United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
EPA/530/SW-91/054
June 1991
Technical Resource
Document
Design, Construction, and
Operation of Hazardous and
Non-Hazardous Waste
Surface Impoundments
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EPA/530/SW-91/054
June 1991
TECHNICAL RESOURCE DOCUMENT
DESIGN, CONSTRUCTION, AND OPERATION
OP
HAZARDOUS AND NON-HAZARDOUS WASTE
SURFACE IMPOUNDMENTS
Robert P. Hartley
Cincinnati, OH 45230
EPA Purchase Order No. 1C6081 NATX
Project Officer
Robert E. Landreth
Waste Minimization, Destruction, and
Disposal Research Division
Risk Reduction Engineering Laboratory
Cincinnati,. OH 45268
RISK REDUCTION ENGINEERING LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
Printed on Recycled Paper
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DISCLAIMER
The information in this document has been funded wholly or
in part by the United States Environmental Protection Agency
under Purchase Order No. 1C6081 NATX to Robert P. Hartley. It
has been subjected to the Agency's peer and administrative
review, and it has been approved for publication as an EPA
document. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
11
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FOREWORD
Today's rapidly developing and changing technologies and
industrial products and practices frequently carry with them the
increased generation of solid and hazardous wastes. These
materials, if improperly dealt with, can threaten both public
health and the environment. Abandoned waste sites and accidental
releases of toxic and hazardous substances to the environment
also have important environmental and public health implications.
The Risk Reduction Engineering Laboratory assists in providing an
authoritative and defensible engineering basis for assessing and
solving these problems. Its products support the policies,
programs and regulations of the Environmental Protection Agency,
the permitting and other responsibilities of State and local
governments, and the needs of both large and small businesses in
handling their wastes responsibly and economically.
This report is a Technical Resource Document, summarizing
the state-of-the-art in the design, construction and operation of
hazardous waste and non-hazardous waste surface impoundments.
Details are generally left to referenced materials. Some of the
information, presented in more detail, has not been previously
published. Most; of the information has been gathered in the
course of hazardous waste research, in accord with past
regulatory emphasis. However, it is believed that most of the
technical information will also be applicable to non-hazardous
waste surface impoundments.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
111
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PREFACE
Subtitle C of the Resource Conservation and Recovery Act
(RCRA) requires the U. S. Environmental Protection Agency (EPA)
to establish a Federal hazardous waste management program. The
program must ensure that hazardous wastes are handled safely from
generation until final disposal. EPA issued a series of
hazardous waste regulations under Subtitle C of RCRA that are
published in Title 40 Code of Federal Regulations (CFR) Parts 260
through 265 and Parts 122 through 124.
Parts 264 and 265 of 40 CFR contain standards applicable to
owners/operators of all facilities that treat, store, or dispose
of hazardous wastes. Wastes are identified or listed as
hazardous under 40 CFR Part 261. Part 264 standards are
implemented through permits issued by authorized States or EPA
according to 40 CFR Part 122 and Part 124 regulations. Land
treatment, storage, and disposal (LTSD) regulations in 40 CFR
Part 264 issued on July 26, 1982, and July 15, 1985, establish
performance standards for hazardous waste landfills, surface
impoundments, land treatment units, and waste piles. Part 265
standards impose minimum technology requirements on the
owners/operators of certain landfills and surface impoundments.
EPA is developing three types of documents to assist
preparers and reviewers of permit applications for hazardous
waste land disposal facilities. These are RCRA Technical
Guidance Documents (TGDs), Permit Guidance Manuals (PGMs), and
Technical Resource Documents (TRDs). Although emphasis is given
to hazardous waste facilities, the information presented in these
documents may be used for designing, constructing, and operating
non-hazardous waste LTSD facilities as well.
The RCRA TGDs present design, construction, and operating
specifications or evaluation techniques that generally comply
with or demonstrate compliance with the design and operating
requirements and the closure and post-closure requirements of
Part 264. The Permit Guidance Manuals are being developed to
describe the permit application information the Agency seeks and
to provide guidance to applicants and permit writers in
addressing information requirements. These manuals will include
a discussion of each step in the permitting process and a
description of each set of specifications that must be considered
for inclusion in the permit.
IV
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The TGDs and PGMs present guidance, not regulations. They
do not supersede the regulations promulgated under RCRA and
published in the CFR. Instead, they provide recommendations,
interpretations, suggestions, and references to additional
information that may be used to help interpret the requirements
of the regulations. The recommendations of methods, procedures,
techniques, or specifications in these manuals and documents is
not intended to suggest that other alternatives might not satisfy
regulatory requirements.
The TRDs present summaries of state-of-the-art technologies
and evaluation techniques determined by the Agency to constitute
good engineering designs, practices, and procedures. They
support the RCRA TGDs and PGMs in certain areeas by describing
current technologies and methods for designing hazardous waste
facilities or for evaluating the performance of a facility
design. Whereas the RCRA TGDs and PGMs are directly related to
the regulations, the information in the TRDs covers a broader
perspective and should not be used to interpret the requirements
of the regulations.
This document is a Technical Resource Document. It reflects
the considerable research that has been performed in the area of
waste containment and the experience that has been gained in this
technology. It incorporates responses to many comments received
in the peer review of the draft document.
Comments on this publication will be accepted at any time.
The Agency intends to update these TRDs periodically based on
comments received and/or the development of new information.
Comments on any of the current TRDs should be addressed to Docket
Clerk, Room S-269(c), Office of Solid Waste and Emergency
Response (WH-562), U. S. Environmental Protection Agency, 401 M
Street, S.W., Washington, D.C., 20460. Communications should
identify the document by title and report number.
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ABSTRACT
This Technical Resource Document provides current
information on the design, construction, and operation of surface
impoundments used for the treatment, storage, or disposal of
hazardous and non-hazardous wastes. The pertinent regulations
under the Resource Conservation and Recovery Act (RCRA) are
summarized. Surface impoundment structures that will meet the
regulatory requirements are described. RCRA's "minimum
technology requirements" specify double-lined structures with a
leak collection and removal layer between the two liners. Dikes,
liners, and leak collection layers may be constructed of
combinations of soils and synthetic materials in multilayer
systems. Other components, also described in the document,
include leak detection systems, secondary containment, and liquid
level monitoring systems. Methods for closing surface
impoundments, either in-place or by waste removal, are described.
In-place closure requires waste treatment and stabilization and
the installation of a landfill cover that also must meet minimum
technology regulatory requirements. Cover technology is also
summarized. Details of the technologies summarized in this
document may be found in the many references cited.
This report was submitted in fulfillment of EPA Purchase
Order No. 1C6081 NATX by Robert P. Hartley under sponsorship of
the U. S. Environmental Protection Agency. This report covers a
period from October 1990 to February 1991, and work was completed
as of February 15, 1991.
VI
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TABLE OF CONTENTS
SECTION
PAGE
DISCLAIMER ii
FOREWORD iii
PREFACE iv
ABSTRACT vi
LIST OF FIGURES xiii
LIST OF TABLES xvi
ACKNOWLEDGMENTS xvii
CHAPTER 1. INTRODUCTION 1
1.1 PURPOSE AND SCOPE 1
1.2 BACKGROUND INFORMATION AND PREVIOUS STUDIES 1
1.3 SURFACE IMPOUNDMENT REGULATIONS 4
1.3.1 Design and Operating Requirements 4
1.3.2 Monitoring and Inspection Requirements.... 6
1.3.3 Emergency Repairs and Contingency Plans ... 6
1.3.4 Response Action Plans 7
1.3.5 Closure and Post-closure Care 8
CHAPTER 2. PRE-DESIGN CONSIDERATIONS 9
2.1 TOPOGRAPHY 9
2.2 SURFACE AND SUBSURFACE HYDROLOGY 9
2.3 GEOLOGY AND SUBSURFACE SOIL CONDITIONS 11
2.4 LAND USE 14
2.5 CLIMATE 16
2.5.1 Flooding 16
2.5.2 Precipitation vs. Evaporation 18
2.5.3 Soil Freezing and Thawing 19
2.6 AIR QUALITY 19
CHAPTER 3. DESIGN 21
3.1 SELECTION OF BASIC CONFIGURATION 21
3.1.1 Impoundment Type 21
3.1.2 Number, Size, and Position 21
3.1.3 Impoundment Surface Area. 24
3.1.4 Impoundment Depth 25
3.1.4.1 Normal Operating Level 26
3.1.4.2 Maximum Operating Level without
External Runoff Input 26
VII
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TABLE OF CONTENTS (continued)
SECTION PAGE
Water Budget Approach 27
Design Storm Approach 30
3.1.4.3 Maximum Operating Level with
External Watershed 31
3.1.4.4 Freeboard Determination 32
3.2 STRUCTURAL COMPONENT DESIGN 33
3.2.1 Foundation Analysis 34
3.2.1.1 Settlement 34
3.2.1.2 Bearing Capacity 35
3.2.2 Dike Design 35
3.2.2.1 Shear Strength 37
3.2.2.2 Slope Stability Analysis 39
Minimum Factory of Safety 42
3.2.3 Liner Systems 44
3.2.3.1 Regulatory Constraints and
Guidance 44
3.2.3.2 Geomembrane/Composite Double Liner .46
3.2.3.3 Geomembrane/Compacted Soil
Double Liner 47
3.2.3.4 Double Composite Liner 48
3.2.3.5 Multiple-Layer Liner
Materials and Specifications . . . .49
Geomembranes 49
Geomembrane Protective
Layers 52
Low-Permeability Soil
Liners 54
Leak Detection and
Collection Systems 57
Geotextile Filter 59
Gas-Venting Layer 60
3.3 LIQUID LEVEL CONTROL 60
3.4 SECONDARY CONTAINMENT 65
3.5 LEAK DETECTION SYSTEMS 65
3.6 SURFACE WATER MANAGEMENT 68
3.7 CONTROLS FOR VOLATILE ORGANIC COMPOUND EMISSIONS . .71
3.8 CONSTRUCTION QUALITY ASSURANCE (CQA) PLAN 73
viii
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TABLE OF CONTENTS (continued)
SECTION PAGE
CHAPTER 4. CONSTRUCTION 74
4.1 SITE PREPARATION 74
4.2 CUT-SLOPE AND FOUNDATION CONSTRUCTION 74
4.2.1 Cut-slopes 75
4.2.2 Dike Foundation 76
4.3 DIKE AND SOIL LINER CONSTRUCTION 77
4.3.1 General Construction Process 77
4.3.2 Pre-placement Soil Preparation 79
4.3.3 Soil Material Placement 80
4.3.4 Soil Compaction 80
4.4 GEOMEMBRANE LINER INSTALLATION 82
4.4.1 Storage of Materials and Equipment 83
4.4.2 Construction Quality Assurance/Inspection . .83
4.4.3 Subgrade Preparation 83
4.4.4 Geomembrane Liner Placement 84
4.4.5 Sealing Around Structures and Anchoring
the Geomembrane 86
4.5 LEAK COLLECTION AND REMOVAL SYSTEMS 88
4.6 TESTING THE LINER SYSTEM 90
4.7 PROTECTIVE COVERINGS 91
4.7.1 Liner Protection 91
4.7.2 Dike Protection 91
4.8 LIQUID LEVEL CONTROL SYSTEMS 92
4.8.1 Active Liquid Level Control 92
4.8.2 Passive Liquid Level Control 93
4.9 SECONDARY CONTAINMENT 93
CHAPTER 5. OPERATION, MAINTENANCE, AND MONITORING 94
5.1 OPERATION AND MAINTENANCE ACTIVITIES 94
5.1.1 Facility Start-up 94
5.1.2 Routine Inspections and Maintenance 95
5.1.2.1 Regulatory Inspections of
Facility 95
ix
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TABLE OF CONTENTS (continued)
SECTION PAGE
5.1.2.2 Operator Inspections of Dike
Slopes, Faces, and Crest 97
5.1.2.3 Operator Inspections of Ancillary
Site Facilities 97
5.1.2.4 Liner Systems 99
Detecting and Measuring
Liner Leakage 99
Determining the Cause of
Liner Leakage 102
Liner Repair 102
Solids and Liquid Removal. . . . 103
5.1.3 Record-keeping 104
5.2 SAMPLING AND ANALYSIS MONITORING ACTIVITIES. . . . 104
5.2.1 Hazardous Waste Monitoring 104
5.2.2 Air Monitoring 107
5.2.2.1 Estimating Emissions from
Surface Impoundments 107
5.2.2.2 Air Sampling and Analyses 109
5.2.3 Ground-water Monitoring 110
5.2.4 Soil-vapor Monitoring 112
5.2.5 Leak Collection and Removal System
Monitoring 113
CHAPTER 6. CONTINGENCY PLANNING 115
6.1 LIQUID-LOSS RESPONSE PLANS 115
6.1.1 Contingency Plan 115
6.1.2 Response Action Plan 116
6.1.3 Corrective Action Program 118
6.2 TYPES OF FAILURE 119
6.3 RESPONSE PLAN IMPLEMENTATION 120
6.3.1 Contingency Plan Implementation 120
6.3.1.1 Immediate Actions 121
6.3.1.2 Contamination Assessment 122
6.3.1.3 Selection and Implementation of
Remedial Actions 123
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TABLE OF CONTENTS (continued)
SECTION PAGE
6.3.1.4 Cleanup Verification 124
6.3.2 Implementation of Response Action Plan. . . 125
6.3.2.1 Leak Correction Verification . . . 127
6.3.3 Implementation of Corrective Action
Program 127
6.3.3.1 Ground-water Cleanup
Verification 128
6.4 PERSONAL SAFETY DURING REMEDIAL OPERATIONS .... 128
CHAPTER 7. CLOSURE AND POST-CLOSURE CARE 130
7.1 ASSESSMENT OF CLOSURE OPTIONS 132
7.1.1 Waste Characteristics 134
7.1.2 Site Location Features 134
7.1.3 Cost 134
7.1.4 Intended Future Site Use 135
7.1.5 Environmental Risk 135
7.2 CLEAN CLOSURE (CLOSURE BY REMOVAL) 135
7.2.1 Free Liquids 136
7.2.2 Residual Sludges 137
7.2.3 Subsoils, Liners, and Other Contaminated
Materials 137
7.2.4 Verification Sampling 137
7.2.4.1 Sampling Schemes 138
7.2.4.2 Indicator Parameters 139
7.2.4.3 Quality Assurance/Quality Control. 139
7.2.5 Regulatory Variance 140
7.2.6 Backfilling 141
7.3 IN-PLACE CLOSURE 141
7.3.1 Removal of Free Liquids 142
7.3.2 Sludge Dewatering 142
7.3.3 Waste Residuals 143
7.3.3.1 Stabilization 143
7.3.3.2 Treatment of Residues 145
xi
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TABLE OF CONTENTS (concluded)
SECTION PAGE
Extraction 146
Immobilization 146
Biodegradation 148
Chemical Degradation 148
7.3.4 Final Cover System 149
7.3.4.1 Protective Surface Layer 151
Vegetation 151
Topsoil Layer 151
7.3.4.2 Drainage Layer 152
7.3.4.3 Biotic Barrier 153
7.3.4.4 Hydraulic Barrier Layer 153
7.3.4.5 Gas-Vent Layer 154
7.3.4.6 Hydraulic Barrier Support Layer. . 155
7.4 POST-CLOSURE ACTIVITIES 155
7.4.1 Monitoring 157
7.4.2 Maintenance 157
7.4.3 Use of the Site 158
7.4.4 Delisting 158
REFERENCES 161
XII
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LIST OF FIGURES
FIG. NO. PAGE
1. Area of net evaporation in the United States 19
2. Maximum anticipated depths of freezing 20
3. Components of maximum operating level for (a) treatment,
(b) surge, and (3) evaporation/disposal 22
4. Example of one large vs. four small impoundments 22
5. Components making up impoundment design depth 25
6. Example of frequency distribution of monthly
climatic data 29
7. Percent confidence that design return period will
not be exceeded during design life 30
8. Example of the effect of differential foundation
compressibility on a surface impoundment dike 34
9. Forces and displacements in bearing capacity analysis . .35
10. Surface impoundment dike and liner interfaces
and layers 36
11. Mohr-Coulomb failure envelopes for clays and sands. . . .38
12. Typical compaction curves showing (a) dry unit
weight - water content relation and (b) variation
of shear strength with water content for a
cohesive soil 39
13. Types of slope instability 40
14. Factor-of-safety contours for slope stability 43
15. Cross section of double liner with composite
bottom liner 45
16. Cross section of double liner with soil-only
bottom liner 45
17. Cross section of double-composite bottom liner 45
18. Impoundment dike cross section showing optional
protective layers 53
Xlll
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LIST OF FIGURES (continued)
FIG. NO. PAGE
19. Junction of side-wall geonet and bottom granular
layers of leak detection, collection, and removal
system 58
20. Example access for leak collection system and
liquid removal 59
21. Liner system showing gas vent layer and exit
through top of dike 61
22. Dike spillway with protective apron 62
23. Impoundment overflow discharge pipe through dike 62
24. Example of liquid-level monitoring setup; manually
read staff gauge 63
25. Example of liquid-level recording and alarm system. . . .63
26. Primary and secondary containment dikes 65
27. System to detect and locate leaks in top
(primary) liner 66
28. System to detect leakage through top (primary) liner. . .67
29. System to detect leakage to the vadose zone 67
30. Lysimeter for leak detection beneath bottom liner . . . .68
31. Runoff diversion past surface impoundment 69
32. Typical diversion ditch and berm cross sections 69
33. Wind diversion fences for VOC control 72
34. Cut-slope, dike, and side-wall cross section 75
35. Idealized schematic showing effects of slope
on compactive effort 76
36. Dike cross section showing lifts and final slope cuts . .78
37. Compactor foot designs 82
38. Geomembrane liner panel layout 85
39. Geomembrane anchor designs at top of dike 87
xiv
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LIST OF FIGURES (concluded)
FIG. NO. PAGE
40. Seals at geomembrane penetrations 88
41. Example of leak collection system layout 90
42. Example of geotextile use in leak collection
system drain layer 90
43. Cutaway view of emission sampling apparatus 109
44. Example layout of ground-water monitoring wells .... Ill
45. Schematic of soil-gas sampling probe 113
46. Flow chart of closure options and requirements 131
47. USEPA-recommended landfill cover design 150
48. USEPA-recommended landfill cover design with
optional layers 152
xv
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LIST OF TABLES
TABLE NO. PAGE
1. Distribution of Surface Impoundment Applications 2
2. Geotechnical Soil Properties Used to Characterize
Surface Impoundment Site Soils 14
3. Geotechnical Soil Properties Used to Characterize
Borrow Material Sources 15
4. Sources of Climatic Data Used in Surface Impoundment
Design and Analysis 17
5. Cost Comparisons of Different Surface Impoundment
Positions with Respect to Grade
(Geomembrane/Composite Liner) 24
6. Current Procedures for Stability Analysis 41
7. Typical Optimum Soil Liner Design Specifications 56
8. Average Leak Rates (mVyr) from Different Size
and Shape Flows in 0.08-cm HOPE Liner over Gravel
at Two Liquid Heads 101
9. Calculated Leak Rate (mVyr) for a Range of Hole
Sizes in Geomembrane Liners over Soils of
Different Conductivities and for Three Heads 101
10. Recommended Air Emission Models for Hazardous
Waste Disposal Facilities 108
11. Outline of Contingency Plan Response Data Sheet .... 117
12. Innovative Investigation Technologies to Assess
Site Contamination 123
13. Advantages and Disadvantages of Closure Options .... 133
14. Test Procedures for Stabilized Wastes 145
15. Compatibility of Surface Impoundment Features and
Various Site Uses for In-place Closure 159
16. Compatibility of Surface Impoundment Features and
Various Site Uses after Hazardous Waste Removal i . . . 160
xvi
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ACKNOWLEDGMENTS
This document was prepared in draft form by PEI Associates,
Cincinnati, OH, K. W. Brown and Associates, College Station, TX,
and E. C. Jordan Co., Portland, ME. After updating and peer
review, it was rewritten and prepared in final form under EPA
Purchase Order No. 1C6081 NATX with Robert P. Hartley. The
document was prepared under the supervision of Robert E. Landreth
of the Risk Reduction Engineering Laboratory. Dr. Robert M.
Koerner, Dr. Gregory N. Richardson, Dr. David E. Daniel, and Dirk
Brunner were especially helpful in the final review of the
document.
xvii
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CHAPTER 1
INTRODUCTION
A surface impoundment is an excavation or diked area
typically used for the treatment, storage, or disposal of liquids
(e.g., wastewater) or materials containing free liquids (e.g.,
sludges). The hydraulic barriers in surface impoundments are
usually constructed of low-permeability soil or polymeric
membranes or both.
Liquids and solids typically separate in a surface
impoundment by gravity settling. Liquids can be removed by
draining, evaporation, or flow from an outlet structure.
Accumulated solids may be removed by dredging during impoundment
operation or when it is closed. Alternatively, solids may be
left in place, as a landfill, when the surface impoundment is
closed.
1.1 PURPOSE AND SCOPE
This document summarizes and supplements existing sources of
information on the state-of-the-art in the design, construction,
operation, and closure of surface impoundments used for waste
containment. In addition, relevant regulations are briefly
summarized, and post-closure activities are discussed. The
document reflects the fact that most of the available
information, ongoing research, and pertinent regulations deal
with surface impoundments used to contain hazardous waste.
However, much of the technical information should be applicable
to non-hazardous waste impoundments, which comprise the majority
of waste impoundment sites.
1.2 BACKGROUND INFORMATION AND PREVIOUS STUDIES
Two major baseline studies by the U.S. Environmental
Protection Agency (USEPA) have gathered background information on
surface impoundments. The first study (USEPA, 1983a) was a
national survey to determine the number, size, and location of
hazardous and non-hazardous waste surface impoundments and their
potential influence on ground-water quality. The survey
characterized over 180,000 surface impoundments. Nearly 30,000
are used by industry, including chemical manufacturers, food
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processors, oil refineries, primary and fabricated metals
manufacturers, paper plants, and commercial waste facilities.
Most of the remainder are used in sewage and wastewater
treatment. Organic chemical manufacturers are the largest
industrial contributors to impounded hazardous waste. Most
surface impoundments are not used for waste disposal but rather
for waste treatment processes (i.e., neutralization, settling,
anaerobic or aerobic digestion, pH adjustment, and polishing).
The industrial surface impoundments ranged in size from less than
0.1 acre (29 percent) to greater than 100 acres (1 percent), with
the majority less than 5 acres. (One acre = 0.405 hectare.)
The EPA national survey categorized surface impoundment
applications into five groups with the percentages in each group
used for storage, disposal, or treatment. The results are shown
in Table 1. Note that the majority of agricultural surface
impoundments were used for waste storage, the majority of oil and
gas surface impoundments for disposal, and the majority of
municipal, industrial, and mining impoundments were used for
treatment.
TABLE 1. DISTRIBUTION OF SURFACE IMPOUNDMENT APPLICATIONS.*
Storage Disposal Treatment
Agricultural
Municipal
Industrial
Mining
Oil & Gas
55
5
17
18
29
26
31
31
27
67
19
64
52
56
4
— TTT-TT: — rnrsa:-"— ••-'—' arsrari
*EPA (1983a).
The second major study (USEPA, 1984a), reviewed the design
of surface impoundments and operating and maintenance practices,
based on nine case studies and interviews with technical experts,
Topics included site selection, liner material selection and
performance, construction quality assurance (CQA) programs,
operation and maintenance programs, and leak detection systems.
USEPA concluded that, in many cases, inappropriate design
criteria, CQA programs, and operation and maintenance procedures
have resulted in release of impounded waste constituents to
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ground water and/or surface water. The study identified areas in
need of further research and development, including evaluation of
new design concepts, compatibility of wastes and liner materials,
and monitoring systems.
USEPA (1982a) has identified typical industrial surface
impoundment applications as follows:
1. Mining and Milling Operations — Surface impoundments
are used to contain various wastewaters such as acid mine water,
solvent wastes from solution mining, and wastes from dump
leaching. Surface impoundments may also be used for separation
settling, washing, sorting of mineral products from tailings, and
recovery of valuable minerals by precipitation.
2. Oil and Gas Industry — one of the largest users of
surface impoundments. Surface impoundments may contain salt
water associated with oil extraction and deep-well repressurizing
operations, oil-water and gas-fluids to be separated or stored
during emergency conditions, and drill cuttings and drilling
muds.
3. Textile and Leather Industries -- Surface impoundments
are primarily used for wastewater treatment and sludge disposal.
Organic species impounded include dye carriers such as
halogenated hydrocarbons and phenols; heavy metals impounded
include chromium, zinc, and copper. Tanning and finishing wastes
may contain sulfides and nitrogenous compounds.
4. Chemical and Allied Products Industries — Surface
impoundments are used for wastewater treatment, sludge disposal,
and residuals treatment and storage. Waste constituents are
process-specific, including phosphates, fluoride, nitrogen, and
assorted trace metals.
5. Other Industries — Surface impoundments are found at
petroleum-refining, primary metals production, wood-treating, and
metal-finishing facilities. Surface impoundments are also used
for the containment and/or treatment of air pollution scrubber
sludge and dredging spoils sludge.
Waste impoundments may contain complex mixtures of
materials, often aggressive to lining materials, capable of rapid
migration to ground water, and producing harmful emissions to the
surrounding air. The design engineer must understand and
minimize these potentials.
Much of the information needed in the design and
construction of surface impoundments can be found in Technical
Resource Documents published by the USEPA that deal with
structural components common to both landfills and surface
impoundments. The more significant of these include "Lining of
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Waste Containment and Other Impoundment Facilities" (USEPA,
1988a), "Design, Construction, and Evaluation of Clay Liners for
Waste Management Facilities" (USEPA, 1988b), and "Technical
Guidance Document: Construction Quality Assurance for Hazardous
Waste Land Disposal Facilities" (USEPA, 1986a). The USEPA1s
Office of Solid Waste has also issued technical guidance relevant
to hazardous waste surface impoundments. Included are "Draft
Minimum Technology Guidance on Double Liner Systems for Landfills
and Surface Impoundments — Design, Construction, and Operation"
(USEPA, 1987a) and "Technical Guidance Document: Final Covers on
Hazardous Waste Landfills and Surface Impoundments" (USEPA,
1989). The cited documents are supported by a significant amount
of research and field experience, most of which is referenced in
this document. Some of the later information has not yet been
published, but has been cited here for completeness.
1.3 SURFACE IMPOUNDMENT REGULATIONS
The design, construction, operation, and closure of
hazardous waste surface impoundments are regulated under
authority of the 1984 Hazardous and Solid Waste Amendments (HSWA)
to the Resource Conservation and Recovery Act of 1976 (RCRA).
The most pertinent regulations are contained in 40 CFR 264,
Subpart K, dealing specifically with new hazardous waste surface
impoundments. They are summarized below.
1.3.1 Design and Operating Requirements
The rules for design and operation of liner systems for
hazardous waste surface impoundments, provided in 40 CFR 264.221,
are summarized as follows:
(1) A new or expanded surface impoundment must have a liner
system that prevents migration of wastes to the adjacent
subsurface soil, ground water, or surface water at any time
during the active life of the surface impoundment. The
liner system must be:
• constructed of materials with appropriate chemical
properties and sufficient strength and thickness to
prevent failure;
• placed on a foundation or base capable of supporting the
liner and resisting pressure gradients above and below
the liner to prevent failure; and
• installed to cover all surrounding earth likely to be in
contact with the waste or leachate.
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(2) New surface impoundments, and replacement or lateral
expansion of existing surface impoundments, must have two or
more liners and a leachate collection system between the
liners. The liner and leachate collection system must
include the following:
• a top (primary) liner that will prevent migration of any
chemical constituent into such liner during the active
life and post-closure care period;
• a lower (secondary) liner, that will prevent the
migration of any chemical constituent through such liner
during the active life and post-closure care period,
(this requirement may be satisfied by a layer at least 3
feet [0.91 meter]* thick of recompacted clay or other
natural material with a permeability equal to or less
than 3.9 x 10'8 in./sec [1 x 10'7 cm/sec]); and
• a leachate collection system that must detect and remove
liquids that leak through the primary liner (without
clogging) during the active life and post-closure care
period.
(3) Double-liner requirements may be waived by the Regional
Administrator for any monofill (i.e., surface impoundment
containing one type of waste) if the following requirements
are met:
• the wastes are from foundry furnace emission controls or
metal-casting molding sand, and contain no constituents
that would render them hazardous for reasons other than
EP Toxicity characteristics;
• the monofill has at least one liner for which there is no
evidence of leakage;
• the monofill is located more than 0.25 miles (0.4
kilometers) from an underground source of drinking water;
• the monofill is in compliance with generally applicable
ground-water monitoring requirements for RCRA-permitted
facilities; and
* Throughout this document, English units of measurement are
used, followed by metric equivalents in parentheses.
English units are primary because nearly all cited work was
originally done using English units and because most
engineers still use the English system, except in referring
to hydraulic conductivity.
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• the monofill is located, designed, and operated so as to
assure that there will be no migration of hazardous
constituents into ground water or surface water at any
future time.
(4) Surface impoundments must prevent overtopping resulting from
normal or abnormal operations; overfilling; wind and wave
action; rainfall; runon; malfunctions of level controllers,
alarms, and other equipment; and human error.
(5) Surface impoundments must have dikes with sufficient
structural integrity to prevent massive failure; dike
stability should be analyzed with the presumption that
liners will leak during the active life of the unit.
(6) The Regional Administrator will specify in the permit, the
design and operating practices that are necessary to ensure
compliance with the requirements of this section.
1.3.2 Monitoring and Inspection Requirements
The rules for monitoring and inspecting surface
impoundments, provided in 40 CFR 264.226, are summarized as
follows:
• During construction and installation, liners and cover
systems (including synthetic, soil-based, and admixed
systems) must be inspected for uniformity, damage, and
imperfections. (Detailed construction quality assurance
requirements have been proposed in 40 CFR 264.19 and
264.20 covering most structural features of a surface
impoundment [USEPA, 1987i]).
• During operation, surface impoundments must be inspected
weekly and after storms for evidence of overtopping,
sudden drops in liquid level, and deterioration of dikes
or other containment devices.
• Before a permit is issued or after an extended period of
downtime, a qualified engineer must certify the
structural integrity of the surface impoundment dike.
(Note: it is assumed that a "qualified engineer" is a
registered professional engineer.)
1.3.3 Emergency Repairs and Contingency Plans
The rules pertaining to emergency repairs and contingency
plans for hazardous waste surface impoundments, provided in 40
CFR 264.227, are summarized as follows:
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• Surface impoundments must be removed from service when
the liquid level suddenly drops for reasons other than
changes in the inflow or outflow (e.g., when the dike or
liner(s) leak).
• To remove a surface impoundment from service, the
owner/operator must cease adding wastes to it; contain
surface leakage and stop the leak; stop or prevent
catastrophic failure; empty the impoundment if the leak
cannot be stopped; and notify the Regional Administrator
within seven days of detecting the problem.
• The owner/operator must provide a contingency plan, as
required in 40 CFR 264, Subpart D, that specifies
procedures for complying with the requirements for
removing a surface impoundment from service.
• Surface impoundments removed from service in accordance
with these requirements may be restored to service only
if the failed portion is repaired and the following steps
are taken, if appropriate: the structural integrity of
the dike is recertified; a new liner is installed; or the
repaired liner system is certified by a qualified
engineer to comply with the permitted design
specifications.
• Surface impoundments removed from service and not
repaired must be closed in accordance with the closure
and post-closure care provisions of 40 CFR 264.228.
1.3.4 Response Action Plans
USEPA has proposed rules in 40 CFR 264.222 and 265.222
detailing the requirements for a response action plan to be
implemented in case of leakage from a hazardous waste impoundment
into the leak collection system. The proposal is summarized as
follows:
• Two action leakage levels are proposed: (1) an "action
leakage rate," (ALR) of between 5 and 20 gallons/acre/
day (47 and 187 liters/hectare/day), which requires
Agency notification and correction, but does not
constitute an emergency; and (2) "rapid and extremely
large leakage rate" (RLL) which must be determined for
each site, and is the rate that exists when the fluid
head is greater than the thickness of the leak collection
layer.
• Response to a RLL must be immediate and major, because of
the increased risk of escape of contaminants in a swamped
system. Response to an ALR may be less immediate,
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because the leak collection system can still handle the
leak rate.
• The Response Action Plan for a RLL must accompany the
permit application for a new surface impoundment. It may
be submitted for ALR at a later time, but not later than
the leak event.
USEPA (1987i) in 40 CFR 264.301 and 265.301 has also
proposed that leak collection systems be capable of detecting a
leak in the top liner as low as one gallon/acre/day (9.35
liters/hectare/day).
1.3.5 Closure and Post-closure Care
Rules for closure and post-closure care of hazardous waste
surface impoundments are provided in 40 CFR 264.228. There are
currently two closure options available:
• clean closure, which may be accomplished by removing or
decontaminating all waste residues, contaminated
containment system components, contaminated subsoils, and
contaminated structures and equipment; or
• in-place closure (landfill or disposal unit closure),
which may be accomplished by dewatering and stabilizing
the wastes and placing an impermeable cover over the
surface impoundment to minimize infiltration.
If clean closure is accomplished, no post-closure care is
required. If in-place closure is performed, some waste residues
or contaminated materials are left in place at final closure
because it is technically or economically impractical to remove
all waste materials; a 30-year (or other appropriate period)
post-closure program is required. This program includes
monitoring and maintaining the cover; monitoring the ground water
and maintaining the ground-water monitoring system; monitoring,
maintaining, collecting, and removing liquids in the leachate
collection system; and preventing runon and runoff from damaging
cover integrity.
Also included in 40 CFR 264.228 are rules that stipulate if
clean closure of an surface impoundment is intended without
satisfaction of the liner requirements of 40 CFR 264.221(a), then
the closure plan must include both a plan for clean closure and a
contingent plan for in-place closure, in case all contaminated
subsoils cannot be practicably removed at closure.
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CHAPTER 2
PRE-DESIGN CONSIDERATIONS
Factors affecting impoundment design include topography,
geology, surface and subsurface hydrology, soil conditions, land
use, and climate. Some environments should simply be avoided
without further consideration of other factors. These
environments include populated areas, flood-prone areas, karst
terrain, unstable soils, wetlands, and aquifer recharge areas.
2.1 TOPOGRAPHY
Topographically,' an ideal surface impoundment site is one
that requires minimal physical modification to accommodate
construction. This generally means an area of low relief.
Wherever possible, the site should be above the 100-year flood
elevation to minimize the height of dikes and the threat of
flooding and washout.
A detailed topographic map is fundamental to site selection
and impoundment design. The topographic survey can be conducted
by aerial or field surveying methods; however, all sites will
require some field surveying. Although published topographic
maps may not provide sufficient detail for design, they may be
used as base maps for the topographic survey and as sources of
information on land use of the surrounding areas.
2.2 SURFACE AND SUBSURFACE HYDROLOGY
The proximity of surface and subsurface water is directly
related to the risk of contamination from surface impoundment
releases. High water tables, in addition to being high risk, may
interfere with surface impoundment construction if they are above
the bottom of the impoundment. The subsurface risk is higher in
porous materials and less in dense clays. Desirable subsurface
conditions include great depth to the uppermost aquifer and a
massive clay soil zone enclosing the impoundment.
The direction of ground-water movement must be known, since
downstream property is at risk if a release should occur. The
flow velocity is also important as it may be directly related to
the time available for remediation before significant
environmental damage.
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Information to be obtained during the hydrogeologic
investigation includes the following:
presence of a perched water table;
depths to the uppermost saturated zones;
ground-water flow direction;
ground-water flow rates;
vertical components of ground-water flow;
potentiometric surface attitude;
effects of seasonal and temporal factors on ground-water
flow; and
• aquiclude characteristics.
Existing descriptions of the occurrence, location, hydraulic
characteristics, and temporal and spatial behavior of shallow
aquifers and aquicludes are unlikely to be sufficient. Gathering
the necessary hydrogeologic information may require exploratory
borings, installation of piezometers and observation wells, and
aquifer testing (e.g., pumping tests, slug tests, and tracer
studies). In most cases, placement of borings and sampling
efforts can be coordinated with those conducted for the geology
and subsurface soil survey. Interpretation of the data obtained
from this investigation can be presented as potentiometric maps,
geologic cross sections, boring logs, geologic maps, field and
analytical test results, and a narrative description of the
hydrogeologic conditions. This information is useful in the
design of a ground-water monitoring system (see Section 5.2.3).
During the drilling of test borings, the location of
saturated layers should be recorded. Piezometers and/or
monitoring wells should be installed in selected boreholes to
enable the determination of horizontal and vertical ground-water
flow directions and for aquifer pumping tests. Core samples
collected from saturated zones should be analyzed in the
laboratory for particle-size distribution, porosity, and
permeability.
The surface drainage pattern and flows and downstream water
uses also must be known and related to potential releases from a
surface impoundment. The associated risks should be considered
and minimized in the design.
The surface water hydrology investigation should include
determining stream flow characteristics, peak flood stages, and
water quality. If sufficient design-related information is not
available in existing sources, a field investigation should be
conducted. Stream-flow characteristics can be measured with
current meters and weirs or flumes. These devices allow
measurement of stream discharge under weather conditions at the
time of the survey. However, to estimate flow rates and volumes
for 25-, 50-, and 100-year flood events, historical flood and
precipitation data must be used.
10
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The scouring and sedimentation effects of surface drainage
may affect the stability of a surface impoundment. Any evidence
of recent nearby stream meandering should be noted. If
impoundment construction involves rerouting of surface drainage
networks, the potential for scouring and/or sedimentation must be
carefully evaluated.
The surface-water hydrology investigation should also
include collecting samples of runoff and nearby streams, during
periods of high and low flow, for water quality analysis.
Analysis of these samples will be used to provide background data
to determine the effect of potential releases to the surface
water.
2.3 GEOLOGY AND SUBSURFACE SOIL CONDITIONS
The geologic makeup (rock lithology and structure) of a
potential site should be determined in detail, for it can
significantly influence the design of a surface impoundment and
the environmental risk associated with it. Seismically active
(earthquake) and unstable (sliding) areas should be avoided.
Areas of differential subsidence, which may be suggested by
intensive subsurface mining activity, are likely to be
unattractive impoundment locations. Porous rock could allow
rapid migration of any released liquids from an impoundment.
Larger voids, either natural or man-made, would be of similar
concern. Limestone, for example, is often characterized by
solution-caused natural voids and channels. On the other hand,
thick, dense, undisturbed shales may be nearly impermeable, a
desirable characteristic.
The depth to bedrock is important. Fractured, porous rock
near the surface may eliminate a site from consideration, while
it might be acceptable at deeper levels under dense soil
overburden.
The characteristics of the soil underlying the site affect
the strength and long-term integrity of a surface impoundment
structure and the risk associated with impoundment operation. As
noted earlier, great thicknesses of clay and great depths to the
upper aquifer, say 20 feet (6.1 meters) or more beneath the
impoundment in both cases, are ordinarily most desirable. When
such is true, impoundment releases will be naturally slowed and
attenuated, thus adding to the time available for remedial
actions before significant ground-water contamination occurs.
At the other extreme, coarse-grained, highly porous soils
and shallow water tables are generally the least desirable site
conditions for a surface impoundment. Perhaps most often,
subsurface soil conditions will lie between the extremes of dense
clay and highly permeable coarse materials. Layering and lensing
11
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are likely in many, if not most, situations. It is important to
determine the subsurface soil characteristics in three-
dimensional detail, so that the design may correct for
deficiencies and take advantage of soil attributes. Several
types require special design treatment, if present at the surface
impoundment site. These include expansive and sensitive clays,
acid sulfate soils, sodic soils, poorly consolidated soils,
saturated soils, and wind-erosive soils.
On-site or near-site soil materials suitable for dike and
liner construction are desirable. Ordinarily, soils with a high
clay content are sought that can be compacted to a high density
and low permeability. Measurement and interpretation of several
engineering properties generally reveal the soil's adequacy for
construction purposes. The properties include grain size, water
content, plasticity, permeability, bearing capacity, and shear
strength. Since the soils will be used to retain waste liquids,
chemical characteristics of the soils should also be determined.
At a minimum, the measured properties should include pH, cation
exchange capacity, and resistivity.
A comprehensive subsurface soils and rock investigation
should assess the following:
• character, distribution, and thickness of the soil and
the surficial geologic units;
• zones of saturation and ground-water surface elevations;
• pertinent physical and engineering properties of site
soils and their horizontal and vertical variability;
• unstable conditions, including slopes, active or
potentially active faults, seismic activity, and heave
potential;
• ground response to excavation and the effect of stress
increases from the facility loading; and
• suitability of on-site soil materials for construction of
dikes, berms and low-permeability liners.
Several methods of soils and rock investigation are
available. Prior to constructing a surface impoundment that
covers a large area of land or is located in an area with
difficult access, geophysical methods (e.g., seismic refraction
surveys) can be used to obtain general information on subsurface
conditions. Test pits can be used to observe shallow (i.e., zero
to 15 feet [zero to 4.6 meters]) soil and ground-water conditions
and to obtain soil and rock samples for laboratory testing. Test
borings provide a means for assessing subsurface conditions at
deeper depths (i.e., down to 100 feet [30 meters] or more), in-
12
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situ testing, collection of soil samples for laboratory testing,
and installation of ground-water observation wells. Test pits
and borings should be excavated or drilled on a grid pattern
across the site with grid density depending on site uniformity.
Test borings made to assess foundation conditions for
structures, dikes, or fills should extend to a depth where the
anticipated increase in stress from the proposed facilities will
be less than 5 percent of the estimated overburden pressure.
Large-diameter borings are sometimes preferable to smaller ones,
especially when drilling in expansive soils or known earthslide
areas, because it is often possible to locate the slide plane by
examining the recovered soil samples. Thin-walled tube samples
(i.e., "undisturbed" samples) should be taken for laboratory
analysis at selected depths in strata suitable for such sampling.
Soil samples collected from test borings or test pits should
be analyzed to determine engineering properties (e.g., hydraulic
conductivity, compressibility, and shear strength) and to
classify the soils. Shear strength analysis is necessary for
surface impoundment design and is discussed in Subsection
3.2.2.1. Classification of soil should be done in accordance
with the Unified Soil Classification System (USCS) (ASTM D2488-
84). Representative samples from borrow material sources should
be analyzed to determine if they meet design specifications.
Soil test methods to characterize the geotechnical properties of
site soils and potential borrow materials are listed in Tables 2
and 3, respectively. Laboratory test methods are also described
in a number of geotechnical references (Lambe, 1951; U.S. Army
Corps of Engineers [USAGE], 1970a; Head, 1980; 1982; and 1986).
During the pre-design phase, the soils investigation should
enable the designer to make the most economical use of the
available on-site or nearby borrow materials. The following
factors should be considered in selecting the borrow materials:
• expansion of soils of medium to high plasticity when
placed under low applied pressure;
• compaction difficulties of plastic soils with high
natural moisture;
• necessity of extensive mixing and/or separation of
naturally stratified soils to be used for borrow
material;
• design specifications for soils to be used for low-
permeability soil liners, dikes, foundations, etc.; and
• available quantity of usable borrow material.
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TABLE 2. GEOTECHNICAL SOIL PROPERTIES USED TO CHARACTERIZE
SURFACE IMPOUNDMENT SITE SOILS.*
Property
Method
Soil Type
Visual-Manual Procedure
Particle Size Analysis
Atterberg Limits
Soil Classification
Strength
Unconfined Compressive Strength
Triaxial Compression
Moisture Content
Moisture-Density Relations
Standard Proctor
Modified Proctor
Hydraulic Conductivity (lab)
Fixed Wall
Flexible Wall
Hydraulic Conductivity (field)
Large Diameter Single-Ring and
Double-Ring Infiltrometers
Sai-Anderson Infiltrometer
ASTM D 2488
ASTM D 422
ASTM D 4318
ASTM D 2487
Soil Survey Staff (1975)
ASTM D 2166
ASTM D 2850
ASTM D 2216
ASTM D 698
ASTM D 1557
EPA, SW-870 (1983b)
Daniel et al (1985)
EPA, SW-846 (1982b),
Method 9100
Daniel & Trautwein (1986)
Anderson et al (1984)
'Minimum of 3 tests per soil type.
2.4 LAND USE
Hazardous waste surface impoundments are not normally
desirable features of the landscape. They have a high potential
for interfering with other land uses. Land use considerations
can be the most significant controlling factors in siting a new
surface impoundment. For this reason, most new surface
impoundments are located in heavy industry areas or at existing
waste treatment-disposal areas. They are most often constructed
in areas where impoundments already exist. Outside these areas,
remoteness from dense population is not only desirable but
probably mandatory.
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TABLE 3. GEOTECHNICAL SOIL PROPERTIES USED TO CHARACTERIZE
BORROW MATERIAL SOURCES.
Property
Frequency of
Measurement
Method
Soil Type
Visual-Manual Procedure
Particle Size Analysis
Atterberg Limits
Soil Classification
Strength
Unconfined Compressive
Strength
Triaxial Compression
Moisture Content
Moisture-Density Relations
Standard Proctor
Modified Proctor
Hydraulic Conductivity
Laboratory-Undisturbed Cores
Fixed Wall
Flexible Wall
each 1000 yd3
(836 m3)
each 5000 yd3
(4180 m1)
each 1000 yd3
(836 m3)
:h 5000 y
(4180 m5)
each 5000 yd3
each 5000 yd3
(4180 m5)
Volume Change
Consolidometer
COLE
Susceptibility to
Frost Damage
Classification
each 5000 yd3
(4180 m1)
ASTM D 2488
ASTM D 422
ASTM D 4318
ASTM D 2487
Soil Survey Staff
(1975)
ASTM D 2166
ASTM D 2850
ASTM 2216
ASTM D 698
ASTM D 1557
EPA, SW-870
(1983b)
Daniel et al
(1985)
EPA, SW-846
(1982b)
Method 9100
Holz, 1965
Grossman et al
(1968)
Chamberlain (1981)
Future land use may be a consideration at an existing or
proposed hazardous waste surface impoundment. Uses will probably
be restricted, depending upon the degree of contaminant removal
15
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and the character of the waste remaining at the time of closure.
Restrictions are likely to parallel post-closure monitoring
requirements, with intensively monitored sites being the most
restricted. Future use considerations should be integrated with
planned uses of adjacent properties.
2.5 CLIMATE
Climate (e.g., excessive rainfall or low evaporation rate)
may eliminate a particular type of surface impoundment as a
waste-handling option. Otherwise, climate will seldom directly
influence the selection of a surface impoundment site.
Climate plays a direct role in sizing a surface impoundment.
The unit must be large enough to contain the planned waste volume
plus some amount of precipitation. Other climatic factors may
affect the operation of the impoundment. For example, air
circulation (wind), temperature, and radiation affect waste
volatilization rates and dispersion of emissions. These factors
also affect the evaporation rate from the unit and thus the
unit's size and need for periodic pumpout and treatment.
The most extensive, continuous, and reliable climatic data
are available from National Weather Service reporting stations.
If a station is not close enough to the planned impoundment,
there may be other data sources, as shown in Table 4.
Factors directly related to climate that may influence site
selection are discussed in the following subsections.
2.5.1 Flooding
The potential for flash floods and large-scale floods should
be considered during the design of a surface impoundment. A
flash flood results from localized, very heavy rainfall in a
short time acting on a relatively small watershed. In this case,
it would be the watershed containing the surface impoundment.
Flash flooding occurs very quickly after, or perhaps even during,
the precipitation event, and the flow may be high-velocity. It
can occur in watersheds not subject to large-scale, longer-term
flooding. Predicting peak flow from a flash flood is necessary
for the design of runon control and diversion structures.
Hydrologic analysis, using methods such as those developed by the
Soil Conservation Service (SCS) (Kent, 1973), can be used to
predict the maximum discharge and flood levels.
Large-scale flooding is the result of precipitation, often
combined with snowmelt, acting on a regional watershed. Lag
times are greater and flood duration longer than for flash
floods. The events causing large-scale flooding usually occur
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TABLE 4. SOURCES OF CLIMATIC DATA USED IN SURFACE IMPOUNDMENT
DESIGN AND ANALYSIS.
Parameter
Type
Reference
Rainfall
Atlas of U.S. - Duration to
1 day and return periods to
100 years
Atlas of Western U.S. - Dura-
tions to 1 day and return
periods to 100 years
Monthly totals and normals
Hershfield (1961)
Miller et al
(1974)
NOAA (no date)
Evaporation
Atlas of U.S. - Average
monthly Class A Pan Evaporation
Class A pan evaporation -
Monthly totals
Atlas of U.S. - Class A pan
and pond evaporation
Atlas of Western U.S. - Class
A and pond evaporation
Mean annual Class A pan-to-
lake coefficients
Monthly mean Class A pan
coefficients
Monthly totals
Brown et al
(1983)
NOAA (no date)
Kohler et al
(1959)
Nordenson (1962)
NOAA (1968)
Kohler (1954)
Jenson (1973)
State
Climatologist
or Agricultural
Experiment
Stations
over a sizable area that may include the locale of the surface
impoundment, but they may take place completely outside it.
While flash flooding may occur almost anywhere near a
stream, large-scale flooding ordinarily occurs in the valleys of
larger, slower-moving streams. Units in areas prone to large-
17
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scale floods must be designed to prevent washout from at least a
100-year flood.
Several sources of flood data are available (see Table 4).
Because watershed characteristics may be altered by natural
forces or by man's activities, flood maps may be inaccurate or
outdated, and direct field evidence should be acquired to
supplement or revise data from published sources.
Siting an impoundment within the 100-year flood plain or in
an area prone to flash floods is generally not advisable and
makes the acquisition of a permit more difficult. However, it
does not preclude the construction of a surface impoundment that
is designed accordingly. The erosive action of flowing flood
waters or wave action, in addition to the potential for dike
overtopping (externally and into the impoundment), must be
addressed in the surface impoundment design in these situations,
using estimates of flood depth and flow rate. Additionally, dike
stability analyses should consider rapid drawdown effects, which
occur when flood waters on outer dike slopes quickly recede.
Larger safety factors are likely to be required.
2.5.2 Precipitation vs. Evaporation
Precipitation adds an unwanted liquid volume to a surface
impoundment. In effect, it reduces the impoundment's waste-
holding capacity. On the other hand, evaporation has the
opposite effect, and is generally desirable. Precipitation and
evaporation rates rank with the rate of waste input in design
consideration. Together, they control the design capacity of the
surface impoundment, and the design of the berms, discharge
pumps, inflow/outflow structures, secondary containment
structures, and surface water controls.
In humid regions, and in cases where the impoundment's
purpose is to collect contaminated storm runoff, provisions for
the storage and periodic discharge must be included in the
freeboard design and in the design of inflow/outflow structures.
Long-life, non-discharging surface impoundments (evaporation
ponds) are feasible in the drier western U.S. where the
evaporation rate exceeds precipitation (see Figure 1). Even
there they are feasible only when storage capacity is adequate to
handle extremes of precipitation and seasonal periods of excess
precipitation. Short-life (say, less than five years), non-
discharging storage impoundments may be designed in excess
precipitation areas, with input being terminated when a pre-
determined level is reached. Further discussions on surface
impoundment water budgets and sizing considerations are included
in Subsection 3.1.
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Figure 1. Area (shaded) of net evaporation in the U.S,
(derived from NOAA data).
2.5.3 Soil Freezing and Thawing
Cycles of freezing and thawing, particularly in the northern
states, can change the chemical and physical properties of soil
from the surface down to the maximum freezing depth. Critical
properties, such as permeability and shear strength, of
constructed soil dikes and liner components may be adversely
affected. Therefore, in evaluating some sites, particularly in
northern areas, it may be necessary to add a protective cover
over the liner or dike to the depth of maximum frost penetration.
Figure 2 shows the maximum anticipated depth of freezing for
the contiguous United States. Local sources should be sought to
determine the soil freezing depth for a specific site.
The freeze-thaw phenomena, as they affect soil-based
structures, are summarized in an EPA publication on clay liner
construction (USEPA, 1988b).
2.6 AIR QUALITY
Background air quality should be tested before surface
impoundment construction to avoid potential distortion by later
site activities. Background air measurements are particularly
important in industrial areas where other emission sources are
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Depth contours are in feet.
(1 ft = 30.5 cm)
Figure 2. Maximum anticipated depths of freezing.
(Spangler and Handy, 1982)
common. Testing should be conducted under a variety of weather
conditions. Without background data, it may be difficult or
impossible to confirm or defend against later accusations of
emissions violations. Several air sampling techniques have been
described by the USEPA (1985f).
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CHAPTER 3
DESIGN
Fundamental impoundment design decisions include the basic
configuration and the selection, justification, and arrangement
of the structural components and ancillary facilities. Certain
components of the configuration (e.g., a double-liner system) are
now required by minimum technology regulations.
3.1 SELECTION OF BASIC CONFIGURATION
Among the first design decisions are the type of impoundment
based on specific use; the number, size, and position of
impoundment units; the impoundment shape and areal dimensions;
and the selection of liner type.
3.1.1 Impoundment Type
The type of surface impoundment required depends on waste
composition, waste-generation rate, and the purpose of the
impoundment. A surface impoundment can be classified as one of
three generic impoundment types: (1) treatment; (2) surge or
equalization (i.e., storage); and (3) non-discharging
(evaporation or disposal). Figure 3 depicts the three types.
The greatest number in use are of the treatment type.
Waste inputs and treated waste discharges from treatment
impoundments may be steady, fluctuating, or intermittent. Except
for some surge or equalization impoundments that are intended to
collect runoff, the only external water input is direct
precipitation on the impoundment surface and interior dike
slopes. Non-discharging surface impoundments generally rely
strictly on natural evaporation to maintain liquid level.
3.1.2 Number, Size, and Position
More than one impoundment may be required where several
incompatible liquid wastes are to be stored. Multiple
impoundments may also be desirable for single or compatible
wastes in some situations. Unger et al. (1985) compared a large
single surface impoundment to multiple small impoundments, as
depicted in Figure 4, with respect to cost and the implications
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waste inflow
(a)
precipitation
evaporation
discharge
waste inflow
(b)
precipitation
discharge —-7
evaporation c==^^^^^\ r
design storm surge
waste inflow
(c)
precipitation
evaporation
Figure 3. Components of maximum operating level for
(a) treatment, (b) surge, and (c)
evaporation/disposal.
perimeter smaller for single-cell impoundment
for same depth and volume
total liner area larger on multiple-cell impoundment
for same depth and volume
Figure 4. Example of one large vs. four small impoundments
22
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of dike and liner failure. Each was optimized for the highest
"fill efficiency ratio," which is defined as the most cost-
effective dimensions for a given volume. The side slopes were
3H:1V in each case. It was found that four small impoundments
would cost nearly twice as much as a single large impoundment.
Other factors were also considered. For example, there is a risk
of greater leakage due to more geomembrane liner seam length and
greater bottom surface area in multiple impoundments. On the
other hand, higher head, and higher hydraulic gradients in clay
liners, make can increase the risk in a single large impoundment.
The risk of a dike failure occurrence is greater in multiple
impoundments due to more dike length, but greater environmental
damage would ensue from dike failure of a single large
impoundment. All things considered, it appears that a single
large impoundment and multiple small impoundments, constructed to
serve the same purpose, are approximately equal in environmental
risk.
According to the USEPA (1983b) the most common and most
economical shape for a surface impoundment is rectangular with
straight-sloped sides. Other shapes increase the cost of
grading, dike construction, and liner installation. The
rectangular shape was confirmed in the later study by Unger et al
(1985). That study did not consider the cost differences between
impoundments excavated entirely below grade, those diked entirely
above grade, and combinations of excavation and above-grade
dikes. Unger's study assumed the combination type.
Orientation of a surface impoundment with respect to compass
direction is probably relatively unimportant as a design
consideration. However, if the primary geomembrane liner is
exposed (not covered with a protective soil layer), weathering
effects are likely to be the most severe on south-facing slopes
(those slopes exposed to direct sunlight for the longest periods)
(USEPA, 1985J). The designer may want to consider this potential
in orienting a rectangular impoundment, if he has the freedom to
do so. Another possible consideration in larger impoundments is
the fetch, or the orientation of the long dimension compared to
higher-velocity prevailing winds.
The position of the surface impoundment with respect to
natural grade can vary from one excavated below-grade, to one
entirely above grade and surrounded by containment dikes, to a
combination of the two. The most common is the combination type,
probably because it is the most economical to construct. For
this type, the excavated material can often be used as dike
material. Below-grade surface impoundments have the
environmental advantage of being less prone to catastrophic
failure, since no dike is involved. Above-grade impoundments, on
the other hand, could lose all their waste to the surrounding
area if the dike were breached. Table 5 is a cost comparison for
a surface impoundment with respect to the three positions.
23
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TABLE 5. COST COMPARISONS OF DIFFERENT SURFACE IMPOUNDMENT
POSITIONS WITH RESPECT TO GRADE
(GEOMEMBRANE/COMPOSITE LINER)
Cost
Operation Above Grade Below Grade Combination
Geotechnical Invest.
Clear and Grub
Excavation
Foundation Preparation
Berm Construction
Soil Liner
Sand Drain Layer
Geosynthetic Drain Layer
Geotextile Layer
Primary Geomembrane Liner
Geotextile Layer
Geomembrane in Composite
$13,805
2,325
0
1,670
41,095
126,982
26,928
27,182
6,313
18,151
6,313
21,825
$13,805
2,325
40,562
3,463
0
123,657
26,170
28,478
5,869
16,873
5,869
20,452
$13,805
2,325
20,602
2,708
8,556
125,734
26,639
27,432
6,175
17,754
6,175
21,397
Liner
Leak Detection
Main 421 372 408
Lateral 1,234 728 1,196
Riser 0 82 425
Pump 8,955 8,955 8,955
Sump 4,964 4,964 4,964
Riprap 8,186 0 3,314
Level Controller 5,000 5,000 5,000
TOTAL $321,349 $307,624 $303,564
============================================:====:==:==:====:========:=
Note: All impoundments are designed to hold 9,940 m3 (2.63
million gallons) of waste liquid (Unger et al, 1985).
Construction materials assumed to be the same in all 3
cases. Actual site-specific costs of construction may be
grossly different.
3.1.3 Impoundment Surface Area
Standard reservoir tables (Kays, 1977) can be used to
determine volume and liner areas for various surface impoundment
configurations. Usually, the surface area and storage capacity
requirements are known beforehand or are determined during pre-
design. The surface area may be limited by available land area.
If so, it can usually be compensated, within reason, by
increasing the impoundment depth.
24
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Surface area is one of the more important factors in
impoundments used for evaporation. The rate of evaporation must
equal or exceed the rate of precipitation plus the liquid
(aqueous waste) inflow rate. If, for example, evaporation is 40
inches (102 cm) per year and precipitation is 30 inches (76 cm),
then 10 inches (25 cm) per year is available for waste liquid
input. Thus, the larger the surface area, the larger the amount
of waste liquid that can be accommodated.
The potential for slope damage by wave action increases with
impoundment size and is directly related to both area and depth.
3.1.4 Impoundment Depth
The surface impoundment depth is a function of the waste
volume to be handled, its eventual disposition, and the increased
depth necessary to contain foreseeable and unforeseen events.
Assuring adequate depth by accurately predicting liquid levels
for all circumstances is a very important design operation.
The liquid level in a surface impoundment will change due to
waste inflow and outflow, storm surges, precipitation, runon (if
applicable), wind speed, and dike slope. Figure 5 is a cross-
sectional representation of a surface impoundment, showing the
components which cumulatively make up its design depth. The
bottom liquid level shown on the figure is the normal operating
level which takes into account only waste inflows and discharges.
The next higher liquid level on the figure is the maximum
operating level. The maximum operating level is the resultant
level caused by the addition of water from a major climatic event
to the normal operating liquid level. The major climatic event
may be, for example, the wettest month in 25 years, or a 100-year
24-hour rainstorm. Additional impoundment depth allowances
(freeboard) are included for wind set-up, wave run-up, and
finally a safety factor.
spillway level
max. wave run-up level —^— ' *—• • * waves
max. wind set-up level
maximum operating level —"^
s— maximum precipitation event
normal operating level —*^s~
waste liquid volume
Figure 5. Components making up impoundment design depth.
25
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3.1.4.1 Normal Operating Level —
For flow-through impoundments, normal operating liquid level
can be determined from waste inflow and outflow rates, liquid
surface area, and detention time of wastes in the impoundment,
using the following formula:
where:
d = normal liquid level or depth in ft. (m)
t = average detention time (hr)
Qt = maximum rate of inflow during t in ftVhr (m3/hr)
Qo = average rate of outflow during t in ftVhr (mVhr)
A = liquid surface area in ft2 (m2)
For non-flowthrough (storage) impoundments, where inflow
exceeds evaporation over the long term, the liquid level will
rise. Reaching the design level would call for an end to
operations, or for cleanout and resumption of operations. In
this case, the above equation would be solved for t, for a given
design depth (d) and surface area (A).
The normal operating level in a surface impoundment used for
evaporation may be considered, in some cases, to be the highest
liquid level reached during a normal year, for example during a
"rainy season," when precipitation exceeds evaporation. If the
waste inflow is variable, the highest expected waste inflow would
be added to the higher precipitation.
3.1.4.2 Maximum Operating Level without External Runoff Input —
Designing the surface impoundment's maximum operating level
is typically done using one of three methods: hydrologic models;
water budget approach; or design storm approach.
The additional storage capacity calculated as being
necessary to contain climatic extremes will vary depending upon
the method used. Geographic areas having high temporal vari-
ability in rainfall amounts (e.g., the southwestern U.S.),
generally have larger storm surcharges that are calculated using
a recorded rare return-period storm rather than using a water
budget or hydrologic simulation approach. The choice of approach
for determining the maximum operating level will depend on the
availability of climatic data and hydrologic models, and the type
of impoundment operation, waste characteristics, and the risks
associated with an uncontrolled release of hazardous waste from
the surface impoundment.
26
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Water Budget Approach — The water budget can determine the
feasibility of using a surface impoundment at the given locale
and can help determine the needed storage volume for non-
discharging disposal or evaporation units. Storage requirements
are a function of precipitation and evaporation and waste inputs.
A basic assumption in a water budget for most non-discharging
surface impoundments is that there is no net change in the volume
of liquid in the impoundment over the long term.
Regardless of the intended impoundment use, the water budget
for determining the maximum operating level (and storage
capacity) can be derived from the following expression:
S = P + W-E-D-L
where:
S = change in storage
P = precipitation
W = waste input
E = evaporation
D = discharge
L = leakage (negligible for a functional liner)
The water budget can be computed using the climatic record,
watershed properties (for units that collect runoff), waste input
rates, and discharge rate, if applicable. For the purpose of
these calculations, it is assumed that adequate volume will be
available to contain all events (i.e., no overtopping of passive
level controls, such as spillways).
The approach taken is a frequency analysis to determine the
amount of excess precipitation (i.e., rainfall minus evaporation)
that can accumulate in the impoundment in a given period, at
least equal to the active life of the impoundment. Long-term
records (preferably more than 20 years, but at least equal to the
impoundment life) of precipitation and lake evaporation are
required for these water budget calculations. Measurements of
pan evaporation can also be used, although pan-to-lake
coefficients will have to be applied (Saxton and McGuinness,
1982). The data should be obtained from National Oceanic and
Atmospheric Administration (NOAA) weather stations or other local
weather stations located as close to the impoundment site as
possible, and from hydrologically similar areas. Long-term
precipitation records are available for weather stations across
the U.S. (NOAA, 1983). While stations collecting evaporation
data are fewer in number, evaporation rates are reasonably
similar over regional areas (Saxton and McGuinness, 1982);
therefore, evaporation data collected at the nearest weather
station recording that parameter can often be used.
27
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Usually an empirical probability distribution is used with
observations of precipitation and lake evaporation data to
determine excess precipitation amounts. Monthly totals are
commonly used to simplify data requirements, although data
compiled for shorter increments (e.g., the design retention time)
may lead to a more accurate estimate of storage requirements.
The occurrence frequency of excess precipitation amounts is
determined by ranking the observed data, computing a plotting
position, and plotting the excess precipitation amounts and
positions on probability paper. A widely used, consistent
plotting position formula is the Weibull Relationship (Haan,
1977). Haan (1977) and Osborne et al (1972) present information
on frequency analysis and probability plotting. The Weibull
Relationship is expressed as follows:
•m
P(y) =
N + 1
where: y = the excess precipitation value
P(y) = the plotting position
m = the rank of the excess precipitation value
N = the number of observations
The return period, or recurrence interval, is given by the
inverse of P(y). There is no general agreement as to which
statistical distribution should be used for this type of
frequency analysis. The National Weather Service (NWS) used a
modified version of the Gumbel extreme value distribution to
develop a rainfall frequency atlas (Miller et al., 1974) for the
U.S. Pruitt and Snyder (1984) recommend using the normal
distribution for determining surface impoundment storage needs
for reclaimed wastewater irrigation projects.
Regardless of the distribution chosen, if a data set plots
as a straight line on the probability paper, the data are said to
be distributed as the distribution corresponding to the
probability paper. Because it is rare for a set of data to plot
exactly on a line, a decision must be made as to whether the
deviations from the line are random or true deviations,
indicating that the data do not follow the given probability
distribution. Often, several types of frequency plots must be
evaluated to select the appropriate distribution. Figure 6 is an
example probability plot for excess precipitation which follows a
normal distribution.
In this example, excess precipitation was greater than six
inches (15 cm) in 10 percent of the months recorded. This
additional six-inch (15-cm), one-month storage requirement
corresponds with a 10-year return period.
28
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I «
t2
; o
Q -4
* fi
g. -6
'o
£-•
-10
-12
1.05
return period (yrs.)
1.4 2 4
10 25 100
MONTHLY DATA
90%
12 5 10 20 30 50 70 80
percent less than
90 95 98 99
99.9
Figure 6. Example of frequency distribution of
monthly climatic data.
Once the probability plot of excess precipitation is
constructed, the next step in establishing the maximum operating
level is selecting the maximum climatic event (monthly
precipitation minus evaporation) that can be tolerated, and the
associated precipitation amount. Because surface impoundments
may contain hazardous waste, the probability of occurrence of a
climatic event that results in exceedance of the maximum
operating level (and possible release) must be very small. The
maximum probability recommended is a 1 in 25, or 4%, probability
that the maximum operating level will be exceeded in any year.
This probability level represents a return period of 25 years for
the maximum monthly excess precipitation calculated using the
water balance approach. However, this does not mean that the
maximum operating level will not be exceeded during a 25-year
design life.
Figure 7 shows the design return period required so that the
maximum operating level will not be exceeded during the expected
facility life for various degrees of confidence. As shown on the
graph, to be 90% confident that the maximum operating level will
not be exceeded in a surface impoundment with a 10-year design
life, the impoundment must be designed using a 100-year climatic
event.
29
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1(
600
400
200
*^x
£ 10°
•^**
~ 50
25
T3
o> 10
c
O)
w 5
X
X
2 5 10 25 50 100
design life - T. (yrs.)
Figure 7. Percent confidence that design return period will
not be exceeded during design life.
Design Storm Approach — The simplest approach for
determining the maximum operating level is to make it equivalent
to the additional storage capacity, over the normal operating
level, that is needed to contain precipitation from a single
design storm. The minimum acceptable probability of a discharge
for hazardous waste surface impoundments is the 100-year, 24-hour
storm. This value represents a 1-percent probability that the
given amount will be equaled or exceeded in any given year. It
also says that if 1,000 surface impoundments are in operation
throughout the U.S., using the 100-year, 24-hour storm would
result in about 10 units (1% x 1,000) equaling or exceeding this
amount each year.
Figure 7 should be consulted to evaluate the risk or percent
chance of a precipitation event in excess of the design storm
during the facility design life. For the 100-year return period
and 10-year design life, there is a about a 10% chance that the
maximum operating level will be exceeded.
Adding to the confidence that the maximum operating level
will not be exceeded is the fact that the storm surge amount is
30
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added to the normal operating liquid level, which already
accounts for extremes in waste inputs. The likelihood that the
extreme events would coincide is slight. The further inclusion
of freeboard that accounts for wind set-up, wave run-up, and a
safety factor adds still more assurance that wastes can be safely
contained under any conditions.
3.1.4.3 Maximum Operating Level with External Watershed —
Computing the maximum operating level of a surface
impoundment having an external watershed (e.g., an impoundment
used for detention of runoff) requires adding watershed runoff to
the direct precipitation on the impoundment surface. The volume
contributed by runoff will be directly related to the watershed
area and is likely to be much larger than the volume contributed
by rainfall directly on the impoundment surface.
Available hydrologic models were not developed for surface
impoundment design, but can be modified to meet the need. Most
rainfall models are designed to predict runoff and are suited for
estimating storm surges to surface impoundments that act as
catchment basins. To date, none of the many available models has
been widely used. One of the better suited models is STORM
(Storage, Treatment, Overflow, Runoff Model), developed for the
USAGE Hydrologic Engineering Center (HEC). STORM is a stochastic
rainfall model used to predict thunderstorm activity and to
generate a synthetic rainfall record for watersheds (Corotis,
1976). Stochastic rainfall models such as STORM are particularly
useful in geographic areas where the weather record may not be
sufficiently long for use in impoundment design.
The Soil Conservation Service developed a method for
estimating volume and rate of runoff from U.S. agricultural
watersheds (Kent, 1973). A more detailed description of the SCS
method has been provided by McCuen (1982). The "Rational
Formula" is another widely used method for predicting peak runoff
from a small watershed for a given design storm intensity:
Q = CIA
where
Q = runoff volume (cubic feet per second)
C = runoff coefficient (dimensionless); the percentage
of rainfall that is surface runoff
I = average rainfall intensity (inches per hour)
A = watershed area (acres)
Introductions to the SCS and Rational Formula methods and
others have been provided by Whipple et al (1983) and Osborne et
al (1972). The hydrologic models discussed earlier will commonly
estimate runoff volumes directly or provide synthetic rainfall
records to be used in a runoff model.
31
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3.1.4.4 Freeboard Determination —
Freeboard is the distance from the water line on a structure
to the top of the structure. In the case of a surface
impoundment it is the distance between the maximum operating
level and the liquid level (e.g., spillway level) which would
result in the release of stored liquid. Determining necessary
freeboard in impoundment design requires considering wind speed,
fetch, maximum liquid depth, and the slope and roughness of the
embankment. These factors influence the development of waves,
wave run-up, and wind set-up. Wave run-up and wind set-up, plus
a safety factor, are accounted for in the calculation of
freeboard.
Wind speeds used in freeboard calculations should be based
on local historical data for maximum sustained winds. In the
absence of such data, a value of 75 mph (120 km/hr) is suggested
for areas that are not subject to hurricanes, and a value of 100
mph (161 km/hr) for areas that are hurricane-prone. In this
document, a "hurricane-prone area" is any area within 50 statute
miles (80 km) of a coast subject to hurricanes (USEPA, 1986b).
Fetch is the distance that the wind blows over the water (or
waste liquid) surface. For a surface impoundment, it will vary
with the wind direction vs. the orientation and dimensions of the
impoundment. In a rectangular impoundment, the maximum fetch
will be along the diagonal.
Depth of the impoundment will ordinarily be uniform, or
nearly so, over most of the liquid area, with relatively small
slope areas along the edges, if the total area is more than, say,
an acre. For calculation of wave height and set-up, the depth
can be considered essentially uniform. For small impoundments,
where the slopes make up most of the area, fetch, and thus wave
action and runup, are likely to be insignificant.
Techniques for predicting wave action in shallow water are
provided by USAGE (1984). Maximum wave height and wave length
are directly related to fetch and water depth. Using a fetch
(distance that the wind blows across the impoundment) of 300 feet
(91 m), a liquid depth of 10 feet (3 m) and a wind speed of 75
mph (120 km/hr), the maximum wave height would be 0.9 feet (0.3
m). Waves would not be expected to break under these conditions,
or in any normally designed surface impoundment.
Wind set-up is the rise in liquid level on the downwind side
of a surface impoundment caused by the wind pushing the liquid to
that side (Figure 5). A corresponding drop in level occurs on
the upwind side. In the past, wind set-up has largely been
ignored in determining freeboard requirements for surface
impoundments. Two methods considered acceptable for set-up
calculation have been presented by Keulegan (1951) and Van Dorn
32
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(1953). Given the same wind conditions and impoundment
dimensions shown above, the set-up would be 1.11 feet (0.34 m) .
Wave run-up is the vertical height to which approaching
waves force water above the still water level on a sloped
embankment (see Figure 5). Wave height, embankment slope and
slope roughness affect wave run-up height. Typically, the
highest run-up occurs on low smooth slopes. Most investigations
conducted to accurately define the parameters that control run-up
have been directed toward run-up on beaches, seawalls, and
breakwaters (USAGE, 1984; Machemehl and Herbich, 1970; Saville,
1956; Toyoshima et al, 1966). Roughness factors will vary from a
value of 1.0 for smooth synthetic liners to 0.45 for coarse
surfaces (e.g., riprap) (USAGE, 1984). Curves that relate slope
roughness to other freeboard parameters are convenient for
estimating roughness effects and can be found in Saville (1956)
and USEPA (1986b). Saville (1956) looked at run-up on smooth
surfaces, which applies to defining run-up on smooth synthetic
liners used in many surface impoundment designs. In surface
impoundments, the highest run-up can be expected on slopes of 3:1
to 5:1 lined with impermeable synthetic membranes (Saville,
1956). Therefore, the designer should consider, as protective
cover, rough-surfaced materials such as riprap, to reduce run-up,
or he should consider, generally as a lesser option, designing
steeper slopes.
More detail on freeboard calculation can be found in
references by Herbich (1986) and those cited above.
3.2 STRUCTURAL COMPONENT DESIGN
The following subsections discuss the structural
considerations necessary in the design of a surface impoundment.
References are presented that may be consulted for detailed
information on specific mechanics of the design process. The
design components discussed include the following:
dikes and foundations
liner systems
liquid level controls
inflow and outflow
protective coverings
secondary containment
leak detection systems
surface water management
controls for emissions of volatile organic compounds
(VOC)
construction quality assurance
33
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3.2.1 Foundation Analysis
The foundation underlying a surface impoundment is the
native soil. The soil must be capable of supporting the added
load of an overlying dike. It also must be capable of supporting
the load imposed by the impounded waste. Although it is not
likely, that load may exceed the load removed by excavation. The
potential for soil compression and the soil's bearing capacity
are the properties of principal concern.
3.2.1.1 Settlement —
Foundation compressibility is an important consideration if
sections of a dike will be placed on soils that are soft or
loose. Such soils will consolidate (compress) under the
additional load imposed by the dike. If the dike height,
foundation thickness, and/or foundation compressibility vary,
differential settlement and perhaps cracking of the dike and
liner could result as shown in Figure 8.
dike settlement
and cracking
compressible zone
of foundation soil
Figure 8. Example of the effect of differential foundation
compressibility on a surface impoundment dike.
The compressibility of fine-grained soils can best be
determined in the laboratory by consolidation tests on
undisturbed tube or block samples obtained during field
exploration. Methods for predicting consolidation settlement are
presented in several introductory geotechnical texts (Spangler
and Handy, 1982; Terzaghi and Peck, 1967; Bowles, 1977).
Settlement of granular soils can be determined by a method
devised by Schmertmann (Schmertmann, 1970; Winterkorn and Fang,
1975).
34
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If the foundation soils are granular, loose, uniformly
graded, and saturated, and could be subjected to earthquake
loading, liquefaction (loss of foundation support) could result.
Liquefaction-potential assessment is also discussed in geo-
technical texts (Winterkorn and Fang, 1975). Low-density
granular soils are best identified during field exploration,
using standard penetration tests or other in-situ testing and/or
field density determinations made in test pits.
3.2.1.2 Bearing Capacity —
The foundation soil that will underlie a dike or other
massive structure associated with the surface impoundment should
be evaluated for its bearing capacity. The bearing capacity of a
foundation is the maximum loading to which the foundation soil
may be subjected without permitting shear displacements
detrimental to the function of the structure (Spangler and Handy,
1982). Figure 9 shows the forces and displacements that occur
when the bearing capacity is exceeded. The design bearing
capacity should have a safety factor of 2.5 to 3.0. Methods of
determining a soil's bearing capacity are presented in several
introductory geotechnical texts (Spangler and Handy, 1982;
Terzaghi and Peck, 1967; Bowles, 1977). Input variables include
the dike and foundation geometry, and the weight and shear
strength of the soil materials.
LOAD
Figure 9. Forces and displacements in bearing capacity
analysis (adapted from Scott, 1980)
3.2.2 Dike Design
Surface impoundments may range from completely above grade
to completely below grade, but most will be a combination with an
excavated portion below grade and a diked portion above grade.
The excavated materials may be used to build the dike. Figure 10
is a cross-section of one side of the combination type showing
the various dike and liner interfaces.
35
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geomembrane liners
with protective
geotextiles
protective soil layer
leak collection
layer
cut-slope
_ low-permeability
soil layer
Figure 10. Surface impoundment dike and liner
interfaces and layers.
A surface impoundment dike and the associated double liner
comprise a relatively complex structure. Stability of the native
foundation soil and internal stability of the dike proper must be
considered, along with the stability of the double liner
components on the inside slope of the dike and excavation. Each
of the liner slope interfaces may be a plane of weakness.
Dike and foundation stability must be evaluated for the
construction and operating conditions, using expected in situ
engineering properties of the foundation and berm materials,
pertinent geologic information (see Section 3.3), and planned
dike slope and height. Side slopes of 2H:1V or flatter are
usually considered adequate for maintaining dike stability.
However, the slope limits for the soil liner component may be
controlling for the dike. USEPA's Minimum Technology Guidance
(USEPA, 1987a) for surface impoundments recommends a
demonstration to show that the low-permeability soil liner on the
side walls can be compacted effectively at the maximum slope used
in the design (USEPA, 1987a). Soil liner slopes steeper than
3H:1V are not recommended due to the difficulty in constructing
them to meet current USEPA performance standards.
Dike stability is determined primarily by the ability of the
dike and foundation soils to resist shear stress. Shear stresses
result from externally applied artificial loads (e.g., the
impoundment contents), the internal body forces caused by the
weight of the soil and dike slopes, and external forces (e.g.,
earthquakes). When the ability of the soils to resist shear
stresses is exceeded, the result can be (1) slope failure,
characterized by a failure plane or surface along which a portion
36
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of the embankment slides; (2) excessive settlement; or (3) a
bearing capacity failure. The following sections describe design
against these failure modes.
3.2.2.1 Shear Strength —
The principal parameter in analyzing dike and foundation
stability is soil shear strength (or shearing resistance). It is
generally determined by field or laboratory tests. To obtain
meaningful shear strength results from a laboratory test program,
structurally undisturbed samples, representative of the in-situ
soils or compacted soils proposed for the project, must be
obtained for testing. For native, cohesive foundation soils,
samples of "undisturbed" materials are typically used. (Soil is
always disturbed to some extent when the sampling operation is
performed. "Undisturbed" samples are those taken in a way to try
to retain as many of, or as near to, the in-place physical
properties as possible.) To determine the shear strength of
soils to be used as compacted fills (dikes and liners), the soils
must simulate as-constructed field conditions, including method
of compaction, dry unit weight, and water content. These samples
may be prepared in the laboratory, or, preferably, they are
"undisturbed" samples taken from a field test fill prepared under
construction conditions.
Test methods for determining shear strength in the
laboratory include triaxial tests, unconfined compression tests,
direct shear tests, and laboratory vane shear tests. Procedures
and descriptions for such testing, and interpretation of the test
results, are provided by several sources (Lambe, 1951; USAGE,
1970; Head, 1980; 1982; and 1986).
Methods for determining in-situ shear strength include
standard penetration, vane shear, pressure meter, cone
penetrometer, and dilatometer. In-situ testing is described in
several sources (ASCE, 1986; FHWA, 1980).
The shearing resistance of cohesive soils differs from that
of cohesionless soils due to the following factors:
• particle size — cohesive soils are finer
• particle shape — cohesive soils are platy; cohesionless
are rounded to angular
• permeability — cohesive soils are less permeable and
drain more slowly
• internal friction angle — lower in cohesive soils
• plasticity — cohesive soils more plastic
The Mohr-Coulomb failure envelopes for clays and sands are
shown in Figure 11 for comparison. The value C indicated on the
shear stress axis, also referred to as the "cohesion intercept,"
37
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CO
CO
CD
is
-------
(a)
(b)
0)
T3
water content, w
t
0)
<
(A
compaction
curve
,B
water content, w
Figure 12.
Typical compaction curves showing (a) dry unit
weight-water content relation and (b) variation
of shear strength with water content for cohesive
soil.
hydrostatic pressure present in the water contained in the soil
voids, affects shear strength determined by effective stress
parameters. During stress application to a soil mass, the
applied stress tends to compress the soil, causing an increase in
the pore water pressure. This pore water pressure buildup may
reduce shear strength.
The low permeability of clays causes pore-water pressure to
build up during laboratory or field tests, thereby controlling
the shear strength properties that are measured and used in
design computations. Therefore, it becomes necessary to
understand the mechanisms of pore-water pressure changes in
cohesive soils when making an attempt to understand the shear
strength behavior of cohesive soils. Discussions of pore
pressure and shear strength relationships can be found in most
soil mechanics texts (e.g. Bowles, 1984; Scott, 1980; Sowers,
1979, etc.).
3.2.2.2 Slope Stability Analysis —
A dike that includes a conservatively specified height and
slope may require only a simple stability chart analysis or a
39
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basic computer analysis. Guidelines for such conservative design
embankment heights and slopes are provided by the U.S. Bureau of
Reclamation (1974). A dike design that includes relatively high
embankments and steeper slopes, however, may require a complex
computer analysis.
Slope stability analysis techniques are well-developed.
They are described in most soil mechanics text books and various
references (Perloff and Baron, 1976; USAGE, 1970b; Wright, 1969).
The conventional stability analysis failure modes can be
classified as either "circular arc" (rotational slide) or
"sliding wedge" (translational slide). A third type of
instability is soil "flow." The three types are depicted in
Figure 13.
a. translational slide
(sliding wedge)
b. rotational slide
(circular arc)
c. flow
Figure 13. Types of slope instability.
40
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Stability analysis should be performed considering the
following load cases:
• end-of-construction or short-term
• steady seepage or long-term
• rapid drawdown
• seismic
Descriptions of these load cases and appropriate strength
parameters for each are presented in several sources (USAGE,
1970b; Sherard, 1963; Perloff and Baron, 1978). Depending on the
subsurface conditions, embankment geometry, operational
characteristics, and regional location of the impoundment, one or
more of the noted load cases may be eliminated from the analysis.
More than 30 methods for performing slope stability analysis
exist. Among these, Bishop's Modified Method and the Ordinary
Method of Slices are the most commonly used for circular failure
surfaces, and Spencer's Method for non-circular failure surfaces.
Johnson (1975) recommends using the simplest suitable procedure,
because even the most detailed of the conventional or limiting
equilibrium methods is relatively crude and neglects stress-
strain relationships. Several of the methods available and
applicable stability conditions are listed in Table 6.
TABLE 6. CURRENT PROCEDURES FOR STABILITY ANALYSIS.
Equilibrium Conditions Satisfied Shape of Computation
Procedure Overall Slice Vertical Horiz. Sliding by
Moment Moment Force Force Surface Computer Hand
Ordinary
method of
slices - Yes
Swedish
method
No No No Circular Yes Yes
Simplified
Bishop Yes No Yes No Circular Yes Yes
Janbu Yes Yes Yes Yes Any Yes Yes
Morgenstern
and Price Yes Yes Yes Yes Any No Yes
Spencer Yes Yes Yes Yes Any Yes Yes
Sarma Yes Yes Yes Yes Any Yes Yes
41
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These methods are applied to dike stability evaluations
using computer programs and stability charts. Generally,
embankments less than 10 to 15 feet (3 to 4.6 m) high can be
analyzed effectively using stability charts. For higher
embankments or more sensitive situations, computer analysis may
be recommended and verified by the design engineer.
Software programs for personal computers are available from
USEPA and other sources (e.g., Jeyapalan, 1986; Geo-Slope
Programming, Ltd., 1985; and USEPA, 1986c). These programs
typically include various options to handle even the most
difficult site conditions.
"Geotechnical Analysis for Review of Dike Stability (GARDS)"
software, developed for the USEPA (1986c), is a menu-driven
software package for design review of earth dike stability. It
can be used to analyze site hydraulic conditions, dike slope and
foundation stability, dike settlement, and soil liquefaction
potential. The program was designed expressly for the evaluation
of dikes at hazardous waste disposal facilities. It is intended
to help determine whether a design meets minimum safety criteria
under service conditions.
Slope stability can be analyzed quickly using stability
charts. The charts generally assume smooth, straight-line slopes
and uniform soil conditions. They can be used to obtain
approximate safety factors for most complex problems if irregular
slopes are approximated by a straight line, and average values of
unit weight, cohesion, and friction angle are used. Stability
charts are presented in a number of geotechnical references
(USAGE, 1970b; Naval Facilities Engineering Command, 1982;
Winterkorn and Fang, 1975).
Minimum Factor of Safety — The recommended safety factor
for a slope depends on the following:
• the degree of uncertainty in the shear strength
measurements, slope geometry, and other conditions;
• the costs of flattening or lowering the slope to
increase stability;
• the costs and consequences of a slope failure; and
• whether the slope is temporary or permanent.
When detailed analyses of slope stability are performed, a
number of circles must be examined to locate the most critical
circle, that is, the one with the lowest factor of safety. The
factors may be plotted, on a dike cross-section, at the circle
centers and then contoured to find the location of the minimum.
Figure 14 shows two examples of the results of this procedure.
42
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+50 -
+40 -
«j +30 -
c +20 -
*
-10
-20 -
f=1.23
all circles tangent
to elevation -20 ft.
R = 80 feet
_sz_
(a) Contours of_ffor circles tangent to elevation -20 ft.
+60 -
+50 -
+40 -
« +30 -
c +20 -
| +10 ^
Si 0
-10
-20
all circles tangent
to elevation -10 ft.
(b) Contours of_ffor circles tangent to elevation -10 ft.
Figure 14. Factor-of-safety contours for slope stability.
(10 feet =3.05 meters)
Factors of safety for design slopes may range from 1.4 to
2.0. The typical range for a hazardous waste surface impoundment
is 1.75 to 2.0, with the actual value depending on consideration
of the influence of the four items noted above.
43
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3.2.3 Liner Systems
The liner system includes the barrier layer, the leak
collection layer, and any soil protective layers.
3.2.3.1 Regulatory Constraints and Guidance —
Minimum technology requirements have not been specified
under the Resource Conservation and Recovery Act for non-
hazardous waste surface impoundments. USEPA is currently
proposing minimum technology for non-hazardous waste landfills.
but not surface impoundments, based upon a single composite liner
or its performance equivalent. Similar technology would seem
reasonable for non-hazardous waste impoundments, but it has not
been officially proposed at this time.
The "minimum technology requirements" specified in HSWA
Section 3004(o) for new hazardous waste landfills and surface
impoundments, require a double liner with a leak detection and
collection layer placed between the liners. The double-liner
system must prevent the migration of hazardous constituents
through the liner during the impoundment's active life and post-
closure care period. USEPA's guidance on the minimum technology
requirements (USEPA, 1987a) identified two acceptable double-
liner system designs that would meet the HSWA requirements:
• Geomembrane/Composite Double-liner System (Figure 15)
• Geomembrane/Soil Double-liner System (Figure 16)
A double-composite liner may also be used (Figure 17).
A "composite" liner in the guidance is a two-component
liner, a geomembrane installed directly on a smooth-surfaced low-
permeability soil component, with the two components acting as a
unit. The two-component composite liner is the bottom secondary
liner of a double-liner system. The top primary liner of the
double-liner system is a geomembrane. The drainage (leak
detection) layer separates the two liners.
Careful consideration must be given to the relatively low
friction angle between the geomembrane and the low-permeability
soil, especially for the possibility of moisture collecting
between the layers. At the same time, attention must be given to
the contact between the geomembrane and the low-permeability
soil. The soil surface should be smooth and the contact with the
geomembrane as continuous as possible to minimize any potential
flow along the interface.
The double-liner system must prevent leakage through the
secondary liner of the system for the active life of the
impoundment and the post-closure care period. More
44
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protective soil layer
geomembrane liners
with protective geotextiles
leak detection/collection layer
composite liner
Figure 15. Cross section of double liner with composite
bottom liner.
protective soil layer
geomembrane liner
with protective geotextiles
^« — -j- _ - - - «^. - - -
r^_-ir^_—/ compacted soil
geotextile filter -
leak detection/collection layer
Figure 16. Cross section of double liner with soil-only
bottom liner.
protective soil layer
geomembrane liners
with protective geotextiles
composite liners'
geotextile filter
leak detection/collection layer
Figure 17. Cross section of double composite bottom liner.
45
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specifically,the double-liner system must prevent migration of
contaminants into the primary liner and through the secondary
liner during the active life and post-closure monitoring period.
The hydraulic head of liquid above the secondary liner
(within the drainage layer) of a double-liner system typically
has a non-zero value (Giroud, 1985). This indicates that the
primary liner does, in fact, leak. A leak-collection (drainage)
layer between the primary and secondary liners, that allows
liquids to drain freely to a pumpout unit, will minimize the
hydraulic head on the bottom liner and thus will lessen any
potential leakage through it.
USEPA has described the interaction of the leak-collection
system and the composite bottom liner in the process of
detecting, collecting, and removing any contaminant leakage
through the top liner (USEPA, 1987g). It was shown that the
composite (geomembrane/clay) liner significantly improved the
leak detection capability for small leaks (leaks less than 1
gal/acre/day [9.35 liters/hectare/day). Even with small holes in
the geomembrane component, the collection efficiency is not
significantly reduced. On the other hand, the EPA report
suggests that a clay liner alone, meeting the 3.9 x 10"8 in./sec
(1 x 10"7 cm/sec) criterion, would absorb a leak on the order of
80 gal/acre/day (748 liters/hectare/day). It is possible then
that only a leak in excess of that rate would be detected in the
sump of the leak-collection system above a clay liner. Further,
USEPA believes that the liquid absorbed in a soil liner will
eventually be released to the environment.
Even though a soil liner may not be effective in preventing
contamination from eventually escaping the facility, it will
minimize leakage through a breach in the adjacent geomembrane
component. In addition, it will attenuate migrating waste
constituents, provide protective bedding for the geomembrane, and
provide a long-lasting stable foundation for the impoundment.
Careful attention must be given to the sliding potential of
the geomembrane on the low-permeability soil layer. It is not
appropriate to include a geotextile at the interface, because it
would destroy the compression seal intended between the
geomembrane and soil.
3.2.3.2 Geomembrane/Composite Double Liner —
As noted above, the geomembrane/composite double-liner
system consists of two liners — a top primary geomembrane liner
and a bottom secondary geomembrane/soil composite liner,
separated by a leak-collection system (Figure 15). USEPA's
minimum technology guidance (USEPA, 1987a) recommends that the
primary geomembrane be at least 30 mils (0.76 mm) thick where
46
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covered by a protective soil and/or geotextile layer. For an
uncovered geomembrane, a thickness of at least 45 mils (1.14 mm)
is recommended. The guidance suggests that thicknesses of 60 to
100 mils (1.52 to 2.54 mm) may be required to resist various
stresses. In any case, the design engineer should recognize that
some geomembrane materials may require greater thicknesses to
prevent failure or to accommodate unique seaming requirements.
It may be possible to use combinations of geotextiles and
geomembranes in lieu of increasing the geomembrane thickness in
some instances.
All liner and leak-collection components must be chemically
resistant to the waste handled at the facility. Chemical
compatibility of geosynthetic materials should be tested using
EPA Method 9090. Past test data or actual performance data may
be used in lieu of new testing if the materials and wastes can be
shown to be the same in both the past and new applications.
As noted above, in the composite secondary liner, the
geomembrane upper component and a low-permeability-soil lower
component should be in direct and uninterrupted contact. The
interface should be smooth and designed and constructed to
provide a compression connection between the two components to
minimize lateral flow between them, should a hole develop in the
geomembrane. Attaining the desired contact is difficult because
of the near impossibility of completely flattening all
imperfections on the soil surface. It requires careful
compaction and construction quality control to minimize the
imperfections (Giroud & Bonaparte, 1989).
The low-permeability-soil component of the composite liner
should be at least 36 inches (90 cm) thick and have an in-place
hydraulic conductivity of 3.9 x 10"8 inches/sec (1 x 10~7 cm/sec)
or less. These characteristics have been chosen by USEPA as
those that will prevent contaminant breakthrough during the
active and post-closure period, when the liner is constructed
ideally (USEPA, 1987a).
The leak-collection system is generally a layer of highly
permeable sand or synthetic material extending across the bottom
and up the sides of the impoundment between the liners. The
hydraulic conductivity of the leak-collection system should be at
least 2 ft/min (1 cm/sec) for sand. The transmissivity of a
geosynthetic layer should be at least 3.3 x 10"2 ftVsec (3 x 10"3
mVsec). A series of drain pipes, a sump, and pumps are also
included in the system to facilitate rapid removal of liquid.
3.2.3.3 Geomembrane/Compacted Soil Double Liner —
In accord with USEPA's minimum technology guidance, the
geomembrane/compacted soil double-liner design consists of a
47
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primary geomembrane liner placed above a secondary low-
permeability-soil liner, separated by a leak-collection system
(Figure 16). It is very similar to the geomembrane/composite
double-liner system described above, except that it does not
contain the geomembrane component in the secondary liner. The
primary geomembrane liner has the same recommended minimum
thickness and chemical compatibility as the primary geomembrane
liner of the geomembrane/composite liner system. The low-
permeability-soil secondary liner thickness depends on site- and
design-specific conditions, but should not be less than 36 inches
(90 cm) thick, according to the guidance.
Although USEPA (1985b) recommends a 36-inch (90-cm) minimum
thickness, the Agency believes that a low-permeability-soil liner
of that thickness with a hydraulic conductivity of 3.9 x 10"8
in./sec (1 x 10~7 cm/sec) may not prevent constituents from
migrating through the liner prior to the end of the post-closure
care period. Note here again that USEPA has concluded that a
well-constructed soil liner will allow contaminated liquid to be
absorbed, and eventually released, at a rate on the order of 80
gal/acre/day (750 liters/hectare/day) or more (USEPA, 1987g).
Therefore, documentation supporting the use of a particular
thickness is required and should include responses to the
reservations expressed by USEPA (1987a) that such a design
meeting the no-breakthrough requirement is neither economically
or technically feasible. USEPA states that conservative assump-
tions should be used to estimate the necessary low-permeability-
soil liner thickness, due to the lack of precision with which
such estimates can be made (USEPA, 1987a).
The leak-collection (drainage) layer in the geomembrane/soil
double liner system is essentially the same as the leak-
collection layer in the geomembrane/composite double-liner system
described above. In this case it is placed directly on the low-
permeability-soil layer, preferably with a geotextile separating
them.
3.2.3.4 Double Composite Liner —
A further variation of the double liner is a system
comprised of two composite geomembrane/compacted soil layers
(Figure 17). USEPA (1987h) recently noted that many owners and
operators of landfills and surface impoundments have indicated
that they planned to use a composite liner for the top liner as
well as the bottom liner.
In a double-composite-liner system, the liners are separated
by a leak-collection layer, no different than those of other
double-liner designs, except for the insertion of a separation
layer at the top of the leak-collection layer. The separation
48
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layer would prevent the migration of the primary soil liner
component material into the underlying leak-collection layer.
The layer also serves to stabilize the surface of the drainage
layer, facilitating compaction of the overlying soil component.
Methods for evaluating the effectiveness of geosynthetic filter
and separation layers have been discussed in at least three
documents (USEPA, 1987c; FHWA, 1985; and Koerner, 1990).
While a double-composite liner should offer further
assurance against the escape of contaminated liquid, it is not
without its own risks. It appears possible for the primary
geomembrane liner to develop a leak that would not drain freely
through the soil component to the leak-collection layer. Liquid
might thus accumulate between the geomembrane and soil components
and go undetected for a long period. On the other hand, this is
not altogether contrary to the purpose of the top composite
liner's soil component, which would be to impede leakage to the
drainage layer.
The following sections discuss design criteria for various
components of multiple-layer liner systems.
3.2.3.5 Multiple-Layer Liner Materials and Specifications —
The principal materials used, and recommended by the USEPA
for use, in the construction of liner systems for hazardous waste
surface impoundments are geosynthetic materials and soils. Types
of each are used to impede liquid movement. Other types are used
to facilitate liquid movement and removal. Still others are used
to prevent one layer from intruding upon another, or to protect
other layers from damage by construction, operation, or weather.
Geomembranes — The USEPA believes that surface impoundment
structures must include the use of geomembranes. They are
considered by the Agency to be the only practicable means of
preventing the migration of chemicals into and through the
impoundment liners during the active life and post-closure
periods. However, they must be carefully selected for waste
compatibility, strength, and constructability in the specific
design situation. Geomembranes are not intended to be stress-
bearing members, and the design should avoid stressing the
material as much as possible.
Since hazardous waste regulations and proposed regulations
for municipal solid waste require that the final cover be as
impermeable as the liner, geomembranes are also recommended in
the final cover for surface impoundments (see Section 7.0), when
they are closed as landfills. In this case, a geomembrane is
used as the barrier to prevent entry of percolating water to the
underlying contaminated material. USEPA's minimum technology
guidance for landfill covers describes cover designs using a
49
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geomembrane as the principal barrier (USEPA, 1989a). The
geomembrane need not necessarily be of the same composition in
both the liner and cover.
Geomembranes are manufactured using low-permeability
synthetic polymers, either non-reinforced or reinforced with
fabric material. Geomembranes are made with various base
polymers and additives, thus varying considerably in their
physical and chemical properties and their interactions with
different wastes. No geomembrane is applicable to all wastes; a
particular type must be selected for each application.
The base materials of geomembranes are high-molecular weight
compounds (polymers). Some common polymers presently in use as
base products for geomembranes follow:
• thermoplastics (e.g., polyvinyl chloride [PVC])
• crystalline thermoplastics (e.g., high density
polyethylene [HOPE])
• thermoplastic elastomers (e.g., chlorosulfonated
polyethylene [CSPE])
• elastomers (e.g., butyl rubber); limited availability
Kays (1987), Giroud (1985), and USEPA (1983b) detail the
processing and manufacturing techniques used in producing
geomembranes. USEPA has provided detailed technical information
about geomembrane materials used to contain specific types of
hazardous wastes (USEPA, 1983b and 1988a). Geomembranes and
their polymers continue to improve as their manufacturers and
users gain experience in waste containment. Indestructability
and constructability are two general goals difficult to achieve
in the same material.
The compatibility of a liner material with a specific waste
is an important consideration in planning a surface impoundment.
Liquid wastes in surface impoundments may vary from complex
mixtures of variably concentrated constituents to highly
concentrated contaminants contained in a relatively simple
matrix. Additionally, waste constituents may change over time.
During the liner design process, a representative sample of the
liquid waste should be analyzed for waste properties that could
potentially cause damage to liner material. Several methods for
obtaining samples of hazardous wastes have been discussed by
USEPA (1982). Potentially detrimental waste properties include
the following:
• acidity;
• alkalinity (greater than pH 10);
• temperature extremes; and
• hydrocarbons.
50
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For hazardous waste surface impoundments, effective waste
containment is required for an extended length of time, including
the active impoundment life plus (if the unit is closed in place)
a 30-year post-closure period. Testing may be necessary to
determine the long-term effect of the impounded wastes on the
liner. Performance history and existing test data may preclude
site-specific testing in those cases where the waste has not
changed characteristics and the same geomembrane material
(essentially identical formulation) is used.
Test and/or performance data must be provided with the
permit application. Tests should be conducted using
representative samples of waste and leachates to which the liner
is to be exposed. USEPA has developed Test Method 9090,
"Compatibility Test for Wastes and Membrane Liners." This test
is intended to facilitate the estimation of the long-term
compatibility of liner material with wastes by immersing liner
samples in the waste for 30, 60, 90, and 120 days at 23 ± 2°C (73
+ 3.6°F) and at 50 + 2°C (122 + 3.6°F). A comparison of specific
physical properties of the geomembrane determined before and
after immersion is then made to determine suitability of the
liner. This test does not address all relevant variables, but
does provide a uniform method of testing geomembrane materials.
USEPA has published other compatibility test methods and a list
of liner manufacturers and material sources (USEPA, 1988a).
Geomembranes used in hazardous waste surface impoundments
are susceptible to failure during facility operation. Giroud and
Ah-Line (1984) discuss two basic types of geomembrane failure:
(1) excessive geomembrane displacement (i.e., vertical or lateral
movement) and (2) an unacceptable leak. Geomembrane liner design
should specify material components and construction practices to
prevent failures by:
• protecting the geomembrane from puncture, scratching,
abrasion, or other damage (from above and below);
• demanding that great care be taken to prevent damage to
the geomembrane sheet during all installation processes;
• providing gas venting in the drainage layer;
• avoiding bridging, rippling, stretching, or other
stressed conditions;
• allowing slack for shrinkage;
• avoiding nonessential penetrations;
• eliminating tensile stresses as much as possible, by
design;
51
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• providing detailed and rigorous seaming instructions;
• providing slip-preventing anchorage the tops of slopes;
• requiring well-trained and experienced installation
personnel; and
• providing a detailed quality assurance plan.
Design details for geomembranes and other geosynthetics to
resist various stresses have been provided by USEPA (1987c) and
Koerner (1990). Protective layers such as soil or geotextile can
help guard against damage to the geomembranes.
Geomembrane Protective Layers — A protective layer of
soil or a soil/geotextile layer may be used on the surface of the
liner system to protect the underlying geomembrane from
construction damage during installation, loads imparted by the
waste, weathering, erosion and abrasion, to increase friction,
and to dampen potential chemical attack. Above the liquid level
of the impoundment, coarser-grained (sometimes rubble) material
placed over a geotextile protective layer is often used. The
coarse material will generally be more stable on steeper slopes
and will dampen wave action and run-up. An even coarser layer
(e.g., riprap) may be applied over the geomembrane-protective
layer to prevent erosion in larger impoundments. Below the
liquid level, sand is often used to protect the geomembrane
against puncture, and to dampen the effects of strong chemical or
high-temperature waste inputs. A cross section showing some
protective layer options is shown in Figure 18.
Based on field experience, USEPA (1983b) recommends that
protective soil on a liner be placed at no greater than a 3H:1V
slope and that it be at least 18 inches (45 cm) thick before
allowing any construction equipment over the liner. Slope
stability may be a problem with soil placed directly on the
geomembrane. Mitchell et al (1989) have examined geomembrane-
soil interfacial stability and have concluded that the design
should incorporate the effect of pore-water pressure. A
geotextile can reduce pore-water pressure at this interface. The
recommended general equation for determining the stability is:
[( yz cos /3) - ju] tan
FS =
7 z sin /3
where FS = factor of safety
7 = soil density (including moisture)
z = soil depth perpendicular to the geomembrane
jS = slope angle
(f> = interfacial friction angle
H = pore-water pressure
52
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A factor of safety of at least 1.5 is recommended.
Geotextiles may be used at various places in the liner
system as protective layers and, where appropriate, as slide-
resistant interface materials between soil and geomembranes. On
the top (primary) geomembrane liner, a geotextile may be used to
shield the geomembrane from any larger, sharper particles in the
overlying soil protective layer. A geotextile layer can be used
to reduce the thickness of the soil protective layer from 18
inches (45 cm) to perhaps 12 inches (30 cm). Geotextiles may
also be used between that geomembrane and the drain layer
material, and again at the bottom of the drain layer atop the
second geomembrane as shown in Figure 18. Nonwoven geotextiles
ordinarily have excellent protection properties, making them
ideal for these applications. In general, the greater the mass
per unit area, the greater the protection afforded by the
geotextile.
The exterior dike slope may be protected by riprap,
vegetation, or other stabilizing material to prevent erosion.
Vegetation requires planting a suitable plant species and
maintaining it. Several references are available to assist in
establishing vegetation (e.g., USEPA, 1979b and 1983g; Lee et al,
geotextiles
riprap armor
soil protective
layer
native soil
foundation
compacted low-
permeability soil
leak collection layer
Figure 18. Impoundment dike cross section showing
optional protective layers.
53
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1985). Riprap consists of graded rock courses and a geotextile
filter placed directly on the dike. Riprap has many advantages,
including flexibility, ease of repair, simple construction, and
salvageability. Battelle Pacific Northwest Labs (1982), the U.S.
Bureau of Reclamation (1974), and Barren (1977) discuss the
design of riprap coverings. FHWA (1985 and 1988) and Koerner
(1986) have reviewed geotextile design for erosion protection.
Although not common, other materials may be used for dike
protection. They include concrete, asphalt, and soil cement.
Weather resistance, and resistance to the contained waste where
in contact with it, must be carefully considered before designing
with these materials.
Low-Permeability Soil Liners — The purposes of the low-
permeability soil component of the secondary liner are (1) to
minimize the migration of hazardous liquids through the
geomembrane component if a breach in the geomembrane should occur
and (2) to attenuate constituents that might leak through the
geomembrane.
Soil liners are not impervious, but they do control seepage
and have been used, because of that and their low cost, in the
past as the sole liner in surface impoundments and landfills.
Soil may be treated, remolded, and/or compacted to achieve
prescribed flow-impeding and contaminant-attenuating
specifications. However, the USEPA does not believe that a soil
liner can ordinarily be constructed to meet, by itself, the
requirement of no contaminant breakthrough during the active life
and post-closure monitoring period for a surface impoundment.
Despite USEPA's wariness of the impermeability of soil
liners, they are still recommended in USEPA's minimum technology
guidance as backup to the geomembrane component of the secondary
liner in the required double-liner system. Further, low-
permeability soil may be used as the secondary liner without the
geomembrane if it can be shown that it will provide equivalent
performance and not allow contaminant breakthrough during the
active life and post-closure monitoring period. The Agency
believes that such a showing will be very difficult.
Waste fluids or leaked fluids may substantially increase the
overall permeability of a soil liner, which then will not meet
the design requirements (USEPA, 1983b). USEPA (1983b) provides
guidance on evaluating the effects that waste fluids may have on
the permeability of soil liners. Selection of soil liner
material should include testing the material with a standard
aqueous permeant (e.g., site ground water or a 0.01 Normal
solution of CaS04) and a representative sample of the waste
liquid to be impounded. Distilled water is not a good aqueous
54
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permeant because it tends to react with clay particles, resulting
in an unnaturally low permeability (Olsen and Daniel, 1981).
The material chosen for the low-permeability soil component
should be compactable to uniform hydraulic conductivity values no
greater than 3.9 x 10~8 in./sec (1 x 10~7 cm/sec). Elsbury et al
(1985) found from a survey of hazardous waste disposal facilities
that liner soils are generally selected on the bases of grain-
size distribution, liquid limit (LL), plasticity index (PI), and
laboratory hydraulic conductivity. In that survey, soils used to
construct low-permeability soil liners consisted predominantly of
the clay groups designated by CL (low-plasticity clay) and CH
(high-plasticity clay) in the USCS. LL and PI values were
generally in the range of 20 to 45 percent and 5 to 30 percent,
respectively. Table 7 lists typical soil liner design
specifications.
The USEPA uses the term "low-permeability soil" so as not to
imply that there is a narrow restriction on the type of soil that
may be used. The term "clay liner" is used casually to refer to
any soil liner, but the liner may, in fact, be comprised for the
most part of materials other than clay. Also, a sandy soil may
be made to meet very low permeability requirements with the
addition of only a small percentage of bentonite.
The performance of the soil selected for the design can be
verified in a test fill. The test fill data should demonstrate
whether a structurally stable soil liner component with uniform
hydraulic conductivity of 3.9 x 10"8 inches/sec (1 x 10"7 cm/sec)
or less is attainable using the soil, construction equipment, and
methodology specified in the design. USEPA (1988b) describes
some of the techniques used in constructing and testing a test
fill. Data from test fills can be used to evaluate the
relationships between density, water content, and hydraulic
conductivity, and to validate design and construction procedures
(Mundell and Bailey, 1985).
USEPA's minimum technology guidance (1987a) recommends that
the soil component of the secondary liner have a thickness of at
least 36 inches (90 cm) and be chemically resistant to the
impounded waste. The guidance also states that the soil should
be compacted in lifts of 6 inches (15 cm) or less, after
compaction, and be free of rocks, roots, and rubbish.
The finished surface of the soil component must be as smooth
as possible to facilitate continuous contact with the overlying
geomembrane.
55
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TABLE 7. TYPICAL OPTIMUM SOIL LINER DESIGN SPECIFICATIONS.
Property
Specification
Test Method/Reference
Thickness
Grain Size
Hydraulic
Conductivity
(laboratory)
Hydraulic
Conductivity
(field)
Liquid Limit
Plasticity
Index
Compaction
Maximum
Clod Size
Frost
Susceptibility
Volume Change
General
Minimum 36 in. (90 cm)
measured perpendicular
to slope and compacted
in 6-in. (15-cm) lifts.
>50% by weight passing
200 mesh; >25% clay
content (0.002 mm)
<1 x 10"7 cm/sec (3.9 x
10"8 in./sec) when
compacted to anticipated
field density and moisture
content.
<1 x 10"7 cm/sec
(3.9 x 10"8 in./sec)
24 - 60
10 - 30
>95% Standard or
>90% modified
Proctor density
< 1 in. (2.5 cm)
0.039 - 0.31 in./day
(1-8 mm/day)
<10%
Liner should be free of
all organic matter (e.g.,
vegetation, wood), large
stones, and construction
debris.
EPA (1987a)
ASTM D 422-63
Daniel et al (1985)
EPA (1983c)
Day and Daniel (1985)
Anderson et al (1984)
ASTM D 4318-84
ASTM D 4318-84
ASTM D 698-78 or
ASTM D 1557-78
Observational
Chamberlain et al
(1982)
Holtz (1965)
56
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Leak Detection and Collection System -- 40 CFR 264.221
requires a leak detection, collection and removal system between
the liners, including the sidewalls, of the double-liner system.
It must perform its function during the active life and post-
closure care period of the impoundment.
The leak collection system should incorporate a granular or
synthetic drainage layer designed to rapidly detect, collect, and
remove liquid that migrates into the space between the liners.
The system should be designed so that little or no liquid head
will ever be present to impinge on the bottom (secondary) liner.
Excessive head in the leachate collection system can result
in liquid "whales" in the upper liner (USEPA, 1989b). The head
would be caused, at least partially, by ground water infiltrating
from a high water table. After the collection system along the
bottom has been saturated and pumpout is inadequate, water can
rise rapidly along the side slopes. Whales may also be caused by
gas buildup beneath the primary liner. Either type is less of a
concern with double liners than with older single liners.
A minimum bottom slope of 2% should be specified for the
secondary liner to facilitate drainage in the collection layer to
the pumpout sump. That layer should have a minimum hydraulic
conductivity of 2 ft/min (1 cm/sec) and should operate without
physical or biological clogging. Granular materials should be
USCS-classified as "SP", without fines that may clog the layer,
and with grain sizes no larger than 3/8-inch (0.95 cm) if placed
directly on the secondary geomembrane liner. A geotextile layer
can be placed between the drainage layer and geomembrane to
facilitate placement of the granular layer and to reduce the
potential for damage to the geomembrane.
The drainage layer materials should be chemically resistant
to the waste contained in the surface impoundment, and of
sufficient strength and thickness to prevent collapse from the
expected overburden pressures. This makes granular drainage
especially attractive. Drain pipes of appropriate size and
spacing must be located in the bottom of the unit to remove
liquids that may collect there. Larger diameter pipes have been
recommended (e.g., 6 inches [15.2 cm]) because they offer greater
protection against clogging and are simpler to inspect and
maintain. Guidance concerning appropriate drain-pipe sizing and
spacing has been provided by USEPA (1983a).
Geosynthetic drainage layer materials, available in a
variety of configurations, can be used in place of the granular
layer. Several of the available materials have been described by
Koerner (1990). If used, they must be demonstrated as equivalent
in performance to the conventional granular system with pipes.
Equivalence in flow capacity equates to a transmissivity of 3 x
57
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10"3 mVsec (0.032 ft2/sec) for the geosynthetic material.
Geosynthetic drainage materials must also be chemically resistant
to the waste and leachates, have no detrimental effect on the
geomembrane liners, and withstand the designed loads.
Geosynthetic drainage and filter materials may creep and
deform under load, resulting in gradual restriction of flow
capacity over time. Design should be based upon laboratory
transmissivity tests of the proposed material over time under
loads equivalent to and exceeding the expected overburden. A
minimum design ratio (factor of safety) of 3.0 for loading has
been suggested by USEPA (1987c) to maintain the required
transmissivity.
Some impoundment designs use synthetic (e.g., "geonet")
drain layers on the side slopes and granular layers on the bottom
section (Figure 19). Geosynthetic material is easier to install
and may be more stable (with anchorage) on the side slopes and,
because it is thinner than a granular layer, allows greater waste
containment capacity within an impoundment of the same overall
dimensions. A geotextile layer may be placed on either side of
the drain layer to prevent creep of the geomembrane into the
drainage material, lowering the capacity (see Figure 42).
Anchorage for geosynthetic drainage material is usually similar
to the anchorage used for the geomembranes and may perhaps be the
same trench used for one of them.
soil protective
layer
geomembrane liners
with protective
geotextiles
low-permeability
soil layer
granular drain layer
Figure 19. Junction of side-wall geonet and bottom granular
layers of leak detection, collection, and removal
system.
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The system design should include an appropriately sized
sump, with a depth of at least 12 inches (30 cm) below the
drainage layer grade, to collect and remove liquids. If
possible, the pumpout and sampling system should be routed up the
side walls and not penetrate the top liner until well above the
liquid level (Figure 20). Again, all system components should be
designed with materials resistant to the waste being contained in
the impoundment. Provision should be made for measuring and
recording the liquid volumes collected in the sump, and for
disposal of the liquid. Operating procedures for handling
liquids collected in leak detection/collection systems are
discussed in Section 5.1.2.
Access to all parts of the system should be provided to
facilitate inspection and maintenance of components. Manholes
and cleanouts should be placed so that maintenance equipment can
reach any pipe section. Bass (1985) summarizes the state-of-the-
art information on system failure, concerning leak collection
system design, construction, inspection, maintenance, and repair.
Geotextile Filter — Geotextiles are used as filters to
prevent fine-grained soils and solids from the waste from
entering and clogging the leachate collection system.
leak collection system
pumpout and sampling access
geomembrane liners
with protective
geotextiles
low-permeability
soil layer
leak collection layer
Figure 20. Example access for leak collection system
and liquid removal.
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Geotextiles, as filters, are placed adjacent to both synthetic
and granular drainage layers. Adequate performance will depend
on proper selection of the geotextile in relation to its
compatibility with the material that is to be filtered. Detailed
design of geotextile filters is described by USEPA (1987c), FHWA
(1985), and Koerner (1990). Design basically consists of
selecting a geotextile whose largest opening, as measured by its
"apparent opening size" (AOS), is smaller than the larger of the
particles to be filtered, as measured by the d85 of that material
times a retention factor (\), i.e.,
AOS = d85/factor of safety
The flow rate through the geotextile, as measured by its
permittivity, ^ , is then selected to be greater than the flow
from the soil times a factor of safety, usually 10 or greater.
The clogging potential of the filter is then evaluated based on
the fines in the material to be filtered and the type of
geotextile. Clogging potential is best evaluated through long-
term flow tests using a simulated cross section and leachate.
Other important considerations include physical durability
(it must survive construction activities) and chemical and
biological resistance to the environment of the impounded waste.
One reference for the minimum physical requirements for
separation and drainage applications is the AASHO-AGC-ARBTA Task
Force 25 set of specifications, as published in "Geotextile
Design and Construction Guidelines" (FHWA, 1990).
Gas-Venting Layer — A gas-venting layer should be
considered in the design beneath the bottom (secondary) liner,
between it and the foundation. It would be appropriate where the
foundation material contains biodegrading organic material or
other sources of gas. The material used for the venting layer
should be similar to that used for the leak collection layer. It
may be either granular or geosynthetic material. Thick
geotextiles, with the capacity for in-plane transmission, may be
considered where gas generation is anticipated to be low. Geonet
strips may be inserted at regular intervals to increase flow
capacity and facilitate venting. Gas exits from the venting
layer should not penetrate the liners. Instead, wherever
possible, they should be brought to the surface outside the side-
slope liner (Figure 21).
3.3 LIQUID LEVEL CONTROL
Perhaps the greatest environmental risk in surface
impoundment operation is unintentional liquid overflow.
Reliable liquid level controls are critically important. Two are
60
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gas vent
soil protective
layer
geomembrane liners
with protective
geotextiles
low-permeability
soil layer
gas vent layer
(geosynthetic or granular)
geotextile filters
Figure 21.
Liner system showing gas-vent layer and
exit through top of dike.
required, passive (spillway or overflow pipe) and active (level-
actuated pump or valve). For detailed information on liquid
level controls, the following references are recommended: Linsley
and Franzini (1979), Considine (1974), and Hicks (1972), and
Shiver et al (1985).
Unless a passive outfall is part of an intended flowthrough
process, it should be designed only as an emergency outlet to
prevent a breach in the dike in case of an uncontrolled liquid
rise. Spillways or weirs are preferred (Figure 22). Pipe or
conduit outlets through the dike (Figure 23) are not generally
recommended because seepage may occur around the conduit, leading
to soil displacement and possible dike failure. A leak plate, as
shown in Figure 23, may be placed within the dike to interrupt
possible flow and channel creation along the pipe-soil interface.
Liquid releases from a hazardous waste surface impoundment
via a passive outfall operating as a safety valve constitute an
emergency situation. Potential releases should be considered in
the design. Containment for such releases, such as a secondary
perimeter dike, should be provided.
The surface impoundment design must also include a system to
monitor liquid level. The system may vary from a manually-read
guage (Figure 24) to a liquid-level detection, monitoring, and
control system designed to operate as an automated system that
detects, monitors, and controls the liquid level with little
manual assistance, other than routine maintenance and
calibration.
61
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impoundment
Figure 22. Dike spillway with protective toe apron.
Figure 23.
overflow
discharge pipe
\
apron
Impoundment overflow discharge
pipe through dike.
62
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gauge support and access
staff gauge
DIKE
Figure 24.
Example of liquid-level monitoring setup;
manually-read staff gauge.
In an automated system, the attainment of a specified liquid
level may trigger a device to stop input to the impoundment or to
open a discharge line, or both. Another option is to trigger a
warning signal indicating that some other action is needed.
Other options, or combinations of options, may be activated with
an automatic level-sensing system. Figure 25 is a sketch of a
liquid-level recorder and alarm system.
automatic
recorder and alarm
gauge support and access
Figure 25. Example of liquid-level recording and alarm system.
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The major components governing the flow into and out of a
surface impoundment are the inflow and outflow structures.
Normally, the inflow structure will be a pipe equipped with a
flow valve. Typical outflow structures are pumps, weirs,
spillways, and pipes.
The valves used at hazardous waste surface impoundment
should be capable of being integrated into the level control
system, and acting as a manual override for adjusting flow if the
automated system fails. Valves used in hazardous waste service
must be constructed of materials that are compatible with the
hazardous liquids to which they are exposed. Considine (1974)
provides a complete review of the types of valves available.
The selection of a specific pump type should be based on the
characteristics of the liquid to be moved (e.g., corrosivity,
ignitability, specific gravity, density, and total solids
content). Aid in selecting a pump can be found in a reference by
Karassik et al (1986).
A protective membrane or apron must be designed to prevent
erosion at outfall and inflow structures where the discharge is
onto a surface. The design of the protective apron will largely
depend on the types of inflow and outflow structures, flow rates,
and liner protection. The construction material for the apron is
often concrete, but may be geomembrane, or, in some cases,
velocity-impeding riprap placed over a geotextile.
When passive outfalls are used, the protective apron should
extend from the outflow structure to some distance away from the
toe of the dike slope (see Figures 22 and 23). If the outfall is
designed for regular process use rather than for emergency
overtopping prevention, apron design must prevent erosive flow
and reduce terminal velocities. Common methods for reducing flow
velocities and erosive flow employ baffled aprons (U.S. Bureau of
Reclamation, 1974) and properly designed sloping aprons (Linsley
and Franzini, 1979).
In many cases, pipes will be used for both inflow and
outfall structures. When used for inflow, the primary concern is
to prevent erosion in soil liners and tears in geomembranes
caused by impinging liquid. Enlarging the outlet of the pipe to
a size several times larger than the average diameter of the
inflow pipe will act to dissipate energy, reducing the potential
for causing liner failure (Kays, 1977). The outlet may be
extended out to the deeper part of the impoundment and located
near the liquid surface so that the energy is dissipated into the
liquid. If a pipe is used as an impoundment outfall structure,
the preferred design is to place the pipe so that the outlet is
at or near the same elevation as grade, and extends beyond the
outside toe of the berm (see Figure 23).
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3.4 SECONDARY CONTAINMENT
Secondary containment structures are not specifically
required under current RCRA regulations. However, they can
prevent widespread damage if the impoundment should be breached,
particularly if the impoundment has been constructed entirely
above grade.
Secondary containment structures usually consist of low
dikes with a much larger plan area than the primary containment
dikes (see Figure 26). The secondary containment structure
should be able to withstand a sudden surge of released liquid and
should retain the released waste for a period sufficient to allow
detection and recovery of the material. Engineering
considerations (e.g., soil stability) used in the design,
construction, and operation of the primary surface impoundment
dike are applicable to secondary containment structures.
Hydrologic considerations also should be included in the
secondary containment design.
primary impoundment dike
secondary
containment dike
Figure 26. Primary and secondary containment dike layout.
3.5 LEAK DETECTION SYSTEMS
"Leak detection system" is a term applied to a monitoring
device or technique which monitors the integrity of an
impoundment liner in a non-destructive manner. In a sense, the
conventional leak collection system serves that purpose. It can
detect the presence of liquid in the leak collection system and
thus the occurrence of a leak in the primary liner. If a leak
collection system is installed beneath the secondary liner, it
could serve the same purpose for the secondary liner. However,
detection does not occur in either case until the liquid reaches
the sump. Nor can the leak location be pinpointed.
A leak detection system, as the term is used here, should be
capable of detecting liquid moving through the liners essentially
as soon as a hole appears. In addition, it is desirable that it
have the capability of locating very small leaks to within an
area of less than 1 ft2 (0.09 m2) (USEPA, 1984b).
65
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Leak detection techniques employ either sensors (1) located
within the potentially leak-affected zone or (2) remotely located
outside the impoundment area. These are referred to as in-situ
and remote systems, respectively. An in-situ system is arrayed
at the time of construction, while a remote system may be
installed at an existing site. Remote sensors may be located at
the surface or in a borehole at some distance from the leak. The
output of either a remote or in-situ system would be recorded at
some convenient surface site. In situ instrumentation can be
continuously monitored throughout the active life of the
impoundment and during the post-closure period. Remote systems
are ordinarily activated periodically during routine monitoring
or when liner failure is suspected.
Leak detection techniques that may be adaptable to the
surface impoundment situation include (1) electrical resistivity,
(2) electromagnetic techniques, (3) acoustic emissions, and (4)
seismic methods. Of these techniques, electrical resistivity
appears to show the most promise with respect to performance and
reliability (USEPA, 1984h).
A leak detection system may be arrayed to detect leakage
into the leak collection system and the leak location in the
primary liner (Figure 27). This is usually a system that is
operated intermittently, because it uses a transducer floating
across the impoundment surface. A system may be designed to
detect leakage into the leak collection system, but without the
capability of locating the leak (Figure 28). One may also be
arrayed to detect leakage through the geomembrane component of
the secondary liner, or through the entire composite liner into
the vadose zone (Figure 29). All of these systems are based on
electrical resistivity.
recording station
geomembrane liner
leak collection layer
Figure 27. System to detect and locate leaks in top (primary)
geomembrane liner.
66
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recording station
conductors
geomembrane liner
leak collection layer
Figure 28. System to detect leakage through top (primary)
geomembrane liner.
recording station
Figure 29. System to detect leakage into vadose zone.
Lysimeters (soil-pore liquid samplers) have been used
successfully to monitor leachates migrating through the
unsaturated zone at land treatment facilities and other disposal
sites (USEPA, 1988b). Lysimeters have also been placed below
liners at surface impoundments to collect samples of leaking
waste liquid (Figure 30). The advantage of placing lysimeters in
the unsaturated zone beneath a liner system is that liquid
samples can be collected and analyzed for the presence of
hazardous constituents, their rate of degradation, and
decomposition by-products (USEPA, 1988b). Unfortunately,
lysimeters cannot be used to determine the rate or absolute
amount of leachate moving through the soil.
Using a liquid mass balance to identify leaks in some
situations may be a reasonable backup method for detecting
surface impoundment leaks (USEPA, 1984b). It is not proposed as
a primary leak detection method. The liquid mass balance process
for leak detection requires long-term measurements of waste input
and environmental parameters. Leaks will not be detected in a
period of days, weeks, or possibly even months, unless the leak
is fairly large.
67
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lysimeter manhole
geomembrane
protective layer
liners
£
y^ggy —I
» h double liner
low-permeability
lysimeter soil
Figure 30. Lysimeter for leak detection beneath bottom liner.
The liquid mass balance method is suited for covered surface
impoundments with known or controllable input and output
parameters. In open systems, where the parameters cannot be
controlled, it is considerably more difficult. Nearly all
surface impoundments are open systems where evaporation,
precipitation, wind speed, and temperature have not been recorded
or have been measured only at low levels of accuracy.
Perhaps the largest source of error in the liquid mass
balance is the evaporation estimate. Due to the many
uncertainties in applying the mass balance to an open impoundment
and the potentially large errors in the parameters, the mass
balance should not be considered as a primary leak detection
method for open impoundments (USEPA,1984b).
3.6 SURFACE WATER MANAGEMENT
Diversion structures (berms and ditches) intercept and
redirect flow of surface water away from a surface impoundment
(Figure 31). Surface impoundments, unless they are intended to
collect runoff, need to be hydrologically isolated from the
surrounding terrain to function properly. Diversion structures
must be designed to prevent flow into the impoundment from at
least the runoff from a 25-year storm (USEPA, 1982a).
Rather than moving water away from an impoundment, diversion
structures can also be used to divert runoff, where contaminated,
into a holding pond or surface impoundment.
Runoff diversion is accomplished by constructing a berm of
moderately compacted soil or by excavating ditches to divert
water around the impoundment. Excess material from constructing
the surface impoundment or from ditch excavation is often
suitable for constructing diversion berms. Figure 32 shows three
different ditch and berm cross sections.
68
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surface water flow
direction
diversion ditch
and berm
secondary
containment dike
Figure 31. Runoff diversion past surface impoundment,
design flow depth
freeboard
design flow depth —
freeboard
design flow depth —I
freeboard
Figure 32. Typical diversion ditch and berm cross sections,
Berms are embankments of earth or other suitable materials
constructed to protect a surface impoundment from spreading
surface waters. They are designed to provide short-term
protection by intercepting storm runoff and diverting the flow to
natural or manmade drainageways. Sufficient freeboard should be
provided to prevent overtopping of the berm by the design flood.
Berm sloughing can be reduced by using proper side slopes and
construction methods. A berm should have a minimum top width of
4 feet (120 cm) and a freeboard of 3.5 to 12 inches (9 to 30 cm)
(USEPA/1982a).
69
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Berms should be placed across the slope at an angle to the
horizontal so that water moving downslope is intercepted and
moved laterally around the impoundment at a low velocity to
minimize erosion. Longer slopes may need more than one berm.
A diversion ditch should be designed to accommodate flows
from a 25-year frequency storm (USEPA, 1982a), and must intercept
and convey the flows at non-erosive velocities. Diversion
ditches are sized based on the Mannings Formula (USEPA, 1982h):
Q = 1>49 AR2/3S1/2
n
where:
Q = discharge in cfs (mVsec)
n = coefficient of roughness
A = cross-sectional area of channel in ft2 (m2)
R = hydraulic radius of the channel in ft (m)
S = longitudinal slope of the channel in ft/ft (m/m)
Design and construction criteria to use in diversion ditch
design follow:
• Location should be based on outlet conditions,
topography, soil type, slope length, and grade.
• Capacity should be sufficient to carry the peak discharge
from a 25-year design storm using an assumed Mannings'
coefficient (n).
• Channel should be trapezoidal, V-shaped, or parabolic,
with side slopes no greater than 3H:1V.
• Flow velocity must not exceed the flow for the assumed
Mannings coefficient (n) during the storm period.
The USDA (1979) provides information on maximum allowable
velocities for an unlined drainage ditch excavated in various
soils. However, channels must be grass-lined or riprapped over a
geotextile to minimize erosion. Riprapped channels are
appropriate for sites with heavy runoff volumes. The maximum
allowable velocity for a grass-lined channel can also be found in
the SCS manual (USDA, 1979). Channel shape may be based on
construction and maintenance equipment available (e.g., graders
are suitable for V-shaped channels).
Erosion is a function of velocity, depth, and time. The
runoff peaks for the site must be determined to find which of
these are key design parameters. Manuals (e.g., Urban Drainage
and Flood Control District, 1979) can be used to find the design
flow depth of channels.
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3.7 CONTROLS FOR VOLATILE ORGANIC COMPOUND (VOC) EMISSIONS
The loss of VOCs from hazardous waste surface impoundments
has been the focus of recent research. Investigated methods for
reducing VOC emissions include complete enclosure, surface
barriers, wind diversion fences, and adsorbents (EPA, 1985e).
Complete enclosure of the impoundment involves the use of an
air-supported structure, or an inflated membrane, covering the
entire impoundment (USEPA, 1985e). The membrane would be sealed
around the edges, and the vapors would be bled off for treatment
or incineration. This method could be expected to be nearly 100%
effective in preventing the escape of VOCs. Compatibility of the
vapors with the synthetic cover material must be considered.
Isolating volatile liquids from the air using a surface
barrier involves floating a cover of foam-like material, oil,
geomembrane, or other material, on the liquid surface. The
objective is to eliminate, or at least reduce, the exposed liquid
surface area, thereby reducing VOC emissions. Among the many
potential problems associated with this method are dislocation of
the surface barrier by wind and wave action, thereby uncovering
the waste. In the case of an oil barrier, VOC emission from the
oil itself may occur. Valsaraj et al (1985), in laboratory
simulations, examined the ability of a mineral oil layer in
controlling emissions of acetone, n-propanol, ether, and benzene.
It was much more effective in controlling the first two.
If a floating geomembrane is to be used, its compatibility
with the waste liquid and vapors must first be considered.
Wind diversion fences have been researched in laboratory
simulations by Thibodeaux et al (1985). Study results indicate
that various arrangements of perimeter and network grid fences
can reduce VOC emissions up to 80 percent (Figure 33). The
research is based on the premise that reducing the flow of air
across a surface impoundment will cause a subsequent reduction in
VOC emissions. This is because the volatilization process which
increase with wind speed. (Wind fences are ineffective at zero
wind speed.) Because the resistance on the liquid side of the
interface is typically much greater than the resistance of the
gas side, the liquid controls the volatilization process.
Adsorbents (e.g., activated carbon) appear to be the least
attractive of the methods. They are expensive and must be
replaced or regenerated. The use of adsorbents is based on the
adsorbent "trapping" a VOC, which is then no longer available to
volatilize. Increasing the amount of adsorbents in the
impoundment reduces the effective concentration of VOCs in the
depends on chemical gradients across the liquid/air interface,
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perimeter wind fence
network wind fence
I I I I I I I I I I I I I
Figure 33.
Wind diversion fences for VOC control.
(from Thibodeaux et al, 1985)
liquid phase, thereby decreasing the emission rate. Further
information on this technique is available in Cudahy and Sandifer
(1980).
It is unlikely that these methods will be perfected to a
point that they can reduce VOC emissions from surface
impoundments by 100%. Therefore, an alternate procedure to
reduce the VOCs reaching the impoundment (e.g., by pretreating
the waste stream) may become important if regulatory guidelines
are adopted that impose stringent VOC emission standards. At the
least, VOC monitoring is likely to become a requirement for
surface impoundments handling hazardous wastes. Waste-stream
pretreatment is a design alternative which should be considered.
Waste pretreatment methods to control VOC emissions may
include stripping the waste stream with air or steam, using
adsorbents (i.e., activated carbon) or chemical oxidation. Each
system has its advantages and disadvantages, and none is best
suited for use in all situations. Information on stripping
methods, adsorption technology, and chemical oxidation is
available in the literature (USEPA, 1984c and 1984d).
Much of the research into methods to reduce VOC emissions
has resulted in models that attempt to estimate VOC losses.
Unfortunately, these models involve complex mathematical
equations and are not easily adapted for practical use. One
exception is a procedure suggested by Shen (1982), which provides
practical engineering solutions and offers a simple technique for
estimating the volatilization rate of VOCs from waste lagoons.
Shen emphasizes that this is an estimate and should only be used
to evaluate the potential for VOC emissions.
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Other methods and related information available for
estimating VOC emissions include the following: USEPA (1985c and
1985f), Mackay and Yeun (1983), and Thibodeaux et al. (1982).
3.8 CONSTRUCTION QUALITY ASSURANCE (CQA) PLAN
A Construction Quality Assurance Plan should be developed as
part of the surface impoundment design process. The plan
describes the programs and principles employed to control quality
of construction. Quality Assurance (QA) principles include the
following:
• sufficient autonomy and authority for the designated QA
personnel or QA/Quality Control (QC) organization;
• establishing procedures to control activities affecting
construction quality;
• documenting evidence that these activities are performed
satisfactorily; and
• systems to identify, correct, and report nonconforming
items.
USEPA (1986a) has provided guidance for preparing a CQA plan
to ensure, with a reasonable degree of certainty, that completed
disposal facilities meet or exceed design criteria, plans, and
specifications. The guidance covers CQA for the following
structural components of surface impoundments:
foundations
dikes
low-permeability soil liners
geomembranes
geotextiles
leak collection systems
final cover systems
CQA activities must be coordinated with construction
activities to ensure that specifications and performance
standards are achieved. This coordination requires that the
contractors establish and modify on a routine basis a schedule of
construction activities. An accurate schedule is essential if
CQA goals are to be achieved.
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CHAPTER 4
CONSTRUCTION
The surface impoundment construction phase includes the
following tasks: site preparation; excavating and preparing
foundations and cut-slopes; constructing dikes, soil liners, and
secondary containment structures; installing geomembrane liners;
constructing leak collection systems; placing protective
coverings over dikes and liners; and installing liquid level
control systems. This section discusses each of these tasks.
4.1 SITE PREPARATION
The surface impoundment site is prepared for construction by
clearing trees, vegetation, topsoil, stones, and unwanted
structures. Support facilities for work crews and equipment are
then installed. They include a management office, field
laboratory, and equipment storage and maintenance areas. Safety
and spill containment devices are installed. Finally, borrow
areas are cleared and established as sources of material for
constructing dikes, liners, and other earthen structures.
Surface drainage at the site should be diverted or otherwise
controlled to prevent runon to the borrow and construction areas.
Runon can adversely impact the workability of clay soils and the
time required to complete construction.
Attention to site security is generally required to prevent
theft, vandalism, or unauthorized entry. The degree of security
depends on the type and amount of equipment and materials stored,
the amount of time personnel are off-site, the remoteness of the
site, and other site-specific factors. Measures that may be
taken include the installation of fencing and locked storage
facilities, and providing security personnel.
4.2 CUT-SLOPE AND DIKE FOUNDATION CONSTRUCTION
Dikes and cut-slopes are constructed so that they form the
continuous interior slopes of a surface impoundment (Figure 34).
Dikes are situated above grade and are constructed of compacted
fill material. Cut-slopes are excavation side slopes cut into
the native soil. The final interior surface of dikes and cut-
slopes, along with the excavated bottom surface, will act as a
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geomembrane liners
and protective
geotextiles
protective soil layer
leak collection
layer
\/X"^\V *
cut-slope
low-permeability
soil layer
Figure 34. Cut-slope, dike, and side-wall cross section.
uniform foundation for the impoundment liner, and, therefore,
must have constructed strength and compressive properties
compatible with foundation support requirements.
4.2.1 Cut-slopes
Cut-slopes are required where all or part of the impoundment
is to be below grade. They are constructed by removing the soil
with standard earth-moving equipment. After the cut forming the
sidewall is completed, the cut-slope may require moisture content
adjustment and/or compaction to bring the soil to desired
specifications. If the moisture content is too high, simply
allowing the slope to dry may be adequate. The cut-slope should
not be exposed to rain unless runoff will not adversely affect
the foundation characteristics of the slope soil.
Some sensitive soils can actually lose strength as a result
of attempted compaction. If the design engineer considers
compaction appropriate, it can be performed using the same
equipment and procedures used to compact dikes and soil liners.
Most equipment can adequately compact side-wall slopes up to
3H:1V. Steeper slopes or wet conditions may require that towed
compactors be used, or even that towing assistance be provided
for self-propelled equipment. Towing may require another piece
of heavy equipment or a winch positioned at the top of the slope.
The steepness of the side slope also affects the soil
compactive effort. As the slope increases, the effect of the
compacting weight applied to the soil decreases due to the
increasing force vector directed down the slope (see Figure 35).
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W = weight of compactor = 2000 Ibs (909 kg)
E = effective compacting
weight = 1414 Ibs (644kg)
P = weight parallel to
slope = 1414 Ibs (644 kg)
effective compaction on 45° slope
W = 2000 Ibs (909 kg)
E = 1897 Ibs (862 kg)
P = 635 Ibs (289 kg)
effective compaction on 18.5 °slope
Figure 35.
Idealized schematic showing effects
of slope on compactive effort.
This effect is overcome by increasing the number of equipment
passes required to reach the specified soil density.
The impoundment bottom is also a cut surface that should
possess the same characteristics as the side slopes when
completed. Most of the same considerations apply.
4.2.2 Dike Foundation
The dike foundation is the base upon which the dike is con-
structed. The condition of this foundation is as important to
dike stability as the strength of the dike material itself. As
impoundment construction begins, the foundation area is cleared
and the topsoil is removed.
If the foundation soil consists of soft or sensitive fine-
grained soil, final excavation to subgrade elevation should be
done using a straight-edged excavator bucket (no teeth) to avoid
remolding of the subgrade soils and the resulting strength loss.
If zones of weak or undesirable soil are suspected, proof-rolling
may verify their existence. Proof-rolling consists of passing a
piece of heavy compaction equipment, such as a pneumatic roller,
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over the soil. Poor or weak areas will be revealed by the
development of rutting or ground heaving. If weak areas are
exposed, poor soils should be removed and the area filled and
recompacted with acceptable material.
Scarification of the foundation soil may be required to
provide the soil with an adequate bonding surface for placing the
initial soil lift during dike construction (Creager et al, 1945;
Hammer and Blackburn, 1977). Scarification is accomplished using
a ripper attachment on a bulldozer, disc harrow, or similar
implement. Foundations composed of firm soil should be easily
scarified, while scarifying soft soils may be less successful.
The correct combination of moisture content, density, and
scarification should promote bonding between the foundation soil
and the initial lift of dike soil by facilitating intermixing of
the two materials during compaction. This action will minimize
horizontal permeability at the foundation-dike interface.
4.3 DIKE AND SOIL LINER CONSTRUCTION
The construction of dikes and the construction of soil
liners have many similarities. The following discussion
addresses them together where appropriate.
4.3.1 General Construction Process
Dikes and low-permeability soil liners are constructed of
soil which is placed in lifts and compacted at a specified
moisture content to a specified density. Successive lifts are
placed and compacted until the design height or total thickness
is achieved. Standard recommended lift thicknesses are 9 inches
(23 cm) for loose lifts; and 6 inches (15 cm) for compacted
lifts, when compacting with penetrating-foot rollers on cohesive
soils (Church, 1981; Creager et al., 1945; Hammer and Blackburn;
1977). The general sequence for dike construction is as follows:
(1) Scarify and adjust moisture content (if required) of
the surface on which the lift is to be compacted; first
lift placed on foundation soil.
(2) Place the loose soil over the surface and level to the
appropriate thickness.
(3) If the soil has not been processed in the borrow area
or if further processing is required, process before
compaction.
(4) Compact the soil to the specified density using an
appropriate number of passes with the compaction
equipment.
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(5) Repeat the above steps until a sufficient number of
lifts has been placed.
A crest should be maintained along the dike centerline to
promote surface drainage (Figure 36).
lifts
final dike slope
foundation soil
Figure 36.
Dike cross section showing lifts and final slope
cuts.
Proper soil compaction should provide the dike with
sufficient strength and sufficiently low permeability to perform
satisfactorily. Further information on soil compaction is given
in Section 4.3.2.1.3, as compaction procedures for soil liners
and dikes are essentially identical, CQA is critical to properly
constructing the dike and other surface impoundment components.
CQA monitors the construction process and ensures that design
specifications are achieved. The elements of the CQA Plan are
discussed in Section 3.11.
Constructing a low-permeability soil liner involves many of
the same procedures as dike construction: preparing the soil for
compaction, placing the material in loose lifts, compacting the
soil to the required density and thickness, and testing to ensure
that specifications are met. The soil-liner construction process
has been described by USEPA (1986b) in its Technical Resource
Document addressing clay liners.
Technical guidance provided by USEPA (1986a) on construction
quality assurance recommends constructing a representative test
fill before constructing the full-scale soil liner to verify that
performance standards can be consistently achieved using
specified materials and equipment. A test fill is also
recommended as a prerequisite to dike construction. Potential
scheduling delays during dike and liner construction due to
compaction or material problems can be avoided if a test fill is
constructed and problems corrected during the surface impoundment
design phase. The test fill should be constructed using the same
soil, equipment, procedures, and specifications to be used in
constructing the full-scale facility.
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Strict control of the test fill construction process and
documentation of the test fill data are required for useful
results. The test fill data can be compared with data obtained
during impoundment construction to indicate acceptable criteria.
USEPA (1986a) provides guidance on constructing and utilizing
test fills.
4.3.2 Pre-placement Soil Preparation
Preparing or processing soil materials prior to soil liner
or dike construction improves compaction efficiency. Processing
activities include reducing clod size, removing unwanted
material, and adjusting moisture content. Removing unwanted
material and adjusting moisture content are often performed in
the borrow areas, while clod size reduction may be performed in
the construction area. Homogenization thoroughly mixes the soil
to dilute small quantities of undesirable materials (e.g., sand
and gravel) which tend to increase the hydraulic conductivity of
soil liners. Clod-size reduction is best accomplished at water
contents at or near the plastic limit. At low water contents,
the clod strength is often high; at high water contents, the soil
may become sticky and have reduced workability.
Mechanical equipment, (e.g., rototillers, pugmills, and
pulverizers) may be used to reduce clod size, homogenize the
soil, and mix amendments or moisture prior to soil placement and
compaction. These processes may also be accomplished using a
disc, blade, or other implement towed by a tractor or dozer.
Proper soil processing enhances compaction and uniformity of
desired liner or dike properties.
Moisture-content adjustments are often required prior to
soil compaction. The moisture content is usually specified with
respect to optimum, as determined from standard Proctor (ASTM D
698-78) or modified Proctor (ASTM D 1557-78) compaction tests
(ASTM, 1986c). A specific acceptable moisture content range
(e.g., 1% less to 3% greater than optimum for liner construction;
drier, perhaps up to 2% less than optimum, for dike construction)
is generally stated in the design specifications. Moisture is
either added or removed to maintain the recommended moisture
content. Moisture reduction is normally performed by air-drying
the soil or by adding and mixing drier soil. Moisture addition
is usually performed using water trucks or sprinklers to add
water to the soil. Dozers, disk harrows, or rototillers are then
used to mix and incorporate the water into the soil. The
preferred procedure suggests that water content be adjusted in
the storage or borrow area before the soil is placed for
compaction.
Moisture may also be added or removed during construction,
or between specific activities, as required. For example, slight
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desiccation of the surface of a compacted soil lift may occur due
to delays, and CQA may require that moisture be added before
continuing construction. Moisture is added or removed in the
same or similar manner as in the borrow area before placement for
compaction. The moisture content of the soil during and after
compaction is monitored according to CQA procedures using
equipment such as a neutron moisture-density probe. Actions
should be taken to prevent or minimize moisture changes due to
construction delays or other factors. For example, a temporary
protective cover of soil may be applied to reduce evaporation, or
the surface may be sealed (rolled smooth) to hasten runoff and
lessen the potential for moisture addition from rainfall.
4.3.3 Soil Material Placement
A survey of liner construction sites and personnel by
Elsbury et al (1985) indicates that soils are generally placed in
parallel strips over the liner area using earth movers or trucks,
and then leveled with bladed equipment to the desired loose lift
thickness. Strips may be stepped up the side slopes in liner
construction in order to maintain a horizontal working surface
and facilitate compaction.
Control of lift thickness is important because it influences
compaction effectiveness and the resulting hydraulic conductivity
of the compacted soil liner. Horizontal and vertical survey
control points should be established to inspect the areal extent
and thickness of liner lifts throughout construction. Loose lift
thickness is commonly specified at 9 inches (23 cm), which
results in a compacted lift of approximately 6 inches (15 cm),
whether a dike or a liner is being constructed. However,
compacted lift thickness depends on soil, equipment, and
operating characteristics (Church, 1981; Creager et al, 1945;
Elsbury et al, 1985; Hammer and Blackburn, 1977).
4.3.4 Soil Compaction
Effective compaction imparts strength to dikes, foundations,
and other earth structures, and reduces the permeability of the
low-permeability soil component of the liner. A proper
compaction procedure densities the loose lifts and bonds the
upper lift to the underlying lift. Bonding of layers is
essential for eliminating zones of higher horizontal hydraulic
conductivities at the lift interfaces. Bonding is achieved by
scarifying (roughening) the upper surface of a compacted lift
before placing and compacting the next lift, and by compaction
equipment that will penetrate through the loose lift into the
compacted lift and a zone of intermixed material between the two.
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The critical performance standard for surface impoundment
soil liner components is low hydraulic conductivity. This
characteristic is obtained by efficient compaction at specified
water contents, which are generally wetter than optimum (e.g., 1
to 3 percent, dependent on soil type and compactive effort).
Preferred compaction induces a homogeneous dispersed or
unoriented structure within the soil material which, in turn,
provides a lower hydraulic conductivity.
Stability, rather than permeability, is the prime concern
for surface impoundment dikes. Thus, dike construction strives
for soil strength, which is accomplished by compacting dry of
optimum to maximum density.
After a number of passes are completed, in-situ moisture and
density measurements should be made with a moisture-density probe
or similar equipment, according to CQA requirements. Compaction
should continue if measurements show that density specifications
have not been achieved. In-place compaction density is normally
required to be in the range of 90 to 95 percent of standard
Proctor maximum density obtained in laboratory testing.
Soil compaction must be accomplished with equipment that is
best suited for the particular soil type and situation. Several
equipment types are available, including the following:
(1) Penetrating-foot rollers:
• sheepsfoot
• pad foot
• tamping foot
(2) Pneumatic or rubber-tired rollers
(3) smooth wheel or steel drum rollers
(4) grid rollers
Compaction equipment is manufactured in both towed and self-
propelled models. The better-suited model will be based on soil,
site, and design characteristics, and equipment model
availability. Compactor choice depends on site-specific
compaction needs and on soil grain sizes. Dikes and liners are
most often compacted with penetrating-foot rollers (Figure 37).
Recommendations are provided by Holtz and Kovacs (1981).
Equipment can often compact satisfactorily outside the range of
prescribed use. Additional information on applicability of
compaction equipment is given by USEPA (1988b and 1989b). (1975).
Several older but still useful references discuss compaction
with penetrating-foot or kneading compactive force (Hilf, 1975;
Johnson and Sallberg, 1960; Lambe, 1958; Mitchell et al, 1965;
and Sowers and Gulliver, 1955). The theory of compaction of
cohesive soils was developed by R. R. Proctor and presented in a
series of articles published in 1933 (Proctor, 1933a;b;c; and d).
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sheepsfoot
pad foot
tamping foot
Figure 37. Compactor foot designs.
Steel-wheel (also known as steel drum) rollers are effective
in final preparation of the low-permeability soil surface for
installing the geomembrane liner. The equipment provides the
soil with a smooth, even surface, free of clods that can damage
the geomembrane. The smoothest possible surface is required for
installation of the overlying geomembrane in a composite liner
(Giroud and Bonaparte, 1989).
4.4 GEOMEMBRANE LINER INSTALLATION
Geomembrane liner installation involves placing and seaming
panels of the synthetic material over a prepared subgrade to form
a barrier against liquid migration. The geomembrane must be
inspected and tested throughout installation to ensure that the
seams and panels are free of wrinkles, blemishes, holes,
inadequate seams or other defects which may allow the escape of
liquid. Workmanship, experience of installers, unstinting care
in installation, and a commitment to quality control are all
critical to a successful, leakproof installation. Giroud (1985)
and USEPA (1989b) present information on constructing liners with
geomembranes. All facets of geomembrane selection, testing, and
installation have been comprehensively addressed by USEPA
(1988a). More specific information is being prepared by USEPA on
the important topic of geomembrane seaming; the first of these
addresses the seaming of polyethylene geomembrane (USEPA, 1989c).
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4.4.1 Storage of Materials and Equipment
Geomembrane materials and installation equipment delivered
to the site should be stored in a secure location to protect them
from vandalism and theft. An existing secured area or a
temporary storage area can be used to provide this protection.
The impoundment area should also be protected from intrusion and
tampering that could damage the geomembrane during construction.
Provisions should be made for appropriate equipment to
unload and transfer the membrane rolls or panels. The rolls are
heavy and may require special or modified equipment to move them
without damaging the material.
Unless the geomembrane is applicable for exposed service, it
should be stored out of the sunlight to prevent ultraviolet
degradation and to minimize blocking. Blocking occurs when liner
materials are heated, causing them to stick together. The
material may then tear when unrolled on the subgrade.
4.4.2 Construction Quality Assurance/Inspection
CQA inspection during installation is essential if the
geomembrane is to be an effective barrier against waste migration
to the underlying soils. CQA assures planned review and tracking
of construction activities.
The subgrade condition, geomembrane placement and seaming,
and sealing of penetrations through the liner require
considerable CQA. A CQA Officer representing the surface
impoundment owner/operator is required to assure that
installation specifications and the contractual obligations of
the installing contractor are met.
An adequate CQA program results in surface impoundments
being built as designed. Guidance for preparing CQA programs is
given by USEPA (1986a). More information on CQA plan preparation
is provided in Section 3.11 of this document.
4.4.3 Subgrade Preparation
Subgrade preparation provides a firm base for the
geomembrane liner. In the USEPA-recommended double-liner design,
the drain or leak collection layer is the subgrade for the
primary or top geomembrane, while the low-permeability soil
component is the subgrade for the geomembrane component of the
composite secondary liner. Rocks, clods, or irregularities with
sharp edges should be eliminated from the finished subgrade by
fine-finishing (rolling smooth) the surface before installing the
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gee-membrane. Fine-finishing of sand layers is accomplished using
vibratory rollers and drags on a slightly wet surface. Smooth-
wheel rollers may be required to finish the surface in both the
low-permeability-soil and sand subgrades. During the fine-
finishing stage, grasses and other vegetation must be removed
from the subgrade layer, or vegetation killer applied, to prevent
their penetrating the geomembrane. Care must be taken to attain
a smooth and flawless surface on the low-permeability soil layer
of the composite liner to facilitate continuous contact with the
geomembrane installed directly on it.
Proper timing of construction activities is essential to
maintaining proper moisture content of the subgrade. The
geomembrane should be placed on the finished subgrade soon after
the finishing process is completed. Uncovered, fine-finished
subgrades can be easily disturbed by rain or wind. If rainfall
occurs during or after fine-finishing work on a slope, rills,
ruts, and ravines may be eroded into the surface. Sealing or
covering the fine-finished subgrade with a protective layer will
prevent soil erosion by surface runoff. A protective covering is
also required to prevent moisture loss and desiccation of the
soil layer during extended dry periods. The protective covering,
if on the low-permeability soil layer, must be removed before
applying the geomembrane.
4.4.4 Geomembrane Liner Placement
The geomembrane liner is secured at the top of the
impoundment dike, usually in an anchor trench, extending around
the impoundment perimeter. The trench should be excavated and
ready to receive the geomembrane before the panels are brought to
the installation location. More information on geomembrane
anchoring is given in Section 4.4.5.
The geomembrane panels will be placed in a pre-determined
pattern, an example of which is shown in Figure 38. Placement
begins by arranging (or spotting) the geomembrane rolls or folded
panels at the top of the dike or on the impoundment floor.
Special or modified equipment may be required to move and locate
the geomembrane without damage. The panels are unfolded or
unrolled down the side slope or across the floor. The panels are
then placed in the proper position and secured with sandbags to
protect them from wind damage or displacement. The geomembrane
should lie relatively flat and smooth after placement on a smooth
compacted subgrade. Sufficient slack should be left in the
material to accommodate shrinkage due to temperature changes.
Depending on the characteristics of the geomembrane material,
shrinkage can occur by temperature increases and decreases.
Temperature increases can cause shrinkage by loss of volatiles
and by release of manufacturing stresses (Giroud and Peggs,
1990). Shrinkage also occurs by contraction induced by lowering
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perimeter anchor trench
toe of slope
Figure 38.
Example of geomembrane liner panel layout.
Other patterns may be used.
the temperature. Temperature increases can also expand a
geomembrane during installation, creating wrinkles where cracking
can later occur. It is a very important aspect of design and
construction to accommodate all shrinkage- and expansion-causing
factors so that failure-inducing stresses, particularly at seams,
are minimized.
The success or failure of a geomembrane installation depends
largely on seam integrity and continuity. USEPA (1983b) lists a
number of seaming methods and factors which affect field seaming,
not the least of which are the skill and experience of the
installer. Seaming methods vary among geomembrane materials.
Common field seaming methods include the following:
• Thermal (hot air gun, hot wedge and dielectric)
• Heat (extrusion fillet or flat welding, and
extrusion/wedge welding)
• Chemical (cement, solvents and vulcanizing adhesive)
• Adhesive Tapes
Recommended panel overlap, where the seam is located, varies
from 4 to 12 inches (10 to 30 centimeters). However, the
installation contractor must follow the surface impoundment
design specifications, which, in turn, should include the
manufacturer's recommended overlap and bonding systems. The
following factors must be considered in constructing field seams
of high integrity:
• Manufacturers' guidelines for adhesives must be followed.
The adhesive system must be compatible with the
geomembrane and be applied under acceptable ambient
conditions.
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• Cold temperatures can prevent successful bonding of
panels. Many manufacturers recommend that adhesive
bonding take place only when temperatures are above 15°C
(59°F).
• Preparation of panel edges for seaming must be carefully
done to prevent damage. For example, grinding can create
deep scratches that may result in cracks (see USEPA,
1989c).
• The seam surface should be clean and dry. The presence of
moisture interferes with the curing and bonding
characteristics of the adhesive, while the presence of
dust creates voids that provide a path for fluid
migration through the seam.
• The geomembrane should rest on a dry, hard, and flat
surface to facilitate applying pressure rollers.
• Panels should be placed and seamed on the same day to
minimize the risk of geomembrane damage by wind, and soil
erosion under the geomembrane due to rain; unfinished
panels should be anchored by bags of sand.
The finished seams should be free of large wrinkles and the
surface should be rolled flat. Many manufacturers recommend that
field seaming begin at the panel center, lengthwise, and continue
to each seam end; this procedure minimizes the potential for
large wrinkles.
As in soil liner compaction, proper placement of the
geomembrane on the impoundment side slope is essential to
successful liner construction. Generally, the panels should be
of sufficient length so that, when placed the field, seams run up
and down the side slopes, with no horizontal seams on the slopes
(Figure 38). This orientation reduces gravitational stress on
field seams, particularly desirable when seams have not yet
cured. Corner patterns should be cut to fit where necessary, and
in a way that minimizes horizontal seams on the slope.
4.4.5 Sealing Around Structures and Anchoring the Geomembrane
The geomembrane is anchored at the top of the dike in a
trench, V-shaped or U-shaped in cross section (Figure 39(a) and
(b)). This technique is often recommended by manufacturers, due
to its simplicity and economy. Excavating the anchor trench is
done using a trenching machine or a bulldozer blade tilted at an
angle. The excavated soil should be spread away from the trench
and smoothed to facilitate unrolling and spotting of panels.
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trench
(a)
trench
(b)
soil fill
bolted anchor
(c)
concrete
anchor beam
geomembrane
liner
geomembrane
liner
geomembrane
liner
Figure 39. Geomembrane anchor designs at top of dike.
The panels should be anchored in the trench following the
field-seaming operation unless other components (e.g., synthetic
drainage layer) are also to be anchored in the trench. Once the
seams are completed for an individual panel, the trench should be
backfilled with soil to anchor the panel. The trench should not
be backfilled until the panels have been seamed, to allow
positioning for optimum seaming.
The geomembrane can also be anchored to concrete structures
along the berm top by securing the liner with batten strips
attached to anchor bolts embedded in the concrete (Figure 39(c)).
This technique can also be applied to bonding the geomembrane to
metal structures (e.g., pipes). A common method places the
anchor bolts on 6- to 12-inch (15- to 30-cm) centers. The
geomembrane is placed over the bolts, an adhesive is applied to
the membrane, and the batten strip is secured and bolted in
place. Compatibility of the adhesive/sealant with the synthetic
and the impounded liquid must be determined during impoundment
design to assure the integrity of the seal. An extruded polymer
strip may be cast in the concrete and the liner welded to it as
an alternative to the batten strip and bolts. Details of
anchoring techniques are discussed by USEPA (1984d) and Kays
(1987) .
Manufacturers' recommendations for using specific materials
and procedures must be followed to establish an effective seal
around penetrations through the geomembrane. Bonding synthetic
materials (e.g., the geomembrane and a plastic pipe) is typically
accomplished using solvents, adhesive, or welding techniques.
Where components of different materials are to be bonded, it must
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be verified before installation that the materials can be
successfully bonded using one of these techniques. For example,
the thermal-expansion characteristics of each material must be
evaluated to determine if temperature-driven expansion and
contraction cycles will permit an effective bond.
Wherever possible, pipes, level control equipment, and other
structures should be placed entirely above or beneath the
geomembrane to avoid penetration. If penetrating structures are
included in the impoundment design, sealing the geomembrane
effectively around the structure is critical to liner integrity.
Standardized designs include pre-engineered seals installed in
the plane of the geomembrane, and pre-formed boots for
installations around penetrations (Figure 40). If pipes are
installed in the impoundment through a concrete structure, the
seal can usually be made in the plane of the liner.
geomembrane
steel clamp
boot
- mastic
boot seal at geomembrane liner
gasket
geomembrane
flange seal at geomembrane liner
Figure 40. Seals at geomembrane liner penetrations.
4.5 LEAK COLLECTION AND REMOVAL SYSTEMS
The leak collection and removal system is designed to drain
and pump out liquids accumulating in the liner system.
Typically, the leak collection system consists of a granular
material (sand) immediately overlying a hydraulic barrier
(liner). The system's ability to drain away moisture is enhanced
by constructing the system at a minimum bottom slope of 2
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percent, using highly permeable drainage media and engineered
geotextile filters, where needed, to prevent migration of fine-
grained soils into the media, and by spacing drain pipes
properly.
The leak collection layer is normally constructed to extend
up the side slopes. The development of synthetic drainage nets
(geonets) has resulted in many surface impoundments being
constructed with side-slope leak collection layers of synthetic
materials and bottom drainage layers of granular material and
drain pipes. Synthetic drainage net material is used on side
slopes instead of granular systems because it is more easily
installed on steep side slopes. Steep side slopes cause granular
drainage material to slump down, while the synthetic drainage
material tends to remain in place if anchored properly at the top
of the slope (refer to Figure 18).
A typical leak collection and removal system is installed as
follows. A layer of sand (about 2 inches [5 cm] thick) is spread
over the underlying layer (e.g., a geomembrane) as a protective
soil covering. It should consist of material which is free of
clods, stones, or other sharp objects that can puncture the
geomembrane. Optionally, a geotextile layer may be used. The 2-
inch (5-cm) protective layer will also provide bedding for the
drain pipes according to the design layout. If a geotextile
protective layer is used, a bedding layer is placed only along
the pipe alignments for pipe support. Typically, perforated
pipes of 4 to 6 inches (10 to 15 cm) in diameter are used. Pipe
perforations are placed face down to prevent clogging by the
drainage media. After the pipes are placed, the remaining
granular material is spread over the area in a single loose lift
to the required thickness, and compacted with a vibratory roller
into a firm base for the primary geomembrane liner. If a
gravelly drainage layer is used, a geotextile protective layer
should be placed over the surface prior to placing the primary
geomembrane. Figure 41 is an example of a leak collection system
layout.
If synthetic drainage materials (e.g., geonet and
geotextiles) are used, they should be unrolled and spotted as in
geomembrane installation, except that the panels are not
overlapped and seamed. The panels are placed edge-to-edge and
connected according to the manufacturer's suggested procedures,
so that the lower portion of the side-slope panel extends into
the granular or other bottom layer to enhance continuity between
the drain layers (refer to Figure 18). A geotextile is placed on
both sides of the drain panels to prevent intrusion of the
geomembranes due to compression or creep phenomena (Figure 42).
The synthetic drain system is then secured in the anchor trench
as in the geomembrane liner installation.
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perforated drain pipes
granular leak collection layer
Figure 41. Example leak collection system layout,
geomembrane
liners
geotextiles
granufar leak collection layer
geotextiles
leak collection sump
Figure 42. Geosynthetic uses in leak collection layer.
4.6 TESTING THE LINER SYSTEM
After the liner system has been installed, it should be
tested for leaks due to pin holes, inadequate seams, and
punctures. Methods for liner testing prior to operational start-
up include filling the impoundment with water and monitoring the
leak collection system for liquid. This method or procedures
that use in-situ and remote (e.g., electrical resistivity) leak
detection techniques permit the identification and repair of
leaks in the primary liner before waste is placed in the surface
impoundment. Unless a leak detection system has also been
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installed below the secondary liner, the integrity of the
secondary liner cannot be tested.
4.7 PROTECTIVE COVERINGS
As discussed in Section 3.7, and shown there in Figure 21,
protective coverings are constructed or installed as preventive
measures against erosion, weathering, or other degradation of the
impoundment's structural components by mechanical and
environmental forces. Coverings may consist of natural material
(e.g., soil, stone, and vegetation) or synthetic materials
(geotextiles). The following discussion covers construction and
installation of protective coverings for liner systems and dikes.
4.7.1 Liner Protection
Protection of the primary liner is often provided by a soil
cover of sufficient thickness to prevent environmental or
mechanical damage. If the liner is a geomembrane, placement of
the soil cover must be accomplished so that the synthetic
material is not damaged. Geotextile bedding may be placed
between the soil and geomembrane (Figure 21). The soil cover
should be placed using low-contact-pressure equipment (to avoid
puncturing the liner) in a thickness of no less than 18 inches
(45 cm). Though not necessary, placing the soil at or near
optimum moisture allows slight soil compaction by equipment
traffic. The cover soil material must be free of clods, stones,
or other sharp objects which can puncture the geomembrane.
Synthetic materials may also be used in some cases for liner
protection instead of soil; for example, the liner may be covered
with a geotextile for short periods of time. The synthetics must
be placed according to manufacturers' suggested methods, and the
engineering plans and specifications for the facility. The
synthetic materials are generally less thick than a soil cover
and, therefore, may be layered (e.g., several layers of
geotextile, drainage layer, or both) to provide a thickness which
will protect the liner from weathering or puncture. As noted
earlier, a reduced-thickness soil protective layer may be placed
on the geotextile for added protection.
4.7.2 Dike Protection
Dike protection consists primarily of vegetation and/or
riprap (Figure 21). Establishing a vegetative cover on the dike
exterior and top surfaces is typically accomplished by
hydroseeding, broadcasting, grass or grain drills, blowers, or
hand planting. The best-suited seeding method depends on
topography, type of vegetation, soil condition, and equipment
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availability (Lee et al., 1985). The soil usually requires
physical and chemical treatment before vegetation can be
established. Physical treatment includes tilling and adding
topsoil. Chemical treatment includes applying fertilizer or
lime. After seeding, the soil should be watered and mulch
applied to prevent erosion until the vegetation is established.
Seeding the dike slope may also be accomplished by placing
degradable nets on or just below the soil surface to provide soil
stability until the vegetation becomes established.
Riprap may be used as dike protection on the inside slope or
on both sides (Figure 21). Riprap placement is discussed by the
Bureau of Public Roads (1967) and USAGE (1977). Riprap should be
placed over a geotextile filter selected to prevent washout of
the underlying protective soil layer. The stone may be placed
using a dragline or similar equipment, or dumped on the dike
slope by transport vehicles, spread into place, and compacted
with a grid roller if required. Riprap should not be placed
directly on a geomembrane, but should be separated from it by a
buffer material of soil as in Figure 21. Other materials that
may be used for dike protection include synthetics or geogrids,
concrete, and asphalt. These should be placed according to
manufacturers' accepted methods, and the facility engineering
plans and specifications.
4.8 LIQUID LEVEL CONTROL SYSTEMS
Active liquid level controls are installed equipment that
control daily level changes within the impoundment. Passive
controls are engineered and constructed into the dike. Passive
controls function in emergencies to prevent liquid overtopping
and catastrophic failure.
4.8.1 Active Liquid Level Control
Construction associated with active liquid level control
systems consists primarily of erecting mounting structures (e.g.,
piers or pilings) needed to suspend a sensor over the liquid
surface. The preferred arrangement is to locate the mounting
structure outside the impoundment and extend it over the liquid
surface (see Figures 24 and 25). This arrangement has no contact
with liner-support components and no liner perforations as
potential leak sources. An alternative design is to construct a
mounting structure that rests on the impoundment bottom, but does
not penetrate or abrade the liner. Both approaches preserve
liner system integrity and are preferable to mounting structures
that penetrate the liner. If the design requires that the
structure penetrate the liner, QC should be diligently exercised
to maintain the integrity of the seal at the liner penetration.
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Regardless of the mounting structure selected, installing
the liquid level sensing system properly, according to the
manufacturer's specifications, is essential. The system will be
integrated with facility inflow and/or outflow devices,
monitoring devices, and alarms to control the liquid level in the
impoundment. The installed system should be tested to verify
that its components function properly. Testing should include
deliberate attempts to test the fail-safe aspects of the system.
A record of testing procedures should be maintained for reference
during routine system maintenance.
4.8.2 Passive Liquid Level Control
Passive outfall structures allow the release of liquid when
the freeboard level is exceeded. Release is accomplished by one
of two general means: (1) through an opening at the dike crest
(e.g., weir, spillway, or flume); or (2) through a conduit (e.g.,
pipe) extending through the dike and liner system (see Figures 22
and 23). Generally, outfalls provided by an opening in the dike
crest are preferred for a surface impoundment because no liner
system penetration is required.
The primary concern associated with constructing spillways,
weirs, flumes, or conduits is ensuring an effective seal between
the outfall structure and the liner. The surface impoundment
liner system generally includes synthetic materials which require
bonding to the outfall. The outfall may possibly be constructed
from the same synthetic material used to line the impoundment,
thereby minimizing the bonding problem. However, the material
used for the outfall will often differ and a special bonding
procedure will be required.
4.9 SECONDARY CONTAINMENT
The secondary containment structure for a surface
impoundment is typically a low dike or berm surrounding the
facility (Figure 25). The secondary containment berm is
constructed in the same manner as the primary dike and consists
of compacted lifts of soil. Given the inherent stability of its
typically low design, a secondary containment dike can often be
compacted effectively with several passes of transportation or
dozer equipment. However, field density tests should be
conducted to determine that compaction using this method is
adequate.
The surface of a secondary dike should not require armored
protection from erosion, unless it is in an area of potentially
rapid-flowing flood waters. Vegetation should be adequate to the
task, similar to vegetation on the primary dike, as discussed in
Sections 4.7.2 and 3.2.3.2.4.
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CHAPTER 5
OPERATION, MAINTENANCE, AND MONITORING
This section presents guidance on routine operation and
monitoring of surface impoundments. Most aspects are similar for
all surface impoundments.
5.1 OPERATION AND MAINTENANCE ACTIVITIES
The information in this section emphasizes current industry
practice for operating and maintaining surface impoundments for
waste treatment, storage, and disposal. The discussions expand
upon previous work (e.g., USEPA, 1983a and 1984a).
5.1.1 Facility Start-up
The start-up procedures for a new surface impoundment should
lay the framework for future facility inspections. The initial
inspection should document baseline conditions against which
future operating decisions are made.
A dormant period usually occurs between the time that
construction is completed and the time that final approval for
use is obtained from regulatory agencies. During this period,
several "dry-run" inspections can be made by the operating
personnel to perfect procedures, to train personnel conducting
the inspections, and to become familiar with various structures
and equipment before use. Before the surface impoundment is
placed in service, structures and equipment should be maintained
and protected from weather damage. Storms present the potential
for erosion of dikes and side walls, and to cause flow, level,
and volume gauges to operate. These components must be checked
and maintained until the facility is ready to accept waste. A
small pool of water should be maintained in the impoundment to
prevent the protective soil layer covering the liner from drying
out. Dike side slopes and top surfaces should be watered
regularly to prevent desiccation and to facilitate the initial
growth of newly planted vegetation.
A final inspection of the entire facility must be made
before start-up. If inflow will enter the impoundment through
pipes, all piping, pumps, valves, controls, gauges, and manual
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controls must be inspected and shown to be in working order
before they are used to accept waste. If discharge to the
surface impoundment is directly from tank trucks, the discharge
area or ramp must be thoroughly inspected before use. Emergency
discharge pipes or spillways must also be inspected to ensure
that they are free of debris and ready for use. These items
should be covered in the CQA Plan discussed in Section 3.11.
5.1.2 Routine Inspections and Maintenance
The RCRA Part B permit application describes the frequency
and detail of inspection for each hazardous waste surface
impoundment system component. When conducting an inspection, the
inspector should document the nature and extent of problems
noted. The inspector should follow up the inspection with a
report noting remedial actions taken to correct deficiencies, the
time required for correction, special problems encountered, and
suggestions for preventing recurring problems. Inspections can
be expected to be conducted periodically by the regulatory agency
and more frequently by the impoundment's operating personnel.
5.1.2.1 Regulatory Inspections of Facility —
Agencies responsible for inspecting surface impoundments
usually have an inspection checklist which emphasizes those areas
that serve as "trigger" points; that is, a problem noted in a
particular area which usually reflects the existence of a larger
problem, which may be less apparent, and triggers a more in-depth
investigation. Prudent facility operators will include a similar
checklist for routine inspections so that problems can be
corrected before more troublesome conditions develop.
Particular questions to be answered during routine
inspections include, but are not limited to, the following:
• Does the surface impoundment appear to provide at least the
design freeboard?
To determine this for a nearly full impoundment, the
inspector should look for erosion on protective soil
cover due to wave action. On geomembranes, the inspector
should look for lines of algae growth, salt deposits, and
oil and grease marks. If evidence exists of erosion, or
markings are evident on the geomembrane, less than the
design freeboard vertically below the lowest point on the
dike summit (e.g., a spillway), then adequate freeboard
is not being maintained. The inspector should also
examine outfall structures for evidence of overtopping.
The inspector should ask for records of impoundment
inflow and discharge volumes and compare these to level-
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recorder data to obtain an impoundment volume estimate
based on a simple mass balance. Large discrepancies in
the balance volume estimate may suggest possible leakage
and should be followed by a more detailed evaluation,
including checks for possible leaks.
Do the dike's crest, side slopes, abutment, and toe areas
have protective cover (e.g., rock or grass) to minimize wind
and water erosion?
In addition to inspecting these components for signs of
erosion, the inspector should look for dead vegetation,
oil stains, salt or mineral deposits, seep areas,
slumping, irregular settling, sink holes, slides,
sloughs, bulges, cracks, animal burrows, roots of shrubs
or trees, and other visible evidence of problems.
Although some of these items may reflect merely the need
for simple maintenance, most indicate the potential for
leakage through the dike.
Is the emergency spillway or overflow structure clean?
The inspector should look for animal nests, collections
of leaves or other detritus that could impede flow, and
signs of an overflow (e.g., oil and rust stains).
Do the emergency (manual) shutoff valves on the influent
line work properly?
The inspector should determine if these valves can be
shut off quickly in an emergency.
Does the emergency and safety equipment work? Is it clearly
labeled and accessible?
This is normally a fairly quick check and is an
indication of the impoundment management's attitude
toward overall safety and emergency preparedness.
Is the leak collection/detection system operational?
Pumps and detection devices should be checked to assure
that they function. Routine operational records should
be examined for frequency of operation and operational
results. Sumps should be checked for liquid level.
Are geomembrane liner problems apparent?
If the geomembrane liner is visible (no protective soil
cover), the inspector should look for signs of abrasion
or tearing along the top edge due to vehicular traffic or
other activities. The inspector must also determine if
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"whales" are present; that is, gasses forming under the
geomembrane which cause it to rise. If the liquid is
sufficiently clear, the inspector should look for ripples
on the impoundment bottom, or stretch folds along the
side slopes. This unevenness is frequently accompanied
by liquid beneath the geomembrane. Such liquids should
generally be intercepted by the leak collection/detection
system. Visible seams and geomembrane anchoring should
be checked for signs of weakness.
5.1.2.2 Operator Inspections of Dike Slopes, Faces, and Crest —
The dike system should be inspected by operating personnel
weekly and after rain, ice, or wind storms for the following:
• liquid level exceeding the design freeboard;
• wave erosion on the interior embankment;
• erosion due to precipitation or irrigation runoff;
• seepage along toe areas; evidence includes wet or spots,
mineralization spots, discoloration or dying vegetation;
• soil movement; evidence includes irregular bank
alignment, slides, sloughs, bulges, or depressions;
• animal burrows and shrub or tree growth on dikes;
• fractures or cracks in the crest, embankment slopes, toe
areas, or around structures such as spillways and
leachate sump pipes;
• damage to crest and interior wall due to traffic when
discharge to the impoundment is from vehicles;
• spillage on the crest or outside the embankment at
unloading dock or ramp; and
• damage from vehicles or other agents to geomembrane
anchorage along the crest.
5.1.2.3 Operator Inspections of Ancillary Site Facilities —
The surface impoundment design normally includes a liquid
level indicator and hard copy recorder; leak collection/detection
system; emergency spillway or other discharge structure; a system
to discourage birds and other animals from using the impoundment;
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survey reference points; a weather station (if needed); inlet
pipes and valves and/or a vehicle unloading area; security
equipment; and health, safety, and emergency response equipment.
A routine inspection and maintenance program for these
facilities should be incorporated into the owner/operator's
operation and maintenance protocol. The following should be
included in the inspection program:
• The liquid-level indicator should be checked for freedom
of movement and response. Grease or scum accumulations
can adversely affect response. The chart recorder should
be checked for calibration and ink supply, as specified
in the manufacturer's guidelines.
• Leak collection systems should be checked for component
deterioration (e.g., pipes and locks). Pumps should be
inspected for proper operation. The sump should be
checked for presence of liquid and, if any is present, a
sampling program should be initiated to characterize the
material. Liquid-level indicators or recorders, if
present, in leak-collection sumps should be inspected and
maintained as necessary.
• The emergency discharge outfall structure should be
checked for cracks, and damage due to mowing or other
vehicular contact. Debris or excessive plant growth that
may impede its operation should be removed.
• Devices to ward off animals (e.g., plastic banners,
fences, and electrically charged wires) should be
inspected routinely.
• Elevation reference points or survey markers should be
inspected for damage. These reference points should be
surveyed annually to monitor soil stability.
• Weather stations (if present) should be inspected per the
manufacturer's operation and maintenance protocol.
• Impoundment inflow structures must be inspected to assure
that valve systems and recorders work properly, and that
piping is not deteriorating. If an unloading dock is
present, the liner immediately below the unloading point
should be checked for impingement or other damage. The
crest area and outside dike wall should also be checked
for waste spills.
• Security equipment (e.g., locks and/or locking caps on
leak collection sump lines, monitoring wells, gates, and
fencing) should be checked to verify that the equipment
operates and tampering has not occurred.
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• Health, safety, and emergency response equipment (e.g.,
air sampling systems, safety showers, eye-wash stations,
respiratory protective equipment, fire-fighting
equipment, caustic and acid neutralization materials,
protective clothing, first-aid kits, air horns or
flashing lights, and radio systems) should be inspected
and maintained according to the manufacturer's
recommended procedures. In addition, personnel must
receive periodic training in proper equipment use.
5.1.2.4 Liner Systems —
Routine facility operations pertaining to liners include
detecting and locating leaks, determining causes of liner
failure, maintaining and operating collection systems and pumps,
and periodically removing accumulated solids. These topics are
discussed in the following paragraphs.
Detecting and Measuring Liner Leakage — USEPA (19871)
has proposed that the impoundment operator have the capability of
detecting a leak through the primary liner as small as 1
gal/acre/day (9.35 liters/hectare/day). Also proposed has been
an "action response level" and a "rapid and extremely large
leakage" rate. The first is a low rate (to be established)
between 5 and 20 gal/acre/day (47 and 187 liters/hectare/day) and
the second is the rate, when exceeded, that leads to a hydraulic
head greater than the thickness of the leak collection layer.
These rates are used as bases for certain leak response actions,
as described in the proposed requirements for Response Action
Plans, called for under RCRA (USEPA, 1987i).
Continuous monitoring of the leak-collection system should
be considered good practice. This can be accomplished by
continuous liquid-level recording in collection sumps. An
increase in level would be prima facie evidence of a leak in the
primary liner. Monitoring for leaks can also be accomplished by
electronic leak detection methods as described in Section 3.5,
and backed up by sump level monitoring.
If leakage through the primary liner is suspected, one
procedure for establishing its existence and magnitude is to pump
the leak collection system dry, measure the amount of recovered
liquid, and note the rate at which the system refills. Recovery
of all the material leaked is not necessarily expected because
the drainage layer will retain some liquids (up to the system's
"field capacity"). If the underlying secondary liner is
compacted soil, it, too, will absorb and retain some liquid. By
estimating the total volume of leak-saturated material, the
liquid volume retained by the various soil components can be
estimated.
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If leakage occurs in a small area, its location may be
determined by a remote or in-situ leak detection system before
the leak-detection layer becomes fully saturated. It is thus
possible for the leak to be detected before it reaches the
collection sump.
The liquid pumped from the leak-collection system can be
managed as a "pump back" problem by returning the leaked liquid
to the surface impoundment. This allows the operator additional
time to plan and devise appropriate remedial actions. Criteria
for determining the type of remedial action required when leakage
occurs through surface impoundment liner systems is discussed in
Section 6.3.3.
If the amount of recovered leakage indicates a significant
loss, or if pump-back rates and leakage rates differ, then the
secondary liner may also be leaking and contaminants escaping to
the environment. This may be cause for cessation of operations
and emptying (and perhaps closing) the impoundment.
Leak-detection technologies should be used to locate the
source point of small leaks in the primary geomembrane liner.
Similar technologies may be used for early detection of releases
to the subsurface soil beneath the secondary liner. Several
reliable methods are available to detect and locate geomembrane
leaks larger than 0.1 to 0.2 inches (2 to 5 mm), but smaller pin-
hole leaks are more difficult to find. Remote and in-situ
techniques to locate leaks are discussed in Section 3.5.
The flow of liquids through flaws in geomembranes depends on
the size and shape of the flaw, the liquid head, and the
hydraulic characteristics of the sub-base. In a recent study of
the characteristics of leaks through geomembrane flaws, Brown et
al (1986) found that leakage rates through slits and seam flaws,
as might logically be expected, were much more variable than
those through round holes. This is due to the variable hole
sizes which can result when a seam or one side of a slit becomes
displaced relative to the other side. The magnitude of leak
rates for various geomembrane flaws is shown in Table 8.
Brown et al (1986) also investigated the effects of subbase
hydraulic conductivity on the leakage rate through a defective
geomembrane liner. Less-permeable soils containing greater
amounts of clay had lower leakage through the geomembrane
component because a better seal was formed between the
geomembrane and soil components, allowing less lateral flow of
liquids at the interface. Table 9, taken from Brown et al
(1986), gives predicted leakage rates from various hole sizes in
geomembranes overlying soils of varying hydraulic conductivities.
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TABLE 8. AVERAGE LEAK RATES (mVyr) FROM DIFFERENT SIZE AND
SHAPE FLAWS IN 30-mil (0.08-cm) HDPE LINER OVER GRAVEL
AT TWO LIQUID HEADS. (1 m3 = 264.2 gal)
Hole Shape and Size
Round - 0.16 cm (.06 in.) diam.
Round - 0.64 cm (.25 in.) diam.
Round - 1.27 cm (.5 in.) diam.
Slit - 5 cm (1 in.) long
Slit - 15 cm (6 in.) long
Seam - 5 cm (1 in.) long
Seam - 15 cm (6 in.) long
===============================:=:=
50
110
1482
4257
—
3866
404
4702
Head (cm)
100
m3/yr
145
2208
6780
79
5623
325
7244
TABLE 9. CALCULATED LEAK RATES (mVyr) FOR A RANGE OF HOLE SIZES IN
GEOMEMBRANE LINERS OVER SOILS OF DIFFERENT CONDUCTIVITIES
AND FOR THREE HEADS (H) . (1 m3 = 264.2 gal)
Hole Diameter (cm)
K.at (cm/s)
0.08
(1/32 in. )
0.16
(1/16 in. )
0.64
(1/4 in.)
1.27
(1/2 in.)
-4
3.40 x 10
3.40 x 10'5
3.40 x 10'6
3.40 x 10"7
167.00
84.60
14.30
1.80
H = 0.3 m (1 ft)
3.40 x 10~4
3.40 x 10'5
3.40 x 10'6
3.40 x 10'7
19.30
4.30
0.54
0.066
31.50
4.88
0.60
0.072
43.20
6.28
0.77
0.095
50.60
7.30
0.89
0.107
H = 1.0 m (3 ft)
3.40 x 10'4
3.40 x ID'5
3.40 x 10'6
3.40 x ID'7
42.30
12.80
1.66
0.20
87.80
14.80
1.83
0.22
128.00
18.70
2.29
0.28
147.00
21.40
2.61
0.32
H = 10.0 m (3.1 ft)
438.00
123.10
15.60
1.90
1030.00
153.50
18.80
2.30
1170.00
171.30
21.00
2.60
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Determining the Cause of Liner Leakage — If the cause of
liner leakage is known, it may affect the type of repair that is
made (i.e., strengthen a weak area). In addition it can provide
information toward preventing a repetition of the same failure.
An unacceptable geomembrane leakage rate may result from
imperfect seaming; rips, punctures, and tears that occur during
installation, or later as a result of operational error; failures
resulting from subsidence or shear failure of the supporting soil
after installation; or exposure to incompatible wastes (e.g.,
concentrated organic solvents), which can dissolve either the
plastic or plasticizer (Brown et al, 1986) of a geomembrane.
Solvent attack can also permanently alter the fabric of a low-
permeability soil liner should the leak extend to that component.
Leakage does not necessarily result from some events that
may be considered liner failures. For example, holes above the
liquid level caused by equipment misuse or by tension separation
at seams, or anchor pullout, are failures, without immediate
leakage. Leakage may be expected later in these situations as
such areas of weakness propagate. The cause of liner failure
above the liquid level may be observable and relatively obvious,
especially if there is no protective soil layer. Below the
liquid level, pumpout may be required, followed by a thorough
inspection of the primary geomembrane liner. If the failure
appears to be chemical-caused, an evaluation of the waste liquids
and sludges contained in the impoundment and their compatibility
with the liner may be in order.
When a liner is repaired or replaced, it goes without saying
that the cause of the failure should not be allowed to repeat
itself. In other words, repair or replacement should always be
accompanied by elimination of the cause of failure before the
operation of the impoundment is resumed.
Liner Repair — Since liners meeting minimum technology
requirements will have a geomembrane as the top or primary liner,
a failed (leaking) system will certainly have a leak in that
geomembrane. In the recommended system, the secondary liner will
also have a geomembrane top component. Thus, the first two
barriers are geomembranes. If the secondary liner is leaking,
the primary liner is likely to be leaking also. With both
leaking, repair becomes much more complicated and much less
practical, than if only the primary liner is involved.
The options available for repairing the primary geomembrane
liner depend on the type and cause of failure. Mechanical
failures are often repairable by patching, re-seaming, or
returning the geomembrane to its original position. If the
primary geomembrane liner has not been significantly affected by
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the waste, such as above the liquid level, or even below the
liquid level if the waste and geomembrane are compatible, repair
may be very practical. However, if the damage is below the
liquid level, and particularly if the primary geomembrane liner's
integrity or chemical structure has been compromised by the
waste, repair may not be practical or even possible. In this
case, the correction is to cease operations and close the
impoundment or replace the entire liner. Repair and replacement
and the difficulties encountered have been explored (USEPA,
1987d).
Solids and Liquid Removal — Sludges that accumulate in the
surface impoundment will need to be removed periodically, and
disposed or temporarily stored. Also, solids removal is likely
to be required following leak detection because maintenance or
repair of leaking liners cannot be performed with liquid sludges
and solids present.
The method selected to remove sludge from the surface
impoundment must be non-destructive to the geomembrane liner and
underlying leak detection/collection system, particularly if the
impoundment is to be put back into operation. The use of
shovels, scrapers, backhoes, etc. would not ordinarily be
advisable for the removal of sludge in contact with the liner.
Pumping would be more appropriate.
Several sludge pumping systems are available. If the liquid
contained in the surface impoundment is to be removed and treated
beforehand, the intake line should be mounted on a float to
prevent pumping of bottom sludge. Sufficient liquid must be
retained, however, to form a pumpable sludge slurry. Some of the
liquid may be stored in tanks for washing sludge from the
geomembrane. This eliminates the need to obtain and use
additional water for liner washing, which would then need to be
treated before disposal or discharge.
After sludge removal, a solidification-stabilization agent,
such as fly ash, lime, portland cement, or a combination, may be
used to stabilize the sludge. A pug mill or other mixing system
should be installed in the immediate vicinity and sufficient
space provided to allow easy vehicle access. Methods for
stabilizing and/or solidifying hazardous wastes have been
described in a handbook published by USEPA (1986g).
If sludge cannot be disposed of on-site, the owner/operator
has two disposal options. Some sludges can be "delisted" (re-
classified as non-hazardous) following mixing with Type II fly
ash. If this can be achieved, the sludge may gain approval for
disposal at a sanitary landfill. If the sludge cannot be
delisted, it must be disposed in an approved hazardous waste
disposal facility. In any case, loaders and transportation
equipment will be required to handle the solidified material.
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Waste liquid removed from an impoundment must be treated, or
perhaps pre-treated and discharged to a municipal wastewater
treatment system. The treatment must be consistent with the
liquid composition and will vary depending on that composition.
Treatment may be expected to produce more sludge that may require
handling and disposal similar to that of the impoundment bottom
sludge. Techniques to dewater and solidify sludges are discussed
in Section 7.3.
Repairing the secondary liner, or replacing it, will require
removal of waste, all or most of the primary liner, and all or
most of the leak collection layer. Thus, in many, if not most or
all, cases of secondary liner leakage, the entire liner system
will require re-building. In these cases, closure may be the
most practical solution.
5.1.3 Record-keepinq
A complete inspection and maintenance program requires
complete, comprehensive records of operating and maintenance
actions. The date and nature of the activity, the details of the
action taken, the time required to achieve the desired result,
and the name of the person responsible should be recorded.
Documenting problems and solutions provides valuable
knowledge for future designs and for solving recurring problems
that may plague a waste disposal facility. Recurring problems
indicate that a change in equipment, personnel training,
supervisory practices, operational procedures, or some
combination thereof may be needed.
Inspection and maintenance records should be kept in a safe
place. Such records are required to be maintained and available
for inspection by federal and state regulators.
5.2 SAMPLING AND ANALYSIS MONITORING ACTIVITIES
Routine monitoring activities applicable to a surface
impoundment include sampling and analysis of hazardous waste, air
emissions, ground water, and liquid from the leak collection and
detection systems. The following sections discuss those
activities required under current regulations, and those that are
either recommended or pending with regulatory agencies.
5.2.1 Hazardous Waste Monitoring
Monitoring waste streams and stored wastes is an essential
part of hazardous waste surface impoundment operating procedures.
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Because hazardous wastes vary in their composition, degree of
hazard, and treatability, the impoundment owner/operator must be
familiar with the industrial processes that generate the waste;
the physical, chemical, and biological properties of the waste;
and any pretreatment processes used on the waste. This knowledge
helps the owner/operator determine potential hazards associated
with handling and storing the wastes, and aids in developing
proper waste handling, isolation, and storage practices.
Federal regulations require that the impoundment
owner/operator provide a list to the regulating agency of
hazardous wastes that are stored, treated, or disposed of in the
impoundment. This list must include the following information
for each waste received (USEPA, 1984e):
• common name of each waste;
• chemical analysis of each waste;
• USEPA ID number of each waste;
• location of each waste at the facility;
• volume of each waste received per month (estimates from
inflow structures or from transport vehicles such as
trucks, trains, and barges);
• physical form of each waste received (i.e., liquid,
sludge, or slurry);
• approximate moisture or solids content and other
significant characteristics of each waste; and
• special handling requirements for each waste.
In addition, the owner/operator must record the physical
characteristics of each waste according to the following
classification (USEPA, 1984e):
• aqueous: inorganic and aqueous-organic (water is the
solvent and inorganics or organics are the solutes);
• organic: the predominant liquids are organic in
composition and the solutes are organic compounds
dissolved in the organic solvent; or
• solids, sludges, and slurries.
After the waste is placed in the surface impoundment,
routine monitoring (sampling and analysis) should continue to
assess the physical and chemical behavior of the waste over time.
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If the impoundment serves to temporarily store and physically,
chemically, or biologically treat the waste, a routine sampling
and analysis program enables the owner/operator to evaluate
current waste status and to monitor the treatment process. This
monitoring program is essential when an effluent is to be
discharged under the facility's National Pollutant Discharge
Elimination System (NPDES) discharge permit. Any effluent from a
surface impoundment complex must be sampled and analyzed before
and during its release into a stream, river, lake, or ocean via
spillway or discharge pipe.
Collecting representative samples from a surface impoundment
is often difficult due to inaccessibility. If the impounded
waste is well-mixed, samples collected from any depth will be
representative. However, if the wastes are stratified (i.e.,
waste density increases with depth), samples should be collected
at different depth intervals at each sample location and the
sample containers labeled accordingly. The number and locations
of samples should be determined by a statistically valid random
sampling plan. USEPA has established a protocol for
statistically valid sampling (USEPA, 1986a).
A variety of sampling devices exists for collecting
hazardous waste samples (USEPA, 1980a and 1986a; Franson, 1985).
Liquid or semi-liquid waste samplers include the following:
• pond sampler or rod-and-clamp sampler (beaker attached to
a telescopic aluminum pole);
• Kemmerer sampler (vessel with a trigger-activated closure
at either end);
• subsurface grab sampler (bottle with a closing lid
attached to an aluminum pole); and
• weighted bottle sampler (weighted bottle with corked
lid) .
The type of equipment selected depends on site-specific
conditions. USEPA recommends using a pond sampler for sampling
impounded liquid. If the physical form of the waste is a solid,
sludge, or slurry, sampling equipment such as spoons, scoops,
shovels, hand augers, or small-diameter push tubes should be used
(USEPA, 1986). A boat or crane should be used to sample areas
beyond reach from dikes (Crawley et al, 1985).
Sampling safety procedures should be tailored to the
specific situation and documented in the sampling plan. Sampling
may require special safety equipment, clothing, and personal
precautions to guard against injury from the impounded waste.
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Due to the complexity of wastes often handled in a surface
impoundment, waste analyses may be complex. The owner/operator
will usually have a written waste analysis plan from the RCRA
Part B Permit (standard operating procedures) which describes
sampling schedules, sampling methods, analytical methods, and a
QA/QC procedure. Information on analytical procedures for
analyses of wastes and wastewaters is given by Considine (1974),
USEPA (1979a and 1982b), Franson (1985), and 40 CFR 261.
5.2.2 Air Monitoring
Hazardous VOCs, odors, and particulates emitted into the
atmosphere during the handling and storage of hazardous wastes
can adversely affect air quality. The influent liquid must be
properly screened so that the emission potential can be predicted
and the appropriate management plans developed to detect and
control the emissions. Air quality over the impoundment must be
continuously monitored to detect and quantify the VOCs being
emitted. Current regulations do not specify which techniques
should be employed for air monitoring.
5.2.2.1 Estimating Emissions from Surface Impoundments —
Several predictive mathematical models exist for estimating
emission rates of VOCs at hazardous waste disposal facilities
(USEPA, 1985c). Table 10 summarizes the recommended air
emissions models for various types of facilities. The two models
that apply to surface impoundment sites are the Mackay and
Leinonen Model (ML) (1975) and the Thibodeaux, Parker, and Heck
Model (TPH) (1982). Of these, the TPH model is most widely
recognized by regulators. This model applies to surface
impoundments under steady-state conditions (i.e., inlet rates,
biodegradation rates, and waste constituent concentrations remain
constant) and assumes that different waste species do not
interact.
USEPA (1985f) determined rates of gaseous emissions from
surface impoundments using several sampling techniques and
compared the results to predictions from the TPH Model. The
study concluded that the TPH Model appears to be generally
applicable to several classes of compounds contained in surface
impoundments that have no oily surface films or mechanical
spraying devices. The presence of oily films on the impoundment
surface caused the model to grossly overestimate the rate of
gaseous emissions for several VOCs.
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TABLE 10. RECOMMENDED AIR EMISSIONS MODELS FOR HAZARDOUS WASTE
DISPOSAL FACILITIES.*
Source
Models
Landfill
Land Treatment
Surface Impoundment
Open Tank
Storage Pile
Fixed Roof Tanks
Floating Roof Tanks
Farmer et al (1978) - for covered landfills
Thibodeaux (1980) - landfill equation w/o
internal gas generation
Thibodeaux (1981) - landfill equation with
internal gas generation
Hartley model (1969)
Thibodeaux - Hwang (1982)
Mackay & Leinonen (1975) - Unsteady-state
predictive model for nonaerated surface
impoundments
Thibodeaux, Parker & Heck (1981) - Steady-
state predictive model for nonaerated and
aerated surface impoundments
Thibodeaux (1980) - Aerated surface
impoundment (ASI) model
Hwang (1970) - Activated sludge surface
aeration (ASSA) model
Freeman (1980) - Diffused air activated
sludge (DASS) model
Midwest Research Institute emission factor
equations for storage piles
API (1962), modified by TRW/EPA — Fixed-
roof tank breathing losses
API (1962) - Fixed-roof tank working losses
API (1980) - Evaporation loss from external
floating-roof tanks
EPA/API (1981) - Standing storage losses
from external floating-roof tanks
EPA/API (1981) - Standing storage losses
from internal floating-roof tanks
*from EPA (1984c)
Emission models for surface impoundments have been described
in some detail by USEPA (1987e) in a volume dealing with
treatment, storage and disposal facilities. Estimation models
are described for quiescent, mechanically aerated, diffused-air,
and oil-film impoundments for flow and non-flow situations.
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5.2.2.2 Air Sampling and Analysis —
Air sampling generally involves collecting air over a given
area via detection/concentration measurement devices or
collection instruments. Direct sampling using an emission
isolation flux chamber in contact with the liquid surface (Figure
43) has been used by researchers to measure emission fluxes of
sulfur and VOCs from surface impoundments and other waste-
handling facilities (Schmidt et al, 1982). USEPA (1985f)
reported that the variability in emission rates using the flux
chamber was typically much less than the variability using
indirect sampling techniques, or predicted emission rates. The
flux chamber appears to be the sampling device best suited to
measuring emission rates from surface impoundments, provided a
sufficient number of sampling stations are placed in appropriate
locations.
temperature
readout
carrier
gas
Figure 43
sample collection
and/or analysis
J- flow controls
grab sample
port
— plexiglass
dome
stainless
steel collar
Cutaway view of emission sampling apparatus
(USEPA, 1985f).
The flux chamber determines gaseous emission rates by
passing clean, dry-sweep air through the chamber at a fixed
controlled rate. The carrier gas flow carries with it VOCs
present in the collection chamber. The carrier gas volumetric
flow rate through the chamber is recorded and the concentration
of the species of interest is measured at the chamber exit
(USEPA, 1985f).
While the flux sampler is in operation, total hydrocarbon
concentrations (THC) are monitored continuously in the chamber
outlet gas stream using a photoionization detector (PID) or a
portable flame ionization detector (FID). Once steady-state
emission rates are obtained from the sampling chamber, gaseous
samples can be collected for subsequent gas chromatographic (GC)
separation and analyses.
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Sampling containers using various solid sorbents can be used
to trap low-molecular weight VOCs, while heavier VOCs are sorbed
by Florisil, glass fiber filters, or polyurethane foam (USEPA,
1985f). Teflon bags and stainless steel containers have also
been used to collect air (emission) samples. If the air
monitoring program requires sampling of liquids or solids from
the impoundment, the collection containers should be designed to
minimize headspace to prevent loss of VOCs from the liquid.
Analyses of collected samples may be performed on- or off-
site, depending on site conditions and available instrumentation.
Two analytical methods described by USEPA (1985f) include the use
of (1) a field-portable GC-FID, and (2) a laboratory-based
capillary column GC-FID/PID/Electron Capture Detector (GC-
FID/PID/ECD) with cryogenic concentration and subambient
temperature programming. When liquid or solid waste samples are
collected in conjunction with air samples, the analyses are
normally conducted using purge and trap techniques followed by
GC-FID/PID/ECD.
5.2.3 Ground-water Monitoring
Under current RCRA regulations, a ground-water monitoring
and protection program must be implemented at all surface
impoundment sites. The ground-water monitoring program has two
parts:
• routine collection and analysis of ground-water samples
to detect contaminants that may have leaked from the
surface impoundment;
• if releases are detected, the collection and analysis of
more ground-water samples over a wider zone to evaluate
concentrations of contaminants, plume characteristics,
and contaminant migration rates.
Intensive monitoring is implemented when contaminants have
entered the ground water. It is continued through the course of
remediation in order to evaluate the progress of that activity.
More information on this type of monitoring is given by Canter
and Knox (1985) and USEPA (1982e; 1983b; and 1986d).
The design of a ground-water monitoring program is based on
an evaluation of site-specific conditions. These conditions
include the characteristics of the wastes handled by the
facility, and the subsurface geology and hydrology and the
potential fate of the hazardous constituents. Guidance may be
found on a ground-water monitoring design in a recently published
handbook on ground water by USEPA (1987f).
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A minimum of four monitoring wells is required to meet the
objectives of a detection monitoring program (EPA, 1985a). One
well is located hydraulically upgradient of the surface
impoundment and three wells downgradient. Due to the often
complex nature of a facility layout (e.g., more than one
impoundment unit) and the complexity of subsurface hydrogeology,
it may be necessary to install additional wells (Figure 44).
ground-water
flow direction
upgradient well
upgradient well
D D D D
^^ downgradient
0 wells
4 •
L— nnti
impoundment
B
D D n D D
optional downgradient wells
Figure 44. Example layout of ground-water monitoring wells.
As with leak detection in the impoundment liner system, the
main objective of ground-water monitoring is the early detection
of leakage after a failure of the liner system. Because many
existing surface impoundments are not equipped with a sensory
leak detection system, the monitoring wells located directly
downgradient of the impoundment act as the "early warning"
system. The number and location of downgradient wells, and
placement of the screened intervals in each well, depend directly
on the location of ground-water pathways along which migrating
contaminants are transported. General regulatory guidelines for
determining the number and spacing of wells have been provided by
USEPA (1985a).
Guidance in selecting the best-suited drilling method for a
given application (e.g., air rotary, water rotary, cable tool,
etc.) has been provided by USEPA (1980b; 1985a; 1987f), Minning
(1982), and Scalf et al (1981).
The type of construction materials used for well casings and
screens depends on local hydrologic conditions and the chemical
properties of the ground water and suspected pollutants (USEPA,
1985a; Lewis, 1982). Pettyjohn et al (1981) discuss well casing
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materials and their suitability in the presence of various
organic contaminants. Stainless steel, black steel, galvanized
steel, polytetrafluoroethylene (teflon), PVC, polyethylene, epoxy
biphenol, and polypropylene are commonly used materials.
In most cases, 2- to 4-inch (5- to 10-cm) inside-diameter
(ID) well casing is sufficient for monitoring purposes. However,
designing for possible future pumping activities (e.g., ground-
water recovery) requires a larger-diameter casing. Some large-
volume pumping units require up to 36-inch (90-cm) diameter
casing.
Once constructed, a well must be developed by overpumping,
surging, or jetting. The objective is to produce a clean,
debris-free environment in the well bore. At the time of
completion, the well should contain only formation water.
For all ground-water monitoring wells, documentation of
design, construction, and completion is required by enforcement
officials. USEPA (1985a) discusses the information that must be
presented as a written record.
The RCRA-required Part B Permit Application for a hazardous
waste surface impoundment includes a comprehensive written
ground-water sampling and analysis plan, which discusses the
following topics:
sampling equipment
sampling techniques
sampling schedules
sample handling and preservation
sample analysis procedures
field and laboratory QA/QC programs
data collection and statistical analysis
Information on these topics is contained in references by USEPA
(1984f and 1985a), Gillham et al (1983), Scalf et al (1981), and
USGS (1985).
Each component of the written sampling and analysis plan
must be closely followed to ensure sample integrity and data
quality. Ground-water sampling and analyses must be conducted by
trained personnel, and procedures should follow the techniques
recommended by USEPA ground-water monitoring regulations.
5.2.4 Soil-vapor Monitoring
Monitoring soil vapors in the vadose zone just above the
water table may be useful in detecting the leakage of volatile
contaminants from the impoundment. Use of soil vapor probes is
an inexpensive and rapid means to collect data that may indicate
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a problem. Soil-vapor probes are designed to collect organic
vapors that accumulate in, or migrate through, unsaturated soil
zones (Figure 45). Field studies using soil-vapor sampling and
analysis at ground-water contamination sites have indicated that
the concentrations of VOCs in soil gases correlate strongly with
the concentrations of those compounds in the ground water
(Kerfoot et al, 1986). Soil-gas sampling should be conducted at
surface impoundment sites by experienced operators of soil-gas
equipment, where ground-water contamination has occurred or is
suspected, provided suitable geologic conditions exist for using
the soil-gas technique.
vacuum pump
sampler
n access
or
NOT TO SCALE
evacuated sample vial
may be lowered to here
small-diameter hole
1 inch (2.5 cm) or less
disposable tip (hand-driven)
Figure 45. Schematic of soil-gas sampling probe,
5.2.5 Leak Collection and Removal System Monitoring
A leak collection system collects leakage accumulating above
the secondary liner and channels the flow through a porous medium
(i.e., sand and gravel) and piping networks to an outflow sump or
collection basin. Primary liners tend to allow some release, and
the intent is to have that waste liquid migrate through the
collection system rather than enter the underlying secondary
liner. The leak collection system serves another purpose, and
that is to monitor the volume and chemical composition of liquid
that may enter it (USEPA, 1983a).
The leak collection system generally consists of sampling
and pumpout sumps along lateral lines or one sump at the system
outflow. For sampling, small-diameter tubing may be inserted
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into a sump and extended to the ground surface through a
protective riser pipe (Figure 20). Samples are collected by
pumping liquid up through the tubing. If excessive leakage
occurs in the primary liner and several sumps are available, the
general point-source area may be identified from increases in
liquid volumes in one or more lateral sumps.
The sump at the system outflow, if any, can be sampled in a
similar manner, although some outflow sumps are connected to the
ground surface by a manhole and samples can be collected using
bailers, pond samplers, grab samplers, or similar devices.
The sampling and analysis schedule for leak-collection
systems depends largely on site conditions and the liquid volume
moving through the collection system. If the system also serves
as the primary method to detect liner leakage, liquid volumes
should be monitored at least on a weekly basis. Continuous
monitoring of the liquid level is preferred. Sampling for
chemical analyses of the liquid should be performed periodically.
Samples should be taken immediately upon the occurrence of
changes in pumping rates that indicate possible leaks. In cases
where the liquid is to be treated and either returned to the
impoundment or released under an NPDES discharge permit, frequent
analyses must be conducted on samples before and after treatment.
Sampling techniques, sample handling and preservation, and
analytical procedures are the same as those used in ground-water
and waste monitoring.
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CHAPTER 6
CONTINGENCY PLANNING
Owners and operators of surface impoundments must take
precautions to protect human health and the environment from
impoundment failures. In general, RCRA regulations and/or
proposed RCRA regulations address three kinds of events that call
for corrective or remedial actions. These are (1) sudden drops
in impoundment level or dike leaks; (2) leaks through the top
liner into the leak detection, collection, and removal system;
and (3) whenever a ground-water protection standard is exceeded
at a compliance monitoring point.
6.1 LIQUID-LOSS RESPONSE PLANS
All three release event possibilities must be addressed as
parts of the facility permit and approved by the Regional
Administrator. Sudden drops in impoundment level, and dike
leaks, must be addressed as part of the facility's "contingency
plan." Liner leaks are addressed in a "response action plan,"
and ground-water standards violations by "corrective action
program" descriptions in the permit. Summaries of the pertinent
regulations are provided in Subsection 1.3 of this document.
6.1.1 Contingency Plan
Contingency plans are required for all hazardous waste
facilities. They must be submitted and approved as part of the
facility permit. The general requirements regarding the plan
content are provided in 40 CFR Parts 264 and 265, Subpart D.
Additional requirements, specific to surface impoundments, are
given in 40 CFR 264.227 and 265.227. Contingency plans address
potential unintended, large, sudden releases of hazardous
materials. These are the most threatening releases, measured in
perhaps tens of liters or more per second, that require immediate
response actions to minimize risk to human health and the
environment. Immediate response actions might include evacuation
of the affected area and mobilization of emergency response teams
and spill mitigation equipment. The decision-making process and
response should proceed according to the contingency plan.
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The general components of a contingency plan are as follows:
• names, addresses, and phone numbers of emergency
coordinators;
• locations, amounts, and characteristics of impounded
wastes;
• potential hazards to human health or the environment
caused by fire, explosion, or uncontrolled release of
impounded wastes;
• list of emergency equipment, including descriptions and
locations;
• evacuation plan for the surface impoundment facility;
• emergency response procedures (preplanned and discussed
with local authorities);
• procedures to reduce and prevent exposure caused by
sudden releases, non-sudden releases, fires, and
explosions;
• procedures for removing the surface impoundment from
service, containing leakage, shutting off inflow,
preventing catastrophic failure, and emptying the
impoundment;
• potential remedial actions, associated health hazards,
decontamination procedures, and methods to provide
personal safety for personnel carrying out remedial
actions;
• location of contingency plan copies; and
• procedures for updating the contingency plan in the event
of system modifications.
Table 11 shows an outline for a "response data sheet,"
summarizing many of the above points, which should be prepared
for each impoundment and made a part of the contingency plan.
6.1.2 Response Action Plan
In proposed regulations, 40 CFR 264.222 and 265.222,
published May 29, 1987, the USEPA proposed requirements for
response action plans as part of the permit process for hazardous
waste surface impoundments. These plans would deal with leaks of
waste liquid through the top liner and into the leak collection
and removal system.
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TABLE 11. OUTLINE OF CONTINGENCY PLAN RESPONSE DATA SHEET.
1. Emergency Coordinator(s)
Name:
Address:
Phone:
2. Surface Impoundment Design
Volume:
Flow Controls:
Flood Routing Data:
3. Properties of Impounded Hazardous Substances
CIS Class:
CERCLA Categorization (class and subclass):
Physical/Chemical Properties:
Hazardous Characteristics:
Protection Level Required:
4. Spill Countermeasures
Physical Countermeasures:
Chemical Countermeasures:
Land/Soil:
Surface Water:
Ground Water:
5. Emergency Equipment
Type:
Capabilities:
Limitations:
Location:
USEPA has proposed two types of response action plans. The
first, required to be submitted with the Part B permit
application, would address a "rapid and extremely large leak." A
rapid and extremely large leak is defined as the maximum that the
leak collection and removal system can remove without the fluid
head on the bottom liner exceeding one foot in a granular leak
collection system. The head limit in a geosynthetic leak
collection system would be equivalent to the geosynthetic
thickness. The actual leakage rate for a rapid and extremely
large leak would be site-specific, but might be expected to be
several hundreds or thousands of gallons per acre per day.
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The second type of response action plan, that may be
submitted at a later date, even as late as the leak event, would
address a leak through the top liner exeeding an "action leakage
rate." USEPA has not determined that rate, but suggests in the
preamble to the proposed regulation that it may be established at
between 5 and 20 gallons/acre/day (47 and 187 liters/hectare/
day). Based upon measurements of actual leakage through top
liners at facilities that have been built under rigid quality
control, Bonaparte and Gross (1990) have suggested an action
leakage rate of 200 liters/hectare/day or about 21
gallons/acre/day for landfills. For surface impoundments, they
found little leakage through a top geomembrane liner, and they
suggest that an action leakage rate of 200 liters/hectare/day can
be met if ponding tests or leak location surveys are carried out.
The proposed regulations would require that a response
action plan include the following:
• description of the unit and the planned method of
closure;
• hazardous constituents in the waste;
• events that may lead to a rapid and extremely large leak;
• factors affecting leakage into the leak collection
system;
• mechanisms to prevent leakage out of the unit;
• assessment of responses that decrease leakage into leak
collection system; responses may include limiting waste
receipt, expeditious repair, or operational changes; and
• correlation of a range of leakage rates with different
responses, indicating why others were not chosen.
For leaks above the action leakage rate but less than rapid
and extremely large, the plan must include similar information,
but the range of responses could be broader, depending on the
severity of the situation. The options could include
continuation of liquid removal, treatment and ground-water
monitoring, or the maintenance of current operations.
6.1.3 Corrective Action Program
Either a spill or a liner leak could result in contamination
of the ground water in the vicinity of the impoundment. In order
for such a liner leak to occur in an engineered double-liner
system, the response action plan would not have been effective,
and both liners will have leaked.
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system, the response action plan would not have been effective,
and both liners will have leaked.
When a release has escaped a surface impoundment, and a
ground-water protection standard has been exceeded at a
compliance monitoring point, a corrective action program must be
implemented. The corrective action program is required under 40
CFR 264.100 and must be initiated and completed "within a
reasonable period of time." The terms of the program are
specified by USEPA's Regional Administrator in the facility
permit. In the permit, he establishes the ground-water
protection standard for the site.
Corrective actions, such as removal or treatment of the
ground-water contaminants, must be adequate to bring the ground-
water at and downgradient of the compliance point to levels that
meet or exceed the standard.
6.2 TYPES OF FAILURE
A surface impoundment failure is manifested by a loss of
impounded waste liquid. Ordinarily, the loss will be one of two
types, (1) a sudden catastrophic loss, or spill, or (2) a liner
leak. As noted above, the two types are reflected in the
different requirements for contingency plans and response action
plans, respectively. The spill type would be characterized by a
large volume lost in a short time, with the flow perhaps measured
in tens of liters per second, whereas the leak type might be
measured in cubic centimeters or smaller per second.
A surface impoundment spill situation would be expected to
be associated with a foundation or dike failure. Causative
factors might include unstable soil on dike slopes, very high
precipitation and dike overtopping, or subsurface weakness
(solution channel or sink hole). The last of these might result
in a spill to ground water, whereas the first two would result in
surface spills. Spills, because of inherently higher volumes,
are usually emergency situations. The real possibility of spills
justifies the contingency plan.
The hazard associated with a sudden release or spill
involves immediate, short-term exposure to hazardous liquids. In
this situation, the appropriate emergency response involves
attention to the safety of persons, immediate spill control,
cessation of process operations, notification of regulatory and
local authorities, and implementation of remedial activities for
treating, storing, or disposing of contaminated soil and water.
Leaks can be associated most often with weaknesses in the
impoundment liner, such as pipe penetrations and liner attachment
to structures. Leaks may also be attributed to accidental
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puncture, parted seams, tensional tears, and low chemical
resistance of the liner. Most impoundment failures appear to be
of this type without a discernible sudden onset. Leaks would be
expected to permeate to ground water, but they could also
permeate the dike to surface water. Leaks generally constitute
serious, but not emergency, instant-response situations.
The risk associated with an impoundment leak, or non-sudden
release, involves the potential for long-term exposure to low
concentrations of hazardous substances in drinking water.
Additionally, a non-sudden release may result in potential long-
term, lower-level exposure to aquatic organisms in surface water
or terrestrial biota in contact with contaminated soil.
6.3 RESPONSE PLAN IMPLEMENTATION
The primary difference in response to a spill and response
to a leak lies in the amount of time, and therefore the degree of
detail, available for investigating the distribution of
contamination and selecting and implementing the remedial
actions. In general, response to a sudden release requires quick
application of containment and protective measures. Responses to
a non-sudden release (a leak detected in the leak collection
system or at a ground-water compliance point) are generally less
urgent and allow more time to assess the contamination and select
appropriate remedies.
6.3.1 Contingency Plan Implementation
When a large sudden release or spill is detected at a
surface impoundment, the contingency plan is implemented by the
Emergency Coordinator (designated in the plan), who has
independent authority to initiate and carry out the response.
Immediate response actions include controlling the source,
containing the leaked liquid, and notifying the authorities.
When immediate threats have been controlled, the response process
proceeds with the contamination assessment; determining the
necessary response objectives; screening available remedial
options; selecting and implementing an appropriate remedy; and
sampling to verify cleanup effectiveness.
Quick implementation of the contingency plan before
contaminants are detected in the ground water could minimize the
magnitude of contamination and necessary remedial action. Just
as importantly, it could help reduce owner/operator liability.
A hazard classification of over 150 compounds, which
provides information on the toxicity of these chemicals to human
health and the environment, was published by Clements and
Associates (1985). Hazardous chemicals can also be categorized
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flaimnability/combustibility, persistence, and corrosiveness. The
information from the response data sheet and the general
contingency plan will enable the coordinator to quickly evaluate
the emergency situation, review the available countermeasures,
and assess the feasibility of each approach. The four basic
steps in a contingency plan are:
1. Immediate Actions
2. Contamination Assessment
3. Selection and Implementation of Remedial Actions
4. Cleanup Verification
Each of these steps is discussed in the following sections.
6.3.1.1 Immediate Actions —
In the event of a sudden release, initial precautionary
emergency measures must be implemented to minimize the risk to
workers and the general public, and to prevent additional fires,
explosions, or uncontrolled releases. Basic steps to reduce the
potential exposure are as follows:
notify on-site facility personnel
control the source of hazardous release
mobilize emergency response teams and equipment
isolate the sudden release area
eliminate ignition sources
restrict water use
prevent commingling of reactive wastes
control fires
The Emergency Coordinator must also initiate efforts to
contain the released waste. Temporary berms may be constructed.
The Emergency Coordinator has the authority to stop process
operations, if necessary. If facility operations are halted,
monitoring should commence in order to detect leaks, pressure
buildup, gas generation, or ruptures in valves, pipes, or other
components of the surface impoundment.
The local police, fire departments, and hospitals must be
notified of the release, and advised on the magnitude of
potential health hazards and the potential need for evacuation of
local areas. The Emergency Coordinator must notify either the
government official designated as on-scene coordinator for that
geographic area or the National Response Center. These response
actions contained in 40 CFR 264.56 are consistent with RCRA
guidelines for emergency procedures at hazardous waste
facilities. Prompt actions during the initial phase of emergency
response can minimize potential hazards to workers and the
general public and reduce the overall cost of containment and
clean-up operations.
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6.3.1.2 Contamination Assessment —
The purpose of the contamination assessment is to determine
the relative severity of the release and the potential safety
hazard to workers and the public. This is done by evaluating the
fate and transport of the contaminants, identifying the exposure
pathways, and assessing the risks posed to public health and the
environment. Additionally, an assessment conducted immediately
after a sudden release from a surface impoundment may allow
reconstruction of the events that caused the release, and
possibly prevention of future releases. The assessment also
provides information needed for screening and selecting remedial
actions, and defines resources available for countermeasure
activities (Melvold et al, 1984). The contamination assessment
should result in defining the response objectives for
remediation. Factors to consider in the contamination assessment
follow:
• surface impoundment(a) involved;
• human exposures and/or injuries;
• potential for fire, explosion, or continued release;
• name and class of substance(s) released;
• physical and chemical characteristics of the released
substance;
• approximate volume and concentration of released
substance;
• media affected by release;
• permeability of affected soils;
• anticipated direction and speed of migration;
• proximity to natural barriers;
• local terrain/topography;
• proximity to environmentally sensitive areas and
likelihood of impact; and
• weather conditions (i.e., wind speed and direction,
air/ground/water temperature, and precipitation).
Some of the innovative technologies that may be used in
assessing site contamination are listed in Table 12.
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TABLE 12. INNOVATIVE INVESTIGATION TECHNOLOGIES TO ASSESS
SITE CONTAMINATION.
Technology/Instrument
Information Provided
Soil Vapor Detector/Analyzer
(Nadeau et al, 1985)
Aerial Photographs
(Finkbeiner & O'Toole, 1985)
Backscatter Adsorption Gas
Imaging
(McRae, 1984)
Acoustic Mapping
(Meyer et al, 1984)
Ground-Penetrating Radar
(Stanfield & McMillan, 1985)
Migration of volatile organic
compound (VOC) plume in soil or
ground water.
Documentation of site conditions,
assessment of potential hazards,
assessment of off-site impacts,
progress of clean-up operations,
and evaluation of post-clean-up
conditions.
Detection and tracking of VOCs.
Delineation of the location and
thickness of contaminant layers
in ground-water aquifers or
surface water sediments.
Location of subsurface obstacles,
contamination, faults, and
fracture zones in clay liners.
6.3.1.3 Selection and Implementation of Remedial Actions —
The Emergency Coordinator will use information from the
contamination assessment and knowledge of facility operations to
evaluate the available remedial response alternatives. This
evaluation should be based on performance, practicality, and
effectiveness of the response alternatives (Melvold et al, 1984).
Availability of the required resources, speed of application, and
effectiveness of the response methodology in different weather
and topographic conditions are also considered.
Several remedial action technologies are available for use
in the event of a sudden release from a surface impoundment.
Available technologies can be divided into three categories: (1)
removal and disposal; (2) in-place treatment; and (3)
containment. For preparation of the contingency plan, resource
documents describing remedial action technologies should be
reviewed. The contingency plan should contain a thorough
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description of each response strategy or alternative in order to
screen and select the appropriate response measures. Information
on each remedial alternative should include, at a minimum, the
following (Unterberg et al, 1984):
• technical description;
• applicability for field use;
• mitigation of human and environmental risks;
• environmental, demographic, and legal constraints;
• requirements for, and availability of, adequate power
supply, manpower, special equipment, and supplies; and
• cost.
Several important references describe available response
strategies for hazardous substance releases (USEPA, 1985k; 1983d;
1984J; and 1986e).
6.3.1.4 Cleanup Verification —
Once a cost-effective remedial action alternative is
selected and implemented, the Emergency Coordinator should begin
evaluating the effectiveness of site cleanup efforts. Factors
that may affect the performance of response efforts include
weather, equipment malfunctions, detection of additional hazards,
and changing response objectives. The remedial action program
should continue until the following criteria have been met,
documented, and verified:
• risk to human health and the environment posed by the
release of hazardous substances has been eliminated or
reduced to an acceptable level;
• potential for recurring hazards (e.g., fire, explosion,
and chemical release) has been eliminated; and
• response equipment and facilities have been
decontaminated.
Environmental sampling should be conducted to determine if
the released contaminants have been successfully treated or
removed. Although the purposes differ, the methods for
verification sampling of environmental media for facility closure
are similar to those for response action; they are discussed in
Section 7.2.4. Decontamination techniques for emergency response
equipment are presented by Meade and Ellis (1985).
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6.3.2 Implementation of Response Action Plan
When a leak through the top liner of a surface impoundment
is detected, its severity should be assessed. If it exceeds the
"rapid and extremely large leakage rate," the owner/operator
should implement the response action plan. A smaller leak, but
exceeding the "action leakage rate," also calls for
implementation of the response action plan, but the response will
be of less immediacy. The response action plan should have
foreseen, and included in its range of potential events, the type
of leak and appropriate responses. Any leak greater than the
"action leakage rate" in a double-lined impoundment requires a
response and written notification to the USEPA Regional
Administrator.
One advantage of a double liner system is that there is
likely to be more time available to respond to a leak than to one
from a single-liner system, because the former allows for early
detection and collection of leaks without contaminant loss to the
environment.
If the leak is through both liners, or through a single
liner as exists in some, especially older, impoundments, the
response may require quicker reaction, with fewer options
available. In this case, a response action plan and a ground-
water corrective action program may overlap, because the leaked
contaminants may have migrated to the ground water before
detection.
If the leakage is greater than the "action leakage rate" but
less than the "rapid and extremely large leakage" rate, the leak
collection and removal system should remain effective, allowing
time to choose among the corrective options available. One of a
variety of responses may be appropriate. At one extreme the
response may be simply a continuation of normal operations with a
greater alertness to contamination release potential. At the
other extreme, impoundment closure may be implemented. If the
response action plan has been prepared and approved, it should
contain the range of options available. It should also indicate
the preferable response option for the type of leak that has
occurred. If the continuation of normal operations is chosen,
increased monitoring is likely to be undertaken. Even though a
leak above the action leakage rate may be relatively easily
handled by an early decision to continue operations, perhaps with
increased monitoring, at some point leak correction will probably
be undertaken as part of the response plan. The correction may
be to repair the leak, change operations to reduce leakage, or
terminate receipt of waste. The choice may be dependent on the
rate of leakage. The choice of response must be approved by the
Regional Administrator.
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The first reponse may be similar for a leak greater than the
"rapid and extremely large leakage" rate. Because it is a leak
of much greater severity, the response must be more immediate and
decisive. By definition, the leak collection and removal system
alone cannot handle a rapid and extremely large leak, although
its operation should continue. The proposed regulations suggest
three options: (1) limiting or terminating waste receipts; (2)
expeditious repair; or (3) operational changes to reduce leakage
to less than rapid and extremely large. At the same time,
mechanisms must be implemented to prevent leakage out of the
unit. In all cases, leaked liquids must continue to be collected
and removed.
When liquids are found in the leak collection system between
two liners of a double-liner system, several possible courses of
action are available, depending on the potential site-specific
impact. In contrast, when leakage occurs in a conventional
single-liner impoundment, the only appropriate response is to
identify and eliminate the source of leakage, and clean up the
contamination. It is apparent that by increasing the level of
protection afforded by the surface impoundment components, the
potential for contamination of the environment decreases, the
number of possible remedial response alternatives increases, and
the flexibility of selecting and implementing the response
actions increases.
Finding the location of a leak in the top liner is likely to
be advisable if liner repair is contemplated. When found, and it
is a relatively small, discrete leak, the liquid may be drawn
down to the leak location, and the damage patched. This may
prove to be difficult if the liner has been exposed to waste for
some time and waste absorption has occurred.
If it is a large leak, or one caused by general
deterioration of the liner, it may be decided to cover the old
liner with a new one. In most cases, this may require emptying
the impoundment. In some, it may be possible to emplace a new
liner without drawing down the waste liquid (Cooper and Schultz,
1983; Shultz et al, 1985). The emplacement would involve pulling
an assembled liner across the impoundment beneath the liquid. An
untried method might be to pull an assembled liner across the
impoundment's surface and then sink it to the bottom.
If both liners of a double liner are leaking sufficiently to
demand action, repair of the system may not be practical. The
bottom liner will likely not be accessible without removal of the
top liner and at least part of the leak detection and collection
layer. It may be most cost-effective to close the impoundment.
USEPA conducted a nationwide study (Cochran et al, 1983) of
the remedial actions being used at uncontrolled hazardous waste
sites. Remedial actions at surface impoundments are also
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discussed in other USEPA documents (USEPA, 1985k and 1983d).
Modeling of a remedial technology using site-specific data may be
used to determine the applicability of a remedial strategy for a
particular site. General guidance in selection of models for
evaluating remedial actions is provided by USEPA (1985g).
6.3.2.1 Leak Correction Verification —
The owner or operator of the surface impoundment must submit
a report (as proposed in 264.222) on the effectiveness of the
leak response action to the Regional Administrator within 60 days
after the action has been taken. Verification of the
effectiveness, if the impoundment has not been closed, requires
the leak collection system to be operating under pre-leak
conditions, that contaminants are not entering the collection
system above the action response level, that contaminants are not
being released to the soil and ground water, and that the liner
system has been returned to USEPA-acceptable design conditions.
One of the leak-detection methodologies described in Section 3.5
may be used to confirm that a leak no longer exists.
If the site has been closed, ground-water sampling must
verify that the ground water does not contain hazardous
constituents exceeding health-based standards.
6.3.3 Implementation of Corrective Action Program
A corrective action program is generally implemented after a
release of contaminants to ground water. The program is directed
at the cleanup, by removal or treatment, of the contaminated
ground water.
In general, a specific sequence of events will have occurred
that triggers a corrective action program. First, the presence
of a monitored hazardous constituent will have been detected at
the compliance monitoring point. When that occurs, a compliance
monitoring program must be implemented. If the constituent(s)
exceed the ground-water protection standard for the constituent,
or a concentration limit established by the Regional
Administrator, a corrective action program must be implemented.
The owner or operator may be relieved of program implementation
only if he can show that the source of the hazardous
constituent(s) is not from his impoundment.
Several reporting requirements are involved in detection and
compliance monitoring and during and after completion of a
corrective action program. All reports are required in writing
to the Regional Administrator. Compliance monitoring and
corrective action programs, once implemented, must be completed
and the contamination cleaned up, regardless of the activity or
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inactivity stage of the impoundment. Cleanup must include ground
water both on and off the site, to the extent necessary to bring
the ground water to within the limits of the established
concentration limits or the ground-water standards.
Cleanup of ground water may be a long and difficult task.
The available options include: (1) extraction (may include
flushing) and removal of the contaminated water from the site,
(2) pumping, treating, and returning the contaminated water to
the aquifer, (3) treating the aquifer in place (may include
flushing after treatment). Treatment alternatives include
immobilization and degradation techniques. Many of the
techniques have been described by USEPA (1984m and 1985k).
Ground-water cleanup may be facilitated by the construction
of barrier walls. Barrier walls may confine the contaminated
zone to a more manageable area, prevent the spread of
contaminated water, and reduce the need to treat uncontaminated
water that would otherwise enter the zone. Slurry walls are
commonly used for this purpose (USEPA, 1984n).
6.3.3.1 Ground-water Cleanup Verification —
A corrective action program must include ground-water
monitoring throughout and after completion of the program.
Monitoring may be a continuation of the compliance monitoring
program initiated before the corrective action program. The
corrective action program may be terminated when the hazardous
constituent concentration has receded to the compliance
concentration limit. When the corrective action is taken after
the impoundment's active life, the action may not be terminated
until the owner/operator can demonstrate that the ground-water
protection standard has not been exceeded for three years.
The owner-operator must report periodically on the progress
of the the corrective action. In addition, he must attest to the
outcome of the program upon its completion. The reports are made
to the Regional Administrator. During the action, if it is not
showing success, the Regional Administrator may require a change
in the program including a change in the permit conditions.
6.4 PERSONAL SAFETY DURING REMEDIAL OPERATIONS
The contingency plan for remedial actions at surface
impoundment sites should identify health hazards and potential
risks and give appropriate methods to provide personal safety.
In light of the possible health hazards that may occur during a
remedial investigation and subsequent cleanup operation, it is
important that the contingency plan provide the means for
assuring worker protection. The required level of worker
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protection during remedial actions will depend on site and waste
characteristics which affect the fate and transport of
contaminants, toxicology of the waste, environmental features of
the site, and physical hazards. Additional information on worker
health and safety hazards has been provided by NIOSH (1985).
There are four basic parts to worker protection (Lippitt et al,
1984) :
• site management procedures to control access and minimize
exposure;
• engineering safeguards to contain the waste and isolate
workers from hazardous areas;
• personal protective clothing and equipment to minimize
direct contact and inhalation; and
• decontamination procedures and practices to remove and
control the spread of contamination.
Each of these facets should be discussed fully in the
contingency plan, and all of the necessary equipment and personal
safety gear should be available and accessible at all times at
the site.
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CHAPTER 7
CLOSURE AND POST-CLOSURE CARE
Owners/operators are required by 40 CFR 264.111 to close
hazardous waste surface impoundments in a manner that minimizes
the need for further maintenance and prevents threats to human
health and the environment. Closed impoundments must control,
minimize, or eliminate the post-closure escape of hazardous
waste, hazardous constituents, contaminated runoff, and waste
decomposition products to ground water, surface water, and the
atmosphere. Closure regulations have not been promulgated or
proposed by USEPA for non-hazardous waste surface impoundments.
Two basic closure options are available for a hazardous
waste surface impoundment under RCRA regulations: clean closure
(complete removal), and in-place closure (40 CFR 264.228[a][1] &
[2]). Figure 46 presents a flowchart for each of the two
options.
Clean closure of a hazardous waste surface impoundment
includes (1) removal or decontamination of all waste residues and
contaminated liner system components and subsoils; (2) sampling
to verify decontamination; and (3) backfilling. USEPA and RCRA
regulations tend to encourage clean closure, because the site is
essentially restored to its pre-impoundment condition, and
further attention is not required. However, the removed
hazardous materials must still be treated and disposed of at some
location, which may be a landfill.
In-place closure of a hazardous waste surface impoundment is
similar to closure of a landfill or other disposal unit. It
involves the solidification and/or treatment of all contaminated
media in the surface impoundment (i.e., liquids, sediments, and
subsoils) by chemical, physical, and/or biological techniques.
No free liquid may remain in the unit after closure. Hazardous
wastes must be treated to minimize their toxicity. The treated
wastes left in place must have a final cover structure that meets
the minimum technology requirements for a hazardous waste
landfill cover (USEPA, 1989a).
The primary goal of in-place closure is stabilization of all
residuals, providing the material with a bearing capacity to
support a final cover. While treatment to reduce the impounded
contaminants may be required, this approach does not necessarily
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closure
assess closure
options - in place
or removal
removal
disposal of free liquids,
residual sludges, contaminated
liners, soils, subsoils, etc.
verification sampling
and analysis
petition for 'clean closure*
waiver and establish
alternative concentration
limits
are all
analytical
data below
background
levels?
backfill Impoundment
with uncontamlnated
material
requirements completed
remove free liquids
sediment dewatering
stabilization/treatment
of residues
Is
solidified
material strong
enough to
support
cover?
construct final cover
petition for post-closure
variance if applicable
post-closure care
period
requirements completed
Figure 46. Flow chart of closure options and requirements.
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eliminate all hazardous constituents. Thus, a 30-year post-
closure care period is required. Post-closure activities are the
same as for a closed hazardous waste landfill. They include
long-term operation and maintenance of the final cover, leachate
collection system operation and maintenance, and ground-water
monitoring.
If the impoundment is older and does not have a non-leaking
liner, all contaminated materials (waste, liner, soils, etc.) may
be removed and managed as hazardous waste. An alternative is to
eliminate free liquids, stabilize the remaining waste, provide an
effective cover, and provide monitoring and maintenance
throughout the post-closure care period.
A critical aspect of in-place closure is constructing the
final cover system with an hydraulic conductivity at least as low
as that of the liner system. If the liner is ineffective, USEPA
may still require a cover that meets the performance objectives
of a "minimum technology" cover. The final cover acts to
minimize infiltration and leachate formation by diverting surface
water with slopes, drainage layers, and low-permeability barriers
(USEPA, 1986a). USEPA (1989a) has published minimum technology
guidance for hazardous waste landfill covers. The guidance
recommends a top soil layer with a vegetated or armored surface,
a drainage layer, and a two-component (geomembrane and compacted
soil) barrier layer. Other layers, such as geotextiles, may be
required for specific purposes.
7.1 ASSESSMENT OF CLOSURE OPTIONS
The anticipated use of the surface impoundment (i.e.,
treatment, storage, or disposal) influences the design and,
eventually, the type of closure option implemented. Surface
impoundments that are used to store and treat wastes usually
require removal and disposal elsewhere of all contaminated
material following the active life of the facility. Disposal
impoundments, on the other hand, are usually (but not always)
closed in place and require a post-closure care period. Table 13
presents advantages and disadvantages of the two closure options.
Several factors should be evaluated in choosing the closure
option for a particular site:
• waste characteristics, including toxicity, mobility,
leachability, reactivity, biodegradability, and
degradation by-products;
• site location features, including topography, geology,
climate, geohydrology, and proximity of highly populous
or environmentally sensitive areas;
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TABLE 13. ADVANTAGES AND DISADVANTAGES OF CLOSURE OPTIONS.
Advantages
Disadvantages
No post-closure monitoring
and care period
Simpler in concept, requires
less administrative time
May be suited for environ-
mentally sensitive areas
May be best option for
highly toxic materials
Land may be used for other
purposes after clean closing
-Removal-
Containment-
Stabilization/Solidification
Stabilization technology
available for most wastes
Work completed on site,
generator keeps control
May offer generators a cost-
effective option, especially
those not near a commercial
RCRA facility
May petition for shortened
post-closure period
Work completed on site,
generator keeps control
May offer generators a cost-
effective option, especially
those not near a commercial
waste-disposal facility.
May file delisting petition
May petition for shortened
post-closure period
Current regulations require
removal of all contaminated
liner and subsoils containing
levels of waste constituents
above background conditions.
Transportation costs often high
Risks associated with trans-
portation
Generator keeps long-term
liability for waste removed to
other management facilities
• 30-yr post-closure monitoring
• Liability associated with
potential threat to environment
as a result of leaking landfill
• Land use is limited
• State administration fees for
hazardous waste programs
• Financial assurance required
for post-closure period
-Treatment-
30-yr post-closure monitoring
if no variance is received
Land use limited if monitor-
ing variance not received
Financial assurance required
for post-closure period
State fee for administration
of hazardous waste program
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• cost;
• intended future site use; and
• environmental risk.
7.1.1 Waste Characteristics
The removal option is advantageous for surface impoundments
with subsoils or residual materials that cannot be treated
sufficiently, or within reasonable cost, to remove environmental
threats. For example, a surface impoundment containing leachable
or volatile hazardous wastes or liquids that cannot be solidified
or stabilized to yield consolidated wastes of sufficient strength
to support the final cover might be better closed by materials
removal (USEPA, 1982a). Inorganic wastes easily treated to a
low-mobility state might be better suited to in-place closure.
7.1.2 Site Location Features
In-place closure of surface impoundments near
environmentally sensitive areas (e.g., wetlands, fault zones,
flood-prone areas, areas with shallow ground water, or areas with
highly permeable subsoils) may pose a significant environmental
risk. Similarly, in-place closure of surface impoundments near
major population centers may increase the future risk to human
health. In these cases, removal and treatment or remote disposal
are preferred. On the other hand, some hazardous wastes (e.g.,
containing a significant radiation component) may pose less of a
threat if left in place and secured there.
7.1.3 Cost
Closure of a surface impoundment involves immediate closure
expenses, plus (if it is in-place closure) longer-term costs for
maintenance and monitoring. Closure costs depend on factors such
as availability and complexity of in-place treatment
technologies; labor and equipment costs; and proximity to an
alternative off-site hazardous waste treatment facility. If both
removal and in-place closure are viable alternatives, the closure
decision may be based on a comparison of cost estimates.
Complete removal or clean closure eliminates long-term
liability and the need for post-closure maintenance and
monitoring costs. However, if soils have been contaminated, the
necessity for removing them may increase the closure costs
dramatically. In-place closure requires a 30-year post-closure
monitoring and care period and associated costs. A post-closure
financial assurance bond is required at the beginning of the
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post-closure care period (40 CFR 264.140). In addition, further
costs will result if remedial actions are required due to
improper in-place closure operations (e.g., faulty design or
construction of the cover system), which lead to migration of
hazardous leachate from the impoundment.
7.1.4 Intended Future Site Use
The potential future use of the site should be considered
during the assessment of closure options. Clean closure is the
better choice if unlimited use is desired. If in-place closure
is an option being considered, the planned site use should be
such that it poses no problems with maintenance of the final
cover or functioning of monitoring systems. Compatibility of
impoundment and cover system components with various site uses is
discussed in Section 7.4.3.
7.1.5 Environmental Risk
Both short-term and long-term environmental risks are
associated with surface impoundment closure. Short-term risks
occur during closure and involve handling and transporting
wastes, worker safety, and mobilizing hazardous constituents by
disturbing the wastes. These risks are potentially greater for
closure through removal than for in-place closure.
Long-term risks are generally associated with in-place
closure, and include the potential for slow release of hazardous
constituents into ground water or nearby surface water through
cover or liner deterioration. Long-term risks may also be
associated with off-site disposal which may be involved during
clean closure. Because of the long-term costs and liabilities
associated with surface impoundment closure, it is important to
evaluate the potential environmental risk which may result from
cover failure during or after in-place or clean closure.
7.2 CLEAN CLOSURE (CLOSURE BY REMOVAL)
Clean closure of a surface impoundment requires removal or
decontamination of all wastes, contaminated liner system
components, structures, subsoils, and equipment (USEPA, 1984a).
As stated earlier, this option requires the removal of all
hazardous constituents to background levels. The owner/operator
of a RCRA-permitted surface impoundment may find that it is not
possible to comply with the clean closure plan because waste
constituents have migrated to great depths and removal to
background levels is not feasible. In that case, current federal
policy requires that the surface impoundment comply with in-place
closure requirements (USEPA, 1984e). For clean closure of
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interim status facilities, it may not be necessary to remove all
subsoils containing hazardous constituents at concentrations
above background levels, if the levels can be shown to be
nonhazardous via 40 CFR Part 261.3(d). A plan describing the
sampling and analysis procedures to verify decontamination of the
site should be submitted as part of the closure plan.
7.2.1 Free Liquids
Regulations (40 CFR 264.314) disallow the disposal of free
liquids in a hazardous waste landfill. This has special
significance for surface impoundments since landfilling is
ordinarily the ultimate disposal method, regardless of whether
clean closure or on-site closure is chosen.
Several options may be considered in dealing with the
impounded hazardous liquid during clean closure. The options
include (1) natural evaporation; (2) forced evaporation; (3)
other on-site treatment (e.g., chemical neutralization); (4)
solidification or stabilization; (5) off-site treatment; and (6)
recovery and process re-use. Some consideration should have been
given to treatment methodology in the impoundment planning and
design process, in anticipation of closure.
Criteria used to select a treatment option include technical
feasibility, reliability, effectiveness, regulatory acceptance,
and cost. Methods used to evaluate these options include
literature review of treatment technologies and vendor
interviews, bench-scale studies, pilot tests, cost estimation,
and consultation with regulatory agencies (Stevens, 1986).
On-site liquid treatment can use technologies developed by
the wastewater treatment industry in recent years, including
mobile or temporary treatment units (USEPA, 1982f). In many
cases, the treated liquids are released either to a publicly
owned treatment works (POTW) or directly to surface waters if
they meet effluent requirements of federal, state, and local
agencies (Crawley et al, 1984; Hale et al, 1983).
Disposing of liquids through natural evaporation is perhaps
the least expensive method, but requires a suitable climate and
can be subject to air emission regulations. However, the
impoundment often will have been operating as an evaporation
impoundment through its active life. With the cessation of waste
liquid input, it would be allowed to dry up.
Solidification and stabilization by the addition of a
solidification agent will ordinarily be one of the less expensive
alternatives. After solidification, the waste material can be
removed and transported to a permitted landfill.
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Other on-site and off-site treatment can be very costly,
with off-site usually being the most expensive. Much of the cost
can be allotted to transport. Transport of hazardous liquid
wastes is controlled by the U.S. Department of Transportation,
which requires the use of proper containment vessels, manifests,
placards, and other handling procedures (40 CFR Part 172).
Alternatives to treatment and disposal of free liquids
(e.g., reuse or waste reduction) are often more cost-effective;
however, these technologies must be implemented during or prior
to the facility's active life rather than during closure.
7.2.2 Residual Sludges
Residual sludges in a hazardous waste surface impoundment
will always be considered hazardous materials and require removal
or treatment. The primary removal option for residual solids is
transport to an off-site, RCRA-approved disposal facility (e.g.,
an incinerator or landfill). To reduce transportation and
disposal costs, the sludges may be dewatered or stabilized before
off-site transport, provided the separated liquids can be treated
and properly disposed. Alternatively, residual solids can be
transported and stabilized at the receiving facility. Methods to
remove surface impoundment sludges for stabilization or
dewatering are discussed in Section 5.1.2.4. Stabilization
techniques are discussed in Section 7.3.3.1.
7.2.3 Subsoils, Liners, and Other Contaminated Materials
Contaminated liner system components and subsoils must also
be treated or removed and transported to a permitted hazardous
waste disposal facility. This is the final step in waste removal
from a surface impoundment. Documentation showing that all
contaminated material has been removed is essential.
All equipment and support structures that are not removed
must be decontaminated (USEPA, 1986f; Meade and Ellis, 1986).
These structures include unloading areas, outfalls, leak
collection sumps, pumps, level detection stations, and other
structures that have contacted the waste.
7.2.4 Verification Sampling
The owner/operator should provide analytical data to verify
that closure was completed satisfactorily and that no significant
environmental hazard remains. Therefore, a plan to sample and
analyze the residual materials, subsoils, adjacent areas, and
equipment should be developed and submitted as part of the
closure plan. This plan should include sampling procedures,
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location and depth of soil samples, indicator parameters,
analytical procedures, and sampling methods to document the
decontamination of equipment and site soils.
Owners/operators who plan to close an impoundment using the
removal option should submit procedures in the sample plan to
establish chemical constituent levels for soils and ground water
at or below which no significant threat is anticipated.
Normally, these are background levels. For new surface
impoundments, background samples should be collected before
construction. For existing surface impoundments, background
values for chemical constituents can be established by collecting
samples in an adjacent area with soil conditions similar to those
at the impoundment site before construction. Efforts should also
be made to establish constituent background levels for borrow
materials used to construct liners, foundations, and dikes.
Establishing constituent background values is important because
the clean-closure option may use these values as standards to
verify adequate treatment or removal.
Soil and ground-water background sample data should be
statistically evaluated to assess the data's spatial variability.
The closure plan should contain a method for comparing the
background data set with the post-closure data set. Information
on this subject can be found in publications by Keith et al
(1983), Mausbach et al (1980), and Beckett and Webster (1982).
In most cases, verifying clean closure is limited to
collecting and analyzing samples from the liner materials and
immediately adjacent subsoils. For most surface impoundments,
ground-water monitoring wells have already been installed and are
being monitored routinely at the time of closure. Ground water
should be sampled and analyzed during verification sampling.
7.2.4.1 Sampling Schemes —
Verification sampling schemes applicable to closed surface
impoundments include simple random sampling, stratified random
sampling, systematic sampling, and judgmental sampling (USEPA,
1983b). Each scheme requires developing protocols for sampling,
sample handling and analyses, and adherence to a QA/QC program.
Each sampling scheme must also include provisions for collecting
soil samples at a number of specified depths to develop a
constituent concentration profile with depth.
The systematic sampling approach uses grids or transects to
provide sample locations. Because surface impoundments typically
are regularly shaped structures, this procedure is an effective
sampling method and is recommended for use in the process of
delisting hazardous wastes at surface impoundments (USEPA,
1985h). The procedure requires that the entire facility be
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divided into unit areas of equal size which do not exceed a
specified surface area (e.g., approximately 10,000 ft2 [930 m2]).
Each unit area is further divided using a grid, representing
potential sampling locations, which are selected using random
numbers. The number of samples collected from each unit quadrant
depends on the degree of spatial variability within the facility.
The samples collected from within a unit area are composited to
form one homogeneous sample for analyses. During the planning
stage, it is recommended that a reference be consulted (e.g.,
USEPA, 1983f and USEPA, 1984k).
7.2.4.2 Indicator Parameters —
Ground-water and soil samples suspected of being
contaminated should be analyzed for certain basic parameters
indicative of contamination. Analysis for indicator parameters
screens samples and identifies those samples requiring more
detailed testing to determine the type and concentration of
specific contaminants. Choice of indicator parameters depends on
the wastes stored, but commonly includes the following:
• total organic carbon (TOG): indicator of organic
compounds;
• electrical conductivity: indicator of soluble ions;
• pH: indicator of acidic or basic conditions;
• oil and grease: indicator of petroleum-based products;
• total organic halogens (TOX): indicator of halogenated
organic compounds; and
• toxicity characteristic leaching procedure (TCLP):
indicator of leachability of hazardous organic and
inorganic compounds.
Soils and ground water suspected of metals contamination
should be analyzed for total quantities of the specific metals.
7.2.4.3 Quality Assurance/Quality Control —
The QA/QC program is implemented to minimize or eliminate
error associated with the sampling and analytical programs. Each
item outlined in the sampling protocol must be defined clearly,
especially those concerning specific sampling locations.
Sampling protocol should include the following points:
• methods used to determine the number of samples to be
collected and the sampling point locations;
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• depth(s) for sample collection;
• type(s) of equipment for sample collection;
• methods for decontaminating sampling equipment;
• compositing procedures; and
• a list of all analytical parameters, preservation
methods, and handling and shipping procedures.
Sampling error may be introduced into the sampling procedure
regardless of the precautions taken. Thus, auditing the sampling
procedures may be worthwhile to determine the magnitude of
sampling error; for example, replicate samples can be analyzed to
monitor sampling procedures. Detailed information on preparing a
QA/QC program has been presented by USEPA (1980c).
The QA/QC program includes laboratory procedures for
chemistry analyses, analytical verification techniques, data
handling and analysis, and personnel responsibilities.
7.2.5 Regulatory Variance
In some if not most cases, removing all subsoil materials
containing hazardous constituents above background levels is not
feasible. This situation is likely to occur in the case of an
existing surface impoundment which either does not have a liner
or has a liner that has been ineffective. In such a situation,
closure leaving the remaining materials in place is necessary to
comply with federal regulations. In some instances, the
materials remaining after free liquids, sludges, contaminated
liners, and subsoils are removed may have hazardous constituent
concentrations higher than background values, but below levels
that present a hazard to the environment or human health.
Current RCRA guidance on facility closure (USEPA, 1982a)
does not provide specific guidance on closure options for surface
impoundments with hazardous waste constituents present in
subsoils or liners at less than hazardous levels. USEPA
proposed on March 19, 1987, in 40 CFR 265.310(c), conditions for
allowing "alternate closure requirements." If the conditions are
met, the requirement could be waived for removing all materials
having contamination above background levels. A waiver would
require that the owner/operator show that the closure performance
standard in 40 CFR 265.111 would still be met. It may require
that "alternate concentration limits" (ACL) be established for
the waste constituents in the soils at the site.
Selecting ACLs for inorganic compounds may follow guidelines
for determining whether materials are considered hazardous, such
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as EP Toxicity testing (40 CFR 261.24) or past experience at
Superfund remediation (CERCLA) sites. Selecting acceptable
values for organic compounds may be extremely difficult due to
the lack of information concerning the persistence or long-term
environmental hazard presented by these chemicals. In situations
where some level of organic compounds is proposed to be left in
place, it is likely that the responsibility lies with the
owner/operator to demonstrate acceptable limits.
7.2.6 Backfilling
The area can be backfilled after verification that the
closure area has been decontaminated. No specific guidelines are
available for backfilling materials; however, a few
considerations are advisable. Soil used for backfilling should
not be contaminated, and the selected soils should be analyzed if
contamination is possible. Soils placed into the impoundment
area should be compacted to a dry bulk density at least as high
as the surrounding soils to prevent subsidence and differential
settlement. Additionally, the backfilled soils should be
compatible with intended future site use. Soil characterization
and engineering tests on the soils may be required before use.
The area should be contour-graded to enhance and control
site runoff and reduce percolation and erosion. Surface grading
and compacting should result in a surface sufficiently sloped to
prevent ponding. Rishel et al (1984) present cost estimates for
contour-grading at surface impoundments.
The surface soil (12 inches [30 cm] or more) should be of
sufficient nutritive capacity to support vegetation. The area
should then be seeded with a suitable grass to prevent erosion.
7.3 IN-PLACE CLOSURE
In-place closure means closing the surface impoundment as a
landfill with a subsequent 30-year post-closure care period.
Facilities with intact liners at closure can petition the USEPA
Regional Administrator to reduce the post-closure period. The
in-place closure option consists of four basic phases:
• solidification or removal, treatment, and disposal of
free liquids;
• treatment and stabilization of residual sludges and
contaminated soils;
• decontamination of equipment; and
• construction of a final cover;
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USEPA recommends that permit applicants address the following
items in their surface impoundment closure plans (USEPA, 1984e):
• detailed information on how free liquids will be removed
or solidified;
• detailed plans describing how the wastes will be
stabilized;
• determination of the unconfined compressive strength and
consolidation characteristics of the stabilized waste;
and
• analysis results showing that the stabilized wastes will
provide sufficient, permanent support for the cover and
other expected loadings.
7.3.1 Removal of Free Liquids
Treatment and removal of free liquids for in-place closure
is similar to that for clean closure (discussed in Section
7.2.1). An alternative to removal is solidifying the liquids in
place, and eliminating the need for off-site discharge or
removal. Care must be taken during this activity to prevent
damage to the impoundment liner. Solidification is discussed in
Section 7.3.3.1.
7.3.2 Sludge Dewaterinq
Remaining sediments may require temporary removal from the
impoundment and dewatering to meet consistency requirements and
to ensure adequate handling characteristics for landfilling. In
some situations, portable processing systems (e.g., clarifiers,
centrifuges, thickeners, or belt-filter presses) may be installed
at the closure site to accomplish active dewatering.
Solidification agents may also be added instead of or in
addition to dewatering. In arid or semi-arid climates, sediments
can be dewatered using passive means (evaporation or free
drainage). This technique usually involves constructing a drying
area within the impoundment or drying beds outside the berms.
Collected water requires proper treatment and disposal (see
Section 7.2.1). Once sufficient liquids are removed so that the
sediments have a semi-solid consistency (i.e., will not flow),
the sediments can be mechanically transported (by clamshell,
auger, dragline, or dozer) and spread in the surface impoundment.
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7.3.3 Waste Residuals
Surface impoundments closed in-place require stabilization
or treatment of the residual solids. The residual solids may be
a combination of the impoundment's accumulated bottom sediment
and the residual from treating the liquid portion. Stabilization
of these solids increases the bearing capacity to a degree
sufficient to support the final cover. Treating certain waste
residuals can render them nonhazardous, thereby increasing the
flexibility of the post-closure care program.
7.3.3.1 Stabilization ~
Sludges and other solids in surface impoundments are likely
to possess poor physical and structural properties; therefore,
stabilizing them to a semi-solid consistency is usually required
to provide adequate containment of waste constituents.
Stabilizing waste residuals improves their physical properties by
two primary means: (1) increasing the density, which reduces the
compressibility of waste residuals and the resultant potential
for settlement of the final cover; and (2) decreasing the
permeability of the waste residuals and thus the mobility of
pollutants in the residuals (Anderson and Jones, 1982).
Stabilization techniques have become important remedial
operations, largely due to CERCLA (Superfund) remediation
activities. The goal in applying these techniques is to produce
a solid, chemically non-reactive material. Several methods exist
for mixing the wastes with stabilizing agents, including in-drum
mixing, in-situ mixing, mobile mixing plants, and area mixing.
These methods and techniques have been outlined by USEPA (1986g).
Most stabilization techniques are proprietary processes that
involve adding absorbents and solidifying agents to the residual.
These processes generally involve one of the following:
• sorption
• pozzolan formation
• encapsulation
Detailed discussions of these processes are contained in reports
by USEPA (1982b, 1982f, 1986g, and 1989e), Spooner (1985), and
Anderson and Jones (1982).
Sorption involves adding a dry, solid substance to a liquid
or semiliquid waste to take up free liquid and improve waste-
handling characteristics. Common sorbents include fly ash, lime
kiln dust, cement kiln dust, zeolites, and soil. This method can
be implemented using readily available equipment at relatively
low cost. The disadvantages include increased material for
disposal and relatively high leaching loss of potential
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contaminants from the stabilized wastes, thus requiring a secure
disposal (Spooner, 1985).
Lime/fly ash pozzolan and Portland cement pozzolan processes
use solidifying agents (e.g., hydrated lime or cement) along with
pozzolans to increase strength and durability. The suspended
solids in a waste slurry become incorporated into the hardened
concrete matrix, increasing the strength and decreasing the
permeability of the mixture. However, a number of compounds or
materials (e.g., oil and grease, calcium sulfate, sodium borate,
and organic matter) can weaken the waste/cement bond and decrease
the physical strength.
Encapsulation isolates wastes by completely surrounding them
with a durable, impermeable coating. Containment of the waste is
complete and assured for the life of the coating material.
Thermoplastic microencapsulation involves mixing dried wastes
with materials (e.g., asphalt, paraffin, polyethylene, and
polypropylene) and placing the mixture in a mold. Some of these
processes are adaptable to highly soluble toxic substances which
are not amenable to lime or cement-based techniques. The
disadvantages of this process include the cost of materials, and
the need for specialized equipment, skilled labor, and energy.
A bench-scale study, in which the wastes are mixed with
various stabilizing agents, is usually performed to select the
appropriate stabilization technique. Standards for testing the
stabilized wastes have not been developed; however, a testing
scheme should be developed for each individual situation. The
tests should be conducted on representative waste residual
samples that have been subjected to the proposed stabilization
technique. Various tests are described by USEPA (1989e).
The two most important engineering properties of stabilized
wastes that must be addressed in the closure plan are unconfined
compressive strength and compressibility. These engineering
properties are used in engineering calculations to demonstrate
that the stabilized waste has sufficient strength to support the
maximum anticipated loadings that may result from overburden,
cover, and equipment that will be used to close the facility
(USEPA, 1984e). Further, these properties are used to estimate
the magnitude of long-term settlement of the cover as a result of
consolidation of the stabilized waste. Unconfined compressive
strength and consolidation properties, as well as other
characteristics of the stabilized waste, should be determined
using procedures listed in Table 14.
Other tests may be required for specific situations (e.g.,
closing existing surface impoundments with inadequate liners).
These tests include leaching rate, permeability, free liquid
content, and biological consolidation (see Table 14).
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TABLE 14. TEST PROCEDURES FOR STABILIZED WASTES.
Parameter
Reference
Comments
Unconfined
Compressive
Strength
Consolidation
ASTM D 2166
ASTM D 2435
Biological Oxygen
Demand
Leaching Potential
Hydraulic
Conductivity
tests
American Public
Health Assoc.
(1985)
Chan et al (1978)
Means & Parcher
(1963)
Adjust water content
and density to the same
as dewatered and
compacted sludge.
Adjust water content
and density to the
same as dewatered and
compacted soil.
Biological consoli-
dation estimated by
assuming 1 unit of
organic matter will be
destroyed for each 2
units of oxygen demand.
EPA considers this
shake test to be the
best currently
available.
Laboratory K tests
typically give lower
values than field.
This method also allows
measurement of
compressibility and
rate of settlement.
Free Liquid Content Spooner (1985)
7.3.3.2 Treatment of Residues —
The primary objective of treating residues is to render them
nonhazardous, thus limiting potential long-term liability and
possibly the duration of the post-closure care period. In-situ
treatment uses chemical, biological, or physical mechanisms to
degrade, remove, or immobilize contaminants. Treating the
residual sediments is similar to treating soils. Therefore, the
following discussions apply to dried sludges and contaminated
soils.
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Contaminated materials that are treated to render them
nonhazardous must be managed as hazardous wastes, unless they can
be shown to be exempt (delisted) via 40 CFR 261.3(d). Wastes
considered characteristically hazardous must be formally delisted
via petition to the USEPA Regional Administrator (see Section
7.4.4) .
Extraction (Soil-flushing) — Extraction entails washing
contaminants from the soil with a suitable solvent (e.g., water,
surfactants, dilute acids or bases, or chelating agents). During
elutriation, sorbed contaminants are solubilized, emulsified, or
reacted chemically with the flushing solution and are mobilized.
The washing solution is injected into the area of contamination,
and the contaminated elutriate is pumped to the surface for
removal or on-site treatment and reinjection. This technique
probably has limited applicability for treating sediments in
lined surface impoundments, because of the difficulty of
establishing a subsurface circulation in the shallow confines of
the impoundment. Care must be exercised in using dilute acids
and bases to prevent the undesired result of dissolving already-
fixed contaminants. This technology is derived from the mining
industry, where it has been used for in-place extraction of
metals from ores (USEPA, 1984b; Wagner and Kosin, 1985; Ellis et
al, 1985).
Immobilization -- Immobilization techniques are designed to
reduce the rate of contaminant release from the soil so that
concentrations along exposure pathways are maintained within
acceptable limits. The primary techniques include precipitation,
sorption, ion exchange, and solidification (USEPA, 1984f).
Precipitation is primarily applicable to heavy metal
contaminants. It can be accomplished by adding carbonates,
sulfides, phosphates, and hydroxides. Within the correct pH
range, these compounds are very insoluble. Potential additives
include calcium phosphate, calcium carbonate, calcium sulfide,
sodium sulfide, iron hydroxide, alum, and ferrous sulfate. The
two major considerations influencing metal precipitation are pH
and rate of complexing agent application. The chemistry of the
potential compounds must be evaluated to prevent creating soluble
compounds that will leach, and to prevent changes in the
chemistry (e.g., pH) over the long term that may lead to leaching
(USEPA, 1984f; Wagner and Kosin, 1985; Malone et al, 1983).
Chemical neutralization is a type of immobilization which
lends itself to acidic, metal-containing wastes. Neutralization
lowers corrosivity and immobilizes metals, rendering the waste
nonhazardous. Calcium and magnesium hydroxides are the additives
most commonly used.
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Application of additives to waste sediments can often be
accomplished with agricultural equipment. In some cases, it is
possible to treat the sludges during the dewatering process.
Where application into deeper subsoil layers is required,
grouting equipment may be used to inject the additive. Grouting
is a technique in which an aqueous suspension containing the
additive is pressure-injected into the subsoil, where it forms a
gel or solidifies in place. This procedure has been used
successfully to treat acidic, metal-contaminated soils beneath
surface impoundments (Crawley et al, 1984).
Sorption is potentially applicable to organic and inorganic
contaminated sediments. A variety of natural and synthetic
materials can be added to soils to increase their natural ability
to sorb ions. Suitable materials that have potential for sorbing
metals include various agricultural products and by-products
(e.g., peanut hulls, straw, bark, sawdust, and wastewater
sludge). The pH of the residuals should be maintained above 6.5
for maximum adsorption efficiency. Tetraethylenepentamine
(tetren) has been used successfully to form metal chelates.
These complexes are strongly sorbed by soil clays and are not
sorbed by organic matter. Tetren must be applied to a soil
relatively high in clay to be effective. The long-term stability
of tetren-metal complexes against decomposition or degradation is
not yet known (Wagner and Kosin, 1985; USEPA, 1984J).
Certain natural clays, zeolites, synthetic resins, and other
colloids have the capacity to adsorb ions preferentially through
exchange reactions or to capture certain ions through stearic
hindrance in the crystal lattice. The ability of a colloid to
sorb metals chemically is related to its cation exchange capacity
(CEC) and surface area. Cation exchange capacity is an
electrical property of colloids and is defined by the number of
positively charged ions (cations) which can potentially be
adsorbed by the colloid. Naturally occurring zeolites typically
have the highest CEC values among natural earthen materials,
followed by high organic (peat) soils, and soils high in
vermiculite or smectic clay content. Zeolites behave as
"molecular sieves," capturing certain ions in the crystal
framework while allowing others to pass through. Increasing the
clay content or CEC of a dried sludge can increase capacity to
immobilize metallic cations (USEPA, 1984j).
Synthetic resins that can adsorb or chelate both cations and
anions have been developed. However, resins are generally
expensive and have limited availability (USEPA, 1984j; Wagner and
Kosin, 1985).
Techniques used to stabilize residues can be used to render
them nonhazardous if the waste constituents are adequately
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immobilized. Sorption, pozzolan formation, and encapsulation
processes are discussed in Section 7.3.3.1.
Biodeqradation — Biodegradation is the breakdown of organic
compounds by microorganisms. Biodegradation techniques include
land treatment and composting sludges (USEPA, 1983a; Overcash and
Pal, 1979). Important factors to be considered in biodegradation
by land treatment include (1) soil conditions (i.e., pH, redox
potential, moisture content, texture, nutrient supply, and
temperature); (2) presence of toxic compounds; (3)
biodegradability of the contaminant; and (4) breakdown products.
Certain hazardous wastes may have limited data available on
degradation rates or by-products. Therefore, a laboratory or
greenhouse treatment demonstration study may be required to
demonstrate treatability of the waste and to determine optimum
degradation conditions. If materials high in organic matter are
closed in place without adequate treatment, biodegradation will
occur during post-closure. This can lead to reduction of the
total mass of solids, consolidation, excessive production of
gases, and differential settling of the cover.
Chemical Degradation — Chemical treatment of soils and
other solids in situ should be carefully considered beforehand.
It is very difficult to treat the materials homogeneously. The
mass may contain zones where treatment has been very effective
while containing other zones where virtually no reaction has
taken place. The following discussion should be taken in light
of these probabilities.
Oxidation and reduction reactions may be carried out in
place to transform soil contaminants into less toxic or less
mobile products. Introducing chemical oxidants (e.g., ozone and
hydrogen peroxide) into the soil system will often promote
oxidation of organics. These agents are applied as aqueous
solutions directly onto the soil surface or injected into vadose
zone subsoils or ground water. Laboratory and field treatability
studies will be required to assess the reactions that occur and
to develop data for design of a full-scale treatment system if
results are favorable (USEPA, 1984j).
In-situ reduction may be accomplished by adding chemical
reducing agents (USEPA, 1984j). This process has been shown to
degrade toxic organic constituents. Organic wastes that are
amenable to treatment include chlorinated organics, unsaturated
aromatics and aliphatics, and other organics susceptible to
reduction (USEPA, 1984j). Possible reducing agents include
catalyzed metal powders of iron, zinc, or aluminum. These agents
can be applied to the soil surface and mixed with contaminated
soil using conventional agricultural equipment. Alternatively, a
grout slurry of the metal powder can be injected into the
subsurface via closely spaced wellpoints.
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Certain inorganic constituents can be immobilized or
transformed to less toxic compounds through reductions. For
example, hexavalent chromium and selenium have been successfully
treated in soils using reduction technology (USEPA, 1984j). The
resulting reduced compounds are trivalent chromium, elemental
selenium, and selenite (Se[IV]). These reduced compounds are
less toxic and less mobile in soil systems than their oxidized
analogues.
7.3.4 Final Cover System
Federal regulations require constructing a cover system over
a hazardous waste disposal impoundment upon final closure.
Regulatory statutes (40 CFR 264.228) require that the cover:
• provide long-term minimization of liquid migration
through the closed facility;
• function with minimum maintenance;
• promote surface water drainage and minimize erosion or
abrasion of the cover;
• accommodate settling and subsidence so that cover
integrity is maintained; and
• have a hydraulic barrier with a permeability less than or
equal to that of the lowest-permeability bottom liner or
natural subsoil layer present immediately beneath the
facility.
USEPA (1989a) provides guidance on designing and
constructing a final cover system that will exhibit these
characteristics. The following discussion of cover design is
condensed from that publication.
The recommended cover design is a multi-layered system
intended to minimize leachate formation by promoting surface
water drainage using slopes, water-retaining topsoil layers,
geotextile filters, and porous drainage layers. The ultimate
resistance to water percolating into the waste is provided by a
composite hydraulic barrier layer, which consists of a compacted,
low-permeability soil component in direct contact with an
overlying geomembrane. For underlying waste materials of low
strength, a geotextile or geogrid stabilization/ reinforcement
layer may also be required to support the hydraulic barrier
layer. Figure 47 provides a schematic of the recommended cover
design, which consists of the following components, top to
bottom:
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vegetation/soil
top layer
drainage layer
low-permeability
geomembrane/soil layer
waste
°°° °<> ° °° °°o ° °/o° o,°0 °o=;
0
o
O
0
o o
60cm
-^— granular or geotextile filter
30cm
— -^— 20-mil (0.51 mm) geomembrane
60 cm w/overlying protective geotextile
— ~*— geotextile separation layer
(for low-bearing-strength waste)
(30 cm = approx. 1 ft)
Figure 47. USEPA-recommended landfill cover design.
• protective surface layer (vegetation component and
topsoil layer)
• geotextile or granular filter
• drainage layer
• geotextile or sand protection layer
• hydraulic barrier layer (geomembrane and compacted soil
component)
• gas vent layer (granular or geosynthetic)
• stabilization layer (if required)
The cover should be constructed with a convex (high in the
center) surface topography that has gentle slopes, preferably
between 3 and 5 percent, after full settlement. This shape
should effectively drain surface waters from the cover without
erosion of the protective surface layer (USEPA, 1979b).
Geosynthetic materials, such as geotextiles, may be incorporated
in and on the surface layer to prevent erosion, particularly
where slopes are necessarily steeper or where water flows
persistently or at higher velocities.
150
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7.3.4.1 Protective Surface Layer —
The upper layer of the final cover system is a protective
surface layer composed of vegetative and topsoil components. A
layer of armoring material, such as cobbles, may be substituted
under some (e.g., arid) conditions (Figure 48). An armoring
layer is designed to prevent erosion and abrasion of the
underlying cover components, while functioning with minimum
maintenance. If cobbles are used as an armor layer, a geotextile
or granular layer placed below it will aid in preventing erosion.
Vegetation — The uppermost component of the protective
surface layer, the vegetative cover (see Figure 47), reduces
percolation into the cover system, shields the topsoil from rain,
stabilizes the soil against erosive and abrasive forces, binds
and anchors the soil to form a stable mass, increases evaporation
rates, and enhances site aesthetics.
Selecting the vegetation species is important and depends on
factors such as climate, site characteristics, and soil
properties. Deep-rooted plant species (especially shrubs and
trees) should be avoided to minimize root extension into the
drainage and hydraulic barrier layers, thus compromising cover
integrity. The vegetation selected should provide a self-
maintaining cover that does not require perennial fertilization
or irrigation, and minimizes the maintenance required at the
closed facility. Several references discuss available plants and
site selection criteria (USEPA, 1979b; 1983d; 1983g; and 1985i),
and Lee et al (1985). A local extension service, consulting
agronomist, or SCS agent should be contacted for recommendations
on locally adaptable plant species and information on area crop
cultivation.
In some regions of the country (e.g., those with arid and
semi-arid climates), establishing a vegetative cover is difficult
or impossible. In these areas, a rock, cobble, or other armor
mulch layer approximately 5 to 10 centimeters (2 to 4 inches)
thick may be substituted for the vegetation (see Figure 48).
Again, geotextiles should be considered under rock or cobble
layers to aid in preventing soil erosion.
Topsoil Layer — The topsoil component (see Figure 47)
should be designed and constructed to support the vegetative
cover by allowing sufficient surface water to infiltrate the
topsoil and by retaining sufficient water to sustain plant growth
through drought periods. Particle-size distribution, structure,
and organic matter content influence the quantity of available
water a given soil can supply and should be considered in
selecting the topsoil material. In general, medium-textured
soils (e.g., loam) have the best overall characteristics for seed
germination and plant root system development.
151
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cobbles/soil
top layer
biotic barrier
(cobbles)
drainage layer
low-permeability
geomembrane/soil layer
gas vent layer
waste
0 0
0 0
0 0
0 0
I0//" °°°°°: °°.°°°°°y>Vi
^^^^-^rr^^^^
•>°« ° " ° °°o ° o °°° °o°°°o ° -
o'Oo o C? 0 0
o o (7 <0 Cbc
0 ^ ° °0)0(o °
0 o° Cb ° (7° o
0
0
0
T^Sr'Sc^
V^I7
°"°'»0,S%
^oG
o0^
60cm
30cm
30cm
60cm
30cm
geotextile filter
geotextile filter
20-mil (0.51 -mm) geomembrane
w/overlying protective geotextile
geotextile filter
geotextile filter
(30 cm = approx. 1 ft)
Figure 48.
USEPA-recommended landfill cover
design with optional layers.
The USEPA-recommended minimum thickness for the topsoil
component is 24 inches (60 cm). Some geographic regions and
climatic conditions may require a thicker component to provide
adequate plant-available water. A professional soil scientist
experienced in designing cover systems should be consulted for
guidance on soil material selection.
The topsoil is placed in uncompacted layers over a graded
granular or geotextile filter overlying the drainage layer. The
filter reduces penetration of soil particles into the drainage
layer and, therefore, clogging of the layer.
7.3.4.2 Drainage Layer —
The final cover system includes a drainage layer (see Figure
47), located below the protective topsoil layer, which intercepts
percolating water. When the precipitation rate is sufficient,
the percolating water migrates downward through the topsoil layer
and into the drainage layer. When it meets the hydraulic barrier
layer, it flows horizontally through the drainage medium to an
outlet located at the cover perimeter.
If the drainage layer is constructed of soil, the USEPA
recommends a minimum thickness of 12 inches (30 cm) with a
minimum hydraulic conductivity of 3.9 x 10"3 inches/sec (1 x 10"2
cm/sec) and a minimum bottom slope of 2 percent after allowance
for settlement. The layer may be constructed of granular
drainage material classified by the USCS as SP (poorly graded
sand).
152
-------
Alternatively, the drainage layer may be constructed of
synthetic materials, such as geonets or prefabricated drainage
boards. The layer must have a transmissivity equivalent to, or
greater than, the soil drainage layer described above.
Constructing the drainage layer consists of placing a 12-
inch (30-cm) layer of granular material or an equivalent
synthetic directly over the geomembrane component of the
hydraulic barrier layer. Protection against damage to the
geomembrane by equipment or personnel should be provided. Fines,
if present, should be removed from granular material before
construction to prevent clogging of the drainage layer. A
synthetic drainage layer may require a geotextile filter between
it and the overlying cover soil.
7.3.4.3 Biotic Barrier —
In some locations, although not required, a biotic barrier
(Figure 48) may be installed to reduce potential intrusion by
burrowing animals or plant roots, which may damage the hydraulic
barrier layer and increase percolation through it. Hokanson
(1979) found that a biotic barrier of 28 inches (70 cm),
consisting of cobbles, overlain by 12 inches (30 cm) of gravel
was effective. The cobbles had sufficient mass to deter
burrowing animals and the large void spaces, which lacked water
and nutrients, acted as a barrier to downward plant root
development.
Information is not yet available on an optimum thickness for
a barrier layer. Until then, a biotic barrier thickness of 24
inches (60 cm) may be considered sufficient in most
circumstances, unless evidence is available to justify a
different thickness. The biotic barrier is located between the
drainage layer and overlying topsoil.
7.3.4.4 Hydraulic Barrier Layer —
The USEPA-recommended hydraulic barrier layer of the final
cover system (Figure 47) consists of two components: a smooth-
surfaced, compacted soil component with a maximum field hydraulic
conductivity of 3.9 x 10"8 inches/sec (1 x 10"7 cm/sec) overlain
by a geomembrane. The geomembrane is placed in direct contact
with the soil, a compression seal is created by the overburden
pressure, and the two components form a composite barrier to
percolating liquid flow.
The recommended minimum thicknesses of the two components
are 24 inches (60 cm) for the compacted soil and 20 mils (0.51
mm) for the geomembrane. The actual thicknesses can be
153
-------
considerably greater, based on site characteristics, soil,
synthetic material, and expected external forces (e.g.,
settlement and overburden pressures). Constructing the compacted
soil component and installing the geomembrane are analogous to
the practices used in liner construction (see Section 4.5.1).
Similar techniques, along with appropriate CQA procedures, should
be used to construct the hydraulic barrier. Additional
recommendations on barrier design and construction are given by
USEPA (1986a; 1987b; and 1989), and information on developing a
CQA program is also given by USEPA (1986a).
The hydraulic barrier geomembrane component may be one of
several available synthetic materials. It need not be of the
same material as the impoundment liner. It is placed directly
above the compacted and smoothed soil component and at the bottom
of the drainage layer. The compacted soil acts as a buffer and
foundation for the geomembrane, and the drainage layer provides
protection from overlying materials. The drainage layer should
be inspected for materials that may damage the geomembrane. If
the drainage layer contains gravel-sized particles, a geotextile
should be used to protect the geomembrane. Appropriate CQA
procedures, as discussed in Section 3.12, should also be
maintained to ensure the integrity of the geomembrane liner
installation.
Care in placing and compacting the soil barrier component is
essential to achieving a low-permeability hydraulic barrier. The
compacted soil component serves as the hydraulic barrier to
leakage through the geomembrane. Appropriate construction proce-
dures (see Sections 4.4.1 and 4.4.2) and the CQA Plan (see
Section 3.12) must be followed to ensure that the completed two-
component hydraulic barrier achieves performance standards.
As outlined in the CQA Plan, field-testing of the completed
soil component of the barrier layer is also strongly recommended.
Because these tests are time-consuming and can cause construction
delays, constructing a test barrier section analogous to the test
fill for liner construction (see Section 4.4.1.1), using the same
soil material and equipment to be used in the actual barrier, is
recommended. Field and laboratory hydraulic conductivity can be
measured on the test section. If the results are satisfactory,
the number of field measurements required on the actual barrier
may be reduced.
7.3.4.5 Gas-Vent Layer —
The gas-vent layer (Figure 48) is recommended in facilities
where gases may be generated by decay of biodegradable organic
matter buried within the closed facility. The gases produced in
biodegradation are usually methane and carbon dioxide and,
without a venting system, may present a fire or explosion hazard.
154
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Other gases emanating from volatile materials may be malodorous
or toxic. The recommended gas vent consists of a 12-inch (30-cm)
layer of porous granular or geosynthetic material similar to that
used to construct the drainage layer.
Horizontal, perforated pipes placed laterally within the
layer collect the gas and vent it to the surface through vertical
riser pipes. The gas may be vented to the atmosphere or
collected for disposal or treatment. Adequate soil compaction
and geomembrane seals must be provided around the vertical riser
pipes. Design and guidance information for gas-vent layers has
been provided by USEPA (1979b; 1985i; and 1989a).
The gas-vent layer provides an exit for gases generated
within the facility, and should provide a buffer between the
waste and compacted soil barrier. The vent layer granular
material may be placed over the waste and brought to design
elevation (minimum thickness of 12 inches (30 cm) before placing
and compacting the soil component of the hydraulic barrier layer.
A vent layer of geosynthetic drainage material may be
substituted for a granular layer, if it can provide equivalent
performance. Performance requirements should be based on actual
measured gas generation times a factor of safety.
7.3.4.6 Hydraulic Barrier Support Layer (Optional) —
In some applications, the waste material may not provide a
suitable foundation, by itself, for the construction of the
hydraulic barrier. In these cases, a geotextile may be used to
help support construction equipment and to provide increased
structural support for the hydraulic barrier and overlying
layers. Several failure conditions require analysis for the
design of a geosynthetic support system. Design guidance may be
found in several references (Bonaparte & Christopher, 1987;
Christopher, 1990; Christopher & Holtz, 1989; Fowler, 1982;
Fowler & Loemer, 1987; Koerner, 1990; and Koerner, 1988)
7.4 POST-CLOSURE ACTIVITIES
Post-closure care and monitoring of the facility are
required if closure does not remove or decontaminate all wastes,
waste constituents, and contaminated components contained in the
surface impoundment. Regulations in 40 CFR 264.117 require the
post-closure care period to continue for 30 years after final
closure. The USEPA Regional Administrator may reduce or extend
the post-closure care period where conditions warrant.
Throughout the post-closure care period, the owner/ operator is
required (40 CFR 264.228[b]) to:
155
-------
• maintain the integrity and effectiveness of the final
cover, including repairing the cover as necessary to
correct the effects of settling, subsidence, erosion, or
other events;
• maintain the ground-water monitoring system and comply
with other requirements of 40 CFR, Subpart F;
• maintain and monitor the leak detection/collection
system, where such a system is present in a double liner;
and
• prevent runon and runoff from eroding or otherwise
damaging the final cover.
Therefore, post-closure care at a minimum includes a periodic
inspection and maintenance program, especially of the final
cover, to assure the integrity of the system and prevent
migration of hazardous constituents into the environment.
Additionally, a ground-water monitoring and leak detection
program, which is generally a continuation of monitoring
activities from the active life of the facility, is required to
detect the migration of waste constituents and/or their presence
in the ground water. Financial assurance will also be required
by the regulatory agency to ensure that funds are available for
the 30-year post-closure care activities. Financial assurance
may be provided by means of a trust fund, surety bond, or letter
of credit (Rodensky, 1985).
A written post-closure care activities plan must be
submitted to the regulatory agency prior to beginning post-
closure care. The plan must include, at a minimum (USEPA,
1984e):
• a list of the components to be inspected and frequency of
inspections;
• description of remedial action to repair damaged facility
components;
• frequency of monitoring device sampling and analysis; and
• the design of runon controls to prevent erosion of final
cover.
The plan should achieve a balance between the specific site
and design limitations and the need for post-closure care (e.g.,
sufficient monitoring and maintenance to ensure the integrity of
the closed unit). The climate, soil type, and cover design
should be considered in planning maintenance activities for the
final cover system. Further discussion on the post-closure care
plan has been provided by USEPA (1982a and 1984e).
156
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7.4.1 Monitoring
Monitoring activities during the post-closure period include
sampling and analysis of ground water from wells located
upgradient and downgradient of the closed impoundment (see
Section 5.2.3). Sampling methodology, sample-handling protocol,
frequency of collection, analysis, and analytical parameters
should be outlined in the post-closure care plan, based on
regulatory requirements and waste characteristics. USEPA has
published guidance on ground-water monitoring well installation,
sample collection, and preservation (USEPA, 1985a). Methods for
sample analysis are given by USEPA (1982b).
Additional monitoring is required for leak-detection and
collection systems or other components designed to detect
releases from the facility. Specific monitoring requirements for
these components should also be detailed in the written post-
closure plan.
7.4.2 Maintenance
Maintenance during post-closure care consists of scheduled
inspections, general site maintenance (e.g., mowing vegetation),
and remedial repairs in the event of disturbances to the cover or
other components. Inspections of the following components of the
surface impoundment will be required to ensure the continued
effective performance of the facility:
• the final cover system for signs of surface erosion,
settlement and subsidence, vegetation damage, and clog-
ging of drainage outlets;
• the leak-detection and collection system, gas vents, and
other facility components for potential performance
problems and evidence of leachate;
• ground-water monitoring wells for evidence of damage,
tampering, or subsidence;
• runon diversion structures for effectiveness; and
• the overall facility condition.
Maintenance of the vegetation is required to ensure that it
remains sufficient to protect the cover from erosion. Mowing,
watering, revegetation, fertilizer application, and herbicide or
insecticide application may be needed to promote plant growth.
Remedial activities are required if components deteriorate.
They include repairing eroded areas, damage from settlement or
subsidence, stressed vegetation, runon control structures, or
157
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other facility components damaged by wind, water, and weathering.
Armoring with cobbles or with geosynthetic materials may often be
appropriate to repair and prevent further damage.
Leak collection systems must be maintained in operating
condition. These systems require pump maintenance and continued
liquid detection and disposal.
7.4.3 Use of the Site
Post-closure site use must be limited to activities that
will not disturb the integrity of the final cover, liner, or
other components of the closed facility. Cover system design
must consider site conditions, surrounding land use, and
compatibility of the intended land use with maintenance of the
closed unit. A discussion of post-closure site use is given by
USEPA (1982c). Tables 15 and 16 summarize the compatibility of
features with various future site uses for impoundments closed
in-place and by waste removal (clean closure), respectively.
7.4.4 Delistincr
Delisting of the waste contained in the closed impoundment
is a potential option for eliminating or reducing the
requirements for post-closure care and monitoring. If the wastes
contained in the surface impoundment prior to closure were listed
wastes (as defined in 40 CFR 261) and closure operations rendered
them nonhazardous (also defined by 40 CFR 261), the waste may be
delisted or otherwise considered nonhazardous. If so, post-
closure requirements may be eliminated or reduced, as well as the
requirement to notify local land authorities. Delisting of
wastes, however, is considered on a site-by-site basis and USEPA
delisting authorities should be consulted before submitting the
delisting petition. Guidance for submitting a delisting petition
is provided by USEPA (1985h).
If hazardous waste or constituents remain in the closed unit
after closure (i.e., in-place closure), a. notice must also be
given to the local land use authority indicating the location and
dimensions of the unit. The notice must contain a survey plan
prepared by a professional land surveyor, with a note that states
the owner's obligation to restrict disturbance of the site (40
CFR 264.116). Additionally, a notice must be given in the deed
to the property stating that the land has been used to manage
hazardous waste, that land use is restricted, and that a survey
plan and record have been filed with local land authorities and
regulatory agencies (40 CFR 264.119).
158
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TABLE 15. COMPATIBILITY OF SURFACE IMPOUNDMENT FEATURES AND
VARIOUS SITE USES FOR IN-PLACE CLOSURES.*
Design Features
Bldgs
Site Uses Upon Closure
Recre-
ation Parking
Areas Areas
Agri-
culture
Open
Spaces
Subsurface Water Control
Extraction C RDA RDA
Well point system C RDA RDA
Cut-off walls C C C
.'Subsurface drainage C C C
Surface Water Control
Cover NC C C
Grading NC C" C
Surface water diversion C C NC
Levees/floodwalIs NC NC NC
Drainage/erosion control RDA RDA RDA
Air Factors
Passive gas control NC C C
Active gas control NC C C
Control of bird hazard C C C
to aircraft
Surface Area Factors
Covers C C C
Access C C C
Land Buffers NC C C
C
C
C
C
C
C
C
NC
C
C
NC
C
C
C
NC
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C = compatible, NC = not compatible, RDA = requires design alteration
* Jirom EPA (1982a) .
except ballparks
159
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TABLE 16. COMPATIBILITY OF SURFACE IMPOUNDMENT FEATURES AND
VARIOUS SITE USES AFTER HAZARDOUS WASTE REMOVAL.*
Site Uses Upon Closure
Design Features
Recre-
ation Parking Agri- Open
Bldgs Areas Areas culture Spaces
Subsurface Water Control
Wells
Subsurface drainage
Surface Water Control
Cover
Surface water diversion
Levees /floodwal Is
Air Factors
Surface Area Factors
CIR
C
RDC
RDC
CIR
C
RDC
CIR
C
C
C
CIR
C
C
CIR
C
C
NC
CIR
C
C
CIR
C
C
C
CIR
C
C
CIR
C
C
C
CIR
C
C
CIR = compatible if removed & dismantled, C = compatible, RDC =
requires consideration.
* from EPA (1982a).
160
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* U.S. G.P.O.:1991-548-187:41705
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