duration, high volume rainstorms,
the less likely that precipitation will
infiltrate to waste, and the more likely
that most will run off as surface flow.
o Higher the mean annual temperature, the
greater the amount of precipitation that
will evaporate and not infiltrate into
waste, reducing the volume of leachate
generation.
o Increasing insolation will increase
evaporation, reducing the quantity of
leachate produced.
o Increasing slope at a disposal site will
promote surface runoff, reducing the volume
of precipitation infiltrating into the
waste and thus reduce the volume of
leachate generated.
o Low permeability cover materials promote
runoff, reducing infiltration and the
volume of leachate generated.
Vegetation
Subsurface Characteristics
Water Content of Waste
Volume of Water Flow o
Through Wastes
Rate of Flow Through Wastes o
Heavy vegetative cover of grasses will
increase transpiration. Heavy vegetative
cover of grasses can also impede runoff
thus increasing infiltration.
Heavy vegetative cover of trees, bushes and
shrubs may increase infiltration by opening
up channels of flow into waste because of
deep root penetration.
An increased water content within waste
will decrease time till leachate generation
begins.
Increased water flow through waste will
increase the volume of leachate generated.
Increased flow rates through waste will
decrease time till start of leachate
generation, but may decrease the
concentration of contaminates present.
2-42
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2*.2.1.2 Leachate Viscosity
The resistance of a leachate to internal flow caused by external forces
is termed its viscosity. Solutions with a higher viscosity tend to flow more
slowly than less viscous ones, thus, they would be expected to disperse less,
and to migrate more slowly, than solutions of lower viscosity. In addition,
leachates exhibiting a viscosity substantially higher than groundwater should
decrease the hydraulic conductivity of the system. Contrary to expectations
based on hydraulic conductivity equations alone, field experience has indi-
cated that the viscos-ity of a leachate has a minor effect on hydraulic
conductivity (Jackson, 1980).
2.2.1.3 Leachate Density
If the density of a plume is significantly greater or lower than that of
groundwater, plume migration patterns can be altered dramatically. Low
density wastes tend to float at the top of an aquifer and spread out to form a
lens of contaminants that may be several feet thick. In Mechanicsburg,
Pennsylvania, for example, a gasoline spill resulted in formation of a
four-foot thick gasoline layer above the water column. A fuel oil spill in
Southeastern Pennsylvania produced a contaminant lens that was five feet in
thickness (Lindorff and Cartwright, 1977). Low density plumes also tend to
remain more concentrated than other leachates that disperse more readily in
the groundwater.
High density plumes can form dense or concentrated layers near the base
of aquifers. For example, chloride contaminants from oil field brines entered
an aquifer in New Mexico and sank to the bottom of the aquifer because they
were more dense than the native groundwater (Lindorff and Cartwright, 1977).
If dense wastes contain substances that degrade or dissolve minerals, they
could eventually produce a breach in an aquitard separating an upper aquifer
from an underlying one. In some instances, dense plume constituents can
migrate downward against the direction of regional groundwater flow. An
example of this situation is shown in Figure 2-15. In this figure, the upper
clay layer splits the plume into two sections as it flows. A small portion of
2-43
-------
FIGURE 2-15.
EFFECTS OF A PERCHED ZONE ON THE FLOW OF A HIGH DENSITY PLUME
^__— _^ ~y
-------
the dense plume sinks to the base of the lower clay layer and moves against
the direction of groundwater flow.
Density is also important because of the effects density differences, and
subsequent pressure differences, can have on plume migration. Flow equations
should be formulated in terms of pressure when variable density conditions
(e.g., oilfield-related brine plumes) are apparent.
Some leachates contain both low and high density constituents. This can
lead to the formation of a two-plume system with widely differing chemical and
physical characteristics, as illustrated in Figure 2-16. Leachates from dis-
posal facilities accepting many waste types will commonly contain constituents
with various densities. In these instances, the leachate constituents will
tend to seek levels of equivalent density, and thus become spread throughout
the water column with light components at the top, and heavy components lower
in the water column.
2.2.1.4 Leachate Solubility in Groundwater
The ability of leachate constituents to dissolve in groundwater has dis-
tinctive effects on plume migration patterns. Water soluble (miscible)
leachates tend to form larger, more dispersed plumes, whereas water insoluble
(immiscible) materials tend to create small and highly concentrated plumes.
In addition, less water soluble constituents tend to migrate more slowly than
more soluble ones (Wilson, et al., 1980).
Solubility characteristics interact with leachate densities to produce
modifications in the classical plume migration profile. Examples of plume
migration patterns for various density and solubility combinations are shown
in Figure 2-17. Typical waste constituents exhibiting the solubility and
density combinations that are shown in Figure 2-17 are listed in Table 2-6.
The characteristics of leachates containing any of these constituents will
2-45
-------
FIGURE 2-16.
EFFECT OF DENSITY ON LEACHATE PLUME MOVEMENT
ro
i
cn
-------
FIGURE 2-17.
CONFIGURATIONS BASED ON SOLUBILITY AND DENSITY
ro
i
-p.
Low Density Plumes
Moderate Density
Plumes
High Density
Plumes
Water
Soluble
Plumes
Water
Insoluble
Plumes
-------
TABLE 2-6
EXAMPLES OF PLUME CONSTITUENTS BASED ON SOLUBILITY ANN DENSITY
Constituents
Low Density
Constituents
Moderate Density
Constituents
High Density
Constituents
Water
Sol uble
Acetone
Acrylonitrile
Ammonia
Benzene
Acetic Acid
Anil ine
Most metal sal ts
Phenol
Chi oroform
Halogenated ethanes
Halogenated phenols
Nitrobenzene
Water
Insoluble
Cyclohexane
Ethylbenzene
Gasoline and some
Toluene
Vinyl chloride
Xyl ene
oil s
T-butyl acetate
Butyl cellosolve
Hexanol ester-alcohol
Carbon tetrachl oride
Chiorobenzene
PCBs
Most pesticides
Most polyhal ogenated benzenes
and phenols
Most polycyclic aromatic
hydrocarbons
-------
depend on constituent concentrations and may be substantially different than
anticipated from the properties of the pure chemicals.
Although a leachate may be immiscible in groundwater, it could form an
emulsion if emulsifiers are present. In these situations, the leachate would
most likely behave as a moderate density solution.
2.2.1.5 Degree of Dispersion
Contaminant transport can occur via three processes in groundwater;
advection, diffusion and dispersion. Contaminant transport through the motion
of groundwater flow is known as advection. Solutes carried by advection only
move at a rate equal to the average linear velocity of groundwater (Freeze and
Cherry, 1979). However, mechanical mixing and molecular diffusion can cause
contaminants to spread out from the path they would follow under advection
only. Diffusion is the process whereby constituents move under the influence
of their kinetic activity in the direction of their concentration gradient.
Diffusion is a factor in contaminant transport at very low velocities, and is
generally ignored during initial studies of contaminant transport.
Dispersion caused entirely by fluid motion is known as mechanical
dispersion, and is best understood in microscopic terms. Freeze and Cherry
(1979) list three mechanisms involved in dispersion:
• Variations in molecular velocities caused by drag executed on fluids
in close contact with particle surfaces
• Variations in pore size which affect groundwater velocities in
different pore channels
• Variations in possible flow channels caused by branching and inter-
fingering.
Dispersion causes contaminants to "spread out" while moving by advection. The
amount of plume dispersion that occurs is greatest in granular or fractured
aquifers where turbulent flow predominates (Pettyjohn, et al., 1982). In
2-49
-------
general, dispersion does not reduce the concentration of hazardous plume con-
taminants to acceptable levels. The processes of dilution and dispersion are
too slow and ineffective to be relied upon for renovation of contaminated
groundwater (Lindorff and Cartwright, 1977).
2.2.1.6 Leachate Temperature
A final physical characteristic that influences plume migration patterns
is temperature. High temperatures can be generated during aerobic degradation
of organic materials, or during exothermic chemical reactions that occur with-
in the waste site. High temperature leachates are less dense than the same
leachate at a lower temperature, thus they tend to rise within the aquifer.
They also react more rapidly with aquifer and groundwater constituents than do
cooler leachates.. For example, hot (175°F) wastes from a furfural plant in
South Florida were injected into a slightly-saline, confined carbonate
aquifer. The wastes collected near the top of the aquifer, where they rapidly
dissolved portions of the upper aquitard (McKenzie, 1976).
Although a knowledge of the physical characteristics of contaminant
plumes in aquifers can assist in predicting overall migration patterns, data
on a plume's chemical characteristics are also necessary to understand the
overall effects of the plume on an aquifer and on groundwater, and its
subsequent migration.
2.2.2 Effects of Geochemical Interactions
Data on the types, concentrations, and interactions of chemical con-
stituents within a leachate are of extreme importance in evaluating the degree
of hazard from the leachate and its effects on the aquifer and the ground-
water. Information on possible leachate hazards is summarized by Mackison, et
al. (1978) and Sax (1980). Leachates having a low concentration of hazardous
constituents generally have a less pronounced impact on aquifer geochemistry
2-50
-------
than highly concentrated plumes. The concentration of the plume is influenced
by a number of factors, including:
• Volume and rate of infiltrate percolating through the waste site
• Solubility and density of plume constituents
• Length of contact time between infiltrate and wastes
• Initial moisture content of wastes
t Age of the waste site
• Anthropogenic factors and human influence
• Rate of groundwater flow receiving percolating leachate.
As groundwater passes through an aquifer, numerous chemical reactions
occur. Although groundwater and aquifer interactions are complex, the
reactions that occur as a result of leachate flow through an aquifer can be
many times more intricate. Rather than attempt to enumerate the vast number
of reactions possible, brief descriptions of the major processes that affect
migration are given.
A number of processes interact to control the mobility of various wastes,
so that not all of the waste placed in a site can leach to groundwater. Some
wastes can be precipitated, biochemically degraded, or temporarily sorbed.
The net result of these processes is to retard the movement of some constit-
uents through the aquifer. This effect is termed attenuation. Important
factors in contaminant attenuation include:
• Form and concentration of each contaminant
• Physical and chemical properties of the leachate mixture
• Physical and chemical properties of groundwater
• Surface area and porosity of geologic materials
• Groundwater and leachate flow rates
• Types and orders of geologic materials encountered.
Not all contaminants are attenuated to the same extent under a given set
of conditions. While contaminant attenuation is a highly complex and site-
specific process, certain generalizations can be made, as described below.
2-51
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2.2.2.1 Attenuation of Inorganic Constituents
Many factors interact to detain various components of leachates as plumes
migrate through aquifers. For cationic (positively charged ions) metals such
as lead, cadmium, zinc, copper, mercury and chromium III, most of the
attenuating effects are caused by sorption and precipitation. In general, the
amount of cationic metal attenuation increases as the pH, cation exchange
capacity, free lime content, free iron oxide content, and surface area of the
aquifer's particles, increases. An increase in the ionic strength of the
leachate decreases the amount of cationic sorption that can occur (Fuller,
1982).
The amount of cation exchange that takes place within an aquifer is also
affected by other reactions including the weathering of clay minerals,
decomposition of aquifer constituents, and occurrence of redox (reduction-
oxidation) reactions. Some cations such as copper and zinc can become tightly
sorbed, and then undergo secondary reactions that render them unexchangable.
The types of cations in the leachate and in the original aquifer constituents
also affect the amount of cation exchanges that occur. In most aquifer
2+ 2+
materials, the relative degree of affinity for specific cations is Ca > Mg
> K = NH. > Na . Other metallic cations are held less strongly than the
above mentioned common cations (Kurtz and Melsted, 1973).
For anionic (negatively charged) metals, such as chromium VI, arsenic,
and selenium, sorption increases as pH decreases. The ionic strength of the
leachate does not seem to influence the degree of anionic metal sorption
(Griffin and Shimp, 1978).
Of the anions commonly found in leachates, phosphate is the least mobile.
Nitrate and chloride have approximately the same high mobility and are used as
indicators of plume migration because very little attenuation of these
leachate constituents occurs in most aquifers (Lindorff and Cartwright, 1977;
Kurtz and Melsted, 1973; Roberts, et al., 1980).
2-52
-------
In addition to the effects of ionic strength and pH on the attenuation of
metals, the presence of certain organic constituents can substantially
increase metal mobility. Under some conditions, hydrolyzable metals such as
lead can become chelated by certain organic constituents even when the lead is
originally undissolved. When chelated, the metal can remain in solution but
still be unavailable for sorption or precipitation. For this reason, chelated
metals can be highly mobile (Griffin and Shimp, 1978). Chelated metals may
also be readily removed from solution if the chelating agent is itself an
organic chemical (e.g., humic and fulvic acids).
Many non-ionic contaminants also exhibit considerable sorption. Sorption
and attenuation can also occur because of van der Waals attractions, weak
hydrogen bonding, covalent bonding, polar attraction, and other processes.
Plume migration rates can also affect attenuation. Laboratory tests of
leachate attenuation in soil columns indicated that, in addition to the soil
and leachate factors mentioned above, the amount of metal attenuation
decreased as flow rate increased (Alessi, et al., 1980). Aquifer physical
properties such as the presence of low permeability layers and the degree of
compaction were also found to influence metal migration rates, probably as a
result of the higher surface areas and lower flow rates associated with such
conditions.
2.2.2.2 Attenuation of Organic Constituents
When organic constituents enter groundwater, one of four processes can
occur. The constituent can be (Roberts, et al., 1980):
• Rapidly and completely degraded or precipitated
• Completely unaffected by groundwater conditions
• Partially sorbed or precipitated with slowly decreasing amounts of
sorption occurring
• Slowly degraded, with gradually increasing amounts of degradation to a
final steady level.
2-53
-------
Attenuation of organic chemicals in the subsurface is an extremely
complex problem in plume dynamics, and the subject of considerable ongoing
research. Nevertheless, several generalizations can be made concerning the
overall behavior of organic constituents within aquifers. These generaliza-
tions are as follows (Wilson, et al., 1980; Roberts, et al., 1980; Lindorff
and Cartwright, 1977).
• Volatilization of low molecular weight organics is inhibited by
aquifer particles, both by the distance volatile organic compounds
must travel before they reach a free water surface, and as a result of
the slow turnover of soil pore air in the unsaturated zone.
• Some organic constituents can be tightly sorbed onto aquifer particles
t Organics tend to be attenuated less than inorganics
t Most of the attenuation mechanisms are biochemical, rather than
geochemical in nature
• The degradation of simpler, less toxic constituents is more pronounced
than that of complex toxic substances
• Degradation is primarily anaerobic within aquifers, although some
aerobic degradation has been found to occur
• The establishment of microbial degradation may occur after a phase
during which the microbes become acclimated to the new organic
substrate
• The presence of one organic constituent can influence the sorption and
attenuation of another
• Degradation of organic leachate constituents can make tracing sources
of leachates more difficult
• Low solubility or hydrophobic constituents tend to move more slowly
than high solubility or hydrophilic substances
• Some sorption of dissolved organics onto organic constituents within
the aquifer may occur.
The result of geochemical processes and attenuation is to reduce the rate
of migration of certain constituents, remove the constituents from ground-
water, or increase the mobility of certain constituents under certain
conditions. A summary of these processes is presented in Table 2-7. The
2-54
-------
TABLE ?-7
PROBABLE EFFECT OF VARIOUS PROCESSES ON THE
MOBILITY OF CONSTITUENTS IN SUBSURFACE WATERS
CONTAMINATED BY WASTE DISPOSAL
(Jackson, 1980)
Physical Processes
Dispersion - Causes dilution of wastes and smearing of the plume front.
The dispersive capacity of a porous or fractured medium is directly
dependent on the groundwater velocity and the heterogeneity of the
aquifer materials, and is inversely proportional to the porosity.
Filtration - Favors reduction in amounts of substances associated with
colloidal or larger-sized particles (e.g., sediments and microbes).
Most effective in clay-rich materials, least effective in gravels or
fractured or cavernous rock.
Gas movement - Requires unsaturated conditions and high porosity if not
dissolved in groundwater. Where gas movement can occur, favors
aerobic breakdown of organic substances and increased rates of
decomposition. Constituents mobile under oxidized conditions (e.g.,
chromium) will then predominate. Restriction of gas movement by
impermeable, unsaturated materials or by saturated materials can
produce an anaerobic state and reduced rates of organic decay. This
will mobilize substances soluble under anaerobic conditions (e.g.,
iron, manganese)
Geochemical Processes
Complexation and ionic strength - Complexes and ion pairs most often form
by combination of ions including one or more multivalent ions and
increase in amount with increased amounts of ions involved. Ionic
strength is a measure of the total ionic species dissolved in
groundwater. Both ionic strength and complexation increase the
total amounts of species in solution that would otherwise be limited
by processes such as oxidation, precipitation, or sorption.
Acid-base reactions - Most constituents increase in solubility and thus
in mobility with decreasing pH. In organic rich waters, the lower
pH's (4 to 6) are associated with high values of carbonic acid and
often also of organic acids. These will be most abundant in
moisture saturated soils and rock.
2-55
-------
TABLE 2-7 (continued)
Oxidation-reduction - Many elements can exist in more than one oxidation
state. Contaminants will often be oxidized or only partially
reduced in unsaturated soils and groundwater recharge areas, but
will become reduced under saturated conditions when excess organic
matter is present. Mobility depends on the element and pH involved:
chromium is most mobile under oxidizing conditions, whereas iron
and manganese are most mobile under those reduced conditions in
which dissolved oxygen and hydrogen sulfide are absent.
Precipitation-dissolution - The abundance of anions such as carbonate,
phosphate, silicate, hydroxide, or sulfide may lead to precipi-
tation, especially of multivalent cations as insoluble compounds.
Dilution or a change in oxygen content, where precipitation has
resulted from oxidation or reduction, may return such constituents
to solution.
Sorption-desorption - Ion exchange can withhold, usually temporarily,
cations and to a lesser extent anions, on the surfaces of clays or
other colloidal-sized materials. Amounts of sorbed metal cations
will increase with increasing pH. Molecular species may be weakly
retained on colloidal size materials by physical sorption. The much
stronger binding forces caused by chemical action result in the
formation of surface compounds involving metal ions and mineral
grains. Sorbed species may return to solution when more dilute
solutions come in contact with the colloidal material, depending on
the nature of the sorption bond and sorption of organic chemicals by
chemical interactions such as bonding and polar attraction.
Biochemical Processes
Decay and respiration - Microorganisms can break down insoluble fats,
carbohydrates, and proteins, and in so doing release their
constituents as solutes or particulates to subsurface waters.
Cell synthesis - N, C, K, and P, and some minor elements are required for
growth or organisms, and can thus be retarded in their movement away
from a waste disposal site because they are temporarily incorporated
within microbial cells.
2-56
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types of processes that influence specific aquifer constituents are listed in
Table 2-8 and descriptions of the overall effects of the various processes on
the mobility of leachate components are given in Table 2-9. The reactions
governing geochemical processes and attenuation are so complex and dependent
upon site specific conditions, that pollutant attenuation is often ignored in
evaluating leachate plume movement.
2.2.2.3 Estimating Leachate, Groundwater, and Aquifer Interactions
Several approaches have been used to estimate the effects of attenuation
and other processes that occur within aquifers on the migration rates of
inorganic and organic leachate constituents. Three of these approaches are
described-briefly below.
The amount of a constituent that is detained by earth materials has been
estimated using laboratory studies of leachate migration through soils or
aquifer materials under aquifer like conditions. One method used to measure
the attenuative properties of various earth material involves saturating the
pores of the sample with a pollutant laden solution. Additional amounts of
the solution are allowed to flow through the sample, under anaerobic
conditions, displacing the original pore volume of solution. This continues
until the concentration of pollutant entering the sample (C ) is equal to the
concentration of pollutant in the solution flowing out of the sample (C). The
point at which C/C equals one is termed the breakthrough point (Fuller,
1982). When a contaminant concentration reaches the breakthrough point, the
aquifer's ability to sorb that particular contaminant is exhausted (Roberts,
et al., 1980).
A curve of C/C concentrations plotted for various numbers of pore volume
displacements indicates the relative amount of pollutant that will be
attenuated by the soil at any particular point in the test (Fuller, 1982).
Typical breakthrough curves are shown in Figure 2-18. Curve A in Figure 2-18
indicates little attenuation in occurring, while Curve E indicates that all of
the pollutant entering the soil is being attenuated.
2-57
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TABLE 2-8
PROCESSES WHICH MAY CONTROL AMOUNTS OF CERTAIN CONSTITUENTS IN
SUBSURFACE WATERS CONTAMINATED BY WASTE DISPOSAL*
(After Jackson, 1980; Fenn, et al., 1977)
ro
en
00
Constituent
Physical
Dis-
persion
Organic Solutes
Microorganisms
Heavy Metal Anions
(Cr, V, Se, B, As)
Heavy Metal Cations
(Pb, Cu, Ni,
Zn, Cd, Hg)
N03
so/
po43'
Na+, K+
NH4*
?+ ?+
Ca' , Mg^
Fe , Mn
X
X
X
X
X
X
X
X
X
X
X
Filtra- Complex-
tion ation
X
X
X
X
(X)
X
X
(X)
X
Geochemical
Ionic Acid- Oxid.- Precip-
Strength Base Red. Solution
(X) X X (X)
X
(X) X X
(X) X X
X
(X) (X) X
XX X
(X)
(X) X X
X (X)
X XXX
Biochemical
Adsorp.- Decay, Cell
Desorp. Respir- Synthesis
ation
(X) X X
X X
X
X
X X
(X) (X)
XXX
X
XXX
X
X
"(BRACKETS DENOTE MINOR CONTROLS)
-------
TABLE 2-9
BEHAVIOR OF SPECIFIC CHEMICAL WASTES AT LANDFILLS
(Jackson, 1980)
Chemical Waste
Behavior
Halogenated Organic
Compounds
Phenols
PCBs
Mineral Oils
Organic Solvents
Acids
Cyanide (CN)
Heavy metals
(e.g., Cr, Zn, Hg, Pb, Cd)
Evaporation (because of volatility) and sorption
onto other solid wastes present may reduce the
release of halo-solvents to groundwater.
Phenols are relatively soluble in groundwater,
undergo biodegradation slowly, and are reversibly
sorbed.
Readily soluble in hydrocarbons, but not in
water. Biodegradation inhibited by anaerobic
conditions. Readily sorbed. PCBs are relatively
inert within landfills and are not leached by
water in high concentrations.
Sorption of oils onto solid fill material is an
important attenuation process. Floats on
groundwater because of low density relative to
groundwater.
Some sorption on landfill materials and some
biodegradation in sands and gravels have been
observed.
Leaches sorbed contaminants and dissolves
precipitates, thereby causing deterioration of
groundwater quality. Inhibits microbiological
activity. Releases harmful gases.
Most CN either volatilizes as HCN, or
precipitates as cyanoferrate II compounds.
+2
+2
Cd+2)
Heavy-metal cations (e.g., Pb"", Hg
controlled primarily by their insolubility as
metal sulfides, carbonates, hydroxides or
phosphates, and secondarily by sorption. Anionic
forms (e.g., Cr90 ~ ) very mobile.
2-59
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FIGURE 2-18.
TYPES OF BREAKTHROUGH CURVES GENERATED BY THE SOIL COLUMN TECHNIQUE
(FULLER, 1982)
ro
i
CD
O
C/Co
Time
-------
Curve types vary depending on soil texture, pollutant type, and other
factors. In general, clays attenuate pollutants much more effectively than
sands, and inorganic cations are attenuated more effectively than are
inorganic anions (Fuller, 1982). However, while dispersed clay particles in
the aquifer matrix will very effectively undergo ion exchange with some
contaminants, relatively impermeable clay liners and beds do not readily
transmit flow and therefore provide very little attenuation. Care should be
taken when attempting to relate breakthrough curve test data to actual
situations, as test conditions rarely mimic site-specific situations. For
example, reports of clay liner failure have shown that volatile solvents and
fuels will strip the waters of hydration from clay and cause dessication
cracks through which seepage occurs.
Another method used to estimate the rates of pollutant migration involves
a more mathematical approach. For some specific chemicals, data have been
assembled that can be used to estimate the migration distances under a
specific set of conditions. One method used to express the relative degree of
a chemical's mobility is to use a value termed the distribution coefficient
(kd). As kd increases, the mobility of a substance decreases. Values of k,
for several pesticides are listed in Table 2-10. Values of k. can be used in
d
conjunction with mathematical models to estimate the size and shape of
leachate plumes after a given period of time.
Many highly complex computer models have been developed to numerically
simulate pollutant migration patterns. These models can be used to assist in
a more comprehensive interpretation of the information on the site. These
modeling programs, however, typically require a considerable amount of
hydrogeologic data obtained over a long time period.
2-61
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TABLE 2-10
PESTICIDE MOBILITY BASED ON DISTRIBUTION COEFFICIENTS
(After Jackson, 1980)
Relative Mobil ity
Highly Mobile
Somewhat Mobile
Relatively Immobile
No Significant
Leaching
Pesticides k . Val
Name
Picloram
2,4-D
Atrazine
Diuron
Ethirimol
Lindane
Parathion
DDT
Paraquat
Use Range
Herbicide 0-10
Herbicide
Herbicide
Herbicide 10-100
Fungicide
Insecticide
Insecticide 100-1,000
Insecticide +1,000
Herbicide
ues
Loam Soil -3%
organic matter
0-3
1
3
10
15
50
200
1,000
10,000+
When models have been used to predict plume behavior under field
conditions, data from field studies have sometimes been found to contradict
computer predictions (Perlmutter and Lieber, 1970; Roberts, et al., 1979).
For this reason, many researchers recommend extreme caution when interpreting
the data from mathematical modeling studies (Cole, 1982; Fenn, et al., 1980;
Roberts, et al., 1979). Despite the disadvantages of various computer models,
numerical simulation can be a useful tool to assist in organizing and
evaluating the complex geochemical, physical, and biochemical interactions
that take place within aquifers. Models useful for plume delineation are
discussed further in Section 3.1.
2-62
-------
CHAPTER 3
PLUME DELINEATION
Controlling the migration of a leachate plume requires a thorough under-
standing of the distribution of contaminants in the subsurface. Because
contaminant movement is usually dominated by lateral groundwater movement, the
direction and rate of groundwater flow must be known in order to approximate
plume boundaries. Initial estimates of plume boundaries are made assuming
that subsurface materials are homogeneous, isotropic, and non-reactive (i.e.,
degradation and attenuation of plume constituents does not occur), and that
contaminant movement is controlled only by groundwater flow. Initial
estimates of plume location can then be improved by making use of known con-
taminant characteristics to estimate how the plume will react in groundwater
(e.g., denser contaminants will tend to sink in the water column) and by
determining preferential flowpaths using a knowledge of subsurface geologic
conditions (e.g., faults and fractures in consolidated rock or sand and gravel
lenses in lower hydraulic conductivity clay matrices).
In general, the more data that are available on a site, the greater the
probability that an approximation of plume boundaries will approach actual
conditions. At a minimum, information on groundwater flow direction and rate,
contaminant characteristics and release rates, and subsurface geologic
profiles are required. Because groundwater plumes cannot be readily observed
(unlike an oil spill on surface water), monitoring wells that provide direct
groundwater quality data will always be required in plume delineation studies.
However, because monitoring wells are fairly expensive to install and monitor,
well data can be optimized by the careful application of less resource inten-
sive data collection techniques that do not provide direct groundwater quality
data. The purpose of this chapter is to provide generalized procedures for
delineating leachate plumes using direct and indirect methods.
3-1
-------
3.1 Plume Delineation Procedures
A generalized approach to identifying the boundaries of a contaminant
plume is illustrated in Figure 3-1. This approach involves a number of
activities which can be summarized in three major steps:
0 Calculate possible plume boundaries based on site hydrogeology and
contaminant release information
t Modify calculated boundaries qualitatively based on site and contam-
inant characteristics, and verify these boundaries, calculations and
modifications using groundwater sampling data and supporting
information
• Extrapolate future plume movements using the verified groundwater flow
data and computer models, if needed.
The objective of this approach is not only to delineate current and
future plume positions but also to ensure that information required for
implementing plume control measures will be available. These steps are
described in further detail below.
3.1.1 Calculate Possible Plume Boundaries
The first step in calculating possible plume boundaries is to identify
the direction(s) of plume movement and what aquifer, aquifers, or portions of
aquifers may be contaminated. Because information on groundwater contamina-
tion will usually be limited at this point in the process, hydrogeologic data
on the site (e.g., groundwater levels) are generally used to identify water
bearing zones that may lie in the path of a migrating plume. This evaluation
should consider both horizontal and vertical plume movement as discussed in
Chapter 2.0. If enough data are available, a potentiometric surface map of
the potentially affected aquifers should be constructed.
3-2
-------
FIGURE 3-1.
GENERALIZED APPROACH TO DELINEATING LEACHATE PLUMES
Determine Flow Direction and Gradient
JL
Determine Aquifer Properties
Determine Leachate Generation History
Calculate Possible Plume Limits
Modify Calculated Limits Qualitatively
Verify Plume Limits with Supporting Data
Does
Estimation
Appear
Reliable?
Reassess Values Used
in Flow Calculations and
Recalculate Plume Limits
Develop and Implement Groundwater Sampling Program I
'No
Yes
Reassess Values Used
in Flow Calculations and
Recalculate Plume Limits
Delineate Current Plume Position and Extrapolate Future Plume Movement
3-3
-------
In evaluating hydraulic head data, the following points are important to
remember:
• Any well pair used to calculate a hydraulic gradient should be located
along the same flowpath. Wells used to establish horizontal gradients
should be screened in the same stratigraphic horizon; wells used to
establish vertical gradients should be in close proximity to each
other.
• Water levels should be measured from an accurately surveyed, fixed
reference point (e.g., top of well casing). Calculated water level
elevations and potentiometric surface contours should not exceed the
precision of the original survey and the water level measuring device.
Calculated gradients should not exceed the precision of the elevation
and distance-between-wells measurements.
• Water level data should be collected from all wells in a monitoring
system at the same time to minimize temporal variations (e.g.,
seasonal water level fluctuations). Temporal variations should be
assessed by measuring system head levels over the course of several
cycles.
In the absence of reliable hydrogeologic information on the site, the
usual assumptions are that (1) horizontal flow predominates over vertical
flow, (2) contamination is restricted to unconfined aquifers, and (3) contami-
nation levels will be highest in the uppermost unconfined aquifer. Subsequent
field investigation activities are then planned to ascertain the validity of
these assumptions. Excellent discussions of methods for determining the
direction of groundwater flow are given in Fetter (1981); Pinder, et al.,
(1981); and Abriola and Pinder (1982).
The second step in calculating possible plume boundaries is to estimate
the rate of groundwater and contaminant movement using Darcy's Law (refer to
Section 2.1.1). Data required by this equation can be obtained using a
variety of methods.
Initial calculations typically result in ballpark estimates based on
indirect data. Indirect data includes generalized data not based on actual
site conditions and data functionally related to the data required by Darcy's
Law. Published reports, aerial imagery, and geophysical surveys are examples
3-4
-------
of indirect data sources which are described in more detail in Section 3.2.
More precise calculations typically require using di rect data on site condi-
tions. Direct data for a site is developed using hydrologic and groundwater
sampling methods described in Section 3.3. Table 3-1 summarizes the
availability and reliability of several information sources for Darcy's Law
calculations.
Darcy's Law is an attractive means for calculating groundwater flow
because it is theoretically simple, not based on many limiting assumptions,
applicable to most hyd-rologic systems, and uses parameters that can be
estimated readily. One major problem, however, is that Darcy's Law loses
validity under conditions of very low and very high hydraulic gradients and
hydraulic conductivities. The reason for this is that Darcy's Law is designed
to predict fluid movement through saturated media under conditions of laminar
flow. Under these conditions, flow paths are assumed to be layered and mini-
mal intermolecular mixing is expected to occur. As flow rates and hydraulic
gradients increase, however, mixing becomes more predominant and flow less
laminar. When velocity increases sufficiently turbulent flow begins. Under
these conditions, flow pathways are nonlinear and a great deal of mixing may
occur. In most aquifers flow rates are normally low enough to allow for
accurate determinations of velocity (Davis and DeWiest, 1966). Under certain
conditions, however, turbulent flow in aquifers can occur. Conditions causing
turbulent flow include aquifers with extremely porous areas such as along
fracture zones, fissures, and large openings in subsurface units (Freeze and
Cherry, 1979).
Calculations of contaminant movement using Darcy's Law assumes water
soluble, conservative contaminants in non-dispersive and unattenuative porous
media (Popkin, 1983). Fried (1975) and Freeze and Cherry (1979) present
analytical methods for addressing dispersion and attenuation. In many cases,
data necessary to use the dispersion and attenuation equations will not be
available.
The final step in calculating possible plume boundaries is to determine
the history of a site's leachate generation. Based on the amount of time (t)
3-5
-------
TABLE 3-1
INFORMATION SOURCES FOR CALCULATING VELOCITY
USING DARCY'S LAW (V = Kl/n)
Parameter
Source
Hydraul ic
Conductivity
(K)
On site aquifer
testing results
Laboratory testing
results
CO
I
01
Site related reports
Regional reports
Hydraulic
Gradient
(I)
General guidelines
On site surveys of
water levels in wells
Availability
Data Reliability
Not generally available
unless previous site work
has been undertaken
Not generally available
unless previous site work
has been undertaken
Commonly available from
a variety of sources such as
feasibility studies for
water supply or construction
projects at neighboring
sites
Readily available in a
variety of forms from such
sources as the USGS, state
geological surveys, and
universities
Readily available
(see Table 2-2)
Not generally available
unless previous site work
has been undertaken
Excellent if planned and
implemented properly
Generally good, but are less
reliable for unconsolidated tests
of compressible material such
as clay. False low values are a
common problem. Sample represen-
tativeness is also a concern (see
Section 3.3.1)
Generally good depending
on proximity of study area and
variability of hydrogeology
Moderate to low unless site
data was used as part of the
regional database
Generally low
Excellent if seasonal and
other temporal variations are
taken into account. Generally
good otherwise if the guidelines
given in Section 3.1.1 are
followed.
-------
TABLE 3-1 (continued}
Parameter
Hydraulic
Gradient
(I)
(continued)
GO
i
Porosity
(n)
Source
Regional and site-
related reports
Estimation from site
topography
Laboratory testing
results
General guidelines
Availability
Data Reliability
Commonly available from
the USGS, state geological
surveys, universities,
and other geotechnical or
engineering sources
Topographic maps are
readily available from
the USGS. Larger
scale topographic maps
may also be available
from local government
agencies.
Not generally available
unless previous site
work has been undertaken
Readily available
(see Table 2-1)
Generally moderate to low
because of the highly site
specific nature of hydraulic
gradients
Generally low
Generally moderate to good
but can be highly variable
because of problems
associated with sample
extraction and representa-
tiveness
Generally low
-------
that contaminants have been within the saturated zone and the estimated
velocity of groundwater and contaminants (V ) from Darcy's Law, the maximum
distance (D ) a plume could migrate from a source area is given by:
°P - V
However, determining the timing of contaminant releases from a site is
commonly the most difficult and least reliable step in calculating possible
plume boundaries. This is especially true of abandoned sites which lack
adequate operation records.
The approach generally taken to estimate release time, t, is to assume
that leachate generation is simultaneous with the initiation of site
activities and then try to pinpoint the initiation of site activities. This
approach is usually valid because the time between waste disposal and leachate
generation is typically small relative to the age of the site. Unfortunately,
determining when hazardous waste related site activities were initiated is not
always possible. Some possible sources of this type of information in the
absence of owner or operator records include historical aerial photographs,
county tax records, chamber of commerce records, and waste generator and
transporter records.
Table 3-2 provides an example to illustrate how possible plume limits can
be calculated. Other methods for calculating plume limits are described by
Pettyjohn, et al. (1982).
3.1.2 Modify and Verify Calculated Boundaries
Once the initial calculations of possible plume limits are completed,
they should be drawn on map view and on several cross sections of the site.
The calculated boundaries should then be assessed and modified as needed based
on the factors affecting plume movement as described in Chapter 2. The
purpose of this step is to help ensure that optimal use will be made of each
3-8
-------
TABLE 3-2
EXAMPLE CALCULATIONS OF MAXIMUM PLUME LIMITS (D )
CALCULATION
METHOD
INITIAL Given that 0 = t(KI/n) where:
t = 19 years based on county records showing purchase data by
disposal facility operator and assumption that onsite
disposal and leachate generated began immediately
K = 10 ft/day based on regional geologic reports that the site
is underlain by a thick deposit of silty sand (see Table
2-2)
I = 0.01 based on an estimate using site topography and surface
water elevations
n = 35% based on published guidelines for silty sand.
Maximum plume limits (assuming zero attenuation) can be
estimated
by:
D = (19 years) (365 days/year) (10 feet/day) (0.01)/(0.35)
D = 2,000 feet.
REVISED Given that D = t ((T/m) I/n) where:
(Note: K = T/m)
t = 17.5 years based on more detailed site operation information
provided by former employees and a preliminary water balance
calculation for leachate generation
T = 900 gpd/foot based on a pumping test
m = 65 feet based on borings indicating a clay layer 90 feet
deep and an average depth to water of 25 feet.
I = 0.008 predominantly in the horizontal direction based on
water levels taken on site
n = 40% based on borings that indicate a lower proportion of
sand than previously thought and published guidelines
Maximum plume limits (assuming zero attenuation) can be
estimated
by:
D = (17.5 years) (365 days/year) ((900 gpd/ft/7.48g/ft3)/65
p feet) (0.008)7(0.40)
D = 236.4 ft
3-9
-------
well and other sampling points in the site monitoring program. The key
factors to consider in evaluating a calculated plume boundary are:
• Groundwater flow patterns (Section 2.1)
• Leachate or contaminant characteristics (Section 2.2.1)
• Plume and geologic media interactions (Section 2.2.2).
Of the three, groundwater flow patterns will generally he the most important
and least difficult factor to evaluate in assessing the calculated plume
limits. Should certain values used in the initial flow calculation appear to
be inappropriate based on this step, the values should be modified and the
maximum plume limits recalculated.
There are a variety of non-sampling methods for checking the validity of
groundwater and plume movement calculations including preliminary site inspec-
tions, aerial imagery, and geophysical surveys. The types of information that
can be obtained by these and other means are outlined later in this chapter.
However, to describe succinctly how this information should be used on a site
specific basis to adjust initial estimates is very difficult. Therefore,
personnel well qualified to assess waste site hydrogeology should be involved
in delineating the location of a site's plume.
Based on the final estimates of the plume's anticipated boundaries, a
groundwater monitoring program is developed and implemented. Plume
delineation by groundwater sampling is discussed in Section 3.3.2. If
utilized properly, groundwater monitoring results will establish plume
boundaries with the accuracy needed to develop remedial action plans. In
addition, the groundwater monitoring data should be used to reassess the
values used in the flow calculations to ensure their accuracy. This is
particularly important if long term predictions of plume movement are to be
developed.
3-10
-------
3.1.3 Extrapolate Future Plume Movement
Most plume management strategies seek to contain, extract, or treat the
plume so that it does not pose a threat to the environment. Some strategies,
however, seek to control the plume's movement so that it will not affect
sensitive receptors. Examples of this latter strategy include diverting the
plume away from drinking water wells, counterpumping to prevent the plume from
advancing in an unwanted direction, and not taking action because an immediate
environmental threat is not evident. In cases where plume movement is being
controlled, or the "no action alternative" is an issue, extrapolation of
future plume movement trends is imperative. Predicting plume movement can be
accomplished manually using calculations such as those described in Section
3.1.1 or through the use of computer models.
There are many commonly accepted models that can be used in forecasting
leachate plume movements and the effects of plume management plans. These
models can be divided into two major groups — release rate models and solute
transport models. Typically, estimates of leachate quantity and quality
released from a site are obtained from a release rate model or through a
groundwater sampling program, and are used as input to a solute transport
model. The theory behind some of the models is fairly complex and can be
found in sources such as Bachmat, et al. (1980), Mercer and Faust (1981),
Anderson (1979), Weston (1978), and Repa, et al. (1982).
The first and probably the most crucial step in waste site modeling is to
obtain accurate estimates of the quantity and quality of leachate that has
been released into the subsurface environment. Only after adequate determina-
tion of leachate release can a solute transport model be used. Most release
rate models are based on dividing the problem of prediction into three
separate components--leachate generation rates, constituent concentrations,
and leachate release rates from the site. Combining the three separate
components allows for prediction of the quantity and quality of leachate that
can be expected to be released from the site. Because of the unknowns
included in estimating values for these components, data are generally
obtained through a groundwater monitoring program and input directly into a
3-11
-------
solute transport model. Table 3-3 summarizes six computer models that can be
used for predicting leachate release rates.
Solute transport models utilize a set of equations, based on explicit
assumptions, to describe the physical processes affecting pollutant transport
from a site. These models can be divided into two types—deterministic and
stochastic. Deterministic models attempt to define the shape and concentra-
tion of leachate plunes using the physical processes (e.g., groundwater flow)
involved, while stochastic models attempt to define causes and effects using
probabalistic methods.
Deterministic mathematical models can be further divided into analytical
and numerical models. Analytical models simplify mathematical equations,
allowing solutions to be obtained by analytical methods (i.e., functions of
real variables). Numerical models, on the other hand, approximate equations
numerically and result in a matrix equation that is usually solved by computer
analysis. Both types of deterministic models address a wide range of physical
and chemical characteristics but the analytical models usually simplify the
characteristics by assuming steady-state conditions. The physical and
chemical characteristics considered by these models include:
• Boundary Conditions -- hydraulic head distribution, recharge and
discharge points, and locations and types of boundaries
• Material Constants -- hydraulic conductivity, porosity,
transmissivity, extent of hydrogeologic units
• Attenuation Mechanisms -- sorption-desorption, ion exchange,
complexing, nuclear decay, ion filtration, gas generation,
precipitation-dissolution, biodegradation, chemical degradation
• Hydrodynamic Pi spersion -- diffusion and dispersion (transverse and
longitudinal)
• Waste Constituent Concentration -- initial and background
concentrations, boundary conditions.
Both mathematical model types incorporate two sets of equations to define
transport—a groundwater flow equation (generally consisting of a water
3-12
-------
TABLE 3-3
RELEASE RATE MODELS
Model Reference Calculation Method
Drainmod/ Skaggs (1982) t Water bal ance method
Drainfil) • Drain equations
Advantages
• Calculates head levels
within site
• Predicts quantity of
drainage to leachate
collection system
« Predicts quantity of
leachate moving through
underlying clay
Disadvantages
• Data intensive
* Roes not consider process
within cell that affects
leachate quantity or
qual ity
• Untested model
OJ
I
OJ
HELP/
HSSWDS
LSIPE
PCLTF
LTTM
Rel ease
Kate
Computation
Perrier and
Gibson (1980)
Moore (1980)
USEPA (1982c)
Pope-Reid Associates
(1982)
SCS Engineers
(1982)
• Water balance method
• Linearized equation with
simpl ified boundary
conditions
• Analytical equation series
• Water balance method
• Analytical equations
• Minimal data requirements
* Estimated horizontal and
vertical drainage through
a maximun of 8 layers
• Estimates impingement rates
on collection system
• Evaluates numerous linear
design types
• Allows for nonlinear equation
with complex boundaries
• Predicts release rates
• Addresses all three
components necessary to
predict release rate
and quality
• Assesses 7 generic sites
• Calculates leachate volume,
leachate head, containment
time, seepage rates, travel
times
• Easy to use
• Series of simple calculations
t Predicts volume of leachate
generated over time
t Not field tested
• Roes not evaluate leachate
quality
• Rata intensive
• Assumes landfill or surface
impoundment are designed
properly
t Estimates through single
cell only
• Does not predict leachate
qual ity
• Data intensive
• Untested model
• Does not address leachate
quality
• Untested model
* Equations may be over
simpl ified
• Roes not predict concen-
tration of ronstltuents
• Untes.ted model
-------
balance equation coupled with Darcy's Law) and a mass balance equation (which
describes the concentrations of a chemical species in a flow pattern). These
equations are coupled to provide predictions of solute transport in the
groundwater system with chemical reactions considered. For analytical models,
these equations are simplified to explicit expressions. For either type of
model, a sensitivity analysis of model results can be performed by varying the
input characteristics singularly or in combination.
One type of sensitivity analysis that can be performed involves changing
a single parameter (within known values of occurrence) to demonstrate the
effects that variations in individual parameters have on model output. This
analysis helps identify those parameters that have the greatest influence on
model results. A second type of sensitivity analysis involves a series of
trial runs using an array of input parameters which vary in accordance with
the expected errors associated with each parameter (i.e., Monte Carlo
simulation techniques). This method provides a general assessment of the
overall model sensitivity and intrinsic precision by providing a range of
variations of the model outputs as a function of the error bars associated
with the input parameters (e.g., mean values, maximum values, minimum values).
Analytical models provide estimates of waste constituent concentrations
and distributions using simplified, explicit expressions generated from
partial differential equations. The mathematical expressions are usually
simplified by assuming steady state conditions relative to fluid velocity,
dispersion dynamics, and other physical parameters. For example, groundwater
flow equations can be simplified if the aquifer is assumed to have infinite
extent. Governing equations characterize both groundwater flow and mass
transport, and may also address dilution, dispersion, and attenuation. These
models can simulate plume migration from the source to a utilized groundwater
system allowing for attenuation and dispersion. The method provides a quick
and inexpensive solution with minimal amounts of data as long as the
simplifying assumptions do not render results invalid.
Numerical models characterize groundwater contamination processes without
the simplification of complex physical and chemical characteristics required
3-14
-------
by analytical models. Numerical models reduce the partial differential
equations to a set of algebraic equations that define hydraulic head at
specific points (i.e., grid points). These equations are solved through
linear algebra using matrix techniques.
The numerical methods most commonly used to simulate groundwater
transport problems can be divided into four groups: finite difference (FD);
finite element (FE); method of characteristics (MOC); and discrete parcel
random-wal k (OPRW). In each method, the governing equations (e.g.,
groundwater flow equations) are solved by subdividing the entire problem
domain into a grid system of polygons. Each polygon block has assigned
hydrogeologic properties (e.g., transmissivity) associated with it that define
the aquifer at that point. Accompanying each block is a node point that
represents a position in the aquifer with an equation having unknown values
(e.g., head). For the finite difference method, the derivatives of the
partial differential equations are approximated by linear interpolation (i.e.,
the differential approach). In the finite element method, the partial
differential equations are transformed to integral form (functionals) and
minimized to solve the dependent variables. The algebraic equations for each
node point, derived by the FD or FE methods, are then combined to form a
matrix equation which is solved numerically. The FE method is better suited
for solving complex two- and three-dimensional boundary conditions than the FD
method. When using FD or FE methods for solving contaminant transport
problems, results are subject to numerical dispersion or numerical oscilla-
tion. Numerical dispersion causes answers to be obscured because of
accumulated round-off error at alternating time steps. Numerical oscillation
causes answers to overshoot and undershoot the actual solution at alternating
time steps. Numerical oscillation is generally associated with FE methods,
while numerical dispersion is generally associated with FD methods.
The method of characteristics and discrete parcel random-walk models were
developed to minimize the numerical difficulties associated with the FE and FD
methods. Both the method of characteristics (MOC) and discrete parcel random
walk method (DPRW) analyze temporal changes in concentrations by tracking a
set of reference points that flow with the groundwater past a fixed grid
3-15
-------
point. In the MOC method, points are placed in each finite difference block
and allowed to move in proportion to the groundwater velocity at the point and
the time increment. Concentrations are recalculated using summed particle
concentrations at the new locations. The DPRU varies from the MOC method
because, instead of solving the transport equation, a random process defines
dispersion. Reference points move as a function of groundwater flow,
consistent with a probability function related to groundwater velocity and
dispersion (longitudinal and transverse). The methods provide comparable
results but the MOC method is time consuming, expensive, and requires
considerable computer storage.
Tables 3-4 and 3-5 list some of the available models that can be used to
predict the movement of contaminants in groundwater. As with the release rate
models listed in Table 3-3, each model contains elements that would make it
more applicable to certain site and waste characteristics. Only eight
analytical and nine numerical models are presented in these tables. Many more
models exist that can be used for the prediction of chemical migration such as
Weston (1978) and Thomas, et al. (1982).
3.2 Indirect Data
Indirect data includes generalized data not based on actual site
conditions and data only functionally related to the information required.
Field investigations involving monitoring well installation, aquifer testing,
groundwater sampling and analyses, and associated activities, are typically
the most critical and costly elements of groundwater quality assessment
projects (Popkin, 1983). The purpose of indirect data, therefore, is to
optimize the usefulness of monitoring wells by identifying the most effective
locations based on anticipated plume limits. This can help minimize costs as
fewer wells may be necessary. In certain instances, risks to personnel
installing monitoring wells at hazardous waste sites can also be reduced
(i.e., in the case where wells must be located in close proximity to buried
waste).
3-16
-------
TABLE 3-4
ANALYTICAL SOLUTE TRANSPORT MODELS (Repa, et al., 1982)
Model
Reference
Dimensions
Madeled
Advantages
Disadvantages
SESOIL Bonazountas and
Wagner (1981)
PESTAN En field, et al.
(1982)
PLUME Wagner (1981)
• Ease of use
• Long term'fate simulations
• Includes numerous degradation
processes
• Models organics and
inorganics
Predicts pollutant
velocity, length of pollu-
tant slug, and concentra-
tions
Screening model-rapid
evaluations
Inexpensive to run
Field verified: DDT and
aldicarb predictions
Two dimensional plume
traces in groundwater
Incorporates degradation
and sorption terms
Can be coupled with PESTAN
Field verified: chromiun
• Unsaturated zone only
• Not field or analytically verified
• May be overly simplistic
• Models only organic species
• Unsaturated zone only
• May be overly simplistic
• May be overly simplistic
-------
TABLE 3-4 (continued)
Model Reference
Dimensions
Model ed
Advantages
Di sadvantages
LPMP
Kent (1982)
oo
i
co
AT123D Yeh (1981)
1-2-3
PATHS
Nel son and
Schur (1980)
• Predicts migration and
mixing of contaminants in
groundwater zone
• Incorporates degradation
terms, and dispersion and
diffusion
• Ease of use
• Field verified: chromiun
• Predicts migration of
contaminant in saturated
and unsaturated zone
• Incorporates wide range
of transport mechanisms
• Example problems provided
• Numerous source release,
aquifer and waste options
• Predicts plume movements
in saturated zone
• Inexpensive, first cut
evaluations
• Can be run with data
generally available for
site
• Analytically verified
• May be overly simplistic
• Not field or analytically
verified
• Not field veri fied
• Degradation and dispersion
options not included
• May be overly simplistic
-------
TABLE 3-5
NUMERICAL TRANSPORT MODELS (Repa, et al., 1982)
Model
Reference
Dimensions
Modeled
Advantages
Di sadvantages
MMT/VTT
Battelle (1982)
1-2
CFEST/
UNSAT1D
Battelle (1982)
2-3
GO
I
Pollutant
Movement
Simulator
FEMWASTE
Khaleel and
Reddell (1977)
2-3
Yen and Ward
(1981)
o Predicts contaminant trans-
port using discrete parcel
random-walk method
o Incorporated degradation terms
o Provides graphic displays
o Field verified: tritium
o Predicts contaminant trans-
port using finite element
techniques
o Models complex flow systems
o Analytically verified
Predicts contaminant trans-
port using finite difference
techniques
Incorporates convection and
dispersion equations using MOC
Field tested: NaCl
Predicts contaminant trans-
port using finite element
methods
Incorporates convective
dispersion, sorption, and
degradation equations
Field verified
Requires intensive data to run
model accurately*
Costly to run*
Degradation and sorption term
presently being incorporated
In the process of field
verification: arsenic and
pharmaceutical chemicals
Model documentation in
preparation
Attenuation processes not
considered
In the process of being
verified for mine wastes
*These two disadvantages apply to all numerical models
-------
TABLE 3-5 (continued)
Model
Reference
Dimensions
Modeled
Advantages
Disadvantages
Random
Walk
GO
I
Solute
Transport,
Dispersion
Model
SWIFP
Prickett, et al.
(1981)
1-2
Konikow and
Bredehoeft (1974)
INTERA
o Predicts contaminant trans-
port using random walk
methods
o Incorporated dispersion and
attenuation equations
o Field verified: fertilizers
o Accounts for varying dis-
charge and recharge rates
o Predicts contaminant trans-
port using method of
characteristics
o Allows for varying rechar-
ges and discharges, and
aquifer properties
o Field verified: radioactives
o Predicts contaminant trans-
port using finite difference
method
o Allows for dispersion and
attenuation effects
o Field verified: sea-water,
geothermal , rad ioacti ves ,
and coal tar
Does not incorporate
attenuation of chemical species
-------
TABLE 3-5 (continued]
Model
Reference
Dimensions
Modeled
Advantages
Di sadvantages
00
i
ro
LOMAM
Sykes , et al,
(1982)
1-2-3
Predicts contaminant trans-
port using finite element
techniques
Incorporates attenuation
and dispersion equations
Field verified: Cl, K,
radioactives, and aldicarb
Model specific to sanitary
landfills
Currently under review
-------
This section discusses several methods of obtaining indirect data useful
in estimating the limits of groundwater contamination. They are:
• Previously collected data
o Aerial imagery
o Geophysical (surface and subsurface) surveys.
Indirect data will not be useful for all sites and each method has
theoretical and practical limitations which affect the data obtained. The
potential benefits to be gained from indirect data collection efforts
(primarily geophysical surveys) must be weighed against their cost.
3.2.1 Previously Collected Data
Some data will always be available concerning sites at which CERCLA funds
are being used because these sites must be rated using the Uncontrolled
Hazardous Waste Site Ranking System (40 CFR 300 Appendix A), which requires
specific data on hydrogeologic conditions, waste types, and site conditions.
The type of information necessary to rank a site and therefore generally
available in EPA files is shown as Table 3-6. While the EPA and state
hazardous waste management agency files and inspection reports may provide
much of the data required to plan a remedial investigation, some data items
may not always be available. In these cases, supplemental information can
often be obtained from published maps and reports, computer data banks, and
other sources.
Published maps and reports on various topics are available from a number
of sources besides EPA and state environmental agencies. Table 3-7 lists
several sources in the Federal government. Many of the information items
listed in Table 3-7 are also available from sister agencies at the state and
local levels of government. Of these sources, probably the most useful are
the U.S. Geological Survey (USGS) and the state geological surveys. Both the
USGS and state surveys can provide information on studies currently being
conducted in a certain geographic area and typically have on-staff experts in
3-22
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TABLE 3-6
TYPE OF INFORMATION GENERALLY AVAILABLE IN
USEPA SITE INVESTIGATION REPORTS
WASTE DATA
- Type of compounds
- Physical state (solubility, viscosity, specific gravity)
- Amount
- Hazard assessments (toxicity, biodegradability, persistence,
ignitable, etc.)
POPULATION AREA
- Population within 1, 2, 3, or 4 mile radii of site
- Population served by drinking water within 3 miles of site
- Population served by surface water within 3 miles of site
HYDROLOGIC DATA
- Depth to groundwater
- Depth to aquifer
- Direction of groundwater flow
Hydraulic conductivity of aquifers
- Net precipitation and 1 year, 24-hour rainfall
- Groundwater use
- Distance to nearest drinking water well
- Groundwater sampling results
- Surface water sampling results
GEOLOGIC DATA
- Type of overburden and bedrock
- Area topography
- Location of sensitive environments
Soil sampling results
SITE HISTORY
- Type and extent of disposal operations
- Original and subsequent owners
- Observed contamination or release incidents
Regulatory responses
3-23
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TABLE 3-7
TYPES AND SOURCES OF MAPS, REPORTS, AND RELATED INFORMATION
(ADAPTED FROM CLARK, ET AL., 1981)
Topic Source*
Climate NCC, NOAA, NOS, NWS
Clinometric (Slope) GS, SCS
Coal Investigations GS
Earthquake Hazards GS
Floodplains CE, FIA, GS, NOS, SCS
Geodetic Control CE, GS, NOS
Geologic GS
Geophysical GS, NOAA
Groundwater GS
Land Cover PS, SCS
Land Use GS, SCS
Mineral Investigations GS
Mines BOM
Navigable Waterways CE, GS, TVA
River Basin/Watershed Surveys GS, SCS, TVA
River Surveys BR, TVA
Soils SCS
Topography GS
Water Resources GS
Wildlife and Scenic Rivers BLM, FS, PS
*Many of these types of maps and reports are also available from State and
local government agencies and universities:
BLM: U.S. Bureau of Land Management
Office of Public Affairs
Washington, DC 20240
BOM: U.S. Bureau of Mines
2401 E Street, NW
Washington, DC
BR: U.S. Bureau of Reclamation
P.O. Box 25007
Denver, CO 80225
CE: U.S. Army Engineer District
Corps of Engineers, Chicago
219 South Dearborn St.
Chicago, IL 60604
(continued)
3-24
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TABLE 3-7 (continued)
CE: U.S. Army Engineer District
Corps of Engineers, Nashville
Post Office Box 1070
Nashville, TN 37202
U.S. Army Engineer District
Corps of Engineers, Omaha
6014 U.S. Post Office and Courthouse Bldg.
Omaha, NE 68102
U.S. Army.Engineer District
Corps of Engineers, Vicksburg
Post Office Box 60
Vicksburg, MS 39180
FIA: U.S. Federal Insurance Administration
Engineering Division
451 7th Street, SW
Washington, DC 20514
FS: U.S. Forest Service
Information Office, Rm. 3238
Post Office Box 2417
Washington, DC 20013
GS: Eastern National Cartographic Information Center
U.S. Geological Survey
Reston, VA 22092
Mid-Continent National Cartographic Information Center
U.S. Geological Survey
1400 Independence Road
Roll a, MO 65401
Rocky Mountain National Cartographic Information Center
U.S. Geological Survey
Federal Center, Building 25
Denver, CO 80225
Western National Cartographic Information Center
U.S. Geological Survey
345 Middlefield Road
Menlo Park, CA 94025
(continued)
3-25
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TABLE 3-7 (continued)
NCC: U.S. National Climatic Center
Federal Building
Asheville, NC 28801
NOAA: U.S. National Oceanographic and Atmospheric Administration
Office of Public Affairs
14th Street, NW
Washington, DC
NOS: U.S. National Ocean Survey
Distribution Division (C-44)
6501 Lafayette Avenue
Riverdale, MD 20840
NWS: U.S. National Weather Service
Gramax Building
8060 13th Street
Silver Spring, MD 20910
PS: U.S. National Park Service
Office of Public Inquiries, Room 1013
Washington, DC 20240
SCS: U.S. Soil Conservation Service
Information Division
Post Office Box 2890
Washington, DC 20013
TVA: Tennessee Valley Authority
Mapping Services Branch
111 Haney Building
Chattanooga, TN 37401
3-26
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local geology. State surveys will probably provide more detailed hydro-
geologic information about a specific locality than would the USGS but
coverage is typically more limited. Data which geologic surveys are able to
provide include regional and local subsurface profiles, aquifer
characteristics, and potentiometric surface maps.
The USGS also coordinates NAWHEX—the National Water Data Exchange-
consisting of over 400 organizations concerned with water and water programs,
which have agreed to share their water data for maximum use. While the type
and quality of information varies between organizations, the service can
locate local sources of water data including well locations, geologic
profiles, potentiometric surfaces, water quality data, and aquifer
characteristics.
NAWDEX services are readily available. Requests may be made at one of 60
NAWDEX offices or the USGS National Headquarters (703/860-6031) by phone,
letter, or personal visit. The request is reviewed by NAWDEX for clarity and
completeness and then is processed. NAWDEX either provides the requested data
from its files or the inquiry is referred directly to organizations that hold
specific data. NAWDEX does charge a fee for the service (USGS, 1980).
While the primary function of NAWDEX is not to provide data storage and
retrieval services, NAWDEX does have direct access to several large data banks
including:
• WATSTORE -- The USGS maintains a water data storage and retrieval
system for geologic, hydrologic, and chemical data on surface water
and groundwater in the U.S. Data is available from approximately
16,000 stream gaging stations, 1,000 lakes and reservoirs, 5,200
surface water quality stations, 30,000 water level observation
stations and 12,500 groundwater quality wells. A groundwater site
inventory file contains information for nearly 700,000 wells
(Kilpatrick, 1981).
• STORET -- EPA maintains a storage and retrieval system which contains
over 40 million individual observations of water quality parameters
for both surface and groundwater. Information is collected from
several Federal organizations as well as more than 40 state
organizations.
3-27
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Despite the large number of data sources, WATSTORE, STORET, and the other
data banks available through NAWDEX will probably not contain information from
sources close enough to a particular site to be of great value. The infor-
mation they contain may provide useful regional geohydrologic data and the
service should be considered as an inexpensive source of background hydro-
geologic data.
In addition to the sources described above, there are also a variety of
other sources of information that can be used in the plume delineation
process. For example, well drillers familiar with local conditions can
provide a wealth of information on subsurface conditions and aquifer
characteristics. Local geotechnical contractors and testing laboratories can
be a valuable source of data on porosity, hydraulic conductivity, and other
subsurface characteristics. Geologists on the staff of local colleges and
universities should also be familiar with regional geology and hydrology.
Examples of other potential information sources are listed in Table 3-8.
A major type of previously collected data other than maps, reports,
computer data banks, and local contacts is aerial imagery. Aerial imagery is
the subject of the next section.
3.2.2 Aerial Imagery
Aerial imagery refers to pictorial representations produced by electro-
magnetic radiation that is emitted or reflected from the earth and recorded by
aircraft-mounted sensors. The simplest, most common form of imagery is the
photograph, which uses only the visible part of the electromagnetic spectrum.
Oblique photos are taken at angles to the earth's surface and thus distort the
scale of the picture (i.e., objects in the foreground are larger and objects
in the background are smaller than they should be). Perpendicular, or
stereophotos, which are taken from directly above the site so that there is
minimal distortion, can be used in pairs to show the topography of the site in
three dimensi ons.
3-28
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TABLE 3-8
EXAMPLES OF ADDITIONAL SOURCES OF INFORMATION
FOR REMEDIAL INVESTIGATIONS
Source
Information
State and Local Government Agencies
Health
Planning and Zoning
Tax Assessor
Engineer
Fi re Department
Law Enforcement
Water and Sewer
Utility Companies
Gas, Electric, Water, and
Petroleum or Natural Gas
Pipelines
Reports of unusual health problems
related to the site, drinking water
analyses, locations of contaminated
wells
Land use
Plat maps and land ownership
Foundation and inspection reports,
survey benchmark locations
History of fires or explosions at a
facility
Complaints and violations of local
ordinances during site operation
Geotechnical studies related to water
supply or construction, locations of
buried mains and lines, data related to
quality of influent to treatment plant
Location of buried lines
3-29
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TABLE 3-8 (Continued)
Source
Information
Contractors
Building
Soil exploration and
foundation
Water supply and wel 1
dril 1 ers
Trade Associations
National Water Well Association
500 W. Wilson Bridge Rd.
Worthington, OH 43085
Chamber of Commerce
Universities
Geology Engineering, Biology,
and Agronomy Departments, and
Medical Schools
Local soils, geology, and shallow water
1evels
Local soils, geology, hydrogeology,
water levels, regulations, and
equipment availability
Local hydrogeology, aquifer properties,
water quality, and locations of
existing wells
Local well drillers and related
contractors, well design and
construction information, and water
supply and aquifer restoration
i nformation
Information on local industries
including waste generators,
transporters, and disposers
Various types of information
on local conditions
3-30
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The second type of aerial imagery uses wavelengths of light outside the
visible spectrum. The most common and useful type for detecting the effects
of leachate is the infrared image. Infrared imagery indicates areas that are
hotter or cooler than the general surroundings. This is useful as areas of
dead vegetation having different radiant heating (albedo) characteristics than
vegetated areas (JRB, 1980).
Indicators of leachate can be both spatial and spectral. Spatial
indicators refer to physical changes to systems caused by leachate. These
include gaps in vegetation or snow cover, and areas of wetness indicating an
outbreak of groundwater or a leachate seep. Spectral indicators refer to
color changes in either water or in vegetation, and may make use of wave-
lengths other than in the visible range. If historical aerial imagery is
available, comparisons between new and old imagery can indicate changes in an
area that may be caused by leachate movement. The gradual development of a
barren area in a forest for unknown reasons may indicate the presence of
hazardous wastes. In general, spatial indicators are more useful and
consistent than spectral indicators (Sangrey and Philipson, 1979).
Gaps can occur in snow or vegetative cover because of leachate wetness,
toxicity or heat. Gaps caused by heat would appear in close proximity to
landfills because heat dissipates quickly with distance and would tend to
radiate from the landfill. Gaps caused by excessive wetness or toxic
substances can occur at greater distances and may radiate from a site or may
occur as isolated patches. Vegetative gaps can be detected in photographs or
near-infrared images. Gaps can also be distinguished in thermal infrared and
microwave images because emissions from vegetation and base soil are
different. Active microwave sensing by radar should detect most vegetative
soil gaps not obscured from direct view. Gaps in snow are easily detected in
photographs, or near-infrared, thermal infrared, and microwave images. The
problem with using gaps as indicators of leachate is that gaps are not limited
to leachate-affected sites. Also, gaps are not guaranteed to occur at the
limits of plume extent (Sangrey and Philipson, 1979).
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Damp and saturated zones or seeps at waste sites can be distinguished
quite easily by remote sensing. The spectral characteristics of seeps are
sufficiently distinct so that they are observable over the entire electro-
magnetic spectrum. Increased soil moisture decreases soil reflectance in the
visible and near-infrared wavelengths, allowing for detection of near-surface
water. During dry periods, these damp areas can be detected because of
anomalous spectral returns caused by moisture stressed vegetation. The
spectral anomaly most often used as a leachate indicator is vegetative stress.
Vegetative stress can be noted by the use of color infrared scanners (Aronoff
and Ross, 1982). The variations caused by vegetative stress can be a result
of death or defoliation from toxic effects, reduction in foliages, or
temperature changes. However, increased nutrient and moisture levels caused
by leachate have been known to enhance vegetative growth. Spectral variations
caused by leachate seeps, oils, or other fluid coatings on water can be
detected in ultraviolet, thermal, or microwave images.
Aerial images may indicate where contamination occurs, but will not
identify the boundary of the contaminated zone because gaps, wetness, and
stressed vegetation are not limited to occurring only at the edges of a plume.
However, these contaminated zones serve as additional data points that are
useful in site investigations. For example, if calculations of flow rate
indicate that the maximum extent of contamination should be X yards from a
landfill, yet stressed vegetation is readily apparent at X + 300 yards,
several explanations are possible. First, the input data to Darcy's Law may
be incorrect: hydraulic conductivity may be greater than calculated, the time
from the start of contamination may be longer than estimated, the gradient may
be steeper, or porosity lower. Second, a preferential migration pathway which
allows contamination to move much faster than through the majority of the
geologic system may have been overlooked. Third, the constituents of the
plume may be more mobile than originally estimated (i.e., dispersion in this
system has a large effect on migration rates). Fourth, there may be multiple
sources of contamination. Finally, the stress may not be caused by the
leachate.
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Geohydrologic conditions can also be determined or verified using aerial
imagery. Aerial image interpretation can be used to:
• Identify rock and soil types, geomorphological features, the nature of
sediments, joint and fault patterns, and outcrop areas
9 Approximate stream flow, evapotranspiration, infiltration, and runoff
values
e Map topography; streams, seeps, and other surface waters; and
vegetation not readily apparent from ground level.
One final use of aerial imagery is in the analysis of historial images to
analyze disposal site operations. Historical data can set the time of the
start of disposal operations for the calculation of migration rates. The
lateral extent and timing of disposal operations can also be estimated using
historic air photos (Erb, et al., 1981). Finally, changes in surface
topography over time can be determined. This last item was of value during
the investigation at Love Canal, where historical aerial photos were used to
map old swales and drainage areas since filled and thought to act as areas of
preferential migration routes (Kufs, et al., 1981).
EPA has three offices that can provide information on all aspects of
aerial photography and photo interpretation:
• Environmental Monitoring Systems Laboratory (EMSL)
P.O. Box 15027
Las Vegas, Nevada 89114
• Environmental Photographic Interpretation Center (EPIC)
P.O. Box 1587
Vint Hill Farm Station
Uarrenton, Virginia 22186
9 National Enforcement Investigation Center (NEIC)
Building 53, Box 25227
Denver, Colorado 80225
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Aerial imagery is available from a number of sources besides EPA.
Government agencies that also perform aerial surveys include:
• Agricultural Stabilization and Conservation Service
• U.S. Geological Survey (USGS)
• U.S. Forest Service
o National Air and Space Administration (NASA)
• National Oceanic and Atmospheric Administration (NOAA)
e National Archives and Record Service
0 National Weather Service
t Soil Conservation Service (SCS)
• Commodity Stabilization Service
• U.S. Air Force (USAF)
• Corps of Engineers
t Bureau of Reclamation
• U.S. Coast and Geodetic Survey
• Tennessee Valley Authority (TVA)
Government sources of aerial imagery and related data are listed by state
by the USGS (1976). In general, the majority of non-military aerial
photography acquired by Federal agencies can be obtained from:
• National Archives and Records Service
Cartographic Branch
8 Pennsylvania Avenue, N.W.
Washington, D.C. 20408
(for photographs taken prior to 1942)
o EROS Data Center
U.S. Geological Survey
Sioux Falls, South Dakota 57198
• Agricultural Stabilization and Conservation Service
U.S. Department of Agriculture
205 Parley's Way
Salt Lake City, Utah 84109
3-34
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o Soil Conservation Service
U.S. Department of Agriculture
Cartographic Section
6505 Delcrest Road
Hyattsville, Maryland 20782
Private sources of aerial imagery can be obtained from the American
Society of Photogrammetry, 105 North Virginia Avenue, Falls Church, Virginia,
22046.
3.2.3 Geophysical Methods
Over the last 10 years, geophysical equipment, survey techniques, and
data processing have been refined to the point where geophysical methods can
provide valuable data during hazardous waste site characterizations (Evans, et
al., 1982). Geophysical methods can be divided into two groups:
• Surface Surveys -- where instruments used in collecting data are
located on the land surface
i
o Borehole Methods -- which require direct access to the subsurface
through boreholes or wells.
Surface surveys provide data that indirectly determine the subsurface geology.
Data may be obtained concerning (Fetter, 1980; Glaccum, et al., 1982; Sendlien
and Yazicigil, 1981 and 1982):
o Thickness of unconsolidated strata
e Depth to water table
• Fault location and joint patterns
9 Location of solution channels
• Location, thickness, and extent of subsurface bodies such as clay or
gravel deposits
e Presence, distribution, and location of buried wastes
« Presence and extent of certain types of contaminants and leachate
plumes.
3-35
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Borehole methods provide data on (Fetter, 1980):
• Areas of high porosity and hydraulic conductivity
o Water flow rate and direction
• Subsurface stratigraphy
0 Lithology of subsurface units
• Chemical and physical characteristics of water.
Surface geophysical methods appear to have more application to hazardous
waste site investigations than borehole methods. Proponents of surface
geophysical methods (e.g., Sendlein and Yazicigil, 1981; Pease and James,
1981; Evans, et al., 1982; Glaccum, et al., 1982) correctly point out that
monitoring wells may poorly define contaminant plumes for a variety of
reasons. The attractiveness of surface geophysical surveys is that they are
able to provide a theoretically continuous profile of the subsurface and,
therefore, are much better suited to detecting preferential migration routes.
Several surface geophysical methods measure changes in the physical parameters
of water that may be caused by the presence of leachate or contamination, and
thus serve to outline the possible boundaries of a plume. Surface geophysical
methods, when used in conjunction with conventional drilling programs, can
have application to many waste site investigations (Evans, et al., 1982).
Two things should be mentioned at this point. First, surface geophysical
methods are not a panacea. They are sources of valuable information but they
do not have application in all situations and are not always the most
effective method of obtaining information (USGS, 1977). Second, geophysical
methods do not provide direct data on groundwater contamination. Those
methods that outline possible plume boundaries measure changes in physical
parameters (e.g., specific conductivity) that may be caused by the presence of
contaminants in groundwater, but they do not provide indications of contami-
nant type and concentration (e.g., toluene at X ppb). Thus, groundwater
samples from monitoring wells will always be necessary to provide direct data
on contaminant type and concentration.
3-36
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The surface geophysical methods rnost applicable to plume delineation
include:
• Ground penetrating radar
e Electromagnetic conductivity
« Galvanic electrical resistivity
• Sei smic methods.
The theory, application, data use, and limitations of each of these methods is
discussed below. Borehole geophysical logging methods are also summarized.
As with all investigation techniques, a careful definition of the problem and
identification of the types of information required should be made prior to
selecting any particular geophysical method or suite of methods.
3.2.3.1 Ground Penetrating Radar (GRP)
GPR systems generate electromagnetic radiation that reflect from various
objects because of changes in the object's ability to conduct the electro-
magnetic wave. The reflections are recorded and interpreted to provide a
continuous subsurface profile, much like a geologic cross-section, along a
traverse path. GPR systems can be used to locate buried drums, detect inter-
faces between different geologic strata, and identify plumes of high chemical
concentration (Pease and James, 1981). The depth of penetration and resolu-
tion are affected by site conditions, thus the applicability of GPR must be
decided on a site specific basis.
GPR systems consist of four elements:
o Electromagnetic radiation source
• Transmitting antenna
• Receiving antenna
o Data processing and display equipment.
GPR power units and pulse transmitters feed the pulse to the transmitter
antenna, which radiates the pulse into the subsurface. Reflected signals are
3-37
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received, usually by the same antenna, and fed into a receiver electronically
isolated from the transmitter. The reflection is amplified and processed to
produce a waveform similar to the original but with a longer time base. This
new signal can be processed further and displayed. The most common type of
display system uses a graphic recorder. Most GPR systems mount all the
necessary equipment, except for the antenna, within the survey vehicle
(Horton, et al., 1981; Benson and Glaccum, 1979). Antennas may be externally
mounted on the same vehicle or on an independent frame capable of being towed.
Prior to running a GPR survey across a site, several preliminary steps
are usually completed. Stakes are driven into the ground to outline the
survey grid and the traverse path is cleared of brush, tree limbs, tall
grasses and weeds, and any other obstacles that may hinder the smooth passage
of the survey vehicle. Several preliminary scans are made to calibrate the
system with the specific electromagnetic conditions at the site. In addition,
the velocity of the signal in the medium must be calculated, along with the
dielectric constant of the soil. Once these values are calculated, the pulse
repetition frequency is set, along with frequency and energy output.
Different settings may be used and several surveys run to obtain maximum
resolution and depth of penetration.
Surveys are performed by slowly moving the antenna over the ground
surface. The survey vehicle is moved across the grid at a set speed (usually
around 5 mph). Precise speed must be maintained such that the area! location
of targets can be positioned within a few feet. For more detailed studies,
discrete readings can be taken at selected points along a traverse (Benson and
Glaccum, 1979). After the survey is run, the data is usually taken from the
recorder for further processing. A graphic display can be generated on-site,
and examined in the field to provide a general indication of the subsurface
conditions (i.e., the general locations of drums) and the effectiveness of the
survey. Fence diagrams can be developed from the subsurface profiles.
Usually GPR cannot be used to map anomalous groundwater conditions that
may be indicative of contamination. However, GPR can be used very effec-
tively, in certain conditions, to outline the water table and indicate the
3-38
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direction of groundwater flow. GPR can be used to outline the boundaries of
disposal sites, which is important in selecting monitoring well locations.
GPR can also map the presence of many soil features including cementation,
clay lenses, organic layers, sediment layering, water infiltration zones,
depth to bedrock, top of bedrock topography, and fractures and cavities in
bedrock.
Because most GPR systems are vehicle mounted, they are limited to sites
that allow access to and over the area of concern. Sites with swamp-like
conditions would probably not be good candidates for GPR surveys because of
access problems and because of the presence of silt, clays, and high water
content reducing the depth of penetration. Likewise, sites with large
boulders, pits, or high topographic relief would not allow the required access
across the site. The type of geologic material present at a site and the
presence of water can dramatically influence the effectiveness of GPR. Depths
up to 30 feet are commonly attained using GPR and in certain cases up to
60 feet penetration has been reported (Evans, et al., 1982). However, high
clay concentrations rapidly attenuate radar waves reducing penetration to less
than 3 feet (Benson and Glaccum, 1979). Lower transmission frequencies can
yield greater penetration, but resolution is sacrificed (Lord, et al., 1980).
The use of GPR in areas with high proportions of silt and clay size particles
should be questioned.
3.2.3.2 Electromagnetic Methods (EM)
EM methods are available which measure the electrical conductivity of
subsurface materials. The conductivity (reciprocal of resistivity) of a
substance is a measure of the ease with which a current passes through it.
Conductivity methods have been used to (McNeill, 1980):
• Determine rock lithology and bedrock depth
• Locate and map aggregate and clay deposits
c Map groundwater extent and salinity
• Detect pollutant plumes
• Locate geothermal areas.
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The theory behind using EM fields to induce currents in conductive media
is well established. A magnetic current is created by passing an alternating
current through a wire loop. When the wire loop is in close proximity to
conductors (i.e., earth materials), a current is induced in the conductor.
The strength of the current flowing through the conductor is dependent upon
its conductivity. Since any current generates a magnetic field, the induced
current produces a secondary magnetic field with the same frequency as the
primary field, but of reduced intensity. The secondary field induces a
current in another wire loop, the receiver. By measuring the reduction in
voltage between the primary and secondary fields, the conductivity
(resistivity) of the material can be determined.
The ability of soils and rocks to conduct electrical currents depends
primarily on the amount, salinity, and distribution of water (USGS, 1977).
Saturated soil and rock can be considered insulator particles immersed in a
conductive fluid. The total resistivity of the system depends on the
conductive fluid and the impedence placed on current flow by the insulators
(McNeil 1, 1980). Minerals in sand and silt fractions of soils are generally
excellent insulators. Dry clay is also an excellent insulator, but moisture
allows certain ions adhering to the surface of clay particles to transmit
electrical current decreasing the resistivity. The effect of organic matter
on resistivity has not yet been determined. Gases dissolved in water tend to
inhibit electrical conductance and thus increase resistivity. Water is by far
the most important influence on conductivity (McNeill, 1980).
There are two types of EM survey techniques:
• Profiling -- where lateral changes in conductivity along a set depth
from the surface are measured
• Sounding -- where vertical changes in conductivity are measured.
In profiling, depth of exploration is set by separating the primary
(generator) and secondary (receiver) coils. A traverse is surveyed and
conductivity at the set depth of exploration can be recorded continuously or
can be measured at discrete points. Contours of equal conductivity are
3-40
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plotted. The location and shape of subsurface features with anomalous
conductivity values can be described, but only at the depth of exploration.
Since profile surveys can be run almost as fast as a survey team can walk,
data from several surveys along the same traverse but with varying depths of
exploration can be evaluated to provide a three dimensional profile of
conductivity anomalies.
In sounding surveys, vertical changes in conductivity are measured by
adjusting electrode spacing and measuring changes in vertical conductivity
with depth. Using available EM equipment for sounding is inconvenient because
most EM setups have set electrode spacings. Sounding surveys are generally
performed using galvanic electrical resistivity equipment.
In order to map contaminant plumes, a contrast in conductivity must exist
between the contaminant plume and local geohydrologic background values.
Leachates with high total dissolved solids usually have high conductivity, but
an uncontaminated saturated clay lens may exhibit the same response. In
mapping depth to water table and groundwater flow direction, the great change
in conductivity between saturated and unsaturated materials is usually
sufficient to allow the water table to be mapped.
Limitations of EM conductivity include (Pease and James, 1981):
• Ability to detect non-conductive pollutants is limited
• Ability to detect plumes is limited if there is not a sharp contrast
between plume and natural groundwater
e Lateral variations in stratigraphy complicate interpretation
• Buried conductive objects may result in anomalous readings.
Horizontal profiling surveys using EM conductivity and performed under
suitable conditions can rapidly trace the outlines of contaminant plumes up to
200 feet deep. Vertical sounding can be performed using EM conductivity, but
is usually performed using Galvanic electrical resistivity.
3-41
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3.2.3.3 Galvanic Electrical Resistivity
Galvanic electrical resistivity methods are among the most widely used
methods in groundwater studies (Yazicigil and Sendlein, 1982; Fetter, 1980).
Resistivity surveys involve applying a direct electrical current into the
subsurface and measuring voltage passing between electrodes. In applying the
current, four electrodes are used—two to generate the current (a positive and
negative electrode) and two potential electrodes to measure the resultant
current flow through the earth materials. By knowing the current flowing
through the ground and the voltage between electrodes, the apparent
resistivity of earth materials between the electrodes can be calculated
(difference between current applied and current measured). The same
properties affecting conductivity also affect resistivity surveys and they may
be used in the same manner to detect contaminant plumes and provide data on
subsurface geology.
There are three electrode arrays commonly used in direct electrical
resistivity surveys (Fetter, 1980; Lord, et al., 1981; Raghunath, 1982):
• Wenner Array -- where the positive and negative electrodes and the
potential electrodes are spaced evenly apart
• Schlumberger Array -- where the potential measuring electrodes are
spaced close together and the current electrodes are spaced far apart
• Dipole-dipole Array -- where the potential and current electrode pairs
are spaced a mutual distance (x) apart and the pairs are an integral
multiple of x from each other. This array is usually limited to very
deep surveys.
Both sounding and profiling can be performed using electrical resistivity
equipment. In sounding, the spacing between the potential and current
electrodes is increased. This causes the current to move deeper into the
subsurface and the apparent resistivity measured incorporates the resistivity
of the deeper material. Either the Wenner or Schlumberger Array may be used;
but the Schlumberger Array is usually much more convenient because the two
inner electrodes are spread only occasionally (Fetter, 1980). Sounding begins
with electrodes positioned close to each other. After a reading is made, the
3-42
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electrodes are positioned farther apart and new readings made. A plot of
calculated apparent resistivities taken at different electrode spacings, but
centered on a single location, is termed a sounding curve. This curve
reflects the changes in electrical characteristics with depth. The curve is
generally interpreted as a series of horizontal subsurface layers each with a
specific resistivity (Urish, 1983).
In horizontal profiling, electrode spacing remains constant, but the
electrodes themselves are moved in a grid pattern over the land surface. The
Wenner Array is used most often in horizontal profiling. If subsurface
conditions are uniform, apparent resistivity should not change across the
study area. Any changes can be interpreted as being caused by subsurface
heterogeneity (e.g., clay lenses in sands) or by contaminated groundwater. In
setting up the grid pattern, the spacing between stations can conveniently be
equal to, or an integral multiple of, the spacing between electrodes. This
facilitates movement of the array. A decision must be made as to the effort
involved in increasing the number of stations and the risk of bypassing
contaminant plumes if stations are farther apart.
Direct electrical resistivity methods, while not as mobile as EM methods,
allow any depth (up to limits of the equipment) to be explored. However,
there are limits to use including (Pease and James, 1981; Urish, 1983):
• Ability to detect non-conductive pollutantsis limited
t Ability to detect plumes is limited if sharp resistivity contrasts do
not exist
• Lateral variations in stratigraphy complicate interpretation
• Equipment range (dependent upon electrode spacing, power output and
subsurface resistivity) is site specific
t Rocks, trees, buildings, and other obstacles may restrict grid
patterns
• Electrical interferences, power lines and buried cables, in general,
must be kept at least one electrode spacing from the grid.
3-43
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Nevertheless, direct electrical resistivity profiling is a relatively
fast and inexpensive method of collecting subsurface data and may be used to
outline plume boundaries prior to monitoring well installation. The advantage
of direct electrical resistivity over EM methods is in the ability to more
precisely set exploration depth.
3.2.3.4 Seismic Refraction Surveys
In a seismic refraction survey, shock waves generated by small explosive
charges or by heavy blows on a metal plate travel downward through the earth
and are refracted back to the surface from the interfaces between different
layers. The refracted waves are picked up at various points on the ground
surface by geophones and recorded. By knowing the arrival times of different
waves at set distances from the energy source, the velocity of propagation of
the waves through each rock layer can be calculated. Particular rocks under
specific conditions have characteristic velocities, thus subsurface profiles
can be developed. Depths to several hundred feet can be probed using seismic
refraction techniques (Ragnuhath, 1982).
The most common use of seismic methods is in mapping bedrock surface.
Buried valley aquifers, for instance, have been mapped in this way. Water
table depth can usually be determined using seismic methods, but only if the
water table lies in unconsolidated deposits. Furthermore, thin layers of
saturated sediments lying above bedrock may not be detected nor will layers
underlying a higher velocity layer. In certain cases, lateral lithologic
changes in aquifers can be mapped. However, large amounts of seismic data and
correlation with borehole data is usually required. Vertical strati graphic
profiles are more easily developed than horizontal profiles using seismic
methods.
While seismic refraction cannot delineate plume boundaries, determining
the depth to groundwater (in alluvium) and fault locations are two major uses
in groundwater studies. The great depth of penetration of this method makes
seismic refraction useful in areas where other geophysical methods are not
applicable.
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3.2.3.5 Geophysical Well Logging
Geophysical well logs are an important tool in defining the subsurface
environment, and often provide the only practical measurement of undisturbed
subsurface sediments. Geophysical logs are useful in determining (USGS,
1977):
0 Lithology
• Geometry
• Resistivity
• Bulk density
• Porosity
• Hydraulic conductivity
0 Moisture content
o Specific yield of water-bearing rocks
o Chemical and physical characteristics of water.
Borehole logging complements drillers and geologists logs, and soil and
rock samples. Cased wells can often be logged through the casing. Borehole
logs correlated with surface geophysical methods usually increase the value of
data obtained from both sources (USGS, 1977). There are many logging methods
in use today, several of which are described below. Geophysical logs are
usually run as pen and ink strip charts. In general, borehole systems consist
of a probe which is lowered into the borehole on a cable containing powerlines
to the probe, transmitter cables to the recorder, and a support cable. The
probe contains the required electronics, energy or nuclear sources, and
detectors. Types of borehole logs are summarized below.
Caliper Logs -- Caliper logs measure the diameter of uncased boreholes in
bedrock units. In plume delineation studies, they can be used to indicate
solution cavities, bedding planes, faults and joints, and other preferential
routes of water movement.
Temperature Logs -- A temperature log is a continuous vertical record of
fluid temperature in the borehole. Temperature logs are usually the first log
3-45
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run on a borehole to minimize mixing effects. The assumption implicit in
temperature logging is that fluid is in equilibrium with the geologic
materials. Temperature logs are useful in locating points of groundwater
entrance or loss from the borehole. Temperature logs are also important in
correcting resistivity and spontaneous potential logs in deeper wells (USGS,
1977).
Single-Point Resistance -- Single-point resistance is one of several
methods for measuring electrical resistance. A single electrode is lowered
into the borehole on an insulated cable while the second electrode remains on
the surface. As the electrode is lowered into the borehole, the resistance of
the material from land surface is measured (Fetter, 1980). The single-point
resistance method has a very limited depth of investigation and does not
measure the resistance of specific formations.
Resistivity -- Resistivity can be measured by spaced electrodes of
various configurations. The short-normal configuration measures resistivity
of the zone closest to the borehole. The long-normal method measures
resistivity farther from the borehole. Lateral configurations can be spaced
to measure set distances from the borehole. However, the wider the spacing
the less likely that thin beds of different material may be found. Resis-
tivity is an important indication of stratigraphy, lithology and water
quality.
Spontaneous Potential -- Spontaneous potential (SP) logs measure the
natural electric potential that develops from contact between the formation
and borehole fluids. SP logs are used chiefly for correlating geologic units,
determining bed thickness, calculating formation water quality, and separating
porous and non-porous rocks such as shale-sandstone and shale-carbonate
sequences (USGS, 1977). SP logs consist of a surface electrode and a borehole
electrode connected to a voltmeter to measure potential.
Nuclear Logging — Nuclear logging can be done in cased or uncased holes.
Measurements can be made of either natural radiation or of the ability of
subsurface materials to attenuate induced radiation. Natural-gamma logs are
3-46
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records of natural gamma radiation emitted by all rocks. They are used
primarily to identify lithology and correlate stratigraphy, especially in
detrital sediments. Neutron logging utilizes a probe with a radioactive
source of neutrons and a detector. Neutron logs respond to water. Below the
water table they measure porosity, while above they measure moisture content.
Gamma radiation logs measure the absorption or scattering of cobalt 60.
Absorption is proportional to bulk density of earth materials and is an
indirect reading of porosity.
Table 3-9 summarizes the applications of various borehole logging
techniques.
3.3 Direct Data
Direct data includes any information which can be applied directly and
precisely to delineating a leachate plume. Direct data can be qualitative
(e.g., observing a leachate seep is direct qualitative evidence of groundwater
contamination at a point) or quantitative (e.g., chemical analysis results).
The three primary means of collecting direct data include:
t On-site and near-site inspections
• Hydrologic testing
e Sampling.
These activities should be undertaken at all sites prior to beginning aquifer
restoration design and implementation activities.
On-site inspections serve three purposes. First, the accuracy of the
background data collected prior to the site visit must be verified. Second,
observations that only a trained investigator can make at a site must be
compiled. Third, a decision must be made on what follow-up work should be
performed at the site and how and when this work should be accomplished.
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TABLE 3-9
SUMMARY OF BOREHOLE LOG APPLICATIONS (KEYS AND MacCARY, 1971)
Information Required
Applicable Logging Techniques
Lithologic and stratigraphic correla-
tion of aquifers and associated
rocks
Total porosity and bulk density
Effective porosity and true resistivity
Clay and shale content
Secondary porosity--fractures,
solution openings
Electric, sonic, or caliper logs
made in open holes. Nuclear logs
made in open or cased holes.
Calibrated sonic logs in open holes;
calibrated neutron or gamma-gamma
logs in open or cased holes.
Calibrated long-normal resistivity
logs.
Gamma logs and resistivity log.
Caliper, sonic, or borehole
television logs.
Specific yield of unconfined aquifers Calibrated neutron logs.
Location of water levels and saturated
zones
Moisture content
InfiItration
Dispersion, dilution, and movement
of waste
Source and movement of water in
a well
Chemical and physical characteristics
of water including salinity,
temperature, density, and viscosity
Electric, temperature or fluid con-
ductivity in open hole or inside
casing. Neutron or gamma-gamma
logs in open hole or outside
casing.
Calibrated neutron logs.
Time-interval nuetron logs under
special circumstances or
radioactive tracers.
Fluid conductivity and temperature
logs and gamma logs for some
radioactive wastes.
Injectivity profile. Flowmeter or
tracer logging during pumping or
injection. Temperature logs.
Calibrated fluid conductivity and
temperature in the well. Neutron
chloride logging outside casing.
Multielectrode resistivity.
3-48
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TABLE 3-9 (continued)
Information Required Applicable Logging Techniques
Construction of existing wells; Gamma-gamma, caliper, collar,
diameter and position of perforation locator, and borehole
casing, perforations, and screens television.
Guide to screen setting All logs providing data on the
lithology, water-bearing
characteristics, and correlation
and thickness of aquifers.
Cementing Caliper, temperature, or gamma-gamma.
Acoustic for cement bond.
Casing corrosion Under some conditions caliper or
collar locator.
Casing leaks or plugged screen Tracer and flowmeter.
Table 3-10 highlights the usefulness of selected site inspection observations
to plume delineation. General site inspection procedures have been described
in several sources including Sisk (1981), JRB (1980) and numerous shorter
references which will not be detailed here. The following two sections
describe the roles of hydrologic testing and sampling in plume delineation.
3.3.1 Hydrologic Testing
Improving the precision of plume boundary calculations generally requires
hydrologic tests of aquifer properties. However, use of aquifer testing
results is secondary to designing plume migration control measures such as
pumping systems. The most important aquifer properties to determine for plume
control technology design are:
o Hydraulic conductivity (K) -- a coefficient of proportionality that
describes the rate at which water, of a prevailing density and
viscosity, can move through a porous medium
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TABLE 3-10
SIGNIFICANCE OF SELECTED ON-SITE OBSERVATIONS TO PLUME DELINEATION
Site Feature
Observation
Significance to Plume Delineation
Facility
Information
OJ
o
Evidence of
Contamination
Groundwater Flow
Locations of waste transfer, treatment,
storage, or disposal activities
Facility design, construction, and
operation
Locations of water supply or monitoring
wel 1 s
Settling cracks in foundations or
basement seepage
Waste types, volumes, forms, and modes
of disposal
Spills
Dead, sparse, or stunted vegetation, and
barren areas
Discolored, disturbed, or odorous soil
Leachate seep
Springs
Possible sources of contamination
Indicative of facility's ability to
control releases
Possible future sampling points
Possible high water table; possible
preferential routes of leachate
migration (backfill for foundations)
Indicative of the possibility,
mobility, size, and hazardousness of
the leachate plume
Possible source of contamination
Possible effects of near-surface
contaminated groundwater
Possible source or result of
contaminant release
Presence of contaminated qroundwater
Intersection of water table with la:id
surface; flow rate related to water
table gradient and hydraulic conductivity
(Continued)
-------
TABLE 3-10 (Continued)
Site Feature
Observation
Significance to Plume Delineation
Groundwater Flow
(continued)
oo
I
CJ1
Surface Water
Flow
Depth to water in wells
Land surface topography
Variations in stream discharge through
site
Locations, discharges, and schedules
of pumping wells
Surface water elevations
Locations
Chemical quality and evidence of
contamination
Drainage patterns
Elevation and gradients of the water
table
Possible directions of flow in
unconfined aquifer
Possible indication of whether streams
recharge groundwater (termed losing or
influent streams) or whether ground-
water recharges streams (termed
gaining or effluent steams)
Possible effects of groundwater with-
drawal on plume movement; possible
future sampling points
Can indicate directions of ground-
water and plume flow
Possible locations of seeps; possible
sampling points; relation of site to
discharge and recharge areas in
drainage basin
Possible indication of subsurface
leachate discharge
Possibly indicative of bedrock structure
and controls on groundwater and contaminant
flow (Howard, 1967)
-------
TABLE 3-10 (Continued)
Site Feature
Observation
Significance to Plume Delineation
Surface Water
Flow
(continued)
Geomorphology
OJ
i
en
ro
Vegetation
Drainage density
Slopes
Sinkholes/karst topography
Floodplains, meander scars, and
other stream features
Dunes
Wetlands
Tidal flats
Glacial deposits
Type of plants (e.g., phreatophytes
such as willow, ash, and cottonwood)
Possibly indicative of infiltration
capacity of soil and leachate generation
potential
Possibly indicative of infiltration capacity
and leachate generation potential; possibly
indicative of recharge and discharge areas
Usually indicative of solution channel
control of groundwater and contaminant flow
Possibly indicative of buried stream
channels and other preferential routes
of leachate migration
Indicative of unconsolidated sand deposits
of high permeability
Near-surface water table; many seeps
1ikely
Near-surface water table; possible
changes in groundwater flow directions
because of the influences of tides
Possibly indicative of highly
stratified materials of varying permeability;
clay deposits may be fractured
Possibly indicative of general
availability of water, depth to water,
moisture retention, and other hydrologic
properties
-------
TABLE 3-10 (continued)
Site Feature
Observation
Significance to Plume Delineation
Vegetation
(continued)
Surface material
OJ
i
en
CO
Distribution of plant types
Stressed, dead, or absent vegetation
Texture (grain size mix), sorting,
angularity, organic content and other
properties
Munsell color
Soil structure
Classification and distribution
Bedrock outcrops (i.e., attitude of
bedding, joints, faults and other
discontinuities; rock types)
Possibly indicative of soil or bedrock
types and structures, and site
hydrogeology
Possibly indicative of near-surface
contamination
Indicative of the hydraulic conductivity and
porosity of the material , and
possibly the rate of groundwater
and leachate movement
Possibly indicative of moisture
conditions and leachate generation
potential
Indicative of infiltration capacity
and leachate generation potential
Possibly indicative of bedrock
conditions and groundwater movement routes
Possibly indicative of rates of ground-
water and contaminant movement
-------
• Transmissivity (T) -- the rate at which water is transmitted through a
unit saturated thickness (m) of an aquifer or confining bed under a
unit hydraulic gradient
« Storativity (S) -- the volume of water an aquifer releases from or
takes into storage per unit surface area of the aquifer per unit
change in head.
Storativity must be determined in the field. Hydraulic conductivity can
either be determined in the laboratory or be derived in the field. Estimates
of T can be made in the field or using the relationship:
T = Km
where m is the saturated thickness of the aquifer. The following sections
describe some of the different types of aquifer tests for determining K, T,
and S.
3.3.1.1 Laboratory Tests
Many methods are available to determine hydraulic conductivity in the
laboratory. In all cases a sample of the earth material must be collected in
the field and returned to the laboratory for testing. The typical method for
collecting samples of unconsolidated material is with a tube sampler (e.g.,
Shelby tubes, split spoons) and for collecting rock samples is core drilling.
Numerous references are available from the American Society for Testing and
Materials which describe the techniques and tools available for obtaining
samples so that disturbance to the sample is minimized. Samples collected in
this manner are commonly labeled "undisturbed samples." In reality,
"undisturbed samples" is a misnomer because certain sample properties (e.g.,
porosity, sorting) will change to some extent, regardless of the care taken in
sample extraction. Furthermore, gross aquifer characteristics (e.g.,
fractures, bedding planes, cavities) are seldom represented intact and in the
proper proportion to the rest of the sample. As a result, laboratory and
field studies of the same geologic unit can, under certain conditions, produce
significantly different estimates for aquifer properties.
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Two basic methods are available for determining hydraulic conductivities
in the laboratory—the constant head method and the falling head method. For
the constant head method, inflow fluid level is maintained at a constant head
and outflow rate is measured as a function of time. Hydraulic conductivity is
then calculated using:
K = QLs/hsAs
where:
K = hydraulic conductivity (ft/day)
Q = outflow rate (ft3/day)
L = length of sample (ft)
h = fluid head difference across sample (ft)
2
A = cross-sectional area of sample (ft )
For the falling head method, the head of the inflowing fluid is allowed
to decrease over time, while the fluid head at the outflow is held constant.
Assuming a large standpipe, hydraulic conductivity for this method can be
calculated using:
K = (2.3ApLs/Ast) log (h./he)
where:
K = hydraulic conductivity (ft/day)
o
A = cross-sectional area of standpipe (ft )
L = length of sample (ft)
2
A = cross-sectional area of sample (ft )
t = time for head level decline (day)
h. = initial head level (ft)
hg = final head level (ft)
Several other methods are available for determining hydraulic conduc-
tivities but all are generally based in part on these two simple methods.
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Table 3-11 summarizes the more common methods utilized to obtain hydraulic
conductivities and their applications. In practice, the test performed on a
sample sent to a laboratory will probably be dependent upon the instrumen-
tation available, not on the applicability of the test to the soil type.
Roberts and Nichols (1981) provide additional information on methods for
determining hydraulic conductivity.
Uhen a laboratory hydraulic conductivity test is performed, there are
numerous potential sources for error in the estimates. Table 3-12 lists some
of these potential sources and the effect they have on the calculated
hydraulic conductivity. If a laboratory measured hydraulic conductivity does
not appear to be consistent with field observations, a second test should be
performed keeping in mind the areas where potential error sources can be
introduced. Laboratory measurements of hydraulic conductivity can be many
orders of magnitude lower than field tests of a sample for a variety of
reasons. However, most comparisons are within an order of magnitude (Olson
and Daniel , 1979).
3.3.1.2 Field Tests
The determination of hydraulic conductivity by a well test is highly
dependent on the design, construction, and development of the well to be
tested. Tests performed on improperly constructed wells will reflect
conditions caused by well construction rather than those of the earth
material. In some cases, the well to be tested will already be in place. For
these instances, a well log and as-built drawing should be obtained for the
well so that elements that may affect the test can be identified. Newly
drilled wells should be designed, installed, and developed properly to ensure
accurate test results (refer to Chapter 5).
Methods available for determining an aquifer's properties based on an
in-situ field test can be divided into single well tests and multiple well
tests (i.e., a test well with observation wells). All of these tests either
require the removal of water from the well or the injection of water into the
well.
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TABLE 3-11
LABORATORY METHODS FOR DETERMINING HYDRAULIC CONDUCTIVITIES
oo
CJ1
Method
Constant head
Falling head
Modified constant
head
Triaxial cell
Pressure-chamber
•
•
•
•
•
•
•
•
•
•
Application Mathematics
Best for samples with high conductivities (i.e., K = QL /h A
coarse grained) s s
Can be used with fine grained samples but test
times may be prohibitively long
Any soil type K = (2.3 A L /A t) log (h^h )
Best suited to materials having a low permeability p
Any soil type K = QL /h A
Best suited for fine-grained soils
Any soil type K = QL /h A
Especially suited for fine-grained, compacted cohesive
soils in which full fluid saturation is difficult to
achieve
Any soil type K = (2.3 A L/At) log (h./h )
Remolded samples p s s i e
-------
TABLE 3-12
EFFECTS OF VARIOUS TYPES OF ERRORS ON LABORATORY
MEASURED VALUES OF HYDRAULIC CONDUCTIVITY (EPA, 1982d)
Source of Error Measured K
Voids formed in sample preparation High
Smear zone formed during trimming Low
Use of distilled water as a permeant Low
Air in sample Low
Growth of microorganisms Low
Use of excessive hydraulic gradient Low or High
Use of temperatures other than the Varies
test temperature
Ignoring volume change caused by stress High
change (confining pressure not used)
Performing laboratory rather than Usually low
in situ tests
Impedance caused by the test Low
apparatus, including the resistance
of the screen or porous stone used
to support the sample
Single well tests can be utilized to measure hydraulic conductivities
when only one well is present at a site or when other wells are located too
far away to make them usable for observation purposes. Most of these methods,
termed slug tests, require the instantaneous removal or addition of water to
the well. These instantaneous changes in head levels can be caused by pumping
or bailing water out of the well; by adding water into the casing very
rapidly; or by lowering a plug with known water displacement volume into the
well, waiting for the water level to stabilize, and then removing the plug
quickly. In all slug test methods, the recovery of head level to original
levels is measured over time. A single well pump test, on the other hand,
3-58
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does not require the instantaneous change in head within the well which
permits a slower extraction of water. Pumping, drawdown, and recovery rates
must be measured during the pump test.
Table 3-13 presents some of the commonly accepted single well tests for
determining hydraulic conductivity. This table also presents the appli-
cability of the test to various aquifer conditions (i.e., confined vs.
unconfined aquifers, expected hydraulic conductivity) and possible sources of
error when the test is performed. Detailed descriptions of the methods used
in performing the tests or calculating the results are available in the
references cited.
Multiple well tests require either the constant withdrawal or injection
of water into a test well and the measurement of head level changes in nearby
observation wells. Multiple well tests measure aquifer properties over a
larger test section than either laboratory tests or single well tests, and are
considered to be far more accurate in characterizing aquifer properties.
These tests can also be used to determine the presence of hydraulic barriers
and boundaries with the proper placement of observation wells.
Performing these tests can be expensive because special well drilling may
have to be performed (i.e., test well with observation wells), and data
acquisition and interpretation is more intensive. Furthermore, large volumes
of contaminated groundwater may be pumped during the test requiring appropri-
ate procedures for treatment and disposal. However, the data generated from
such tests can save time and money because of the better understanding one can
gain of possible plume paths. This understanding can maximize resource use in
terms of proper planning of monitoring and remedial actions.
Table 3-14 summarizes applicability, procedures, and calculation method
for major classes of multiple well tests. The Jacob method is generally much
easier to use and obtain results from than the Theis method but requires
meeting certain additional test conditions. USGS (1977) provides a listing of
test methods that have been developed for specific site conditions and subject
emphases, and can be used as a reference guide to aid in selecting an appro-
3-59
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TABLE 3-13
SINGLE WELL TESTS
Test Method
Applicability
Procedure
Calculation
Error Sources
Conventional Slug
Test: Confined
Aquifer (Lohman, 1972)
Pressurized Slug Test
(Bredehoeft and
Papadopulos, 1980)
Conventional Slug
Test: Unconfined
Aquifer (Bower and
Rice, 1976)
Piezometer Test
(Hvorslev, 1951)
Pumping Test
• Moderately permeable formations
• Confined aquifers
* Entire aquifer open to well
• Aquifer uniform in all directions
t Low to extremely low hydraulic
conductivities (silts, clays,
shales)
• Confined aquifers
• Moderately permeable formations
• Unconfined aquifers
• Fully or partially penetrating
well
t Measured value principally in
horizontal direction
• Any formation
• Unconfined aquifers
• Fully or partially penetrating
wells
• Any formation
• Confined and unconfined aquifers
• Slug removal of
water
• Measure recovery
rate to 85% initial
head
Sudden pressuriza-
tion of packed-
off area within
well with water
Pressure decay
rate monitored
Sudden change in
head level by
removal of
submersed weight
Measurement of
water level
recovery rates
Sudden change in
head levels caused
by removal or
injection of water
Measurement of
recovery rate
over time
Removal of water
over time by
pumping
Measurement of
drawdown, pumping,
and recovery
rates
• Plot of h/h vs t on
semi log papeV
t Superimpose on type
curve
t Calculation of trans-
missivity and storativity
• Plot of pressure decay
vs time
• Superimpose plots on type
curve
• Calculation of transmissivity
i Analytical equation based on
measured parameters
• Calculation of hydraulic
conductivity and effective
radius of well
• Plot of head level change
vs time
• Analytical equation based on
collected data
• Calculation of hydraulic
conductivity
* Semi log plot of draw-
down vs time
• Analytical equation based
on data collected
• Calculations of transmissivity
and storage
• Entire thickness of
aquifer not open to
well
• Flow to well not radial
t Measurement errors in
recovery rates and times
• Improper type curve
matching
• Hydraulic properties
throughout aquifer
are same as packed-
off section
I Measurement errors in
recovery rates and
times
• Measurement errors in
recovery rates and
times
• Significant flow of
water from above the
test zone
• Measurement errors
recovery rates and
times
Measurement errors
in drawdown rates,
recovery rates, and
times
Recovery storage not
equal to discharge
storage
-------
TABLE 3-14
MULTIPLE-WELL PUMP TESTS
Test Method
Applicability
Procedure
Calculation Method
Theis Method
(Theis, 1935)
oo
i
Jacob Method
(Jacob, 1950)
Any formation type; although low
hydraulic conductivity formations are
difficult to pump at constant rates
Confined and unconfined aquifers
Hell fully penetrates aquifer
Requires the use of one or more
observation wells
Any formation (same as Theis)
Confined and unconfined aquifer
Fully penetrating well
Requires the use of one or more
observation wells
• Pump well at constant
rate
• Measure drawdown verses
time in observation wells
• Measure discharge rate
• Pump well at constant rate
• Measure drawdown versus
time in observation wells
or recovery rates
versus time in observa-
tion wells
• Measure discharge rate
• Log-log plot of drawdown versus
distance to observation well
divided by time
• Match plot with type curve
Semi log plots of:
- Drawdown versus time
- Drawdown versus observation well
distances from test wells
- Recovery versus time
Calculate transmissivity and
storatlvity with appropriate
equations
-------
priate test when site conditions are known. Other discussions of the design
and interpretation of multiple well pump tests are provided in Powers (1981),
Lohman (1972), and Stallman (1971).
3.3.2 Sampling
Direct sampling of groundwater (and possibly other media) will always be
required during studies concerning delineation and management of leachate
plumes. Sampling provides data on constituent type and concentration in
groundwater. Also, sampling is required to verify or identify inaccurate
calculations or assumptions made concerning plume extent, to identify
unexpected problems or factors affecting plume dynamics, and to develop and
monitor remedial actions. Procedures involved in completing a sampling
program include:
• Planning -- Identification of data needs and development of proper
system design and necessary activities to collect the required data
• Implementation -- Completion of activities necessary to collect the
required data, including well installation and development, purging,
sample collection and handling, and laboratory analysis
• Data Evaluation -- Reduction of raw data to assess its reliability and
integration into the overall program to satisfy sampling program
objectives. Typically, this may include the calculation of concen-
tration gradients and the development of isoconcentration maps.
The following sections discuss each phase involved in a sampling program,
along with factors affecting sample validity. Procedures related to ground-
water monitoring are discussed where appropriate. Publications providing more
details concerning sampling programs include Sisk (1981), Scalf, et al.
(1981), USGS (1977), and EPA (1982c).
3-62
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3.3.2.1 Sampling Program Plan
A sampling program plan should include descriptions of:
0 Objectives -- describing the questions to be answered by the sampling
program and the data needed to address the questions
o Program Design -- including sample analysis parameters, frequency,
QA/QC, and integration of data with other media sampling programs
• System Design -- detailing sample site locations, numbers, depths, and
well configuration
o Component Design -- describing well construction materials, screen
type and setting, and security measures
• Implementation Procedures -- specifying well installation and
development procedures and other field activities relating to sample
collection and analysis activities
• Data Evaluation Procedures -- including how data will be integrated
with other data to attain sample program objectives.
Clearly defined program objectives should be developed prior to other
planning activities. Identification of program objectives is important so
that the reason for monitoring is clearly understood. The program can be
planned to meet objectives with a minimum amount of time and resources, when
all activities are focused. Monitoring program objectives should reflect site
conditions and available data, and should be modified as needed as new data is
compiled. If objectives are modified during a study, the monitoring program
should be reevaluated to keep activities focused on meeting the new
objectives. A monitoring program designed to meet one set of objectives may
be totally inadequate to accomplish subsequent revisions.
All data necessary to attain objectives should be identified once the
objectives are clearly defined. For example, if sampling objectives include
determination of the extent of contamination plus migration potential, then
data needs would include constituent types and concentrations over time, as
well as vertical and horizontal groundwater gradients, vertical and horizontal
3-63
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concentration gradients, and aquifer characteristics. Data needs should be
evaluated as to completeness and used to develop a data acquisition strategy.
The acquisition strategy includes general procedures or components that
must be accomplished to collect the necessary data. For example if surface
water and groundwater relationships appear to be important, a sampling program
addressing only groundwater is inadequate. The applicability of the data
acquisition strategy to the situation can be addressed by listing data
resulting from each step or component of the system and comparing this to the
required data list developed previously. The acquisition strategy, in
addition to data requirements and program objectives, will set standards for
program, system, and component (well) design.
Program design refers to how the monitoring system will be implemented to
meet the stated objectives. Items to be specified include:
• Media to be sampled (e.g., groundwater, surface water)
• Sample types and frequency (e.g., groundwater grab samples at three
month intervals)
• Parameters to be measured (e.g., priority pollutants)
• Integration with other media sampling programs
o Activities scheduled (e.g., well installation, development, purging,
sample collection and handling, laboratory analyses, and data
evaluation)
t QA/QC plans to ensure sample representativeness and validity.
One of the more critical elements of program design is Quality Assurance/
Quality Control (QA/QC). QA/QC procedures are essential to produce data that
meet user requirements in terms of completeness, accuracy, representativeness,
and comparability. QA/QC enters into each study phase and specific procedures
should be specified in the program plan. For example, QA/QC plans for ground-
3-64
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water sampling and analysis would incorporate the use of prepared samples such
as:
e Field Blanks -- All procedures normally followed during sampling are
performed but deionized or distilled water is added to the sample
bottles instead of groundwater or surface water. This procedure
checks for contamination resulting from sampling apparatus that come
into contact with the sample.
• Spiked Samples — Known amounts of a particular constituent are added
to blank samples to determine the accuracy of laboratory procedures.
Spiked samples can be prepared in the field or at the laboratory.
• QA Samples — Duplicate or triplicate samples (taking samples from the
same source and placing portions into two or more containers) are
collected using normal sampling procedures. Samples are identified
independently and sent to the same or different laboratories to check
for procedure precision.
t Trip Blanks -- Blank or spiked samples are prepared in the laboratory
and sealed. These accompany the sample containers from the labora-
tory, through field sampling and back to the laboratory. These
provide a check for interferences and contamination from external
sources not related to sampling.
These QA/QC checks provide indications of sample method accuracy and
precision, as well as possible cross-contamination resulting from sample
collection, handling, or analysis.
QA/QC plans should be a part of any sampling program plan. Several items
of concern for groundwater sampling program components include:
• Sampling Program Design -- Does the design result in all necessary
data? Are schedules planned to take advantage of common activities?
t Well Installation -- Will the installation procedures avoid cross
contamination between aquifers in multiaquifer systems and between
wells?
• Sample Collection -- Will the planned procedure prevent cross
contamination between samples and from sampling equipment? Are there
adequate procedures for purging and handling purge water, calibrating
field measurement equipment, and decontaminating equipment?
3-65
-------
o Sample Handling -- Will there be proper procedures for cleaning and
preparing bottles, and for sample preservation and shipment?
• Analysis -- Is there a complete and adequate laboratory QA/QC plan?
All these factors should be considered when developing or evaluating a QA/QC
plan.
System design refers to the relationships between the individual
components of the program. For example, in groundwater monitoring studies
system design would specify:
• Number of wells
• Well locations
• Screen settings (depth)
• Well configurations.
Well configuration refers to the relation of a well to adjacent wells and
to individual aquifers. Figure 3-2 illustrates five types of well
configurations that have been used for groundwater monitoring. Table 3-15
lists some of the advantages and disadvantages of each configuration.
Component design refers to the proposed construction of individual wells
in the monitoring systems. Typical well design specifications are listed in
Table 3-16. Although monitoring well design differs significantly from the
design of wells for plume extraction, the procedures used for well installa-
tion and development are similar. These procedures are described in Chapter
5.
After the program and individual components are designed, three
activities can be developed to carry out the program. These implementation
procedures should be detailed and in a "cookbook" fashion such that sample
3-66
-------
FIGURE 32.
WELL CONFIGURATIONS USED FOR GROUNDWATER MONITORING
(AFTER MASLANSKY, 1982)
Single
Zone
Wei!
Fully
Screened
Well
Multiple
Sampling
Point
Well
00
I
en
Single
Borehole
Well Nest
Multiple
Borehole
Well Nest
-------
TABLE 3-15
ADVANTAGES AND DISADVANTAGES OF VARIOUS TYPES OF MONITORING WELL CONFIGURATIONS
Well Configuration
Advantages
Disadvantages
Single Zone
Well
Fully Screened
Well
CTl
oo
Multiple
Sampl ing Point
Well
• Relatively simple to install
• Can be installed by a variety of methods
• Can provide discrete samples from a precise
interval thus aiding data interpretation
• Easy to prevent interaquifer contamination
if designed and installed properly
• Relatively simple to install
• Can be installed by a variety of methods
• Can provide composite samples of large
intervals thus reducing the number of samples
• Produces relatively higher yields and thus
are more amenable to pump testing
Can provide information on the vertical
distribution of contaminants and hydraulic
gradients
Installation is rapid and simple, although
construction takes longer then for wells
with a single screen
Can be used to obtain composite
samples
Fewer wells are needed in a monitoring system
thus reducing costs
Vertical distribution of contaminants
or hydraulic gradients cannot be
determined
Many wells are needed to delineate
plume increasing costs and
the time required to install and
sample the system
Highly contaminated waters may be
diluted by less contaminated waters
during sampling biasing results
Vertical distributions of contaminants
and hydaulic gradients cannot be
determined
Vertical migration of contaminants may
occur over screened interval
spreading contaminants to clean zones
Impossible to prevent interaquifer mixing
if screened over more than one aquifer
Preventing interaquifer contamination
is difficult if not impossible
Sampling is complicated, time
consuming, and requires specialized
equipment
Cost per well is fairly high
-------
TABLE 3-15 (continued)
Well Configuration
Advantages
Disadvantages
Single-
Borehole
Well Nest
CTl
Multiple-
Borehole
Well Nest
Provides information on the vertical
distribution of contaminants and hydraulic
gradients
Preventing interaquifer contamination is
generally not difficult
Sampling is not difficult but may require
specialized equipment depending on well
diameters
Provides information on the vertical
distribution of contaminants and hydraulic
gradients
Installation simple by a variety of methods
Preventing aquifer cross contamination is
not difficult
Sampling is simple and usually does not
require specialized equipment
0 Number of suitable installation
methods is restricted
• Improper construction can reduce
effectiveness and cause vertical
movement of contaminants
t Installation is fairly time consuming
but not difficult
• Cost per nest is fairly high
although cost per well is low
• Installation is fairly time consuming
but not difficult
• Cost per nest is very high
-------
TABLE 3-16
DESIGN SPECIFICATIONS FOR MONITORING WELLS
Design Item
Example Specification
Significance
i
-~j
o
Borehole diameter
Borehole depth
Well diameter
Casing length
Screen length
Screen setting
Screen openings
Casing type
Screen type
Sump
Joints
6-inch outside diameter
25 feet below land surface (BLS)
4-inch
25 feet
10-foot (two 5-foot sections, threaded)
25 to 20 feet BLS
10 slot/inch; micromesh
PVC or high carbon steel
PVC or stainless steel (match with
casing type)
2-foot
Threaded couplings, welded or glued
(depending on casing type)
Must be large enough to accommodate well
and sufficiently thick sand pack
Must accommodate well and sump (if
specified)
Based on required yields and sampling
equipment to be used
Equal to the total borehole depth plus
stickup minus the screen length
Based on required yields and aquifer to
be sampled
Based on aquifer to be sampled
Based on aquifer particle sizes,
gradations of sand pack designs, and
aquifer to be sampled
Based on type of contamination expected
and costs
Based on type of contamination expected
and costs
Used as a collection basin for fine
particles passing through screen
Based on materials and installation
procedures to be used
(Continued)
-------
TABLE 3-16 (continued)
Design Item
Example Specification
Significance
OJ
—i
Sand pack depth
Sand pack type
Bentonite setting
and thickness
Bentonite type
Grout thickness
Grout type
Well finish
Protective casing
Security
Identification
At least 3 feet over top of screen
Silica sand; Q-rok fl
3 feet over sand pack
Sodium bentonite pellets
5-foot minimum
Portland cement and bentonite mix at
a ratio of 20:1
3-foot stickup
12-inch oversize casing
sunk 3 feet BLS
Locking cap and traffic guards
Numbering system with color coding
Must allow for settlement and an adequate
thickness for the seal
Based on particle sizes and gradations of
aquifer to be sampled
Used to separate sand pack from grout
Less likely to mix with sand pack than
fluid grouts
Based on total well depth
Based on groundwater quantity and quality
Based on traffic loads and needs for easy
location
Used to protect well head from most types
of damage
Used to protect well from accidental
damage or intentional tampering
Used to identify specific well locations
especially in well nests
-------
team members can use the plan to complete their activities. The sampling plan
should also include a data evaluation section specifying:
« Data resulting from program activities
• Evaluation procedures (including references) for reducing the raw data
into a usable form
0 Integration of the usable data to achieve the specified objectives.
3.3.2.2 Sample Program Implementation
Implementation of groundwater sampling programs generally involves four
major elements—well installation and development, sample collection and
direct measurement, sample handling, and sample analysis. Well installation
typically consists of the following steps:
• Opening a borehole to the desired depth
• Installing a well constructed in accordance with the component design
• Filling the annulus ( i.e., the space between the well and the
borehole wall) with a sand pack
• Sealing the sand pack from surface water infiltration by installing a
bentonite seal
• Grouting the well in place
• Installing security measures to prevent tampering.
Drilling procedures and well finishing are discussed in Chapter 5.
One very important concern in drilling at hazardous waste sites is to
prevent contaminant migration into an uncontaminated aquifer or zone within an
aquifer. This is especially important in areas consisting of multiaquifer
systems. A drilling program to install wells into an aquifer separated from
contaminated groundwater must incorporate methods to seal off the contaminated
3-72
-------
zone from the uncontaminated zones. One method for accomplishing this
objective consists of:
• Boring through the contaminated aquifer and into, but not through, the
separating less permeable layer
0 Installing an outer casing to seal off the contaminated aquifer
0 Drilling through the casing plug and separating layer into the lower
aquifer
• Installing the monitoring well as described previously.
A double based well installed using this procedure is illustrated in Figure
3-3. The use of the outer casing and seal prevents contaminants from
migrating between aquifers. Drilling equipment should be throughly
decontaminated before drilling into the lower uncontaminated zone.
Wells should be designed and constructed to avoid possible interference
with sample analysis. In general, well materials made of steel are more
durable, subject to less chemical degradation than plastics (e.g., PVC), and
cost more. If PVC or other plastic material is to be used, couplings should
be threaded and sealed with Teflon tape and not glued. PVC glues contain
organic solvents and other constituents which could affect sample results.
There is some concern at present as to the effect of PVC well material on
groundwater samples in areas of high concentrations of organic contaminants.
Studies are currently underway to determine the leaching of contaminants from
PVC in these situations. If metals are of concern, high carbon steel or
stainless steel wells may adversely affect sampling results for chromium,
nickel , iron, and zinc.
Screens should be installed that are compatible with the chemical
contaminants anticipated and the aquifer grain size distribution. If
information is not available concerning aquifer grain size, 10 slot screens
(i.e., screens having slot widths of 0.01 inch) and appropriate sand-pack
materials can usually be used with good results (Sisk, 1981).
3-73
-------
FIGURE 3-3.
WELL DESIGN FOR PREVENTING INTERAQUIFER CONTAMINATION
(WICKLIIME, et al., 1983)
Land
iirfare
10"
W
-
P
\\
\S.
\\
-Jx
mmmm
p
\^
I
\\
IOv
Sf
bi
8" Ste
4" Steel
Casinq
Cement a
C
f.
. .
Grout
Bentonite Seal
Grade
Well Gravel
6" or 8"
Borehole
4" x 10' Stainless
Screen, 10 Slot
4" x 2' Plugged
Sump
3-74
-------
Sample collection includes the following activities:
0 Purging -- which involves removing a predetermined volume of water
from a well prior to sampling to ensure that "stagnant" water is
removed
• Field Measurements -- which involves obtaining values for certain
physical parameters at the sample site using direct reading equipment
• Sample Withdrawal -- which involves methods to remove a specific
volume of water from a well and introducing this water into specified
sample containers.
Groundwater sampling should only be performed after wells have been
purged and the groundwater system has returned to equilibrium. The amount of
water to be purged is typically specified in terms of multiples of the casing
volume (i.e., the volume of standing water in a well). The number of casing
volumes to purge is a topic of discussion. EPA recommends three to five
casing volumes, while the U.S. Army (USATHAMA) recommends five casing volumes
(where "casing" includes the sand pack). The merits of purging between 10 and
20 casing volumes, and in some cases, up to 100 casing volumes have also been
presented (Schmidt, 1982). The USGS recommends pumping until certain water
quality parameters (e.g., pH, temperature and specific conductance) stabilize.
However, the focus of most USGS studies is basin wide as opposed to site
specific. Pumping large quantities (i.e., more than 10 to 20 casing volumes)
of contaminated groundwater can distort plume boundaries and movement
patterns, dilute the actual sample with groundwater from some distance away
from the well, and cause significant purge water disposal problems. Reports
have noted that organic chemical concentrations changed continuously over
several hours of pumping despite stabilization of the USGS indicator param-
eters within only a few minutes of pumping (Keely and Wolf, 1983). Therefore,
unless there is a specific reason for pumping large volumes of water (e.g.,
time series sampling; Keely, 1982) this practice should be avoided. The EPA
recommendation of three to five casing volumes is probably acceptable for most
situations.
There is a special case where purging less than three casing volumes is
considered adequate. Low-yield monitoring wells, usually of shallow depths,
3-75
-------
can be pumped dry with the proper equipment. In cases where recovery is
adequate, the well is usually pumped three times, allowing partial recovery
between pumping. Where recharge is slow, the well is emptied twice.
The selection of equipment for purging depends on the well to be sampled.
Limitations include well diameter (e.g., a 4-inch submersible pump will not
fit into a 3-inch well), depth (e.g., centrifugal pumps cannot be used if the
water table is more than 25 feet deep), and the volume of water to be purged
(e.g., a 1 gpm pump is not appropriate if 200 gallons must be purged).
Equipment decontamination is an important step in preventing cross
contamination (i.e., transfer of contaminants from one well to another) and as
a health and safety consideration. Pump decontamination is usually easier to
accomplish as components are pulled from a well. Distilled water (or other
approved water) and nonresidue detergent should be used for decontamination.
Direct measurements provide additional data points for subsequent
evaluation. They can also indicate if there are changes in water type
sampled. These measurements are taken both before and after well purging, and
before and after sampling. Usually, a volume of water is emptied into a
Nalgene (or other suitable material) bucket for measurement, although flow-
through systems have recently been developed. Precise values, both before and
after sampling, usually indicate that external influences have not affected
the sample. Direct measurements are obtainable in the field for a variety of
parameters; however, the four most often measured are:
• Specific Conductance -- a measure of the water's ability to carry ar
electrical current under specific conditions. Ionized salts are
measured, giving an indication of the concentration of dissolved
solids in the sample and an indication if a different source of water
is being sampled.
• Temperature -- important in groundwater sampling because temperature
may indicate that different water sources are being sampled. Digital
electric probes or two calibrated thermometers can be used to collect
data.
• pH -- a measure of the effective hydrogen-ion concentration which can
indicate the solution of certain metals and the presence of
3-76
-------
contaminants or other water sources. pH can be measured using pH
paper but more precisely with a calibrated pH meter.
• Uater Level Measurements -- used in subsequent analyses to derive the
direction of groundwater flow and calculate groundwater gradients.
Water level measurements are typically obtained using either a tape
measure with some water level marking device (i.e. wetted tape
method), or an electrical probe where an electrical circuit is
completed by contact with the water and is registered on a meter.
There are a variety of marking methods which can be used with tape
measures including marking the tape with chalk or a water soluble
marker and noting the depth at which the mark is washed off by water
in the well. By holding the tape at a reference point on the well
(i.e., marked point on the well casing that has been surveyed), the
depth to water is easily determined. Because of the possibility that
samples may be affected by the tape marking, the electrical probe
method is generally recommended. Water level measurements have limited
usefulness unless there is a surveyed reference point from which to
measure. Water levels should be taken a minimum of two times or until
close agreement with measurements is obtained.
There are a variety of sample withdrawal methods currently available.
Some of these methods are listed in Table 3-17 along with their advantages and
disadvantages. In most instances, using a Teflon bailer is preferable because
of ease of use and decontamination, thus minimizing the potential for cross
contamination.
Sample preservation, packaging and storage is a very important step in
maintaining sample validity. Preservatives serve to maintain the physical,
chemical, and biological or bacteriological integrity of samples from
collection to analysis. Preservative methods usually include pH adjustment,
chemical addition, and refrigeration. Table 3-18 lists recommendations for
sampling containers, preservatives, and holding times.
In monitoring wells, siltation may cause minute quantities of suspended
solids to be collected in samples. Introduction of suspended solids into
containers with acid preservatives may cause the dissolution of metal ions
from the solids because of the pH change. This would increase the concentra-
tion of dissolved metals and would indicate that groundwater contains higher
than actual concentrations. Therefore, EPA recommends filtration of samples
for dissolved solids. Filtration in the field is performed using a vacuum
3-77
-------
TABLE 3-17
SELECTED GROUNDWATER SAMPLE WITHDRAWAL METHODS
Method
Advantages
Disadvantages
GO
I
CO
Bailers (all types)
Suction Lift Pumps
Submersible Pumps
Gas Lift
Positive Displacement
Pumps
Quick and easy to use
Minimizes cross contamination
Useful In almost all situations
Inexpensive
Easy to use
Fairly rapid to use
• May be used at any depth
• Fairly rapid to use
t May be used at any depth
• Can sample from various depths
• May become time consuming for very deep wells
• Sample exposed to air during withdrawal
• Maximizes potential outgassing of samples
• Can only be used to depths of 30 feet
• Check valve must be used to prevent cross
contamination
Outside power source required
Maximizes potential outgassing
Difficult to prevent cross contamination
May not work in cold weather or silted wells
Limited to wells 2-inches in diameter or over
Outside source of compressed gas required
Complex equipment required
Difficult to prevent cross contamination
Maximizes potential outgassing of samples
Difficult to prevent cross contamination
Outside air or gas source required
-------
TABLE 3-18
CONTAINERS, PRESERVATION, AND HOLDING TIMES (U.S. ARMY, 1982)
Measurement
Acidity
Alkalinity
Ammonia
Biochemical oxygen demand
Biochemical oxygen demand,
carbonaceous
Bromide
Chemical oxygen demand
Chloride
Chlorine, total residual
Color
Cyanide, total and
amenable to chlorination
Dissolved oxygen
Probe
Winkler
Fl uoride
Hardness
Hydrogen ion (pH)
Kjeldahl and organic
nitrogen
Metal sd
Chromium VI
Mercury
Metals except above
Container3
P
P
P
P
P
P
P
P
P
P
P
G bottle & top
G bottle & top
P
P
P
P
P
P
P
Preservative
Cool to 4°C
Cool to 4°C
Cool to 4°C
H2S04 to pH<2
Cool to 4°C
Cool to 4°C
None required
Cool to 4°C
H2S04 to pH<2
None required
Determine on site
Cool to 4°C
Cool to 4°C
NaOH to pH>12,f
0.008% Na2S203
Determine on site
Fix on site
None required
HN03 to pH<2
Determine on site
Cool to 4°C,
H2S04 to pH<2
Cool to 4°C
HNO., to pH<2,
0.05% K?Cr?07
HN00 to pH<2
o
Maximum
Holding Time0
14 days
14 days
28 days
48 hours
48 hours
28 days
28 days
28 days
2 hours
48 hours
14 days
1 hour
8 hours
28 days
6 months
2 hours
28 days
48 hours
28 days
6 months
(continued)
3-79
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TABLE 3-18 (continued)
Measurement
Nitrate
Nitrate and nitrite
Nitrite
Oil and grease
Container3
P
P
P
G
Preservative
Cool to 4°C
Cool to 4°C,
H-SO. to pH<2
COorto 4°C
Cool to 4°C,
Maximum
Holding Time
48 hours
28 days
48 hours
28 days
Organic carbon
Organic Compounds G, teflon
Extractables (including lined cap
phthalates, nitrosamines,
organochlorine pesti-
cides, PCBs, nitroaro-
matics, isophorone,
polynuclear aromatic
hydrocarbons, haloethers,
chlorinated hydrocarbons
and TCDD)
Extractables (phenols)
Purgeables (halocar-
bons, aromatics,
acrolein, and
acrylonitrile)
Orthophosphate
Pesticides
Phenols
Phosphorous, elemental
Phosphorous, total
G, teflon
lined cap
G, teflon
lined septum
G, teflon
lined cap
G
P,G
H?SO. to pH<2
COorto 4°C,
H2S04 to pH<2
Cool to 4°C
0.008% Na0S00-
28 days
7 days
r(until extraction)
30 days
(after extraction)
Cool to 4°C 7 days
f(until extraction)
0.008% Na?S?03 30 days
(after extraction)
Cool to 4°C, f 14 days
0.008% Na0S000
Filter on site, 48 hours
Cool to 4°C
Cool to 4°C 7 days
f(until extraction)
0.008% Na?S?0- 30 days
(after extraction)
Cool to 4°C, 28 days
H?SO. to pH<2
Cool to 4°C 48 hours
Cool to 4°C, 28 hours
H9SO, to pH<2
(continued)
3-80
-------
TABLE 3-18 (continued)
Measurement
Container0
Preservative
Maximum
Holding Time
Residue, total
Residue, filterable
Residue, nonfilterable
Residue, settleable
Residue, volatile
Silica
Specific conductance
Sulfate
Sulfide
Sulfite
Surfactants
Temperature
Turbidity
a - Polyethylene (P) or
b - Sample preservation
P
P
P
P
P
P
P
P
P
P
P
P
P
Glass (G).
should be perfc
Cool to 4°C
Cool to 4°C
Cool to 4°C
Cool to 4°C
Cool to 4°C
Cool to 4°C
Cool to 4°C
Cool to 4°C
Cool to 4°C,
Zinc Acetate
Cool to 4°C
Cool to 4°C
Determine on site
Cool to 4°C
)rmed immediately upon samp
14 days
14 days
7 days
7 days
7 days
28 days
28 days
28 days
28 days
48 hours
48 hours
Immediately
48 hours
le collection.
For composite samples each aliquot should be preserved at the time of
collection. When use of an automatic sampler makes preservation of each
aliquot impossible, samples may be preserved by maintaining at 4°C until
compositing and sample splitting is completed.
c - Samples should be analyzed as soon as possible after collection. The times
listed are the maximum times that samples may be held. Samples may be held
for longer periods only if the laboratory has data on file to show that the
specific types of samples under study are stable for the longer time. Some
samples may not be stable for the maximum time period given in the table. A
laboratory is obligated to hold the sample for a shorter time if knowledge
exists to show this is necessary to maintain sample integrity.
d - Samples should be filtered immediately on site before adding preservative for
dissolved metals.
e - Guidance applies to samples to be analyzed by GC, LC, or GC/MS for specific
organic compounds.
f - This should only be used in the presence of residual chlorine.
(Compounds not found on table should be preserved at 4°C; storage: 1 week).
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pump, appropriate glassware, and filter paper. Approved EPA filtration
holders and media are described by EPA (1982c).
Confusion has arisen over filtered versus unfiltered samples because os
the increased use of "Priority Pollutant Scans" in hazardous waste investiga-
tions. The confusion is a result of several items, the first being that the
priority pollutant values for metals specifies total metals and EPA procedures
do not call for field filtration. Priority pollutant scans therefore do not
require filtration which may result in elevated metal concentrations being
detected for groundwater. To avoid this, filtration can be conducted for one
set of samples for dissolved metals analysis, while another unfiltered sample
is collected for total metals. Thus, values for total, dissolved, and
suspended metals are obtained.
Organic chemical concentrations may be affected by filtering. Phthalates
or other binders may be washed out of filters, thus indicating the false
presence of these compounds in groundwater. Nonfiltering may allow sorption
of organics to suspended particles and sediments. One compromise is to decant
the clearest portion of a sample while discarding the cloudy portion of the
sample containing the highest concentration of sediments and suspended solids.
All samples must be shipped according to DOT standards. Groundwater
samples are usually not considered hazardous materials and may be shipped
counter to counter via priority air freight to the identified laboratory.
Chain of custody (COC) procedures should be followed during transportation.
The purpose of chain of custody procedures is to document the identity of the
sample and sample handling from the point of collection until laboratory
analysis is complete and laboratory QA/QC procedures confirm accuracy.
Documentation supports this by providing a written record of procedures
followed and data collected during sampling.
Sample analyses should follow approved EPA procedures wherever possible.
EPA has recently published the second edition of "Test Methods for Evaluation
of Solid Waste" (EPA, 1982c) which contains procedures that may be used to
determine chemical composition of wastes or the presence of hazardous wastes
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in other matrices. However, many of these methods have not yet been fully
tested.
QA/QC procedures followed in the laboratory should be in accordance with
those outlined in EPA (1979), EPA (1980), and EPA (1982c). Laboratories
participating in EPA's performance evaluation and quality assurance review
program should have these procedures in place.
Appropriate QA/QC data must accompany sample results so that sample
validity can be determined. The following QA/QC checks are typically provided
by laboratories:
• Surrogate Recovery Data -- Surrogates are chemical compounds similar
in chemistry to the compound under analyses that are added to samples
to monitor method efficiency. Surrogate recovery results outside of
established limits may indicate poor quality data but may also reflect
interferences with similar chemical compounds (Gurka, et al., 1982).
• Duplicate Analyses Results -- Duplicates are reanalyses of individual
samples. Close agreement between sample and duplicate results is a
good indication of method precision.
o Field and Trip Blanks, and QA and Spiked Samples Results -- These
samples have been discussed previously.
Laboratory QA/QC performance should be evaluated prior to laboratory
selection and before any samples are taken. Performance evaluation can be
performed by sending split samples to several laboratories for comparison.
3.3.2.3 Evaluation
The first step in the evaluation process is to assess the validity of the
data. Because leachate constituent concentrations are typically in the parts
per million (ppm) or part per billion (ppb) range, even a seemingly innocuous
event can have a major impact on the resulting data. Table 3-19 summarizes
some of the factors that can affect the validity of groundwater samples. This
assessment helps to place the results and subsequent evaluations and
conclusions in the proper context.
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Sample results are typically reported as concentrations of specific
chemicals in mg/1 (ppm) or ug/1 (ppb). These data can he used to plot isocon-
centration maps and cross sections in which areas having equal concentration
of selected contaminants are connected by contour lines. These maps and cross
sections are drawn in much the same manner as potentiometric surface maps and
cross sections, but use concentration results instead of water levels. The
problem inherent in interpreting these isoconcentration figures is that a
three dimensional phenomenon (i.e., a leachate plume) is being presented in
two dimensions. Furthermore:
0 The data typically represents only one point in time and may become
obsolete quickly depending on the rate of plume movement
• Samples taken from wells designated as points on a map or cross
section may actually represent large three dimensional surfaces within
the aquifer
0 Analytical results must be extrapolated between wells to produce
contours
• There is a certain degree of natural and analytical variability that
cannot be expressed easily in a figure.
These concerns should be kept in mind when interpreting sampling results.
Initial calculations of concentration gradients generally assume conser-
vative and nonreactive leachate constitutents, and use linear interpolation.
For example, if sampling results indicate that well M-l contains 500 ppb of
some constituent and well M-2 contains 100 ppb and these wells are screened in
the same aquifer 200 feet apart along the same groundwater flowpath, then the
linear concentration gradient can be calculated by:
(500 ppb - 100 ppb)/200 feet = 2 ppb/foot
This gradient can then be used to estimate the location of the plume boundary.
In the example described above, the boundary would be 50 feet beyond well M-2
along the same groundwater flowpath.
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TABLE 3-19
POTENTIAL SOURCES OF FACTORS AFFECTING GROUNDWATER
SAMPLE VALIDITY (After Fetter, 1983; and Nacht, 1983)
Source or Activity
Drilling Equipment
Techniques
and
Uell Construction
and Materials
Sampling Equipment
and Techniques
Effect
t Equipment can be a source of organic solvents,
diesel fuel and gasoline, oils and greases,
and cross contaminants from other drill sites
• Drilling fluids can add contaminants or affect
certain physical parameters (e.g., COD, BOD,
pH)
• PVC casing and elements used to join PVC
casing can leach organic chemicals and may
sorb certain heavy metal compounds
• Stainless steel or high
release metals to water
organic compounds
carbon steel may
and sorb certain
• Bentonite used in drilling fluids or in
annulus seals may sorb heavy metals and
organics
• Inadequate purging can introduce stagnant
water into sample reducing concentrations of
volatile organics
• Excessive purging can dilute contaminants
below detectable limits by mixing major
concentrations with large amounts of
uncontaminated formation water
• Pumps can sorb metals, organics, or induce
outgassing of volatile organics through
turbulence
• Gas lift methods can induce outgassing and
oxidation
• Field filtering of samples may add dissolved
solids if dissolution of organic solvents
occurs or cause oxidation and outgassing if
contact with air is allowed
• Improper cleaning may introduce contaminants
from one well to another
(continued)
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TABLE 3-19 (continued)
Source or Activity
Effect
Sampling Equipment
and Techniques (cont.)
Sample Preservation
and Analysis
• Excess turbulence caused while placing the
sample in containers can result in outgassing
and oxidation
• Samples that are not preserved properly
or that are held too long prior to analysis
can undergo a variety of chemical changes
• Bottles cleaned with organic solvents (e.g.,
methylene chloride) can introduce a variety of
contaminants into the sample depending on the
purity (i.e., grade) of the solvent and the
degree to which the solvent was volatilized
from the bottle prior to sealing
• Certain organic compounds (phthalates,
methylene chloride, trichlorof luoromethane )
are common contaminants in laboratories and
may affect sample results
Both concentration and hydraulic gradients are not always linear, but
frequently exhibit logarithmic changes. Rather than calculate logarithmic
gradients (such as radioactive decay rates), gradients are generally easier to
approximately locate if isoconcentration contours or plume boundaries are
plotted by using log paper as a reference in the same manner that a ruler is
used in linear interpolation.
Nonlinear concentration gradients can also be a result of pulsed releases
of contaminants. In general, differences of a few 10's of parts per billion
are most often meaningless. Orders of magnitude changes in concentration are
usually more reliable as plume indicators, but may not be suitable to easy
interpretation in complex hydrogeologic situations.
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Plume migration rates are calculated simply by dividing the maximum
distance the plume has moved from the source by the time since leachate
generation began. This calculation assumes that the duration of leachate
generation is known (Section 3.1.1), the boundaries of the plume have been
correctly delineated, and the rates of leachate generation and movement are
constant over time.
Another use of sample results involves comparing this direct data on
plume boundaries and migration rates to hydrologic calculations made with
previously collected data. If results differ widely, some input to the
calculations must be wrong or some natural factor is affecting plume movement.
If sample results indicate the plume has migrated farther than had been
calculated initially, then one or more of the following is true:
• There are preferential routes of migration that are not accounted for
in the calculations
• The transmissi vity (or hydraulic conductivity), hydraulic gradient,
porosity, or time from the beginning of leachate generation has been
underestimated
t The average saturated thickness of the aquifer has been over
estimated.
If sampling results indicate the plume has not migrated as far as had been
calculated initially, then one or more of the following is true:
• Pollutant attenuation is significant and the assumption cannot be made
that leachate constituents are conservative and nonreactive
o Natural or artificial factors have at some time or times reversed or
changed flow directions or patterns
• Leachate generation volumes and rates have been over estimated, or
leachate generation has not been constant over time
• Unidentified natural barriers to contaminant migration are present
• Plume migration patterns are more complex than previously supposed
because of either subsurface hydrogeologic conditions or the physical
and chemical properties of the leachate
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• The transmissivity, hydraulic gradient, porosity, or time from the
beginning of leachate generation has been over estimated
• The average saturated thickness has been under estimated.
All calculations should be checked to try to identify the source of the
discrepancy so that modifications can be made to the hydrogeology data base
prior to planning plume control measures. If the sampling information
verifies the initial calculations, then aquifer restoration planning can
begin.
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CHAPTER 4
PLUME CONTROL TECHNOLOGIES
Methods for controlling the migration of a leachate plume can be placed
into one of four categories -- groundwater pumping, subsurface drains, low
permeability barriers, and innovative technologies.
Groundwater pumping technologies involve the extraction or injection of
groundwater, through wells, to alter the direction of leachate plume movement.
In pumping a groundwater extraction well, a cone of depression is created
which causes groundwater to flow toward the well. A cone of impression or
mound is created around a groundwater injection well which causes groundwater
to flow away from the well. Groundwater injection and extraction wells can be
used separately, or in combination, to change the flow of groundwater in order
to contain or remove a leachate plume.
Subsurface drains are continuous, permeable barriers designed to inter-
cept groundwater flow. As such, they can be used much like pumping systems.
Groundwater collected in subsurface drains flows to a sump where collected
water can be pumped to a treatment system.
Subsurface barriers consist of a vertical wall of low permeability
material constructed underground for the purpose of redirecting groundwater
flow. Impermeable barriers can be used to contain contaminant plumes
associated with waste sites by completely surrounding a waste site or plume,
or they can be used to lower a groundwater table to prevent contact with
wastes.
In addition to these more established techniques, innovative technologies
are being developed to control leachate plume movements. These include
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in situ biological treatment (i.e., bioreclamation), in situ chemical treat-
ment, permeable treatment beds, block displacement, and others. In situ
treatment techniques have already been used on a limited basis for aquifer
restoration.
The purpose of this chapter is to present an overview of available
techniques for controlling the migration of leachate plumes and to present
methods for evaluating and selecting a technique. These techniques are
described in more detail in the following chapters.
4.1 Control Techniques
4.1.1 Groundwater Pumping
Well systems can be designed to perform several functions with or without
the assistance of other technologies (e.g., barrier walls). The main applica-
tions in plume management are groundwater level adjustment, plume containment,
and pi ume removal.
Well systems to adjust groundwater levels can be designed using extrac-
tion wells to lower water levels or using injection wells to create ground-
water mounds. By adjusting groundwater levels, plume migration can be stopped
at the source or the speed and direction of the plume can be altered. In
either case, contaminated water is not extracted from the groundwater system
as is the case with containment and removal techniques.
Well systems used to contain a plume may incorporate extraction wells or
extraction and injection wells in combination. Containment differs from
removal in that the source of contamination generally is not stopped, so that
contamination is an ongoing process. Because containment requires removing
contaminated groundwater, a treatment or disposal method must be developed to
handle the system discharge.
Plume removal implies a complete purging of the groundwater system of
contaminants. Removal techniques are suitable when contaminant sources have
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been stopped (e.g., by waste removal or site capping) or contained (e.g., by
barrier walls) and aquifer restoration is desired. Extraction or extraction
and injection well systems can be used in plume removal. Numerous arrays and
patterns are available for injection and extraction wells depending on site-
specific suitability. Extraction and injection techniques can also be used
with flushing compounds to accelerate contaminant removal. As with
containment strategies, treatment of pumped water is necessary.
Previous utilization of pumping technologies to manage plumes has shown
that these methods are most effective at sites where underlying aquifers have
high hydraulic conductivities (e.g., coarse grained sands) and where the
contaminants move readily with groundwater (e.g., benzene). Pumping methods
have also be utilized with some effectiveness at sites where pollutant move-
ment is occurring along fractured or jointed bedrock. However, the fracture
patterns must be traced in detail to ensure proper well placement.
The shape and size of a well's cone of depression is dependent upon the
pumping rate and cycle, slope of the original water table, location of
hydraulic barriers, aquifer characteristics, and location of recharge zones.
Two aquifer characteristics important in determining the cone's configuration
are the coefficients of transmissivity (T) and storage (S). Both of these
parameters are inversely related to drawdown and the deepening of the cone.
The extent of the cone under equilibrium pumping conditions is termed the
radius of influence. The radius of influence can be used in determining well
spacing, pumping rates, pumping cycles, and screen lengths when flow rate is
low. However, when moderate flow rates exist at a site, distortion occurs in
the flow lines such that a plume may pass between two adjacent wells even
though their cones of depression overlap. For these cases velocity plots must
be developed to properly design a well system.
Selecting and designing an appropriate well system requires that adequate
information be available on the site's hydrogeologic conditions and the
plume's characteristics. Four basic types of wells are used in plume
management: well points, suction wells, ejector wells, and deep wells. Using
the site data, selection of an appropriate well type can be made.
Table 4-1 lists selection criteria for these well types.
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TABLE 4-1. CRITERIA FOR WELL SELECTION
(Powers, 1981)
Parameters Well points Suction Wells Ejector Wells Deep Wells
Hydrology
• Low hydraulic conductivity Good Poor Good Fair to Poor
(e.g., silty or
clayey sands)
• High hydraulic conductivity Good Good Poor Good
(e.g., clean sands
and gravel)
• Heterogeneous materials Good Poor Good Fair to Poor
(e.g., stratified soils)
• Proximate recharge Good Poor Good to Fair Poor
• Remote recharge Good Good Good Good
Depth of well Shallow < 20 ft Shallow < 20 ft Deep > 20 ft Deep > 20 ft
Normal spacing 5-10 ft 20-40 ft 10-20 ft > 50 ft
Normal range of 0.1-25.0 gpm 50-400 gpm 0.1-40.0 gpm 25-3000 gpm
capacity (per unit)
Efficiency Good Good Poor Fair
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The installation of wells varies with geologic materials, well type, and
diameter, but most procedures include opening the borehole, installing casing,
completing the well, and developing the well. The first of these steps
consists of dislodging and removing the earth materials located in the path of
the well. Screens and casings may be installed simultaneously with the
opening of the hole or after the hole has been completed. The sequence of
events depends upon the technique used to open the hole and the geolog-ic
characteristics in the immediate vicinity of the hole. Installation of
screens, filters, pumps, and grout completes well construction. Well develop-
ment is the last step and consists of removing material that has built up on
the well screen and walls of the borehole during the previous steps. Removal
of these particles maximizes well yield and prevents pump damage.
4.1.2 Subsurface Drains
Subsurface drains include any type of buried conduit used to collect
liquid discharges (e.g., contaminated groundwater) by gravity flow. The major
components of a subsurface drainage system include: drain pipes, envelope or
filter, backfill, manholes or wetwells, and pumping stations. Subsurface
drains function similarly to an infinite line of extraction wells. That is,
they create a continuous zone of depression which runs the length of the
drainage trench.
Although subsurface drains perform many of the same functions as pumping
systems, drains may be more cost effective in certain circumstances. For
example, they may be particularly well suited to sites with relatively low
hydraulic conductivities where the cost of pumping may be prohibitively high
because of the need to locate wells very close together. However, there are a
number of limitations to the use of subsurface drains as a remedial technique.
They are not well suited to areas of high hydraulic conductivity and high flow
rate. Also, contamination at great depth may cause construction costs to be
prohibitive, particularly if a substantial amount of hard rock must be
excavated. Subsurface drains are also not suitable when the plume is viscous
or reactive because this type of leachate may clog the drain system.
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Functionally, there are two basic types of drains--relief drains and
interceptor drains. Relief drains are installed in areas where the hydraulic
gradient is relatively flat. They are generally used to lower the water table
beneath a site or to prevent contamination from reaching a deeper, underlying
aquifer. Relief drains are installed in parallel on either side of the site
such that their areas of influence overlap and contaminated groundwater does
not flow between the drain lines (i.e., same principle of flow apply to drains
as for wells). They can also be installed completely around the perimeter of
the site. Interceptor drains, on the other hand, are used to collect ground-
water from an upgradient source in order to prevent leachate from reaching
wells or surface water located hydraulically downgradient from the site. They
are installed perpendicular to groundwater flow. A single interceptor located
at the toe of the landfill, or two or more parallel interceptors may be
needed depending upon the circumstances.
Drainage system are classified by function (i.e., as either relief or
interceptor drains), by the type of drain pipe, and by their configuration.
Perforated pipe, available in chemically resistant concrete, vitrified clay,
and various plastics, is the most widely used drain pipe material for remedial
action work, particularly if the drains are to be placed deep in the sub-
stratum. Other types of drainage conduits include jointed clay and concrete
tiles, and flexible corrugated plastic pipe. The configuration of the drain-
age system depends on the site configuration and size, and the groundwater
flow rate. The system may be singular, consisting only of lateral pipes
discharging to a collection sump, or composite where laterals discharge to
larger collector pipes which may in turn discharge into a main before reaching
a collection sump.
In a subsurface drainage system containing parallel relief drains, depth
and spacing are interdependent design variables. In theory, the deeper the
drains are the greater the spacing that can be used to obtain the same zone of
depression. This relationship between depth and spacing is critical to the
design of effective parallel drainage systems. In designing a system for
hazardous waste sites, the distance between drains located on either side of
the site must be determined so that their drawdowns intersect and thereby
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capture the entire plume. This spacing, as with wells, will be dependent on
flow rates. The higher the flow rates the closer the drains will have to be
placed (holding other factors constant). Velocity plots for determining
capture zones are probably the best method available to adequately calculate
the spacing on drains. Also, drain depth and spacing may need to be
manipulated in order to capture the entire plume. However, in designing
parallel drains for hazardous waste sites, a minimum spacing is often imposed
by the boundaries of the waste since excavation through the waste material can
be extremely hazardous. A maximum depth may be imposed by the prohibitive
cost of trench excavation.
The shape of the drawdown curve upgradient of the site is independent of
hydraulic conductivity but is a function of head or hydraulic gradient. The
upgradient influence or drawdown extends for a distance which is inversely
proportional to the water table gradient. The distance to which the water
table is lowered downgradient of the interceptor drain is directly propor-
tional to the depth of the drain. Theoretically, a true interceptor drain
lowers the water table downgradient to a depth equal to the depth of the
drain. The distance downgradient to which the drain is effective in lowering
the water table is infinite provided there is not recharge. This however is
never the case since infiltration from precipitation recharges the ground-
water. The quantity of flow is also proportional to the depth of the drain.
If an interceptor drain is placed at the midpoint between the water table and
an impervious layer, a little less than 50 percent of the flow will be inter-
cepted. The upgradient and downgradient influence of an interceptor can be
determined theoretically or in the field.
In addition to designing the subsurface drainage system for appropriate
depth and spacing, the hydraulic design must also be developed. The drain's
pipe size and gradient must be adequate to cause the water to flow after it
enters the pipe. The gradient of the drain pipes should be great enough to
result in a flow velocity that prevents siltation, yet will not cause turbu-
lence. The Soil Conservation Service (1973) has published data on minimum
recommended grades for various pipe diameters and maximum velocities
recommended for various soil types. The diameter of the drain for a given
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capacity is dependent on flow, hydraulic gradient and the roughness
coefficient, which in turn is a function of the hydraulic resistance of the
drain material. The formula for hydraulic design is based on the Manning
formula for rough pipes. SCS (1973) and Cavelaars (1974) have developed
monographs for estimating pipe diameter based on the Manning formula when
hydraulic gradient, flow rate, and drainage area are known. Roughness
coefficients have been published for all drain types.
The other criterion for successful hydraulic design is to ensure that the
pipe will accept the drainage water when it arrives at the drainline. To meet
this criterion, the relationship between the hydraulic conductivity of the
gravel envelope or filter material, the perforations in the drain pipe, and
the hydraulic conductivity of the base soil material must be evaluated. The
primary function of a filter is to prevent soil particles from entering and
clogging the drain. The function of an envelope is to improve water flow into
the drains by providing a material that is more permeable than the surrounding
soil. Although filters and envelopes have distinctly different functions,
requirements of both a filter and an envelope can be met. SCS (1973) has
developed distinct design criteria for gravel filters and envelopes, whereas
the Bureau of Reclamation (1978) has developed one set of standards for a
well graded envelope which meets the requirements of both a filter and an
envelope. For tile drains, the criteria are based on a comparison of the
grain sizes of the envelope and base soil material. Where perforated pipe
drains are being used, the minimum size of the envelope material is based on
the size of the perforations. Geotextile fabrics offer an alternative to the
more conventional sand and gravel filters provided the fabrics are compatible
with the waste components in the leachate so that the drains do not clog.
Design of the drainage sump and pumping plant are also considered part of the
total hydraulic design.
Construction and installation of subsurface drains can be divided into
two major phases—trench excavation and drain installation. Trench excavation
is often the most complex and costly aspect of construction and installation.
The ease or difficulty of excavation can have a dramatic effect on the cost of
the total installation. Difficult excavation may result in exclusion of
subsurface drainage as a viable technique because of prohibitive costs. Once
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trench excavation is completed the components of the subsurface drain can be
installed. This process consists of installing drain pipe bedding, drainage
pipe, a gravel or soil envelope, filter fabric, backfill, and auxiliary
components including manholes and pumping stations.
4.1.3 Low Permeability Barriers
Low permeability barriers can be used to divert groundwater flow away
from a waste disposal site or to contain contaminated groundwater leaking from
a waste site. There-are three major types of low permeability barriers that
are applicable to leachate plume management—slurry walls, diaphragm walls,
and grout curtains.
A slurry wall is formed by excavating a trench using a bentonite and
water slurry to support the sides. The trench is backfilled with materials
having lower permeability than the surrounding soils. The slurry backfill
trench or slurry wall reduces or redirects the flow of groundwater.
A diaphragm wall is designed for structural strength and integrity in
addition to low permeability. Diaphragm walls can be made of cast in place
concrete or precast panels with cast in place joints.
A grout curtain is formed by pressure injecting one of a variety of
special grouts to seal and strengthen a rock or soil body. Once in place,
these grouts set or gel in the rock or soil voids. This greatly reduces the
permeability of and imparts increased mechanical strength to the grouted mass,
and results in a grout wall or curtain. Because a grout curtain can be three
times as costly as a slurry wall, grout curtains are rarely used when ground-
water has to be controlled in soil or loose overburden. Grout is used
primarily to seal voids in porous or fractured rock when other methods of
controlling groundwater are impractical.
Barrier walls are classified by the materials of which they are composed
and the position in which they are placed with respect to the pollution
4-9
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source. There are three major types of barrier walls categorized according to
material used to backfill the trench:
t Soil-bentonite
• Cement-bentonite
t Diaphragms.
Soil-bentonite walls are composed of soil materials (often the trench
spoils) mixed with small amounts of bentonite slurry from the trench. Cement-
bentonite walls are composed of a slurry of portland cement and bentonite.
Diaphragm walls are composed of precast or cast in place reinforced concrete
panels (diaphragms) installed by excavating a short, slurry supported trench
section, using a clamshell bucket or other suitable piece of equipment. Upon
completion of excavation, the trench section is filled with a precast, rein-
forced concrete panel or tremied concrete around a reinforcement cage.
Alternate or primary panels are installed first, followed by the secondary
panels. Joints between panels are formed using stop end tubes that are
concreted after adjacent panels are completed. Another method involves using
a cement-bentonite slurry as the exavation fluid which forms the joint between
panels when set.
In general, soil-bentonite walls can be expected to have the lowest
permeability, the widest range of waste compatibilities, and the lowest cost.
They also offer the least structural strength (highest elasticity), usually
require the largest work area, and are restricted to a relatively flat
topography unless the site can be terraced.
Cement-bentonite walls can be installed at sites where there is
insufficient work area to mix and place soil-bentonite backfill. By allowing
wall sections to harden and then continuing the wall at a higher or lower
elevation, they can be installed in a more extreme topography. Although
cement-bentonite walls are stronger than soil-bentonite walls, they are at
least an order of magnitude more permeable, resistant to fewer chemicals, and
more costly.
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Diaphragm walls are the strongest of the three types, and are the most
costly. Provided the joints between panels are installed correctly, diaphragm
walls have approximately the same permeability as cement-bentonite walls and
because of a similarity of materials, about the same chemical compatibilities.
Diaphragm walls are most typically used in situations requiring structural
strength and relatively low permeability.
Configuration refers to the vertical and horizontal positioning of a
barrier wall with respect to the location of the pollution source and ground-
water flow. There are two types of configuration—vertical and horizontal.
The wall configuration, combined with any other remedial measures (e.g.,
pumping, capping), determine barrier wall effectiveness in controlling
leachate plume migration.
Vertical configuration refers to the depth of the wall with respect to
both geologic formations and the water table. Based on vertical positioning,
walls are either keyed into a low permeability formation below the aquifer or
placed to intercept only the upper portion of the aquifer. The latter type,
known as a hanging barrier wall, can be used to control contaminants such as
petroleum products which do not mix with the groundwater but float on top of
it. In these situations, the barrier need only extend to a depth in the water
table sufficient to intercept the contaminants. Keyed barrier walls are
excavated through the water table to a confining layer to contain contaminants
that mix with or sink to the bottom of the aquifer. The connection between
the wall and the confining layer is very important to the overall
effectiveness of the barrier.
Horizontal configuration refers to the positioning of the barrier
relative to the location of the pollution to be controlled and the direction
of groundwater flow (i.e., the gradient). Based on horizontal configuration,
barrier walls may completely surround the pollution source or be placed
upgradient or downgradient from it.
Circumferential placement refers to placing a barrier wall completely
around the site. Although this requires a greater wall length and higher cost
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than either upgradient or downgradient placement alone, some advantages are
offered. A circumferential barrier wall, when used with a surface infil-
tration barrier such as a cap, can greatly reduce the amount of leachate a
site will release to the environment. If a leachate pumping or drainage
system is used, as often is the case, a waste site can be virtually dewatered.
In addition to vastly reducing the amount of leachate to be treated,
dewatering can help increase the longevity of the wall.
Upgradient wall placement refers to the positioning of a wall on the
groundwater source side of a waste site. This type of placement theoretically
can be used where there is a relatively steep gradient across the site in
order to divert uncontaminated groundwater around the wastes. In such cases,
clean groundwater is prevented from becoming contaminated thereby reducing
leachate treatment requirements.
Placement of a barrier wall on the side opposite the groundwater source
is referred to as downgradient placement. This placement configuration does
nothing to limit the amount of leachate being generated and so is practical
only in situations where there is a limited amount of groundwater or contami-
nant flow, such as near drainage divides. This type of barrier theoretically
can contain leachate so the leachate can be recovered for treatment. Although
this wall configuration may be keyed into a confining layer for miscible or
sinking contaminants, most often the wall is hanging to contain and recover
floating contaminants.
Grout curtains are the most practical and efficient method for sealing
fissures, solution channels, and other voids in rock. There are four basic
techniques for installing a grout curtain. These are:
• Stage-up method
• Stage-down method
0 Grout-port method
• Vibrating beam method.
The first three methods are injection methods in which the grout is
injected from either the bottom of a borehole to the top (stage-up), or the
4-12
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top of a borehole to the bottom (stage-down), or through a slotted injection
pipe that has been sealed into the borehole with a brittle portland cement and
clay mortar jacket (grout-port). The grout port method utilizes rubber
sleeves which cover the outside of each slit (or port) permitting grout to
flow only out of the pipe. The vibrating beam method is not a true injection
technique, rather, the method is a way of placing the grout in which an I-beam
is vibrated into the soil to the desired depth and then raised at a controlled
rate. As the beam is raised, grout is pumped through a set of nozzles mounted
on the base of the beam, thus filling the newly formed cavity. When the
cavity is completely fil led, the beam is moved in the direction of the wall,
leaving a suitable overlap to ensure continuity. The process is continued
until a wall of the desired length is constructed (Harr, et al., unpublished).
Grouting materials fall into three basic groups—cement, bituminous, and
chemical. Some specific grout mixtures include portland cement, sand and
cement, clay and cement, clay and bentonite, bituminous emulsions, sodium
silicate, and acrylamide. The applicability of each material is based on the
size of the openings in the soil or rock formation and the anticipated area of
grout penetration. The major grouts in use are cement and clay which make up
approximately 95 percent of all grouts used.
Cement grouts utilize materials that set, harden, and do not disintegrate
in water. Because of their large particle size, cement grouts are more
suitable for rock than for soil applications. Materials may be added to
cement grouts to improve their applicability. Sand may be added to portland
cement to create a grout suitable for coarse materials, while bentonite may be
added to improve the penetration of cement in alluvial soils.
Clays have been used widely as grouts either alone or in formulations.
In general, coarse sands and gravels are initially grouted using clay or clay
and cement because they are inexpensive. Bentonite is an excellent clay
grouting material because of its swelling and gel formation properties.
As with slurry walls, placing a grout curtain upgradient from a waste
site can redirect flow so that groundwater does not contact the wastes that
4-13
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are creating the leachate plume. However, placement of a grout curtain
downgradient from a hazardous waste site may not be successful because of
grout and leachate interactions. For example, in many instances grout setting
time is difficult to control, increasing the difficulty of emplacing a curtain
of reliable integrity. Additional problems can occur in attempting to grout a
horizontal curtain or layer beneath a waste site. Injection holes must be
drilled either directionally from the site perimeter or directly through the
wastes. The first case may not always be feasible and may be quite costly,
and the second might be quite hazardous to construction crews.
4.1.4 Innovative Technologies
Groundwater pumping, subsurface drains, and low permeability barriers are
well established technologies that were originally developed for nonhazardous
applications and have been adapted for use in controlling leachate plume
migration. A variety of other techniques for plume management are under
development, many of which were designed specifically for application at
hazardous waste release sites. Two technologies in particular, bioreclamation
and in situ chemical treatment, appear especially promising and have been
demonstrated on a limited basis for site cleanup. These two techniques are
described briefly below; other innovative technologies are described in
Chapter 8.
4.1.4.1 Bioreclamation
Bioreclamation is an in situ groundwater treatment technique based on the
concept of utilizing microorganisms, combined with aeration and the addition
of nutrients, to accelerate the biodegradation rate of groundwater
contaminants.
Many species of bacteria, actinomycetes, and fungi have been found to
degrade hydrocarbons associated with petroleum. Bacteria are the prime
microorganisms involved with biodegradation of organics in groundwater.
Naturally occurring species of the genera Pseudomonas, Arthrobacter, Nocardia,
Achromobacterium, and Flavobacterium have been found to attack petroleum
4-14
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hydrocarbons and other organic chemicals (Raymond, et al., 1976). These
bacteria can be stimulated by adding nutrients and oxygen to develop a
population that is adapted to readily degrade organic chemicals present in
groundwater.
An alternative to developing adapted populations from naturally occurring
bacteria is to inoculate the subsurface with microorganisms developed in the
laboratory to degrade specific organic chemicals or chemical groups. The
particular advantages of this alternative are that overall biodegradation
rates of specific organics may be increased and the time required for
adaptation of a naturally occurring population is eliminated. In this method,
the parent microorganisms are collected from a naturally occurring source
containing the specific chemical groupings, such as oil refinery treatment
plant sludge for phenol and cyanide treatment. Mutant strains are developed
from irradiation and these mutant bacteria are cultured in large amounts and
packaged for use. The mutant bacteria can be cultured in even larger amounts
in holding ponds at the site and introduced into the subsurface by spray
irrigation, surface flooding, or subsurface injection.
Cell nutrients required for proper cell growth and respiration include
nitrogen, phosphorus, and trace elements (e.g., potassium, sulfur, sodium,
calcium, magnesium, iron, copper). Groundwater usually only contains
sufficient amounts of nitrogen and phosphorous for degradation of 10 to
20 milligrams per liter (mg/1) of organic material. More nutrients must be
added for biodegradation of higher contaminant levels. Oxygen is also
required for the aerobic decomposition of organics by bacteria. Roughly 3 or
4 mg/1 of oxygen are required for every mg/1 of organic constituent degraded.
A constant supply of dissolved oxygen must be supplied to the contaminated
groundwater in order to maintain biological activity. Subsurface aeration has
been the method most commonly used during previous bioreclamation projects of
groundwaters contaminated with gasoline. However, since this method can only
provide a maximum of about 10 mg/1 of dissolve oxygen, biodegradation of high
levels of organics in the subsurface can be limited. Alternative oxygenation
techniques include the use of pure oxygen systems, ozone, and low concentra-
tions of hydrogen peroxide.
4-15
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The selection of bioreclamation as a plume management technique depends
on the biodegradability of the components in the contaminant plume. Bio-
degradabilities of various organic substances can be estimated using the ratio
of Biochemical Oxygen Demand (BOD) to Chemical Oxygen Demand (COD). Compounds
with a BOD/COD ratio of less than 0.01 are considered relatively undegradable,
compounds with a ratio of 0.01 to 0.1 are considered moderately degradable,
and componds with a ratio of 0.1 or greater are considered degradable (Lyman,
et al., 1974).
Implementation of the bioreclamation process involves the placement of
extraction wells to control migration of the contaminant plume by pumping.
Groundwater pumped to the surface is mixed with nutrients and reinjected
upgradient of the extraction wells. Specialized bacteria may also be added
along with the nutrients. The groundwater may be oxygenated with air, oxygen,
or hydrogen peroxide. The bioreclamation technique has been used successfully
in a number of cases to treat contaminated groundwater plumes from underground
gasoline and hydrocarbon leaks.
4.1.4.2 In Situ Chemical Treatment
In situ chemical treatment techniques involve the injection of a chemical
into a leachate plume to neutralize, detoxify, precipitate, or otherwise
affect the contaminant materials. These techniques are highly dependent on
the contaminant and have in the past been used only for spills of specific
chemicals. Dilute solutions of acids or bases, such as nitric acid or sodium
hydroxide, could theoretically be used to neutralize acidic or basic ground-
water contaminants. A system of extraction and injection wells could be used
to disperse the neutralizing agent, and contain and cycle groundwater until
the appropriate pH was attained. Similarly, chemical agents could be used in
this manner to detoxify plume contaminants. For instance, sodium hypo-
chlorite, has been used to oxidize cyanide contaminated groundwaters. Other
oxidizing chemicals such as hydrogen peroxide or ozone may find potential
application in this type of remedial approach. Solutions of sodium sulfide
have also been proposed to precipitate toxic metals from groundwater, thereby
resulting in their immobilization. Recently, an underground spill of acrylate
4-16
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monomer was treated by the injection of a catalyst which caused the plume of
acrylate monomer to polymerize and solidify (Williams, 1982).
4.2 Technology Evaluation and Selection
Identifying the most appropriate technique for managing a leachate plume
can be an extremely complex process. The selection process involves the
acquisition, evaluation, and application of data that vary in reliability,
applicability and depth of detail. Although data development is relatively
straightforward, the .evaluation and use of this data can be difficult because
of the many interrelated elements involved. Furthermore, developing and
evaluating site remediation plans requires exercising a great deal of
technical judgement.
Under the Comprehensive Environmental Response, Compensation, and
Liability Act of 1980 (CERCLA or Superfund), EPA developed the National
Contingency Plan (NCP) for evaluating site remediation alternatives and their
costs. The NCP (47 FR 31180, July 16, 1982) specifies a three step procedure
for selecting long term site remediation alternative consisting of:
• Develop alternatives for resolving site problems
• Screen the alternatives based on site and waste characteristics
• Conduct a detailed analysis of the alternatives based on expected
implementation and performance problems.
EPA, et al ., (1983) provides general guidance for using this procedure to
evaluate and select site remediation alternatives. This chapter describes how
the procedure can be applied to selecting leachate plume management alterna-
tives. Other sources of information on procedures for selecting among
remedial action alternatives include EPA (1982a) and RADIAN (1983).
4.2.1 Development of Alternatives
The first step in developing a plume management plan involves identifying
and prioritizing the problems at a site, setting response objectives, and
4-17
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developing remedial action alternatives. These alternatives may consist of
variations of the same basic technology (e.g., keyed, circumferential,
cement-bentonite walls; hanging, upgradient, soil-bentonite walls) or combina-
tions of different technologies (e.g., a low permeability barrier combined
with a groundwater pumping system).
Typically, hazardous waste release sites pose many types of environmental
threats which must be prioritized and translated into a series of corrective
actions. For example, a site may pose fire and air contamination hazards from
ignitable and volatile wastes present at the surface in addition to ground-
water contamination. The first action at such a site might be to mitigate the
most immediate hazard by removing surface waste to safeguard the local
populace. Once the threats of fire and noxious emissions are alleviated, a
thorough investigation can be implemented to plan long term corrective meas-
ures for groundwater contamination. If the investigation were to identify
drinking water contamination, providing alternative supplies to local resi-
dents would take priority over implementation of plume management plans. The
need for identifying, prioritizing, and addressing site hazards often con-
tinues throughout the response process as new data is developed and evaluated.
Based on the assessment of a site's problems and the priorities set for
mitigating these problems, specific response goals can be established.
Response objectives can be stated in specific terms, like "Reduce the concen-
tration of substance X in the groundwater at point Y to less than 3 times
background levels within one year." The objectives can also be presented in a
more general manner, such as by stating the degree of plume control that
should be achieved. Establishing the goals of a remediation effort prior to
the technology screening and detailed analysis steps is extremely important to
focus control efforts on the most critical problems of the site.
Response objectives can be used in a number of ways. First, they can be
used to schedule plume migration control efforts in order of priority. One
example of such a ranking could be (1) Protect nearby municipal wells,
(2) Reduce or eliminate plume movement, and (3) Remove the plume. Second,
response objectives can be used during design preparation and review to
4-18
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compare the projected efficiency of a remedial action alternative to the
actual efficiency required to control the plume. For example, if the response
objective was to collect 90 percent of a plume and a groundwater pumping
system was projected to collect only 75 percent of the plume, then that
alternative would be considered to be 83 percent effective. Third, response
objectives can be used to set the standards by which the efficiencies of the
various technologies are judged after implementation. If, for example, a
pumping system were designed to collect at least 60 percent of a plume but
only collected 40 percent of the plume during operation, the pumping system
would be considered to be 66 percent effective. In developing response
objectives for evaluating site remediation alternatives, a number of criteria
should be addressed including:
• Priorities -- Do the response objectives focus on the most critical
problems at the site?
• Scope -- Do the response objectives address all site problems?
• Effectiveness -- To what extent can a given response objective be
fulfilled? To what extent can all response objectives be fulfilled?
• Feasibility -- How difficult will fulfilling a given response
objective be? How difficult will fulfilling all response objectives
be?
• Timing -- How long before beneficial changes are apparent and how long
will the changes 1 ast?
• Requirements -- What resources will be needed to fulfill a given set
of response objectives?
• Benefits -- If fulfilled, will the response objectives produce
environmental changes that are significantly preferable to not taking
action?
Developing plume management alternatives based on the established
response objectives requires exercising a considerable amount of technical
judgement. While there are basically four plume management technologies
available (i.e., well systems, subsurface drainage systems, low permeability
barriers, and in situ treatment techniques), variations and combinations of
these technologies can lead to a large number of possible plume management
alternatives to be considered.
4-19
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To further complicate matters, many plume management technologies cannot
be used by themselves to effectively control the contamination caused by
leachate plume migration. One example of this situation is the use of extrac-
tion wells to remove contaminated water from an aquifer. Once contaminated
water has been removed, the extracted water must be stored in a secure area
and treated to reduce contaminant concentrations to acceptable levels.
Residues from the treatment process must also be disposed of properly and the
treated water must be either released to surface waters, placed in seepage
basins to recharge the aquifer, or reinjected into the aquifer. Thus, the use
of extraction well systems also requires the use of contaminated water
storage, treatment, and disposal facilities.
Once the technologies and their required auxiliary measures have been
identified and developed into a site restoration alternative, a preliminary
evaluation of each alternative can be initiated. This screening step involves
assessing the suitability of each alternative relative to specific site
conditions.
4.2.2 Screening of Alternatives
Screening involves making a preliminary comparison of the plume manage-
ment alternatives to identify those alternatives that are precluded by site
and waste conditions, thereby, reducing the number of alternatives for
detailed analysis to a manageable few. There are three fundamental criteria
to consider in screening remedial action alternatives--technical feasibility,
environmental and public health impacts, and costs.
Technical feasibility screening consists of a review of all pertinent
site and waste characteristics to identify any conditions that would preclude
or otherwise effect the use of a certain alternative. Tables 4-2 through 4-5
highlight the effects that selected site and waste characteristics can have on
the applicability and performance of the four basic plume management tech-
nologies. Additional information on technology applications is detailed in
Chapters 5 through 8. In conducting a technical feasibility screening of
alternatives, a great deal of technical judgement is required to extrapolate
4-20
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TABLE 4-2
THE INFLUENCE OF SITE GEOLOGY ON THE SELECTION AND
PERFORMANCE OF LEACHATE MIGRATION CONTROL TECHNOLOGIES
Geologic
Characteristic
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Soil or overburden
thickness or depth
to bedrock
Little or no effect
Ease of excavation Little or no effect
Soil texture and
1ithology
Stratigraphy,
horizonation,
structure
Mineralogy;
geochemistry;
soil chemistry
Will affect the
design of well
screens and gravel
packs
Will affect well
design but not
necessarily
performance
No effect as long as
groundwater chemistry
is favorable (i.e.,
well materials will
not be corroded or
encrusted)
Drains are generally
restricted to appli-
cations in unconsoli-
dated materials less
than 100 feet deep
Difficult excavation
may preclude the use
of drains
Will affect the design
of the envelope and
filter
Will affect drain
design but not neces-
sarily performance
No effect as long as
groundwater chemistry
is favorable (i.e.,
drains are not
clogged by mineral
deposits)
Slurry walls are generally
restricted to applications
in unconsolidated materials
less than 100 feet deep;
grouts can be used at any
depth if injected
Difficult excavation may
preclude the use of barrier
walls
May be a factor in slurry
loss during installation
No effect as long as key-in
is effective and excavation
is easy
No effect as long as
groundwater chemistry is
favorable (i.e., wall
materials are not
degraded)
Little or no effect
depending on the
delivery system
Little or no effect
depending on the
delivery system
Will affect the design
of the delivery system
No effect as long as
delivery system func-
tions and biochemical
conditions are
favorable
Can have a major
affect on treatment
reaction rates and
extents
-------
TABLE 4-3
THE INFLUENCE OF SITE HYDROLOGY ON THE SELECTION AND
PERFORMANCE OF LEACHATE MIGRATION CONTROL TECHNOLOGIES
Hydro-logic
Characteristic
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Ji
ro
Aquifer type
Aquifer depth and
thickness
Aquifer extent
and configuration
Aquifer homogeneity
and isotropy
Aquifer properties
(e.g., transmis-
sivlty, hydraulic
conductivity,
porosity)
Can be used in both
confined and uncon-
fined aquifers
Little or no adverse
effect
May affect design
but generally will
not influence selec-
tion or performance
May affect perfor-
mance 1f system is
not designed
appropriately
Will affect design
but generally not
performance, although
well systems may not
be the most efficient
alternative for low
transm1ss1v1ty
aquifers
Generally restricted
to unconflned aquifers
Drain depth is
generally limited
to less than 100 feet
May affect design but
generally will not
influence selection
or performance
May affect performance
if system is not
designed appropriately
Generally restricted
to aquifers having
a low to moderate
transm1ss1v1ty
Generally restricted to
unconfined aquifers
Wall depth is generally
limited to less than
100 feet
Little or no effect
Little or no effect
Excess slurry loss may
occur in highly permeable
aquifers; grouting 1s
generally not effective
in low transmissivity
aquifers
Depends on the treat-
ment technology and
delivery system used
Depends on the
delivery system used
Aquifer configuration
may affect the design
of the delivery system
May effect performance
of delivery systems
May affect the selec-
tion of a delivery
system
(continued)
-------
TABLE 4-3 (continued)
Hydrologic
Characteristic
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Hydraulic gradient
J=»
OJ
Will affect design
but not selection
or performance
Hydraulic barriers Will affect design
(i.e., recharge and and performance if
discharge boundaries) barriers are not
Identified adequately
Aquiclude effective-
ness and adjacent
aquifers
Groundwater
chemistry
May affect perfor-
mance if system is
not designed
appropriately
Can be a factor in
corrosion and
encrustation of well
screens and pumps
Will affect the
selection of inter-
cepter drains (high
gradients) versus
relief drains (low
gradients)
Will affect design
and performance if
barriers are not
identified adequately
May affect performance
if system Is not
designed appropriately
Can be a factor in
envelope and drain
clogging
Generally no effect
although very high
hydraulic gradients across
a barrier may induce
failure
Little or no effect
especially if site is
capped effectively
Keyed systems wi11 be
ineffective if aquiclude
leaks
Can be a factor in wall
degradation
May affect delivery
system
Will affect design and
performance of the
delivery system
May affect design and
performance of the
delivery system
Can affect the rate
and extent of some
treatment systems as
well as the delivery
system
-------
TABLE 4-4
THE INFLUENCE OF PLUME CHARACTERISTICS ON THE SELECTION AND
PERFORMANCE OF LEACHATE MIGRATION CONTROL TECHNOLOGIES
Plume
Characteristic
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Flow direction
Flow rate
i
ro
Volume and extent
Contaminant types
Concentrations
Density
Little or no effect
because the system
will readjust flow
directions
Important in system
design and operation
but generally does
not affect selection
or performance
Little or no effect
Can affect the
selection of system
materials and
components; some
contaminants can
clog systems
Little or no effect
if the system is
designed and
operated properly
Will influence the
placement of screens
in individual wells
Little or no effect
because the system
will readjust flow
directions
High flow rates
generally preclude
the use of subsurface
drains
Generally not
practical for very
large plumes
Can affect the
selection of system
materials and
components; some
contaminants can
clog systems
High concentrations
of some contaminants
will clog drainage
systems
Can reduce system
effectiveness if
drain is not situated
properly within the
aquifer
Very important for placing
noncircumferential walls
especially if flow direc-
tions change seasonally
Can be very important
for noncircumferential
walls especially down-
gradient walls
Generally not practical
for isolating very
large plumes
Can affect wall
placement and performance
if contaminants degrade
wall materials
High concentrations
of some contaminants
can degrade wall
materials
Very important for
hanging walls
Little or no effect
Can be very important
depending on the
treatment system used
Generally not
practical for very
large plumes
Generally the primary
consideration in
selecting a treatment
system
High contaminant con-
centrations cannot be
treated effectively by
some treatment systems
May be a factor in
the design of the
delivery system
(continued)
-------
TABLE 4-4 (continued)
Plume
Characteristic
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Viscosity
Solubility in
groundwater
ro
en
Reactiyeness,
explosiveness,
corrosiveness,
volatility
Toxicity
Highly viscous lea-
chate may clog
screens, pipes,
and pumps thus
reducing well
efficiency
Pumping systems can
be designed for both
soluble and insoluble
contaminants
Care must be taken
during installation
and the operation
of pumps and treat-
ment systems; some
contaminants may
degrade well
materials
Little or no effect
except for treatment
system
Highly viscous lea-
chate may clog
envelope materials
resulting in drain
failure
Drainage systems can
be designed for both
soluble and insoluble
contaminants, although
drains tend to function
more effectively when
contaminants are
soluble in groundwater
Care must be taken
during installation
and the operation
of pumps and treat-
ment systems; some
contaminants may
degrade drain
materials
Little or no effect
except for treatment
system
Little or no effect
Little or no effect
Care must be taken
during Installation;
some contaminants
may degrade wall
materials
Little or no effect
Effect depends on the
delivery system used
Effect depends on
treatment technology
used
Care must be taken in
implementing in situ
treatment activities.
The ability of con-
taminants to enter
into chemical and
biochemical reactions,
however, is a require-
ment of in situ
treatment technologies.
Effect depends on the
treatment technique
used. Some highly
toxic contaminants can
not be bioreclaimed.
-------
TABLE 4-5
THE INFLUENCE OF SURFACE CONDITIONS ON THE SELECTION AND
PERFORMANCE OF LEACHATE MIGRATION CONTROL TECHNOLOGIES
Surface
Condition
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Climate
Topography,
slopes
Vegetation
Existing well
pumpage
Locations of
facilities
(e.g., buildings,
pipelines,
utilities, roads)
Generally no effect
Generally no effect
Generally no effect
except to restrict
access or require
clearing
May reduce system
effectiveness if
not accounted for
in design
Generally little
or no effect
Excess precipitation
may cause a reduction
in system effective-
ness
Installation may be
difficult or
impossible in
rough terrain
Generally no effect
except to restrict
access or require
clearing
May reduce system
effectiveness if
not accounted for
in design
May require extra
care in installation
and special design
considerations
Freeze-thaw cycles and
drought may cause wall
failure
Installation may be
difficult or
impossible in
rough terrain
Generally no effect
except to restrict
access or require
clearing
Generally no effect
May require extra
care In installation,
special procedures
to prevent slurry loss,
and special design
considerations
Some treatment
reactions are
inhibited at low
temperature
Generally no effect
depending on the
delivery system
Generally no effect
except to restrict
access or require
clearing
May effect delivery
system performance
Little or no effect
except as it
influences the
delivery system
(continued)
-------
TABLE 4-5 (continued)
i
ro
Surface
Condition
Receptors
Leachate Migration Control Technologies
Groundwater Pumping
Site security
required
Subsurface Drains
Site security
usually required
Low Permeability Barriers
Site security
usually advisable
even if site is
capped
In Situ Treatment
Site security
required
Potential for
future land
usefulness
Good if waste
source and plume
are removed
Good if waste
source and plume
are removed
Not advisable
Good if waste source
is removed and
treatment process
was effective
-------
general guidelines on technology applications (such as those listed in Tables
4-2 through 4-5) to site specific plume management alternatives. Furthermore,
certain site and waste conditions may not preclude a given alternative when
considered singly but would if considered together. Therefore, reconsidering
some decisions made during the screening process may be necessary in subse-
quent detailed analyses.
Screening alternatives for environmental and public health impacts
involves two major issues. First, the anticipated effectiveness of the
alternative in reducing risks related to the site must be determined. This
evaluation generally uses both the response objectives described previously
and the no action alternative as points of reference for the effectiveness of
the alternatives. Second, all potential effects of implementing the
alternative which are adverse to the environment or the public health must be
•
identified. For example, excavation of a low permeability barrier or subsur-
face drain may create dust or waste volatilization problems. The results of
this screening step is a summary of the benefits of implementing a given
remedial action alternative.
Cost screening is based on a review of projected capital and operation
and maintenance (O&M) costs. Capital costs of a remedial action alternative
can include (EPA, et al., 1983):
• Construction costs -- includes equipment, labor (including fringe
benefits and workman's compensation), and materials required to
install a remedial action.
t Equipment costs -- includes remedial action and service equipment.
• Land and site development costs -- includes land related expenses
associated with purchase of land and development of existing property.
• Buildings and service costs -- includes process and nonprocess
buildings and utility hook-ups.
t Relocation expenses -- includes costs for temporary or permanent
accommodations for affected residents.
• Engineering expenses -- includes administration, design, construction
supervision, drafting, and testing of remedial action alternatives.
4-28
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• Legal fees, license and permit costs -- includes administrative and
technical costs necessary to obtain licenses and permits for facility
installation and operation.
• Start up and shake down costs -- includes costs incurred during
remedial action start up.
t Contingency allowances -- Contingency allowances should correspond to
the reliability of estimated costs and experience with the remedial
action technology.
O&M costs can include (EPA, et al., 1983):
• Operating labor costs -- includes all wages, salaries, training,
overhead, and fringe benefits associated with the labor needed for
post construction operations.
• Maintenance materials and labor costs -- includes the cost for labor,
parts, and other materials required to perform routine maintenance of
facilities and equipment for the remedial alternative.
• Auxiliary materials and energy -- includes such items as chemicals and
electricity needed for treatment plant operations, water and sewer
service, and fuel costs.
• Purchased services -- includes such items as sampling costs,
laboratory fees and professional services for which the need can be
predicted.
• Disposal costs -- includes transportation and disposal of any waste
materials, such as treatment plant residues generated during remedial
operations.
• Administrative costs -- includes all costs associated with administra-
tion of remedialaction operation and maintenance not included under
other categories such as labor overhead.
• Insurance, taxes, and licensing costs -- includes such items as
liability and sudden and accidental insurance, real estate taxes on
purchased land or right-of-way licensing fees for certain technol-
ogies, and permit renewal and reporting costs.
• Maintenance reserve and contingency funds -- represents annual
payments into escrow funds to cover anticipated replacement or
rebuilding of equipment and any large unanticipated O&M costs,
respectively.
t Other costs -- includes all other items which do not fit into any of
the above categories.
4-29
-------
These costs should be updated to current values using standard costs indices.
If the alternatives have significantly different time periods for implementa-
tion, the costs should be adjusted by conducting a present worth analysis
(EPA, et al., 1983).
The cost screening evaluation involves comparing the costs of competing
remedial action alternatives to establish the relative costs of alternatives
producing similar environmental, public health, and public welfare benefits.
Competing alternatives should be eliminated if they are deemed expensive
(i.e., an order of magnitude or more) and offer similar or lesser environ-
mental and public health benefits. Alternatives that are more expensive but
offer greater benefits should not be eliminated.
Alternatives that have passed this final stage of preliminary screening
will undergo a more detailed evaluation using the parameters outlined in
Section 4.2.3.
4.2.3 Detailed Analysis of Alternatives
The detailed analysis of the remaining alternatives is similar to screen-
ing in the criteria assessed but is different in that technical judgement
plays a less significant role. Instead, additional data are collected bearing
on the technical feasibility, environmental and public health impacts, and
costs of each alternative. These data are analyzed and used as a basis for
eliminating inappropriate alternatives and selecting the best site remediation
alternative.
Assessing an alternative's technical feasibility involves examining its
anticipated reliability and implementability. The reliability of a remedial
action alternative depends on the alternative's:
• Performance history -- The degree of success experienced with the same
or a similar remedial action alternative under similar site or waste
conditions or both (CERCLA funds can be used only if the technologies
included in an alternative have been demonstrated effective for waste
site restoration).
4-30
-------
• Effectiveness -- The extent to which the alternative is expected to
meet or exceed each response objective.
• Durability -- The projected period of effectiveness and the amount of
maintenance required.
• Flexibility -- The ability of the alternative to be adapted to changes
in site and waste conditions without causing environmental damage.
• Sensitivity -- The ability of the alternative to accommodate transient
upset conditions without a major reduction in effectiveness.
• Reliance on companion measures -- The extent to which the primary
plume control technology depends on associated remedial measures to
achieve its maximum effectiveness.
able 4-6 summarizes the general reliability of the four basic leachate migra-
ion control technologies.
The implementability of a remedial action alternative depends on the
following alternatives:
• Ease of installation -- The difficulty anticipated in installing and
making an alternative operational because of site conditions (e.g.,
topography, geology, hydrology, soils, climate).
• Ease of construction -- The difficulties anticipated because of the
alternative's special technical requirements (e.g., access, avail-
ability of equipment and materials, special technical skills required,
QA)
• Time to implementation -- The anticipated time necessary to collected
additional data required, design the system, install the system, make
the system operational, and note an improvement in environmental
conditions.
t Ease of operation -- The difficulty anticipated in operating the
system over its design life to fulfill the response objectives and in
monitoring the system's performance.
• Ease of repair -- The difficulty anticipated in repairing, replacing,
or rehabilitating key components of the system.
• Safety requirements -- The equipment and procedures needed to protect
the health and safety of site workers and local inhabitants.
Table 4-7 summarizes the general implementabil ity of the four basic leachate
migration control technologies.
4-31
-------
TABLE 4-6
FACTORS AFFECTING THE RELATIVE RELIABILITY OF LEACHATE MIGRATION CONTROL TECHNOLOGIES
Reliability
Criterion
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Performance
history
i
00
ro
Effectiveness
Durability
Used successfully at
many hazardous
waste sites and for
related dewatering
applications
Generally very
effective if
designed, operated,
and maintained
properly. Poor
operation and
maintenance are
the chief reasons
for poor
effectiveness.
Generally very
durable and easy
to maintain. Wells
can generally last
the ful1 period of
operation of the
system although pumps
require periodic
replacement.
Used successfully at
many sites but not
as commonly as well
systems; also used
frequently for
related dewatering
applications
Can be very effective
and more efficient
than pumping systems
if designed and
maintained properly.
Drain clogging and
pipe failure are the
chief reasons for
poor effectiveness.
Not as durable as
pumping systems
because drain
envelopes tend to
clog. Maintenance
is not as simple as
for well systems.
Used successfully at
selected hazardous
waste sites; use of
slurry trenching
techniques is
increasing; grouting
applications are
relatively minor
Very effective if
installed properly
although some leakage
will generally occur
Durability will be highly
variable depending on the
hydrologic and chemical
stresses on the wall.
Maintenance is generally
minimal.
Certain treatment
systems have been
demonstrated at
hazardous waste sites
including biorecla-
mation, flushing, and
polymerization; used
primarily for single-
chemical spills
Variable effectiveness
depending on site and
leachate characteris-
tics and the treatment
technology used
Not applicable if the
leachate source is
removed
(continued)
-------
TABLE 4-6 (continued)
Reliability
Criterion
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Flexibility
Sensitivity
CO
CO
Very flexible.
System can be
adapted to changes
simply by adding
wells, revising
pumping rates and
schedules, or
changing treatment
operations.
Generally can
accommodate minor
upset conditions
such as high water
levels resulting
from heavy precip-
itation. Pump
failure can render
portions of the
system ineffective,
however.
Not very flexible
because drain
influence cannot be
control led without
changing system design
Generally can
accommodate upset
conditions except
for major system
failure (e.g.,
clogged drainage
main)
Not flexible once
installed, although
this is typically
not restrictive
Generally can
accommodate upset
conditions except
for major wal1
failures
Somewhat flexible
depending on the
delivery system and
treatment technology
used, although this
is typically not
restrictive
Variable depending
on the type of upset
condition and the
delivery system and
treatment technology
used
Reliance on
companion
measures
Generally requires
leachate storage
and treatment
facilities; may
require source
removal or isolation
Generally requires
leachate storage
and treatment
facilities; may
require source
removal or isolation
Generally requires site
capping for circumferential
walls, and wells or drains
for downgradient and
floating walls; may
require source or leachate
removal or treatment
Requires a delivery and
recovery system such
as wells or drains;
generally requires
source removal and
treatment or disposal
-------
TABLE 4-7
FACTORS AFFECTING THE RELATIVE IMPLEMENTABILITY OF LEACHATE MIGRATION CONTROL TECHNOLOGIES
Implementabillty
Criterion
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Ease of
instal lation
-Pi
i
OJ
Ease of
construction
Time to
implementation
Generally very easy
to install but can
be somewhat diffi-
cult to fine tune
operations
Most systems can
be constructed by
qualified local
drillers and pump
installers
Fairly rapid to
install and affect
changes in plume
migration patterns
Can be very difficult
to install if hard
rock excavation and
shoring and dewater-
ing is needed, if the
design drain depth is
large, and if the
topography is very
uneven; these
problems are not
insurmountable
Some systems may
require the use of
specialized equip-
ment and materials;
QA is an important
consideration; local
contractors may not
be qualified
Moderately time
consuming to install
but will affect
changes in plume
migration patterns
fairly rapidly
Can be difficult or
impossible to instal1
in areas of extreme
topography or climate
Some systems may require
the use of specialized
equipment and materials;
QA is an important
consideration; local
contractors may not be
qualified
Moderately time consuming
to install and may not
affect any improvement
in site conditions for
some time depending on
system placement
Variable depending
on the delivery
system and treatment
technology used.
Climate, hydrology,
and soil geochemistry
are the most signif-
icant factors.
May require special-
ized equipment and
materials. Most
in situ treatment
is undertaken by
specialty firms
Highly variable
depending on site
size and treatment
effectiveness
(continued)
-------
TABLE 4-7 (continued)
Implementability
Criterion
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Ease of
operation
i
GO
CJ1
Ease of
repair
Safety
requirements
Can be fairly complex
to fine tune system
and operate at
maximum efficiency;
treatment may also
be fairly complex
Generally, fairly
easy to rehabilitate
or replace wells,
pumps, and surface
piping; treatment
system repair may
be more complex
Generally the least
stringent of the
technologies because
subsurface distur-
bances are minimal;
can be significant
if explosive wastes
are present or if
wells must be
installed through
waste piles
Fairly simple since
groundwater collec-
tion is a function
of gravity flow;
treatment may be
fairly complex
Pipe and envelope
clogging requires
excavation and
reinstallation;
pump and treatment
system repair are
the same as for
well systems
More stringent than
well systems because
of the greater extent
of subsurface
excavation and the
potential for waste
volatilization and
dust generation;
trench cave-in is a
primary safety concern
Implementation limited
to monitoring and
operating associated
remedial action
technologies
Slurry trench sections
that have failed generally
cannot be repaired but
instead are replaced with
new wall sections; grout
failure usually cannot
be repaired effectively
Slurry trench requirements
are similar to those for
drains; grouting require-
ments are similar to those
for wells
Can be very complex
because of the
variables involved
in system operation
Generally, fairly easy
to rehabilitate or
replace system
components
Variable, but can be
as stringent or more
stringent than for
drains depending on
the delivery system
and the treatment
agents used
-------
Conducting a detailed environmental analysis of each alternative involves
predicting all changes (both adverse and beneficial) from existing conditions.
This applies to both sensitive biotic environments (e.g., endangered and pro-
tected species, wildlife habitats and breeding grounds, vulnerable ecosystems)
and human resource use patterns (e.g., recreational areas, fishing and hunting
grounds, historic sites, traffic patterns, water supplies). These changes can
be evaluated by comparing them to the response objectives, the no action
alternative, or published environmental criteria such as those given in:
• "Quality Criteria for Water" (Federal Register November 28, 1980) --
Criteria documents are available for each of 64 toxic pollutants.
• "Quality Criteria for Water" (Red Book, July 1976) -- Contains some
additional criteria not preempted by above (e.g., temperature,
aesthetics, dissolved oxygen).
• Safe Drinking Water Act (40 C.F.R. 141) -- Maximum levels set for six
contaminants. Suggested No Adverse Response Levels (SNARLS) are given
for others.
• Pesticide Residue Amendment to the Food, Drug and Cosmetic Act (40
C.F.R. 180) -- Safe levels of 301 pesticides in soil based on crop
tolerance levels.
The degree of actual or potential harm caused by the plume must also be
addressed. Contamination of some aquifers, notably saline ones, may be con-
sidered a relatively low priority problem, whereas the presence of a hazardous
waste plume in an aquifer that is the sole drinking water source for numerous
people is a high priority issue.
Anticipating and preventing adverse impacts from site remediation
activities is a key step in the detailed environmental assessment. Areas that
are disturbed during excavation of subsurface drainage trenches, construction
access roads, sumps, slurry walls, recharge basins, slurry mixing areas, and
leachate storage facilities are extremely susceptible to erosion unless
erosion prevention measures are used. The soil carried off of the site as a
result of erosion becomes deposited in surface water bodies and can clog water
ways, interfere with wildlife, and contribute to flooding downstream. If the
eroded sediments contain chemical contaminants, the degradation of water
4-36
-------
quality is considerably more serious. The quality of the air surrounding the
site can also become degraded when disturbance of the waste site results in
fires or explosions, volatilization of waste constituents, or production of
toxic gases, mists, or dusts.
When technologies that remove the plume from the aquifer are used, the
extracted plume must be treated to reduce its contaminant concentrations to
acceptable levels. In some cases, the treated water will still contain
higher than background levels of contaminants. If this water is discharged to
surface waters, the treated water may result in a reduction in surface water
quality. Furthermore, removal of uncontaminated groundwater upgradient of the
site without recharging this water downgradient of the site can lower the
water table downgradient and result in modifications in both groundwater and
surface water flow and use, and vegetation patterns. These conditions would
be most pronounced if the extracted groundwater was diverted to a different
watershed. Hydrophillic (water loving) plants in areas that formerly had a
high water table may become replaced by less water tolerant species. This
could then cause a modification in wildlife populations or distribution in the
affected areas.
Also important in the detailed environmental assessment is an evaluation
of the hazards that will be posed until a site restoration alternative can be
implemented and the risks and hazards associated with the failure of an
alternative to fulfill the response objectives. In addition to the technical
feasibility and environmental impact aspects of each alternative, institu-
tional and health and safety requirements should be evaluated.
Local, state and federal laws require provisions for regulating certain
activities. Because of these laws, special permits, bonds or licenses may be
required to conduct certain activities at some sites. For example, removal of
groundwater from some aquifers, particularly in arid and semiarid areas, may
be prohibited without special permission, and transfer of water from one
watershed to another is illegal in some areas. Table 4-8 highlights regula-
tions that may be applicable to site remediation activities. Other important
institutional issues to be addressed include community relations and the need
4-37
-------
TABLE 4-8
SELECTED REGULATORY REQUIREMENTS FOR
AQUIFER RESTORATION ACTIVITIES
Source of Requirement
Requirement or Prohibition
FEDERAL PERMITS AND REGULATIONS
Resource Conservation and Recovery Act
Federal Water Pollution Control Act
(also called the Clean Water Act)
Safe Drinking Water Act
Wild and Scenic Rivers Act
Endangered Species Act
Fish and Wildlife Coordination Act
Coastal Zone Management Act
RCRA permit or interim status
authorization may be needed if
hazardous wastes are treated,
stored, or disposed
RCRA manifest needed if wastes are
to be transported from the site
National pollutant discharge
elimination system (NPDES) permit
needed if wastewater is to be
discharged into navigable waters
Dredge or fill permit needed if
these materials are to be
discharged into navigable waters
Underground injection control
permit may be needed if treated
groundwater is reinjected into the
subsurface
Sole source aquifer permit may be
required if reinjection involves a
sole source aquifer
Prohibits any action that will
adversely affect the wild, scenic,
or recreation status of a river on
the National Inventory
Prohibits jeopardizing any endan-
gered or threatened species or its
habitat
Prohibits modifying a body of
water without state approval if it
will affect wildlife resources
Prohibits actions inconsistent
with Federally approved state
coastal zone management programs
(continued)
4-38
-------
TABLE 4-8 (continued)
Source of Requirement
Requirement or Prohibition
Archaeological and Historic
Preservation Act
Atomic Energy Act; Low Level
Radioactive Waste Policy Act
Flood Disaster Protection Act;
National Flood Insurance Act;
Executive Order 11988
National Environmental Protection Act
Executive Order 11990
Prohibits actions resulting in the
loss or destruction of significant
scientific, historical, or archae-
ological data
Prohibits the transport and
disposal of radioactive wastes
in certain areas under certain
conditions
Requires Federal agencies to
evaluate potential effects of
actions planned for floodplains
Requires flood insurances in some
cases
May require the preparation of
environmental impact statements
and environmental assessments
Requires that activities in
wetlands include all practical
means of minimizing harm to the
environment
STATE AND LOCAL PERMITS AND REGULATIONS
Federal delegation of authority or
state and local requirements analogous
to Federal requirements
Requires permits, rulings, or
approvals for the treatment,
storage, transport, and disposal
of hazardous wastes; for the devel-
opment, operation, and closure of
waste management facilities; for
wastewater discharges to surface
waters (NPDES); for activities
likely to result in hazardous air
emissions; for activities involving
underground injection; for dredge
or fill, wetlands, and sensitive
resources; and for activities that
may require state or local funding
or support
(continued)
4-39
-------
TABLE 4-8 (continued)
Source of Requirement
Requirement or Prohibition
Zoning regulations, building codes
(fire, water, electrical, sewer),
water use regulations
Public safety regulations
Requires permits or the preparation
of approved plans for construction
or demolition activities, erosion
and dust control, right-of-way,
easements, well installation, and
water use and discharge
Generally requires advanced plan-
ning and coordination with local
police, fire departments,
hospitals, and rescue squads
to coordinate activities with site owners and governmental agencies (e.g.,
EPA, USGS, COE, and state environmental agencies).
Safeguarding the health and safety of both the personnel installing the
plume management technologies and the people living near the site is a primary
factor in evaluating alternatives. In addition to the safety hazards asso-
ciated with any major construction activity, construction at hazardous waste
sites involves other hazards, including potential fires and explosions, and
possible release of chemicals from buried drums or storage tanks. Personnel
working at hazardous waste sites and local residents can also be exposed to
numerous health hazards. These hazards can reach the worker through direct
dermal exposure, inhalation of toxic constituents, or indirect ingestion of
dusts from the site. Direct dermal exposure can be prevented by requiring
workers to wear personal protective gear when working in hot zones at the
site. This gear can also assist in preventing inhalation of toxic
constituents, if respirators or self contained breathing apparatus (SCBA) are
used.. Indirect ingestion of toxins can be prevented by using strict site
security to prevent public access, by prohibiting eating, drinking or smoking
in hazardous areas of the site, and by requiring the workers to wash prior to
eating or smoking.
4-40
-------
Indirect ingestion can be a particular problem at sites where contami-
nated dusts are present. If dusts are inhaled, the majority will be captured
in the upper respiratory tract so very little will reach the lungs. However,
the dust that enters the upper respiratory tract will accumulate there until
the individual coughs to dislodge it. Most individuals will then swallow, and
the contaminated dust will enter the digestive tract. Site workers can be
protected from this hazard by using SCBA or respirators with properly selected
filters. Local inhabitants can be protected by controlling dust generation
and if necessary, by evacuation.
In evaluating the health and safety aspects of a site restoration alter-
native, precautions for safeguarding site workers and the public can increase
dramatically the time and resources required to implement the alternative.
Therefore, assessing both alternatives that minimize health and safety risks
and those that attempt to control risks is important.
The cost assessment portion of the detailed analysis of alternatives is
intended to provide measures of both the total costs and cost effectiveness
over time associated with each alternative under consideration. The most
reliable method for comparing the costs of proposed options is to prepare
preliminary designs for each of the alternatives that are being considered for
a site. These designs can then be used to compile site specific cost
estimates. These cost estimates generally have an accuracy of +50 to -30 per-
cent (EPA, et al., 1983). Key cost items and the relative costs of the four
basic leachate migration control technologies are summarized in Table 4-9.
The procedure recommended for conducting a cost assessment of remedial
action alternatives consists of four steps (EPA, et al., 1983):
• Estimate costs -- Determine capital and annual operating costs for
remedial alternatives.
• Standardize costs -- Calculate stream of payments and present worth
for each remedial action alternative using estimated costs.
• Assess cost sensitivity -- Evaluate risks and uncertainties in cost
estimates.
4-41
-------
TABLE 4-9
RELATIVE COSTS AND KEY COST ITEMS OF LEACHATE MIGRATION CONTROL TECHNOLOGIES
Cost
Criterion
Leachate Migration Control Technologies
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In S1tu Treatment
I
-p.
CAPITAL COSTS
Relative cost
Key items
O&M COSTS
Relative cost
Key items
Generally low
although groundwater
treatment, storage,
and disposal can
have a major Impact
t Well Installation
and materials
• Pumps
• Surface piping
• Effluent storage
and treatment
facilities
High to very high
depending on treat-
ment requirements
Fairly high depending
on site size, depth
and ease of excava-
tion, and system
design
• Drain excavation
and Installation
• Drain materials
• Pumps and piping
• Effluent storage
and treatment
facilities
High to very high
depending on treat-
ment requirements
t Effluent treatment • Effluent treatment
• Pump operation
• System maintenance
• Monitoring
• System maintenance
and operations
Moderate to high depending
on site size, depth and
ease of excavation, and
system design
• Wall excavation and
Installation
• Wall materials
Fairly low depending
on associated
technologies used
• Monitoring
• System maintenance
Highly variable
depending on site
size, treatment method
used, and associated
technologies required
• Treatment agents
• Delivery systems
• Associated
technologies
Monitoring
Highly variable
depending on associ-
ated technologies
used
• Monitoring
• System maintenance
• System operation
-------
• Assess cost effectiveness — Judge the importance of the poential
benefits for each alternative relative to its prospected costs.
Detailed descriptions of this procedure and a matrix based methodology for
assessing remedial action cost effectiveness are given in Radian (1983) and
EPA, et al., (1983). Based on the cost effectiveness analysis, the best site
remediation alternative should be selected and developed in detail. Table
4-10 presents a general summary of the advantages and limitations of the four
basic leachate migration control technologies.
4.3 Design and Implementation of Alternatives
Once the best (or best few) remedial action alternative(s) has been
selected, a conceptual design should be prepared. The conceptual design
should include:
• Engineering drawings showing present topography and proposed
topographic changes; site boundaries and property lines; locations of
buildings, waterways, and utility lines, and other surface features;
locations of sewer lines, pipelines, and other subsurface features;
and locations of remediation systems (such as wells) to be installed
• Estimated dimensions of subsurface drains or barriers, number of wells
or other specific size related data
• Lists of required auxiliary measures
• Estimations of equipment and materials needs
• Estimated implementation periods and milestones
• Other factors such as requirements for worker safety and public
protection, waste removal, transport and disposal (if necessary) and
potential for adverse environmental impacts
• Budget cost estimates.
During preparation of these designs, additional factors or considerations may
be identified. These should also be incorporated into the conceptual design
document.
4-43
-------
TABLE 4-10
SUMMARY OF THE FOUR BASIC LEACHATE MIGRATION CONTROL TECHNOLOGIES
Criteria
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Performance
history
Effectiveness
Durability
Flexibility
Sensitivity
Reliance on
companions
measures
Applied successfully
at both hazardous and
nonhazardous sites;
poor system operation
1s the chief cause
of performance
problems
Generally good
Excellent with
occasional main-
tenance
Very good
Good
Usually requires
leachate treatment
Applied successfully
at both hazardous and
nonhazardous sites;
system clogging is the
chief cause of per-
formance problems
Generally good
Dependent on leachate
and groundwater
chemistry
Very good
Good
Usually requires
leachate treatment
Applied successfully
at both hazardous and
nonhazardous sites;
wall degradation
caused by chemical and
hydrologlc conditions
Is the chief cause of
performance problems
Generally good
Dependent on leachate and
groundwater chemistry
Limited
Very good
Usually requires site
capping
Limited number of
applications at
hazardous sites and
no applications at
nonhazardous sites;
effectiveness of the
delivery system is
the chief cause of
performance problems
Variable depending on
site conditions
Not applicable
Usually very limited
Limited
Usually requires
source removal
(continued)
-------
TABLE 4-10 (continued)
Criteria
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
i
-P»
tn
Ease of
installation
Ease of
construction
Time to
implementation
Ease of operation
Ease of repair
Safety requirements
Very good
Very good
Relatively short
Fairly complex
Fairly simple
Minimal relative to
other technologies
Variable depending on
shoring, dewatering,
and excavation
requirements
Good; some special-
ized equipment may
be required
Moderate to short
Relatively simple
Fairly complex
Very stringent
because of the need
to control dust,
leachate, and ground-
water during
construction
Variable depending on
topography and formation
permeability
Generally good; some
specialized equipment may
be required
Moderate to short
Very simple
Fairly complex
Fairly stringent because
of the need to control
dust and leachate during
construction
Variable depending on
treatment scheme and
the delivery system
Requires contractor
with specialized
experience, equipment,
and materials
Fairly long to system
operation but short to
completion of
treatment
Complex
Complex to simple
Very stringent
depending on the
treatment scheme and
the delivery system
(continued)
-------
TABLE 4-10 (continued)
Criteria
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
Capital costs
O&M costs
General comments
Relatively low; high
If on site treatment
is needed
Relatively high;
very high 1f on site
treatment 1s needed
Relatively high
Relatively high; very
high if leachate
treatment Is needed
t Can be designed
for nearly any
hydrogeologic con-
dition and leachate
type, although
pumping systems
tend to be less
efficient in low
transmlsslvity
aquifers and with
viscous leachates
Relatively low to
very high if leachate
treatment Is needed
t Especially useful i
in low transmls-
slvlty aquifers and
for collecting
groundwater mounds
beneath sites
Use may be restricted
by high flow rates,
difficult excavation,
and leachates that
can clog drain
components
Relatively low
Slurry trenches can be
used in both high and
low transmissivity
aquifers although slurry
loss can be a problem in
some cases; grouts can
generally only be used
in high permeability
formations
Variable depending on
the treatment scheme
and the delivery
system
High to low depending
on system selected
Treatment system
selection and
implementation is
greatly influenced
by groundwater and
leachate chemistry.
The success of the
system will depend
largely on these
factors: the
effectiveness of
the delivery
systems, and the
ability of the
operator to adjust
to changes in site
conditions.
(continued)
-------
TABLE 4-10 (continued)
Criteria
Groundwater Pumping Subsurface Drains
Low Permeability Barriers In Situ Treatment
General comments
(continued)
i
-fi
—i
• Pumping systems
can be Installed
rapidly and
inexpensively by
qualified local
well drillers,
pump installers,
and general
contractors
• System design can
be modified easily
to accommodate
changes in site
conditions
• Pumping systems
are relatively
simple to repair
but difficult
and expensive to
operate, especially
if on site leachate
treatment is needed
• Operation is
generally simpler
than for pumping
systems, although
maintenance and
repair is more
difficult
• OSM costs can be
very high if
on site leachate
treatment is needed
• Circumferential walls,
the most commonly used
type of barrier; require
site capping and effec-
tive aquiclude key-in
to be successful
• The most significant
limitation of barriers
1s the ability of wall
materials to withstand
chemical attack by
reactive leachates
• Operation and main-
tenance Is generally
simple and relatively
Inexpensive
• Implementation
generally requires
using specialty
firms as opposed to
local contractors
• No long term site
presence required
if leachate source
is removed and
treatment is
effective
-------
Engineering specifications generally provided in the design of a
remediation alternative include detailed descriptions of material requirements
and local availability, work practices, and product performance. Other items
described in these specifications include quality control and quality
assurance procedures and equipment requirements. Schedules should also be
provided which allow for segmenting the operations so that convenient
stopping-points are identified in advance. If the operation is set up in this
manner, the difficulties associated with work stoppages, because of supply
shortages, strikes, funding problems or adverse weather conditions can be
minimized. Phasing plans that allow maximum coordination between contractors
working at the site also can reduce costs and speed construction.
Once an acceptable leachate migration control plan is developed, site
remediation efforts can begin. Details related to designing and implementing
plume management systems are described in Chapters 5 through 8.
4-48
-------
CHAPTER 5
GROUNDWATER PUMPING
5.1 Introduction
This section describes the theory, applications, design considerations
and costs associated with groundwater pumping methods to contain and remove
plumes originating from hazardous waste disposal facilities. Plume con-
tainment is defined here as arresting the further spread of groundwater
contamination by altering hydraulic gradients. Containment methods do not
necessitate stopping contamination at the source. Plume removal is defined as
removing the contaminants from the groundwater system so that the aquifer is
purged of harmful constituents. Both approaches emphasize the active
diversion and removal of groundwater rather than the passive approach of
installing hydraulic barriers.
An associated problem arises from utilizing groundwater pumping methods
(or subsurface drains) to control plumes, that is, the treatment or disposal
of the removed contaminated groundwater. Numerous treatment processes (e.g.,
carbon absorption) are available to remove contaminants from the water.
However, these techniques are beyond the scope of this document and will not
be discussed. Injection of treated water back into the groundwater system
will however be discussed as it relates to altering groundwater gradients.
Previous utilization of pumping technologies to manage plumes has shown
that these methods are most effective at sites where underlying aquifers have
high intergranular hydraulic conductivities (e.g., coarse grained sands) and
with contaminants that move readily with the groundwater flow (e.g., benzene).
Pumping methods have also been utilized with some effectiveness at sites where
pollutant movement is occurring along fractured or jointed bedrock. However,
the fracture patterns must be traced in detail to ensure proper well
piacement.
5-1
-------
5.2 Well Theory
The economic and effective withdrawal of contaminated water from a
groundwater system is highly dependent on the proper design and construction
of a well. In order to make proper design decisions, an understanding of the
principles of well hydraulics is needed. Applications and extensions of these
theories permit solutions of problems related to flow towards wells under a
variety of situations. This section provides some of the basic well hydraulic
equations describing the relationships among the various hydraulic character-
istics of aquifers and the response of aquifer systems to pumping wells.
5.2.1 Darcy's Law
Darcy's Law describes flow through a porous medium and thus provides the
foundation for all groundwater flow theory. Common forms of Darcy's law are:
Q = KIA = VAn
where:
V = Kl/n
I = (hrh2)/L
and the variables are defined as:
Q = flow (ft3/day)
K = hydraulic conductivity of the porous medium (ft/day)
I = hydraulic gradient (dimensionless)
o
A = area normal to flow direction (ft )
V = velocity of groundwater flow (ft/day)
n = effective porosity (dimensionless)
h,-h? = water levels at two points along the same
groundwater flow path (ft)
L = length of the flow path between h, and h« (ft)
Darcy's Law is only valid when conditions of laminar flow exist.
5-2
-------
Based on Darcy's Law, the hydraulic gradient varies directly with flow
velocity and indirectly with area. This relationship can be used to explain
the formation of a cone of depression around a pumping well (Figure 5-1).
When a well is pumped, the water level in the vicinity of the well is lowered
and the greatest depth of lowering occurs at the well. Depth of lowering or
drawdown is less at greater distances from the well and at some point in the
aquifer lowering is nonexistent. Because the water level is lowest at the
well, water moves from the aquifer into the well to replace the withdrawn
water. Gravity and water pressure (head) drive the water towards the well in
the direction of decreasing head. As the flow converges towards the well
under an increased hydraulic gradient, the velocity increases according to
Darcy's Law. This process results in the typical cone of depression
associated with pumped wells (Figure 5-2).
The shape and size of the cone is dependent upon the pumping rate,
pumping period (i.e., cycles), slope of the original water table, hydraulic
barriers, aquifer characteristics, and recharge zones. Two aquifer character-
istics that are important in determining the cone's configuration are of
transmissivity (T) and coefficients of storage (S). Transmissivity indicates
the amount of water that can move through an aquifer. The coefficient of
storage indicates the amount of water which can be removed by pumping.
Transmissivity (T) is related to the hydraulic conductivity (K) of an aquifer
by:
T = Km
where m is the saturated thickness of the aquifer. The effects of trans-
missivity and storage on the shape of the cone of depression is illustrated in
Figure 5-3. This illustration shows that drawdown is inversely related to the
coefficients of storage and transmissivity.
Values of transmissivity and storage are generally determined for an
aquifer by drilling wells and conducting pumping tests. Once these values
5-3
-------
FIGURE 5-1.
DEVELOPMENT OF FLOW DISTRIBUTION ABOUT A
DISCHARGING WELL IN A FREE AQUIFER - A FULLY
PENETRATING AND 33-PERCENT OPEN HOLE
(BUREAU OF RECLAMATION, 1977)
Static water table
5-1a. Initial stage in pumping a free aquifer Most water follows a path
with a high vertical component from the water table to the
screen
5-1b. Intermediate stage in pumping a free aquifer Radial
component of flow becomes more pronounced but contribution
from drawdown cone in immediate vicinity of well is still
important
Drawdown _ •*• '
cone
5-1c. Approximate steady state stage in pumping a free aquifer.
Profile of cone of depression is established Nearly all water
originating near outer edge of area of influence and stable
primarily radial flow pattern established.
. — ~+ — Flow Lines
Equipotential Lines
5-4
-------
FIGURE 5-2.
FORMATION OF CONE OF DEPRESSION
FOR A PUMPED WELL
Cone of Depression
5-5
-------
FIGURE 5-3.
EFFECT OF STORAGE AND TRANSMISSIVITY ON THE SHAPE
OF THE CONE OF DEPRESSION (BUREAU OF RECLAMATION, 1977)
S, = 50S2
All Other Factors Constant
Static Water Level
I 40'
•
60
80
\ »
1200 800
400
400
8QO 1200
Radial Distance from Well Feet
Figure 5-3a. Influence of storativity on drawdown in a well.
T, = 3T2
All Other Factors Constant
Static Piezometric Surface
Not Coincident, but Too
Close to Be Separated at
Scale of Drawing
VY
<
\
Figure 5-3b. Influence of transmissivity on drawdown in a well.
5-6
-------
have been obtained, predictions can be made which are critical in designing
and constructing effective pumping systems for plume management. Some of the
predictions which can be made are (Johnson Division, UOP Inc., 1975):
• Specific capacity of wells with differing diameters
• Drawdown of an aquifer at varying distances from the well
• Drawdowns with varying pumping rates and times.
In plume management the radius of influence (i.e., the extent of the cone of
depression) and the resulting change to the groundwater velocity will be the
most important unknowns. By controlling the radius of influence, the flow of
contaminants in the groundwater system can be controlled.
5.2.2 Equilibrium Well Formula
C. Theim and P. Forchheimer independently derived equations for
equilibrium (steady state) radial flow to wells under confined and unconfined
aquifer conditions. Both equations assume recharge at the edge of the cone so
that the cones dimensions remain constant as long as pumping rates remain
constant.
The basic formula for an unconfined aquifer (Figure 5-4) is:
H2-h^ = (QArK) In (RO/PW)
where:
H = saturated thickness of the aquifer (ft)
hw = height of water in the well (ft)
Q = pumping rate (ft /day)
K = hydraulic conductivity (ft/day)
RQ = radius of influence of the cone of depression (ft)
r = radius of the well (ft).
5-7
-------
FIGURE 5-4.
UNCONFINED AQUIFER FLOW
(DAVIS AND DeWIEST, 1966)
Q = Constant
= (Q/nK) In (R0/rJ
The height (h) of the water table at any distance r from the well where r is
greater than 1.5H, can be estimated by:
h = (H2-(Q/77K)ln (R0/rw))0'5
For solutions to h where r is less than 1.5H, the empirical relationships
developed by Boreli can be applied. Drawdown (H-h) close to a water table
well can be estimated for ratios of r/H between 0.3 and 1.5 by (Powers, 1981)
H-h = Q(0.13 In (RQ/r))(ln 10(RQ/H))/TrKH
5-8
-------
and for ratios less than 0.3 by:
H-h = Q(0.13 ln(RQ/r) -0.0123 ln(RQ/10r) )(ln (10R0/H))/7rKH
Equations for drawndown resulting from a well pumping in a confined
aquifer are simpler than those for an unconfined aquifer. Drawdown (H-h) at
any distance r from the well in a confined aquifer (Figure 5-5) is given by:
H-h = (Q/277T)ln (RQ/rw)
Plots of H-h versus r' on semilogarithmic paper can be developed for both the
unconfined and confined conditions, so that predictions of R and the wetted
screen length can be made. Figure 5-6 shows two such plots.
FIGURE 5-5.
CONFINED AQUIFER FLOW
(DAVIS AND DeWIEST, 1966)
Q = Constant
Depression
Cone
Confined .
Aquifer
Bedrock
H-h =
In
5-9
-------
FIGURE 5-6.
PLOTS OF H-h VERSUS r FOR UNCONFINED
AND CONFINED AQUIFERS (POWERS. 1981)
1000
rift)
= 1.5H
Figure 5-6a. Equilibrium plot for a water table aquifer. Q - 500 gpm.
R0 = 1000 ft. K = 300 gpd/ft.2 H = 100 ft. rw = 0.5 ft.
_ 1-°
§2.0
I 3,
a: 4,
I5
6'
8
9.
10
0
0
,0
0
.0
,0
0
.0
1.0
10
100
1000
10,000
Figure 5-6b. Equilibrium plot for a confined aquifer. Q = 500 gpm.
K = 300 gpd/ft.2 m = 100 ft. RQ = 2000 ft. rw = 0.5 ft.
5-10
-------
The equilibrium equations presented here are based on the simplifying
assumptions presented in Table 5-1. Utilization of these equations appears to
be severely limited by the simplifying assumptions, but in practice, the
assumptions customarily do not limit the equation's use.
5.2.3 Non-Equil ibrium Well Formula
Non-equilibrium well formulas take into account the effects of varying
rates of pumping on well yield. Using these equations, drawdown at a
specified distance from a well can be predicted for any time after pumping has
started assuming the coefficients of storage and transmissivity for a confined
aquifer and specific yield for an unconfined aquifer are known. These values
are usually determined by pumping tests.
The Theis Equation describes non-equilibrium flow in confined aquifers.
This formula is:
H-h = (Q/47rT)W(M)
where is given as:
M= r2S/4Tt
and:
H-h = drawdown (ft)
o
Q = pumping rate (ft /day)
P
T = coefficient of transmissivity (ft /day)
= well function of n (dimensionless)
r = distance from well center to point where drawdown is
measured (ft)
S = coefficient of storage (dimensionless)
t = time elapsed since pumping started (day).
5-11
-------
TABLE 5-1. SIMPLIFYING ASSUMPTIONS FOR STEADY STATE EQUATIONS
(Johnson Division, UOP Inc., 1975)
Simplifying Assumption
Practice
The water-bearing materials are
of uniform hyraulic conductivity
within the radius of influence
of the well.
The aquifer is not stratified.
For a water-table aquifer, the
saturated thickness is constant
before pumping starts; for an
artesian aquifer, the aquifer
thickness is constant.
The pumping well is 100 per cent
efficient.
Neither the water table nor
piezometric surface has any
slope; both are horizontal
surfaces.
Uniform hydraulic conductivity is seldom
found in a real aquifer, but the average
hydraulic conductivity as determined from
aquifer pumping tests has proved to be
reliable for predicting well performance.
For artesian wells where most of the
aquifer thickness is penetrated and
screened, the assumption of no stratifi-
cation is not an important limitation. For
water-table aquifers, where drawdown reduces
the saturated thickness considerably, the
situation can be handled when the stratifi-
cation is known and taken into account in
applying the formula.
The assumption of constant thickness is not
a serious limitation because variations in
aquifer thickness within the cone of depres-
sion in most real situations is relatively
small. Where changes in thickness are
important, they can be taken into account.
The assumption that the well is 100 per cent
efficient can cause the calculated well
yield to be seriously in error where the
real well may be inefficient as the result
of improper design or construction.
The assumption that the water table or the
piezometric surface is horizontal before
pumping is never fulfilled. The slope or
hydraulic gradient, however, is usually very
flat and the effect on calculations of well
yield is negligible in most cases. The
slope of the water table or the piezometric
surface does cause distortion of the cone of
depression, making it more elliptical than
circular.
(Continued)
5-12
-------
TABLE 5-1. (Continued;
Simplifying Assumption
Practice
Laminar flow exists throughout
the aquifer and within the
radius of influence of the well
The pumping well penetrates to
the bottom of the aquifer.
The cone of depression has
reached equilibrium so that
both drawdown and the radius of
influence of the well do not
change with continued time of
pumping at a given rate.
Flow at all places in the aquifer is
considered to be laminar. Some invest-
igators have theorized that turbulent flow
near a well could result in relatively high
head losses. Actual laboratory and field
tests, however, shows that some departure
from laminar flow near a well causes
additional head loss of only minor
proportions.
Other equations given later describe
partially penetrating wells.
Other equations given later describe
non-steady state pumping.
For a calculated value of ^, a value of W(^) can be obtained from Table
5-2. As 1/jU becomes large, the value of W(M) becomes asymptotic and nearly
constant. Simplifying assumptions for the Theis formula are essentially the
same as for equilibrium formula except the dimensions of the cone of
depression need not have reached equilibrium.
A well pumping an unconfined aquifer under non-equilibrium conditions
does not follow the Theis equation because water flowing to the well is
derived from storage release and specific yield. A solution developed by
Numen for determining drawdown in unconfined aquifers that takes into account
the dual release mechanism is:
H-h = (Q/477T)W(MA,MB,r)
,T) are defined as:
= r S/4Tt (for early drawdown)
where W(M.,Mg,r) are defined as:
5-13
-------
TABLE 5-2. VALUES OF W(M) FOR VARIOUS VALUES OF M (Ferris et al. as cited by Lohman, 1972)
-------
9
= r S//4Tt (for later drawdown
r = r2Kz/H2Kh
and the variables not previously defined are:
S = specific yield
K, - horizontal hydraulic conductivity
K = vertical hydraulic conductivity.
Once M« or Mg and have been calculated, the function W(MA» T ) or
can be derived from Tables 5-3 and 5-4. Type M/\ values are used for early
drawdown times when waters are derived from storage and type MD values are
used for later drawdown when gravity drainage (specific yield) is predominant
Simplifying assumptions for the formulas are similar to those for the Theis
equations.
5.2.4 Semiconfined Aquifers
Generally confined aquifers are not truly confined. They receive
vertical recharge through the semipervious layers above or below (when the
hydraulic gradient is favorable). Two types of semipervious layers exist;
leaky confining layers without storage and leaky confining layers with
storage. Figure 5-7 depicts a semiconfined aquifer.
The Hantush-Jacob formula can be applied to leaky confining layers
without storage. The basic equation is given by:
H-h = (2.3Q/477T)W(/i,r/B)
where:
M= r2S/4Tt
r/B = r(Tm'/K')°-5
and the new variables are defined as:
m1 = thickness of the leaky confining layer
K' = hydraulic conductivity of the leaky confining layer.
5-15
-------
en
TABLE 5-3. VALUES OF THE FUNCTION W(//A,T) FOR WATER-TABLE AQUIFERS (Fetter, 1980)
r= 0.001 r= 0.01 r= 0.06 r= 0.2 r= 0.6 r =1.0 r=2.0 T=4.0 T=6.0
4.0 x IO"1
8.0 x 10"1
1.4 x 10°
2.4 x 10°
4.0 x 10°
8.0 x 10°
1.4 x IO1
2.4 x IO1
4.0 x IO1
8.0 x IO1
i)
1.4 x 10^
y
2.4 x 10^
4.0 x 102
8.0 x IO2
1.4 x IO3
2.4 x IO3
4.0 x IO3
8.0 x IO3
1.4 x IO4
2.48
1.45
3.58
6.62
1.02
1.57
2.05
2.52
2.97
3.56
4.01
4.42
4.77
5.16
5.40
5.54
5.59
5.62
5.62
x
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
lO"2
io-1
io-1
ID'1
10°
10°
10°
10°
10°
10°
n
10°
n
10°
10°
10°
10°
10°
10°
10°
10°
2.
1.
3.
6.
9.
1.
1.
2.
2.
3.
3.
3.
3.
3.
3.
3.
41
40
45
33
63
46
88
27
61
00
23
37
43
45
46
46
x
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
io-2
io-1
io-1
io-1
io-1
10°
10°
10°
10°
10°
A
10°
A
10°
10°
10°
10°
10°
2.
1.
3.
5.
8.
1.
1.
1.
1.
1.
1.
1.
1.
30 x
31 x
18 x
70 x
49 x
23 x
51 x
73 x
85 x
92 x
93 x
94 x
94 x
io-2
io-1
io-1
io-1
ID'1
10°
10°
10°
10°
10°
A
10°
A
10°
10°
2.14 x 10"2 1.88x 10"2 1.70 x 10"2 1.38 x 10"2 9.33 x 10"3 6.39 x 10"3
1.19 x 10"1 9.88 x 10"2 8.49 x 10"2 6.03 x 10'2 3.17 x 10"2 1.74 x 10"2
2.79 x 10"1 2.17 x 10"1 1.75 x 10"1 1.07 x 10'1 4.45 x 10"2 2.10 x 10"2
4.83 x 10"1 3.43 x 10"1 2.56 x 10"1 1.33 x 10"1 4.76 x 10"2 2.14 x 10"2
6.88 x 10"1 4.38 x 10"1 3.00 x 10"1 1.40 x 10"1 4.78 x 10"2 2.15 x 10"2
9.18 x 10"1 4.97 x 10'1 3.17 x 10"1 1.41 x 10"1
1.03 x 10° 5.07 x 10"1
1.07 x 10°
1.08 x 10°
1.08 x 10° 5.07 x 10"1 3.17 x 10'1 1.41 x 10"1 4.78 x 10"2 2.15 x 10'2
-------
TABLE 5-4. VALUES OF THE FUNCTION W(Mg,r) FOR WATER-TABLE AQUIFERS
01
I
I/UB T» o.ooi
4.0 x
8.0 x
1.4 x
2.4 x
4.0 x
8.0 x
1.4 x
2.4 x
4.0 x
8.0 x
1.4 x
2.4 x
4.0 x
8.0 x
1.4 x
2.4 x
4.0 x
8.0 x
1.4 x
2.4 x
4.0 x
8.0 x
1.4 x
10"4 5.62 x 10°
10-"
.3
ID'3
io-3
io-3
io-2
io-2
io-2
io-2
io-1
io-1
io-1
10"1 5.62 x 10°
10° 5.63 x 10°
10° 5.63 x 10°
10° 5.63 x 10°
10° 5.64 x 10°
IO1 5.65 x 10°
IO1 5.67 x 10°
IO1 5.70 x 10°
IO1 5.76 x 10°
IO2 5.85 x 10°
T' 0.001 T« 0.
3.46 x 10° 1.94 x
1.94 x
1.95 X
1.96 x
3.46 x 10° 1.98 x
3.47 x 10° 2.01 x
3.49 x 10° 2.06 x
3.51 x 10° 2.13 x
3.56 x 10° 2.31 x
3.63 x 10° 2.55 x
3.74 x 10° 2.86 x
3.90 x 10° 3.24 x
4.22 x 10° 3.85 x
4.58 x 10° 4.38 x
.06
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
T=0,
1.09 x
1.09 x
1.10 x
1.11 x
1.13 x
1.18 x
1.24 x
1.35 x
1.50 x
1.85 x
2.23 x
2.68 x
3.15 x
3.82 x
4.37 x
,2
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
10°
r= 0.6
5.08 x
5.08 x
5.09 x
5.10 x
5.12 x
5.16 x
5.24 x
5.37 x
5.57 x
5.89 x
6.67 x
7.80 x
9.54 x
1.20 x
1.68 x
2.15 x
2.65 x
3.14 x
3.82 x
4.37 x
io-1
io-1
ID"1
io-1
io-1
io-1
io-1
io-1
ID'1
ID'1
io-1
io-1
io-1
10°
10°
10°
10°
10°
10°
10°
T= 1.0
3.18 x 10'1
3.18 x 10"1
3.19 x 10"1
3.21 x 10'1
3.23 x 10"1
3.27 x 10"1
3.37 x 10"1
3.50 x 10"1
3.74 x 10"1
4.12 x 10"1
5.06 x 10'1
6.42 x 10"1
8.50 x 10'1
1.13 x 10°
1.65 x 10°
2.14 x 10°
2.65 x 10°
3.14 x 10°
3.82 x 10°
4.37 x 10°
T= 2.0
1.42 x 10'1
1.42 x 10"1
1.43 x 10"1
1.45 x 10"1
1.47 x 10"1
1.52 x 10'1
1.62 x 10"1
1.78 x 10'1
2.05 x 10"1
2.48 x 10"1
3.57 x 10"1
5.17 x 10"1
7.63 x 10"1
1.08 x 10°
1.63 x 10°
2.14 x 10°
2.64 x 10°
3.14 x 10°
3.82 x 10°
4.37 x 10°
T= 4.0
4.79 x 10"2
4.80 x 10-2
4.81 x 10'2
4.84 x 10'2
4.78 x 10"2
4.96 x 10"2
5.09 x 10"2
5.32 x 10"2
5.68 x 10"2
6.61 x 10~2
8.06 x 10'2
1.06 x 10"1
1.49 x 10'1
2.66 x 10"1
4.45 x 10"1
7.18 x 10'1
1.06 x 10°
1.63 x 10°
2.14 x 10°
2.64 x 10°
3.14 x 10°
3.82 x 10°
4.37 x 10°
T = 6.0
2.15 x 10'2
2.16 x 10'2
2.17 x 10'2
2.19 x 10'2
2.21 x 10'2
2.28 x 10'2
2.39 x 10"2
2.57 x 10"2
2.86 x 10'2
3.62 x 10"2
4.86 x 10"2
7.14 x 10"2
1.13 x 10'1
2.31 x 10"1
4.19 x 10'1
7.03 x 10"1
1.05 x 10°
1.63 x 10°
2.14 x 10°
2.64 x 10°
3.14 x 10°
3.82 x 10°
4.37 x 10°
(Continued)
-------
TABLE 5-4. (Continued)
1/fig T= 0.001
2.4 x 102
4.0 x 102
8.0 x 102
en 1.4 x 103
oo 2.4 x 103
4.0 x 103
8.0 x 103
1.4 x 104
2.4 x 104
4.0 x 104
5.99
6.16
6.47
6.67
7.21
7.72
8.41
8.97
9.51
1.94
x
x
x
x
X
X
X
X
X
X
10°
10°
10°
10°
10°
10°
10°
10°
10°
101
T =
5.00
5.46
6.11
6.67
7.21
7.72
8.41
8.97
9.51
1.94
0.001
x 10°
x 10°
xlO°
x 10°
x 10°
x!0°
x!0°
x 10°
x 10°
x 101
r=
4.91
5.42
6.11
6.67
7.21
7.72
8.41
8.97
9.51
1.94
0.06
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 101
T =
4.91
5.42
6.11
6.67
7.21
7.72
8.41
8.97
9.51
1.94
=0.
x
x
X
X
X
X
X
X
X
X
.2
10°
10°
10°
10°
10°
10°
10°
10°
10°
101
T -
4.91
5.42
6.11
6.67
7.21
7.72
8.41
8.97
9.51
1.94
= 0.6
x 10°
x 10°
x 10°
x 10°
x 10°
xlO°
x 10°
x 10°
x 10°
x 101
r= 1.0
4.91
5.42
6.11
6.67
7.21
7.72
8.41
8.97
9.51
1.94
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 101
T =
4.91
5.42
6.11
6.67
7.21
7.72
8.41
8.97
9.51
1.94
= 2.0
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 10°
x 101
T = 4.0
4.91 x
5.42 x
6.11 x
6.67 x
7.21 x
7.72 x
8.41 x
8.97 x
9.51 x
1.94 x
10°
10°
10°
10°
10°
10°
10°
10°
10°
101
T =
4.91
5.42
6.11
6.67
7.21
7.72
8.41
8.97
9.51
1.94
= 6.0
x 10°
x 10°
x 10°
x 10°
x 10°
xlO°
xlO°
x 10°
x 10°
x 101
SOURCE: Adapted from S. P. Neuman, Water Resources Research, 11(1975):329-42 as cited by Fetter, 1980.
-------
FIGURE 5-7.
SEMICONFINED AQUIFER
(DAVIS AND DeWIEST, 1966}
Ground Surface
t . . .... ,.
Initial Watertable
V
*
Sealed Well
Ponded Water
. h = Piezometnc Head in Main Aquifer
X\X\X\N
Semiconfining
Stratum
Well
Screen
•/.vV-AylvftvtMain Aquifer F
Bedrock
Leaky Confining Bed Without Storage
H-h = (2.3Q/4*77 Wfa,r/B)
Leaky Confining Bed with Storage
H-h =(Q/4nT) Hlp.B)
Values of W(M,r/B) for calculated values of Mand r/B are given in Table 5-5.
Underlying assumptions for these equations are similar to the Theis formula
with the addition of (Fetter, 1980):
• confining bed leakage is vertical and proportional to drawdown
• hydraulic head in the leaking layer is constant
• storage in the confining bed is negligible
5-19
-------
TABLE 5-5. VALUES OF FUNCTIONS W(M,r/B) AND W(MA, r/B) FOR VARIOUS VALUES OF^ORM
CJl
I
ro
o
U or u .
0.000001
0.000006
0.00001
0.00005
0.0001
0. 0005
0.001
0.005
0.01
0.05
0.1
0.5
1.0
5.0
0.01
9.4425
9.4413
9.4176
8.8827
8.3983
6.9750
6. 3069
4.7212
4.0356
2.4675
1.8227
0.5598
0.2194
0.0011
0.015
B.6319
"
8.6313
8.4533
8.1414
6.9152
6.3765
4.7152
4.0326
2.4670
1.8225
0.5597
0.2194
0.0011
0.03
7.2471
"
"
7.2450
7.2122
6.6219
6.1202
4.6829
4.0167
2.4642
1.8213
0. 5596
0.2193
0.0011
0.05
6.2285
•
"
"
6.2282
6.0821
5.7965
4.6084
3.9795
2.4576
1.8184
0. 5594
0.2193
0.0011
0.075
5.4228
•
"
•
11
5.4062
5.3078
4.4713
3.9091
2.4448
1.8128
0. 5588
0.2191
0.0011
0.10
4.8541
"
"
"
"
4.8530
4.8292
4.2960
3.8150
2.4271
1.8050
0.5581
0.2190
0.0011
0.15
4.0601
"
"
"
"
"
4.0595
3.8821
3.5725
2.3776
1.7829
0.5561
0.2186
0.0011
0.2
3.5054
"
"
"
11
"
3.4567
3.2875
2.3110
1.7527
O.S532
0.2179
0.0011
0.3
2.7449
"
"
"
"
"
2. 7428
2.7104
1.9283
1.6704
0.5453
0.2161
0.0011
r/B
0.4 0.5
2.2291 1.8488
" "
" "
«
"
"
2.2290
2.2253 1.8486
1.7075 1.4927
1.5644 1.4422
0.5344 0.5206
0.2135 0.2103
0.0011 0,0011
0.6
1.5550
"
"
"
"
"
"
"
1.2995
1.3115
0.5044
0.2065
0.0011
0.7
1.3210
"
"
"
"
"
"
"
"
1.2995
1.1791
0.4860
0. 2020
0.0011
0.8
1.1307
"
"
"
"
"
"
"
1.1210
1.0505
0.4658
0.1970
0.0011
0.9
0.9735
"
"
"
"
"
"
"
"
0.9700
0.9297
0.4440
0.1914
0.0011
1.8 1.5 2.0 2.5
0.8420 0.4276 0.2278 0.1247
" " " "
" " " "
" " " "
" " " "
" "
" " "
" " "
"
0.8409
0.8190 0.4271
0.4210 0.3007 0.1944 0.1174
0.1855 0.1509 0.1139 0.0803
0.0011 0.0010 0.0010 0.0009
SOURCE- After M. S. Hantush, "Analysis of Data from Pumping Tests in Leaky Aquifers," Transactions, American Geophysical Union, 37 (1956): 702-14 as cited by Fetter, 1980.
-------
• t >(30r2 S/T)(l-(10rw/m)2)
• r/B<0.1.
W
If significant storage occurs in the confining layer, then flow is
derived from storage in the layer during initial time periods. As pumping
continues drawdown will occur in the unpumped aquifer. If the confining layer
has finite storage, drawdown does not occur in the unpumped layer, and -time
intervals are short (S'm'/lOK1), then the drawndown equation for leaky
aquifers with storage becomes:
H-h = (Q/47rT)H(M,P)
where:
M= r2S/4Tt
P= (r/4B)(S'/S)0'5
B = (Tm'/K1)0'5
Values at H(^,p) can be obtained from Table 5-6 after /LI and [3 are calculated.
5.2.5 Partially Penetrating Wells
The equations presented previously assume that the well penetrates the
entire thickness of the aquifer. In many cases, wells do not penetrate the
total thickness such that true radial flow to the well does not exist.
Figure 5-8 shows two examples of flow to partially penetrating wells in
confined aquifers. These figures show that water moving from the lower part
of an aquifer must move along curved lines to reach the well. The result is
increased flow path lengths and increased head loss at the well. Therefore,
for a given yield drawdown will be greater in a partially penetrating well
than in a fully penetrating well in the same aquifer.
Formulas for partial penetration are difficult to solve under simple
conditions (i.e., homogeneous isotropic aquifers) and extremely difficult to
solve for complex conditions (i.e., stratified aquifers). Kozeny developed an
5-21
-------
TABLE 5-6. VALUES OF THE FUNCTION
ro
ro
0.000001
0.000005
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
0.1
0.5
1.0
5.0
0.001
11.9842
10.8958
10.3739
9.0422
8.4258
6.9273
6.2624
4.6951
4.0163
2.4590
1.8172
0.5584
0.2189
0.00115
0.005
10.5908
9.7174
9.3203
8.3171
7.8386
6.6024
6.0193
4.5786
3.9334
2.4243
1.7949
0.5530
0.2169
0.00114
0.01
9.9259
9.0866
8.7142
7.8031
7.3803
6.2934
5.7727
4.4474
3.8374
2.3826
1.7677
0.5463
0.2144
0.00112
0.05 -
8.3395
7.5284
7.1771
6.3523
5.9906
5.1223
4.7290
3.7415
3.2752
2.1007
1.5768
0.4969
0.1961
0.00104
0.10
7.6497
6.8427
6.4944
5.6821
5.3297
4.4996
4.1337
3. 2483
2.8443
1.8401
1.3893
0.4436
0.1758
0.00093
P
0.20
6.9590
6.1548
5.8085
5.0045
4.6581
3.8527
3.5045
2.6891
2.3325
1.4872
1.1207
0.3591
0.1427
0.00076
0.50
6.0463
5.2459
4.9024
4.1090
3.7700
2.9933
2.6650
1.9250
1.6193
0.9540
0.6947
0.2083
0.0812
0.00042
1.0
5.3575
4.5617
4.2212
3.4394
3.1082
2.3601
2.0506
1.3767
1.1122
0.5812
0.3970
0.1006
0.0365
0.00017
2.0
4.6721
3.8836
3.5481
2.7848
2.4658
1.7604
1.4776
0.8915
0.6775
0.2923
0.1789
0.0325
0.00993
0.00003
5.0
3.7756
3.0055
2.6822
1.9622
1.6704
1.0564
0.8271
0.4001
0.2670
0.0755
0.0359
0.00288
0.00055
10.0
3.1110
2.3661
2.0590
1.3943
1.1359
0.6252
0.4513
0.1677
0.0955
0.0160
0.00552
0.00015
0.00002
20.0
2.4671
1.7633
1.4816
0.8994
0.6878
0.3089
0.1976
0.0493
0.0221
0.00164
0.00034
SOURCE: Condensed from M. S. Hanfush, "Modification of the Theory of Leaky Aquifers,: Journal of Geophysical Research, 65 (1960): 3713-25 as
cited by Fetter, 1980.
-------
FIGURE 5-8.
FLOW TO PARTIALLY PENETRATING WELLS
(BUREAU OF RECLAMATION, 1977)
Figure 5-8a. Just Penetrating Top of Aquifer
Static Piezometric Surface
/ ////////////// / 77 7 7 7 7 777 7
Lower Confining Bed
— — — - Flow Lines
Equipotential Lines
Figure 5-8b. Penetrating 50% of Aquifer
Static Piezometric Surface
Lower Confining Bed
— — • — Flow Lines
— Equipotential Lines
5-23
-------
equation for partially penetrating wells in confined aquifers that are nearly
homogeneous. His equation can be written as (Johnson Division, UOP Inc.,
1975):
(Q/sn)/(Q/s) =
p
w
/2ml)°-5cos(7rl/2)
where:
Q/sn = specific capacity of a partially penetrating well
P (ft/day)
Q/s = specific capacity of a fully penetrating well (ft/day)
r = well radius (ft)
m = aquifer thickness (ft)
1 = well screen length as a fraction of aquifer thickness
(dimensionless)
Kozney's formula is valid when the aquifer thickness is large, the percent
penetration is small, and the well radius is small.
A major problem associated with the use of partially penetrating wells
for the containment of plumes is that if the well does not fully penetrate the
aquifer and pumping is not sufficient, contaminants can flow under the well
(Figure 5-9). Partially penetrating wells may be most useful in collecting
floating contaminants.
5.2.6 Cumulative Drawdown
The formulas presented thus far have been for single wells pumping an
aquifer. When more than one well is pumping the same aquifer, their cones of
depression will possibly intersect. For containing and removing plumes from
an aquifer, this principle is important because it allows for the determ-
ination of well spacing intervals, pumping rates, and pumping times.
Theoretically, if the cones of depression (radii of influence) intersect each
other, flow will not occur between the wells and any contaminants in the
groundwater system will be extracted. This theory is probably valid for
5-24
-------
FIGURE 5-9.
FLOW IN CONFINED AQUIFER WITH PARTIALLY
PENETRATING WELL
Piezometric
Surface
V/////////////////////////////////I
V///////////////////7
aquifers with low natural flow velocities, however, for aquifers with high
natural flow velocities overlapping cones may not be sufficient to capture the
plume (Keely and Tsang, 1983).
For confined aquifers with several wells tapping the same system, the
composite pumping cone can be calculated by summing the individual drawdowns
caused by each well (Figure 5-10). This can be accomplished by graphical
superposition (i.e., adding the drawdown curves, H-h, for each well) or by
5-25
-------
FIGURE 5-10.
COMPOSITE DRAWDOWN IN A CONFINED AQUIFER
(FREEZE AND CHERRY, 1979 AS CITED BY EPA, 1982a)
-- Drad*kj«Yn due to Qj - - - Drawdown due to Q,
Total Drawdown
calculation. For a system of n wells pumping at any rate, the drawdown at a
radius r from each well is given for equilbrium conditions by:
H-h = (
and for non-equilibrium conditions by:
H-h = (
where:
= 2
rS/4Tt.
for i = 1,2,...n
Linear superposition is only valid for confined aquifers because the value of
transmissivity does not change with drawdown.
For unconfined aquifers, linear superposition of individual well
drawdowns results in predicted composite drawdowns that are less than they
will actually be because a decrease in saturated thickness reduces the
transmissivity. Composite drawdowns for unconfined aquifers under equilibrium
conditions can be calculated from;
H2-h2 = (Q1ln(Ro/r1)/7rK)+(Q2ln(Ro/r2)/7rK)....(Qn1n(Ro/rn)/^K)
Alternatively, composite drawdowns can be derived by graphical superposition
2 2
using the calculated H-h for each well (Figure 5-11). In this case the
2 2
H -h are added and the drawdowns are found by taking square roots.
5-26
-------
I
INS
FIGURE 5-11.
COMPOSITE DRAWDOWN IN AN UNCONFINED AQUIFER
(DAVIS AND DeWlEST, 1966)
Figure 5-11a. Plane View
"W K I Composite drawdown
Figure 5-11 b. Cross-sectional View
Initial WaterTable = Ground Surface
0
0 50 100 200 300 400 500
x in ft
1000
-------
Determining well interference or intersecting cones of depression is a
necessary exercise in designing well-field layouts for plume management. For
aquifers that have high flow velocities, a velocity plot may also need to be
developed to determine well field layout. Based on these analyses the
following parameters can be determined:
• Well spacing to contain contaminants
• Length of pipe needed to carry discharge from the well to the
treatment plant
• Well pump characteristics (e.g., type, horsepower)
• Time interval for pumping, if not equilibrium pumping.
5.2.7 Hydrogeologic Boundary Effects
Typically, wells are not located in aquifers that have infinite area!
extent and therefore drawdown cones extend until they intercept a recharge
boundary or a barrier boundary. Recharge boundaries (e.g., streams) are areas
where aquifers are replenished with water and barrier boundaries (e.g.,
impermeable zones) are where aquifers terminate. The effect of the boundary
conditions on drawdowns is shown in Figure 5-12. Total drawdown and the rate
of drawdown are less than theoretical predictions when recharge boundaries are
present and greater when barrier boundaries are present.
Calculating the effect of barriers on wells is usually performed using
the method of images. Where recharge boundaries are present, rechargee image
wells are constructed and when barrier boundaries are present discharge image
wells are utilized. Figure 5-13 shows an example for each type of boundary.
In both cases, image wells are drawn about the axis where the boundary occurs.
For recharging images, cones of depression are added to obtain the resultant
cone and for discharging images, cones of depression are subtracted.
5-28
-------
FIGURE 5-12.
EFFECT OF RECHARGE AND BARRIER BOUNDARIES
ON DRAWDOWN (FETTER, 1980)
s
Q
10
102
103
\
\
104
105
10 102 103 104
Time Since Pumping Began
10b
5-29
-------
FIGURE 5 13. METHOD OF IMAGES FOR DETERMINING
RESULTANT CONE OF DEPRESSION
(FERRIS et al., 1962, AS CITED BY LOHMAN, 1972)
Discharging Well
Nonpumpmg Water Level
Pumping Water Level
Figure 5-13a. Hydraulic Barrier
Q
/L.
.•X'-V'V Impermeable or „-;•',»•
jf.^.fVc/ Confining Mateial ,\*,'\
•'•••I'v'v Aquifer •';••'.'•;•
Average or Effective Position
of Line of Zero Flow
GO
O
REAL SYSTEM
Figure 5-13b. Recharge Barrier
Discharging Well
Perennial
Stream
Land Surface
' Nonpumpmg
Water Level M
-_rir:r^ Confining Material nr"-r^r-I:
REAL SYSTEM
Drawdown Component
of Image Well
Discharging
Real Well
Drawdown Component
of Real Well
Discharging
Image Well
Nonpumpmg
Watei Level
Confining Material fI-£;-£-£;-^-£>i:-£I-£I-£l- I-E>EHI-EHHH>£I- I^^^-EI^-£I-£}£H>EHHH}T_-£H--u-^
« a •• a p-|
NOTE Aquifer thickness M should be very large
compared to resultant drawdown near real well
HYDRAULIC COUNTERPART OF REAL SYSTEM
Zero Drawdown
Boundary \
Buildup Component
of Image Well
Discharging
Real Well
,, Recharging
(Cone of f/ lmaae We"
Impression) I'lx
V--TK,
H *-^
ii --~._
Nonpumpmg [ j Water Level
Drawdown Component
-""of Real Well
I r
.1 t
T
M
lif :
(Cone of Depression)
NOTE: Aquifer thickness M should be very large
compared to resultant drawdown near real well
HYDRAULIC COUNTERPART OF REAL SYSTEM
-------
5.2.8 Flow Between Discharge and Recharge Wells
Injection well theory is identical to the discharge well theory
previously described except that cones of depression and drawdowns are
inverted to above the water table or piezometric surface and become cones of
impression and additional head. The use of recharge (injection) wells in
combination with discharging wells allows contaminants to be flushed more
rapidly or diverted (i.e., against the normal flow path) to recovery wells.
Injection wells can also be utilized to create hydraulic barriers to stop the
migration of contaminants, or to change the hydraulic head so that contam-
inants can be diverted or made to flow against the original flow path.
An example of an extraction and recharge well operating in the same
confined aquifer is given in Figure 5-14. In the example, the recharge and
discharge wells are pumping at the same rate (QR = QT). To estimate the shape
of the piezometric surface formed by pumping these wells, the cone of
depression and the cone of impression are plotted and then the two are added.
Because the aquifer is confined, the cones can be added directly (refer to the
section on cumulative drawdown). This technique cannot be applied directly to
unconfined aquifers because the saturated thickness changes resulting in
2 2
transmissivity changes. Plotting the H -h for each well and then super-
imposing them will result in correct answers for unconfined aquifers.
5.2.9 Radius of Influence
Determining the radius of influence for a well in a given aquifer is
critical in remedial action design because it can be used for determining well
spacing, pumping rates, pumping cycles, and screen lengths. *\s can be seen
from the equations presented previously, the radius of influence of a well
increases as pumping continues until equilibrium conditions are reached (i.e.,
when aquifer recharge equals the pumping rate or the discharge rate).
Recharge can occur through one or more of the following situations (Johnson
Division, UOP Inc., 1975):
• Storage release from the aquifer
5-31
-------
FIGURE 5-14.
RECHARGE AND DISCHARGE WELLS IN A CONFINED
AQUIFER
(DAVIS AND DeWIEST, 1966)
a.
T
h = H
s „-•
M/iW/////s/ss/sssss/s/s/js,s.
•:>:/' «;$.£:•.
y
ia.\..'^ '.>'•'••.: ••%:.•'
-V :•.•:•••.••:••;••• .::'». ^.:
N ! -i
N_K_I_
•;';.".*.'.•.. '•'• '*"_"!•'•*'•''..'.'.* ;
IsSL
h
IW
;v. ... m-,:\
irf
Figure 5-14a. Cross-sectional View
Eauipotential
Line
Streamline
Figure 5-14b. Plane View
5-32
-------
• Surface water entering the aquifer
• Recharge from ugradient flow and from precipitation within the radius
of influence
0 Leakage through overlying or underlying aquifers.
Once equilibrium conditions have been reached, the shape of the cone does not
change with increased pumping time (as long as the other factors remain
constant). The time required to reach equilibrium conditions after pumping is
started can be very short, e.g., within 2 hours, or may take as long as years.
Typically, the radius Of influence for a confined aquifer is larger than that
for an unconfined aquifer and increases as hydraulic conductivity increases.
Estimates of the radius of influence can be made by using several tech-
niques. Generalized values for the radius of influence for various porous
media under unconfined conditions are given in Table 5-7. These values are
estimates and should be treated as such.
TABLE 5-7. RADIUS OF INFLUENCE FOR VARIOUS UNCONSOLIDATED MATERIALS
(i.e., BASED ON TYPICAL HYDRAULIC CONDUCTIVITY RANGES)
Porous Medium Radius of Influence (R )
(ft) °
Coarse Gravel 5,000
Fine Gravel 1,300-1,650
Coarse Sand 650-1,300
Medium Sand 300-650
Fine Sand 150-300
Very Fine Sand 50-150
Silty Fine Sand <50
5-33
-------
Rough approximations of R without recharge can be made by adapting the
Jacob formul a to:
0.5
RQ = rs + (Tt/SC)
where :
r = well radius or equivalent radius
T = coefficient of transmi ssivity
t = time
S = coefficient of storage
C = constant (based on the units used).
The above equation is valid only for confined aquifers and the value obtained
should be adjusted downward on the basis of possible recharge. Using the
above relationship for unconfined aquifers provides reasonable results as long
as the drawdown (H-h ) is not a large percentage of the saturated thickness
(H).
Si chart and Kyrieleis (1930) developed an empirical relationship for
calculating the radius of influence (RQ) as a function of drawdown (H-hw) and
hydraulic conductivity (K):
R = 3 (H-h )(0.47K)°-5
o x w'v '
f\
where H-h is in feet, K is in gpd/ft and RQ is in feet. Theoretically, the
radius of influence does not depend on drawdown, but rather, depends on the
pumping rate which does not appear in this formula.
For exact solutions, distance-drawdown diagrams can be constructed using
the equations given previously. Distance-drawdown diagrams are plotted on
semil ogarithmic paper with the distance being plotted on the logarithmic
scale. Two such diagrams are shown in Figure 5-15. The radius of influence
can be obtained from the intercept of the plots with the distance axis at zero
drawdown. For cases where pumping rates are variable but all other parameters
5-34
-------
FIGURE 5-15.
DISTANCE DRAWDOWN DIAGRAMS FOR A) VARYING PUMPING RATING
AND B) VARYING PUMPING TIMES (JOHNSON DIVISION, UOP INC., 1975)
o
12
16
20
24
AS = 5.3 ft
T = 300 mm.
Drawdown = 18.8 ft.
Q = 400 gpm
2 3
10 20 30 50 100 200 300 500
Distance from Pumped Well, in Feet
Figure 5-15a. Pumping Rates
c 12
I
Q 16
20
24
Drawdown in Observation
Well "A" after 300 min.
Drawdown in Observatoin
Well "A" after 1000 min.
Curves Constructed for Q = 200 gpm
2 3
10 20 30 50 100 200 300 500
Distance from Pumped Well, in Feet
Figure 5-15b. Pumping Times
5-35
-------
pumping times are varied, however, the radius of influence is changed such
that as pumping time increases, the radius of influence increases. This
relationship holds true until equilibrium is reached after which the radius of
influence does not increase with continued pumping.
Two useful relationships for understanding the effects of drawdown on the
pumping volume and the radius of influence are:
Q/H-h = 2 7rT/ln(R0/r) = Constant (for confined aquifers)
and
o p
Q/H-h = 7rK/ln(R /r) = Constant (for unconfined aquifers)
Thus, the radius of influence will not change with the pumping rate under
equilibrium conditions. The factor that affects the radius of influence
greatest under non-equilibrium conditions is the pumping time.
Table 5-8 summarizes the equations that can be used to determine the
js of influence, R . An example R calculation for
being pumped at steady state conditions is given below.
radius of influence, R . An example RQ calculation for an unconfined aquifer
An unconfined aquifer is pumped at a rate of 500 gpm and the system has
2
reached equilibrium. The aquifer's hydraulic conductivity is 300 gpd/ft ,
drawdown in the well is 35 feet and the saturated thickness is 100 feet. The
radius of the pumping well is 0.5 feet. The radius of influence of the well
is calculated by:
In RQ = (K(H2-hw2)/458Q) + lnrw
= (300(1002-652) / 458 (500)) + In0.5
= 6.872
RQ= e6.872
= 965 feet.
5-36
-------
TABLE 5-8. METHODS FOR CALCULATING THE RADIUS OF INFLUENCE (R )
Pumping Water Table Aquifer Confined Aquifer
Condition (Unconfined)
Equilibri um
- Exact lnRQ =(K(H2 -hw2)/(458Q)) + lnrw 1 nRQ =(T(H-hw)/229Q))+ Inr
Non-equil ibri um
- Exact Drawdown vs. log distance plots or Theis Method
Approximate R = r +(Tt/4790S)
RQ = 3(H-hw)(0.47K)0'5
RQ = Radius of influence (ft)
o
K = Hydraulic conductivity (gpd/ft )
H = Total head (ft)
h = Head in well (f)
w '
Q = Pumping rate (gpm)
r = Well radius (ft)
w
T = Transmissivity (gpd/ft)
t = Time (min)
S = Storage coefficient (dimensionl ess)
5-37
-------
5.3 Applications
This section briefly describes the applications of well systems for plume
management. The general types of wells that can be used are wellpoints,
suction wells, ejector wells, and deep wells, which can fully penetrate or
partially penetrate the aquifer. Well placement and use (i.e., extraction or
injection) is highly variable and depends on the hydrogeologic characteristics
of the site and the plume's characteristics. Well systems can be designed to
perform almost any function from plume containment to plume removal with or
without the assistance of other technologies (e.g., slurry walls). The
applications for which well systems are typically designed are:
• Groundwater level adjustments
• Containment of plumes to prevent their migration
t Removal of plumes from the groundwater system.
5.3.1 Groundwater Level Adjustment
Well systems for adjusting groundwater levels can be designed using
extraction wells to lower water levels or using injection wells to create
groundwater mounds (i.e., barriers). By adjusting groundwater levels, plume
development can be stopped at the source (e.g., lower water table under a
site) or the speed and direction of the plume can be altered. In either case,
contaminated water is not extracted from the groundwater system as in the case
with containment and removal techniques described in following sections.
Groundwater level adjustments are not usually practical, however, because of
problems associated with operation and design so as not to allow contaminants
to escape or be misdirected. Also, contaminated water may inadvertently be
extracted if care is not taken during adjustments.
Plume development can be controlled at sites where the water table
intercepts disposed wastes by lowering the water table with extraction wells.
In order for the pumping technique to be effective, infiltration into the
waste pile must be eliminated and liquid wastes must be completely removed.
If these conditions are not met, the potential exists for development of a
5-38
-------
contamination plume. The major drawback to using well systems for lowering
water tables is the continued costs associated with the maintenance of the
system after site closure.
Groundwater barriers can be created using injection wells to change both
the direction of the plume and the speed of plume migration. Figure 5-16
shows an example of plume diversion using a line of injection wells to protect
domestic water sources. By creating an area with a higher hydraulic head, the
plume can be forced to change direction. This technique may be desirable when
short term diversions are needed or when the plume will have sufficient time
to naturally degrade so that containment and removal is not required.
5.3.2 Plume Containment
Well systems can be used to contain a plume's migration (i.e., prevent
further migration and contamination) using extraction wells or extraction and
injection wells in combination. Containment differs from removal in that the
source of contamination is not stopped and groundwater contamination is an
ongoing process. Because containment requires removing contaminated ground-
water, a treatment or disposal method must also be developed.
Figure 5-17 depicts the use of a line of extraction wells to contain a
plume and protect a domestic water supply. The distance between wells is
determined such that the plume is captured, i.e., their radii of influence
overlapped for low natural flow velocities or velocity plots developed for
high natural flow velocity, thus preventing contaminated groundwater from
flowing between adjacent wells. Wells do not necessarily have to be drilled
in front of the plume's leading edge but can be drilled behind it as long as
the edge is contained within the stagnation zone developed by the pumping
well.
Extraction and injection well systems can also be designed to contain a
plume. In this type of system, injection wells are used to direct contam-
inants to extraction wells. Injection well water can be derived from treated
extraction well water thus allowing for a convenient disposal method. This
5-39
-------
FIGURE 5-16.
PLUME DIVERSION USING INJECTION WELLS
Future Plume
Movement
"\
;
Injection Wells
Domestic Wells
5-40
-------
Impermeable Bedrock
K via Cross
Figure 5-1/»• ^
..sectional View
Extract.cn Wells
w,th Radius of
influences
Figure
5_V7b.
5-41
-------
type of system may be most advantageous in situations where complete
dewatering of a local groundwater system is not desired.
Well systems can also be used in conjunction with other plume management
technologies. The use of wells with barrier walls, for instance, is
potentially the most useful technology combination. However, the barrier wall
used would have to be designed to be compatible with the waste so that wall
degradation would not occur.
5.3.3 Plume Removal
Plume removal implies a complete purging of the groundwater system to
remove contaminants. Plume removal techniques are utilized after contaminant
sources have been fixed, encapsulated, or removed, and aquifer restoration is
desired. Plumes of this type may originate from spills, pipeline breaks
(e.g., gas line), or other accidental discharges. Extraction, and extraction
and injection well systems can be used in plume removal. As with containment,
plume removal typically requires treatment of the extracted groundwater before
disposal.
Plume removal through the use of extraction wells is most appropriate
where:
• Hydraulic gradients are steep (i.e., >0.2 ft/ft)
t Contaminants are flowing with velocities equal to or greater than the
groundwater
• Removal times can be long (i.e., quick removal is not necessary).
As with containment, wells are placed downgradient of the plume near the
plume's leading edge with their radii of influence or capture zones over-
lapping so as not to allow contaminant escape.
Extraction and injection well systems may be designed to remove contam-
inants from the groundwater as shown in Figure 5-18. The major difference
5-42
-------
FIGURE 5-18.
EXTRACTION AND INJECTION WELLS PATTERNS
FOR PLUME REMOVAL
Extraction Well
Injection Well
Plume Boundary
Radius of Influence
GW Flow
a.
b.
GW Flow
5-43
-------
between the two removal plans is the placement of wells in relationship to
each other. In Figure 5-18a, extraction wells surround the injection wells in
a staggered array. In Figure 5-18b, extraction wells alternate with injection
wells in straight lines. Both methods are effective in plume removal and the
choice of pattern would depend on the size of the plume to be removed and
required number of wells. However, dead spots (i.e., areas where water
movement is very slow or nonexistent) can occur when these configurations are
used. The size of a dead spot is directly related to the amount of overlap
between adjacent radii of influence (i.e., the greater the overlaps the
smaller the dead spots). The system's efficiency should be monitored in these
dead spots. Another problem with extraction and injection systems is
operational problems associated with injection wells. These wells can suffer
from many operation problems including air locks, and frequent maintenance and
well rehabilitation.
Extraction and injection systems are most appropriate where:
• Hydraulic gradients are low (i.e., <0.2 ft/ft)
• Contaminants are flowing at velocities less than that of the
groundwater
• Required removal time is short.
A special case where these techniques are especially applicable is with
contaminants that move slowly or cling to geologic materials. Extraction and
injection well systems can also be designed to allow for flushing (e.g., using
surfactants) and recovering contaminants that could not be recovered using
normal pumping methods.
5.4 Design and Construction
5.4.1 Well Design
Designing the most appropriate type of well and well system requires
having adequate information on the site's hydrogeology and the plume's
5-44
-------
characteristics. The types of data that are typically required are presented
in Table 5-9. As with any design program, the more accurate the data the more
likely the system designed will perform its intended function.
There are four basic well types that can be used in plume management;
wellpoints, suction wells, ejector wells, and deep wells. Using the site
data, selection of an appropriate well type can be made. Table 5-10 lists the
conditions under which these well types would be most applicable to a
particular site.
Wellpoint systems are the most versatile method, being effective in
almost any hydraulic situation. They are best suited for shallow aquifers
where extraction is not needed below more than 15 to 20 feet. Beyond this
depth suction lifting, the standard pumping technique for wellpoints, is
ineffective. Suction wells operate in a similar fashion and are also depth
limited. The only advantage of suction wells over wellpoints is that they
have higher capacities. For extraction depths greater than 20 feet, deep
wells and ejector wells are typically utilized. Deep well systems are better
suited to homogeneous aquifers with high hydraulic conductivities and where
large volumes of water may be pumped. Ejector wells perform better than deep
wells in heterogeneous aquifers with low hydraulic conductivities. A problem
with ejector systems is that they are inefficient and are sensitive to
constituents in the groundwater which may cause chemical precipitates and well
clogging (Powers, 1981).
The following section gives descriptions of the basic components of wells
and the design factors in developing a system. Wellpoints and suction wells
have been combined because of the similarities between the two.
5.4.1.1 Deep Wells
The typical components of a deep well are a screen, casing, filter pack
and seal, and turbine pump. Motors for pumps can be submersible and attached
to the pump, or at ground level driving the pump with a shaft. Components of
a typical deep well are shown in Figure 5-19 and described in Table 5-11. Not
5-45
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TABLE 5-9. DATA REQUIREMENTS FOR WELL SYSTEM DESIGN
Geology (consolidated and unconsolidated materials)
• Structure
• Stratigraphy
• Lithology
Aquifer
9 Types (i.e., confined, unconfined, perched)
• Thicknesses, depths, and formational designations
• Boundaries
• Hydraulic conductivities, storativities, transmissivities
0 Discharge and recharge zones
t Groundwater and surface water relationships
• Locations of existing wells
0 Pump test data (e.g., drawdowns, pumping rates, times)
Plume
• Areal extent and depth
t Types of contaminants
• Location in aquifer
5-46
-------
TABLE 5-10. CRITERIA FOR WELL SELECTION (Powers, 1981)
Parameters
Hydrology
• Low hydr
aulic conductivities
Wellpoints
Good
Suction Wells
Poor
Ejector Wei Is
Good
Deep
Fair
Wells
to Poor
(e.g., silty or clayey sands)
• High hydraulic conductivities
(e.g., clean sands and gravel)
• Heterogeneous materials
(e.g., stratified soils)
• Proximate recharge
• Remote recharge
Depth of Well
Normal Spacing
Normal Range of Capacity
(per unit)
Efficiency
Good
Good
Good
Poor
Poor
Good
Good
Fair to Poor
Good Poor Good to Fair Poor
Good Good Good Good
Shallow <20 ft Shallow <20 ft Deep >20 ft Deep >20 ft
5 - 10 ft 20 - 40 ft 10 - 20 ft >50 ft
0.1 - 25 gpm 50 -400 gpm 0.1 -40 gpm 25 - 3000 gpm
Good
Good
Poor
Fair
-------
FIGURE 5-19.
COMPONENTS OF TYPICAL DEEP WELL
(POWERS, 1981)
fl A Q
i Valve
T
Height x
i
Depth z
Detail of Air Line for
Measuring Operating
Level
5-48
-------
TABLE 5-11. DEEP WELL COMPONENTS AND SELECTION CRITERIA (Powers, 1981)
Item Description
(See Figure 3-22)
Selection Criteria
A Throttle Valve
B Check Valve
C Turbine Pump
D Electric Motor
E Control Panel
F Air Line
G Filter Piezometer
H Pressure Gauge
I Seal
J Gravel Pipe
K Well Screen
L Fi Her Material
M Casing
Prevents damage caused by pumping surges when the pump is operating at less then
the design capacity (typically after initial drawdown), also ensures maintenance
of a constant discharge
Prevents backflow (recharge) from occurring if the pump fails in systems where a
number of wells are connected to a common manifold
Sized to suit aquifer conditions, may need to be corrosion resistant
Sized to operate within its service range under any pump load that may be
encountered
May need to be in a tamper proof enclosure
Used to measure the operating level of the well ; monitors performance of the pump
and the well
Used to measure screen loss during development and fractional increases caused by
encrustation
Used to monitor pump wear
Prevents surface inflow of water into well, typically cement grout or bentonite
Used to introduce new gravel in sealed wells during subsequent redevelopment
Size in accordance to needs, may need to be corrosion resistant
-------
en
o
TABLE 5-11. (Continued)
Item Description Selection Criteria
(See Figure 3-22)
N
0
P
Discharge Column
Meter Device
Manifold
Sized for maximum discharge with acceptable fraction; may need to
resistant, must also carry weight and hydraulic thrust of motor
Used for monitoring well discharge
Used to collect water pumped from wells and carry pumped water to
be corrosion
treatment
-------
all components listed will be utilized or are necessary for individual wells,
but incorporation should be based upon such factors as the life expectancy of
the well, the well's design, the treatment facilities available, and the
corrosiveness of the contaminated groundwater.
5.4.1.1.1 Pumps
Two types of pumps are commonly utilized in deep well applications, the
turbine submersible pump and the vertical line shaft pump. Turbine submers-
ible pumps were originally designed for water supplies but have been used
extensively in construction for dewatering. Submersible pumps have the
advantage of being relatively slender for their capacity allowing their use in
small diameter wells. Commercially available pumps can be obtained with
capacities greater than 100 gallons per minute and with motors of several
hundred horsepower.
Vertical lineshaft pumps are similar to submersible pumps except that the
motor is located on the surface rather than being attached directly to the
pump. This type of pump is used for extracting high volumes of groundwater
and is available in sizes ranging from a few horsepower to over a thousand
horsepower. The cost of a lineshaft pump is typically greater than for a
submersible pump of similar horsepower sizes.
Pump impellers and diffusers (components that drive water) are available
in a variety of materials (e.g., plastic, cast iron, bronze) and can be made
of special metals for wastes containing corrosive chemicals. Abrasive matter
such as sand can cause premature wearing of these parts and lose of well
efficiency. To avoid this, wells should be developed (i.e., pumped free of
fines as discussed in a later section) and cleaned to remove particles after
drilling. Also, the pump should be placed above the bottom of the well (i.e.,
>2 ft) to minimize pump plugging caused by fines.
5-51
-------
Numerous problems can arise when using deep well pumps for plume
management. These include the following:
• Corrosion of pump parts by contaminated waters
• Electrical malfunction (submersible pumps)
• Lightning damage
• Excessive part wear (e.g., impeller wear caused by intake of fines)
These problems should be factored into the design specification to minimize
operational problems.
Before selecting a pump for installation, accurate information on the
required capacity of the well and the total head is necessary. Required
capacity can be determined from the equations presented previously for the
radius of influence (R ) and drawdown (H-h). Factors such as pumping
cycles, peak loads, dynamic head, and future needs should also be considered
in design capacity. Total dynamic head (h.) can be calculated using the
formula:
ht = he + hf + hv
where:
h = total vertical lift, from the pumping level in the well to
e the water discharge point (ft)
h,: = total frictional losses expressed as head (ft)
h = velocity head required to produce the desired flow (ft).
Total vertical lift (h ) is the sum of the vertical distance from the
pumping level in the well to the outlet and the hydrostatic pressure head at
the discharge outlet. Velocity head (h ) is determined using the formula,
9 •
h = 0.155V . Usually h is very small and only taken into account at
installations where:
t Very high discharges are expected at very low lift and discharge head
• Velocity of flow is very high.
5-52
-------
Frictional losses in pipes and fittings are calculated using tabulated values.
Tables 5-12 and 5-13 give values for smooth pipe and fittings.
Once the total dynamic head is obtained and the capacity is determined, a
pump and motor can be chosen using performance curves. These curves typically
plot capacity versus total head and percent efficiency of the system. An
example of performance curves for 325 gallon per minute pumps is given in
Figure 5-20. The most efficient operating range for these pumps is between
250 gallons per minute and 380 gallons per minute. If, for example, the total
calculated head is 200 feet and the desired capacity is 300 gallons per
minute, a 25 horsepower, 5 stage pump would be selected for use (i.e., a pump
that exceeds the expected use).
Another criteria in selecting a pump that should be considered is the
type of drawdown anticipated and the pumping cycle. If the pump is to be
utilized under varying head pressures and cyclic pumping conditions, a steep
head pump should be chosen (i.e., steep drawdowns). If the pump will be used
for steady pumping, resulting in a flat drawdown, a flat head pump should be
chosen. Proper selection of either a flat or steep head pump will reduce O&M
cost and improve system efficiency.
5.4.1.1.2 Well Casing and Depth
Casing size (diameter) is chosen to satisfy two requirements. The casing
must be large enough to (Johnson Division, UOP Inc., 1975):
• Accommodate the pump with enough clearance for effective operation
t Ensure proper hydraulic efficiency of the well at the intake section.
The controlling factor is usually the size of the pump selected for the
desired capacity and head. A good rule of thumb is that the casing size
should be two standard pipe sizes larger than the nominal diameter of the pump
or pump bowl, and not less than one size larger. Recommended well casing
diameters are given in Table 5-14. For example, a 300 gallon per minute pump
5-53
-------
I
cn
TABLE 5-12. LOSS OF HEAD DUE TO FRICTION IN SMOOTH PIPE*
(approximate head loss in ft. per 1,000 ft. of pipe)
(Johnson Division, UOP Inc, 1975)
Flow
Rate
in gpm
10
15
20
25
30
40
50
60
75
100
125
150
175
200
250
300
350
400
500
600
750
1,000
1,500
2,000
1-1/4 1-1/2 2
20 9 2
44 20 6
79 35 10
123 55 16
178 79 22
142 40
222 64
90
140
258
Nominal
2-1/2
4
6
9
16
25
37
57
102
159
228
Pipe Size, in
3
1
2
3
5
8
11
18
30
46
68
90
122
4
2
3
4
8
12
17
23
30
47
70
93
123
191
inches
5
1
2
3
5
7
9
14
20
27
35
54
78
122
6
2
2
3
5
8
11
14
21
31
48
86
194
8 10
1
2
3
5 2
7
11 3
19 6
43 15
76 22
12
2
5
10
*For Rough Pipe, add 50%
-------
TABLE 5-13. APPROXIMATE HEAD LOSS EQUIVALENTS FOR PIPE FITTINGS
(Johnson Division, UOP Inc, 1975) (expressed in fit of straight pipe)
en
i
en
en
Type of F itt ing
90° elbow
45° elbow
90° elbow ( long radius)
Tees
Reducer (1 step) large end
Gate valve (open)
Gate valve (1/2 open)
Globe valve (open)
Angle valve (open)
Swing check
Nominal Pipe Size, in
1.25
3.5
1.5
2.5
See
2
1
20
35
18
9
1.5
4.
2
3
note
2.
1
25
45
24
11
2
5 5.5
2.5
4
below
5 3
1.5
35
55
30
13
2.5
6.5
3
4.5
3.5
1.5
42
67
35
16
3
8
4
5
4
2
50
80
40
20
3.5
9.5
4.5
6
5
2
58
100
50
23
4
11
5
7
6
2.5
65
115
55
25
inches
5
14
6
9
8
3
80
140
70
32
6
16
8
11
10
4
100
160
80
45
8
21
10
14
12
5
130
215
110
52
10
26
13
17
15
6
170
285
140
65
12
32
15
20
18
7
200
340
175
80
Tees vary with the direction of flow through them. For straight-through flow, the head loss is about equal to that of a long
radius 90° elbow; when flow is through the branch, the head loss is approximately 3 times as great.
-------
FIGURE 5-20.
PERFORMANCE CURVES
(FLINT Er WALLING INC.)
Most Efficient
Operating Range
700
600
500
400
tt>
I 300
200
100
80
70
60
50
.2
100 200 300
Gallons per Minute
400
500
5-56
-------
TABLE 5-14. RECOMMENDED WELL CASING DIAMETERS
(JOHNSON DIVISION, UOP INC., 1975)
Well Yield
(gpm)
<100
75-175
150-400
350-650
600-900
850-1,300
1200-1,800
1600-3,000
Nominal Size
of Pump Bowl
(in)
4
5
6
8
10
12
14
16
Optimum Size
of Well Casing
(in)
6 ID
8 ID
10 ID
12 ID
14 OD
16 OD
20 OD
24 OD
Minimum Size
of Well Casing
(in)
5 ID
6 ID
8 ID
10 ID
12 ID
14 OD
16 OD
20 OD
ID = inside diameter
OD = outside diameter
with a 6 inch diameter, an optimum casing size would be 10 inches (inside
diameter).
Well depth is usually determined from the geologic data (e.g., aquifer
thickness) available for the site and the plume's characteristics. In most
cases wells will be completed to the bottom of the aquifer. This is typically
the case for plumes that contain contaminants that are miscible with ground-
water or are denser than groundwater or both. The exception to full
penetration of the aquifer would be where contaminants are less dense than
groundwater such that they float at the top of the aquifer. In this case
partially penetrating wells may be desirable.
5-57
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5.4.1.1.3 Well Screens
The purpose of a well screen is to (Department of the Interior, 1977):
• Stabilize the sides of the hole
t Keep soil particles out of the well
• Facilitate flow into and within the well.
Screens can range from blank pipe that is perforated inplace to manufactured,
continuous-slot well screens with accurately sized openings. The types of
screens commercially available and commonly used for dewatering are listed and
described in Table 5-15. Most are made in lengths ranging from 5 to 20 feet
so that any length of screen can be created by joining individual sections.
The options in selecting well screens are the type, length, diameter, and
opening size. The following sections provide information for making these
choices.
The type of well screen selected is generally based on the availability
of screens having the proper diameter and opening size that are made of a
suitable material. The selection of screen diameter and opening size is
controlled by several factors, which are discussed later. Material selection
is governed by the corrosive and incrusting properties of the groundwater and
contaminant plume, and the strength requirements imposed by column load and
collapse pressure. Corrosion of the screen causes enlargement of openings,
which could permit excessive sand entrance and premature pump wear. If
corrosion is severe, collapse could occur. Incrusting is caused by mineral
deposits on the screen surface which tend to plug the openings of the screen
and formation. Removal by acids is often used to remedy incrustation
problems. Bacteria growth can also cause well incrustation in the same manner
as mineral deposits. Removal of deposits is typically done using strong
chlorine solutions. These problems necessitate the use of screens (and
casing) made of corrosion resistant materials. Table 5-16 lists some common
materials and their applications.
5-58
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TABLE 5-15. WELL SCREEN TYPES (After Powers, 1981)
Type
Diameters
(in)
Openings
(in)
Advantages
Disadvantages
Slotted
in
i
Continuous Slot
Louvered
Mire Mesh
4 - 12
4-36
6 - 48
4 - 24
0.010 - 0.100
0.003 - 0.250
(vertical rods
not accounted
for in area
calculation)
0.032 - 0.250
>0.020
• Reasonable cost
• Easy installation
t Resistant to corrosion
• Screen has high open
area
• Precise slot dimensions
• More effective develop-
ment
Can be reused
Reasonable cost
Reusable
Effective in gravel
Effective in jetted
wells
Effective in fine soils
Precise slot dimensions
• Type I PVC is brittle
• PVC susceptible to
degradation by chlor-
inated hydrocarbons
• Can have loading
probelms with thinner
walled pipe
• Slots can become partly
clogged by sand
t Generally minimal
opening area available
• Must be welded to casing
• Corrosion problem can
occur depending upon
material used
• Susceptible to corrosion
• Limited open area
• Not suitable for drilled
wells
• Limited open area
because of holes behind
screen
-------
TABLE 5-16. WELL SCREEN MATERIALS AND APPLICATIONS (Johnson Division, UOP Inc., 1975)
01
o
Material
Monel
Stainless Steel
Everdur
Silicon Red
Brass
Armco Iron
Steel
Nominal Composition
70% Nickel
30% Copper
74% Steel
18% Chromium
8% Nickel
96% Copper
3% Silicon
1% Manganese
82% Copper
16% Zinc
1% Silicon
99.84% Pure Iron
(double galvanized)
99.35/99.72 Iron
Cost
Factor
1.5
1.0
1.0
0.9
0.6
0.5
Suggested Applications
Waters with high sodium chloride and dissolved oxygen
levels such as sea water
Waters containing hydrogen sulfide, dissolved oxygen,
carbon dioxide, or iron bacteria; excellent strength
Waters having a high total hardness, high sodium
chloride content where dissolved oxygen is absent, or a
high iron content; extremely resistant to acid treatment
Same as Everdur but nor quite as good nor as strong;
used in relatively inactive waters
Not corrosion resistant; used in neutral water
Not corrosion resistant; used for temporary wells
PVC
0.09/0.15 Carbon
0.20/0.50 Manganese
(double galvanized)
Polyvinylchloride
0.25 Highly corrosion resistant; Type I PVC is brittle and
will crack when strained; not useful with chlorinated
organics and some organic solvents
-------
The well screen choice is also determined partly on strength require-
ments. Two loads are imposed on the screen which must be considered in screen
selection; column load and collapse pressure (Johnson Division, UOP Inc., 1975)
Column load results from the screen supporting the weight of the casing.
These forces can be considerable in deep wells. Collapse pressure results
from earth pressure and caving materials squeezing the screen. Resistance
to these loads is directly proportional to the modules of elasticity of the
screen material. For example, the strength of a stainless steel screen is
twice that of a copper alloy screen of the same size (steel modules = 30
million psi , copper alloy modul es = 15 mill ion psi). Screens shoul d be chosen
to have:
t Strength to resist load and collapse
• Maximum open area consistent with the strength requirements.
The optimum length of a screen is based on the type and thickness of the
aquifer, the available drawdown, and the stratification of the aquifer. Table
5-17 gives some general rules that should be applied when determining screen
length. The criteria given in Table 5-17 is for contaminants that completely
mix in the aquifer or are denser than water. For contaminants that are less
dense than water (i.e., floating contaminants), screen length and placement
should be determined on the basis of plume boundaries and the desired
drawdown.
The opening size of the well screen requires that the entrance velocity
of the groundwater be less than a critical value. Some references suggest
using a critical value equal to or less than 0.1 feet per minute (Johnson
Division, UOP Inc., 1975), while others suggest choosing the critical value of
entrance velocity based on the hydraulic conductivity of the filter material
in contact with the screen (Powers, 1981). Table 5-18 gives recommended
5-61
-------
TABLE 5-17. CRITERIA FOR WELL SCREEN LENGTH SELECTION
(After Johnson Division, UOP Inc., 1975)
Aquifer
Type
Homogeneity
Criteria
Water Table
(unconfined)
Homogeneous
Heterogeneous
Artesian
(confined)
Homogeneous
Heterogeneous
• Screen bottom one third of aquifer
• Drawdown should not exceed the top
of the screen or a point slightly
above
• Screen the most permeable
sections, especially in lower part
of the aquifer
• Screen sections that contain
contaminants regardless of
permeability
• Drawdown should not exceed the top
of the screen
• 70% to 80% of the aquifer's total
thickness
<25 ft - 70%
25-50 ft - 75%
>50 ft - 80%
• Maximum allowable drawdown should
be the top of the aquifer
t Screen as in a homogeneous aquifer
to extract contaminants
• Drawdown should not be lower than
the top of the screen
5-62
-------
TABLE 5-18. RECOMMENDED ENTRANCE VELOCITIES FOR VARIOUS FILTERS
(Powers, 1981)
Hydraulic Entrance Velocity
Conductivity (ft/mi n)
>6,000
6,000
5,000
4,000
3,000
2,500
2,000
1,500
1,000
500
<500
12
11
10
9
8
7
6
5
4
3
2
entrance velocities for various filters. Regardless of the method chosen to
obtain critical entrance velocities, the open area can be calculated from:
= 19.2 Q/Vs
where:
A = open area of well screen (in/ft)
Q = expected or desired yield of well per unit length of screen
(g/min-ft)
Vs = entrance velocity (ft/min)
Once the open area has been determined, the only parameter needed to correctly
select a well screen is the slot size. Typical open areas of some commer-
cially available screens are given in Table 5-19.
5-63
-------
TABLE 5-19. OPEN AREAS OF COMMERCIALLY AVAILABLE WELLSCREENS
(Powers, 1981)
Nominal
Diameter
(in.)
4
6
8
12
18
24
Slot
Size
(in.)
0.015
0.030
0.060
0.090
0.120
0.015
0.030
0.060
0.090
0.120
0.015
0.030
0.060
0.090
0.120
0.015
0.030
0.060
0.090
0.120
0.015
0.030
0.060
0.090
0.120
0.015
0.030
0.060
0.090
0.120
Continuous
Wire
27.6
46.6
71.2
86.4
96.7
41.2
60.0
84.8
106.9
123.2
39.3
69.3
113.0
142.6
164.3
59.2
81.5
117.4
155.9
186.5
53.5
99.2
169.6
228.1
271.3
83.2
117.5
172.3
235.1
289.4
Approximate
Double
Louvre
5.9
—
—
—
_ «
6.9
13.9
20.7
28.7
. _
9.1
18.6
27.6
38.3
_ _
13.7
27.8
41.3
57.4
_ _.
19.8
40.2
59.7
82.9
— _
25.9
52.6
78.1
108.4
Open Area
Slotted
PVC
8.9
14.4
27.1
43.4
57.6
10.1
20.2
40.3
60.5
80.6
13.0
25.9
51.8
77.8
103.7
20.2
40.3
80.5
121.0
161.3
28.8
57.6
115.9
172.8
217.5
_ w
__
__
__
— -
(in2/ft)*
Wire
Mesh
76.3
__
__
__
--
112.4
__
__
__
--
146.3
__
—
__
--
216.3
—
—
__
—
305.4
—
__
__
--
407.2
__
__
__
--
*0pen area may be less than manufacturer's specifications because of design;
refer to Table 5-14.
5-64
-------
Determining the required slot size for the screen depends on the well
development method to be used and the filter pack. A mechanical analysis
(i.e., sieve analysis) must be performed on a sample representing the water-
bearing formation to select the proper slot size. This approach differs from
that used for monitoring wells, i.e., the use of a 10-slot screen (Sisk,
1981), because wells used in remedial actions are typically pumped exten-
sively. Extensive pumping with improper screens and development can result in
low yields, premature pump wear, and possible well failure.
Most wells used for remedial actions will probably be drilled, which
normally requires the area around the well screen to be filled with a filter
or gravel. This filter performs several important functions (Powers, 1981):
• Fills the annular space preventing the uncontrolled collapse of the
formation against the screen
t Retains a sufficient percentage of fines thus preventing them from
being pumped continuously
• Passes some amount of fines and mud cake that have built-up on the
sides of the hole
• Transmits water freely from the aquifer to the screen during pumping.
Procedures for selecting filters and screens have been developed by
numerous groups, however, a method developed by Powers (1981) is described
below. The first step in the procedures is to obtain a sample at the aquifer
and conduct a mechanical analysis of it. Results of the analysis are plotted
as shown in Figure 5-21a. A filter range is thus established such that:
• The filter is of uniform material, preferably with a uniformity
coefficient (C ) of C <3.0
• The filter C is not greater than the aquifer C unless it is a graded
filter u u
• The filter D™ is four to eight times greater than the aquifer D™.
5-65
-------
FIGURE 5-21. RANGE OF FILTER SELECTIONS
(POWERS, 1981)
3" 2"
Sieve Numbers
V 3/4" 1/2" 1/4" 4 6 10 16 20 30 40 50 60 100 140 200 270
I I II II ill; I I i I I I I
Mechanical Analysis
of Soil
C. - 4.5
|9 7 | 3 | |9 7 | 3
10 5.0 2.0 1.0 0.5 0.25
Grain Size (mm)
250 160 100 80 60 40 30 20 To Slotted Screen
11 1
1
4
Gravel
1 1 1 1 1 I 1
' 3" 1" 3" 1" 1" 1" Louvred Screen
16 8 32 16 32 64 Si|,
Coarse Sand
Medium
Sand
Fine Sand
and
Clay
Figure 5-21 a.
MA Classification
Sieve Numbers
1/4" 4 6 10 16 20 30 40 606080100140200270
t
^
100
90
80
70
60
50
40
30
20
10
K
•1 '
' —
9 7
0 i
\\
V1
\
x
\ i
1
v\ ^\
(&C
N\V
\\\
Filter
Specrfi
\vi
cation ^A ,'
\]
\
3 1
> 2 1
i
\
s
9 7
\
V
-. Baa
Lu
V
V
\l
0
ft
c Filter Curve
- 2.5 DM -
! !
1
5 X 0.4 * 2.0
i
i
. — Mechanical Analysis
1 of Sou '
OBO - 0.4 c - 6 ;
\
\
!
3 | |9 7 3
5 0.25 .1 .05 .0
0.025 in. 0.63mm H
, Medium ,
Gravel Coarte Sand Sand I Fme Sand
Silt and Clay
Figure 6-21 b.
Classification
5-66
-------
The range of suitable filters is then plotted as shown in Figure 5-21a.
A rather broad range of filters is established for a soil D5Q and selection of
the proper filter within this range is based on the gradation of the soil and
the desired yield of the well. Table 5-20 gives criteria for determining
filter requirements more precisely.
Once the range of acceptable filters has been narrowed to fit the
specific application, the results of the mechanical analysis is replotted as
shown in Figure 5-21b. Typically, a set of limits within which the filter
material must fall is furnished to a supplier. The mechanical analysis of the
supplied filter material is then plotted. Well screen slot size is finally
selected to pass about 10 percent at the fine limit and 0 percent of the
coarse limit (Figure 5-21b). While this procedure appears to be fairly
simple, a significant amount of judgement is required in applying it to
specific site conditions.
For naturally developed wells (i.e., wells without a gravel pack), well
screen opening size is selected from a mechanical analysis of the water-
bearing formation (Figure 5-22). The correct slot opening size is chosen so
that the screen will retain from 40 percent to 50 percent of the particles
(Johnson Division, UOP Inc., 1975). The 40 percent size is chosen for wells
in non-corrosive water and where sample analysis is considered reliable. The
50 percent size is chosen for wells in corrosive waters (i.e., to allow for
some enlarging of screen openings) and where sample analysis reliability is in
doubt. For the grain size analysis shown in Figure 5-22, the screen opening
selected would be between 0.040 inch and 0.050 inch (50 percent and 40 percent
retention, respectively).
Where stratified soils are encountered, the screen slot opening is chosen
according to the gradation of materials of the different strata. Table 5-21
gives some criteria for selecting and placing screens in stratified soils.
The examples given in the table are generalizations and should be viewed as
such. For example, cutting off silty zones with blank casing may be
preferable. The use of variable or graded filters may also be necessary to
achieve good results. However, a rule of thumb used by drillers is to allow
5-67
-------
TABLE 5-20. FILTER SELECTION CRITERIA (AFTER POWERS, 1981)
Characteristic
Criteria
Gradation of Soil
For uniform soil s (C < 3): the D™ of the fil ter
u — b(J
should be in the low range, about 4 to 5 times the D
of the soil.
50
For graded soils (4 < C < 6): the Dco of the filter
— U — bl)
can be 5 to 6 times the Dr_ of the soil .
bi)
For very well graded soils (C >_7) where it may be
desirable to develop some fines from the soil : the
D&0 of the filter can be as great as 8 times the D
of the soil.
50
Yield of Well
For low well yields per linear foot of screen in
relation to hydraulic conductivity: the multiples of
the Drg of the filter can be increased by a factor of
1 or perhaps 2 because pore velocities are low enough
to preclude the movement of fines.
For high well yields per linear foot of screen in
relation to hydraulic conductivity: the multiplier of
the D filter should not be increased.
bL)
5-68
-------
en
i
CTl
IO
Cumulative Percent Retained
& S
o
x o
(O >
IP
o —
^ m £)
£>S
z c
-------
TABLE 5-21. WELL SCREEN SELECTION FOR STRATIFIED SOILS
(Johnson Division, UOP Inc, 1975)
Development
Type
Strati fication
Scheme
Criteri a
Filter Pack
Coarse material over fine
Fine material over coarse
Natural ly
Coarse material over fine
Fine material over coarse
Filter and screen selected
should be suitable for the
finer materi al
Filter and screen selected
should be suitable for the
coarse material because the
coarse material will
probably draw the fine
material
Screen should be selected
according to each soil
materi al
Screen should be selected
according to each soil
material; screen in the
fine material should extend
at least 2 feet into the
coarse material and the
slot si ze in the coarse
material should not be
twice that of the screen in
the fine material
the finer material to dictate screen size over the various strata because
mixing packs in the field is difficult and graded packs can differentiate in
pi ace.
5.4.1.2 Ejector Wells
Ejector wells or systems have certain advantages over wellpoints and deep
wells because they are not typically depth limited as wellpoints are and they
are less expensive than deep wells when close spacing is required. The
biggest drawback to the use of ejector wells is that they are very inefficient
(typically less than 15 percent efficiency). Ejector wells can be used
5-70
-------
independently of each other or arranged so that they utilize a common pumping
system.
The typical components of a two-pipe and a single-pipe ejector well
system are illustrated in Figure 5-23. The lift principle for the two-pipe
model is (Powers, 1981):
• High pressure supply water (Q ) moves down the supply pipe through
ports in the ejector body to the tapered nozzle where the pressure
head is converted to water velocity
• Supply water exits the nozzle at less than atmospheric pressure
creating a vacuum in the suction chamber
• Groundwater (Q?) is drawn into the chamber through the foot valve
because of the pressure differential
• Supply water and groundwater (Q + Q?) are mixed in the suction
chamber
• The mixed water enters the venturi where the velocity decreases
because of divergence resulting in increased pressure
• The increase in pressure develops sufficient head to return the
combined flow to the surface.
The lift principle for the single-pipe model, illustrated in Figure 5-23,
i s (Powers, 1981):
• Supply water under pressure (Q ) flows downward between the well
casing and the inner ejector return pipe
• The nozzle, suction chamber, and venturi perform as described for the
two-pipe system
• A packer assembly separates the supply water from the groundwater so
that different pressures are developed
• Foot valves located below the packer are used as in the two-pipe
ejector system.
Casing and screens used by ejector systems are the same as those utilized
by deep wells or wellpoints. Design recommendations for casing and riser
sizes are given in Table 5-22. Single-pipe ejector wells are most commonly
used because they require less piping and yields are greater for smaller
diameter casings.
5-71
-------
FIGURE 5-23.
COMPONENTS OF ONE-PIPE AND TWO-PIPE EJECTOR WELLS
(POWERS, 1981)
Overflow
Strainer
Return Header
Well Seal
2 in. (50-mm)
Riser—
-X
Supply Header
Packer
2 in. (50-mm)
Return Line
1% in. (32-mm) Riser
with Turned Couplings
tl
•2-in (50-mm|
wcllpomt
Screen.
,Well Casing
and Screen
• 1 % in.
137mm)
Supply Line
(Q
• Ejector
Body
• Foot
Valve
Typical Single-Pipe
Ejector Installed
in a 2-m. (50-mm)
Wellpoint
Typical Two-Pipe
Ejector Installed in
a 6-in. (150-mm)
Well
5-72
-------
TABLE 5-22. RECOMMENDED CASING AND RISER SIZES FOR EJECTOR SYSTEMS (Powers, 1981)*
en
GO
Groundwater
Flow (Q )
(gpmr
12
20
40
70
Single Pipe
Well Casing
(in)
2.0
2.5
4.0
5.0
Ejector
Return Pipe
(in)
1.25
1.50
2.00
2.50
Well Casing
(in)
4
5
5
6
Two Pipe Ejector
Supply Pipe
(in)
1.00
1.25
1.50
2.00
Return Pipe
(in)
1.25
1.50
2.00
2.50
*Recommended pipe sizes are for a setting of 40 feet with a supply pressure of 120 psi; deeper
settings or lower pressures require larger pipe sizes.
-------
Ejector pumps consist of a water tank and a pump with the required valves
and piping. Each well can have its own pump or one pump can be used by a
number of wells. Water from the storage tank is pumped under pressure through
a header pipe that supplies the ejector wells. Return flow, a mixture of
supply water and groundwater, recharges the tank through the header system.
Excess water in the tank is discharged to a treatment system.
Selecting the proper ejector pump and educers (i.e., nozzle and venturi)
can be made using manufacturer's data or through calculations. An example of
manufacturer's data is given in Table 5-23. The procedures for calculating
the size of an ejector are to calculate head ratios, estimate the capacity
ratios, and calculate the diameters of the nozzle and the venturi. The
procedure is fairly complex and is described in detail by Powers (1981).
Materials for risers, swings, headers, tanks, screens, and pumps should
be selected to minimize corrosion and encrustation. Selection criteria are
the same as for deep well systems, however, ejectors must also be protected to
maintain well performance. Ejectors used in corrosive environments can be
made of plastic to eliminate the problem. Of greater importance is the
ejectors' sensitivity to clogging especially if clogging results from
encrustation,caused by the pressure reduction. Reduction in pressures at the
throat of the venturi can cause accelerated rates of chemical precipitation.
A continuous water supply to eliminate recirculation and filters may be
necessary to prevent encrustation and clogging. A water analysis prior to
designing the system is advisable to test for problems that may limit ejector
well use.
5.4.1.3 Wellpoints
Well point systems consist of a group of closely spaced wells connected to
a header pipe and pumped by a suction pump. Wellpoints are best suited for
groundwater extraction in stratified soils where total lift or drawdown will
not exceed approximately 22 feet. The advantages to using wellpoints are that
the system design is flexible and the wellpoints are relatively inexpensive
even when closely spaced.
5-74
-------
TABLE 5-23. EJECTOR SYSTEM PERFORMANCE SPECIFICATIONS (Burks HNAZ Series Pumps)
One-Pipe Educer
Pump Well
Dia.
HNAZ Series
5 HNAZ . 2"
1/2 Up
7 HNAZ 2"
3/4 Up
10 HNAZ 2"
1 np
Educer Pressure
No. Switch
Setting
22183 20-40
22184
22185 30-50
22171
22185 30-50
22186
22176
Control
Valve
Setting
38
35
32
35
35
42
50
Capacity in Gallons Per Hour
At Vertical Depth To Pumping Level
30'
675
515
840
495
965
550
40'
660
480
800
480
920
520
50' 60'
545 425
450 400
680 560
465 450
790 660
505 485
350 350
70'
330
350
425
420
530
450
330
80'
190
290
310
390
400
410
320
90'
250
330
285
360
295
100'
200
270
140
320
260
110' 120' 130' 140' 150' 160'
150
210 150
285 155
230 200 175 150 125 100
Max. Shutoff
Pressure At
Deepest Set'g
55
62
55
62
55
66
72
Two-Pipe Educer
Pump Min.
Well
Dia.
5 HNAZ 4"
1/2 hp
7 HNAZ 4"
3/4 hp
10 HNAZ 4"
1 hp
Educer Pressure
No. Switch
Setting
22199 20-40
22200
22196 30-50
22197
22201 30-50
22194
22202
Control
Valve
Setting
32
32
28
30
33
38
38
Capacity in
Gallons Per Hour
At Vertical Depth To Pumping
30'
585
405
860
990
575
385
40'
570
400
850
970
570
380
50' 60'
475 380
395 390
730 610
480 480
820 665
565 555
375 370
70'
305
365
505
455
525
525
365
80'
225
320
385
395
375
465
360
90'
275
330
405
355
100'
230
270
335
345
Level
110' 120' 130' 140' 150' 160'
190 150
220 180 140
275 210 155
315 260 210 175 140 105
Max. Shutoff
Pressure At
Deepest Set'g
58
61
58
63
53
59
62
-------
A suction (vacuum) pump is typically used in wellpoint systems to lift
water. Suction pumps accomplish lift by developing a negative pressure head
at the pump intake rather than by applying force to the water source as in
ejector pump systems. Four factors limit the maximum lift attainable by
suction: atmospheric pressure, vapor pressure, head losses caused by
friction, and the required inlet head of the pump. Because of these factors,
the maximum lift theoretically attainable is about 25 feet. In practice,
however, lifts between 15 and 22 feet are more common when centrifugal pumps
are used.
Two types of suction pumps are commonly used, the oil sealed and the
water sealed vacuum pump. Pumps are usually rated by the volume of air
handled at various vacuums under standard conditions of temperature and
pressure. Oil sealed pumps are capable of producing vacuums of 30 to
100 cubic feet per minute at 25 inches of mercury; water sealed pumps produce
vacuums of 50 to 500 cubic feet per minute at 25 inches of mercury.
Wellpoints are specially made well-screens that are typically 1.5 to
3.5 inches in diameter and are capable of yields up to 35 gallons per minute.
Large wellpoints are available, up to 8 inches in diameter, which have
capacities greater than 35 gallons per minute. These large wells are
generally called suction wells rather than wellpoints. Wellpoints can be
installed using a variety of methods including jetting and driving.
Figure 5-24 shows the basic types of wellpoints. Wellpoint screens can be
made of heavy wire mesh, continuous wire, slotted plastic, or perforated
plates. The materials selected should minimize the potential for corrosion
and encrustation. The guidelines for wellpoint materials are the same as for
deep wells.
The depth at which wellpoints are set is dependent upon the hydrogeology
of the site. Drawdown is limited to approximately 22 feet below land surface,
but the wellpoint can be set at almost any depth depending on the situation.
Where contaminants float at the top of the aquifer, wellpoints can be set at
shallow depths. Where contaminants sink or mix with the groundwater, the
wellpoints can be set deeper. The only criterion for wellpoint depth is to
5-76
-------
FIGURE 5-24.
DRIVEN WELLPOINT (a), JETTED WELLPOINT (b),
AND DRILLED WELLPOINT (c)
(JOHNSON DIVISION, UOP INC., 1975 a & b; POWERS, 1981c)
en
i
Well Casing
(from pump)
Figure 5-24a. Weilpoint Driven
Well Casing
Ring Seal of
Semi-rigid Plastic
Temporary
Wash Pipe
X— Well Screen
-- Coupling on Wash Pipe -V
Rests in Conical Seal
-~ Combination Back-pressure
Valve in Open Position
,--,;.'-. ;Varved
.v. "-;.'• siit
•
-------
avoid dewatering below the screen. If this occurs, air enters the system and
reduces the vacuum and therefore the drawdown. During normal operation of the
system, dewatering below the top of a well screen can sometimes occur. The
problem can be minimized in the field by adjusting the values that control
individual wells.
Because well points are typically installed in oversized boreholes, filter
sands are placed around the wellpoint to fill the annular space. Besides
filling the annular space, the sands perform other functions, including
(Powers, 1981):
• Increasing the effective diameter of the wellpoint
• Decreasing the entrance velocities of water
0 Preventing clogging of the screen with fines
• Providing vertical drainage from overlying layers.
Commercially available wellpoints are typically designed with openings
suitable for use with washed concrete sand filters. This type of filter
performs well when the soil penetrated is finer than the concrete sand. If
soils are very fine and have little cohesion, however, they may migrate
towards the wellpoint when concrete sand filters are used. In this instance,
mortar sand filters may improve well yields and prevent clogging. For some
applications, selecting the filter material and the screen opening specific-
ally for each wellpoint's application may be necessary as described in the
deep well section.
5.4.1.4 Recharge Wells
Recharge wells are sometimes used in plume management to create ground-
water mounds or to act as depositors for treated water. The design of a
recharge well is similar to an extraction well (Figure 5-25) with a few
variations. Pumps, casing, filter packs, and screens should be designed in a
5-78
-------
FIGURE 5-25.
BASIC INJECTION WELL (POWERS, 1981)
Air Vent-
Meter
Control
Valve
Recharge l
Header
Sand
Aquifer
- Air Vent
Filter Replacement
-Tube
Casing
. Concrete or
Grout Seal
• Downspout
. Filter Pack
- Wellscreen
5-79
-------
similar manner as outlined in the previous sections. The additional features
that are normally needed are (Powers, 1981):
• A downspout to prevent air entrapment from cascading water when the
well is operated at a low level
t An air vent to release trapped air at start-ups
t A concrete or grout seal to prevent water from flowing along the
casing to the surface when the well is pressurized.
Development and periodic redevelopment of an injection well is necessary to
maintain efficiencies because they do not continue to develop themselves with
use as extraction wells do.
5.4.2 System Design
System design includes a determination of the number of wells needed, the
patterns and spacing of the wells, the design of the individual wells, the
pumping cycles and rates needed, and the method of handling discharges. The
following sections provide some general guidelines for well system design.
5.4.2.1 Preliminary Requirements
Prior to designing the well system, a complete hydrogeologic under-
standing of the site must be established. A potentiometric surface map (i.e.,
a map depicting contours of equal head) and a geologic cross-section of the
site should be developed. Development of these two tools is very important to
well system design. Figure 5-26 gives an example of these for a gasoline
pipeline leak. Other parameters that are required for system design are the
coefficients of transmissivity (T) and storage (S) and the discharge (Q) and
drawdowns (H-h ) from the pump tests. Once data on these parameters are
W
established, the design process can proceed.
5-80
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FIGURE 5-26.
POTENTIOMETRIC SURFACE MAP AND GEOLOGIC
CROSS-SECTION OF GASOLINE POLLUTION SITE
(WILLIAMS AND WILDER, 1971)
Strong Taste
and Odor Area
Figure 5-26a. Plane View
-Brown Clayey Silt
Grey Organic Clay
Figure 5-26b. Cross-sectional View
5-81
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5.4.2.2 Equilibrium vs. Non-Equilibrium Pumping
A choice has to be made whether equilibrium or non-equilibrium pumping is
to be utilized at the site because this affects the extent of the radii of
influence. As pumping time decreases the radii of influence also decrease
(refer to Figure 5-15) and this will result in a larger number of wells needed
for a given area. Using the example given in Figure 5-15, increasing the
pumping time from 300 minutes to 1,000 minutes causes the radius of influence
to increase from 350 feet to 650 feet.
Equilibrium pumping will probably be utilized in most plume management
sytems, but non-equilibrium pumping does have advantages in some cases. Table
5-24 lists the advantages and disadvantages of both approaches. Typically,
cyclic pumping systems cost more because of the need for a larger number of
wells with greater capacities and the greater operation and maintenance
requirements.
5.4.2.3 Well Spacing
Determination of the proper spacing of wells to completely capture a
groundwater plume is probably the most important item in system design. Field
practitioners have long had a standing "rule of thumb" for estimating well
spacing; adjacent cones of depression should overlap (i.e., radius of
influence should overlap). This method is valid for aquifers that have low
natural flow velocities but will not be valid for aquifers with large natural
flow velocities. For these latter cases (and probably all cases) velocity
distribution plots should be developed to determine well spacing and ensure
capture of the plume (Keely and Tsang, 1983). Both of these methods are
discussed in the following sections.
5.4.2.3.1 Radius of Influence
The most accurate method for estimating the radius of influence is by
pumping test analysis. Pumping tests can identify recharge boundaries,
barrier boundaries, and slow-storage release conditions. The pumping test
5-82
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TABLE 5-24. ADVANTAGES AND DISADVANTAGES OF PUMPING METHODS
Method
Advantage
Disadvantage
Equilibrium
Non-equilibrium
• Greater well spacing is
feasible, thus reducing
the number of wells
• Constant discharges to
treatment
• Reduced pumping rates may
be possible
• Better suited to
aquifers with high
hydraulic conductivities
• Operation and maintenance
cost may be lower because
cycling is hard on system
components
• Easier to design and operate
effectively
• Better suited to aquifers
with low hydraulic
conductivities
• May be more applicable to
floating and sinking plumes
• May be more suitable for
sites that have groundwater
barriers and scant recharge
t May require long pumping
times before cones
intersect, which may be
a problem with fast
moving contaminants
• Not as workable in
aquifers with low
hydraulic conductivities
t May not work well where
the flow of contaminants
is low, where the plume
floats, or sinks, or
where recharge is scant
• Closer spacing of wells
is required resulting in
a greater number of
well s
• Design is fairly
complicated
• May have higher
operation and
maintenance costs
because of cycling
• Greater capacity pumps
are usually required
5-83
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should be performed until equil ibriun conditions are reached. Typical test
durations for a confined aquifer are about 24 hours, while for an unconfined
aquifer they may be several days. Once equilibrium conditions have been
reached, the radius of influence for equilibrium or non-equilibrium conditions
can be estimated using the methods described previously and the equations in
Table 5-25.
When pumping test data are lacking or incomplete, rough estimates of the
radius of influence can be obtained from the values of transmissivity or
hydraulic conductivity, pumping times, and coefficient of storage and the
equations presented in Table 5-25. Coefficients of storage values typically
range from 0.01 to 0.35 for water-table aquifers and from 0.00001 to 0.001
for confined aquifers. Typical values are 0.2 for water-table aquifers and
0.001 for confined aquifers.
When using equilibrium formulas, the radius of influence appears as a
logarithmic function so that percision in estimating is not necessary.
However, values of RQ measured in the field can vary from 100 to 100,000 feet
and greater orders of magnitudes are possible. Therefore, gross errors in
actual R values are possible.
When estimates are made without pumping data, the values obtained are
rough approximations that do not take recharge into account. These estimates
should be adjusted on the basis of judgments made on the locations of possible
recharge and discharge barriers. Adjusting values of R downward is advisable
in most cases because the greater overlap of the cones of depression will
result in a lower probability of containment escape between wells. The
tradeoff, however, is a greater number of wells and higher costs.
A comparison of the results obtained for R when pumping test data are
used and when estimates are made is given below. A well is an unconfined
aquifer pumped under equil ibriun conditions provided the following
information:
• Q = 500 gpd
• K = 500 gpd/ft2
5-84
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TABLE 5-25. RADIUS OF INFLUENCE EQUATIONS
Pumping Unconfined Aquifer Confined Aquifer
Method
Equilibria 1 nRQ = (K(H2-hw2)/458Q) + lnrw 1 nRQ = (T(H-hw)/229Q) + 1
Non-equil ibriun distance - drawdown plots or Theis Method
Estimates Rn = r + (Tt/4790S)°'5*
(j w
f ••'
Ro = 3(H-hw)(0.47K)°-5
R = radius of influence (ft)
2
K = hydraulic conductivity (gpd/ft )
H = total head (ft)
h = head in wel 1 (ft)
Q = pumping rate (gpd)
r = well radius (effective radius) (ft)
W
T = transmissivity (gpd/ft)
t = time (min)
S = storage coefficient
*For unconfined aquifers H-h cannot be a large percentate of H
5-85
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• r = 0.5 ft
w
e H = 100 ft
• h = 80 ft.
w
Using the exact equation for R given in Table 5-25:
lnRQ = [K(H2-hw2)/458Q] + lnrw
= [500(1002-802)/458(500)] + In0.5
R = 1296 ft.
o
Using the first estimation equation for R given in Table 5-25:
R0 = rw + (Tt/4790S)0'5
t = 7 day or 10080 min (assumed)
C = 0.2 (assumed)
= 0.5 + [50000(10080)74790(0.2)]°'5
R = 725 ft.
o
Using the second estimation equation for R given in Table 5-25:
R = 3(H-hw)(0.47K)°'5
= 3(100-80)(0.47(500))°'5
R = 920 ft.
0
This comparison illustrates the importance of exercising good judgment in
selecting well spacing.
5.4.2.3.2 Velocity Plots
Well spacing for complete plume removal can more accurately be determined
by developing velocity plots for the pumping well (Keely and Tsang, 1983).
5-86
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Velocity of flow through an aquifer can be expressed as (i.e., from Darcy's
Law: Section 5.2.1):
V = Q/An = Kl/n
where:
V = pore velocity (ft/day)
Q = flow rate (gpd)
2
A = area normal to flow direction (ft )
n = effective porosity (dimensionless)
2
K = hydraulic conductivity (gpd/ft )
I = hydraulic gradient (dimensionless)
Using Darcy's Law for calculating pore velocity, two important deri-
vations can be made: water velocity toward a pumping well and natural fl
velocity in the aquifer. Velocity toward a well can be represented by:
ow
V . = Q/An = Q/2vrrhn
pumping
where the new terms are:
r = distance from well center to point where drawdown is measured
(ft)
h = drawdown (ft)
This expression is typically used for pumping wells because Q is usually known
for the well and A can be easily estimated. The area, A, in the above
equation represents the area of the curved face of an imaginery cylinder
(refer to Figure 5-2) for some distance away from the well. This area can be
calculated by A = 2 TT rh and substituted into the original Darcy equation.
5-87
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For calculating natural flow velocities the following equation is
typically used:
Natural = KI/n
This equation is chosen because, for the aquifer under investigation, the
average hydraulic conductivity (K) and hydraulic gradient (I) are usually
known or can be estimated with a relative degree of accuracy.
Utilizing these two equations the downgradient stagnation point, i.e.,
the distance downgradient the pull of waters back toward the well by pumping
is exactly countered by the natural flow velocity of water away from the well
(Keely and Tsang, 1983), can be calculated by setting the expression for
Vpumping equal to the value of VnaturaV This equa1ity is represented by
(Keely and Tsang, 1983):
V = V
pumping natural
Q/2?r rhn = V . ,
natural
and then solving for r to provide the stagnation point:
r = (Q/27Thn) V . ,
' natural
where h is represented by saturated thickness of the aquifer. This
relationship shows that the stagnation point is directly related to the
pumping rate of the well (i.e., the higher the pumping rate the further
downgradient the stagnation point) and inversely related to the natural flow
velocity (i.e., the greater the natural flow velocity the closer the
stagnation point is to the well).
The maximum distance to either side of the well perpendicular to
groundwater flow, and therefore the well spacing, that contaminants are not
drawn completely into the well has been defined by Todd (1980) as * times the
downgradient stagnation point ( Trr). Hence, any contaminants that lie outside
5-88
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the TT r distance will not be captured by the well. These boundaries, -nr
downgradient and it r perpendicular to flow, define the areal limits of a
pumping well's capture zone. Keely and Tsang (1983) have noted that for only
the extremely rare case of zero natural flow are the areal boundaries of the
capture zone identical to the calculated cone of depression. This means that
even though the cones of depression of two pumping wells intersect they may
not be completely capturing the plume unless their capture zones intersect.
The ramifications of the this finding on well spacing for plume removal is
paramount.
The use of the preceding equation can be used to calculate the capture
zone of a pumping well and well spacing ( irr) using a simple hand-held
calculation. However, numerous computer models are available that can perform
these calculations and provide velocity plots. Two such models are the radial
flow time series model and the RESSQ model. Readers are referred to Keely and
Tsang (1983) for a detailed explanation of these models.
An example of the use of the equality equation is provided below. Assume
that the following data are available for the aquifer under investigation:
Q = 100,000 ft3/day
K = 600 ft/day
h = 100 ft
I = 0.0015 (15 ft/10,0000 ft)
n = 0.30
Using these data in the equality equation for the downgradient stagnation
point provides the following:
r = (Q/27Thn) Vnatupal = Q/27T hn(KI/n)
= (100,000/2 7T (100)(0.30))[(600)(0.0015)/0.30)]
= (100,000/27T (100)(0.30))(2.6)
= 204 ft
5-89
-------
The well spacing can therefore be calculated (as previously defined) by ?r r =
604 ft. This means that a line of extraction wells placed perpendicular to
groundwater flow would have to be spaced at distances of 1208 ft (i.e., 2 ?rr)
to capture the plume.
5.4.2.4 Pumping Rates
Pumping rates and drawdowns are related by:
2 2
• Q/H -h = constant for unconfined aquifers
• Q/H-h = constant for confined aquifers.
This means that increasing the pumping rates will not affect the radius of
influence, but will affect the time of pumping (refer to Figure 5-15).
However, in reality, this may or may not be true for a field situation.
For unconfined aquifers, the optimum operating characteristics have been
shown to occur at about 67 percent of the maximum drawdown. This can be
demonstrated mathematically from plots of specific yield versus maximum
drawdown, and specific capacity versus maximum drawdown (Figure 5-27). When
the product of yield and capacity is a maximum, optimal operating conditions
are obtained. Drawdowns greater than 70 percent of the maximum are
uneconomical. This means that for an unconfined aquifer with a saturated
thickness of 100 feet, the drawdown should be approximately 67 feet for
optimum operating conditions to be obtained. Drawdowns less than 67 percent
of the maximum may be desirable in cases when steady state, long term pumping
is anticipated because the cones of depression will be the same.
For confined aquifers, the drawdown is directly proportional to the
discharge as long as the drawdown does not exceed the top of the aquifer.
Therefore, optimum operating conditions vary. Because pumping rates are
directly related to drawdowns and will not affect the cone of depression,
pumping rates can be obtained to suit the situation. In situations where the
contaminate plume floats, drawdowns and pumping rates will probably be small.
Large drawdowns and high pumping rates are desirable where contaminants are
5-90
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FIGURE 5-27.
DRAWDOWN VERSUS YIELD AND SPECIFIC CAPACITY
FOR A WATER TABLE AQUIFER
(JOHNSON DIVISION, UOP INC., 1975)
100
90
80
70
>-
'o
(D
a
(U
O
o
60 'o
50 |
40 J
"o
30 -
a>
S
20 i£
10
10 20 30 40 50 60 70 80
Percent of Maximum Drawdown
90 100
dispersed throughout the aquifer, where quick removal is desired and natural
groundwater flow rates are large. Close spacing of wells with large drawdowns
and high pumping rates can very quickly dewater an aquifer.
5.4.2.5 System Integration
Once the well spacing, pumping rate, and drawdown have been determined,
the system can be designed as a unit. At this point, a decision must be made
on the pattern and type (i.e., injection or extraction) of wells to be
installed. Numerous patterns of extraction wells or injection wells or both
are available. The choice is typically based on whether the design is for
containment or removal, the time available for recovery, and the amount of
dewatering that is allowable. Straight line extraction or injection patterns
would probably be used in containment, while the more complex patterns of
extraction and injection wells would be used for removal. Patterns that
combine extraction and injection wells allow for more rapid contaminant
removal without greatly affecting groundwater levels. These patterns are also
advantageous because the treated water extracted can be reinjected.
5-91
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After a well pattern is chosen, the number of wells needed to control the
plume must be determined. This is based on the estimated well spacing and the
drawdown required. In spacing wells for plume control, it is necessary to
have the well's capture zones intersect each other so that contaminants will
not flow between wells and escape. Where low natural flow velocities exist,
cone overlap may be small; where large natural flow velocities exist, cone
overlap should be greater to ensure capturing the plume.
The number of wells needed can be determined by plotting the chosen map
pattern of wells with their required spacing on the potentiometric surface map
of the site. After this is done, the drawdowns within the radii of influence
should be plotted and cumulative drawdowns determined. This will result in a
new potentiometric surface map of the site that can be used to identify dead-
spots or where contaminants can escape. Monitoring the system's effectiveness
in contaminant removal should also be performed utilizing wells. The location
of these wells can be determined from the revised potentiometric surface map
by identifying deadspots or areas where cones of depression overlap. An
example of a system design for an ideal, low natural flow velocity, water-
table aquifer (i.e., homogeneous, anisotropic, unlimited extent) is given
below and shown in Figure 5-28.
A contaminant plume originating from a landfill has been discovered and
delineated as shown in Figure 5-28. Removal of the existing plume and
containment of any new contaminants is desired. A pumping test conducted at
the site determined the hydrogeologic parameters to be:
t K = 25 gpd/ft2
• Q = 100 gpm
t H = 100 ft
• hw = 46 ft
• rw = 0.5 ft.
5-92
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Impermeable Bedrock
Figure
-------
Continuous pumping is to be used at the site because the source of contam-
inants cannot be eliminated effectively. Based on this, the radius of
influence for a pumping well is determined by:
where x = [K(H2-h 2)/458(Q)] + Inr
w w
= [25(1002-462)/458(100)] + lnO.5
= 4.996
= 150 feet (approximately).
Because pumping is to be continuous, the optimum operating conditions for the
well should be approximately 67 percent of the total drawdown. Using this
drawdown (H-h^), pump rating (Q ) and screen length can be estimated by:
(H-hwm) = °'67(H)
= 0.67(100)
= 67 feet
h - 33 feet
win 999 9
= (100/(1002-462)(1002-332)
= 113 gpm
Using the calculated information, a single 6-inch well with a radius of
influence (R ) equal to 150 feet would provide plume removal and containment
at the site (Figure 5-28). Optimum pumping conditions can be achieved with a
continuously operated pump rated for approximated 113 gallons per minute. The
length of screen installed should be less than 33 feet because drawdown will
be approximately 67 feet. The material used for the well would be determined
by the type of contaminants present.
5-94
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5.4.3 Installation and Maintenance of Wells
Well installation typically consists of four steps:
• Opening the borehole
t Installing the casing and screen (if used)
• Completing the well
• Developing the well.
The first of these steps consists of opening a borehole for the well by dis-
lodging and removing earth materials. Variations in the method used for
dislodging and removing the earth materials are the main differences in well
construction techniques. Screens and casings may be installed simultaneously
with the opening of the borehole or after the borehole has been completed.
The sequence of events depends on the technique used to open the hole and the
geologic characteristics of the materials in the immediate vicinity of the
hole. Installation of screens, filters, pumps, and grout completes the
construction of the well. Well development is the last step and consists of
dislodging and removing fines that have built up on the well screen, the
filter, and walls of the aquifer during the previous steps. Removal of these
particles maximizes well yield and prevents damage to the pump.
Well maintenance generally is performed periodically after installation
of the well to maintain yields. Maintenance programs consist of recording
performance, evaluating the need for maintenance, and treating the well to
improve yields. The three basic causes of reduced yield that require well
maintenance are pump failure, corrosion, and incrustation.
5.4.3.1 Opening Boreholes and Installing Casing
There are numerous methods for opening holes and installing casing which
can be classified according to the well depth:
• Shallow wells (<50 feet)--dug, bored, driven, and jetted wells
5-95
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t Deep wells (>50 feet)--drilled (e.g., hollow-core, hydraulic-rotary,
air-rotary), cable tooled, and jetted wells.
The method chosen depends on the geologic characteristics of the soil and
rock, the well diameter, and the hydrologic characteristics of the aquifer.
Uells can also be classified by the type of geologic materials they penetrate
(i.e., consolidated and unconsolidated). Regardless of the method chosen for
well installation, exploration techniques should be employed prior to and
during the opening of the borehole. While installation is occurring,
continuous testing to monitor changes in the soil and water being removed
should be performed so that installation techniques can be modified as
necessary. The various means of opening a borehole and installing casing and
screen are described in Table 5-26.
5.4.3.1.1 Hand Methods
Shallow wells up to six inches in diameter can be pushed into soft soils
by hand methods. Depths of these wells is generally limited to between 20 and
30 feet, but 90 foot wells have been installed in this manner. Wells
installed by hand methods are usually wellpoints attached to a riser pipe.
The maximum depth to groundwater in these wells is generally less than 20 feet
because suction pumps are utilized. Manual methods for opening a hole are
typically driving and augering. The use of augers is limited because they are
ineffective in water saturated sands and coarse silts. A combination of the
two methods is often used successfully to reach greater depths. Deeper and
larger diameter wells can be installed if heavier driving block assemblies are
utilized. These blocks can weigh 75 pounds or more and can be operated by a
block and tackle or with a drill rig.
5.4.3.1.2 Boring
Wells can either be bored with large diameter auger buckets or with
continuous flight, spiral augers. Rotary bucket augers are capable of opening
holes up to 48 inches in diameter and 90 feet deep in very soft soils.
Diameters are typically limited to about 24 inches in most materials.
5-96
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TABLE 5-26. METHODS OF WELL-INSTALLATION
Method
Basic
Hand
Boring
Jetting
Variations
Augers
Driving Points
Rotary Auger
Bucket
Spiral Auger
Self jetting
Well point/Riser
Unit
Separate Tem-
porary Jett-
ing Pipe
Separate Per-
manent
Jetting Pipe
Applications
Geologic Material Max. Well Max. Well
Dia. (in) Depth (ft)
Soft, without excess 6 20
sand and water, no
boulders
Soft soil free of 3 30
boulders
Soft soils without 48 90
excess boulders
Soft soils without 6 90
excess sand and
water, no boulders
Soft soils free 8 50
of boulders
Soft soils free 8 50
of boulders
Soft soils free 8 100
of boulders
Soft soils free 8 100
of boulders
Casing/Screen
Treatment
Driven after
opened
Driven as
hole proceeds
Driven after
hole opened
Driven after
hole opened
Drives as hole
opened
Driven as hole
opened
Placed after
hole opened
Driven as hole
opened
Comments
Wider and deeper holes
can be machine driven
Typically limited to
24-in diameter wells
or less
Can not be used below
water table, must be
used in combination
with other techniques
Jetted wells require
less development
Method uses large
quantities of water
(Continued)
-------
TABLE 5-26. (Continued)
en
i
00
Method
Basic
Jetting
(cont.)
Rotary
Drilling
Cable Tool
Variations
Rotary Hole-
puncher
Conventional
Hydraulic
Reverse
Hydraulic
Air Rotary
Ai r Rotary with
Pneumatic
Hammer
Conventional
California
Stove Pipe
Appl ications
Geologic Material Max. Well Max. Well
Dia. (in) Depth (ft)
Soft soil, sandstone, 24 200
schist
Any type
Any type, boulders 60
may be a problem
Any type 12
Any type 8
Any type
Any type
Casing/Screen
Treatment
Driven as
hole opened
Placed after
hole opened
Placed after
hole opened
Placed after
hole opened
Placed after
hole opened
Driven during
or after hole
opened
Pushed with
jacks as hole
opened
Comments
Typically used for
wells, quickest
drilling method
Requires large
quantities
of water
Very fast method
Typically used for
wells, provide abi
to closely monitor
log well
deep
deep
lity
and
Quicker and less
expensive than con-
ventional cable tool
-------
However, this type of auger can be used in soils with minor caving problems if
the boreholes are filled with light drilling mud. The mud not only prevents
caving but helps keep porous materials in the bucket. The use of drilling
muds is not recommended for use in contaminant monitoring wells because of
possible analytical water quality testing interferences.
Spiral screw augers are typically limited to diameters between 4 and
6 inches and depths less than 90 feet. They can be ineffective when used
below the water table or in sandy soils because the screw will not lift the
soil. Spiral augers are used most commonly in conjunction with other methods.
Tools used for rotary bucket augers include the bucket, drilling pipe or
rod, a kelley bar, mechanical pull downs, and boulder handling equipment. The
cylindrical bucket has cutting blades on the bottom and is attached to the
kelly bar by rods. Drill rods supply the length necessary to reach into deep
holes and can be eliminated in shallow holes. The kelly bar transfers the
circular motion from the rotary table and the downward force of the mechanical
pull-downs to the rods and the bucket. Any large boulders in the path of the
borehole must be removed using orange peel buckets, stone tongs, or ram horn
tools.
Spiral augers are less complicated and easier to use than bucket augers.
The bucket, rods, and kelly bar are replaced by a screw auger which does the
cutting. Cuttings are pushed to the surface by spiral action. Spiral augers
are attached directly to the rotary table through the drill rods.
Casing installation for augered wells can normally be done after the hole
has been opened to its entire depth. One exception is when bucket augers and
drilling mud are used in sandy soils. In this case, casing should be driven
closely behind the progress of the bucket. For applications where the
borehole is not in danger of caving, the casing can be installed after the
hole is completed. Casing is normally installed by pushing it with a drive
block similar to the ones used for driving wells. The top of the casing is
protected with a drive head and the bottom with a drive shoe.
5-99
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5.4.3.1.3 Jetting
Jetting involves opening the borehole using the force of a water jet.
Water jets typically have flow rates of 100 to 250 gallons per minute and
develop pressures of 50 to 300 pounds per square inch. The force of the water
jet not only opens the hole but also conveys the waste materials back to the
surface. Jetted wells can range in diameter from a few inches to about
24 inches and can reach a depth of up to 200 feet. Jetting can be done in
geological materials as hard as sandstone and schist, but most often is used
in soil (unconsolidated) materials. Since the equipment required for jetting
is fairly sophisticated and equipment set-up times are long, jetting is most
commonly used for installing many closely spaced wells (e.g., wellpoint
systems).
There are six basic jetting methods. Four of the jetting methods are
used for installing small diameter, shallow wells such as wellpoints. These
four methods are:
t Self jetting, permanent drop tube—The entire wellpoint is jetted down
and remains in place. The permanant drop tube eliminates the need for
temporary jetting pipes. Riser tubes are used to extend the length of
the well as jetting proceeds and to act as casings.
• Self jetting, separate drop tube—Same as the self jetting, permanant
drop tube method except that the drop tube is removed after jetting is
completed.
• Separate temporary jetting pipe—A capped pipe with cutting teeth at
the bottom is jetted down by water forced through openings in the cap.
If soil becomes difficult to remove, the pipe is rotated back and
forth to aid the jetting process. Once the well is pushed to the
final depth, the wellpoint and riser are placed in the jetting pipe
and the jetting pipe is removed.
• Separate permanent jetting pipes—Similar to the separate temporary
method except that the wellpoint with screen is packed into the
jetting pipe, the pipe is lifted to expose the screen, and the jetting
pipe then becomes the riser casing.
Deeper wells can be installed using two jetting methods; jet-percussion
and rotary holepunchers. In the jet-percussion method, a chisel bit is used
5-100
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that has jetting nozzles attached. The nozzles direct water onto the bit for
cleaning and at the bottom of the hole to loosen material. The chisel bit is
lifted and dropped with a block and tackle or motorized assembly. The
combined action of the bit and the water jet open the wellhole. Drilling
water is circulated in a closed loop to minimize water consumption. Once the
water emerges from the bit, it rises along the outside of the drilling pipe
carrying cuttings with it. Cuttings are settled out in a pit and the water is
recycled. Casing is driven as the hole progresses, and well screens are
installed through the casing. The rotary holepuncher method is similar to the
jet-percussion method .except that the bit is rotated by rotary drilling
methods instead of being lifted and dropped. Rotary drilling methods are
described in the next section.
5.4.3.1.4 Rotary Drilling
There are two basic components of rotary drilling--the drill bit and
stem, and the drilling fluid. The rotary action of the drill bit and stem
being forced down by mechanical or gravitational means opens the borehole. A
continuous flow of drilling fluid prevents the hole from collapsing and
removes the cuttings. Casings and screens are typically installed after the
borehole has been completed. Conventional rotary drilling and its variations
can be used to install wells of any diameter and depth in any geologic
material. While this is one of the fastest drilling methods, equipment set-up
time and labor costs can prohibit its use for shallow wells.
In conventional hydraulic rotary drilling, the flow of the drilling fluid
is similar to that for jet-percussion where fluid flows down the drilling rod,
out the drill bit, and up the sides of the hole. Besides removing cuttings
and keeping the hole from collapsing, the drilling fluid:
• Prevents the bit from sticking when operations are interrupted
• Seals the well hole to prevent fluid loss
• Cools and cleans the drill bit
• Lubricates bits, pumps, and pipes.
5-101
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The properties of the drilling fluid that can be controlled to ensure
satisfactory performance are density, viscosity, sand content, gel strength,
and filtration properties. The disadvantages of using a drilling fluid is
greater drill rig decontamination time and well development expense.
Reverse circulation, rotary drilling is a variation of the conventional
method that is appropriate for soft soils where water tables are greater than
10 feet below the surface. Reverse rotary is the least expensive method for
wells ranging from 18 to 60 inches in diameter. The drilling fluid is more of
a water than a mud in this method and fluid losses can be as great as 500
gallons per minute. Because of this, an ample water supply must be available
and a settling pit of three times the final borehole volume should be
designed. The major difficulties with the method are high fluid loss and
caving when drilling in loose, permeable soils.
Another variation of conventional rotary methods is the air rotary
method. The drilling fluid in this case is high pressure air rather than a
water-based fluid. Air rotary methods are good for consolidated rock drilling
and will allow faster penetrations and longer bit life as long as water
infiltration into the hole is small. Typical hole diameters are 12 inches or
less but larger bits are available. This method is frequently used in
combination with conventional hydraulic rotary methods to complete a single
hole.
A variation of the air rotary method is air rotary with pneumatic
hammers. This method may be the quickest when drilling through very hard
geologic formations. Well diameters with this method are typically 8 inches
or less. The pneumatic hammer requires air pressures greater than 300 pounds
per square inch when drilling larger holes. Both air rotary methods have the
advantage of being able to observe water inflows into the well as drilling
progresses.
5-102
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5.4.3.1.5 Cable-Tool
The cable-tool method (also known as the "percussion," "solid-tool,"
"standard," and "churn" method) opens a hole by lifting and dropping a heavy
string of tools which loosen and crush the geologic material. The resulting
particles or cuttings are mixed with water from the well or water pumped into
the well and the resulting slurry is removed with a bailer. When drilling in
soft soils, the casing closely follows the bit to prevent caving. In hard
rock formations the casing can be installed after the hole has been completed.
The cable tool drilling method is typically used for deep well installation,
is relatively slow (5 to 10 ft/day in solid rock, 50 to 100 ft/day in soft
soil), and is high in labor costs. Conventional cable-tool methods and a
single variation are discussed below.
Conventional cable-tool equipment consists of a bit, a stem for length
and weight, jars to loosen stuck stem and bits, and a rope socket which
connects the entire string to the drill cable. The up and down motion of the
tools results from the motorized lever called the walking beam or the spudder
(i.e., winch). The bailer is on a separate cable called the sandline. The
bailer is comprised of a pipe with a check valve that opens as the pipe enters
the hole and closes as it is lifted, thereby removing cuttings.
One variation on the standard method is the California Stovepipe. The
major differences are:
• A mud scow acts as both bit and bailer
• A thin pipe within a pipe ("stovepipe") is used as a casing as opposed
to standard line pipe
• The casing is pushed with jacks rather than hammered with tools.
A unique feature of the casing is that it is too weak to pull back out of the
hole to expose the well screen. Therefore, perforations are blasted into the
bottom of the casing to allow water flow and act as a screen.
5-103
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5.4.3.2 Well Completion
Once the borehole has been opened installing a screen, filter pack, and
grout is necessary to complete the well prior to well development. The order
that these installations occur is dependent on whether the borehole is over-
sized and requires a filter pack or not. The majority of the wells that are
drilled for contaminant removal will require filter packs.
5.4.3.2.1 Filter Pack Wells
The method typically utilized for completing a well with an artificial
filter is the double-casing method. In this method, a string of outside
casing, corresponding to the size of the outside diameter of the filter pack
(i.e., the borehole), is installed as the hole is drilled or after it is
completely opened. A second string of casing containing the well screen is
then centered within the outer casing. The selected filter material is then
placed between the inner and outer casings. After placement of a few feet of
filter material, the outer casing is pulled back an equal amount and the
procedures is repeated until the filter material is brought to the desired
level above the screen.
The outer casing may be removed completely or left in place above the
level of the screen. In either case, the top of the annular space above the
filter is sealed with grout (e.g., cement, clay). If the outer casing is to
be permanent, the inner casing above the screen may be removed as long as the
two overlap a few feet. The top of the inner casing should be sealed using a
lead slip packer. The annular space left between the outer casing and the
aquifer should also be sealed with grout. Pumps are then installed into the
inner casing and the well is developed.
Jetted wellpoints are completed in a similar manner as a drilled well.
In this case, filter sands are packed around the wellpoint and grout is
installed from the top of the filter to the surface. The grout prevents
surface water infiltration into the well and minimizes the chances of air
entering the wellpoint.
5-104
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5.4.3.2.2 Naturally Developed Wells
The method used for completing a naturally developed well is the opposite
of the method for filter pack wells. Grouting and sealing, if necessary,
occur first followed by installation of the well screen. The two basic
approaches to grouting are:
• Slurry placement method in which grout is pumped or gravity fed into
the annular space
0 Casing method in which well casing is used to keep the borehole open
while installing grout.
Regardless of the method used, the grout must always be placed from the bottom
of the well up. If a temporary outside casing has been installed during
drilling, the casing must be removed while the grout is still fluid. This
allows for a good seal between the borehole walls and the grout. Once the
grout has set, the plug can be drilled out and the well screen installed.
Numerous methods are available for installing well screens depending on
the type of well screen used, the drilling equipment, the geologic material,
and the presence of grout. The types of well screen installation methods are:
• Pull back method—Casing is sunk to the full depth of the borehole,
the well screen is lowered inside the casing and the casing is pulled
back to expose the screen. The casing can not be grouted.
• Setting in open hole—The casing is grouted in place, a hole is
drilled through the grout to below the level of the casing and the
well screen is installed inside the casing in the drilled hole.
t Bail-down procedure—The well screen is fitted with a bail down shoe
attached to a bailing pipe, the assembly is sunk into the formation
below the well casing by operating the bailer and drilling tools
through the screen, the screen moves into the hole formed by dis-
placing the soil, and a packer at the top of the screen is expanded
after the pipe is removed. This method is good only in soft, sandy
soils.
• Wash-down method—The well screen is installed using the same method
as for jetting techniques and a packer is used to seal the screen and
the casing.
5-105
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Once the well screen is installed the pump can be placed within the casing.
Well development can then take place to ensure adequate yield.
5.4.3.3 Well Development
Well development is the process where fine soil materials are removed
from in and around the screen allowing water to flow freely. This process is
accomplished by surging water or air through the well screen and into or out
of the surrounding material. The well development process:
• Removes materials that have built-up in the openings of the screen
during the well drilling and installation processes
t Removes fines from the sides of the borehole that resulted from the
drilling procedure, e.g., drilling mud
• Increases the hydraulic conductivity of adjacent geologic materials
and the filter pack by removing fine materials
• Stabilizes the fine materials that remain in the vicinity of the well
and retards their movement into the well.
The benefits of well development are increased yields, reduced pumping of
fines which can damage pumps, and decreased corrosion and encrustation. The
results of the development process is a layer of coarse particles adjacent to
the screen. The percentage of finer particles increases with distance away
from the well. Well development is necessary in any well because clogging can
occur regardless of the drilling method used or the formation being
penetrated.
Wells can be developed by natural groundwater flow or by artificial
means. Prior to developing a well, the well should be bailed to remove sand
that has accumulated during construction. Bailing also ensures that water
will flow into the well. Development procedures can last from about two hours
to two days depending on well depth and formation properties.
Natural development of wells is accomplished by alternately pumping and
allowing the well to recover. This method does not cause much of a surging
5-106
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action and is relatively ineffective in alleviating particle bridging and in
removing fines. There are seven additional methods that can be used in well
development. They are:
t Overpumping—This method develops the aquifers by pumping at high
rates and assumes that the system will be stable at normal pumping
rates. When the pumps are stopped, backwash helps overcome bridging.
While this method is the simplest and quickest, it is also the least
effective.
• Rawhiding--This method involves pumping intermittently to lift water
out of the well and then adding water back into the well causing a
surging action. Rawhiding is limited to wells without check valves
that prevent backwashing. The method is not normally recommended as a
primary development method but is used commonly as a finishing method.
• Recirculation—In this method, water is pumped from a high point in
the well and discharged at a low point in the well screen. This
results in a mild turbulence at the screen which frees fines. It is
not generally used as a sole well development method.
• Surge blocking—In this method, a surge block or surge plunger is
pushed in and pulled out of the well in a plunger-like fashion. The
plunger can be solid or valved. Valved plungers allow action on the
downstroke and strong action on the upstroke, but care must be taken
on the upstroke because screens can be collapsed. This method is the
most common and highly effective method of development.
• Hydraulic jetting—In hydraulic jetting, a device with two or more
nozzles is pushed to the well screen and water is jetted through the
screen openings. The water reverses direction returning through the
screen and carries fines into the well for removal. Pumping the well
while jetting helps flow reversal and fine removal, and provides water
for jetting.
• Compressed air surging—This method is a combination of air jetting
and pumping with water. Air is gently pumped into the well and water
is pumped out of the well to start the circulation. The airline is
then closed off and pushed down into the screen area while air
pressures build to 100 to 150 pounds per square inch. An air blast is
then released surging through the system. This cycle is repeated
several times.
• Sonic development—In this method, high frequency sound waves are sent
through the well screen and the surrounding formation where they
vibrate and loosen particles. Because the waves effectively penetrate
beyond the well, this method is very attractive for filter packed
wells.
5-107
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The methods previously discussed can be aided with the use of dispersing
agents and acids. Dispersing agents (such as polyphosphates) act as
deflocculants and disperse clay. Mild acids can be used to dissolve limestone
and open crevices. In rock formations, blasting has been used to enhance well
yields. However, blasting is not a recommended procedure for most hazardous
waste release sites.
5.4.3.4 Well Maintenance
Pumps, casing, and screens must be maintained to ensure a constant
reliable flow of water from the well. Proper well maintenance is especially
important in plume management because the loss of a well could result in
contaminant escape. The causes of well yield loss and failure are:
• Encrustation—The build-up of scale or other chemical or biological
material
t Corrosion—A breakdown of well hardware by turbulent flow, chemical
reactions, galvanic reactions, or fatigue stress
t Pump failure—Typically caused by sand intrusion, wear, or mechanical
or electrical failure.
Prior to beginning any well maintenance, a preliminary evaluation must be
conducted to identify whether or not the problem can be corrected.
Operational records will determine the normal operating conditions of the well
and aid in evaluating the problem. Removing the pumps and checking the casing
and screens may be necessary. This can also be accomplished by downhole video
equipment.
General maintenance procedures that can be used for encrustation,
corrosion, and related pump problems include:
• Chemical treatment of casing and screens
0 Redevelopment of the well
• Chemical treatment of pumps, and mechanical and electrical
maintenance.
5-108
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Chemical treatments are designed to dissolve encrustation and corrosion that
has formed or have been deposited on the well components. Three classes of
chemicals are generally used; acids, biocides, and phosphates. Acids are used
to dissolve inorganic substances that have formed, such as calcium, magnesium,
and iron. Biocides are used to remove organic materials that may have devel-
oped such as iron bacteria. The effectiveness of both of these treatments is
directly related to retention times on the affected components. Additives are
available to aid in the treatment including:
• Inhibitors—Allow for the selective removal of some substances
• Chelates--Keep metals in solution to prevent redeposition
• Wetting agents—Reduce surface tension allowing chemicals to penetrate
smaller openings.
Phosphates or surfactants act as dispersing agents which help break-up clays,
colloids, and some metals. The effectiveness of surfactants depends only on
initial contact; prolonged contact does not increase their effectiveness.
whenever surfactants are used (especially phosphates), chlorine should be used
to prevent bacteria and algae growths.
The best and most common application method used in chemical treatment is
the double surge block. This method allows the application of concentrated
chemicals directly to the required areas. If chemicals are necessary to clean
the surrounding aquifer they can be injected through the screen using
compressed air or surge blocking techniques.
Mechanical redevelopment of the well can also be used to increase yields
and remove encrustation. Redevelopment should always follow chemical
treatments. The seven development methods described previously can be used.
5.5 Costs of Well Systems
Costs of well systems for plume management can vary greatly from site to
site. Some of the factors that detemine these costs are the geology, the
5-109
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characteristics of the contaminant and naturally occurring groundwater, the
extent of contamination, the periods and durations of pumping, local wage
rates, the availability of supplies and equipment, and the electrical power
required. Costs associated with a well system can be grouped into the
following categories (Powers, 1981):
• Mobilization costs
• Installation and removal costs
• Operation and maintenance costs.
Mobilization costs include all costs incurred in obtaining equipment and
having it available at the site. Some of the items included in mobilization
costs are (Powers, 1981):
• Well components—including well screens, casing, well pumps, motors,
controls, discharge columns, well heads, fittings, collector pipes,
and power lines (Figure 5-29)
• Installation equipment—including drilling rigs, jetting equipment,
and well development equipment
• Pumping equipment—including pumps, contacts, hoses, and cables
• Standby equipment—including generators, switches, pipes, and cables
• Equipment rental and repair
• Delivery and handling charges for equipment
• Utility installation
• Enclosures for storing equipment
t Engineering and geotechnical services—including the design of the
system, submittal preparation, field testing, and on-site supervision
during installation
• Waste, water, and soil treatment—including transport, treatment, and
disposal
t Decontamination of drill rigs and tools
• Health and safety precautions.
5-110
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FIGURE 5-29.
TYPICAL WELL SYSTEM COMPONENTS
(POWERS, 1981)
Discharge Line
Discharge Collector
Standby
Generator
High Line
Transformer
Switchgear
Discharge
Collector
Discharge
Column '
Wellhead and
Fittings
Pump-
Deep Well (typical)
Casing
Screen
Installation and removal costs include the costs for crews and equipment
necessary to install the well system at the site. These costs should also
include allowances for set-up, clean-up, weather, and other miscellaneous
delays that typically occur. Removal costs will probably be incurred at all
sites at some point when pumping is no longer required. Removal costs can be
off-set somewhat by salvage costs of the removed equipment. However, decon-
tamination of the equipment may be more costly than salvage value.
Operation and maintenance costs are typically high for pumping systems.
In some cases, these costs can be greater than the initial installation and
mobilization costs. The following items should be considered in preparing
cost estimates (Powers, 1981):
t Operating labor—continuous or intermittent manning of equipment
5-111
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• Maintenance labor--includes servicing for pumps, well cleaning, and
maintaining engines and electric equipment
• Supervision
• Energy—includes fuel and electricity
• Maintenance materials--includes materials for surface components and
for well rehabilitation
t Repair and overhaul of equipment.
The above list of cost items is not all inclusive; some sites will
require additional items. If long term operations are expected, operation and
maintenance, and removal costs should include escalating factors. Tables 5-27
through 5-29 give some typical costs that may be incurred for the items
mentioned above. However, because the costs of many of these items will vary
with site location, specific estimates from local contractors and suppliers
should be obtained.
5-112
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TABLE 5-27. RANGE OF COSTS FOR SELECTED PUMPS AND ACCESSORIES
(l/ STA-RITE INDUSTRIES, INC.)
Pump/Accessory
Description
Cost Range (1981 $)
Jet Pump '
- shallow well
- deep well
- jets and valves
- seals
- foot valves
- air volume controls
Submersible Pumps
- 4 in. pump
- control boxes
- magnetic starters
- check valves
- well seals
Vacuum Pumps
- diesel motors
- electric motor
pumping depths 25 ft
horsepowers 1/3 to 1-1/2 HP
capacities 60 to 27000 gph
pumping to depths 320 ft
horsepowers 1/3 to 2 HP
capacities 60 to 1000 gph
single pipe jets
double pipe jets
single or double pipe
pumping depths 900 ft
horsepowers 1/3 to 3 HP
capacities 50 to 2000 gph
800 - 7000 gpm (48,000 —
420,000 gph)
800 - 7000 gpm (48,000 —
420,000 gph)
$220 to $430
$220 to $570
$40 to $80
$30 to $50
$15 to $25
$10 to $40
$10.00
$350 to $1400
$60 to $120
$160 to $250
$14 to $330
$15 to $100
$13,000 to $40,000
$9,000 to $31,000
5-113
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TABLE 5-28. TYPICAL RANGE OF COSTS FOR WELLSCREENS AND WELLPOINTS
(Johnson Division, UOP Inc., and Gator Plastics Inc.)
Type
Description
Costs (1981 $)
Drive Well points
Well screens
Jetting Screens
(fittings)
Bail down Shoe
(fittings)
stainless steel
1-1/4 to 2-in ID
low carbon steel
1-1/4 to 2-in ID
PVC plastic
1-1/4 to 2-in ID
stainless steel
1-1/4 to 36-in ID
low carbon steel
1-1/4 to 36-in ID
PVC plastic
1-1/4 to 12-in ID
cast i ron or mild steel
2 to 12-in ID
mild steel
4 to 12-in ID
$28.00 to $40.00/ft
$15.00 to $35.00/ft
$5.00 to $6.00/ft
$28.00 to $610.00
$15.00 to $170.00/ft
$10.00 to $60.00/ft
$30.00 to $270.00
$180.00 to $800.00
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TABLE 5-29. AVERAGE DRILLING COSTS (1981) FOR UNCONSOLIDATED MATERIALS
(Stang Drilling and Exploration)
Drilling Technique
Drilling Costs ' Average Production Drilling Cost
($/hr) Rates (ft/hr) ($/ft)
Conventional Hydraulic
Rotary
Reverse Hydraulic
Rotary
Air Rot ray '
Air with Pneumatic
Hammer
Auger
Bucket Auger
Cable-Tool
Hole Punchec,
(jetting)*7
Self jetting2/
$100/hr
$220/hr
$175/hr
$180/hr
$90/hr
$100 to $140/hr
$60-$70/hr
40 ft/hr
40 ft/hr
50-60 ft/hr
40-50 ft/hr
20-40 ft/hr
50 ft/hr
4 ft/hr
$2.50/ft
$5.50/ft
$3.50 -
$4.50 -
$4.50 -
$2.00 -
$15.00 -
$35/ft
$18/ft
$3.00/ft
$3.60/ft
$2.50/ft
$2.80/ft
$17.50/ft
'consolidated material
2 /
includes rental of all necessary equipment; e.g., wellpoints, pumps and
headers
'drilling costs approximately equal mobilization costs
5-115
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CHAPTER 6
SUBSURFACE DRAINS
6.1 Introduction
Subsurface drains include any type of buried conduit used to collect
liquid discharges (e.g., contaminated groundwater) by gravity flow. The key
components of a typical subsurface drainage system are:
t Drain pipe or gravel bed—for conveying flow to a storage tank or wet
well
• Envelope—for conveying flow from the aquifer to the drain pipe or bed
• Filter—for preventing fine particles from clogging the system
i Backfill—to bring the drain to grade and prevent ponding
• Manholes or wet wells—to collect flow and pump the discharge to a
treatment plant.
The theory of groundwater flow and the effects of various aquifer proper-
ties on flow are similar for drains and wells. Well theory is discussed in
Chapter 5. Specific differences and similarities between flow toward drains
and wells are described in Section 6.2.
Section 6.3 discusses site specific variables which determine the
applicability and performance of subsurface drains. Soil properties, leachate
characteristics, size and flow rate of the plume, aquifer properties, and site
geology are considered relative to the design and performance of subsurface
drains. The design considerations discussed in this section include drain
depth and spacing, hydraulic design of pipes (pipe diameter and gradient, and
flow velocity), and envelope and filter materials. This section also includes
6-1
-------
a discussion of design criteria for a sump and pumping station for collection
and transfer of leachate to a treatment system.
Equipment, methods, and unit costs for the construction, installation
and maintenance of subsurface drainage systems are described in Section 6.4.
Construction and installation can be divided into two major activities:
(1) excavation of the trench to the required depth and gradient, and
(2) installation of drain materials. The discussion of trench excavation in
this section includes methods and equipment for trench excavation, wall
stabilization, dewatering, and grade control. Drain installation includes
materials and procedures for installing bedding, drain pipes, envelopes, and
filter material. This subsection also includes procedures for backfilling the
trench. Unit costs are provided throughout this section for equipment,
methods, and procedures. The section also provides guidance on the selection
of equipment and methods based on depth requirements, subsurface geology, and
soils.
Section 6.4.3 addresses procedures and methods for maintenance and
inspection of subsurface drains. This section includes a discussion of
problems which might arise during construction and operation of the system.
Inspection, preventative maintenance, and corrective action procedures to
ensure against system failure are also discussed. Potential problems which
can impact the performance of drains include clogging of envelopes and filters
by siltation; chemical build-up or incompatibility; clogging as a result of
sediment build-up in manholes; root clogging of tile drains; and development
of sinkholes.
Subsurface drainage can be used to control leachate plume migration for a
wide range of site conditions. They essentially function like an infinite
line of extraction wells. That is, they create a continuous zone of
depression in which groundwater within this zone of influence flows toward the
drain. Accordingly, subsurface drains can perform many of the same functions
as pumping technologies. The decision to use one or the other should be based
upon a cost-effective analysis. However, drains may be more cost-effective
than pumping at sites with substrata of low or variable hydraulic conductivity
6-2
-------
and where pumping systems cannot provide a continuous hydrologic barrier.
They are limited to relatively shallow applications.
Subsurface drains can be used to contain or remove a leachate plume or to
lower the groundwater table near a disposal site in order to prevent contact
of water with waste materials. Containing or removing a plume located
hydraulically downgradient of a site typically requires less extensive
drainage than lowering the groundwater table. However, containment or removal
generally requires deeper drains than systems used to lower the groundwater
table in order to ensure that the entire plume is intercepted. This can be
seen by comparing Figures 6-1 and 6-2.
Subsurface drains can be applied with liners or other barrier materials
to restrict inflow to one side of the drain. A typical situation where this
technique would be particularly applicable is when a surface water body
located near a subsurface drain was contributing a substantial flow of clean
water into the drainage system. The use of conventional subsurface drainage
would result in large flow capacity requirements for the drain and high costs
for treating the additional water. One sided drainage could be used primarily
to collect the contaminated portion of groundwater and cutoff recharge flow
from the surface water body. The use of one sided subsurface drainage
requires that the system be keyed into a low permeability formation so that
groundwater does not travel under the barrier material (Figure 6-3). One
sided subsurface drainage may be applicable to many other conditions where the
main objective is to restrict the flow of clean water, and thus, minimize
water treatment requirements.
Subsurface drains are also used in conjunction with groundwater cutoff
barriers to prevent the buildup of groundwater upgradient of the barrier. In
this application, they are not the main control technique, but rather, serve
an ancillary function. When used with barriers, subsurface drains may be
designed to handle the reduced flow conditions of a totally encapsulated site
(Figure 6-4) or to limit the up-gradient head increase preventing overflow and
minimizing differential hydraulic pressures on the barrier (Figure 6-5).
6-3
-------
FIGURE 6-1.
THE USE OF SUBSURFACE DRAINAGE TO CONTAIN A LEACHATE PLUME
Waste Disposal
Site
• Groundwater Flow
Direction
•'Contaminated
' Groundwater
Plume
, Subsurface Drainage
-ST'
Collected groundwater
. is pumped to treatment
r**~\/ system
Cross Section
Waste Disposal
Site
Contaminated
Groundwater
Plume
V
V
* ~^>-
/ ^j
J
•^
V
*
<»
>>
>
M
, Original Water
x^^- .3
6-4
-------
FIGURE 6-2.
THE USE OF SUBSURFACE DRAINAGE TO LOWER GROUNDWATER LEVELS
Map View
-Waste Disposal Site
• Subsurface Drain
Collected Groundwater
Pumped to Receiving
Stream
Cross Section
Waste Disposal Site
Subsurface
Dram
Original Water Table
Lowered Groundwater Table
Under Disposal Site
6-5
-------
\
wD
-------
FIGURE 6-4.
THE USE OF SUBSURFACE DRAINAGE IN A COMPLETELY ENCAPSULATED SITE
Backfill
Clay Cap
Barrier Wall
Subsurface Dram
FIGURE 6-5.
THE USE OF SUBSURFACE DRAINAGE TO PREVENT
OVERFLOW AND PONDING
Groundwater would
overflow or seep to
surface without the
use of subsurface drainage
6-7
-------
There are a number of limitations to the use of subsurface drains as a
remedial technique. For example, an aquifer with a high hydraulic
conductivity and high flow rate may preclude the use of a subsurface drain
because of the requirement for a large system capacity. However, use of a
barrier containment system together with subsurface drains, to reduce flow,
may be effective under conditions of high flow. Also, contamination at great
depths may cause construction costs to be prohibitive, particularly if a
substantial amount of hard rock would have to be excavated to install the
drain.
Other limitations to the use of drains may include the presence of
viscous or reactive chemicals in the contaminant plume which could clog the
drain. Also, high concentrations of iron and manganese in the plume or in the
groundwater could cause the eventual clogging of the drain system because of
the buildup of insoluble compounds. Drains are fairly difficult and costly to
rehabilitate.
6.2 Theory
The principle of flow toward a subsurface drainage trench or pipe is
similar to that discussed in Chapter 5 for flow toward a well. However,
instead of the cone of depression which is observed around a pumping well, a
water table trough develops which runs the length of the drainage trench.
This difference is illustrated in Figure 6-6.
The similarities and differences in flow between a well and a drain can
be better appreciated from the theoretical expressions for flow. Flow in an
unconfined aquifer, for example, can be most simply described by the following
formulas:
(1) Flow toward a drain (two sided flow)--
Q = xK (H2-hd2)/L
(2) Flow toward a well--
Q =TT K(H2-hw2)/(ln RQ/rw)
6-8
-------
FIGURE 6-6.
DIFFERENCES BETWEEN GROUNDWATER FLOW TOWARD DRAINS VERSUS WELLS
Flow
f
rrm
"TTTT
nrrrr
rrrn
t '. '. *. '
i
j
Tnr.
i
TTTT
(
rTTTT
TTTTT
TTTT
TTTT
TTTT
.
~~^-jL
~~f\\/'
nn
yJJ
h-\^/^
t—^\
X
Flow to a Perforated Drain Pipe
Flow to a Slotted Well Screen
6-9
-------
where:
Q = discharge rate (ft /sec)
K = hydraulic conductivity (ft/sec)
H = total head at aquifer prior to pumping (ft)
hj = desired drawdown (ft)
h = height of water in well after pumping (ft)
R = radius of influence at well (ft)
rw = radius at well, including gravel pack (ft)
L = influence of drain (ft)
x = unit length of pipe (ft)
2 2
Figure 6-7 illustrates how h. in the equation Q = xK(H -h, )/L and h in the
•o o Q Q W
equation Q = TT K(H -HW )/(ln RO/TW) are similar and how in both cases H-h , or
H-h is equal to the drawdown.
Whereas flow from a given point to a well is inversely proportional to
the radius of influence divided by well radius (R./r ), flow to a drain is
o w
inversely proportional to the influence of the drain (L) only. That is, the
diameter of the drain does not significantly affect flow as does the radius of
the well. This is because the diameter of the drain is only a matter of
inches whereas the length of the flowlines are many feet. Therefore total
frictional resistance to groundwater flow through the entire system caused by
the pipe is usually ignored.
Flow to drains in a confined aquifer is also similar to flow to wells,
and can be described by the following formulas:
(1) Flow toward a drain--
Q = 2Km (H-hd)/L
(2) Flow toward a well —
Q = 27rKm(H-hw)/(ln RQ/rw)
where m is the thickness of the confined aquifer. Figure 6-7 illustrates the
similarities between flow to drains and wells under confined and unconfined
conditions.
6-10
-------
FIGURE 6-7.
RELATIONSHIP BETWEEN DRAWDOWN (H-h) IN A DRAIN AND A WELL
(AFTER POWERS, 1981)
Drains
Wells
Confined
Unconfined
\VOsV\\\V\\
Confined Flow from a
Line Source to a
Drainage Trench
T
hd
-H
Water Table Flow
from a Line Source
to a Drainage Trench
H
-T
Radial Flow,
Confined Aquifer
Radial Flow,
Water Table Aquifer
6-11
-------
6.2.1 Drainage System Terminology
Functionally, there are two types of subsurface drains which can be used
for collecting leachate--interceptor drains and relief drains. Interceptor
drains are installed perpendicular to groundwater flow and are used to collect
groundwater from an up-gradient source. Interceptor drains can be used to
prevent contaminated leachate or groundwater from reaching wells or surface
waters hydraulically downgradient from the site. Relief drains are installed
in areas where the water table is relatively flat. Relief drains can be used
to lower the water table beneath a waste site and are usually installed around
the perimeter of the site or parallel on either side of the site such that
their areas of influence overlap. Figure 6-8 illustrates the function of
relief drains and interceptor drains in altering the configuration of the
groundwater table. Interceptor drains can also be used where infiltration
from a landfill or a surface impoundment results in a groundwater mound which
induces lateral flow at the edges of the mound. Figure 6-9 illustrates this
situation. In general, interceptor drains are more widely used than relief
drains.
The major difference in the way these two types of drains function is
that the drawdown created by interceptor drains is proportional to the
hydraulic gradient, whereas the drawdown created by relief drains is a
function of the hydraulic conductivity and depth to the impermeable barrier
below the drain. Hydraulic gradient is the key consideration in choosing
which type of drain system to install. Relief drains are used if the
hydraulic gradient is slight (about three percent or less), whereas inter-
ceptor drains are used if the gradient is steep. The rationale for the way
these drains function is discussed in Section 6.2.2.
The components of a relief or interceptor drainage system are classified
by their size and function as laterals, collectors, or mains. Laterals are
the smallest diameter component of drainage systems and collect groundwater
directly. Collectors are larger than laterals and receive water from one or
more lateral drains. Mains are the largest of the three components and
6-12
-------
X?
r»-
*A
'*
«sr
-------
FIGURE 6-9.
THE USE OF INTERCEPTOR DRAINS TO COLLECT FLOW INDUCED BY
GROUNDWATER MOUNDING
Local
Groundwater
Flow
Impoundment
Location for Drains
receive water from one or more collector drains. These components are
illustrated in Figure 6-10.
Subsurface drains can be designed to use either pipe or a gravel layer to
transmit flow. Drains that use clay or cement tiles or plastic pipes are
called tile or pipe drains. Drains that use a high permeable bed only are
called french or gravel drains. Pipe drains are somewhat more difficult and
expensive to install than french drains but are less susceptible to clogging
in most instances. While both pipe and french drains have been used for waste
site remediation, pipe drains are much more common.
In summary, drainage systems are classified by function (i.e., as relief
or interceptor systems), by their components (i.e., as pipe or gravel drainage
systems), and by their configuration (i.e., as a single component or singular
system, or a multi-component or composite systems). In a singular system,
each lateral drains into its own sump or collection tank. In a composite
6-14
-------
-------
system, each lateral drains into a collector, which may in turn drain into
either a main or a collection tank. Selection of a singular or a composite
system design depends upon the size of the leachate plume, the hydraulic
conductivity of the subsurface strata, the hydraulic gradient, and other
factors.
When a drainage system is being designed, the following four elements
must be determined:
• Drain depth (i.e., depth of water table lowering)
0 Parallel drain spacing
• Drain pipe diameter and gradient
t Envelope and filler materials and design.
Drainage system design generally requires using a combination of theoretical
design considerations, field test results, and practical experience.
Theoretical equations not involving the use of computer programs or
electric analogues are based on various idealized conditions. For example,
some equations are based on the assumption of steady state conditions (i.e.,
where the hydraulic head does not vary with time). Others assume unsteady
state conditions (i.e., a fluctuating water table). Steady state conditions
require that recharge rates equal the drain discharge rates so that the water
table remains relatively constant. Non-steady state conditions take into
account some fluctuation of the water table with time under the influence of
non-steady recharge. Since solutions to drainage problems using the steady
state assumption are simpler to derive, most systems are designed based on
these conditions. Steady state assumptions are generally valid when
considering average discharges over extended periods of time. However, in
areas subject to periodic, high-intensity rainfalls followed by long dry
periods, the use of steady state equations can result in over design (i.e.,
larger diameter pipes or narrower drain spacing than is actually necessary).
Another idealized condition often assumed in drainage formulas is that the
earth materials are homogeneous and isotropic. Such conditions rarely exist,
but the use of average hydraulic conductivities generally results in
6-16
-------
reasonable estimates for drain design. Experience has shown however that the
efficiency of subsurface drains is generally less than predicted by the
theoretical equations. As a result, systems are often overdesigned on purpose
by placing drains closer together and by using larger drain pipe than
estimated from the equations.
6.2.2 Depth and Spacing
This section discusses both theoretical approaches and field methods for
determining the depth and spacing of relief and interceptor drains. In many
instances where interceptor drains are used, a single drain is adequate and
spacing is not a consideration.
6.2.2.1 Relief Drains
When two parallel drain lines are installed, each one exerts an influence
(L) on the water table (see Figure 6-7). If the drains are designed properly
such that their influences intersect each other, the drawdown each exerts is
additive, resulting in an increased total drawdown of the water table. This
effect is greatest midway between the two drains. For a site where the drain
will not rest on an impervious clay layer, the zone of influence and the depth
are interdependent design variables, which depend upon hydraulic conductivity,
depth to an impervious layer, and the discharge rate. If the drains could be
placed deeper, the effective drawdown would increase, assuming homogeneous
earth materials, or the drains could be placed further apart while maintaining
a constant effective drawdown. If the drains can not be placed deeper, for
example because of shallow bedrock, the effective drawdown could be increased
by spacing the drains closer together. The relationship between depth and
spacing is critical to the design of effective parallel drainage systems.
In designing a subsurface drain system for a hazardous waste site, it
must be determined whether two parallel lateral drains on either side of the
site will be able to effectively lower the groundwater beneath a site. The
importance of this is illustrated in Figures 6-lla through 6-llc. If the
drains were designed with the depth and spacing shown in Figure 6-lla, the
6-17
-------
FIGURE 6-11.
THE RELATIONSHIP OF DRAIN DEPTH AND SPACING TO WATER TABLE DRAWDOWN
Original
Water
Table
New Water
Table
(b)
(c)
6-18
-------
zone of influence of the drains would not be adequate to dewater the waste
pile. By placing the drains closer together as illustrated in Figure 6-llb,
their combined drawdown would be below the level of the waste pile so that it
would effectively dewater the pile. However, a minimum spacing is often
imposed by the boundaries of the waste because excavation through the waste
material can be extremely hazardous.
If a drainage system has been designed so that the drains are as closely
spaced as possible and the plume is not totally intercepted, there are two
possible alternatives to improve the design. First, the drains may be placed
deeper (Figure 6-llc), provided the depth to bedrock and hydraulic
conductivity of soil layers is adequate. Alternatively, the system can be
redesigned as a composite drainage system. The choice between these two
alternatives depends on the depth to an impermeable barrier, the boundaries of
the plume and waste site, and the cost.
As mentioned previously, drain spacing is also influenced by the
hydraulic conductivity of the earth material in which it is installed and the
depth to the impervious barrier. In theory, the deeper the impervious barrier
below the drain the greater the thickness of the water transmitting layer, and
consequently, the wider the drain spacing can be to achieve the same drawdown.
This phenomenon can be explained by the fact that there is radial flow (i.e.,
flow from all sides) to the drain from underlying soils, and the extent of
this flow is dependent upon the distance beneath the drain to the impervious
layer. The effect of the depth to the impermeable barrier on drain spacing is
illustrated in Figure 6-12. Determining this depth as part of field testing
is essential.
The effect of hydraulic conductivity on drain spacing and the importance
of placing drains in a permeable layer can be appreciated by the example shown
in Figure 6-13. If drains are laid in the layer of lower hydraulic conduc-
tivity (K^ = 1.64 feet/day) a drain spacing of only 591 feet could be used.
If, on the other hand, the drains are placed in the uppermost part of a sandy
aquifer having a hydraulic conductivity of 32.8 feet/day, the drains may be
6-19
-------
FIGURE 6-12.
THE EFFECT OF DEPTH TO A LOW PERMEABILITY BARRIER ON DRAIN SPACING
(DE RIDDER AND VAN AART, 1974)
. L - 164 0 ft .
Low Permeability -
|V.V."-'.'..'.V.1
[.'••y>:V:::.::| K = 1 64 ft /day
-L = 311 7 ft
203ft
Low Permeability ~
-H
Low Permeabilitv i-~
6-20
-------
FIGURE 6-13.
THE EFFECT OF HYDRAULIC CONDUCTIVITY DRAIN SPACING
(DE RIDDER AND VAN AART. 1974)
" Low Permeability '
spaced 2034 feet apart (DeRidder and Van Aart, 1974). This example illus-
trates the importance of investigating both the layer at and below the
envisioned drain depth.
Drain spacing can be determined based either on field test data using
experimental plots or on theoretical design formulas. Numerous equations and
models have been developed for determining drain spacing for various idealized
conditions. Drain spacing equations for relief drains generally relate the
following factors:
• Precipitation and other sources of recharge
• Evaporation
t Hydraulic conductivity
• Depth to the impermeable barrier
t Cross-sectional area of the drain
• Water level in the drain.
6-21
-------
In areas with a large natural flow velocity, spacing the drains so that
their zones of influence just overlap may not be sufficient. High flow
velocity is not accounted for by these equations and it may be necessary to
place the drains closer together than predicted by theoretical equations to
intercept the entire plume.
6.2.2.1.1 Flow to Drains Reaching an "Impermeable" Barrier
Drains are often used where the depth to a low permeability barrier is
relatively shallow and -the drains can be laid just above the barrier. In
developing and using drain spacing formulas for this case, an underlying soil
layer is considered to be "impermeable" if the hydraulic conductivity is less
than one tenth that of the above soil layer (Wesseling, 1973).
An equation for flow to parallel ditches resting on an impermeable
barrier was developed by Donnan (1946) and described by Wesseling (1973). The
flow conditions illustrated in Figure 6-14 can be described by the equation:
q = (8KDH + 4KH2)/L2
where: q = drain discharge rate per unit surface area (ft/sec)
K = hydraulic conductivity (ft/sec)
H = height of the water table above the water level in the drain
midway between two drains (ft)
D = distance between the water level in the drain and the
"impermeable" layer (ft)
L = drain spacing (ft).
When D is very small compared to H, as it is in the case of pipe drains
resting on an impermeable barrier, the formula can be simplifed to:
q = 4KH2/L2
6.2.2.1.2 Flow to Drains Not Reaching an Impermeable Barrier
In many instances, relief drains will not be installed at the top of the
impermeable barrier. This may be because the plume can be completely
intercepted by shallow drains or because of the high cost of installing drains
to the full depth.
6-22
-------
FIGURE 6-14.
FLOW TO A DRAIN RESTING ON A LOW PERMEABILITY BARRIER
ro
oo
T
-------
The Herman (1946) equation described in the previous section does not
adequately describe the flow to a drain which does not reach an impermeable
barrier. If the drain does not reach an impermeable barrier (Figure 6-15),
the flowlines will not be parallel and horizontal (as shown in Figure 6-14),
but will converge towards the drain (i.e., radial flow). This convergence
causes a more than proportional head loss in the groundwater system which must
be accounted for in the drain spacing formula. Hooghoudt (1940), as described
by Wesseling (1973), developed a modified drain spacing formula for a two
layered soil given by:
q = (8K2dH + 4K1H2)/L2
where: K, ;K? = hydraulic conductivity above (K,) and below (K?) the
drain (ft/day)
d = equivalent depth of the aquifer below the drain (ft).
FIGURE 6-15.
FLOW TO DRAIN NOT RESTING ON A LOW PERMEABILITY BARRIER
(VAN SCHILFGAARD. 1974)
Water Table
K, D I
~*1 Low Permeability
Layer
In the Hooghoudt (1940) equation both drain spacing, L, and equivalent
depth, d, are unknowns. The value of d is typically calculated from a
specified value for L, so that the Hooghoudt (1940) equation cannot be solved
6-24
-------
explicitly in terms of L. The use of this equation as a drain spacing formula
involves either a trial and error procedure of selecting d and L until both
sides of the equation are equal or the use of nomographs which have been
developed specifically for equivalent depth and drain spacing. Table 6-1
gives values of the equivalent depth (d) as a function of drain spacing (L)
and saturated thickness below the drains (D). This table shows values of d
for a drain pipe with a radius (r,) of four inches. Similar tables have been
prepared for other values of rd (Wesseling, 1973). For saturated thicknesses
(D) greater than 32.8 feet, the equivalent depth can be calculated from drain
spacing using the following equation (Repa et al., 1983):
d = 0.057 (L) + 0.845
This equation was developed by linear regression from the values given in
Table 6-1.
6.2.2.1.3 Flow to Drains in a Two-layered Soil
Although Hooghoudt1s equation is widely used to approximate drain
spacing, it is only accurate in two-layered soils when the level of the drain
corresponds with the interface between the two soil layers. Ernst (1962), as
described by Wesseling (1973), developed an equation which offers a con-
siderable improvement over the Hooghoudt equation for two layered soils
insofar as the interface between the two soil layers can be above or below the
drains. This formula is especially useful when the upper layer has a consid-
erably lower hydraulic conductivity than the lower layer. The flow to drains
as described by Ernst is illustrated in Figure 6-16 and written as follows:
H = (qD /K ) + (qL2/82(K.m.)) + (qL/7rK) In («D/u)
V V 11 r r
where:
H = total hydraulic head or water table height above drain
level midway between drains (ft)
q = drain discharge rate per unit surface area (ft/day)
L = drain spacing (ft)
K = hydraulic conductivity of the layer with radial flow
r (ft/day)
6-25
-------
TABLE 6-1
VALUES FOR EQUIVALENT DEPTH d(m) FOR r, = 4 inches
CALCULATED FOR DIFFERENT VALUES OF DRAIN SPACING (L)
AND SATURATED THICKNESS BELOW DRAINS
(Wesseling, 1973)
(D)
I
ro
L(m) 5 7.5
0(m)
0.5 0.47 0.48
0.75 0.60 0.65
1.00 0.67 0.75
1.25 0.70 0.82
1.50 0.88
1.75 0.91
2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
4.50
5.00
5.50
6.00
7.00
8.00
9.00
10.00
0.71 0.93
10
0.49
0.69
0.80
0.89
0.97
1.02
1.08
1.13
1.14
15
0.49
0.71
0.86
1.00
1.11
1.20
1.28
1.34
1.38
1.42
1.45
1.48
1.50
1.52
1.53
20
0.49
0.73
0.89
1.05
1.19
1.30
1.41
1.50
1.57
1.63
1.67
1.71
1.75
1.78
1.81
1.85
1.88
1.89
25
0.50
0.74
0.91
1.09
1.25
1.39
1.5
1.69
1.69
1.76
1.83
1.88
1.93
1.97
2.02
2.08
2.15
2.20
2.24
30
0.50
0.75
0.93
1.12
1.28
1.45
1.57
1.69
1.79
1.88
1.97
2.04
2.11
2.17
2.22
2.31
2.38
2.43
2.48
2.54
2.57
2.58
35
0.75
0.94
1.13
1.31
1.49
1.62
1.76
1.87
1.98
2.08
2.16
2.24
2.31
2.37
2.50
2.58
2/65
2.70
2.81
2.85
2.89
2.91
40
0.75
0.96
1.14
1.34
1.52
1.66
1.81
1.94
2.05
2.16
2.26
2.35
2.44
2.51
2.63
2.75
2.84
2.92
3.03
3.13
3.18
3.23
3.24
45
0.76
0.96
1.14
1.35
1.55
1.70
1.84
1.99
2.12
2.23
2.35
2.45
2.54
2.62
2.76
2.89
3.00
3.09
3.24
3.35
3.43
3.48
3.56
50
0.76
0.96
1.15
1.36
1.57
1.72
1.86
2.02
2.18
2.29
2.42
2.54
2.64
2.71
2.87
3.02
3.15
3.26
3.43
3.56
3.66
3.74
3.88
L(m)
D(m)
0.5
1
2
3
4
5
6
7
8
9
10
12.5
15
17.5
20
25
30
35
40
45
50
60
50
0.50
0.96
1.72
2.29
2.71
3.02
3.23
3.43
3.56
3.66
3.74
3.88
75
0.97
1.80
2.49
3.04
3.49
3.85
4.14
4.38
4.57
4.74
5.02
5.20
5.30
5.38
80
0.97
1.82
2.52
3.08
3.55
3.93
4.23
4.49
4.70
4.89
5.20
5.40
5.53
5.62
5.74
5.76
85
0.97
1.82
2.54
3.12
3.61
4.00
4.33
4.61
4.82
5.04
5.38
5.60
5.76
5.87
5.96
6.00
90
0.98
1.83
2.56
3.16
3.67
4.08
4.42
4.72
4.95
5.18
5.56
5.80
5.99
6.12
6.20
6.26
ino
0.98
1.85
2.60
3.24
3.78
4.23
4.62
4.95
5.23
5.47
5.92
6.25
6.44
6.60
6.79
6.82
150
0.99
1.00
2.72
3.46
4.12
4.70
5.22
5.68
6.09
6.45
7.20
7.77
8.20
8.54
8.99
9.27
9.44
9.55
200
0.99
1.92
2.70
3.58
4.31
4.97
5.57
6.13
6.63
7.09
8.06
8.84
9.47
9.97
10.7
11.3
11.6
11.8
12.0
12.1
12.2
250
0.99
1.94
2.83
3.66
4.43
5.15
5.81
6.41
7.00
7.53
8.68
9.64
10.4
11.1
l?.l
12.9
11.4
13.8
13.8
14.3
14.6
14.7
-------
FIGURE 6-16.
SYMBOLS FOR THE ERNST EQUATION FOR FLOW IN A TWO-LAYERED SOIL WITH
(A) THE DRAIN IN THE LOWER LAYER AND IB) THE DRAIN IN THE UPPER LAYER
-K,
-K,
///A\\V//A\\ W/A\W//AUVv/\X\V//A\\\////\\VV///\\\\///
T
6-27
-------
K = hydraulic conductivity of the layer with vertical flow
(ft/day)
D = thickness of the layer over which vertical flow is
v
D = thickness of the layer in which radial flow occurs or
thickness below the drain level of the layer in which the
considered (ft)
thickness of th<
thickness below
drain is located (ft)
2(K.m.) = Transmissivity of the soil layers through which horizontal
flow is considered (ft /day)
u = wet perimeter of the drain (ft)
a = geometry factor for radial flow which depends upon flow
conditions (dimensionless).
Values of u,Z(K.m.), and a require further discussion. For pipe drains in an
area of high hydraulic conductivity, u is determined from:
u = (b + 2)(2rd)
where the new terms are:
b = width of the trench (ft)
r. = radius of the drain (ft)
If envelope material is used, it is advisable to replace 2r . by the height of
the envelope. 2(K-m.) is equal to the transmissivity of the entire aquifer
for a two-layered soil or l(K.m-) = K-, m, + K? m-. Values for the geometry
factor, o, vary depending upon the relative hydraulic conductivity of the two
soil layers and whether the drains are installed in the upper or lower layers.
The value,«, for various conditions is discussed further below.
The Ernst equation can be used to determine drains spacing in homogeneous
or two-layered soils. However, under some conditions, drain spacing estimates
using the Ernst equation will not necessarily be any more valid then those
obtained using Hooghoudt's formula. These situations include:
t Where all soil layers are homogeneous.
• Where drains are installed in the upper layer of a two-layer aquifer
such that 0.1 K, is greater than K~. In this case the lower layer is
considered to be an impervious barrier and the soil is considered
homogeneous.
6-28
-------
• Where the depth from the drain bottom to the impermeable barrier is
larger or suspected to be larger than 0.25L. When this occurs, the
hydraulic conductivity and thickness of the second layer does not have
an effect on flow and the two layers may be considered homogeneous.
The Hooghoudt equation should be used under these circumstances to simplify
the calculations.
Conditions where the Ernst equation is most useful include:
t Where the drains are to be situated in the lower layer of a two-
layer soil and K, < K2 (the vertical resistance in the second layer
can be neglected against that in the first), so that (1) The geometry
factor, a, can be neglected, (2) l(K,m,) is approximately equal to
K2m?, (3) K = K? and K = K, (K = hydraulic conductivity of radial
flow and K = hydraulic conductivity of vertical flow), (4) Dy = 2m1
(average thickness below the water table of layer with permeability
K-, ) and (5) D = D . In this case, the formula can be expressed as
f 6l lows: r °
H - (q2m./K-) + (qL/8K2m2) + (qL/7rK2) In (DQ/u)
where D is the distance from the water level in the drain to the next
soil layer below having different hydraulic properties.
Where the drain is entirely in the upper layer of a two-layer soil and
K? >20 K,, such that «= 4, K and K = K,, and D = H. In this case,
tne formula becomes:
H = (qH/TrK^ + (qL/8 2(1^.))+ (qL/ir 1^) In (4DQ/u)
o Where the drain is entirely in the upper layer of a two-layer soil and
0.1 K, < K2 < 20 K,. In this case, the geometry factor, a , has to be
determined from the nomograph in Figure 6-17 and introduced into the
Ernst equation (Wesseling, 1973).
6.2.2.1.4 Drain Spacing Estimates Based on Field Experiments
Experimental or trial drains can be used to determine or verify drain
spacing calculations. They are also used at times to determine drain spacing
where theoretical estimates are difficult to obtain because of variable soil
conditions. For example, in areas where soils are heterogeneous, reliable
data on hydraulic conductivity may be difficult to obtain from conventional
sampling methods. In such cases, determination or verification of these
6-29
-------
FIGURE 6-17.
NOMOGRAPH FOR DETERMINING THE GEOMETRY FACTOR a,
IN THE ERNST EQUATION (WESSELING, 1973)
30 40 50
factors may be more economical or efficient using experimental drain fields.
A necessary prerequisite to using experimental drain fields is that a site can
be located which has similar hydrologic, pedologic, geologic, and topographic
conditions to the waste site.
The dimensions of the test site are governed by:
• The drain spacing to be tested--Spacings which are narrower and wider
than those calculated or estimated should be included and the inter-
vals should be chosen in distinct steps. Thus, if the spacing is
estimated by calculations to be 164 feet, spacings of 82 feet and 328
feet should be included as well.
• A length to width ratio of at least 5 and preferably 7 to 10--Thus,
when the test spacing is 164 feet, the test length should be at least
820 feet. If the plots are too wide in relationship to the length,
then boundary effects (i.e., inflow from an undrained part of the
area) may be significant.
6-30
-------
The depth to which experimental drains are installed can be approximated
from theoretical estimates and information on the depth of the plume and depth
to permeable and impermeable layers.
Water observation wells should be installed at various points in the
experimental field to aid in the evaluation as follows (Dieleman, 1974):
• Midway between drains to measure the hydraulic head or the water table
height above the drain
• Near one or more of the drains in each field plot to measure the shape
of the water table. Shape should be measured at varying points (e.g.,
10, 20, and 50 feet from the drain)
• At the upper and lower ends of some plots to observe boundary effects
• On top of drain tubes to determine drain function.
The adequacy of the drain spacing can be determined from the measured
hydraulic head and shape of the water table. These data are also used to
determine hydraulic conductivity using the Hooghoudt or other appropriate
equation. In using the Hooghoudt equation:
q = (8K2dH + 4K1H2)/L2
the discharge rate per unit area drained (q=Q/A), is measured directly in the
field using a discharge recorder attached to the drain outlet.
6.2.2.2 Interceptor Drains
Interceptor drains are designed to cut off the flow of groundwater
originating from an up-gradient source and are generally more applicable to
plume management than relief drains.
6-31
-------
There are basically two major applications for interceptor drains in
plume management. In the first situation (Figure 6-18a), the natural gradient
of the water table causes significant contaminant movement away from the
site. Here, interceptor drains could be installed downgradient of the
contaminant source to cut off the contaminants flow to streams or wells.
In the second situation (Figure 6-18b), contaminants flow outward in
several directions from a site because of a groundwater mound which has built
up beneath the site. This situation would require installation of parallel
interceptor drains located on either side of the site or a drainage system
which completely encircles the site.
6.2.2.2.1 Interceptor Drains to Cut Off Natural Groundwater
Flow (Figure 6-18a)
The design of interceptor drains is more often based on practical
experience than on theoretical design. Preliminary field investigations are
undertaken to determine the direction of flow, the hydraulic gradient, the
hydraulic conductivity, location of subsurface strata, and the boundaries of
the plume. These data are typically obtained through an on site boring
program. With the results of the field survey and a knowledge of how inter-
ceptors capture flow, the depth and position of the drains are estimated.
After the line is staked on the site, additional borings should be taken along
and across the staked line, and the alignment shifted, if needed, to obtain
proper interception.
Interceptor drains are generally installed perpendicular to groundwater
flow and parallel to the contours of the land. Figure 6-19 illustrates the
positions of an interceptor drain relative to groundwater flow and topography.
If soil borings indicate that stratified soils having greatly different
hydraulic conductivities exist, the drain should be installed with a sand and
gravel envelope resting on the layer of lower hydraulic conductivity, if
possible. If the trench line is cut through an impervious stratum, there is
danger that a significant percentage of the water moving laterally will bridge
6-32
-------
FIGURE 6-18.
SITE CONDITIONS REQUIRING INTERCEPTOR DRAINS FOR PLUME MANAGEMENT
Waste Disposal
Site
Private
Well
Waste Disposal
Site
Contaminated Groundwater
6-33
-------
FIGURE 6-19.
LOCATION OF A SUBSURFACE DRAIN WITH RESPECT TO TOPOGRAPHY AND
THE DIRECTION OF GROUNDWATER FLOW
Groundwater
Flow 730
Land
Surface
Elevations
720
Interceptor
Drain
over the drain and continue downgradient. This is especially important where
a permeable layer sandwiched between two relatively impermeable layers
outcrops along the side of the slope such that surface seepage is a problem.
When this situation occurs, the flow can be intercepted by a drain located
just upgradient of the seep.
For the purpose of installing an interceptor drain, a layer can be
considered "impermeable" if its hydraulic conductivity is less than one tenth
that of an adjacent, more permeable layer.
In order to decide where to position a drain (or drains) to intercept the
entire leachate plume, the relationship between depth and flow, and the up-
gradient and down-gradient influence of the interceptor drain must be known.
6-34
-------
If other boundary conditions are fixed, the shape of the drawdown curve up-
gradient of the site is independent of hydraulic conductivity, but is a
function of head. The upgradient influence on drawdown extends for a distance
which is greater the more gradual the water table gradient. In general, the
upgradient influence is small compared to the downgradient influence.
The depth to which the water table is lowered downgradient of the
interceptor is proportional to the depth of the drain. Theoretically, a true
interceptor drain lowers the water table downgradient to a depth equal to the
depth of the drain. The distance downgradient to which it is effective in
lowering the water table is infinite provided recharge is not occurring. This
however is never the case since infiltration from precipitation always
recharges the groundwater.
If a leachate plume extends from the water table to the depth of the
impermeable layer, and an interceptor drain is placed at the midpoint between
the water table and the impervious layer, a little less than 50 percent of the
flow will be intercepted. If the drain is placed at the top of the imperme-
able layer nearly all of the flow will be intercepted (Van Schilfgaarde, 1974;
SCS, 1973).
In summary, the major considerations in the design of an interceptor
drain are:
• To place the drains deep enough to intercept the entire plume,
preferably to the top of an impermeable barrier
• To ensure that the upgradient influence of the drain is adequate to
intercept the plume
• To ensure that the downgradient influence is adequate to intercept the
plume, if necessary. In many cases it will be adequate to cut off the
upgradient source of the plume and allow any contaminated water
already downgradient to continue on its course. This will depend upon
the size of the plume and the use of the groundwater downgradient.
6-35
-------
Interceptor drains must be properly designed so that their upgradient and
downgradient influences completely capture and remove the contaminant plume.
If after installation, a drain is not operating properly, effective plume
management can still be achieved by:
• Installing a cut-off wall downgradient of the drain to limit plume
migration
• Installing a second drain up- or downgradient of the first to cause
further reductions in head levels
• Installing a second drain above or below the first to intercept
leachate which bypasses the original drain.
The upgradient and downgradient influence of an interceptor drain can be
determined theoretically or in the field. Evaluating the upgradient and
downgradient influence in the field requires that the first interceptor drain
be installed at a position and depth determined from soil borings and ground-
water monitoring. A period of time should be allowed for the system to come
to equilibrium before piezometers are monitored to determine the drawdown at
various points upgradient and downgradient of the interceptor.
The theoretical determination of the upgradient influence of an inter-
ceptor drain involves the use of an equation developed by Glover and Donnan
(1959), as described by Van Hoorn and Vandermolen (1974):
i
Du = 1.33 msl "
where: D = Effective distance of drawdown upgradient (ft)
ms = Saturated thickness of the water bearing strata not
affected by drainage (ft)
I = hydraulic gradient (dimensionless).
The theoretical determination of the downgradient influence can be
obtained from the following equation (Figure 6-20):
Dd = (Kl/q) (de - hd - D2)
6-36
-------
FIGURE 6-20.
SYMBOLS FOR THE GLOVER AND DONNAN EQUATION FOR CALCULATING THE
DOWNGRADIENT INFLUENCE OF AN INTERCEPTOR DRAIN
Original Water Table
Water Table
After Drainage
where: K = hydraulic conductivity (ft/day)
I = hydraulic gradient (dimensionless)
q = drainage coefficient (ft/day)
d = depth of drain (ft)
h. = desired depth of drawdown (ft)
Dp = distance from ground surface to water table prior to
drainage at the distance D, downgradient from the
drain (ft)
D, = downgradient influence (ft).
In the equation given above, D, and Dp are interdependent variables. In
obtaining the solution to the equation, estimating the value of D? is
necessary and then trial computations are made. If the actual value of Dp at
distance D. is appreciably different, a second calculation is necessary.
Where I is uniform throughout the area, Dp can be considered equal to D,
(i.e., the distance from the ground surface to the water table measured at the
drain). If a second interceptor is needed to lower the water table to the
desired depth, it would be located D. feet downgradient from the first.
6-37
-------
6.2.2.2.2 Interceptor Drains for Controlling a Groundwater
Hound (Figure 6-18b)
The design of subsurface drains for managing a plume generated from a
site containing a groundwater mound depends on the direction(s) of flow from
the site, the direction(s) of natural groundwater flow, the hydraulic
gradient(s) and the depth to a low permeability barrier. For this situation,
drains can be placed completely encircling the mound, or in parallel on either
side of a mound.
If the drains are installed so that they rest on top of a low permeabil-
ity barrier and the water table is drawn down to the level of the drain
piping, drain spacing is not important because any part of the plume that lies
outside the influence of the drains will nevertheless be contained beneath the
site boundaries. If the site overlies fractured bedrock or if the cost of
construction precludes installation of the drains to the top of the low
permeability layer, then spacing becomes an important design criteria. This
is because the drawdowns of the drains may not intersect, and leachate could
flow beneath the drains or through fractured bedrock. If the hydraulic
gradient is not too steep, the drain spacing formulas presented for relief
drains can be used to approximate spacing. These formulas (Hooghoudt and
Ernst equations) can provide a reasonable approximation of spacing where the
hydraulic gradient is less than three percent. Gradient becomes a much more
important factor in determining flow and drain spacing when gradients are
greater than three percent. Under these circumstances, the formulas for
determining the upgradient and downgradient influence of the interceptor
drains provide a more accurate approach for drain spacing. Computer models
are generally required for these situations to ensure proper design.
6.3 Design
In designing a closed pipe drain, the pipe is assumed to be able to
accept the drainage water when it arrives at the drain!ine and that the pipe
will carry away the water without a build up of pressure. To meet the first
criterion, the relationship between the hydraulic conductivity of the gravel
6-38
-------
envelope, the perforations in the drain pipe, and the base material must be
assessed as described in the following subsection on filters and envelopes.
To meet the the second criterion, the pipe size and drain slope must be
adequate to carry away the water after it enters the pipe.
6.3.1 Flow Capacity
6.3.1.1 Total Drainage Discharge
In order to estimate drain diameters (i.e., the hydraulic design) and
volume of storage required by the sump, the total discharge (Q) from the
laterals, collectors, and mains must be determined. Estimates of total
discharge can be obtained using the water balance method. This method
provides an estimate of the amount of percolation that will recharge the water
table between the lines of the drain. Once the percolation rate has been
calculated, discharge can be obtained by multiplying the rate times the
•3 p
drainage area: Q (ft /day) = q (ft/day) x area (ft ). Where water balance
data are not available, the discharge from parallel relief drains can be
approximated from the following formulas (Bureau of Reclamation, 1978):
Q = 27rKHdx/L (for drains above a barrier)
2
Q = 4KH x/L (for drains on a barrier)
o
where: Q = total discharge from two sides (ft /day)
H = maximum height of the water table above the drain, midway
between the drains (ft)
K = weighted average hydraulic conductivity (ft/day)
d = equivalent depth (ft)
L = drain spacing (ft)
x = length of pipe (ft).
Guidance for calculating H, d, and L is given in section 6.2.2.1.
The rate of flow (Q) from an interceptor drain can be estimated
quantitatively using the equation:
Q = KIA
6-39
-------
where:
Q = unit flow (ft3/second)
K = hydraulic conductivity (ft/second)
I = slope (dimensionless) ?
A = area through which flow occurs (ft ).
The cross sectional area intercepted by an interceptor drain is equal to the
effective depth of the drain (i.e., the vertical distance from the bottom of
the drain to the water table) times the length of the drain:
Q = KI dex
where:
d = average effective depth of the drain (ft)
x = length of the drain (ft).
The use of the above equation should be reserved for situations in which
the hydraulic conductivity, soil profile, and cross sectional area are uniform
and accurately known. In some instances constructing a pilot drainage system
may be more desirable to more accurately determine discharge and drain size
(SCS, 1973).
6.3.1.2 Gradient and Velocity
The proper installation and function of pipe drains requires rigid
control of grade and alignment in order to prevent siltation. The minimum
grades for a closed pipe drain where siltation is not likely to be a hazard
are given in Table 6-2. Steeper grades are generally more desirable. With
steeper grades, the control required during construction is less exacting and
there is also less chance of the drains clogging. The selected grade should
be great enough to result in a flow velocity that prevents siltation yet will
not cause turbulence. Where the velocity is less than 1.4 feet/second and
siltation is a hazard, preventative measures including filters and silt traps
should be considered. The velocity (i.e., critical velocity) at which
turbulence results varies with soil type. Critical velocities of various soil
types are shown in Table 6-3. Table 6-4 gives the grades for various drain
sizes which result in the critical velocity discussed above. (SCS, 1973;
Bureau of Reclamation, 1978).
6-40
-------
TABLE 6-2
MINIMUM GRADES FOR VARIOUS PIPE SIZES (SCS, 1973)
Pipe diameter (in) Grade (%)
4 0.10
5 0.07
6 0.05
TABLE 6-3
CRITICAL VELOCITY OF VARIOUS SOIL TYPES (SCS, 1973)
Soil Types Velocity (ft/sec)
Sand and Sandy Loam 3.5
Silt and Silt Loam 5.0
Silty Clay Loam 6.0
Clay and Clay Loam 7.0
Course Sand and Gravel 9.0
6-41
-------
TABLE 6-4
DRAIN GRADES FOR SELECTED CRITICAL VELOCITIES
Drain Size
Inches 1.4
VELOCITY (ft/sec)
3.5 5.0 6.0 7.0
9.0
Grade—feet per 100 feet
For drains with "N" =
Clay Tile, Concrete Tile, and Concrete Pipe (with good alignment)
4
5
10
12
.28
.21
.17
.11
.08
.07
1.8
1.3
0
1.
0.
0.
0.4
3.6
2.7
2.1
1.4
1.1
0.8
5.1
3.9
3.1
2.1
1.5
1.2
7.0
5.3
4.1
2.8
2.1
1.6
11.5
8.7
6.9
4.6
3.5
2.7
For drains with "N" = 0.013
Clay Tile, Concrete Tile, and Concrete Pipe (with fair alignment)
4
5
6
8
10
12
.41
.31
.24
.17
.12
.09
2.5
1.9
1.5
1.0
.6
5.2
3.9
3.1
2.1
1.6
1.2
7.5
5.6
4.4
3.0
2.2
1.8
10.2
7.7
6.0
4.1
3.0
2.4
16
12.
10.0
6.8
5.0
3.9
For drains with "N" = 0.015
Corrugated Plastic Pipe
4
5
6
8
10
12
.53
.40
.32
.21
.16
.13
3.3
2.5
2.0
1.3
1.0
.8
6.8
5.1
4.0
2.7
2.0
1.6
9.8
7.3
5.8
3.9
2.9
2.3
13.3
9.9
7.9
5.3
4.0
3.1
21.9
16.6
13.2
8.8
6.6
5.1
(a)--"N" is the roughness coefficient.
6-42
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6.3.1.3 Drain Diameter
The size of the drain for a given capacity is dependent on the flow,
hydraulic gradient, and the roughness coefficient, N, which is a function of
the hydraulic resistance of the drain material. The formula for hydraulic
design is based on the Manning formula for pipes, which is written as follows:
Q - AV - kmAR0-67 I °'5
in which: Q = discharge (ft /sec)
R = hydraulic radius (feet) equal to the wetted
cross-sectional area, A , divided by the wetted
perimeter,
(1/4 the diameter for full flowing pipes)
I = hydraulic gradient
k = roughness factor (1/N)
V = velocity,., (ft/sec)
A = area (fr).
To apply the Manning formula in the hydraulic design of drain pipes, use is
made of the equation:
Q = AV = qAd
in which q is the specific discharge of the drain (feet/sec) and A, is the
2
area affected by the drain (ft ).
Using the calculated discharge and knowing the gradient of the drain line
and the roughness coefficient, the pipe size can be determined from nomographs
based on the Manning Formula. Figures 6-21 and 6-22 are nomographs for
estimating drain size for values of N = 0.013 and N = 0.015 respectively.
These nomographs provide a means of selecting proper drain size at the
starting point in the drainage system. However, in drainage systems with long
laterals or mains, common practice is to change to a larger pipe size after
some distance to allow for the increased quantity of water to be carried. The
following example illustrates a method for determining at what distance a
change to the next larger pipe diameter should be made.
6-43
-------
FIGURE 6-21.
CAPACITY CHART FOR N = 0.013
Drain Capacity Chart-N =0.013
- - r-r-1 1—
Drain Diameter
(Flowing Full)
N = 0.013
velocity
I III
o o o oooooo
Hydraulic Gradient (Feet per Foot)
Source: SCS, 1973
Note: The shaded area indicates where the velocity of ftow is less than 1.4 feet per second
to indicate where drain filters may be required.
6-44
-------
FIGURE 6-22.
CAPACITY CHART FOR N = 0.015
Drain Capacity Chart-N=0.015
30
Drain Diameter
(Flowing Full)*;
N = 0.015 :H:
o o o o o
O o O O O
Hydraulic Gradient (Feet per Foot)
-o
o
o
-------
Assume that the length of a drain is 6000 feet and that the total
discharge at a constant grade of 0.2 percent is 2.40 feet /second. The
increase in flow or accretion would be given by:
2.40 (ft3/sec)/6000 (ft) = 0.0004 (ft2/sec)
Assuming that a 6 inch drain pipe is used at the upper end of the system,
one can determine to what length down drain the 6 inch pipe would be adequate
by referring to Figure 6-22 (N=0.015). From Figure 6-22, a 6 inch diameter
pipe on a grade of 0.2Q percent has a maximum capacity of 0.22 ft /sec.
Letting X equal the distance downdrain that the 6 inch pipe would be adequate,
then:
X = 0.22 (ft3/sec)/0.0004 (ft2/sec) - 550 ft
The 6 inch drain is therefore adequate for 550 feet of line. At this
point, the switch to an 8 inch pipe is made. These computations should be
continued progressively for the total 6000 feet of drain pipe. If laterals
enter the drain, the estimated yield of these should be added at the proper
section (SCS, 1973).
6.3.2 Filters and Envelopes
Performance of a drainage system is based on the assumption that the
drainage system will accept drainage water when it arrives at the drain!ine.
Filters and envelopes are used to ensure that this requirement is met.
6.3.2.1 Function of Filters and Envelopes
The primary function of a filter is to prevent soil particles from
entering and clogging the drain. The function of an envelope is to improve
water flow into the drains by providing a material that is more permeable than
the surrounding soil. Envelopes may also be used to provide suitable bedding
for a drain and to stabilize the soil material on which the drain is being
placed.
6-46
-------
The filter's function and the envelope's function are somewhat contradic-
tory. Whereas filtering is best accomplished by fine materials, coarse
materials are more appropriate for envelopes.
As water approaches a subsurface drain, the flow velocity increases as a
result of convergence towards the perforations or joints in the pipe. This
increase in velocity is accompanied by an increase in hydraulic gradient. As
a result, the potential for soil particles to move towards the drain is
increased. By using a highly permeable material such as gravel around the
pipe, the number of pore connections at the boundary between the soil and the
envelope will increase, thereby decreasing the flow velocity (Wesseling,
1973).
A filter should prevent the entry of soil particles, which could result
in sedimentation and clogging of the drains, blocking of perforation or tile
joints, or blocking of the envelope. The filter materials should not, how-
ever, be so fine that they prevent all soil particles from passing through.
If silts and clays are not permitted to pass through, they may clog the
envelope resulting in increased entrance resistance, which can cause the water
level to rise above the drain (Wesseling, 1973).
Although filters and envelopes have distinctly different functions, well
graded sands and gravels can be used to meet the requirements of both a filter
and an envelope. Depending on the design manual that is consulted, there may
or may not be a distinction made between envelopes and filters.
6.3.2.2 Types of Filters and Envelopes
Well graded sands and gravels can function both as an envelope and a
filter. The specifications for granular filters, however, are more rigid than
those for envelopes. Usually filter materials must be screened and graded to
develop the desired gradation curves. Envelope materials, on the other hand,
may have a wide range of allowable sizes and gradings (SCS, 1973).
6-47
-------
A variety of soil stabilizers have been developed as a substitute for
sand and gravel envelopes. They are added to local soils in low concentra-
tions to make stable aggregates which function as envelopes when backfilled
around the trench. Artificial aggregates have been produced using Portland
cement, asphalt emulsions, and various polymer solutions and emulsions
(Wesseling, 1973). With the exception of portland cement aggregates, which
have been used successfully at some sites, soil stabilizers are generally not
an economical substitute for sand and gravel. Also in many instances, these
aggregates can react unfavorably with some components of the leachate thus
altering the hydraulic conductivity of the envelope materials and increasing
the hydraulic resistance.
Synthetic materials are used mainly for filtration. The most widely used
materials for synthetic filters are nylon, polypropylene, polyvinyl chloride,
polyethylene, and polyester fibers. The most common manufacturing processes
for synthetic filters include weaving, which produces a window screen type of
material and bonding in which the density and open spaces are controlled by
fiber diameter and the number of fiber layers. Spun bonded filters have a
large number of openings with a range of opening sizes throughout, whereas
woven filters have openings of a fixed size. Spun bonded fabrics are most
widely used in drainage systems and are recommended for gravel and sand soils
but not for silts and clays.
6.3.2.3 Design
6.3.2.3.1 Sand and Gravel Filters and Envelopes
Detailed design procedures are available for both gravel and sand
envelopes. SCS (1973) has distinct design criteria for filters and envelopes,
whereas the Bureau of Reclamation (1978) has developed one set of standards
for a well graded envelope which meets the requirements of both a filter and
an envelope. The separate SCS (1973) design criteria will be considered below
for the following reasons:
• Site specific conditions may warrant the use of only a filter or an
envelope, but not both
6-48
-------
• Where both a filter and an envelope are needed, the SCS design
criteria for a filter can generally be used
• Use of fabric filters may be desirable with a gravel envelope.
To provide general guidelines for the use of gravel filters, SCS has divided
soils into three groups depending on their need for a filter (Table 6-5).
Soils with a high percentage of fines (the first grouping in Table 5-5) always
require a filter. The second group may or may not need a filter and the last
group seldom needs a filter.
The general procedure for designing a gravel filter is to: (1) make a
mechanical analysis of both the soil and the proposed envelope material; (2)
compare the two particle distribution curves; and (3) decide by some set of
criteria whether the envelope is satisfactory. The Corps of Engineers and the
Soil Conservation Service have adopted similar criteria which set size limits
for a filter material based on the size of the base material. These limits
are as follows:
DKn filter/Debase = 12 to 58%
oU bU
D15 filter/D15 base = 12 to 40%
Multiplying the 50 percent grain size (D5Q) of the base material by 12 and 58
percent gives the limits the 50 percent grain size of the filter should fall
within. Multiplying the 15 percent grain size (D15) of the base material by
12 and 40 percent gives the limits the 15 percent grain size of the filter
should fall within. Figure 6-23 shows the upper and lower limit curves of
drain envelope suitability for a specific soil. In this example, only filter
"No. 3" falls within the 15 percent and 50 percent limits. When filter and
base materials are more or less uniformly graded, a generally safe filter
stability ratio is given by:
D1C filter/DQC filter <5
ID ob
6-49
-------
TABLE 6-5
A CLASSIFICATION TO DETERMINE THE NEED
FOR DRAIN FILTERS OR ENVELOPES (SCS, 1973)
Unified Soil
Classification
Soil Description
Filter
Recommendation
Envelope
Recommendation
SP (fine)
SM (fine)
ML
MH
(diatomaceous
fine)
Poorly graded sands,
gravelly sands.
Silty sands, poorly graded
sand-silt mixture.
Inorganic silts and very
fine sands, rock flour, Filter
silty. or clayey fine sands needed
with slight plasticity.
Inorganic silts, micaceous
sandy or silty soils, elastic
silts.
Not needed where
a sand and gravel
filter is used but
may be needed
with flexible
drain tubing and
other type
filters.
GP
SC
GM
SM (coarse)
GC
CL
SP,GP (coarse)
SW
CH
OL
OH
Pt
Poorly graded gravels,
gravel-sand mixtures,
little or no fines.
Clayey sands, poorly
graded sand-clay mix-
tures.
Silty gravels, poorly
graded gravel-sand-
silt mixtures.
Silty sands, poorly
graded sand-silt
mixtures.
Clayey gravels, poorly
graded gravel-sand-clay
mixtures.
Inorganic clays of low to
medium plasticity, gravelly
clays, sandy clays, silty
clays, lean clays.
Same as SP & GP above.
Well graded gravels, gravel -
sand mixtures, little or no
fines.
Well graded sands, gravelly
sands, little or no fines.
Inorganic, fat clays.
Organic silts and organic
silt-clays of low plasticity.
Organic clays of medium to
high plasticity.
Peat.
Subject to
local on-site
determination.
Not needed where
a sand and gravel
filter is used but
may be needed with
flexible drain
tubing and other
type filters.
None
Optional.
May be needed
with flexible
drain tubing.
6-50
-------
FIGURE 6-23.
MECHANICAL ANALYSIS OF A GRAVEL FILTER MATERIAL
(SCS, 1973)
State
Property
Area
District
John Jones
Location T/3&. /?./#£.. SI** S
-------
For perforated pipe drains, the requirement for the minimum size of the
envelope material is affected by the size of the perforations. SCS recommends
that the 85 percent grain size (Dgc) of the envelope material should not be
smaller than half the diameter of the perforations, and not more than 10
percent of the filter material should pass the No. 60 sieve. SCS (1973) also
recommends a minimum filter thickness of three inches or more.
6.3.2.3.2 Sand and Gravel Envelopes
Envelopes are generally recommended in order to provide a permeable path
for water to move into the pipe openings. They are also recommended to
improve the stability of certain soils during pipe installation and to provide
stability and support for corrugated flexible pipe. The design approach
recommended by SCS (1973) is to first determine whether the drainage requires
a filter and then to determine the need for an envelope. This is because a
well graded filter can generally also serve the function of an envelope.
The first requirement of sand and gravel envelopes is that the envelope
have a hydraulic conductivity higher than that of the base material. SCS
(1973) generally recommends that all of the envelope material should pass the
1.5-inch sieve, 90 percent should pass the 0.75-inch sieve, and not more than
10 percent should pass the No. 60 sieve (0.01-inch). This minimum limitation
is the same for filter materials, however, the gradation of the envelope is
not important since it is not designed to act as a filter.
The optimum thickness of envelope materials has been a subject of con-
siderable debate. Theoretically, by increasing the diameter of the pipe, the
inflow is increased. If the permeable envelope is considered to be an
extension of the pipe, then the larger the envelope's thickness the greater
the inflow. There are, however, practical limitations to increasing envelope
thickness. The perimeter of the envelope through which flow occurs increases
as the first power of the diameter of the envelope, while the amount of
envelope material required increases as the square of the diameter. Doubling
the diameter of the envelope (and consequently decreasing the inflow velocity
6-52
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at the soil- envelope interface by half) would require four times the volume
of envelope material.
Recommendations for drain envelope thickness have been made by various
agencies. The Bureau of Reclamation recommends a minimum thickness of four
inches around the pipe, while SCS (1973) recommends a three-inch minimum
thickness.
6.3.2.3.2 Synthetic Filters
For synthetic materials the suitability of a filter can be determined
from the ratio of the particle size distribution to the pore size of the
fabric. The accepted design criterion for geotextile filters is (Dupont,
1981):
PQC (85% pore size of the filter fabric) <^ j
TJor (85% grain size of the subgrade material) ^
nr P -^ n
or P85 ^ U85
6.3.3 Design and Selection of Pipes
Materials commonly used for subsurface drains include ceramic and
concrete tile and plastic pipe. Clay and concrete tiles may be perforated, or
they may have openings or joints between the segments through which water may
enter. Jointed concrete tiles may be manufactured with plain, tongue-and-
groove, or bell-and-spigot ends as shown in Figure 6-24. Tongue-and-groove
and bell-and-spigot joints interlock, making them easier to place and hold
alignment. Perforated pipe is easier to install than jointed pipe and is more
widely used for subsurface drainage at waste disposal sites. Plastic pipe is
available as perforated and flexible corrugated pipe. Subsurface drains may
be installed at considerable depth and the ability of the pipe to carry the
load of the backfill will be an important consideration in some cases. The
American Society for Testing and Materials (ASTM) standards lists allowable
crushing strengths of rigid pipe drains. These standards are listed below
under the discussion of specific pipe types. For corrugated plastic tubing,
6-53
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FIGURE 6-24.
JOINT DESIGN FOR RIGID DRAIN TILES
(BUREAU OF RECLAMATION, 1978)
Tongue and Groove Type for Concrete Pipe
Bell and Spigot Type for Clay or Concrete Pipe
Use 4 Wedges and Lugs for Concrete Pipe (Clay Pipe Shown)
Plain End Type for Clay or Concrete Pipe
6-54
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the strength depends upon the bedding material. All plastic drains should be
installed in at least a four-inch gravel envelope. Flexible pipe deflects
when loaded, which results in a transfer of the load to the bedding material.
Safe loads are those that will cause 10 percent or less deflection. Equations
for determining deflection are presented in the Bureau of Reclamation's
Drainage Manual (1978).
6.3.3.1 Clay Pipes
There are basically two types of clay drains manufactured for subsurface
drainage—clay drain tile, which is no longer widely used, and perforated,
vitrified clay pipe. Standard specifications can be found in ASTM C4 and C700
(ASTM, Part 16, 1982), respectively. Clay drain tile is considerably less
expensive than vitrified clay pipe, but vitrified clay is stronger, more
durable, and more chemically resistant than clay tile. Vitrified clay piping
tested over a six month exposure period was found to be resistant to acids,
bases, chlorinated and aromatic solvents, chromic acid, copper sulfate, and
numerous other chemicals. However, vitrified clay piping was found not to be
resistant to hydrofluoric acid (Logan Clay, 1983). Vitrified clay pipe is
available in standard strength and extra strength, ranging in diameter from 3
to 42 inches. Standards require that perforations be 0.25 inch in diameter.
6.3.3.2 Concrete Pipes
Unreinforced concrete drain pipes suitable for drainage of industrial
wastes must comply with ASTM C14, C412, or C444 (ASTM, Part 16, 1982).
Standard C412 covers drain tile and C14 and C444 deal with perforated concrete
pipe. Where concrete pipe will be exposed to a pH of less than six or to
sulfate concentrations in excess of 400 parts per million or both, acid and
sulfate resistant cement should be used since regular types of cement are
subject to disintegration by these materials. Perforated asbestos cement pipe
(C-508-81) is also resistant to acids and may be used for drainage.
6-55
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6.3.3.3 Plastic Pipes
The most commonly used materials in the manufacture of plastic pipes are
polyvinyl chloride (PVC) and high density polyethylene (HOPE). At an equiva-
lent weight and size, PVC pipes have a somewhat higher resistance to outside
pressure than HOPE pipes. PVC is generally recommended for large pipes in
excess of six inches because of its greater strength. However, HOPE is less
impact resistant especially under low temperatures (Cavelaars, 1974). PVC and
HOPE have good resistance to dilute acids and bases. PVC is not recommended
for chlorinated and aromatic solvents.
Plastic pipes come in smooth and corrugated varieties and there are
advantages and disadvantages to each. Corrugated wall plastic tubes are
stronger, lighter weight, less expensive, and easier to handle because of
their flexibility than are smooth-walled plastic pipes. Smooth pipes, on the
other hand, have a considerably lower hydraulic resistance than do corrugated
pipes. The outside diameter of smooth pipes can be about 25 percent less than
that of corrugated pipes (Cavelaars, 1974).
Flexible corrugated tubes have gained considerable popularity in agri-
cultural drainage in recent years. A flexible drain tube gains part of its
vertical soil load carrying capacity by lateral support from the soil at the
sides of the conduit. Thus, the stiffness of the conduit wall as well as the
rigidity of the soil surrounding the tube are both structural parameters (Van
Schilfgaarde, 1974). This implies that the stability of the surrounding soil
is an important factor in the performance of flexible corrugated tubing. For
conventional ceramic or concrete tile, conduit wall rigidly is the principal
parameter.
6.3.3.4 Manholes
Manholes are located in pipe drains to serve as junction boxes, silt and
sand traps, observation wells, discharge measurement points, access sites to
the drain for maintenance, and for the easy location of drains. There are not
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set criteria for spacing manholes. In general, they should be used at junc-
tion points on a drain and at major grade changes. Sometimes a manhole is not
required at a junction and a simple "Y" or "T" section may be adequate.
6.3.4 Drainage Sump and Pumping Plant
The main steps in the design of a drainage sump and pumping plant (Figure
6-25) are to (Bureau of Reclamation, 1978):
• Determine the maximum inflow to the sump
• Determine the amount of storage required
• Calculate the pumping rate
• Determine the start, stop, and discharge levels
t Determine the type and size of the storage tank
• Select the pump and the motor.
The maximum inflow to the sump is determined from the drainage coefficient and
the area served by a drain or drains discharging to the sump. As a precau-
tion, a 20 percent allowance should be made for flow that may occur in excess
of the design rate. Therefore:
QP - l'2 qg
o
where: Q = pumped discharge capacity (ft /day)
Qq = gravity discharge capacity (ft /day)
The amount of storage required depends on the inflow and the cycling
operation of the pump. The pump and motor are most efficient when run
continuously, although an 8 to 12 minute cycle is almost as efficient.
Assuming a 12 minute cycle, for example, during maximum inflow there will
be 5 cycles per hour, or 5 starts per hour, with equal on and off times of
6 minutes each. During low flow periods, the off time will be much longer
than the running time. For a motor to have equal on and off times, the
storage capacity must be equal to the amount of water that would run into the
6-57
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FIGURE 6-25.
TYPICAL DESIGN OF AN AUTOMATIC DRAINAGE PUMPING PLANT
(BUREAU OF RECLAMATION, 1978)
Shelter
Door
Stop Collar
Start Collar
Float Switch
Ground Surface, El. 1306.0 -
Pump Supports—
Start Level
Pipe Collector
El. 1296.0 -
Stop Level
Collector
Plug
Round Sump
Stilling Chamber
Concrete Base-
sump in one-half the cycling time (t ), or 6 minutes for the example used
above. Therefore:
Sv = *c Qp
Q
where: S is the volume of storage required (ft ).
The pumping rate, Q (ft /min), can then be determined using the following
expression:
where: t is the running time of the pump for maximum inflow (min)
6-58
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In order to estimate the size of the storage sump required, the inflow and the
minimum and maximum water levels in the sump must be estimated. In general,
the maximum water level for starting the pump should be at the top of the pipe
drain discharging into the sump. Maximum water level should never exceed
one-half of the pipe diameter over the top of the drain. The mimimum
elevation should be from 2 to 4 feet above the base of the sump. The
difference in elevation between the water level in the sump and the discharge
elevation is called the pump lift (Figure 6-25).
The volume required for storage plus the requirement for a minimum water
level above the bottom of the sump determines the size of the sump. The
distance between the pump cut-off and cut-on elevation (D ) should be small to
keep the depth of the sump reasonable. Assuming a D of 2 feet and an inflow
3
volume (S ) of 150 feet , the required diameter (D ) for a vertical
V /\
cylindrical sump can be estimated as follows:
Dx = (Sv/0.7854De)0'5
DY = (150 ft3/(0.7854(2))°'5
A
Dx = 10 ft
The next step in the design is to determine pump type, number, and size.
The type and size of the pump and motor can be selected from reliable pump and
motor manufacturers. Suitable pumps must be able to handle relatively large
flows and substantial amounts of sediment. The performance of the pump varies
with head (static lift plus frictional losses in the pump), speed, discharge,
and horsepower. Axial flow, mixed flow, or radial flow centrifugal pumps are
generally recommended.
The size and number of pumps are determined from the system's required
capacity. If the discharge volume is large, having two pumps may be advan-
tageous to provide more efficient pumping over a wider range of pumping rates.
Also, if a breakdown of one pump occurs the system operation will not be
upset. In a plant with two pumps, one pump operation is generally recommended
to have about half the capacity of the other. When the pumps are not of equal
6-59
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capacity, storage should be allowed for the capacity of the largest pump
(Bureau of Reclamation, 1978; SCS, 1973).
6.4 Installation and Maintenance
The following section describes the equipment and procedures used in the
construction and maintenance of subsurface drain systems. A large portion of
this section is devoted to trench excavation methods and equipment because
this portion of the drain installation process is often the most complex and
costly. Included are sections on excavation equipment, wall stabilization,
dewatering, and grade control. Drain installation is the next major section,
which includes the material and procedures most commonly used for placing
bedding, drain pipe, the gravel envelope, geotextile filter fabric, and
backfil1 material.
6.4.1 Trench Excavation
Trench excavation is the most significant step in the construction of a
subsurface drain. The ease or difficulty of excavation can have a dramatic
effect on the cost of the total installation. A difficult trench excavation
could even result in the exclusion of subsurface drainage as a viable
technique because of prohibitive costs.
A number of different work elements may be involved in excavating a
trench for a subsurface drain. Preliminary exploration may be carried out by
direct excavation, drilling, or seismic testing. The presence of rock may
require that the rock be mechanically ripped, blasted, or otherwise frag-
mented. Actual excavation of earth or rock may be carried out by a variety of
excavation equipment, the optimum being determined by the depth, width, and
length of the trench and the material being removed. Also, a number of
alternatives are available to prevent wall failure. These alternatives are
related to the size of the trench and the stability of the surrounding soil.
Provisions for dewatering and proper grading are also important aspects of
trench excavation.
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6.4.1.1 Subsurface Exploration
The selection of an excavation technique is intimately related to the
characteristics of the unconsolidated material or rock being removed. Some
form of subsurface exploration should be conducted to define the various earth
layers so that proper excavation techniques can be selected, accurate cost
estimates can be developed, and proper planning can occur.
Generally, three methods exist for subsurface exploration—direct
excavation, drilling, and seismic testing. Direct excavation is carried out
with a backhoe or ripper. Drilling will most probably be occurring at a
hazardous waste site for the purposes of determining the local geology and the
extent of groundwater and soil contamination. For subsurface exploration
purposes, only drilling techniques in which a drilling log can be prepared are
appropriate. Techniques for drilling are discussed in Chapter 5.
Seismic techniques have been used for about 75 years as a standard method
for oil exploration and for construction planning. Seismic testing is based
on measuring the velocity of a shock wave through earth formations at various
depths. The shock wave velocity varies considerably with respect to the
condition of the earth materials through which the wave passes. In seismic
testing, the shock wave can be generated by a sledge hammer and striking disk
(for limited explorations) or by blasting for larger explorations. The sledge
hammer-disk method can be used for distances up to 300 feet and at working
depths of about 100 feet. Blasting, although the preferred techniques for
large scale seismic studies, should only be used with extreme caution near
hazardous waste sites, if at all. The shock wave generated from either
technique is measured by a sensing device called a geophone, which is located
a designated distance from the generation point. The time interval between
shock wave generation and measurement and the distance between the two points
determines velocity.
Seismic profiles are used in the construction industry to classify the
type of excavation required at various depths. Generally, excavation can be
classified into three categories: common (non-ripping), mechanical ripping,
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and blasting. Ripping is further divided into soft, medium, hard, and
extremely hard ripping, which helps to describe the types of equipment
required, production rates, and unit costs. General guidelines for classify-
ing the type of excavation and specifying the equipment required with respect
to seismic velocities have been developed over the years by many correlative
studies. The guidelines are summarized in Figure 6-26.
As shown in Figure 6-26, if seismic velocities are over 7,000 feet per
second, rock can not be mechanically ripped and must be fragmented in another
manner. Blasting and other techniques greatly increase the cost of trench
excavation. Thus, the distance to the "drill and shoot" or blasting zone and
the amount of rock requiring blasting are major considerations with respect to
the feasibility of installing a subsurface drain.
FIGURE 6-26.
CORRELATION OF REQUIRED EXCAVATION METHODS WITH SEISMIC TESTING DATA
(DERIVED FROM CHURCH, 1961)
7000-
6000 —
- 5000 —
.e 4000-
8
3000
to
2000 —
1000
For Heavy-Weight
Tractor-Rippers
300-525 HP
100,000-160,000 Ibs.
For Medium-Weight
Tractor Rippers
200-300 HP
60,000-90,000 Ibs.
No
Ripping
Soft
Ripping
Medium
Ripping
Hard
Ripping
Ex. Hard
Ripping
or Blasting
Blasting
6-62
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The operations of mechanical ripping and blasting are obviously quite
different, and excavation by either means mandates a different set of
considerations. The equipment, methodology, and limitations with respect to
each of the techniques is briefly discussed in later sections.
6.4.1.2 Rock Fragmentation
6.4.1.2.1 Ripping
Mechanical ripping can now fragment rock of considerable consolidation
where previously only blasting was feasible. The ripper is basically composed
of a shank with a replaceable alloy steel tip attached to a tool bar. The
tool bar can be raised, lowered, or inclined by a series of arms and hydraulic
cylinders known as the power assembly. The ripper is attached to and pulled
by a tractor, which usually doubles as a push dozer. Rock ripping usually
takes place by lowering and angling the tip into the rock while the tractor
moves slowly along at a range of 0.75 to 1.50 miles per hour. Ripping occurs
in igneous and metamorphic rocks by a crushing and breaking action and in
sedimentary rocks by splitting and cleaving.
Rippers for trench excavation are composed of a single shank and tip
assembly, while rippers for other types of applications may have two or three
assembles mounted on the tool bar.
The maximum vertical reach of the largest trench ripper available is a
little over six feet (Alban Tractor Co., 1982). Therefore, the use of ripping
is limited to a trench depth of six feet. However, if the ripper can enter
the trench to rip the lower lifts, a greater depth can be reached. At some
trench depths, ripping may become relatively uneconomical to continue to use
because the trench width needed for clearance increases the volume of material
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to be excavated. Analysis of alternate construction techniques as a function
of depth is essential to minimize project costs.
6.4.1.2.2 Impact Methods
Impact methods can be used on a smaller scale in trench work to break
rock. The hourly outputs of impact methods are only about one percent of the
output of a ripper. However, they may be preferred in some cases even though
they are a more expensive option. Three types of equipment are available for
impact or percussive rock fragmentation—the hand-held jackhammer, the backhoe
mounted Hobgoblin, and the Darda rock and concrete splitter.
The hand held jackhammer is a small percussion drill operating on about
90 pounds per square inch of air pressure and imparting about 2,000 blows per
minute (Church, 1981). A jack hammer's use is limited to a depth of 16
inches. The average output of a jackhammer in limestone is about six cubic
yards per hour (Richardson, 1980). The Hobgoblin is a pneumatically driven
impact tool that is mounted on a backhoe arm and acts similar to a jackhammer.
A Hobgoblin also has a relatively low production rate (i.e., six cubic yards
per hour). The Darda rock and concrete splitter uses water as the percussive
force in splitting rock. The splitter exhibits deeper penetration and higher
output than the jackhammer or Hobgoblin, however, the addition of water could
exacerbate pollution from a disposal site. The Darda splitter has a
penetration depth of 26 inches and an average production rate of 13 cubic
yards per hour (Richardson, 1980).
6.4.1.2.3 Blasting
Blasting is not recommended for rock excavation purposes around hazardous
waste sites for a number of reasons. Blasting, even if well controlled, could
result in unwanted fracturing below the subsurface drain such that a bypass
for the contaminant plume is created. Ground vibration from blasting may also
cause unwanted fracturing of impermeable layers such as a previously installed
clay cap. The compression wave set off by blasting could cause secondary
detonation in a hazardous waste site that contains explosive materials.
6-64
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Blasting also requires extensive safety provisions, causes general citizen
concern (because of noise and flyrock and air pollution) and may cause damage
or failure of trench side slopes and shoring.
Blasting to excavate rock for a trench would be conducted using the
blasthole technique. In this technique, holes are drilled in rock in a
specified arrangement and explosives are loaded into the holes. The
percussive force of the detonation fractures the rock between the holes.
Controlled blasting is a technique to reduce unwanted fracturing of nearby
rock. This method involves drilling a line or lines of open holes that are
not loaded with explosives between the blastholes so to create a plane of
weakness for rock fragmentation. This action helps to contain fracturing.
However, even controlled blasting does not completely eliminate unwanted
fracturing. Blasting ranges from $38 to $44 per cubic yard of trench
(Godfrey, 1981).
6.4.1.2.4 Non-explosive Demolition
Non-explosive demolition is a viable alternative to blasting as a means
of fragmenting rock. In non-explosive demolition, holes are drilled into the
rock in a preselected pattern. The dry demolition agent is mixed with the
appropriate amount of water and is poured into the holes. As the material
dries it expands and exerts an expansive stress on the rock of more than 3,000
metric tons per square meter (A.M. Harris and Sons, Inc., 1982). The rock
then fractures because of stress induced by the material's expansion and the
pattern of the drill holes. The fracture mechanism is composed of three
phases—crack initiation, crack propagation, and the increase of crack width.
The direction of cracking can be planned by appropriately arranging the hole
spacing, depth, and inclination. Maximum hole depth is approximately 33 feet.
Non-explosive demolition is a process free from flyrock, noise, ground
vibration, gas, dust, and the other environmental and safety problems associ-
ated with blasting. However, the method is expensive compared to other
fragmentation methods because of the amount and cost of the materials needed.
Costs range from $300 to $470 per cubic yard of fragmented material. The time
6-65
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needed for crack initiation and propagation also limits the alternative's
feasibility.
6.4.1.3 Excavation Equipment
The following sections describe equipment that can be used to remove
earth or fragmented rock for the purposes of excavating a trench. Equipment
includes trenches, backhoes, clamshells, cranes, and bulldozers. In each of
the following sections, the specific applications, limitations, and production
costs are discussed.
6.4.1.3.1 Trenchers
Trenchers or ditchers are designed to provide continuous excavation in
soil and well fragmented or weathered rock. They consist of a series of
buckets mounted on a wheel (bucket-wheel type) or a chain sprocket and ladder
(bucket-ladder type). In continuous trenching, the wheel or ladder is lowered
as the revolving buckets excavate the trench to the appropriate depth. The
trench assembly may be mounted on wheels or on semi-crawler or full-crawler
frames. The trencher moves forward simultaneously as the trench is excavated
resulting in a trench of neat lines and grades. The bucket wheel types are
generally used to dig shallow trenches for agricultural drainage. The maximum
depth for a large wheel trencher is about 8.5 feet (Church, 1981). Bucket-
ladder type trenchers can excavate trenches up to 27 feet deep and about 6
feet wide, although 4 feet is the maximum economical width (Church, 1981).
Generally, continuous trenching in suitable materials is much faster than
trenching via backhoe. Hourly production rates for wheel and ladder trenchers
operating at 100 percent efficiency in various materials is given in Table
6-6. Actual efficiencies for the entire spectrum of job conditions may range
from 20 to 90 percent (Church, 1981). Trenchers are much more efficient in
rural settings where fewer obstructions are present.
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TABLE 6-6
APPROXIMATE HOURLY PRODUCTION IN CUBIC YARDS FOR LADDER AND
WHEEL TRENCHERS OPERATING AT 100 PERCENT EFFICIENCY (CHURCH, 1981)
Rock-earth formation
Alluvium, sand-gravels, lightly cemented
Weathered rock-earth:
Maximum weathering
Minimum weathering
Engine
50
210
180
120
60
horsepower
100
420
360
240
120
of
150
630
540
360
180
trencher
200
840
720
480
240
Trenchers can be equipped with back-end modifications to provide shoring,
install a geotextile envelope, lay either tile or flexible piping, blind the
piping, and backfill with gravel or with excavated soil. Thus, they can be
designed to perform practically all drain installation functions. Costs for
trenching will vary according to the specific site conditions. Costs for
deep-trenching using a ladder trencher at 31 percent efficiency (urban
setting) were estimated at $1.25 per cubic yard in 1978, including the cost of
the trencher, operation and maintenance costs, and manpower (Church, 1981).
6.4.1.3.2 Backhoes
The backhoe is an inverted shovel operated mechanically, hydraulically,
or by cable and used for trenching and other subsurface work. A backhoe is
used at job sites where the excavator must be kept at ground level and where
subsurface structures are numerous. Backhoes can excavate earth and
fragmented rock up to one-half the bucket diameter. Large modified backhoes
can have a digging depth range of over 70 feet; however, their large size may
exclude their use in urban areas.
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Production rates for various sizes of backhoe dipper buckets are given in
Table 6-7. These rates are based on 100 percent working efficiency. Actual
working efficiencies range from 33 to 83 percent depending on the particular
site conditions (Church, 1981).
Unit costs for excavating trenches at various depths using various sizes
of backhoes are given in Table 6-8.
6.4.1.3.3 Clamshells and Draglines
Clamshells and draglines are crane mounted buckets which are sometimes used in
trench excavation operations. Clamshells are used when the trench depth is
below the limit of other equipment (about 70 feet) or when access is a problem
that prevents the use of other equipment. The use of clamshells is limited to
loose rock or earth materials. Production rates for clamshells are low
compared to other methods. Typical production rates are from 20 to 35 cubic
yards per hour. Costs for clamshell excavation have been estimated to be
$2.93 to $4.34 per cubic yard. If excavation is taking place along sheeting
or a cofferdam, costs could rise to above $14 per cubic yard excavated.
Draglines are sometimes used to remove loose rock and earth in trench-
work. The dragline bucket is cast outward by the crane and pulled back as it
scrapes the rock or earth into the bucket. Draglines are generally used when
a large reach is required for loading or casting. Hourly production rates at
100 percent efficiency are on the same order as a backhoe, as shown in Table
6-9. Costs for excavating by dragline for 0.75 and 1.5 cubic yard buckets
have been estimated to be $2.47 and $2.76 per cubic yard respectively
(Godfrey, 1981).
6.4.1.4 Wall Stabilization Methods
Deep trenching requires that some form of wall stabilization technique be
used to prevent cave-in during installation of drain pipe. The most common
technique used to reinforce trench openings is to use shoring, which is a
network of wood or steel braces or both. Freezing the ground making up the
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TABLE 6-7
THEORETICAL HOURLY PRODUCTION OF A HYDRAULIC BACKHOE (EPA, 1982)
Soil
Moist
Material
1
loam, 85
Bucket
1.5 2
125 175
Size (cubic yards)
2.5 3 3.5
220 275 330
4
380
sandy clay
Sand
and gravel 80
Common earth 70
Clay,
hard dense 65
PRODUCTION COSTS FOR
Trench Depth
(feet)
8
10
12
14
16
18
20
120 160
105 150
100 130
TABLE 6-8
TRENCHING USING
205 260 310
190 240 280
170 210 252
BACKHOES (GODFREY, 1982)
365
330
300
Hoe Bucket Size $/cu.yd.
(cubic yards)
0.5
0.75
1
1.25
2
2.5
3.5
2.92
2.34
1.95
1.88
1.57
1.53
1.27
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TABLE 6-9
THEORETICAL HOURLY PRODUCTION RATE
OF A DRAGLINE EXCAVATOR (EPA, 1976)
Type of soil
Bucket size (cubic yards)
Moist loam,
sandy clay
Sand and gravel
Common earth
Clay, hard
dense
1
130
(6.6.)
130
(6.6)
110
(8.0)
90
(9.3)
1.5
180
(7.4)
175
(7.4)
160
(9.0)
130
(10.7)
2
220
(8.0)
210
(8.0)
190
(9.9)
160
(11.8)
2.5
250
(8.5)
245
(8.5)
220
(10.5)
190
(12.3)
3
290
(9.0)
280
(9.0)
250
(11.0)
225
(12.8)
3.5
325
(9.5)
315
(9.5)
280
(11.5)
250
(13.3)
4
385
(10.0)
375
(10.0)
375
(10.0)
280
(12.0)
Note: Numbers in parentheses represent the maximum digging depth.
trench wall is another common technique usually applied to larger underground
construction jobs. In many cases the trench is cut with walls sloped to the
angle of soil stability.
6.4.1.4.1 Shoring
Shoring involves supporting the trench walls with wood or steel
structures. A number of different types of shoring exist. Many construction
firms use slip shields, which are also known as sliding caissons. Slipshields
are constructed on-site by welding I-beams between two parallel pieces of
sheet steel.
The slipshield is constructed for a specific trench width. When a
section of trench is excavated, the slipshield is lowered into the trench
using a crane. Pipe installation then begins. After piping is installed and
6-70
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the trench is filled with gravel, the slipshield is eased out of the trench
with the crane and moved to the next excavated section. Use of the slipshield
is limited to the load capacity of the crane.
Adjustable aluminum bracing can also be used for trench wall stabiliza-
tion. Bracing is available for trenches up to 3 feet wide at depths from 5 to
10 feet and spaced on 4 foot centers. Costs for rented aluminum bracing has
been estimated to be $1.95 to $2.75 per linear foot of trench (Richardson,
1980). Aluminum bracing is much more economical than using wooden braces,
which for the same type excavation would cost about $2.50 to $3.25 per linear
foot of trench.
For deeper trenching, other methods of shoring are available. Steel
sheet piling can be driven and braced to support the trench walls. Also,
steel H piles can be driven with horizontal wooden beams inserted between
them.
Solid wooden shoring is also used for deeper trenches. Wooden shoring
consists of vertical wooden planks supported by cross pieces called wales,
which are braced across the trench opening. Costs for the above methods are
summarized in Figure 6-27.
6.4.1.4.2 Ground Freezing
Ground freezing is a method of wall stabilization and groundwater cutoff
using the technique of refrigeration. The method has been practiced for a
number of years by some specialty firms, but has not been applied widely to
earth or rock excavation (Church, 1981). Groundwater freezing involves
installing a series of vertical refrigeration pipes into the ground next to
the area of trenching. Heat energy is removed from the soil via recirculating
brine in the refrigeration pipes until the ground freezes. Granular soils
achieve excellent strength only a few degrees below the freezing point. Clay
soils may require substantially lower temperatures (down to -20°F) to attain
the required strength. Although very expensive, ground freezing can also
serve as a groundwater cut-off wall. The combination of both these functions
6-71
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FIGURE 6-27.
COSTS OF SHORING FOR DEEP TRENCHES
(GODFREY, 1961)
01
I
«j
W
II
26
24
22
20
18
16
14
12
10
8
6
4
2
Soldier Beams and Lagging
Sheet Piling and Braces
Solid Wooden Shoring
10
15
20
25
Trench Depth (ft.)
30
35
40
45
may make it a suitable technique for trenchwork depths of 25 feet and greater
(Freezwall, Inc., 1980).
6.4.1.4.3 Open Cuts
In shallow trenches, trenches may possibly be dug with sloped walls so
that a stable angle is attained and shoring is not required. This is called
an open cut. The application is of course limited to shallow trenches in most
areas, since the width of the trench at the surface will usually be limited by
other construction. Railroad and highway design standards specify a 1.5
(horizontal) to 1 (vertical) slope for most conditions and a 2 to 1 slope for
very soft soils. Open cuts require that more earth is excavated, so costs for
6-72
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excavation and backfilling will be higher than for a vertical wall trench.
However, there may be an overall savings because of the lack of shoring, which
is generally a more expensive endeavor.
6.4.1.5 Dewatering
Construction of a subsurface drain to intercept or divert groundwater
requires excavation to a depth below that of the water table. Maintaining
proper grade, placement, and alignment of drainage pipe are operations in
subsurface drain installation which are best carried out in a "dry" environ-
ment. Three basic options are available for dewatering the subsurface:
• Open pumping
• Predrainage using well points or well systems
• Groundwater cutoff.
These techniques may be used separately or in combination to accomplish
dewatering. Open pumping and predrainage require the pumping of water from
the trench excavation. In the case of installing a subsurface drain down-
gradient of a leachate plume, there is a strong likelihood that these
dewatering operations will be handling contaminated groundwater. Treatment of
contaminated groundwater is required for those dewatering technologies
requiring pumping. This may include pumping or trucking to an existing
wastewater treatment plant, using on-site treatment equipment, reinjecting the
contaminated groundwater into the waste site, or installing and operating the
permanent on-site treatment plant which will treat the total flow when the
drainage system is operational. Selection of the proper treatment scheme
depends on many factors, including the size of the excavation, the volume of
contaminated pumpage, the duration of excavation, the concentrations and kinds
of contaminants, and the costs for the various alternatives.
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6.4.1.5.1 Open Pumping
Open pumping is the direct removal of water that has seeped into the
excavation. This method is the least expensive of the dewatering technolo-
gies. Open pumping requires the construction of a sump hole or pit at the
lowest point of the excavation, so that water can flow towards and collect in
the pit. A sump pump, which may be one of several types, pumps the
accumulated water from the sump out of the excavation.
Open pumping is applicable to shallow trench excavations with stable
soils of low hydraulic conductivity where groundwater seepage into the
excavation is minimal (Powers, 1981). There is a possibility of side wall
slumping in open dewatering, therefore, the method is most applicable in
excavations near open, undeveloped areas. Open dewatering is used frequently
with groundwater cutoff techniques. Open dewatering should not be used in
areas where soils are unstable, such as uniform granular soils without plastic
fines or soft granular silts and clays because of the danger of slope
instability. The flow of water in an excavation can also cause erosion, which
may limit the use of open dewatering. Conditions of high flow caused by
materials with moderate to high hydraulic conductivities or a large head also
limit the use of open dewatering as a viable method (Powers, 1981).
As mentioned above, there are two basic components in an open pumping
system—the sump hole and the pump. The sump consists of a pit dug several
feet below subgrade that is large enough to provide ample settling time to
remove sediments. A submersible pump or inlet line from a surface pump is
placed in the pit about a foot above the pit bottom. The pit is surrounded by
a large diameter corrugated steel pipe which has been perforated. The sump
area around the corrugated steel piping is packed with gravel or crushed rock
to provide protection against erosion and to trap sediments that can damage
the pump. Cleaning and maintenance of sumps is an ongoing chore during
excavation. In large trench excavations, there may be multiple sumps, each
handling a portion of the total flow.
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Three types of pumps are available for removing water from a sump: cen-
trifugal suction, centrifugal submersible, and diaphragm pumps. Centrifugal
suction pumps are used most commonly for removing large volumes of water, such
as during springtime, after flooding, and after shutdown operations. Specifi-
cations for various sizes of centrifugal suction pumps are given in Table
6-10.
Centrifugal submersible pumps operate underwater and are used commonly in
sump holes. Specifications for various size submersible pumps are shown in
Table 6-11. Diaphragm pumps have the ability to handle many materials ranging
from clean water to water containing muds, sands, small rocks, and miscellane-
ous trash. They are used commonly in the most difficult dewatering operations
because of their ability to handle a wide range of materials. Specifications
for diaphragm pumps are given in Table 6-12. Costs for sump pit construction
and pumping are given in Table 6-13.
6.4.1.5.2 Predrainage Using Well Points or Wells
Well points and deep wells can be used to lower the water table near a
trench excavation. Well points are one of the most widely used and most
versatile dewatering technologies. The use of well points and deep wells and
associated costs are discussed in Chapter 5.
6.4.1.5.3 Groundwater Cutoff
Groundwater cut-off barriers are used to reduce groundwater flow into an
excavation. Open pumping is generally used in conjunction with cutoff
barriers to handle the low rate of seepage through the barrier. Cutoff
barriers may consist of steel sheet piling, concrete, or a bentonite slurry.
Frequently these techniques are used as leachate control technologies in
themselves as discussed in Chapter 7. Ground freezing is also considered a
groundwater cutoff technique, and although expensive, may be a viable
technique considering it serves the dual purpose of shoring and acting as a
groundwater cutoff.
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TABLE 6-10
TYPICAL CHARACTERISTICS OF SELF-PRIMING CENTRIFUGAL PUMPS
(DELIVERY IN GALLONS PER MINUTE ACCORDING
TO TOTAL HEAD; CHURCH, 1981)
Total Head Height of Pump
Including Friction Above water
15 ft
20 ft
25 ft
30 ft
40 ft
50 ft
55 ft
25 ft
30 ft
40 ft
50 ft
60 ft
70 ft
30 ft
40 ft
50 ft
60 ft
70 ft
30 ft
40 ft
50 ft
60 ft
70 ft
80 ft
10 ft
Model 4-M, 1 1/2 in
67
66
65
63
54
37
25
Model 10M, 2 in
166
164
157
145
122
85
Model 15-M, 3 in
250
230
200
160
110
Model 20-M, 3 in
333
310
275
220
160
90
20 ft
47
47
44
34
25
115
1 1 1
107
97
75
170
165
155
138
100
235
230
220
195
155
90
Model 30-M, 4 in light
30 ft
40 ft
50 ft
60 ft
70 ft
80 ft
90 ft
100 ft
NOTE: Shaft or
culated by the
hp = gal/rain x
500
495
475
450
415
355
250
100
brake horsepower must
equation:
8.34 x head
350
345
340
325
300
270
215
100
be taken
Total Head Height of Pump
Including Friction Above Water
25 ft
30 ft
40 ft
50 ft
60 ft
70 ft
80 ft
90 ft
100 ft
110 ft
25 ft
30 ft
40 ft
50 ft
60 ft
70 ft
80 ft
90 ft
100 ft
25 ft
30 ft
40 ft
50 ft
60 ft
70 ft
80 ft
90 ft
100 ft
110 ft
10ft
Model 40-M, 4 in heavy
665
660
645
620
585
535
465
375
250
65
Model 90-M, 6 in
1500
1480
1430
1350
1225
1050
800
450
100
Model 125-M, 8 in
2100
2060
1960
1800
1650
1460
1250
1020
800
570
from manufacturer" s performance curves
20 ft
475
465
455
435
410
365
300
195
50
1050
1020
970
900
775
600
265
100
1570
1560
1520
1450
1360
1250
1110
940
710
500
or cal-
efficiency x 33,000
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TABLE 6-11
REPRESENTATIVE SPECIFICATIONS AND PERFORMANCES OF
CENTRIFUGAL SUBMERSIBLE PUMPS (CHURCH, 1981)
Specification
Nominal Discharge Diameter (inches)
Pump:
Discharge, diameter, in 2 4 68
Impeller, diameter, in 11 23 8 28
Speed, r/min 3450 1750 1750 1750
Motor, electric, AC, 60 cycle:
Horsepower 2 25 60 95
Speed, r/min 3450 1750 1750 1750
Phase Single Three Three Three
Voltage 115-230 230-460 230-460 230-460
Complete machine:
Cable length, ft 50 50 50 50
Strainer: ?
Open area, in 38 48 48 48
Openings size, in 1/4 3/8 1 1
Dimensions, in:
Height 23 46 57 57
Diameter 16 32 39 39
Weight, approximate, Ib 50 150 450 800
Performance in gal/min
for total head of:
20 ft 130 930 2160 2800
40 ft 100 870 1950 2720
60 ft 70 780 1720 2570
80 ft ... 680 1480 2370
100 ft ... 550 1100 2120
120 ft ... 360 400 1800
6-77
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TABLE 6-12
REPRESENTATIVE SPECIFICATIONS AND PERFORMANCES
OF DIAPHRAGM PUMPS (CHURCH, 1981)
Specification
Nominal Discharge Diameter (inches)
Pump, single-acting:
Strokes per minute
Length of stroke, in
Usable diameter of diaphragm, in
Displacement, in
Prime mover:
Diesel or gasoline engine, hp
Electric motor, as 60-cycle:
Horsepower
Speed, r/min
Phase and voltage
Complete machine:
Weight, wheels-tires-mount, Ib
Diesel engine
Gasoline engine
Performances, gal/min versus total head:
5-ft suction head with discharge
head of:
5 ft
15 ft
25 ft
15-ft suction head with discharge head
5 ft
15 ft
25 ft
25-ft suction head with discharge head
5 ft
15 ft
25 ft
60
2 1/2
7
71
3
1/2
1750
115-230
230
150
150
26
22
18
of:
21
19
17
of:
18
16
15
60
2 13/16
11 1/2
221
6
1 1/2
1750
115-230
350
200
200
78
66
64
68
64
60
56
60
56
52
3 3/4
12 3/4
381
10
3
1750
115-230
500
400
350
125
113
104
97
74
70
82
66
52
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TABLE 6-13
COST FOR OPEN PUMPING (GODFREY, 1981)
Item
Unit Cost
Sump hole construction,
includes excavation, and gravel
Corrugated pipe collar (installed)
12" pipe
15"
18"
24"
Pumping 8 hr., attended 2 hrs. per day,
including 20 feet of suction, hose, and
100 feet discharge hose (rental)
2" diaphragm
4" diaphragm
Pumping 8 hr., attended 8 hrs. per day (rental)
2" diaphragm
4" diaphragm
3" centrifugal
6" centrifugal
Automatic Submersible sump pumps (centrifugal)
Bronze
TTT74~"inch, 26 GPM at 8 foot head
1-1/4 inch, 45 GPM at 8 foot head
1-1/4 inch, 63 GPM at 8 foot head
1-1/4 inch, 87 GPM at 8 foot head
Cast Iron
1-1/4 inch, 26 GPM at 8 foot head
1-1/4 inch, 45 GPM at 8 foot head
1-1/4 inch, 63 GPM at 8 foot head
$0.80/cu ft
$18/ft
$22/ft
$25/ft
$35/ft
$77/day
$85/day
$310/day
$340/day
$315/day
$380/day
$180/ea
$190/ea
$280/ea
$375/ea
$115/ea
$250/ea
$280/ea
Cast iron diaphragm pumps (starter & level control included)
2" discharge
10 GPM at 20 foot head
60 GPM at 20 foot head
120 GPM at 20 foot head
160 GPM at 20 foot head
3" discharge
220 GPM at 20 foot head
$370/ea
$460/ea'
$850/ea
$1450/ea
$1950/ea
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6.4.1.6 Grade Control
Proper grade in a subsurface drain ensures against ponding of water and
provides for a non-silting velocity in the drainage pipe. Specifications for
grading were discussed previously. The control of grade during trench
excavation is important so that the grade of the as-built drainage system is
as close as possible to the design grade. Poor grade control can result in
siltation or the need for extensive backfilling and leveling, which drives up
costs.
There are two categories of grade control that are presently used with
mechanical trench excavators—visual and automatic laser control. The visual
method is known as target grade control. In target control, grade stakes (of
equal length) are driven to the design subgrade along every 100 feet of the
trench line. A line drawn through the tops of the grade stakes would be
parallel to the design grade of the trench. Targets are driven next to the
grade stakes and are adjusted to a fixed distance above the elevation of the
grade stakes. The selection of this distance depends on the depth of the
trench and the line of sight between the machine operator and a reference
sighting rod on the machine. When the trenching machine is cutting on grade,
the targets will align with the reference sighting rod.
Accuracies of plus or minus 0.1 foot are easily obtainable with the
target method. If the depth of the trenches is also checked with respect to
the design grade at each target point, the target grade control method can
attain an accuracy of within plus or minus 0.02 foot of the design grade
(Taylor and Willardson, 1971).
Target grade control enables the early detection of errors in setting
grade stakes or targets because any misalignment will be apparent. Changes in
grading can be accomplished quickly by resetting targets. Target grade
control always proceeds trench excavation and even with modifications, does
not hinder the progress of conventional trench excavation machines.
6-80
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Target grading has a few disadvantages. The precision and accuracy of
grade control depends on the machine operators skills and alertness in
checking grades constantly to ensure proper control. Fog and other visual
obstructions may require that targets be placed closer together. Large
numbers of targets are difficult to handle and they will tend to have very
limited lifespans because of breakage and accidental burial.
An alternative to the target control system is the use of automatic grade
control with laser beams. Many conventional trenchers (including some back-
hoes) are equipped with this control system. The basic system consists of a
portable, tripod-mounted, low-power laser beam projector and a machine-mounted
electronic tracker-receiver (Van Schilfgaarde, 1974). The laser beam
projector emits a beam set to an elevation or grading datum, or both. The
receiver-tracker adjusts the trenching unit hydraulically to automatically
control depth and trench grade. A number of commercial systems are available,
some eliminating the need to ever reset the laser projector, thus enabling
change of grade at any point along the trench excavation.
Costs for automatic grade control vary widely. A laser beam sending unit
with tripod mounting costs approximately $8,500 (Lazer Plane Corp., 1982). A
machine-mounted receiver for a backhoe, which simply indicates depth, is
estimated to cost approximately $3,500 (Lazer Plane Corp., 1982). The most
sophisticated receiver-tracker for a continuous trencher, enabling grade
change at any point during excavation, without resetting the sender would cost
from $7,500 to $11,500 (Lazer Plane Corp., 1982).
6.4.2 Drain Installation
Once trench excavation is completed, the components of the subsurface
drain can be installed. The process consists of the installation of drain
pipe bedding, the drainage pipe or bed, the gravel or soil envelope, the
filter fabric, the backfill, and any auxiliary components. Modified contin-
uous trenching machines exist which accomplish all excavation and drain
installation operations simultaneously. However, simultaneous excavation/
installation machinery is limited to small diameter drains in which the pipe
6-81
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can be inserted into an automatic pipe feeder and can be carried along on
rolls or wagons. Larger diameter pipe drains require separate operations.
Installation of the various components of a subsurface drain are discussed
below.
6.4.2.1 Drain Pipe Bedding
The bedding for a subsurface drain depends somewhat on the type of
drainage pipe to be installed and the expected loading on the pipe from the
backfill. The simplest bedding material used is gravel or crushed rock.
Laying a gravel bed simply consists of bringing the trench to design grade by
excavation and backfilling and then spreading an even layer of gravel over the
trench floor. The pipe is set by placing additional gravel around the pipe.
In the case of partial dewatering of an unstable trench floor, the trench is
often over excavated and backfilled with a larger sized stabilizing gravel, as
specified in Table 6-14. This enables proper drainage to the sump without
soil loss and instability. The bedding material can then be laid on top of
the stabilized floor.
TABLE 6-14
SPECIFICATIONS FOR STABILIZING GRAVEL
Gradation of stabilizing material Percent
Retained on 5-inch screen 0
Retained on 4-inch screen 0 to 20
Retained on 3-inch screen 0 to 30
Retained on 2-inch screen 20 to 50
Retained on 3/4-inch screen 20 to 50
Passing No. 4 sieve Less than 8
6-82
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Gravel beddings are used with larger diameter drainage pipes of perfo-
rated concrete, vitrified clay, and rigid plastic construction. For alignment
purposes, trench floors are often cut in a semicircle so that the gravel fill
forms a cradle for the pipe. Costs for gravel are approximately $8.60 to
$10.10 per cubic yard (Godfrey, 1981).
In tile drainage systems where proper alignment is crucial, the trench
floor is brought up to grade with soil compacted to its original density. A
small V-notch is then excavated along the trench bottom to serve as an align-
ment guide for laying the tile. Soil must be removed around bell-and-spigot
joints for proper alignment. Bedding for corrugated flexible pipe consists of
digging a semi-circular groove instead of a V-notch to prevent pipe collapse
during backfilling. A groove may not be required if backfilling is very well
controlled, such as on continuous excavation and installation machines. As
mentioned earlier, bedding may be part of a continuous excavate, install drain
and backfill procedure or it may be conducted separately from the other
operations.
When unstable soils are encountered (oftentimes because of dewatering
problems), timber cradles may be used to ensure proper grade and alignment.
Timber cradles consist of wooden supports on which the pipe is laid. These
cradles are supported by piles or long stakes driven into a solid base. This
is an expensive operation since each pile or stake must be checked to be sure
it is aligned, driven into a solid base, and cutoff at the proper grade.
Timber cradles are used mostly for concrete and clay pipe. Concrete pipe
cradles are also used for these purposes.
6.4.2.2 Installation of Drain Pipe
Installation procedures for drain pipe vary widely according to the type,
diameter, and length of pipe being used. Pipe installation is usually
initiated at the lowest trench elevation and proceeds upgrade. Flexible
plastic pipe can be installed from rolls mounted on a trenching machine and
continuously fed into the trench as the excavation proceeds. Flexible pipe
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can also be fed by hand from rolls mounted on wagons. Lengths of pipe can be
joined at the surface between rolls.
The installation of flexible plastic pipe has some inherent disadvan-
tages. Stretching the pipe will cause a loss of structural strength and
possible collapse. Stretching should be guarded against, especially in
periods of hot weather. Some drain contractors use sun shields over their
pipe rolls in hot weather work. Brittleness and difficulty in unbending are
problems that are encountered in cold weather. Also, flexible plastic pipe
tends to float and partial dewatering may present problems.
The installation of rigid pipe can not be done automatically. Laying
tile pipe can be a semi-automatic operation in which a worker feeds tile into
a tile chute which properly aligns, spaces, and holds each tile until the
backfill is added. Where bell-and-spigot type joints are used, the bell end
should always be installed upgrade. When tiling manually, care must be taken
to ensure proper spacing between tiles. A tile should always be held by hand
or other means until the next tile is ready to be placed. Cracked or broken
tile should be discarded (Van Schifgaarde, 1974). If a tile is warped, the
tightest fit should be made on the top. Tile should be cut to maintain even
spacing when working around curves.
Long lengths of rigid pipe are either hand carried or lowered by crane
into the trench for installation depending on their size. Perforated pipes of
PVC, polyethlene, concrete, bituminous fiber, and vitrified clay commonly come
in long lengths.
When extending drainage systems under roadways, structures, root zones,
or areas not requiring drainage, unperforated pipe (or in the case of tile,
cemented joints) should be specified.
Installed costs for various types and sizes of pipe, excluding excavation
and backfilling, are given in Table 6-15.
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TABLE 6-15
INSTALLATION COSTS FOR DRAINAGE PIPE
(GODFREY, 1981)
Item
$ Per Linear Foot
Asbestos Cement, class 4000 underdrain, perforated
4" diameter
6" diameter
8" diameter
10" diameter
12" diameter
Unperforated, 4" diameter
Bituminous fiber, perforated underdrain
3" diameter
4" diameter
5" diameter
6" diameter
Corrugated steel or aluminum perforated, asphalt coated
6" diameter, 18 ga.
8" diameter, 16 ga.
10" diameter, 16 ga.
12" diameter, 16 ga.
18" diameter, 16 ga.
Plain steel or aluminum, corrugated
6" diameter, 18 ga.
8" diameter, 16 ga.
10" diameter, 16 ga.
12" diameter, 16 ga.
18" diameter, 16 ga.
3.30
4.33
6.40
7.25
9.60
2.57
2.10
2.32
3.37
3.94
4.16
5.45
6.90
8.60
12.15
3.78
5.10
6.50
8.15
11.35
Porous wall
concrete underdrain, std. strength
4" diameter
6" diameter
8" diameter
12" diameter
15" diameter
18" diameter
3.17
3.35
4.80
7.70
9.15
12.30
(Continued)
6-85
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TABLE 6-15 (continued)
Item $ Per Linear Foot
Porous wall concrete underdrain, std. strength (continued)
Extra strength
6" diameter 3.57
8" diameter 4.96
10" diameter 7.90
12" diameter 9.10
15" diameter 10.35
18" diameter 14.00
Vitrified clay, perforated, 2' lengths (C-211)
4" diameter 3.09
6" diameter 4.45
8" diameter 7.10
12" diameter 10.10
Vitrified clay sewer pipe, premium joint (C-200)
4' and 5' lengths, 4" diameter 4.10
4' and 5' lengths, 6" diameter 5.60
8" diameter 7.20
10" diameter 9.45
12" diameter 12.25
15" diameter 19.75
18" diameter 26
24" diameter 55
30" diameter 73
36" diameter 110
3' lengths, add 30% to above
2' lengths, add 60% to above
PVC, perforated underdrain , ,
3" $0.64}aj
4" $0.82
6" $1.85
Flexible PVC, perforated, 4" $0.29
(b)
;?
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6.4.2.3 Gravel or Soil Envelopes
Gravel envelopes are installed around the pipe drain to increase flow
into the drain and reduce the buildup of sediments in the drain line. Gravel
envelopes may be placed by hand, backhoe, or by a hopper cart or truck. In
continuous trencher-drain installation machines, gravel filling may be ongoing
along with other operations.
When placing the gravel envelope, the gravel must be in complete contact
with the pipe. Specifications for gravel envelopes around clay pipe call for
a minimum of 12 inches of gravel envelope above the pipe for structural
support. The horizontal thickness of envelopes varies according to design
considerations, as discussed previously.
When fine grained soils are encountered, a dual layered aggregate gravel
envelope can be installed. To install the dual layered envelope, fine
aggregate is placed to form the bottom of the outer envelope. A narrow strip
of coarse aggregate is then placed to form the bottom of the inner envelope.
After the drainage pipe is installed, the sides and tops of the two layer
system are then completed.
t^f,f- ^
Costs for placing gravel fill are $8.60 and $10.10 per cubic jfpot, for
bank run and screened gravel, respectively (Godfrey, 1981).
If the existing soil hydraulic conductivity is sufficient, the existing
soil may be backfilled as the envelope material. This practice is commonly
used in agricultural drainage, however, a greater degree of control is desired
for drainage at waste disposal sites. Therefore, for leachate plume control,
envelope materials should consist of a hydraulic conductivity significantly
higher than the surrounding strata.
6.4.2.4 Filter Fabrics
Filter fabrics are sometimes installed around the gravel envelope to
prevent fines from clogging the envelope and drain pipe. Fabrics function by
6-87
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creating a graded soil filter against the fabric. Non-woven filter fabrics
can be used successfully in sand and gravel base materials containing little
or no silt or clay, i.e., up to 1 percent (Wolbert-Master, Inc., 1982). When
soils contain substantial percentages of silt or clay, the non-woven fabric
eventually forms a filter cake and prevents the groundwater from entering the
envelope. Woven fabrics can be used in strata with significant concentrations
of silt, i.e., 5 to 10 percent (Wolbert-Master, 1982).
When constructing a drain using a fabric filter wrapping, the fabric is
installed first, followed by the bedding, the pipe, and the envelope in that
order. The fabric filter is then wrapped around the top of the envelope prior
to backfilling with soil. A schematic of an installed pipe drain with filter
fabric is shown in Figure 6-28. Fabric filters can be installed manually or
by machine.
FIGURE 6-28.
PIPE DRAIN WITH FILTER FABRIC
Perforated
Pipe
Backfill
Envelope
Geotextile
Fabric Filter
The costs for installing geotextile fabrics vary somewhat depending on
the difficulty of installation. Material costs are typically $0.75 to $0.85
per square yard of fabric for non-woven, spun bound synthetics, and about
6-88
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$1.60 to $2.70 for the heavier, traditional woven-type fabric (Wolbert-Master,
Inc., 1982). Costs for simple installations are typically $0.10 to $0.15 per
square yard of material. In deep trenches, where installation is more
difficult, rolls of material may have to be sewn together and installation
costs have been known to range from $2.00 to $5.00 per square yard
(Wolbert-Master, Inc., 1982).
6.4.2.5 Backfilling
After the gravel envelope has been installed, the trench must be back-
filled to the original grade. Almost any type of excavation equipment can be
used to backfill trenches. In addition to the previously mentioned excavation
equipment, bulldozers, bucket loaders, scrapers, or front end loaders can be
used. Many times a backhoe combined with a front end loader will be used on
the job site, with excavation being done by the hoe end (i.e., the back end)
and loading or backfilling being done by the front end. Backfilling is done
simultaneously in some continuous trenchers. The use of a geotextile fabric
on the top of the envelope may be advisable to prevent siltation of the
envelope from the backfill material. In order to prevent settling of the
backfill after construction, periodic compaction of soil lifts is also
required. This may be accomplished using air tamping, or a vibrating or
sheepsfoot compactor. An additional compaction technique developed for
agricultural drainage is called puddling. Puddling involves inundating a soil
lift with water to cause compaction. Dams are built across the trench every
200 feet to provide suitable conditions for puddling (Van Schilfgaarde, 1979).
Puddling may not be advisable because the water applied may contact contam-
inated water or soil near the trench and thus require treatment.
The top two feet of trench should be backfilled with topsoil (if origi-
nally present) to reestablish the ambient vegetation. Estimated costs and
daily outputs for backfilling by dozer with and without compaction is given in
Table 6-16.
6-89
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TABLE 6-16
ESTIMATED COSTS AND OUTPUTS FOR BACKFILLING BY DOZER
(Godfrey, 1981)
Item
Dozer backfilling in trench,
up to 300 foot haul , no
compaction
Air tamped
Compacted Backfill, 6" to
12" lifts, vibrating roller
Sheepsfoot roller
Daily Output (yd3)
825
235
460
410
Cost/yd3
$1.09
$4.92
$2.11
$2.39
6.4.2.6 Manholes and Wet Wells
Manholes are used in subsurface drainage systems to serve as junction
boxes between drains, silt and sand traps, observation wells, and access
points for pipe location, inspection, and maintenance. Manholes should be
located at junction points, changes in alignment or grade, and other
designated points. There is not a set criteria for manhole spacing.
A manhole should extend from 12 to 24 inches above the ground surface for
ease of location. The base of the manhole should be a minimum of 18 inches
below the lowest pipe to provide a trap for sediments. Manholes are typically
designed to have a drop in elevation between the inlet and outlet pipes, to
compensate for head losses in the manhole (Bureau of Reclamation, 1978). A
typical manhole design is shown in Figure 6-29.
A manhole is not required for the upper end of a drainage line, however,
the end should be plugged to prevent debris from entering. Riser pipes may be
put at the upper end of a drainage pipe for cleanout purposes. These should
be installed at an angle enabling access for pipe cleaning equipment. Costs
for the installation of concrete manholes is given in Table 6-17.
6-90
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FIGURE 6-29.
TYPICAL MANHOLE DESIGN FOR A CLOSED DRAIN
(BUREAU OF RECLAMATION, 1978)
M Bars « 12"
OC Both Ways
Onter of Cover
Handle - M Bar
Note Use chain or other locking
device between handles
Plan
Cover
'- }t£j&w^
f)
12" Mm
24" Max
Ground Surface
36" Mm for dram pipe up
to and including 12" diameter
Manholes receiving three
or more large size pipe and
all boxes receiving larger
than 12" pipe should have
a dimension of 42"
Standard Precast Unrein-
forced Concrete Pipe
M Bars O 12"
O.C Both Ways
Canter of Base
Loops, if us«d, should be placed J
close to trwde of manhole
Base
Break tower section of manhote in the
fiekJ so that rough circular opening is
formed to receive pipe After sections are
fitted in place, grout carefully to bring
pipe to grade and place gravel packing
around pipe as directed
Vertical Section
Concrete Base. Precast or
Cast in Place, Square or
Round
6-91
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TABLE 6-17
INSTALLED COSTS FOR MANHOLES
(Godfrey, 1981)
Item
Cost ($)
Concrete slab, cast in place,
8" thick,
6' deep
8' deep
12' deep
16' deep
20' deep
Precast concrete riser pipe,
4' inside diameter,
6' deep
8' deep
12' deep
16' deep
20' deep
6' inside diameter,
6' deep
8' deep
12' deep
16' deep
20' deep
Slab tops, precast, 8" thick,
4' diameter
5' diameter
6' diameter
Frames and covers,
watertight,
24" diameter
32" diameter
light traffic,
24" diameter
36" diameter
630
865
1,345
1,825
2,305
510
690
781
872
963
1,075
1,475
1,665
1,855
2,045
122
136
170
310
405
185
355
6-92
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Manholes are also used as wetwells and as silt traps in conjunction with
a wetwell, so that pumps can be protected from abrasive sediments. A typical
design of a combined sediment trap and wet well for pumping collected
groundwater to the surface is shown in Figure 6-30.
FIGURE 6-30.
TYPICAL DESIGN OF SEDIMENT TRAP AND WETWELL
• Pump
Pump
Screen
6.4.3 Inspection and Maintenance
Proper and frequent inspection of the subsurface drain during construc-
tion will often uncover potential problems that might otherwise remain
undetected for years. The following items should be checked during
construction:
o Quality of tile, tubing, pipe and other materials
o Alignment, depth, and grade of drain and bedding, (although minor
variations are acceptable, there should never be a reverse grade)
6-93
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• Trench width at the top of the drain
o Spacing of tile joints and integrity of other connections
• Bedding, filter, and envelope materials and installation
• Backfilling and compaction
0 Auxiliary structures including manholes, cleanout riser pipes,
sediment traps and wet wells, and pumps and piping.
After all components of the drain are in place the drain should be tested
for obstructions. This can be done visually by shining a high powered flash-
light through a drain from one manhole and observing the beam in another. If
an unobstructed view cannot be obtained by this method because of an actual
obstruction, slight misalignment, or great distance, an air filled rubber
ball or plug about one inch less than the drain diameter should be flushed
or pulled through the drain (Bureau of Reclamation, 1978). Only a moderate
amount of water pressure or pulling force should be used so as not to deform
the ball. Any obstruction which causes the floating ball to stop or which
significantly increases the pulling force (greater than 25 Ibs; Van
Schilfgaarde, 1979) will justify digging up and replacing the pipe, removing
the obstruction, or taking some other corrective action. In large diameter
drains, TV-camera inspection can be used after construction and periodically
during operation.
Manholes and silt traps should be checked frequently for the first year
or two of operation for sediment buildup. Less frequent inspection is
required as the system ages.
Piezometers may be installed in the various parts of the drainage system
to identify operational problems with the filter, envelope, pipe, or other
components of the system. Piezometers can measure the loss of head through a
medium, and thus, can identify obstructions to flow, such as a clogged
envelope or filter.
6-94
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There are a number of problems that can develop in drainage systems that
will require maintenance or corrective action, including:
• Clogging by sediment build-up in manholes or drains
• Root clogging of tile drains
• Clogging by build-up of chemical compounds
• Sinkholes developing in the soil above the drain.
Problems caused by the above conditions are usually apparent at the surface
above the drain. Inspection of the area will reveal soft or ponded surface
conditions, areas of subsidence, and areas of accelerated vegetative growth.
Sediments that enter the system will usually be scoured through the drain
to the manhole where they collect. If manholes are not cleaned out periodi-
cally, sediments can build-up to the point of clogging the drains. Drains can
also clog because of breaking or partial collapse of pipe, a partial obstruc-
tion, or an adverse change in alignment caused by settling in a poorly
compacted soil or inadequately installed bedding.
Subsurface drainage pipes can also be clogged by plant roots, particu-
larly trees and shrubs. The roots of a single plant may extend over a
distance of several meters in a drainage pipe, given the right conditions.
Cottonwoods and willows are particularly troublesome.
The build-up of chemical compounds may also occur in a drain pipe. Iron
and manganese can form insoluble precipitates with sulfur via microbial oxida-
tion. If drains are located in areas with high deposits of reduced iron and
manganese and the flow to the drains is sufficiently constant for a long
period of time, these compounds can build-up to the point of clogging the
drain.
Sinkholes, or blowouts, may also develop over the drain. These may be
caused by improper spacing of tile drains, pipe breakage, insufficient cover
material, or high pressures within the drain.
6-95
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Clogged drainage pipe can be corrected by either hydraulic jetting or
mechanically scraping the drain. Flushing methods use high pressure water
jetting equipment that can travel inside the drain pipe. The water jets can
remove sediment and chemical compounds from the walls as well as from perfor-
ation in the drainage pipe. Specially designed water jets can cut through
dead roots up to one-half inch in diameter (Winger, 1979). Some water jetting
apparatus also contain special, closed-cage, rotating root cutters for cutting
larger roots. Cleaning speed by water jetting is about 1,000 feet per hour,
with few roots, to 800 feet per hour, with a moderate number of roots,
(Winger, 1979).
Mechanical scraping removes deposits from the pipe wall using such tools
as augers, pull or push blades, brushes, and hollow pipe bailers. Costs for
mechanical and hydraulic pipe cleaning were about $2.00 per foot in 1980
(USEPA, 1982).
Chemical means are often used for removing iron and manganese deposits
where hydraulic or mechanical methods are ineffective. Usually, sulfur
dioxide gas or a solution composed mainly of sulfuric acid is used to soften
the deposit to the point where hydraulic jetting can be used. Non-corrosive
sulfuric acid pellets have recently been developed which prevent the initial
formation of iron and manganese compounds when added to the gravel envelope.
In cases where there is a structural problem, such as drain breakage or
improper drain spacing causing a sinkhole, the drain must be dug up and the
condition corrected. Malfunctioning perforated pipe drains located near root
systems should be dug up and replaced with non-perforated pipe (or in the case
of tiles, sections of cemented joints).
6-96
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CHAPTER 7
LOW PERMEABILITY BARRIERS
Low permeability barriers can be used to divert groundwater flow away
from a waste disposal site or to contain groundwater contaminated by a waste
site. There are two major types of low permeability barriers which find
application in leachate plume control — slurry walls and grout curtains. A
slurry wall is a subsurface barrier that is formed through the excavation of a
trench using a high density slurry, typically bentonite and water, to support
the sides. The trench is then backfilled with materials having lower perme-
ability than the surrounding soil. This backfilled trench or slurry wall
reduces or redirects the flow of qroundwater. A grout curtain is a subsurface
barrier formed through the pressure injection of one of a variety of special
fluids known as grouts into a rock or soil body to seal and strengthen it.
Once in place, these fluids set or gel in the rock or soil voids to greatly
reduce the permeability of and impart increased mechanical strength to the
grouted mass. Because a grout curtain can be three times as costly as a
slurry wall and is incapable of achieving uniformly low permeability, it is
rarely used when groundwater has to be controlled in loose overburden. Grouts
are primarily used to seal voids in porous or fractured rock when other
methods of controlling groundwater flow are impractical.
Impeded groundwater flow may cause an increase in the upgradient
hydraulic head, which may in turn affect the rate of vertical movement of the
water. Therefore, the probable effects of a locally heightened water table
should be carefully considered before deciding to apply a barrier wall. Also,
unless measures are taken to reduce infiltration, a site surrounded by a
barrier wall could easily fill up in the so-called "bathtub effect."
7-1
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Several types and variations of slurry wall and grout implacement methods
can be used to control or reverse leachate plume migration. The following
sections describe the applications and limitations, the theory, the design and
construction, and the cost of slurry walls and grouts.
7.1 Slurry Walls
Slurry walls are formed through a slurry trench construction process.
Slurry trenching is a means of placing a low permeability, subsurface cut-off
or wall near a polluting waste source in order to capture or contain leachate
plumes. A trench is excavated through or under a slurry of bentonite clay and
water. The trench is then backfilled with an engineered mixture of earth
materials having the desired permeability properties. The width of slurry
trenches vary, but are typically from two to five feet (D'Appolonia, 1979).
Slurry trenches are usually excavated down to and often into a natural
low permeability layer (called an aquiclude) in order to shut off groundwater
flow. However, when only lowering the water table is required, the trench may
not have to be keyed into an impervious layer.
7.1.1 Applications and Limitations
Slurry walls are classified by the materials of which they are composed
and by the position in which they are placed with respect to the pollution
source. Slurry wall material and configuration determine the applications and
limitations of different slurry wall alternatives for leachate plume
management.
There are three major types of slurry walls that are categorized
according to the material used to backfill the trench. These three types are:
• Soil-bentonite
• Cement-bentonite
t Diaphragms.
7-2
-------
Soil-bentonite walls are composed of soil materials (often the trench
spoils) mixed with small amounts of the bentonite slurry. Cement-bentonite
cut-off walls are excavated using a slurry of Portland cement and bentonite
which is left to set or harden to form the final wall. Diaphragm walls are
composed of pre-cast or cast-in-place reinforced concrete panels (diaphragms)
installed using slurry trenching techniques. Each of these, as well as
hybrids of the three, has different characteristics and applications.
In general, soil-bentonite walls can be expected to have the lowest
permeability, the widest range of waste compatibilities, and the lowest
installation cost. They also offer the least structural strength (highest
compressibility), usually require the largest work area, and are restricted to
sites that can be graded nearly level.
Cement-bentonite walls can be installed at sites where there is
insufficient work area to mix and place soil-bentonite backfill. Also, by
allowing wall sections to harden and then continuing the wall at a higher or
lower elevation, they are more adaptable to an extreme topography. Although
cement-bentonite walls are stronger than soil-bentonite walls, they are at
least an order of magnitude more permeable, resistant to fewer chemicals, and
more costly.
Diaphragm walls are the strongest of the three types as well as the most
costly. Provided the joints between panels are installed correctly, diaphragm
walls have lower permeability and about the same chemical compatibilities as
cement-bentonite walls. Because of the higher expense of diaphragm walls and
the rarity of a need for a high strength barrier, they are seldom used for
pollution control, thus they will not be treated in detail here.
Combinations of these three major backfill types may be included within
the same wall. For example, a soil-bentonite backfill may be used for the
majority of a wall with cement-bentonite being used for a portion, such as a
road or rail crossing that requires greater strength. Being able to combine
7-3
-------
the various types of walls makes this technique adaptable to a wider range of
site characteristics.
The vertical and horizontal positioning of a slurry wall with respect to
the location of the pollution source and groundwater flow characteristics is
known as configuration. There are two types of configurations--vertica1 and
horizontal. Wall configuration, combined with associated remedial measures,
determine in theory how effective a slurry wall will be in controlling
leachate migration. Although configuration depends greatly on geologic and
topographic setting, waste characteristics, and the nature of the environ-
mental problems caused by the site, generalizations on the applications of
different slurry walls can be useful in understanding and evaluating a slurry
wall as a part of a remedial action.
7.1.1.1 Vertical Configuration
Vertical configuration refers to the depth of the slurry wall with
respect to both geologic formations and the water table. Based on vertical
positioning, walls are either "keyed" into a low permeability formation below
an aquifer or placed to intercept only the upper portion of the aquifer. This
latter type is commonly referred to as a "hanging" slurry wall.
Keyed-in slurry walls are excavated to a confining layer below, to
contain contaminants that mix with groundwater or sink to the bottom of the
aquifer. This layer may be a low permeability formation such as a clay or
silty clay or may be the underlying bedrock (Figure 7-1). In either case, the
connection between the wall and the low permeability zone is critical to the
overall effectiveness of the wall. If the low permeability zone is some
easily excavated material, such as a clay layer or weathered rock, basic
construction quality control should be sufficient to ensure a good key-in. In
cases were the low permeability zone is hard bedrock, however, the excavation
process may be much more complicated and costly.
7-4
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FIGURE 7-1.
HANGING SLURRY WALL (SPOONER et al., 1984)
Hanging slurry walls are not keyed into a low permeability layer. This
configuration is used to control contaminants, such as petroleum products,
which do not mix with the groundwater but float on top of it. In such cases,
the slurry wall need only extend into the water table to intercept the
contaminants (Figure 7-2). The exact depth of the wall will depend on the
thickness of the floating contaminant layer and the lowest water table
elevation. Other considerations include the extent to which the weight of the
contaminant might have depressed the water table and the effect removal of the
contaminants would have on the water table.
7.1.1.2 Horizontal Configuration
Horizontal configuration refers to the positioning of the wall relative
to the location of the pollution to be controlled and the direction of ground-
water flow. Depending on desired horizontal configuration, slurry walls may
7-5
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FIGURE 7-2.
HANGING SLURRY WALL (SPOONER et al., 1984}
Leaky
• Fuel Tank
Extraction Traffic
We!!
Cap
L-
Water
Table
Floating
Contaminants
Slurry
Wall
»%»" **" • - - * f *
;-/.; Bedrock :^W::
completely surround the polluted area or be placed upgradient or downgradient
from it.
Circumferential placement refers to placing a slurry wall completely
around a contaminated area (Figure 7-3). Although this requires a greater
wall length than either upgradient or downgradient placement alone, it does
offer many advantages and is a common practice. A circumferential slurry
wall, when used with a surface infiltration barrier (cap), can greatly reduce
both leachate movement and generation. If a leachate collection system is
used, the surrounded area can be virtually dewatered. This offers the
advantages of vastly reduced leachate amounts and can help increase the
longevity of the wall by reducing the amount of leachate and wall contact. As
shown in Figure 7-4, groundwater levels can be adjusted to maintain the
direction of groundwater flow inward, and thus, prevent the escape of
contaminants.
7-6
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FIGURE 7-3.
PLAN OF CIRCUMFERENTIAL WALL PLACEMENT
(SPOONER et al., 1984)
Slurry Wall
Extraction Wells
FIGURE 7-4.
CUT-AWAY CROSS-SECTION OF CIRCUMFERENTIAL
WALL PLACEMENT (SPOONER et al., 1984)
7-7
-------
Upgradient placement refers to the positioning of a wall on the
groundwater source side of a leachate source. This type of placement can be
used where there is a relatively steep gradient across the site such that
groundwater flow can be diverted around the wastes. In such cases, leachate
generation is reduced. As can be seen by Figures 7-5 and 7-6, a high water
table gradient is required for upgradient placement to be effective.
Depending on the site setting and the contaminants involved, an upgradient
wall may be keyed-in or left hanging. In either case, drainage and diversion
structures are likely to be needed to successfully alter the flow of clean
groundwater. The use of upgradient barrier walls is not common.
Placement of a slurry wall at a site on the side opposite the groundwater
source is referred to as downgradient placement. This placement configuration
does nothing to limit the amount of groundwater entering the site and is
practical only in situations where there is a limited amount of groundwater
flow, such as near drainage divides (Figures 7-7 and 7-8). This positioning
does not reduce the amount of leachate being generated, but acts as a barrier
to contain the leachate so it can be recovered. Although this placement may
be used as a keyed-in wall for miscible or sinking contaminants, it is most
often used to contain and recover floating contaminants. In either case, the
compatibility between the leachate and the wall backfill is important because
of contact between the two. In addition, care must be taken in designing a
downgradient wall installation to ensure that contaminated groundwater does
not overtop the wall.
As outlined above, slurry walls can be applied to a plume migration
problem in several ways. The possible combinations of configurations and
their typical uses are summarized in Table 7-1.
7.1.2 Theory
During construction of slurry walls, a slurry containing bentonite is
placed in the open trench to support the trench walls. After excavation is
completed, a mixture of bentonite slurry and soil, or a mixture of cement,
bentonite, and water is placed in the trench to form the completed wall. This
7-8
-------
FIGURE 7-5.
PLAN OF UPGRAOIENT PLACEMENT WITH DRAIN
(SPOONER et al.. 1984)
FIGURE 7-6.
CUT-AWAY CROSS-SECTION OF UPGRADIENT
PLACEMENT WITH DRAIN (SPOONER et al., 1984)
7-9
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FIGURE 7-7.
PLAN OF DOWNGRADIENT PLACEMENT
(SPOONER et al., 1984)
Groundwater Divide
«- Extraction Welte
Slurry Wall
FIGURE 7-8.
CUT-AWAY CROSS-SECTION OF DOWNGRADIENT
PLACEMENT {SPOONER et al., 1984)
To Treatment
-^*
Extraction Wall
7-10
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TABLE 7-1. SUMMARY OF SLURRY WALL CONFIGURATIONS (Spooner et al., 1984)
Vertical
Configuration
Horizontal Configuration
Ci rcumferential
Upgradient
Downgradient
Keyed-in
Most common and
expensive use
Most complete
containment
Vastly reduced
leachate
generation
Hanging
Used for floating
contaminants
moving in more
than one direction
(such as on a
groundwater divide)
Not common
Used to divert
groundwater
around site in
high gradient
situations
Can reduce
leachate
generation but
not movement
Compatibil ity
not critical
Not common
May temporarily
lower water table
behind it
Can stagnate
leachate but
halt flow
not
Used to capture
miscibl e or sinking
contaminants for
treatment
Inflow not
restricted, may
raise water table
Compatibility
very important
Used to capture
floating contami-
nants for treatment
Infl ow not
restricted, may
raise water table
Compatibility very
important
7-11
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section presents the current-held theories on the mechanisms by which the
slurry and the backfill perform this function.
7.1.2.1 Slurry Materials and Functions
To properly construct a barrier wall for pollution migration control, a
high quality slurry is necessary which:
o Has a high viscosity when allowed to stand and a low viscosity when
agitated (termed thixotrophy)
o Forms a low permeability layer called a filter cake on the trench
sides
o Has a density that is lower than that of the backfill.
Slurry specifications, along with ranges normally encountered during
construction, are presented in Table 7-2.
7.1.2.1.1 Clay
Both soil-bentonite and cement-bentonite slurries rely on bentonite clay
to maintain the required slurry properties. Bentonite is a soft, soapy-
feeling deposit composed primarily of the clay mineral montmorillonite and
about 10 percent impurities, such as iron oxides and native sediments (Boyes,
1975). Montmorillonite's structure and chemical composition give bentonite
its unique properties.
Crystals of montmorillonite are composed of three distinct layers. The
outer layers consist of a tetrahedral arrangement of silicon and oxygen
molecules. Some of the silicon atoms in these layers have been replaced by
aluminum. Sandwiched between the silica layers is a layer of aluminum atoms
surrounded by six hydroxyls or oxygen atoms in an octahedral configuration.
Some of the aluminum atoms in this layer have been replaced by magnesium.
Because of the substitutions in the three layers, unsatisfied bonds exist
within the crystal, resulting in a net negative charge. To satisfy this
charge, cations and water molecules are adsorbed onto the internal and
external surfaces of the clay crystals. These surfaces comprise the exchange
7-12
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TABLE 7-2. SPECIFIED PROPERTIES OF BENTONITE AND CEMENT BENTONITE SLURRIES (Spooner et al., 1984)
Parameter
Density (g/cm )
(p.s.f.)
Viscosity, apparent
(Seconds Marsh)
(centipoise)
Viscosity, plastic
Filtrate Loss, ml
PH
Fluid Content, %
of Total Wt.
Bentonite Content, %
Other Ingredients, %
by weight
Gel Strengths
10 seconds, Pascals
10 minutes, Pascals
10 minutes,,
lb/100 ft* 7
(24-72 dynes/cnr)
Bentonite Slurry
Fresh-Hydrated
1.01-1.04
65
38-45
15
<20*
<30
range 15-30
7.5 to 12
93-97
4-7
sand <1*
solids 2
7-30
5-15
During Excavation
1.10-1.24
69-84
38-68
range 15-70
apparent average
40-60
10.5-12
78-82
6
sand <5*
solids 3-16
20-40
Cement-Bentonite Slurry
Fresh-Hydrated
1.03 to 1.4
40-45
15
9
100-300
12-13
76
3-6
cement 18
solids 15-30
15
18
During Excavation
38-80
>130
30-50
55-70
3-6
30-45
10
22
*Specification for construction of tremied concrete diaphragm walls.
Adapted from Case, 1982; Xanthakos, 1979; Millet and Perez, 1981; U.S. Army Corps of Engineers, 1976;
Guertin and McTigue, 1982b; Boyes, 1975; Jefferis, 1981a; Ryan, 1976
-------
complex of the clay. The types of cations adsorbed on the exchange complex
have a great influence on the properties of the clay (Brady, 1974).
There are two major types of montmorillonites; sodium-saturated and
calcium-saturated. Sodium montmorillonites are preferable to calcium mont-
morillonites because sodium montmorillonites swell more and flocculate less
than calcium montmorillonites when exposed to water. Not all of the
bentonites used for slurry trench construction, however, consist chiefly of
sodium montmorillonites because there are limited quantities of natural sodium
bentonites. In some cases, calciim bentonites are used after being exposed to
sodiim-containing materials such as sodium hydroxide to force some of the
calcium ions off of the exchange complex of the montmorillonite and then
replace them with sodium ions (Grim, 1968).
Where high concentrations of calcium salts occur in the soil or
groundwater and where cement-bentonite is used, calcium ions can easily
displace sodium ions on the clay. The ease of replacement of sodium by
calcium increases as the concentration of calcium in the solution and on the
clay exchange site increases. Once about 30 percent of the exchange sites
become occupied by calcium, the clay acts like calcium montmorillonite rather
than the sodium variety (Grim, 1968).
Clays other than montmorillonite have occasionally been used in slurries
to maintain trench stability, particularly in areas where bentonite is
difficult to obtain. Table 7-3 lists several properties of montmorillonite as
compared to other common clay minerals. The primary drawback to the use of
other clays in the slurry during trench construction is the inability of these
clays to form a filter cake under relatively low hydraulic heads and short
formation times.
An exception to this general rule concerns situations where a high
concentration of salts in the slurry trench construction area is anticipated.
When sodium bentonite is exposed to salt water, undesirable property changes
occur; the slurry begins to flocculate, viscosity decreases, fluid loss
7-14
-------
TABLE 7-3. COMPARISON OF SELECTED PROPERTIES OF CLAYS (Spooner et al., 1984)
I
I—"
en
Parameter
Volume change
caused by hydration
Hydration rate
Particle shape
Theoretical
specific surface
area, m /g
Cation Exchange
Capacity,
meg/lOOg
Liquid limit
Plastic limit
Montmorlllonite Kaolinlte 11 lite
2-11 cm3/g
Water sorption
continues for
about 1 week
thin, flat, irregular irregular, flat
plates hexagonal s
700-800 5-20 100-200
60-150 3-15 10-40
150-700 29-75 59-90
65-97 26-35 34-43
Other Clays or
Sheet Silicates
vermiculite >montmorillonite
>beidellite >kaolinite
>ha Hoy site
water sorption for
most colloidal clays is
complete in 1 to 3 days
attapul gi te-f 1 brous ,
sepiolite-fibrous,
most others irregular
and flat
vermiculite 300-500
vermiculite 100-150,
attapulgite/sepiolite
3-15
--
Continued
Adapted from Baver, Gardner and Gardner, 1972; Grim, 1968; Grim and Guven, 1978; Xanthakos, 1979; Spangler and
Handy, 1973
-------
TABLE 7-3 (continued)
Parameter
Montmorillonite
Kaolinite
Illlte
Other Clays or
Sheet Silicates
en
Percentage of clay 5.5-12
by weight In
water to produce
a 15 centlpolse
colloidal suspension
Approximate 0.5-1.3
negative charge
per formula
unit
1.2-2.0
25-36 for typical
native clays
attapulgite-same as
montimorillonite
vermiculite 1.2-1.8,
muscovite 2.0
Density2of chacge
meq/m x 10"
Layer thickness
in angstroms
Particle density,
g/cnr
1.1-1.9
expansive >10
air dry 15
2.5 Wyoming bentonite
2.2 Japanese bentonite
6-7.5
7.15
1.0-2.0 vermiculite 3.0-3.3
10 vermiculite 14,
muscovite 10,
biotite 10,
halloysite 10,
mica 2.8-3.2
Adapted from Baver, Gardner and Gardner, 1972; Grim, 1968; Grim and Guven, 1978; Xanthakos, 1979; Spangler and
Handy, 1973
-------
increases, and the slurry may gel completely and lose its thixotropic
properties (Xanthakos, 1979). In these situations, a type of clay called
attapulgite has been used.
Attapulgite is not composed of plates as is montmoril lonite. Instead,
the attapulgite crystals are composed of linked silica chains that form a
fibrous structure; chains of water molecules fill voids between the
tetrahedral and octahedral silica layers (Grim, 1968). Attapulgite is
apparently not negatively affected by the electrolyte concentration in the
water to which it is added and does not require any initial hydration time as
does montmoril lonite.
Wyoming bentonites are the most commonly used bentonites for slurry
trench construction. They contain about 60 percent sodium and 40 percent
calcium and magnesium cations on the exchange complex of the clay (Connybear,
1982). These bentonites are mixed at a rate of 4 to 7 percent bentonite in 93
to 96 percent water (Boyes, 1975).
7.1.2.1.2 Filter Cake
When trench excavation is initiated, the slurry is pumped into the trench
to maintain a slurry level at or near the initial ground level (Millet and
Perez, 1981). As the slurry is introduced into the trench, it flows into
pores in the strata through which the trench is cut. Leakage of slurry into
these voids continues until the flat clay particles in the slurry begin to
accumulate in layers, which grow large enough to bridge the gaps between the
soil particles or until gelation of the slurry within the pores occurs.
Figure 7-9 illustrates the relatively rapid initial slurry loss, followed by a
reduction in the rate of loss. This reduction is caused by the formation of a
layer of clay particles "plastered" on the trench sides, which reduces lateral
liquid flow out of the trench and into the adjacent soil. This layer is
called a filter cake.
7-17
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FIGURE 7-9.
FLUID LOSS DURING FILTER CAKE FORMATION
(HUTCHINSON et al., 1983 AS CITED BY SPOONER et al., 1984)
I
s
a
Time for Initial Cake Formation
(TimeT)V4
The filter cake is a thin membrane composed of plate-like bentonite
particles (Case, 1982). The solids content of the newly formed filter cake
ranges from 10 to 50 percent, with higher solids contents found in filter
cakes from calcium than from sodium bentonites (Grim and Guven, 1978). The
filter cake from a slurry containing 5 percent bentonite typically contains 15
percent bentonite (Hutchinson et al . , 1975). During active excavation, the
filter cake is usually less than three millimeters thick. This thick layer of
clay is, however, an effective barrier to water movement as the permeability
of the filter cake can be as low as 10~6 ft/day (Xanthakos, 1979).
Formation of the filter cake is of critical importance in slurry trench
construction. This membrane performs numerous functions including:
0 Minimization of slurry loss into surrounding soils
• Stabilization of the soil that is in contact with the slurry by
gelling in the soil pores and by plastering the particles against the
trench wall s
7-1!
-------
0 Providing a plane on each trench wall against which the hydraulic
pressure and dead weight of the slurry can act to stabilize the
excavation.
Desirable characteristics of filter cakes include rapid formation and
reformation when necessary, resistance to shearing, and low permeability.
Experience has shown that a thin filter cake is an indication of a tough,
impermeable membrane, but a thick "flabby" filter cake is likely to allow high
fluid losses (Boyes, 1975).
Filter cakes must be formed rapidly when the initial soil contact occurs
in order to avoid excessive slurry losses. These membranes must resist
mechanical disruption by the backhoe bucket or clamshell during trench
excavation. If inadvertently scraped off the trench wall during excavation,
the slurry must be of such composition as to allow rapid formation of a new
filter cake to avoid possible collapse of the trench.
A high gel strength is desirable in the filter cake because the gel
structure contributes to shear strength, and the filter cake must resist
shearing forces from both the excavation equipment agitated slurry and the
soil comprising the trench wall. In addition, high gel strength indicates
rapid formation of a gel structure. When the slurry penetrates soil pores,
rapid gelation assists in restricting further slurry flow and thus minimizes
slurry losses (Xanthakos, 1979).
A final desirable filter cake characteristic is low permeability. The
movement of water through the filter cake should be minimized to:
o Avoid wetting and thus softening and lubricating unstable layers that
may be present in the soil surrounding the trench (Boyes, 1975)
• Avoid increasing pore water pressure, because this increases the total
stress on the system and reduces the angle of friction in the soil
surrounding the trench (Hutchinson et al., 1975).
• Maintain the slurry level in the trench well above the groundwater
level. This sustains the thrust of the slurry on the trench side
walls by restricting the pressure losses caused by filter cake leakage
(Xanthakos, 1979).
7-19
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7.1.2.1.3 Cement-Bentonite
Cement-bentonite (CB) slurries normally contain less than 6 percent by
weight bentonite, 18 percent ordinary Portland cement (o.p.c.), and 76 percent
water (Jefferis, 1981b). Typical ranges of CB slurry contents are presented
in Tabl e 7-4.
TABLE 7-4. TYPICAL COMPOSITIONS OF CEMENT-BENTONITE SLURRIES
(Jefferis, 1981b as cited by Spooner et al., 1984)
Constituent Amount of Slurry
(Percent by weight)
Bentonite 4-7
Water 68-88
Cement
without replacements 8-25
when blast furnace slag added, minimums 1-3
when fly ash added, minimums 2-7
Blast furnace slag, maximums 7-22
(not commonly available in U.S.)
Fly ash, maximums, if used 6-18
When bentonite slurries are compared to CB slurries, the differences
become evident. Most of these differences are caused in part by the effects
of the calcium from the cement on the sodium montmorillonite in the bentonite.
The three most important differences between the properties of CB slurries and
bentonite slurries are:
• Physical properties, including viscosity and filter cake formation
• Setting times
• Filter cake permeability.
7-20
-------
Because of the calcium in the cement, the properties of the sodium
bentonite in CB slurries are permanently altered. For example, the initial
viscosity is somewhat higher because of the flocculation of the slurry and the
higher solids concentration; 15 to 30 percent in CB slurry, opposed to about 6
percent in bentonite slurries (Millet and Perez, 1981; Jefferis, 1981b). The
results of the filtrate loss test are also higher. This indicates that the
time required for filter cake formation is longer and the permeability of the
cake formed is higher (Hutchinson et al., 1975; Millet and Perez, 1981).
The primary differences betwen CB and soil-bentonite (SB) slurries that
is of practical importance in slurry trench construction is the fact that CB
slurries begin to harden within two to three hours after mixing, while
bentonite slurries do not set (Case, 1982). This often necessitates the use
of construction techniques different from those used during construction of SB
walls, as described below.
CB walls may be constructed in a series of panels but are more commonly
installed as a continuous trench. When CB panels are constructed, alternate
panels are excavated under a CB slurry, then allowed to partially set. When
they have obtained a sufficient shear strength, the intervening panels are
excavated also under a slurry. A portion of the initial panel ends are
removed during this second stage to ensure continuity between the initial
panels and the intervening ones. This technique is used only for very deep
installations.
7.1.2.2 Backfill Materials and Functions
Two major types of backfill have been used in barrier walls: soil-
bentonite and cement-bentonite. Soil-bentonite (SB) backfills are composed of
bentonite slurry and soil materials, often the spoils excavated from the
trench. Cement-bentonite (CB) backfills are the result of the in-place
hardening of cement-bentonite slurries. The design factors affecting the
permeability of completed cement-bentonite and soil-bentonite walls are
backfill composition and slurry properties.
7-21
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7.1.2.2.1 Soil-Bentonite Walls
Soil-bentonite walls are excavated under a bentonite slurry in a
continuous trench. The trench is then backfilled with a soil-bentonite
mixture having the desired properties.
Two primary components make up the backfill: the bentonite slurry and a
suitable soil mixture. The soil mixture used in backfilling is often soil
material from the trench excavation, augmented with selected soil from a
suitable borrow source. Regardless of the source, the soil material must meet
three requirements:
• Must be free of deleterious materials
• Must have a suitable particle size distribution
• Must mix with the slurry to form a more-or-less homogeneous paste.
The backfill should not contain materials that would react with the
pollutants being contained. For example, calcium-containing materials
(gypsum, chalk, or caliche) should not be used in walls intended to contain
highly acidic groundwater.
Another requirement for backfill material is that it contain a suitable
particle size distribution. To ensure a low permeability, the backfill must
have from 20 to 40 percent fine particles, preferably plastic fines. As shown
in Figure 7-10, backfill permeability will be lower when the backfill material
contains a higher proportion of fines (D'Appol onia, 1980). Fine particles,
particularly clays, contribute to low permeability by assisting in bridging
the pores between larger particles.
A final requirement of the soil material used in the backfill is that it
be mixed with the slurry to form a more-or-less homogeneous paste. This is
largely a function of water content and blending technique. According to
D'Appolonia (1980), this requirement is relatively easy to meet as a wide
variety of materials have been successful ly mixed at slurry trench construc-
tion sites. In general, clayey soils will require a greater blending effort
and higher water content than more coarse materials.
7-22
-------
FIGURE 7-10.
RELATIONSHIP BETWEEN PERMEABILITY AND
QUANTITY OF BENTONITE ADDED TO SB BACKFILL
(D'APPOLONIA, 1980 AS CITED BY SPOONER et al., 1984)
10
-2
10
10-
-3
10
-5
10
-6
10
-7
10
-9 '
Well Graded
Coarse Gradations
(30-70% + 20 Sieve)
w/10 to 25% NP Fines
Poorly Graded
SitySand
w/30-50% NP Fines
Clayey Saty Sand
w/30 to 50% Fines
1234
% Bentonite by Dry Weight of SB Backfill
Once the backfill material has been selected, it is mixed with the slurry
until the slurry attains the necessary slump. The slump of the backfill is
the vertical distance a cone-shaped mass of concrete or other plastic material
will settle and is a measure of the backfill's shear strength. The slump
should range from two to seven inches on the ASTM C143-74 "Slump of Portland
Concrete" Test (D'Appolonia, 1980). The shear strength of the backfill
mixture, as indicated by the slump cone test, should be high enough that the
backfill easily displaces the slurry, but not so high that it folds over
itself rather than flowing into the trench. The density of the backfill
should be at least 15 pounds per cubic foot (240 kilograms per cubic meter)
greater than that of the slurry in the trench (D1Appolonia, 1980). Prefer-
ably, the shear strength should be lower than that of the filter cake, yet
high enough to allow it to stand on a 5:1 to 10:1 slope (Millet and Perez,
1981).
7-23
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The primary requirement of a slurry wall for controlling leachate plumes
is low permeability. Permeabilities of completed soil-bentonite walls can be
as low as 1.5 x 10 ft/day, although higher permeabilities are more common
(Xanthakos, 1979). Several factors affect the permeability of the completed
wall including:
• Quality of the filter cake
• Fines content of the backfill
t Bentonite concentration of the backfill
• Plasticity of the fines in the backfill
0 Voids ratio of the backfill.
D'Appolonia (1980) found that plastic fines inhibit water movement more
effectively than non-plastic fines. This is logical since the plastic fines
are composed mostly of clays, which are less pervious than the non-plastic
fines, which are primarily silts. Since bentonite has a very small particle
size, it also contributes to reduced permeability. The effect of plastic and
non-plastic fines on backfill permeability is shown in Figure 7-11.
FIGURE 7-11.
EFFECT OF PLASTIC AND NON-PLASTIC FINES
CONTENT ON SOIL-BENTONITE BACKFILL PERMEABILITY
(D'APPOLONIA. 1980 AS CITED BY SPOONER et al., 1984)
80
70
60
SO
40
30
20
10
0
O Non-Ptottic or Low
Plasticity Fines
i o
0 10~9 10-* 10-7 10-« 10-8
SB Backfill Permeability, cm/sec.
10-*
7-24
-------
The void ratio of a soil material is dependent on pore size and particle
size. Small particle sizes and a low void ratio (high packing density) will
yield low permeability.
7.1.2.2.2 Cement-Bentonite Walls
In contrast to soil-bentonite walls, cement-bentonite walls are normally
used where strength, rather than low permeability is the primary consideration
(Guertin and McTigue, 1982). The requirements for cement-bentonite wall
performance are the same as for soil-bentonite walls.
Cement-bentonite wall strength is designed to be slightly greater than
that of the surrounding ground and is typically comparable in strength to
stiff clay (Jefferis, 1981a; Millet and Perez, 1981). Although strengths of
cement-bentonite walls can range from 10 to 100 pounds per square inch,
ultimate strengths are generally about 5 to 20 pounds per square inch and are
achieved after 90 days (Xanthakos, 1979; Case, 1982).
Cement-bentonite walls can usually withstand compressive strains of about
2 percent without cracking. This is because they are not as brittle as
typical concrete walls (Ryan, 1976). Moreover, they can withstand relatively
high hydraulic gradients and are basically non-erodable. A cement-bentonite
wall only 2 to 3 feet wide can satisfactorily withstand at least 100 feet of
hydrostatic head because of its cohesive nature (Millet and Perez, 1981).
Despite the fact that cement-bentonite walls are both non-brittle and
cohesive, they are not indestructable and hydro fracturing has been reported
(Millet and Perez, 1981).
The continuity of cement-bentonite walls is an especially important
factor when these walls are constructed in panels rather than in a continuous
trench. Thus, there is a possibility for gaps to remain between the panels
which can allow for leachate escape. The time required for cement-bentonite
walls to harden depends on the presence of set time retarders and cement
replacements, among other factors. The speed of set time is of interest when
panel construction techniques are employed.
7-25
-------
•5
The permeability of cement-bentonite walls is normally about 10 ft/day
(Case, 1982). This can be decreased slightly by adding blast furnace slag or
additional bentonite. Jefferis (1981a) reports that the permeability can also
be decreased as much as an order of magnitude because of consolidation of the
completed wall. This, however, can be deleterious as it is accompanied by
cracking.
There are many factors which affect the performance of cement-bentonite
walls including:
• Slurry contents
t Mixing methods
• Use of additives
• Cement content
• Bentonite content
• Use of cement replacements.
As the cement content in the backfill is increased, a stronger, more
brittle wall is formed (Millet and Perez, 1981). Higher cement contents,
however, may allow higher wall permeabilities because of consolidation
cracking. The ultimate strength of cement-bentonite walls is low, on the
order of 15 to 20 pounds per square inch, and the permeability is relatively
high, usually around 10"3 ft/day (Case, 1982).
The ratio of the water to cement in the slurry also affects the
characteristics of cement-bentonite walls. Generally, higher ratios produce
weaker walls. Typical water-cement ratios range from 3:1 to 5:1. These are
much higher than the ratios found in concrete mixes. The cement is kept in
suspension while the slurry is liquid because of the presence of the bentonite
in the mixture. The bentonite content in the cement-bentonite slurry walls
contributes to their low permeability and resistance to chemical attack.
Where very low permeability or resistance to aggressive chemicals are
required, the bentonite content of the slurry can be increased (Jefferis,
1981a). However, as the bentonite content is increased, the viscosity goes
up, and the slurry becomes unworkable.
7-26
-------
Cement replacements, such as blast furnace slag or fly ash, can be used
to replace up to one-third of the cement in the slurry (Jefferis, 1981a).
Typical percentages of these constituents in cement-bentonite slurries are
shown in Table 7-4. Cement replacements have several effects on the
characteristics of the completed wall. The most significant effect is the
reduction in the rate at which water separates from the cement mixture before
and during hardening (bleeding rate).
When conventional cement-bentonite walls or walls with fly ash repl ace-
_3
ment are allowed to set for seven days, permeabilities of 2.8 x 10 to 1.4 x
10~2 ft/day (1 to 5 x 10~6 cm/sec) result. In contrast, cement-bentonite
walls containing from 2.5 to 7.5 percent blast furnace slag exhibited
permeabilities of about 2.8 x 10"4 ft/day (10 cm/sec) after seven days of
hardening (Jefferis, 1981a). Fly ash replacements, on the other hand, can
effectively increase a wall's chemical resistance while blast furnace slag
replacements cannot (Jefferis, 1981a).
7.1.2.2.3 Diaphragm Walls
Precast concrete panels and cast-in-pl ace concrete walls are known as
diaphragm walls. Unlike soil-bentonite and cement-bentonite walls, these
walls develop a great deal of strength over time and can be used as structural
components.
Precast concrete panel walls are cast offsite in segments from 1 to 3
feet wide, from 10 to 20 feet long, and from 30 to 50 feet deep. The panels
are lowered into a trench containing bentonite slurry and secured in place.
Because of their dimensional limitations, concrete panel walls are usually
only employed where depths of 50 feet or less are required (Guertin and
McTigue, 1982). An exception to this general rule occurs where a cement-
bentonite slurry that remains in place is allowed to set up around the panel.
Using this technique, the trench can be extended lower than 50 feet and the
panel suspended in the cement-bentonite slurry. The slurry then forms a
cut-off both below the panel and on either side of it.
7-27
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Cast-in-place concrete walls are constructed by excavating a short trench
(or slot) under a bentonite slurry. The maximum allowable slurry density is
75 pounds per square inch and the maximum allowable sand content is 5 percent
(Millet and Perez, 1981). Once the density and the sand content of the slurry
in the trench are appropriate, the reinforcing bars are lowered into place.
Concrete is then tremied into place using a funnel-like apparatus that directs
the concrete to the trench bottom. The apparatus is raised as the concrete
level rises. Slurry is pumped out as the concrete is tremied in and then
filtered and used for the next panel. When set, the wall is composed of a
concrete panel sandwiched between two bentonite filter cakes. One side of the
wall is normally excavated during later construction thus destroying one of
the filter cakes. The other side, however, retains its filter cake which
serves as a low permeability seal (Guertin and McTigue, 1982).
Diaphragm walls are not normally used for pollution migration control
because of their susceptibility to leakage through panel connections and their
greater expense.
7.1.2.3 Failure Mechanisms
There are several mechanisms or processes that can affect the construc-
tion or functioning of slurry walls and cause failure. These failures may be
the result of excavation and installation procedures or subsurface conditions.
The failure may occur during excavation and installation thus requiring
re-excavation of the slurry trench. Once the wall is in place, a variety of
factors can affect the integrity of the wall and its impermeability.
One failure mechanism is trench collapse caused by the loss of stability
in the trench walls during excavation and before backfilling or cement-
bentonite slurry hardening. Direct causes of trench collapse are (U.S. Corps
of Engineers, 1978; Duguid et al., 1971; Nash and Jones, 1963).:
• Insufficient slurry head above groundwater
• Sudden or rapid loss of slurry caused by contact with gravel , large
pores, or fissures
7-28
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• Vibrations
• Surface runoff into open cracks
• Surcharges
• Insufficient agitation of the slurry.
A number of mechanisms can cause failure of the completed wall. Among
these fail ure mechanisms are:
• Cracking
• Hydrofracturing
• Tunnelling
• Piping
• Chemical disruption.
Each of these mechanisms is discussed below.
Soils containing an appreciable concentration of clay can shrink when
allowed to dry, forming cracks. Even cement bentonite walls can crack
"drastically" when allowed to dry (Jefferis, 1981b). Some soils can, however,
shrink and crack even when nearly fluid enough to flow. Nash (1976) and
Tavenas et al. (1975) noted the formation of cracks in silty clay soil
mixtures only slightly dry of their liquid limits.
These cracks can be caused by several processes, including:
• Consolidation
• Hydraulic fracturing
• Syneresis
• Tunnelling and piping
• Chemical disruption.
7.1.2.3.1 Consolidation
Consol idaton of soils occurs when water is squeezed from the soil pores
(Hirschfeld, 1979). This process is accompanied by a decrease in the volume
7-29
-------
of the soil mass caused by a decrease in the volume of voids in the soil
(Baver, Baver and Gardner, 1972). The amount of consolidation is maximal in
fine-textured soils and minimal in coarse-textured materials. The rate of
consolidation depends on the soil permeability, the thickness of the layer
being loaded and the magnitude of the volume decrease. In fine textured
soils, consolidation occurs at a much slower rate than in coarse tex.tured
ones. Most consolidation occurs rather quickly, however, fine textured soils
can continue slow minor consolidation for several months (Hirschfeld, 1979).
The soil-bentonite backfill in slurry walls has been found to continue
consolidating for about 6 months, with minimal decreases in volume thereafter
(Xanthakos, 1979).
The amount of consolidation in slurry walls is limited by the backfill's
arching action along the trench walls and by grain to grain contact in the
backfill (Xanthakos, 1979; D'Appolonia, 1980). The amount of slurry wall
consolidation depends on trench width as well as on the amount of fines in the
backfill. As the fines content increases, consolidation increases because
fine particles are more compressible than coarse ones (Mitchell, 1976). Wider
trenches have been found to consolidate more than narrow ones. An eight-foot
wide trench, for example, was reported by Xanthankos (1979) to have
consolidated from one to six inches during the first few months after
construction. In contrast to soil-bentonite walls, cement-bentonite walls
consolidate very rapidly (Ryan, 1976).
The process of consolidation in soil-bentonite walls can produce many
small cracks along shear zones in the soil mass (Mitchell, 1976). Horizontal
cracks that extend through the backfill can also be produced by the arching
action mentioned earlier (Nash, 1976). This cracking can occur even when the
backfill is still quite fluid. Normally, the backfill contains from 25 to 30
percent water. This moisture content is slightly more than the liquid limit
of the backfill (D'Appolonia, 1980a). When soil is at its liquid limit, it
contains so much water that it will flow under the influence of an applied
stress (Baver, Baver and Gardner, 1972). Even so, Nash (1976) observed the
formation of horizontal cracks in a silty clay backfill that was only slightly
below its 1iquid limit.
7-30
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The arching that accompanies consolidation can also lead to the formation
of another type of cracking called hydrofracturing. The relationship between
hydrofracturing and consolidation is described below.
7.1.2.3.2 Hydrofracturing
When soils or rocks are subjected to excessive hydraulic pressures,
cracks may form through which the excess water can flow. The pressure at
which this cracking occurs can be less than the effective overburden pressure
on the rock or soil in situ. This phenomenon has been used by petroleum
geologists to fracture petroleum-containing strata and thereby increase the
yield of oil wells (Bjerrum et al., 1972).
The amount of pressure required to induce fracturing depends on the depth
of the point receiving the pressure and the ratio of vertical to horizontal
pressures at that point. Other factors are also likely involved. Values used
by petroleum geologists are 1 psi per foot of depth where the vertical
pressures are less than the horizontal pressures, and 0.64 psi per foot of
depth where the vertical pressures are greater than the horizontal pressures
(Bjerrum et al., 1972).
The cracks caused by this pressure extend vertically where the vertical
pressures are greatest and horizontally where the horizontal pressures are
greatest (Tavenas et al., 1975). These cracks can continue to increase in
length as long as the excess head is applied until they reach an area having
greater permeability (Bjerrum et al., 1972). When the pressure is decreased,
the cracks will partially close but will reopen when the pressure is again
increased (Tavenas et al., 1975).
The effects of hydrofracturing on the functioning of a slurry wall can be
severe. Where the vertical pressure exceeds the horizontal pressure (which is
normally the case) the pressure exerted on the aquielude can cause it to
fracture. The vertical cracks will be immediately filled with slurry if this
occurs during construction. Continued fracturing can, however, occur during
backfilling. The backfill is less likely than the slurry to completely flow
7-31
-------
in and fill the cracks in the aquiclude, and the aquiclude's permeabil ity may
thus be increased. The overall effect of this type of fracturing may be
minimal except at sites where the aquiclude is thin or overlies a very
permeable strata or both.
A more detrimental effect of hydrofracturing may occur where the
horizontal pressures on the wall are greater than the vertical pressures. If
more than 1 psi of pressure per foot of depth is applied to the wall,
horizontal cracks may form. These could allow significant amounts of water or
leachate to flow through the wall. Hydrofracturing has been reported in both
soil-bentonite and cement-bentonite walls (Bjerrum et al ., 1972; Millet and
Perez, 1981).
The likelihood of slurry wall damage caused by hydraulic fracturing is
highest where:
• Significant amounts of consolidation occur
t Piezometers are installed in the wall to monitor the wall's
permeability using constant head tests
• Large vertical loads are applied to the soils on either side of the
trench
• Large hydraulic gradients are allowed to develop across the wall.
When a great deal of consolidation and subsequent arching occur after
backfilling, the vertical loadings on the wall can be reduced to levels less
than the horizontal loadings. This allows the wall to become susceptible to
horizontal cracking (Bjerrum et a!., 1972). The amount of consolidation can
be minimized by reducing the content of fine particles in the backfill
(Mitchell, 1976). However, when additional coarse material is included in the
backfill, the wall's permeability is likely to be increased (D"Appolonia,
1980b). The amount of permeability increase expected because of the inclusion
of additional coarse material in the backfill should be weighed against the
risk of hydrofracturing caused by the anticipated pressure differential across
the trench.
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Where piezometers are placed in the wall to test the wall's permeability,
hydrofracturing may be induced when the excess head is applied. Hydro-
fracturing caused by use of piezometers occurred when permeability tests were
being conducted on a series of dikes with soil-bentonite cores that were
installed in Israel. The constant head permeability tests were designed so
that the maximum head applied did not exceed the effective weight of the soil
above the piezometer. However, even at very low hydraulic pressures,
hydrofracturing occurred. This fracturing resulted in a thousand-fold
-4
increase in the measured permeability, i.e., from 10 ft/day initially to
10"1 ft/day after fracturing (Bjerrum et al., 1972).
Tests on an in situ, normally consolidated clay were conducted to see if
hydrofracturing was caused by arching along the walls of the soil bentonite
cores. The normally consolidated clay that was not influenced by arching
along trench walls also fractured under the influence of an applied hydraulic
pressure that was less than the effective overburden pressure (Bjerrum et al.,
1972). This indicates that the occurrence of hydrofracturing is not dependent
upon arching (and consolidation) along the walls of slurry trenches.
When large vertical loads are applied along the trench walls, the
horizontal stresses on the slurry wall can be greatly increased. The vertical
loading can occur because of placement of stockpiles or heavy equipment along
the sides of the trench. Depending on the other stresses acting on the wall
at the site, the horizontal stress may become greater than the vertical
stress, thus making the wall susceptible to horizontal hydrofracturing.
A fourth potential cause of hydrofracturing is the presence of an exces-
sive hydraulic gradient across the wall. If the pressure on the upgradient
wall exceeds 1 psi per foot of depth and the horizontal pressures acting on
the wall are greater than the vertical pressures, horizontal fracturing of the
wall could occur. Excessively high hydraulic gradients could be induced by:
• Failing to provide subsurface drains or extraction wells upgradient of
the wall
• Installing extraction or injection wells too close to the wall
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• Dewatering a site without deflecting groundwater around the site and
away from the wall (via drains, ditches, or extraction wells).
For this reason, the proper use and placement of auxil iary measures, such as
wells, should receive careful attention during the wall's design stages.
7.1.2.3.3 Syneresis
Another process that can result in the formation of cracks in a slurry
wall is syneresis. . According to Mitchell (1976), syneresis is a "mutual
attraction between clay particles" that causes the particles "to form closely
knit aggregates with fissures between." Contractions that occur in the gel
result in the extrusion of liquid (water). Syneresis is often observed in
gelatin after aging (Mitchell, 1976). Syneresis may take place in slurry
walls, however, the extent to which this phenomenon affects the performance of
slurry walls is not known.
7.1.2.3.4 Tunnelling and Piping
Two processes that can result in extensive breaching of a slurry wall are
tunnelling and piping. Both of these processes involve the formation of
channels through the wall. However, the causes and solutions for the two
problems are different.
Several dams have failed because of the formation of very large pores
that extend completely through the dam from the upstream to the downstream
face. These failures occurred where the earthen dams were constructed of low
to medium plasticity native clays that contained appreciable amounts of sodium
montmorillonite (Mitchell, 1976). The process by which the failures occurred
is termed tunnelling and can be described as a series of interrelated steps,
as are listed below (Mitchell, 1976):
1. Differential consolidation of the wet and dry portions of several
earthen dams led to the formation of stress cracks below the water
1 ine.
7-34
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2. Water that contained calciun ions flowed into the cracks.
3. Calcium ions from the water replaced sodiun ions to the exchange
complexes of the clay particles in the dam.
4. The calcium ions caused the clay particles to decrease in size
(shrink) and to form packets or "floes."
5. As the clay particles formed floes, they became less dispersed and
the space formerly occupied by the dispersed clay particles became
filled with water.
6. As the sizes of water-filled spaces (pores) increased, the rate of
water movement in the pores increased.
7. The increased rate of water movement allowed the water to carry more
particles in suspension.
8. As the particle-carrying capacity of the water increased, the number
of clay particles eroded by the flowing water increased.
9. As the number of clay particles eroded from the dams increased, the
sizes of the pores in the dams increased.
10. As the pore space size increased, the speed of the tunnelling process
increased until extensive tunnelling had occurred.
This tunnelling process has been found to take place in earthen dams and
P
embankments that had initial permeabilities as low as 10" ft/day. One method
that has been used to reduce the likelihood of tunnelling is to mix the soil
with lime prior to dam construction. This causes the clay particles to shrink
and become less easily dispersed before any cracking or particle erosion can
occur (Mitchell , 1976).
The risk of tunnelling failures in slurry walls is greatest where ground-
waters contain high concentrations of calcium. At these sites the calcium
ions from the groundwater can disrupt the soil bentonite backfill and cause
tunnelling failures that are similar to the ones experienced with the earthen
dams described above. High calciun concentrations in groundwater are most
commonly found in sedimentary aquifers, particularly limestone ones. Water
from these aquifers can contain over 50 ppm dissolved calcium (Freeze and
Cherry, 1979).
7-35
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The presence of stress cracks, high hydraulic gradients, and permeable
backfills are not prerequisites for the tunnelling process, but these factors
are likely to speed failure rate considerably. The causes of stress cracking
were described previously. The hydraulic gradients across slurry walls at
hazardous waste sites are not normally as high as the gradients across dams,
so the rate of tunnelling in slurry walls should be much slower than in
earthen dams. Another factor that operates in favor of slurry walls is their
low permeability, which is normally one to three orders of magnitude less than
the minimum permeability of the materials through which tunnelling had been
reported (Mitchell, 1976; D'Appolonia, 1980b). This low permeability
indicates that the initial rate of water movement through a slurry wall will
be much less than through the earthen dams, consequently the particle carrying
capacity will also be severely restricted and the erosion rate will be
minimal.
Despite the fact that tunnelling failures, if they occur in a soil-
bentonite wall, are expected to require long time periods to develop, the
potential for slurry wall disruption caused by the presence of calcium ions
should be kept in mind when evaluating the feasibility and design criteria for
a slurry wall installation at a particular site.
On the other hand, piping begins at the downstream face and proceeds
toward the upstream face (Mitchell, 1976). Piping occurs because of the use
of improper backfill materials or procedures. Variable wall thicknesses,
poorly mixed backfill, or extensive quantities of coarse materials in the
backfill can all contribute to piping failure.
Piping occurs where a high hydraulic gradient causes the rate of water
movement through the wall to increase as the water nears the downgradient side
of the wall. If the water movement is rapid enough, the water could
conceivably force the downstream filter cake into the pores in the soil along
the trench wall. As the rate of water movement out of the wall increases, the
water can begin to erode the easily-dispersed backfill, creating even larger
pores and allowing the water movement rate to increase further (Anderson and
Brown, 1980). To avoid piping failures, the quality of the filter cake should
7-36
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be maximized, the backfill materials should be properly selected and mixed,
the backfill should be carefully placed to avoid fold-overs and permeable
areas, and the hydraulic gradient across the wall should be monitored and kept
within designed levels.
7.1.2.3.5 Chemical Disruption
Chemical substances in soil and groundwater can affect the durability of
the slurry wall once it is in place. Chemical destruction can affect the
cement in cement bentonite slurry walls as well as the bentonite in cement
bentonite and soil bentonite walls. The effects of alkali salts on bentonite
slurries were described previously.
The action of the chemicals on cement or bentonite are similar to that of
the tunnelling process. The cement may become slurry solubilized and the
bentonite may become entrained in the solution as the chemicals eventually
create a solution channel through the wall into surrounding soil. Thus,
chemical destruction processes may create, as well as accelerate, the
tunnelling process. Chemicals may also prevent the slurry from forming an
adequate filter cake along the sides of the slurry trench by interfering in
the slurry gelation process.
7.1.3 Design and Construction
This section discusses design and construction considerations for
pollution control slurry walls. The section also shows how the design must
consider site-specific factors and discusses design components. Construction
techniques for soil- and cement-bentonite and diaphragm walls are also
covered.
7.1.3.1 Design Procedures and Considerations
The process of designing a slurry wall requires assessment of site-
specific data and consideration of numerous design variables to determine the
feasibility of a slurry wall and to select the most appropriate wall type for
use at the site. This section describes these processes.
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7.1.3.1.1 Feasibility Determination
Using the data from the site investigations, the designer must determine
the feasibility and applicability of installing a slurry wall at the site.
Factors to consider include:
• Potential waste incompatibility
t Anticipated hydraulic gradients and maximum allowable permeability in
the completed wall
• Aquiclude characteristics, e.g., depth, permeability, continuity, and
hardness
• Wall placement relative to wastes and leachates
• Cost and time considerations.
Each of these factors is discussed below.
Waste and leachate compatibility with proposed slurry wall backfill
mixtures can be determined using laboratory tests. Where long-term
permeability is crucial, clay mineralogy and geochemical testing is advisable.
These tests provide an indication of which proposed backfill mixtures show the
greatest resistance to long- and short-term permeation by the pollutants at
the site.
Data on the anticipated hydraulic pressures on either side of the wall
indicate the range of hydraulic gradients to which the wall may be exposed.
When projected permeabilities and wall areas are known, the rate of subsurface
movement through the wall can be determined using Darcy's Law. Darcy's Law
states that
Q = KIA
where Q is the volume of water flowing through the wall, K is the hydraulic
conductivity of the wall, I is the hydraulic gradient, and A is the
cross-sectional wall area.
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To illustrate how Darcy's Law can be used to estimate a slurry wall's
effects on groundwater flow at a site, consider the following hypothetical
situation: a proposed slurry wall is designed to be 164 feet (50 meters)
long, 81 feet (25 meters) deep, and 3.2 feet (1 meter) thick. The hydraulic
gradient at the site is estimated at 2, and the wall's hydraulic conductivity
is designed to be less than 2.12 x 10" gpd/ft (1 x 10" cm/sec). According to
Darcy's Law, the amount of water that will move through the wall is 57 gpd
2
(0.216 m /day). Before the slurry wall was installed, the hydraulic
2 -4
conductivity of the same area was about 2.12 gpd/ft (1 x 10 cm/sec), a low
permeability for undisturbed soils. The amount of water flowing through the
2
area each day prior to slurry wall installation was 57,000 gpd (216 m /day).
Thus, this particular slurry wall would reduce the volume of water flow
through this area by 99.9 percent. In areas having higher initial hydraulic
conductivity the effect of the slurry wall would be even greater.
Another factor to consider when evaluating the feasibility of a slurry
wall is the aquiclude at the site. Ideally, the aquiclude should be thick,
impermeable, and unfractured, and should be soft enough for a backhoe or
clamshell to excavate a 1- to 3-foot key-in to prevent seepage under the
slurry wall. In areas where aquicludes are very hard, either slurry walls
will be more expensive to install or the aquiclude wall union will be less
certain. Where the aquiclude is thin, discontinuous, or fractured, slurry
walls can be expected to be less efficient in pollution migration control
because of seepage through the aquiclude. Consequently, other remedial
measures may be called for.
An early assessment of possible wall locations and configurations can
indicate the overall amount of wall exposure to wastes and leachates, the
types and placements of auxiliary measures, and the actual length and depth of
wall required. These factors, along with an estimate of the necessary wall
durability, can assist designers in projecting the cost of the construction
effort.
The need for rapid response at some sites necessitates an evaluation of
the construction time required. According to Miller (1979), soil-bentonite
7-39
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slurry walls are normally installed at a rate of 25 to 100 linear feet per
day. Thus, the wall described previously could be installed in 2 to 7 days,
assuming the work is accomplished in a nonhazardous environment. Hazardous
conditions can more than double on-site work time.
7.1.3.1.2 Selection of Slurry Wall Type
If a slurry wall is determined feasible, the type of wall (soil- or
cement-bentonite) that is required should be established. To decide whether a
soil- or cement-bentonite cut-off wall should be installed at a particular
site, several factors need to be considered. The following factors affect the
suitability of soil- and bentonite-cement walls:
• Required permeability and hydraulic pressure
• Leachate characteristics
• Availability of backfill material
• Required wall strength
• Aquicl ude depth
• Site terrain
• Cost.
These factors are discussed below.
Where low permeability is required, soil-bentonite walls are used and the
wall width is determined by the hydraulic head across the trench. Case (1982)
recommends that the trench should "have a width of 0.5 feet to 0.75 feet per
10 feet of hydrostatic head on the wall. Thus, for a 100-foot head loss, wall
thickness should range from 5 to 7.5 feet." In comparison, a cement-bentonite
wall only 2 to 3 feet thick will stand up to the same hydrostatic force (i.e.,
100 feet). Deeper walls are generally wider then shallower ones because
larger excavation equipment is used for deep walls, and this equipment
generally digs a wider trench (Millet and Perez, 1981). Generally, cement-
bentonite walls are designed to be narrower than soil-bentonite walls because
of the greater shear strength and the higher cost of cement-bentonite walls
(Millet and Perez, 1981; Ryan, 1976).
7-40
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Soil-bentonite walls exhibit a lower permeability and a greater
resistance to chemical attack, particularly to acids, than cement-bentonite
walls. For this reason, soil-bentonite walls are favored for use as pollution
migration cut-offs (Jefferis, 1982b; Xanthakos, 1979). Where floating rather
than sinking contaminants are encountered, the slurry wall does not have to be
extended down into the aquiclude. Instead, a "hanging" wall is installed.
These walls are usually soil-bentonite types.
In some sites, the material excavated from the trench is contaminated
because of contact with polluted groundwater. This contaminated soil may be
unsuitable for use in the backfill. Samples of this material should be mixed
with bentonite slurry and tested to determine the effects of the contaminants
on the soil-bentonite wall permeability. In some cases, the contaminants will
increase the permeability of the completed wall , but the increase may be less
than the increase that could be caused by the sudden exposure of the wall
to the polluted groundwater. At these sites, use of the contaminated material
in the backfill may be advisable, providing the gradation is adequate
(D'Appolonia, 1980b). If, however, the contaminated soil is discovered to be
inappropriate for use as backfill material, a suitable borrow area should be
found. Where such a borrow area is not available nearby, cement-bentonite
walls may be more appropriate.
Generally, cement-bentonite walls are used where heavy vertical loadings
are anticipated and large lateral earth movements are not expected. This is
because cement-bentonite walls have a higher shear strength and lower
compressibility than soil-bentonite walls. Cement-bentonite walls are,
however, more likely to crack than relatively plastic soil-bentonite (Millet
and Perez, 1981). If the wall must be extended beneath roads, rail tracks, or
in close proximity to existing foundations, cement-bentonite walls can be
used. In addition, cement-bentonite walls can be used in localized areas
requiring strength and tied into soil-bentonite walls for the rest of the
trench di stance.
Cement-bentonite walls are more expensive than soil-bentonite walls
because of the cost of the cement. For this reason, cement-bentonite walls
7-41
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are not generally used where the aquielude is deep or where very long cut-off
walls are required (Ryan 1977).
At sites where slopes are steep, the areas for backfill mixing are
limited or non-existent, and low permeability is not critical, cement-
bentonite walls may be preferred. In general, soil-bentonite walls are limited
to areas where the maximum slope along the trench line is on the order of
2 percent or less. At many sites, hills can be leveled and depressions
backfilled with compacted soil prior to trench construction. The lack of
sufficient backfil 1 .mixing areas can be overcome by hauling trench spoils to a
central backfill mixing area, then hauling mixed backfill back to the trench.
Pug mills can also be used for backfill mixing. These operations, site
leveling use of pug mills and central backfill mixing, result in slower
construction rates and higher costs.
In contrast to soil-bentonite walls, cement-bentonite walls can be
constructed in areas of steeper terrain by utilizing the cement-bentonite
panel construction technique described later in this section.
As mentioned earlier, a cement-bentonite wall is typically more expensive
than a soil-bentonite wall of the same volume because of the cost of the
cement. Where thick or deep walls are planned, cement-bentonite walls will,
in most cases, be more expensive than soil-bentonite walls. Where wall
thickness can be minimized and very low permeability is not essential,
cement-bentonite walls can be considered.
7.1.3.2 Soil Bentonite Wall Construction
Following award of the construction contract, the selected firm will
proceed with construction of the slurry wall. The major activities include:
• Reconstruction assessment and mobilization
• Site preparation
• Slurry preparation and control
• Slurry mixing and hydration
7-42
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t Slurry placement
• Trench excavation
• Backfill preparation
• Backfill placement
• Site cleanup and demobilization.
Discussions of these activities follow.
7.1.3.2.1 Reconstruction Assessment and Mobilization
Three major activities occur during the mobilization phase of slurry
trench construction. These are:
• Layout of the site plan
• Determination of the the equipment, type, amounts of materials, and
facilities required.
• Determination of the number and source of personnel required.
These three activities are discussed below.
A preliminary layout is prepared based on drawings supplied by the
engineer. Once the preliminary layout is developed, a close examination of
the proposed construction site must be performed in order to ensure that all
details of the plan are practical. After the on-site examination, a final
layout of the worksite can be prepared. A diagram of a typical slurry wall
construction site is shown in Figure 7-12.
The specifications and drawings, test boring records, subsurface explora-
tion reports, and records of utility lines are the first sources of informa-
tion for determining equipment, materials and facilities needs. In addition,
an on-site inspection may be required to gain the detailed understanding
needed for planning the construction activities.
7-43
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FIGURE 7-12.
TYPICAL SLURRY WALL CONSTRUCTION SITE
Bemonite
Storage
Backhoe
Area of Active
Excavation
Propoaed Line
of Excavation
Hydration
Pond
\
Slurry
Storage
Pond
Slurry
Pumps
Slurry
Preparation
Equipment
Bentonite
Storage
ooo
Water Tanks
Access
Road
7-44
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The major work elements and equipment and facilities typically associated
with each work element are:
t Excavation
hydraulic backhoe
- mechanically operated clamshell
- hydraul ically operated kelly-mounted clamshell
• Slurry preparation and control
high speed colloidal batch mixer for small projects
- flash mixer for large projects
- pumps, valves, pipes, and tools
- hydration ponds
- desanders, hydrocyl ones, or screens
o Slurry placement
pumps
placement hoses and piping
• Backfill preparation
dozer or grader
• Backfill placement
- dozer
- mechanically operated clamshell
- trucks and trough
• Supervision and quality assurance
shed or trailer
- Marsh funnel
- mud balance
- standard sieves.
Numerous factors influence the types of equipment required as well as the
final plan layout and the relative difficulty of construction activities.
Some of these factors, along with their potential effects on slurry trench
construction operations are listed in Table 7-5.
Determining the correct equipment applications for a particular project
is based upon construction requirements and the constraints imposed by the job
site. For example, choice of excavation equipment depends upon the depth of
the slurry wall and the soil in which the wall is placed. The maximum excava-
tion depth for a standard backhoe is about 50 feet, but larger, extended
models are available to reach up to 80 feet in depth (D'Appolonia, 1980a).
Both clamshell excavation systems will reach depths of more than 150 feet,
7-45
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TABLE 7-5.
TYPES OF PHYSICAL CONSTRAINTS AND THEIR EFFECTS ON SLURRY WALL CONSTRUCTION
(Ryan, 1980a; U.S. Army Corps of Engineers, 1978; Xanthakos, 1979;
Wetzel, 1982; Tamaro, 1980; Ryan, 1980b; Namy, 1980)
Physical Constraints
Possible Affected Areas
Approach Required
Topography: Irregular contours • Necessary equipment
Steeply sloping terrain
• Site access and work space
• Type of wall selected
Ol
Site Access Site congestion/
and Work traffic
Space: Steep terrain
Dense vegetation
Lack of head room
Insufficient space
for mixing
• Extent of site prepara-
tion and pre-construction
• Type of equipment selected
• Type of wall selected
• Wall construction process
• Selection of equipment capable of
operating in site specific terrain
• Extensive site preparatory work -
leveling of areas for site entry and
work space
• Use of cement-bentonite wall in panels
or diaphragm wal 1
Special equipment needs; construction
of access road; leveling of working
area; clearing of dense vegetation
Amount of head room affects type of
equipment selected or needed to
relocate obstruction
Amount of work space affects wall type
selected; soil-bentonite wall requires
space for mixing; cement-bentonite
wall requires less area for opera-
tions, but is more expensive; soil-
bentonite can be mixed away from
trench but this approach may mean
cement-bentonite is cheaper for the
site
(continued)
-------
TABLE 7-5. (Continued)
Physical Constraints
Possible Affected Areas
Approach Required
Site Access
and Work
Space:
(continued)
• Extra time needed for site prepara-
tion and construction
t Appropriate easement clearances
Utilities: Abandoned sewers
Pipelines
Leakage from water
mains, sewers
Power/telephone cables
• Equipment selection
t Construction process
(operations)
• Problem control methods
• Sudden slurry loss and
possible trench collapse
if unanticipated pervious
zone, i.e., sewer piping
is entountered and
ruptured
• Special equipment necessary for
excavation around piping and sewer
lines, or need for manual excavation
• Sequence of trench segment excava-
tion may change if utility discovered;
excavate other areas first
• Watermain or sewer leakage may cause
slurry contamination and loss of
trench stability; a control plan
necessary at outset of project
t Sudden slurry loss requires immediate
placement of solid materials (soil,
debris) into trench
Cultural
Features:
Old foundations
Nearby structures
Overhead structures
• Equipment selection
• Construction process
(operations)
• Problem control methods
• Foundation penetration to isolate site
• Excavation around foundations, or
incorporate foundation into wall; if
foundation support needed cement
bentonite or diagraphm may be required
(continued)
-------
TABLE 7-5. (Continued)
Physical Constraints
Possible Affected Areas
Approach Required
-J
00
Cultural Features:
(continued)
• Headroom
Other: Availability of water
Time of year; water
table fluctuations,
temperature
Subsurface geology;
large subsurface
bounders
Type of wall backfill
• Equipment selection
• Slurry mixing
• Time needed and available
for project completion
• Site preparation
• Problem control methods
• Special equipment needed if breaking
old foundations
• Tall equipment may be restricted,
e.g., cranes
• More time may be necessary for
operations
• Experienced problem control personnel
necessary
• Equipment selection for boulder
destruction or excavation
• Site may need de-watering system if
water table is high or is expected
to rise
• Soil-bentonite backfill cannot be
mixed in subfreezing temperatures
t Cement-bentonite will not set in
certain temperature ranges
t Experienced problem control-personnel
necessary
• Transport of water to site if none
available
-------
with the hydraulic model sometimes preferred in more difficult digging
conditions (Guertin and McTigue, 1982). Table 7-6 lists the typical equipment
used for slurry trench construction.
Consideration must also be given to site access and obstructions. Access
road's might limit the size equipment that can be brought to the site, while
obstructions at the site might preclude the use of some types of equipment.
TABLE 7-6
EXCAVATION EQUIPMENT USED FOR SLURRY TRENCH CONSTRUCTION
(Case, 1982; D'Appolonia, 1980; Guertin and McTigue, 1982; Shallard, 1983)
Type
Trench Width
(feet)
Trench Depth
(feet)
Comments
Standard
backhoe
Modi fied
backhoe
Cl amshel 1
Dragline
Rotary drill ,
percussion
drill or large
chisel
1-5
2-5
1-5
4-10
50 Most rapid and least costly
excavation method
80 Uses an extended dipper stick,
modified engine & counter-
weighted frame; is also rapid
and relatively low cost
>150 Attached to a kelly bar or
crane; needs >_ 18 ton crane;
can be mechanical or hydraulic
>120 Primarily used for wide, deep
soil-bentonite trenches
Used to break up boulders and
to key into hard rock
aquicludes. Can slow construc-
tion and result in irregular
trench walls
7-49
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When determining the sources of personnel for use at slurry trench
construction site, two choices face the construction firm. The firm can send
their own equipment and personnel to the construction site or they can rent
equipment and hire personnel locally. Most firms will use varying combina-
tions of each approach. For larger jobs and critical small jobs, equipment
and personnel are more frequently sent directly from the construction firm.
Small jobs can often be handled effectively by using only specialized
company-owned equipment, such as an extended backhoe arm, accompanied by
supervisory personnel. Other equipment such as bulldozers, cranes, clam-
shells, and large ba'ckhoes can be rented near the job site. Laborers and
certain equipment operators can be hired locally for the specific job. How-
ever, there are not set rules and each construction contractor will tailor his
approach on a site-by-site basis.
Slurry trench construction contractors are best able to judge appropriate
equipment and personnel needed at the job site. However, site owners and
their representatives should be aware of the approach to be taken by the
construction contractor and should be satisfied that appropriate equipment and
personnel are available at the job site.
7.1.3.2.2 Pre-excavation Site Preparation
Once site planning has been completed, necessary permits and clearances
have been obtained, and required utility, water, and other services have been
arranged, preparation of the construction site can proceed. The work site can
then be cleared if necessary, security fences erected, utility and water
hook-ups made, equipment and facilities brought in and set-up, and construc-
tion materials delivered. At this time, work can proceed to move or remove
obstructions if necessary.
7.1.3.2.3 Slurry Preparation and Control
Before excavation begins, the slurry must be prepared. To do this,
bentonite and water quality must be tested, hydration ponds must be con-
structed, lines laid, pumps placed, and the mixing area prepared. The slurry
7-50
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is then mixed in a venturi or paddle mixer and allowed to hydrate fully prior
to placement in the trench for soil-bentonite slurry trench cut-off construc-
tion or mixing with cement for cement-bentonite cut-off construction.
Bentonite quality is critical to the quality of the slurry. Bentonite is
usually shipped to the job site accompanied by laboratory test results showing
that it meets quality criteria. These criteria include physical and chemical
purity, pH, gel strength, dry fineness (percent passing the number 200 sieve)
and filtrate loss. At the job site, these criteria are checked frequently,
such as by testing every truckload of bentonite delivered. Requiring field
testing of delivered bentonite is important for the site owner because
deliveries are occasionally rejected by field testing. At a minimum, testing
of pH, viscosity, and fluid loss should be conducted in the field for
bentonite delivered to the site.
Slurry quality decreases significantly if the quality of make-up water is
poor. Make-up water should be relatively low in hardness, near neutral or
slightly higher pH, and low in dissolved salts. Water suitable for drinking
is not necessarily suitable for mixing with bentonite. Water of inadequate
quality will result in higher bentonite consumption and a lumpy slurry that is
difficult to mix and contains above average amounts of free water. In some
instances, poor quality water can be chemically treated to make it suitable
for mixing (Ryan, 1977).
Two types of mixing systems are most frequently used. These are batch
mixing and flash mixing. In the batch system, specified quantities of water
and bentonite are placed in a tank and mixed at high speeds with a circulation
pump or paddle mixer. Mixing continues until hydration is complete and the
batch is ready for use in the trench. Hydration is usually complete in a
matter of minutes for the two- to five-cubic-yard batch produced by this
system. Because of the low output of the batch system, its use is limited to
small jobs.
The second type of mixing system is the flash or venturi mixer. For this
system, bentonite is fed at a predetermined rate into a metered water stream
7-51
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as it is forced through a nozzle at a constant rate. The slurry is subjected
to high shear mixing for only a fraction of a second, which is not always
adequate for hydration. Therefore, the slurry is often stored until hydration
is complete. This is determined by periodically measuring the Marsh Funnel
vi scosity.
When Marsh Funnel viscosity readings stabilize, hydration is considered
complete. Flash mixing is a process that can be operated at high production
rates. Because a majority of cut-off walls require continuous production of
large amounts of slurry, flash mixing is the more common of the two mixing
methods (Ryan, 1977). The type of mixing system used has been found to affect
the quality of the slurry produced. High shear (or high speed batch mixers)
produce slurries with higher gel strengths (Xanthakos, 1979).
The grade of bentonite dictates the percent needed for a given slurry and
hydration time. For example, a grade 90 (bbl/ton) bentonite may have to be
mixed at a 6.3 percent concentration, while a grade 125 would require 4.5
percent concentration for an equivalent slurry. Hydration times for higher
grades are likewise lower, which may result in higher slurry production rates
for a given slurry preparation facility (Ryan, 1977). However, the higher
cost of the higher grade bentonites requires that the selection of bentonite
grade be made on a site-by-site basis. Frequently a mud balance test is run
on slurry from the hydration pond as a quality control check of the bentonite
content. Viscosity and pH are also checked frequently (Cavalli, 1982).
7.1.3.2.4 Slurry Placement
From the hydration pond, slurry is pumped on an as-needed basis to the
excavated slurry trench. Slurry level in the trench must be maintained at
least several feet above the water table and normally within a foot or two of
ground level. This slurry level is maintained to provide the hydrostatic
pressure necessary to hold open the trench.
Once a slurry trench installation is under way, backfill and excavation
are being performed simultaneously, with a minimum amount of trench remaining
7-52
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open under the slurry. Figure 7-13 illustrates the excavation and backfill
placement operations. The amount of trench remaining open at any one time
depends on the properties of the backfill material and the characteristics of
the excavation equipment, which are discussed in the following sections.
Samples removed from the trench for QC checks must be representative, that is,
they should not be taken only from the top surface of the slurry but should be
taken at various depths in the trench. The slurry, designed to keep the
trench open during excavation and backfilling, must also allow displacement by
the backfill material. That is the reason for stipulating maximum and minimum
values for parameters such as density, viscosity, and sand content. Those
requirements are listed in Table 7-7. During the excavation operation, some
of the spoil becomes incorporated in the slurry. This increases slurry
density and sand content. A high sand content indicates a high density and a
likelihood of problems with eventual displacement of the slurry by the
backfill (D'Appolonia and Ryan, 1979).
FIGURE 7-13.
CROSS-SECTION OF SLURRY TRENCH, SHOWING
EXCAVATION AND BACKFILLING OPERATIONS
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TABLE 7-7.
MATERIALS QUALITY CONTROL PROGRAM FOR SOIL-BENTONITE WALLS
(Federal Bentonite, 1981 as cited by Spooner et al., 1984}
Duality Subject Standard Name
Control Item
Materials Water
Type of Test
-pH
-Total hardness
Frequency
Per water source
or as changes
occur
Specified Values
As required to properly
hydrate bentonite with
approved additives.
Determined by slurry
viscosity and gel strength
tests
Slurry
Additives
Bentonite
Backfill
soils
Prepared
for place-
ment Into
the trench
In Trench
API Std 13:
Standard Procedure
for Testing
Drilling Fluids
Backfill
Mix
At Trench
API Std 13:
Standard Procedure
for Testing
Drilling Fluids
API Std 13B1:
Standard Procedure
for Testing
Drilling Fluids
ASTM C 143
Slump Cone Test
Manufacturer certificate
of compliance with
stated characteristics
Manufacturer certificate
of compliance
Selected soils obtained
from a borrow area
approved by the engineer
Roll to 1/8" thread
- Unit Weight
- Viscosity
1 set per shift or
per batch (pond)
- Filtrate Loss
- pH
- Unit Weight
- Slump
- Gradation
1 set per shift at
point of trenching
1 set per 200 cu
yds
As approved by engineer
Premium grade sodium cation
montmorlllonite
65 to 100% passing 3/8" Sieve
35 to 85% passing 120 Sieve
15 to 35% passing 1200 Sieve
Unit Weight = 1.03 gm/cc
v = 15 centipose of
40 sec-Marsh 8 6°F
Loss = 15 cc to 25 cc 1n
30 min 3 100 psi
pH = 8
unit weight = 1.03 to 1.36
gm/cc
Slump 2 to 6 inches
65 to 100% passing 2/8" Sieve
35 to 85% passing #20 Sieve
15 to 35% passing 1200 Sieve
-------
7.1.3.2.5 Trench Excavation
Excavation of a slurry trench parallels standard trench excavation
procedure except that only the portion of the trench above slurry level can be
visually inspected for continuity. Trench excavation is usually accomplished
with backhoes with appropriate boom length and bucket capacity. Frequently,
boom lengths are extended by construction contractors to meet the needs of the
trench installation. Counterweights are often required to offset the movement
created by the long boom lifting a full bucket from the trench. The backhoe
is the favored means of excavating a slurry trench because it is much faster
than other equipment, such as the crane and clamshell. However, boom lengths
are currently limited to 70 to 90 feet. For greater depths, the crane and
clamshell are normally used. Drag lines have been used in the past, but have
been used rarely for recent installations (D' Appolonia, 1982).
Trench continuity is critical to a successful installation. For checking
continuity of slurry trenches, several approaches have been used. All of them
may be employed at a given site to ensure that the trenching is continuous
from the ground surface to the aquiclude key-in.
The field inspector should have boring logs and a cross-sectional drawing
of the trench so that visual inspection of the excavated material and the
degree of extension of the backhoe boom will indicate approximate depth and
whether the aquiclude has been reached. Soil boring data can also be used to
quantify the aquiclude key-in. By watching excavated material for a change in
color or texture, construction personnel can determine when the subsurface
layer which is to be keyed into is reached.
Sounding of depth with a weighted line or a rod should be performed
frequently to ensure an even trench bottom and to detect any irregularities.
Finished trench depths should be recorded for preparation of a drawing showing
trench cross section.
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7.1.3.2.6 Backfill Preparation
Standard practice during backfill preparation for soil bentonite walls is
to use excavated material mixed with slurry from the trench for backfill. In
this case, the slurry provides moisture necessary for backfill mixing. When
trench spoils are used, the material excavated from the trench is usually
placed nearby, slurry is added, and a bulldozer is used to track and blade the
material until it is thoroughly mixed.
A relatively level working surface is needed for backfill mixing. At
sites that are too steep, backfill mixing areas can be excavated. These
should be at least as wide as the width of the excavating equipment track.
Where backfill mixing areas are not available, batch mixers or pugmills can be
used, although these are slower than using a bulldozer for backfill mixing.
A number of quality control checks are necessary for backfill preparation
activities. These include tests of the:
• Fines content
• SI ump
• Wet density
• Presence of contaminants.
A key parameter in the design of a backfill is the sieve analysis and
particularly the amount of fines. The content of fines in the backfill is
directly related to the permeability of the finished soil-bentonite wall and
its ability to withstand chemical attack. A standard practice is to perform
permeability testing in the pre-construction phase to define the range of
acceptable grain size distributions that will provide the design permeability.
During construction, frequent grain size distributions and less frequent
permeability testing is performed on the backfill material to ensure that the
design permeability requirements will be met by the completed slurry trench.
Grain size distributions can be performed in a field laboratory and are much
less expensive to run than permeability tests, which are usually run in a
1aboratory.
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Slump cone testing should be performed frequently on backfill material
after mixing to make sure that backfill is wet enough to slump in the trench
without trapping pockets of slurry, yet dry enough to displace the slurry
easily. High slump also indicates a gentler slope of backfill in the trench,
which would require keeping more of the trench open. A slump of 2 to 6 inches
is adequate (D'Appolonia , 1980).
Mud balance testing should be performed frequently on the backfill before
it is placed in the trench. Mud balance tests indicate the wet density of the
backfill. This shows for certain that the backfill will or will not readily
displace the slurry. The wet backfill sample used for the mud balance test
can be dried and reweighed to determine the water content. A very high water
content can result in excess water infiltration into the trench walls and
excess settlement of the backfill. The separation of water from an exces-
sively wet backfill can also dilute the slurry in the trench.
7.1.3.2.7 Backfill Placement
Once trench excavation has proceeded for a distance that will not result
in backfill material being re-excavated, backfill placement can begin. First,
samples of slurry at the base of the trench are collected and tested for wet
density. The slurry should be at least 15 pcf less dense than the backfill
mixture (D'Appolonia, 1980b). If the slurry is too dense, it will not be
displaced properly during backfill placement. The dense slurry or coarse
material on the trench bottom must be removed via airlift pumps, a clamshell
bucket or other method. This slurry can be used for backfill mixing or be
desanded via desanders, hydrocycl ones, or screens, and returned to the trench.
To place the initial layer of backfill, a clamshell is often used. The
backfill must not be allowed to drop freely through the slurry, as this may
cause segregation of the backfill particles or entrapment of slurry pockets
within the backfill. For this reason, the clamshell lowers the initial
grabfull of backfill to the trench bottom. The next grabfull is placed on top
of the first, and so on until the backfill is visible at the ground surface.
Thereafter, the backfill is pushed into the trench by bulldozers or graders.
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The point of trench backfilling progresses toward the area of active
excavation (D'Appolonia and Ryan, 1979).
The slope of the emplaced backfill is normally 5:1 to 10:1. The distance
to be maintained between the toe of the backfill and the area of active
excavation varies greatly, depending on soil types and backfill slopes.
Ideally this distance is kept to a minimum to avert trench stabil ity problems
(D'Appolonia, 1980b). Some specifications have set the distance at from 30 to
200 feet (U.S. Army Corps of Engineers, 1975; Ryan, 1976).
Soundings of the placed backfill should be taken to show the slope at
which the backfill is coming to rest and to indicate possible problems with
trench wall collapse and entrapment of pockets of slurry. Depths should be
recorded and plotted on a cross-sectional drawing of the trench. Trench
excavation and backfilling progress can also be recorded in this manner.
When a slurry wall is constructed to entirely surround a waste site, the
excavation must end with enough overlap to ensure that all material designated
for excavation is removed. The inspector must determine that the backhoe
bucket is removing backfill materials from the full trench depth to verify
that the trench is continuous for its entire length.
7.1.3.2.8 Capping
To protect the finished soil-bentonite wall, either a dessication cap or
a traffic cap is generally applied to the slurry wall surface.
Once backfill has been completed, cracking is soon observed on the top of
the slurry wall unless it stays wet. To complete the installation, the top 1
to 3 feet of wall is removed to eliminate the cracks and a high quality
backfill material replaces it. The material used is usually required to have
a high clay content and to be compacted in lifts over the trench. This forms
a low permeability cap to protect the cut-off wall from excessive dessication
(U.S. Army Corps of Engineers, 1976). This is followed by topsoil and seeding
or a gravel layer to prevent water and wind erosion.
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Where traffic over the wall is anticipated, a traffic cap can be con-
structed to reduce the load on the completed cut-off. To do this, lifts of
compacted clay are interspersed with geotextile layers. Gravel can be used
over the final geotextile layer at the surface. At one facility where a soil
bentonite cut-off wall was close to a heavily traveled gravel road, the cap
consisted of an 18-inch thick compacted clay lift topped by a geotextile
sheet. This was overlain by another 18-inch thick compacted clay layer topped
by another geotextile sheet. Several inches of gravel were placed over the
final sheet of geotextile to distribute the weight and bear the load of the
vehicles (Coneybear, 1982).
7.1.3.2.9 Cleanup Activities
After wall construction is complete, the excess slurry and mixed backfill
must be disposed of in a manner that avoids erosion and disruption of sewer
lines. The slurry should not be allowed to enter sanitary or storm sewer
lines because of the potential for pipe blockage from the slurry. Excess
slurry also should not be left as a thick layer on the soil surface, as this
may result in excessive ponding of surface water. One method of slurry
disposal would be to mix the slurry with dry, coarse soil to produce as dry a
mixture as possible. This material could then be either buried or spread in a
thin layer over disturbed areas, then fertilized and seeded. Any contaminated
soil from the excavation must be disposed of in accordance with site require-
ments. All disturbed areas should be stabilized and site maintenance
procedures should be instituted.
There are a number of differences in construction activities that vary
with construction materials. The following is a brief summary of cement
bentonite and diaphragm wall construction techniques.
7.1.3.3 Cement Bentonite Wall Construction
The discussion presented above is an outline of a soil-bentonite cut-off
wall construction. Modifications to the construction specifications are
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necessary when constructing a cement-bentonite cut-off wall. These
modifications include the following:
0 The requirement for backfill materials are eliminated
• A description of the standards for cement and cement storage is added
in the materials section
• A description of the cement-bentonite slurry requirements and the
cement/water (c/w) ratio to be maintained for desired compressive
strength is added to the materials section
• The methods to be used for tie in of adjacent cement-bentonite panels
(if used) are addressed in the performance section
• Under the Quality Control section, a requirement is added for the
manufacturer's certification of the cement and the testing of
cement/water ratio for each batch of mix is required.
Cement bentonite slurry walls involve the use of a slurry consisting of
water, bentonite, and cement. The advantages of cement-bentonite walls are
that backfill material is not needed and they exhibit some structural
strength. In addition, by excavating one section (panel) at a time, a
cement-bentonite wall can be installed on a site with more extreme topography.
Two types of cement-bentonite walls are being used. The in-place method
involves simply excavating under a cement-bentonite slurry and leaving the
slurry in place. The slurry eventually sets to provide some structural
strength. For the replacement method, excavation takes place under a
bentonite slurry. After excavation of the section of wall is complete, the
bentonite slurry is pumped out of the trench and the cement-bentonite slurry
is pumped in and allowed to set. The replacement method is used only when
setting of the cement-bentonite slurry could possibly occur while excavation
is being completed.
Cement-bentonite slurries begin to set within 2 to 3 hours after the
cement and slurry are mixed. If the slurries are agitated for over 48 hours,
they lose their ability to set (Jefferis, 1981b). For this reason, when it
appears that the excavation of a single cement-bentonite panel will take
longer than a day or so to complete, cement retarders are added to the slurry
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or the replacement method is used. Examples of this situation would be very
deep excavations, when rock is encountered in the excavation or when keying
the trench into bedrock.
Quality control procedures for cement-bentonite walls are identical to
those for soil bentonite walls. However, composition of the cement-bentonite
slurry is more critical, therefore, care must be taken when weighing and
mixing components of the slurry. This is because the calcium in the cement
causes an irreversible decrease in slurry quality. Table 7-8 shows typical
materials quality control standards for cement-bentonite cut-off walls.
7.1.3.4 Diaphragm Wai 1 Construction
Construction of diaphragm walls also involves the use of bentonite or
cement-bentonite slurries, although these walls are not normally used for
pollution migration cut-offs, except where high strength is required.
Diaphragm walls are composed of either precast concrete panels or cast-in-
place concrete sections. Unlike soil-bentonite and cement-bentonite walls,
these walls develop a great deal of strength over time and can be used as
structural components. A brief description of the techniques used for
construction of diaphragm walls is given in Section 7.1.2.2.3.
7.1.4 Completed Wall Costs
Contractors have provided average costs for completed slurry cut-off wall
construction and installation. These estimates may vary widely, however, as
they are based upon a number of site-specific factors. Some of these factors
include the following:
• Type of backfill (either cement- or soil-bentonite)
t Distance materials must be transported
• Presence of contamination or high salt content in groundwater
requiring special bentonite and excavation procedures
• Health and safety considerations
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TABLE 7-8.
MATERIALS QUALITY CONTROL PROGRAM FOR CEMENT-BENTONITE WALLS
(Federal Bentonite, 1981 as cited by Spooner et al., 1984)
Subject
Standard
Type of Test
Frequency
Specified Values
Water
-^1
CT)
Materials Bentonite API STD 13A
Cement
ASTM C 150
- pH
- Total hardness
(Ca & Mg)
Manufacturer
certificate of
compliance
Manufacturer
certificate of
compliance
Per water source
or as changes
occur
As required to properly
hydrate bentonite with
approved additives.
Determined by slurry
viscosity and gel strength
tests.
Unaltered sodium cation
montmorillonite
Portland, Type 1 (Type V or
Type II for certain applica-
tions)
Bentonite Prior to API STD 13B
Slurry addition of
cement
- Viscosity
1 set per shift or
per batch (pond)
v = 34 sec-Marsh @ 68°F
pH = 8
Cement -
Bentonite
Slurry
Upon
introduc-
tion in
the trench
API STD 13B
API STD 10B
- C/W ratio
- Viscosity
Each batch
5 per shift
C/W = 0.20
v = 40 to 50 sec-Marsh
-------
• Type of overburden being excavated
• Depth of excavation
t Presence of physical constraints upon working area (i.e., buildings or
other structures which must be worked around)
• Suitability of native materials for use as backfill constituents.
Ressi di Cervia (1980) developed a chart (Table 7-9) which relates
cut-off wall construction costs to type of backfill used, depth of excavation,
and soil type present. Costs are given in terms of square foot of wall since
the width of the excavation is generally determined by the excavation
equi pment.
7.2 GROUTING
Grouting is a process in which a fluid material is injected into a soil
or rock mass in order to reduce water movement or to impart increased strength
to the formation. Once in place within the formation voids, the fluids
solidify or gel thus greatly reducing the permeability and imparting increased
mechanical strength to the grouted mass (JRB, 1982; Herndon and Lenahan ,
1976a & b).
Grout barriers can be many times as costly as slurry walls and are
generally incapable of attaining truly low permeabilities in unconsolidated
materials. Therefore, they are rarely used when groundwater control in
unconsolidated materials is desired. The primary use of grouting is to seal
voids in porous or fractured rock when other groundwater control methods are
not practical. The installation of grout barriers has not been as common a
practice for controlling leachate plume migration from hazardous waste sites
as has slurry wall installation. Hence, there is not a great deal of
information available on the application of grouting techniques at hazardous
waste sites for controlling leachate plume migration.
Grouting is the most practical and efficient method for sealing fissures,
solution channels, and other voids in rock. Where rock voids allow the
passage of large volumes of water, a grout can often be formulated to set with
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TABLE 7-9
RELATION OF SLURRY CUT-OFF WALL COSTS PER SQUARE FOOT
AS A FUNCTION OF MEDIUM AND DEPTH
(Ressi di Cervia, 1980 as cited by Spooner et al., 1984)
Geologic Material
Slurry Trench Prices
in 1979 Dollars
Soil Bentonite Backfill
(Dollars/Square Foot)
Unreinforced Slurry Wall
Prices in 1979 Dollars
Cement Bentonite Backfill
(Dollars/Square Foot)
Depth Depth Depth
30 30-75 75-120
Feet Feet Feet
Soft to Medium Soil
N 40 2-4 4-8 8-10
Hard Soil
N = 40 - 200 4-7 5-10 10-20
Occasional Boulders 4-8 5-8 8-25
Soft to Medium Rock
N 200 Sandstone, Shale 6-12 10-20 20-50
Boulder Strata 15-25 15-25 50-80
Hard Rock
Granite, Gneiss, Schist* — — —
Depth Depth
60 60-150
Feet Feet
15-20 20-30
25-30 30-40
20-30 30-40
50-60 60-85
30-40 40-95
95-140 140-175
Depth
> 150
Feet
30-75
40-95
40-85
85-175
95-210
175-235
Notes:
N is standard penetration value in number of blows of the hammer per foot
of penetration (ASTM D1586-67)
*Normal Penetration Only
For standard reinforcement add $8.00 per sq. ft.
For construction in urban environment add 25% to 50% of price.
Costs do not reflect work in a contaminated environment.
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sufficient speed to shut off the flow. In theory, placement of a grout
curtain upgradient or downgradient from, or beneath, a hazardous waste site is
possible. In practice, however, this can be a very difficult task to
accomplish successfully.
As with slurry walls, placing a grout barrier upgradient from a waste
site can redirect the flow so that groundwater does not flow through the
wastes that are creating the hazardous leachate. Given normal groundwater
chemistry, most grouts could be expected to function well in this capacity.
Placement of a grout barrier downgradient from or beneath a hazardous
waste site is quite another matter. A variety of problems typically occur
when attempting to grout in the presence of leachate or very impure ground-
water. In many instances, controlling the set time, and consequently,
ensuring a barrier of reliable integrity is difficult or impossible. Little
information exists in the literature on the resistance of grouts to chemical
attack. Should a case arise where a grout might contact leachate or very
impure groundwater, extensive testing must be conducted to determine possible
effects on the grout. Additional problems occur in attempting to grout a
horizontal curtain or layer beneath a waste site. In order to inject grout in
such a case, injection holes must be drilled either directionally from the
site perimeter or through the wastes. The first situation can be very
difficult and expensive and the second could be very dangerous. In either
case, effective barrier placement is very difficult and virtually impossible
to ensure (JRB, 1982).
7.2.1 Theory
Successful grouting depends on the selection of the proper grouting
material for the specific area to be grouted. Thus, the physical and chemical
characteristics, the geology and hydrology, and the groundwater chemistry of
the site must be evaluated. The factors that will determine the groutability
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of the site include (Bowen, 1981; Sommerer and Kitchens, 1980; Herndon and
Lenahan, 1976a & b):
• Soil and rock physical characteristics: The permeability, porosity,
particle distribution, and pore size distribution of the formation
will control the physical properties, such as viscosity and particle
size, that the grout must possess as well as the quantity of grout
that will be needed.
• Groundwater characteristics: The hydrology and contaminants contained
in the groundwater will determine the chemical properties such as set
time and structure, that the grout must possess.
• Soil and rock chemical characteristics: The contaminants contained in
the soil and rock structure will control the chemical properties that
the grout must possess.
Based on these factors, an appropriate grout can be selected from
(Guertin and McTigue, 1982; Bowen, 1981):
• Bituminous grouts: These grouts, either as emulsions or asphalts, can
be used to waterproof soils or rock cavities.
• Suspension grouts: These are the most common types of grout and
include coarse grouts which contain particles in suspension such as
cement, clay and cement-clay. These materials are usually the more
viscous of the available grouting materials as well as having the
largest particle size. Thus, these grouts are best used in the
grouting of rock or coarse material.
• Chemical grouts: These grouts rely on chemical reactions to form
gels. They initially have low viscosities and thus can be used in
finer-grained cohesionless soils and as a secondary treatment for
grouting coarse soils and rock fissures. The common types of chemical
grouts include silicates, acryl amides, and various polymers.
The following sections discuss the chemical composition and reaction
theory of the major types of grout.
7.2.1.1 Bitumen Grouts
Bitumen or asphalt emulsions consist of bitumen, water, and an emulsi-
fier. Bitumens are viscoelastic materials containing high molecular weight
hydrocarbons (Kirk-Othmer, 1978a). When dispersed in water, bitumens yield an
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emulsion with a low viscosity suitable for injection (Tallard and Caron,
1977a). The viscosity of the emulsion is controlled by adjusting the ratio of
bitumen to water. Typical emulsions include bitumens; bitumen, soap, and
casein; and bitumen with a filler such as clay (Bowen, 1981).
Emulsions are stabilized by the emulsifiers, which delay mol ecular
aggregation and increase viscosity (Ki rk-Othmer, 1978a). The emulsifiers are
polar and determine whether the resulting emulsion is cationic or anionic
(Bowen, 1981). For example, amine chains are often used as acid emulsifiers
to yield cationic emulsions (Bowen, 1981; Kirk-Othmer, 1978a). Emulsifiers
should be water soluble and have properly balanced hydrophilic and lipophilic
properties (Koehmstedt, Hartley and Davis, 1977).
Bitumen emulsions break up upon contact with earth materials so that the
more viscous emulsion components settle out and fill the pores and fissures of
the earth material. The breakdown of bitumen emulsions can occur through
direct breakdown, the addition of destabilizing agents, or the adsorption of
emulsifying water. In direct breakdown, a stabilizing agent is eliminated
through decomposition or absorption by fine soil material. Direct breakdown
is difficult to control and often occurs too quickly or too slowly (Tallard
and Caron, 1977a). A second method involves adding destabilizing agents
(electrolytes or hydrolyzable esters) to the emulsion either before or after
the emulsion is injected to promote breakdown and flocculation of the emulsion
(Bowen, 1981; Tallard and Caron, 1977a). When destabilizing agents are added,
a one-step method (addition of additives prior to injection) is preferable
(Tallard and Caron, 1977a).
The set time of bitumen grout will vary depending on the method used to
achieve a breakdown of the emulsion. Set time is further controlled by the
emulsifier present in the grout (Koehmstedt, Hartley and Davis, 1977).
Bitumen grout is known to have long term stability (Tallard and Caron,
1977a). Oxidation and aqueous leaching of oxidation products are the primary
causes of degradation. Oxidative processes include microbial action and
sunlight. There is little evidence of anaerobic bacterial oxidation (Hartley,
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et al., 1981). Upon aging, viscous bitumen material can develop an internal
structure and the viscosity may increase (Kirk-Othmer, 1978a).
The bitumen used as grout typically consists of a coal tar or asphalt
base (Tallard and Caron, 1977a). These materials consist of complex hydro-
carbons which can be toxic if leached.
7.2.1.2 Cement Grouts
Cement grout consists of Portland cement and water. Several types of
Portland cement are available. The types most often used in soil grouting
include Type I (ordinary Portland cement), Type II (modified Portland cement,
moderate sulfate resistance), and Type V (low alumina, sulfate resistant).
Fillers such as clay, sand, or pozzolans, and additives such as chemical
polymers may be added to the cement to vary the characteristics of the grout
and improve its resistance to deleterious chemicals (Littl ejohn, 1982; Bowen,
1981).
Portland cement is made from limestone, clay, and iron oxide and consists
chiefly of tricalcium silicate (45 percent), dicalcium silicate (27 percent),
tricalcium aluminate, tetracalcium al umino-ferrite, magnesium oxide, and other
minor constituents (Littlejohn, 1982; Ingles and Metcalf, 1973). The calcium
silicates are the major cementation compounds in cement (Neville, 1973).
Various materials (organic and inorganic) may be added to cement grout to
achieve special characteristics or to control grout properties. Special
additives include anti-bleed agents, accelerators, retarders, and expansion
agents (Littlejohn, 1982). Latexes or water-soluble polymers may be added to
achieve special properties (Kirk-Othmer, 1979). Calcium chloride may be added
as an accelerator, however, calcium chloride may cause increased shrinkage
upon drying (Littlejohn, 1982). Sand may be added so that cement grouts may
be used to treat coarse materials (Bowen, 1981). Clays can be added to
stabilize the cement while pozzolans or clay may be added to improve alkali
resistance (Littlejohn, 1982; Bowen, 1981). Polyhydric alcohols can be added
to provide acid-resistance (Farkas and Szwere, 1949). Colloids such as
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gelatin, agar, and ammonium stearate may be added as stabilizers (Bowen,
1981).
Upon addition of water, the silicates and aluminates in cement form
hydration products which have low solubilities in water (Neville, 1973). The
calcium silicates form gels of mono- and di-calcium silicate hydrate (Ingles
and Metcalf, 1973). The insoluble calcium silicate hydrate crystallizes to
form a matrix within which the remaining hydration products form (Ingles and
Metcalf, 1973; Neville, 1973). The resulting cement gel is considered to be a
fine physical mixture of copolymers of hydrates (Neville, 1973).
The properties of fresh and cured concrete depend primarily upon the
water-cement ratio and the degree of hydration of the cement (Kirk-Othmer,
1979). Excess water will result in poor durability, increased shrinkage, and
bleeding (Littlejohn, 1982). High water-cement ratios also cause large
numbers of capillary spaces in the matrix (Kirk-Othmer, 1979).
The viscosity of cement grouts depends on the amount of water added to
the cement. As the water-cement ratio increases from 0.4 to 0.7, the
viscosity decreases from 5,000 to 500 centipoise. The viscosity can also be
reduced through the addition of certain organic admixtures (Kirk-Othmer,
1979).
The setting process occurs in two stages. First, fluidity of the grout
decreases until it is not pumpabl e--this is called the set time. Second, the
grout hardens and increases in strength--thi s is called the hardening time
(Littlejohn, 1982). The various components of cement grouts set at different
rates. These rates vary from a "flash set" (tricalciim aluminate) to several
hours (tricalcium silicate and dicalcium silicate) (Littlejohn, 1982).
Complete setting takes approximately six hours (Kirk-Othmer, 1979).
The set time may be increased by the action of a number of substances
including: organic materials, silt, clay, coal, lignite, sulfates, sodium
salts, metals, sugar, and tartaric acid (Fung, 1980; Littlejohn, 1982;
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Thompson, Mai one and Jones, 1980). Increasing the water-cement ratio will
also increase the set time.
While concrete is durable under normal conditions, concrete is subject to
deterioration as a result of deficiencies in grout quality, chemical attack,
drying, and temperature fluctuations (Littlejohn, 1982). Increasing the
water-to-cement ratio can greatly increase the permeability of the concrete
through an increase in the number of capillary spaces which permit penetration
of solutions (ACI Committee 515, 1979; Kirk-Othmer, 1979). Furthermore, all
cement grouts expel bleed water which can lead to accumulations of water and a
decrease in strength and lateral impermeability (Bowen, 1981). Cement may
also shrink causing the formation of microfi ssures and cracks unless it is
moist-cured (Littlejohn, 1982).
Concrete is vulnerable to chemical attack because of its alkalinity,
reactivity, and permeability. Penetration of fluids may be accompanied by
chemical reactions between the fluid and concrete constitutents (ACI Committee
515, 1979). Concrete will deteriorate because of sul fates, chemical wastes,
and organic acids (Tomlinson, 1980). Furthermore, cement hydration compounds
may leach from the matrix and this can also cause deterioration (ACI Committee
515, 1979).
The basic components of cement grouts are lime, silica, alumina, iron
oxide and water, which are all nontoxic. However, substances such as chemical
polymers (e.g., acryl amide) may be added to the grout to modify its
properties, and these materials may be toxic.
7.2.1.3 Clay Grouts
Clay grouts are composed primarily of bentonite. The basic reaction
theory and chemical composition of bentonite is described in section 7.1.2.1.
Bentonite grouts will start to set as soon as the injection pressure is
decreased. Once this process starts, the viscosity and then the gel formation
and strength will increase with time. The final strength of the gel will
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depend on the setting time, colloid concentration, and the composition of the
suspending fluid (Xanthakos, 1979). The final strength obtained, though, is
very low compared to other grouts (Talland and Caron, 1977b). The set time
can be controlled through the addition of silicates to the bentonite grout,
resulting in set times from a few minutes to five hours (Bowen, 1981).
The basic ingredient in this type of group, bentonite, is essentially
nontoxic. The toxicity of most of the additives, such as sodium silicate, are
also low, although agents used to gel the sodium silicate could pose a risk
(Tallard and Caron, 1977b).
The presence of organic or inorganic compounds in the groundwater can
have a detrimental effect on the ability of bentonite grouts to contain
pollutants. These chemicals can affect the physical and chemical properties
of the bentonite, result in fl occulation, reduce swelling of the bentonite, or
destroy the bentonite's crystalline structures.
If the bentonite is injected into groundwater which contains high con-
centrations of electrolytes, such as sodium, calcium, and heavy metals, the
bentonite could flocculate. This will result in particles that can exceed
10 microns in diameter, thus hampering the grout's ability to penetrate into
the soil structure (Tallard and Caron, 1977b; Matrecon, Inc., 1980; Alt her,
1981b).
Various organic and inorganic compounds can cause a change in the amount
of swelling that bentonite particles undergo (Alther, 1981b). This can lead
to increased porosities and permeabilities. Strong organic and inorganic
acids and bases can dissolve alumina and silica or alter the bentonite, and
also lead to large permeability increases (D'Appolonia and Ryan, 1979; Alther,
1981b). Undiluted alcohols increase grout permeability by extracting water
from the clay interlayers, thus reducing the amount of particle swelling.
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7.2.1.4 Silicate Grouts
Silicate grouts consist of alkali silicates, water, a gelling or setting
agent, and sometimes an accelerator. Typically, sodium silicate is used in
the grout although potassium silicate may be used instead. The gelling or
setting agent used varies depending on the desired properties of the gel. In
general, acids, acid-forming compounds, polyvalent cations, and some organics
may be used as setting or gelling agents (Kirk-Othmer, 1979; Hurley and
Thornburn, 1971). Accelerators include chlorides, aluminates, or bicarbonates
(Johnson, 1979). Other substances, such as organic esters, may be added to
delay gelling time (Bowen, 1981). Grout compositions can vary considerably
with the grouting method and are often proprietary (Herndon and Lenahan,
1976a).
The viscosity of sodium silicate grouts varies from 1.5 to 50 centipoise
and may be as high as 260 centipoise (Sommerer and Kitchens, 1980; Tall and and
Caron, 1977a). The viscosity of the grout depends on the ratio of SiO_ to
Na20. The higher the ratio of silica to sodium, the lower the viscosity. The
set time of silicate grouts varies from less than a minute to several hours.
Factors affecting the set time include silicate concentrations, setting agent
concentrations, and temperature. Increasing any of these three factors will
decrease the set time (Tall and and Caron, 1977a). The accelerator concentra-
tions may also be varied to control set time. Soil conditions can also affect
the set time with acid soils reducing gel time and alkaline soils potentially
preventing gel formation (Office of the Chief of Engineers, 1973).
Tallard and Caron (1977a) report that silicate grout is quite durable.
However, long-term strength and impermeability are of concern because silicate
grouts are subject to deterioration via syneresis (water expulsion), shrinkage
(dessication), and solution erosion by groundwater (Hurley and Thornburn,
1971).
Sodium silicate grouts are essentially nontoxic. The set grout has a
toxicity (oral) of 15 g/kg, while sodium silicate has a toxicity (oral) of 1.1
g/kg. Anides frequently used in formulating the grout are skin irritants
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(Tallard and Caron, 1977b). Other organic substances, such as formamide, are
toxic, possibly carcinogenic, and require special precautions when preparing
and injecting the grout (Karol , 1982a; Ki rk-Othmer, 1979; Tallard and Caron,
1977a). Heavy metal salts, which may be used as gelling agents, are also
toxic and could potentially be leached from the gel. In addition, sodium
salts may be expelled from the grout and under certain circumstances they may
be an environmental hazard (Karol, 1982a).
7.2.1.5 Organic Polymer Grouts
Organic polymer grouts represent only a small fraction of the grouts in
use. These grouts consist of organic materials (monomers) that polymerize and
cross-link to form an insoluble gel. This section addresses acryl amide,
phenolic, urethane, urea-formaldehyde, epoxy, and polyester grouts and their
properties.
7.2.1.5.1 Acryl amide Grouts
Acryl amide grouts consist of a base material (typically a monomer or
mixture of monomers), a cross-linking agent, an initiator or catalyst, and an
accelerator or activator. Persul fates or peroxides, typically ammonium
persulfate, are used as an initiator. Accelerators or activators include
dimethyl aminopropionitrile , diethyl aminopropionitrile, or triethanolamine
(Tallard and Caron, 1977a). The substances used vary with the particular
product. Gel time may be controlled through the addition of a reaction
inhibitor, typically potassium ferricyanide (Karol, 1982a; Tallard and Caron,
1977a). Buffers may be required to maintain the pH of the grout solution
around 8.
The set time of acryl amide grouts can vary from a few seconds to several
days (Karol, 1982a; Cues, Inc., 1982; Tallard and Caron, 1977a; Office of the
Chief of Engineers, 1973). The primary factors controlling set time are
concentrations of reaction inhibitors, catalysts, and activators (Karol,
1982a; Ki rk-Othmer, 1979; Tallard and Caron, 1977a). An acidic grout solution
or acidic grouting conditions (groundwater or earth material) can lengthen the
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set time and may even prevent gelation (Office of the Chief of Engineers,
1973). The set time may be shortened through the action of metals, tri-
ethanol amine, or ammonium persulfate by increasing the pH, or by increasing
the dry matter in the grout (Clarke, 1982; Avanti International, 1982; Tallard
and Caron, 1977a; Office of the Chief of Engineers, 1973).
Once an acryl amide grout is set, it is fairly stable chemically and is
not subject to slow deterioration or syneresis (Karol, 1982a; Berry, 1982;
Avanti International, 1981; Tallard and Caron, 1977a). Over time, however,
the grout may shrink or some salts, unreacted materials, and hydrolysis
products may leach out (Karol, 1982a; Tallard and Caron, 1977a).
7.2.1.5.2 Phenolic Grouts
Phenolic grouts, commonly referred to as phenoplasts, are polycondensates
of phenols and aldehydes (Sommerer and Kitchens, 1980; Tallard and Caron,
1977a). Atypical phenolic grout consists of a phenol, an aldehyde, water,
and a catalyst. Formaldehyde is used exclusively because of its reactivity
(Billmeyer, 1971). Sodium hydroxide or other alkaline materials (hydroxides,
carbonates, phosphates) are typically used as catalysts, although acids can
al so be used.
Phenolic grouts may be mixed in a one or two solution system. The
proportions of phenol, formaldehyde, and catalyst are fixed by reaction
requirements so the only variable is the amount of water added (Tallard and
Caron, 1977a). Polymerization begins as soon as the solutions or grout
components are mixed. In general, polyvalent cations from the alkali catalyst
initiate and promote polymerization (Chung, 1973). The polymerization
(polycondensation) process results in the formation of a three dimensional
network of polymer chains that are joined and cross-linked by formaldehyde
(Karol, 1982a; Bowen, 1981; Tallard and Caron, 1977a). The catalyst is also
attached to the polymer resin and may form secondary linkages within the resin
network or bond with soil. The resulting resin is insoluble and retains all
constituent material s (Tallard and Caron, 1977a).
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Phenolic grouts made with resorcinol and formaldehyde have viscosities
between 1.2 and 3.0 centipoise (Karol , 1982a; Tallard and Caron, 1977b).
Commercial products consisting of tannin or polyphenols typically have higher
viscosities: "Geoseal" ranges from 2 to 12 centipoise, "Terranier" ranges
from 4 to 10 centipoise, "Rocagil" ranges from 5 to 10 centipoise (Bowen,
1981; Tallard and Caron, 1977a; Tallard and Caron, 1977b). The viscosity of
resorcinol-formaldehyde grouts, like acryl amides, remains constant until
gelation occurs. In tannin-based grouts, the viscosity gradually increases
after the components are mixed (Karol, 1982a; Tall ard and Caron, 1977b).
The gel or set time of phenolic grouts is proportional to the solution
diluteness and may vary from several minutes to several hours (Bowen, 1981;
Karol, 1982a; Tallard and Caron, 1977a). With very dilute solutions, the gel
time increases so much that gelation never occurs and the grout becomes
unusable (Tallard and Caron, 1977a). If all other factors remain constant,
the choice of catalyst will affect the set time because different bases have
different reactivities. Sodium hydroxide, the most common catalyst, provides
approximately a 20 minute gel time as do some carbonates and calcium
hydroxide. Other bases (hydroxides and carbonates) provide longer set times
(Tallard and Caron, 1977b).
The length of set time affects the strength of the grout. Short set
times give strong gels while long set times give weak gels. Strong grouts are
not critical to waterproofing, but short set times are often important (Karol.,
1982a). To decrease the set time, phenolic grouts may be combined with
another grout such as a silicate that has a shorter set time. This grout will
set first and provide a "false set." The phenolic grout is retained in the
grout matrix and sets at its normal rate. The final properties of the grout
mixtures are determined by the phenolic grout (Tallard and Caron, 1977a).
Wet cured phenolic grouts (under groundwater) are generally durable,
however, there may be a slight weakening over time potentially caused by the
gradual swelling of the resin (Tallard and Caron, 1977b). After setting is
complete, the phenolic resin contains water that is not chemically bound in
the matrix. Under dry conditions, this water can evaporate and the gel may
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shrink and crack (Sommerer and Kitchens, 1980; Tall ard and Caron, 1977a;
Tallard and Caron, 1977b). Unlike acryl amides and silicates, this dehydration
is irreversible and can lead to disintegration of the gel (Karol , 1982a;
Tallard and Caron, 1977a).
Proper proportioning of phenol and formaldehyde in the grout will produce
a complete reaction and the resulting gel is insoluble and inert (Tall ard and
Caron, 1977a; Tallard and Caron; 1977b). If the materials are not properly
proportioned, excess materials may leach out of the resin. In either case,
some of the catalyst may remain partially soluble and leach out of the matrix
(Karol, 1982a; Tallard and Caron, 1977a; Tallard and Caron, 1977b).
7.2.1.5.3 Urethane Grouts
Urethane, or polyurethane , grout consists primarily of a polyi socyanate
and a polyol or other hydroxy compound such as a polyether, polyester, or
glycol (Karol, 1982a; Vinson and Mitchell, 1972). A dii socyanate is often
®
used (Vinson and Mitchell, 1972). RokLok , a polyurethane grout, consists of
polymethylene polyphenyl isocyanate (containing diphenylmethane diisocyanate)
and poly(oxyal kyl ene) polyether polyol resin (Mobay Chemical Corporation,
1982). Other substances such as catalysts, surfactants, dilution agents,
pi asticizers, and stabilizers may be added to control the reaction of the
grout and its properties before and after setting.
Urethane grouts set through a polymerization process. The initial
reaction occurs between excess isocyanate and the polyol compound to form a
polyurethane prepolymer (Jiacai, et al., 1982; Karol, 1982a). The reaction
may be stopped at this step and be completed later. To complete the reaction
sequence, the prepolymer is reacted with water, carboxylic acid, or other
hydroxyl-containing compounds to form polyurethane foam (Karol , 1982a; Vinson
and Mitchell, 1972). This foam consists of cross-linked polyurethane chains
with the cross-linking occurring through the formation of urea linkages and
the generation of carbon dioxide gas (Vinson and Mitchell, 1972; Billmeyer,
1971).
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Additional materials may be added to the grout. Catalysts such as
tertiary amines (triethylamine, triethanolamine, triethylenediamine) or tin
salt may be added to control the rate of gellation and foaming (Karol , 1982a;
Jiacai, et al., 1982). Stabilizers and surfactants may be added to control
the surface tension of the grout as well as the size of the bubbles (Karol,
1982a; Jiacai, et al., 1982). Since the prepolymer has a high viscosity, the
grout may be diluted with acetone, xylene, ethyl acetate, or dichl oroethane.
Plasticizers such as dibutyl phthal ate may also serve as dilution agents
(Jiacai , et al., 1982).
The set time varies from several seconds to several minutes or hours
(McCabe, 1982; Avanti International, 1982; Tall and and Caron, 1977a). The set
time can be shortened by increasing the water content, moving from a primary
to a secondary or tertiary alcohol, or increasing the catalyst (amine) content
(Vinson and Mitchell, 1972; Tallard and Caron, 1977a) . The set time can be
lengthened by decreasing the size of the polyol or by adding acid (Sommerer
and Kitchens, 1980; Vinson and Mitchell, 1972).
7.2.1.5.4 Urea-Formaldehyde Grouts
Urea-form aldehyde resin grouts consist of urea, formaldehyde, catalyst,
and water. The urea-formaldehyde mixture may be in monomer or prepolymer
form. The catalyst is an organic acid, inorganic acid, or acid salt.
The urea-formaldehyde resin is formed in a two-step reaction process.
First, the urea and formaldehyde monomers react through methyl ol ation or
hydroxymethylation to form low molecular weight polymers. This reaction may
be either acid or base catalyzed (Kirk-Othmer, 1978b). Second, further
polymerization occurs through condensation of the polymers with water being
generated. This reaction will only occur with an acid catalyst (Kirk-Othmer,
1978b; Billmeyer, 1971). The resulting resin is a stable network of cross-
linked urea-formaldehyde polymers (Tallard and Caron, 1977a).
There are two mechanisms for achieving this reaction sequence. Where the
grout mixture uses urea and formaldehyde monomers, the two reactions occur
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rapidly and the overall reaction is difficult to control (Karol , 1982a). The
second mechanism involves stopping the reaction sequence after methyl ol ation.
At this point, precondensates or prepolymers have formed that are soluble in
water and are prevented from further reaction by the use of inhibitors or by
pH control (Karol, 1982a; Tallard and Caron, 1977a). Commercially, the
methyl ol ation reaction is base catalyzed (Kirk-Othmer, 1978b; Tallard and
Caron, 1977a). The second reaction, polymerization, may occur later by
lowering the pH of the prepolymer solution. By introducing an intermediate
stage in the reaction, better gel time control is achieved and sudden setting
of the grout may be avoided (Karol, 1982a; Tallard and Caron, 1977a).
In either case, an acid medium is required for final polymerization to
occur. Urea-formaldehyde resins cannot be used in alkaline media because the
acid catalyst would react with the media and be destroyed before it could
react with the grout (Sommerer and Kitchens, 1980; Tallard and Caron, 1977a;
Rensvold, 1968). In addition to pH control, the urea-formaldehyde reaction is
also controlled by the mole ratio of the reactants and the dilution of the
mixture (Kirk-Othmer, 1978b; Tallard and Caron, 1977a).
Urea-formaldehyde grouts have a low vi scosity. Urea solutions
(unpolymerized) have viscosities similar to acryl amides and phenol ics (Karol,
1982a). Solutions of urea-formaldehyde prepolymers are more viscous, having
typical viscosities of 10 to 13 centipoise (Karol, 1982a; Sommerer and
Kitchens, 1980).
The set time varies with the type of grout formulation and the type of
catalyst. Monomer grouts have a very short set time because the reaction is
abrupt. To achieve more control over the set time, prepolymer grouts are
used, however, this increases the viscosity of the grout and makes the grout
unsuitable for fine soils. Depending on the catalyst, the set time, may vary
from several minutes (hydrochloric acid) to almost an hour (sulfuric acid).
The proportion of the catalyst and the dilution of the grout will also affect
the set time (Tallard and Caron, 1977a).
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For urea-formaldehyde grouts there is both a gel time and a cure time.
The gel time refers to the time required to form a soft gel similar to
acryl amide. Following gellation, the grout cures to a sti f f er consistency.
This occurs over a few hours to as long as a day depending on the gel time
(Karol , 1982a). The setting of urea-formaldehyde grouts may be slowed by
organic solvents and oils (Malone, Jones and Larson, 1980). Cement will
prevent the setting of these grouts (Kirk-Othmer, 1979). In general, alkaline
materials will inhibit the polymerization reaction through destruction of the
acid catalyst.
7.2.1.5.5 Epoxy Grouts
Epoxy grouts are resins that consist of an epoxide, a hydroxy compound,
and a hardener. The epoxide is typically epichl orohydrin while the hydroxy
compound is typically bis-phenol-(2,2-bis(4-hydroxyphenol )propane (Modern
Plastics Encyclopedia, 1981; Tallard and Caron, 1977a; Billmeyer, 1971).
Epoxies are generally cured through the addition of a hardener (Tallard
and Caron, 1977a). These hardeners are cross-linking agents that react with
epoxy and hydroxyl groups (Modern Plastics Encyclopedia, 1981). The resulting
epoxy resins are polyethers (Billmeyer, 1971). A number of hardeners may be
used, typically, amines, polycarboxyl ic anhydrides, or monocarboxyl ic acids.
Each hardener reacts differently and imparts different properties to the
resin. Different proportions between the resin and the hardener will also
provide different types of resins. However, the resin-hardener ratio cannot
be varied greatly (Tallard and Caron, 1977a).
As a result of the amine hardening process, amine-terminated polyamide
resins are generated. These resins replace water on wet surfaces creating a
water-free interface between the resin and the material covered (Engineering
News-Record, 1965). For this reason, epoxy resins are useful in applications
in wet areas or under water.
The viscosity of epoxy grouts varies with the molecular weight
(Billmeyer, 1971). The most fluid of these resins has a viscosity of at least
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400 centipoise. This viscosity may be lowered to 100 centipoise by adding a
fluid hardener (Tallard and Caron, 1977a). Organic solvents and reactive
dilution substances may also be used to change the viscosity (Sommerer and
Kitchens, 1980; Tallard and Caron, 1977a). Ethers such as butyl glycidyl
ether can decrease the viscosity of epoxy grouts to 20 centipoise (Tallard and
Caron, 1977a).
The set time of epoxy grouts varies depending on the choice of hardener.
In general, the set time is difficult to regulate (Tallard and Caron, 1977a).
7.2.1.5.6 Polyester Grouts
Polyester grouts consist of a resin base and a catalyst (Office of the
Chief of Engineers, 1973). The resin is unsaturated and consists of a
polyester produced from the reaction of a polyacid and a polyalcohol.
Typically, this reaction involves the condensation of an unsaturated diacid
(maleic acid or fumaric acid) with a dialcohol. In commercial products, a
reticulant is included with the polyester resin. These products may contain
30 to 40 percent reticulant, typically styrene (Tallard and Caron, 1977a;
Billmeyer, 1971).
Polymerization is achieved through the addition of the catalyst which is
generally a peroxide. The catalyst causes the polyester resin to polymerize
as well as copolymerize with the reticulant. A gel forms which eventually
hardens to a solid material (Tallard and Caron, 1977a; Office of the Chief of
Engineers, 1973). The hardening process is accompanied by shrinkage of the
resin by as much as 10 percent (Office of the Chief of Engineers, 1973).
Accelerators may be added to speed up the setting by facilitating the
decomposition of the catalyst into free radicals (Tallard and Caron, 1977a;
Office of the Chief of Engineers, 1973). Accelerators include cobalt,
manganese, or vanadium salts, mercaptans, tertiary amines, and quaternary
ammonium salts (Tallard and Caron, 1977a).
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The quantity and type of polyester, reticulant, and catalyst may each be
varied. Each variation will produce different resins with different
characteristics (Tallard and Caron, 1977a).
Commercial polyesters vary in viscosity from several hundred to several
thousand centipoise. The minimum viscosity is between 200 to 250 centipoise,
however, this is too high to grout fine earth materials such as sand. By
adding reactive diluent, the viscosity may be reduced to 10 to 50 centipoise
(Tallard and Caron, 1977a).
The set time of polyester grouts varies from a few minutes to several
days. The resins may contain volatile compounds that make long set times
uncertain (Tallard and Caron, 1977a). The set time is dependent on resin
volume, ambient temperature, catalyst selection, and heat dissipation
(polymerization is exothermic). In addition, excessive moisture may inhibit
the setting of polyester grouts (Office of the Chief of Engineers, 1973).
7.2.2 Design and Construction
7.2.2.1 Types of Grouts
Different types of grouts are currently available including emulsion,
polymers, and particle suspensions. These grouts are generally water
solutions with low viscosity which allow for easy penetration of formation
voids. There are five major grout types:
• Bitumen grouts
• Cement grouts
t Cl ay grouts
t Silicate grouts
• Organic polymer grouts.
The most common grouts in use are cement and clay, constituting
approximately 95 percent of all grouts used. Silicates represent the majority
of the remaining 5 percent. Bitumen and organic polymer grouts are of
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relatively limited use in plume management. A brief description of these
different grout types and their applications follows.
7.2.2.1.1 Bitumen Grouts
Bitumen or asphalt emulsions are direct emulsions in which water is the
continuous phase. These emulsions have a variety of uses including surface
and subsurface waterproofing applications. Emulsions are typically used for
fine materials such as sand or in finely fissured materials. Hot bitumen
grout has been used in coarse soil formations, however, it is difficult to
handle (Tallard and Caron, 1977a). Bitumenous materials appear to have
1imited use as grouts.
Bitumen or asphalt is resistant to most chemicals. Inorganic chemicals
(except concentrated acids), dilute acids, simple alcohols, glycols, and
aldehydes will not affect bitumen (Puzinaurkas, 1982). ASPEMIX®, an asphalt-
based slurry used in vibrating beam constructed slurry walls, has been found
in the short term to be resistant to paint thinner and hazardous waste site
leachate containing chemicals or brine (Slurry Systems, 1982). In particular,
ASPEMIX appears to be a resistant coal tar, which contains dimethyl-
naphthalene, methyl naphthalene, and pyrene (Drozda, 1981).
Asphalt is not compatible with concentrated mineral acids. Most polar
and nonpolar solvents will dissolve asphalt as will chlorinated, aliphatic,
and aromatic hydrocarbons. Ketones and phenols may also slowly degrade
asphalt (Puzinaurkas, 1982). Salts and organic matter in the earth materials
will prevent proper formation of a seal between bitumen or asphalt and soil.
In addition, some salts will cause bitumen to effloresce (Ingles and Metcalf,
1973).
Some liners for hazardous waste sites consist of an emulsified asphalt
membrane. These membranes are not compatible with nitric acid or oils. In
general, materials reported to be detrimental to emulsified asphal t membranes
include organic substances, highly ionic wastes, and waste containing salts,
strong acids, or strong bases (Haxo, 1980).
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The primary area of bitumen grout usage is for reducing the permeability
of fine sands, fine soils (e.g., clayey sands), or finely fissured rock. This
type of grout is not generally suitable for coarse soils because a poorly
gelled grout may draw away from the coarse material resulting in an
ineffective seal (Tall and and Caron, 1977a). Bitumen grout may be applied in
combination with cement (Bowen, 1981).
7.2.2.1.2 Cement Grouts
Cement has probably been used longer than any other type for grouting
applications (Bowen, 1981). Cement grouts utilize hydraulic cement which
sets, hardens, and does not disintegrate in water (Kirk-Othmer, 1979).
Because of their large particle size, cement grouts are more suitable for rock
than for soil applications (Bowen, 1981). However, the addition of clay or
chemical polymers can improve the range of usage. Cement grouts have been
used for both soil consolidation and water cut-off applications, but their use
is primarily restricted to more open soils. Typically, cement grouts cannot
be used in fine-grained soils with cracks less than 0.1 millimeter wide
(Bowen, 1981).
Cement grout may be applied to fractured rock (w'ith voids of sufficient
size to ensure penetration of the grout) for underpinning and constructing a
variety of structures (Bowen, 1981; Ki rk-Othmer, 1979). Type I Portland
cement may be used for materials with large voids. Resin and gypsum cements
are used by the oil well cementing industry for rapid water seal-off
applications (Bowen, 1981).
Materials may be added to cement grouts to improve their applicability.
Sand may be added to portl and cement to create a grout suitable for coarse
materials (Bowen, 1981). Bentonite may be added to improve the penetration of
cement in alluvial soils (Sol entanche, no date).
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7.2.2.1.3 Clay Grouts
Clays have been widely used as grouts, either alone or in formulations
because they are inexpensive (Guertin and McTigue, 1982). Only certain types
of clay minerals, however, possess the physical and chemical characteristics
favorable for use in grouting. These characteristics include the ability to
swell in the presence of water and to form a gel structure at low solution
concentrations. These properties are possessed most markedly by the
montmoril lonites. Other types of clay mineral s, such as kaolinite and illite,
can be used as fillers in grout formulations, such as clay-cement mixtures
(Greenwood and Raffle, 1963).
Local deposits of clay will contain a mixture of clay minerals which,
depending on their proportions, can be utilized as a grout material either
directly or in mixtures. Many local clays require some type of treatment in
order to remove large particles that would reduce the effective distance that
the clay could penetrate into a formation. Instead of using locally derived
material, pure montmorillonite or bentonite can be utilized.
Bentonite grouts alone can be used as void sealers in coarse sands with
a permeability of more than 10 ft/day (10 cm/sec). Bentonite-chemical
2
grouts can be used on medium to fine sands with a permeability between 10
ft/day (10"1 cm/sec) and 1 ft/day (10 cm/sec). Both of these grout types
can also be utilized to seal small rock fissures (Guertin and McTigue, 1982).
Because of their low gel strengths, bentonite grouts are not able to support
structures and therefore can only be used as void sealers (Tallard and Caron,
1977b).
7.2.2.1.4 Silicate Grouts
Alkali silicates are the largest and most widely used type of chemical
grouts. Sodium, potassium, and lithium silicates are available with sodium
silicates being used most frequently. Chemical grouts (i.e., silicates and
organic polymers) constitute less than 5 percent by volume of the grouts used
in the United States although they represent almost 50 percent in Europe
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(Kirk-Othmer, 1979). In addition to their use as a grout, sodium silicates
may be used as additives to other grouts, such as Portland cement, to improve
strength and durability (Hurley and Thornburn, 1971).
Silicate grouts are used for both soil consolidation and void sealing
applications. These grouts are suitable for subsurface applications in soils
2
with a permeability of less than 10 ft/day (10 cm/sec). Silicate grouts are
not suitable for open fissures or highly permeable materials, because of
syneresis, unless they are preceded by cement grouting (Karol , 1982a; Sommerer
and Kitchens, 1980). Furthermore, tests conducted by the Waterways Experiment
Station found silicate grouts to be ineffective in waterproofing fine-grained
soils (Hurley and Thronburn, 1971).
Silicate grouts are resistant to moderate amounts of acids or alkalies
(Kirk-Othmer, 1979). These grouts are also resistant to high concentrations
of chromic, nitric, and sulfuric acid (Boova, 1977). Organic esters have
little effect on silicate grout (Bowen, 1981). Sil icate mortar is similar to
silicate grout but contains fillers such as silica, quartz, or ganister. This
mortar is resistant to most acids (except hydrofluoric acid) as well as
neutral salt solutions (ASTM, 1982).
Silicate grouts gel through the action of acids or acid salts. Hurley
and Thornburn (1971) and Karol (1982a) report that the setting time of
silicate grouts may be significantly decreased in the presence of soils or
groundwater with appreciable salt contents. These effects may be mitigated by
using the groundwater to mix the grout. Acidic soils may also decrease the
gel time (Karol , 1982a).
Silicate grouts are not compatible with a number of materials. Gel time
and grout strength may be affected by large amounts of acid or alkali (Kirk-
Othmer, 1979). Additionally, organic materials and high concentrations of
some metals will slow the setting time (Malone, et al ., 1980).
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7.2.2.1.5 Organic Polymer Grouts
Organic polymer grouts represent only a small fraction of the grouts in
use. These grouts consist of organic materials (monomers) that polymerize and
cross-link to form an insoluble gel. The organic polymer grouts include:
• Acryl amide grouts
t Phenolic grouts
• Urethane grouts
t Urea-formaldehyde grouts
• Epoxy grouts
• Polyester grouts.
Acryl amide Grouts--Ac ryl amide grouts have been in use for about 30 years,
and were the first of the organic chemical polymer grouts to be developed.
Acryl amide grouts have the largest use among the organic polymer grouts and
are the second most widely used chemical grouts (Karol , 1982a). They may be
used alone or in combination with other grouts such as silicates, bitumens,
clay, or cement (Tallard and Caron, 1977a).
AM-9® was the first acryl amide grout developed, however, it was removed
from the market in 1978 because of the toxicity of one of its components
(Karol, 1982a). There are only a few acryl amide or acryl amide-based grout
products available and a number of them are imported. These grouts include
acryl ates, polyacryl amides, and acryl amide derivates. Some grouts, such as
Rocagil 1295®, are not used in the United States (Karol, 1982a). Although
AM-9 was removed from the market, it has been used as recently as 1980 in
water cut-off applications (Berry, 1982). A number of acryl amide grouts that
were similar to AM-9, have been imported and marketed after AM-9's removal
(Karol, 1982a). AM-9 is probably the most studied of the acryl amide grouts
and because of the similarity of the materials and the reaction mechanisms,
much of the AM-9 data is valid to other acryl amide grouts (Karol, 1982a).
The use of acryl amide and acryl amide-based grouts is greater in the
United States than in Europe where phenolic grouts are more common. Acrylic
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and polyacryl amide grouts are typically used in ground surface treatment,
ground treatment for oil well drilling, and subsurface applications (e.g.,
waterproof concrete structures). Acryl ate grouts are more commonly used for
ground surface treatment than for soil injection where acryl amide grouts are
more frequently used. Acryl amide applications include structural support and
seepage control for mines, soil consolidation for foundations of structures
and dams, and water control and soil consolidation for tunnels, wells, and
mines (Tallard and Caron, 1977a). Specific applications include grout
curtains, loose sand stabilization, artesian flow shut-off, and water seepage
control in jointed and fissured rock (Office of the Chief of Engineers, 1973).
Based on AM-9 applications, acryl amide grouts may be used in a variety of soil
materials such as fine gravel; coarse, medium, or fine sand; coarse silt; and
some clays (Herndon and Lenahan, 1976b).
All of the acryl amide grout formulations contain substances that are
toxic and require special handling (Berry, 1982; Geochemical Corporation,
1982; Tallard and Caron, 1977a). While the gelled grouts or polymerized
acrylamides are reported to be non-toxic, unreacted toxic monomer or other
substances can leach from the grout matrix if the polymerization process is
not complete (Tallard and Caron, 1977a).
Many groundwater contaminants (e.g., acids, alkalies, salts) will have
little effect on acryl amide grouts, particularly if dilute and the groundwater
is used to mix the grout (Clarke, 1982; Avanti International, 1981; Kirk-
Othmer, 1979). Acryl amide grouts are impermeable to gases and hydrocarbon
solvents such as kerosenes (Office of the Chief of Engineers, 1973). Toluene,
heptane, and dilute hydrochloric acid (2 percent) also do not have effect on
acryl amide grouts (Berry, 1982). They are also unaffected by 10 percent
solutions of alcohols, ketones, hydrocarbons, acids, and metal salts (Clarke,
1982).
Some salts and pH will affect the setting time of acryl amide grouts
(Caron, 1963). Low pH conditions (less than 6.5) can prevent acryl amide
grouts from setting. Polymerization inhibitors such as sodium nitrates and
metallic salts can also delay gelation (Avanti International, 1981; Office of
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the Chief of Engineers, 1973). Alkaline conditions or metal ions, such as
iron, copper, zinc, or tin can shorten the gelation time (Avanti Inter-
national, 1981; Tallard and Caron, 1977a). Other gel time accelerators
include hydrogen sulfide and soluble salts such as Nad , CaCl , sul fates, and
phosphates (Karol , 1982a; Office of the Chief of Engineers, 1973).
The durability of acryl amide grout is affected by highly alkaline media.
Such media can promote saponi fication of the grout, particularly the monomer
(Sommerer and Kitchens, 1980; Tallard and Caron, 1977a), which will affect the
performance of the grout (Tallard and Caron, 1977a). Herndon and Lenahan
(1976b) report that sul fides affect AM-9 grout, but the effect is not
specified. Strong or concentrated hydrating and dehydrating agents will have
the greatest impact on acryl amide grouts. For example, acryl amide grouts will
swell in the presence of sulfuric acid, sodi in chloride, sodium sulfite,
sodiun hydroxide, and laundry detergent. Alcohols and glycols will cause the
grout to shrink by drawing out the water (Berry, 1982).
Phenolic Grouts—The use of phenolic resin grouts in underground and
foundation construction began in the 1960's (Kirk-Othmer, 1979; Tallard and
Caron, 1977a). These grouts may be used in fine soils and sands for a variety
of water control and ground treatment applications. However, phenolic grouts
are not widely used aTone but are typically used in conjunction with other
grouts (Tallard and Caron, 1977a). Phenolic resin mortars are recommended for
use with organic acids, wet gases (reducing), nonoxidizing and nonreducing
gases, and nonoxidizing mineral acids except hydrofluoric acid and highly
concentrated sulfuric acid. These recommendations are based on immersion of
the mortar (ASTM, 1982).
Phenolic resins are not resistant to alkali (Boova, 1977). Both strong
acids and bases will attack phenol-formaldehyde resins (Billmeyer, 1971).
Phenolic resin mortars are not recommended for use with bleaches or wet gases
(oxidizing), but they are recommended for limited use with oxidizing mineral
acids, inorganic alkali, and organic solvents (ASTM, 1982). Phenol-
formaldehyde resins are resistant to most organic solvents (Billmeyer, 1971).
7-88
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Phenolic grouts are used in soils of high permeability such as fine
gravel and coarse, medium, and fine sand (Sommerer and Kitchens, 1980; Herndon
and Lenahan, 1976b). These grouts have been used to stop leaks in railway
tunnels and for ground surface treatment (Flatau, Brockett and Brown, 1972;
Tall ard and Caron, 1977a). Phenolic grouts find their greatest use in
combination with silicate grouts in treating fine sands and silts (Tallard and
Caron, 1977a).
Different phenolic grout products have been formulated to meet different
needs. For example, Geoseal MQ-4® was designed for resistance to saline
groundwater, Geoseal MQ-14 was designed for treating low permeability
materials, and Terranier C® was designed for silts (Bowen, 1981).
Phenolic grouts are limited however in that they contain toxic and
caustic materials which require special handling (Karol , 1982a; Tallard and
Caron, 1977b).
Urethane Grouts--Urethane grouts are the second most commonly used type
of organic polymer grout (Jacques, 1981). Urethane grouts were developed in
Germany for consolidation applications and are now used in Europe, South
Africa, Australia, and Japan (Sommerer and Kitchens, 1980). These grouts are
used for water and soil applications and can penetrate finely fissured
material.
If properly formulated, urethane grouts resist most chemical and other
degradative processes. Urethane grouts are reported to have good resistance
to oxidation, shrinkage from drying, and biological agents, although some
shrinkage of the grout may occur in response to water table fluctuations
(Avanti International, 1982; Billmeyer, 1971).
While the prepolymer used in urethane grouts is flammable, this type of
grout is relatively nontoxic (Berry, 1982; Avanti International, 1982; Mobay
Chemical Corporation, 1982). Most of the grouting formulations contain some
free toluene diisocyanate, hence, special handling and protective equipment
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are needed when working with the grout materials because they are eye, skin,
and respiratory irritants (Berry, 1982; Avanti International, 1982).
Urethane grouts have been used for a number of water and soils applica-
tions. Some formulations have been used for consolidation in coal mines and
railroad tunnels (Mobay Chemical Corporation, 1982). Other formulations are
primarily used for sewer grouting and pipe sealing (Avanti International,
1982). CR-250® has been used for potable water applications as well as for
soil sealing and waterproofing applications (Jacques, 1981).
TACSS®, a urethane grout produced in Japan, has been used for treating
rivulets in karstic materials and for consolidating and waterproofing ground
through which large volumes of water circulate (Tallard and Caron, 1977a).
Because of the high viscosity of this formulation (and by analogy, other
urethane grouts), TACSS cannot be used to treat fine-grained soils (Karol ,
1982a).
Urea-Formaldehyde Grouts--Urea-formaldehyde resins are frequently
referred to as aminoplasts. The idea for the use of these resins as grouts
came from their use as glue in the oil industry (Tallard and Caron, 1977a).
Although urea-formaldehyde grouts have been available since the 1960's, they
have found limited usage (Karol, 1982b; Sommerer and Kitchens, 1980). These
grouts can only set-up in an acid environment, therefore, they cannot be used
in basic formations.
Urea-formaldehyde grouts are considered to have good stability (Karol,
1982a). If the grout is properly formulated and a good polycondensation
reaction is achieved, the resulting resin should be inert and insoluble to
most solvents although it will contain some free formaldehyde (Karol, 1982a;
Tallard and Caron, 1977a). However, these grouts may quickly break down when
subjected to cyclic wet and dry or freeze and thaw cycles (Karol, 1982a).
Fung (1980) reports that some urea-formal dehyd e resins are also biodegradable.
Solutions of urea-formaldehyde grout are both toxic and corrosive because
they contain formaldehyde and an acid catalyst. However, solutions using
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prepolymers have less free formaldehyde (Karol , 1982a; Tall ard and Caron,
1977a). The cured resin has low toxicity and considered inert, but it does
contain some unreacted formaldehyde (Kirk-Othmer, 1978b; Tall ard and Caron,
1977a).
Because of the acidity requirements, urea-formal dehyde grouts have under-
gone little development (Tall ard and Caron, 1977a). They have been used for
ground stabilization and for sealing coal mines but long-term applications
have not been reported (Karol, 1982a; Tall ard and Caron, 1977a). These grouts
can generally be used only in ground and groundwater with a pH less than 7.
Their development has been further limited because of their toxic constituents
and the production of ammonia during condensation (Sommerer and Kitchens,
1980).
Most of the descriptions of urea-formal dehyde grout applications come
from Eastern Europe, the USSR, and Japan. Products containing prepolymers
have been used in Poland and Hungary. Injection of an acid solution to
destroy carbonates before grouting has been used in the USSR to minimize high
pH media problems. However, this technique is not used widely because it is
costly and increases the size of soil voids. Further, more soil imbalances
may be created through the destruction of soil components (Tallard and Caron,
1977a).
Epoxy Grouts--Epoxy grouts and other glue-like grouts have been in use
since 1960. These grouts have had limited use in soil grouting primarily
because of their high cost (Tall ard and Caron, 1977a). Most of the applica-
tions reported in the literature involve the use of epoxy resins in mortars
and for sealing cracks. Epoxy resins can adhere to and seal submerged
concrete, steel, or wood surfaces and are useful in water applications
(Engineering News-Record, 1965). They have been used for grouting cracked
concrete for structural repairs and grouting fractured rock to improve its
strength (Office of the Chief of Engineers, 1973).
Epoxy grouts exhibit good durability. In the ground, their properties
are similar to those of polyester grouts in that they may be subject to
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hydrolysis (Tallard and Caron, 1977a). Epoxy mortars tend to have little
shrinkage and limited water absorption (U.S. Grout Corporation, 1981; Boova,
1977).
Epoxy grouts consist of substances requiring special handling precau-
tions. If the grouts are properly formulated, the set resin should incor-
porate all of the materials such that the toxicity of the gel or its
components is minimized (Tallard and Caron, 1977a).
Polyester Grouts—Polyester grouts have been in use since the 1960's.
They have been used in a variety of construction applications, principally to
treat cracks in buildings and structures (Tallard and Caron, 1977a). These
grouts have also been used in mines as well as to stabilize and strengthen
porous and fissured rock (Tallard and Caron, 1977a; Office of the Chief of
Engineers, 1973). Polyester grouts have been used infrequently to treat sand
(Tallard and Caron, 1977a).
The long-term behavior of polyester grouts is reported to be good,
but there is a long-term risk of hydrolysis particularly in alkaline media
(Tallard and Caron, 1977a). Furthermore, these gels shrink as much as
10 percent during curing (Office of the Chief of Engineers, 1973).
The components of the polyester grouts are toxic and often require
special handling during grout preparation. After polymerization, the risks
are lower although unreacted grout constituents may leach out (Tallard and
Caron, 1977a).
7.2.2.1.6 Other Grouts
There are several other grout types that have limited application in
soil and rock. The two major types are lignochrome and furan grouts.
Lignochrome grouts are also referred to as 1 ignosul fonate or chrome! ignin
grouts. This type of grout consists of a 1 ignin-containing material and a
hexavalent chromium salt (Kirk-Othmer, 1979; Ingles and Metcalf, 1973).
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Calcium 1 ignosul fates provide better waterproofing and stability than sodium
1 ignosul fates (Rogo shews ki , et al., 1980). Ammonium 1 ignosul fonate is also
used occasionally (Sommerer and Kitchens, 1980). Potassium dichromate may be
used as the hexavalent chrome salt (Ingles and Metcalf, 1973).
Lignochrome grouts typically have low viscosity (2 to 15 centipoise) and
moderately short setting times -(3 to 300 minutes). The durability of these
grouts is questionable. Their strength generally decreases over time in
water-saturated environments (Kirk-Othmer, 1979). Also, chromium can be
leached from the set grout depending on the age of the grout, the chrome-
lignin ratio, the acidity (pH), and the curing time (Sommerer and Kitchens,
1980; Office of the Chief of Engineers, 1973).
The application of 1 ignochrome grouts is limited because the chromium
salts used in 1 ignochrome grouts are toxic and the lignin materials can cause
skin irritation. Lignochrome grouts should not be used with Portland cement
because the pH of the materials conflict (Kirk-Othmer, 1979). Furthermore,
1 ignochrome grouts are not compatible with fly ash because the fly ash's
alkalinity can cause trivalent chromiun to precipitate from the dichromate
catalyst (Chung, 1973).
Lignochrome grouts have been used primarily for water cut-off and
consolidation of fine, granular soils (Office of Chief of Engineers, 1973).
However, they can be used in sands with a permeability between 102 to 1 ft/day
-1 -3
(10 to 10 cm/sec) (Sommerer and Kitchens, 1980). Although, 1 ignochrome
grouts had been used in water cut-off applications for dams, their use is rare
and they are not manufactured in the U.S. (Engineering News Record, 1953).
Furan grouts consist of simple polymers of furfuryl alcohol dissolved in
excess furfuryl alcohol. They are used primarily as mortars in which the
liquid resin is mixed with an inert powder filler (usually carbon) containing
an acid catalyst. This catalyst promotes further polymerization to form a
cross-linked, infusible material (Boova, 1977). Furan mortar grouts have
excellent resistance to a broad range of chemicals including organic and
inorganic acids, alkalies, salts, greases, and solvents (Boova, 1977).
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However, in general they are not recommended for use with oxidizing acids or
with chromic acid (20%), nitric acid, sodium hypochlorite (10%), or
concentrated sulfuric acid (Boova, 1977; ASTM, 1982).
Furan resin grouts are used primarily in applications where resistance to
corrosive media is important. They are also used for polymer concrete, in
which alkali-free aggregate (silica or quartz) is bonded with furan resins and
cured with an acidic catalyst. This concrete is resistant to acids, salts,
solvents, and bases (Modern Plastics Encyclopedia, 1981).
Several other types of grouts have been referenced with regard to soil
applications, the majority of which are polymers. One such grout, Polythixon
FRD®, is an oil-based unsaturated fatty acid polymer. Polythixon FRD has low
viscosity (10 to 80 centi poise) and a gel time of 25 to 360 minutes. This
grout is recommended for high strength consolidation rather than waterproofing
applications (Neelands and James, 1963).
Another polymer grout is PWG® sealant, a polymerized cross-linked gel of
an unspecified polymer. This grout has a very low viscosity (1.5 centi poise)
and a very short set time (several seconds to a few minutes). The set gel is
insoluble in water, kerosene, and oil, and impermeable to water, oil, and gas.
If the gel dehydrates, it can rehydrate in the presence of water to regain its
original size. In addition, this gel may undergo "wicking," i.e., if one face
of the gel dehydrates, moisture can move from the hydrated face to the
dehydrated face (Lenahan, 1973).
Anil ine-furfural resins may be used for the stabilization of cohesion-
less sand. These resins are catalyzed by pentachl orophenol or ferric chloride
(Bowen, 1981). Emulsions such as latex and salt water, styrene butadiene
latex, and pitch polyurethane mixtures may also be used as grouts (Bowen,
1981). Base-cured materials have been investigated for use as grouts,
however, these materials do not have low viscosities or other characteristics
that lend them to pumping or injection at pressures (Rensvold, 1968).
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7.2.2.2 Grouting Preparation
This section discusses some of the considerations that must be addressed
prior to actual grout injection. Because grouting is seldom considered for
restricting leachate flow in unconsol idated materials, particular emphasis is
placed on preparation for rock grouting. Many of these concerns also apply to
soil grouting.
As with other types of barrier construction, the ultimate success of a
grouting project depends on thorough site characterization. The ability to
seal water bearing voids or zones is dependent on being able to locate them.
In many remedial grouting operations, only a small portion of the rock mass
will transport water and must be sealed. Consequently, the exploratory
investigation must be very thorough. Detailed geologic mapping of the site,
aided by ranote sensing techniques and extensive rock coring, is required.
Even with extensive investigation, the complexity of groundwater flow in
fractured and fissured bedrock can make a grouting project impossible to plan
in advance.
Based on the background and ex pi oratory data, the location for a pattern
of primary injection holes is chosen and injection at one or more zones is
identified. The first few primary holes are then drilled and pressure washed
with water and air (Millet and Engelhardt, 1982). This step removes drill
cuttings and other debris from the hole to allow better grout penetration.
Each hole is then pressure tested, often using a non-setting fluid of the same
viscosity as the grout to be used. These tests are used to determine the
initial grout mixture and are often conducted using the grout plant and other
equipment to be used for the actual grouting (Millet and Engelhardt, 1982;
Karol , 1982).
Each zone within each primary hole is then injected with the grout
mixture until a predetermined amount is pumped (grout take), or a
predetermined flow rate at maximum allowable pressure is reached. Maximum
allowable pressure is typically around 1 pound per square inch (psi) per foot
of overburden (Millet and Engelhardt, 1982). Data from the drilling and
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injection of the first primary holes is analyzed, and if necessary, the grout
mixture or injection pressure modi fied before completing the remaining primary
holes. Following completion of the primary hole grouting, the program is
again analyzed, necessary changes made, and a pattern of more closely spaced
secondary holes drilled and injected.
The analysis and evaluation of the completed grouting becomes, in
essence, another pressure test. Close quality control during drilling and
grouting identifies areas that require tertiary hole grouting to complete
sealing. Such areas are identified by faster than expected drilling rates and
higher than expected grout takes (Millet and Engelhardt, 1982). For a
successful grouting program, each hole series (i.e., primary, secondary) will
have lower grout takes than the previous one. Many projects will require that
proof holes be drilled and injected. A very low grout take on tertiary or
proof holes indicates that most voids are grout filled and the grouting
program was successful.
A variety of methods are in use for actual grout injection, and many
types of grout are in use. The following sections describe grout injection
techniques and selection techniques.
7.2.2.3 Grout Injection
Grouting methods combine two processes—mixing and injection. Chemical
grouts are prepared by either batch mixing or by continuous mixing systems
based on metering or proportioning pumps. Continuous mixing systems permit
better control over the injection process since short gel times can be used.
Typical gel times used with batch systems are several hours, whereas gel times
used with continuously mixed systems are usually 10 to 20 minutes. Thus, the
formation of large pools of ungelled grout in the ground is avoided.
Additionally, long gel time grouts can become diluted and wash away before
gellation occurs. Batch mixing can probably be used without difficulty in
most soil grouting projects, but greater control is afforded by the use of
short gel times (Hayward Baker et al., 1980). Mixing methods are dependent
upon the size of the project in relationship to the grouting materials used
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and the equipment available. A larger project would most likely require a
continuous mixing system as opposed to a batch system to ensure that correct
grout specifications are met throughout the site in a relatively short period
of time.
Construction of a grout barrier is accomplished by pressure injecting the
grouting material through a pipe into the strata to be waterproofed. The
injection points are usually arranged in a triple line of primary and
secondary grout holes (Figure 7-14). A predetermined quantity of grout is
pumped into the primary holes. After the grout in the primary holes has had
time to gel , the secondary holes are injected. The secondary grout holes are
intended to fill in any gaps left by the primary grout injection (Hayward
Baker et al ., 1980). The primary holes are typically spaced at 20- to 40-foot
intervals (Guertin and McTigue, 1982).
There are several basic techniques that are utilized to form the grout
wall. These include (Hayward Baker et al., 1980; Guertin and McTigue, 1982):
t Stage-up method
• Stage-down method
• Grout-port method
0 Vibrating beam method.
In the stage-up method, the borehole is drilled to the full depth of the wall
prior to grout injection. The drill is withdrawn one "stage," leaving several
feet of borehole exposed. Grout is then injected into this length of open
borehole until the desired volume has been injected. When injection is
complete the drill is withdrawn further and the next stage is injected
(Hayward Baker et al ., 1980).
Stage-down grouting differs from stage-up grouting in that the injections
are made from the top down. Thus, the borehole is drilled through the first
zone that is to be grouted, the drill is withdrawn, and the grout injected.
Upon completion of the injection, the borehole is redrilled through the
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FIGURE 7-14.
GROUTING PIPE LAYOUT FOR CONSTRUCTING A BARRIER WALL
(HAYWARD BAKER et al., 1980 AS CITED BY SPOONER et al, 1984b)
i
10
CO
o-—^
Basic Cell
Primary Grout Pipe
o
o
Secondary Grout Pipe
Secondary Grout Ball
Primary Grout Ball
-------
grouted layer into the next zone to be grouted and the process is repeated
(Guertin and McTigue, 1982).
The grout-port method utilizes a slotted injection pipe that has been
sealed into the borehole with a brittle Portland cement and clay mortar
jacket. Rubber sleeves cover the outside of each slit (or port) permitting
grout to flow only out of the pipe. The injection process begins by isolating
the grout port in the zone to be injected using a double packer. A brief
pulse of high pressure water is injected into the port to rupture the mortar
jacket. Grout which is pumped between the double packers, passes through the
ports in the pipe, under the rubber sleeve, and out through the cracked mortar
jacket into the soil (Guertin and McTigue, 1982). This grouting process is
illustrated in Figure 7-15 and a detailed diagram of the double packer is
shown in Figure 7-16.
The vibrating beam method is not an injection technique as described
above, but instead is a way of placing grout in such a way as to generate a
wall. In this method, an I-beam is vibrated into the soil to the desired
depth and then raised at a controlled rate. As the beam is raised, grout is
pumped through a set of nozzles mounted in the beam's base filling the newly
formed cavity. When the cavity is completely filled, the beam is moved along
the direction of the wall, leaving a suitable overlap to ensure continuity
(Harr, Diamond and Schmednecht, unpublished). Figure 7-17 shows the steps
involved in forming a vibrating beam cut-off wall.
7.2.2.4 Grout Selection
The physical and chemical properties of grouts that determine the type
that should be used at a specific site include (Herndon and Lenahan, 1976a):
• Viscosity
• Setting time
• Permeability
• Strength
7-99
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FIGURE 7-15.
STAGE-UP GROUT-PORT INJECTION PROCESS
(HAYWARD BAKER et al., 1980)
Grout Rod
Grout Pipe
(PVC Pipe
with Grout
Ports)
Double Packer
on Grout Rod
Mortar Jacket
Rubber
Sleeves
over Grout
Ports on
Grout Pipe
7-100
-------
FIGURE 7-16.
DIAGRAM OF A DOUBLE PACKER USED IN
GROUT PORT INJECTION
(GUERTIN AND McTIGUE, 1982)
Double Packer
Wall of Grout Hole
Semi-plastic Sealing
Sheath
Pipe Sealed into Hole
Rubber "Manchette"
Grouting Orifice
Grouting Pipe
Double-Packer
7-101
-------
FIGURE 7-17.
VIBRATING BEAM GROUP PLACEMENT PROCESS
(BOYES, 1971, AS CITED BY GUERTIN AND McTIGUE, 1982)
Grouted .
Sections
(shaded)
1
1
1
1
— *
1 r
'Q^sXXV
fc
r
P
^
w\ w\\\\\. \
y
i
' (typ.)
Tfa&fr
a. Insertion of Single Injection Beam
10-12 cm
Extracting
Last Beam
Grouted
Zone
(shaded)
1-
Inserting
Lead Beam
b. Use of Multiple Injection Beams
Grout Tubes
LZIiq^tJMHMH
Grouted
Zone
Inserted Clutch
of Piles
7-102
-------
• Stability
t Toxicity.
The viscosity of a grout will control its ability to penetrate the voids
in the soil or rock structure (Tall and and Caron, 19775). Grout viscosity can
also contribute to the effectiveness of the groundwater cut-off wall. For
example, if the initial viscosity is too low, all of the soil or rock voids
may not be completely filled and holes in the wall may develop. Conversely,
if the viscosity is too high, the grout may not be able to penetrate all of
the strata, potentially resulting in wall openings (Tallard and Caron, 1977b).
Low viscosity grouts, such as chemical grouts, can be used in fine soils
? c
having a permeability greater than 10 ft/day (10 cm/sec). High viscosity
grouts can generally only be used in coarse-grained soils or in fractured rock
_2
having a coefficient of permeability greater than 10 ft/day (10 cm/sec)
(Sommerer and Kitchens, 1980).
After a grout is mixed, its viscosity will generally increase with time
until it gels or solidifies. This change in viscosity will control both the
time period during which the grout can be pumped and the distance which the
grout can travel through the soil or rock formation. If the setting time of
the grout is too short, it will be too viscous to move far enough into the
soil or rock structure. Conversely, if the grout has too long a setting time,
the grout may filter down past the target zone (Tallard and Caron, 1977b).
For suspension grouts, the setting time is a function of the water to
particulate ratio and the temperature. As the water to particul ate ratio is
increased, the grout viscosity, set time, and pumpabil ity will also increase.
For chemical grouts, the set time will be a function of the ratio of chemicals
used and the temperature (Sommerer and Kitchens, 1980).
Grout strength can determine the long term mechanical stability of a
grout cut-off wall. The grouted soil or rock mass must have enough strength
to resist hydrostatic forces (Tallard and Caron, 1977b). The long-term
durability of the wall is critical if the groundwater barrier is to be
maintained over extended periods of time. Any chemicals contained within the
soil mass or in the groundwater must not affect the grout's set time or its
7-103
-------
ability to set fully. If the set time is varied, the finished wall may not be
continuous or will contain weak spots. Also, the grout must be able to
withstand long-term contact with chemicals in the groundwater without any
deterioration (Sommerer and Kitchens, 1980).
Since grouts used to form waterproof barriers come in contact with
groundwater, the toxic properties of their ingredients should be considered.
The toxicity of the grout has to be examined from two aspects—the toxicity of
the grout's components and the toxicity of the hardened grout (Sommerer and
Kitchens, 1980; Tallard and Caron, 1977b). If the compounds contained in the
grout do not react fully, they can contaminate the groundwater. This happened
in Japan in 1974 when groundwater was contaminated by acryl amide monomer from
a chemical grout used in the construction of a sewer system. This contami-
nation led to subacute poisoning of local inhabitants (Ando and Makita, 1977).
Hardened grout can also release potentially toxic compounds through syneresis
of the grout (Sommerer and Kitchens, 1980).
The selection of the proper grout will depend not only on the physical
and chemical properties of the soil or rock strata but also on such factors as
grout availability, costs, local experience, and groundwater flow and
chemistry (Guertin and McTigue, 1982). Thus, grout selection is a very site
specific process. In the selection of a grout to be used at hazardous waste
disposal sites, the most important factor to be considered is its
compatabil ity with chemicals found in the groundwater and in the soil at the
disposal site.
7.2.3 Grouting Costs
The drawbacks to grout usage stem from the fact that grouting is
conducted by a limited number of firms in the United States and involves
special techniques and equipment. In most cases, a substantial equipment
mobilization fee must be paid. Equally important is the cost of
characterization and testing that must be performed to ensure effective
grouting. A final consideration is the cost of the grout itself. Approximate
costs of grouts are found in Table 7-10.
7-104
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TABLE 7-10
APPROXIMATE COSTS OF GROUTS (Spooner et al ., 1984)
Grout type Approximate cost
$/gallon of solution (1979)
Portland cement 0.95
Bentonite 1.25
Silicate - 20% 1.25
- 30% 2.10
- 40% 2.75
Epoxy $30.00
Acryl amide 6.65
Urea formaldehyde 5.70
Individual grout costs can show a wide variation, whereas, grout costs
for a complete job show much less variation. This is because the cheaper,
particulate grouts are used to seal large voids, thus using more grout, while
the more expensive, chemical grouts are commonly used to seal small voids.
7-105
-------
-------
CHAPTER 8
INNOVATIVE TECHNOLOGIES
8.1 Introduction
The techniques presented in this chapter are relatively new or novel
ideas that have seen limited use at hazardous waste sites with contaminated
groundwaters, hence the use of the term "innovative technologies." The
innovative technologies presented in this chapter include bioreclamation, in
situ chemical treatment (e.g., soil flushing, polymerization, permeable
treatment beds), and block displacement. Because this group of technologies
has not been widely used or accepted, the amount of information available for
each varies widely. Therefore, attempts have not been made to make the
contents of this chapter parallel the other technology volumes (Chapters 5, 6,
and 7). The reader is advised that because these technologies are relatively
untested and unproven compared to the more established techniques presented
previously, they will require more extensive research and development before
they can be confidently implemented at hazardous waste sites.
Most of the techniques presented originated as methods to treat con-
taminated surface waters or spill sites, and have been modified for the in
situ treatment of groundwater. In general, these technologies have many
limitations associated with their use, and they may be most useful in the
treatment of groundwater residuals and in cases where a prevalence of one type
of contaminant exists, such as a biodegradable organic compound or a toxic
metal.
Although bioreclamation techniques may be the furthest along in develop-
ment and testing to fulfill this need, using any one of these innovative
techniques as a sole method to control or mitigate the migration of a leachate
8-1
-------
plume probably is not warranted at this time. Most of these technologies
could be used with one of the more accepted technologies to enhance or
accelerate contaminant recovery. The result of the combined technologies
could be faster cleanups at lowered costs. Even within the group of
innovative technologies, a combination of these technologies nay prove to be
more effective than a single technology alone. For example, oxidation (a
chemical treatment) may be used to break down high molecular weight organic
compounds so that they are more readily biodegraded.
This chapter is divided into three main sections: Bioreclamation,
Chemical Treatment, and Block Displacement. Bioreclamation, presented first
because it appears to have the greatest potential for treating contaminant
plumes, is stirring the most interest and controversy, and is furthest along
in development and testing.
The bioreclamation process involves the addition of nutrients and oxygen
to the contaminated groundwater so that either the naturally occurring soil
bacteria or introduced mutant species are provided with an optimized environ-
ment for the biodegradation of the contaminants in the plume. Bioreclamation
techniques for cleaning up contaminated groundwaters originated with work
performed to clean up gasoline. Since then, the technique has been used at
numerous sites with gasoline contaminated groundwaters with varying degrees of
success. With the discovery of naturally occurring microorganisms that are
capable of degrading refractory chlorinated organics and the advent of
genetically engineered microorganisms developed for specific compounds and
chemical classes, bioreclamation is expected to see increasing applications to
hazardous waste site cleanups and spills.
The second section of the chapter deals with those techniques that
require the injection of a specific chemical or chemicals into the plume to
either degrade, immobilize, or increase the mobility of the contaminants. The
techniques reviewed include soil flushing (solution mining), oxidation/
reduction, precipitation/polymerization, neutralization/hydrolysis, and
permeable treatment beds.
8-2
-------
Soil flushing or solution mining is a technique used to remove residual
contamination from soils. The method involves the introduction of a solvent
to the soil to solubilize substances adsorbed to the soil surface. Solvents
that can be used are water, water plus a surfactant, dilute acids or bases,
and complexing agents. Soil flushing has the potential to be used at
hazardous waste sites in the removal of organics and metals from soils.
Because soil flushing is used for removing residual contaminants from soils
rather than from groundwater, it is not a true leachate plume control
technique.
The remaining techniques--oxidation/reduction, precipitation/
polymerization, neutralization/hydrolysis, and permeable treatment beds--
involve injecting or placing specific chemicals in the groundwater to, in
theory, destroy or otherwise render harmless contaminants in the plume. They
are applicable only to specific chemical contaminants and have been proposed
mainly for hazardous substance spill cleanup. Actual use of these chemical
techniques at hazardous waste sites with contaminated groundwater has been
rare.
Oxidation/reduction involves the injection of an oxidizing or reducing
substance into the contaminated groundwater. The oxidizing agent may alter
the oxidation state of a metal or decompose an organic to a less toxic, more
soluble, or more biodegradable form. Reducing agents alter the oxidation
state of metals to a less toxic or soluble form, and they may cause a
precipitate to be formed.
Precipitation involves the introduction of a substance which will form an
insoluble precipitate with the contaminant substance (usually a metal), there-
by removing the substance from the groundwater flow regime. Similarly,
polymerization involves injection of a catalyst into a groundwater plume to
cause polymerization of an organic monomer. The polymerization reaction
transforms the once fluid substance into a gel-like, non-mobile mass.
Neutralization/hydrolysis involves injecting acids or bases into the
groundwater plume to adjust the pH of the groundwater. This method may be
8-3
-------
useful as a preliminary treatment for bioreclamation, as a means to degrade
organic chemicals, as a means to prevent toxic gas formation (such as
cyanide), as a preparation for other chemical treatments (polymerization under
acid conditions), or as a post-treatment technique to restore natural
groundwater pH.
Permeable treatment beds are essentially excavated trenches placed
perpendicular to groundwater flow and filled with an appropriate material to
treat the plume as it flows through the material. Some of the materials that
may be used in the treatment bed are limestone, crushed shell, activated
carbon, glauconitic greensands, and synthetic ion exchange resins. Permeable
treatment beds have the potential to reduce the quantities of contaminants
present in leachate plumes.
The final section in this chapter deals with a relatively new technique
that had its origin in mining: block displacement. This technique involves
vertically lifting a large mass of earth and isolating the mass completely
from the groundwater flow regime using a physical barrier. The barrier is
formed by pumping slurry into a series of notched injection holes which are
physically connected underground. As the slurry is pumped in, the soil mass
is displaced upward resulting in the block's displacement.
Prior to discussing the individual in situ treatment techniques, a
general presentation of the types of wastes that are found at hazardous waste
sites and the potentially useful in situ treatments that could be utilized may
aid the reader. A survey of the 114 top priority Superfund sites (EPA, 1981)
was performed by Ellis, et al. (1982) to determine the contaminants that are
likely to be found and be amenable to in situ treatment. Table 8-1 presents
the results of this survey. The occurrence of heavy metals, acids, alkalis,
aromatics, phenols, PCB's, and other halogenated hydrocarbons is widespread
among sites.
Ellis, et al. (1982) went further and suggested potential in situ
treatments for each of the chemical groups based on available literature.
This information is presented in Table 8-2. The table lists the five
8-4
-------
TABLE 8-1.
HAZARDOUS SOIL CONTAMINANTS AT SUPERFUND SITES (Ellis, et al., 1982)
Contaminants
Heavy Metal Wastes
Chromium
Arsenic
Lead
Zinc
Cadmium
Iron
Copper
Mercury
Selenium
Nickel
Vanadium
Fly ash
Plating wastes
Other Inorganics
Cyanides
Acids
Alkalis
Radioactive wastes
Number of Total Examples
Sites Sites
47
9
8
7
5
4
3
2
2
2
1
1
1
2
26
6
7 sulfuric acid
6 lime, ammonia
3 uranium mining and purifica-
Miscellaneous
Hydrophobic Organics
Polychlorinated biphenyls 15
Oil, grease 11
Volatile hydrocarbons 6
Chlorinated hydrocarbon 5
pesticides
Polynuclear aromatics 1
tion wastes, radium, tritium
beryllium, boron hydride,
sulfides, asbestos
38
Varsol, hexane
endrin, lindane,
diel drin
DDT, 2,4,5-T,
(Continued)
8-5
-------
TABLE 8-1. (continued)
Contaminants
Number of
Sites
Total
Sites
Examples
Slightly Water Soluble Organics
Aromatics
Benzene 9
Toluene 8
Xylene 5
Other aromatics 3
Halogenated hydrocarbons
Trichloroethylene 11
Ethylene dichloride 6
Vinyl chloride 4
Methylene chloride 3
Other halogenated 15
hydrocarbons
Hydrophilic Organics
Alcohols 4
Phenols 12
Other hydrophilics 4
Organic solvents (unspecified)
and other organics
64
styrene, napthalene
20
30
chloroform, trichloroethane,
tetrachloroethylene,
trichlorofluoromethane
methyl, isopropyl, butyl
picric acid, pentachloro-
phenol, creosote
dioxane, bis(2-chloroethyl)
ether, urethane, rocket fuel
dioxin, dioxane, dyes,
pigments, inks, paints,
nitrobenzene
8-6
-------
TABLE 8-2.
POTENTIAL USEFUL SOIL IN SITU TREATMENT PROCESSES (Ellis, et al., 1982)
Hazardous Waste
Heavy metals
oo
i
Hydrophobia organics
In Situ Treatment
• Sulfide precipitation
• Fixation with municipal refuse
• Aqueous leaching to dissolve and/or
flush with injection/recovery system
• Water injection/recovery system with:
Surfactants
Oxidizing reactants (NaOCl, H-CL)
Micellar-polymer, e.g., petroleum
sulfonates and polyacrylamides
Solvents
• Sodium polyethylene glycol reactant
• Clay immobilization: injection/
fixation process
Selected References
Pohland, et al. (1981, 1982)
U.S. EPA (1979)
Huibregtse and Kastman (1978)
Myers, et al. (1980)
Phunq, et al. (1982)
Kinman, et al. (1982)
Jones and Malone (1982)
Epstein, et al. (1978)
Fuller and Korte (1976)
Huibregtse et al. (1978)
Hill, et al. (1973)
Chou, et al. (1982)
Klins, et al. (1976)
Anderson, et al. (1982)
Griffin and Chou (1980)
Pytlewski, et al. (1980)
(continued)
-------
TABLE 8-2. (continued)
Hazardous Waste
In Situ Treatment
Selected References
oo
i
oo
Hydrophobic organics
(continued)
• Biostimulation
Slightly water-soluble • Water injection/recovery system
organics (e.g., benzene, with surfactants
toluene., trichloroethylene)
• Chemical and aerobic oxidation
Hydrophilic organics
(e.g., aniline, phenol)
• Anaerobic oxidation
t Water injection/recovery system with:
pH adjustment (buffering)
• Biostimulation
Wilkinson, et al. (1978)
Kobayashi and Rittman (1982)
Zitrides (1982)
Texas Research Institute (1979, 1982b)
Dragun and Helling (1982)
Texas Research Institute (1982a)
Kinman, et al. (1982)
Laguros and Robertson (1978)
Wilkinson (1978), Kobayashi and
Rittmann (1982), Zitrides (1982)
-------
categories of hazardous waste materials previously identified as significant,
several potential in situ treatment techniques for each hazardous waste
category, and key citations to the source of information. These information
sources should be referred to for elucidation of the processes and situations
under which they were utilized.
8.2 Bioreclamation
Bioreclamation is an in situ groundwater treatment technique based on the
concept of enhancing microbial activity by altering the physical or chemical
environment of the aquifer. Methods currently under investigation include
aeration and the addition of nutrients to accelerate the biodegradation of
groundwater contaminants. Typically, in bioreclamation, groundwater extrac-
tion wells are strategically placed downgradient to control the migration of
the contaminant plume. Contaminated groundwater pumped to the surface is
mixed with nutrients needed for organism growth, and the treated contaminated
groundwater is reinjected upgradient. Specialized microorganisms may be
injected along with the nutrients, or the indigenous soil bacteria may be
stimulated with nutrients to adapt to the groundwater contaminants. The
groundwater is aerated above ground by in-line injection or underground using
a series of wells. A simplified view of the process is shown in Figure 8-1.
The bioreclamation process, as described above, results in aerobic
decomposition of groundwater contaminants in the subsurface. Bioreclamation
claims a number of advantages such as low capital costs, minimal worker safety
and air pollution control requirements, and contaminant destruction rather
than containment.
The bioreclamation technique has been used by a number of specialized
firms to treat contaminated groundwater plumes resulting from underground
gasoline and hydrocarbon leaks. The technique has not yet been demonstrated
for groundwater treatment at an uncontrolled hazardous waste site. However,
bioreclamation1s potential to treat organics which are or can be made
biodegradable in contaminated groundwaters establishes it as a viable
technique.
8-9
-------
FIGURE 8-1.
SIMPLIFIED VIEW OF GROUNDWATER BIORECLAMATIOIM
Subsurface Aeration Wells
Injection Well
Extraction Well
Direction of Groundwater Flow
Nutrients c
In-line
Injection
Well
Aeration Zone
Direction of Groundwater Flow '—^
Extraction Well
8-10
-------
8.2.1 Applications and Limitations
Bioreel amat ion techniques are applicable to leachate plumes which contain
biodegradable organics. Relative biodegradabilities of selected compounds
have been reported based on ratios of laboratory parameters associated with
the oxygen requirement for decomposition. These include 5-day or 21-day
biochemical oxygen demand (BODr, BOD^i), chemical oxygen demand (COD), and
ultimate oxygen demand (UOD). Table 8-3 presents relative biodegradabilities
by adapted sludge cultures of various compounds in terms of a BODr/COD ratio.
A higher BOO,-/COD means a higher relative biodegradability. Table 8-4
presents relative biodegradabilities in terms of an alternate ratio, i.e.,
the BOD?,/UOD ratio, termed refractory index (RI). Again, the higher the RI,
the more biodegradable. The data presented in Tables 8-3 and 8-4 provide a
general guideline for determining the biodegradability of certain compounds,
but do not preclude the need for a laboratory treatability study. The
degradative capability of different microbial populations can vary
considerably; and so, to accurately predict the treatability of a specific
leachate plume, site-specific laboratory treatability studies are essential.
Additionally, many compounds previously thought to be refractory have been
found to be degraded by certain naturally-occurring bacteria as well as
constructed strains.
In addition to the biodegradability of the leachate plume components, the
applicability of bioreclamation also depends on groundwater temperature. The
temperature range for optimal organism growth in aerobic biological wastewater
treatment processes has been found to range from 68°F to 99°F (20°C to 37°C).
Figure 8-2 gives groundwater temperatures throughout the United States. A
comparison of Figure 8-2 to the optimal temperature ranges for aerobic
biological wastewater treatment processes suggests that only the southern
portion of the country is within the optimal range. In areas of the country
where the groundwater temperatures are below optimum, slower rates of
biodegradation should be expected. Studies of 2,4-D breakdown in surface
water indicated that a 75 percent reduction in degradation rates occurred when
temperature was reduced by 64CF (Lyman, et al., 1982). This reduction in
degradation rates may be attributable to a drop in temperature from that which
8-11
-------
TABLE 8-3. BOD../COD RATIOS FOR VARIOUS ORGANIC COMPOUNDS
9 (Lyman, et al., 1982)
1
Compound
Relatively Non-degradable
Butane
Butyl ene
Carbon tetrachlonde
Chloroform
1,4-01 oxane
Ethane
Heptane
Hexane
Isobutane
Isobutylene
Liquefied natural gas
Liquefied petroleum gas
Methane
Methyl bromide
Methyl chloride
Monochlorodifluoromethane
Nitrobenzene
Propane
Propylene
Propylene oxide
Tet rachl oroethyl ene
Tetrahydronaphthalene
1-Pentene
Ethyl ene d1 chloride
1-Octene
Morpholine
Ethylened1am1netetracet1c acid
Triet Hanoi ami ne
o-Hylene
m-Xylene
Ethyl benzene
Moderately degradable
Ethyl ether
Sodium alkylbenxenesulfonates
Monoisopropanoalmine
Gas oil (cracked)
Gasolines (various)
Mineral spirits
Cyclohexanol
Acrylonitrile
Nonanol
Undecanol
Methyltthylpyrldine
1 -Hexane
Methyl IsobutyUetone
01 ethanol ami ne
Formic add
Styrene
Heptanol
sec -Butyl acetate
n -Butyl acetate
Methyl alcohol
AcetonltMle
Ethyl ene glycol
Ethyl ene glycol monothyl ether
Sodium cyanide
Linear alcohols (12-15 carbons)
Allyl alcohol
Dodecanol
Relatively Degradable
Valeraldenyde
n-Decyl alcohol
p-Hylene
Urea
Toluene
Ratio
Compound
Ratio
Relatively Degradable (continued)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
<0.002
0.002
>0.003
<0.004
0.005
<0.006
<0.008
<0.008
<0.009
0.012
0.017
<0.02
0.02
0.02
0.02
0.03
0.031
>0.033
<0.04
0.04-0.75
<0.044
<0.044
<0.049
0.05
>0.06
<0.07
0.07-0.23
0.07-0.24
0.07-0.73
0.079
0.081
<0.09
<0.09
>0.09
0.091
0.097
<0.10
>0.10
<0.11
0.11
<0.12
Potassium cyanide
Isopropyl acetate
Amyl acetate
Chlorobenzene
Jet fuels (various)
Kerosene
Range oil
Glycerine
Mlponitrile
Furfural
2-Ethyl -3-propylacrole1n
Methylethylpyrldine
Vinyl acetate
Diethylene glycol
monomethyl ether
Naphthalene (molten)
D1 butyl phthalate
Hexanol
Soybean oil
Paraformaldehyde
n-Propyl alcohol
Methyl methacrylate
Acrylic acid
Sodium alkyl sulfates
Triethylene glycol
Acetic acid
Acetic anhydride
Ethyl enedi ami ne
Formaldehyde solution
Ethyl acetate
Octanol
SorbHol
Benzene
n-Butyl alcohol
Propionaldehyde
n-Butyraldehyde
Ethyleneimine
Monoethanolamine
Pyridine
D1 methyl formamide
Dextrose solution
Corn syrup
Maleic anhydride
Proplonic add
Acetone
Aniline
Isopropyl alcohol
n-Amyl alcohol
Isoamyl alcohol
Cresols
Crotonaldehyde
Phthallc anhydride
Benzaldehyde
Isobutyl alcohol
2,4-D1ch1orophenol
Tallow
Phenol
Benzole add
Carbolic acid
Methyl ethyl ketone
Benzoyl chloride
Hydrazine
Oxalic acid
0.12
<0.13
0.13-0.34
0.15
0.15
0.15
0.15
<0.16
0.17
0.17-0.46
<0.19
<0.20
<0.20
<0.20
<0.20
0.20
0.20
0.20
0.20
0.20-0.63
<0.24
0.26
0.30
0.31
0.31-0.37
>0.32
<0.35
0.35
<0.36
0.37
<0.38
<0.39
0.42-0.74
<0.43
<0.43
0.46
0.46
0.46-0.58
0.48
0.50
0.50
>0.51
0.52
0.55
0.56
0.56
0.57
0.57
0.57-0.68
<0.58
0.58
0.62
0.63
0.78
0.80
0.81
0.84
0.84
0.88
0.94
1.0
1.1
BODj values were not measured under the same conditions for all chemicals
8-12
-------
TABLE 8-4.
REFRACTORY INDICES FOR VARIOUS ORGANIC COMPOUNDS
(Adapted from Lyman, et al., 1982)
Compound RI
High Degradability
Biphenyl 1.14
Antifreeze 1.12
Sevin 1.0
d-Glutamic acid 1.00
d-Glucose 0.93
1-Valine 0.93
Acetone 0.93, 0.71
Phenol 0.87
Sodium butyrate 0.84
1-Aspartic acid 0.81
Sodium prop ionate 0.80
Propylene glycol 0.78, 0.52
Ethylene glycol 0.76
Medium-High Degradability
Potato Starch 0.72, 0.64
1-Arginine 0.65
Acetic acid 0.61
Aniline 0.58
Soluble starch 0.54
1-Histidine 0.52
1-Lysine 0.52
Hydroquinone 0.41
Low Degradability
Benzene 0.23
Gasoline 0.21
Adenine 0.14, 0.12
Vinyl chloride 0
Carboxymethyl
cellulose 0
Humics 0
DDT with carrier 0
p-Chlorophenol 0
Dichlorophenol 0
Bipyridine 0
Chloroform 0
Cyanuric Acid 0
8-13
-------
00
47°
52°
62°
67° 72°
FIGURE 8-2.
TYPICAL GROUNOWATER TEMPERATURES <°F) AT 100 FT. DEPTH
IN THE CONTERMINOUS UNITED STATES (JOHNSON DIVISION,
UOP INC., 1975)
-------
The organism is accustomed. Ladd, et al. (1982) reported high heterotrophic
activities in groundwater samples from Alberta, Canada. Therefore,
biorecTarnation may not be as applicable to sites in the extreme north because
of reduced biodegradation rates caused by low groundwater temperatures.
However, organisms adapted to cold water will still grow and metabolize at
appreciable rates if stimulated with oxygen and nutrients.
Site geology is also an important factor that may affect the feasibility
of using the bioreclamation process. Optimum geologic conditions for bio-
reclamation require substrata with moderate to high hydraulic conductivities.
Substrate materials with the above hydraulic conductivity characteristics may
include gravels, coarse sands, sandstones, and highly fractured rocks.
Other factors may also limit the applicability of bioreclamation. The pH
of the leachate plume is one such factor. Microbial growth can occur within a
relatively wide pH range, about 5.0 to 9.0. However, optimum growth occurs
within a pH range of about 6.0 to 8.0, with slightly alkaline conditions being
more favorable. Leachate plumes having pH values outside this range may not
provide suitable conditions for rapid biodegradation of contaminants. How-
ever, in some cases adjusting pH in the process may be possible.
The availability of oxygen and nutrients for aerobic decomposition
enhances to the bioreclamation process, and if not present, would limit
biodegradation. Introducing enough soluble oxygen into the groundwater has
been the major challenge in applying bioreclamation to leachate plumes. This
is because the demand for oxygen is high in this process and the solubility of
oxygen in groundwater is relatively low. The problem is compounded by the
difficulty of distributing the gas within the substrate. The design section
of this chapter presents several approaches to solving the oxygen availability
problem.
8.2.2 Theory
The bioreclamation process is based on the capability of many species of
bacteria to degrade a wide variety of organic compounds. Species have been
8-15
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isolated which are capable of degrading hydrocarbons, aromatics, and
halogenated aliphatics and aromatics. Naturally occurring species of
Pseudomonas and Arthrobacter have been associated with the biodegradation of
gasoline contaminated groundwater (Raymond, et al., 1976). Other major genera
known to attack organic contaminants include Achromobacterium, Flavobacterium,
and Nocardia (Kincannon, 1972). In general, more rapid and more complete
biodegradation is carried out by microorganisms in an aerobic (oxygenated)
environment. However, for certain compounds, most notably the halogenated
alkanes and certain pesticides such as DDT, more rapid breakdown occurs in
anaerobic (oxygen-free) environments (Suflita, et al., 1982; Bouwer and
McCarty, 1983).
In the aerobic decomposition of organic matter, bacteria respire oxygen
while catabolizing the organic compound, producing metabolic by-products, cell
biomass, carbon dioxide, and water. The generalized process is given by the
following equation:
Bacteria + 02 + Organics + Nutrients •-CCL + hLO + Byproducts + Cell Biomass
Approximately 5 to 50 percent of the organic material metabolized will be
transformed into cell biomass. The more refractive a compound, the less
carbon there is available for cell growth. Therefore, an increase in cell
number is directly related to the biodegradability of the compound.
Inorganic nutrients such as nitrogen, phosphorus, and trace elements are
required for proper cell respiration, growth, and reproduction. Oxygen serves
as the terminal electron acceptor in aerobic cell respiration, and thus, must
be present for aerobic biodegradation. The organic matter, known as the
substrate, is the organic carbon containing energy for the microorganisms.
8.2.2.1 Nlicrobial Populations
In bioreclamation, specialized microbial populations are promoted to
degrade the organic materials contaminating the groundwater. The total
microbial population may originate from naturally occurring species in the
8-16
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environment, enriched strains, or genetically manipulated organisms developed
in the laboratory. Supporting evidence is available that naturally occurring
species of bacteria survive in the soil subsurface and that these can be
stimulated to proliferate with the introduction of oxygen and a suitable
substrate. Also, introduced strains, developed in the laboratory, have been
shown to be able to survive in the subsurface environment.
Many types of bacteria have been isolated from soil cores, subsurface
oil, and water from deep wells (Dunlap and McNabb, 1973). The data suggest
that subsurface microbial colonization may occur down to several thousand
feet, however, the studies did not take into account the possibility of
contamination from the surface during sampling. A number of other studies
identified microbial populations in the lower unsaturated zone and in upper
regions of the groundwater table. Sulfate-reducing bacteria were found in
concentrations of 10 to 10 organisms per milliliter in well waters in
Montana (Dockins, et al., 1980). Nitrate-reducing bacteria were found to
depths of 120 feet (40 meters) in the unsaturated zone of the Chalk Formation
in Great Britain in a concentration of 10 to 10 organisms per gram of soil
(Whitelaw and Edwards, 1980). In other studies of two water table aquifers in
Oklahoma, a variety of bacteria were found with typical concentrations of 10
to 10 cells per gram of soil. The bacteria were relatively small, with both
gram-negative and gram-positive forms being present. Many double cells were
found suggesting that the bacteria were in the process of cell division
(Ghiorse and Balkwell, 1981). The movement of bacteria through considerable
distances of soil, particularly during saturated flow conditions, has been
well established (Hagedorn, et al., 1981). Thus, there is strong evidence
that the subsurface and water table aquifers are constantly inoculated with
organisms from the surface during recharge events. Also, subsurface
sedimentary formations may contain bacteria that were once surface organisms
which have maintained significant populations.
Some species of these naturally occurring bacteria have the ability to
feed on hydrocarbons and other organics, as evidenced by the studies of McKee
(as cited by Litchfield and Clark, 1973), and Litchfield and Clark (1973).
8-17
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McKee found that high bacterial populations of Pseudomonas; and Arthrobacter
were associated with gasoline contaminated groundwater, while low counts of
bacteria were associated with odor-free water (Raymond, et al., 1976).
Litchfield and Clark (1973) confirmed that significant populations of bacteria
were present in groundwater contaminated with gasoline, fuel oil, and other
petroleum products. They found that waters containing less than 10 parts per
million (ppm) of hydrocarbons generally had populations of less than 103
organisms per milliliter, while hydrocarbon concentrations in excess of 10 ppm
generally supported populations of 10 organisms per milliliter. Species were
identified as belonging mostly to the genera Pseudomonas and Arthrobacter.
Firms specializing in the bioreclamation process have found that the
naturally occurring organisms in the subsurface can, in fact, be stimulated by
adding nutrients and oxygen to produce a thriving colony which uses the
groundwater contaminants as substrates for growth (Williams, 1982). Alterna-
tively, mutant strains developed for the treatment of industrial wastewaters
have been used in the bioreclamation process. In this method, a thriving
microbial colony, acclimated to the contaminants, is introduced rather than
developed, and the time requirement for adaption of a microbial population is
significantly reduced or eliminated (Thibault and Elliot, 1980).
The simplified selective adaptation and mutation process is illustrated
in Figure 8-3. Original strains are collected from industrial or
environmental sources, such as an oil well, coke oven, or contaminated
aquifer, where natural populations have been exposed to the compound of
interest. Adaptation can, sometimes, be accelerated by irradiation-induced
mutation and growth on selective media. In the bioreclamation process, the
mutant bacteria can be cultured with contaminated groundwater in a surface
holding pond and then inoculated into the subsurface via spraying or injection
wells.
8.2.2.2 Nutrient Requirements
Besides the carbon-containing substrate, inorganic nutrients are also
required for proper cell growth and therefore are essential elements of the
8-18
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(Pure Culturesl
Long-term
Storage
Vacuum
Vials
Nutrients
(Pure Cultures)
Isolated Adapted Mutants
Growth and
Selection
Scale Up
Shake Flasks
FIGURE 8-3.
SIMPLIFIED SELECTIVE ADAPTION/MUTATION PROCESS
(MCDOWELL, et ai., 1982)
A.4
Radiation
Dry
Blend
Store
8-19
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bioreel amat ion process. Cell nutrients include nitrogen, phosphorus, and
potassium. Trace elements are also required and include sulfur, sodium,
calcium, magnesium, iron, and copper (Rogoshewski, 1977). Nitrogen and phos-
phorus are needed in the greatest quantities for cell growth and can limit
cell growth if they are not present at sufficient levels. Trace elements
usually are present in sufficient quantities in groundwater environments.
The quantity of phosphorus occurring naturally in groundwater usually is
controlled by the presence and solubility of the mineral apatite. Phosphorus
concentrations in solution rarely exceed 0.1 milligrams per liter (mg/1)
(Bouwer, 1978). Bacteria contain about one percent phosphorus on a dry-weight
basis (Doetsch and Cook, 1973), suggesting that groundwater containing 0.1
mg/1 of phosphorus could support a maximum bacterial population of 10 mg/1.
Assuming a 50 percent conversion efficiency, contaminant plumes of concen-
trations greater than 20 mg/1 of substrate may become phosphorus deficient and
require the addition of phosphate.
Similarly, bacteria were found to contain about 10 percent nitrogen on a
dry-weight basis, while normal groundwater contains from 0.1 to 10.0 mg/1
(Doetsch and Cook, 1973; Davis and DeWiest, 1966). These concentrations would
permit degradation of 2 to 200 mg/1 of substrate, assuming a 50 percent
conversion of nitrogen-free organic matter to new cells. Chemical analysis of
nitrogen in groundwater should provide a reasonable estimate of the amount of
organic contamination that can be degraded according to the above relation-
ships. This has been confirmed in a study by Kappler and Wuhrman (1978) in
which 0.17 mg/1 of bound nitrogen in groundwater permitted degradation of only
2.0 to 2.1 mg/1 of water soluble organics. When ammonium chloride was added,
further degradation took place. The relationships observed in this study
indicate that organic concentrations in groundwater above 120 mg/1 probably
will require additional nitrogen. This addition of nutrients may cause
problems by further contaminating the aquifer. Therefore, only the amount
needed to sustain biological activity should be added.
8-20
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8.2.2.3 Oxygen Requirements
Theoretical quantities for the amount of oxygen required to degrade
organic chemicals can be determined from stoichiometric analysis. The major
assumption in this approach is that all organic materials break down into
carbon dioxide and water. Degradation of 1.0 mg of a simple organic acid,
such as acetic acid, theoretically would require only 1.1 mg of oxygen, while
a fully saturated aliphatic hydrocarbon would require about 3.4 mg. Thus,
groundwater with an organic contaminant concentration of 100 mg/1 may require
oxygen concentrations .as high as 340 mg/1 if saturated organics are involved.
Dissolved oxygen concentrations in groundwater are generally considered
to be quite low for aerobic degradation of compounds. This presumes that any
dissolved oxygen in water that percolates through the soil column is used up
by microbial oxidation of soil organics (Johnson Division, UOP Inc., 1975).
Therefore, oxygen generally must be added to support biodegradation of
organics in a contaminated groundwater plume. Different techniques of adding
oxygen and the various advantages and disadvantages of each system are
discussed in Section 8.2.3.3: Oxygen Supply.
8.2.2.4 Substrate Characteristics
The key to the success of bioreclamation of contaminated groundwaters is
the ability of the microorganisms to use the pollutants as substrates or food
for their metabolism and growth. Previously mentioned bioreclamation studies
of groundwater contaminated with gasoline and fuel oil have shown that many
hydrocarbons can be biodegraded. However, in a leachate plume associated with
a hazardous waste site, many toxic compounds with different biodegradability
rates may be present. In determining whether bioreclamation can be a viable
process for treating a specific leachate plume, consideration must be given to
the biodegradabilities of all substances present, their concentrations in the
plume, and the interactive effects of the contaminants on the microbial
community.
8-21
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The biodegradability of a specific substance can be related to its
chemical structure. However, there is much disagreement centered around the
development of generalized structure/biodegradability relationships for
predictive purposes (JRB Associates, 1982). Empirical approaches have been
developed in which the relative biodegradability of specific chemical sub-
stances is obtained from laboratory measurements. One such approach uses the
ratio of the five day biochemical oxygen demand (600^) to chemical oxygen
demand (COD). Compounds with a BODg/COD ratio of less than 0.01 are defined
as relatively nondegradable, whereas compounds with a BODg/COD ratio of 0.01
or greater are defined as relatively degradable. BOD,-/COD ratios for specific
chemicals have been given previously in Table 8-3. A second empirical
approach involves the use of a ratio of ultimate biochemical oxygen demand
(BODU) to ultimate oxygen demand (UOD). This ratio is known as the refractory
index (RI). An RI over 0.5 indicates that a compound biodegrades readily to
carbon dioxide, water, and other associated mineralization products.
Compounds with an RI value of 0.0 to 0.5 are considered to be of low to
moderate biodegradability. Table 8-4 gives refractory indices for a number of
organic substances.
The classification of a substance as having a moderate or low biode-
gradability does not mean it cannot be biodegraded. Gasoline, which is listed
as moderately biodegradable in Table 8-3 and only slightly biodegradable in
Table 8-4, has been the substance most frequently treated successfully by the
bioreclamation process. Studies indicate that certain bacterial species can
degrade compounds considered to be fairly refractory (Vandenbergh, et al.,
1981). Also, pretreating a compound of low biodegradability may be possible
with ozone for example, to partially oxidize it to a more biodegradable form.
In leachate plumes consisting of many components which are biodegradable,
organisms may preferentially degrade one compound before another (SCS
Engineers, 1979). The following generalizations apply concerning these
preferences:
• Non-aromatics are biodegraded preferentially over aromatics
t Substances with unsaturated bonds are biodegraded preferentially over
substances with saturated bonds
8-22
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• Straight chain compounds are biodegraded preferentially over branched
isomers and complex, polymeric substances
• Soluble compounds are usually more biodegradable than insoluble
compounds
0 The presence of various functional groups affects biodegradability:
i.e., alcohols, aldehydes, acids, esters, amides, and amino acids are
more biodegradable than corresponding alkanes, olefins, ketones,
dicarboxylic acids, nitriles, and chloroalkanes (Lyman, et al., 1982)
• Halogen substituents generally make a compound more biorefractory.
Concentrations of substances present in the leachate plume can also
affect biodegradability. When substances are present at levels below 0.1
mg/1, the assimilative processes of microorganisms are sometimes not stimu-
lated, thus adaption to the particular substrate will not occur and the
substance will generally not be degraded (SCS Engineers, 1979). Conversely,
high concentrations of organic substances may cause inhibition of normal
microbial processes because of their toxic nature, and thus may appear to be
of low biodegradability. Table 8-5 presents a list of compounds that have
been found to be problematic in this respect during industrial wastewater
treatment. Problem concentrations are listed for two conditions: substrate
limiting—in which the subject compound is the sole source of microorganism
food; and non-substrate limiting--in which other carbon substrates are
present. In a leachate plume of mixed organics, the non-substrate limiting
condition would be expected to prevail. If the leachate plume contains
inhibiting concentrations of organics, the bioreclamation process can be
designed to treat a diluted portion of the contaminated groundwater to
circumvent inhibitory effects.
8.2.3 Design and Operation
The design of an in situ groundwater bioreclamation system must take into
account biodegradation kinetics, hydraulic design, oxygen supply, and the need
for nutrient addition. In the past, bioreclamation projects have been con-
ducted largely on a trial and error basis. Even though the basic scientific
principles of the system may be fairly well understood, the application of
these elements for cleanup at a hazardous waste disposal site is not yet fully
8-23
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TABLE 8-5.
PROBLEM CONCENTRATIONS OF SELECTED CHEMICALS
(SCS Engineers, 1979)
Chemical
Problem Concentration (mg/1)
Substrate
Limiting
Non-Substrate
Limiting
n-Butanol
sec-Butanol
t-Butanol
Allyl alcohol
2-Ethyl-l-hexanol
Formaldehyde
Crotonaldehyde
Acrol ein
Acetone
Methyl isobutyl ketone
Isophorone
Di ethyl amine
Ethyl enediamine
Acrylonitril e
2-Methyl-5-ethylpyridine
N,N-dimethylaniline
phenol
Ethyl benzene
Ethyl acrylate
Sod i im ac ryl ate
Dodecane
Dextrose
Ethyl acetate
Ethyl ene glycol
Diethylene glycol
Tetralin
Kerosene
Cobalt chl oride
>1000
500-1000
200
>1000
>1000
>1000
>1000
>1000
>1000
>1000
600-1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
50-100
50-100
>1000
100-300
300-1000
100-300
100
100
300-1000
300-600
>500
XLOOO
>900
>1000
>500
>1000
8-24
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developed. Furthermore, most private companies specializing in bioreclamation
of polluted groundwater have not always been willing to disclose information
about bioreclamation technology for fear of revealing trade secrets. Never-
theless, enough design information for the bioreclamation process is available
for adopting reliable bioreclamation treatment at hazardous waste sites. The
following sections describe biodegradation kinetics, hydraulic design, oxygen
supply, nutrient addition, and other design aspects.
8.2.3.1 Biodegradation Kinetics
In situ bioreclamation of highly contaminated groundwater (i.e., over
100 mg/1) generally involves aerobic microbial degradation at ambient tempera-
tures. In this case, the rate equation describing biodegradation of organic
material in the aqueous environment is based on Monod kinetics (Lyman, et al.,
1982):
- d[C]/dt = (Um)(Cm)(C)/Yd(Cu/2 + C)
where: d[C]/dt = the rate of disappearance of substrate
U = maximum growth rate of the microorganism
C = the concentration of microorganisms
C = concentration of the substrate
C i2 = concentration of the substrate supporting a half-maximum
growth rate (Um/2); can be assumed to range from 0.1 to
10 mg/1 for most substances
Yj = the yield coefficient, which is -dCm/dC, can be assumed to
be 0.5 for non-dilute systems.
The equation is a variable order rate equation, however, it can be
simplified to first and second order approximations for two conditions—when
C is much greater than C ,„ and when C is much less than C /,,. At the start
of bioreclamation of contaminated groundwater, the first condition will apply
because substrate concentrations will be substantial. However, when the
concentration of contaminating substance has been lowered by bioreclamation to
a residual level, the second condition will apply.
8-25
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When C is much greater than Cu/2, the expression (Cu/2+C) is approx-
imately equal to C. By substituting C for (Cu/2+C) and the expression 0.5C+C
for C (which is derived from integrating the yield coefficient equation YJ =
0.5 + -dC /dC), the equation reduces via integration to a first order
equation:
where: C,: = final concentration
C- = initial concentration
t = time
U = maximum growth rate of microorganisms; equal to the first
order rate constant k
e = base of natural logarithms.
This equation is applicable only for large values of C in relation to
C /2. If C is at least ten times C,2, less than a 10 percent error in the
calculated final concentration can be expected.
In a residual concentration, when C is much less than C/2, the value
C /?+C is approximately equal to C /2 and the equation reduces to the second
order expression:
- d[C]/dt = f(Cm)(C),
where: f = WCu/2>-
When the substrate concentration is of the same order as the C /2 value,
the rate expression is a hybrid between first and second order.
Biodegradation rates in groundwater can be calculated more accurately if
sorption/desorption effects of the earth material comprising the aquifer are
taken into account. In this case, the rate order constant (k) can be related
8-26
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to an apparent constant (k ) by the partition coefficient of the substrate
(k ) and the earth-to-water mass ratio (p) as (Maki, et al., 1980):
kapp ' k/<1+Pkp'
Unfortunately, much of the data compiled on biodegradation rate constants
are not generally useful for assessing compound degradation in an in situ
groundwater bioreclamation situation because the data have been developed for
river die-away predictions based on concentrations of organics which are too
dilute. Some rate data have been developed based on aerobic biodegradation of
organic materials by an adapted activated sludge process, in which the rate is
expressed in mg of COD degraded per mass of the microbial inoculum per unit
time (Pitter, 1976). However, these rate data may not be directly applicable
to assessing in situ biodegradation rates for a combination of organics
because of synergistic and antagonistic effects such as preferential bio-
degradation and co-metabolism.
In properly assessing the applicability of bioreclamation as a means of
treating contaminated groundwater, bench and pilot scale testing should be
conducted to obtain the necessary values of the variables described in the
above equations. Test results for in situ biodegradation studies may prove to
be more accurate if they are conducted in a system mimicking the groundwater
system to be treated (i.e., physical and chemical properties).
Degradation rates obtained from controlled bench and pilot scale studies
can be extrapolated to ambient temperature rates by using the Arrhenius
equation (Lyman, et al., 1982):
Rt = R.e- a
where: R. = temperature-corrected reaction rate
R. = initial rate of reaction
E = activation energy
a
R = gas constant
T_ = absolute temperature
a
e = base of natural logarithms.
8-27
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This equation also demonstrates the relationship between temperature and
reaction rate (i.e., lower temperatures result in lower reaction rates and
higher temperatures result in higher reaction rates).
8.2.3.2 Hydraulic Design
Hydraulic design is an essential element of the bioreclamation process
pertaining to the collection and containment of the groundwater plume. Proper
design of groundwater injection systems is important for the prevention of
pollutant migration away from treatment areas. In addition, hydraulic design
relates to placement of the extraction system to collect contaminated ground-
water .
Injection and extraction of contaminated groundwater may be accomplished
by using either a pumping system (Chapter 5) or a subsurface drainage system
(Chapter 6). In either case, the groundwater extraction and injection system
must be properly designed to contain the groundwater plume and minimize back-
flow as a result of groundwater reinjection.
A subsurface trench was used to reinject treated groundwater during the
cleanup of an underground spill in Waldwick, N.J. (Jhaveri and Mazzacca,
1983). In this case, the bioreclamation process was augmented by surface
biological treatment. A diagram of the injection trench is shown in
Figure 8-4. The trench is 10 feet deep by 4 feet wide by 100 feet long. The
trench has a 15 mil plastic liner installed on the bottom, back, ends, and top
such that reinjected water can only flow out of the front (downgradient) face
of the trench. About 40 feet of slotted steel pipe is installed horizontally
in the trench to carry reinjected water into the trench system. As water
flows into the injection trench, the water is forced to exit only from the
front face. Back flow is minimized by this design feature. Barriers can also
be used behind the trench and extended to a point where backflow is further
minimized. In extreme cases, total control of backflow and plume containment
can be obtained by installing a circumferential barrier wall.
8-28
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CO
i
ro
Plane View
FIGURE 8-4.
CONFIGURATION OF REINJECTION TRENCHES
(JHAVERI AND MAZZACCA, 1983 AS CITED BY
COCHRAN, et al., 1984)
Edge of Earth Mound
2" Well
Front View
4" Diameter Washed Stone
2" Diameter Well
Hand Slotted
4" Diameter Plastic
Well Hand Slotted
Plastic Liner
(Overlapped 1-3 Times)
4" Diameter
r Washed Stone
i*.V--vft - ?"/ •.••{•••-••/MvYf-• -:• •••- v- •-;• ••.• •H-: "tf-\v •>V":--:--:Y! frT-yft -^-!
^i^::-S«ty.:/r.^)^^
6" ^i»i\.•.-.'...y:.'.'"iV-
.71 ~ sv-J !•' *•>:*»• ?• • •:."'
Approx 10' ^
^\
Grade
Plastic Liner
(Overlapped 2-3 Times)
4" Diameter
Washed Stone
Plastic Liner
Sand Base
on Top
of Liner
Sand Base
Below Liner
(Approx. 1 Ft. Thick
No Less Than 6")
Note: Treated water exits only from one side of the trench.
-------
8.2.3.3 Oxygen Supply
The supply of oxygen to subsurface microorganisms is essential to the
effectiveness of the bioreclamation process. Oxygen can be supplied in a
number of ways, including aeration, oxygenation, and the use of hydrogen
peroxide and other oxygen containing compounds.
8.2.3.3.1 Aeration
Air has been used for many years in conventional wastewater treatment to
provide the necessary oxygen for microbial activity and growth. In an in situ
bioreclamation process, air can either be added to the extracted groundwater
before reinjection or injected directly into the contaminated plume. The
first method involves adding the air into the pipeline and mixing it with a
static mixer (Figure 8-5). The aerated water can then be reinjected to the
subsurface. Air injection can provide a maximum of about 10 mg/1 of dissolved
oxygen at a temperature sufficient to carry out bioreclamation (15°C). Table
8-6 shows saturated dissolved oxygen concentrations in equilibrium with air
for other temperatures.
A higher oxygen concentration can be attained in a pressurized line or
a confined aquifer. The equilibrium oxygen concentration in water increases
with increased air pressure according to Henry's Law (Sawyer and McCarty,
1967):
CL - « PHk
where: CL = concentration of oxygen in liquid (mg/1)
a = volume fraction (0.21 for 02 in air)
P = air pressure (atm)
H^ = Henry's Law Constant for oxygen.
The value of Henry's Law constant is 43.8 mg/1-atmosphere at 68DF (20°C).
Pressure increases with groundwater depth at the rate of 0.0294 atmospheres
per foot.
8-30
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FIGURE 8-5.
CONFIGURATION OF STATIC MIXER
Air
Flow
TABLE 8-6. SOLUBILITIES OF OXYGEN IN WATER AT SELECTED TEMPERATURES
(After Davis and DeWiest, 1966)
Temperature °F (°C)
Dissolved Oxygen Concentration (mg/1)
in Equilibrium with the
Air at 760 mm
32 (0)
50 (10)
68 (20)
96 (30)
122 (50)
15
11
9
8
6
8-31
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Oxygen solubility in a non-polar organic compound, such as hexane,
is approximately ten times higher than the solubility of oxygen in water
(Texas Research Institute, 1982b). Therefore, in leachate plumes consisting
of an aqueous phase and a non-aqueous organic phase, concentrations of
dissolved oxygen may be higher than that attainable with ordinary water.
The theoretical amount of oxygen required to degrade 1 mg/1 of hydro-
carbon substrate can be calculated by performing a stoichiometric analysis for
the given substance, as shown by the following equation:
CxHy + (x+(y/4))02 ^x C02 + (y/2)H20
Usually, about 3 to 4 mg/1 of oxygen is required to degrade 1 mg/1 of a
medium-length hydrocarbon compound. If 50 percent of the organic material is
converted to bacterial cell matter and the other half oxidized to carbon
dioxide and water, only 4 to 6 mg/1 of organic material can be converted and
oxidized under oxygen saturation conditions. Thus, for contaminated ground-
waters having organic concentrations significantly higher than the above
values, in-line aeration prior to injection is insufficient, because only
about 10 mg/1 dissolved oxygen can be attained on a single pass, and the
reinjected groundwater will use up all available oxygen in a very short period
of time.
The use of in situ aeration wells is a much more feasible approach to
treating highly contaminated leachate plumes. A bank of aeration wells can be
installed to provide a zone of continuous aeration through which the contam-
inated groundwater would flow. Oxygen saturation conditions can be maintained
for degrading organics during the residence time of groundwater flow through
the aerated zone. The required time for aeration can be derived from bench
scale studies. Residence time (t ) through the aerated zone can be calculated
8-32
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from Darcy's equation (Freeze and Cherry, 1979) using groundwater elevations
(i.e., head) and hydraulic conductivity as follows:
tr = (La)2/K(hrh2)
where: t = residence time
K = hydraulic conductivity
L = length of aerated zone
a
K, = groundwater elevation at beginning
of aerated zone
h2 = groundwater elevation at end of aerated zone
In the design of an in situ aeration well zone system, the zone must be
wide enough to allow the total plume to pass through. The flow of air must be
sufficient to give a substantial radius of aeration while small enough not to
cause an air barrier to the flow of groundwater.
Data based on de-icing of waste lagoons by bubbling has shown that a
ratio of about 1:1 exists between the depth of aeration and the diameter of
the zone of aeration (Rothman, 1983). Based on this logic, an aeration well
20 feet into pure water would give a radius of aeration of about 10 feet.
However, the effect of aeration wells in a geological formation is not known
and the effect on the zone of aeration is not well understood but would
probably be less than in water. Pilot studies would be necessary to determine
the relationship accurately.
At greater groundwater depths, greater pressure is present (2 atms) at
33 feet) and oxygen solubility and mass transfer is increased. A possible
configuration for an aeration well bank is given in Figure 8-6. Much of the
information presented needs further bench and pilot scale investigation to
determine optimum design criteria.
8-33
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FIGURE 8-6
POSSIBLE CONFIGURATION OF IN SITU AERATION WELL BANK
Surface Contours-
Zone of Aeration
Plane View
Direction of Groundwater Flow
. Air Injection Wells
h~'/2d*
1 »««•..
*eO '.
d V
1
V
1 \1
• * • •
• 9
'•'."•
*.' 0 0
' ',* o .
e o •
tt ^
• *» *
/ •';•;"
* •
0 0 °
-/ ^
<>'»•' ''•
«'• o • *
• • *. '
'.' '* .'*
•* * V
ff ^
• • 0 0
•••"/
• o • r*
* 0 ^
-------
A blower can be used to provide the flow rate and pressure for aeration.
At the groundwater bioreclamation project in Waldwick, N.J., 5 pounds per
square inch pressure was maintained in nine 10-foot aeration wells, each with
an air flow of 5 cubic feet per minute (Groundwater Decontamination Systems,
Inc., 1983).
8.2.3.3.2 Oxygenation
Oxygenation systems, either in-line or in situ can also be installed to
supply oxygen to the bioreclamation process. Their advantage over conven-
tional aeration is that higher oxygen solubilities can be attained and hence,
more efficient oxygen transfer to the microorganisms. Oxygen solubilities in
water in equilibrium with gaseous oxygen at one atmosphere are given in
Table 8-7.
TABLE 8-7. OXYGEN SOLUBILITY IN WATER IN EQUILIBRIUM WITH
OXYGEN GAS AT ONE ATMOSPHERE (After Davis and DeWiest, 1966)
Temperature (°F) (°C) Oxygen Solubility (mg/1)
32 (0) 70
50 (10) 54
68 (20) 44
86 (30) 37
122 (50) 27
Solubilities of oxygen in various liquids are four to five times higher
under pure oxygen systems than with conventional aeration. Therefore, in-line
injection of pure oxygen will provide sufficient dissolved oxygen to degrade
20 to 30 mg/1 of organic material, assuming 50 percent cell conversion. This
value is still probably insufficient for treating contaminant plumes with
higher levels of contamination, since many recyclings would be required.
However, according to Henry's Law, oxygen solubilities will significantly
8-35
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increase at groundwater depths greater than 30 feet or in pressurized
aquifers.
As with aeration, in situ wells can be installed to provide pure oxygen
to the groundwater. The higher oxygen solubilities may provide some flexi-
bility in the design of cell banks, especially at greater pressures, since the
oxygen may not be used up immediately, as with aeration, well distances
parallel to groundwater flow may then be designed on the basis of time for
oxygen consumption if groundwater flow rates are compatible. Configuration of
an in situ oxygenation well system may be similar to Figure 8-7.
FIGURE 8-7.
CONFIGURATION OF IN SITU OXYGENATION WELL SYSTEM
AND DISSOLVED OXYGEN CONCENTRATIONS AS A
FUNCTION OF DISTANCE FROM WELL
GROUNDWATER
FLOW
OXYGENATED ZONE
OXYGEN WELLS
50 •
'm 40
20 -
10 -
0 -1
A - AERATED ZONE
R - RESIDUAL ZONE
8-36
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Pure oxygen systems are expensive. Two types of systems are available:
stored liquid oxygen and on-site generation. Costs for the stored system are
about $100,000 for a 45 ton storage installation and supply costs are about
$100/ton within a 50 mile radius of the supplier (additional $16 for each
50 mile increment). An on-site oxygen generation plant costs about $2.5 mil-
lion for a 30 ton per day (tpd) plant and about $1.5 million for a 10 tpd
plant. Power and operation and maintenance (O&M) costs are about $55 -to $75
per ton, respectively (Lamparella, 1983).
For a temporary installation such as bioreclamation, the stored system is
recommended over the on-site generation system because of lower costs. A
typical installation may be a bank of 50 aeration wells, supplying 5 cubic
feet per minute at 1 bar or 250 cubic feet per minute. This converts to 12
tons of oxygen per day using a conversion factor of 24,000 cubic feet per ton
of oxygen (Lamparella, 1983). Supply costs for oxygen would run about $1,200
per day.
8.2.3.3.3 Hydrogen Peroxide
Classically, hydrogen peroxide (H?0?) has been used as a bactericide in
medicine and as a chemical oxidant in industrial wastewater treatment. How-
ever, in dilute concentrations, H202 may be feasible to use as a source of
oxygen for the microorganisms associated with bioreclamation. Bacterial cells
normally produce H?0? during respiration, and though H^Op is cytotoxic at
higher concentrations (3%), bacteria have developed enzymatic defenses against
H202 toxicity, known as hydroperoxidases (Texas Research Institute, 1982a).
This phenomena suggests that a threshold concentration exists below which
microorganisms can tolerate hydrogen peroxide. The cells may then be free to
use the oxygen provided by the decomposition of H^Op to aerobically degrade
organic material.
Studies conducted in flasks indicate that toxicity threshold levels for
hydrogen peroxide are dependent on cell populations. H202 concentrations
higher than 1000 mg/1 have not been found to elicit a toxic response from
established microbial populations. In fact, maximum cell mass was measured at
8-37
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concentrations of about 1000 mg/1 H202 (Texas Research Institute, 1982a). A
much lower toxicity threshold of 100 mg/1 H202 was exhibited with non-
established, smaller populations. Measurements were not conducted on hydro-
carbon utilization rates.
A major concern in the use of hydrogen peroxide as an oxygen source is
that it may be completely decomposed in a matter of hours, thereby releasing
oxygen all at once instead of under more controlled conditions. Then, oxygen
may bubble out of the system instead of being available for areas farther
downgradient in the aquifer.
Decomposition of H^ can be catalyzed either biologically or chemically.
Enzymatic decomposition by hydroperoxidases (catalases and peroxidases) is the
defense mechanism of bacteria mentioned earlier. The reactions are as follows
(Texas Research Institute, 1982a):
catalase
peroxidase
H202 + XH2 - ^2H20 + X
where X is reduced nicotinamide adenine dinucleotide (NADH), glutathione or
another biochemical reductant.
Enzymatic decomposition of H202 to oxygen would occur only with catalases
and not with peroxidases. Decomposition of hLO^ to oxygen also is catalyzed
by metal salts, particularly ferrous iron. The mixture of HLO,, and ferrous
salts is called Fentons reagent, and has been widely used as a hydroxylation
reagent (Texas Research Institute, 1982a). Reduced iron present in the
substrata may catalyze the decomposition reaction and cause immediate
decomposition and release of 0?. Similarly, an alkaline pH will also
accelerate decomposition of the H202 molecule.
8-38
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Decomposition of H202 may be slowed by the addition of mineral acid,
dissolution in an acidic solution, or addition of a stabilizing agent,
such as acetanilide or sodium pyrophosphate (Texas Research Institute, 1982a).
However, the effect of stabilizing agents is limited to conditions of low
metal contamination (FMC Corporation). Introduction of H202 to soil would
cause contact with metals and thus probably catalyze the decomposition
reaction. Other stabilizers, which slow the rate down considerably, may be
developed . Studies to measure decomposition rates and dissolved oxygen
retention are required to properly assess the feasibility of H202.
Hydrogen peroxide is available commercially for industrial uses in
35 percent, 50 percent, and 70 percent solutions (FMC Corporation). A highly
stabilized product mix is recommended for bioreclamation; however, even this
may not be adequate because of the high susceptibility to catalytic decom-
position when in contact with metal-containing substances, such as soils. An
H202 storage and metering facility is required for use. Drum or tank storage
may be used. Drum storage has inherent labor costs associated with it, while
a tank facility would incur large capital costs. Since H202 is a powerful
oxidizer, only specific materials of construction can be used such as
aluminium alloys, white chemical porcelain, pyrex, teflon, and Kel- F® 81
resin (FMC Corporation). Polyethylene, stainless steel, and polyvinyl chloride
also can be used for limited contact application.
8.2.3.3.5 Other Oxygen Sources
Other oxygen containing compounds have been proposed, including potassium
permanganate, barium and strontium peroxides, and urea-peroxide (Texas
Research Institute, 1982b). All but the latter introduce undesirable metals
into the system. Urea-peroxide has been used in conjunction with phosphate
solutions to treat plants suffering from oxygen starvation in the root zone
(Texas Research Institute, 1982b).
8-39
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8.2.3.4 Pretreatment by Ozone of Other Chemical Oxidants
In some cases, improving the biodegradability of refractory substances in
the leachate plume may be possible by pretreating with ozone or other chemical
oxidants prior to nutrient addition, reinjection, and aeration. The role of
an oxidative pretreatment step would be to partially oxidize the refractory
compounds to a form more usable-by microorganisms. Oxidative pretreatment, if
conducted at excessively high oxidant concentrations, could destroy bacteria
in the pumped leachate feed stream. Also, residual concentrations of oxidant
could linger in the pretreatment effluent at levels toxic to the micro-
organisms. However, such deleterious effects are lessened by two factors:
• Much of the active bacterial population will remain in the substrata
attached to or filtered by earth materials, and thus will not be
carried off in the pumped leachate and destroyed by pretreatment
* Groundwater will dilute reinjected water containing oxidant residuals,
and thus will help to bring down oxidant concentrations below toxic
levels.
These factors suggest that oxidative pretreatment could be carried out in a
bioreclamation scheme without significant adverse effects on the active
microbial population. Any deleterious effects could be further attenuated by
conducting pretreatment at subtoxic oxidant levels (to be determined by
laboratory studies), providing a retention or aeration basin to remove
residuals, or by pretreating at a lower flow rate, so that the dilution effect
of groundwater is more pronounced.
Potential chemical oxidants include ozone, hydrogen peroxide (^Og),
ozone and HLOp mixes and other free radical forming mixtures, potassium
permanganate, and chlorine dioxide.
Classically, ozone has been used for disinfection and chemical oxidation
of organics in water and wastewater treatment. In commercially available
ozone-from-air generators, ozone is produced at a concentration of one to
two percent in air (Nezgod, 1983). In bioreclamation, this ozone-in-air
mixture could be contacted with pumped leachate using in-line injection and
8-40
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static mixing or using a bubble contact tank. A dosage of 1 to 3 mg/1 of
ozone can be used to attain chemical oxidation (Nezgod, 1983). However,
German research on ozone pretreatment of contaminated drinking waters
indicates that the maximum ozone dosage should not be greater than 1 mg/1 of
ozone per mg/1 total organic carbon. Higher concentrations may cause
deleterious effects to microorganisms (Rice, 1983). Contact with the ozone-
in-air mixture will increase the dissolved oxygen in the leachate stream,
similar to in-line aeration. However, additional aeration may be required,
depending on the concentration of organic material in the leachate (refer to
Section 8.2.3.3.1).. Partial oxidation should be noted as only causing an
increase in biodegradability of a substance. Partial oxidation does not
decrease the amount of substance present in the leachate.
Estimated costs for ozone pre-treatment, based on an ozone-from-air
generator and an air preparation unit to remove moisture, are shown in Table
8-8 as a function of flow rate. Power requirements and average power costs
for ozone-from-air systems are shown for different size units in Table 8-9.
TABLE 8-8. CAPITAL COSTS FOR OZONE TREATMENT
(Nezgod, 1983)
Gas Flow Rate
Cost ($)
scfm1 m3/s2
0.5 2 x 10"4
7.0 3 x 10"3
25 0.01
250 0.1
7,090
28,000
65,000
290,000
scfm = standard cubic feet per minute
m /s = cubic meter per second
8-41
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00
I
ro
TABLE 8-9.
POWER COSTS FOR OZONE GENERATOR SYSTEM (Nezgod, 1983)
Gas Flow Rate
scfm
0.5
7.0
25
250
m3/s
2 x 10"4
3 x 10"3
0.01
0.1
Ozone
Ibs/day
1
14
50
500
Rate
kg/day
2.2
31
110
1,100
Power Requirements
KWhr/lb 03
27.5
16.5
9.7
3.5
KWhr/kg 03
12.5
7.5
4.5
1.6
Cost
Dollars per day at
$0.08/KWhr
2.2
18.5
38.8
740
-------
Hydrogen peroxide, previously discussed in section 8.2.3.3.3, can also be
used in higher concentrations as a chemical oxidant. Hydrogen peroxide can
partially oxidize a limited number of substances including alcohols, alde-
hydes, secondary amines, acryl chlorides, nitriles, dicarboxylic acids,
polynuclear hydrocarbons, and various unsaturated compounds. A weight ratio
of two to four units of H^CL per unit of organic has been recommended for com-
plete oxidation to carbon dioxide and water (FMC, 1978). Partial oxidation
would most likely require a lesser concentration.
Hydrogen peroxide can also be used in combination with ozone or ferrous
salts to produce hydroxyl (OH-) free radicals which will attack a greater
number of substances (Rice, 1983). Incorporating these combined treatments to
partially oxidize the more refractory compounds may be possible.
Other oxidants, such as potassium permanganate and chlorine dioxide, may
also be considered for use in an oxidative pretreatment. However, the use of
such chemicals introduces undesirable substances to the groundwater.
Potassium permanganate introduces manganese to the groundwater, which
besides being a contaminant, may clog well screens and soil pores upon
precipitation.
8.2.3.5 Nutrients and Other Additives
Nitrogen and phosphorus are usually the limiting nutrients in biodegrada-
tion processes and must be added if not present in sufficient quantities to
support an active microbial population. Analysis of cell protoplasm indicate
that the carbon: nitrogen: phosphorus weight ratio is 100:15:3 (Thibault and
Elliot, 1980). The total amount of nitrogen and phosphorus required could be
calculated if the following were known:
• The total volume of contaminated water to be treated
• The amount of elemental carbon available in the organic compounds
present
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• Nitrogen and phosphorus associated with the organic compounds present
in the leachate plume.
Usually the above quantities are not known, therefore nutrient levels
cannot be accurately calculated. Determining the optimum amount of these
nutrients should be based on previous research from similar projects and
treatability studies conducted in-the laboratory, if available. Other
nutrients required for sustaining microbial populations include potassium,
sulfur, sodium, calcium, magnesium, iron, and manganese. With the exception
of potassium, most of these substances are present in sufficient quantities
in the groundwater and need not be supplemented. As with nitrogen and
phosphorus, potassium supplements will most likely need to be added to the
leachate plume.
In previous bioreclamation projects not utilizing in situ methods,
required levels of nutrients have been derived empirically through plate
counts and shaker studies. Growth studies conducted by Suntech Inc. on a
gasoline-contaminated groundwater incorporated a basal salts medium.
Composition of the medium is given in Table 8-10 (Jamison, et al., 1976).
Once microbial activity was established, Suntech, Inc. applied bulk
quantities of ammonium sulfate [(NH4)2S04], disodium phosphate (Na2HP04), and
monosodium phosphate (NaH2P04) to the groundwater in the form of a 30% con-
centrate in water which was metered into test wells. They determined that
other nutrient substances were present in sufficient quantities in the aquifer
and did not need to be added. The solution was made up in a 2200 gallon tank
truck and contained 1.7 tons (NH4)2S04, 0.5 tons) Na2HP04, and 0.4 tons
NaH2P04 (Raymond, et al., 1976). Suntech Inc., in flask studies, found that
the forms of nitrogen and phosphorus were not critical. However, they did
find from this initial investigation that diammonium phosphate could not be
used because of excessive precipitatio-n (Jamison, et al., 1976).
Groundwater Decontamination Systems (CDS) Inc. used a variation of the
basal salt medium in a combined surface and in situ microbial degradation
system. Concentrations of nutrients added to the surface biotreatment system
are given in Table 8-11. GDS Inc. (1982) has also used the Davis medium,
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TABLE 8-10. COMPOSITION OF BASAL SALTS MEDIUM
(Jamison, et al., 1976)
Salt Type
KH2P04
N cl/-» n r U H
r A-
(NH4)N03
MgS04.7H20
Na2C03
CaCl2.2H20
MnS04.H20
FeS04.7H20
Concentration (mg/1)
400
600
10
200
100
10
20
5
TABLE 8-11. BASAL SALT MEDIUM USED BY GDS INC,
(Groundwater Decontamination Systems, Inc.,
1983 as cited Cochran, et al., 1984)
Salt
NH4C1
KH2P04
K2HP04
MgS04
Na2C03
CaCl9
2
MnSO.
4
FeS04
Concentration (mg/1)
500
270
410
1.4
9
0.9
1.8
0.45
8-45
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which contains citrate to hold trace metals in solution, for growth studies.
Adding citrate if alkaline groundwater is encountered may be advisable when
metals would tend to precipitate. Exact composition of the nutrient mix may
be varied to accommodate pH adjustment and buffering capabilities.
Adding low concentrations of readily metabolized protein-type compounds
(such as peptone, yeast extract, nicotinamide, riboflavin, pyridoxine, and
thiamine) have been found to often promote biodegradation of other target
organics. However, high concentrations of these substances may hinder
degradation of the target hydrocarbons (Texas Research Institute, 1982b). Use
of these protein sources may become important when low levels of target
organics are reached and some substrate is required to sustain an active
microbial population.
The use of biodegradable emulsifiers to "pseudo-solubilize" hydrocarbons
also has been proposed to increase the availability of the nonsolubles to the
microorganisms (Thibault and Elliot, 1980). However, emulsifiers have been
found to cause foaming in aeration wells, which may present mechanical
problems. This is especially true if surfactant emulsifiers are to be used
(Raymond, et al., 1976). Non-toxic and biodegradable anti-foaming agents
possibly may be added to control this.
As mentioned earlier, pH control can be attained by adjusting the
nutrient salt composition to produce a specific pH (around 7) and serve as a
buffer to maintain this pH. Also, mineral acids (preferably nitric or
phosphoric) and alkaline substances (sodium hydroxide or lime solutions) can
be used for pH control. Acid conditions (pH 6 to 7) should be maintained if
hydrogen peroxide is being proposed as the oxygen supply to retard catalytic
decomposition. Biological systems are vulnerable to sudden changes in
environment and addition of nutrients, additives, and pH control reagents
should be performed gradually. In operations, batch and slug doses should be
avoided to prevent toxic, osmotic, and pH shock conditions, especially if the
more susceptible engineered microorganisms are used. Since nutrient additions
may be potentially contaminating the groundwater, consideration should be
given to minimize the applied concentrations.
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8.2.3.6 Other Design Aspects
Optimum extraction and injection flow rates will many times be pre-
determined by aquifer yield limits or hydraulic design for plume containment.
The factors affecting aquifer flow rates are described in Chapter 5,
Groundwater Pumping.
Aquifer flow rates should be sufficiently high so that the aquifer is
flushed several times over the period of operation. Thus, if the cleanup
occurs over a three-year period, flow rates between injection and extraction
wells should be such that a residence time of one-half year or less occurs
between the well pairs. This corresponds to six or more flushes. Several
recycles would cause flushing of soils containing organics, preventing the
clogging caused by microorganism build-up (because of increased flow rate);
more even distribution of nutrients and organic concentration within the
plume; and better and more controlled degradation. Flow rates and the number
of recycles should not be high enough to incur excessive pumping costs, loss
of hydraulic containment efficiency because of turbulent conditions, or cause
corrosion, excessive deposition on system components, flooding, or well blow
out. The operating period will depend on the field biodegradation rate of the
contaminants in the plume and the number of recycles. If the period of
operation is excessively long, for example more than 5 years, the operating
costs of bioreclamation may outweigh the capital costs of another remedial
alternative.
8.2.3.7 Operation and Maintenance
Operation and maintenance of a bioreclamation process involve aspects of
the hydraulic system as well as the biological system. The hydraulic aspects
relate to pumps, extraction wells, injection wells, and injection trenches,
which have been discussed previously in Chapters 5 and 6.
In a biological system, pH should be maintained in a range between 6 and
8 and concentrations of both nutrients and organics should be kept as uniform
as possible to protect against shock loading. Dissolved oxygen should be
8-47
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maintained above the critical concentration for the promotion of aerobic
activity, which ranges from 0.2 to 2.0 mg/1, with the most common being
0.5 mg/1 (Hammer, 1975).
Clogging of the aquifer, injection wells or trenches, and extraction
wells by microbiological sludge is a possibility. GDS Inc. installed two
wells in each of their injection trenches in case flushing was ever required
to remove sludge. After 1-1/2 years of operation clogging had not occurred
(Groundwater Decontamination Systems Inc., 1982). Microorganisms growing on
toxic organics have characteristically low growth rates compared to those
growing on other substrate systems. Therefore, they are not expected to
produce much sludge (Wilson, et al., 1982). The above suggests that clogging
may not be a usual occurrence in bioreclamation of aquifers contaminated with
toxic organics.
Maintenance of the bacterial population at their optimal levels is also
important, especially for selective mutant organisms which tend to be more
sensitive than naturally occurring species. There is a possibility that
predators could destroy the cultured bacteria; however, this has not been
reported in the literature. A continuous incubation facility operating at
higher temperatures and under more controlled conditions could be used to
maintain the microbial population. The high biomass containing stream-formed
organisms from such a facility could then be reinjected via wells or trenches
so as to reinoculate the subsurface continuously.
Aeration wells may be susceptible to operational problems. If injected
gas fluidizes the surrounding material, soil substrata shifts can occur which
may cause a well blowout (free passage of air to the surface). The cone of
influence in a blown-out well will be greatly reduced, therefore, requiring
the installation of a new well. The best method to prevent blowouts is to
keep gas velocities below those necessary to cause fluidization or to place
wells deep enough so that overburden pressure prevents excessive fluidization,
or both (Sullivan, 1983). Suntech, Inc. stated that a number of aeration
wells became inoperative becasue of blowout during their groundwater clean up
in 1972 and had to be replaced (Raymond, et al., 1976). This suggests that
8-48
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aeration well blowout could become a commonly encountered problem if attention
is not paid to the design criteria.
8.3 Chemical Treatment
This section summarizes some of the basic information available on five
types of in situ chemical treatments:
• Soil flushing/solution mining
t Oxidation/reduction
• Precipitation/polymerization
• Neutralization/hydrolysis
• Permeable treatment beds.
In general, the in situ chemical treatment technologies presented here are not
as developed as other currently available technologies for restoring contam-
inated aquifers. This is true for a number of reasons. In situ chemical
treatment is typically very pollutant or pollutant group specific. Also, a
large number of possible chemical reactions can occur for each type of
chemical treatment, making applied research and development of a technique a
large and complex task. Nevertheless, a considerable amount of work is under-
way that could prove fruitful in the future and some successful demonstrations
of in situ chemical treatment have been reported (Williams, 1982).
8.3.1 Soil Flushing/Solution Mining
Soil flushing or solution mining is the process of flooding contaminated
soils with water or a water and chemical mixture (e.g., solvents, surfactants)
and collecting the elutriated solution. During elutriation, contaminants from
the sorbed phase are mobilized into solution by reason of solubility,
formation of an emulsion, or by chemical reaction with flushing material.
Soil flushing techniques are typically applied to the latter stages of
leachate removal as a method of accelerating the removal of sorbed residual
contaminants. Injection treatments such as soil flushing are most applicable
8-49
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to coarse-grained, permeable materials such as sands and gravels because the
flushing material employed has a better chance of contacting the sorbed
contaminants. Fine grained, less permeable soils such as clays and silts will
not be as effectively treated because of the difficulty in having the solution
come in contact with the contaminants.
Solutions with the greatest potential use for soil flushing fall in the
following classes:
0 Water
• Surfactants
• Acids-bases
• Complexing and chelating agents.
Water is the most feasible and cost effective flushing solution. In
fact, the most basic soil flushing process is natural flushing in which clean
groundwater is allowed to gradually flush the aquifer of contaminants. Water
flushing can be very effective for organic compounds that are hydrophilic and
for soluble inorganics.
Dilute aqueous solutions of acids or bases have been used widely to
extract metal ions. Rogoshewski, et al., (1982) has suggested the use of
sulfuric, hydrochloric, nitric, phosphoric, and carbonic acids to dissolve
metal salts. Sulfuric acid is the most widely used acid in solution mining
operations, but its use is restricted to metal cations that do not form
insoluble sulfates. Solutions of hydrochloric acid can be used in conjunction
with metal complexing agents, such as thiourea, to keep metals in solution.
Carbon dioxide may be used with ammonia for the leaching of copper and nickel
(Rogoshewski, et al., 1982). Also, acids may serve to flush some basic
organics, such as amines, ethers and anilines, from soils (Weininger, 1972).
Sodium hydroxide solutions can be used to solubilize aluminum, zinc, tin,
lead, and other metals (Rogoshewski, et al., 1982). Also, they may serve as a
solvent for substances such as acidic sulfur compounds, phenols, organic
acids, and cyanides (Huibregtse and Kastman, 1979).
8-50
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Complexing and chelating agents also may be used to hold solubilized
metals in solution. Commonly used substances include ammonia and ammonium
salts, citric acid, ethylene diamine tetraacetic acid (EDTA), and thiourea
(Rogoshewski and Carstea, 1980).
Surfactant solutions appear to be very promising for emulsifying non-
soluble organics sorbed on the soil phase. The ability to mobilize organic
compounds is desirable because much of the hazardous waste disposed of in
landfills falls into this class. The use of surfactants also appears
desirable at the present time because numerous environmentally safe and
relatively inexpensive surfactants are commercially available. Because of the
great potential for the use of surfactants in accelerating and enhancing
contaminant recovery, the remainder of this section deals with surfactants.
Surfactants are a general class of chemicals whose amphipathetic
molecular structures typically contain a hydrophobic structural group that has
little affinity for the carrier solvent (water) and a hydrophilic structural
group that is readily soluble in the solvent phase (Shaw, 1976). Because of
their amphipathetic character, surfactants have a tendency to preferentially
concentrate at phase interfaces (e.g., liquid-solid). This results in their
unique ability to alter certain aqueous solution properties. In theory,
therefore, surfactants can improve the effectiveness of contaminant removal by
improving both the detergency of aqueous solutions applied and the efficiency
by which organics may be transported by aqueous solutions (Envirosphere
Company, 1983).
Theoretically, surfactants will increase the detergency of the flushing
solution, and therefore its cleaning power, through the following processes
(Envirosphere Company, 1983):
• Preferential Wetting—surfactants improve the wetting properties of an
aqueous phase by decreasing the interfacial tension between the
aqueous phase and the solid phase, e.g., water and a soil particle
(Rosen, 1978).
8-51
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• Solubilization--surfactants can enhance the ability of the aqueous
solution to solubilize the contaminant by causing micelle formation,
i.e., clustering of surfactant molecules within an aqueous phase such
that surfactant hydrophobic groups are directed toward the interior of
the micelle clusters and hydrophilic groups toward the solvent water
molecules.
• Emulsification—surfactants can promote the dispersion of an insoluble
organic phase within the aqueous phase (emulsification), resulting in
enhanced detergency.
Besides improving the detergency of the flushing solution, surfactants
can potentially increase the efficiency with which aqueous solution may
entrain and transport contaminants through soils (Envirosphere Company, 1983).
Contaminants which do not preferentially wet soil particles try to assume
geometrical configurations with a minimum surface area and a minimum free
surface energy, i.e., spherical droplets. In order to move these contaminants
out of and through the soil, the geometry of the droplet must be distorted.
This requires overcoming the interfacial energy barrier, or capillary
pressure, which is directly related to the interfacial tension between the
aqueous and contaminant phase. The addition of surfactants can lower the
interfacial tension between phases and therefore increase contaminant mobility
and recovery.
Use of surfactants to date has mainly been restricted to laboratory
research. The major thrust of most of this research has been performed by the
petroleum industry for tertiary oil recovery (Doe, et al ., 1977; Wilson and
Brandner, 1977; Cash, et al., 1977; Barakat, et al., 1983). Aqueous surfac-
tants have been proposed for gasoline cleanup. In one study performed by the
Texas Research Institute (1979) for the American Petroleum Institute, a sur-
factant mixture of anionic Richonate YLA and nonionic Hyponic PE-90 was shown
to improve the effectiveness of contaminant recovery by up to 40 percent.
A similar laboratory study was performed by Ellis and Payne (1983) in
which a contaminated sandy soil was packed into soil columns and washed with a
mixture of 2 percent Adsee 799 and 2 percent Hyonic NP90 (both non-ionic
surfactants). Soil in the columns was contaminated with either a Murban Crude
Oil Distillation Fraction, PCB, or chlorinated phenol mixture (di-, tri-, and
8-52
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pentachlorophenol). For the Murban Crude Oil contaminated soil, recovery
efficiencies were increased from less than 1 percent with pure water to
86 percent with the surfactant mixture. PCB recovery using the surfactant
increased from less than 1 percent with pure water to 68 percent. In the case
of the chlorinated phenol mixture, water washing extracted 70 percent of the
contaminants and futher washing with the surfactant had a negligible effect.
Based on the study results, surfactant flushing of contaminated soils can
greatly increase the efficiency of recovery of some contaminants over water
flushing alone.
Because of the lack of data, pertaining to surfactant use in enhancing
organic contaminant recovery, a listing of contaminants and usable surfactants
for increasing extraction efficiency cannot be compiled at this time. How-
ever, guidelines for selecting an appropriate surfactant can be made. The
factors that should be considered in the selection process are (Ellis and
Payne, 1983):
t High solubility of the surfactant in water
• Ablility of the surfactant to disperse hydrophobic hydrocarbons
• Minimal suspension of fine soil particles
• Environmentally safe (i.e., low toxicity and easily biodegraded)
• Low cost.
Tables 8-12 and 8-13 list some of the characteristics of surfactants and their
environmental and chemical properties. These tables can be used to aid in the
preliminary selection of a surfactant. However, after the surfactant or
mixture of surfactants has been chosen, laboratory testing of the surfactant
in shaker table and column tests should be done to verify the properties of
the surfactant.
8.3.2 Oxidation/Reduction
Chemical oxidation is the process by which the oxidaton state of a
compound is increased by the loss of electrons. Conversely, reduction is the
process by which the oxidation state of a compound is decreased by the
8-53
-------
TABLE 8-12.
SURFACTANT CHARACTERISTICS (Envirosphere Company, 1983)
Surfactant
Type
Selected Properties
and Uses
Solubility
Reactivity
CO
i
en
ANIONIC 1) Carboxylic Acid Salts
2) Sulfuric Acid Ester Salts
3) Phosphoric 8 Polyphosphoric
Acid Esters
4) Perfluorinated Anionlcs
5) Sulfonic Add Salts
CATIONIC
• Good Detergency • Generally Water Soluble •
• Good Wetting Agents •
• Strong Surface Ten- • Soluble in Polar Organics •
sion Reducers
1) Long Chain Amines
2) Diamines & Polyamines
3) Quaternary Ammonium Salts
4) Polyoxyethylenated Long
Chain Amines
NON IONIC 1) Polyoxyethylenated Alkyl-
phenols Alkylphenol
Ethoxylates
2) Polyoxyethylenated Straight
Chain Alcohols and Alcohol
Ethoxylates
3) Polyoxyethylenated Poly-
oxypropylene Glycols
4) Polyoxyethylenated Mercaptans
5) Long-Chain Carboxylic Acid
Esters
6) Alkylolamine "Condensates",
Alkanolamides
7) Tertiary Acetylenic Glycols
• Good Oil In Water
Emulsifiers
• Emulsifying Agents • Low or Varying Water
Solubility
Electrolyte Tolerant
Electrolyte Sensitive
Resistant to Biodegradation
High Chemical Stability
Resistant to Acid and Alkaline
Hydrolysis
t Corrosion Inhibitor
• Water Soluble
• Emulsifying Agents • Generally Water Soluble
t Acid Stable
Surface Adsorption to
Silicaeous Materials
• Good Chemical Stability
• Detergents
• Wetting Agents
• Dispersents
• Foam Control
Water Insoluble Formulations •
Resistant to Biodegrad-
ation
t Relatively Non-Toxic
Subject to Acid and
Alkaline Hydrolysis
(Continued)
-------
TABLE 8-12. (CONTINUED)
Surfactant
Type
Selected Properties
and Uses
Solubility
Reactivity
00
l
en
AMPHOTERICS
1) pH Sensitive
2) pH Insensitive
• Solublizing Agents
t Wetting Agents
• Varied (pH dependent)
• Non-Toxic
• Electrolyte Tolerant
t Adsorption to Negatively
Charged Surfaces
-------
TABLE 8-13.
ENVIRONMENTAL CHEMICAL PROPERTIES OF SELECTED COMMERCIAL SURFACTANTS (Envirosphere Company, 1983)
00
1
en
a\
SURFACTANT CLASS
Fluorocarbons
Sulfonates (Anlonfcs)
Alcohol Ethoxylates
(Nonlonic)
Sulfosucd nates
(AniofHc)
Alkyl Sul fates
(An1on1c)
EXAMPLE
• Lodyne Series
(CIBA-GEIGr)
t Zonyl Series
t Alkanol Series
(DuPont)
• Merpol Series
(OuPont)
• OT 8 Aerosol
Series (Cyanamld)
• Duponol Series
(DuPont)
MATER SOLUBILITY ELECTROLYTE
TOLERANCE
(HARDNESS - PPM)
t Soluble t <300
• >2gns/100gms
• Soluble
• Generally • Electrolyte
>30t Tolerant
• l-60gms/100ml • 500-2500
• Soluble • Electrolyte
Tolerant •
PH OF AQUEOUS BIODEGRADATION
SOLUTIONS1
5.0 - 8.5 (1J) • Slow
• Slow
7.5 - 10.0 (1J) • Blodegraded
6.0-9.0 (It) • Blodegraded
(20-60% In 20 days
with acclimated
bacteria)
5.0 - 8.0 • 50-100t 1n <8 days
(CSMA-Shake
Culture Test)
7.5 - 11.0 (31) • Blodegraded
(days to weeks)
TOXICITY2
• 3-10 (gm/kg)
• Acute Dermal LD50
(Rabbit) 3-10 gm/kg
t 1-25 (gm/kg)
• Acute Oral
Toxiclty for F1sh
1-6 (mg/L)
• 1-10 (ml/kg)
• 2-20 (gm/kg)
• Acute Oral
Toxiclty to Fish
(5-20 mg/1)
Parentheses Indicate concentration of surfactant.
Toxiclty reported as acute oral for rats unless otherwise specified.
-------
addition of electrons. Oxidizing or reducing agents can be injected into a
leachate plume to detoxify or otherwise beneficially alter the groundwater
contaminants (e.g., oxidizing or reducing contaminants to make them more
readily soluble or biodegradable). Many hazardous substances including
various organics, sulfites, soluble cyanide and arsenic containing compounds,
hydroxylamine, and chromates may be able to be oxidized or reduced to forms
which are removed more readily from the groundwater (Huibregtse and Kastman,
1979).
The choice of an -oxidizing or reducing agent is dependent on the
substance or substances to be detoxified. A single oxidation or reduction
agent potentially can detoxify a number of different contaminants in a plume.
However, the rate of oxidation or reduction will in all probability be
different; therefore, detoxification of some chemicals may be rapid while
others may not show signs of detoxification. As with the other chemical
injection methodologies, oxidation or reduction techniques are most applicable
in coarse-grained deposits and not as effective in fine-grained materials.
Chemical oxidation and reduction techniques have been widely used in
treating aqueous waste streams and in wastewater treatments. In this appli-
cation, undesirable chemical species are treated, rendering them harmless.
These types of application are well documented in the literature. However,
oxidation or reduction methods for the treatment of hazardous waste piles or
contaminant plumes are scant. Therefore, most of the discussion that follows
is based upon extrapolation of methods used in other treatment areas and is
not based upon actual field applications.
Numerous oxidizing agents are available that can detoxify a variety of
compounds. Of these agents, probably three—hydrogen peroxide, ozone, and
hypochlorite--have potential for use in detoxifying contaminant plumes. These
oxidizing agents appear to be environmentally safe (i.e., easily degraded and
tending not to form toxic compounds or residuals) and are relatively
inexpensive.
8-57
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Hydrogen peroxide (Hp02) is a moderate strength chemical oxidant (i.e.,
compared to chlorine) that strongly reacts exothermically with high concentra-
tions of some organic and inorganic wastes; e.g., amines, cyanides, formalde-
hyde, phenols, ferrous ions, and hypochlorite. Hydrogen peroxide is com-
mercially available in aqueous solutions of a wide concentration range and is
miscible with water at all concentrations. These properties make hydrogen
peroxide ideally suited for chemical treatment of a contaminant plume using
injection well technology.
The major chemical reaction of hydrogen peroxide is oxidation; however,
some applications involve molecular additions, substitutions, and reductions
as shown in the following expressions (Envirosphere Company, 1983):
t Oxidation -- H202 + W —^WO + H20
• Molecular addition -- H202 + Y—»-YH202
t Substitution — H202 + RX "-ROOH + HX
• Reduction -- H202 + Z »-ZH2 + 02
These reactions may occur directly after the hydrogen peroxide has ionized or
dissociated into free radicals.
Hydrogen peroxide has been used to detoxify both municipal and industrial
waste and wastewater. Hydrogen peroxide is used in municipal wastewater
treatment to control hydrogen sulfide generation, reduce BOD and COD, and
eliminate the need for identification and bulking in activated sludge plants
(Envirosphere Company, 1983). For industrial wastewater treatment, hydrogen
peroxide has been used to detoxify cyanide and organic pollutants including
formaldehyde, phenol, acetic acid, lignin sugars, surfactants, amines and
glycol ethers, aldehydes, dialkyl sulfides, dithionate, and certain nitrogen
and sulfur compounds (Envirosphere Company, 1983; FMC Corp., 1979). Table
8-14 indicates chemical compound classes that may be degraded using hydrogen
peroxide. However, this table is based upon wastewater treatment experience
and available literature, not actual use for in situ treatment. Laboratory or
pilot scale testing, or both, should be performed using the actual contaminant
prior to instituting an in situ treatment program at a waste site.
8-58
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TABLE 8-14.
HASTE CHEMICAL CLASSES
ABILITY TO REACT WITH HYDROGEN PEROXIDE (Envirosphere Company, 1983)
Chemical Compound
Yes No Unknown
Comments
oo
tn
vo
Aliphatic Hydrocarbons x
Alkyl Halldes
Ethers
Halogenated Ethers and Epoxldes
Alcohols x
Glycols, Epoxldes x
Aldehydes, Ketones x
Carboxyllc Acids x
Amides x
Esters
Nitrlles x
Amines x
Azo Compounds, Hydrazlne
Derivatives x
Nltrosamlnes x
Thlols x
Sulfldes, D1sulf1des x
Sulfonlc Acids, SuIfoxides
Benzene and substituted Benzene x
Hal ogenated Aromatic Compounds
Nltrophenollc Compounds x
Fused Polycycllc Hydrocarbons x
Fused Non-Alterant Polycycllc Hydrocarbon x
Heterocycllc Nitrogen Compounds x
Heterocycllc Oxygen Compounds x
Heterocycllc Sulfur Compounds
Organophosphorus Compounds
saturated alkanes unreactlve; unsaturated compounds form epoxldes
and poly-hydroxy compounds
,0
require Fe catalyst
may require catalyst
may require catalyst
may require catalyst
require Fe
-------
Ozone is a strong oxidizing agent (gas) that is very unstable and
extremely reactive. Because ozone is unstable and reactive, ozone cannot be
shipped or stored; rather, it must be generated on-site immediately prior to
application. Ozone decomposes back to oxygen rapidly in solutions containing
impurities. Ozone's half life in distilled water at 68°F is 25 minutes, while
in groundwater it drops to 18 minutes (Envirosphere Company, 1983). The solu-
bility of ozone in water is dependent on the temperature of the water, where
the solubility ranges from 0.64 liter/liter water at 32°F to 0.00 liter/liter
water at 140°F.
Oxidation reactions with ozone can occur along three different pathways.
These pathways are (Masschelein, 1982):
• Direct oxidation of the solute by ozone
• Oxidation of the solute by OH radicals formed from decomposed ozone
• Oxidation reaction induced by the solute.
Each of these oxidation pathways will result in different types of end
products; therefore, the oxidation pathway should be known so that undesirable
chemical species are not produced and detoxification of the pollutant occurs.
Ozone has been used in the treatment of drinking water, municipal waste
water, and industrial waste. Ozonation has been successfully used on drinking
water for the following applications (Rice and Netzen, 1982, as cited by
Envirosphere Company, 1983):
• Disinfection and inactivation of bacteria and virus
• Oxidation of soluble iron and manganese, and organically bound
manganese
• Removal of color, taste, and odor
t Removal of algae and organics (phenols, detergents, pesticides)
t Microflocculation of dissolved organics
• Oxidation of inorganics (cyanides, sulfides, nitrites)
8-60
-------
• Removal of turbidity and suspended solids
• Pretreatment for biological processes (on sand, anthracite, and
granular activated carbon).
Industrial wastewaters have been successfully treated by ozonation in the
following areas (Rice and Browning, 1981 as cited by Envirosphere Company,
1983):
• Oxidation of cyanide from electroplating wastes
• Decolorization t)f dye stuffs
• Removal of phenolic compounds
• Recovery and reutilization of spent iron cyanide photoprocessing
bleach waters
• Treatment of acid mine discharge
• Treatment of processing wastewaters of mixed ore
• Treatment of organic waste streams.
Based on the literature and past applications of ozone in wastewater
treatment, Envirosphere Company (1983) developed a listing of chemical classes
that have the potential to be treated by ozonation. These data are given in
Table 8-15. Before ozonation can be used to treat chemicals in situ, thorough
laboratory and pilot testing should be performed to determine (Envirosphere
Company, 1983):
• The competition for ozone among the various compounds in the aqueous
solution, so that detoxification rates are known
• The pathway ozonation will take for the various compounds present, so
that oxidation by-products can be evaluated for toxicity.
Hypochlorites are a class of chemicals that act as strong oxidizing
agents. They are almost always used in an aqueous solution, an ideal form for
in situ chemical treatment use. Two forms of hypochlorite are generally
available in commercial quantities; calcium hypochlorite (CA^ClK) and sodium
8-61
-------
TABLE 8-15.
WASTE CHEMICAL CLASSES ABILITY TO REACT WITH OZONE
(Envirosphere Company, 1983)
Chemical Compound Yes No Unknown
Aliphatic Hydrocarbons
Saturated
Unsaturated X
Alkyl Hal ides
Ethers X
Halogenated Ethers and Epoxides
Alcohols X
Glycols, Epoxides
Aldehydes, Ketones X
Carboxyl ic Acids X
Amides
Esters X
Nitriles
Amines X
Compounds, Hydrazine
Derivatives
Nitrosamines
Thiols
Sul fides, Disul fides X
Sulfonic Acids, Sulfoxides X
Benzene and Substituted Benzenes X
Halogenated Aromatic Compounds X
Aromatic Nitro Compounds X
Phenols X
Halogenated Phenolic Compounds X
Nitrophenol ic Compounds X
Fused Polycyclic Hydrocarbon X
Fused Non-Al terate Polycyclic
Hydrocarbon
Heterocyclic Nitrogen Compounds
Heterocyclic Oxygen Compounds
Heterocyclic Sulfer Compounds
Organophosphorus Compounds X
8-62
-------
hypochlorite (NAOC1). Calcium hypochlorite is available in a form containing
about 70 percent available Cl, and sodium hypochlorite is available in 5.25,
10 and 13.03 weight percent of NAOC1 (Envirosphere Company, 1983).
Hypochlorite can act as either an oxidizing or chlorinating agent
depending on the pH of the organic solution treated. For weakly acidic
solutions at moderate pH, the oxidation reaction prevails, while for strong
solutions at low pH, the chlorination reaction prevails. Three basic reaction
mechanisms exist between hypochlorite and organic compounds:
• Addition
• Substitution
• Oxidation.
Addition and substitution result in the production of chlorinated organic
compounds which may or may not be toxic. Oxidation reactions are only effec-
tive on a limited number of organic compounds.
Hypochlorite has been used in treating drinking water, municipal waste-
water, and industrial waste. Uses in the drinking water and municipal waste-
water treatment include control of algae and biofouling organisms, and
bleaching of textile and paper products. Industrial waste treatments include
the oxidation of cyanide, ammonium sulfide,and ammonium sulfite (Huibregtse
and Kastman, 1979). Sodium hypochlorite solutions at concentrations of
2500 mg/1 have also been used to detoxify (by oxidation) cyanide contamination
from indiscriminate dumping (Farb, 1978).
Potential uses of aqueous solutions of hypochlorite in treating organic
compounds in situ are given in Table 8-16 (Envirosphere Company, 1983).
Because the principal products from chlorination of organic contaminants are
chlorinated organics which can be as much of a problem as the original com-
pound, hypochlorite in situ treatment will be very limited. Based on this
information, Envirosphere Company (1983) concluded that the greatest potential
lied in the treatment of phenols and phenolic compounds with hypochlorite.
However, as with the other two oxidation treatments presented, extensive
8-63
-------
TABLE 8-16.
WASTE CHEMICAL CLASSES
ABILITY TO REACT WITH HYPOCHLORITES (Envirosphere Company, 1983)
Chemical Compound
Yes No Unknown
Comments
oo
a>
Aliphatic Hydrocarbons x
Alkyl Hal ides x
Ethers
Halogenated Ethers and Eposides
Alcohols x
Glycols, Epoxides
Aldehydes, Ketones x
Carboxylic Acids x
Amides x
Esters
Nitriles
Amines x
Azo Compounds, Hydrazine Derivatives x
Nitrosamines x
Thiols
Possible chlorination and formation of chloramines
Chlorinated product possible
Forms alkylhypochlorites, hazardous and explosive
Used in preparation of Epoxides and Glycols from Halohydrin
Reaction of acetaldehyde yielding Chloroform (CHC1.J
Chlorinated byproducts possible
Forms chloramines, hydrolysis of C-N bond, possible NCK formation
Will not react unless unsaturated bonds are available for
chlorohydrin formation
Will not react unless unsaturated bonds are available for
chlorohydrin formation
Forms chloramines
Forms chloramines
Forms chloramines
(Continued)
-------
Table 8-16. (continued)
Chemical Compound
Yes No Unknown
Comments
co
i
01
Sulfides, Disul fides x
Sulfonic Acids, Sulfoxides
Benzene, Substituted Benzene x
Halogenated Aromatic Compounds x
Aromatic Nitro Compounds x
Phenols x
Halogenated Phenolic Compounds x
Nitrophenolic Compounds x
Fused Polycyclic Hydrocarbons (PNA's) x
Fused Non-Alterant Polycyclic Hydrocarbons x
Heterocyclic Nitrogen Compounds x
Heterocyclic Oxygen Compounds x
Heterocyclic Sulfur Compounds x
Organophosphorous Compounds x
Sulfides oxidize to sulfoxides without forming sulfones
Forms chlorinated aromatic
Forms chlorinated aromatic, possible oxidation
Forms chlorinated aromatic or chloramine
Forms chlorinated phenols, oxidized to aliphatic acid
Oxidized to aliphatic acid
Chlorination of aromatic ring
Chlorinated and oxidized products (e.g., phenols and quinolines)
Chlorinated product formed
Chlorinated product formed
Chlorinated product formed
-------
laboratory and pilot scale tests would be required before actual field use of
the method could be performed.
Reducing agents have been proposed to detoxify wastes and plumes, but
their application does not appear to have the potential that oxidizing agents
have. For example, sodium sulfites have been proposed to treat groundwater
contaminated by sodium hypochlorite, i.e., the reverse of that proposed in the
above section (Huibregtse and Kastman, 1979). Ferrous sulfate in conjunction
with hydroxides has been proposed to detoxify and insolubilize hexavalent
chromium (Tolman, et al., 1978; Metcalf and Eddy, Inc., 1972). Little work
has been done in the use of reducing agents for organic wastes.
Major disadvantages are inherent (e.g., costs, control of chemical
reaction) with the use of oxidizing and reducing agents for in situ treatment
of contaminant plumes, making their use at a hazardous waste site rather
unlikely. This technology is generally compound specific making treatment of
heterogeneous mixtures of contaminants a difficult problem. Also, the
introduction of chemicals into the groundwater system may create a pollution
problem in itself. As with soil flushing, uncertainty exists with respect to
obtaining adequate contact with the contaminants in the plume.
Costs for oxidation or reduction include the costs for the distribution
and collection system, chemical and feed system, and fees for probing, excava-
tion and drilling. Costs for laboratory and pilot scale studies would also
have to be considered in performing this type of cleanup. Tolman, et al.
(1978) estimated the cost (excluding preliminary studies) of cleaning-up soils
at a hypothetical 10-acre disposal site that received a single load of cyanide
salts in drums. Cost for the oxidation treatment using sodium hypochlorite
may be representative of costs for other oxidation or reduction treatments as
well as for other chemical injection technologies. These costs are shown in
Table 8-17.
8-66
-------
TABLE 8-17.
COSTS FOR IN SITU DETOXIFICATION OF CYANIDE
(Tolman, et al., 1978 as cited by Rogoshewski, et al., 1982)
Item Cost ($)
Exploratory probing, excavation, and drilling 15,000
Development of water supply well, 90 ft; 5,000
pump and piping
Installation of 45 well points 10,000
Cost of chemical feed pump 2,000
Cost of chemical (sodium hypochlorite) 5,400
Labor for chemical injection, raising of well points 48,000
to flood successive elevations (assumed 4 wells handled
simultaneously), and general labor (1,600 hours)
Power (assumed electrical supply available) 500
Total $85,900
Assumed 10-acre landfill with a total of 1,566 Ibs of cyanide distributed
within a fill volume of 4.9 million cubic feet. Chemical application rate of
68 gallons of sodium hypochlorite per pound of cyanide.
8-67
-------
8.3.3 Precipitation/Polymerization
Precipitation involves the injection of substances into the leachate
plume which form insoluble products with the contaminants, thereby reducing
the potential for migration in the groundwater. This technique is mainly
applicable to dissolved metals, such as lead, cadmium, zinc, and iron. Some
forms of arsenic, chromium, and'mercury, and some organic fatty acids can also
be treated by precipitation (Huibregtse and Kastman, 1979). The most common
precipitation reagents include hydroxides, oxides, sulfides, and sulfates.
Also, calcium salts have been suggested to precipitate free fluorides (Tolman,
et al., 1978).
Sodium sulfide used in conjunction with sodium hydroxide has shown wide-
spread applicability for precipitation of metals. Precipitation takes place at
a neutral or slightly alkaline pH. Resolubilization of sulfides is low.
Addition of sodium hydroxide minimized the formation of hydrogen sulfide gas
by assuring an alkaline pH. Experiments with sulfide precipitation of zinc
indicate that a high residual of unreacted sulfide may remain in solution.
As with other in situ techniques, precipitation is only applicable to
sites with aquifers having high hydraulic conductivities. Major disadvantages
include the application to a narrow, specific group of chemicals (mainly
metals), the injection of a potential groundwater pollutant, formation of
toxic gases (in the case of sulfide treatment), and the possibility of
resolubilization of the precipitate. The design and economics are similar to
other chemical treatment techniques.
In situ polymerization involves the injection of a polymerization
catalyst into a non-aqueous organic phase of a leachate plume to cause
polymerization. The resulting polymer is gel-like and non-mobile in the
groundwater flow regime. Polymerization is a very specific technique that is
applicable to organic monomers such as styrene, vinyl chloride, isoprene,
methyl methacrylate, and acrylonitrile, for example (Huibregtse and Kastman,
1979). In a hazardous waste site situation where groundwater pollution has
occurred over time, any organic monomers originally present would most likely
8-68
-------
have already polymerized in containers or upon contact with the soil. There-
fore, in situ polymerization is a technique most suited for groundwater
cleanup following land spills or underground leaks of pure monomer. Applica-
tions to uncontrolled hazardous waste sites are unlikely. Major disadvantages
include very limited application and difficulty of initiating sufficient
contact of the catalyst with the dispersed monomer (Huibregtse and Kastman,
1979).
In situ polymerization was successfully performed to remedy an acrylate
monomer leak, in which.4,200 gallons of acrylate monomer leaked from a
corroded underground pipeline into a glacial sand and gravel layer. Soil
borings indicated that as much as 90 percent of the monomer had been
polymerized by injection of a catalyst, activator, and wetting agent
(Williams, 1982).
The in situ treatment of a leachate plume using a precipitation or
polymerization technique probably has very limited application. Problems
associated with these techniques include:
• Chemicals must come in contact with the contaminants to ensure
complete precipitation or polymerization, thus limiting the
technique's applicability to coarse-grained aquifers
• Closely spaced injection wells are required, even in coarse-grained
deposits, because the action of precipitation or polymerization will
lower hydraulic conductivities near the injection wells, thus reducing
effectiveness of treatment
• Contaminants are not removed from the aquifer and some chemical
reactions can be reversed, allowing contaminants to again migrate with
groundwater flow
• Additional contamination can be caused by injecting a treatment agent
that is a potential groundwater pollutant or by the formation of toxic
by-products.
Therefore, prior to the application of an in situ precipitation or polymeriza-
tion technique at a hazardous waste site, thorough testing in the laboratory
and on the pilot scale should be initiated to determine possible deleterious
8-69
-------
affects and ensure complete precipitation or polymerization of the chemical
compounds.
8.3.4 Neutralization/Hydrolysis
In situ neutralization is the treatment technique that involves injecting
dilute acids or bases into a leachate plume for the purpose of adjusting the
plume's pH. Neutralization has a number of applications as presented below:
t Pretreating the leachate plume where bioreclamation is to be imple-
mented; bioreclamation requires a pH range from 6 to 8; acids or bases
may also act as cell nutrients (ammonia, nitric acid, phosphoric acid)
• Preventing the formation of hydrogen sulfide and hydrogen cyanide
during oxidation or reduction treatments or sulfide precipitation
• Adjusting pH of a plume to optimum conditions prior to in situ
oxidation or reduction treatment
• Restoring groundwater to an acceptable pH after using another treat-
ment technique, using neutralizing materials that do not seriously
affect groundwater quality (e.g., carbonic acid, calcium hydroxide)
• Neutralizing plumes that are either acidic or basic in composition,
and that do not require another type of treatment.
In theory, strong bases are the most economical solutions to treat strong
acids and strong acids are the most economical to treat strong bases. How-
ever, because of the problems associated with handling strong acids or bases
and the potential for the neutralization agent to move away from the affected
area, the use of weaker acids and bases that will not cause the pH to move
outside the range of 6 to 9 if an excess volume is used are recommended.
Prior to trying to neutralize a plume with an acid or base, laboratory and
bench scale testing should be performed to determine the effectiveness of
reagents in neutralizing the chemicals present and ensure that unwanted toxic
by-products are not generated.
8-70
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Besides using acid and bases for adjusting the pH of a plume, they can be
used to increase the hydrolysis rate of some organic chemicals. Hydrolysis is
the chemical reaction of water with an organic molecule and can be expressed
by the following displacement reaction:
RX + H20 —*• ROH + HX
where R is the organic moiety and X is the leaving group in the hydrolysis
reaction. The hydrolysis reaction can occur along a variety of pathways which
can result in different by-products that can be more toxic than the parent
compound. Therefore, before a hydrolysis reaction is promoted, the pathways
for reactions should be determined to ensure that toxic by-products are not
produced.
The rate of the hydrolysis reaction is dependent on:
t pH
• Temperature
• Solvent composition
t Catalysts.
Of the parameters listed above, changing the pH of the plume has the
greatest potential for adjusting the rate of in situ hydrolysis, since this
factor can be controlled by the injection of chemicals. Because the hydroly-
sis rate of a given compound may be the sum of the neutral, acid-catalyzed,
and base-catalyzed processes, the effect of pH on the acid- or base-catalyzed
hydrolysis rate can be pronounced (Envirosphere Company, 1984). Changes of up
to one order of magnitude for a one standard unit of change in pH can be
realized. Envirosphere Company (1984) classified numerous organic chemicals
that could be expected to show increased hydrolysis rates, when the pH of the
solution is adjusted. Tables 8-18 through 8-24 summarizes their work, listing
only those chemicals with hydrolysis rates that can be affected by pH changes.
Attempts have not been made to classify by-products produced during
hydrolysis.
8-71
-------
CO
I
TABLE 8-18.
HYDROLYSIS OF ALKYL HALIDES (Envirosphere Company, 1984)
Class/Compound
Methyl Fluoride
Methyl Iodide
Methylene Chloride
CH2CHCH2C1
CHC13
CHBrCl2
CHBr2Cl
CHBr3
CHIC12
CHFIC1
kA kN kB
7.44E-10 5.82E-7
7.28E-8 6.47E-5
3.2E-11 2.13E-8
1.16E-7 6.24E-5
6.9E-5
1.6E-3
8.0E-4
3.2E-4
8.0E-4
2.2E-1
t/1/2 at pH
5
30y
HOd
704y
69d
350,000y
13,700y
27,400y
6,600y
27,500y
~
6
30y
llOd
704y
69d
35,000y
1370y
2740y
6860y
2750y
••
7
30y
llOd
704y
69d
3500y
137y
27 4y
686y
275y
l.Oy
8
30y
llOd
704y
69d
350y
13. 7y
27. 4y
68. 6y
27. 5y
36. 5d
9
30y
llOd
704y
69d
35y
1.37y
2.74y
6.86y
2.8y
3.7d
10
30y
llOd
704y
69d
3.5y
50d
lOOd
250d
0.28y
0.37d
11
15y
55d
350y
44. 9d
0.35y
5d
lOd
25d
lOd
0.037d
Note: All values reported for 25°C. Rate constants in sec-1.
kA = acid-catalyzed hydrolysis constant
kN = neutral hydrolysis constant
kg = base-catalyzed hydrolysis constant
t/1/2 » half life time
y = year
d = day
h = hour
m = minute
s = second
-------
00
TABLE 8-19.
HYDROLYSIS OF EPOXIDES (Envirosphere Company, 1984)
Compound k.
1,2-Epoxy ethane 1E-2
1,2 Epoxy-2-methyl
propane 7.3
1,2 Epoxy-3-hydroxy
propane 2.5E-3
l,2-Epoxy-2-methyl-3-
hydroxy propane 1.1E-2
l,2-Epoxy-2-methyl-3-
chloropropane 1.84E-3
Trans-2,3 epoxy butane 1.2E-1
Cis-2,3-epoxy butane 2.4E-1
Note: All values reported at 25°C.
kN kB
5 6
6.7E-7 - 12d 12d
1.1E-6 - O.ld .95d
2.84E-7 26d 28d
4E-7 - 13d 16d
5E-7 - 15d 16d
5E-7 - 4.7d 13d
5E-7 - 2.8d lid
Rate constants in sec-1
t/1/2 at pH
7 8 9 10 11
12d 12d 12d 12d lid
4.4d 6.8d 7.2d 7.3d 7.3d
28d 28d 28d 28d 28d
16d 16d 16d 16d 16d
16d 16d 16d 16d 16d
15. 7d 16d 16d 16d 16d
15.3d 16d 16d 16d 16d
k. = acid-catalyzed hydrolysis constant
kN = neutral hydrolysis constant
kg = base-catalyzed hydrolysis constant
t/1/2 = half life time
y = year
d = day
h = hour
m = minute
s = second
-------
CO
I
TABLE 8-20.
HYDROLYSIS OF ESTERS (Envirosphere Company, 1984)
Compound
Ethyl Acetate
Isopropyl Acetate
Butyl Acetate
Vinyl Acetate
Allyl Acetate
Benzyl Acetate
0-acetyl phenol
2,4-dinitrophenyl acetate
C1CH2C(0)OCH3
C12CHC(0)OCH3
C12CHC(0)OC6H5
F2CHC(0)OC2H5
CH3SCH2C(0)OC2H5
CH3S(0)CHC(0)C2H5
(CH3)2SCH2C(0)-
OC2H5
C2H5C(0)OC2H5
C3H7C(0)OC2H5
(CH3)2CHC(0)OC2H5
CH2CHC(0)OC2H5
kA kN
1.1E-4 1.5E-10
6.0E-5 -
1.3E-4 -
1.4E-4 1.1E-7
-
1.1E-4 -
7.8E-5 6.6E-8
1.1E-5
8.5E-5 2.1E-7
2.3E-4 1.5E-5
1.8E-3
5.7E-5
-
-
-
3.3E-5 -
1.8E-5 -
-
1.2E-6 -
kB
1.1E-1
2.6E-2
1.5E-3
l.OE+1
7.3E-1
2.0E-1
1.4
9.4E+1
1.4E+2
2.8E+3
1.3E+4
4.5E+3
9.2E-1
1.3E+1
2.0E+2
8.7E-2
3.8E-2
2.3E-2
7.8E-2
5
16y
35y
1.6y
73d
30y
17y
119d
17h
23d
llh
6.4m
3.3h
24y
1.7y
40d
52. 3y
lOOOy
960y
244y
6
16y
68y
78y
67d
3.0y
10. 4y
lOOd
16h
5.0d
4.5h
6.1m
1.9h
2.4y
62d
96h
24. 4y
55y
96y
35y
t/1/2
7
2.0y
8.4y
140y
7.3d
llOd
i.iy
38d
9.4h
14h
38m
3.7m
23m
87d
6.2d
9.6h
2.5y
5.8y
9.6y
3.5y
at pH
8
0.2y
308d
1.5y
0.8d
lid
40d
5.5d
1.8h
1.4h
4.1m
1+lm
2.6m
8.7d
0.62d
Ih
91d
21 2d
350d
128d
9
7.3d
31d
01 5y
1.9h
l.ld
4.0d
0.57d
0.2h
8.4
1+lm
1+lm
1+lm
0.87d
1.5h
6m
9. Id
21d
35d
13d
10
0.73d
3. Id
5.5d
12m
2.6h
9.6h
Ih
1.2m
1m
1+lm
1+lm
1+lm
2.1h
9m
1+lm
0.9d
2. Id
3.5d
1.3d
11
0.073d
0.31d
0.5d
1.2m
0.26h
Ih
5m
1+lm
1+lm
1+lm
1+lm
1+lm
13m
1m
1+lm
2.2h
5h
0.35d
0.13d
-------
TABLE 8-20. (Continued)
00
~-J
en
Compound k. k,.
trans-CH3CHCHC(0)-
OC2H5 6.3E-7 -
CHCC(0)OC2H5
CgH5C(0)OCH3 4.0E-7 -
CgH5C(0)OC2H5
CgH5C(0)OCH(CH3)2
CgH5C(0)OCH2CgH5
p-N02-CgH4C(0)OCH3 4.3E-7 -
p-N02-CgH4C(0)OCH3
p-N02-CgH4C(0)-
OC2H5 1.4E-7 -
1-C5H4NC(0)OC2H5
o-C6H4[C(0)OC2H5]2
o-CgH4[C(0)OCH2-
W2
p-CgH4(CCO)OCHH3]2
p-CgH4[C(0)OC2H5]2
kB
1.3E-2
4.68
1.9E-3
3.0E-2
6.2E-3
8.0E-3
7.4E-2
6.4E-1
2.4E-1
5.4E-1
l.OE-2
1.7E-2
2.5E-1
6.9E-2
5
1140y
4.7y
3720y
730y
3500y
2700y
282y
34y
92y
41y
2200y
1300y
88Qy
320y
6
160y
170d
1160y
73y
350y
270y
30y
3.4y
9.2y
4.1y
220y
130y
88y
32y
t/1/2
7
I7y
17d
118y
7.3y
35y
27y
3.0y
0.34y
0.92y
0.41y
22y
I3y
0.88y
3.2y
at pH
8
1.7y
1.7d
11. 8y
0.73y
3.5y
2.7y
0.3y
12.4d
34d
15d
2.2y
1.3y
32d
117d
9
62d
0.17d
1.2y
27d
128d
99d
lid
1.2d
3.4d
1.5d
80.3d
47. 5d
3.2d
11. 7d
10
6.2d
24m
0.12y
2.7d
12. 8d
9.9d
l.ld
2.6h
0.34d
3.6h
8.0d
4.8d
7.7h
1.2d
11
0.62d
2.4m
4.4d
0.27d
1.3d
l.Od
2.6h
0.26h
0.8h
0.36h
0.8d
0.48d
0.77h
2.6h
Note: All values reported for 25°C. Rate constants in sec-1.
k. = acid-catalyzed hydrolysis constant
k,. = neutral hydrolysis constant
kg = base-catalyzed hydrolysis constant
t/1/2 = half life time
y = year
d = day
h = hour
m = minute
s = second
-------
TABLE 8-21.
HYDROLYSIS OF AMIDES (Envirosphere Company, 1984)
00
I
-•J
CT1
Compound
Ace t ami de
Valeramide
Isobutyamlde
Cyclopentanecarboxamide
Methoxy acetamide
Chloroacetamide
Di chloroacetamide
Tri chl oroacetami de
Bromoacetaml de
N-methyl acetami de
N-ethyl acetamide
kA kN
8.36E-6
5.43E-6
4.63E-6
2.34E-5
7.84E-6
1.1E-5.
-
-
-
3.2E-7
9.36E-8
kB
4.71E-5
1.41E-5
2.40E-5
1.67E---5
3.95E-4
1.5E-1
3.0E-1
9.4E-1
1.03E-5
5.46E-6
3.10E-6
t/1/2 at pH
5
262y
404y
47 Oy
93. 9y
280y
84. 5y
73y
23y
2x1 0°y
6900y
23,000y
6
2490y
3950y
4500y
931y
1860y
14.6y
7.3y
2-3y«-v
2x1 Oby
58,600y
1.8xl05
7
3950y
ll.SOOy
7700
5500y
500y
1.46y
0.73y
0.23y
21,200y
38,000h
70,000y
8
465y
1560y
915y
1300y
55. 6y
0.1 5y
26. 6d
8.4d
2120y
4020y
7090y
9
46. 5y
156y
91 5y
1.32y
5.6y
5.5d
2.7d
0.84d
21 2y
402y
709y
10
4.65y
15. 6y
9.2y
13. 2y
0.56y
0.55d
6.5h
2. Oh
21. 2y
40y
7iy
11
0.47y
l.Oy
0.92y
132y
20d
1.3h
0.6h
12m
2.iy
4.0y
7.iy
Note: All values at 25°C. Rate constants in sec-1.
k. = acid-catalyzed hydrolysis constant
k,, = neutral hydrolysis constant
kg = base-catalyzed hydrolysis constant
t/1/2 = half life time
y = year
d = day
h = hour
m = minute
s = second
-------
TABLE 8-22.
HYDROLYSIS OF CARBAMATES (Envirosphere Company, 1984)
oo
i
Compound k.
Ch3OC(0)N(H)C6H5
C2H50(CO)N(CH3)C6H5
C6H50(CO)N(H)C6H5
CgH50(CO)N(CH3)C6H5
P-CH3OCgH4(0)N(H)-
C6H5
m-C!CgH4OC(0)N(H)-
C6H5
p-N02CgH4OC(0)N(H)-
C6H5
p-N02CgH4OC(0)N-
(CH3)CgH5
1-C10H9OC(0)M(H)CH3
(0)N(H)C6H5
(0)N(H)C6H3(CH3)3
(CH3)3NCfiH4OC(0)N-(H)
CH3
(CH3)3NC5H4OC(0)N-
(CH3)2
C1CH2CH2OC(0)N(H)-
C6H5
Cl2CHCH2OC(0)NHCgH5
CCl3CH2OC(0)NICgH6
CF3CH2OC(0)NHC6H5
kN kg t/1/2 at pH
5.
5.
5.
4.
2.
1.
2.
8.
9.
2.
9.
6.
2.
1.
5.
3.
1.
5E-5
OE-6
42E1
2E-5
5E1
8E3
7E5
-OE-4
4EO
6E-5
4E-7
7E-1
8E-4
6E-3
OE-2
2E-1
OE-1
5
4x1 05y
4.4xl06hr
150d
5.2xl05y
320d
4.5d
43m
27,500y
2.3y
8.5xl05y
2.3xl07y
33y
89,500y
24,000y
440y
69y
220y
6
40,000y
44.4xl05y
15d
52,000y
32d
llh
4.3m
2750y
85d
85,000y
2.3xl06y
3.3y
7850y
1400y
My
6.9y
22y
7
4.000y
44,000y
1.5d
5200y
3.2d
l.lh
26s
275y
8.5d
8400y
240,000y
120d
785y
140y
4.4y
25 2d
2.2y
8
400y
4,400y
3.6h
520y
7.7h
6.4m
2.5s
27. 5y
20h
850y
23,000y
12d
78. 5y
Hy
160d
25d
80d
9
40y
440y
21m
52y
46m
19s
0.3s
2.7y
2h
85y
2300y
1.2d
7-9y
1.4y
16d
2.5d
8d
10
4y
44y
2m
5.2y
4.6m
3.9s
0.03s
lOOd
1.2m
8.5y
230y
2.9h
268d
50d
1.6d
6h
20h
11
146d
4.4y
13s
191d
28s
0.4s
0.003s
lOd
74s
31 Od
23y
1.7m
29d
5d
3.9h
36m
Note: All values at 25°C. Rate constants 1n sec-1.
k, = acid-catalyzed hydrolysis constant
k.. - neutral hydrolysis constant
kg = base-catalyzed hydrolysis constant
t/1/2 « half 11ft time
y • year
d * day
h * hour
m = minute
s = second
-------
00
I
OD
TABLE 8-23.
HYDROLYSIS OF PHOSPHORIC AND PHOSPHONIC ACID ESTERS (Envirosphere Company, 1984)
Compound k, k^
CH3P(0)(PCH3)2 1.36E-9
CH3P(0)(OC2H5)2 1.7E-9
CH3P(0)(OCH(CH3)2)2 6.4E-9
Ch3P(0)(OC2H5)(0-p-
CgH4N02) 1.2E-7
C2H5P(0)(OCH(CH3)2)2 3.2E-9
CgH5P(0)(OC2H5)2 1.1E-9
(CH30)3PO - 1.8E-9
(C2H50)3PO - 4.0E-9
(C2H5S)3PO - 1.4E-9
(CgH50)3PO - 2.7E-11
(C2H5))0(0)(-p-
CgH4N02) - 3.3E-6
(p-CgH4N02)3PO - l.OE-3
(CH30)2P(S)p-
C6H4N02 " 1.1E-7
CH3OP(S)SCHCH-
(C02C_H,.)2
(C2H50)2P(S)(p-
CgH4N02) - 3.0E-9
Note: All valves reported for T=25°C. Rate
k, = acid-catalyzed hydrolysis constant
k,. = neutral hydrolysis constant
kD = base-catalyzed hydrolysis constant
D
t/1/2 = half life time
y = year
d = day
h = hour
m = minute
s * sec
kB
2.5E-3
2.2E-4
3.2E-9
4.0E-2
3.7E-8
5.0E-4
1.3E-4
8.2E-6
1.2E-2
1.7E-2
5.3E-1
3.43E-1
5.95E-3
4.3
2.2E-4
constants
t/1/2 at pH
5
8700y
93,000y
3.4xl05y
530y
6.9xl05y
43,000y
i.zy
5.5y
165
550y
2d
12m
73d
5y
7y
in sec-1.
6
880y
9980y
2.3xl06y
55y
6.2xl06y
4400y
1.2y
5.5y
14y
112y
2d
12m
73d
187d
7y
7
88y
990y
663,000y
5.5y
5.5xl06y
440y
1.22y
5.5y
8.5y
13y
2d
llm
72d
18d
7y
8
8.8y
lOOy
69,000y
200d
5.9xl05y
44y
i.Zy
5.5y
1.6y
1.3y
2d
llm
69d
1.9d
7y
9
321d
lOy
6900y
20d
59,000y
4.4y
i.iy
5.4y
66d
47d
9h
9m
47d
4.5h
4y
10
32d
iy
690y
2d
5900y
160d
260d
4.6y
7d
5d
3h
3m
lid
27m
321d
11
3.2d
3bd
69y
4.8h
590y
16d
54d
1.8y
16h
llh
22m
20s
1.3d
3m
-------
TABLE 8-24.
HYDROLYSIS OF MISCELLANEOUS COMPOUNDS INCLUDING PESTICIDES (Envirosphere Company, 1984)
oo
i
Compound k.
Dimethyl sulfate
Methoxychlor
Captan
Malathion 4.8E-5
Parathion
Paraoxon
Diazinon 2.1E-2
Diazoxon 6.4E-1
Chloropyrifos
Sevin
Sevin
Baygon
Pyrolam
Dimetilan
P-Nitrophenyl -N-methyl
carbamate
2,4-D,m-butoxyethylester 2.0E-5
Methoxychlor
DDT
2,4-D.methylester
kN
1.
2.
1.
7.
4.
4.
4.
2.
1.
4.
2.
2.
1.
66E-4
99E-9
87E-5
7E-9
5E-8
1E-8
3E-8
8E-7
OE-7
_
-
-
-
-
OE-5
OE-5
8E-8
9E-9
—
kB
1.
3.
5.
5.
2.
1.
5.
7.
1.
7.
3.
4.
1.
5.
3.
3.
2.
9.
1.
48E-2
64E-4
7E+2
5EO
3E-2
3E-1
3E-3
6E-6
OE-1
7
4
6E-1
1E-2
7E-5
OE+3
02E+1
8E-4
9E-3
7E+1
5
1.2h
270d
lOh
1.6h
178d
195d
32d
1.2
80d
2.9h
6.5y
48y
2000y _
3.9xlOby
4.5h
9.6h
286d
I2y
1.3y
6
1.2h
270d
8h
128d
177d
190d
125d
9d
79d
104d
236d
4.8y
200y
39,000y
2.8h
9.5h
286d
iiy
47d
t/1/2
7
1.2h
270d
3h
14d
170d
149d
176d
23d
73d
lOd
24d
174d
20y
3900y
34m
8.4h
286d
7.6y
4.7d
at pH
8
1.2h
267d
20m
1.5d
118d
47d
165d
28d
40d
Id
2.4d
17d
2y
390y
3.8m
4.4h
283d
1.9y
llh
9
1.2h
241d
2m
3.5h
29d
6d
14d
29d
7d
2.5h
5.7h
1.7d
73d
39y
23s
36m
252d
79d
l.lh
10
l.lh
121d
12s
21m
3.4d
15h
14d
29d
19h
15m
34m
4.2h
7.3d
3.9y
2.3s
4m
122d
8d
6.8m
11
l.lh
20d
Is
2m
8.4h
1.5h
1.5d
29d
2h
1.5m
3.4m
25m
18h
141d
0.23s
23s
20d
19h
41s
Note: All values for 25°C. Rate constants in sec-1.
k. = acid-catalyzed hydrolysis constant
k.. = neutral hydrolysis constant
kg = base-catalyzed hydrolysis constant
t/1/2 = half life time
y = year
d = day
h = hour
m = minute
s = second
-------
Based upon Envirosphere Company's work, the following classes of
compounds have the potential for in situ degradation by hydrolysis:
• Esters
• Amides
• Carbamates
• Phosphoric and phosphonic acid esters
• Pesticides.
Because actual field application to plume treatment is not available, thorough
laboratory and pilot-scale testing should be performed prior to using
hydrolysis as an in situ treatment technique.
Both neutralization and hydrolysis suffer from the same disadvantages as
the other chemical treatment techniques, such as possible incomplete contact
with the leachate plume, application limited to coarse-grained strata, and
possible production of toxic by-products. However, these techniques are more
applicable than some of the other in situ methods and can be applied to almost
any leachate plume requiring pH adjustment.
8.3.5 Permeable Treatment Beds
Permeable treatment beds are a potential in situ treatment technique
applicable to sites with relatively shallow groundwater tables containing a
plume. In contrast to the conventional approaches of containing or removing a
plume, the concept of a permeable treatment bed involves (Figure 8-8) exca-
vating a trench, filling the trench with permeable treatment material, and
allowing the plume to flow through the bed thus physically removing or
chemically altering the contaminants. The conceptual function of a permeable
treatment bed is to reduce the quantities of contaminants present in a
leachate plume to acceptable levels. To date, permeable treatment beds have
not been used at hazardous waste sites. However, bench- and pilot-scale
testing has provided preliminary quantification of treatment bed effective-
ness. Numerous potential problems exist in using a permeable treatment bed.
These include saturation of bed material, plugging of bed with precipitates,
8-80
-------
cP
\
\
V
-------
and short life of the treatment material. Therefore, permeable treatment beds
should probably be considered a temporary remedial action rather than a
permanent one.
Selecting appropriate bed material to treat the contaminants present and
designing the bed to function properly are two elements that partially
determine the effectiveness of an installed permeable treatment bed. Types of
currently available treatment bed fill material include limestone, crushed
shell, activated carbon, glauconitic greensands, and synthetically produced
ion exchange resins. Ensuring proper physical design of the treatment bed
requires a knowledge of the hydrogeology of the site (e.g., groundwater flow
rate and direction, hydraulic conductivities) and the chemical characteristics
of the plume (i.e., types and concentrations of constituents).
8.3.5.1 Bed Fill Material
Relatively few fill materials exist that can feasibly be considered for
permeable beds to control contaminated groundwater. These materials include:
t Limestone
t Activated carbon
• Glauconitic greensands
t Zeolites and synthetic ion exchange resins.
Knowledge of the type and concentration of contaminants targeted for treatment
or removal by the bed is essential to selecting appropriate fill material. The
four potentially useful bed materials listed are described in detail in the
following sections.
8.3.5.1.1 Limestone
A crushed limestone bed may be applied in cases where neutralization of
acidic groundwater flow is needed. Limestone beds have also been reported to
be effective in removing certain metals such as cadmium, iron, and chromium
(EPA, 1978). Crushed shell, which has similar chemical characteristics as
8-82
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limestone, can also be used as a material for permeable treatment beds. In
most coastal regions where the availability of shells is good, crushed shell
can become a very useful material for permeable treatment beds, and may
compete with limestone in the control of acidic groundwater by this method.
Fuller (EPA, 1978) studied the neutralization effect that a crushed
limestone bed had on acidic water passed through the bed. Results of the
study showed that the contact time required to change the acidic water 1 pH
unit was in the range of 8 to 15 days. Extrapolating these results to a
permeable treatment bed application would result in a permeable bed having
either very low hydraulic conductivities or a large width to attain adequate
contact time.
Fuller and other researchers (EPA, 1978) have discussed the use of
crushed limestone as an effective low-cost landfill liner to aid in the
attenuation of the migration of certain heavy metals from solid waste
leachates. According to the authors, dolomitic limestone (containing signif-
icant amounts of magnesium carbonate) is less effective in removing ions than
purer limestone containing little magnesium carbonate. Therefore, in the
design of a limestone treatment bed, limestone with high calcium content is
recommended to remove heavy metals and to neutralize contaminated groundwater.
Little information is presently available on determining the contact time
needed for the optimum removal of heavy metals. According to Fuller (EPA,
1978), the efficiency of limestone in removing heavy metals from leachate
depends heavily on contact time, leachate concentration, and leachate pH.
Limestone has been shown to be very effective in attenuating the migration
rate of chromate, but an explanation for the mechanism of removal of this
metallic salt is not available. Removal of metallic cations such as
beryllium, cadmium, nickel, and zinc from leachate were also studied, but
conclusive results were not obtained. Thus, more studies are needed to
determine the effectiveness of limestone in removing heavy metals from
landfill leachate and the optimum contact time for the removal process.
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In regard to designing vertical permeable treatment beds, the particle
size of limestone used should be selected dependent on the type of soil in
which the groundwater flows (i.e., which controls flow rates) and the level of
contamination. In general, a mixture of gravel size and sand size limestone
should be used to minimize settling through dissolution. Where excessive
channelling through the bed by rapid groundwater movement is expected or where
improved contact time between the contaminated groundwater and the treatment
bed is required, a higher percentage of sand size particles is more
appropriate.
Tests to quantify conclusively the effectiveness of limestone as a
permeable treatment bed material for plume control have not been performed.
However, advantages in the use of limestone as a treatment bed material can be
identified. Generally, these advantages include the ability to neutralize
acidic groundwater, the ability to remove certain heavy metals (good potential
for removal of chromate anions), and the ready availability and low expense
associated with the use of limestone. Limitations to the effectiveness of
limestone as a permeable treatment bed are its potential for cementation,
plugging, and channelization, and its inability to remove organic
contaminants.
8.3.5.1.2 Activated Carbon
Activated carbon has been considered as a possible treatment bed material
because of its ability to effectively remove nonpolar organic compounds such
as PCB and CC1.. The mechanism of removal associated with these nonpolar com-
pounds is adsorbtion resulting from van der Waals forces and other chemical
attractive forces. Activated carbon also can remove certain heavy metals, but
is not very practical for actual use. Polar organic compounds, such as
alcohols and ketones, do not appear to be effectively removed by activated
carbon because of their electrical charges. This limits the effective use of
activated carbon to removal of nonpolar organic compounds in a plume.
8-84
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Although activated carbon is readily available, easy to install and
handle, and has significant potential to remove nonpolar organics, many dis-
advantages are associated with its use as a permeable treatment bed material.
These disadvantages include:
0 Plugging of the bed
• Presence of other chemicals in the groundwater which may reduce the
effectiveness of bed adsorption
• Desorption of hazardous adsorbed materials may occur, resulting in
recontamination
t Life of the bed may be very short in the presence of complex organic
compounds
• Removal and disposal of spent activated carbon is difficult and
hazardous
• Not very effective in removal of polar organic compounds
• Cost of the material is very high.
The numerous disadvantages associated with the use of activated carbon as
a treatment bed material have limited interest in and testing of this
material. However, where nonpolar organics are principal contaminants,
activated carbon may eventually become a suitable bed material.
8.3.5.1.3 Glauconitic Greensands
Glauconitic greensand deposits of the Atlantic Coastal Plain have high
potential for the removal of a number of heavy metals from contaminated
waters. Glauconite is a hydrous aluminosilicate clay mineral, which is rich
in ferric iron and contains significant amounts of potassium (Spoljaric and
Crawford, 1979). Glauconite occurs as dark, light, or yellowish-green pellets,
as casts of fossil shells, as coatings on other grains, and as a clayey matrix
in coarse-grained sediments.
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Studies by Spoljaric and Crawford (1979) indicated that glauconitic
greensand had good retention of heavy metal cations in bench-scale studies
with leachate from the Pigeon Point Landfill in northern Delaware. Highest
removal efficiencies were reported for copper, mercury, nickel, arsenic, and
cadmium, as shown in Table 8-25. The chemical composition of glauconites from
the Delaware Coastal Plain that were used in the study is given in Table 8-26.
The authors reported increased efficiencies with increased contact time.
Contact time for bench-scale testing was estimated at two minutes, suggesting
that with contact times used in the field being in the order of days, metal
removal efficiencies may be extremely high for many of the metals listed
(Spoljaric, 1980). The glauconitic sand treatment was also found to reduce
odors, suggesting that adsorption of volatile organics may occur as well.
Only minute amounts of metal cations were released from the charged
greensand upon flushing with distilled water and solutions of pH 2 and pH 12
(Spoljaric and Crawford, 1979). The results of these experiments suggest that
greensands have a high capacity for heavy metal cation retention, and thus
seem very applicable as a material for permeable treatment beds (Spoljaric,
1980). Saturation points for heavy metal adsorption by glauconitic sands have
not been determined and sorptive capacity has yet to be assessed through
further experimentation. Also, the applicability of greensands in treating
higher concentrations of heavy metals has not been determined. While they
appear promising, more experimental work is required before glauconitic
greensands can be thoroughly assessed as a permeable treatment bed material.
Ross (1980) conducted an in situ experiment near Alfred's Hill in
Uffington, England, involving leachate interactions in the Lower Greensand
(Cretaceous Age). The composition of the Lower Greensand is described as
containing quartz as the dominant mineral, with 15 to 20 percent calcium
montmorillonite, 8 percent calcite, and minor amounts of aragonite, pyrite,
glauconite, and goethite. Synthetic leachate solutions were introduced into
the Lower Greensand with known concentrations of heavy metals. The emphasis
of the study was to determine the sequential attenuation of heavy metals by
the Lower Greensand.
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TABLE 8-25.
RESULTS OF CHEMICAL ANALYSES OF GREENSAND FILTRATION
OF PIGEON POINT LANDFILL LEACHATE1
(Spoljaric and Crawford, 1979 as cited by Rogoshewski, et al., 1982)
Cation
Al uminum
Arsenic
Cadmium
Calcium
Chromium
Cobalt
Copper
Iron
Lead
Magnesium
Manganese
Mercury
Nickel
Potassium
Silver
Sodium
Zinc
pH
Cation concentration (ppm)
Before
filtration
0.68
0.0022
0.006
129
0.03
0.015
0.38
8.1
0.13
62
4.1
0.0087
0.074
122
0.0014
275
0.49
7.65
After
greensand
0.17
0.0002
0.001
48
0.01
0.003
ND
1.1
ND
20
0.48
ND
0.003
74
0.0007
175
0.16
6.29
Percent retained
by greensand
75
91
83
63
66
80
100
86
100
67
88
100
96
39
50
36
67
1
Flow rate: 100 ml/min.
ND = Not detected.
8-87
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TABLE 8-26.
CHEMICAL COMPOSITION OF GLAUCONITES FROM
THE DELAWARE COASTAL PLAIN
(Spoljaric and Crawford, 1979)
Oxide
Si02
A1203
Fe2°3
FeO
MnO
MgO
CaO
Na 0
K20
Cr2°3
H20 total
co2
Total
Fetotal as Fe;
1
49.6
0.84
9.6
14.0
2.26
0.04
3.1
Trace
0.11
6.2
0.045
10.13
95.9
203 16.5
Sample (wt/1/2)
2
47.3
0.28
7.1
17.6
1.89
0.03
3.7
2.1
0.11
8.0
0.034
8.99
1.0
98.1
19.9
3
47.3
0.27
7.2
17.5
2.30
0.03
3.6
2.2
0.05
8.0
0.056
8.2
1.1
97.8
20.1
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On the basis of the concentration of heavy metals extracted by lysimeters
from the sediments, the order of retention of the metals was found to be
Ni
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excavating a trench to intercept the flow of the contaminated groundwater,
filling the trench with the appropriate materials, and capping the trench
(refer to Chapter 6).
An important consideration in the design of a permeable treatment bed is
the size and shape of the trench as it relates to the specific problem of
groundwater contamination. The trench should be long enough to contain the
plume and deep enough to stop the groundwater from flowing underneath the bed.
For practical design purposes, the depth of the bed is the distance between
the ground surface and the bedrock or other impermeable layer (such as a clay
layer). The width of the trench is determined by the velocity of the ground-
water flow, the permeability of the material used for treatment, and the
contact time required for effective treatment.
Trench excavation will typically require the use of conventional shoring
such as sheet piling, and bracers and struts. Because the trench will
intersect the water table, dewatering will be required during excavation.
Groundwater pumped from the trench will likely be contaminated and, therefore,
will probably require treatment.
Prior to designing the permeable treatment bed, the residence time, or
contact time, must be estimated. Residence time is a function of the level of
contamination of groundwater and the treatment characteristics of the material
used for the decontamination process. The residence time of the contaminated
groundwater flow through the bed must be sufficient to ensure optimum treat-
ment conditions. Thus, the determination of the optimum contact time, tc,
requires a knowledge of the chemistry of the contaminated groundwater and of
its reaction to treatment by the material that will be present in the bed.
Once the residence time is estimated, the bed width required for treatment can
be calculated.
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To properly design a permeable treatment bed, a working knowledge of the
mechanisms of groundwater flow is essential. The flow of water through the
ground or its filtration through sand may be determined by using Darcy's Law:
V = Kl/n
where V is the pore velocity of water through a unit cross-sectional area
(ft/day), I is the gradient or loss of elevation head per unit of length in
the direction of the flow, K is the hydraulic conductivity (ft/day), and n is
the effective porosity of the material. Thus, by knowing the hydraulic con-
ductivity of the soil through which the groundwater flows and the difference
in hydraulic head between two points, flow velocity can be calculated for the
aquifer using estimates of appropriate effective porosity (refer to Chapter 3
for detailed discussions).
To ensure that disturbance of the groundwater flow by the treatment bed
is minimized, flow velocities within the treatment bed (Vb) should be equal to
or greater than flow velocities in the undisturbed soil (V ). Assuming the
hydraulic gradient at the bed and soil are the same, the ratios of hydraulic
conductivity to effective porosity must be similar between the two media.
Hydraulic conductivity can be determined either in the field (for the aquifer)
or in the laboratory (for the aquifer and bed material) using constant or
falling head methods. Effective porosity can be estimated from published data
or calculated from laboratory tests.
Consequently, the particle size distribution of the bed material can be
selected according to the desired hydraulic conductivity and porosity. Once
the bed material is selected, a series of bench scale tests can be performed,
varying particle size distribution to determine what sorting of particle sizes
provides an appropriate K/n ratio.
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Once the flow velocity of water through the bed (V, ) is calculated for
the appropriate treatment bed fill material and the residence time (t )
\*
estimated, the thickness (W, ) of the permeable treatment bed can be determined
from:
wk = V.t
b be
where the units are: W, in feet, V, in feet per day, and t in days.
D D C
To summarize, in the actual design of a treatment bed, the hydraulic
conductivity and porosity of the soil and the hydraulic gradients in the field
must be studied in order to estimate the actual velocities of the groundwater
flow. Then, the highest groundwater flow velocity is chosen as the minimum
design velocity of the flow through the bed to ensure optimum operational
conditions. To obtain the same velocity of groundwater through the bed as
through the aquifer, the K/n ratio of the bed must be the same as the K/n
ratio of the soil. Consequently, the particle size of the material used in
the bed must be chosen to achieve the desired K/n ratio of the bed. This can
be accomplished by conducting a series of bench-scale tests with bed materials
of different particle size distribution. To design an effective treatment
bed, an optimum contact time must be determined on the basis of expected rates
of interaction between the groundwater and the bed material . When the
velocity of the flow through the bed and the optimum contact time are
estimated, the bed thickness can be determined.
Unit costs for the installation of a permeable treatment bed were
estimated by the Rogoshewski , et al. (1982) and are shown in Table 8-27.
Costs can be assumed to have increased marginally since estimates included in
the report are somewhat dated.
8.4 BLOCK DISPLACEMENT
The Block Displacement (BD) method is a technique that was developed to
completely isolate a large mass of contaminated soil containing a waste site
or plume, or both. The BD concept relies on producing a fixed underground
8-92
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TABLE 8-27.
ESTIMATED COSTS FOR INSTALLATION OF A PERMEABLE TREATMENT BED1
(Rogoshewski, et al., 1982)
Item Cost ($)
Unit Total
Trench excavation costs (approximately 3,000 cubic yards) 3,000
Cover spreading costs 1,980
Well point dewatering system 75,000
Sheet piling costs 365,000
Water, connection and structure costs 6,100
Total cost of trench construction 451,080
Limestone/materials and installation costs 34,500
Total cost using limestone 485,580
Activated carbon/materials and installation costs 4,080,000
Total cost using activated carbon 4,531,080
o
Glauconitic Greensand cost Not provided
Zeolite/Synthetic ion exchange resins cost Not provided
Costs based on trench dimensions of 20' deep, 4' wide, 1000' long, costs for
contractors and materials compiled during 1980
2
Could entail possible land purchase, borrow excavation, and hauling to
disposal site
Considered economically infeasible
8-93
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barrier around and beneath the contaminated zone to essentially encapsulate a
block of earth (Brunsing, 1983). The method involves the construction of two
separate barriers, a perimeter barrier and a bottom barrier, which are inter-
connected to allow block displacement.
The perimeter barrier can be constructed using various conventional means
including slurry wall , vibrating beam, or jet grouting techniques (refer to
Chapter 7). The perimeter barrier can be constructed in conjunction with the
bottom barrier either prior to, during, or following bottom barrier construc-
tion. If constructed prior to bottom barrier construction, the perimeter
barrier can be used to aid in bottom separation.
The bottom barrier is formed by pumping slurry composed of local soil,
bentonite, and water, into a series of notched injection holes. Continued
pumping of slurry under pressure theoretically produces an uplift force
against the bottom of the block that is capable of creating upward vertical
displacement of the block. Atypical Block Displacement barrier is shown
schematically in Figure 8-9.
FIGURE 8-9.
SCHEMATIC DIAGRAM OF BLOCK DISPLACEMENT
(BRUNSING, 1983)
Groundwater
Level
' } Perimeter
Barrier
Positive Seal Through
Injected Bentonite
Mixture
Bottom Barrier
8-94
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The Block Displacement method was developed for applications where an
unweathered bedrock or low permeability stratum does not occur at shallow
enough depths below a contaminated zone for a perimeter (e.g., slurry wall)
alone to provide cost-effective isolation of the contaminant plume. In situ
isolation using a slurry wall or other conventional vertical barriers require
keying the barrier into a naturally occurring impermeable stratum. Also,
Block Displacement is conceptually designed to minimize the volume of soil or
earth material to be isolated. Thus, Block Displacement would theoretically
isolate a contaminated area through the creation of a man-made confining layer
immediately surrounding the contaminated zone.
Construction principles of the Block Displacement method require the use
of barrier slurry material that is hydraulically and chemically compatible
with in situ soils and contaminated groundwater. Construction of the bottom
barrier proceeds in four phases:
1. Formation of notches at the base of the injection holes
2. Initial bottom separation at the notched holes during initial slurry
pumping
3. Coalescence of local separations at each injection point to form a
single large bottom separation
4. Generation of a complete bottom barrier by controlled vertical
displacement of the earth mass using low pressure slurry injection
into the horizontal separation.
The perimeter barrier can be constructed by various means including a
slurry wall, vibrating beam or jet grouting technique. If constructed prior
to bottom separation, the perimeter wall can be used to ensure a favorable
horizontal stress field for proper orientation of the propagating bottom
separation. Also, the injection pressure required to achieve bottom
separation is dependent not only on block weight and geometry, but the shear
resistance in the previously constructed perimeter as well. This relationship
8-95
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The perimeter barrier can be constructed by various means including a
slurry wall, vibrating beam or jet grouting technique. If constructed prior
to bottom separation, the perimeter wall can be used to ensure a favorable
horizontal stress field for proper orientation of the propagating bottom
separation. Also, the injection pressure required to achieve bottom
separation is dependent not only on block weight and geometry, but the shear
resistance in the previously constructed perimeter as well,, This relationship
is shown in the following equation (Brunsing, 1983), which also takes into
account the slurry pressure drop across the bottom separation:
P0 = PI + A PB
where P = required injection pressure
Pi = pressure in the bottom separation at its intersection with
the perimeter
APg = the pressure drop across the bottom separation caused by flow
resistance in the slurry.
The width of an initially thin perimeter barrier, such as might be con-
structed by vibrating beam or jet grouting techniques, can be increased during
block lift or continued bottom separation. Upward displacement of the block
(d) resulting from injection along the bottom barrier should increase the
initial perimeter barrier thickness (W ), to a desired barrier thickness (W)
if the perimeter is constructed at a slight angle off vertical (0). This
principle is expressed in the following equation offered by Brunsing (1983):
W = d sin 0 + WQ
Hence, if perimeter barrier construction is performed prior to bottom
separation and barrier construction, favorable horizontal stress fields may be
ensured to help properly orient bottom separation propagation and thinly con-
structed perimeter barriers may be increased to a desired thickness. Figure
8-10, shows in cross-section, the schematic configuration of the perimeter
barrier, injection wells, and hypothetically coalescing bottom separation.
8-96
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Perimeter
Surcharge
(when required)
FIGURE 8-10.
SCHEMATIC CROSS-SECTION OF BLOCK DISPLACEMENT
SHOWING SEPARATING TO INDUCE DISPLACEMENT
(BRUNSING, 1983)
Slurry
Injection
Fracture Bedrock
8-97
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The notching operation at the base of the injection hole requires a
high-pressure rotating slurry jet (Figure 8-11). The jetting slurry should be
designed to optimize notch erosion, remove cuttings, and minimize leakage into
the soils. A large notch diameter is desired to reduce initiation slurry
pumping pressure requirements during bottom barrier emplacement and to reduce
the tendency of separations to propagate upward. The slurry injection
pressure PQ required to initiate bottom separation is defined by (Brunsing,
1983):
P0 = Prgh + AP
where Pr = the average earth mass density
g = gravitational constant
h = depth to bottom separation
AP = pressure in excess of the overburden.
The pressure in excess of the overburden is a function of (Brunsing, 1983):
• Soil strength or strength of the material being fractured
t Notch diameter which affects the initial slurry pressure required to
propagate fractures at the tip of the notch
• Slurry properties that should be designed to inhibit filter cake
formation at the end of the notches, thus preventing deflected
fracture propagation
t Slurry injection rate which should minimize the time available for
filter cake formation.
Coalescence of the separations is then a function of adding slurry volume and
by gradually increasing the gel strength and viscosity of the slurry. The
increased area over which the slurry is in contact should decrease the slurry
pressure required to propagate the horizontal separation.
The concept of block displacement for hazardous waste problems has not
been substantially documented. However, a demonstration of the Block
8-98
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FIGURE 8-11.
SLURRY JET IN AIR NOTCHING OPERATION
(BRUNSING, 1983)
PUMP SUCTION
BACK PRESSURE VALVE
HIGH
PRESSURE
PUMP
SETTLING TANK
AIR BACK
PRESSURE
SUPPLY
ROTATING NOZZLE
BACK PRESSURIZED
RETURN LINES
SLURRY LEVEL
8-99
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Displacement method was conducted in Whitehouse, Florida, during 1982
(Brunsing and Grube, 1982). The demonstration block was approximately 60 feet
in diameter and 23 feet in depth, and composed of unconsolidated marine
sediments. Although the results of the test did not conclusively demonstrate
complete block isolation, maximum uplift was measured to be 12 inches in
portions of the block, and corings of the bottom barrier documented slurry
thicknesses ranging from 1.5 to 4.75 inches. However, areas of the block were
measured as not having undergone uplift and the varied thicknesses of the
bottom barrier in the test corings did not match uplift measured directly at
the surface. The varied thickness in the bottom barrier slurry also raises
questions regarding complete lateral continuity of the bottom barrier beneath
the block.
The subsurface geologic conditions to which BD would be an appropriate
remedial action are restricted. Variable fluid flow characteristics of soils
or soft sediments, and discontinuities within the vicinity of bottom barrier
construction could inhibit effective slurry propagation and coalescence. In
situ stresses control separation orientation, since separations beyond the
notch diameter will propagate in a plane normal to the direction of minimum
principal stress. Vertical stress, which is determined by overburden weight,
is in many cases greater than horizontal stresses. In such cases, fractures
initiated horizontally will have a tendency to turn upward. Also, block
displacement would be considered infeasible in areas where faults, fractures,
or solution cavities may be present.
Problems associated with the use of Block Displacement to encapsulate a
contaminated block include not only difficulties in ensuring bottom barrier
continuity, but also the health and safety, environmental, and construction
risks of drilling injection holes through a contaminated zone, and potentially
through hazardous materials. Even if injection holes could be constructed
safely, they could potentially serve as conduits for increased rates of
vertical contaminant migration prior to slurry injection.
8-100
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B-26
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COPYRIGHT NOTICE
Table 2-7
Table 2-8
Table 2-9
Table 4-1
Figure 5-6
Figure 5-19
Table 5-11
Table 5-18
Table 5-19
Figure 5-21a
Figure 5-21b
Table 5-20
Figure 5-23
Table 5-22
Figure 5-24c
Figure 5-25
Figure 5-29
Figure 6-7
Table 5-1
Figure 5-15a
Figure 5-15b
Table 5-12
Table 5-13
Table 5-14
Table 5-16
Figure 5-22
Table 5-21
Figure 5-24a
Figure 5-24b
Figure 5-27
Figure 8-2
Table 5-3
Figure 5-12
Figure 5-4
Figure 5-5
Figure 5-7
Figure 5-lla
Figure 5-llb
Figure 5-14a
Figure 5-14b
From Aquifer Contamination and Protection, by Jackson.
Copyright (c) 1980, UNESCO. Used by permission of
UNESCO.
From Construction Dewatering, A Guide to Theory and
Practice, by J. Patrick Powers. Copyright (c) 1981,
John Wiley $ Sons, Inc. Used by permission of John
Wiley & Sons, Inc.
From Groundwater and Wells. Reprinted by permission
of Johnson Division, UOP, Inc., Copyright (c) 1975,
Saint Paul, Minnesota.
From Applied Hydrogeology, by C.W. Fetter.
Copyright (c) 1980, Charles E. Merrill Publishing
Company. Used by permission of Charles E. Merrill
Publishing Company.
From Hydrogeology, by Stanley N. Davis
and Roger J.M. DeWiest. Copyright (c)
1966, John Wiley & Sons, Inc. Used
by permission of John Wiley A Sons, Inc.
C-l
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COPYRIGHT NOTICE (continued)
Table 5-23
Table 6-1
Figure 6-17
Table 6-8
Figure 6-12
Figure 6-13
Figure 6-15
Figure 6-26
Table 6-6
Table 6-10
Table 6-11
Table 6-12
Figure 6-27
Table 6-13
Table 6-15
Table 6-16
Table 6-17
Table 8-3
Figure 8-3
From Jet Pumps Technical Information Brochure, Form
No. 927. Reprinted by permission of Burks Pumps.
Copyright (c) 1982, Burks Pumps, Decatur, Illinois.
From Drainage Principles and Applications, by
Wesseling. Copyright (c) 1973, International
Institute for Land Reclamation and Improvement.
by permission of the International Institute for
Reclamation and Improvement.
Used
Land
From Building Construction Cost Data, 40th Annual
Edition, by R.R. Godfrey. Copyright (c) 1982, Robert
Snow Means Company. Used by permission of Robert Snow
Means Company.
From Procedures in Drainage Surveys, in Drainage
Principles and Applications, Vol. IV: Design and
Management of Drainage Systems, Publication 16, by De
Van Aart. Copyright (c) 1974,
Institute for Land Reclamation and
Used by permission of the International
Land Reclamation and Improvement.
Ridder and R.
International
Improvement.
Institute for
From Drainage for Agriculture, Agronomy Monograph 17,
by Van SchiIfgaarde. Copyright (c) 1974, American
Society of Agronomy. Used by permission of the
American Society of Argonomy.
From Excavation Handbook, by H.K. Church.
Copyright (c) 1981, McGraw-Hill Book Company.
Used by permission of McGraw-Hill Book Company.
From Building Construction Cost Data, 39th
Annual Edition, by R.R. Godfrey. Copyright (c) 1981,
Robert Snow Means Company. Used by permission
of Robert Snow Means Company.
From Handbook of Chemical Property Estimation Methods,
by Lyman, Reehl, and Rosenblatt. Copyright (c) 1982,
McGraw-Hill Book Company. Used by permisson of
McGraw-Hill Book Company.
From Biodecontamination: The Neglected Alternative,
Hazardous Material Spills Conference Proceedings, by
C.S. McDowell, J. Zikipoulas, and T.G. Zitrides.
Copyright (c) 1982, Government Institutes, Inc. Used
by permission of Government Institutes, Inc.
C-2
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COPYRIGHT NOTICE (continued)
Table 8-26 From Removal of Contaminants from Landfill Glauconic
Green Sands, in Environmental Geology, Vol. 2, No. 6,
by N. Spoljaric and W.A. Crawford. Copyright (c)
1978/79, Springer-Verlag New York, Inc. Used by
permission of Springer-Verlag New York, Inc.
C-3
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