OSWER DIRECTIVE 9502.00-6D
INTERIM FINAL
RCRA FACILITY INVESTIGATION (RFI) GUIDANCE
VOLUME II OF IV
SOIL, GROUND WATER AND
SUBSURFACE GAS RELEASES
EPA 530/SW-89-031
MAY 1989
WASTE MANAGEMENT DIVISION
OFFICE OF SOLID WASTE
U.S. ENVIRONMENTAL PROTECTION AGENCY
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ABSTRACT
On November 8, 1984, Congress enacted the Hazardous and Solid Waste
Amendments (HSWA) to RCRA. Among the most significant provisions of HSWA are
§3004(u), which requires corrective action for releases of hazardous waste or
constituents from solid waste management units at hazardous waste treatment,
storage and disposal facilities seeking final RCRA permits; and §3004(v), which
compels corrective action for releases that have migrated beyond the facility
property boundary. EPA will be promulgating rules to implement the corrective
action provisions of HSWA, including requirements for release investigations and
corrective measures.
This document, which is presented in four volumes, provides guidance to
regulatory agency personnel on overseeing owners or operators of hazardous waste
management facilities in the conduct of the second phase of the RCRA Corrective
Action Program, the RCRA Facility Investigation (RFI). Guidance is provided for the
development and performance of an investigation by the facility owner or operator
based on determinations made by the regulatory agency as expressed in the
schedule of a permit or in an enforcement order issued under §3008(h), §7003,
and/or §3013. The purpose of the RFI is to obtain information to fully characterize
the nature, extent and rate of migration of releases of hazardous waste or
constituents and to interpret this information to determine whether interim
corrective measures and/or a Corrective Measures Study may be necessary.
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DISCLAIMER
This document is intended to assist Regional and State personnel in exercising
the discretion conferred by regulation in developing requirements for the conduct
of RCRA Facility Investigations (RFIs) pursuant to 40 CFR 264. Conformance with this
guidance is expected to result in the development of RFIs that meet the regulatory
standard of adequately detecting and characterizing the nature and extent of
releases. However, EPA will not necessarily limit acceptable RFIs to those that
comport with the guidance set forth herein. This document is not a regulation (i.e.,
it does not establish a standard of conduct which has the force of law) and should
not be used as such. Regional and State personnel must exercise their discretion in
using this guidance document as well as other relevant information in determining
whether an RFI meets the regulatory standard.
Mention of company or product names in this document should not be
considered as an endorsement by the U.S. Environmental Protection Agency.
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RCRA FACILITY INVESTIGATION (RFI) GUIDANCE
VOLUME II
SOIL, GROUND WATER AND SUBSURFACE GAS RELEASES
TABLE OF CONTENTS
SECTION
ABSTRACT i
DISCLAIMER M
TABLE OF CONTENTS m
TABLES xii
FIGURES xiii
LIST OF ACRONYMS
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VOLUME II
SOIL, GROUND WATER AND SUBSURFACE GAS RELEASES
TABLE OF CONTENTS
SECTION PAGE
9.0 SOIL 9-1
9.1 OVERVIEW 9-1
9.2 APPROACH FOR CHARACTERIZING RELEASES TO SOIL 9-2
9.2.1 General Approach 9-2
9.2.2 Inter-media Transport 9-8
9.3 CHARACTERIZATION OF THE CONTAMINANT SOURCE AND 9-9
THE ENVIRONMENTAL SETTING
9.3.1 Waste Characterization 9-9
9.3.2 Unit Characterization 9-17
9.3.2.1 Unit Design and Operating Characteristics 9-17
9.3.2.2 Release Type (Point or Non-Point Source) 9-19
9.3.2.3 Depth of the Release 9-20
9.3.2.4 Magnitude of the Release 9-22
9.3.2.5 Timing of the Release 9-23
9.3.3 Characterization of the Environmental Setting 9-24
9.3.3.1 Spatial Variability 9-25
9.3.3.2 Spatial and Temporal Fluctuations in Soil 9-26
Moisture Content
9.3.3.3 Soil, Liquid, and Gaseous Materials in 9-28
the Unsaturated Zone
IV
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VOLUME II CONTENTS (Continued)
SECTION PAGE
9.3.4 Sources of Existing Information 9-38
9.3.4.1 Geological and Climatological Data 9-38
9.3.4.2 Facility Records and Site-Investigations 9-39
9.4 DESIGN OF A MONITORING PROGRAM TO CHARACTERIZE 9-39
RELEASES
9.4.1 Objectives of the Monitoring Program 9-39
9.4.2 Monitoring Constituents and Indicator Parameters 9-43
9.4.3 Monitoring Schedule 9-43
944 Monitoring Locations 9-44
9.4.4.1 Determine Study and Background Areas 9-44
9.4.4.2 Determine Location and Number of Samples 9-45
9.4.4.3 Predicting Mobility of Hazardous Constituents 9-48
in Soil
9.4.4.3.1 Constituent Mobility 9-49
9.4.4.3.2 Estimating Impact on Ground-Water 9-51
Quality
9.5 DATA PRESENTATION 9-60
9.5.1 Waste and Unit Characterization 9-60
9.5.2 Environmental Setting Characterization 9-60
9.5.3 Characterization of the Release 9-61
9.6 FIELD METHODS 9-64
9.6.1 Surficial Sampling Techniques 9-66
9.6.1.1 Soil Punch 9-66
9.6.1.2 Ring Samplers 9-66
9.6.1.3 Shovels, Spatulas, and Scoops 9-67
9.6.1.4 Soil Probes (tube samplers) 9-67
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VOLUME II CONTENTS (Continued)
SECTION PAGE
9.6.1.5 Hand Augers 9-67
9.6.2 Deep Sampling Methods 9-68
9.6.2.1 Hollow-Stem Augers 9-68
9.6.2.2 Solid-Stem Augers 9-68
9.6.2.3 Core Samplers 9-68
9.6.2.3.1 Thin-Walled Tube Samplers 9-69
9.6.2.3.2 Split-Spoon Samplers 9-69
9.6.2.4 Trenching 9-69
9.6.3 Pore Water Sampling 9-70
9.7 SITE REMEDIATION 9-72
9.8 CHECKLIST 9-73
9.9 REFERENCES 9-75
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VOLUME II CONTENTS (Continued)
SECTION PAGE
10.0 GROUND WATER 10-1
10.1 OVERVIEW 10-1
10.2 APPROACH FOR CHARACTERIZING RELEASES TO 10-2
GROUND WATER
10.2.1 General Approach 10-2
10.2.2 Inter-media Transport 10-8
10.3 CHARACTERIZATION OF THE CONTAMINANT SOURCE 10-9
AND THE ENVIRONMENTAL SETTING
10.3.1 Waste Characterization 10-9
10.3.2 Unit Characterization 10-12
10.3.3 Characterization of the Environmental Setting 10-13
10.3.3.1 Subsurface Geology 10-55
10.3.3.2 Flow Systems 10-58
10.3.4 Sources of Existing Information 10-63
10.3.4.1 Geology 10-64
10.3.4.2 Climate 10-64
10.3.4.3 Ground-Water Hydrology 10-64
10.3.4.4 Aerial Photographs 10-65
10.3.4.5 Other Sources 10-65
10.4 DESIGN OF A MONITORING PROGRAM TO CHARACTERIZE 10-65
RELEASES
10.4.1 Objectives of the Monitoring Program 10-66
10.4.2 Monitoring Constituents and Indicator Parameters 10-68
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VOLUME II CONTENTS (Continued)
SECTION PAGE
10.4.3 Monitoring Schedule 10-69
10.4.3.1 Monitoring Frequency 10-69
10.4.3.2 Duration of Monitoring 10-70
10.4.4 Monitoring Locations 10-71
10.4.4.1 Background and Downgradient Wells 10-75
10.4.4.2 Well Spacing 10-76
10.4.4.3 Depth and Screened Intervals 10-79
10.5 DATA PRESENTATION 10-84
10.5.1 Waste and Unit Characterization 10-84
10.5.2 Environmental Setting Characterization 10-85
10.5.3 Characterization of the Release 10-92
10.6 FIELD METHODS 10-94
10.6.1 Geophysical Techniques 10-94
10.6.2 Soil Boring and Monitoring Well Installation 10-95
10.6.2.1 Soil Borings 10-95
10.6.2.2 Monitoring Well Installation 10-99
10.6.3 Aquifer Characterization 10-101
10.6.3.1 Hydraulic Conductivity Tests 10-101
10.6.3.2 Water Level Measurements 10-103
10.6.3.3 Dye Tracing 10-105
10.6.4 Ground-Water Sample Collection Techniques 10-105
10.7 SITE REMEDIATION 10-108
10.8 CHECKLIST 10-110
10.9 REFERENCES 10-113
viii
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VOLUME II CONTENTS (Continued)
SECTION PAGE
11.0 SUBSURFACE GAS 11-1
11.1 OVERVIEW 11-1
11.2 APPROACH FOR CHARACTERIZING RELEASES OF n_2
SUBSURFACE GAS
11.2.1 General Approach 11-2
11.2.2 Inter-media Transport 11-7
11.3 CHARACTERIZATION OF THE CONTAMINANT SOURCE 11.7
AND THE ENVIRONMENTAL SETTING
11.3.1 Waste Characterization 11-11
11.3.1.1 Decomposition Process 11-11
11.3.1.1.1 Biological Decomposition 11-11
11.3.1.1.2 Chemical Decomposition 11-12
11.3.1.1.3 Physical Decomposition 11-12
11.3.1.2 Presence of Constituents 11-13
11.3.1.3 Concentration 11-14
11.3.1.4 Other Factors 11-14
11.3.2 Unit Characterization 11-15
11.3.2.1 Landfills 11-17
11.3.2.2 Units Closed as Landfills 11-17
11.3.2.3 Underground Tanks 11-17
11.3.3 Characterization of the Environmental Setting 11-17
11.3.3.1 Natural and Engineered Barriers 11-17
11.3.3.1.1 Natural Barriers 11-18
11.3.3.1.2 Engineered Barriers 11-18
ix
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VOLUME II CONTENTS (Continued)
SECTION PAGE
11.3.3.2 Climate and Meteorological Conditions 11-19
11.3.3.3 Receptors 11-20
11.4 DESIGN OF A MONITORING PROGRAM TO CHARACTERIZE 11-20
RELEASES
11.4.1 Objectives of the Monitoring Program 11-22
11.4.2 Monitoring Constituents and Indicator Parameters 11-23
11.4.3 Monitoring Schedule 11-24
11.4.4 Monitoring Locations 11-25
11.4.4.1 Shallow Borehole Monitoring 11-25
11.4.4.2 Gas Monitoring Wells 11-26
11.4.4.3 Monitoring in Buildings 11-28
11.4.4.4 Use of Predictive Models 11-29
11.5 DATA PRESENTATION 11-30
11.5.1 Waste and Unit Characterization 11-30
11.5.2 Environmental Setting Characterization 11-30
11.5.3 Characterization of the Release 11-31
11.6 FIELD METHODS 11-31
11.6.1 Above Ground Monitoring 11-37
11.6.2 Shallow Borehole Monitoring 11-38
11.6.3 Gas Well Monitoring 11-38
11.7 SITE REMEDIATION 11-38
11.8 CHECKLIST 11-42
11.9 REFERENCES 11-44
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VOLUME II CONTENTS (Continued)
SECTION
APPENDICES
Appendix C:
Appendix D:
Appendix E:
Appendix F:
Geophysical Techniques
Subsurface Gas Migration Model
Estimation of Basement Air Contaminant
Concentrations Due to Volatile Components in
Ground Water Seeped Into the Basement
Method 1312: Synthetic Precipitation Leach Test
for Soils
PAGE
C-1
D-1
E-1
F-1
xi
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TABLES (Volume II)
NUMBER PAGE
9-1 Example Strategy for Characterizing Releases 9.3
to Soil
9-2 Release Characterization Tasks for Soils 9-7
9-3 Transformation/Transport Processes in Soil 9-10
9-4 BODs/COD Ratios for Various Organic Compounds 9-14
9-5 Potential Release Mechanisms for Various Unit Types 9-18
9-6 Relative Mobility of Solutes 9-55
10-1 Example Strategy for Characterizing Releases to 10-4
Ground Water
10-2 Release Characterization Tasks for Ground Water 10-6
10-3 Summary of U.S. Ground-Water Regions 10-17
10-4 Default Values for Effective Porosity 10-51
10-5 Factors Influencing the Intervals Between Individual 10-77
Monitoring Wells Within a Potential Migration Pathway
10-6 Applications of Geophysical Methods to Hazardous 10-96
Waste Sites
10-7 Factors Influencing Density of Initial Boreholes 10-97
11-1 Example Strategy for Characterizing Releases of 11-4
Subsurface Gas
11-2 Release Characterization Tasks for Subsurface Gas 11-6
11-3 Summary of Selected Onsite Organic Screening 11-32
Methodologies
11-4 Summary of Candidate Methodologies for Quantification 11-33
of Vapor Phase Organics
11-5 Typical Commercially Available Screening Techniques for 11-36
Organics in Air
11-6 Subsurface Sampling Techniques 11-39
XII
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FIGURES (Volume II
NUMBER PAGE
9-1 Hydrogeologic Conditions Affecting Soil Moisture 9-27
Transport
9-2 Soil Terms 9-30
9-3 Hypothetical Adsorption Curves for A) Cations and B) 9-56
Anions Showing Effect of pH and Organic Matter
9-4 Fields of Stability for Aqueous Mercury at 25°C 9-59
and Atmospheric Pressure
9-5 Example of a Completed Boring Log 9-62
9-6 Typical Ceramic Cup Pressure/Vacuum Lysimeter 9-71
10-1 Occurrence and Movement of Ground Water and 10-15
Contaminants Through (a) Porous Media, (b) Fractured
or Creviced Media, (c) Fractured Porous Media
10-2 Ground-Water Flow Paths in Some Different 10-16
Hydrogeologic Settings
10-3 Western Mountain Ranges 10-18
10-4 Alluvial Basins 10-20
10-5 Columbia Lava Plateau 10-21
10-6 Colorado Plateau 10-23
10-7 High Plains 10-24
10-8 Non-glaciated Central 10-26
10-9 Glaciated Central 10-28
10-10 Piedmont and Blue Ridge 10-31
10-11 Northeast and Superior Uplands 10-32
10-12 Atlantic and Gulf Coastal Plain 10-34
10-13 Southeast Coastal Plain 10-37
10-14 Hawaiian Islands 10-39
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FIGURES (Volume n-Continued)
NUMBER PAGE
10-15 Alaska 10-41
10-16 Alluvial Valleys 10-43
10-17 Monitoring Well Placement and Screen Lengths in a 10-54
Mature Karst Terrain/Fractured Bedrock Setting
10-18 Monitoring Well Locations 10-67
10-19 Example of Using Soil Gas Analysis to Define Probable 10-73
Location of Ground-water Release Containing Volatile
Organics
10-20 Vertical Well Cluster Placement 10-81
10-21 General Schematic of Multi phase Contamination In a 10-83
Sand Aquifer
10-22 Potentiometric Surface Showing Flow Direction 10-89
10-23 Approximate Flow Net 10-90
11-1 Subsurface Gas Generation/Migration in a Landfill 11-9
11-2 Subsurface Gas Generation/Migration from Tanks and 11-10
Units Closed as Landfills
11-3 Schematic of a Deep Subsurface Gas Monitoring Well 11-27
X IV
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LIST OF ACRONYMS
AA Atomic Absorption
Al Soil Adsorption Isotherm Test
ASCS Agricultural Stabilization and Conservation Service
ASTM American Society for Testing and Materials
BCF Bioconcentration Factor
BOD Biological Oxygen Demand
CAG EPA Carcinogen Assessment Group
CPF Carcinogen Potency Factor
CBI Confidential Business Information
CEC Cation Exchange Capacity
CERCLA Comprehensive Environmental Response, Compensation, and Lability
Act
CFR Code of Federal Regulations
CIR Color Infrared
CM Corrective Measures
CMI Corrective Measures Implementation
CMS Corrective Measures Study
COD Chemical Oxygen Demand
COLIWASA Composite Liquid Waste Sampler
DNPH Dinitrophenyl Hydrazine
DO Dissolved Oxygen
DOT Department of Transportation
ECD Electron Capture Detector
EM Electromagnetic
EP Extraction Procedure
EPA Environmental Protection Agency
FEMA Federal Emergency Management Agency
FID Flame lonization Detector
Fraction organic carbon in soil
us Fish and wildlife Service
G C Gas Chromatography
GC/MS Gas chromatography/Mass Spectroscopy
GPR Ground Penetrating Radar
HEA Health and Environmental Assessment
HEEP Health and Environmental Effects Profile
HPLC High Pressure Liquid Chromatography
HSWA Hazardous and Solid Waste Amendments (to RCRA)
HWM Hazardous Waste Management
ICP Inductively Coupled (Argon) Plasma
ID Infrared Detector
Kd Soil/Water Partition Coefficient
Koc Organic Carbon Absorption Coefficient
Kow Octanol/Water Partition Coefficient
LEL Lower Explosive Limit
MCL Maximum Contaminant Level
MM5 Modified Method 5
MS/MS Mass Spectroscopy/Mass Spectroscopy
NFIP National Flood Insurance Program
NIOSH National Institute for Occupational Safety and Health
NPDES National Pollutant Discharge Elimination System
OSHA Occupational Safety and Health Administration
foe
FWS
xv
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LIST OF ACRONYMS (Continued)
OVA Organic Vapor Analyzer
PID Photo lonization Detector
pKa Acid Dissociation Constant
ppb parts per billion
ppm parts per million
PUF Polyurethane Foam
PVC Polyvinyl Chloride
QA/QC Quality Assurance/Quality Control
RCRA Resource Conservation and Recovery Act
RFA RCRA Facility Assessment
RfD Reference Dose
RFI RCRA Facility Investigation
RMCL Recommended Maximum Contaminant Level
RSD Risk Specific Dose
SASS Source Assessment Sampling System
SCBA Self Contained Breathing Apparatus
SCS Soil Conservation Service
SOP Standard Operating Procedure
SWMU Solid Waste Management Unit
TCLP Toxicity Characteristic Leaching Procedure
TEGD Technical Enforcement Guidance Document (EPA, 1986)
TOC Total Organic Carbon
TOT Time of travel
TOX Total Organic Halogen
USGS United States Geologic Survey
USLE Universal Soil Loss Equation
UV Ultraviolet
VOST Volatile Organic Sampling Train
VSP Verticle Seismic Profiling
WQC Water Quality Criteria
XVI
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SECTION 9
SOIL
9.1 Overview
The objective of an Investigation of a release to soil is to characterize the
nature, extent, and rate of migration of a release of hazardous waste or
constituents to that medium This section provides:
An example strategy for characterizing releases to soils, which includes
characterization of the source and the environmental setting of the
release, and conducting a monitoring program that will characterize the
release.
Formats for data organization and presentation;
Field methods that may be used in the investigation; and
A checklist of information that may be needed for release
characterization.
The exact type and amount of information required for sufficient release
characterization will be site-specific and should be determined through interactions
between the regulatory agency and the facility owner or operator during the RFI
process. This guidance does not define the specific data needed in all instances;
however, it identifies possible information that might be necessary to perform
release characterizations and methods for obtaining this information. The RFI
Checklist, presented at the end of this section, provides a tool for planning and
tracking information for release characterization. This list is not meant to be a list
of requirements for all releases to soil. Some release investigations will involve the
collection of only a subset of the items listed, while others may involve the
collection of additional data.
9-1
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9.2 Approach for Characterizing Releases to Soil
9.2.1 General Approach
A preliminary task in any soil investigation should be to review existing site
information that might help to define the nature and magnitude of the release.
Information supplied by the regulatory agency in permit conditions or an
enforcement order will indicate known or suspected releases to soil from specific
units at the facility needing investigation; and may also indicate situations where
inter-media contaminant transfer should be investigated.
A conceptual model of the release should be formulated using all available
information on the waste, unit characteristics, environmental setting, and any
existing monitoring data. This model (not a computer or numerical simulation
model) should provide a working hypothesis of the release mechanism, transport
pathway/mechanism, and exposure route (if any). The model should be
testable/verifiable and flexible enough to be modified as new data become
available. For soil investigations, this model should account for the ability of the
waste to be dissolved by infiltrating precipitation, its affinity for soil particles (i.e.,
sorption), its degradability (biological and chemical), and its decomposition
products. Unit-specific factors affecting the magnitude and configuration of the
release should also be incorporated (e.g., large area releases from land treatment
versus more localized releases from small drum storage areas). The conceptual
model should also address the potential for transfer of contaminants in soil to other
environmental media (e.g., overland runoff to surface water, leaching to ground
water, and volatilization to the atmosphere).
Characterizing contaminant releases to soils may employ a phased approach.
Data collected during an initial phase can be evaluated to determine the need for or
scope of subsequent efforts. For example, if a suspected release was identified by
the regulatory agency, the initial monitoring effort may be geared to release
verification. Table 9-1 presents an example of a release characterization strategy.
The intensity and duration of the investigation will depend on the complexity of the
environmental setting and the nature and magnitude (e.g., spatial extent and
concentrations) of the release.
9-2
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TABLE 9-1
EXAMPLE STRATEGY FOR CHARACTERIZING RELEASES TO SOIL*
INITIAL PHASE
1- Collect and review existing information on:
Waste
Unit
Environmental setting
Releases, including inter-media transport
2. Identify additional information necessary to fully characterize release.
Waste
Unit
Environmental setting
Releases, including inter-media transport
3. Develop monitoring procedures:
Formulate conceptual model of release
Determine monitoring program objectives
Select constituents and indicators to be monitored
Plan initial sampling based on unit/waste/environmental setting
characteristics and conceptual model. May include field screening
methods, if appropriate.
Define study and background areas
Determine sampling methods, locations, depths and numbers
Sampling frequency
Analytical methods
QA/QC procedures
9-3
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TABLE 9-1 (continued)
EXAMPLE STRATEGY FOR CHARACTERIZING RELEASES TO SOIL*
4. Conduct initial monitoring phase:
Employ field screening methods, if appropriate
Conduct initial soil sampling and other appropriate field
measurements
Collect geologic data
Analyze samples for selected constituents and indicators
5. Collect, evaluate, and report results:
Compare monitoring results to health and environmental criteria and
identify and respond to emergency situations and identify priority
situations that may warrant interim corrective measures - Notify
regulatory agency
Evaluate potential for inter-media contaminant transfer
Summarize and present data in an appropriate format
Determine if monitoring program objectives were met (e. g.,
monitoring locations, constituents and frequency were adequate to
characterize release (nature, rate and extent)
Report results to regulatory agency
SUBSEQUENT PHASES (if necessary)
1. Identify additional information necessary to characterize release:
Determine need to expand or include further soil stratigraphic and
hydrologic sampling
Information needed to evaluate potential for inter-media
contaminant transfer (e.g., leaching studies to evaluate potential for
ground-water contamination)
2. Expand monitoring network as necessary:
Expand area of field screening, if appropriate
Expand sampling area and/or increase density
Add or delete constituents and parameters of concern
Increase or decrease monitoring frequency
9-4
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TABLE 9-1 (Continued)
EXAMPLE STRATEGY FOR CHARACTERIZING RELEASES TO SOIL*
3. Conduct subsequent monitoring phases:
Perform expanded monitoring and field analyses
Analyze samples for selected constituents and parameters
4. Collect, evaluate, and report results/identify additional Information
necessary to characterize release:
Compare results to health and environmental criteria and identify and
respond to emergency situations and identify priority situations that
warrant interim corrective measures - Notify regulatory agency
Summarize and present data in appropriate format
Determine if monitoring program objectives were met
Determine if monitoring locations, constituents, and frequency were
adequate to characterize release (nature, extent, and rate)
Determine need to expand monitoring system
Evaluate potential for inter-media contaminant transfer
Report results to regulatory agency, including results of inter-media
transfer evaluation, if applicable.
The possibility for inter-media transfer of contamination should be
anticipated throughout the investigation.
9-5
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The owner or operator should plan the Initial characterization effort with all
available information on the site, including wastes and soil characteristics. During
the initial phase, constituents of concern as well as indicator parameters should be
identified that can be used to characterize the release and determine the
approximate extent and rate of migration of the release. Table 9-2 lists tasks that
can be performed to characterize a release to soils and displays the associated
techniques and outputs from each of these tasks. Soil characteristics and other
environmental factors include 1) surface features such as topography, erosion
potential, land-use capability, and vegetation; 2) stratigraphic/hydrologic features
such as soil profile, particle size distribution, hydraulic conductivity, pH, porosity,
and cation exchange capacity; and 3) meteorological factors such as temperature,
precipitation, runoff, and evapotranspiration. Relevant soil physical and chemical
properties should be measured and related to waste properties to determine the
potential mobility of the contaminants in the soil.
As monitoring data become available, both within and at the conclusion of
discrete investigation phases, it should be reported to the regulatory agency as
directed. The regulatory agency will compare the monitoring data to applicable
health and environmental criteria to determine the need for (1) interim corrective
measures; and/or (2) a Corrective Measures Study. In addition, the regulatory
agency will evaluate the monitoring data with respect to adequacy and
completeness to determine the need for any additional monitoring efforts. The
health and environmental criteria and a general discussion of how the regulatory
agency will apply them are supplied in Section 8. A flow diagram illustrating RFI
decision points is provided in Section 3 (see Figure 3-2).
Notwithstanding the above process, the owner or operator has a continuing
responsibility to identify and respond to emergency situations and to define priority
situations that may warrant interim corrective measures. For such situations, the
owner or operator is directed to obtain and follow the RCRA Contingency Plan
requirements under 40 CFR Part 264, Subpart D, and Part 265, Subpart D.
As indicated above, depending on the results of the initial phase, the need for
further characterization will be determined by the regulatory agency. Subsequent
phases, if necessary, may involve expansion of the sampling network, changes in the
study area, investigation of contaminant transfer to other media, or other
9-6
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TABLE 9-2
RELEASE CHARACTERIZATION TASKS FOR SOILS
Investigatory Tasks
Investigatory Techniques
Data Presentation
Formats/Outputs
I Waste/Unit
Characterization
Refer to Sections 3 and 7
Table of monitoring
constituents and their
chemical/physical properties
Table of unit features
contributing to soil releases
2. Environmental Setting
Characterization
Determine surface
features and
topography
Characterize soil
stratigraphy and
hydrology
Aerial photography or
mapping (See Appendix A
Soil core examination
Measurement of soil
properties
Meteprological
Conditions
On-site meteorological
monitoring
- Soil survey map
Topographic map
Photographs
- Soil boring logs
- Soil profNe, transect, or
fence diagram
Particle size distribution
Table of unsaturated
hydraulic conductivities for
each soil layer
Table of soil chemistry and
structure (e.g., pH, porosity)
for each soil type
- Temperature charts
Tables of monthly and
annual precipitation,
runoff, and evapo-
transplration
3. Release Characterization
Field Screening
Sampling and Analysis
Soil Transport Modeling
Maps and tables showing
results of soil gas surveys
Tables and graphs showing
results of chemical analyses
performed in the field
Map of sampling points
Table of constituent
concentrations measured at
each sampling point
Area and profile maps of
site, shown distribution of
contaminants
Table of input values,
boundary conditions,
output values, and
modeling assumptions
Maps of resent or future
extent of contamination
9-7
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objectives dictated by the initial findings. The owner or operator may propose to
use mathematical models (e.g., chemical, physical) to aid in the choice of additional
sampling locations or to estimate contaminant mobility in soil. The results of all
characterization efforts should be organized and presented to the regulatory
agency in a format appropriate to the data.
Case Study Numbers 2, 3, 15, 16 and 17 in Volume IV (Case Study Examples)
illustrate various aspects of soil investigations.
9.2.2 Inter-media Transport
As mentioned above, the potential for inter-media transfer of releases from
the soil medium to other media is significant. Contaminated soil can be a major
source of contamination to ground water, air, subsurface gas and surface water.
Hazardous wastes or constituents, particularly those having a moderate to high
degree of mobility, can leach from the soil to the ground water. Volatile wastes or
constituents can contribute to subsurface gas and releases to air. Contaminated
soils can also contribute to surface water releases, especially through run-off during
heavy rains. Application of the universal soil loss equation (See Section 13.6) can
indicate whether inter-media transport from soil to surface water as a result of
erosion can act as a source of contamination. The owner or operator should
recognize the potential for inter-media transport of releases to soil and should
communicate as appropriate with the regulatory agency when such transport is
suspected or identified during the investigation.
Similarly, the potential for inter-media transport of constituents from other
media to the soil also exists. For example, hazardous waste or constituents may be
transported to the soil via atmospheric deposition (especially during rain or
snowfall events) through the air medium, and also through releases of subsurface
gas. The guidance provided in this section addresses characterization of releases to
soil from units and also can be used to characterize releases to soil as a result of
inter-media transport through other media. A key to such characterization is
determining the nature of the contaminant source, which is described in Section 9.3.
It is also important to recognize that where multiple media appear to be
contaminated, the investigation can be coordinated to provide results that can
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apply to more than one of the affected media. For example, soil-gas analysis (e.g.,
using a portable gas chromatography during the subsurface investigation) can be
used to investigate releases to soil and subsurface gas releases, and may also
provide information concerning the spatial extent of contaminated ground water
9.3 Characterization of the Contaminant Source and the Environmental
Setting
9.3.1 Waste Characterization
The physical and chemical properties of the waste or its constituents affect
their fate and transport in soil; and, therefore affect the selection of sampling and
analytical methods. Identification of monitoring constituents and the use of
indicator parameters is discussed in Section 3 and Appendix B. Sources of
information and sampling techniques for determining waste characteristics are
discussed in detail in Section 7.
Chemicals released to soil may undergo transformation or degradation by
chemical or biological mechanisms, may be adsorbed onto soil particles, or may
volatilize into soil pore spaces or into the air. Table 9-3 summarizes various physical,
chemical, and biological transformation/transport processes that may affect waste
and waste constituents in soil.
The chemical properties of the contaminants of concern also influence the
choice of sampling method. Important considerations include the water volubility
and volatility of the contaminants, and the potential hazards to equipment and
operators during sampling. For example, water soluble compounds that are mobile
in soil water may be detected by pore-water sampling and whole soil sampling.
Volatile organic contaminants require specialized sampling and sample storage
measures to prevent losses prior to analysis. Viscous substances require different
sampling techniques due to their physical properties.
Reactive, corrosive, or explosive wastes may pose a potential hazard to
personnel during soil sampling. High levels of organic contamination may also
cause health problems due to toxicity. For example, landfills can produce methane
gas that can explode if ignited by sparks or heat from the drilling operation,
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TABLE 9-3
TRANSFORMATION/TRANSPORT PROCESSES IN SOIL
Process
Biodegradation
Photodegradation
Hydrolysis
Oxidation/reduction
Volatilization
Adsorption
dissolution
Key Factor
Waste degradability
Waste toxicity
Acclimation of microbial community
Aerobic/anaerobic conditions
PH
Temperature
Nutrient concentrations
Solar irradiation
Exposed surface area
Functional group of chemical
Soil pH and buffering capacity
Temperature
Chemical class of contaminant
Presence of oxidizing agents
Partial pressure
Henry's Law Constant
Soil porosity
Temperature
Effective surface area of soil
Cation exchange capacity (CEC)
Fraction organic content (Foc) of soil
Octanol/water partition coefficient (Kow)
Solubility
Soil pH and buffering capacity
Complex formation
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Corrosive, reactive, or explosive wastes can also damage soil sampling equipment or
cause fires and explosions, Appropriate precautions to prevent such incidents
include having an adequate health and safety plan in place, using explosimeters or
organic vapor detectors as early-warning devices, and employing geophysical
techniques to help identify buried objects (e.g., to locate buried drums). All
contaminated soil samples should be handled as if they contain dangerous levels of
hazardous wastes or constituents.
identity and composition of contaminants-The owner or operator should
identify and provide approximate concentrations for any constituents of concern
found in the original waste and, if available, in leachate from any releasing unit.
Identification of other (non-hazardous) waste components that may affect the
behavior of hazardous constituents or may be used as indicator parameters is also
recommended. Such components may form a primary leachate causing transport
behavior different from water and may also mobilize hazardous constituents bound
to the soil. Estimations of transport behavior can help to focus the determination of
sampling locations.
Physical state of contaminants-The physical state (solid, liquid, or gas) of the
contaminants in the waste and soil should be determined by inspection or from site
operating records. Sampling can then be performed at locations most likely to
contain the contaminant.
Viscosity--The viscosity of any bulk liquid wastes should be determined to
estimate potential mobility in soils. A liquid with a lower viscosity will generally
travel faster than one of a higher viscosity.
pH -Bulk liquid pH may affect contaminant transport in at least two ways:
(1) it may alter the chemical form of acids and bases, metal salts, and other metal
complexes, thereby altering their water volubility and soil sorption properties, and
(2) it may alter the soil chemical or physical makeup, leading to changes in sorptive
capacity or permeability. For example, release of acidic (low pH) wastes in a karst
(e.g., limestone) environment can lead to the formation of solution channels. See
Section 10.3 for more information on karst formations.
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Dissociation constant (pKa) For compounds that are appreciably ionized
within the expected range of field pH values, the pKa of the compound should be
determined. Ionized compounds have either a positive or negative charge and are
often highly soluble in water; therefore, they are generally more mobile than in
their neutral forms when dissolved. Compounds that may ionize include organic
and inorganic acids and bases, phenols, metal salts, and other inorganic complexes.
Estimated contaminant concentration isopleths can be plotted with this
information and can be used in determining sampling locations.
Density --The density of major waste components should be determined,
especially for liquid wastes. Components with a density greater than water, such as
carbon tetrachloride, may migrate through soil layers more quickly than
components less dense than water, such as toluene, assuming viscosity to be
negligible. Density differences become more significant when contaminants reach
the saturated zone. Here they may sink, float, or be dissolved in the ground water.
Some fraction of a "sinker" or "floater" may also be dissolved in the ground water.
Water volubility-This chemical property influences constituent mobility and
sorption of chemicals to soil particle surfaces. Highly water-soluble compounds are
generally very mobile in soil and ground water. Liquid wastes that have low
volubility in water may form a distinct phase in the soil with flow behavior different
from that of water. Additional sampling locations may be needed to characterize
releases of insoluble species.
Henry's Law constant-This parameter indicates the partitioning ratio of a
chemical between air and water phases at equilibrium. The larger the value of a
constituent's Henry's Law Constant, the greater is the tendency of the constituent
to volatilize from water surrounding soil particles into soil pore spaces or into
above-ground air. The Henry's Law Constant should be considered in assessing the
potential for inter-media transport of constituents in soil gas to the air. Therefore,
this topic is also discussed in the Subsurface Gas and Air sections (Sections 11 and
12, respectively). Information on this parameter can help in determining which
phases to sample in the soil investigation.
Octanol/Water partition coefficient (K^J -The characteristic distribution of a
chemical between an aqueous phase and an organic phase (octanol) can be used to
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predict the sorption of organic chemicals onto soils. It is frequently expressed as a
logarithm (log Kow). In transport models, Kowis frequently converted to Koc, a
parameter that takes into account the organic content of the soil. The empirical
expression used to calculate Kocis: Koc= 0.63 Kowf0cwhere focis the fraction by
weight of organic carbon in the soil. The higher the value of Kow (or Koc) the
greater the tendency of a constituent to adsorb to soils containing appreciable
organic carbon. Consideration of this parameter will also help in determining which
phases to sample in the soil investigation.
Biodegradability --There is a wide variety of microorganisms that may be
present in the soil, Generally, soils that have significant amounts of organic matter
will contain a higher microbial population, both in density and in diversity.
Microorganisms are responsible for the decay and/or transformation of organic
materials and thrive mostly in the "A" (uppermost) soil horizon where carbon
content is generally highest and where aerobic digestion occurs. Because some
contaminants can serve as organic nutrient sources that soil microorganisms will
digest as food, these contaminants will be profoundly affected within organic soils.
Digestion may lead to complete decomposition, yielding carbon dioxide and water,
but more often results in partial decomposition and transformation into other
substances. Transformation products will likely have different physical, chemical or
toxicological characteristics than the original contaminants, These products may
also be hazardous constituents (some with higher toxicities) and should therefore
be considered in developing monitoring programs. The decomposition or
degradation rate depends on various factors, including:
t The molecular structure of the contaminants. Certain manmade
compounds (e.g., PCBs and chlorinated pesticides) are relatively
nondegradable (or persistent), whereas others (e.g., methyl alcohol) are
rapidly consumed by bacteria. The owner or operator should consult
published lists of compound degradability, such as Table 9-4, to estimate
the persistence of waste constituents in soil. This table provides relative
degradabilities for some organic compounds and can be an aid to
identifying appropriate monitoring constituents and indicator
parameters. It may be especially useful for older releases where
degradation may be a significant factor. For example,, some of the
parent compounds that are relatively degradable (see Table 9-4) may
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TABLE 9-4. BODs/COD RATIOS FOR VARIOUS ORGANIC COMPOUNDS*
Compound
RELATIVELY UNDEGRADABLE
Butane
Butylene
Carbon tetrachloride
Chloroform
1,4-Dioxane
Ethane
Heptane
Hexane
Isobutane
Isobutylene
Liquefied natural gas
Liquefied petroleum gas
Methane
Methyl bromide
Methyl chloride
Monochlorodifluoromethane
Nitrobenzene
Propane
Propylene
Propylene oxide
Tetrachloroethylene
Tetrahydronaphthalene
1 Pentrene
Ethylene dichloride
1 Octene
Morpholine
Ethylenediaminetetracetic acid
Triethanolamine
o-Xylene
m-Xylene
Ethyl benzene
MODERATELY DEGRADABLE
Ethyl ether
sodium alkylbenzenesulfonat.es
Monoisopropanol amine
Gas oil (cracked)
Gasolines (various)
Ratio
~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
Compound
MODERATELY DEGRADABLE
(CONT'D)
Mineral spirits
Cyclohexanol
Acrylonitrile
Nonanol
Undecanol
Methylethylpyridine
1-Hexene
Methyl isobutyl ketone
Diethanolamine
Formic acid
Styrene
Heptanol
sec-Butyl acetate
n-Butyl acetate
Methyl alcohol
Acetonitrile
Ethylene glycol
Ethylene glycol monoethyl ether
Sodium cyanide
Linear alcohols (12-1 5 carbons)
Allyl alcohol
Dodecanol
RELATIVELY DEGRADABLE
Valeraldehyde
n-Decyl alcohol
p-Xylene
Urea
Toluene
Potassium cyanide
Isopropyl acetate
Amyl acetate
Chlorobenzene
Jet fuels (various)
Kerosene
Range oil
Glycerine
Adiponitrile
Ratio
-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
0.12
<0.13
0.13-0.34
0.15
-0.15
-0.15
-0.15
<0.16
0.17
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TABLE 9-4. (Continued)
Compound
RELATIVELY DEGRADABLE
(CONT'D.)
Furfural
2-Ethyl-3-propylacrolein
Methylethylpyridine
Vinyl acetate
Diethylene glycol monomethyl
ether
Napthalene (molten)
Dibutyl phthalate
Hexanol
Soybean oil
Paraformaldehyde
n-Propyl alcohol
Methyl methacrylate
Acrylic acid
Sodium alkyl sulfates
Triethylene glycol
Acetic acid
Acetic anhydride
Ethylenediamine
Formaldehyde solution
Ethyl acetate
Octanol
Sorbitol
Benzene
n-Butyl alcohol
Propionaldehyde
n-Butyraldehyde
Ratio
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
<0.24
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
Compound
RELATIVELY DEGRADABLE
(CONT'D.)
Ethyleneimine
Monoethanolamine
Pyridine
Dimethylformamide
Dextrose solution
Corn syrup
Maleic anhydride
Propionic acid
Acetone
Aniline
Isopropyl alcohol
n-Amyl alcohol
Isoamyl alcohol
Cresols
Crotonaldehyde
Phthalic anhydride
Benzaldehyde
Isobutyl alcohol
2,4-Dichlorophenol
Tallow
Phenol
Benzole acid
Carbolic acid
Methyl ethyl ketone
Benzoyl chloride
Hydrazine
Oxalic acid
Ratio
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
*Source: U.S. EPA 1985. Handbook: Remedial Action at Waste Disposal Sites (Revised).
EPA/625/6-85/006. NTIS PB82-239054. Office of Emergency and Remedial Response.
Washington, D.C. 20460.
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have been reduced to carbon dioxide and water or other decomposition
products prior to sampling. Additional information on degradability can
be found in Elliott and Stevenson, 1977; Sims et al, 1984; and U.S. EPA,
1985. See Section 9.8 for complete citations for these references.
Moisture content. Active biodegradation does not generally occur in
relatively dry soils or in some types of saturated soils, such as those that
are saturated for long periods of time, as in a bog.
The presence or absence of oxygen in the soil. Most degradable
chemicals decompose more rapidly in aerobic (oxygenated) soil.
Although unsaturated surficial soils are generally aerobic, anaerobic
conditions may exist under landfills or other units. Soils that are
generally saturated year round are relatively anaerobic (e.g., as in a bog);
however, most saturated soils contain enough oxygen to support active
biodegradation. Anaerobic biodegradation, however, can also be
significant in some cases. For example, DDT degrades more rapidly under
anaerobic conditions than under aerobic conditions.
Microbial adaptation or acclimation. Biodegradation depends on the
presence in the soil of organisms capable of metabolizing the waste
constituents. The large and varied population of microorganisms in soil
is likely to have some potential for favorable growth using organic
wastes and constituents as nutrients. However, active metabolism
usually requires a period of adaptation or acclimation that can range
from several hours to several weeks or months, depending on the
constituent or waste properties and the microorganisms involved.
The availability of contaminants to micro-organisms. Releases that occur
below the upper 6 to 8 inches of soil are less likely to be affected because
fewer micro-organisms exist there. In addition, compounds with greater
aqueous solubilities are generally more available for degradation.
However, high volubility also correlates directly to the degree of
mobility. If relatively permeable soil conditions prevail and constituents
migrate rapidly, they are less likely to be retained long enough in the soil
for biodegradation to occur.
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Other factors. Activity of organisms is also dependent on favorable
temperature and pH conditions as well as the availability of other
organic and inorganic nutrients for metabolism.
Rates of Hydrolysis, Photolysis, and Oxidation-Chemical and physical
transformation of the waste can also affect the identity, amounts, and transport
behavior of the waste constituents. Photolysis is important primarily for chemicals
on the land surface, whereas hydrolysis and oxidation can occur at various depths.
Published literature sources should be consulted to determine whether individual
constituents are likely to degraded by these processes, but it should be recognized
that most literature values refer to aqueous systems. Relevant references include
Elliott and Stevenson, 1977; Sims et al, 1984; and U.S. EPA, 1985. Chemical and
physical degradation will also be affected by soil characteristics such as pH, water
content, and soil type.
9.3.2 Unit Characterization
Unit-related factors that may be important in characterizing a release include:
Unit design and operating characteristics;
Release type (point-source or nonpoint-source);
Depth of the release;
Magnitude of the release; and
Timing of the release.
9.3.2.1 Unit Design and Operating Characteristics
Information on design and operating characteristics of a unit can be helpful in
characterizing a release. Table 9-5 presents important mechanisms of contaminant
release to soils for various unit types. This information can be used to identify areas
for initial soil monitoring.
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TABLE 9-5
POTENTIAL RELEASE MECHANISMS FOR VARIOUS UNIT TYPES
Unit Type
Release Mechanisms
Surface Impoundment
Loading/unloading areas
Releases from overtopping
Seepage
Landfill
Migration of releases outside the unit's runoff collection
and containment system
Migration of releases outside the containment area from
loading and unloading operations
Leakage through dikes or unlined portions to surrounding
soils
Waste Pile
Migration of runoff outside the unit's runoff collection and
containment system
Migration of releases outside the containment area from
loading and unloading operations.
Seepage through underlying soils
Land Treatment Unit
Migration of runoff outside the containment area
Passage of leachate into the soil horizon
Container Storage Area
Migration of runoff outside the containment area
Loading/unloading areas
Leaking drums
Above-ground or
In-ground Tank
Releases from overflow
Leaks through tank shell
Leakage from coupling/uncoupling operations
Leakage from cracked or corroded tanks
Incinerator
Routine releases from waste handling/preparation activities
Leakage due to mechanical failure
Class I and IV Injection
Wells
Leakage from waste handling operations at the well head
Waste transfer Stations and waste recycling operations generally have mechanisms of
release similar to tanks.
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9.3.2.2 Release Type (Point or Non-Point Source)
The owner or operator should establish whether the release involved a
localized (point) source or a non-point source. Units that are likely sources of
localized releases to soil include container handling and storage areas, tanks, waste
piles, and bulk chemical transfer areas (e.g., loading docks, pipelines, and staging
areas). Non-point sources may include airborne particulate contamination
originating from a land treatment unit and widespread leachate seeps from a
landfill. Land treatment can also result in widespread releases beyond the
treatment zone if such units are not properly designed and operated; refer to EPA's
Permit Guidance Manual on Hazardous Waste Land Treatment Demonstration, July,
1986 (NTIS PB86-229192) for additional information on determining contamination
from land treatment units. This manual also discusses use of the RITZ model
(Regulatory and Investigation Treatment Zone Model), which may be particularly
useful for evaluating mobility and degradation within the treatment zone. This
model is discussed in more detail in Section 9.4.4.2.
The primary characteristic of a localized release is generally a limited area of
relatively high contaminant concentration surrounded by larger areas of relatively
clean soil. Therefore, the release characterization should focus on determining the
boundaries of the contaminated area to minimize the analysis of numerous
uncontaminated samples. Where appropriate, a survey of the area with an organic
vapor analyzer, portable gas chromatography, surface geophysical instruments (see
Appendix C), or other rapid screening techniques may aid in narrowing the area
under investigation. Stained soil and stressed vegetation may provide additional
indications of contamination. However, even if the extent of contamination
appears to be obvious, it is the responsibility of the owner or operator to verify
boundaries of the contamination by analysis of samples both inside and outside of
the contaminated area.
Non-point type releases to soil may also result from deposition of particulate
carried in the air, such as from incinerator "fallout". Such releases generally have a
characteristic distribution with concentrations often decreasing logarithmically
away from the source and generally having low variability within a small area. The
highest contaminant concentrations tend to follow the prevailing wind directions
(See also Section 12 on Air). Non-point releases occurring via other mechanisms
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(e-g., land treatment) may be distributed more evenly over the affected area. In
these situations, a large area may need to reinvestigated in order to determine the
extent of contamination. However, the relative lack of "hots pots" may allow the
number of samples per unit area to be smaller than for a point source type release.
9.3.2.3 Depth of the Release
The owner or operator should consider the original depth of the release to soil
and the depth to which contamination may have migrated since the release. Often,
releases occur at the soil surface as a result of spillage or leakage. Releases directly
to the subsurface can occur from leaking underground tanks, buried pipelines,
waste piles, impoundments, landfills, etc.
Differentiating between deep and shallow soil or surficial soil can be
important in sampling and in determining potential impacts of contaminated soil.
Different methods to characterize releases within deep and surficial soils may be
used. For example, sampling of surficial soil may involve the use of shovels or hand-
driven coring equipment, whereas deep-soil contamination usually requires the use
of power-driven equipment (see Section 9.6 for more information). In addition,
deep-soil and surficial-soil contamination may be evaluated differently in the health
and environmental assessment process discussed in Section 8. Assessment of
surficial-soil contamination will involve assessing risk from potential ingestion of
the contaminated soil as well as assessing potential impacts to ground water. The
assessment of deep-soil contamination may be limited to determining the potential
for the soil to act as a continuing source of potential contamination to ground
water.
For purposes of the RFI, surficial or shallow-zone soils may be defined as those
comprising the upper 2 feet of earth, although specific sites may exhibit surficial soil
extending to depths of up to 12 feet or more. Considerations for determining the
depth of the shallow-soil zone may include:
Meteorological conditions (e.g., precipitation, erosion due to high winds,
evaporation of soil-pore gases);
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Potential for excessive surface runoff, especially if runoff would result in
gully formation;
Transpiration, particularly from the root zone, and effects on vegetation
and animals, including livestock, that may feed on the vegetation; and
Land use, including potential for excavation/construction, use of the soil
for fill material, installation of utilities (e.g., sewer lines or electrical
cables), and farming activities.
Land use that involves housing developments is an example of when the
surficial soil depth may extend to 12 feet because foundation excavation may result
in deep contaminated soils being moved to the surface. Deep-soil zones, for
purposes of the RFI, may be defined as those extending from 2 feet below the land
surface to the ground-water surface, if deep-soil contamination is already affecting
ground water (through inter-media transport) at a specific site, consideration
should be given to evaluating the potential for such contamination to act as a
continuing source of ground-water contamination.
The depth to which a release may migrate depends on many factors, including
volume of waste released, amount of water infiltrating the soil, age of the release,
and chemical and physical properties of the waste and soil (as addressed in the
previous section), in a porous, homogeneous soil, contaminants tend to move
primarily downward within the unsaturated zone. Lateral movement generally
occurs only through dispersion and diffusion. However, changes in soil structure or
composition with depth (e. g., stratification), and the presence of zones of
seasonally saturated soil, fractures, and other features may cause contaminants to
spread horizontally for some distance before migrating downward. Careful
examination of soil cores and accurate measurement of physical properties and
moisture content of soil are therefore essential in estimating the potential for
contaminant transport.
Transport of chemicals in the soil is largely caused by diffusion and mass flow.
Diffusion results from random thermal motion of molecules. Mass flow, also known
as convective flow, is transport by a flowing liquid or by a gaseous phase. Mass flow
is typically downward (due to gravity); however, mass flow could also be upward
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due to capillary action (e.g., if significant evaporation occurs at the surface). Mass
flow is a much faster transport mechanism than is diffusion (Merrill et al., 1985).
Other factors that can promote downward contaminant migration include
turnover of soil by burrowing animals, freeze/thaw cycles, and plowing or other
human activities. All factors that may affect the depth of contamination should be
considered. The owner or operator should use available information to estimate
the depth of contamination and should then conduct sampling at appropriate
depths to confirm these estimates.
Approaches to monitoring releases to soil will differ substantially depending
on the depth of contamination. For investigations of both surficial and deep-soil
contamination, a phased approach may be used. Initial characterization will often
necessitate a judgmental approach in which sampling depths are chosen based on
available information (e.g., topography, soil stratigraphy, and visual indication of a
release). Information derived from this initial phase can then be used to refine
estimates of contaminant distribution and transport. This information will serve as
a basis for any subsequent monitoring that may be necessary.
Where the source or precise location of a suspected release has not been
clearly identified, field screening methods (See Section 9.6) may be appropriate.
Subsurface contamination can be detected by using geophysical methods or soil gas
surveying equipment (e.g., organic vapor analyzers). Geophysical methods, for
example, can help in locating buried drums. Soil gas surveys can be useful in
estimating the lateral and vertical extent of soil contamination. Further delineation
of the vertical extent of contamination may necessitate an additional effort such as
core sampling and analysis. Sampling approaches for locating and delineating
subsurface contaminant sources include systematic and random grid sampling.
These approaches are discussed in Section 3. Geophysical methods are discussed in
Section 10 (Ground Water) and in Appendix C (Geophysical Techniques).
9.3.2.4 Magnitude of the Release
information on the magnitude of the release can be estimated from site
operating records, unit design features, and other sources. The quantity (mass) of
waste released to soil and the rate of release can affect the geographical extent and
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nature of the contamination. Each soil type has a specific sorptive capacity to bind
contaminants. If the sorptive capacity is exceeded, contaminants tend to migrate
through the soil toward the ground water. Therefore, a " minor" release may be, at
least temporarily, immobilized in shallow soils, whereas a "major" release is more
likely to result in ground-water contamination. The physical processes of
volatilization and dissolution in water are also affected by contaminant
concentrations and should, therefore, be considered in assessing the potential for
inter-media transport. Section 9.4.4.3 provides additional guidance on estimating
the mobility of constituents within contaminated soils.
9.3.2.5 Timing of the Release
Time-related factors that should be considered in characterizing a release
include:
Age of the release;
Duration of the release;
Frequency of the release; and
Season (time of year).
The length of time that has passed since a release occurred can affect the
extent of contamination, the chemical composition of the contaminants present in
soil, and the potential for inter-media transport. Recent releases tend to be more
similar in composition to the parent waste material and may also be more
concentrated within the original boundaries of the release. If a recent release
occurred at the land surface, contaminant volatilization to air or dissolution in
overland runoff may be important transport mechanisms. Older releases are more
likely to have undergone extensive chemical or biological changes that altered their
original composition and may have migrated a considerable distance from their
original location. If the contaminants are relatively mobile in soil, transport to
ground water may be a concern; whereas soil-bound contaminants may be more
likely affected by surface transport, such as overland runoff or wind action. These
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factors should be considered in the selection of monitoring constituents and
sampling locations.
The duration and frequency of the release can affect the amounts of waste
released to the soil and its distribution in the soil. For example, a release that
consisted of a single episode, such as a ruptured tank, may move as a discrete "slug"
of contamination through the soil. On the other hand, intermittent or continuous
releases may present a situation in which contaminants exist at different distances
from the source and/or have undergone considerable chemical and biological
decomposition. Therefore, the design of monitoring procedures and estimations of
contaminant fate and transport should consider release duration and frequency.
The time of year or season may also affect release fate and transport. Volatile
constituents are more likely to be released to the air or to migrate as subsurface gas
during the warmer summer months. During the colder winter months, releases may
be less mobile, especially if freezing occurs.
9.3.3 Characterization of the Environmental Setting
The nature and extent of contamination is affected by environmental
processes such as dispersion and degradation acting after the release has occurred.
Factors which should be considered include soil physical and chemical properties,
subsurface geology and hydrology, and climatic or meteorologic patterns. These
factors are discussed below.
Characteristics of the soil medium which should be considered in order to
obtain representative samples for chemical or physical analysis include:
The potentially large spatial variability of soil properties and
contaminant distribution;
Spatial and temporal fluctuations in soil moisture content; and
The presence of solid, liquid, and gaseous phases in the unsaturated
zone.
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9.3.3.1 Spatial Variability
Spatial variability, or heterogeneity, can be defined as horizontal and vertical
differences in soil properties occurring within the scale of the area under
consideration. Vertical discontinuities are found in most soil profiles as a result of
climatic changes during soil formation, alterations in topography or vegetative
cover, etc. Soil layers show wide differences in their tendency to sorb contaminants
or to transmit contaminants in a liquid form; therefore, a monitoring program that
fails to consider vertical stratification will likely result in an inaccurate assessment of
contaminant distribution. Variability in soil properties may also occur in the
horizontal plane as a result of factors such as drainage, slope, land use history, and
plant cover.
Soil and site maps will aid in designing sampling procedures by identifying
drainage patterns, areas of high or low surface permeability, and areas susceptible
to wind erosion and contaminant volatilization. Maps of unconsolidated deposits
may be prepared from existing soil core information, well drilling logs, or from
previous geological studies. Alternately, the information can be obtained from new
soil borings. Because soil coring can be a resource-intensive activity, it is generally
more efficient to also obtain samples from these cores for preliminary chemical
analyses and to conduct such activity concurrent with investigation of releases to
other media (e.g., ground water).
The number of cores necessary to characterize site soils depends on the site's
geological complexity and size, the potential areal extent of the release, and the
importance of defining small-scale discontinuities in surficial materials. Another
consideration is the potential risk of spreading the contamination as a result of the
sampling effort. For example, an improperly installed well casing could lead to
leakage of contaminated water through a formerly low permeability clay layer. The
risks of disturbing the subsurface should be considered when determining the need
for obtaining more data.
Chemical and physical measurements should be made for each distinct soil
layer, or boundary between layers, that may be affected by a release. During
drilling, the investigator should note on the drilling log the depths of soil horizons,
soil types and textures, and the presence of joints, channels, and zones containing
9-25
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plant roots or animal burrows. Soil variability, if apparent, should generally be
accounted for by increasing the number of sample points for measurement of soil
chemical and physical properties. Determination of the range and variability of
values for soil properties and parameters will allow more accurate prediction of the
mobility of contaminants in the soil.
9.3.3.2 Spatial and Temporal Fluctuations in Soil Moisture Content
As described earlier in this section, there are several mechanisms for transport
of waste constituents in the soil. Release migration can be increased by the physical
disturbance of the soil during freeze/thaw cycles or by burrowing animals.
Movement can also be influenced by microbial-induced transformations. In
addition, movement can occur through diffusion and mass flow of gases and liquids.
Although all of these mechanisms exist, movement of hazardous waste or
constituents through soil toward ground water occurs primarily by aqueous
transport of dissolved chemicals in soil pore water. Soil moisture content affects the
hydraulic conductivity of the soil and the transport of dissolved wastes through the
unsaturated zone. Therefore, characterizing the storage and flow of water in the
unsaturated zone is very important. Moisture in the unsaturated zone is in a
dynamic state and is constantly acted upon by competing physical forces.
Water applied to the soil surface (primarily through precipitation) infiltrates
downward under the influence of gravity until the soil moisture content reaches
equilibrium with capillary forces. A zone of saturation ( or wetting front) may occur
beneath the bottom of a unit (e.g., an unlined lagoon) if the unit is providing a
constant source of moisture. In a low porosity soil, such a saturation front may
migrate downward through the unsaturated zone to the water table, and create a
ground-water or liquid "mound" (see Figure 9-1). In a higher porosity soil, the
saturation front may only extend a small distance below the unit, with liquid below
this distance then moving through the soil under unsaturated conditions toward
ground water (see Figure 9-1). In many cases, this area will remain partially
saturated until the capillary fringe area is reached. The capillary fringe can be
defined as the zone immediately above the water table where the pressure is less
than atmospheric and where water and other liquids are held within the pore
spaces against the force of gravity by interracial forces (attractive forces between
different molecules).
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HAZARDOUS WASTE DISPOSAL IMPOUNDMENT
UNSATURATED
ZONE I
SATURATION FRONT
ISOLATED SATURATION LAYER
NJ
SEEPAGE
PATH
I
I i
CLAY LENS
MOUNDING /S[[[^ CAPILLARY
FRINGE
WATER TABLE
SATURATED ZONE
-------
In certain cases, soil moisture characterization can also be affected by the
presence of isolated zones of saturation and fluctuations in the depth to ground
water, as illustrated in Figure 9-1. Where there is evidence of migration below the
soil surface, these factors should be considered in the investigation by careful
characterization of subsurface geology and measurement of hydraulic conductivity
in each layer of soil that could be affected by subsurface contamination.
9.3.3.3 Solid, Liquid, and Gaseous Materials in the Unsaturated Zone
Soil in the unsaturated zone generally contains solid, liquid, and gaseous
phases. Depending upon the physical and chemical properties of the waste or its
constituents, contaminants of concern may be bound to the soil, dissolved in the
pore water, as a vapor within the soil pores or interstitial spaces, or as a distinct
liquid phase. The investigation should therefore take into consideration the
predominant form of the contaminant in the soil. For example, some whole-soil
sampling methods may lead to losses of volatile chemicals, whereas analysis of soil-
pore water may not be able to detect low volubility compounds such as PCBs that
remain primarily adsorbed in the solid phase. Release characterization procedures
should consider chemical and physical properties of both the soil and the waste
constituents to assist in determining the nature and extent of contamination.
Soil classification--The owner or operator should classify each soil layer
potentially affected by the release. One or more of the classification systems
discussed below should be used, based on the objectives of the investigation.
USDA Soil Classification System (USDA, 1975)-Primarily developed for
agricultural purposes, the USDA system also provides information on
typical soil profiles (e.g., l-foot fine sandy loam over gravelly sand, depth
to bedrock 12 feet), ranges of permeabilities for each layer, and
approximate particle size ranges. These values are not generally accurate
enough for predictive purposes, however, and should not be used to
replace information collected on site. Existing information on regional
soil types is available but suitable for initial planning purposes only. U.S.
Department of Agriculture (USDA) county soil surveys may be obtained
for most areas.
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Unified Soil Classification Systems (USCS) (Lambe and Whitman, 1979) -A
procedure for qualitative field classification of soils according to ASTM
D2487-69, this system should be used to identify materials in soil boring
logs. The USCS is based on field determination of the percentages of
gravel, sand and fines in the soil, and on the plasticity and compressibility
of fine-g rained soils. Figure 9-2 displays the decision matrix used in
classifying soils by this system.
The above classification systems are adequate for descriptive purposes and for
qualitative estimates of the fluid transport properties of soil layers. Quantitative
estimation of fluid transport properties of soil layers requires determination of the
particle size distribution for each soil layer, as described below.
Particle size distribution--A measurement of particle size distribution should
be made for each layer of soil potentially affected by the release. The
recommended method for measurement of particle size distribution is a
sieve/hydrometer analysis according to ASTM D422 (ASTM, 1984).
The particle size distribution has two major uses in a soils investigation: (1)
estimation of the hydraulic conductivity of the soil by use of the Hazen (or similar)
formula, and (2) assessment of soil sorptive capacity.
1. The hydraulic conductivity(K) may be estimated from the particle size
distribution using the Hazen formula:
K = A (dj2
where d10is equal to the effective grain size, which is that grain-size
diameter at which 10 percent by weight of the particles are finer and
90 percent are coarser (Freeze and Cherry, 1979). The coefficient A is
equal to 1.0 when K is in units of cm/sec and d10is in mm. Results should
be verified with in-situ hydraulic conductivity techniques.
2. Particle size can affect sorptive capacity and, therefore, the potential for
retardation of contaminants in the soil. Sandy soils generally have a low
sorptive capacity whereas clays generally have a high affinity for heavy
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Figure 9-2. Soil Terms
UNIFIED SOIL CLASSIFICATION ( USCS )
COARSE GRAINED SOILS
More lhan half ol material Is LARGER than No 200 sieve size
FIELD IDENTIFICATION PROCEDURES
(Excluding particles large' lhan 3" & basing tractions
on estimated weights)
GRAVELS
50*O)> 1/4* »
«
*r
*
CLEAN
GRAVE-LS
Low %
tirws
3«*
>5£»
!S«
G =; *s
z £S!
< "-.?£
w » I"~
Wide range in grain size and substantial
amounts ol all Intermediate particle sizes
Predominantly one size or a range ol sizes
with some intermediate sizes missing
Non-plastic lines (lor Identification
procedures see ML)
Plastic lines (lor Identification procedures
see CL)
Wide range In grain size and substantial
amounts of all Intermediate panicle sizes
Predominantly one size or a range of sizes
with some intermediate sizes missing
Non-plastic lines (for identification
procedures see ML)
Plastic lines (tor Idenlllicallon procedures
see CL)
GROUP
SYM -
BOLS
GW
GP
GM
GC
SW
SP
SM
SC
TYPICAL NAMES
Well graded gravels, gravel-sand mixtures.
little or no fines
Poorly graded gravels, gravel-sand
mixtures, little or no fines
Silly gravels, poorly graded gravel-sand-
slll mixtures
Clayey gravels, poorly graded gravel-
sand-clay mixtures
Well graded sand, gravelly sands, lillle or
no lines
Poorly graded sands, gravelly sands, little
or no fines
Sllty sands, poorly graded land-sill
mixtures
Clayey sands, poorly graded sand-clay
mixtures
FINE GRAINED SOILS
More lhan hall ol malarial Is SMALLER than No. 200 sieve size
FIELD IDENTIFICATION PROCEDURES
(Excluding particles larger than 3" & basing fractions
on estimated weights)
Idenlillcallon procedures on fraction smallor than No. 40 stove size
to o
5 v
S !
I \
-------
metals and some organic contaminants. This is due in part to the fact
that small clay particles have a larger surface area in relation to their
volume than do larger sand particles. This larger surface area can result
in stronger interactions with waste molecules. Clays may also bind
contaminants due to the chemical structure of the clay matrix.
Porosity-Soil porosity is the percentage of the total soil volume not occupied
by solid particles (i.e., the volume of the voids). In general, the greater the porosity,
the more readily fluids may flow through the soil. An exception is clayey soils that
tightly hold fluids by capillary forces. Porosity is usually measured by oven-drying an
undisturbed sample and weighing it. It is then saturated with liquid and weighed
again. Finally, the saturated sample is immersed in the same liquid, and the weight
of the displaced liquid is measured. Porosity is the weight of liquid required to
saturate the sample divided by the weight of liquid displaced, expressed as a
decimal fraction.
Hydraulic conductivitv--An essential physical property affecting contaminant
mobility in soil is hydraulic conductivity. This property indicates the ease with which
water at the prevailing viscosity will flow through the soil and is dependent on the
porosity of the soil, grain size, degree of consolidation and cementation, and other
soil factors.
Measurement of hydraulic conductivity in soil within the saturated zone is
fairly routine. Field and laboratory methods to determine saturated conductivity
are discussed in the section on ground-water investigations (Section 10).
Measurement of unsaturated conductivity is usually more difficult because the
value changes with changing soil moisture content. Therefore, conductivities for a
range of moisture contents may need to be determined for each type of soil at the
facility.
Techniques for determining saturated hydraulic conductivity are provided in
Method 9100 (Saturated Hydraulic Conductivity, Saturated Leach ate Conductivity,
and Intrinsic Permeability) from SW-846, Test Methods for Evaluating Solid Waste.
EPA. 3rd edition. September, 1986. Method 9100 includes techniques for:
Laboratory
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constant head methods; and
falling head methods.
Field
sample collection;
well construction;
well development;
single well tests (slug tests); and
references for multiple well (pumping tests).
A detailed discussion of field and laboratory methods for determining
saturated and unsaturated hydraulic conductivity is also contained in Soil Properties
Classification and Hydraulic Conductivity Testing (U.S. EPA, 1984). In general, field
tests are recommended when the soil is heterogeneous, while laboratory tests may
suffice for a soil without significant strati graphic changes. Estimation of hydraulic
conductivity from the particle size distribution may be used as a rough estimate for
comparison purposes and if precise values are not needed.
Relative permeability--The hydraulic conductivity of a soil is usually established
using water as the infiltrating liquid. However, at sites where there is the likelihood
of a highly contaminated leachate or a separate liquid waste phase, the owner or
operator should also consider determining conductivity with that liquid. The ratio
of the permeability of a soil to a non-aqueous solution and its permeability to water
is known as relative permeability.
The importance of determining this value is due to the potential effects of
leachate on soil hydraulic properties. Changes in conductivity from infiltration of
leachate may result from differences in the viscosity or surface tension of the waste,
or the leachate may affect the soil structure so as to alter its permeability. For
example, studies of waste migration through landfill liners made of clay have
demonstrated that certain wastes may cause shrinking or expansion of the clay
molecular structures, dissolve clays and organic matter, clog soil pores with fine
particles, and cause other changes that affect permeability.
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Soil sorptive capacity and soil-water partition coefficient (Kd)--The mobility of
contaminants in soil depends not only on properties related to the physical structure
of the soil, but also on the extent to which the soil material will retain, or adsorb,
the hazardous constituents. The extent to which a constituent is adsorbed depends
on chemical properties of the constituent and of the soil. Therefore, the sorptive
capacity must be determined with reference to particular constituent and soil pairs.
The soil-water partition coefficient (Kd) is generally used to quantify soil sorption.
Kd is the ratio of the adsorbed contaminant concentration to the dissolved
concentration, at equilibrium.
There are two basic approaches to determining Kd: (1) soil adsorption
laboratory tests, and (2) prediction from soil and constituent properties. The Soil
Adsorption Isotherm (Al) test is widely used to estimate the extent of adsorption of
a chemical (i. e., constituent) in soil systems. Adsorption is measured by
equilibrating aqueous solutions containing varying concentrations of the test
chemical with a known quantity of uncontaminated soil. After equilibrium is
reached, the distribution of the chemical between the soil and water (Kd) is
measured by a suitable analytical method.
The Al test has several desirable features. Adsorption results are highly
reproducible. The test provides excellent quantitative data that are readily
amenable to statistical analysis. In addition, it has relatively modest reagent, soils,
laboratory space and equipment requirements. The ease of performing this test will
depend on the physical/chemical properties of the contaminant and the availability
of suitable analytical techniques to measure the chemical.
The Al test can be used to determine the soil adsorption potential of slightly
water soluble to infinitely water soluble chemicals. In general, a chemical having a
water volubility of less than 0.5 mg/l is not tested with this method because these
chemicals are relatively immobile in soil. The U.S. EPA Office of Pesticides and Toxic
Substances (U.S. EPA 1982a, 1982b) has compiled information on the use of the Al
test, including a detailed discussion of apparatus, procedures, sources of error,
statistical requirements, calculation methods, and limitations of the test.
A second approach for determining Kd is to estimate the value from soil and
waste properties. Soil properties that should be considered when using this
9-33
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approach are particle size distribution, cation exchange capacity, and soil organic
carbon content. The waste properties that should be determined will vary
depending on the type of waste. Lyman et al. (1981) discuss several methods for
estimating Kdfrom chemical properties of the constituent (e.g., Kowand water
volubility) and the soil organic content. Data collection needs for waste properties
were discussed earlier in this section.
Cation exchange capacity (CEC)-This parameter represents the extent to
which the clay and humic fractions of the soil will retain charged species such as
metal ions. The CEC is an important factor in evaluating transport of lead,
cadmium, and other toxic metals. Soils with a high CEC will retain correspondingly
high levels of these inorganic. Although hazardous constituents may be
immobilized by such soils in the short-term, such conditions do not rule out the
possibility of future releases given certain conditions (e.g., action of additional
releases of low pH). A method for the determination of CEC is detailed in SW-846,
Method 9081 (U.S. EPA, 1986).
Organic carbon content-The amount of natural organic material in a soil can
have a strong effect on retention of organic pollutants. The greater the fraction by
weight of organic carbon (Foe), the greater the adsorption of organics. Soil Foc
ranges from under 2 percent for many subsurface soils to over 20 percent for a peat
soil. An estimate of Focshould be made based on literature values for similar soils if
site-specific information is not available.
Soil pH-Soil pH affects the mobility of potentially ionized organic and
inorganic chemicals in the soil. Compounds in these groups include organic and
inorganic acids and bases, and metals.
Depth to ground water -The thickness of the unsaturated zone may affect the
attenuation capacity of the soil and the time taken for contaminants to migrate to
ground water. If significant, seasonal fluctuations in ground-water elevations
should be identified as well as elevation changes due to pumping or other factors
(e.g., tidal influences).
Pore-water velocity-Pore water velocity affects the time of travel of
contaminants in unsaturated soil to ground water. For steady state flow and a unit
9-34
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hydraulic gradient (i.e., moisture content does not change with depth), the pore-
water velocity can be calculated by the following equation:
V = q/e
where: V = pore water velocity, cm/day
q = 'volumetric flux/unit area, cm/day
e = volumetric water content, dimensionless
A simple approximation of volumetric flux (q) can be made by assuming that it
is equal to percolation at the site. Percolation can be estimated by performing a
water balance as described below. This approach for calculating pore-water
velocity is limited by simplifying assumptions; however, the method may be used to
develop an initial estimate for time of travel of contaminants. More detailed
methods, which account for unsteady flow and differences in moisture content are
described in the following reference:
U.S. EPA. 1986. Criteria for Identifying Areas of Vulnerable Hydroqeoloqy
Under the Resource Conservation and Recovery Act. NTIS PB86-224953. Office
of Solid Waste. Washington, D.C. 20460.
Percolation (volumetric flux per unit area) -Movement of contaminants from
unsaturated soil to ground water occurs primarily via dissolution and transport with
percolating soil water. It is important, therefore, to determine the volume of water
passing through the soil. The percolation rate, or volumetric flux, must be
determined in order to calculate pore-water velocity through the unsaturated zone.
The rate of percolation can be estimated from the water balance equation:
PER = P- ET-DR
where: PER = Percolation/recharge to ground water
P = Precipitation and irrigation
ET = Evapotranspiration
DR = Direct surface runoff
9-35
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Annual averages for P, ET and DR should be obtained from existing local sources.
Sources of information to estimate PER include:
State or Regional water agencies;
Federal water agencies (Geological Survey, Forest Service); and
National Weather Service stations.
It is recommended that site-specific ET and DR data be used if possible, because local
conditions can vary significantly from regional estimates. More information on
percolation and ground-water recharge can be found in standard ground-water
texts, such as Freeze and Cherry, 1979. Information on evapotranspiration and
direct surface runoff may be found in the following references:
U.S. EPA. 1975. Use of the Water Balance Method for Predicting Leachate
Generation from Solid Waste Disposal Sites. EPA/530/SW-168. Office of Solid
Waste. Washington, D.C. 20460.
U.S. Geological Survey. 1982. National Handbook of Recommended Methods
for Water Data Acquisition.
Volumetric water content-The volumetric water content is the percent of
total soil volume that is filled with water, it is equal to the amount of water lost
from the soil upon drying to constant weight at 105°C, expressed as the volume of
water/bulk volume of soil. This parameter affects the unsaturated hydraulic
conductivity and is required for calculation of pore-water velocity. At saturation,
the volumetric water content is equal to the porosity of the soil.
Additional soil conditions-Additional soil conditions that may require special
consideration in investigating releases to soil are discussed below.
In certain dense, cohesive soils, water may move primarily through
narrow solution channels or fracture zones rather than by permeating
the bulk of the soil. This condition can sometimes be recognized by dark-
9-36
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colored deposits indicating the fractures or by the tendency of soil cores
to break apart at the discontinuity.
Decomposed rock (e.g., transitional soils) may have a low primary
porosity but a high secondary porosity due to relict joints or fractures or
solution channels. Therefore, most flow may occur through these cracks
"and channels rather than through the soil pores. As a result, the rate of
fluid flow is likely to be high, and the low surface area within the joint or
fracture system generally results in a low sorptive capacity. Because field
conditions are highly variable, the characterization of soil structure
should be sufficiently detailed to identify such joints or fractures that
may provide contaminant pathways.
Certain clay soils known as vertisols, or expandable clays, may fracture
into large blocks when dry. These cracks can be a direct route for
ground-water contamination. Soil surveys should be consulted to
determine whether these soils are present at the site. They occur in, but
are not limited to, eastern Mississippi and central and southern Texas.
Other clay soils may also develop desiccation cracks to a lesser degree. In
these cases, it may be advisable to sample during both wet and dry
seasons.
Sampling saturated soils may be accomplished with the same drilling
techniques used for unsaturated soil sampling. Particular care must be
taken to prevent contamination between soil layers. Methods of
telescoping smaller diameter casing downward through larger diameter,
grouted casing are useful for minimizing cross-contamination between
soil layers (See Section 9.6 for additional information on telescoping
methods).
Frequently, the choice of sampling technique is dictated by mechanical
factors. Hard, rocky, or dense soils may prevent the use of manual tube
samplers or augers. Power-driven auger drill rigs equipped with split-
spoon samplers can penetrate most soils. Power augers can penetrate
most unconsolidated materials, but will not drill through rock, for which
an air-driven rotary drill is the recommended method. Loose sandy soils
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will fail to be retained in a tube sampler; therefore a sampler equipped
with a retaining device should be used in such cases. Core sampling
should generally be carried out under the supervision of an experienced
driller, in order to avoid poor results or damaged equipment.
Where unfavorable soil conditions interfere with a proposed sampling
location, the sampling point may have to be moved to a nearby location.
In the event that such conditions are encountered, new locations should
be chosen that are adequate to characterize the release.
9.3.4 Sources of Existing information
Considerable information may already be available to assist in characterizing a
release. Existing information should be reviewed to avoid duplication of previous
efforts and to aid in scoping the RFI. Any existing information relating to releases
from the unit and to hydrogeological, meteorological, and environmental factors
that could influence the persistence, transport, or location of contaminants should
be reviewed. This information may aid in:
Delineating the boundaries of the sampling area;
Choosing sampling and analytical techniques; and
Identifying information needs for later phases of the investigation, if
necessary.
Information may be obtained from readily available sources of geological and
meteorological data, waste characteristics, and facility operating records. (See also
Sections 2,3,7 and Appendix A).
9.3.4.1 Geological and Climatological Data
The Federal government and most state governments compile geological data,
soil surveys, land use records, and Climatological information. These sources should
be consulted for local geology, soil types, historical precipitation, ground-water
elevation records, and other useful data. Sources which may be consulted for soils
9-38
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data include the Soil Conservation Service (SCS), Agricultural Stabilization and
Conservation Service (ASCS), the U.S. Geological Survey (USGS), state soils bureaus
and agricultural extension services, university soil science departments, and private
consultants. Additional sources of geologic information include geotechnical
boring logs for foundation studies, well logs made during drilling of water supply
wells, and previous hydrogeologic investigation monitoring wells. These logs
should indicate the depth, thickness, and character of geologic materials, and the
depth to the water table. Climate and weather information can be obtained from:
National Climatic Center
Department of Commerce
Federal Building
Asheville, North Carolina 28801
Tel: (704)258-2850
9.3.4.2 Facility Records and Site Investigations
The owner or operator should plan investigation activities by focusing on the
conditions specified in the permit or enforcement order. Facility records, the
facility's RCRA permit application, and any previous site reports (e.g., the RFA
report) should also be examined for any other information on unit characteristics,
wastes produced at the facility, and other factors relevant to releases to soil. Facility
operating records should have data on wastes treated, stored, or disposed of at the
facility. Wastes regulated under RCRA are identified by a waste code that may also
aid in identifying constituents of concern (see 40 CFR Part 261), Wastes originating
within the facility may be identified through analysis of process control records.
Unit releases (e.g., losses from leaking tanks) can sometimes be estimated from
storage records.
9.4 Design of a Monitoring Program to Characterize Releases
9.4.1 Objectives of the Monitoring Program
Monitoring procedures that specify locations, numbers, depths, and collection
techniques for soil samples should be prepared by the owner or operator prior to
each sampling effort. These procedures should provide the justification for the
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proposed samples, in terms of their expected contribution to the investigation.
Examples of soil monitoring objectives include:
Describing soil contamination in a drainage channel where a release is
known to have occurred;
Establishing a random or systematic grid sampling network to determine
soil contamination concentrations in all zones of a large area affected by
airborne deposition; and
Filling in data gaps concerning the transport of waste constituents within
a permeable soil layer.
In preparing soil monitoring procedures, the owner or operator should take
into consideration those factors discussed in Sections 9.3. I through 9.3.4 that apply
to the facility. Also see Section 9.4.4.3 (Predicting Mobility of Hazardous
Constituents in Soil).
As discussed previously, the release characterization may be conducted in
phases. The objectives of the initial soil characterization are generally to verify
suspected releases or to begin characterizing known releases. This characterization
should use relevant soil physical and chemical measurements and other information
as described earlier. In developing the approach, the owner or operator should
determine the following:
Constituents and indicator parameters to be monitored;
Role of field screening methods, if any;
Sampling methods;
Approximate study and background areas;
Sampling locations and approach (e.g., judgmental or systematic); and
Number of samples to be collected.
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The owner or operator may propose the use of field screening methods to aid
in delineating the zone affected by a contaminant release to soil and/or ground
water. Such methods may be applied just below the land surface or at greater
depths, as within soil bore holes. An increasingly used method to detect organic
vapors is generally known as a soil gas survey. Such a survey can yield qualitative
and relative quantitative data on volatile constituents present in the soil gas,
depending on the instrumentation used. For example, a total photoionization
detector will provide an integrated value for the volatile organics present; whereas
a portable gas chromatography can identify and quantitate specific compounds
present in the soil vapor. Field screening can also include chemical analyses of soil
samples performed onsite in mobile laboratories.
When conducting a soil gas survey, it should be realized that any measured soil
vapor concentrations of specific compounds cannot be directly correlated with their
actual concentrations in the soil zone of concern. The concentrations in soil vapor
resulting from a soil with given volatile contaminant concentrations will vary,
depending on several factors, including barometric pressure, relative humidity in
the soil, weather conditions (e.g., precipitation events, soil inhomogeneities, and
temperature). Therefore, the results of a soil gas survey can reveal the relative
abundances of volatile compounds in the soil gas, but not their actual
concentrations in the soil.
The soil gas survey technique may also be applied when drilling boreholes to
characterize site geology or when drilling to install ground-water monitoring wells.
Soil samples taken at various depths within the borehole can be placed in separate
sample bottles with septums.
A sample of the gas in the headspace can then be withdrawn with a syringe
and injected into a portable gas chromatography to identify the presence and
relative abundances of specific volatile compounds in the soil gas. Analysis of drill
cuttings in the open air is not as effective as the headspace technique in detecting
volatile organic compounds; therefore, the headspace method is preferred.
Additional information on soil gas monitoring may be obtained from the
following reference:
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U.S. EPA. 1987. Soil Gas Monitoring Techniques Videotape. National Audio
Visual Center. Capital Heights, Maryland 20743.
Screening methods may help to reduce the number of soil and/or ground-
water samples needed to characterize a release by better delineating the area of
concern in a relatively rapid manner. However, due to limitations (e.g., relatively
high detection limits and inability to identify all the potential hazardous
constituents of concern), some screening methods may not be adequate to verify
the absence of a release. For such verification, an appropriate number of soil
samples would need to be analyzed in the laboratory. Additional information on
field screening methods is presented later in this section and in the Compendium of
Field Operations Methods, (EPA, 1987).
Depending on the outcome of the initial characterization effort, the owner or
operator may be required to obtain additional data to characterize the release. The
findings of the initial phase will dictate the objectives of any later phases. Such
subsequent phases will generally involve the following:
Expanding the number of sampling locations to a wider area and/or
depth, or increasing sampling density where data are sparse;
Institution of a refined grid sampling approach to further assess releases
identified by judgmental sampling (see Section 3);
Addition or deletion of specific monitoring constituents or indicator
parameters; and
Sampling in areas of interest based on previous sampling or model
predictions to confirm the suspected extent of the release.
There is no specified or recommended number of phases to complete a soil
investigation. The owner or operator should determine through consultation with
the regulatory agency whether the collected data are sufficient to meet the
objectives of the investigation.
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9.4.2 Monitoring Constituents and Indicator Parameters
The owner or operator should propose hazardous constituents for monitoring
based on the composition of wastes known or suspected to be present or released
to soils at the site (see Sections 3 and 7 and Appendix B). Additional measurements
may include nonhazardous chemicals that could serve as indicators of the presence
of hazardous constituents or that could mobilize or otherwise affect the fate and
transport of hazardous constituents. Chemical and physical properties of the soil
that can be measured from soil samples should also be included in the list of
parameters (see Section 9.3.3.3).
Justification of monitoring constituent selection may be provided through
detailed facility records or waste analyses, as explained in Section 3. If such
justification is inadequate, it may be necessary to perform a broader analytical
program (See Section 3 and Appendix B).
During or after the selection of monitoring constituents, the owner or
operator should review guidance on compound-specific requirements for sampling
and sample preservation. The laboratory should use EPA protocols and analytical
procedures when available, and accepted QA/QC practices. Guidance and specific
references in these areas are provided in Sections 2,3,4, and 7.
9.4.3 Monitoring Schedule
Monitoring frequency and duration determinations should be based primarily
on the type of release to the soil. A single episode or intermittent release, as with
any release, would require monitoring until the nature and extent of contamination
has been characterized. This may be accomplished with one or two sample sets in
some cases. Longer-term releases will usually necessitate a greater duration of
sampling. Soil-pore liquid may require more frequent monitoring than in soil solids
because changes generally occur faster in these fluids. Frequency may also be
adjusted, if appropriate, as sampling results become available. As with single
episode releases, longer-term releases are monitored until the nature and extent of
contamination has been adequately characterized.
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9.4.4 Monitoring Locations
9.4.4.1 Determine Study and Background Areas
Determination of the area of interest will depend on the facility layout,
topography, the distribution of surface soils, soil stratigraphy, and information on
the nature and source of the release. The size and type of unit may affect the area
under consideration. For example, a small land-fill may only require monitoring of
the surrounding soil whereas an inactive land treatment facility may require
sampling over the entire unit surface and beyond.
High variability in the chemical composition of soils makes determination of
background levels for the constituents of concern essential. This is particularly
important for quantification of toxic metals, because such metals commonly occur
naturally in soil. Background areas not affected by any facility release should be
selected based on their similarity to the study area in terms of soil type, drainage,
and other physical factors. Background soil samples should be taken from areas
that are not near a suspected source of contamination and from the same
stratigraphic layer as the study area samples, if possible. Selection and sampling of
appropriate background areas may be important because verification of a release in
a contaminated area may involve a comparison of study and background
concentrations.
The owner or operator may increase efficiency in the initial characterization
effort by using rapid, field-screening methods (e.g., soil gas surveys using HNu, OVA
or portable gas chromatography) or through indicator parameter measurements to
establish the extent of the study area. Subsurface soil contamination can sometimes
be identified by geophysical techniques such as electromagnetic and resistivity
techniques (See Section 10 and Appendix C). Indicator parameters can also be
helpful in establishing the extent of the monitoring area. For example, Total
Organic Halogen (TOX) or Total Organic Carbon (TOC) analysis may be useful in
detecting total chlorinated and nonchlorinated organic solvents. Such parameters
may be used to characterize the nature and extent of a release but should always be
verified by an adequate number of specific constituent analyses.
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it is generally recommended that a sampling grid be developed for the site,
even for judgmental sampling. Gridding of the area to be sampled prior to the
sampling effort will aid in determining appropriate sampling locations and in
describing these locations. Refer to Section 3.6 for additional information on
gridding of a study site.
9.4.4.2 Determine Location and Number of Samples
The owner or operator should propose monitoring locations and the number
of samples to be collected and analyzed. Samples should be taken from the vicinity
of all units identified in the conditions of the permit or order as suspected or known
sources of soil contamination. The total number of samples necessary for the initial
investigation will depend on the extent of prior information, the suspected extent
and severity of the release, and the objectives of the characterization. However, the
following general guidance should aid the owner oroperator to sample efficiently.
Sampling efficiency may be increased by use of a proportional sampling
approach, which involves dividing the area of concern into zones, based
on proximity to the release source and/or other factors. The number of
samples taken in each zone should be proportional to the area of a zone.
Use of composite samples may be able to allow detection of
contamination over an area of concern with a smaller number of
analyses. Compositing involves pooling and homogenization of multiple
soil samples. The composite is then analyzed to give an average value for
soil contamination in that area. However, as discussed in Sections 3 and
7, composites should have very limited application during the RFI and
should always be accompanied by an appropriate number of individual
grab samples. The following additional limitations on compositing
should be observed:
Compositing is most useful when large numbers of soil samples can
be easily collected (e.g., for surficial contamination). In order to
obtain the maximum information from deep soil coring, individual
grab samples are preferred over composites.
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Compositing should not be used when analyzing soils for volatile
organics because the constituents of interest may be lost during
homogenization and sample handling.
The owner or operator should employ appropriate procedures for the
evaluation and reporting of monitoring data. These procedures can vary
in a site-specific manner but should result in determinations of the
nature, extent, and rate of migration of the release. Where the release is
obvious and/or chemically simple, it may be possible to characterize it
readily from a descriptive presentation of concentrations found.
However, where contamination is less obvious or the release is chemically
complex, a statistical inference approach may be proposed. The owner
or operator should plan initially to take a descriptive approach to data
evaluation in order to broadly delineate the extent of contamination.
Statistical comparisons of monitoring data among monitoring locations
and over time may be appropriate if a descriptive approach does not
provide a clear characterization of the release. Further guidance on use
of statistical methods in soil investigations is provided in the following
documents:
Barth, D.S. and B.J. Mason. 1984. Soil Sampling Quality Assurance
User's Guide. U.S. EPA 600/4-84-043. NTIS PB84-198621 .
Washington, D.C. 20460.
Mason, B.J. 1983. Preparation of a Soil Sampling Protocol:
Techniques and Strategies. NTIS PB83-206979. U.S. EPA 600/4-83-
020. Washington, D.C. 20460.
Characterization of contaminant distribution with depth necessitates
sampling of each distinct soil layer that might be affected by the release
and from boundaries between soil layers. If the soil profile contains thick
layers of homogeneous soil, samples should be taken at regular intervals
(e.g., every 5 feet). In addition, samples should be taken where borings
intersect fracture systems, at interfaces of zones of high and low
permeability materials, or at other features that could affect
contaminant transport. The owner or operator should consider
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remeasurement of soil physical and hydraulic properties in each distinct soil
layer. The objective of such measurements in the initial release
characterization effort is to identify properties that vary with depth. This
approach may indicate the use of stratified sampling in any future
sampling phases. Determination of soil properties will also aid in
refining conceptual models of contaminant transport and can be. input
for mathematical models of soil transport.
Modeling-Prediction of contaminant fate and transport can range from a
"conceptual" model of contaminant behavior in the soil to complex computer
programs requiring extensive input of soil and water budget data. The primary uses
of predictive modeling in soil investigations are to locate appropriate sampling
locations using site-specific input data and to estimate the future rate, extent, and
concentration of contaminant releases.
Modeling of contaminant transport in the unsaturated zone is often difficult
due to the generally high spatial variability in soil physical and hydraulic properties.
Therefore, modeling should not be used to replace actual measured values (e.g.,
when establishing the limits of waste leaching or diffusion in soil). However, if used
with caution, models can act as useful tools to guide sampling efforts by directing
sampling towards site areas identified as preferred soil/water flowpaths (e.g., a
permeable soil layer). The owner or operator should discuss the use of specific
models with the regulatory agency prior to use.
Numerous models, including computer models, have been developed to
calculate water flow and contaminant transport under saturated and unsaturated
soil conditions. In using such models, site-specific data on soils and wastes should be
used. Ground-water (saturated flow) models are discussed in Section 10. A U.S.
Nuclear Regulatory Commission Report (Oster, 1982) may be reviewed for
information on the applicability of 55 unsaturated flow and transport models. Use
of the RITZ Model (found in U.S. EPA. 1986. Permit Guidance Manual on Hazardous
Waste Land Treatment Demonstration. NTIS PB86-229192) may be particularly
appropriate in certain situations. The RITZ model describes a soil column, 1 meter
square, with a depth equal to the land treatment zone (usually 1.5 m). The soil
column consists of a plow zone and lower treatment zone that are made up of four
phases: soil grains, pore water, pore air, and pore oil. Mobilization of constituents
9-47
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within the soil is accounted for by dispersion, advection, and migration between
phases. The constituent may also be degraded by biochemical processes
represented in the model. Output from the model includes the concentration (C) of
a constituent at the bottom of the treatment zone, and the time (T) required for a
constituent to travel a distance equal to the treatment zone depth. Although the
RITZ model was developed for evaluating the effectiveness of land treatment units,
the model may be used for other applications, as appropriate (see above referenced
document).
EPA is in the process of developing a more sophisticated version of the RITZ
model, known as the RITZ-VIP model. The VIP version differs in that it is designed to
provide information for multiple waste loadings in a land treatment situation. The
initial version of the RITZ model only applies where the waste or material in
question is applied to the land once. The RITZ-VIP version is currently in the
review/verification process. More information on this model may be obtained by
writing to EPA at the following address:
U.S. Environmental Protection Agency
Robert S. Kerr Environmental Research Laboratory/0RD
P.O. Box1198
Ada, Oklahoma 74820
Computer models if proposed for use in the RFI should (1) be well-
documented; (2) have been peer reviewed; and (3) have undergone extensive field
testing. As indicated previously, model documentation (e.g., model theory,
structure, use, and testing) should be provided to the regulatory agency for review
prior to use. Access to the relevant data sets should also be available upon request.
The regulatory agency may also recommend that a sensitivity analysis be performed
and that the results of the analysis be submitted with the model results. In selecting
a model, the owner or operator should consider its applicability, limitations, data
requirements, and resource requirements.
9.4.4.3 Predicting Mobility of Hazardous Constituents in Soil
Predicting the mobility of hazardous constituents in soil may be necessary in
an RFI. The prediction may then be used to estimate the probable vertical or lateral
9-48
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extent of contamination, which can be used to identify potential sampling
locations. Mobility predictions may also be used in determining potential inter-
media transfers from the soil to ground or surface water. Finally, mobility
predictions may provide information that can be used during the Corrective
Measures Study to differentiate between contaminated soil that should be removed
from the site and that which may remain at the site without adversely affecting
human health or the environment. Predicting mobility of soil constituents may be
particularly relevant, as indicated in Section 8, for determining whether deep-soil
contamination, or in some cases surficial-soil contamination, can lead to ground-
water contamination at a level above health and environmental criteria (if such an
impact has not already occurred).
There is no universally accepted, straightforward method for predicting the
mobility of all hazardous constituents within soils under all possible sets of
environmental conditions. Nor is there a fully tested method of estimating the
impact of constituents originating in the unsaturated zone on ground-water
quality. Therefore, to avoid unneeded efforts, the first question the owner or
operator should address is whether this task is necessary. For example, the
characterization of ground-water quality (conducted following the guidance in
Section 10) may provide information sufficient to describe the extent of the release
in soils as well, and to determine that a Corrective Measures Study is necessary. This
may be the case in situations where contaminated soils are located solely within the
ground water and when the contaminants are relatively mobile. The most recent
ground-water impact characterization data may not, however, provide information
on the future impact of contaminated soils on ground water (e.g., due to different
leaching rates for different contaminants).
This section presents various approaches for predicting constituent mobility in
both saturated and unsaturated soils; it also discusses how to estimate the impact
on ground-water quality of the constituents leached from unsaturated soils. The
limitations of these methods are also reviewed.
9.4.4.3.1 Constituent Mobility
There are several means of investigating mobility, including a descriptive
approach (i. e., consideration of constituent and soil properties), the use of
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mathematical models, and the use of laboratory models or leaching tests. Leaching
tests have the advantage of being the only approach that integrates soil and
constituent properties in a single evaluation. They may, in certain cases, provide a
conservative (reasonable worst case) estimate of the concentration within leachate
of waste constituents that may eventually impact ground water. Leaching test
results must be coupled with site-specific factors, (e.g., soil cation exchange
capacity, ground-water pH, and depth to ground water) when used to design
monitoring programs, determine potential for inter-media impacts, and evaluate
options for contaminated-soil corrective measures. When assessing leach test
results, specific hazardous constituent concentrations in the leachate will be
compared with the health and environmental criteria concentrations for water
described in Section 8.
The descriptive approach and the use of mathematical models (such as the
RITZ Model, discussed previously) may be appropriate in those cases where
assumptions implicit in the use of leaching tests may not be applicable. For
example, leaching tests may be overpredictive of leachate concentrations where
extensive channeling (e. g., because of root zone or joints) through the
contaminated zone is present; in this case, the contact time between the leaching
fluid (e.g., infiltrating precipitation) and the soil, as well as the surface area of the
soil exposed to the fluid, would be less than that simulated by the leaching test.
Leaching tests may also not be applicable where low redox (reduction/oxidation)
conditions are identified. Consideration of redox conditions is particularly relevant
for inorganic.
The Agency has devised a soils/waste mixture leaching procedure, known as
the Synthetic Precipitation Leach Test (Method 1312) that it generally believes may
be appropriate for evaluating the potential impact of contaminated soils on
ground-water quality. (See Appendix F for a description of this procedure).
Although neither Method 1312 nor any other leaching test (such as the Toxicity
Characteristic Leaching Procedure (Method 1311) have been validated for use on a
wide range of contaminated-soil types, the Agency believes that Method 1312 may
have the broadest applicability. Method 1312 may be particularly appropriate
when no future waste management or other industrial activities likely to produce
an acidic leaching medium are likely to be conducted at the site of the release.
9-50
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However, other leaching tests may be appropriate under certain case-specific
circumstances. For example, a test such as Method 1311 may be appropriate at a
release site that will be used for management of municipal refuse or a similar waste
in the future, because the refuse could produce an acidic leaching medium, which
Method 1311 has been designed to simulate. The evaluation of leaching from
cyanide-containing soils should be performed with neutral water, rather than an
acidic leaching medium, because leaching of cyanide-containing waste under acidic
conditions may result in the formation of toxic hydrogen cyanide gas. Other
leaching test variations may be necessary if interactive effects on mobility are
caused by non-aqueous solvents, for example, or if an aqueous phase leaching
medium may underpredict potential mobility due to site and waste constituent
characteristics.
9.4.4.3.2 Estimating Impact on Ground-Water Quality
In evaluating results obtained using the leach test for the evaluation of
contaminants of concern at a specific release site, the Agency will consider relevant
hazardous constituent properties, the physical and chemical characteristics of the
soil/waste matrix at the site, and local climatological factors. Factors that will be
considered include the following:
Chemical structure, classification, and bonding (organic vs. inorganic,
ionic vs. covalent, etc);
Volubility of the constituents;
Octanol/water or other partitioning coefficients;
Density;
organic carbon adsorption coefficient;
Volatility (e.g., Henry's Law constant);
Dissociation constants (Pk);
9-51
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Degradation potential (hydrolysis, biodegradation);
Soil/waste matrix characteristics;
Cation exchange capacity;
Soil pH and Eh;
Soil classification (e.g., clay, silt, and sand content);
Particle-size distribution;
Porosity;
Unsaturated hydraulic conductivity;
Climatological characteristics;
Precipitation patterns (volume, frequency, etc.); and
pH of local or regional precipitation.
The results obtained from a specific leach test must be supported by an
analysis of the relevant factors, such as those listed above, and considering the likely
future use of the site (industrial, waste management, residential, etc.).
As an alternative approach to the use of a leach test for evaluating
contaminated soil, the owner or operator may propose to perform an analysis of the
waste, soil, and Climatological conditions, considering such factors as are listed
above, to demonstrate that the expected concentrations of any constituents that
could leach from any contaminated section of the subsurface soils would not exceed
the action levels for ground-water. This analysis, which would require appropriate
technical justification and should rely as much as possible on data (such as the
results of published field studies conducted under environmental conditions similar
to those at the release site), must be based on conservative assumptions related to
9-52
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future changes in environmental conditions and land use (e.g., the use of the site
for future non-hazardous waste management).
At the present time, studies are being designed to more fully examine various
methods for evaluating leaching of hazardous constituents from contaminated
soils. Further guidance will be provided by the Agency upon completion of these
studies. It is recommended that the owner or operator review the procedures and
methods described in Sections 8 and 9 and Appendix J of Petitions to Delist
Hazardous waste, EPA/530-SW-85-003, as well as SW-846, to assist in determining
the appropriateness of any particular leaching procedures for evaluating
contaminated soils. Until more definitive guidance is available, the owner or
operator may propose what he believes to be the most appropriate leaching
procedure, and provide technical justification to support the proposed procedure
based on site and waste conditions at the time of the investigation. For additional
assistance on selection of a leaching procedure, the owner or operator may contact
the Technical Assessment Branch of the Office of Solid Waste in Washington, D.C.
(202/382-4764).
As indicated above, waste and site-specific factors should be evaluated,
together with leaching test concentrations, to arrive at predictions of the potential
impacts to ground water. For example, if the depth to ground water is great
enough, and the soil cation exchange capacity is high, the owner or operator may
be able to predict that metal species would be adsorbed by the soil before the soil
leachate reaches the ground water. Particular attention, in this example, would be
needed to ensure that the cation exchange capacity of the soil could not be
exceeded. The characteristics of the metal ions that are displaced from the
exchange sites should also be considered.
As another example, the soil-water partition coefficient (Kd) is useful for
describing chemical mobility in the subsurface environment, and is widely used in
studies of ground-water contamination. For primarily aqueous solutions, the
partitioning between the aqueous solution and the solid medium can be derived
from thermodynamic principles (Freeze and Cherry, 1979).
More commonly, Kd is determined from batch experiments in which the
contaminated solution and geologic material of interest are brought into contact.
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After a period of time has elapsed (e.g., 24-hours), the degree of partitioning of the
contaminant between the solution and the geologic material is determined. The
partition coefficient is then calculated using the following equation:
mass of sorbed chemical/gram of solid
K d =
mass of chemical/ml of solution
The relative mobility of attenuated constituents in ground water can then be
estimated as follows (after Mills, et al., 1985):
(1 + Kdb)/ne
where
v = average linear velocity of attenuated constituent along centerline
of plume, distance/time;
Vs = ground-water velocity, distance/time;
b = soil bulk density, mass/volume;
ne = effective porosity, dimensionless; and
Kd = soil-water partition coefficient, volume/mass.
The relative mobility of selected constituents, based on typical partition
coefficients, is shown in Table 9-6. It is important to note that Kd is a simplified
measure of the relative affinity of a chemical for the solution and the soil. Kdis
highly site-specific, varying as a function of pH, redox conditions, soil characteristics,
and the availability of alternate solution phases (organic and inorganic liquids, or
colloidal solids). The general effect of pH and organic matter content on partition
coefficients for metals is shown in Figure 9-3.
The Kd value selected for use in estimating chemical mobility should reflect the
predominant chemical species in solution. One approach to estimating solution
composition is to use thermodynamic stability diagrams, commonly illustrated as
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TABLE 9-6 RELATIVE MOBILITY OF SOLUTES1
Group
Conservative
Slightly Attenuated
Moderately Attenuated
More Strongly
Attenuated
Examples
Total Dissolved
Solids
Chloride
Bromide
Nitrate
Sulfate
Boron
Trichloro-
ethylene
Selenium
Arsenic
Benzene
Lead
Mercury
Penta-
chlorophenol
Master Variables2
V
V
V
V, Redox Conditions
V, Redox Conditions
V, pH, organic matter
V, organic matter
V, pH, Iron hydroxides,
V, pH, Iron hydroxides,
V, organic matter
V, pH, Sulfate
V, pH, Chloride
V, organic matter
1 Under typical ground-water conditions (i.e., neutral pH and
oxidizing conditions). Under other conditions mobility may differ
substantially. For example, acidic conditions can enhance the
mobility of metals by several orders of magnitude.
2 Variables which strongly influence the fate of the indicated solute
groups. Based on data from Mills et al., 1985 and Rai and Zachara,
1984. (V= Average Linear Velocity)
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100
Percent
Adsorption
by Soil
50
I Shift due
to presence
' of soil organic
I natter
I
.^-Typical adsorption
' ' curve for heavy
I netal x, on silica
I or aluminum silicate
surface coated with
/ soil organic matter
Typical
Adsorption
curve for
heavy metal
x. on a clear
silica or
aluminum
silicate
surface
pH of the Soil Solution
a) Generalized Heavy Metal Adsorption Curve for Cationic Species
(e.g., CuOH*)
100 --
Percent
Adsorption
by Soil
50 -
X
Typical adsorption
curve for heavy
metal species, K,
on iron hydroxide
C "N
%
\
\
V «
\ -A
\Shift \
x due to \
\ presence \
\ of soil \
* organic \
N matter \
\
pH of the Soil Solution
b) Generalized Heavy Metal Adsorption Curve for Anionic Species
i
(e.g.,
Figure 9-3. Hypothetical Adsorption Curves for A) Cations and
B) Anions Showing Effect of pH and Organic Matter
(Mil Is eta I., 1985)
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Eh-pH diagrams. These diagrams represent solution composition for specified
chemicals as a function of redox potential (Eh) and of pH under equilibrium
conditions.
Many metals of interest in ground-water contamination problems are
influenced by redox conditions that result from changes in the oxidation state of
the metal or from nonmetallic elements with which the metal can form complexes.
Garrels and Christ (1965) present a comprehensive treatment of the subject and
provide numerous Eh-pH diagrams that can be used for analysis of geological
systems.
For any particular point in an Eh-pH diagram, a chemical reaction can be
written that describes the equilibrium between the solid and dissolved phases of a
particular constituent. The following equation represents the general form of the
equilibrium reaction:
aA + bB =cC + dD
where: a, b, c, d = number of moles of constituent
A and B = reactants
C and D = products
At equilibrium, the volubility constant (K) expresses the relation between the
reactants and the products following the law of mass action:
K= [C]c[D]d
[A]a[B]b
The brackets signify an effective concentration, or activity, that is reported as
molality (moles per liter). Volubility constants for many reactions in water are
reported by Stumm and Morgan (1981). Alternatively, volubility constants can be
calculated from thermodynamic data (Gibbs free energy) for products and
reactants. Freeze and Cherry (1979) describe the use of thermodynamic data to
calculate volubility constants for several constituents common in ground water.
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An example illustrating the use of Eh-pH diagrams and the influence of redox
conditions on solution composition is shown for mercury (Hg) in Figure 9-4. The
stability diagram shown in Figure 9-4 is constructed for mercury-contaminated
water that contains chloride (Cl) and dissolved sulfur species. The solid lines in the
diagram represent the Eh-pH values at which the various phases are in equilibrium.
For pH values of less than about 7 and Eh values greater than 0.5 volts (strong
oxidizing conditions), HgCIJs the dominant dissolved species. For pH values
greater than 7, and at a high redox potential, Hg(OH)2is the dominant dissolved
species. The main equilibrium reaction in this Eh-pH environment is:
HgO + H20 = Hg (OH)2
From the law of mass action, the volubility relationship for this reaction is
written as follows:
[Hg(OH)2]
K
[HgO] [H20]
At 25°C, the volubility constant (log K) for this reaction is -3.7 (Freeze and
Cherry, 1979). The activity coefficients for a solid (HgO) and H20 are assumed to be
one; therefore, the concentration of Hg(OH)2in solution is calculated as follows:
[Hg(OH)2] = K = 10'37= 1.995 x KTmoles/l = 47 mg/l (mol. wgt. = 235 g/mole)
The Eh-pH diagram can be used to estimate the concentration of mercury in
solution at any particular point in the diagram if the volubility constant for the
appropriate equilibrium reaction is known. For lower redox conditions (pH = 6.0,
Eh = 0.0), the concentration of mercury in solution would be approximately 0.025
mg/l (Callahan et al., 1979).
Several limitations are associated with the use of Eh-pH diagrams to predict
dissolved chemical species, including the accuracy of thermodynamic data, the
assumption of equilibrium conditions, and of other chemical processes such as
adsorption that can maintain concentrations below those that would exist as a
result of only volubility constraints. However, the Eh-pH diagrams serve to illustrate
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1.20
1.00
2 4 6 8 10 12 14
Figure 9-4. Fields of Stability for Aqueous Mercury at 25°C
and Atmospheric Pressure (Callahan et al., 1979)
9-59
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that solution composition depends on redox potential and that chemical mobility
within a ground-water system may vary from one zone to another.
9.5 Data Presentation
The owner or operator will be required to report on the progress of the RFI at
appropriate intervals during the investigation. The data should be reported in a
clear and concise manner, with interpretations supported by the data. The
following data presentation methods are suggested for soil investigations. Further
information is provided in Section 5.
9.5.1 Waste and Unit Characterization
Waste and unit characteristics may be presented as:
Tables of waste constituents and concentrations;
Tables of relevant physical and chemical properties of waste and
constituents;
Narrative description of unit operations; and
Surface map and plan drawings of the facility and waste unit(s).
9.5.2 Environmental Setting Characterization
Environmental characteristics may be presented as:
A map and narrative description of soil classifications;
Soil boring logs;
Measurements of soil physical or hydrologic characteristics; and
Onsite survey results (e. g., OVA, portable gas chromatography,
geophysical techniques).
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Soil and site map(s)-ln addition to the required RCRA permit site topographic
map, the owner or operator should prepare a map(s) displaying the location of
surface soil types (described according to the appropriate classification system),
paved areas, areas of artificially compacted soil, fill or other disturbed soil, and
other features that could affect contaminant distribution. Specific guidance on the
use of maps and other techniques such as aerial photographs and geophysical
surveys is provided in Appendices A and C.
The owner or operator should develop maps of unconsolidated geologic
materials at the site. These maps should identify the thicknesses, depths, and
textures of soils, and the presence of saturated regions and other hydrogeological
features. Subsurface soils should be identified according to accepted methods for
description of soils (See Section 9.3.3.3). Figure 9-5 displays a typical soil boring log.
Graphical methods commonly used to display soil boring data are cross-
sections, fence diagrams, and isopach maps. Cross-sections are typically derived
from borings taken along a straight line through the site. Plotting the stratigraphy
of surficial deposits against horizontal distance between sampling points gives a
vertical profile or transect. Fence diagrams can depict the same type of information
between points that are not in a straight line. An isopach map resembles a
topographic map, however, the isopleth lines on an isopach map represent units of
thickness of a particular soil layer rather than elevations. For example, a map of clay
isopachs may be used to show the thickness in feet of a low permeability layer
below a waste lagoon. Generally, to verify lateral continuity, more than one
transect through a site will be necessary. When it is important to indicate the areal
extent of a layer (e.g., where a clay lens is suspected to cause lateral transport in the
unsaturated zone) both vertical and horizontal presentations may be necessary.
Graphical methods are discussed in detail in Section 5 (Data Management and
Presentation).
9.5.3 Characterization of the Release
Graphical displays of contaminant distributions in soil may include:
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BORING L06
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I I if
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9-62
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BORING L06
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9-63
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Area/site maps with concentrations indicated by numerical values,
symbols, or isoconcentration lines;
Three-dimensional isopleth plots of concentrations (including stack
maps), such as are produced by computer graphics; and
Vertical concentration contours (isopleths) plotted along a transect or
fence diagram.
All graphical displays should be accompanied by data tables showing
concentrations for each sampling location.
9.6 Field Methods
Both soil and soil-pore water sampling may be utilized in the investigation.
Chemical analysis of soil core samples may be used to characterize constituents of
concern that are adsorbed to the solid matrix. Lysimeters can be installed in
boreholes created during core sampling to identify mobile constituents that may
migrate to ground water. In addition, field screening methods may be used to help
determine the presence and extent of releases.
Appropriate sample collection and preservation techniques should be
specified. When a soil sample is removed from its surroundings, chemical and
physical changes can begin immediately. These changes include moisture loss,
oxidation, gas exchange, loss of volatile components, increased or decreased
biological activity, and potential contamination of the sample. Therefore,
appropriate measures must be taken to store and preserve samples to minimize
their degradation. Sampling techniques should not adversely affect analytical
procedures and hence results. For example, use of fluids other than water during
drilling can introduce organic or inorganic contaminants that may make
quantification of the contaminants of concern impossible. The practice of coating
metal parts of drilling equipment with oils or greases to prevent rust will have a
similar effect.
Volatile compounds can sometimes be detected near the soil surface using
rapid, field screening methods (e.g., portable photoionization detector such as HNu
9-64
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or Photovac or an organic vapor analyzer (OVA)). Organic vapors can also be
detected and measured in shallow boreholes or in ground-water monitoring wells.
Vapor sampling is especially useful for initial characterization because it is a rapid,
semi-quantitative technique. Benefits of field screening methods include:
The investigator can, in certain cases, quickly determine whether a
sample is contaminated, thus, aiding in the identification of areas of
concern;
Samples that may undergo chemical changes with storage can be
evaluated immediately; and
These techniques can be used to investigate releases to several media
simultaneously (e.g., subsurface gas, ground water and soil).
However, there are limitations in using field screening methods, including:
They cannot always account for all constituents that may be present in
the release;
They may not be able to quantify concentrations of specific constituents
of concern; and
Constituents may be present at levels below detection capability.
Field-screening methods are described in the Compendium of Field Operations
Methods (EPA, 1987).
Soil sampling methods will commonly vary with the depth of interest. For
purposes of the RFI, these methods are described as "surficial" or "subsurface".
Surficial sampling in the upper 20 cm of soil can usually be accomplished with simple
tools, including shovels, spatulas, soil punches, and ring samplers. Contaminants
that have moved further downward in the soil profile often require tools such as
tube samplers and augers. Manually operated tools are commonly useful to about 1
to 2 meters in depth, depending on the soil type. Below this depth, hydraulically or
mechanically driven equipment is generally needed (See Everett et al, 1984 for
9-65
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additional information on soil sampling techniques, as well as Sections 3 and 7 of
this Guidance for discussions of additional sampling methods and references).
Methods to sample soil-pore water or other fluids are presented in Section
9.6.3.
9.6.1 Surficial Sampling Techniques
Surficial soils may also contain various materials, including rocks, vegetation,
and man-made items. The owner or operator should propose how these materials
will be treated (i.e., whether they will be discarded or analyzed separately). Care
should be taken in choosing sampling equipment that will not adversely affect the
analytical objectives (e.g., painted or chrome/nickel plated equipment may
adversely affect metals analyses). Some commonly used surficial soil sampling
techniques are discussed below.
9.6.1.1 Soil Punch
A soil punch is a thin-walled steel tube that is commonly 15 to 20 cm long and
1.3 cm to 5.1 cm in diameter. The tube is driven into the ground with a wooden
mallet and twisted to free the sample. The punch is pulled out and the soil pushed
or shaken from the tube. This technique is rapid but is generally not useful in rocky
areas or in loose, granular soils that will not remain in the punch. Soil punching is
not useful for soil structure descriptions because the method causes compaction
that destroys natural fractures.
9.6.1.2 Ring Samplers
A ring sampler consists of a 15 to 30 cm diameter steel ring that is driven into
the ground. The soil is subsequently removed for analysis. This technique is useful
when results are to be expressed on a unit area basis, because the soil ring contains
a known area of soil. Ring samplers will generally not be useful in loose, sandy soils
or stiff clays.
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9.6.1.3 Shovels, Spatulas, and Scoops
Collection of grab samples by shovel, spatula, or scoop is not recommended if
sample area or volume determinations are required (the two previous methods are
more accurate). The reproducibility of sample size is limited and subject to sample
bias. The principal advantages of grab sampling are the efficiency of collection and
the fact that samples may indicate the range of contaminant concentrations at the
site.
9.6.1.4 Soil Probes (tube samplers)
Manual soil probes are designed to obtain samples from the upper two meters
of the soil profile. The soil probe is commonly a stainless-steel or brass tube that is
sharpened and beveled on one end and fitted with a T-handle. Soil probes are
common agricultural tools and can be obtained in several diameters. The probe is
pushed into the soil in 20 to 30 cm increments. At the desired depth, the tube is
pulled out and the soil sample extruded. The sample may be considered
"disturbed" or "undisturbed" depending on whether it can be removed intact. The
samples, however, are generally considered to be disturbed for the purposes of
engineering or physical measurements. Loose soils will be difficult to sample with
this tool, and the borehole will tend to collapse when the tube is withdrawn to
obtain samples.
9.6.1.5 Hand Augers
Augers have a spiral cutting blade that transports soil cuttings upwards. Hand-
operated augers are generally used to a depth of approximately 6 feet. Single flight
augers are pulled from the ground periodically and soil samples are taken from the
threads of the auger. Continuous flight augers transport the loosened soil to the
top of the borehole, where it can be collected. Augers provide highly disturbed
samples. Limited information can be obtained on soil structure, bulk density, or
permeability. Cross-contamination between soil layers is likely and depth
information on various soil layers is not reliable. Therefore, reliance on augering as
a sole sampling technique is not recommended. Augering may be used, however, in
conjunction with tube sampling that obtains undisturbed samples.
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9.6.2 Deep Sampling Methods
The subject of deep drilling is discussed more extensively in the section on
ground-water sampling (see Section 10), because deep cores will generally be taken
in conjunction with drilling for monitoring well emplacement. There are some
techniques that are of particular importance to soil sampling and, therefore, a brief
discussion is included here. Procedures for sampling with split-spoon and thin-wall
tube corers and other equipment are presented in Section 7.
9.6.2.1 Hollow-Stem Augers
Hollow-stem augers have a continuous flight-cutting blade around a hollow
metal cylinder. A stem with a plug is ordinarily kept inside the auger barrel to
prevent soil from entering. When core samples are desired, the stem is withdrawn
and a tube sampler may be inserted to the bottom of the borehole. This drilling
method may be used for continuous soil sampling. An additional advantage of
hollow-stem augers is that they do not require drilling fluids.
9.6.2.2 Solid-Stem Augers
Solid-stem augers, as the name implies, are augers that do not have an inner
barrel. As with the manual variety, single-flight augers must be withdrawn each
time a sample is desired, or samples may be taken from the cuttings brought to the
surface by augers of the continuous flight type. Augers may be used in conjunction
with tube samplers by withdrawing the auger and obtaining a sample from the
bottom of the borehole. This sampling approach is only useful with soils that do not
cave in or crumble after drilling.
9.6.2.3 Core Samplers
Soil coring devices that may be used with hydraulically or mechanically- driven
drilling rigs include thin-walled Shelby tubes and split-spoon samplers. These are
two of the most common samplers and are discussed below.
9-68
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9.6.2.3.1 Thin-Walled Tube Samplers
The Shelby tube is a metal cylinder with the end sharpened and beveled for
cutting into the soil. Common sizes used for field investigations are 1 to 3 inches in
diameter. The tube is pushed down into the soil with a smooth even motion by
applying downward pressure from a drilling rig or other apparatus. Thin-walled
tubes produce high quality undisturbed cores that can be used for engineering and
hydraulics testing but are useful only in cohesive soils as loose soils may fall out of
the tube during removal. The soil must be extruded from the tube in a laboratory or
in a field extruding unit because core removal is generally difficult. For rapid
characterization of the soil stratigraphy in the field, split-spoon samplers are
recommended.
9.6.2.3.2 Split-Spoon Samplers
A split-spoon consists of a hollow steel cylinder split in half and screwed into
an "unsplit" outer tube and tip. This assembly can be connected to drill rods. The
tube is commonly forced into the soil by applying a 140 pound sliding hammer,
dropping 30 inches along the drill rod (ASTM, 1986). The number of hammer blows
required to advance the sampler in six inch increments is recorded. The total blow
count number for the second and third increments is related to a standard
engineering parameter indicating soil density. After the tube is pulled from the
soil, the cylinder is removed from the drill rod and opened, exposing the soil core.
Core samples may be used to determine stratigraphy, for chemical and grain-size
analysis, or for pore water extraction. Split-spoons are the preferred method for
obtaining unconsolidated soil samples and may also be used to penetrate some
types of rock.
9.6.2.4 Trenching
Trenches and test pits are useful where detailed examination of soil
stratigraphy and geology is required. Trenching is generally limited for practicality
to the top eight feet of soil. Shallow trenches may be dug manually, but in most
instances, a backhoe will be faster and easier. Bulk soil samples may be obtained
with this method.
9-69
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9.6.3 Pore Water Sampling
When contaminants are suspected of migrating readily through the soil with
infiltrating water, monitoring of water or other fluids in the unsaturated zone may
be appropriate. Sampling soil pore water before it reaches the water table can
provide an early warning of threats to ground water.
Compounds for which pore water sampling may be useful are those that are
moderately to highly water soluble and thus are not appreciably retained on soil
particles. Examples include poorly adsorbed inorganic such as cyanide or sulfate,
halogenated solvents such as TCE, and organic acids. Due to the mobility of these
compounds, pore water sampling will be most useful for current releases.
A common pore water collection technique uses a suction device called a
pressure vacuum lysimeter, which consists of a porous ceramic cup connected by
tubing to a collection flask and vacuum pump (Figure 9-6). The lysimeter cup may
be permanently installed in a borehole of the appropriate depth, and if the hole is
properly backfilled. Suction, from the pump works against soil suction to pull water
out of the silica flour surrounding the cup. This method will not work well in
relatively dry soils.
An advantage of this method is that the installation is " permanent, " allowing
multiple samples from one spot to measure changes in pore water quality with
time. Limitations include:
Measurements cannot be correlated accurately with soil concentrations
because the sample is obtained from an unknown volume of soil;
Lysimeters are subject to plugging and are difficult to install in fractured
or rocky soils;
Some organic and inorganic constituents may be adsorbed by the
ceramic cup (Teflon porous suction lysimeters may overcome this
problem); and
9-70
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PRESSURE-VACUUM
ACCESS TUBE
BENTONITE
BENTONITE
POROUS
CERAMIC
TIP
ACCESS L/NES
'POLYETHYLENE
TUBING)
DISCHARGE TUBE
BACKFILL
POWDERED QUARTZ
BENTONITE
AUGERED HOLE
*" DIAMETER
Figbre 9-6. Typical Ceramic Cup Pressure/Vacuum Lysimeter
9-71
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Volatile organics will be lost unless a special organics trap is installed in
the system.
9.7 Site Remediation
Although the RFI Guidance is not intended to provide detailed guidance on
site remediation, it should be recognized that certain data collection activities that
may be necessary for a Corrective Measures Study may be collected during the RFI.
EPA has developed a practical guide for assessing and remediating contaminated
sites that directs users toward technical support, potential data requirements and
technologies that may be applicable to EPA programs such as RCRA and CERCLA.
The reference for this guide is provided below.
U.S. EPA. 1988. Practical Guide for Assessing and Remediating Contaminated
Sites. Office of Solid Waste and Emergency Response. Washington, D.C.
20460.
The guide is designed to address releases to ground water as well as soil,
surface water and air. A short description of the guide is provided in Section 1.2
(Overall RCRA Corrective Action Process), under the discussion of Corrective
Measures Study.
9-72
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9.8 Checklist
RFI CHECKLIST - SOILS
Site Name/Location.
Type of unit
1. Does waste characterization include the following information? (Y/N)
Identity and composition of contaminants
Physical state of contaminants
Viscosity
PH
pKa
Density
Water Volubility
Henry's Law Constant
__
"^o w
Biodegradability
Rates of hydrolysis, photolysis and oxidation
2. Does unit characterization include the following
information? (Y/N)
Age of unit
Construction integrity
Presence of liner (natural or synthetic)
Location relative to ground-water table
or bedrock or other confining barriers
Unit operation data
Presence of cover
Presence of on/offsite buildings
Depth and dimensions of unit
Inspection records
Operation logs
Presence of natural or engineered barriers
near unit
3. Does environmental setting information include the following
information? (Y/N)
Site soil characteristics
Surface soil distribution map
Soil moisture content
Predominant soil phase to sample (solid, liquid, gaseous)
Soil classification
Particle size distribution
9-73
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RFI CHECKLIST- SOILS
(Continued)
Porosity
Hydraulic conductivity (saturated and unsaturated)
Relative permeability
Soil sorptive capacity
Cation exchange capacity
Organic carbon content
Soil pH
Depth to water table
Pore water velocity
Percolation
Volumetric water content
Have the following data on the initial phase of the release
characterization been collected?
Geological and climatoiogical data
Facility records and site-specific investigations
Area of contamination
Distribution of contaminants within study area
Depth of contamination
Chemistry of contaminants
Vertical rate of transport
Lateral rate of transport in each stratum
Persistence of contaminants in soil
Potential for release from surface soils to air
Potential for release from surface soils to
surface water
Existing soil/ground-water monitoring data
Evidence of vegetative stress
Potential for release to ground water
Potential receptors
Have the following data on the subsequent phase(s) of the
release characterization been collected?
Further soil stratigraphic and hydrologic
characterization data
Expanded sampling data
Geophysical data on release location
(Y/N)
(Y/N)
9-74
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9.9 References
ASTM. 1984. Particle Size Analysis for Soils. Annual Book of ASTM Standards,
Method D422-63. Vol. 4.08. Philadelphia, PA.
ASTM. 1984. Standard Recommended Practice for Description of Soils. Annual
Book of ASTM Standards, Method D2488-69. Vol. 4.08. Philadelphia, PA.
Barth, D. S., and B. J. Mason. 1984. Soil Sampling Quality Assurance User's Guide.
EPA 600/4-84-043. NTIS PB84-198621. U.S. EPA. Las Vegas, Nevada.
Black, C. A. 1965. Methods of Soil Analysis, Part2: Chemical and Microbiological
Properties. American Society of Agronomy. Madison, Wisconsin.
Callahan, M. A., et al. 1979. Water-Related Environmental Fate of 129 Priority
Pollutants. Vol. 1 and 2, EPA 440/4-79-029a. NTIS PB80-204373. U.S. EPA.
Washington, D.C. 20460.
Elliot, L. F., and F. J. Stevenson. 1977. Soils for Management of Organic Wastes
and Waste Waters. Soil Science Society of America, American Society of
Agronomy, Crop Science Society of America. Madison, Wisconsin.
Everett, L. G., L. G. Wilson, and E. W. Hoylman. 1984. Vadose Zone Monitoring
for Hazardous Waste Sites. Noyes Data Corporation. Park Ridge, New Jersey,
Ford, P. J., et al. 1984. Characterization of Hazardous Waste Site - A Methods
Manual, Vol.II, Available Sampling Methods. NTIS PB85-168771. U.S. EPA.
EPA 600/4-84-076. Las Vegas, Nevada.
Freeze and Cherry. 1979. Ground Water. Prentice-Hall, Inc., Englewood Cliffs,
N.J.
Garrels, R.M. and C.L. Christ. 1965. Solutions, Minerals, and Eguilibria. Harper
and Row, New York.
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Lambe, T.W. and R.V. Whitman. 1979. Soil Mechanics, SI Version. John Wiley and
Sons, Inc., New York, New York.
Lyman, W. J. Reehl, W. F. and D. H. Rosenblatt. 1981 Handbook of Chemical
Property Estimation Methods. McGraw Hill.
Mason, B. J. 1983. Preparation of a Soil Sampling Protocol: Techniques and
Strategies. NTIS PB83-206979. U.S. EPA. Las Vegas, Nevada.
Mills, W. B., et al. 1985. Water Quality Assessment: A Screening Procedure for
Toxic and Conventional Pollutants in Surface and Ground Water. EPA/600/6-
85/002a,b, c. Vol. I, II and III. NTIS PB86-122494, 122504 and 162195.
Washington, D.C. 20460.
Merrill, L. G., L. W. Reed, and K. S. K. Chinn. 1985. Toxic Chemicals in the Soil
Environment, Volume 2: Interactions of Some Toxic Chemicals/Chemical
Warfare Agents and Soils. AD-A158-215. U.S. Army Dugway Proving Ground.
Dugway, Utah.
Oster, C. A. 1982. Review of Ground Water Flow and Transport Models in the
Unsaturated Zone. PNL-4427. Battelle Pacific Northwest Laboratory.
Richland, WA.
Rai, D. and J.M. Zachara, 1984. Chemical Attenuation Studies: Data
Development and Use. Presented at Second Technology Transfer Seminar:
Solute Migration in Ground Water at Utility Waste Disposal Sites. Held in
Denver, Colorado. October 24-25,1985. EPRI-EA-3356.
Sims, R. C., et al. 1984. Review of In-Place Treatment Techniques for
Contaminated Surface Soils, Volume 2: Background Information for In Situ
Treatment. EPA-540/2 -84-003b. NTIS PB85-1 24899. U.S. EPA. Washington,
D.C. 20460.
Stumm, W, and J.J. Morgan. 1981. Aquatic Chemistry. An Introduction
Emphasizing Chemical Equilibria in Natural Waters. John Wiley and
Sons. New York, N.Y.
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U.S.D.A. (U.S. Department of Agriculture). 1975. Soil Taxonomy: A Basic System
of Soil Classification for Making and Interpreting Soil Surveys. Soil Survey
Staff, Soil Conservation Service. Washington, D.C.
U.S. EPA. 1975. Use of the Water Balance Method for Prediciting Leachate
Generation from Solid Waste Disposal Sites. EPA/530/SW-I 68. Office of Solid
Waste. Washington, D.C. 20460.
U.S. EPA. 1982. Sediment and Soil Adsorption Isotherm. Test Guideline No. CG-
1710. ITL Chemical Fate Test Guidelines. EPA 560/6-82-003. NTIS PB82-
233008. Office of Pesticide and Toxic Substances. Washington, D.C. 20460.
U.S. EPA. 1982. Sediment and Soil Adsorption Isotherm. Support Document No.
CS-1710. ITL Chemical Fate Test Guidelines. EPA 560/6-82-003. NTIS PB83-
257709. Office of Pesticide and Toxic Substances. Washington, D.C. 20460.
U.S. EPA. 1984. Soil Properties, Classification and Hydraulic Conductivity
Testing. EPA/SW-925. Office of Solid Waste. Washington, D.C. 20460.
U.S. EPA. 1985. Handbook: Remedial Action at Waste Disposal Sites (Revised).
EPA/625/6-85/006. NTIS PB82-239054. Office of Emergency and Remedial
Response. Washington, D.C. 20460.
U.S. EPA. 1986. Criteria for Identifying Areas of Vulnerable Hydrogeoloqy Under
the Resource Conservation and Recovery Act. NTIS PB86-224953. Office of
Solid Waste. Washington, D.C. 20460.
U.S. EPA. 1986. Petitions to Delist Hazardous Wastes. EPA/530-SW-85-003. NTIS PB
85-194488. Office of Solid Waste. Washington, D.C. 20460.
U.S. EPA. 1986. Test Methods for Evaluating Solid Waste. EPA/SW-846. GPO No.
955-001-00000-1. Office of Solid Waste. Washington, D.C. 20460.
U.S. EPA. June 13, 1986. Federal Register. Volume 51, Pg. 21648. TCLP Proposed
Rule.
9-77
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U. S. EPA. 1986. Permit Guidance Manual on Hazardous Waste Land Treatment
Demonstration. NTIS PB86-229192. Office of Solid Waste. Washington, D.C.
20460.
U.S. EPA. 1987. Soil Gas Monitoring Techniques Videotape. National Audio Visual
Center. Capital Heights, Maryland 20743.
U.S. Geological Survey. 1982. National Handbook of Recommended Methods for
Water Data Acquisition.
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SECTION 10
GROUND WATER
10.1 Overview
The objective of an investigation of a release to ground water is to
characterize the nature, extent, and rate of migration of a release of hazardous
waste or constituents to that medium. This section provides:
An example strategy for characterizing releases to ground water, which
includes characterization of the source and the environmental setting of
the release, and conducting a monitoring program which will
characterize the release itself;
Formats for data organization and presentation;
Field methods which may be used in the investigation; and
A checklist of information that may be needed for release character-
ization.
The exact type and amount of information required for sufficient release
characterization will be site-specific and should be determined through interactions
between the regulatory agency and the facility owner or operator during the RFI
process. This guidance does not define the specific data needed in all instances;
however, it identifies possible information necessary to perform release
characterizations and methods for obtaining this information. The RFI Checklist,
presented at the end of this section, provides a tool for planning and tracking
information for release characterization. This list is not meant as a list of
requirements for all releases to ground water. Some release investigations will
involve the collection of only a subset of the items listed, while others may involve
the collection of additional data.
10-1
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10.2 Approach for Characterizing Releases to Ground Water
10.2.1 General Approach
A conceptual model of the release should be formulated using all available
information on the waste, unit characteristics, environmental setting, and any
existing monitoring data. This model (not a computer or numerical simulation
model) should provide a working hypothesis of the release mechanism, transport
pathway/mechanism, and exposure route (if any). The model should be
testable/verifiable and flexible enough to be modified as new data become
available.
For ground-water investigations, this model should account for the ability of
the waste to be dissolved or to appear as a distinct phase (i.e., "sinkers" and
"floaters"), as well as geologic and hydrologic factors which affect the release
pathway. Both the regional and site-specific ground-water flow regimes should be
considered in determining the potential magnitude of the release, migration
pathways and possible exposure routes. Exposure routes of concern include
ingestion of ground water as drinking water and near-surface flow of contaminated
ground water into basements of residences or other structures (see Appendix E).
This "basement seepage" pathway can pose threats through direct contact,
inhalation of toxic vapors and through fires and explosions if the contaminants are
flammable. The model should consider the degradability (chemical and biological)
of the waste and its decomposition products. The conceptual model should also
address the potential for the transfer of contaminants in ground water to other
environmental media (e.g., discharge to surface water and volatilization to the
atmosphere).
Based on the conceptual model, the owner or operator should develop a
monitoring program to determine the nature, extent, and rate of migration of
contaminant releases from SWMUs* to ground water. Three-dimensional
characterization is particularly important. The initial monitoring phase should
* Guidance in this section applies to releases from all solid waste management units, except
releases to ground water from "regulated units" as defined under 40 CFR pan 264.NW).
Releases to ground water from "regulated units" must be addressed according to the
requirements of 40 CFR Parts 264.91 thorugh 264.100 for purposes of detection,
characterization and appropriate response.
10-2
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include a limited number of monitoring wells, located and screened in such a way
that they are capable of providing background water quality and of intercepting
any release. The regulatory agency will evaluate the adequacy of an existing
monitoring system, if proposed for use in the initial monitoring phase. The owner
or operator may be required to install new wells if the existing well system is found
to be inadequate.
Initial ground-water sampling and analysis may be conducted for a limited set
of monitoring constituents. This set should include a subset of the hazardous
constituents of concern, and may also include indicator parameters (e.g., TOX).
Guidance regarding the selection of monitoring constituents and indicator para-
meters is provided in Sections 3 and 7 and in Appendix B. Sampling frequency and
duration should also be proposed in the RFI Work Plan.
Investigation of a suspected release may be terminated based on results from
an initial monitoring phase if these results show that an actual release has not, in
fact, occurred. If, however, contamination is found, the release must be adequately
characterized through a subsequent monitoring phase(s).
Subsequent characterization involves determining the detailed chemical
composition and the areal and vertical (i.e., three dimensional) extent of the
contaminant release, as well as its rate of migration. This should be accomplished
through direct sampling and analysis and, when appropriate, can be supplemented
by indirect means such as geophysical methods (See Appendix C) and modeling
techniques.
Table 10-1 outlines an example of strategy for characterizing releases to
ground water. Table 10-2 lists the specific tasks which may be used in implementing
the strategy, and the corresponding data outputs. The steps delineated in these
tables should generally be performed in sequential order, although some may be
accomplished concurrently. For example, the site's hydrogeology may be
investigated at the same time as waste and unit characterization; soil borings
installed during hydrogeologic characterization may be converted into monitoring
wells; and additional wells may be installed to more accurately characterize a
release while a sampling and analysis program is in effect at existing wells.
10-3
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TABLE 10-1
EXAMPLE STRATEGY FOR CHARACTERIZING
RELEASES TO GROUND WATER1
INITIAL PHASE
1. Collect and review existing information on:
Waste
Unit
Environmental setting
Contaminant releases, including inter-media transport
2. Identify any additional information necessary to fully characterize release:
Waste
Unit
Environmental setting
Contaminant releases, including inter-media transport
3. Develop monitoring procedures:
Formulate conceptual model of release
Determine monitoring program objectives
Plan field screening if appropriate (e.g., geophysical investigations - see
Appendix C)
Select monitoring constituents and indicator parameters
Identify QA/QC and analytical procedures
Appropriate initial area well locations (background and downgradient)
Collection of additional hydrogeologic data (if necessary)
Proper well screen interval selection
Borehole testing and use of test pitting
Sampling frequency and duration of monitoring
Identification of data presentation and evaluation procedures
4. Conduct initial monitoring phase:
Conduct field screening, if appropraite
Collect samples and perform appropriate field measurements
Analyze samples for selected parameters and constituents
5. Collect, evaluate and report results:
Compare monitoring results to health and environmental criteria and
identify and respond to emergency situations and identify priority
situations that warrant interim corrective measures - Notify regulatory
agency
Determine completeness and adequacy of collected data
Summarize and present data in appropriate format
10-4
-------
TABLE 10-1 (Continued)
EXAMPLE STRATEGY FOR CHARACTERIZING
RELEASES TO GROUND WATER'
INITIAL PHASE (Continued)
Determine if monitoring program objectives were met
Determine if monitoring locations, constituents and frequency were
adequate to characterize release (nature, rate, and extent)
SUBSEQUENT PHASES (If Necessary)
1. Identify additional information necessary to characterize release:
Perform further hydrogeologic characterization, if necessary
Add and delete constituents or indicator parameters as appropriate
Employ geophysical and other methods to estimate extent of release and
to determine suitable new monitoring locations
Inter-media transport
2. Expand monitoring network as necessary:
Increase density of monitoring locations
Expand monitoring locations to new areas
Install new monitoring wells
3. Conduct subsequent monitoring phases:
Collect samples and complete field analysis
Analyze samples for selected parameters and constituents
4. Collect, evaluate, and report results/identify additional information necessary
to characterize release:
Compare monitoring results to health and environmental criteria and
identify and respond to emergency situations and identify priority
situations the warrant interim corrective measures - Notify regulatory
agency
Summarize and present data in appropriate format
Determine if monitoring program objectives were met
Determine if monitoring locations, constituents, and frequency were
adequate to characterize release (nature, extent, and rate)
Identify additional information needs
Determine need to expand monitoring
Evaluate potential role of inter-media impact
Report results to regulatory agency
1 The possibility for inter-media transport of contamination should be
anticipated throughout the investigation.
10-5
-------
TABLE 10-2
RELEASE CHARACTERIZATION TASKS FOR GROUND WATER
Investigatory Tasks
Investigatory Techniques
Data Presentation Formats/Outputs
1. Waste/Unit Characterization
Identify waste properties
(e.g., pH, viscosity)
Identify constituents of
concern/possible indicator
parameters
- Determine physical/chemical
properties of constituents
- Determine unit dimensions
and other important design
features and operational
conditions
- Investigate possible uni
release mechanisms to help
determine flow
characteristics
Review existing information and
conduct waste sampling if
necessary (See Sections 3 &7)
Review existing information and
conduct waste sampling if
necessary (See Sections 3 &7)
Review existing information (See
Section 7)
Review existing information and
conduct unit examinations (See
Section 7)
Review existing information am
conduct unit examinations (See
Section 7)
Tabular presentation (See
Section 5)
Tabular presentation (See
Section 5)
Tabular presentation (See
Section 5)
Tabular presentations, facility
maps & photographs & narrative
discussion (See Section 5 and
Appendix A)
Facility maps & photographs&
narrative discussions (See
Appendix A)
2. Environmental Setting
Characterization
Examine surface features &
topography for indications
of subsurface conditions
Define subsurface conditions
& materials, including soil
and subsurface physical
properties (e.g., porosity
cation exchange capacity)
- Review existing information
facility maps, aerial & other
photographs, site history,
conduct surface geological
surveys
- Review of existing geologi
information
- Soil borings and rock coring
- Soil & subsurface material
testing
Geophysical technqiues (Sec
Appendix C)
Facility map & photographs/text
discussion (See Appendix A &C)
- Narrative discussions of geology
- Boring and coring logs
- Subsurface profiles, transects &
fence diagrams (See Appendix A
& Section 5)
- Tabular presentations of soil &
subsurface physical & chemical
properties
- Geologic cross sections &
geologic & soil maps (See Section
5 & 9 & Appendix A)
- Structure contour maps (plan
view) of aquifer & aquitards (See
Section 5 & Appendix A)
10-6
-------
TABLE 10-2
RELEASE CHARACTERIZATION TASKS FOR GROUND WATER (continued)
Investigatory Tasks
Investigatory Techniques
Data Presentation Formats/Outputs
2. Environmental Setting
Characterization (Continued)
- Identification of regional
flow ceils, ground-water
flow paths & general
hydrology, including
hydraulic conductivities &
aquifer interconnections
Review of existing information
Installation of piezometers &
water level measurements at
different depths
Flow cell & flow net analyses
using measured heads
Pumping & slug tests& tracer
studies
Geophysical techniques (See
Appendix C)
Identification of potential
receptors
Review of existing information,
area maps, etc.
Narrative descriptions of
ground-water conditions, flow
cells, flow nets, flow patterns,
including flow rates & direction
Water table or potentiometric
maps (plan view) with flow lines
(See Section 5)
hydrologic cross sectional maps
(See Section 5)
Flow nets for vertical &
horizontal flow
Tabular presentations of raw
data & interpretive analysis
Narrative discussion & area maps
3. Release Characterization
Determine background
levels & determine vertical
and horizontal extent of
release, including
concentrations of
constituents & determine
rate & directions of release
migration
Sampling & analysis of ground-
water samples from monitoring
system
Geophysical methods (See
Appendix C) for detect!ng&
tracking plume
Modeling to estimate extent of
plume & rate& direction of
plume migration
Tabular presentations of
constituent & indicator
parameter analyses (See Section
5)
Iso-concentrations maps of
contamination (See Section 5)
Maps of rates of release
migration &direction showing
locations of possible receptors
(See Section 5)
Narrative discussion &
interpretations of tabular&
graphical presentations
10-7
-------
The specific tasks to be conducted for each release will be determined on a
site-specific basis. It should be noted that some of the characterization tasks may
have been previously accomplished in conjunction with the 40 CFR Parts 264
and 265, Subpart F (ground-water monitoring) regulations.
As monitoring data become available, both within and at the conclusion of
discrete investigation phases, it should be reported to the regulatory agency as
directed. The regulatory agency will compare the monitoring data to applicable
health and environmental criteria to determine the need for (1) interim corrective
measures; and/or (2) a Corrective Measures Study. In addition, the regulatory
agency will evaluate the monitoring data with respect to adequacy and
completeness to determine the need for any additional monitoring efforts. The
health and environmental criteria and a general discussion of how the regulatory
agency will apply them are supplied in Section 8. A flow diagram illustrating RFI
decision points is provided in Section 3 (See Figure 3-2).
Notwithstanding the above process, the owner or operator has a continuing
responsibility to identify and respond to emergency situations and to define priority
situations that may warrant interim corrective measures. For these situations, the
owner or operator is directed to obtain and follow the RCRA Contingency Plan
under 40 CFR Part 264, Subpart D.
Case Study numbers 10, 18, 19, 20, 21 and 22 in Volume IV (Case Study
Examples) illustrate the conduct of various aspects of ground-water investigations.
10.2.2 Inter-media Transport
Indirect releases (inter-media transfer) to ground water may occur as a result
of contaminant releases to soil and/or surface water that percolate or discharge to
ground water. These releases may be recurrent or intermittent in nature, as in the
case of overland run-off, and can vary considerably in areal extent. Direct releases
to ground water may occur when waste materials are in direct contact with ground
water ( e.g., when a landfill rests below the water table).
Releases of contaminated ground water to other media may also occur, for
example, in those cases where ground and surface waters are hydraulically
10-8
-------
connected. Volatilization of contaminated ground water to the air within
residential and other structures may occur via the basement seepage pathway, as
described previously. It is important for the owner or operator to be aware of the
potential for such occurrences, and to communicate these to the regulatory agency
when discovered.
This section provides guidance on characterizing ground-water releases from
units, as well as those cases where inter-media transport has contaminated ground
water. The owner or operator should be aware that releases to several media can
often be investigated using concurrent techniques. For example, soil gas surveys
may help to characterize the extent of soil and subsurface gas releases and, at the
same time, be used to estimate the extent of a ground-water release. Further
guidance on the use of soil gas surveys for investigating releases to soil and ground
water are presented in the Soil Section (Section 9).
10.3 Characterization of the Contaminant Source and the Environmental Setting
10.3.1 Waste Characterization
Knowledge of the waste constituents (historical and current) and their
characteristics at the units of concern is essential in selecting monitoring
constituents and well locations. Waste (source) information should include
identifying volumes and concentrations of hazardous waste or constituents present,
and their physical and chemical characteristics.
Identification of hazardous constituents may be a relatively simple matter of
reviewing records of unit operations, but generally will require direct sampling and
analysis of the waste in the unit. Hazardous constituents may be grouped by similar
chemical and physical properties to aid in developing a more focused monitoring
program. Knowledge of physical and chemical properties of hazardous constituents
can help to determine their mobility, and their ability to degrade or persist in the
environment. The mobility of chemicals in ground water is commonly related to
their volubility, volatility, sorption, partitioning, and density.
Section 3 provides additional guidance on monitoring constituent selection
and Section 7 provides additional guidance on waste characterization. The
10-9
-------
following discussion describes several waste-related factors and properties which
can aid in developing ground-water monitoring procedures:
t The mobility of a waste is highly influenced by its physical form. Solid
and gaseous wastes are less likely to come in contact with ground water
than liquid wastes, except in situations where the ground-water surface
directly intersects the waste, or where infiltrating liquids are leaching
through the unsaturated zone.
The concentration of any constituent at the waste source may provide an
indication of the concentration at which it may appear in the ground
water.
The chemical class (i.e., organic, inorganic, acid, base, etc.) provides an
indication of how the waste might be detected in the ground water, and
how the various components might react with the subsurface geologic
materials, the ground water, and each other.
The pH of a waste can provide an indication of the pH at which it would
be expected to appear in the ground water. A low pH waste could also
be expected to cause dissolution of some subsurface geologic materials
(e.g., limestone), causing channelization and differential ground-water
flow, as in karst areas.
The acid dissociation constant of a chemical (pKa) is a value which
indicates its equilibrium potential in water, and is equal to the pH at
which the hydrogen ion is in equilibrium with its associated base. If
direct pH measurements are not feasible, the concentration of a waste in
combination with its pKa can be used to estimate the likely pH which will
occur at equilibrium (in ground water), at a given temperature. Acid
dissociation values can be found in most standard chemistry handbooks,
and values for varying temperatures can be calculated using the Van't
Hoff equation (Snoeyink and Jenkins, 1980).
Viscosity is a measure of a liquid's resistance to flow at a given
temperature. The more viscous a fluid is, the more resistant it is to flow.
10-10
-------
Highly viscous wastes may travel more slowly than the ground water,
while low-viscosity wastes may travel more quickly than the ground
water.
Water volubility describes the mass of a compound that dissolves in or is
miscible with water at a given temperature and pressure. Water
volubility is important in assessing the fate and transport of the
contaminants in ground water because it indicates the chemical's affinity
for the aqueous medium. High water volubility permits greater amounts
of the hazardous constituent to enter the aqueous phase, whereas low
water volubility indicates that a contaminant can be present in ground
water as a separate phase. Therefore, this parameter can be used to
establish the potential for a constituent to enter and remain in the
ground water.
The density of a substance (solid or liquid) is its weight per unit volume.
The density of a waste will determine whether it sinks or floats when it
encounters ground water, and will assist in locating well screen depths
when attempting to monitor for specific hazardous constituents released
to ground water.
The log of the octanol/water partition coefficient (Kow) is a measure of
the relative affinity of a constituent for the neutral organic and inorganic
phases represented by n-octanol and water, respectively. It is calculated
from a ratio (P) of the equilibrium concentrations (C) of the constituent
in each phase:
P = Coctano1 and Kow = log P
Cwater
The Kowhas been correlated to a number of factors for determining
contaminant fate and transport. These include adsorption onto soil
organic matter, bioaccumulation, and biological uptake. It also bears a
relationship to aqueous volubility.
10-11
-------
The Henry's Law Constant of a constituent is the relative equilibrium
ratio of a compound in air and water at a constant temperature. It can
be estimated from the equilibrium vapor pressure divided by the
volubility in water and has the units of atm-mVmole. The Henry's Law
Constant expresses the equilibrium distribution of the constituent
between air and water and indicates the relative ease with which the
constituent may be removed from aqueous solution.
Other influences of the waste constituents should also be considered.
Constituents may react with soils, thereby altering the physical properties
of the soil, most notably hydraulic conductivity. Chemical interactions
among waste constituents should also be considered. Such interactions
may affect mobility, reactivity, volubility, or toxicity of the constituents.
The potential for wastes or reaction products to interact with unit
construction materials (e.g., synthetic liners) should also be considered.
The references listed in Section 7 may be used to obtain information on the
parameters discussed above. Other waste information may be found in facility
records, permits, or permit applications. It should be noted that mixtures of
chemicals may exhibit characteristics different than those of any single chemical.
10.3.2 Unit Characterization
Unsound unit design and operating practices can allow waste to migrate from
a unit and possibly mix with natural runoff. Examples include surface impound-
ments with insufficient freeboard allowing for periodic overtopping; leaking tanks
or containers; or land based units above shallow, low permeability materials which,
if not properly designed and operated, can fill up with water and spill over. In
addition, precipitation falling on exposed wastes can dissolve and thereby mobilize
hazardous constituents. For example, at uncapped active or inactive waste piles and
landfills, precipitation and leachate are likely to mix at the toe of the active face or
the low point of the trench floor.
Unit dimensions (e.g., depth and surface area) and configuration (e.g.,
rectangular, parallel trenches), as well as volume (e.g., capacity) should also be
10-12
-------
described, because these factors will have a bearing on predicting the extent of the
release and the development of a suitable monitoring network.
10.3.3 Characterization of the Environmental Setting
Hydrogeologic conditions at the site to be monitored should be evaluated for
the potential impacts the setting may have on the development of a monitoring
program and the quality of the resulting data. Several hydrogeologic parameters
should be evaluated, including:
Types and distribution of geologic materials;
Occurrence and movement of ground water through these materials;
Location of the facility with respect to the regional ground-water flow
system;
Relative permeability of the materials; and
Potential interactions between contaminants and the geochemical
parameters within the formation(s) of interest.
These conditions are interrelated and are therefore discussed collectively below.
There are three basic types of geologic materials through which ground water
normally flows. These are: (1) porous media; (2) fractured media; and (3) fractured
porous media. In porous media (e.g., sand and gravels, silt, loess, clay, till, and
sandstone), ground water and contaminants move through the pore spaces
between individual grains. In fractured media (e.g., dolomites, some shales,
granites, and crystalline rocks), ground water and contaminants move
predominantly through cracks or solution crevices in otherwise relatively
impermeable rock. In fractured porous media (e.g., fractured tills, fractured
sandstone, and some fractured shales), ground water and contaminants can move
through both the intergranular pore spaces as well as cracks or crevices in the rock
or soil. The occurrence and movement of ground water through pores and cracks or
solution crevices depends on the relative effective porosity and degree of
10-13
-------
channeling occurring in cracks or crevices. Figure 10-1 illustrates the occurrence and
movement of ground water and contaminants in the three types of geologic
materials presented above.
The distribution of these three basic types of geologic materials is seldom
homogeneous or uniform. In most settings, two or more types of materials will be
present. Even for one type of material at a given site, large differences in
hydrologic characteristics may be encountered. The heterogeneity of the materials
can play a significant role in the rate of contaminant transport, as well as in
developing appropriate monitoring procedures for a site.
Once the geologic setting is understood, the site hydrology should be
evaluated. The location of the site within the regional ground-water flow system,
or regional flow net, should be determined to evaluate the potential for
contaminant migration on the regional scale. Potentiometric surface data (water
level information) for each applicable geologic formation at properly selected
vertical and horizontal locations is needed to determine the horizontal and vertical
ground-water flow paths (gradients) at the site. Figure 10-2(a) and (b) illustrate two
geohydrologic settings commonly encountered in eastern regions of the
United States, where ground water recharge exceeds evapotranspirational rates.
Figure 10-2(C) illustrates a common geohydrologic setting for the arid western
regions of the United States. The potential dimensions of a contaminant release
would depend on a number of factors including ground-water recharge and
discharge patterns, net precipitation, topography, surface water body locations,
and the regional geologic setting.
Table 10-3 and Figures 10-3 through 10-16 illustrate regional, intermediate,
and local ground water regimes for the major ground-water regions in the United
States. Ground-water flow paths, and where possible, generalized flow nets are
shown superimposed on cross-sections of the geological units. Much of the
information presented in the figures and following text descriptions were taken
from Heath et. al., 1984 (Ground Water Regions of the U. S., U. S.G.S. Water Supply
10-14
-------
(a)
(b)
(O
Figure 10-1. Occurrence and movement of ground water and contaminants
through (a) porous media, (b) fractured or creviced media,
(c) fractured porous media.
-------
(a) LOCAL AND REGIONAL GROUND WATER
FLOW SYSTEMS IN HUMID ENVIRONMENTS
(b) TEMPORARY REVERSAL OF IMOUND-WATER FLOW DUE TO
FLOODING OF A RIVER OR STREAM
(c) TYPICAL GROUND-WATER FLOW PATHS IN ARID ENVIRONMENTS
Figure 10-2. Ground-water flow paths in some different hydrogeologic settings.
10-16
-------
TABLE 10-3. SUMMARY OF U.S GROUND WATER REGIONS
Region
I
2
3
4
5
6
7
8
9
10
11
12
13
Region Name
(Heath, 1984)
Western Mountain Ranges
Alluvial Basins
Columbia Lava Plateau
Colorado Plateau
High Plains
Non-glaciated central
Glaciated Central
Piedmont and Blue Ridge
Northeast and Superior
Uplands
Atlantic & Gulf Coastal
Plain
Southeast Coastal Plain
Hawaiian Islands
Alaska
Recharge Area
infiltration in mountains
and mountain fronts
plateau uplands
surface infiltration
infiltration in plateau
uplands; infiltration from
surface waters
surface infiltration
upland infiltration
surf ace infiltration
surface infiltration
upland infiltration
infiltration in outcrop areas
infiltration in outcrop areas
surface infiltration
variable*
Discharge Area
streams and rivers
streams and rivers,
some enclosed basins,
localized springs and seeps
in steeper terrain
rivers and streams
seeps, springs, and surface
waters
rivers and streams, seeps
and springs along eastern
escarpments
springs, seeps, streams and
rivers
springs, streams, rivers, and
lakes
springs, seeps, and surface
waters
surface water
surface water or subsea
leakage
surface water or subsea
leakage
springs, seeps, and surface
waters
variable*
Dimensions
(miles)
<1-5 unconfined
5-60 confined
<1-20 unconfined
5-80 confined
10-200 miles
5-80 miles
2-300 miles
<1-40 miles
<1-20 miles
<1-5 miles
<1-20 miles
10-150 miles
1-80 miles
<1-30 miles
variable*
Example
Wasatch Range, Utah
Nevada
Snake River Plain
Southeast Utah
Nebraska
Ohio Great Miami
Minnesota
West Virginia
Massachusetts
New Jersey
South Georgia
Oahu, Hawaii
North Slope
The recharge area, discharge area, and dimensions of the flow cells within Alaska are highly variable due to the wide range in topography
and geology found in this region.
-------
WESTERN MOUNTAIN RANGES
(Mountains with thin soils over fractured rocks,
alternating with narrow alluvial and, in part,
glaciatad valleys)
A
Potenfometric Surface
of Lower Aquifer
I
Confined
Bed
Silty Clay
Figure 10-3. Western Mountain Ranges
10-18
-------
Recharge Area
(Mountain Range)
Recharge Area
Granite
Weathered Bedrock
Clay
iiJ Alluvial Deposits
,0-3. Western Moun,ainRanges(eontJnued)
-------
ALLUVIAL BASINS
(Thick alluvial deposits in basins and valleys
bordered by mountains)
A1
Vally Fill
Alluvial Deposit
Boundary condition
Alluvial
Deposit
Limes tone
Figure 10-4. Alluvial Basins A
10-20
-------
COLUMBIA LAVA PLATEAU
Thick sequence of laval flows irregulary intebdded
with thin unconsolidated deposits and overlain by thin soils)
^consolidated
Holocene-Piiocene
Sediments
Basalt
- J
rk0JH interbedded Basalt
and Sediments
1^-ij Basement Rocks
Figure 10-5. Columbia Lava Plateau
10-21
-------
Schematic Diagram of
Ground Water Flow Regime Through a Saturated Cross Section
Note: Assume hydraulic heads increase with depth.
Interbed,
Row Top, ^
Row Bottom
Dense Flow
Center with -
Vertical Joints
-High horizontal flow along flow tops
-Low vertical leakage through basalt interiors
Figure 10-5. Columbia Lava Plateau (continued)
10-22
-------
COLORADO PLATEAU AND
WYOMING BASIN
(This soils over consolidated sedimentary
rocks)
Fault scarp
Cliff.
Canyon Extinct v°'<»noes
Ridges
Dome
Metamorphic
rocks
Figure 10-6. Colorado Plateau
10-23
-------
HIGH PLAINS
(Thick Alluvial deposits over fractured
sedimentary rocks
1. Paleovalley Alluvial Aquifers
2. High Plains Aquifer System
3. Niobrara Sandstone Aquifer
4. Pierre Shale Aquitard
5. Dakota sandstone Aquifer
6. Undifferentiated Aquifers
in Crataceous Rocks
Generalized local ground water regime for site within the
High plains Region
Figure 10-7. High Plains
10-24
-------
Ground water flow in
sandstone and clay lenses
B
plow Line
Equipotential Line
Gravel
Sand
Sandsto
ne
Losing Stream
Generalized Regional Flow
Withdrawal Well
Playa
Western Texas.
(Recharge centered at playas)
Figure 10-7. High Plains (continued)
10-25
-------
NONGLACIAED CENTRAL REGION
(Thin regolith over fractured sedimentary rocks)
if' Sandstone
r" J
L J Saltwater
Fractures
Clay So/Is
Solution Enlarged
ts and Fractures and
ne Disolution
Figure 10-8. Non-glaciated Central
10-26
-------
Surface Impoundments
Stream
Fill berm
Alluvial Deposits
" "i Y.V'l ' "~
Figure 10-8.
Shale
Sandstone
Clay Stone
Pittsburgh Coal
Limestone
Flow Line
ntial Line
Example of a surface impoundment site in Non-Glaciated Central
Region (continued)
-------
GLACIATED CENTRAL REGION
(Glacaial deposits over fractured sedimenary rocks)
^~~-3 Shale
Sand
°UtWaShed ^Posits and
A1
Figure 10-9. Glaciated Central
Sandstone
F'ow Line
£9U'Poten6alLine
IO-28
-------
600'
Waste Disposal Unit
500" JE
400'
200'
TJII > k xx,
"" Clay and Silt
100- so-
0'
50" tOO-
-------
PIEDMONT BLUE RIDGE REGION
(Thick regolith over fractured crystalline and
metamorphosed sedimentary rocks)
V-I--T--T-
Bedrock outcrops
Fractures
sfo Saprolite
[::;X-:-X'.j Crystalline Bedrock
Note: In areas of fractured bedrock, flow through fractures is often greater than flow through the bedrock matrix. Flow through these frac-
tures may not conform to Darcy's Law. The above flow lines represent generalized flow paths rather than quantitative flow lines used in
a flow net.
Figure 10-10. Piedmont and Blue Ridge
10-31
-------
NORTHEAST AND SUPERIOR UPLANDS
(Glacial Deposits Over Fractured
Crystalline rocks)
Fractures
Glacio-Fluvial Sand and Gravel
jig] Fluvial Valley Train Deposits
Delta Deposits
Kame Terrace Deposits
-^ri.-^-I Glacio-lacustrine Fine-grained sediments
-'//.','/\ Bedrock
- Equipotential Line
A'
Note: Flow component along
axis of valley, although
not shown in this
cross-section can often
be important.
Figure 10-11. Northeast and Superior Uplands
10-32
-------
Discharge Area
B
Water Table
Generalized local ground water regime within the Northeast and
Superior Uplands Region showing a confining layer of till.
Figure 10-11. Northeast and Superior Uplands (continued)
10-33
-------
ATLANTIC AND GULF COASTAL PLAIN
(Complexly interbedded sand, silt, and day)
& Clay
Sand
' i ' I Limestone
Figure 10-12. Atlantic and Gulf Coastal Plain
10-34
-------
Losing Stream
'Q'*:'*';] Alluvial Deposits
200
*4_>L~?-^^^^5Sfet Sea Level
^"~T ~ ~~^^>r~~^-r-~-
L_ -200
Sandy Shale
Note: Regional flow based on high recharge in hills which are
not shown in this diagram.
Figure 10-12. Atlantic and Gulf Coastal Plain (continued)
10-35
-------
Landfill site near the Savannah River in Georgia.
West
East
UJ
-*"*»-*-^
h=107
Figure 10-12. Atlantic and Gulf Coastal Plain (continued)
-------
SOUTHEAST COASTAL PLAIN
(Thick layers of sand and day over semiconsolidated
carbonate rocks)
Row Line
A" - Equipotential Line
Solution Limestone
Cross-section with highly generalized flow path lines and equipotential
lines. Actual condition in Karst terrain may not be definable due to
fractures and solution channel flow.
Recharge Area
Discharge Area
Sinkhole
Pteistocene/Holocene Sand *
k /
Lake ^ / Limestone Spring
(Artesian Condition)
Hawthorn g>-:->:-".. rr:>: j^g
Figure *\ 0-13. Southeast Coastal Plain
10-37
-------
-Highly permeable due to water-washed sands.
a 8ourc8 °' recha^
to «r» underlying uncorwolidated coastal deposits.
Figure 10-13. Southeast Coastal Plain (continued)
10-38
-------
9C
HAWAIIAN ISLANDS
(Lava flows segmented in part by dikes,
interbedded with ash deposits, and partly
overlain by alluvium)
.8-
HAWAIIAN
, ISIANO
PACIFIC |0 C £ A
Buried s
UNSATURATED ROCK
jj c^^^c? -** ^
i coastal deposits
Figure 10-13. Southeast Coastal Plain (continued)
-------
159*30'
159*20'
1&8W
Keuei
22"
10'
22"
00'
Kekeh?
10M4LES
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156-20' 15Fl6'
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21'
30'
21'
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157-SO'
157'40'
156*00'
20-
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30'
00'
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BartMr* Point
S 10 MILES
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0 S 10 IS KILOMETERS
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10 20 30 KILOMETERS
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0 5 10 IS KILOMETERS
^^^ . EXPLANATION
Ullllllj Ground water impounded by dikM or
other Mructur**
11 i | i | i | Af>« underlain by geologic structure*
uiteble for impounding ground weter
I I Ground water perched on Boil or esh
layera
| JBeeel ground water floeting on Mline
ground weler
[ ) Brecki*h betel ground weter
Approximate outline of [he different ground-water areas on the principal Hawaiian islands. (From Takasaki, 1977 )
Figure 10-14. Hawaiian Islands (Continued)
-------
ALASKA
(Glacial and Alluvial Deposits, Occupied in
Part by Permafrost, and Overlying Crystalline,
Metamorphic, and Sedimentary Rocks)
Water
Permafrost
Figure 10-15. Alaska
10-41
-------
EXPLANATION
Continuous permafrost
Discontinuous permsfrosi
No permafrost
Losing
Stream
Alluvium
°''^V,'::'7r^^v:^
^^^^Vv^-^X: -Tf-; .if- V
^
Figure 10-15. Alaska (Continued)
10-42
Permafrost
Fluvial
Deposits
Till
Alluvial
Deposits
-------
ALLUVIAL VALLEYS
Thick sand and gravel deposits beneath floodplains and terraces
of strams)
MISSISSIPPI RIVER
\\.v\\v^
Naturall^ee
Gravel
Sand
Silt and clay
i i] Limeatone
Figure 10-16. Alluvial Valleys
10-43
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Buried drum site
Existing Ground Surface.
Pre-Excavation Ground Surface
£ Fine Sand, Trace ^ Lake Deposits
1+00 2-fOO 3+00 4+00 5+00 6+00 7+00 8+00 9+00 10+00
Horizontal Distance
Stratified Drift
Figure 10-16. Alluvial Valleys (continued)
10-44
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Paper No. 2242). Following are descriptions of each of the major ground-water
regions illustrated in the Figures (Figures 10-3 through 10-16).
Ground-water flow in the Western Mountain Ranges region is influenced by
melting snow and rainfall at higher altitudes. The thin soils and fractures present in
the underlying bedrock have a limited storage capacity and are filled quickly with
recharging ground water flowing from higher elevations (see Figure 10-3). The
remaining surface water runs overland to streams that eventually may recharge
other areas. Streams that recharge ground water are referred to as "losing
streams." Figure 10-3 also shows local ground-water flow paths influenced by low
permeability bedrock located in intermountain valleys throughout the mountain
ranges.
The Alluvial Basins region consists of deep, unconsolidated sediments adjacent
to mountain ranges. Precipitation often runs rapidly off the mountains and
infiltrates into the alluvium at the valley margins. The water moves through the
sand and gravel layers toward the centers of the basins (Figure 10-4). The presence
of disjointed masses of bedrock in this region is crucial to the hydrogeological
regime. Low permeability igneous bedrock often isolates the ground-water regime
into individual basins with minimal exchange of ground water. Where the bedrock
is composed of limestone or other highly permeable formations, large regional flow
systems can develop, encompassing many basins. Recharge areas in this region are
located in upland areas; lowland stream beds only carry water when sufficient
runoff from the adjoining mountains occurs.
Basaltic bedrock is the major source of ground water within the Columbia Lava
Plateau region. Volcanic bedrock yields water mainly from zones at the contacts of
separate basalt flows. The permeability and hydraulic conductivity are much higher
in these zones at the edges of the flows than in the center of the flows (see Figure
10-5.) This is caused partially by the rapid cooling and consequent fracturing of the
top of each basalt flow.
The Colorado Plateau and Wyoming Basin region is a large plateau consisting
principally of sandstones, shales, and limestones. These sedimentary rocks are
generally horizontal but have been modified by basins and domes in some areas
(see Figure 10-6). Sandstones have significant primary porosity and are the major
10-45
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water-bearing units in this region. Recharge occurs where the sandstones are
exposed. Intermittent losing streams created by sudden summer storms provide
some recharge, but most recharge is caused by snowmelt.
Generally, ground water is unconfined in the recharge areas and confined in
the lower reaches of the aquifers. The storage coefficients and transmissivities in
the confined portions of the aquifers are small, causing extensive drawdown during
even minor pumping. Saline ground water is characteristic of this region and is
caused by the existence of gypsum and halide in the sedimentary deposits.
The High Plains region is underlain by thick alluvial deposits that comprise a
productive and extensively developed aquifer system. The source of recharge to the
aquifer system is precipitation, except in Western Texas where recharge is centered
at playas (see Figure 10-7). In many areas, well discharges far exceed recharge, and
water levels are declining. The dominant features influencing ground-water flow in
this region include the Ogalalla Aquifer, the Pierre Shale, and the complex
interbedding of sand and clay lenses. Figure 10-7 provides generalized flow nets,
showing flow patterns through these features.
Thin regolith over fractured sedimentary rocks typifies the nature of the
geology in the Nonqiaciated Central region (see Figure 10-8). This region extends
from the Rocky Mountains to the Appalachian Mountains. Water is transmitted
primarily along fractures developed at bedding planes. Interconnected vertical
fractures also can store a large portion of the ground water. An example of ground-
water flow on a local scale is shown for karst terrain, where ground water moves
rapidly through solution cavities and fractures in limestone and where the flow
pathways are closely associated with the configuration of fractures. Ground-water
flow in the karst regime does not usually follow Darcy's law because most of the
flow goes through large channels rather than the pores in the rock. Thus,
construction of a flow net may not be appropriate in some cases. An additional
example of localized flow in this region is provided, showing a surface
impoundment site in Pennsylvania. Notice that ground water discharges to surface
water, a phenomenon typical of this region.
The topography of the Glaciated Central region is characterized by rolling hills
and mountains in the eastern portion of the region and by flat to gently rolling
10-46
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terrain in the western portion of the region. Glacial deposits vary in thickness
within the region and are underlain by bedrock. Ground water occurs in the glacial
deposits in pores between the grains and in the bedrock primarily along fractures.
Permeability of glacial deposits ranges from extremely transmissive in gravels to low
transmissivity in poorly sorted tills. The presence of buried valleys, till, deltas,
kames, and other glacial artifacts highly influences the transmission of ground
water within the region. Two examples of localized flow are presented in Figure 10-
9. The first example shows a flow regime in an area where till has the highest
hydraulic conductivity relative to the other formations. In the second example, the
till bed has a much lower hydraulic conductivity than the deltaic outwash deposited
above it.
Thick regolith overlies fractured crystalline and metamorphic bedrock in most
of the Piedmont and Blue Ridge region. The hydraulic conductivities of regolith and
fractured bedrock are similar. However, bedrock wells generally have much larger
ground-water yields than regolith wells because, being deeper, they have a much
larger available drawdown. Fracture-controlled movement of ground water
through bedrock is illustrated by generalized flow paths rather than quantitative
flow lines used in a flow net in Figure 10-10, as is ground-water movement through
saproiite (weathered bedrock) and river alluvium.
The Northeast and Superior Uplands region is characterized by folded and
faulted igneous and metamorphic bedrock overlain by glacial deposits. The primary
difference in the ground-water environment between this region and the Piedmont
and Blue Ridge region is the presence of glacial material rather than regolith. The
different types of glacial material have vastly different storage capacities and
hydraulic conductivities. Examples of ground-water flow through till, delta, and
kame deposits, as well as a generalized ground-water regime with upward
gradients, are illustrated in Figure 10-11.
The Atlantic and Gulf Coastal Plain region is underlain by unconsolidated
sediments that consist primarily of sand, silt, and clay. The sediments are often
interbedded as a result of deposition on floodplains or deltas and of subsequent
reworking by ocean currents. Recharge to the ground-water system occurs in the
interstream areas; most streams in this region are gaining streams (see Figure 10-
12). Encroachment of salt water into well drawdown areas can be a problem in this
10-47
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area if high rates of ground-water withdrawal occur. An example of a regional flow
net based on high recharge in hills shows how regional flow may differ from
localized flow based on local topography. Also shown in Figure 10-12 is a landfill
located in a recharge area near the Savannah River in Georgia.
Ground water in the Southeast Coastal Plain region lies primarily within
semiconsolidated limestone. Sand, gravel, clay, and shell beds overlie the limestone
beds. Recharge in this region occurs by precipitation infiltrating directly into
exposed limestone and by seepage through the permeable soils that partially
mantle the limestone (see Figure 10-13). Coastal environments, such as beaches and
bars, and swamp areas have different ground-water regimes, which are shown in
Figure 10-13. Flow through solution channels and large fractures in limestone is
often rapid, similar to the situation shown in Figure 10-8.
The Hawaiian Islands region consists of many distinct and separate lava flows
that repeatedly issued from several eruption centers forming mountainous islands.
Lava extruded below sea level is relatively impermeable; lava extruded above sea
level is much more permeable, having interconnected cavities, faults, and joints.
Ground-water flow in this region is similar to that of the Columbia Plateau region,
with the central parts of thick lava flows being less permeable and the major
portion of ground-water flow in these thick beds occurring at the edges and
contacts of the different lava flows. Alluvium overlies the lava in the valleys and
portions of the coastal plains.
Ground water in this region can be characterized by one of three ground-
water flow regimes. The first flow regime consists of ground water impounded in
vertical compartments by dikes in the higher elevations near the eruption centers.
The second flow regime consists of fresh water floating on salt water in the lava
deposits that flank the eruption centers. This ground water is referred to as basal
ground water and makes up the major aquifers in the region. In some areas of the
coastal plain, basal ground water is confined by overlying alluvium, which may
restrain seaward migration of fresh water. The third flow regime is where ground
water is perched on soils, ash, or thick impermeable lava flows above the basal
ground water. Figure 10-14 illustrates examples of ground-water flow in this
region.
10-48
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The Alaska region comprises several distinct flow regimes that can be
categorized by ground-water regions in the lower 48 States. For example, Alaska's
Pacific Mountain System is similar to the Western Mountain Range and Alluvial
Basin regions described previously. The major variable causing Alaska to be
classified as a separate region is its climate and the existence of permafrost over
most of the region.
Permafrost has a major effect on the hydraulic conductivity of most geologic
deposits. Hydraulic conductivity declines as temperatures drop below 0°C. This
effect can be severe, causing a deposit that would be an aquifer in another area to
become a low-permeability aquitard in an area of permafrost. In Alaska, ground-
water supplies are drawn from deposits that underlie the permafrost or from areas
where the permafrost is not continuous. See Figure 10-15.
Most recharge in this region occurs in large alluvial deposits, such as alluvial
fans, which streams cross and discharge to. Although the volume of interstream
surface water is large during periods of snow melt, these interstream areas do not
act as recharge areas because they are usually frozen during the snow melts.
The Alluvial Valley region consists of valleys underlain by sand and gravel
deposited by streams carrying sediment-laden melt water from glaciation that
occurred during the Pleistocene. These valleys are considered to be a distinct
ground-water terrain. They occur throughout the United States and can supply
water to wells at moderate to high rates (see Figure 10-16). These valleys have thick
sand and gravel deposits that are in a clearly defined band and are in hydraulic
contact with a perennial stream. The sand and gravel deposits generally have a
transmissivity of 10 or more times greater than that of the adjacent bedrock. Silt
and clay commonly are found both above and below the sand and gravel channels
in the Alluvial Valley region as a result of overbank flooding of rivers. Ground-
water recharge in this region is predominantly by precipitation on the valleys, by
ground water moving from the adjacent and underlying aquifers, by overbank
flooding of the streams, and, in some glacial valleys, by infiltration from tributary
10-49
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streams. An example of a flow net illustrating local ground-water movement
beneath a waste disposal site in Connecticut also is shown in Figure 10-16.
In addition to determining the directions of ground-water flow, it is essential
to determine the approximate rates of ground-water movement to properly design
a monitoring program. Hydraulic conductivity, hydraulic gradient, and effective
porosity data are required to estimate the average linear velocity of ground water
and, therefore, assist in the determination of the rate of contaminant migration.
Hydraulic conductivity data can be determined using single well (slug) test data.
Several hydraulic conductivity measurements can be made on materials penetrated
by individual wells to provide data on the relative heterogeneity of the materials in
question. Measurements made in several wells also provide a comparison to check
for effects of poor well construction. Hydraulic conductivity can also be determined
from multiple-well (pumping) tests. A multiple-well test provides a hydraulic
conductivity value for a larger portion of the aquifer. Hydraulic conductivities
determined in the laboratory have been shown to vary by orders of magnitude from
values determined by field methods and are, therefore, not recommended for use in
the RFI.
Porosity can have an important controlling influence on hydraulic con-
ductivity. Materials with high porosity values generally also have high hydraulic
conductivities. An exception is clayey geologic materials which, although possessing
high porosities, have low hydraulic conductivity values (resulting in low flow rates)
due to their molecular structure. All of the pore spaces within geologic materials
are not available for water or solute flow. Dead-end pores and the portion of the
total porosity occupied by water held to soil particles by surface tension forces, do
not contribute to effective porosity. Therefore, to determine average linear
velocities, the effective porosity of the materials should be determined. In the
absence of measured values, the values provided in Table 10-4 should be used.
Knowledge of the rates of ground-water flow is essential to determine if the
locations of the monitoring wells are within reasonable flow distances of the
contaminant sources. Flow rate data can also be used to calculate reasonable
sampling frequencies. This is particularly important when attempting to monitor
the potential migration of a intermittent contaminant release.
10-50
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TABLE 10-4. DEFAULT VALUES FOR EFFECTIVE POROSITY
Soil Textural Classes
Effective
Porosity of
Saturation
Unified Soil Classification System
GC, GP, GM, GS
SW, SP, SM, SC
ML, MH
CL, OL, CH, OH, PT
USDA Soil Textural Classes
Clays, silty clays,
sandy clays
Silts, silt loams,
Silty clay loams
All others
Rock Units (all)
Porous media (nonfractured
rocks such as sandstone and some carbonates)
Fractured rocks (most carbonates, shales,
granites, etc.)
0.20
(20%)
0.15
(15%)
0.01
0.01
0.10
(10%)
0.20
(20%)
0.15
(15%)
0.0001
(0.01%)
"These values are estimates. There may be differences between similar units.
"Assumes de minimus secondary porosity. If fractures or soil structure are
present, effective porosity should be 0.001 (0.1 %).
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Geochemical and biological properties of the aquifer matrix should be
evaluated in terms of their potential interference with the goals of the monitoring
program. For example, chemical reactions or biological transformations of the
monitoring constituents of concern may introduce artifacts into the results. Physical
and hydrologic conditions will determine whether or not information on chemical
or biological interactions can be collected. If the potential for these reactions or
transformations exists, consideration should be given to monitoring for likely
intermediate transformation or degradation products.
The monitoring system design is influenced in many ways by a site's
hydrogeologic setting. Determination of the items noted in the stratigraphy and
flow systems discussions will aid in logical monitoring network configurations and
sampling activities. For example:
Background and downgradient wells should be screened in the same
stratigraphic horizon(s) to obtain comparable ground-water quality
data. Hydraulic conductivities should be determined to evaluate
preferential flowpaths (which will require monitoring) and to establish
sampling frequencies.
The distances between and number of wells (well density) should be a
function of the spatial heterogeneity of a site's hydrogeology, as is
sampling frequency. For example, formations of unconsolidated
deposits with numerous interbedded lenses of varying hydraulic
conductivity or consolidated rock with numerous fracture traces will
generally require a greater number of sampling locations to ensure that
contaminant pathways are intercepted.
The slope of the potentiometric surface and the slope of the aquitard
formation strongly influence the migration rates of light and dense
immiscible compounds.
The hydrogeology will strongly influence the applicability of various
geophysical methods (Appendix C), and should be used to establish
boundary conditions for any modeling to be performed for the site.
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Analyses for contaminants of concern in the ground-water monitoring
program can be influenced by the general water quality present.
Naturally-occurring cations and anions can affect contaminant reactivity,
solubility, and mobility.
Sites with complex geology will generally require more hydrogeologic
information to provide a reasonable assurance that well placements will
intercept contaminant migration pathways. For example, Figure 10-17
illustrates a cross-sectional and plan view of a waste landfill located in a
mature Karst environment. This setting is characteristic of carbonate
environments encountered in various parts of the country, but especially
in the southeastern states. An assessment of the geology of the site
through the use of borings, geophysical surveys, aerial photography,
tracer studies, and other geological investigatory techniques, identified a
mature Karst geologic formation characterized by sinkholes, solution
channels and extensive vertical and horizontal fracturing in an
interbedded limestone/dolomite. Using potentiometric data, ground-
water flow was found to be predominantly in an easterly direction.
Solution channels are formed by the flow of water through the fractures.
The chemical reaction between the carbonate rock and the ground-
water flow in the fractures produces solution channels. Through time,
these solution channels are enlarged to the point where the weight of
the overlaying rock is too great to support; consequently causing a
"roof" collapse and the formation of a sinkhole. The location of these
solution channels should guide the placement of monitoring wells.
Note that in Figure 10-17 the placement of well No. 2 is offset 50 feet
from the perimeter of the landfill. The horizontal placement of well No.
2, although not immediately adjacent to the landfill, is necessary in order
to monitor all potential contaminant pathways. The discrete nature of
these solution channels dictate that each potential pathway be
monitored.
The height of the solution channels ranges from three to six feet directly
beneath the sinkhole to one foot under the landfill except for the 40-
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l/l
WELL 2
(PROJECTED 100 FEET)
1.120
1.100
1,080-
1.060
1.040-
1.020
FEET
A BACKGROUND/UPGRADIENTWELL
DOWNGRADIENT MONITORING WELL
WELL SCREEN
POTENTIOMETRIC SURFACE
FRACTURE TRACE
OUTLINE OF CAVERN (PLAN VIEW)
PLUGGED BOREHOLE
WELL 4 WELLS
(PROJECTED 155 FEETI (PROJECTED 50 FEET)
LANDFILL
(PROJECTED 5 - 50 FEET)
SOLUTION CHANNEL
LIMESTONE
K=5.0x10 cm/iec
REGOLITH
K=10"3 cm/sec
FOSSIL HASH
K=10'* cm/sec
100
150
250
300
350
400
450
500
550
600
Figure 10-17. Monitoring well placement and screen lengths in a mature
karst terrain/fractured bedrock setting.
-------
foot deep cavern. This limited vertical extent of the cavities allows for full screening
of the horizontal solution channels. (Note the change in orientation of solution
channels due to the presence of the fossil hash layer).
Chapter I of the RCRA Ground Water Monitoring Technical Enforcement
Guidance Document (TEGD) (U.S. EPA, 1986) provides additional guidance in
characterization of site hydrogeology. Various sections of the document will be
useful to the facility owner or operator in developing monitoring plans for RCRA
Facility Investigations.
In order to further characterize a release to ground water, data should be
collected to assess subsurface strati graphy and ground-water flow systems. These
are discussed in the following subsections.
10.3.3.1 Subsurface Geology
In order to adequately characterize the hydrologic setting of a site, an analysis
of site geology should first be completed. Geologic site characterization consists of
both a characterization of stratigraphy, which includes unconsolidated material
analysis, bedrock features such as lithology and structure, and depositional
information, which indicates the sequence of events which resulted in the present
subsurface configuration.
Information that may be needed to characterize a site's subsurface geology
includes:
Grain size distribution and gradation;
Hydraulic conductivity;
Porosity;
Discontinuities in soil strata; and
Degree and orientation of subsurface stratification and bedding.
10-55
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Refer to Section 9 (Soil) for further details.
Grain size distribution and gradation-A measurement of the percentage of
sand, silt, and clay should be made for each distinct layer of the soil. Particle size can
affect contaminant transport through its impact on adsorption and hydraulic
conductivity. Sandy soils generally have low sorptive capacity while clays tend to
have a high affinity for heavy metals and some organic contaminants. This is due in
part to the fact that small clay particles have a greater surface area in relation to
their volume than do the larger sand particles. Greater surface areas allow for
increased interactions with contaminant molecules. Clays may also bind
contaminants due to the chemical structure of the clay. Methods for determination
of sand/silt/clay fractions are available from-ASTM, Standard Method No. D422-63
(ASTM, 1984).
Hydraulic conductivity-This property represents the ease with which fluids can
flow through a formation, and is dependent on porosity, and grain size, as well as
on the viscosity of the fluid. Hydraulic conductivity can be determined by the use of
field tests, as discussed in Section 10.6.
Porosity --soil porosity is the volume percentage of the total volume of the soil
not occupied by solid particles (i.e., the volume of the voids). In general, the greater
the porosity, the more readily fluids may flow through the soil, with the exception
of clays (high porosity), in which fluids are held tightly by capillary forces.
Discontinuities in geological materials-Folds are layers of rock or soil that have
been naturally bent over geologic time. The size of a fold may vary from several
inches wide to several miles wide. In any case, folding usually results in a complex
structural configuration of layers (Billings, 1972).
Faults are ruptures in rock or soil formations along which the opposite walls of
the formation have moved past each other. Like folds, faults vary in size. The result
of faulting is the disruption of the continuity of structural layers.
Folds and faults may act as either barriers to or pathways for ground-water
(and contaminant) flow. Consequently, complex hydrogeologic conditions may be
exhibited. The existence of folds or faults can usually be determined by examining
10-56
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geologic maps or surveys. Aerial photographs can also be used to identify the
existence of these features. Where more detailed information is needed, field
methods (e.g., borings or geophysical methods) may need to be employed.
Joints are relatively smooth fractures found in bedrock. Joints may be as long
as several hundred feet (Billings, 1972). Most joints are tight fractures, but because
of weathering, joints may be enlarged to open fissures. Joints result in a secondary
porosity in the bedrock which may be the major pathway of ground-water flow
through the formation (Sowers, 1981).
Interconnected conduits between grains may form during rock formation
(Sowers, 1981). The permeability of a bedrock mass is often defined by the degree
of jointing. Ground water may travel preferentially along joints, which usually
governs the rate of flow through the bedrock. The degree and orientation of joints
and interconnected voids is needed to determine if there will be any vertical or
horizontal leakage through the formation. In some cases, bedrock acts as an
aquitard, limiting the ground-water flow in an aquifer. In other cases, the bedrock
may be much more productive than overlying alluvial aquifers.
Geologic maps available from the USGS (see Section 7) may be useful in
obtaining information on the degree and orientation of jointing or interconnected
void formation. Rock corings may also be used to identify these characteristics.
Degree and orientation of subsurface stratification and bedding-The owner
or operator should develop maps of the subsurface structure for the areas of
concern. These maps should identify the thickness and depth of formations, soil
types and textures, the locations of saturated regions and other hydrogeological
features. For example, the existence of an extensive, continuous, relatively
horizontal, shallow strata of low permeability can provide a clue to contaminant
routing. In such cases, the contaminants may migrate at shallow depths, which are
above the regional aquifer. Such contamination could discharge into nearby, low-
lying structures (e.g., seepage into residential basements). This "basement
seepage" pathway has been demonstrated to be a significant migration channel in
many cases. This pathway may result from migration of vapors in the vadose zone
or through lateral migration of contaminated ground water. Basement seepage is
more likely to occur in locations with shallow ground water. A method for
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estimating basement air contaminant concentrations due to volatile components in
ground-water seeped into basements appears in Appendix E.
A variety of direct and indirect methods are available to characterize a site
geologically with respect to the above geologic characteristics. Direct methods
utilize soil borings and rock core samples and subsequent lab analysis to evaluate
grain size, texture, uniformity, mineralogy, soil moisture content, bedrock lithology,
porosity, and structure. Combined, these data provide the basis for delineating the
geologic nature of the site and, in turn, provide the data necessary to evaluate the
hydrologic setting.
Indirect methods of geologic investigation, such as geophysical techniques
(See Appendix C) and aerial photography (See Appendix A) can be used to
supplement data gathered by direct field methods, through extrapolation and
correlation of data on surface and subsurface geologic features. Borehole
geophysical techniques can be used to extrapolate direct data from soil borings and
bedrock cores. Surface geophysical methods can provide indirect information on
depth, thickness, lateral extent, and variation of subsurface features that can be
used to extrapolate information gained from direct methods, Applicable surface
geophysical methods include seismic refraction, electrical resistivity, electro-
magnetic, magnetics, and ground penetrating radar.
10.3.3.2 Flow Systems
In addition to characterizing the subsurface geology, the owner or operator
should adequately describe the ground-water flow system. To adequately describe
the ground-water flow paths, the owner or operator should:
Establish the direction of ground-water flow (including horizontal and
vertical components of flow);
Establish the seasonal, temporal, and artificially induced (e.g., offsite
production well pumping, agricultural use) variations in ground-water
flow; and
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Determine the hydraulic conductivities of the hydrogeologic units
underlying the site.
Hydrologic and hydraulic properties and other relevant information needed to
fully evaluate the ground-water flow system are listed and discussed below:
Hydraulic conductivity;
Hydraulic gradient (vertical and horizontal);
Direction and rate of flow;
Aquifer type/identification of aquifer boundaries;
Specific yield (effective porosity)/storage coefficient;
Depth to ground water;
Identify uppermost aquifer;
Identify recharge and discharge areas;
Use of aquifer; and
Aquitard type and location.
Hydraulic conductivity-ln addition to defining the direction of ground-water
flow in the vertical and horizontal directions, the owner or operator should identify
the distribution of hydraulic conductivity within each formation. Variations in the
hydraulic conductivity of subsurface materials can affect flow rates and alter
directions of ground-water flow paths. Areas of high hydraulic conductivity
represent areas of greater ground-water flow and zones of potential migration.
Therefore, information on hydraulic conductivities is needed to make decisions
regarding well placements. Hydraulic conductivity measurement is described in
Section 10.6.
10-59
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Hydraulic gradient-The hydraulic gradient is defined as the change in static
head per unit distance in a given direction. The hydraulic gradient defines the
direction of flow and maybe expressed on maps of water level measurements taken
around the site. Ground-water velocity is directly related to hydraulic gradient.
Both vertical and horizontal gradients should be characterized.
Direction and rate of flow--A thorough understanding of how ground water
flows beneath the facility will aid the owner or operator in locating wells to provide
suitable background and/or downgradient samples. Of particular importance is the
direction of ground-water flow and the impact that external factors (intermittent
well pumping, temporal variations in recharge patterns, tidal effects, etc.) may have
on ground-water flow patterns. In order to account for these factors, monitoring
procedures should include precise water level measurements in piezometers or
observation wells. These measurements should be made in a sufficient number of
wells and at a frequency sufficient to adequately gauge both seasonal average flow
directions and to show any seasonal or temporal fluctuations in flow directions.
Horizontal and vertical components of ground-water flow should be assessed.
Methods for determining vertical and horizontal components of flow are described
in Subsection 10.5.4.
Identification of aquifer boundaries/aquifer type-Aquifer boundaries define
the flow limits and the degree of confinement of an aquifer. There are two major
types of aquifers: unconfined and confined. An unconfined aquifer has a free
water surface at which the fluid pressure is the same as atmospheric. A confined
aquifer is enclosed by retarding geologic formations and is, therefore, under
pressure greater than atmospheric. A confining unit consists of consolidated or
unconsolidated earth materials that are substantially less permeable than aquifers.
Confining units are called aquitards or aquicludes. Aquifer boundaries can be
identified by consulting geologic maps and state geologic surveys. Observation
wells and piezometers can be used to determine the degree of confinement of an
aquifer through analysis of water level data.
Specific vield/storativity --Specific yield and storativity are both terms used to
characterize the amount of water an aquifer is capable of yielding. In an
unconfined system, the specific yield is the ratio of the drainable volume to the bulk
volume of the aquifer medium (some liquid will be retained in pore spaces). The
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storativity of a confined aquifer is the volume of water released from a column of
unit area and height per unit decline of pressure head. Specific yield or storativity
values may be necessary to perform complex ground-water modeling.
Depth to ground water-The depth to ground water is the vertical distance
from the land's surface to the top of the saturated zone. A release from a unit not
in contact with the water table will first percolate through the unsaturated zone
and may, depending upon the nature of the geologic material, disperse
horizontally. Thus, a release of this nature may reach a deep water table with
limited lateral spreading. Depth to ground water can influence the selection of
sampling methods as well as geophysical methods.
A shallow water table can also facilitate releases to other environments via
volatilization of some compounds into the unsaturated zone, seepage into base-
ments of buildings in contact with the saturated zone, or the transport of
contaminants into wetlands where the water table reaches the level of the ground
surface. Sufficient mapping of the water table with particular attention to these
features should provide an indication of where these interactions may exist.
Identification of uppermost aquifer--As defined in 40 CFR §260.10, "aquifer"
means a geologic formation, group of formations, or part of a formation capable of
yielding a significant amount of ground water to wells or springs. "Uppermost
aquifer, " also defined in 40 CFR §260.10, means the geologic formation nearest the
natural ground surface that is an aquifer, as well as lower aquifers that are
hydraulically interconnected with this aquifer within the facility's property
boundary. Chapter one of the Technical Enforcement Guidance Document (TEGD)
(U.S. EPA, 1986) elaborates on the uppermost aquifer definition. It states that the
identification of the confining layer or lower boundary is an essential facet of the
definition. There should be very limited interconnection, based on pumping tests,
between the uppermost and lower aquifers. If zones of saturation capable of
yielding significant amounts of water are interconnected, they all comprise the
uppermost aquifer. Identification of formations capable of "significant yield" must
be made on a case-by-case basis.
There are saturated zones, such as low permeability clay, that may not yield a
significant amount of water, yet may act as pathways for contamination that can
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migrate horizontally for some distance before reaching a zone which yields a
significant amount of water. In other cases, there may be low yielding saturated
zones above the aquifer which can provide a pathway for contaminated ground
water to reach basements, if there is reason to believe that a potential exists for
contamination to escape along such pathways, the owner or operator should
monitor such zones.
For further information on the uppermost aquifer definition, including
examples illustrating the determination of hydraulic interconnection in various
geologic settings, see Chapter One of the TEGD.
Identification of recharge and discharge areas-Ground-water recharge can be
defined as the entry into the saturated zone of water made available at the water
table surface, together with the associated flow away from the water table within
the saturated zone. Ground-water discharge can be defined as the removal of
water from the saturated zone across the water table surface, together with the
associated flow toward the water table within the saturated zone (Freeze and
Cherry, 1979). Ground-water recharge and discharge areas also represent areas of
potential inter-media transport.
Recharge can be derived from the infiltration of precipitation, inter-aquifer
leakage, inflow from streams or lakes, or inadvertently by leakage from lagoons,
sewer lines, landfills, etc. Discharge occurs where ground water flows to springs,
streams, swamps, or lakes, or is removed by evapotranspiration or pumping wells,
etc. Information on the source and location of aquifer recharge and discharge areas
may be obtained from state water resource publications, geologic surveys, or
existing site information. Comparison of aquifer water levels with nearby surface
water levels may also provide an indication of the source and location of aquifer
recharge and discharge areas.
Flow nets can also be used to determine areas of aquifer recharge and
discharge. Section 10.5.2 describes the use of flow nets to determine ground-water
flow patterns.
Use of aquifer-The proximity and extent of local ground-water use (e.g.,
pumping) may dramatically influence the rate and direction of ground-water flow
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possibly causing seasonal or episodic variations. These factors should be considered
when designing and implementing a ground-water monitoring system.
Information on local aquifer use may be available from the USGS, and state and
local water authorities. Aquifer use for drinking water or other purposes may also
influence the location of ground-water monitoring wells, as it may be appropriate
to monitor at locations pertinent to receptors.
Aquitard type and location-Aquitard type refers to the type of geologic
formation that serves to bound ground-water flow for a given aquifer. Such
boundaries may be rock or may be an unconsolidated unit such as clay, shale, or
glacial till. The identification of such formations and their hydraulic characteristics
is essential in determining ground-water flow paths. Aquitard locations can be
determined by consulting geologic maps and boring log information. Although
aquitards are substantially less permeable than aquifers, they are not totally
impermeable and can allow significant quantities of water to pass through them
over time. The location of an aquitard should be used in determining monitoring
well depths.
10.3.4 Sources of Existing Information
A complete review of relevant existing information on the facility is an
essential part of the release characterization. This review can provide valuable
knowledge and a basis for developing monitoring procedures. Information that
may be available and useful for the investigation includes both site-specific studies
and regional surveys available from local, state, and Federal agencies.
Information from the regulatory agency such as the RFA report should be
thoroughly reviewed in developing monitoring procedures, and should serve as a
primary information source. It may also provide references to other sources of
information. In addition, the facility's RCRA Permit Application may contain other
relevant information. These reports and all of the facility's RCRA compliance/permit
files will provide an understanding of the current level of knowledge about the
facility, and will assist in identifying data gaps to be filled during the investigation.
Public information is available from local, state, and Federal governments (see
Section 7) concerning the topics discussed below.
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10.3.4.1 Geology
Knowledge of local bedrock types and depths is important to the investigation
of a site. Sources of geologic information include United States Geological Survey
(USGS) reports, maps, and files; State geological survey records; and local well
drilling logs. See also Section 9 (Soils).
10.3.4.2 Climate
Climate is also an important factor affecting the potential for contaminant
migration from a release source. Mean values for precipitation, evaporation,
evapotranspiration, and estimated percolation will help determine the potential for
onsite and offsite contaminant transport. The investigator should consult monthly
or seasonal precipitation and evaporation (or temperature) records. Climate and
weather information can be obtained from:
National Climatic Center
Department of Commerce
Federal Building
Asheville, North Carolina 28801
Tel: (704)258-2850
10.3.4.3 Ground-Water Hydrology
The owner or operator will need to acquire information on the ground-water
hydrology of a site and its surrounding environment. Ground-water use in the area
of the site should be thoroughly investigated to find the depths of local wells, and
their pumping rates. Sources of such information include the USGS, state geological
surveys, local well drillers, and State and local water resources boards. A list of all
state and local cooperating offices is available from the USGS, Water Resources
Division in Reston, Virginia, 22092. This list has also been distributed to EPA
Regional Offices. Water quality data, including surface waters, is available through
the USGS via their automated NAWDEX system. For further information, telephone
(703)860-6031.
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10.3.4.4 Aerial Photographs
Aerial reconnaissance can be an effective and economical tool for gathering
information on waste management facilities. For this application, aerial recon-
naissance includes aerial photography and thermal infrared scanning. See
Appendix A for a more detailed discussion of the usefulness of aerial photography
in release characterization and availability of aerial photographs.
10.3.4.5 Other Sources
Other sources of information for subsurface and release characterization
include:
U.S. EPA files (e.g., CERCLA-related reports);
U.S. Geological Survey;
U.S. Department of Agriculture Soil Conservation Service;
U.S. Department of Agriculture Agricultural Stabilization and
Conservation Service;
U.S. Department of Interior -Bureau of Reclamation;
State Environmental Protection or Public Health Agencies;
State Geological Survey;
Local Planning Boards;
County or City Health Departments;
Local Library;
Local Well Drillers; and
Regional Geologic and Hydrologic Publications.
10.4 Design of a Monitoring Program to Characterize Releases
Information on waste, unit and environmental characterization can be used to
develop a conceptual model of the release, which can subsequently be used to
design a monitoring program to fully characterize the release. The design of a
monitoring program is discussed below.
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10.4.1 Objectives of the Monitoring Program
The objective of initial monitoring is to verify or to begin characterizing
known or suspected contaminant releases to ground water. To help accomplish this
objective, the owner or operator should evaluate any existing monitoring wells to
determine if they are capable of providing samples representative of background
and downgradient ground-water quality for the unit(s) of concern. Figure 10-18
illustrates three possible cases where existing well systems are evaluated with
regard to their horizontal location for use in a ground-water investigation.
Adequacy is not only a function of well location but also well construction.
Guidance on appropriate well construction materials and methods can be found in
the TEGD (EPA, 1986). If the monitoring network is found to be inadequate for all
or some of the units of concern, additional monitoring wells should be installed.
Further characterization, utilizing both direct and indirect investigative methods, of
the site's hydrogeology should be completed to identify appropriate locations for
the new monitoring wells.
If initial monitoring verifies a suspected contaminant release, the owner or
operator should extend the monitoring program to determine the vertical and
horizontal concentrations (i.e., 3-dimensions) of all hazardous constituents in the
release. The rate of contaminant migration should also be determined. A variety of
investigatory techniques are available for such monitoring programs.
Monitoring procedures should include direct methods of obtaining ground-
water quality information (e.g., sampling and analysis of ground water from
monitoring wells). Indirect methods of investigation may also be used when
appropriate to aid in determining locations for monitoring wells (i.e., through
geologic and/or geochemical interpretation of indirect data). For many cases, the
use of both direct and indirect methods may be the most efficient approach.
Elements to be addressed in the ground-water monitoring program include:
Monitoring constituents and indicator parameters;
Frequency and duration at which samples will be taken;
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Case One
WMA
1
l_
No new wells may be needed
for SWMU if all units are
closely spaced.
Case Two
Wells for RUs not adequate
for SWMU due to geographic
remoteness.
Case Three
WMA
WMA
I I
New wells needed for SWMU
due to presence of a ground-
water divide.
KEY:
SWMU - Solid Waste Management Unit
RU - Regulated Unit
WMA - Waste Management Area
O - Background Monitoring Well
- Downgradient Monitoring Well
Ground-Water Flow Direction
Note: Drawings not to scale.
-------
Sampling and analysis techniques to be used, including appropriate
QA/QC procedures; and
Monitoring locations.
[Note: Permit application regulations in 40 CFR §270.14(C)(2) require appli-
cants to identify the uppermost aquifer and hydraulically interconnected
aquifers beneath the facility property if the facility has any "regulated" units.
The application must indicate ground-water flow directions and provide the
basis for the aquifer identification (e.g., a report written by a qualified
hydrogeologist on the hydrogeologic characteristics of the facility property
supported by at least the well drilling logs and available professional
literature). However, some RCRA permit applications did not require
hydrogeologic characterizations (e.g., storage only facilities) prior to the
HSWA Amendments of 1984. Now, such characterizations may be required
according to RCRA Section 3004(u) when SWMU releases to ground water are
suspected or known. The RCRA Ground Water Monitoring Technical Enforce-
ment Guidance Document (TEGD) (U.S. EPA, 1986), and the Permit Applicant's
Guidance Manual for Hazardous Waste Land Treatment, Storaqe.and Disposal
Facilities (U.S. EPA, 1984) should be consulted for further information on
regulatory requirements.]
10.4.2 Monitoring Constituents and Indicator Parameters
Initial monitoring should be focused on rapid, effective release character-
ization at the downgradient limit of the waste management area. Monitoring
constituents should include waste-specific subsets of hazardous constituents from
40 CFR Part 261, Appendix VIII (see Section 3 and the lists provided in Appendix B).
Indicator parameters (e.g., TOX, specific conductance) may also be proposed as
indicated in Section 3. Such indicators alone may not be sufficient to characterize a
release of hazardous constituents, because the natural background variability of
indicator constituents can be quite high. Furthermore, indicator concentrations do
not precisely represent hazardous constituent concentrations, and the detection
limits for indicator analyses are significantly higher than those for specific
constituents.
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In developing an initial list of monitoring constituents and indicator para-
meters, the following items should be considered:
The nature of the wastes managed at the facility should be reviewed to
determine which constituents (and any chemical reaction products, if
appropriate) are relatively mobile and persistent;
The effects of the unsaturated zone (if present) beneath the facility on
the mobility, stability and persistence of the waste constituents; and
The concentrations and related variability of the proposed constituents
in background ground water.
In the absence of detailed waste characterization information, the owner or
operator should review the guidance presented in Section 3, which discusses the use
of the monitoring constituent lists in Appendix B. As discussed in Section 3, the use
of these lists is contingent upon the level of detail provided by the waste
characterization.
The owner or operator should consider monitoring for additional inorganic
indicators that characterize the general quality of water at the site (e.g., chloride,
iron, manganese, sodium, sulfate, calcium, magnesium, potassium, nitrate,
phosphate, silicate, ammonium, alkalinity and pH). Baseline data on such indicators
can be used for subsequent monitoring phases and for selecting corrective measures
(e.g., in assessing ground-water treatment alternatives). This is also discussed in
Section 3 and Appendix B. Information on the major anions and cations that make
up the bulk of dissolved solids in water can be used to determine reactivity and
volubility of hazardous constituents and therefore predict their mobility under
actual site conditions.
10.4.3 Monitoring Schedule
10.4.3.1 Monitoring Frequency
Monitoring frequency should be based on various factors, including:
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Ground-water flow rate and flow patterns;
Adequacy of existing monitoring data; and
Climatological characteristics (e.g., precipitation patterns),
Generally, the greater the rate of ground-water flow, the greater the
monitoring frequency needed. For example, monitoring frequency in an
intergranular porosity flow aquifer of low permeability materials would likely be
less than for a fracture or solution porosity flow aquifer with unpredictable and
high flow rates. In the case of a fracture or solution porosity flow aquifer, it is
possible that contaminants could migrate past the facility boundary in a matter of
days, weeks, or months; thus requiring frequent monitoring.
The adequacy of existing monitoring data can be a factor in determining the
monitoring schedule. For example, a facility which has performed adequate
monitoring under RCRA interim status requirements may have a good data base
which can be helpful in evaluating initial monitoring results. At the other end of
the spectrum are facilities lacking hydrogeologic data and monitoring systems.
Owners or operators of these facilities will need to design and install an adequate
monitoring system for the units of concern. An accelerated monitoring program is
recommended at such facilities.
10.4.3.2 Duration of Monitoring
The duration of the initial monitoring phase will vary with facility-specific
conditions (e.g., hydrogeoiogy, wastes present) and should be determined through
consultation with the regulatory agency. The regulatory agency will evaluate initial
monitoring results to determine how long monitoring should continue and to
determine the need for adjustments in the monitoring schedule, the list of
monitoring constituents, and other aspects of the monitoring effort. If the
regulatory agency determines that a release to ground water has not occurred, the
investigation process for that release can be terminated at its discretion, if
contamination is found during initial monitoring, further monitoring to fully
characterize the release will generally be necessary.
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10.4.4 Monitoring Locations
If there is no existing monitoring system or if the system is inadequate to
effectively characterize ground-water contamination, the owner or operator should
design and install a well system capable of intercepting the suspected contaminant
plume(s). The system should also be used for obtaining relevant hydrogeologic
data. The monitoring well network configuration should be based on the site's
hydrogeoiogy, the layout of the facility and the units of concern, the location of
receptors, and should reflect a consideration of any information available on the
nature and source of the release. It is important to recognize that the potential
pathways of contaminant migration are three dimensional. Consequently, the
design of a monitoring network which intercepts these potential pathways requires
a three dimensional approach.
in many cases, the initial monitoring system will need to be expanded for
subsequent phases. Additional downgradient wells will often be needed to
determine the extent of the contaminant plume. A greater number of background
wells may also be needed to account for spatial variability in ground-water quality.
Prior to the installation of additional downgradient monitoring wells, a
conceptual model of the release should be made from a review of waste and unit
information and current and past site characterization information. Additional
hydrogeologic investigations may also be appropriate. For example, piezometer
readings surrounding the well(s) showing a release, should be used to determine
the current hydraulic gradient(s). These values should be compared to the
potentiometric surface map developed for the site hydrogeologic characterization
to better describe the direction(s) of release migration. Seasonal (natural or
induced) or regional fluctuations should be considered during this comparison. A
re-evaluation of the facility's subsurface geologic information should be performed
to identify preferential pathways of contaminant migration. In many situations, it
may be appropriate to develop ground-water flow nets to show vertical and
horizontal components of flow. Guidance on construction of flow nets is provided
in Section 10.5.2 and in the Ground Water Flow Net/Flow Line Technical Resource
Document. NTIS PB86-224979. (EPA, 1985). The installation of additional
piezometers may be necessary to verify the accuracy of the flow nets and assist in
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determining whether or not the site hydrogeology has been adequately
characterized.
At facilities where it is known or likely that volatile organics have been
released to the ground water, organic vapor analysis of soil gas from shallow bore
holes may provide an initial indication of the areal extent of the release
(Figure 10-19). An organic vapor analyzer (OVA) may be used to measure the
volatile organic constituents in shallow hand-augered holes. Alternatively, a
sample of soil gas may be extracted from a shallow hole and analyzed in the field
using a portable gas chromatography. These techniques are limited to situations
where volatile organics are present. As discussed previously, it is recommended
that, where possible, concurrent investigations of more than one contaminated
media be conducted. Further, the presence of intervening, saturated, low
permeability sediments strongly interferes with the ability to extract a gas sample.
Although it is not necessarily a limitation, optimal gas chromatography results are
obtained when the analyte is matched with the highest resolution technique, (e.g.,
electron capture for halogenated species). The effectiveness of this approach
should be evaluated by initial OVA sampling in the vicinity of any wells known to be
contaminated.
Other direct methods that may be used to define the extent of a release
include sampling of seeps and springs. Seeps and springs occur where the local
ground-water surface intersects the land surface resulting in ground-water
discharge into a stream, lake, or other surface water body. Seeps and springs may
be observed near marshes, at road cuts, or near streams. As discharges from seeps
and springs reflect the height of the potentiometric surface, they are likely to be
most abundant during a wet season.
To minimize the installation of new wells, the use of applicable geophysical
and modeling methods may be proposed to describe geologic conditions and
contaminant release geometry/characteristics. Such methods can also aid in the
placement of new monitoring wells.
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o
CO
/ LANDFILL ICONIAININO VOLAf lit ODGANICSl
(Plan View)
LEGEND
Monitoring Well
(No Contaminant Detection)
Soil Gas Analysis Probe Point
Monitoring Well
(Contaminant Detected)
Extent of Contaminated Soil
^_^ Extent of Ground-Water Contamination
Plume
Figure 10-19. Example of using soil gas analysis to define probable location of
ground-water release containing volatile organics.
-------
A variety of indirect geophysical methods are currently available to aid in
characterizing geologic conditions and ground-water contamination. Geophysical
methods do not provide detailed, constituent-specific data; however, they can be
useful in investigating geologic conditions and in estimating the general areal
extent of a release. This may reduce speculation involved in determining new well
locations. Details on the use of geophysical methods are presented in Section 10.6
and in Appendix C.
Mathematical and/or computer modeling results may be used in conjunction
with the results of geophysical investigations to assist in well placement decisions.
The owner or operator should not, however, depend solely on such models to
determine the placement of new monitoring wells. Because models may not
accurately account for the high spatial and temporal variability of conditions
encountered in the field, modeling results should be limited to estimating the aerial
extent of a release, and in determining placement of new monitoring wells.
In order to estimate the potential extent of a release in the direction of
ground-water flow, Darcy's law should be applied, if appropriate, to determine the
average linear ground-water velocity (see Section 10.5.3). This velocity should then
be multiplied by the age of the unit of concern (assuming the unit began releasing
immediately) to estimate the potential distance of contaminant migration. This
distance should be used as a "yardstick" in determining well locations. More
complex modeling (e.g., solute transport), may be proposed by the owner or
operator to assist in locating additional monitoring wells. However, modeling
results should not be used in lieu of field monitoring data.
The International Ground Water Modeling Center supported largely by the
U.S. Environmental Protection Agency, operates a clearing-house for ground-water
modeling software, organizes and conducts short courses and seminars, and carries
out a research program supporting the Center's technology transfer and
educational activities. Two major functions of the Center are the dissemination of
information regarding ground-water models and the distribution of modeling
software. The Center maintains computerized data bases, including updated
computer codes and test files, and descriptions of a large number of ground-water
models. By means of a search and retrieval procedure, this information is easily
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inaccessible and readily available. The Center can be contacted at the following
address:
International Ground Water Modeling Center
Holcomb Research Institute
Butler University
Indianapolis, Indiana 46208
Telephone: (317)283-9458
The Center will send, upon request and free of charge, a listing of available
publications, and a copy of its Newsletter.
In selecting and applying models, it is important to remember that a model is
an artificial representation of a physical system used to characterize a site. A model
cannot replace field data, nor can it be more accurate than the available site data.
In addition, the use of computer models requires special expertise. Time and
experience are needed to select the appropriate code and subsequent calibration, if
these resources are not available, modeling should not be attempted. Models are
used in conjunction with scientific and engineering judgment; they are an aid to,
not a surrogate for, a skilled analyst.
If a model is proposed in the monitoring procedures, the owner or operator
should describe all assumptions used in applying the model to the site in question. A
sensitivity analysis of the model should be run to determine which input parameters
have the most influence on model results, and the model's results should be verified
by field sampling. The owner or operator should clear the use of any and all models
through the regulatory agency prior to use. Section 3 provides additional
information on the use of models.
10.4.4.1 Background and Downgradient Wells
Background wells (preferably upgradient) may be installed to obtain samples
that are not affected by the facility, if the owner or operator believes that other
sources are contributing to the releases of concern. These wells should be screened
at the same stratigraphic horizon(s) as the downgradient wells. Background wells,
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if installed, should be sufficient in number to. account for any heterogeneity in
background ground-water quality.
Downgradient wells should be located and constructed to provide samples of
ground water containing any releases of hazardous constituents from the units of
concern. Determination of the appropriate number of wells to be included in an
initial monitoring system should be based on various factors, including unit size and
the complexity of the hydrogeologic setting (e.g., degree of fracturing and
variation in hydraulic conductivity). Downgradient monitoring wells should be
located at the limit of the waste management area of the units of concern and at
other downgradient locations, as appropriate. For example, "old" releases may
show higher constituent concentrations at locations downgradient of the unit. In
such cases, flow nets may be useful in determining additional downgradient well
locations (See Section 10.5.2).
10.4.4.2 Well Spacing
The horizontal spacing between wells should be a design consideration. Site
specific factors as listed in Table 10-5 should be considered when determining the
horizontal distances between initial monitoring system wells. These factors cover a
variety of physical and operational aspects relating to the facility including
hydrogeologic setting, dispersivity, ground-water velocity, facility design, and
waste characteristics. In the less common homogeneous geologic setting where
simple flow patterns are identified, a more regular well spacing pattern may be
appropriate. Further guidance on the consideration of site specific conditions to
evaluate well spacing is described in Chapter Two of the TEGD (U.S. EPA, 1986).
Subsequent phase monitoring systems should be capable of identifying the
full extent of the contaminant release and establishing the concentration of
individual constituents throughout the release. Well installation and monitoring
should concentrate on defining those areas that have been affected by the release.
A well cluster network should be installed in and around the release to define the
horizontal and vertical extent of contamination. Networks of monitoring wells will
vary from site to site, depending upon hydrogeological complexity and
contaminant characteristics. Surface geophysical techniques and modeling may also
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TABLE 10-5. FACTORS INFLUENCING THE INTERVALS BETWEEN INDIVIDUAL
MONITORING WELLS WITHIN A POTENTIAL MIGRATION PATHWAY
Wells intervals May Be
Closer If the Site:
Wells Intervals May be
Wider If the Site:
Manages or has managed liquid waste
Is very small (i.e., the d?wngradient
perimeter of the site is less than 1 so
feet)
as waste incompatible with liner
materials
as fill material near the waste
management units (where preferential
flow might occur)
Has buried pipes, utility trenches, etc.,
where a point-source leak might occur
Has complicated geology
- closely spaced fractures
- faults
- tight folds
solution channej^
discontinuous structures
as heterogeneous conditions
- variable hydraulic conductivity
- variable lithology
ocated in or neara recharge zone
a high (steep) or variablp nydraulic
client e
dispersivity
average linear velocity
Has simple geology
- no fractures
-no faults
- no folds
-no solution channels
-continuous structures
Has homogeneous conditions
-uniform hydraulic conductivity
-uniform lithology
a low (flat) and constant hydraulic
gradient
High dispersivity
Low
average linear velocity
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be used, where appropriate, to help facilitate release definition. The well density or
amount of sampling undertaken to completely identify the extent of migration
should be determined by the variability in subsurface geology present at the site.
Formations such as unconsolidated deposits with numerous interbedded lenses of
varying permeability, or consolidated rock with numerous fracture traces, will
generally require more extensive monitoring to ensure that contamination is
appropriately characterized.
Monitoring should be performed to characterize the interior portion(s) of a
release. This is important because constituents can migrate at differing rates and
may have been released at different times. Monitoring only at the periphery of the
release may not identify all the constituents in the release, and the concentration of
monitoring constituents measured at the periphery of the release may be
significantly less than in the interior portion(s). Patterns in concentrations of
individual constituents can be established throughout the release by sampling
along several lines that perpendicularly transect the release. The number of
transects and the spacing between sampling points should be based on the waste
characteristics, the size of the release, and variability in geology observed at the
site. Sampling locations should also be selected so as to identify those areas of
maximum contamination within the release. In addition to the expected hazardous
constituents, the release may contain degradation and reaction products, which
may also be hazardous.
Results of geophysical methods may be correlated with data from the
monitoring well network. The monitoring program should be flexible so that
adjustments can be made to reflect release migration and changes in direction.
The spacing between initial downgradient monitoring wells should ensure the
measurement of releases near the unit(s) of concern. However, it is possible that the
initial spacings between wells will only provide for measurements in the peripheral
portion of a release. This might result in water quality measurements that do not
reflect the maximum concentration of contaminants in the release. Therefore,
additional downgradient wells may be needed adjacent to the units of concern
during subsequent monitoring phases.
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A similar effect may be observed, even with a closely spaced initial
downgradient monitoring network, if a narrow, localized release migrates past the
limit of the waste management area. Such a plume may originate from a small leak
in a liner and/or from a leak located close to the downgradient limit of the waste
management area, thereby limiting the amount of dispersion occurring in the
release prior to its passing the monitoring wells. Consequently, if relatively wide
spacing exists between wells or there is reason to expect a narrow, localized release,
the installation of additional monitoring wells may be necessary in the immediate
vicinity of those wells in which a release has been measured. Such an expansion of
the monitoring network is recommended when a release has been measured in only
one or two monitoring wells, indicating a localized plume.
10.4.4.3 Depth and Screened Intervals
The depth and screened intervals for initial phase monitoring wells should be
based on: (1) geologic factors influencing the potential contaminant pathways of
migration to ground water; (2) physical/chemical characteristics of the contaminant
controlling its likely movement and distribution in the ground water; and (3)
hydrologic factors likely to have an impact on contaminant movement. The
consideration of these factors in evaluating the design of monitoring systems is
described in the TEGD (U.S. EPA, 1986), including examples of placement in some
common geologic environments. Subsection 10.6 provides guidance on borings and
monitoring well construction.
In order to establish vertical concentration gradients of hazardous
constituents in the release during subsequent monitoring phases, well clusters or
multi-depth monitoring wells should be installed. The first well in a cluster (or
initial sampling interval in a multi-depth well) should be screened at the horizon in
which contamination was initially discovered. Additional wells in a cluster should
be screened, where appropriate, above and below the initial well's sampling
interval until the margins of the release are established.
Several wells should be placed at the fringes of the release to define its vertical
margins, and several wells should be placed within the release to identify
constituents and concentrations. Care must be taken in placing contiguously
screened wells close together because one well's drawdown may influence the next
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and thus change the horizon from which its samples are drawn. Alternating lower
and higher screens should reduce this effect (see Figure 10-20).
The specifications of sampling depths should clearly identify the interval over
which each sample will be taken. It is important that these sampling intervals be
sufficiently discrete to allow vertical profiling of constituent concentrations in
ground water at each sampling location. Sampling will only provide measurements
of the average contaminant concentration over the interval from which that sample
is taken. Samples taken from wells screened over a large vertical interval may be
subject to dilution effects from uncontaminated ground water lying outside the
plume limits. The proposed screened interval should reflect the expected vertical
concentration gradients within the release.
At those facilities where immiscible contaminants have been released and
have migrated as a separate phase (see Figure 10-21), specific techniques will be
necessary to evaluate their migration. The detection and sampling of immiscible
layers requires specialized equipment that must be used before the well is
evacuated for conventional sampling. Chapter 4 of the TEGD (U.S. EPA, 1986)
contains a discussion of ground-water monitoring techniques that can be used to
sample multi-phased contamination. These sampling techniques vary according to
whether the immiscible phase is lighter than water (i.e., floats) or denser than water
(i.e., sinks), and is also dependent on the thickness of the layer.
The formation of separate phases of immiscible contaminants in the
subsurface is largely controlled by the rate of infiltration of the immiscible
contaminant and the solubility of that contaminant in ground water. Immiscible
contaminants generally have limited volubility in water. Thus, some amount of the
immiscible contaminant released from a unit(s) will dissolve in the ground water
and thus migrate in solution. However, if the amount of immiscible contaminant
reaching ground water exceeds the ability of ground water to dissolve it (i.e., the
constituent water solubility), the ground water in the upper portion of the water
table aquifer will become saturated and the contaminant will form a separate
immiscible phase. Hence, the contaminant will be present in the ground water at a
concentration approaching its water volubility, as well as in a separate immiscible
phase. If cosolvents are present, the concentration of the contaminant in the
ground water can exceed the contaminant's water volubility, whether or not a
separate immiscible phase is present.
10-80
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UECENO
ANOSCflMN
10- SCfltCN tINGTM
Illll* WATCA TAiLI
Figure 10-20
Vertical Well Cluster Placement
10-81
-------
At this point, the behavior and migration of an immiscible contaminant will be
strongly influenced by its density relative to ground water. If the immiscible is less
dense than ground water, it will tend to form a separate immiscible layer and
migrate on top of the ground water. If the density of the immiscible contaminant is
similar to that of ground water, it will tend to mix and flow as a separate phase with
the ground water, creating a condition of multiphase flow.
If the density of the immiscible constituent is greater than ground water, it will
tend to sink in the aquifer (see Figure 10-21). As the immiscible layer sinks and
reaches unaffected ground water in a deeper portion of the aquifer, more of the
immiscible contaminant will tend to enter into solution in ground water and begin
to migrate as a dissolved constituent. However, if enough of the dense immiscible
contaminants are present, some portion of these contaminants will continue to sink
as a separate immiscible phase until a geologic formation of reduced permeability is
reached. At this point, these dense contaminants will tend to form a layer that
migrates along the geologic formation (boundary).
Immiscible phase contaminants may migrate at rates different than that of
ground water. In addition, immiscible contaminants may not flow in the same
direction as ground water. However, it is important to re-emphasize that some
fraction of these contaminants may dissolve in ground water and migrate away
from the facility as dissolved constituents.
Light immiscible contaminants tend to migrate downgradient as a floating
layer above the saturated zone (see Figure 10-21). The hydraulic gradient is a major
factor in the movement of this light immiscible layer. Other important factors
involved in the migration rate of a light immiscible phase include the intrinsic
permeability of the medium, and the density and viscosity of the contaminants.
Oftentimes, an ellipsoidal plume will develop over the saturated zone as depicted in
Figure 10-21. While it may be possible to analyze the behavior of a light immiscible
layer using analytical or numerical models, the most practical approach for
determining the rate and direction of migration of such a layer is to observe its
behavior overtime with appropriately located monitoring wells.
10-82
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CONTAMINATION
SOURCE
SAND
AQUIFER
AOUITARO
LEGfNO
LICMI IMMISCIULE PLUME
MAIN PLUME
HEAVY IMMISCIBLE PLUME
Figure 10-21. General schematic of multiphase contamination in a sand aquifer.
-------
The migration of a layer of dense immiscibles resting on a low permeability
geologic formation may be strongly influenced by gravity. Depending on the slope
of the retarding formation, the immiscible layer may move with or in a different
direction from the flow of the ground water. Consequently, the evaluation of the
rate and direction of migration of a dense immiscible layer should include a
determination of the configuration of the retarding formation on which the
immiscible layer is migrating. The direction of migration and estimates of migration
rates of dense immiscibles can then be obtained by including the gravitational
forces induced by the slope of the retarding formation in the gradients used to
calculate contaminant flow rates. If a dense immiscible layer(s) is expected or
known, the monitoring plan should include procedures to verify its direction and
rate of flow.
10.5 Data Presentation
Section 5 of this guidance describes data presentation methods with examples.
In addition to sorted data tables, the methods described for contaminant isopleth
maps, geologic cross-sections, cross-sectional concentration contours, and fence
diagrams should be useful for presenting ground-water investigation findings. The
following presents specific data presentation methods that may be particularly
useful for presenting ground-water investigation data.
10.5.1 Waste and Unit Characterization
Waste and unit characteristics should be presented as:
Tables of waste constituents and concentrations;
Tables of relevant physical and chemical properties of waste and
constituents;
Narrative description of unit dimensions, operations etc.; and
Topographical map and plan drawings of facility and surrounding areas.
10-84
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10.5.2 Environmental Setting Characterization
Environmental characteristics should be presented as follows:
Tabular summaries of annual and monthly or seasonal relevant climatic
information (e.g., temperature, precipitation);
Narratives and maps of soil and relevant hydrogeological characteristics
such as porosity, organic matter content and depth to ground water;
Maps showing location of natural or man-made engineering barriers and
likely migration routes; and
Maps of geologic material at the site identifying the thickness, depth,
and textures of soils, and the presence of saturated regions and other
hydrogeological features.
Flow nets should be particularly useful for presenting environmental setting
information for the ground-water medium. A flow net provides a graphical
technique for obtaining solutions to steady state ground-water flow. A properly
constructed flow net can be used to determine the distribution of heads, discharges,
areas of high (or low) velocities, and the general flow pattern (McWhorter and
Sunada, 1977).
The Ground Water Flow Net/Flow Line Technical Resource Document (TRD).
NTIS PB86-224979. (U. S. EPA, 1985), provides detailed discussion and guidance in
the construction of flow nets. Although the focus of this document is on the
construction of vertical flow nets, the same data requirements and theoretical
assumptions apply to horizontal flow nets. The fundamental difference between
vertical and horizontal flow nets is in their application. A flow net in the horizontal
plane may be used to identify suitable locations for monitoring wells whereas a
flow net in the vertical plane would aid in determining the screened interval of a
well.
10-85
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The following excerpts from the Flow Net Document (U.S. EPA, 1985) explain
data needs for flow net construction. Several assumptions must be made to
construct a flow net:
Ground-water flow is steady state, which means flow is constant with
time;
The aquifer is completely saturated;
No consolidation or expansion of the soil or water occurs;
The same amount of recharge occurs across the system; and
Flow is laminar and Darcy's law is valid.
Knowledge of the hydrologic parameters of the ground-water system is
required to properly construct a flow net. These parameters include:
Head distribution, both horizontally and vertically;
Hydraulic conductivity of the saturated zone;
Saturated zone thickness; and
Boundary conditions.
The distribution of head can be determined using time equivalent water level
measurements obtained from piezometers and/or wells. Plotting the water level
elevations on a base map and contouring these data will provide a potentiometric
surface. Contour lines representing equal head are called lines of equipotential.
Changes in hydraulic head, both horizontally and vertically within an aquifer, must
be known for proper flow-net construction. These changes can be delineated with
piezometers or monitoring wells installed at varying depths and spatially
distributed. The data must be time equivalent because water levels change over
time. Ground-water flow directions can be determined by drawing lines
10-86
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perpendicular to the equipotential lines. Ground water flows from areas of higher
hydraulic head to areas of lower hydraulic head.
The hydraulic conductivity of a material depends on the properties of the fluid
and the media. Clayey materials generally have low hydraulic conductivities,
whereas sands and gravels have high conductivities (U.S. EPA, 1985). Where flow
crosses a boundary between different homogeneous media the ground-water
flowlines refract and flow velocity changes due to an abrupt change in hydraulic
conductivity. The higher permeability formation serves as a conduit to ground-
water flow. This is visually apparent in a properly constructed flow net, because
flow tubes are narrower in layers with higher conductivity because less area is
necessary to conduct the same volume of ground water. In media of lower
conductivity, flow tubes will be wider in order to conduct the same volume of flow
(Cedergren, 1977). Construction of flow nets for layered geologic settings
(heterogeneous, isotropic systems) are discussed in Section 2 of the flow net
document (U.S. EPA, 1985).
The boundary conditions of an aquifer must also' be known to properly
construct a flow net. These boundary conditions will establish the boundaries of the
flow net. The three types of boundaries are: 1) impermeable boundaries;
2) constant head boundaries; and 3) water table boundaries (Freeze and Cherry,
1979). Ground water will not flow across an impermeable boundary; it flows
parallel to these boundaries. A boundary where the hydraulic head is constant is
termed a constant head boundary. Ground-water flow at a constant head
boundary is perpendicular to the boundary. Examples of constant head boundaries
are lakes, streams, and ponds. The water table boundary is the upper boundary of
an unconfined aquifer, and is a line of known and variable head. Flow can be at any
angle in relation to the water table due to recharge and the regional ground-water
gradient. The boundary conditions of an aquifer can be determined after a review
of the geohydrologic data for a site (U.S. EPA, 1985).
Although a complete understanding of the mathematics of ground-water
flow is not necessary for proper flow-net construction by graphical methods, a
general understanding of the theory of ground-water flow is required. For a brief
discussion of ground-water flow theory as applied to flow nets, refer to Section 1 of
the flow net document (U.S. EPA, 1985). Detailed guidance on graphical
10-87
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construction of flow nets is given in Section 2 of that document. Mathematical
techniques can be used to construct flow nets although graphical techniques are the
simplest and most commonly used. It is worth noting that flow nets are
dimensionless.
When a flow net has been constructed for a site, it is advisable to test the
adequacy of the flow net by installing additional piezometers at selected locations.
if the site hydrogeology is adequately characterized by the flow net, the head
values in the new piezometer(s) will not vary significantly from those predicted by
the flow net.
The number of new piezometers needed to check the adequacy of the flow
net would vary depending on a number of factors including size of the site,
complexity of the site hydrogeology, amount of data used to construct the flow net,
and the level of agreement between the site specific flow net and the regional flow
regime. For example, at a site with predominantly horizontal flow and well defined
stratigraphy, such as illustrated in Figure 10-22, a single new piezometer could test
the flow net. For a site with multiple, interconnected aquifers and a significant
vertical component of flow, such as illustrated in Figure 10-23, several nested
piezometers might be necessary to test the flow net.
In evaluating flow nets and the results of flow net tests, several factors should
be kept in mind. The head measurements in a new piezometer may not exactly
match the values predicted by the flow net. Some variation is inherent in this type
of measurement. The owner or operator should evaluate whether or not the
difference between measured and predicted values is significant in the context of
flow direction or flow velocity. A new value which reverses the direction of flow or
redirects flow towards potential receptors would obviously be significant. A change
in flow velocity as indicated by a revised gradient might be significant if the
magnitude of the change is substantial or if an increased velocity suggests that the
characterization needs to be extended to a greater distance.
There are several situations in which extreme caution is needed in evaluating a
flow net test. In many cases, temporal variations will alter the potentiometric
surface between the time the flow net is constructed and a test piezometer is
installed. Examples of this situation would include locations with large seasonal
10-88
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IN
DOWNGRADIENT
3.5-
UPGRADIENT
HAZARDOUS WASTE
LAND APPLICATION
SITES
0 50 100
Scale, Meters
Key:
Equipotential Lines
(in Meters)
Flow Lines
Wells with Well Numbers
Figure 10-22, Potentiometric surface showing flow direction
10-89
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805.8
10
o
^-»
BOUNDARIES
OF THE PROPOSED FACILITY
.WATEHTABLE
rv
743
<*' ^Tja.2 -7fl9 /I79
f ' ' /
/ / / / /
'-' I ///
x / / / i
888
866
827.3* \
' 872
708.8
ASSUME 100 =
Figure 10-23. Approximate flow net
-------
variations in ground-water levels. Another situation that would introduce problems
in interpretation would be a site that is adjacent to tidally influenced surface
waters.
Construction of flow nets is not appropriate or valid in certain instances. As
discussed in the flow net document (U.S. EPA, 1985), these situations occur when
there is a lack of three-dimensional hydrologic data for a ground-water system, and
when ground- water flow in a system does not conform to the principles expressed
by and assumptions made in Darcy's law. Scaling problems occur when the aquifer
and/or geologic layers associated with a particular ground-water system are thin in
relation to the length of the flow net. If a flow net is constructed for this situation,
the flow net will be made up of squares that are too small to work with unless the
scale is exaggerated. For sites where the assumption of steady-state flow is not
valid, the construction of flow nets is very difficult. The flow net must be redrawn
each time the flow field changes to simulate the transient conditions.
Lack of three-dimensional hydrologic data or hydrologically equivalent data
for a ground-water flow system makes proper flow-net construction impossible.
Hydrologic testing at various depths within an aquifer and determination of the
vertical hydraulic conductivity of an aquifer are essential to provide the necessary
data. If these data are not available it will be necessary to obtain them before a
flow net can be constructed.
There are three types of ground-water systems in which the principles
expressed by Darcy's law do not apply. The first is a system in which the flow is
through materials with low hydraulic conductivities under extremely low gradients
(Freeze and Cherry, 1979). The second is a system in which a large amount of flow
passes through materials with very high hydraulic conductivities. The third is a
system in which the porous media assumption is not valid. Darcy's law expresses
linear relationships and requires that flow be laminar (flow in which stream lines
remain distinct from one another). In a system with high hydraulic conductivity,
flow is often turbulent. Turbulent flow is characteristic of karstic limestone and
dolomite, cavernous volcanics, and fractured rock systems. Construction of flow
nets for areas of turbulent flow would not be valid. The use of Darcy's law also
requires the assumption of porous media flow. This assumption may not be valid
for many fractured bedrock and karst environments where fractured flow is
dominant or large solution features are present.
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10.5.3 Characterization of the Release
The objective of monitoring is to estimate the nature, rate, and extent (3-
dimensional) of the release. Data are, therefore, collected from a set of monitoring
wells that will allow characterization of the dimensions and concentrations of
constituents in the plume, as well as the rate of flow.
Subsequent monitoring phases may include the measurement of additional
constituents in a more extensive well network than initial monitoring. This will
necessitate careful data management. Sections 6.8 and 6.9 of the TEGD (U.S. EPA,
1986) provide useful guidance on organizing, evaluating, and presenting
monitoring data. Section 4.7 of the TEGD addresses evaluation of the quality of
ground-water data. Specific data presentation and evaluation procedures are
presented below.
Migration rates can be determined by using the concentration of monitoring
constituents over a period of time in wells aligned in the direction of flow. If these
wells are located both at the edge of the release and in the interior of the release,
subsequent analysis of the monitoring data can then provide an estimate of the rate
of migration both of the contaminant front as a whole and of individual
constituents within the release. This approach does not necessarily provide a
reliable determination of the migration rates that will occur as the contaminant
release moves further away from the facility, due to potential changes in
geohydrologic conditions or degradation of the contaminants. More importantly,
this approach requires the collection of a time series of data of sufficient duration
and frequency to gauge the movement of contaminants. Such a delay is normally
inappropriate during initial characterization of ground-water contamination
because a relatively quick determination of at least an estimate of migration rates is
needed to deduce the impact of ground-water contamination and to formulate an
appropriate reaction.
Rapid estimates of migration rates should be made from aquifer properties
obtained during the hydrogeologic investigation. The average linear velocity (v) of
the ground water should be calculated using the following form of Darcy's law:
-Ki
ne
10-92
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where (K) is hydraulic conductivity, (i) is hydraulic gradient, and (ne) is the effective
porosity. This assumes that contaminants flow at the same rate as ground water.
This equation can be used to roughly estimate the rate of migration, both of the
contaminant front as a whole, and of individual dissolved constituents within the
release.
Rough estimates of migration rates beyond the facility property boundary can
be made based on aquifer properties obtained during the site hydrogeologic
characterization and knowledge of the physical and chemical properties of
contaminants known to be present. By recognizing the various factors which can
affect the transport of monitoring constituents, the owner or operator can
determine approximate migration rates. Continued monitoring of the release over
time should be conducted to verify the rate(s) of migration. Information on rate(s)
of migration should be used in determining any additional monitoring well
locations.
More refined estimates of contaminant migration rates should consider
potential differential transport rates among various monitoring constituents.
Differential transport rates are caused by several factors, including:
Dispersion due to diffusion and mechanical mixing;
Retardation due to adsorption and electrostatic interactions; and
Transformation due to physical, chemical; and/or biological processes.
Dispersion results in the overall dilution of the contaminant; however,
chromatographic separation of the contaminant constituents and differential
dispersion effects can result in a contaminant arriving at a particular location before
the arrival time computed solely on the average linear velocity of ground-water
flow. Alternately, retardation processes can delay the arrival of contaminants
beyond that calculated using average ground-water flow rate(s). Transformation of
waste constituents is a complex process which can be difficult to estimate. While
some contaminants, such as radionuclides, decay at a constant rate over time, most
degradable chemicals are influenced by a variety of factors and the interactions of
10-93
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these factors can be extremely difficult to predict. Local geologic variations will also
affect constituent migration rates. Relating constituent migration rates to ground-
water flow rates is a reasonable and relatively quick way to estimate contaminant
flow rates. Where possible, contaminant-specific migration rates should also be
determined.
Procedures for the evaluation of monitoring data vary in a site-specific
manner, but should all result in determinations of the rate of migration, extent, and
composition of hazardous constituents of the release. Where the release is obvious
and/or chemically simple, it may be possible to characterize it readily from a
descriptive presentation of concentrations found in monitoring wells and through
geophysical measurements. Where contamination is less obvious or the release is
chemically complex, however, the owner or operator may employ a statistical
inference approach. The owner or operator should plan initially to take a
descriptive approach to data analysis in order to broadly delineate the extent of
contamination. Statistical comparisons of monitoring data among wells and/or over
time may be necessary, should the descriptive approach provide no clear
determination of the rate of migration, extent, and hazardous constituent
composition of the release.
10.6 Field Methods
10.6.1 Geophysical Techniques
During the past decade, extensive development of remote sensing geophysical
equipment, portable field instrumentation, field methods, analytical techniques
and related computer processing have resulted in an improvement in the capability
to characterize hydrogeology and contaminant releases. Some of these geophysical
methods offer a means of detecting contaminant plumes and flow directions in
both the saturated and unsaturated zones. Others offer a way to obtain detailed
information about subsurface soil and rock characteristics. This capability to rapidly
analyze subsurface conditions without disturbing the site may provide a better
overall understanding of complex site conditions, with relatively low risk to the
investigative team.
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Various geophysical techniques, including electromagnetic, seismic refraction,
electrical resistivity, ground penetrating radar, magnetic, and several borehole
methods, can be applicable to RCRA Facility Investigations. Table 10-6 suggests
appropriate applications for the various geophysical methods. Appendix C provides
additional information.
10.6.2 Soil Boring and Monitoring Well Installation
10.6.2.1 Soil Borings
Soil borings should be sufficient to characterize the subsurface geology below
the site. Section 1.2 of TEGD (U.S. EPA, 1986) provides criteria for adequate borings.
A summary of these criteria is presented below.
Installation of initial boreholes at a density based on criteria described in
Table 10-7 and sufficient to provide initial information upon which to
determine the scope of a more detailed evaluation of geology and
potential pathways of contaminant migration.
Initial boreholes should be drilled into the first confining layer beneath
the uppermost aquifer. The portion of the borehole extending into the
confining layer should be plugged properly after a sample is taken.
Additional boreholes should be installed in numbers and locations
sufficient to characterize the geology beneath the site. The number and
locations of additional boreholes should be based on data from initial
borings and indirect investigation.
Collection of samples of every significant stratigraphic contact and
formation, especially the confining layer should be taken. Continuous
cores should be taken initially to ascertain the presence and distribution
of small and large scale permeable layers. Once stratigraphic control is
established, samples taken at regular intervals (e.g., five foot) could be
substituted for continuous cores.
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10
TABLE 10-6. APPLICATIONS OF GEOPHYSICAL METHODS TO
HAZARDOUS WASTE SITES
APPLICATION
Mapping of Geohydrologic
Features
Mapping of Conductive Leachates
and Contaminant Plumes (e.g.,
Landfills, Acids, Bases)
Locations and Boundary
Definition of Buried Trenches
with Metal
Location and Boundary Definition
of Buried Trenches without Metal
Location and Definition of Buried
Metallic Objects (e.g., Drums,
Ordinance)
RADAR
1
2
1
1
2
ELECTROMAGNETICS
1
1
1
1
2
RESISTIVITY
1
1
2
2
SEISMIC
1
2
2
METAL
DETECTOR
2
1
MAGNETOMETER
2
1
1. Primary method - Indicates the most effective method
2. Secondary method - Indicates an alternate approach
Source: EPA, 1982, Geophysical Techniques for Sensing Buried Waste and Waste Migration
-------
TABLE 10-7. FACTORS INFLUENCING DENSITY OF INITIAL BOREHOLES
Factors That May Substantiate
Reduced Density of Boreholes:
Simple geology (i.e., horizontal, thick,
homogeneous geologic strata that are
continuous across site that are
unfractured and are substantiated by
regional geologic information).
Use of geophysical data to correlate
well log data.
Factors That May Substantiate
Increased Density of Boreholes:
Fracture zones encountered during
drilling.
Suspected pinchout zones (e.g.,
discontinuous areas across the site).
Geologic formations that are tilted or
folded.
Suspected zones of high permeability
that would not defined by drilling
at 300-foot intervals.
Laterally transitional geologic units
with irregular permeability (e.g.,
sedimentary facies changes).
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Boreholes in which permanent wells are not constructed should be
sealed with materials at least an order of magnitude less permeable than
the surrounding soil/sediment/rock in order to reduce the number of
potential contaminant pathways.
Samples should be logged in the field by a qualified professional
geologist.
Sufficient laboratory analysis should be performed to provide
information concerning petrologic variation, sorting (for unconsolidated
sedimentary units), cementation (for consolidated sedimentary units),
moisture content, and hydraulic conductivity of each significant geologic
unit or soil zone above the confining layer/unit.
Sufficient laboratory analysis should be performed to describe the
mineralogy (X-ray diffraction), degree of compaction, moisture content,
and other pertinent characteristics of any clays or other fine- grained
sediments held to be the confining unit/layer. Coupled with the
examination of clay mineralogy and structural characteristics should be a
preliminary analysis of the reactivity of the confining layer in the
presence of the wastes present.
ASTM or equivalent methods should be used for soil classification, specifically:
ASTM Method D422-63 for the particle size analysis of soils, which
describes the quantitative determination of the distribution of particle
sizes in soils; and
ASTM Methods D2488-69, for the identification and description of soils
based on visual examination and simple manual tests.
An adequate number of geologic cross-sections should be presented by the
owner or operator. These cross-sections should adequately depict major geologic or
structural trends and reflect geologic/structural features in relation to ground-
water flow. Additionally, an owner or operator should provide a surface topo-
10-98
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graphic map and aerial photograph of the site. Details regarding specific means for
the presentation of geologic data are presented in Section 5 and in Section 1.2.3 of
the TEGD (U.S. EPA, 1986).
10.6.2.2 Monitoring Well Installation
The owner or operator is advised to consult Chapter Three of the TEGD (U.S.
EPA, 1986) for guidance on monitoring well installation. This chapter provides
information on the following topics:
Drilling Methods for Installing Wells-Section 3.1 (TEGD) discusses a
variety of well drilling methods and corresponding applicability to the
installation of RCRA monitoring wells. The selection of the actual drilling
method that an owner or operator should use at a particular site is a
function of site-specific geologic conditions. Of utmost importance is
that the drilling method the owner or operator uses will minimize the
disturbance of subsurface materials and will not cause contamination of
the ground water.
Monitoring Well Construction Materials-Section 3.2 (TEGD) discusses the
selection of construction materials for RCRA monitoring wells which are
durable enough to resist chemical and physical degradation, and do not
interfere with the quality of ground-water samples. Specific well
components that are of concern include well casings, well screens, filter
packs, and annular seals.
Design of Well Intakes-Section 3.3 (TEGD) discusses the design and
construction of the intake of monitoring wells so as to: (1) allow
sufficient ground-water flow to the well for sampling; (2) minimize the
passage of formation materials (turbidity) into the well; and (3) ensure
sufficient structural integrity to prevent the collapse of the intake
structure.
Development of Wells-Section 3.4 (TEGD) discusses the requirements for
proper development of the monitoring wells to ensure turbid-free
ground water samples.
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Documentation of Well Construction Activity-Section 3.5 (TEGD) lists the
information required for the design and construction of wells as follows:
date/time of construction;
drilling method and drilling fluid used;
well location (± 0.5 ft);
borehole diameter and well casing diameter;
well depth (± 0.1 ft);
drilling and lithologic logs;
casing materials;
screen materials and design;
casing and screen joint type;
screen slot size/length;
filter pack material/size;
filter pack volume calculations;
filter pack placement method;
sealant materials (percent bentonite);
sealant volume (Ibs/gallon of cement);
sealant placement method;
surface seal design/construction;
well development procedure;
type of protective well cap;
ground surface elevation (±0.01 ft);
top of casing elevation (±0.01 ft); and
detailed drawing of well (including dimensions).
Specialized Well Design-Section 3.6 (TEGD) discusses two cases which
require special monitoring well design: (1) where dedicated pumps are
used to draw ground-water samples; and (2) where light and/or dense
phase immiscible layers are present.
Evaluation of Existing Wells-Section 3.7 (TEGD) discusses how to
evaluate the ability of existing wells to produce representative ground-
water samples.
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Particular attention should be paid to the discussion in Section 3.2.1 regarding
well casing materials (TEGD). It is imperative that well materials are nonreactive to
contaminants that may be present in the ground water. In cases where the facility
has existing monitoring wells which could potentially be used in the RFI, the owner
or operator should evaluate whether these wells are capable of producing
representative ground-water samples. A demonstration involving the installation
of new well(s) near existing wells and the analysis and comparison of samples for
the same monitoring constituents from both wells may be necessary if the existing
wells' integrity is in question.
10.6.3 Aquifer Characterization
10.6.3.1 Hydraulic Conductivity Tests
In addition to defining the direction of ground-water flow in the vertical and
horizontal direction, the owner or operator should identify areas of high and low
hydraulic conductivity within each formation. Variations in the hydraulic
conductivity of subsurface materials can create irregularities in ground-water flow
paths. Areas of high hydraulic conductivity represent areas of greater ground-
water flow and, if contaminants are present, zones of potential migration.
Therefore, information on hydraulic conductivities is generally required before the
owner or operator can make reasoned decisions regarding well placements. It may
be beneficial to use analogy or laboratory methods to corroborate results of field
tests; however, only field methods provide direct information that is adequate to
define the hydraulic conductivity.
Hydraulic conductivity can be determined in the field using single well tests,
more commonly referred to as slug tests, which are performed by suddenly adding
or removing a slug (known volume) of water from a well or piezometer and
observing the recovery of the water surface to its original level. Similar results can
be achieved by pressurizing the well casing, depressing the water level, and
suddenly releasing the pressure to simulate removal of water from the well. Where
slug tests are not appropriate (e. g., in fractured flow aquifers), hydraulic
conductivity can be determined by multiple well (pumping) tests.
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Slug testing is applied by hydrogeologists in many field situations.
Interpretation of the results requires some professional judgement. Slug test
accuracy is reduced when dealing with extreme values of hydraulic conductivity.
Very low values (e.g., less than 10-6 cm/see) are more accurately measured by a
resurg head test after bailing or pumping the well dry. High values (e. g., greater
than 10-2 cm/sec) generally require fast response electronic measurement
equipment. High value cases in fractured rock or karst terrain may be misleading if
the slug test is measuring the most permeable fractures or solution channels. In
such cases, the test results may be misinterpreted to give an artificially high value
for the formation as a whole.
When reviewing information obtained from slug tests, several criteria should
be considered. First, slug tests are run on one well and, as such, the information
obtained from single well tests is limited in scope to the geologic area directly
adjacent to the well. Second, the vertical extent of screening will control the part of
the geologic formation that is being tested during the slug test. That part of the
column above or below the screened interval that has not been tested during the
slug test will not have been adequately tested for hydraulic conductivity. Third, the
methods used to collect the information obtained from slug tests should be
adequate to measure accurately parameters such as changing static water (prior to
initiation, during, and following completion of slug test), the amount of water
added to, or removed from the well, and the elapsed time of recovery. This is
especially important in highly permeable formations where pressure transducers
and high speed recording equipment should be used. Lastly, interpretation of the
slug test data should be consistent with the existing geologic information (e.g.,
boring log data). It is, therefore, important that the program of slug testing ensure
that enough tests are run to provide representative measures of hydraulic
conductivity, and to document lateral and vertical variation of hydraulic
conductivity in the geologic materials below the site.
It is important that hydraulic conductivity measurements define hydraulic
conductivity both in a vertical and horizontal manner across a site. In assessing
hydraulic conductivity measurements, results from the boring program used to
characterize the site geology should be considered. Zones of expected high
permeability or fractures identified from drilling logs should generally be included
in the determination of hydraulic conductivity. Additionally, information from
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coring logs can be used to refine the data generated by slug tests (TEGD, Section
1.3.3).
Techniques for determining hydraulic conductivity are specified in Method
9100, Saturated Hydraulic Conductivity, Saturated Leachate Conductivity, and
Intrinsic Permeability; from SW-846, Test Methods for Evaluating Solid Waste. 3rd
edition. 1986. Method 9100 includes techniques for:
Laboratory
sample collection;
constant head methods; and
falling head methods.
Field
well construction;
well development;
single well tests (slug tests); and
references for multiple well (pumping) tests.
Cedergren, 1977 also provides an excellent discussion on aquifer tests,
including laboratory methods (constant head and falling head), multiple well
(pumping) tests (steady-state and nonsteady-state), and single well tests (open-end,
packer, and others).
10.6.3.2 Water Level Measurements
Water level measurements are necessary for determining depth to the water
table and mapping ground-water contours to determine hydraulic gradients and
flow rates. Depths to water are normally measured with respect to the top of the
casing as in well depth determinations. Several methods are available, including
the electric sounder and the chalked steel tape.
The electric sounder, although not the most accurate method, is
recommended for initial site work because of the minimal potential for equipment
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contamination and simplicity of use. Sounders usually consist of a conductivity cell
at the end of a graduated wire, and a battery powered buzzer. When the cell
contacts the water the increased conductivity completes the circuit and allows
current to flow to the alarm buzzer. The depth to water can then be read from the
graduations on the wire or the wire can be measured directly. This device may not
be suitable for use if a potentially flammable or explosive layer (e.g., due to
methane gas) is present in the well, unless it is an intrinsically safe device.
The chalked steel tape is a more accurate device for measuring static water
levels. The lower 0.5 to 1.0 meters of a steel measuring tape is coated on either side
with either carpenter's chalk or any of the various indicating pastes. A weight is
attached to the lower end to keep the tape taut and it is lowered into the center of
the well (condensate on the casing wall may prematurely wet the tape). A hollow
"plopping" sound occurs when the weight reaches water, then the tape is lowered
very slowly for at least another 15 cm, preferably to an even increment on the
measuring tape. Next, the tape is carefully withdrawn from the well; water depth is
determined by subtracting the wetted length of tape from the total length of tape
in the well. In small diameter wells, the volume of the weight may cause the water
to rise by displacement. In general, the use of indicating paste or chalk should be
discouraged although they may not present a significant problem if water samples
are not collected. As with all depth measurement devices, the wetted section of the
tape and the weight must be thoroughly cleaned before reuse to avoid cross
contamination.
The following sections of the TEGD (U.S. EPA, 1986) should be consulted for
water level measurement requirements, and information on data interpretation:
Ground-water level measurement (1.3.1.1);
Interpretation of ground-water level measurements (1.3.1.2);
Establishing vertical components of ground-water flow (1.3.1.3); and
Interpretation of flow direction (1.3.1.4).
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10.6.3.3 Dye Tracing
Dye tracing is a field method which can be used to measure the velocity of
ground water for highly permeable strata (such as karst terrain and highly fractured
rock media). When the velocity of flowing water and the hydraulic gradient at a
common point are known, the permeability can be estimated. The hydraulic
gradient (i) of an existing water table can be estimated from wells in the area. If
not, observation wells must be installed (Cedergren, 1977).
The procedure used in dye tracing involves the insertion of a dye, such as
fluorescein sodium into a test hole and observation of the time it takes to emerge in
a nearby test pit or on a bank from which seepage is emerging. The average linear
velocity, v, is determined by dividing the distance traveled, L, by the time of travel, t.
The effective porosity, ne, is determined from test data for the in-place soil; if no
tests are available, it is determined using the values in Table 10-4. The hydraulic
conductivity is calculated from the equation:
It should be noted that the time required for tracers to move even short
distances can be very long unless the formations contain highly permeable strata
(Cedergren, 1977). As a result of the limitations of tracer techniques, this type of
study is applied only in highly specialized locations. Uncertainties associated with
the flow path make interpretation of the results difficult. This technique has been
used effectively in conjunction with modeling in complex terrain with the tracer
study serving to calibrate the model.
10.6.4 Ground-Water Sample Collection Techniques
The procedure for collecting a ground water sample involves the following
steps presented in Chapter 4 of TEGD (U.S. EPA, 1986):
Measurement of static water level elevation (4.2.1);
10-105
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Monitoring of immiscible layers (4.2.2);
Well evacuation (4.2.3);
Sample withdrawal (4.2.4);
In situ or field analyses (4.2.5);
Sample preservation and handling (4.3); and
Chain-of-custody procedures (4.4).
Collection of static water level elevations on a continuing basis is important to
determine if horizontal and vertical flow gradients have changed since initial site
characterization, which could necessitate modification of the ground-water
monitoring system. Steps should be taken to monitor for the presence and/or
extent of light and/or dense phase immiscible organic layers before the well is
evacuated for conventional sampling if wastes of this type are present at the
facility.
The water standing in the well prior to sampling may not be representative of
in situ ground-water quality. Therefore, the owner or operator should remove the
standing water in the well so that water which is representative of the formation
can replace the standing water. Purged water should be collected and screened
with photoionization or organic vapor analyzers, pH, temperature, and conductivity
meters. If these parameters and facility background data suggest that the water
may be hazardous, it should be drummed and disposed of properly.
The technique used to withdraw a ground-water sample from a well should be
selected based on a consideration of the parameters which will be analyzed in the
sample. To ensure the ground-water sample is representative of the formation, it is
important to avoid physically altering or chemically contaminating the sample
during the withdrawal process. In order to minimize the possibility of sample
contamination, the owner or operator should:
10-106
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(1) Use only polytetrafluoroethylene (PTFE) or stainless steel sampling
devices; and
(2) Use dedicated samplers for each well. (If a dedicated sampler is not
available for each well, the sampler should be thoroughly cleaned
between sampling events, and blanks should be taken and analyzed to
ensure that cross contamination has not occurred.)
Section 4.2.4 of TEGD (U.S. EPA, 1986) includes specific factors to take into
consideration regarding sample withdrawal.
Some parameters are physically or chemically unstable and must be tested
either in the borehole using a probe (in situ) or immediately after collection using a
field test kit. Examples of several unstable parameters include pH, redox potential,
chlorine, dissolved oxygen, and temperature. Although specific conductivity
(analogous to electrical resistance) is relatively stable, it is recommended that this
characteristic also be determined in the field. Most conductivity instruments
require temperature compensation; therefore, temperatures of the samples should
be measured at the time conductivity is determined.
Many of the constituents and parameters that are included in ground-water
monitoring programs are not stable and, therefore, sample preservation may be
required. Refer to methods from EPA's Test Methods for Evaluating Solid Waste -
Physical/Chemical Methods. 1986 (EPA/SW-846 GPO No. 955-001-00000-1) for
sample preservation procedures and sample container requirements.
Improper sample handling may lead to sample contamination. Samples should
be transferred into their containers in such a way as to minimize any contamination.
Handling methods are analyte dependent. Special handling considerations for
various analyte types are discussed in Section 4.3.3 of the TEGD (U.S. EPA, 1986).
An adequate chain-of-custody program will allow for the tracing of possession
and handling of individual samples from the time of field collection through
laboratory analysis. An owner or operator's chain-of-custody program
requirements are detailed in Section 4 (Quality Assurance and Quality Control).
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Chapter Four of the TEGD (U.S. EPA, 1986) may also be consulted for sample
collection techniques as well as for analytical procedures, field and laboratory
QA/QC requirements, and suggestions for reporting of ground-water data. Section
4 of this guidance presents a general discussion of QA/QC. In addition, the owner or
operator may also find the following publication useful for sampling information:
U.S. EPA. September, 1987. Practical Guide for Ground Water Sampling.
EPA/600/2-85/104. NTIS PB86-137304. Washington, D.C. 20460.
10.7 Site Remediation
Although the RFI Guidance is not intended to provide detailed guidance on
site remediation, it should be recognized that certain data collection activities that
may be necessary for a Corrective Measures Study may be collected during the RFI.
EPA has developed a practical guide for assessing and remediating contaminated
sites that directs users toward technical support, potential data requirements and
technologies that may be applicable to EPA programs such as RCRA and CERCLA.
The reference for this guide is provided below.
U.S. EPA. 1988. Practical Guide for Assessing and Remediating Contaminated
Sites. Office of Solid Waste and Emergency Response. Washington, D.C.
20460.
This guide is designed to address releases to ground water as well as soil,
surface water and air. A short description of the guide is provided in Section 1.2
(Overall RCRA Corrective Action Process), under the discussion of Corrective
Measures Study.
In addition to the above described reference, several ground-water computer
modeling programs are available to assist in designing ground-water remediation
systems, such as the one referenced below. Application of such models should be
based on site-specific considerations, as most models are not applicable to all
situations.
U.S. EPA. 1987. Zone of Capture for Ground Water Corrective Action. IBM
Compatible Computer Program and Users Guide. Federal Computer Products
10-108
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Center, National Technical Information Service. Springfield, VA 22161.
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10.8 Checklist
RFI CHECKLIST - GROUND WATER
Site Name/Location.
Type of Unit
1. Does waste characterization include the following information? (Y/N)
Constituents of concern/supporting indicator parameters
Concentrations of constituents
Physical form of waste
Chemical properties of waste (organic, inorganic,
acid, base) and constituents
pH
pKa
Viscosity
Water volubility
Density
* KOW
Henry's Law Constant
Physical and chemical degradation (e.g., hydrolysis)
2. Does unit characterization include the following information? (Y/N)
Age of unit
Construction integrity
Presence of liner (natural or synthetic)
Location relative to ground-water table or bedrock or
other confining barriers
Unit operation data
Presence of cover
Presence of on/offsite buildings
Depth and dimensions of unit
Inspection records
Operation logs
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RFI CHECKLIST- GROUND WATER (Continued)
Past fire, explosion, or other complaint reports
Existing ground-water monitoring data
Presence of natural or engineered barriers near unit
3. Does environmental setting information include the following information?
(Y/N)
Site Soil Characteristics
Grain size distribution and gradation
Hydraulic Conductivity
Porosity
Discontinuities in soil strata (e.g., faults)
Degree and orientation of subsurface stratification
and bedding
Ground-Water Flow System Characterization (Y/N)
Use of aquifer
Regional flow cells and flow nets
Depth to water table
Direction of flow
Rate of flow
Hydraulic conductivity
Storativity/specific yield (effective porosity)
Aquifer type (confined or unconfined)
Aquifer characteristics (e.g., homogeneous, isotropic,
leaky)
Hydraulic gradient
Identification of recharge and discharge areas
Identification of aquifer boundaries (i.e., areal extent)
Aquitard characteristics (depth, permeability degree of
jointing, continuity)
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RFI CHECKLIST- GROUND WATER (Continued)
Ground-Water Quality Characteristics (Y/N)
Presence of minerals and organics
Background water quality
Monitoring constituents and indicator parameters
4. Have the following data on the initial phase of the release characterization
been collected? (Y/N)
Extent
Location
Shape
Hydraulic gradient across plume
Depth to plume
Chemistry and concentration
Velocity
Potential receptors
5. Have the following data on the subsequent phase(s) of the release character-
ization been collected? (Y/N)
Extent
Location
Shape
Hydraulic gradient across plume
Depth to plume
Chemistry and concentration
Velocity
Potential receptors
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10.9 References
ASTM. 1984. Annual Book of ASTM Standards. Volume 4.08: Natural Building
Stones; Soil and Rock. American Society for Testing and Materials.
Philadelphia, PA.
Balch, A. H., and W. W. Lee. 1984. Vertical Seismic Profilin Technique, Applications
and Case Histories. DE83751260. International Human Resource Development
Corp.
Billings. 1972. Structural Geology. 3rd Edition. Prentice-Hall, Inc. Englewood
Cliffs, New Jersey.
Brady. 1974. The Nature and Properties of Soils. 8th Edition. MacMillan
Publishing Co., Inc. New York, N.Y.
Callahan, et al. 1979. Water-Related Environmental Fate of 129 Priority Pollutants.
EPA-440/4-79-029. NTIS PB80-204373. Washington, D.C. 20460.
Cedergren. 1977, Seepage. Drainage, and Flow Nets. 2nd Edition. John Wiley &
Sons. New York, N.Y.
Freeze and Cherry. 1979. Ground water. Prentice-Hall, Inc. Englewood Cliffs,
New Jersey.
Linsley, R. K., M.A. Kohler, and J. Paulhus. 1982. Hvdroloav for Engineers. Third
Edition. McGraw-Hill, Inc. New York, N.Y.
McWhorter and Sunada. 1977. Ground Water Hydrology and Hydraulics. Water
Resources Publications. Littleton, Colorado.
OkirD.S. and T.W. Giambelluca, "DBCP, EDB, and TCP Contamination of Ground
Water in Hawaii, " Ground Water. Vol. 25, No. 6, November/December 1987.
Snoeyink and Jenkins. 1980. Water Chemistry. John Wiley& Sons. New York, N.Y.
10-113
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Sowers, G. F. 1981. Rock Permeability or Hydraulic Conductivity - An Overview in
Permeability and Ground Water Transport. F. Zimmic and C. 0. Riggs, Eds.
ASTM Special Technical Publication 746. Philadelphia, PA.
Sun, R. J., Editor. 1986. Regional Aauifer-Svstem Analysis Program of the U.S.
Geological Survey. Summary of Projects. 1978-1984. U.S.G.S. Circular 1002.
U.S. Geological Survey. Denver, CO.
Technos, Inc. 1982. Geophysical Techniques for Sensing Buried Wastes and Waste
Migration. Environmental Monitoring Systems Laboratory. NTIS PB84-198449.
U.S. EPA. Washington, D.C. 20460.
U.S. Department of Agriculture. 1975. Soil Taxonomy: A Basic System of Soil
Classification for Making and Interpreting Soil Surveys. Soil Survey Staff, Soil
Conservation Service. Washington, D.C.
U.S. Department of the Army. 1979. Geophysical Explorations. Army Corps of
Engineers. Engineering Manual 1110-1-1802. May, 1979.
U.S. EPA. 1985. Characterization of Hazardous Waste Sites - A Methods Manual.
Volume I - Site Investigations. EPA-600/4-84/075. NTIS PB85-215960. Office of
Research and Development. Washington, D.C. 20460.
U.S. EPA. 1984. Characterization of Hazardous Waste Sites - A Methods Manual:
Volume II: Available Sampling Methods. 2nd Edition. EPA-600/4-84-076. NTIS
PB 85-168771. Office of Research and Development. Washington, D.C. 20460.
U.S. EPA. 1986. Ground Water Flow Net/Flow Line Technical Resource Document
(TRD^ Final Report. NTIS PB86-224979. Office of Solid Waste. Washington,
D.C. 20460.
U.S. EPA. 1985. Guidance on Remedial Investigations Under CERCLA. NTIS PB85-
238616. Hazardous Waste Engineering Research Laboratory, Office of
Research and Development. Cincinnati, OH 45268.
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U.S. EPA. 1982. Handbook for Remedial Action at Waste Disposal Sites. EPA-625/6-
82-006. NTIS PB82-239054. Office of Emergency and Remedial Response.
Washington, D.C. 20460.
U.S. EPA. 1984. Permit Applicant's Guidance Manual for Hazardous Waste - Land
Treatment. Storage, and Disposal Facilities. Office of Solid Waste.
Washington, D.C. 20460.
U.S. EPA. 1985. DRASTIC: A Standardized System for Evaluating Ground-water
Pollution Potential Using Hvdroaeoloaic Settings. EPA/600/2-88/018. Robert S.
Kerr Environmental Research Laboratory. Ada, OK.
U.S. EPA. 1986. Guidance Criteria for Identifying Areas of Vulnerable Hydrogeology
Under the Resource Conservation and Recovery ActJnterim Final.
Washington, D.C. 20460
U.S. EPA. 1986. Permit Writers' Guidance Manual for the Location of Hazardous
Waste Land Storage and Disposal Facilities - Phase II: Method for Evaluating
the Vulnerability of Ground Water. NTIS P886-125580. Office of Solid Waste.
Washington, D.C. 20460.
U.S. EPA. 1985. Practical Guide for Ground Water Sampling. EPA-600/2-85/104.
NTIS PB86-137304. Washington, D.C. 20460.
U.S. EPA. 1985. RCRA Ground-Water Monitoring Compliance Order Guidance
(Final). Office of Solid Waste. Washington, D.C. 20460.
U.S. EPA. 1986. RCRA Ground-Water Monitoring Technical Enforcement Guidance
Document. Office of Solid Waste. Washington, D.C. 20460.
U.S. EPA. 1987. Zone of Capture for Ground Water Corrective Action. IBM
Compatible Computer Program and Users Guide. Federal Computer Products
Center, National Technical Information Service. Springfield, VA 22161.
10-115
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U.S. EPA. 1988. Practical Guide for Assessing and Remediating Contaminated
Ground Water. Office of Emergency and Remedial Response. Washington,
D.C. 20460.
U.S. Geological Survey. 1984. Ground water Regions of the U.S. Heath et. al.,
Water Supply Paper No 2242. Washington, D.C.
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SECTION 11
SUBSURFACE GAS
11.1 Overview
This section applies to units with subsurface gas releases, primarily landfills,
leaking underground tanks, and units containing putrescible organic matter, but
may include other units.
The objective of an investigation of a subsurface gas release is to verify, if
necessary, that subsurface gas migration has occurred and to characterize the
nature, extent, and rate of migration of the release of gaseous material or
constituents through the soil. Methane gas should be monitored because it poses a
hazard due to its explosive properties when it reaches high concentrations, and also
because it can serve as an indicator (i.e., carrier gas) for the migration of hazardous
constituents. Other gases (e.g., carbon dioxide and sulfur dioxide) may also serve as
indicators. This section provides:
An example strategy for characterizing subsurface gas releases, which
includes characterization of the source and the environmental setting of
the release, and conducting monitoring to characterize the release itself;
Formats for data organization and presentation;
Field methods which may be used in the investigation; and
A checklist of information that may be needed for release
characterization.
The exact type and amount of information required for sufficient release
characterization will be site-specific and should be determined through interactions
between the regulatory agency and the facility owner or operator during the RFI
process. This guidance does not define the specific data required in all instances;
11-1
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however, it identifies possible information which may be necessary to perform
release characterizations and methods for obtaining this information. The RF
Checklist, presented at the end of this section, provides a tool for planning and
tracking information for subsurface gas release characterizations. This list is not
meant to serve as a list of requirements for all subsurface gas releases to soil. Some
releases will involve the collection of only a subset of the items listed.
As indicated in the following sections, subsurface gas migrates along the path
of least resistance, and can accumulate in structures (primarily basements) on or off
the facility property. If this occurs, it is possible that an immediate hazard may exist
(especially if the structures are used or inhabited by people) and that interim
corrective measures may be appropriate. Where conditions warrant, the owner or
operator should immediately contact the regulatory agency and consider
immediate measures (e.g., evacuation of a structure).
Case Study Numbers 23 and 24 in Volume IV (Case Study Examples) provide
examples of subsurface gas investigations.
11.2 Approach for Characterizing Subsurface Gas Releases
11.2.1 General Approach
The collection and review of existing information for characterization of the
contaminant source and the environmental setting will be the primary basis for
development of a conceptual model of the release and subsequent development of
monitoring procedures to characterize the release. A conceptual model of the
release should be formulated using all available information on the waste, unit
characteristics, environmental setting, and any existing monitoring data. This
model (not a computer or numerical simulation model) should provide a working
hypothesis of the release mechanism, transport pathway/mechanism, and exposure
route (if any). The model should be testable/verifiable and flexible enough to be
modified as new data become available.
11-2
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The conceptual model for subsurface gas should consider the ability of the
waste to generate gaseous constituents, the conditions which would favor
subsurface migration of the gaseous release, and the likelihood of such a release to
reach and accumulate within structures (e.g., residential basements) at explosive or
toxic concentrations.
Additional data collection to characterize the contaminant source and
environmental setting may be necessary prior to implementing the monitoring
procedures. The subsurface pathway data collection effort should be coordinated,
as appropriate, with similar efforts for other media investigations.
Characterization of subsurface gas releases can be accomplished through a
phased monitoring approach. An example of a strategy for characterizing
subsurface gas releases is shown in Table 11-1.
Development of monitoring procedures should include determining the
specific set of subsurface gas indicators and constituents for monitoring. Methane,
carbon dioxide, and site-specific volatile organics (e.g., vinyl chloride), can be used
to identify the presence of subsurface gas during initial monitoring. Subsequent
monitoring will generally involve these gases, but may also involve various other
constituents. Development of the monitoring procedures should also include
selection of the appropriate field and analytical methods. Selection of these
methods will be dependent on site and unit specific conditions.
An initial monitoring phase should be implemented using screening
techniques and appropriate monitoring constituent(s). A subsurface gas migration
model can be used, as applicable, as an aid in selection of monitoring locations.
Subsequent monitoring will generally be necessary if subsurface gas migration is
detected during the initial survey. This additional monitoring may include a wider
range of constituents.
Characterization of a subsurface gas release can involve a number of tasks to
be completed throughout the course of the investigation. These tasks are listed in
Table 11-2 with associated techniques and data outputs.
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TABLE 11-1
EXAMPLE STRATEGY FOR CHARACTERIZING RELEASES OF SUBSURFACE GAS1
INITIAL PHASE
1. Collect and review existing information on:
Waste
Unit
Environmental setting
Contaminant releases, including inter-media transport
2. Identify any additional information necessary to fully characterize release:
Waste
Unit
Environmental setting
Contaminant releases, including inter-media transport
3. Develop monitoring procedures:
Formulate conceptual model of release
Determine monitoring program objectives
Determine monitoring constituents and indicator parameters
Sampling approach selection
Sampling schedule
Monitoring locations
Analytical methods
QA/QC procedures
4. Conduct Initial Monitoring:
Use subsurface gas migration model to estimate release dimensions (plot
1.0 and 0.25 lower explosion limit isopleths for methane)
Monitor ambient air and shallow boreholes around the site using
portable survey instruments to detect methane and other indicator
parameters
Use results of above two steps to refine conceptual model and determine
sampling locations and depths; conduct limited well installation
program. Monitor well gas and shallow soil boreholes for indicators and
constituents
Monitor surrounding structures (e.g., buildings and engineered conduits)
for other indicator parameters and constituents
5. Collect, evaluate and report results:
Compare methane results with lower explosion limit (LEL) and 0.25 LEL
and report results immediately to regulatory agency if these values are
exceeded
11-4
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TABLE 11-1 (Continued)
EXAMPLE STRATEGY FOR CHARACTERIZING RELEASES OF SUBSURFACE GAS1
Summarize and present data in appropriate format
Determine if monitoring program objectives were met
Determine if data are adequate to describe nature, rate and extent of
release
Report results to regulatory agency
SUBSEQUENT PHASES (If Necessary)
1. Identify additional information necessary to characterize release:
Modify conceptual model and identify additional information needs
Selection of monitoring constituents for subsequent phase
Spatial extent of subsurface gas migration
Concentration levels of methane and other indicators and additional
monitoring constituents
Evaluate potential role of inter-media transport
2. Expand initial monitoring as necessary:
Expand subsurface gas well monitoring network
Add or delete constituents and parameters
Expand number of structures subject to monitoring
Increase or decrease monitoring frequency
3. Conduct subsequent monitoring:
Perform expanded monitoring of area for methane and other indicator
parameters and specific monitoring constituents
Further monitoring of surrounding structures if warranted
4 . Collect, evaluate and report results/identify additional information necessary
to characterize release:
Compare monitoring results to health and environmental criteria and
identify/respond to emergency situations and identify priority situations
that warrant interim corrective measures - notify regulatory agency
immediately
Summarize and present data in appropriate format
Determine if monitoring program objectives were met
Determine if data are adequate to describe nature, rate, and extent of
release
Identify additional information needs
Determine need to expand monitoring system
Evaluate potential role of inter-media transport
Report results to regulatory agency
The possibility for inter-media transport of contamination should be
anticipated throughout the investigation.
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TABLE 11-2
RELEASE CHARACTERIZATION TASKS FOR SUBSURFACE GAS
Investigatory Tasks
Investigatory Techniques
Data Presentation Formats/Outputs
1. Waste/Unit Characterization
Identification of waste
constituents of concern
Identification of unit
characteristics which
promote a subsurface gas
release
See Sections 3,7 and Appendix
B
See Section 7
Listing of potential monitoring
constituents
Description of the unit, if
active, and operational
conditions concurrent with
subsurface gas sampling
2. Environmental Setting
Characterization
Definition of climate
Definition of site-specific
meteorological conditions
Definition of soil conditions
Definition of site-specific
terrain
Identification of subsurface
gas migration pathways
Identification and location
of engineered conduits
Identification and location
of surrounding structures
Climate summaries for regional
National Weather Service
stations
Meteorological data from
regional National Weather
Service stations
See Section 9 (e.g., porosity,
moisture content, organic
carbon content, etc.)
See Sections 7,9 and Appendix
A
Review of unit design and
environmental setting
Review of water level
measurements
Examination of maps,
engineering diagrams, etc.
Ground penetrating radar (See
Appendix C)
Survey of surrounding area
Tabular summaries for
parameters of interest
Tabular listing for parameters
of interest concurrent with
subsurface gas sampling
Soil physical properties
Topographic map of site area
Identification of possible
migration pathways
Depth to water table
Description of the examination
Results of study
Map with structures identified
1. Release Characterization
Model extent of release
Screening evaluation of
subsurface gas release
Measurement for specific
constituents
Gas migration model (See
Appendix D)
Shallow borehole monitoring
and monitoring in surrounding
buildings for indicators and
specific constituent(s)
Selected gas well installation
and monitoring
Monitoring in surrounding
buildings
Estimated methane
concentration isopleths for LEL
and 0.25 LEL
Listing of concentrations levels
Tables of concentrations
Detailed assessment of extent
and magnitude of releases
Tables of concentrations
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As monitoring data become available, both within and at the conclusion of
discrete investigation phases, it should be reported to the regulatory agency as
directed. The regulatory agency will compare the monitoring data to applicable
health and environmental criteria to determine the need for (1) interim corrective
measures; and/or (2) a Corrective Measures Study. In addition, the regulatory
agency will evaluate the monitoring data with respect to adequacy and
completeness to determine the need for any additional monitoring efforts. The
health and environmental criteria and a general discussion of how the regulatory
agency will apply them are supplied in Section 8. A flow diagram illustrating RFI
decision points is provided in Section 3 (See Figure 3-2).
Notwithstanding the above process, the owner or operator has a continuing
responsibility to identify and respond to emergency situations and to define priority
situations that may warrant interim corrective measures. For these situations, the
owner or operator is directed to obtain and follow the RCRA Contingency Plan
requirements under 40 CFR Part 264, Subpart D.
11.2.2 Inter-media Transport
Contaminated ground water and contaminated soil can result in releases of
gaseous constituents via subsurface migration, primarily due to volatilization of
organic constituents. Information collected from ground-water and soil
investigations may provide useful input data for the subsurface gas pathway
characterization. It may also be more efficient to jointly conduct monitoring
programs for such related media (e.g., concurrent ground water and subsurface gas
migration monitoring programs).
Subsurface gas migration also has the potential for inter-media transport (e.g.,
transfer of contamination from subsurface gas to the soil and air media). Therefore,
information from the subsurface gas migration investigation will also provide
useful input for assessing soil contamination and potential air emissions.
11.3 Characterization of the Contaminant Source and the Environmental Setting
The type of waste managed in the unit will determine the conditions under
which the gas can be generated, and the type of unit and characteristics of the
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surrounding environment (e.g., soil type and organic content) establishes potential
migration pathways. Units which may be of particular concern for subsurface gas
releases contain putrescible organic material and generally include below grade
landfills, units closed as landfills (e.g., surface impoundments), and underground
tanks. These types of units may have waste deposited or stored at such depths as to
allow for subsurface gas generation by volatilization or decomposition of organic
wastes and subsequent migration (see Figures 11-1 and 11-2).
The nature and extent of contamination are affected by environmental
processes such as dispersion, diffusion, and degradation, that can occur before and
after the release occurred. Factors that should be considered include soil physical
and chemical properties, subsurface geology and hydrology, and in some cases,
climatic or meteorologic patterns.
The principle components of "landfill gas" are generally methane and carbon
dioxide produced by the anaerobic decomposition of organic materials in wastes.
Methane is of particular concern due to its explosive/flammable properties,
although other gases of concern could be present. The presence of these other
gases in a unit is primarily dependent upon the types of wastes managed, the
volatilities of the waste constituents, temperature, and possible chemical
interactions within the waste. Previous studies (e.g., Hazardous Pollutants in Class II
Landfills, 1986, South Coast Air Quality Management District, El Monte, California
and U.S. EPA. 1985. Technical Guidance for Corrective Measures - Subsurface Gas.
Washington, D.C. 20460) have indicated that the predominant components of
landfill gas are methane and carbon dioxide. Methane is generally of greater
concentration, however, carbon dioxide levels are generally also high, especially
during the early stages of the methane generation process. Concentrations of
subsurface gas constituents which may accompany methane/carbon dioxide are
generally several orders of magnitude less than methane. In some cases (e.g.,
associated with acidic refinery wastes) sulfur dioxide may be the primary subsurface
gas.
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UNSATURATED
GROUNDWATER TABLE
Figure 11-1.
-------
UNDERGROUND TANK
PAVING
file
VOLATILE
LIQUIDS
SURFACE IMPOUNDMENT CLOSED AS LANDFILL
LIQUIDS/SLUDGES
UNSATURATED
SOIL
Figure 11-2. Subsurface Gas Generation/Migration from Tanks and Units Closed
as Landfills (Note: Gas may also migrate slowly through cover
soil.)
-------
11.3.1 Waste Characterization
11.3.1.1 Decomposition Processes
Subsurface gas generation occurs by biological, chemical, and physical
decomposition of disposed or stored wastes. Waste characteristics usually affect the
rate of decomposition. The owner or operator should review unit-specific
information (waste receipts, waste composition surveys, and any other records of
wastes managed) to determine waste type, quantities, location, dates of disposal,
waste moisture content, organic content, etc.
The three decomposition processes known to occur in the production of
subsurface gases are biological decomposition, chemical decomposition, and
physical decomposition. These are discussed below:
11.3.1.1.1 Biological Decomposition
The extent of biological decomposition and subsequent gas generation from a
given waste is related to the type of unit. Biological decomposition, due primarily
to anaerobic microbial degradation, is significant in most landfills and units closed
as landfills which contain organic wastes. Generally, the amount of gas generated
in a landfill is directly related to the amount of organic matter present.
Organic wastes such as food, sewage sludges, and garden wastes decompose
rapidly, resulting in gas generation shortly after burial, with high initial yields.
Much slower decomposing organic wastes include paper, cardboard, wood, leather,
some textiles and several other organic components. Inorganic and inert materials
such as plastics, man-made textiles, glass, ceramics, metals, ash, and rock do not
contribute to biological gas production. At units closed as landfills, waste types that
undergo biological decomposition might include bulk organic wastes, food
processing sludges, treatment plant sludges, and comporting waste.
Waste characteristics can increase or decrease the rate of biological
decomposition. Factors that enhance anaerobic decomposition include high
moisture content, adequate buffer capacity and neutral pH, sufficient nutrients
(nitrogen and phosphorus), and moderate temperatures. Characteristics that
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generally decrease biological decomposition include the presence of acidic or basic
pH, sulfur, soluble metals and other microbial toxicants. The owner or operator
should review the waste characteristic information to document if biological
decomposition and subsequent gas generation may be occurring.
Under anaerobic conditions, organic wastes are primarily converted by
microbial action into carbon dioxide and methane. Trace amounts of hydrogen,
ammonia, aromatic hydrocarbons, halogenated organics, and hydrogen sulfide may
also be present. With regard to subsurface migration, the primary gases of concern
are methane (because of its explosive properties) and constituents that may be
present in amounts hazardous to human health or the environment.
11.3.1.1.2 Chemical Decomposition
Gas production by chemical reaction can result from the disposal or storage of
incompatible wastes. Reactive or ignitable wastes can produce explosive or heat-
producing reactions, resulting in rapid production of gases, and increased pressures
and temperatures. Under acidic conditions, a strong oxidizing agent can react with
organic wastes to produce carbon dioxide and ammonia which can migrate from
the unit, possibly providing a transport mechanism for other gaseous components.
Under typical conditions, gas production from chemical reactions is not
expected to occur at landfills or units closed as landfills. However, volatile liquids
stored in underground tanks may have a significant potential to create a release by
chemical reaction. Good waste management practices, particularly the proper
design and operation (e.g., pressure-relief valves and leak detection systems) of
underground tanks can minimize the potential for gas release.
11.3.1 .1.3 Physical Decomposition
Physical decomposition phenomena include volatilization and combustion.
Volatilization can result in subsurface gas generation in underground tanks if there
is a leak or puncture. The greater a compound's vapor pressure, the greater will be
its potential to volatilize. Maintenance of underground tanks (e.g., pressure-relief
valves and leak detection systems) can minimize volatilization.
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Combustion processes (e.g., underground fires) sometimes occur at active
landfills and result in subsurface gas release. Combustion can convert wastes to
byproducts such as carbon dioxide, carbon monoxide, and trace toxic components.
Combustion processes can also accelerate chemical reaction rates and biological
decomposition, creating greater potential for future subsurface gas generation and
subsequent release. The owner or operator should review facility records to
determine if combustion has occurred and when.
11.3.1.2 Presence of Constituents
Subsurface gas generation and migration of methane is of concern because of
its explosive properties. In addition, methane and other decomposition gases can
facilitate the migration of volatile organic constituents that may be of concern
because of potential toxic effects. Subsurface gas migration due to leaks from
subsurface tanks may also be associated with a variety of volatile organic
constituents.
In determining the nature of a release, it may be necessary to determine the
specific waste constituents in the unit. Two means of obtaining these data are:
(1) Review of facility records. Review of facility records may not provide
adequate information (e.g., constituent concentrations) for RFI purposes.
For example, facility records of waste handled in the unit may only
indicate generic waste information. Knowledge of individual
constituents and concentrations is generally needed for purposes of the
RFI.
(2) Conducting waste sampling and analysis. When facility records do not
indicate the specific constituents of the waste which are likely to be
released and may migrate as subsurface gas, direct waste
characterization may be necessary. This effort, aimed at providing
compound specific data on the waste, can be focused in terms of the
constituents for which analysis should be performed through review of
the waste types in the unit. In some cases, however, the generic waste
description (e.g., flammable liquids) will not give an indication of the
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specific constituents present, and analysis for ail of the constituents of
concern as gaseous releases (See Appendix B, List 2) may be required.
Additional guidance on identification of monitoring constituents is presented
in Section 3.6. Section 7 provides guidance on waste characterization.
11.3.1.3 Concentration
Determination of concentrations of the constituents of concern in the waste
may indicate those constituents which are of prime concern for monitoring. The
concentration of a constituent in a waste (in conjunction with its physical/chemical
properties and total quantity) provides an indication of the gross quantity of
material that may be released in the gaseous form.
11.3.1.4 Other Factors
In addition to the factors described above, determination of the potential for
volatilization of the waste constituents will help determine if they may be released.
The parameters most important when assessing the potential for volatilization of a
constituent include the following:
Water solubility. The volubility in water indicates the maximum
concentration at which a constituent can dissolve in water at a given
temperature. This value can be used to estimate the distribution of a
constituent between the dissolved aqueous phase in the unit and the
undissolved solid or immiscible liquid phase. Considered in combination
with the constituent's vapor pressure, it can provide a relative assessment
of the potential for volatilization.
Vapor pressure. Vapor pressure refers to the pressure of vapor in
equilibrium with a pure liquid. It is best used in a relative sense;
constituents with high vapor pressures are more likely to be released in
the gaseous form than those with low vapor pressues, depending on
other factors such as relative volubility and concentration (i. e., at high
concentrations releases can occur even though a constituent's vapor
pressure is relatively low).
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Octanol/water partition coefficient. The octanol/water partition
coefficient indicates the tendency of an organic constituent to sorb to
organic components of the soil or waste matrices of a unit. Constituents
with high octanol/water partition coefficients will adsorb readily to
organic carbon, rather than volatilizing to the atmosphere. This is
particularly important in landfills and land treatment units, where high
organic carbon contents in soils or cover material can significantly reduce
the release potential of vapor phase constituents.
Partial pressure. For constituents in a mixture, particularly in a solid
matrix, the partial pressure of a constituent will be more significant than
the pure vapor pressure. In general, the greater the partial pressure, the
greater the potential for release. Partial pressures will be difficult to
obtain. However, when waste characterization data is available, partial
pressures can be estimated using methods commonly found in
engineering and environmental science handbooks.
Henry's Law constant. Henry's law constant is the ratio of the vapor
pressure of a constituent and its aqueous volubility (at equilibrium). It
can be used to assess the relative ease with which the compound may be
removed from the aqueous phase via vaporization. It is accurate only
when used in evaluating low concentration wastes in aqueous solution.
Thus it will be most useful when the unit being assessed is a surface
impoundment or tank containing dilute wastewaters. As the value
increases, the potential for significant vaporization increases, and when
it is greater than 0.001, rapid volatilization will generally occur.
Raoult's Law. Raoult's Law can be used to predict releases from
concentrated aqueous solutions (i.e., solutions over 10% solute). This
will be most useful when the unit contains concentrated waste streams.
11.3.2 Unit Characterization
Unit design (e.g., waste depth, unit configuration, and cover materials) also
affects gas generation. Generally, the amount of gas generated increases with
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landfill volume and often with landfill depth. Deeper landfills have a proportionally
larger anaerobic zone, greater insulation and compaction, and are more likely to
confine gas production. Deeper landfills, such as trench fills or canyon fills, can trap
gases along confining sidewalls and bottom bedrock or ground water. Daily,
interim, and final cover soils can confine gases within the landfill. This is particularly
true for low permeability cover soils (e.g., clays) which impede vertical gas
migration. Conversely, mounds or shallow landfills have large surface areas
through which gases can vent more easily.
Unit operations, such as methods and procedures used to segregate and
isolate inert wastes, to prevent moisture infiltration, to compact and increase the
density of the waste, and to minimize or prevent mixing of waste types, can affect
resultant releases of subsurface gases. Daily covering of the unit may inhibit
decomposition and thus gas generation and subsequent migration.
Certain units have a high potential for allowing the movement of subsurface
gas. These units are those that receive and/or store large volumes of decomposable
wastes, volatile organic liquids, or highly reactive materials. Subsurface gas
migration may occur especially when major portions of a land-based unit are below
grade. Gas generated by these units can migrate vertically and laterally from the
unit, following the path of least resistance.
Some units are operated above grade or in relatively shallow soils (e.g., surface
impoundments, land treatment units). The potential for subsurface gas migration
from such units is usually low. Gases generated by such units will generally be
vented to the atmosphere unless prevented by a natural barrier (e. g., frozen
ground) or an engineered barrier (e.g., soil cover).
Information on unit operations will therefore be important in assessing the
potential for subsurface gas migration. Unit operational data may also be required
concurrent with any subsurface gas sampling activities. It is particularly important
to obtain operational data on any gas collection system in use at the time of
sampling. These gas collection systems can significantly affect subsurface gas
migration rates, patterns and constituent concentration levels.
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Generally, the units that pose the greatest potential for subsurface gas
migration include landfills, sites closed as landfills, and underground storage tanks.
These are discussed below.
11.3.2.1 Landfills
Gas generated in landfills can vent vertically to the atmosphere and/or migrate
horizontally through permeable soil, as shown in Figure 11-1. Closure of the landfill
or periodic covering of cells or lifts with impermeable caps may impede the vertical
movement of the gases, forcing them to migrate laterally from the unit. Gas
migration laterally through the subsurface (e.g., through underground utility line
channels or sand lenses) may accumulate in structures on or off the facility property.
11.3.2.2 Units Closed as Landfills
Gas generation and subsequent migration is likely to occur at units closed as
landfills containing organic wastes, as previously discussed. Although surface
impoundments and waste piles may be closed as landfills, they tend to produce less
gas than landfills because they generally contain smaller quantities of
decomposable and volatile wastes and are generally at shallow depths. Closure of
such units with an impermeable cover will, however, increase the potential for
lateral gas movement and accumulation in onsite and offsite structures (see Figure
11-2).
11.3.2.3 Underground Tanks
Subsurface gas release and subsequent migration may occur if an
underground tank is leaking. Underground tanks frequently contain volatile liquids
that could enter the unsaturated zone should a leak occur (see Figure 11-2).
11.3.3 Characterization of the Environmental Setting
11.3.3.1 Natural and Engineered Barriers
Subsurface conditions at the site should be evaluated to determine likely gas
migration routes. Due to the inherent mobility of gases, special attention must be
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paid to zones of high permeability created by man-made, biological, and physical
weathering action. These zones include backfill around pipes, animal burrows,
solution channels, sand and/or gravel lenses, desiccation cracks, and jointing in
bedrock. The presence of dead rodents, snakes and other burrowing animals is
usually a good indication of a potential subsurface gas pathway.
Natural and engineered barriers can also affect gas migration, generally by
inhibiting migration pathways. Natural barriers to gas migration include surface
water, ground water, and geologic formations. Engineered barriers include walls,
onsite structures, underground structures, caps, liners, and other design features.
On the other hand, preferred pathways for subsurface gas migration may result
from previous underground construction (e.g., underground utility lines) that can
facilitate gas flow. Natural and engineered barriers are discussed in more detail
below.
11.3.3.1.1 Natural Barriers
Surface water, ground water, and saturated soils can slow down or control the
direction of subsurface gas migration. Gases encountering these barriers will follow
the pathway of least resistance, usually through unsaturated porous soil,
Geologic barriers can also impede or control the route of subsurface gas
migration. For example, soil type is an important factor in gas migration. Gravels
and sands allow gas to migrate readily, particularly sand/gravel lenses, while clayey
gravels and sandy and organic clays tend to impede gas movement. Underground
utility trenches, backfill with granular materials, filled-in mine shafts, and tunnels or
natural caverns can also serve to channel subsurface gas flow. Climatic conditions
such as precipitation or freezing can reduce the porosity of surface soils, thereby
impeding upward gas movement. Information regarding characterization of soils is
provided in Section 9 (Soils).
11.3.3.1.2 Engineered Barriers
Landfills and units closed as landfills may use caps and liners to prevent
moisture infiltration and leachate percolation to ground water. Caps can
contribute to horizontal gas movement when upward migration to the surface is
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restricted (as shown in Figure 11-1). Liners tend to impede lateral migration into
the surrounding unsaturated soils. The owner or operator should evaluate cap/liner
systems (type, age, location, etc.) to determine potential gas migration pathways.
Similar to liners, slurry walls used to border landfill units can retard lateral gas
movement. With respect to underground tanks, caps and liners are not typically
used. Tanks are often placed into soils with sand or gravel backfill during
installation, followed by paving on the surface. Thus, any escaping gases from a
leaking underground tank may migrate laterally along the path of least resistance
adjacent to the units. The owner or operator should evaluate tank construction,
and age, integrity, and location.
11.3.3.2 Climate and Meteorological Conditions
The climate of the site should be defined to provide background information
for assessing the potential for subsurface gas migration, identifying migration
pathways, and designing the subsurface gas migration monitoring system. Climatic
information, on an annual and monthly or seasonal basis, should be collected for
the following parameters:
Temperature means/extremes and frost season (which indicates the
potential for impeding the upward migration of the subsurface gas, thus
confining the gas within the ground);
Precipitation means and snowfall (which indicates the potential for
"trapping" as well as an indication of soil moisture conditions which
affect subsurface gas migration); and
Atmospheric pressure means (which indicates the potential for gaseous
releases to ambient air from a unit of concern).
The primary source of climate information for the Unites States is the National
Climatic Data Center (Asheville, NC). The National Climatic Data Center can provide
climate summaries for the National Weather Service station nearest to the site of
interest. Standard references for climatic information also include the following:
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Local Climatological Data - Annual Summaries with Comparative Data,
published annually by the National Climatic Data Center;
Climates of the States, National Climatic Data Center; and
Weather Atlas of the United States, National Climatic Data Center.
Meteorological data for the above parameters should also be obtained
concurrently with subsurface gas sampling activities. As previously discussed, these
meteorological conditions can influence subsurface gas migration rates, patterns
and concentration levels. Therefore, these data are necessary to properly interpret
subsurface gas sampling data. Concurrent meteorological data for the sampling
period can be obtained from the National Climatic Data Center for National
Weather Service stations representative of the site area. In some cases, onsite
meteorological data will also be available from an existing monitoring program or
associated with an RFI characterization of the air media (See Section 12).
11.3.3.3 Receptors
Receptor information needed to assess potential subsurface gas exposures
includes the identification and location of surrounding buildings and potential
sensitive receptors (e.g., residences, nursing homes, hospitals, schools, etc.). This
information should also be considered in developing the monitoring procedures.
Additional discussion of potential receptors is provided in Section 2.
11.4 Design of a Monitoring Program to Characterize Releases
Existing data should help to indicate which units have the potential to
generate methane or other gases or constituents of concern. Such information can
be found in construction or design documents, permit and inspection reports,
records of waste disposal, unit design and operation records, and documentation of
past releases.
Units of concern should be identified on the facility's topographic map. The
location and areal extent of these units can be determined from historical records,
aerial photographs, or field surveys. The depths and dimensions of underground
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structures, locations of surrounding buildings, and waste-related information
should be identified. Waste management records may provide information on
waste types, quantities managed, location of waste units, and dates of waste
disposal. Waste receipts, waste composition surveys, and records of waste types
(e.g., municipal refuse, bulk liquids, sludges, contaminated soils, industrial process
wastes or inert materials) should be reviewed. For underground tanks, liquid waste
compositions, quantities, and physical properties should be determined.
Review of unit design and operation records may provide background
information on units of concern. These records may include engineering design
plans, inspection records, operations logs, damage or nuisance litigation, and
routine monitoring data. Also, for landfills and units closed as landfills, data may
include the presence and thickness of a liner, ground-water elevations, waste
moisture contents, type and amount of daily cover, records of subsurface fires, and
in-place leachate and/or gas collection systems. Historical information on
underground tank integrity may be contained in construction and monitoring
records. Records of past releases may provide information on problems, corrective
measures, and controls initiated.
The owner or operator should review records of subsurface conditions to
determine potential migration pathways. Aerial photographs or field observations
should identify surface water locations. Infrared aerial photography or geological
surveys from the USGS can be used as preliminary aids to identify subsurface
geologic features and ground-water location. In addition to obtaining and
reviewing existing information, a field investigation may be necessary to confirm
the location of natural barriers. The local soil conservation service will often have
information describing soil characteristics (e. g., soil type, permeability, particle size)
or a site specific investigation may need to be conducted. (Soil information sources
are discussed in Section 9). Climatic summaries (e. g., temperature, rainfall,
snowfall) can be obtained from the National Climatic Data Center for the National
Weather Service station nearest to the site of interest (Specific climatic data
references are cited in Section 12). Historical records of the site (prior use,
construction, etc.) should also be reviewed to identify any factors affecting gas
migration routes. The monitoring program should also address any engineered
structures affecting the migration pathway.
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In addition to the above, the owner or operator should examine the units and
surrounding area for signs of settlement, erosion, cracking of covers, stressed or
dead vegetation, dead rodents, snakes and other burrowing animals,
contamination of surface waters, odors, elevated temperatures in any existing
monitoring wells, and for venting of smoke or gases. The condition of any existing
gas monitoring systems and containment or collection systems should also be
examined, as well as any structural defects in tanks or liners. Any overflow/alarm
shut off systems, subsurface leak detection systems, secondary containment
structures (e.g., concrete pads, dikes or curbs) or other safety systems for early
detection of potential gas releases should be checked.
By reviewing all existing information, the owner or operator should be able to
develop a conceptual model of the release and design a monitoring program to
characterize the release.
11.4.1 Objectives of the Monitoring Program
Characterization of subsurface gas releases can be accomplished through a
phased monitoring approach. The objective of initial monitoring should be to verify
suspected releases, if necessary, or to begin characterizing known releases.
Monitoring should include methane and other indicators such as carbon dioxide, as
well as individual constituents if appropriate. If initial monitoring verifies a
suspected release, the owner or operator should expand the monitoring program to
determine the vertical and horizontal extent of the release, as well as the
concentrations of all constituents of concern in the release.
The full extent of the release can be determined through additional shallow
borehole and gas monitoring well locations. The goal of this further
characterization will be to identify the boundary of gas migration, including the
leading edge of the migration.
A great deal of the effort conducted during any subsequent phase may involve
investigating anomalous areas where subsurface conditions are non-uniform. In
these situations, the gas migration characteristics may differ from surrounding
areas. Consequently, non-random sampling techniques are generally most
appropriate to monitor these areas. The location of additional gas wells and
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shallow boreholes at the sites of subsurface anomalies will provide information
regarding the migration pattern around these anomalous areas. Also, because gas
well installation may be conducted only to a limited extent under the initial
monitoring phase, additional wells may need to be installed.
The monitoring program should also address the selection of constituents of
concern, sampling frequency and duration, and the monitoring system design.
11.4.2 Monitoring Constituents and Indicator Parameters
As discussed above, the number and identity of potential subsurface gas
constituents will vary on a site-specific basis. Constituents to be included for
monitoring depends primarily on the type of wastes received. For example, if an
underground storage tank contains specific constituents, they should be considered
during subsurface gas monitoring activities. The guidance provided in Section 3 and
the lists provided in Appendix B should be used to determine a select set of
constituents and indicator parameters for subsurface gas monitoring.
Methane should be used as the primary indicator of subsurface gas migration
during the initial and any subsequent monitoring phases. Supplemental indicators
(e.g., carbon dioxide and sulfur dioxide) may also be used as appropriate. Field
screening equipment should be used to detect the presence of methane in terms of
the lower explosive limit (LEL). The LEL for methane is 5 percent by volume, which is
equivalent to 50,000 ppm. Individual constituents should also be monitored. In
addition, oxygen detectors and nitrogen analyses can be used to confirm the
representativeness of all subsurface gas well samples obtained. (The presence of
oxygen and nitrogen in well samples indicates the intrusion of ambient air into the
well during monitoring. Samples containing ambient air would result in an
underestimate of methane and other indicators as well as specific monitoring
constituents.)
Methane concentrations observed during the initial monitoring phase which
exceed the LEL at the property boundary or 0.25 the LEL within surrounding
structures, would warrant initiation of subsequent monitoring phases and, possibly,
consideration of interim corrective measures. Similarly, the presence of individual
constituents would also trigger the need for subsequent monitoring phases.
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Regardless of the degree to which monitoring constituents can be limited by
site-specific data, analyses for all constituents identified as applicable in Appendix B
(List 2) will generally be necessary for the subsurface gas medium at selected
monitoring locations.
11.4.3 Monitoring Schedule
A monitoring schedule should be established and described in the RFI Work
Plan. This schedule should describe the sampling frequency, the duration of the
sampling effort, and the conditions under which sampling should occur.
During initial monitoring, bar punch probe (See Section 11-6) monitoring for
methane and appropriate constituents should be conducted at least twice over the
course of one week. Monitoring the wells for methane and constituents should be
conducted at least once a week for one month. (Subsurface gas wells should not be
monitored for at least 24 hours after installation to allow time for equilibration.)
Surrounding buildings should be monitored at least once a week for one month.
During any subsequent monitoring phases, more extensive sampling may be
needed to adequately characterize the nature and extent of the release. Monitoring
of wells and buildings for methane and constituents should be conducted every
other day for a two week period to account for daily fluctuations in gas
concentrations.
Conditions for sampling should also be defined. Sampling should generally
not be performed if conditions conducive to decreasing gas concentrations are
present (e.g., subsurface gas pressure at less than atmospheric pressure). In these
cases, sampling should be delayed until such conditions pass. Subsurface gas
pressures have a diurnal cycle and are generally at a maximum during the
afternoon.
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11.4.4 Monitoring Locations
11.4.4.1 Shallow Borehole Monitoring
Areas identified for subsurface gas monitoring as a result of characterization
of the contaminant source and the environmental setting should be investigated for
concentrations of methane and constituents during the initial monitoring phase.
Shallow borehole monitoring using a bar punch probe method or equivalent (See
Section 11.6) is recommended. The bar punch is simply a steel or metal bar which is
hand-driven or hammered to depths of 6 feet. Once this hole is made it is covered
with a stopper or seal to confine the headspace in the hole. The hole should be
allowed to equilibrate for up to an hour prior to sampling to provide sufficient time
for subsurface gas to replace the air in the hole. The ease of installation of bar
punch holes and the ability to obtain real-time direct measurements from field
survey instruments combine to make this task a relatively simple operation. It
should be recognized, however, that shallow borehole monitoring is a rapid
screening method and therefore has its limitations. Two major limitations are that
negative findings cannot assure the absence of a release at a greater depth and that
air intrusions can dilute the sampling readings. See also Sections 9 (Soil) and 10
(Ground Water) for additional information.
The number of locations to monitor will vary from site to site. However, due
to the ease of this operation, it is recommended that many locations be surveyed
during the initial monitoring phase. Selection of locations along the perimeter of
the unit of concern and at intervals of approximately 100 feet is an adequate initial
approach. Individual site conditions and anomalies should be considered to
determine whether the number of sampling locations should be increased or
decreased. A large site with homogeneous subsurface conditions could require
fewer sampling locations by increasing the distance between sampling points. A
site with many subsurface anomalies, such as engineered barriers or varying soil
strata, would require a greater number of sampling locations. In general, sampling
locations should be established where conditions are conducive to gas migration,
such as in sands, gravels and porous soils, and near engineered conduits (e. g.,
underground utility lines). The appropriate precautions should be taken when
sampling near engineered conduits so as not to damage such property and to assure
the safety of the investigative team and others.
11-25
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The distance from the unit at which to sample can best be determined through
consideration of site-specific characteristics (e.g., soil conditions), and can be aided
by the use of the gas concentration contour map generated by the predictive model
described in Appendix D. The shallow borehole survey should be fairly extensive,
ranging from sampling locations very near the unit to locations at the property
boundary and beyond.
11.4.4.2 Gas Monitoring Wells
Gas monitoring wells (See Section 11.6) should be installed to obtain data on
subsurface gas concentrations at depths greater than the depth accessible with a
bar punch probe. Wells should be installed to a depth equal to that of the unit.
Multiple probe depths may be installed at a single location as illustrated in Figure
11-3. Where buried material is fairly shallow (e.g., <10-feet), single depth gas
monitoring probes may be sufficient. When buried material exceeds this depth
below ground, multiple depth probes should be installed.
The location and depth of gas monitoring wells should be based on the
presence of highly permeable zones (e.g., dry sand or gravel), alignment with offsite
structures, proximity of the waste deposit, areas where there is dead or unhealthy
vegetation (that may be due to gas migration), and any engineered channels which
would promote the migration of a subsurface gas release (e. g., utility lines). This
information should be gathered during a review of subsurface conditions, as
discussed previously. At a minimum, a monitoring well should be installed at the
location(s) of expected maximum concentration(s), as determined or estimated
during the initial monitoring phase.
Gas monitoring well installation usually requires the use of a drilling rig or
power auger. Once a borehole has been drilled to the desired depth, the gas
monitoring probes can be installed as illustrated in Figure 11-3. Additional
information concerning the installation of subsurface gas monitoring wells is
provided in Section 10 (Ground Water) and in Guidance Manual for the
Classification of Solid Waste Disposal Facilities NTIS PB81 -218505 (U.S. EPA, 1981).
11-26
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1/2 DIA, SCH. 40
PVC PIPE
1/8" DIA.
PERFORATIONS
FIBERGLASS
SCREENING TO
BE WRAPPED
AROUND a TAPED
TO TUBE.
MONITORING PROSE DETAIL
MONITORNG
PROBE
1/2" DIA. SCH. 40
PVC PIPE
SOIL BACKFILL
'-2' 3ENTONITE
PLUG
BACKFILL
PEA GRAVEL
SOIL BACKFILL
'-2' BENIONITE
PLUG
SOIL BACKFILL
2' PEA GRAVEL
SOIL BACKFILL
'-2' BEiNTONITE
PLUG
SOIL BACKFILL
21 PEA GRAVEL
Figure 11-3. Schematic of a Deep Subsurface Gas Monitoring Well
11-27
-------
Equilibration times of at least 24 hours should be allowed prior to collection of
subsurface gas samples for analysis after well installation and between subsequent
collection periods, individual site characteristics or anomalies which can create
significantly different subsurface conditions will require an increased number of
wells to sufficiently determine the presence of gas migration. For example, if the
predominant soil strata along one side of a unit changes from sandy clay to gravel, a
well should be installed in both of these areas. Also, if the amount of gas producing
waste buried at the site varies greatly from one area to another, gas monitoring
wells should be installed near each area of concern.
Subsurface gas monitoring may be done concurrently with ground-water
investigations (Section 10), because results of subsurface gas monitoring may
provide useful information for identifying the overall extent of any ground-water
contamination.
11.4.4.3 Monitoring in Buildings
Monitoring should also be conducted in surrounding structures near the areas
of concern, since methane and other subsurface gas constituents migrating through
the soil can accumulate in confined areas. Use of an explosimeter for methane is
the recommended monitoring technique (See Section 11.6).
Sampling should be conducted at times when the dilution of the indoor air is
minimized and the concentration of soil gas is expected to be at its highest
concentration. Optimal sampling conditions would be after the building has been
closed for the weekend or overnight and when the soil surface outside the building
and over the unit of concern has been wet or frozen for several days. These
conditions will maximize the potential for lateral migration of gas into buildings
rather than vertically into the ambient air. Recommended sampling locations
within the building include basements, crawl spaces, and around subsurface utility
lines such as sewer or electrical connections. Access conduits such as manholes or
meter boxes are good sampling locations for water, sewer, or gas main connections.
Methane and, if appropriate, individual constituents should be monitored for.
The threat of explosion from accumulation of methane within a building
makes this monitoring activity important as well as dangerous. The monitoring of
11-28
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gas concentrations within buildings is a simple process involving a walk through
inspection of areas with portable field instruments (e.g., explosimiter). Such
measurements should begin during the initial monitoring phase. The importance of
identifying potential releases to buildings warrants a complete inspection of all
suspect areas. The inherent danger during these investigatons warrants adequate
health and safety procedures (See Section 6).
If significant concentrations of methane or constituents are measured in
surrounding structures during initial monitoring, subsequent monitoring may need
to be expanded to include buildings at greater distances from the unit(s) of concern
and to include additional constituents of concern. In addition, interim corrective
measures should be considered.
Background indoor air quality levels may be accounted for during the
collection and evaluation of the in-building sampling data. Background levels can
be accounted for by identifying potential indoor air emission sources (e.g., use of
natural gas as a fuel or wood paneling which has the potential for formaldehyde
emissions). Further guidance on this subject is presented in the following reference:
U.S. EPA. 1983. Guidelines for Monitoring Indoor Air Quality. EPA- 600/1-4
83-046. NTIS PB83-264465. Office of Research and Development.
Washington, D.C. 20460.
11.4.4.4 Use of Predictive Models
In addition to monitoring potential gas releases using portable survey
instruments, the owner or operator should consider the use of predictive models to
estimate the configuration and concentration of gas releases. A subsurface gas
predictive model has been developed by EPA to estimate methane gas migration
from sanitary landfills. This model is based on site soil conditions, waste-related
data, and other environmental factors.
As part of the initial monitoring phase, the model provided in Appendix D (or
another appropriate predictive model after consultation with the regulatory
agency), should be used to estimate the extent of subsurface gas migration. Results
from this model can be used in determining appropriate monitoring locations. The
11-29
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methane gas migration model presented in Appendix D yields a methane
concentration isopleth map of a release. The LEL and 0.25 LEL isopleths for methane
should be mapped for the RFI when appropriate. Because predictive models may
not be sensitive to relevant site conditions, however, model results should be used
cautiously for the monitoring program design and to supplement actual field data.
11.5 Data Presentation
Subsurface gas data collected during the RFI should be presented in formats
that clearly define the composition and extent of the release. The use of tables and
graphs is highly recommended. Section 5.2 provides a detailed discussion of data
presentation methods.
11.5.1 Waste and Unit Characterization
Waste and unit characteristics should be presented as:
Tables of waste constituents and concentrations;
Tables of relevant physical and chemical properties of waste and
potential contaminants;
Narrative description of unit dimensions, operations, etc.; and
Topographical map and plan drawings of facility and surrounding areas.
11.5.2 Environmental Setting Characterization
Environmental characteristics should be presented as follows:
Tabular summaries of annual and monthly or seasonal relevant climatic
information (e.g., temperature, precipitation);
Narratives and maps of soil and relevant hydrogeological characteristics
such as porosity, organic matter content, and depth to ground water;
11-30
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Maps showing location of natural or man-made engineering barriers and
likely migration routes; and
Maps of geologic material at the site identifying the thickness, depth,
and textures of soils, and the presence of saturated regions and other
hydrogeological features.
11.5.3 Characterization of the Release
In general, release data should be initially presented in tabular form. To
facilitate interpretation, graphs of concentrations of individual constituents plotted
against distance from the unit should be used to identify migration pathways and
areas of elevated concentrations. Concentration isopleth maps can also be drawn to
identify the direction, depths, and distances of gas migration, and concentrations of
constituents of concern. Specific examples of these and other data presentation
methods are provided in Section 5. Methane concentrations should be presented in
terms of the LEL and 0.25 LEL isopleths. Specific monitoring constituent
concentrations should also be presented.
11.6 Field Methods
Field methods for subsurface gas investigations involve sample collection and
analysis. Sample collection methods are discussed to summarize the monitoring
techniques described above. Because subsurface gas monitoring is similar to air
monitoring, the available methods for the collection and analysis of subsurface gas
samples are presented here only in tabular format with further discussion in the air
section of this document (Section 12). Tables 11-3 through 11-5 summarize various
methodologies available to collect and analyze air samples. These methodologies
range from real-time analyzers (e.g., methane explosimeters) to the collection of
organic vapors on sorbents or whole air samples with subsequent laboratory
analysis.
A portable gas chromatography with a flame ionization detector (calibrated
with reference to methane) can be used to measure methane concentrations in the
field. Methane explosimeters (based on the principle of thermal conductivity) are
11-31
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TABLE 11-4
SUMMARY OF CANDIDATE METHODOLOGIES FOR QUANTIFICATION OF VAPOR PHASE ORGANICS
Collection Techniques
Analytical Technique
Applicability
Positive Aspects
Negative Aspects
. Sorption onto Tenax-GC
or carbon molecular
sieve packed cartridges
using low-volume pump
Thermal Resorption into GC
or GC/MS
adequate QA/QC data
base
widely used on
investigations around
uncontroNteoti waste sites
wide range of
applicability
|jg/m3 detection limits
practicality for field use
possibility of
contamination
artifact formation
problems
t rigorous cleanup needed
no possibility of multiple
analysis
t low breakthrough
volumes for some
compounds
u>
U)
2. Sorption onto charcoal
packed cartridges using
low-volume pump
Resorption with solvent-
analysis by GC or GC/MS
large data base for
various compounds
wide use in industrial
applications
practical for field use
t problems with
irreversible adsorption of
some compounds
high (mg/m3) detection
limits
artifact formation
problems
t high humidity reduces
retention
J. Sorption onto
polyurethane foam (PUF)
using low-volume or
high-volume pump
Solvent extraction of PUF;
analysis by GC/MS
wide range of
applicability
easy to preclean and
extract
very low blanks
excellent collection and
retention efficiencies
reusable up to 10 times
possibility of
contamination
t losses of more volatile
compounds may occur
during storage
-------
TABLE 11-4 (continued)
SUMMARY OF CANDIDATE METHODOLOGIES FOR QUANTIFICATION OF VAPOR PHASE ORGANICS
Collection Techniques
Analytical Technique
Applicability
Positive Aspects
Negative Aspects
1. Sorption on passive
dosimeters using Tenax
or charcoal as adsorbing
medium
Analysis by chemical or
thermal resorption followed
by GC or GUMS
I or
Samplers are small,
portable, require no
pumps
makes use of analytical
procedures of known
precision and accuracy
for a broad range of
compounds
ug/m3detection limits
problems associated with
sampling using sorbents
(see #l and II) are present
uncertainty in volume of
air sampled makes
concentration
calculations difficult
requires minimum
external air flow rate
Ul
5. Cryogenic trapping of
analytes in the field
Resorption into GC
applicable to a wide
range of compounds
artifact formation
minimized
low blanks
requires field use of
liquid nitrogen or
oxygen
sample is totally used in
one analysis-no
reanalysis possible
samplers easily clogged
with water vapor
no large data base on
precision or recoveries
6. Whole air sample taken
in glass or stainless steel
bottles
Cryogenic trapping or direct
injection into GC or GC/MS
(onsite or laboratory)
useful for grab sampling
large data base
excellent long-term
storage
wide applicability
allows multiple analyses
difficult to obtain
integrated samples
low sensitivity if
preconcentration is not
used
7. Whole air sample taken
in TedlareBag
Cryogenic trapping or direct
injection into GC or GC/MS
(onsite or laboratory)
grab or integrated
sampling
wide applicability
allows multiple analyses
long-term stability
uncertain
low sensitivity if
preconcentration is not
used
adequate cleaning of
containers between
samples may be difficult
-------
TABLE 11-4 (continued)
SUMMARY OF CANDIDATE METHODOLOGIES FOR QUANTIFICATION OF VAPOR PHASE ORGANICS
Collection Techniques
Analytical Technique
Applicability
Positive Aspects
Negative Aspects
8. Dinitropheynlhydrazine
liquid Impinger sampling
using low-volume pump
HPLC/UV analysis
IV
specific to aldehydes and
ketones
good stability for
derivatized compounds
low detection limits
fragile equipment
sensitivity limited by
reagent impurities
problems with solvent
evaporation when long-
term sampling is
performed
9. Direct introduction by
probe
Mobile MS/MS
I,II, III, IV
immediate results
field identification of air
contaminants
allows "real-time"
monitoring
widest applicability of
any analytical method
high instrument cost
requires highly trained
operators
grab samples only
no large data base on
precision or accuracy
a Applicability Code
I Volatile, nonpolar organics (e.g., aromatic hydrocarbons, chlorinated hydrocarbons) having boiling points in the
range of 80 to 200° C.
II Highly volatile, nonpolar organics (e.g., vinyl chloride, vinylidene chloride, benzene, toluene) having boiling points
in the range of -15 to + 120° C.
Ill Semivolatile organic chemicals (e.g., organochlorine pesticides and PCBs).
IV Aldehydes and ketones.
-------
TABLE 11-5
TYPICAL COMMERCIALLY AVAILABLE SCREENING TECHNIQUES FOR ORGANICS IN AIR
Techniques
Gas Detection Tubes
Continuous Flow Calorimeter
Calorimetric Tape Monitor
Infrared Analysis
FID (Total Hydrocarbon
Analyzer)
GC/FID (portable)
PID and GC/PID (portable)
GC/ECD (portable)
GC/FPD (portable)
Chemiluminescent
Nitrogen Detector
Manufacturer
Draeger Matheson (Kitagawa)
CEA Instruments, Inc.
MDA Scientific
Foxboro/Wilkes
Beckman
MSA, Inc.
AID, Inc.
Foxboro/Century
AID, Inc.
HNU, Inc.
AID, Inc.
Photovac, Inc.
AID, Inc.
AID, Inc.
Antek, Inc.
Compounds Detected
Various organics and inorganic
Acrylonitrile, formaldehyde,
phosgene
Toluene, diisocyanate, dinitro-
toluene, phosgene, and various
inorganic
Most organics
Most organics
Same as above except that polar
compounds may not elute from
the column
Most organic compounds can
be detected with the exception
of methane
Halogenated and nitro
substituted compounds
Sulfur or phosphorus-
containing compounds
Nitrogen-containing
compounds
Approximate
Detection
Limit
0.1 to 1 ppmv
0.005 to 0.5
ppmv
0.05-0.5
ppmv
1-10ppmv
0.5 ppmv
0.5 ppmv
0.1 to loo
ppbv
0.1 to loo
ppbv
10-100 ppbv
0.1 ppmv (as
N)
Comment
Sensitivity and selectivity highly
dependent on components of
interest.
Sensitivity and selectivity similar
to detector tubes.
Sensitivity and selectivity similar
to detector tubes.
Some inorganic gases (H2, CO)
will be detected and therefore
are potential interferences.
Responds uniformly to most
organic compounds on a carbon
basis.
Qualitative as well as
quantitative information
obtained.
Selectivity can be adjusted by
selections of lamp energy.
Aromatics most readily
detected.
Response varies widely from
compound to compound.
Both inorganic and organic
sulfur or phosphorus
compounds will be detected.
Inorganic nitrogen compounds
will interfere.
-------
also available and provide direct readings of LEL levels and/or percent methane
present by volume.
Table 11-3 provides a list of organic screening methodologies suited for
detection of methane. Commercial monitoring equipment (direct reading) suitable
for screening application are also available specifically for carbon dioxide, and
sulfur dioxide. Similar field screening equipment are available for oxygen in order
to check for the potential for intrusion of ambient air into the subsurface gas
monitoring well. These screening monitors are available from most major industrial
hygiene equipment vendors. Direct reading gas detection (e.g., draeger) tubes are
also available for methane and other subsurface gas indicators for screening
applications.
It is important that all monitoring procedures be fully documented and
supported with adequate QA/QC procedures. Information should include:
locations and depths of sampling points, methods used (including sketches and
photographs), survey instruments used, date and time, atmospheric/soil
temperature, analytical methods, and laboratory used, if any. Also see Section 4
(Quality Assurance and Quality Control).
The three basic monitoring techniques available for sampling subsurface gas;
above ground air monitoring, shallow borehole monitoring, and gas well
monitoring are summarized below.
11.6.1 Above Ground Monitoring
This technique consists of the collection of samples of the subsurface gas after
it has migrated out of the soil or into engineered structures (e.g., within buildings
or along under-ground utility lines.). Basically, there is no difference in the
apparatus from that described for ambient air monitoring (Section 12). The
locations at which sampling is conducted, however, are selected to focus on areas
where gases might accumulate. Sampling methods can utilize various types and
brands of portable direct-reading survey instruments (see Table 11-5). However,
because methane gas is frequently the major component of the soil gas, those
which are most sensitive to methane, such as explosimeters and FID organic vapor
11-37
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analyzers, are the preferred instruments. More selective air sampling methods are
used, however, for constituent analyses (see Section 12- Air Methods).
11.6.2 Shallow Borehole Monitoring
Shallow borehole monitoring involves subsurface gas monitoring to depths of
up to 6 feet below the ground surface. Bar punches or metal rods which can be
hand-driven or hammered into the ground are used to make boreholes from which
gas samples are removed. Table 11-6 provides the basic procedure for shallow and
deep subsurface monitoring techniques. Sample collection should follow the same
methods employed during above ground monitoring.
Shallow borehole monitoring, as previously discussed, is a rapid screening
method and, therefore, has its limitations. Two major limitations are that negative
findings cannot assure the absence of a release at a greater depth and that air
intrusion can dilute the measured concentration levels of the sample. Misleading
results can also be obtained if the surface soil layer is contaminated (e.g., due to a
spill).
11.6.3 Gas Well Monitoring
Monitoring gas within wells will involve either the lowering of a sampling
probe (made of a nonsparking material) through a sealed capon the top of the well
to designated depths, or the use of fixed-depth monitoring probes (see Figure 11-3
and Table 11-6). The probe outlet is usually connected to the desired gas
monitoring instrument. More information on gas well monitoring is provided in
Sections 9 (Soil) and 10 (Ground Water).
11.7 Site Remediation
Although the RFI Guidance is not intended to provide detailed guidance on
sites remediation, it should be recognized that certain data collection activities that
may be necessary for a Corrective Measures Study may be collected during the RFI.
EPA has developed a practical guide for assessing and remediating contaminated
site that directs users toward technical support, potential data requirements and
11-38
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TABLE 11-6
SUBSURFACE SAMPLING TECHNIQUES
SHALLOW (Up to 6 ft deep)
t Select sampling locations based on soil data and existing monitoring
data.
Penetrate soil to desired depth. A steel rod 1/2 to 3/4 inch diameter and a
heavy hammer are sufficient. A bar punch equipped with insulated
handles is better for numerous holes. It is a small, hand operated pile
driver with a sliding weight on the top. Hand augers may also be used.
Insert inert (e.g., Teflon) tubing to bottom of hole. Tubing may be
weighted or attached to a small diameter stick to assure that it gets to
the bottom of the hole. Tubing should be perforated along bottom few
inches to assure gas flow.
Close top of hole around tubing using a gas impervious seal.
t Before sampling record well head pressure.
Readings may be taken immediately after making the barhole.
t Attach meter or sampling pump and evacuate hole of air-diluted gases
before recording gas concentrations or taking samples.
When using a portable meter, begin with the most sensitive range (0-100
percent by volume of the lower explosive limit (LEL) for methane). If
meter is pegged, change to the next least sensitive range to determine
actual gas concentration.
t Tubing shall be marked, sealed, and protected if sampling will be done
later.
11-39
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TABLE 11-6 (Continued)
SUBSURFACE SAMPLING TECHNIQUES
If results are erratic the hole should be plugged and further reading
taken a few minutes later.
Monitoring should be repeated a day or two after probe installation t
verify readings.
DEEP (More Than 6 ft deep)
Same general procedures as above.
Use portable power augers or truck-mounted augers.
For permanent monitoring points, use rigid tubing (e.g., Teflon) and the
general construction techniques shown in Figure 11-4.
CAUTION
When using hand powered equipment, stop if any unusually high
resistance is met. This resistance could be from a gas pipe or an electrica
cable.
Before using powered equipment, confirm that there are no
underground utilities in the location(s) selected (see Appendix C -
Geophysical Techniques).
Use non-sparking equipment and procedures and monitor for methane
explosive limits.
11-40
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technologies that may be applicable to EPA programs such as RCRA and CERCLA.
The reference for this guide is provided below.
U.S. EPA. 1988. Practical Guide for AssessinG and Remediating Contaminated
Sites. Office of Solid Waste and Emergency Response. Washington, D.C.
20460.
The guide is designed to address releases to ground water as well as soil,
surface water and air. A short description of the guide is provided in Section 1.2
(Overall RCRA Corrective Action Process), under the discussion of Corrective
Measures Study.
11-41
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11.8 Checklist
RFI CHECKLIST- SUBSURFACE GAS
Site Name/Location
Type of Unit
1. Does waste characterization include the following information? (Y/N)
Physical form of waste
Chemical composition and concentrations
Presence of biodegradable waste components
Quantities managed and dates of receipt
Location of wastes in unit
Waste material moisture content and temperature
Chemical and physical properties of constituents
of concern
2. Does unit characterization include the following information? (Y/N)
Age of unit
Construction integrity
presence of liner (natural or synthetic)
Location relative to ground-water table or bedrock or
other confining barriers
Unit operation data
Presence of cover or other surface covering to impede
vertical gas migration
Presence of gas collection system
presence of surrounding structures such as buildings
and utility conduits
Depth and dimensions of unit
Inspection records
Operation logs
Past fire, explosion, odor complaint reports
11-42
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RFI Checklist - SUBSURFACE GAS (Continued)
Existing gas/ground-water monitoring data
Presence of natural or engineered barriers near unit
Evidence of vegetative stress
3. Does environmental setting information include the following
information? (Y/N)
Definition of regional climate
Definition of site-specific meteorological conditions
Definition of soil conditions
Definition of site specific terrain
Identification of subsurface gas migration routes
Identification and location of engineered conduits
Identification of surrounding structures
4. Have the following data on the initial phase of the release
characterization been collected? (Y/N)
Extent and configuration of gas plume
Measured methane and gaseous constituent
concentration levels in subsurface soil and
surrounding structures
Sampling locations and schedule
5. Have the following data on the subsequent phase(s) of the release (Y/N)
characterization been collected?
Extent and configuration of gas plume
Measured methane and gaseous constituent
concentration levels in subsurface soil and surrounding
structures
Sampling locations and schedule
11-43
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11.9 References
National Climatic Data Center. Local Climatoloqical Data - Annual Summaries
with Comparative Data. National Oceanic and Atmospheric Administration.
published annually. Asheville, N.C.
National Climatic Data Center. Climates of the States. National Oceanic
and Atmospheric Administration. Asheville, N.C.
National Climatic Data Center. Weather Atlas of the United States,
National Oceanic and Atmospheric Administration. Asheville, N.C.
South Coast Air Quality Management District. 1986. Hazardous Pollutants in
Class II Landfills. El Monte, California.
U.S. EPA. October 1986. RCRA Facility Assessment Guidance. NTIS PB87-107769.
Office of Solid Waste. Washington, D.C. 20460.
U.S. EPA. 1983. Guidelines for Monitoring Indoor Air Quality. EPA-600 14-83-046.
NITS PB83-264465. Office of Research and Development. Washington, D.C.
20460.
U.S. EPA. January 1981. Guidance Manual for the Classification of Solid Waste
Disposal Facilities. NTIS PB81-218505. Office of Solid Waste. Washington, D.C.
20460.
U.S. EPA. 1985. Technical Guidance for Corrective Measures - Subsurface Gas.
Office of Solid Waste. Washington, D.C. 20460.
11-44
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APPENDIX C
GEOPHYSICAL TECHNIQUES
C-1
-------
APPENDIX C
GEOPHYSICAL TECHNIQUES
The methods presented in this Appendix have been drawn primarily from two
sources. The first, Geophysical Techniques for Sensing Buried Wastes and Waste
Migration (Technos, Inc., 1982) was written specifically for application at hazardous
waste sites, and for an audience with limited technical background. All of the
surface geophysical methods discussed below can be found in this document. The
second, Geophysical Explorations (U.S. Army Corps of Engineers, Engineering
Manual 1110-1-1802, 1979) is a more generic application-oriented manual which
contains the borehole methods described in this section.
Caution should be exercised in the use of geophysical methods involving the
introduction or generation of an electrical current, particularly when contaminants
are known or suspected to be present which have ignitable or explosive properties.
The borehole methods are of particular concern due to the possible build up of
large amounts of explosive or ignitable gases (e.g., methane).
ELECTROMAGNETIC SURVEYS
The electromagnetic (EM)* method provides a means of measuring the
electrical conductivity of subsurface soil, rock, and ground water. Electrical
conductivity is a function of the type of soil and rock, its porosity, permeability, and
the fluids which fill the pore space. In most cases the conductivity (specific
conductance) of the pore fluids will dominate the measurement. Accordingly, the
EM method is applicable both to assessment of natural geohydrologic conditions
and to mapping of many types of contaminant plumes. Additionally, trench
*The term "electromagnetic" has been used in contemporary literature as a
descriptive term for other geophysical methods, including ground penetrating
radar and metal detectors which are based on electromagnetic principles.
However, this document will use electromagnetic (EM) to specifically imply the
measurement of subsurface conductivities by low frequency electromagnetic
induction. This is in keeping with the traditional use of the term in the geophysical
industry from which the EM methods originated.
C-2
-------
boundaries, buried wastes and drums, as well as metallic utility lines can be located
with EM techniques.
Natural variations in subsurface conductivity may be caused by changes in soil
moisutre content, ground-water specific conductance, depth of soil cover over rock,
and thickness of soil and rock layers. Changes in basic soil or rock types, and
structural features such as fractures or voids may also produce changes in
conductivity. Localized deposits of natural organics, clay, sand, gravel, or salt- rich
zones will also affect subsurface conductivity.
Many contaminants will produce an increase in free ion concentration when
introduced into the soil or ground water systems. This increase over background
conductivity enables detection and mapping of contaminated soil and ground
water at hazardous waste sites. Large amounts of organic fluids such as diesel fuel
can displace the normal soil moisture, causing a decrease in conductivity which may
also be mapped, although this is not commonly done. The mapping of a plume will
usually define the local flow direction of contaminants. Contaminant migration
rates can be estimated by comparing measurements taken at different times.
The absolute values of conductivity for geologic materials (and contaminants)
are not necessarily diagnostic in themselves, but the variations in conductivity,
laterally and with depth, are significant. It is these variations which enable the
investigator to rapidly find anomalous conditions (See Figure C-1).
At hazardous waste sites, applications of EM can provide:
Assessment of natural geohydrologic conditions;
Locating and mapping of burial trenches and pits containing drums
and/or bulk wastes;
Locating and mapping of plume boundaries;
Determination of flow direction in both unsaturated and saturated
zones;
C-3
-------
Phos«
Sensing
Circuits
Chort
Mog Tap*
Recorders
RANSMITTER
GROUND SURFACE
INDUCED
CURRENT
LOOPS
SECONDARY FIELDS
PROM CURRENT LOOPS
SENSED BY
RECEIVER COIL
Figure C-l. Block diagram showing EM principle of operations.
C-4
-------
Rate of plume movement by comparing measurements taken at dif-
ferent times; and
Locating and mapping of utility pipes and cables which may affect other
geophysical measurements, or whose trench may provide a pathway for
contaminant flow.
Chapter V of Geophysical Techniques for Sensing Buried Wastes and Waste
Migration (Technos, Inc., 1982) should be consulted for further detail regarding use,
capabilities, and limitations of electromagnetic surveys.
SEISMIC REFRACTION SURVEYS
Seismic refraction techniques are used to determine the thickness and depth
of geologic layers and the travel time or velocity of seismic waves within the layers.
Seismic refraction methods are often used to map depths to specific horizons such
as bedrock, clay layers, and the water table. In addition to mapping natural
features, other secondary applications of the seismic method include the locations
and definition of burial pits and trenches.
Seismic waves transmitted into the subsurface travel at different velocities in
various types of soil and rock, and are refracted (or bent) at the interfaces between
layers. This refraction affects their path of travel. An array of geophones
(transducers that respond to the motion of the ground) on the surface measures the
travel time of the seismic waves from the source to the geophones at a number of
spacings. The time required for the wave to complete this path is measured,
permitting a determination to be made of the number of layers, the thicknesses of
the layers and their depths, as well as the seismic velocity of each layer. The wave
velocity in each layer is directly related to its material properties such as density and
hardness. Figure C-2 depicts the seismic refraction technique.
Seismic refraction can be used to define natural geohydrologic conditions,
including thickness and depth of soil and rock layers, their composition and physical
properties, and depth to bedrock or the water table. It can also be used for the
detection and location of anomalous features, such as pits and trenches and for
evaluation of the depth of burial sites or landfills.
C-5
-------
n
Hammer
Source
-i-' i -,-T.in: Bedrock
Figure C-2. Filed layout of a 12-channel seismograph showing the path of direct
and refracted seismic waves in a two-layer soil/rock system.
-------
Specific details regarding the use of seismic refraction surveys, and the
capabilities and limitations of this method can be found in Chapter VII of
Geophysical Techniques for Sensing Buried Wastes and Waste Migration (Technos,
Inc., 1982).
RESISTIVITY SURVEYS
The resistivity method is used to measure the electrical resistivity of the
geohydrologic section which includes the soil, rock, and ground water. Accordingly,
the method may be used to assess lateral changes and vertical cross- sections of the
natural geohydrologic settings. In addition, it can be used to evaluate contaminant
plumes and locate buried wastes at hazardous waste sites. Figure C-3 is a graphical
representation of the concept of a resistivity survey.
Applications of the resistivity method at hazardous waste sites include:
Locating and mapping contaminant plumes;
Establishing direction and rate of flow of contaminant plumes;
Defining burial sites by:
- locating trenches,
- defining trench boundaries, and
- determining the depths of trenches; and
Defining natural geohydrologic conditions such as:
- depth to water table or to water-bearing horizons; and
- depth to bedrock, thickness of soil, etc.
Chapter VI of Geophysical Techniques for Sensing Buried Wastes and Waste
Migration (Technos. Inc., 1982), discusses methods, use, capabilities, and limitations
of the resistivity method.
C-7
-------
-Current Meter
Surface
Current Flow
Through Earth
Current
Voltage
Figure C-3. Diagram showing basic concept of resistivity measurement.
C-8
-------
GROUND PENETRATING RADAR SURVEYS
Ground penetrating radar (GPR)* uses high frequency radio waves to acquire
subsurface information. From a small antenna which is moved slowly across the
surface of the ground, energy is radiated downward into the subsurface, then
reflected back to the receiving antenna, where variations in the return signal are
continuously recorded. This produces a continuous cross-sectional "picture" or
profile of shallow subsurface conditions. These responses are caused by radar wave
reflections from interfaces of materials having different electrical properties. Such
reflections are often associated with natural geohydrologic conditions such as
bedding, cementation, moisture and clay content, voids, fractures, and intrusions,
as well as man-made objects. The radar method has been used at numerous sites to
evaluate natural soil and rock conditions, as well as to detect buried wastes. Figure
C-4 depicts the ground penetrating radar method.
Radar responds to changes in soil and rock conditions. An interface between
two soil or rock layers having sufficiently different electrical properties will show up
in the radar profile. Buried pipes and other discrete objects will also be detected.
Radar has effectively mapped soil layers, depth of bedrock, buried stream
channels, rock fractures, and cavities in natural settings. Radar applications include:
Evaluation of the natural soil and geologic conditions;
Location and delineation of buried waste materials, including both, bulk
and drummed wastes;
* GPR has been called by various names: ground piercing radar, ground probing
radar, and subsurface impulse radar. It is also known as an electromagnetic
method (which in fact it is); however, since there are many other methods which
are also electromagnetic, the term GPR has come into common use today, and is
used herein.
C-9
-------
GRAPHIC RECORDER
ANTENNA
CONTROLLER
TAPE RECORDER
00
Figure C-4. Block diagram of ground penetrating radar system. Radar waves are
relfected from soil/rock interface.
C-10
-------
Location and delineation of contaminant plume areas; and
Location and mapping of buried utilities (both metallic and nonmetallic).
in areas where sufficient ground penetration is achieved, the radar method
provides a powerful assessment tool. Of the geophysical methods discussed in this
document, radar offers the highest resolution. Ground penetrating radar methods
are further detailed in Chapter IV of Geophysical Techniques for Sensing Buried
Wastes and Waste Migration (Technos, Inc., 1982), as are this method's capabilities
and limitations.
MAGNETOMETER SURVEYS
Magnetic measurements are commonly used to map regional geologic
structure and to explore for minerals. They are also used to locate pipes and survey
stakes or to map archeological sites. In addition, they are commonly used to locate
buried drums and trenches.
A magnetometer measures the intensity of the earth's magnetic field. The
presence of ferrous metals creates variations in the local strength of that field,
permitting their detection. A magnetometer's response is proportional to the mass
of the ferrous target. Typically, a single drum can be detected at distances up to 6
meters, while massive piles of drums can be detected at distances up to 20 meters or
more. Figure C-5 shows the use of a magnetometer in detecting a buried drum.
Magnetometers may be used to:
Locate buried drums;
Define boundaries of trenches filled with ferrous containers;
Locate ferrous underground utilities, such as iron pipes or tanks, and the
permeable pathways often associated with them; and
C-ll
-------
Amplifiers
and
Counter
Circuits
Chart and
Mag Tap*
Rtcordart
Ground Surface
Figure C-5. Simplified block diagram of a magnetometer. A magnetometer
senses change in the earth's magnetic field due to buried iron drum.
C-12
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Aid in selecting drilling locations that are clear of buried drums,
underground utilities, and other obstructions.
The use, capabilities, and limitations of magnetometer surveys at hazardous
waste sites are provided in chapter IX of Geophysical Techniques for Sensing Buried
Wastes and Waste Migration (Technos, Inc., 1982).
BOREHOLE GEOPHYSICAL METHODS
There are several different types of borehole geophysical methods used in the
evaluation of subsurface lithology, stratigraphy, and structure. Much of the data
collected in boreholes is analyzed in conjunction with surface geophysical data to
develop a more detailed description of subsurface features. In this section, the
major and most applicable types of borehole geophysical methods are identified
and briefly discussed. They include:
I. Electrical Surveys
a. Spontaneous Potential
b. Resistivity
II. Nuclear Logging
a. Natural Gamma
b. Gamma Gamma
c. Neutron
III. Seismic Surveys
a. Up and Down Hole
b. Crosshole Tests
c. Vertical Seismic Profiling
IV. Sonic Borehole Surveys
a. Sonic Borehole Imagery
b. Sonic Velocity
v. Auxiliary Surveys
a. Temperature
b. Caliper
c. Fluid Resistivity
All of the borehole methods presented in this section are detailed in the Army
Corps of Engineers Geophysical Explorations Manual (Engineering Manual 1110-1-
1802, 1979), with the exception of vertical seismic profiling. This method is
relatively new and further information can be found in Batch and Lee, 1984.
C-13
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Electrical Surveys
The two types of electrical subsurface surveys of geotechnical interest, both of
which involve continuous logging with depth of the electrical characteristics of the
borehole walls, are the spontaneous potential log and the borehole resistivity log.
The spontaneous potential log (also known as self potential) is a record of the
variation with depth of naturally occurring electrical potentials (voltages) between
an electrode at the depth in a fluid filled borehole and another at the surface
The known origins for spontaneous potentials arise from the relative mobility
and concentrations of the different elemental ions dissolved in the borehole fluid
and the fluid in adjacent strata. The electrochemical activities of the minerals in the
strata also cause a component of the measured spontaneous potentials (Figure C-6).
The relative senses and magnitudes of the several causes from which spontaneous
potentials arise are affected by the nature of the borehole fluid, by the
mineralogical characteristics of all the strata the borehole penetrates, and by the
dissolved solid concentration in the ground water in all potential layers.
The second type of electric survey is the electrical resistivity log. The electrical
resistivity of strata is one of the basic parameters that correlates to lithology and
hydrology. Direct access to individual layers of the subsurface materials by means of
the borehole is the primary advantage of electrical resistivity logging over the more
indirect use of apparent electrical resistivity surveys from the surface.
Electrical current can be passed through in situ earth materials between two
electrodes. Electric fields created within the three dimensional earth medium are
related to the medium's structure and the nature of the aqueous fluid in the
medium. Figure C-7 demonstrates the conceptual field configuration for borehole
electrical resistivity survey.
C-14
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E, » BOREHOLE-FORMATION
FLUID POTENTIAL
E$ * SMALE-SAND.POTENTIAL.
E. * MUD FILTRATE-FORMATION
B FLUID POTENTIAL
im * BOREHOLE FLUID RESISTANCE
t,h s SHALE RESISTANCE
r,, « SAND RESISTANCE
rb » FILTRATE RESISTANCE
Figure C-6. Conceptual equivalent circuit for self-potential data (prepared by the
Waterways Experiment Station, U.S. Army Corps of Engineers,
Vicksburg, Mississippi).
C-15
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ANO
RECORDER
INSULATED
SHE A VC
> s'DHAWWQKKS
co r.S
W
i"
*esis-
TANCE
IS
ELECTROOC
L.INCX OF
ELCCTMtCAl.
CURRENT
Figure C-7. Single-point resistance log (preparedly the Waterways Experiment
Station, U.S. Army Corps of Engineers, Vicksburg, Mississippi).
C-16
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Resistivity logging is a valuable tool in correlating beds from borehole to
borehole. In addition, they can be used together with knowledge of ground water
and rock matrix resistivities (obtained from samples) to calculate porosities and/or
water saturations. Also, if porosity is known and a borehole temperature log is
available, contaminant concentrations can be inferred by electrical resistivity
variations.
Nuclear Logging
Nuclear borehole logging can be used quite effectively for borehole depths
ranging from 10 to more than 1,000 feet. At considerable depths, as for large
buried structures, nuclear logging is a very effective means of expanding a small
number of data points obtained from direct measurements on core samples to
continuous records of clay content, bulk density, water content, and/or porosity.
The logs are among the simplest to perform and interpret, but the calibrations
required for meaningful quantitative interpretations must be meticulously
complete in attention to detail and consideration of all factors affecting nuclear
radiation in earth materials. Under favorable conditions, nuclear measurements
approach the precision of direct density tests on rock cores. The gamma-gamma
density log and the neutron water content log require the use of isotopic sources of
nuclear radiation. Potential radiation hazards mandate thorough training of
personnel working around these sources. Strict compliance with U.S. NRC Title 10,
Part 20, as well as local safety regulations, is required. Additional information on
natural gamma, gamma-gamma, and neutron gamma methods is provided below.
The natural gamma radiation tool is a passive device measuring the amount of
gamma radiation naturally occurring in the strata being logged. The primary
sources of radiation are trace amounts of the potassium isotope K40and isotopes of
uranium and thorium. K40is most prevalent, by far, existing as an average of 0.012
percent by weight of ail potassium. Because potassium is part of the crystal lattices
of illites, micas, montmorillanites, and other clay materials, the engineering gamma
log is mainly a qualitative indication of the clay content of the strata.
The natural gamma log is put to its simplest and most frequently used
applications in qualitative lithologic interpretation (specifically identification of
shale and clay layers) and bed correlations from hole to hole. Since clay fractions
C-17
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frequently reduce the primary porosity and permeability of sediments, inferences as
to those parameters may sometimes be possible from the natural gamma log.
Environmentally based surveys may utilize the log for tracing radioactive pollutants.
If regulatory restrictions allow the use of radioactive tracers, the natural gammea log
can be used to locate ground water flow paths. The natural gamma radiation level
is also a correction factor to the gamma-gamma density log.
In the gamma-gamma logging technique, a radioactive source and detector
are used to determine density variations in the borehole. An isotopic source of
gamma radiation can be placed on the gamma radiation tool and shielded so that
direct paths of that radiation from source to detector are blocked. The source
radiation then permeates the space and materials near itself. As the gamma
photons pass through the matter, they are affected by several factors among which
is "Compton scattering." Part of each photon's energy is lost to orbital electrons in
the scattering material. The amount of scattering is proportional to the number of
electrons present. Therefore, if the portion of radiation able to escape through the
logged earth materials without being widely scattered and de-energized is
measured, then that is an inverse active measure of electron density. A schematic
representation of the borehole gamma-gamma tool is shown in Figure C-8.
The neutron water detector logging method is much like the gamma-gamma
technique in that it uses a radioactive source and detector. The difference is that
the neutron log measures water content rather than density of the borehole
material. A composite isotopic source of neutron radiation can be placed on a
probe together with a neutron detector. A neutron has about the same mass and
diameter as a hydrogen nucleus and is much lighter and smaller than any other
geochemically common nucleus. Upon collision with a hydrogen nucleus the
neutron loses about half its kinetic energy to the nucleus and is slowed down as well
as scattered. Collision with one of the larger nuclei scatters the neutron but
does not slow it. After a number of collisions with hydrogen nuclei, a neutron is
slowed, or it is captured by a hydrogen atom and produces a secondary neutron
emission of thermal energy plus a secondary gamma photon. Detectors can be
"tuned" to be sensitive to the epithermal (slowed) neutron or to the thermal
C-18
-------
I
I
I
PHOTONS
o F«OM
ISOTOPIC SOURCS
I
I
Figure C-8. Schematic of the borehole gamma-gamma density tool (prepared by
the Waterways Experiment Station, U.S. Army Corps of Engineers,
Vicksburg, Mississippi).
C-19
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neutron or to the gamma radiation. One of these detectors plus the neutron source
is then a device capable of measuring the amount of hydrogen in the vicinity of the
tool. In the geologic environment, hydrogen exists most commonly in water (H20)
and in hydrocarbons. If it can be safely assumed that hydrocarbons are not present
in appreciable amounts, then the neutron-epithermal neutron, the neutron-
thermal neutron, and the neutron-gamma logs are measures of the amount of
water present if the tool is calibrated in terms of its response to saturated rocks of
various porosities.
The neutron log can be used for hole to hole stratigraphic correlation. Its
designed purpose is to measure water quantities in the formation. Therefore, the
gamma-gamma density, the neutron water detector, the natural gamma, and the
caliper logs together form a "suite" of logs that, when taken together, can produce
continuous interpreted values of water content, bulk density, dry density, void ratio,
porosity, and pecent of water saturation.
Seismic Surveys
The principles involved in subsurface seismic surveys are the same as those
discussed earlier under surface seismic surveys. The travel times for P- and S- waves
between source and detector are measured, and wave velocities are determined on
the basis of theoretical travel paths. These calculated wave velocities can then be
used to complement and supplement other geophysical surveys conducted in the
area of investigation.
Three common types of borehole seismic surveys are discussed in this section.
They include Uphole and Downhole surveys, Crosshole Tests, and Vertical Seismic
Surveys. The applications and limitations are discussed for each of these methods.
In the uphole and downhole seismic survey, a seismic signal travels between a
point in a borehole and a point on the ground near the hole, in an uphole survey
the energy source is in the borehole, and the detector on the ground surface; in a
downhole survey, their positions are reversed. The raw data obtained are the travel
times for this signal and distances between the seismic source and the geophones.
A plot of travel time versus depth yields, from the slope of the curve, the average
C-20
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wave propagation velocities at various intervals in the borehole. Figure C-9 depicts a
downhole seismic survey technique.
Uphole and downhole surveys are usually performed to complement other
seismic tests and provide redundancy in a geophysical test program. However,
because these surveys force the seismic signals to traverse all of the strata between
the source and detector, they provide a means of detecting features, such as a low
velocity layer underlying a higher velocity layer of a "blind" or "hidden" zone (a
layer with insufficient thickness and velocity contrast to be detected by surface
refraction).
Crosshole tests are conducted to determine the P- and S-wave velocity of each
earth material or layer within the depth of interest through the measurement of
the arrival time of a seismic signal that has traveled from a source in one borehole
to a detector in another. The crosshole test concept is shown in Figure C-10.
In addition to providing true P- and S-wave velocities as a function of depth,
their companion purpose is to detect seismic anomalies, such as a lower velocity
zone underlying a higher velocity zone or a layer with insufficient thickness and
velocity contrast to be detected by surface refraction seismic tests.
The vertical seismic profiling technique involves the recording of seismic waves
at regular and closely spaced geophones in the borehole. The surface source can be
stationary or it can be moved to evaluate seismic travel times to borehole
geophones, calculate velocities, and determine the nature of subsurface features in
the vicinity of the borehole.
Vertical seismic profiling surveys are different from downhole surveys in that
they provide data on not only direct path seismic signals, but reflected signals as
well. By moving the surface source to discrete distances and azimuths from the
borehole, this method provides a means of characterizing the nature and con-
figuration of subsurface interfaces (bedding, ground water-table, faults), and
anomalous velocity zones around the borehole.
C-21
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Figure C-9. Downhole survey techniques for P-wave data (prepared by the
waterways Experiment Station, U.S. Army Corps of Engineers,
Vicksburg, Mississippi).
C-22
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RECORDER
ITiMCRI
SOURCE
RECEIVER
Figure C-IO. Basic crosshole test concept (prepared by the Waterways Experiment
Station, U.S. Army Corps of Engineers, Vicksburg, Mississippi ).
C-23
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The interpretation of processed vertical seismic profiling data is used in
conjunction with surface seismic surveys as well as other geophysical surveys in the
evaluation of subsurface lithology, stratigraphy, and structure. Vertical seismic
profiling survey interpretations also provide a basis for correlation between
boreholes.
Sonic Borehole Surveys
In this section, two types of continuous borehole surveys involving high
frequency sound wave propagation are discussed. Sound waves are physically
identical to seismic P-waves. The term sound wave is usually employed when the
frequencies include the audible range and the propagating medium is air to water.
Ultrasonic waves are also physically the same, except that the frequency range is
above the audible range.
The Sonic borehole imagery log provides a record of the surface configuration
of the cylindrical wall of the borehole. Pulses of high frequency sound are used in a
way similar to marine sonar to probe the wall of the borehole and, through
electronic and photographic means, to create a visual image representing the
surface configuration of the borehole wall. The physical principle involved is wave
reflection from a high impedance surface, the same principle used in reflection
seismic surveying and acoustic subbottom profiling. The sonic borehole imagery
logging concept is depicted in Figure C-11.
The sonic borehole imagery log can be used to detect discontinuities in
competent rock lining the borehole. Varying lithologies, such as shale, sandstone,
and limestone, can sometimes be distinguished on high quality records by ex-
perienced personnel.
Another method of sonic borehole logging is referred to as the continuous
sonic velocity logging technique. The continuous sonic velocity logging device is
used to measure and record the transit time of seismic waves along the borehole
wall between two transducers as it is moved up or down the hole. A diagram of the
continuous sonic velocity logging device is provided in Figure C-12.
C-24
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POWER SUPPLY AND
IMAGING DEVICE
COMPASS OK
DIRECTION
SENSOR
COO
ULTRASONIC
ACOUSTIC
ROTATING
PIEZOELECTRIC
TRANSDUCER
N C S W N
RECORD SHOWING
AZIMUTMAL DIRECTIONS
ABOVE "UNWRAPPED"
BOREHOLE IMAGE
WITH IMAGES OF
TWO DIPPING PLANES
Figure C-ll. Sonic imagery logger (prepared by the Waterways Experiment
Station, U.S. Army Corp of Engineers, Vicksburg, Mississippi).
C-25
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ACOUSTIC ISOLATOft
UCtlVtK
ORIIIINC FLUIO _
^
'
^» »
4
4
4
y
I
i
UBI
T
_J
,
F
P ^^j^t
»^^^*^«
^1
'fc
/
''' \
F
' V
f
Figure C-12. Diagram of three-dimensional velocity tool (courtesy of Seismograph
Service Corporation, Birdwell Division).
C-26
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This subsurface logging method provides data on fractures and abrupt
lithology changes along the borehole wall that can be effective in characterizing
the nature of surrounding material as well as borehole correlation in lithology and
structure.
Auxiliary Surveys
An auxiliary survey is the direct measurement of some parameter of the
borehole or its contained fluid to provide information that will either permit the
efficient evaluation of the lithology penetrated by the boring or aid in the
interpretation or reduction of the data from other borehole logging operations. In
most instances, auxiliary logs are made where the property recorded is essential to
the quantitative evaluation of other geophysical logs. In some instances, however,
the auxiliary results can be interpreted and used directly to infer the existence of
certain lithologic or hydrologic conditions.
Discussed here are three different auxiliary logs; fluid temperature, caliper,
and fluid resistivity, that are especially applicable to the logging methods discussed
in this text. A description of each auxiliary log is presented below.
Temperature logs are the continuous records of the temperature encountered
at successive elevations in a borehole. The two basic types of temperature logs are
standard (gradient) and differential. Both types of logs rely upon a downhole
probe, containing one or more temperature sensors (thermistors) and surface
electronics to monitor and record the temperature changes encountered in a
borehole. The standard temperature log is the result of a single thermistor
continuously sensing the thermal gradient of the fluid in the borehole as the sonde
is raised or lowered in the hole. The differential temperature log depicts the
difference in temperature over a fixed interval of depth in the borehole by
employing two thermistors spaced from one to several feet apart or through use of
a single thermistor and an electronic memory to compare the temperature at one
depth with that of a selected previous depth.
C-27
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Temperature logs provide useful information in both cased and uncased
borings and are necessary for correct interpretation of other geophysical logs
(particularly resistivity logs). Temperature logs can also be used directly to indicate
the source and movement of water into a borehole, to identify aquifers, to locate
zones of potential recharge, to determine areas containing wastes discharged into
the ground, and to detect sources of thermal pollution. The thermal conductivity
and permeability of rock formations can be inferred from temperature logs as can
be the location of grout behind casing by the presence of anomalous zones of heat
buildup due to the hydration of the setting cement.
The caliper log is a record of the changes in borehole casing or cavity size as
determined by a highly sensitive borehole measuring device. The record may be
presented in the form of a continuous vertical profile of the borehole or casing wall,
which is obtained with normal or standard caliper logging systems, or as a
horizontal cross section at selected depths, used for measuring voids or large
subsurface openings. There are two basic methods of obtaining caliper logs. One
technique utilizes mechanically activated measuring arms or bown springs, and the
other employs piezoelectric transducers for sending and receiving a focused
acoustic signal. The acoustic method requires that the hole be filled with water or
mud, but the mechanical method operates equally well in water, mud, or air.
Reliable mechanically derived caliper logs can be obtained in small (2 in.) diameter
exploratory borings as well as large (36 in.) inspection or access calyx-type borings.
Caliper or borehole diameter logs represent one of the most useful and
possibly the simplest of all techniques employed in borehole geophysics. They
provide a means for determining inhole conditions and should be obtained in all
borings in which other geophysical logs are contemplated. Borehole diameter logs
provide information on subsurface lithology and rock quality. Borehole diameter
varies with the hardness, fracture frequency, and cementation of the various beds
penetrated. Borehole diameter logs can be used to accurately identify zones of
enlargement (washouts) or construction (swelling), or to aid in the structural
evaluation of an area by the accurate location of fractures or solution openings,
particularly in borings where core loss has presented a problem. Caliper logs also
are a means of identifying the more porous zones in a boring by locating the
intervals in which excessive mud filter cake has built up on the walls of the
borehole. One of the major uses of standard or borehole caliper logs is to provide
C-28
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information by which other geophysically derived raw data logs can be corrected
for borehole diameter effects. This is particularly true for such nonfocused logs as
those obtained in radiation logging or the quantitative evaluation of flowmeter
logs or tracer and water quality work where inhole diameters must be considered.
Caliper logs also can be useful to evaluate inhole conditions for placement of water
well screens or for the selection of locations of packers for permeability testing.
The fluid resistivity log is a continuous graphical record of the resistivity of the
fluid within a borehole. Such records are made by measuring the voltage drop
between two closely spaced electrodes enclosed within a downhole probe through
which a representative sample of the borehole fluid is channeled. Some systems,
rather than recording in units of resistivity, are designed to provide a log of fluid
conductivity. As conductivity is merely the reciprocal of resistivity, either system can
be used to collect the information on inhole fluid required for the correct
interpretation of other downhole logs.
The primary use of fluid resistivity or conductivity logs is to provide
information for the correct interpretation of other borehole logs. The evaluation of
nuclear and most electrical logs requires corrections for salinity of the inhole fluids,
particularly when quantitative parameters are desired for determining porosity
from formation resistivity logs.
C-29
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APPENDIX D
SUBSURFACE GAS MIGRATION MODEL
METHANE MIGRATION DISTANCE PREDICTION CHARTS
Migration distance charts have been developed to estimate methane distances
and to plan the monitoring program. The basic methane migration distance
prediction chart and appropriate corrective factor charts were produced by
imposing a set of simplifying assumptions on a general methane migration
computer model. These charts are based on a number of assumptions that were
made to produce them. Case Study Number 24 (Volume IV) illustrates the use of the
Subsurface Gas Migration Model.
To illustrate the use of the charts, an example landfill is shown in Figure D-1
along with two cross-sections. Conditions along each side of the waste deposit are
typical conditions that could be encountered. A similar sketch or plan of a facility
being evaluated should be prepared. The land use within 1/4-mile of the solid waste
limits, including offsite and facility structures, should be on the map. The property
boundaries and solid waste deposit limits should also be plotted, as has been done
in Figure D-1.
Additional data needs are:
1. The age of the site from the initial deposit of organic waste in years;
2. The average elevation of the bottom of the solid waste;
3. Natural boundaries and topography around the site; and
4. The average elevation below the solid waste of a gas impervious
boundary such as unfractured rock.
D-2
-------
Mudhole Creek
k
Agricultural
'.Area//// ,
CO
Houses
CE3 O SI
J/////////////.
/'Agricultural Area'/
7/////////////
Gatehouse
Equipment
Shed
Landfill
Waste Limits
J
s
O
Wooded Lowland
f,
N
Houses
El
East
W/oft
.. \^ Solid Waste ^S
Agricultural x<« SanH fc\ /
House
M R?l
_ _2« Sand ^
Section A-A
Ground-Water Level
South
North
Section B-B
===- Ground-Water Level
Note: Not to Scale
FIGURE D-1. EXAMPLE LANDFILL
D-3
-------
Two calculations of migration distance from the waste boundary are needed
for each side of the landfill:
1. The 5 percent (Lower Explosion Limit or LEL) distance for property
boundaries.
2. The 1.25 percent (1/4 of the LEL) distance for onsite facility structures.
After preparation of the sketch and cross-sections, the determination of the
estimated migration distances begins with the use of Figure D-2 for the 5 percent
methane (LEL) migration distance and for the 1.25 percent (1/4 LEL) distance. These
distances are then modified, if necessary, with the corrective factors for each depth
and surrounding soil surface permeability (Figures D-3 and D-4). The final distances
of migration for each side of the landfill can then be plotted on the landfill sketch
for comparison to property boundary and structures locations.
UNCORRECTED MIGRATION DISTANCES
The use of Figure D-2 requires the age of the site and the type of soil
extending out from each side of the solid waste deposit. The graph is entered with
the site age, moving up to the appropriate soil type and methane concentration
(1.25 or 5 percent). Interpolations between the sand and clay lines on the graph can
be made for other soils, using the following general guidance:
Soil Name USCS Classification Chart Use
Clean (no fines) GW, GP, SW, SP Sand
gravels and sands
Silty gravels and sands, GM, SM, ML, OL, MH Interpolate
silt, silty and sandy
loam, organic silts
Clayey gravels and GC, SC, CL, CH, OH Clay
sands, lean, fat, and
organic clays
D-4
-------
D
1/1
300
£ 200
u.
01
0
z
1-
5
z
o
(-
cc
:= 100
5
0
^
/
/ Vi
* /
f
/
1
1
1
/
/
'
1 <
' f
+
^
X"
^.^"-"
»"^"
^^.--"'
SAND
CLAY
-1.25%
-1.25%
SAND - 5%
CLAY
5%
246 8 10 12 14 16 18 20
SITE AGE - YEARS
FIGURE D-2. FIVE PERCENT AND 1.25 PERCENT METHANE MIGRATION DISTANCE
-------
CORRECTION FACTOR
r" .-* is) M
w o 01 b b
-
-
-
^
<>''
s
^'
+ ^^
^
^'
""""
---'
60_
25' DEEP
_,..
niCPP
*""
100' DEEP
ABOVE GRADE
-
-
-
-
2 46 8 10 12 14 16 18 2
SITE AGE-YEARS
FIGURE D-3. CORRECTION FACTORS FOR LANDFILL DEPTH BEFORE GRADE
-------
D
1/1
300
£ 200
u.
01
0
z
1-
5
z
o
(-
cc
:= 100
5
0
^
/
/ Vi
* /
f
/
1
1
1
/
/
'
1 <
' f
+
^
X"
^.^"-"
»"^"
^^.--"'
SAND
CLAY
-1.25%
-1.25%
SAND - 5%
CLAY
5%
246 8 10 12 14 16 18 20
SITE AGE - YEARS
FIGURE D-2. FIVE PERCENT AND 1.25 PERCENT METHANE MIGRATION DISTANCE
-------
CORRECTION FACTOR
r" .-* is) M
w o 01 b b
-
-
-
^
<>''
s
^'
+ ^^
^
^'
""""
---'
60_
25' DEEP
_,..
niCPP
*""
100' DEEP
ABOVE GRADE
-
-
-
-
2 46 8 10 12 14 16 18 2
SITE AGE-YEARS
FIGURE D-3. CORRECTION FACTORS FOR LANDFILL DEPTH BEFORE GRADE
-------
3.0
2.0
cc
O
o
2 1.0
o
UJ
cc
cc
o
o
2.0
3.0
-
-
-
-
SAND
**
CLAY
^^-
__ _ .
__ __ IMPE
^^^'
IRVIOUS- 10
«. _^
\tfj
fBlfiS.-*
) ^^0 ^^*
.^--y
IMPERVIOUS -_100J_p£EP_
)' DEEP
MPFRl/ini 10
L_ IR/lf*
"^
| FREE V
50'_DEEP
^.
-
ENTING
-
8 10 12
SITE AGE -YEARS
14 16 18 20
FIGURE D 4. CORRECTION FACTORS FOR SOIL SURFACE VENTING CONDITION AROUND LANDFILL
-------
The unconnected mignation distance fnom the solid waste limit can then be
nead on the left fon the appnopniate site age and soil type.
If the soil along a given boundany is stnatified and the vaniability extends fnom
the waste deposit to the pnopenty boundany, the most penmeable unsatunated
thickness should be used in entening the chants. Fon example, if dny, clean sand
undenlies sunficial silty clays, the unconnected mignation distance should be obtained
using the sand line of the chant. If thene ane questions as to the extent of panticulan
soils along a boundany, helpful infonmation might be obtained fnom Soil
Consenvation Senvices (SCS) Soil Sunvey Maps on the landfill openaton. Field
inspection, SCS maps, and penmit boning infonmation ane sufficient. Additional
bonings ane not necessany as this is only a nanking pnocedune. Whene thene is doubt,
use the most penmeable soil gnoup pnesent.
Fon the example landfill in Figune D-1, the unconnected 5 pencent methane
mignation distances fon a 10-yean old landfill would be (Figune C-2):
Section A-A: East side, 10 yeans, sand = 165'
West side, 10 yeans, sand = 165'
Section B-B: South side, 10 yeans, sand = 165'
North side, 10 yeans, clay = 130'
The connesponding unconnected distances fon the 1.25 pencent methane
mignation would be:
Section A-A: East side, 10 yeans, sand = 225'
West side, 10 yeans, sand = 135'
Section B-B: South side, 10 yeans, sand = 255'
North side, 10 yeans, clay = 200'
The depth to connective mulitpliens fon the example sites would be:
Section A-A: East side, 10 yeans, 20' deep = 1.0
West side, 10 yeans, 20' deep = 1.0
D-8
-------
Section B-B: South side, 10 years, 10' deep = 0.95
North side, 10 years, 50' deep = 1.4
VENTING CONDITIONS CORRECTION
The corrective factors for the surrounding soil venting conditions are obtained
using the chart in Figure D-4. This chart is based on the assumption that the
surrounding surficial soil is impervious 100 percent of the time. Thus, the value read
from the chart must be adjusted, based on the percentage of time the surrounding
surficial soil is saturated or frozen and the percentage of land along the path of gas
migration from which gas venting to the atmosphere is blocked all year (asphalt or
concrete roads or parking lots, shallow perched ground water, surface water bodies
not interconnected to ground water). The totally impervious corrective factor is
only used when the landfill is entirely surrounded at ail times by these conditions.
Both time and area adjustments are necessary, and the percentages are additive.
Estimates to the nearest 20 percent are sufficient, An adjusted corrective factor is
obtained by entering the char-t with site age and obtaining the totally impervious
corrective factor for the appropriate depth and soil type and then entering this
value in the following equation:
Adjusted corrected factor = [(Impervious corrective factor)-1)]
x [5 of impervious time or area] + 1
When free venting conditions are prevalent most of the year, simply use 1.0
(no correction). For depths less than 25 feet deep, use the 25 foot value. For the
example site, the adjusted corrective factors for frozen or wet soil conditions so
percent of the year are:
Section A-A: East side (ignore narrow = (2.1-1)(0.50) + 1 = 1.55
road, sand 20' deep,
10 years old)
West side (sand 20' deep, = (2.1-1)(0.50) + 1 = 1.55
10 years old)
D-9
-------
Section B-B: South side (sand, 10' deep, = (2.1-1)(0.50) + 1 = 1.55
10 years old)
North side (clay, 50' deep, = (i .4-1 )(0.50) + 1 = 1.2
10 years old)
Once the surface venting factors have been tabulated as in Table D-1, the
corrective distance can be obtained by multiplying across the chart for each side of
the landfill. These values can then be plotted on the scale plan to describe contours
of the 5 percent and 1.25 percent methane concentrations or simply compared to
the distance from the waste deposit to structures of concern (Figure D-5).
D-10
-------
APPENDIX E
ESTIMATION OF BASEMENT AIR CONTAMINANT CONCENTRATIONS DUE TO
VOLATILE COMPONENTS IN GROUND WATER SEEPED INTO THE BASEMENT
E-1
-------
APPENDIX E
ESTIMATION OF BASEMENT AIR CONTAMINANT CONCENTRATIONS DUE TO
VOLATILE COMPONENTS IN GROUND WATER SEEPED INTO THE BASEMENT
Ground water can reach the basement and the walls of a house in several
ways. If ground water is contaminated by volatile components, there are several
possibilities that the indoor ambient air can be affected by these constituents.
There are several methods which can be applied to estimating the ambient air
concentrations in the basement into which the contaminants are volatilized from
ground water. The manner in which and the extent to which the ground water
reaches the basement or the walls will dictate the choice of a method.
Two cases are considered as example scenarios: Case 1) Ground water is
seeped inside the basement completely wetting the basement, with a visual
indication of water on the floor, Case 2) The basement is partially wetted without
a visual indication of liquid on the floor. This latter case can be subdivided into two
subcases: Subcase 1) involving a damp floor evident on the surface; Subcase 2)
involving a floor without observable dampness on the floor surface but with ground
water underneath the concrete floor.
The way the emission rates are estimated will be different for the three cases.
If the emission flux rate per unit square area of the exposed surface is denoted by E
(g/m2day), then in all cases the air concentration, C (ug/m3), in the basement can be
estimated from:
C (ug/m3) = Ex106AteA/B (1)
where A = basement floor and wall area exposed to ground water, m2
VB = volume of the basement, m3, and
te = air exchange time for the basement, days.
E-2
-------
The air exchange time should be determined on a site-specific or situation-
specific basis. The tight room will have a longer time per air exchange in the room,
and the room with an exhaust fan will have a shorter time per air exchange. The
default value for a typical house could be te= 0.05 days.
The emission rates in Eq. (1) can be estimated for the various case scenarios
illustrated above.
Case 1. Wet basement with visible liquid,
The volatilization is a mass transfer phenomenon from the liquid phase of
ground water on the floor to the basement air. Emission flux rate can be estimated
from:
E = KOL(CL-CL*) (2)
where KOL = overall mass transfer coefficient in the liquid phase unit, m/day, CL =
concentration of contaminant in water, g/m3, and CL* = liquid phase concentration
in equilibrium concentration with the basement air, g/m3. The equilibrium
concentration C* could be assumed to be approaching a small value compared to
the ground water contaminant concentration when the air exchange rate is high, or
when the time per air exchange is small. But this assumption would not be valid at a
low air exchange rate or at a longer time for a room air exchange. In this case, the
emission flux rate should be estimated by a trial and error method using Equation
(2) in combination with Equation (1), and Henry's Law constant.
It is a well-established scientific principle to use the two-resistance theory to
obtain the overall mass transfer coefficient, KOL, as follows:
KOL= -± + -J (3)
KL Hckq
where KL and kg = individual mass transfer coefficients in liquid and gas phases,
respectively, m/day, and He = dimensionless Henry's Law constant obtained from
E-3
-------
reconcentration units for gas and liquid phase concentrations. The numerical value
for Hccan be calculated from Henry's Law constant given in atm/g-mol.m3by
multiplying by 41. Default values for the individual mass transfer coefficients can be
estimated from:
1
kL = 3 cm . /..44 \ 2 / 24 M hr \
hr 1 MW I \ 100 cm day I (4)
kg = 3000 -^m |^l 24 M
hr I MW I I 100 cm dav I (5)
where MW = molecular weight of the contaminant.
Case 2. Basement partially wetted with no visual indication of liquid.
(a) Subcase 1. Dampness evident on the floor or wall surface. The
volatilization process can be treated as a diffusional process from the air at the
water-air interface through the air pores in the basement floor material and into
the basement air. The diffusional process can be solved using the approach
described in the EPA report Development of Advisory Levels for Polychlorinated
Biphenyls (PCBS) Cleanup (PB86-232774). The final result needed for emission flux
estimation would be:
e = e
'V iraT
E-4
-------
APPENDIX F
METHOD 1312: SYNTHETIC PRECIPITATION
LEACH TEST FOR SOILS
-------
METHOD 1312
SYNTHETIC PRECIPITATION LEACH TEST FOR SOILS
1.0 SCOPE AND APPLICATION
1.1 Method 1312 is designed to determine the mobility of
both organic and inorganic contaminants present in soils.
1.2 If a total analysis of the soil demonstrates that in-
dividual contaminants are not present in the soil, or that they
are present but at such low concentrations that the appropriate
regulatory thresholds could not possibly be exceeded, Method
1312 need not be run.
2 . 0 SUMMARY OF METHOD
2.1 The particle size of the soil is reduced (if necessary)
and is extracted with an amount of extraction fluid equal to 20
times the weight of the soil. The extraction fluid employed is
a function of the region of the country where the soil site is
located. A special extractor vessel is used when testing for
volatiles. Following extraction, the liquid extract is separated
from the soil by 0.6-0.8 urn glass fiber filter.
3.0 INTERFERENCES
3.1 Potential interferences that may be encountered during
analysis are discussed in the individual analytical methods.
4 . 0 APPARATUS AND MATERIALS
4.1 Agitation apparatus - an acceptable agitation apparatus
is one which is capable of rotating the extraction vessel in an
end-over-end fashion at 30 ± 2 rpm (see Figure 1) . Suitable
devices known to EPA are identified in Table 2.
4.2 Extraction vessel - acceptable extraction vessels are
those that are listed below:
4.2.1 Zero Headspace Extraction Vessel (ZHE) - This
device is for use only when the soil is being tested for the
mobility of volatile constituents (see Table 1) . The ZHE is an
extraction vessel that allows for liquid/solid separation within
the device and which effectively precludes headspace (as depicted
in Figure 3). This type of vessel allows for initial liquid/solid
separation, extraction, and final extract filtration without
having to open the vessel (see Step 4.3.1). These vessels shall
have an internal volume of 500 to 600 mL and be equipped to
accommodate a 90-mm filter. Suitable ZHE devices known to EPA
are identified in Table 3. These devices contain viton 0-rings
which should be replaced frequently. For the ZHE to be acceptable
for use, the piston within the ZHE should be able to be moved
1312-1 Revision 0
December 1988
-------
with approximately 15 psi or less. if it takes more pressure
to move the piston, the 0-rinqs in the device should be replaced.
If this does not solve the problem, the ZHE is unacceptable for
1312 analyses and the manufacturer should be contacted. The ZHE
should be checked after every extraction. if the device con-
tains a built-in pressure gauge, pressurize the device to
50 psi, allow it to stand unattended for 1 hour, and recheck
the pressure. If the device does not have a built-in pressure
gauge, pressurize the device to 50 psi, submerge it in water
and check for the presence of air bubbles escaping from any
of the fittings. If pressure is lost, check all fittinqs and
inspect and replace 0-rings, if necessary. Retest the device.
If leakage problems cannot be solved, the manufacturer should
be contacted.
4.2.2 When the soil is being evaluated for other than
volatile contaminants, an extraction vessel that does not pre-
clude headspace (ea. a 2-liter bottle) is used. Suitable
extraction vessels include bottles made from various materials,
depending on the contaminants to be analyzed and the nature of the
waste (see Step 4.3.3)0 It is recommended that borosilicate
glass bottles be used over other types of glass, especially
when inorganic are of concern. Plastic bottles may be used
only if inorganic are to be investigated. Bottles are available
from a number of laboratory suppliers. When this type of ex-
traction vessel is used, the filtration device discussed in
Step 4.3.2 is used for initial liquid/solid separation and final
extract filtration.
4.2.3 Some ZHEs use gas pressure to actuate the ZHE piston,
while others use mechanical pressure (see Table 3). Whereas
the volatiles procedure (see Step 7.4) refers to pounds-per-
square inch (psi), for the mechanically actuated piston, the
pressure applied is measured in torque- inch-pounds. Refer to
the manufacturer's instructions as to the proper conversion.
4.3 Filtration devices - It is recommended that all filtrations
be performed in a hood.
4.3.1 Zero-Headspace Extractor Vessel (see Figure 3)-
When the waste is being evaluated for volatiles, the zero-
headspace extraction vessel is used for filtration. The device
shall be capable of supporting and keeping in place the fiber
filter, and be able to withstand the pressure needed to accomplish
separation (50 psi).
NOTE: When is it suspected that the glass fiber filter
has been ruptured, an in-line glass fiber filter may be
used to filter the material within the ZHE.
4.3.2 Filter holder - when the soil is being evaluated
for other than volatile compounds, a filter holder capable of
1312-2 Revision 0
December 1988
-------
supporting a glass fiber filter and able to withstand 50 psi
or more of pressure shall be used. These devices shall have a
minimum internal volume of 300 mL and be equipped to accommodate
a minimum filter size of 47 mm (filter holders having an
internal capacity of 1.5 liters or greater are recommended).
4.3.3 Materials of construction - filtration devices shall
be made of inert materials which will not leach or absorb soil
components. Glass, polytetrafluoroethylene (PTFE) or type 316
stainless steel equipment may be used when evaluating the mobility
of both organic and inorganic components. Devices made of hiqh
density polyethylene (HDPE), polypropylene, or polyvinyl chloride
may be used only when evaluating the mobility of metals. Boro-
silicate glass bottles are recommended for use over other types
of glass bottles, especially when inorganic are constituents
of concern.
4.4 Filters - filters shall be made of borosilicate glass
fiber, shall have an effective pore size of 0.6 - 0.8 urn and
shall contain no binder materials. Filters known to EPA to meet
these requirements are identified in Table 5. When evaluating the
mobility of metals, filters should be acid-washed prior to use
by rinsing with 1. ON nitric acid followed by three consecutive rinses
with deionized distilled water (a minimum of 1-liter per rinse is
recommended). Glass fiber filters are fragile and should be handled
with care.
4.5 pH meters - any of the commmonly available pH meters are
acceptable.
4.6 ZHE extract collection devices - TEDLAR bags, glass, stain-
less steel or PTFE gas tight syringes are used to collect the volatile
extract.
4.7 Laboratory balance - any laboratory balance accurate to
within + 0.01 g may be used (all weight measurements are to be within
+ 0.1 g).
4.8 ZHE extraction fluid transfer devices - any device capable
of transferring the extraction fluid into the ZHE without changing
the nature of the extraction fluid is recommended.
5 . 0 REAGENTS
5.1 Reagent water - reagent water is defined as water in
which an interferent is not observed at or above the method
detection limit of the analyte(s) of interest. For non-volatile
extractions, ASTM Type II water, or equivalent meets the definition
of reagent water. For volatile extractions, it is recommended
that reagent water be generated by any of the following methods.
Reagent water should be monitored periodically for impurities.
1312-3 Revision 0
December 1988
-------
5.1.1 Reagent water for volatile extractions may be
generated by passing tap water through a carbon filter bed
containing about 500 g of activated carbon (Calgon Corp.,
Filtrasorb 300 or equivalent).
5.1.2 A water purification system (Millipore Super-Q or
equivalent) may also be used to generate reagent water for
volatile extractions.
5.1.3 Reagent water for volatile extractions may also
be prepared by boiling water for 15 minutes. Subsequently,
while maintaining the water temperature at 90 + 5°C, bubble
a contaminant-free inert gas (e.g. nitrogen) through the
water for 1 hour. While still hot, transfer the water to a
narrow-mouth screw-cap bottle under zero headspace and seal
with a Teflon lined septum and cap.
5.2 Sulfuric acid\nitric acid (60/40 weight percent mixture)
H2S04/HN03. Cautiously mix 60 g of concentrated sulfuric acid with
40 g of concentrated nitric acid.
5.3 Extraction fluids:
5.3.1 Extraction fluid #1 - this fluid is made by adding
the 60/40 weight percent mixture of sulfuric and nitric acids
to reagent water until the pH is 4.20 ±0.05.
5.3.2 Extraction fluid #2 - this fluid is made by adding
the 60/40 weight percent mixture of sulfuric and nitric acids
to reagent water until the pH is 5.00 ±0.05.
5.3.3 Extraction fluid #3 - this fluid is reagent water
(ASTM Type II water, or equivalent) used to determine cyanide
leachability.
Note: It is suggested that these extraction fluids be moni-
tored frequently for impurities. The pH should be
checked prior to use to ensure that these fluids are
made up accurately.
5.4 Analytical standards shall be prepared according to the
appropriate analytical method.
6.0 SAMPLE COLLECTION, PRESERVATION, AND HANDLING
6.1 All samples shall be collected using an appropriate
sampling plan.
6.2 At least two separate representative samples of a soil
should be collected. The first sample is used to determine if the
soil requires particle-size reduction and, if desired, the percent
solids of the soil. The second sample is used for extraction
of volatiles and non-volatiles.
1312-4 Revision 0
December 1988
-------
6.3 Preservatives shall not be added to samples.
6.4 Samples shall be refrigerated to minimize loss of volatile
organics and to retard biological activity.
6.5 When the soil is to be evaluated for volatile contaminants,
care should be taken to minimize the loss of volatiles. Samples
shall be taken and stored in a manner to prevent the loss of
volatile contaminants. If possible, it is recommended that any
necessary particle-size reduction be conducted as the sample is
being taken.
6.6. 1312 extracts should be prepared for analysis and
analyzed as soon as possible following extraction. if they need
to be stored, even for a short period of time, storage shall be at
4°C, and samples for volatiles analysis shall not be allowed to
come into contact with the atmosphere (i.e. no headspace) . See
Section 8.0 (Quality Control) for acceptable sample and extract
holding times.
7 . 0 PROCEDURE
7.1 The preliminary 1312 evaluations are performed on a mini-
mum 100g representative sample of soil that will not actually under-
go 1312 extraction (designated as the first sample in Step 6.2).
7.1.1 Determine whether the soil requires particle-size
reduction. If the soil passes through a 9.5 mm (0.375-inch)
standard sieve, particle-size reduction is not required
(proceed to Step 7.2) . If portions of the sample do not
pass through the sieve, then the oversize portion of the
soil will have to be prepared for extraction by crushing
the soil to pass the 9.5 mm sieve.
7.1.2 Determine the percent solids if desired.
7.2 Procedure when volatiles are not involved - Enough
solids should be generated for extraction such that the volume
of 1312 extract will be sufficient to support all of the analyses
required. However, a minimum sample size of 100 grams shall
be used. If the amount of extract generated by a single 1312
extract will not be sufficient to perform all of the analyses,
it is recommended that more than one extraction be performed and
the extracts be combined and then aliquoted for analysis.
7.2.1 Weigh out a representative subsample of the soil and
transfer to the filter holder extractor vessel.
7.2.2 Determine the appropriate extraction fluid to use.
If the soil is from a site that is east of the Mississippi
River, extraction fluid #1 should be used. If the soil is
from a site that is west of the Mississippi River, extraction
fluid #2 should be used. If the soil is to be tested for
cyanide leachability, extraction fluid #3 should be used.
1312-5 Revision 0
December 1988
-------
Note: Extraction fluid #3 (reagent water) must be used
when evaluating cyanide-containing soils because leaching
of cyanide-containing soils under acidic conditions may
result in the formation of hydrogen cyanide gas.
7.2.3 Determine the amount of extraction fluid to add
based on the following formula:
amount of extraction fluid (mL) = 20 x weight of soil (q)
Slowly add the amount of appropriate extraction fluid to the
extractor vessel. Close the extractor bottle tightly (it
is recommended that Teflon tape be used to ensure a tight
seal), secure in rotary extractor device, and rotate at 30
+ 2 rpm for 18+2 hours. Ambient temperature (i.e. temper-
ature of room in which extraction is to take place) shall
be maintained at 22 + 3°C during the extraction period.
Note: As agitation continues, pressure may build up within the
extractor bottle for some types of soil (e.g. limed or
calcium carbonate containing soil may evolve gases such as
carbon dioxide). To relieve excess pressure, the extractor
bottle may be periodically opened (e.g. after 15 minutes,
30 minutes, and 1 hour) and vented into a hood.
7.2.4 Following the 18 ±2 hour extraction, the material in
the extractor vessel is separated into its component liquid and
solid phases by filtering through a glass fiber filter.
7.2.5 Following collection of the 1312 extract it is re-
commended that the pH of the extract be recorded. The extract
should be immediately aliquoted for analysis and properly
preserved (metals aliquots must be acidified with nitric
acid to pH < 2; all other aliquots must be stored under
refrigeration (4°C) until analyzed) . The 1312 extract
shall be prepared and analyzed according to appropriate
analytical methods. 1312 extracts to be analyzed for metals,
other than mercury, shall be acid digested.
7.2.6 The contaminant concentrations in the 1312 extract are
compared to thresholds in the clean closure guidance manual.
Refer to Section 8.0 for Quality Control requirements.
7.3 Procedure when volatiles are involved:
7.3.1 The ZHE device is used to obtain 1312 extracts for
volatile analysis only. Extract resulting from the use of the
ZHE shall not be used to evaluate the mobility of non-volatile
analytes (e.g. metals, pesticides, etc.). The ZHE device
has approximately a 500 mL internal capacity. Although a minimum
sample size of 100 g was required in the Step 7.2 procedure, the
ZHE can only accommodate a maximum of 25 g of solid , due to the
need to add an amount of extraction fluid equal to 20 times the
1312-6 Revision 0
December 1988
-------
weight of the soil. The ZHE is charged with sample only once and
the device is not opened until the final extract has been col-
lected. Although the following procedure allows for particle-
size reduction during the conduct of the procedure, this could
result in the loss of volatile compounds. if possible particle-
size reduction (see Step 7.1.1) should be conducted on the
sample as it is being taken (e.g., particle-size may be reduced
by crumbling). If necessary particle-size reduction may be
conducted during the procedure. in carrying out the following
steps, do not allow the soil to be exposed to the atmosphere for
any more time than is absolutely necessary. Any manipulation of
these materials should be done when cold (4°C) to minimize the
loss of volatiles. Pre-weigh the ejaculated container which
will receive the filtrate (see Step 4.6), and set aside. If
using a TEDLAR°bag, all air must be expressed from the device.
7.3.2 Place the ZHE piston within the body of the ZHE (it
may be helpful firs-t to moisten the piston 0-rings slightly with
extraction fluid). Adjust the piston within the ZHE body to a
height that will minimize the distance the piston will have to
move once it is charged with sample. Secure the gas inlet/outlet
flange (bottom flange) onto the ZHE body in accordance with the
manufacturer's instructions. Secure the glass fiber filter
between the support screens and set aside. Set liquid inlet/out-
let flange (top flange) aside.
7.3.3 Quantitatively transfer 25 g of soil to the ZHE.
Secure the filter and support screens into the top flange of the
device and secure the top flange to the ZHE body in accordance
with the manufacturer's instructions. Tighten all ZHE fittings
and place the device in the vertical position (gas inlet/outlet
flange on the bottom). DO not attach the extraction collection
device to the top plate. Attach a gas line to the gas inlet/out-
let valve (bottom flanqe) and, with the liquid inlet/outlet
valve (top flange) open, begin applying gentle pressure of 1-10
psi to a maximum of 50 psi to force most of the headspace out of
the device.
7.3.4 With the ZHE in the vertical position, attach a
line from the extraction fluid reservoir to the liquid inlet/
outlet valve. The line used shall contain fresh extraction
fluid and should be preflushed with fluid to eliminate any air
pockets in the line. Release qas pressure on the ZHE piston
(from the gas inlet/outlet valve), open the liquid inlet/
outlet valve, and begin transferring extraction fluid (by
pumping or similar means) into the ZHE. Continue pumping
extraction fluid into the ZHE until the appropriate amount of
fluid has been introduced into the device.
7.3.5 After the extraction fluid has been added, immediately
close the inlet/outlet valve and disconnect the extraction fluid
line. Check the ZHE to ensure that all valves are in their closed
positions. Physically rotate the device in an end-over-end fashion
1312-7 Revision 0
December 1988
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2 or 3 times. Reposition the ZHE in the vertical position with
the liquid inlet/outlet valve on top. Put 5-10 psi behind the
piston (if nesessary) and slowly open the liquid inlet/outlet
valve to bleed out any headspace (into a hood) that may have
been introduced due to the addition of extraction fluid.
This bleeding shall be done quickly and shall be stopped at the
first appearance of liquid from the valve. Re-pressurize the
ZHE with 5-10 psi and check all ZHE fittings to ensure that
they are closed.
7.3.6 Place the ZHE in the rotary extractor apparatus (if
it is not already there) and rotate the ZHE at 30 + 2 rpm for
18 ± 2 hours. Ambient temperature (i.e. temperature of the room
in which extraction is to occur) shall be maintained at 22 +_ 3°C
during agitation.
7.3.7 Following the 18+2 hour agitation period, check
the pressure behind the ZHE piston by quickly opening and closing
the gas inlet/outlet valve and noting the escape of gas. If the
pressure has not been maintained (i.e. no gas release observed),
the device is leaking. Check the ZHE for leaking and redo the
extraction with a new sample of soil. If the pressure within
the device has been maintained, the material in the extractor
vessel is separated into its component liquid and solid phases.
7.3.8 Attach the evacuated pre-weighed filtrate collection
container to the liquid inlet/outlet valve and open the valve.
Begin applying gentle pressure of 1-10 psi to force the liquid
phase into the filtrate collection container. If no additional
liquid has passed through the filter in any 2 minute interval,
slowly increase the pressure in 10-psi increments to a maximum of
50 psi. After each incremental increase of 10 psi, if no additional
liquid has passed through the filter in any 2 minute interval,
proceed to the next 10 psi increment. When liquid flow has
ceased such that continued pressure filtration at 50 psi does
not result in any additional filtrate within any 2 minute period,
filtration is stopped. Close the inlet/outlet valve, discontinue
pressure to the piston, and disconnect the filtration collection
container.
NOTE : Instantaneous application of high pressure can
degrade the glass fiber filter and may cause
premature plugging.
7.3.9 Following collection of the 1312 extract, the extract
should be immediately aliguoted for analysis and stored with
minimal headspace at 4°C until analyzed. The 1312 extract will be
prepared and analyzed according to the appropriate analytical
methods.
8.0 QUALITY CONTROL
8.1 All data, including quality assurance data, should be
1312-8 Revision 0
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maintained and available for reference or inspection.
8.2 A minimum of one blank (extraction fluid # 1) for every
10 extractions that have been conducted in an extraction vessel
shall be employed as a check to determine if any memory effects
from the extraction equipment are occurring.
8.3 For each analytical batch (up to twenty samples), it is
recommended that a matrix spike be performed. Addition of matrix
spikes should occur once the 1312 extract has been generated
(i.e. should not occur prior to performance of the 1312 procedure).
The purpose of the matrix spike is to monitor the adequacy of the
analytical methods used on the 1312 extract and for determining
if matrix interferences exist in analyte detection.
8.4 All quality control measures described in the appropriate
analytical methods shall be followed.
8.5 The method of standard addition shall be employed for
each analyte if: 1) recovery of the compound from the 1312
extract is not between 50 and 150%, or 2) if the concentration of
the constituent measured in the extract is within 20% of the
appropriate regulatory threshold. If more than one extraction is
being run on samples of the same waste (up to twenty samples) ,
the method of standard addition need be applied only once and the
percent recoveries applied on the remainder of the extractions.
8.6 Samples must undergo 1312 extraction within the following
time period after sample receipt: Volatiles, 14 days; Semi-
Volatiles, 40 days; Mercury, 28 days; and other Metals, 180 days.
1312 extracts shall be analyzed after generation and preservation
within the following periods: Volatiles, 14 days; Semi-Volatiles,
40 days; Mercury, 28 days; and other Metals, 180 days.
9.0 METHOD PERFORMANCE
9.1 None available.
10.0 REFERENCES
10. 1 None available.
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TABLE 1. -- VOLATILE CONTAMINANTS
Compounds
Acetone
Ac rrylonitzrile -
Benzene
Carbon disulf ide
Chlorobenzene
1 1 Dichloroethylene
Ethyl acetate
Ethyl either
Isobutanol
Methylene chloride
1112 -Tetrachloroethane
Tetrachloroethylene
Tolulene
1,1, 2 -Trichloroethane
Trichlorofluoromethane
l,l,2-Trichloro-l,2,2-trifluoroethane
Xylene
CAS No.
67-64-1
m-13-1
71-43-2
71-36-6
75-15-0
56-23-5
108-90-7
67-66-3
107-06-2
75-35-4
141-78-6
100 41 4
60-29-7
78-83-1
67-56-1
75-09-2
78-93-3
108-10-1
630-20-6
79-34-5
127-18-4
108-88-3
71-55-6
79-00-5
79-01-6
75-69-4
76-13-1
75-01-7
1330-20-7
1312-10
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TABLE 2. -- SUITABLE ROTARY AGITATION APPARATUS1
Company
Location
Model
Analytical Testing and
Consulting Services, Inc.
Associated Design and
Manufacturing Company
Environmental Machine
and Design, Inc.
IRA Machine Shop and
Laboratory
Lars Lande Manufacturing
Millipore Corp.
REXNORD
Warrington, PA
(215) 343-4490
Alexandria, VA
(703) 549-5999
Lynchburg, VA
(804) 845-6424
Santurce, PR
(809) 752-4004
Whitmore Lake, MI
(313) 449-4116
Bedford, MA
(800) 225-3384
Milwaukee, WI
(414) 643-2850
4-vessel device
4-vessel device,
6-vessel device
4-vessel device,
6-vessel device
16-vessel device
10-vessel device
5-vessel device
4-vessel ZHE device
or 4-one litter
bottle extractor
device
6-vessel device
'Any device that rotates the extraction vessel in an end-over-end
fashion at 30 ± 2 rpm is acceptable.
TABLE 3. -- SUITABLE ZERO-HEADSPACE EXTRACTOR VESSELS
Company
Location
Model No.
Analytical Testing & Con-
sulting Services, Inc.
Associated Design & Manu-
facturing Co.
Lars Lande Mfg.
Millipore Corp.
Barrington, PA,
(215) 343-4490
Alexandria, VA
(703) 549-5999
Whitmore Lake, MI
(313) 449-4116
Bedford, MA,
(800) 225-33.84
C102, Mechanical
Pressure Device
3740-ZHB, Gas
Pressure Device
Gas Pressure
Device
SD1 P581 C5, Gas
Pressure Device
1312-11
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TABLE 4. -- SUITABLE ZHE FILTER HOLDERS1
Company
Micro Filtration Systems
Millipore Corp.
Nuc 1 eopor e Corp .
Location
Dublin, CA
(415) 828-6010
Bedford, MA
(800) 225-3384
Pleasanton, CA
(800) 882-7711
Model
302400
YT30142HW
XX1004700
425910
410400
Size
142 mm
142 mm
47 mm
142 mm
47 mm
'Any device capable of separating the liquid from the solid phase of
the soil is suitable, providing that it is chemically compatible with
the soil and the constituents to be analyzed. Plastic devices (not
listed above) may be used when only inorganic contaminants are of con-
cern. The 142 mm size filter holder is recommended.
TABLE 5. -- SUITABLE FILTER MEDIA
Company
Millipore Corp.
Nuc 1 eopor e Corp .
Whatman Laboratory
Products, Inc.
Location
Bedford, MA
(800) 225-3384
Pleasanton, CA
(415) 463-2530
Clifton, NJ
(201) 773-5800
Model
AP40
211625
GFF
Size!
0.7
0.7
0.7
'Nominal pore size
1312-12
Revision 0
December 1988
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Figure 1. Rotary Agitation
Motor
(30 * 2 rpra)
Extraction Vess«l Holder
1312-13
Revision O
December 1988
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Figure 2. Zero-Headspace Extraction Vessel
liquid inlet/outlet valve
f
filter
waste and
extraction
fluid
piston
top flange
-V body
VITON 0-rings
> bottom flange
pressurizing gas inlet/outlet valve
1312-14
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December 1988
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