United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/625/R-94/008
September 1994
&EPA Seminar Publication
Design, Operation, and
Closure of Municipal Solid
Waste Landfills
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EPA/625/R-94/008
September 1994
Seminar Publication
Design, Operation, and Closure of
Municipal Solid Waste Landfills
Center for Environmental Research Information
U.S. Environmental Protection Agency
Cincinnati, OH 45268
Printed on Recycled Paper
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Notice
This document has been reviewed in accordance with the U.S. Environmental Protection Agency's
peer and administrative review policies and approved for publication. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use.
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Contents
Page
Chapter 1 Introduction 1
1.1 Background 1
1.2 Overview of RCRA Subtitle D MSWLF Criteria 2
1.2.1 Applicability 2
1.2.2 Implementation 2
1.2.3 Small Landfill Exemption 2
1.2.4 Major Provisions 2
1.3 Technical Guidance 5
Chapter 2 Landfill Siting 7
2.1 Introduction 7
2.2 Airport Restrictions 7
2.3 Floodplain Restrictions 8
2.4 Wetlands Restrictions 8
2.5 Restrictions in Fault Areas 8
2.6 Restrictions in Seismic Impact Zones 9
2.7 Restrictions in Unstable Areas 9
2.8 Closure of Existing Landfills if Siting Restrictions Cannot Be Met 11
Chapter 3 Design Criteria 13
3.1 Introduction 13
3.2 Liner Design: Point-of-Compliance Method 13
3.3 Liner Design 14
3.3.1 Design and Construction Considerations for Geomembrane Liners 14
3.3.2 Design and Construction Considerations for Compacted Soil Liners 16
3.4 Leachate Collection System 17
3.4.1 Area Collector 18
3.4.2 Collection Laterals 18
3.4.3 Sumps 19
3.4.4 Stormwater/Leachate Removal 20
3.4.5 Biological Clogging 20
Chapter 4 Landfill Operations 23
4.1 Introduction 23
4.2 Waste Identification and Restriction 23
4.2.1 Exclusion of Hazardous Wastes, PCBs, and Liquids 23
4.2.2 Segregating Hazardous Wastes 26
4.2.3 Recordkeeping and Notification 26
4.3 Daily Cover Material 26
4.3.1 Purpose of Daily Cover 26
4.3.2 Soil Covers 26
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Contents (continued)
4.3.3 Alternative Cover Materials 28
4.3.4 Temporary Waivers for Daily Covers 28
4.4 Run-on and Run-off Control. 28
4.4.1 Run-on Control 28
4.4.2 Run-off Control 29
4.4.3 Factors To Consider in Selecting Run-on/Run-off Control Methods 30
4.4.4 Leachate Storage 30
4.5 Safety 30
4.5.1 General Operations 30
4.5.2 Access Restrictions 31
4.5.3 Traffic Control 31
4.5.4 Personnel Equipment 31
4.5.5 Hazardous Waste Inspections 31
4.5.6 Gaseous Conditions 31
4.6 Landfill Gas Monitoring and Management 32
4.6.1 Gas Generation 32
4.6.2 Characteristics and Potential Hazards of Landfill Gases 32
4.6.3 Landfill Gas Migration 33
4.6.4 Landfill Gas Monitoring 34
4.6.5 Gas Collection 35
4.6.6 Gas Treatment 37
4.7 Special Wastes 38
4.7.1 Medical Wastes 38
4.7.2 Sewage Sludge and Industrial Sludge 39
4.7.3 Incinerator Ash 39
Chapter 5 Ground-Water Monitoring 41
5.1 Introduction to Subtitle D Ground-Water Monitoring Requirements 41
5.2 Overview of Ground-Water Movement 42
5.2.1 Hydraulic Head, Hydraulic Gradient, and the Water Table 42
5.2.2 The Ground-Water/Surface-Water Link 42
5.2.3 Factors Affecting Point-of-Compliance Selection 43
5.2.4 Subsurface Heterogeneity 43
5.3 Pollutants at Landfills 44
5.3.1 Overview of Types of Pollutants 44
5.3.2 Pollutant Transport 45
5.4 Selecting Monitoring Well Locations 47
5.4.1 Number of Monitoring Wells 47
5.4.2 Stratigraphic and Other Well Location Considerations 48
5.5 Installation of Monitoring Wells 49
5.5.1 Basic Components of Monitoring Wells 49
5.5.2 Drilling 50
5.5.3 Casings and Screens 51
5.5.4 Joints 52
5.5.5 Filter Packs 52
5.5.6 Grouting 52
5.5.7 Well Surface Considerations 53
5.6 Well Development and Maintenance 54
5.6.1 Techniques To Clean Wells and Control Problems 54
iv
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Contents (continued)
5.6.2 Decontamination 55
5.7 Well Abandonment 56
5.8 Documentation 56
5.9 Ground-Water and Vadose-Zone Sampling 56
5.9.1 Vadose-Zone Sampling Techniques 56
5.9.2 Saturated-Zone Sampling Techniques 57
5.10 Detection Monitoring 60
5.11 Statistical Data Analysis 60
5.12 Assessment Monitoring 61
5.12.1 When Assessment Monitoring Is Not Required 63
5.12.2 Elements of an Assessment Monitoring Program 63
Chapter 6 Release Characterization and Remediation 65
6.1 Introduction 65
6.2 Release Characterization 65
6.2.1 Site Assessment 65
6.2.2 Characterization Methods 66
6.3 Remedy Selection and Implementation 67
6.3.1 Regulatory Requirements 67
6.3.2 Remediation Alternatives 67
6.3.3 Sources for Further Information on Remediation Techniques 70
Chapter 7 Closure and Post-Closure 73
7.1 Introduction 73
7.2 Closure Design Considerations 73
7.2.1 Profile of the Cover 73
7.2.2 Infiltration (Barrier) Layer 74
7.2.3 Drainage Layer 74
7.2.4 Erosion Control Layer 74
7.2.5 Gas Collection System 75
7.2.6 Landfill Cover Slope Stability 75
7.2.7 Subsidence Effects 76
7.2.8 Weather Effects 77
7.2.9 Documentation of Closure 77
7.3 Post-Closure Care 77
7.3.1 Required Post-Closure Care 77
Chapter 8 Financial Assurance Criteria 79
8.1 Introduction 79
8.2 Financial Assurance for Closure 79
8.2.1 Estimating Final Cover Costs 79
8.2.2 Annual Updating of Closure Costs 80
8.3 Financial Assurance for Post-Closure Care 80
8.3.1 Estimating Post-Closure Care Costs 80
8.4 Financial Assurance for Corrective Action 81
8.5 Financial Assurance Mechanisms 81
8.5.1 Trust Funds 82
8.5.2 Surety Bonds 82
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Contents (continued)
8.5.3 Letter of Credit 82
8.5.4 Insurance 82
8.5.5 Corporate and Local Government Financial Tests and Guarantees 82
8.5.6 State-Approved Mechanisms 82
8.5.7 State Assumption of Responsibility 82
8.5.8 Use of Multiple Financial Assurance Mechanisms 83
Chapter 9 References 85
VI
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List of Figures
Figure Page
1-1 Amount of MSW generated in the United States in 1990 1
1-2 Composite liner and leachate collection system design in unapproved states 5
2-1 Seismic impact zones 10
3-1 Various methods available to fabricate geomembrane seams 16
3-2 Seam strength tests 17
3-3 Comparison of the hydraulic conductivity of soil with different clod sizes 18
3-4 Influence of soil moisture content and compactive energy on soil permeability 19
3-5 Moisture-density acceptance criteria for soil compaction 19
3-6 Required capacity of leachate collection pipe 20
3-7 Mounding equation used to calculate horizontal spacing of collection pipes 20
3-8 Depression of composite liner system to create sumps 21
3-9 Leachate-stormwater separation system using interior berms 21
4-1 Hazardous waste inspection decision tree 25
4-2 Daily soil cover for landfill operations 27
4-3 Impacts of daily soil cover on landfill capacity 28
4-4 Example of run-on/run-off control structures 29
4-5 Changes in landfill gas composition over time 32
4-6 Landfill conditions that result in vertical gas migration 33
4-7 Landfill conditions that result in lateral gas migration 33
4-8 Typical single screen gas monitoring probe 35
4-9 Typical multiple screen gas monitoring probe 35
4-10 Typical passive gas collection system for venting of landfill gas 36
4-11 Schematic of gas extraction well 37
4-12 Schematic of a landfill flare system with blower. 38
5-1 Example of a gaining stream 43
5-2 Example of geologic heterogeneity, with one aquifer above another 44
5-3 Movement of LNAPLs in the subsurface 47
5-4 Movement of DNAPLs in the subsurface 48
5-5 Full vertical penetration of DNAPL through an aquifer 49
5-6 Components of a typical ground-water monitoring well 50
5-7 Schematics of the three basic types of vertical well drilling methods 51
5-8 Examples of a shutter-type screen and a wire-wound screen 52
5-9 Various types of casing joints 52
5-10 Void spaces produced by improperly installed annular seals 53
5-11 Correct wedge shape for surface grouting 53
5-12 Examples of a multiport sampler and two types of nested samplers 60
5-13 Subtitle D ground-water detection and assessment monitoring 61
6-1 In situ heating device 69
vii
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List of Figures (continued)
Figure Page
6-2 Soil vapor extraction 69
6-3 Air sparging 70
6-4 Bioremediation system 71
7-1 Minimum requirement for final cover design 74
7-2 Multiaxial stress vs. strain for five geomembrane materials 74
7-3 Schematic of a sideslope drainage layer 75
7-4 Landfill gas vents passing through geomembrane covers 76
VIII
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List of Tables
Table Page
1-1 Summary of Changes to the Effective Dates of the MSWLF Criteria 3
1-2 Maximum Contaminant Levels (MCLs) Point-of-Compliance Performance-Based Criteria 4
2-1 Subtitle D Location Restrictions for MSWLFs 7
5-1 Constituents for Detection Monitoring 62
IX
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Acknowledgments
This seminar publication was developed for the U.S. Environmental Protection Agency (EPA) Center
for Environmental Research Information (CERI) in Cincinnati, OH under contract #68-C1-0040.
Daniel Murray of CERI coordinated the preparation of this publication and provided technical
direction throughout its development.
This publication is the product of the efforts of many individuals. Gratitude goes to each person
involved in the preparation and review of this document. Principal authors were Gregory Richardson,
G.N. Richardson and Associates, Raleigh, NC; Peter Thompson and Roy Koster of ABB Environ-
mental Services, Portland, ME; and David Kreamer of the University of Nevada, Las Vegas.
The following individuals provided invaluable technical assistance during the seminar series and
the development of this publication: John Bove, Hazan and Sawyer, Raleigh, NC; Dirk Brunner, ABB
Environmental Services, Portland, ME; and Allen Geswein, U.S. EPA Office of Soild Waste,
Washington, DC
The following individuals peer reviewed this publication: David Carson, U.S. EPA Office of Research
and Development, Risk Reduction Engineering Laboratory, Cincinnati, OH; Paul Cassidy, U.S. EPA
Office of Solid Waste, Washington, DC; and Susan Schock, U.S. EPA Office of Research and
Development, Center for Environmental Research Information, Cincinnati, OH.
Anne Jones and Linda Stein of Eastern Research Group, Inc. (ERG) in Lexington, MA, directed the
editing and production of the publication.
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Chapter 1
Introduction
1.1 Background
The United States is facing a major municipal solid waste
management challenge. In 1990, a total of 195.7 million
tons of municipal solid waste (MSW) was generated in our
country (U.S. EPA, 1992a), approximately 66 percent of
which was disposed in landfills (see Figure 1-1). Three
factors illustrate the MSW management problems that
must be addressed, especially as they relate to landfills:
e By the year 2000, it is estimated that this nation will
generate over 222 million tons of MSW per year (U.S.
EPA, 1992a).
* Although recycling and composting are expected to
reduce the overall percentage of MSW disposed in
landfills, it is estimated that at least 120 million tons
of MSW will continue to be disposed in landfills in
1995 (U.S. EPA, 1992a).
» Many existing MSW landfills have closed, drastically
reducing the available space for disposal of MSW.
In addition, it is now known that older municipal solid
waste landfills (MSWLFs), often referred to as "dumps,"
historically have accepted a wide array of questionable
wastes that threaten underlying ground-water resources.
Many abandoned "dumps" are now Superfund sites,
facing costly remediation.
Because of these MSW management and environ-
mental crises, the need arose to develop new MSWLFs
to satisfy the nation's disposal needs into the next cen-
tury in an environmentally safe manner. To prescribe
criteria for the design, construction, operation, and clo-
sure of reliable MSWLFs and to allow states the flexibil-
ity to define their individual landfill needs, the U.S.
Environmental Protection Agency (EPA) published final
MSW landfill regulations in the Federal Register on
October 9, 1991, under authority of Subtitle D of the
Resource Conservation and Recovery Act (RCRA) and
Section 405 of the Clean Water Act. These new regula-
tions, 40 CFR Part 258, established minimum design
and operating criteria for all solid waste landfills that:
Receive MSW, as defined in Part 258
Codispose sewage sludge with MSW
Receive nonhazardous MSW combustion ash
Are not regulated under Subtitle C of RCRA
To assist landfill owners and operators in complying with
these new requirements, EPA's Office of Research and
Development, in particular the Center for Environmental
Research Information in Cincinnati, Ohio, developed a
series of 2-day seminars. These seminars were pre-
sented in 14 different locations during the summer of
1992. The goal of the seminars was to present state-of-
the-art information on the proper design, construction,
operation, and closure of MSWLFs.
This seminar publication is a documented summary of the
technical information presented at the seminars. It is in-
tended to supply the seminar information to those individu-
als who could not attend one of the seminars and to serve
as a valuable reference to those responsible for the chal-
lenging task of designing, constructing, operating, or clos-
ing a MSWLF in compliance with federal and applicable
state requirements.
Landfill, other, 66.6%
130.4 million tons
Recovery, 17.1%
33.4 million tons
Combustion, 16.3%
31.9 million tons
(Total weight = 195.7 million tons)
Figure 1-1. Amount of MSW generated in the United States in
1990 (U.S. EPA, 1992a).
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1.2 Overview of RCRA Subtitle D MSWLF
Criteria
The Subtitle D criteria established siting, design, opera-
tion, closure, post-closure care, ground-water monitoring,
and financial assurance requirements for all municipal
solid waste landfills that have received or will receive
MSW after October 9, 1991. This section presents an
overview of these MSWLF criteria. The regulations (40
CFR Part 258) and Agency guidance should be re-
viewed for specific issues and details of the regulation.
1.2.1 Applicability
The MSWLF criteria do not apply to landfills that ceased
receiving MSW on or before October 9,1991. If a landfill
accepted MSW after October 9, 1991, but ceased ac-
cepting MSW before October 9,1993, this landfill must
comply only with the closure requirements of 40 CFR
Part 258.60(a). All MSWLFs that accepted MSW on or
after October 9, 1993, however, must comply with all
applicable criteria. These criteria apply to MSWLFs that
receive MSW, sewage sludge, or nonhazardous munici-
pal waste combustion ash. In addition, these criteria
apply to municipal nonhazardous waste combustion ash
monofills. The criteria do not apply to sewage sludge
monofills.
1.2.2 Implementation
The MSWLF criteria are self-implementing in states
that do not have EPA-approved permit programs. Self-
implementing means that MSWLF owners/operators are
required to implement the requirements of the criteria
and maintain adequate documentation to demonstrate
compliance. The citizen suit provisions of RCRA will be
relied dn primarily for enforcement in these states.
The criteria provide implementation flexibility to those
states that receive EPA approval of their permit pro-
grams. For MSWLFs in these states, the director of the
approved state program has the authority to develop
and implement alternative requirements, provided that
the new requirements meet the intent of the MSWLF
criteria. This provision enables states to consider site-
specific factors and conditions in implementing their
programs. Examples of available state flexibility are pre-
sented below in the discussion of the major provisions
of the criteria (Section 1.2.4).
1.2.3 Small Landfill Exemption1
Some facilities might qualify for an exemption from the
MSWLF design requirements. To be eligible for the ex-
emption, a landfill must receive an average of less than
20 tons of MSW per day, and no evidence of ground-
1 EPA delayed the effective date for these small MSWLFs until
October 9, 1995 (see 58 Fed. Reg. 51536, October 1, 1993).
water contamination can be present at the site. In addi-
tion, the exemption applies only if:
The facility experiences 3 consecutive months of in-
terrupted surface transportation, or
No practicable waste management alternative exists,
and the facility receives less than 25 inches of annual
precipitation.
1.2.4 Major Provisions
The new MSWLF criteria contain six major provisions,
which are discussed below. A summary of changes to the
effective date of each provision is presented in Table 1-1.
1.2.4.1 Location Restrictions
The MSWLF location restrictions involve the proximity
of landfills to:
Airports
Floodplains
Wetlands
Fault areas
Seismic impact zones
Unstable areas
For new MSWLFs and lateral expansions of existing
MSWLFs, all the restrictions listed above apply. For
existing MSWLFs that are not expanding laterally, re-
strictions regarding airports, floodplains, and unstable
areas apply. Existing MSWLFs that cannot meet these
location restrictions must close by October 9, 1996. In
states with EPA-approved programs, the director can
extend the closure deadline up to an additional 2 years
under certain circumstances.
1.2.4.2 Operating Criteria
The MSWLF criteria include the following operational
requirements:
Establishing procedures for excluding hazardous waste
Applying daily cover
Controlling disease vectors (flies, rats, etc.)
Controlling explosive gases
Restricting open burning
Controlling access to the landfill
Controlling run-on and run-off
Protecting surface waters
Restricting liquids
Maintaining operating records
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1.2.4.3 Design Criteria
Performance-based and technology-based criteria govern
the design of new MSWLFs and lateral expansions of
existing MSWLFs. In states with EPA-approved programs,
the director of the program can approve landfill designs
that meet the performance requirements of the criteria.
The performance requirements are:
The contaminant levels in Table 1-2 shall not be ex-
ceeded in the uppermost aquifer at the relevant point
of compliance, as established by the director of the
program, and
The relevant point of compliance shall not be more
than 150 meters from the unit boundary and shall be
on the property of the owner/operator. In states with-
out EPA-approved programs, however, the relevant
point of compliance must be at the unit boundary.
In states without EPA-approved programs; owners/
operators have two options for designs of new MSWLFs
and lateral expansions of existing MSWLFs:
A standard design can be used. This design requires
a composite liner consisting of an upper flexible
membrane liner (FML), commonly referred to as a
geomembrane, at least 30 mil thick (60 mil for high
density polyethylene [HOPE]) and a lower compacted
soil layer with a hydraulic conductivity of no more
than 1 x 10'7 centimeters per second, along with a
leachate collection system (Figure 1-2).
An owner/operator can request that the state petition
EPA for approval of an alternative design based on
meeting the performance requirements discussed
above.
1.2.4.4 Ground-Water Monitoring and Corrective
Action
The MSWLF criteria establish requirements for ground-
water monitoring and corrective action for all landfills.
The criteria include a systematic process that requires
routine ground-water monitoring, referred to as detec-
tion monitoring. In detection monitoring, a minimum
number of indicator parameters must be tested at least
annually. If statistically significant increases above
background concentrations of any of the indicator pa-
rameters are detected, a more comprehensive monitor-
ing program, referred to as assessment monitoring,
must be instituted. If elevated concentrations of pol-
lutant parameters continue or increase, the owner/
operator then is required to develop and implement a
corrective action program.
In states with EPA-approved programs, the director of the
program has the authority to modify the ground-water
Table 1-1. Summary of Changes to the Effective Dates of the MSWLF Criteria (as of October 1,1993) (U.S. EPA, 1993a.)
MSWLF Units Accepting
Greater Than 100 TPD
MSWLF Units Accepting
100 TPD or Less; Are Not
on the NPL; Are Located In
a State That Has Submitted
an Application for Approval
by 10/9/93, or on Indian
Lands or Indian Country.
MSWLF Units That
Meet the Small
Landfill Exemption
In 40 CFR 258.1(f)
MSWLF Units
Receiving Flood-
Related Waste
General effective
date* (This is the
effective date for
location, operation,
design, and closure/
post-closure)
Date by which to
install final cover if
cease receipt of
waste by the general
effective date
Effective date of
ground-water
monitoring and
corrective action
Effective date of
financial assurance
requirements
October 9, 1993
April 9, 1994
October 9, 1995
Octobers, 1994
Prior to receipt of waste
for new units; October 9,
1994, through October 9,
1996, for existing units
and lateral expansions
April 9, 1995
October 9, 1994
October 9, 1993, for new
units; October 9, 1994,
through October 9, 1996,
for existing units and lateral
expansions
April 9, 1995
October 9, 1996
October 9, 1995, for
new units; October 9,
1995, through October
9, 1996, for existing
units and lateral
expansions
October 9, 1995
Up to October 9, 1994,
as determined by state
Within one year of date
determined by state; no
later than October 9,
1995
October 9,1993, for
new units; October 9,
1994, through October
9, 1996, for existing
units and lateral
expansions
April 9, 1995
If a MSWLF unit receives waste after this date, the unit must comply with all of Part 258.
Note: See the final rule and preamble published on October 1, 1993 (58 Fed. Reg. 51536), for a full discussion of all changes and related
conditions. All other versions of this table, including the version in 58 Fed. Reg. 51536, are obsolete.
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Table 1-2. Maximum Contaminant Levels (MCLs)
Point-of-Compllance Performance-Based Criteria
Chemical
MCLs (mg/L)
Arsenic
Barium
Benzene
Cadmium
Carbon tetrachloride
Chromium (hexavalent)
2,4-Dichlorophenoxy acetic acid
1 ,4-Dichlorobenrene
1,2-Dichloroethane
1,1-Dichloroethylene
Endrin
Fluoride
Lindane
Lead
Mercury
Methoxychlor
Nitrate
Selenium
Silver
Toxaphene
1 ,1 ,1 -Trichloromethane
Trichloroethylene
2,4,5-Trichlorophenoxy acetic acid
Vinyl chloride
0.05
1.0
0.005
0.01
0.005
0.05
0.1
0.075
0.005
0.007
0.0002
4.0
0.004
0.05
0.002
0.1
10.0
0.01
0.05
0.005
0.2
0.005
0.01
0.002
Source: Federal Register, October 9, 1991 (40 CFR Part 258.40)
monitoring requirements, including reducing the number
of parameters that need to be monitored.
1.2.4.5 Closure and Post-Closure Care
After receipt of the final delivery of MSW, a landfill is
required to be properly closed, and the owner/operator
must provide post-closure care. The overall goals of
closure and post-closure care are to minimize the infil-
tration of water into the landfill and maintain the integrity
of the cover during the post-closure period by minimizing
cover erosion.
Closure and post-closure plans for existing MSWLFs
must be developed by the effective dates in the regula-
tions. For new MSWLFs, the closure and post-closure
plans must be prepared before the final receipt of MSW.
The closure plan must describe the steps necessary to
close all MSWLF units at any point during the active life
of the landfill. The post-closure plan must include a
description of monitoring and maintenance activities to
be conducted during the post-closure period, as well as
a description of any uses of the property during the
post-closure period.
The MSWLF criteria also establish minimum requirements
for a final landfill cover. At a minimum, the final cover shall
consist of:
An infiltration layer of at least 18 inches of earthen
material that has a permeability less than or equal to
the permeability of any bottom liner system or natural
subsoils present, or a permeability of no greater than 1
x 10'5 centimeters per second, whichever is less, and
An erosion layer of at least 6 inches of earthen ma-
terial that is capable of sustaining native plant growth.
In states with EPA-approved programs, the director of
the program may approve alternative cover designs.
Post-closure care of the landfill and final cover system
includes necessary monitoring and maintenance activi-
ties described in the post-closure plan. For MSWLFs in
states without EPA-approved programs, post-closure
care must be conducted for 30 years. In states with
EPA-approved programs, the director of the program
has the authority to decrease or increase the post-
closure period.
1.2.4.6 Financial Assurance
In general, all entities (including Native American tribes),
except for states and the federal government, are re-
quired to provide financial assurance that a MSWLF will
be properly closed and maintained. The regulations re-
quire that financial assurance be provided for:
Closure
Post-closure care
Corrective action to address known releases
Many financial assurance mechanisms are available for
use, including:
Trust funds
Surety bonds
Letters of credit
Insurance
EPA currently is developing financial tests to determine
the financial assurance capability of municipalities and
corporations. Once developed, these tests will enable
public and private entities to determine their ability to
provide financial assurance and the need to secure
other financial assurance mechanisms.
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Flexible membrane liner (FML)*
Leachate collection system*
0*0
o o
Compacted soil
(permeability
£1 x 10'7cm/s)
2ft
Leachate collection system must maintain leachate level <30 cm.
1 FML must be at least 30 mil thick; FML consisting of high density polythylene (HOPE) must
be at least 60 mil thick (mil = 1,000th of an inch).
Figure 1-2. Composite liner and leachate collection system design in unapproved states (adapted from Federal Register,
October 9, 1991d).
1.3 Technical Guidance
EPA's 1992 seminars on proper MSWLF design and
operation, on which this document is based, were held
prior to publication of EPA's technical manual on Solid
Waste Disposal Facility Criteria (U.S.EPA, 1993a). The
reader should refer to this manual for additional techni-
cal guidance on the Part 258 regulation. The manual
was developed to assist MSWLF owners and operators
in achieving compliance with the revised Part 258 criteria,
and includes information on the purpose, scope, and
applicability of the Part 258 requirements, technical con-
siderations relating to each requirement, and sources
for further information.
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Chapter 2
Landfill Siting
2.1 Introduction
The Subtitle D MSWLF siting restrictions establish mini-
mum national siting standards for landfills. Many state
regulations contain stricter landfill siting requirements,
including considerations not in Subtitle D, such as re-
strictions on development in critical watershed areas,
wellhead protection areas, sole-source aquifers, mini-
mum buffer zones, or agricultural lands. Because states
and localities currently are developing their own landfill
siting programs, both state and local regulations should
be consulted for possible additional requirements be-
yond those required by Subtitle D.
The Subtitle D siting requirements include restrictions
on siting MSWLFs near or in airports, floodplains, wet-
lands, fault areas, seismic impact zones, and unstable
areas. Some of the restrictions apply to all MSWLFs,
whereas others apply to new and laterally expanding
landfills but not to existing facilities, as shown in Table
2-1. These siting restrictions are discussed below.
2.2 Airport Restrictions
Airport safety as it relates to landfill siting is an issue that
has been addressed by Federal Aviation Administration
(FAA) policy for several years. These restrictions were
developed to protect aircraft from collisions with scav-
enger birds that are generally associated with landfill
facilities. Such collisions have caused extensive dam-
age to aircraft and can lead to aircraft crashes during
takeoffs and landings. Owners or operators of existing,
new, or laterally expanding MSWLF units located (1)
within 10,000 feet of the end of any airport runway used
by turbojet aircraft or (2) within 5,000 feet of the end of
any airport runway used only by piston-type aircraft must
demonstrate that the landfill unit does not pose a bird
hazard to aircraft. If this cannot be demonstrated, then
the facility must close by October 9,1996. The FAA must
be notified if a new or laterally expanding landfill site is
closer than 5 miles to a public airport runway.
Certain operational procedures at the landfill site might
deter birds from inhabiting the site. A number of tech-
nologies are available, with variable success rates, to
minimize food sources and discourage nesting. Waste
management techniques to reduce the supply of food to
birds include:
Frequent covering of wastes that provide a source of
food.
Shredding, milling, or baling food-containing wastes.
Eliminating wastes from the landfill that represent a
food source for birds (e.g., through alternative waste
management techniques, such as source separation,
composting, and waste minimization).
Frequent covering of wastes that represent a food source
for birds effectively reduces the availability of the food
supply. Depending on site conditions, such as volume and
types of wastes, waste delivery schedules, and size of
Table 2-1. Subtitle D Location Restrictions for MSWLFs
Restricted Locations
Applies to Existing
MSWLFs?
Applies to New and
Lateral MSWLF
Expansions?
Make Demonstration
to Director or Put
Demonstration In
Operating Record?
Must Existing Units
Close If Cannot Make
Demonstration?
Airports
Floodplains
Wetlands
Fault Areas
Seismic Impact Zones
Unstable Areas
Yes
Yes
No
No
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Operating Record
Operating Record
Director
Director
Director
Operating Record
Yes
Yes
NA
NA
NA
Yes
-------
the working face, the operator might need to apply cover
several times a day to keep the inactive portion of the
working face small relative to the area accessible to
birds. Maintaining a small working face also concen-
trates spreading and compaction equipment in a small
area, which further disrupts scavenging by birds.
Milling or shredding MSW tends to break up food waste
into smaller particle sizes and distributes the particles
throughout nonfood wastes, thereby diluting food
wastes to a level that frequently makes the mixture no
longer attractive as a food supply for birds. Similarly,
baling of MSW reduces the surface area of the waste
available to scavenging birds.
Various deterrents to bird scavenging and nesting have
been used with limited short-term success. Such deter-
rents include the use of loud sounds at random intervals
and visual deterrents, such as realistic models of preda-
tor birds. The use of physical barriers such as a canopy
of fine wires or nets strung around the working face also
have proved effective. Nets have been strung over suf-
ficient landfill acreage such that the weekly operation of
the facility is not affected by the presence of the nets.
These nets use widely spaced wires, commonly 10 to
15 feet apart, that limit the ability of birds such as
seagulls to land on the waste.
2.3 Floodplain Restrictions
Floodplains are defined as lowland and other flat areas
adjacent to inland or coastal waters that are inundated
during a 100-year flood. The Subtitle D regulations limit
the siting of MSWLFs within a floodplain. Under Subtitle
D, a landfill located in a 100-year floodplain cannot
restrict the flow of the 100-year flood, reduce the tem-
porary storage capacity of the floodplain, or result in
washout of MSW. Existing MSWLFs in 100-year flood-
plains must close by October 9, 1996, unless it can be
demonstrated that the landfill will not pose unacceptable
hazards to the floodplain.
A potential problem related to floodplain siting restric-
tions concerns stormwater run-off control. A common
method used to control offsite loss of soils from run-off
is a sedimentation basin (see Section 4.4). The flood-
plain restrictions limit acceptable locations for sedimen-
tation basins at existing landfills located in or adjacent
to floodplains. Implementing proper sedimentation con-
trol at these facilities could be difficult if run-off sedimen-
tation control devices cannot be sited. Nevertheless, the
reasons for keeping MSWLFs and support facilities out
of floodplains outweigh these drawbacks.
2.4 Wetlands Restrictions
Under 40 CFR 232.2, wetlands are defined as those
areas that are inundated or saturated by surface water
or ground water at a frequency and duration sufficient to
support, and that under normal circumstances do sup-
port, a prevalence of vegetation typically adapted for life
in saturated soil conditions. Wetlands generally include
swamps, marshes, bogs, and similar areas. Wetlands are
identified using three criten'a: (1) the presence of charac-
teristic vegetation, (2) inundation of the site by water for a
certain number of days per year, and (3) the presence of
hydric soils.
Subtitle D is consistent with EPA's objective of no net
loss of wetlands in terms of acreage and function. The
regulation prohibits new MSWLF units and lateral ex-
pansions in wetlands unless the owner/operator can
demonstrate that no practical alternative not involving
wetlands exists. Additionally, the owner/operator must
show that construction and operation of the MSWLF unit
will not violate applicable state water quality standards
(WQS) or toxic effluent standards of the Clean Water Act
and does not jeopardize an endangered species. Also,
the MSWLF design must clearly demonstrate the stabil-
ity and erosion potential of both native and fill soils used
to construct the facility.
Subtitle D includes wetlands restoration or creation as a
last option to achieve no net loss of wetlands. Successfully
creating or restoring wetlands, however, is difficult. Wet-
lands restoration or creation is more complicated than
providing a wet area; these tasks require the expertise of
people in a number of broad disciplines, such as agrono-
mists, biologists, and ecological engineers, in addition,
to the civil engineers and geologists who usually are
involved in landfill design. Wetlands creation programs
at MSWLF sites are generally onsite programs imple-
mented during construction of a landfill cell intruding on
wetlands. Federal, state, and local governments are
considering stricter enforcement and interpretation re-
garding wetlands creation.
2.5 Restrictions in Fault Areas
A fault is a fracture or zone of fractures in geologic
material along which strata on one side have been
displaced with respect to strata on the other side. Sub-
title D requires that no new MSWLF or lateral expansion
of a MSWLF be sited within 200 feet (60 meters) of a
fault area that has experienced displacement within the
Holocene Epoch (the last 10,000 years). If differential
movement between the two sides of a fault bridged by
a landfill were to occur, the landfill's liner system may
not be able to resist the movement and could fail. A
sophisticated geologic study is needed to evaluate site
conditions to determine if a proposed new or expanded
facility is located on or near an active fault. A geologist
can determine that a fault has not moved in Holocene
time by examining surficial deposits for displacements.
Potentially active faults can be located based on re-
cords of seismic epicenters and by examining high-al-
titude, high-resolution aerial photographs from the U.S.
-------
Geological Survey (e.g., USGS Preliminary Young Fault
Map MF916). In states with an approved program, an
alternative setback distance of less than 200 feet might
be allowed if the owner/operator can demonstrate that
at this distance ground movement will not damage the
structural integrity of the facility and the setback dis-
tance will be protective of human health and the envi-
ronment.
2.6 Restrictions in Seismic Impact Zones
Seismic impact zones are defined as regions having a
10-percent or greater probability that maximum horizon-
tal acceleration at the site caused by an earthquake will
exceed 0.1 g in 250 years (Earth's gravitational force is
1 g). This ground movement applies to movement of
lithified rock material, not soils or manmade materials
such as concrete. The concern is with the potential
impact of earthquake-induced lateral accelerations on
the stability of the landfill and subgrade soils. Under
Subtitle D, new MSWLFs and lateral expansions of
landfill units cannot be located in seismic impact zones
unless the owner/operator can demonstrate to the direc-
tor of an approved state program that all containment
structures (e.g., liners, leachate collection system) are
designed to resist the maximum horizontal acceleration
and that the site will remain stable.
Seismic impact zones in the continental United States
are shown in Figure 2-1, which is based on ongoing
work by the U.S. Geological Service (Algermissen et a!.,
1982, 1990). This map is based on probabilistic studies
of earthquake recurrence periods and earthquake mag-
nitude. In the western United States, earthquakes of
large magnitude are frequent and can be associated
with specific active faults. Such earthquake events, al-
though large in magnitude, tend to affect a relatively
small geographic area. Thus the probability of seismic
ground movements occurring in a given location in the
West is closely tied to the area's proximity to active
faults. Conversely, very few earthquakes of large mag-
nitude occur in the eastern United States, but those that
do occur affect a large geographic area. Thus the seis-
mic impact zones in the eastern United States are not,
for the most part, defined by the proximity of the site to
active faults. The exact source or mechanism for earth-
quakes in much of the eastern United States is not
understood at present.
Within seismic impact zones, the design of a MSWLF
must consider the stability of the landfill, its support
structures, and the underlying soils. The evaluation of
landfill stability should focus on the effect of earthquake-
induced horizontal accelerations on the slope stability of
the landfill during operation and the post-closure period.
Such evaluations are particularly important given the
low friction angle of the surface of geomembrane liners
used in MSWLF lining systems. The evaluation of the
site's subgrade stability should identify zones of satu-
rated, loose sands that could possibly liquefy during an
earthquake. This liquefaction is caused by the genera-
tion of shear-induced excess pore water pressures
within the sands, which produce a "quick" condition in
the sand. This quick condition can lead to a loss of
bearing capacity in the subgrade soils and subsequent
failure of the MSWLF liner system.
The EPA's Risk Reduction Engineering Laboratory, lo-
cated in Cincinnati, Ohio, is currently developing a tech-
nical guidance document on designing municipal solid
waste landfills in areas affected by seismic activity. The
document, RCRA Subtitle D Seismic Design Guidance
for Municipal Solid Waste Landfill Facilities, should be
available in early 1995.
2.7 Restrictions in Unstable Areas
The final Subtitle D siting restriction for MSWLFs per-
tains to unstable site subgrades. Unstable MSWLF sites
have subgrades susceptible to natural or human-induced
events or forces that could produce settlement or dis-
placement capable of impairing the integrity of the land-
fill. These areas might include poor foundation conditions
(e.g., highly compressible soil layers), sites susceptible
to mass movements (e.g., landslides), and karst terrain
that may have hidden sink holes. Under Subtitle D, if a
MSWLF is located in an unstable area, the owner/operator
of the landfill must demonstrate that engineering meas-
ures have been incorporated into the unifs design to
ensure the integrity of the landfill's structural components.
One example of siting in an unstable area is siting a
MSWLF over a thick, extensive clay layer. A landfill site
with a 60-foot natural clay layer, and therefore a low
permeability site, ordinarily would be considered a good
landfill site because leachate impacts would be mini-
mized. But the design of an MSWLF sited on compress-
ible clays also must include an evaluation of the impact
of long-term settlement on the integrity of the liner and
leachate collection system to ensure that the strains in
the liner system remain acceptable and that flow direc-
tions in the leachate collection system are not reversed.
Additionally, as MSW is placed in the landfill, the weight
acting on the clay increases and water is squeezed from
the compressible clay. This process reduces the volume
of the clay and produces settlement of the landfill. The
amount of settlement in a given area increases as the
weight of the waste increases and as the clay beneath
the area compresses. A highly compressible clay be-
neath the area therefore will increase settlement. Clay
properties are time and moisture dependent and influ-
enced by freezing. Initially, the clay might not be strong
enough to support a large amount of waste, but over
time, as the weight of the waste squeezes the water out
of the clay beneath it, the clay may become stronger and
might be capable of supporting the waste. Thus, the rate
-------
The shaded areas generally indicate
MSWLF owner/operators must comply with th
demonstration requirements in 40 CFR 258.14.
Principal Islands °
of Hawaii
Scale 1:7,600,00
3
o
,<£>
Scale 1:17,000,000
Albera Equal Area Projection
Scale 1:7,600,00
Figure 2-1. Seismic impact zones (U.S. EPA, 1993a).
-------
of waste placement at such a site might be an important
consideration.
A stability analysis should be undertaken when a landfill
is sited in an unstable area to demonstrate that the
subgrade can support additional MSW loads. MSWLF
designers typically use a unit waste weight of 1,400
pounds per cubic yard when performing this stability
analysis. But the contents of many communities' waste
stream have changed since this rule of thumb was
developed (e.g., many elements, such as "white" met-
als, and tires now are recycled rather than disposed).
Unit weights of MSW as high as 2,500 pounds per cubic
yard have been measured, which would significantly
affect the results of a stability analysis. Actual MSW
density measurements in the field might be required to
confirm the stability analysis at a particular site.
A different type of stability concern arises when consid-
ering karst terrain, which can include sink holes, caves,
and large springs that result from the dissolution of
limestone or other soluble rock. Two significant
problems exist with siting an MSWLF in karst terrain:
(1) hidden sink holes can collapse and significantly
damage the waste containment system; and (2) detec-
tion of leakage from the MSWLF unit is difficult, because
leachate can move rapidly through hidden conduits
within the limestone beneath the site. Subtitle D does
not preclude siting MSWLFs in karst areas, but does
require the designer to evaluate karst conditions and
potential impacts.
2.8 Closure of Existing Landfills if Siting
Restrictions Cannot Be Met
Existing landfills that do not meet the airport, floodplain,
or unstable area siting restrictions must close by Octo-
ber 9,1996, and conduct required post-closure activities
unless the owner/operator can demonstrate that no al-
ternative exists for disposal or that the landfill presents
no immediate threat to human health or the environ-
ment. In such cases, the deadline for closure might be
extended for 2 years by the director of an approved state
program.
11
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Chapter 3
Design Criteria
3.1 Introduction
This section discusses the specific MSWLF design cri-
teria contained in Subtitle D and presents guidelines for
meeting the regulatory requirements. These require-
ments are applicable to both new and lateral expansions
of MSWLFs, although some small landfills that receive
less than 20 tons per day (TPD) of waste on average
are exempted. Chapter 1 discusses small landfill ex-
emptions in more detail.
Two specific design criteria are presented in Subtitle D:
(1) in approved states, the liner design must ensure that
the allowable values in Table 1-2 will not be exceeded
in the uppermost aquifer at the relevant point of compli-
ance; or (2) a composite liner system must be used with
a leachate collection system that is designed and con-
structed to maintain less than a 30-centimeter depth of
leachate over the liner. Sections 3.2 and 3.3 below
describe the design criteria that must be met, as well as
the practical and technical considerations that affect
landfill design for these criteria; Section 3.4 discusses the
design issues related to leachate collection systems.
The owner/operator first determines whether the MSWLF
is in an approved state. In an approved state, the
owner/operator would follow the design standards of the
state. In an unapproved state, the owner/operator would
choose the composite liner or use the petition process
provided for in Subtitle D to seek approval of an alternative
liner design.
3.2 Liner Design: Point-of-Compliance
Method
Subtitle D is a major departure from previous hazard-
ous waste regulations that have addressed waste con-
tainment systems. Hazardous waste regulations and
related minimum-technology guidance provide very
specific requirements for liner components (e.g., com-
pacted clay liners and geomembranes). Subtitle D al-
lows for consideration of performance standards when
designing a MSWLF. In this manner, the designer is
able to consider site-specific factors in the design. Such
factors include:
The hydrogeologic characteristics of the facility and
surrounding land.
The volume and physical and chemical characteristics
of the leachate.
The quantity, quality, and direction of ground-water
flow.
The proximity and withdrawal rate of ground-water
users.
The availability of alternative drinking water supplies.
The existing quality of the ground water.
Under Subtitle D, the relevant point of compliance of a
designed MSWLF can be as far as 150 meters from the
waste management unit boundary in a state with an
approved landfill management program, as long as the
area within the 150-meter buffer zone belongs to the
facility. In states without an approved program, the point
of compliance must be at the waste management unit
boundary. Ground water beyond the point of compliance
must comply with either the MCLs for pollutants listed in
Table 1-2 or existing background levels of pollutants,
whichever is greater.
Practical application of the point-of-compliance criteria
to the design of a MSWLF liner system depends on the
designer's ability to model accurately the rate of pollut-
ant movement through the liner system and site strati-
graphy. Contaminant transport at the landfill site (e.g.,
advection, diffusion, soil adsorptionsee Chapter 5)
must be studied carefully to determine the direction,
speed, and concentration of contaminant flow. Because
contaminant transport in ground water can be very com-
plicated, the accurate prediction of contaminant move-
ment will increase significantly the cost of design and
site characterization.
EPA's Environmental Research Laboratory in Athens,
Georgia, is currently developing a computer software
model, MULTIMED (U.S. EPA, 1990a), for use in evalu-
ating liner designs based on point of compliance. This
software model requires contaminant-specific transport
factors that have been applied to only a few of the
13
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contaminants listed in Table 1-2. The point-of-compli-
ance evaluation proposed by EPA uses the Hydrologic
Evaluation of Landfill Performance (HELP) (U.S. EPA,
1984) computer model to estimate the rate of liquid loss
through the bottom of the landfill. The MULTIMED com-
puter model then predicts the rate at which the contami-
nant moves through the partially saturated soil beneath
the liner system.
3.3 Liner Design
If a state does not have an EPA-approved MSWLF pro-
gram, or if the designer cannot justify a point-of-compli-
ance design, newly designed or laterally expanding
municipal landfills must use the composite liner alterna-
tive. This liner system consists of an upper geomem-
brane liner and a lower compacted soil liner. The
geomembrane must be at least 30 millimeters thick,
except for HOPE geomembranes, which must be at least
60 millimeters thick. The compacted soil liner must be at
least 2 feet thick and have a hydraulic conductivity of less
than 1 x 10~7 centimeters per second.
The geomembrane liner (GML) minimizes the exposure
of the compacted soil liner to leachate, thus significantly
reducing the volume of leachate reaching the soil liner.
Reducing membrane penetration is vital to controlling
the escape of leachate into ground water. One way
to reduce membrane penetration is to institute a com-
prehensive construction quality assurance program, which
is discussed later in Section 3.3.1.5. The leakage rate
through a hole in the geomembrane of a composite liner
can be calculated based on the following empirical func-
tion:
Q = 3aa75ha75Kd°-5
where:
Q = Leachate leakage through a hole in the GML
of the composite liner (m3/s)
a = Area of hole (m2)
h = Hydraulic head of liquid applied to the
membrane (m)
Kd = Hydraulic conductivity of the compacted clay
(m/s)
The hydraulic conductivity of the compacted soil liner is
an important factor in leakage control. For example, a
1-square-centimeter hole in a geomembrane with 12
inches of liquid head can have a leakage rate as high
as 3,300 gallons per day. The presence of a compacted
soil liner with a conductivity of 10"7 centimeters per second
underneath the geomembrane can reduce this rate to 0.2
gallons per day. Even if the conductivity changes to 10"6
centimeters per second, the leakage rate still would be less
than 4 gallons per day, providing a dramatic improvement
over the use of a geomembrane alone.
3.3.1 Design and Construction
Considerations for Geomembrane
Liners
Many factors must be considered for a successful
geomembrane design and installation, including:
Selection of proper membrane materials.
Proper subgrade preparation.
Membrane transportation, storage, and placement.
Proper installation conditions (weather, temperature,
etc.).
Seaming and tests.
Application of construction quality assurance (CQA).
The following sections explain in detail the requirements
and considerations associated with these factors.
3.3.1.1 Membrane Materials and Properties
A GML must provide excellent chemical resistance and
reliable seams at a competitive cost. HOPE liners pro-
vide significant chemical resistance at a moderate cost.
HOPE, however, is difficult to seam and requires a rig-
orous CQA program to ensure seam integrity. Alternative
geomembrane polymers include polyvinyl chloride
(PVC) and polypropylene. These polymers have excel-
lent biaxial stress-strain properties, but do not have
substantial chemical resistance. Therefore, they are
useful as cover membranes because their biaxial
strength allows them to withstand significant waste sub-
sidence, while their lack of direct contact with leachate
reduces the need for a liner with substantial chemical
resistance. Available data on MSWLF leachate quality
indicate that leachate typically has a pH range between
5.5 to 7.0 and low concentrations of organic compounds.
Based on these data, the chemical resistance of all the
polymers mentioned above is excellent.
3.3.1.2 Subgrade Preparation
The surface of the compacted soil liner must be smooth
and strong enough to provide continuous support for the
geomembrane. The surface of the soil must be relatively
free of rocks, roots, and excess water. EPA studies (U.S.
EPA, 1988) show that stones at the surface that are
smaller than 3/4 inches and are not angular will not
penetrate most geomembranes.
3.3.1.3 Geomembrane Transportation, Storage,
and Placement
Rolls or pallets of geomembranes are shipped to the
job site by truck. Geomembranes such as PVC are
commonly prefabricated into large panels, folded, and
shipped secured to a pallet. Geomembranes such as
14
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HOPE and polypropylene must not be folded and are
shipped to the job site as rolls.
Once at the job site, the geomembranes should be
stored such that direct contact with the ground is avoided.
This requirement could be met by placing a protective
surface (e.g., a geotextile) over the ground or having the
geomembrane rolls wrapped in plastic at the factory.
The stored geomembrane also should be protected from
excessive exposure to dust, water, and heat.
To limit scratching of the underside of the geomembrane,
rolls of geomembranes should be handled by placing a
steel lifting tube through the center of the roll and lifting
the membrane with a beam that prevents cables from
touching the roll. The geomembrane then should be un-
rolled into its final position with a minimum amount of
dragging and shifting.
3.3.1.4 Geomembrane Seaming and Testing
Most geomembrane liners are seamed thermally. Ther-
mal seaming requires both proper weather conditions
and a clean surface on both membrane surfaces. If the
surface of a membrane is wet, water can vaporize and
form bubbles in the seam, which significantly reduces
the strength of the seam and might lead to leakage.
Ambient temperature also is an important factor that
should be considered during installation. Thermal seam-
ing should be performed when the ambient temperature
is between 40°F and 104°F. The most common reason
for poor geomembrane seaming is the presence of dust.
Therefore, dust control during the seaming process is
critical.
Geomembrane seaming is important in maintaining
membrane integrity, and a seam testing program should
be established for quality control. Seam testing methods
can be categorized into two groups: nondestructive test-
ing, which usually is performed in the field, and destruc-
tive testing, which can be conducted either in the field or
a laboratory. For different seaming styles, different testing
methods can be applied. Many handbooks and manuals
describing these testing methods are available (U.S.
EPA, 1989b, 1991b). Figure 3-1 shows different seam-
ing configurations. The most common thermal seams
currently used are the extrusion and double-wedge
seams. These seams are described in greater detail below.
Double hot-air wedge seams (two parallel seams with
an air channel in between) can be nondestructively
tested by applying air pressure (normally 30 pounds per
square inch) to the channel. If the channel can hold the
pressure for five minutes, the seam is acceptable.
Sometimes when the test is conducted, the pressure
fluctuates in response to ambient temperature variations
(such as when weather conditions change from sunny
to overcast). As long as the pressure fluctuation does
not exceed 3 to 4 pounds per square inch, the seam
should be acceptable. For a long seam (i.e., over 100
feet), additional gages might need to be installed to
measure each section of the seam.
Extrusion seaming requires careful quality control to
prevent long-term problems. During the seaming, the
surface of each membrane must be abraded at the
seaming area. The two sheets are then welded together
to form a proper seam. Overgrinding of the surface must
be avoided. In general, the grinding depth should be
controlled to less than 10 millimeters. If more than 1/4
inch of the abraded surface is visible after seaming, the
finished seam must be rejected. This type of seam can
be nondestructively tested by applying soapy water to
the surface of the seam and then putting a vacuum box
over the seam. A vacuum is applied inside the box; if
bubbles appear at the seam area, the seam must be
redone.
Destructive testing for all seam types includes the shear
test and the peel test, as shown in Figure 3-2. The shear
test demonstrates that the seam develops the full tensile
strength of the parent membrane. A sample is cut across
a seam and placed on an extension machine for testing.
The shear test does not judge the quality of the seam
but instead measures the product of the seam strength
and seam area. A poor-quality seam with a large weld
area might develop the strength required and pass the
test. A peel test, in which the force is focused on the
leading edge of the seam, can truly evaluate the quality
of a seam. Statistically, a minimum of one sample for
every 500 feet of the liner seam must be taken.
3.3.1.5 Construction Quality Assurance (CQA)
To minimize holes in a liner (caused by product defects,
transportation, installation, seaming, etc.) and to meet
the required standards, a CQA program should be es-
tablished for the liner installation (U.S. EPA, 1986,
1992b). The program is a planned system of activities
performed by landfill owners or their representatives
(CQA inspectors) to ensure that the facilities are con-
structed as specified in the design. The program should
be developed during the landfill design stage, and the
state should review a facility's CQA program before a
permit is issued for construction. CQA is distinct from
Construction Quality Control (CQC), which is performed
by the installer to ensure the quality of the work.
Several elements in the CQA program are important to
its overall success, including:
Responsibility and Authority: CQA personnel are
given responsibility and authority by the landfill owner
to represent his or her interests to ensure that the
liner meets design specifications.
Personnel Qualifications: The CQA inspector must
have extensive experience and knowledge about the
work performed in the field. A program administered
15
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Fillet-type
Flat-type
(a) Extrusion seams
Dual hot wedge
(single track is also possible)
(b) Fusion seams
Single hot air
(dual track is also possible)
Chemical
Bodied chemical
(c) Chemical seams
Chemical adhesive Contact adhesive
(d) Adhesive seams
Figure 3-1. Various methods available to fabricate geomembrane seams (U.S. EPA, 1989b).
by the National Institute for Certifying Engineering
Technicians (NICET) gives formal exams and pro-
vides certification of CQA inspectors for membrane
installation. Many large landfill owners/operators now
require CQA personnel to have certification.
Inspection Activities: The CQA program must clearly
define the testing program and acceptance criteria
for all significant components of the MSWLF. For the
liner system, the CQA program should specify the
frequency of testing to be performed on the com-
pacted soil and geomembrane liner, outline the sam-
pling strategy, and define the specific tests to be
performed.
Sampling Strategies: CQA testing is performed using
a combination of statistical and judgmental sampling
strategies. Typical statistical sampling strategies in-
clude defined interval testing, such as one destructive
seam test per 5,000 feet of seam, or one moisture/
density test per 5,000 cubic yards of soil liner. Judg-
mental testing allows the CQA inspector to call for
testing when the quality of workmanship is suspect.
Documentation: Most states now require documenta-
tion that a CQA program was performed before a
permit to operate the MSWLF is issued. AH CQA
activities must be clearly documented so that a third
party can understand and verify the testing and in-
spection program.
An average CQA program for a landfill with a single
composite liner costs approximately $4,500 to $6,000
(1994 dollars) per acre. These costs can vary widely
depending on site conditions.
3.3.2 Design and Construction Considerations
for Compacted Soil Liners
Clay is a difficult engineering material with which to work
because of its highly moisture-dependent physical prop-
erties. As a basic landfill liner, clay must meet certain
16
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Shear test
r
J
Peel test
Figure 3-2. Seam strength tests (U.S. EPA, 1989b).
criteria to protect ground water from leachate contami-
nation. The clay soil liner must be constructed to
provide a minimum 2-foot layer of compacted clay with
a hydraulic conductivity of less than 10~7 centimeters
per second. To meet this requirement, the following
steps should be taken during construction of a com-
pacted clay liner:
Destroy soil clods
Eliminate lift interfaces
Conduct proper compaction
Meet moisture-density criteria
Avoid desiccation
Each of these requirements is discussed below.
3.3.2.1 Soil Clod Destruction
Soil clod size has significant impact on the permeability
of compacted clay. For example, if two clay samples,
one with 3/4-inch soil clods and the second with 1/5-inch
soil clods, are compacted with equal force and have
equal water content, the first sample might have a
hydraulic conductivity of 10'4 centimeters per second
and the second sample might have a hydraulic conduc-
tivity of 10~7 centimeters per second (see Figure 3-3).
Clod size in raw clay liner material can be controlled by
passing the clay through a sieve of the desired size.
Another way to destroy soil clods is to increase soil mois-
ture. Clods will soften and break apart at a high moisture
content.
3.3.2.2 Lift Interface Prevention and Proper
Compaction
Clay liners are constructed by compacting the clay in
horizontal layers commonly called lifts. During construction
of a clay liner, if a new lift is applied directly to the unscari-
fied surface of a previous lift, a zone of high permeability
and low strength forms at the interface. Moisture moving
through the liner could then spread quickly across this
interface to a lower lift. To avoid formation of interface flow
paths, the surface of the previous lift must be scarified
before another compacted clay lift is added. A fully pene-
trating sheepsfoot roller can be used to compact the new
lift intimately to the previous lift. "Fully penetrating" means
that the height of the feet on the compaction wheels is
greater than the thickness of the loose soil placed to form
the new lift.
3.3.2.3 Moisture-Density Requirements
Clay becomes less permeable when it is compacted at a
high moisture density. Its shear strength, however, de-
creases under high moisture-density conditions and might
become so low that the clay cannot support the compac-
tion device. Compaction criteria for landfill liners differ from
compaction criteria for other purposes (e.g., building foun-
dations). Traditionally, compaction criteria are based on
strength for load-carrying capacity. For MSWLF liners,
however, compaction criteria are designed to produce low
permeability. Clay liners must be installed when the mois-
ture content of soils is 2- to 6-percent wetter than soils used
for other construction purposes. Figures 3-4 and 3-5 illus-
trate this concept.
3.3.2.4 Desiccation Prevention
Desiccation of the clay liner is difficult to avoid. A dry
environment or freezing of the liner can cause the liner to
lose moisture. The potential for soil-liner desiccation in-
creases if a capillary break exists beneath the liner. A
capillary break can be formed by a natural sand layer or
a leachate detection layer. The capillary break prevents
the day liner from drawing moisture up from deeper soil
layers to replenish moisture lost through surface evapora-
tion. Therefore, caution should be taken to avoid desicca-
tion when a sand layer, is installed for leak detection under
compacted clays. When a compacted clay liner freezes,
permeability decreases after the liner thaws. Each freezing
cycle reduces the permeability of the clay liner up to an
order of magnitude. A layer of soil can be used to protect
clay liners from desiccation and freezing. The soil layer
can be removed for subsequent work.
3.4 Leachate Collection System
A leachate collection system is designed and con-
str jcted to collect leachate and convey the leachate out
of the landfill for treatment. This system must ensure
17
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135
125
.
0)
5 10 15 20 25
Molding water content (%)
105
95
85
0.2-in. clods
0.75-in. clods
Shitt
I
I
10 15 20 25
Molding water content (%)
Figure 3-3. Comparison of the hydraulic conductivity of soil with different clod sizes (U.S. EPA, 1989b).
that less than 30 centimeters of leachate (the amount of
leachate the liner must be designed to maintain, accord-
ing to Subtitle D) accumulates over the composite liner
to minimize possible contamination of ground water.
When designing and constructing a leachate collection
system, the following components must be considered:
Area collectorthe drain that covers the liner and
collects leachate.
Collection lateralsthe pipe network that drains the
area collector.
Sump designthe low point where the leachate exits
the MSWLF.
Stormwater/leachate separation systema system
for minimizing leachate generation.
These components must be designed to handle larger
leachate flows associated with initial operations and to
resist problems such as biological clogging that can
destroy the long-term flow capacity of the system.
3.4.1 Area Collector
The area collector, also called the blanket drain, covers
the surface of the membrane liner and collects leachate.
The area collector system is commonly built with at least
a 12-inch layer of sand having a hydraulic conductivity
greater than 10"2 centimeters per second. An alternative
type of blanket drain can be constructed using a geonet.
This alternative synthetic system has a high transmis-
sivity (the product of layer thickness and permeability)
and reduces the required thickness of a collection sys-
tem, allowing more space for waste storage. Geonet
systems are especially suitable for use on a side slope
because they eliminate the need to operate heavy
equipment directly on the liner system (such as during
the construction of a sand collection system). Many
types of geonet material are commercially available.
One type, foam net, however, is not recommended be-
cause it can be compressed by the solid waste load and
lose its ability to collect leachate.
One of the disadvantages of geonets is that they have
limited hydraulic storage capacity and, therefore, no
buffer capacity for stormwater flow into the system. A
sand collection system is thus more suitable for a col-
lection system located at the bottom (rather than the
side slope) of the landfill. A sand collection system not
only supplies stormwater buffer capacity but also forms
an operational liner cover that prevents construction or
operation equipment from directly contacting the com-
posite liner underneath.
3.4.2 Collection Laterals
In general, the regulatory limit of a 30-centimeter maxi-
mum liquid head over the liner cannot be achieved using
an area collector alonecollection laterals are needed.
Collection laterals are perforated pipes that direct
leachate to sumps so that the leachate can be removed
from the landfill. During landfill operation, leachate
passes through the area collector, into collection later-
als, and drains to a sump where it is removed from the
MSWLF.
Spacing of the collection lateral pipes depends on the
permeability of the collector, the slope of the liner, and
the assumed impingement rate of leachate (see Fig-
ure 3-6). The lower the permeability, the closer the
18
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10'5
10'6
10"
10"
Optimum w -
Static
compaction
Kneading compaction
15 19 23
Molding w (%)
27
10'
10
5 _
16 20
Molding water content (%)
24
Figure 3-4. Influence of soil moisture content and compactive energy on soil permeability (U.S. EPA, 1989b).
0.95
Zero air voids
Acceptable
range
Acceptable
range
w,
Molding water content
opt
w
Figure 3-5. Moisture-density acceptance criteria for soil compaction (left: conventional criteria; right: permeability criteria) (U.S.
EPA, 1989b).
space should be between pipes. The slope of collection
laterals should be greater than 2 percent to achieve
adequate flow velocity to help clean the pipes and en-
sure that settlement of the foundation caused by the
weight of the waste will not reverse the slope of the pipe.
The horizontal spacing of the collection pipes is calcu-
lated using the mounding equation shown in Figure
3-7. Design impingements (the number of inches of
rainfall per minute) should be based on realistic opera-
tional conditions; a 24-hour, 25-year storm probably will
generate excessive head (greater than the 30 centime-
ters allowed by Subtitle D) on the liner during the actual
storm event.
3.4.3 Sumps
As mentioned earlier, sumps are low points in the
liner constructed to collect leachate. Commonly, the
19
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E
8.120
(0
.£
1
MOO
80
60
40
a
I
20
6
§" 12345
CC
Percolation, in inches per month
Where b = width of area contributing to leachate collection pipe
Figure 3-6. Required capacity of leachate collection pipe (U.S.
EPA, 1989b).
composite liner system is depressed in areas to create
these sumps (see Figure 3-8). It is difficult to test the
seaming in such sumps because of the slopes and
corners on which the seams occur. Because of the
difficulty in seam testing sumps, sump areas often are
designed with an additional layer of geomembrane. Al-
ternatively, many sumps now are being constructed us-
ing premanufactured units made of HOPE, with
large-diameter HOPE pipe or HOPE manholes. Al-
though more costly, the premanufactured sumps can be
thoroughly field-tested.
INFLOW
i ;
DRAINAGE LAYER
tan2a
tan a
where:
c = q/k
k = permeability
q = inflow rate
Figure 3-7. Mounding equation used to calculate horizontal
spacing of collection pipes (U.S. EPA, 1989b).
to the leachate collection system is removed. In this
manner, the volume of leachate requiring treatment over
the life of the cell can be reduced dramatically. This
procedure provides a significant cost savings to the
operator because leachate treatment costs average ap-
proximately $0.15 per gallon. See Section 4.4 for a
further discussion of stormwater collection.
3.4.4 Stormwater/Leachate Removal
During the design of the leachate collection system, the
effect of stormwater on the landfill must be considered.
Stormwater increases the amount of liquids requiring
removal from the landfill and is the largest potential
source of liquid reaching the sump. The stormwater
removal problem is managed by using an impingement
rate (see Section 3.4.2) based on the storm event for
which the landfill was designed. The volume of storm-
water treated as leachate, however, can be reduced
significantly by including a stormwater/leachate separa-
tion system in the MSWLF design. Such systems divide
the MSWLF into subcells using interior berms or using
the slope of the liner as an interim stormwater system
(see Figure 3-9). Rain falling into a subcell that contains
no MSW is removed from the cell as stormwater. Before
MSW is placed in a subcell, the stormwater removal
system is disconnected from that subcell, and the barrier
3.4.5 Biological Clogging
Biological growth on sand drains and geonets can cause
clogging of the leachate collection system (U.S. EPA,
1991 a). This clogging directly affects the liner's ability to
maintain a hydraulic head of less than 30 centimeters.
The biological growth occurs because of the high bio-
logical oxygen demand (BOD) level of the leachate
being removed. This growth does not attack the liner or
drainage system, but does clog the drainage elements.
Current EPA-sponsored research shows that sand
drains and geotextiles are particularly prone to clogging.
The potential for clogging of the leachate collection
system can be reduced using coarse stone around the
collection pipe and providing cleanouts for the primary
leachate collection pipes. Based on EPA research and
similar findings in Germany and Italy, wrapping geotextiles
around gravel drains surrounding the collector pipes is not
recommended.
20
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Figure 3-8. Depression of composite liner system to create sumps (provided by Greg Richardson).
Stormwater
Figure 3-9. Leachate-stormwater separation system using
Interior berms (U.S. EPA, 1992c).
21
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Chapter 4
Landfill Operations
4.1 Introduction
The Subtitle D operational requirements for landfills are
designed to ensure the safety of people on the landfill
sitefacility operators, haulers, and the publicand to
protect the environment. Subtitle D regulations also re-
quire that records be kept of the operation and that these
records be available to regulatory personnel. Further,
Subtitle D regulations require landfill owners or opera-
tors to Implement measures to:
Exclude hazardous waste and PCBs
Provide daily cover
Control onsite disease vectors
Provide routine methane monitoring
Eliminate most open burning
Control public access
Institute run-on/run-off controls
Control discharges to surface waters
Eliminate disposal of most liquid wastes
Keep records that demonstrate compliance
To meet the requirements of liners and leachate collec-
tion systems, landfill design and construction have be-
come increasingly complex. Because of this complexity
in landfill design, facility integrity easily can be jeopard-
ized by careless or inappropriate operations by an
unknowledgeable operator. Therefore, facility operators
should be fully aware of landfill operational requirements
and the reasons for these requirements, particularly
because the reason for some of the procedures might
not be readily apparent. Communication about design
and operation must be maintained in two directions.
Design concepts must be communicated clearly and
understood by operators to ensure the facility is oper-
ated as designed. At the same time, landfill designers
and regulators should obtain feedback from the facility
operators, who must implement the day-to-day opera-
tional requirements. A complex, sophisticated design
that cannot be operated in the field will not achieve its
intended purpose.
This section discusses the Subtitle D operational re-
quirements. It also includes information on procedures
that are not specifically required by the regulation but
might be helpful in efficiently operating the landfill in a
safe and environmentally sound manner. Some of these
suggested procedures might reduce liability and help
prevent problems requiring costly remediation. Because
state regulations vary and might be more restrictive than
Subtitle D, which sets only minimum standards, any
operating plan developed must be coordinated with the
appropriate state agencies.
More specifically, this section discusses waste identifi-
cation and restriction, including inspections, source con-
trol, and segregation of hazardous wastes; daily cover
materials, such as soils, geotextiles, and other materi-
als, as well as cover costs; run-on/run-off controls; op-
erational safety concerns; landfill gases, including gas
accumulation, migration, collection, and treatment; and
the management of special wastes, including medical
wastes, sewage sludge, and incinerator ash.
4.2 Waste Identification and Restriction
Owners/operators of MSWLFs must develop a program
to exclude regulated quantities of hazardous wastes
from the landfill, as described in Section 4.2.1. Even with
such a program, however, the owner/operator still might
find that some unacceptable hazardous wastes have
been delivered to the landfill site. These wastes must be
segregated and handled appropriately, as discussed in
Section 4.2.2. Section 4.2.3 outlines the recordkeeping
and notification requirements for hazardous wastes
found at MSWLFs.
4.2.7 Exclusion of Hazardous Wastes, PCBs,
and Liquids
4.2.1.1 Why These Wastes Must Be Excluded
Hazardous and other inappropriate wastes must be ex-
cluded from MSWLFs for four reasons: (1) regulatory
requirements; (2) protection of ground water from poten-
tial contamination; (3) incompatibility with other materi-
als in the landfill; and (4) potential adverse impact on
leachate treatability. Excluding hazardous and other
23
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inappropriate wastes from the landfill helps ensure that
the wastes coming into the landfill are compatible with
other wastes and materials at the site. If waste that
reacts with water or leachate, for example, contacts the
landfill liner, it could cause the liner material to fail.
Restricting hazardous wastes also helps ensure that
leachate remains treatable. Toxic wastes in the landfill,
for example, could impair biological treatment of leachate
(e.g., by destroying bacteria that normally biodegrade
the leachate). Also, treatment is most effective when
leachate is relatively uniform; if hazardous wastes, with
their numerous components, were disposed in the land-
fill, the leachate quality could become highly variable.
The Subtitle D regulations require that landfill owners/
operators implement a program for detecting regulated
quantities of hazardous wastes and RGBs to prevent
these wastes from being disposed in MSWLFs. Wastes
are classified as hazardous either because they are
listed as hazardous in 40 CFR Part 261 or because they
exhibit hazardous characteristics, including toxicity
(i.e., materials that fail the Toxic Characteristic Leaching
Procedure [TCLP] test, 40 CFR Part 261, Appendix II);
reactivity (materials that may be explosive or react vio-
lently with water); corrosivity; and ignitability. The defini-
tion of hazardous waste includes many specific
compounds and some sludges from various industrial
processes. Hazardous waste does not include material
from: conditionally exempt small quantity generators,
which are those generators that generate less than 100
kilograms per month of hazardous waste; household
waste; and hazardous household waste. PCBs, which
are regulated under the Toxic Substances Control Act,
also must be excluded from municipal solid waste landfills.
Other wastes that must be excluded from MSWLFs in-
clude liquid wastes in bulk containers and uncontained
liquid wastes. Small, household-type containers are ac-
ceptable, as are leachate and landfill gas condensate
liquids returned (i.e., recirculated) to the landfill if the facility
has a composite liner and leachate collection system.
Two methods can be used to exclude hazardous wastes
from the landfill: random inspections and source control.
These methods are discussed below in detail.
4.2.1.2 How To Ensure Wastes Are Excluded
Random Inspections
Unfortunately, the high cost of hazardous waste disposal
at a properly licensed hazardous waste facility can be
an economic incentive for illegal disposal at MSWLFs.
One purpose of waste identification and random inspec-
tions is to discourage illegal dumping.
Subtitle D regulations state that random inspections for
hazardous wastes must be performed unless the landfill
owner takes other measures to exclude hazardous
wastes. The regulations also state that facility operators
must be trained to recognize hazardous wastes and
PCBs. Subtitle D regulations do not specify how often
inspections must be conducted or how they should be
performed. Safety considerations during inspections are
discussed in Section 4.5.4. Suggestions for conducting
inspections are discussed below.
There are different ways of recognizing and identifying
wastes that should be excluded from landfills. One of
the most obvious ways is to look for Department of
Transportation (DOT) and other descriptive labels that
often identify whether the material is hazardous or non-
hazardous and specify what a container holds. Manifest
forms that might accompany the waste also can be
reviewed.
Inspections can be performed in several ways. In a
simple inspection, the operator can visually inspect
waste by looking into an open truck to view its contents.
If a more complete inspection seems warranted, the
hauler should be instructed to dump the load onto a
concrete pad, where normal disposal operations are not
obstructed and such that subsequent handling of the
waste is not inhibited. If some of the materials in the
waste are not acceptable, they should be separated and
managed as restricted waste. A typical flow sheet of
activities to be followed during a waste inspection is
shown in Figure 4-1.
Any unidentified waste could be an excluded waste and
should be handled by properly trained personnel using
appropriate techniques. If any waste is suspected of
being hazardous, it should be stored as a hazardous
waste until proved otherwise. If the contents of a con-
tainer are unknown, proper protection, such as a face
mask and protective clothing, should be worn. The po-
tential risks should be understood when investigating an
unknown waste.
Inspections should focus on loads that are more likely
to contain unacceptable wastes, such as loads from
commercial or industrial establishments and unknown
haulers; other factors also might serve as a warning. For
example, drums or other containers are not normally
used to dispose of municipal solid waste, but are often
used for liquids. Generally, a waste containing at least
20 percent solids would pass the paint filter test and
would be defined as a solid (sludge can be disposed of
in a landfill provided it is sufficiently dewatered and
passes the paint filter test). Another warning sign is a
waste with an oily appearance, which might indicate the
presence of PCBs.
Source Control
Source control can be used as an alternative to conduct-
ing random inspections. With source control, a landfill
receives wastes only from household and other sources
24
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Waste inspected by personnel
trained to recognize
hazardous wastes prior to
delivery at working face
Waste is identified as
nonhazardous
Waste is not readily
identifiable
Waste is identified as
a hazardous waste
Deliver to
working face
Isolate wastes by
moving to temporary
storage area
Record inspection
Refuse waste
Have wastes tested
including unidentified
containerized wastes
Record inspection
Waste determined to
be nonhazardous
Notify state director
Waste determined to
be hazardous
Return to working
face and dispose
Manifest and
transport wastes to
a facility
permitted to handle
the hazardous waste
(e.g., a facility with a
RCRA permit or
interim status)
Record inspection
and notify state
director
Figure 4-1. Hazardous waste Inspection decision tree (U.S. EPA, 1993a).
25
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that have been screened previously. The screening
process would identify potential sources, determine if
and when they might have unacceptable material, and
establish programs to segregate excluded wastes gen-
erated from these sources. Wastes whose charac-
teristics are unknown should be tested. Where possible,
waste characteristics should be identified before the
wastes are brought to the site. Source control is prob-
ably most suitable for very small, rural landfills. If the
source of the waste cannot be controlled, random in-
spections should be conducted.
4.2.2 Segregating Hazardous Wastes
If hazardous wastes are found on site in the possession
of the hauler, the hauler is still responsible for proper
disposal. The landfill operator can reject any waste until
it is identified and determined to be acceptable at the site.
If hazardous wastes are identified on site and the hauler
and/or source of the wastes cannot be identified, the
landfill owner/operator is responsible for proper disposal.
Hazardous wastes identified at a landfill site can be
stored for up to 90 days at the facility without a permit if
the wastes are containerized and the date received is
visibly marked on the container. Further, a temporary
storage area must be designated, and the stored con-
tainers must be marked "Hazardous Waste." In addition,
the landfill owner must designate an employee as an
emergency coordinator whose phone number, along
with the telephone number of the fire department, must
be listed next to the facility phone.
The temporary storage area for hazardous wastes
should be fenced off, with restricted access only. The
area should be designed to protect soil and ground
water, as well as people. Both unidentified and hazard-
ous wastes should be kept in this protected area.
To transport the material off site, the landfill owner
should either obtain an EPA identification number for
transporting the waste or hire a licensed hauler to re-
move the wastes. The wastes should be packaged ac-
cording to DOT regulations and labeled properly. If
hazardous wastes are identified relatively frequently, the
landfill owner might consider having a standing contract
with a licensed hauler. These contractors are available
at short notice to pick up hazardous waste loads. Such
an arrangement might be more economic for very small
facilities than training facility operators to handle hazard-
ous wastes.
4.2.3 Recordkeeping and Notification
If hazardous wastes are found at the site either during
an inspection or afterwards, the operator is required to
record that information and notify the appropriate state
or EPA personnel. The proper authorities must be noti-
fied of the results of any inspections, including when
hazardous waste is rejected or returned to the hauler.
Records must include the date, time the waste was
received, the name of the hauling firm and the driver, the
source of waste, the hauler identification number, and
any observations made, as well as results of the inspec-
tion. Particularly at large facilities, the landfill operator
also should consider instituting additional recordkeeping
for conventional wastes and for haulers that come to the
landfill regularly.
Many landfill owners already collect much of the infor-
mation noted above. Alternatively, the information can
be collected easily at the weigh station. These records
are valuable not only to meet regulatory requirements,
but also for long-term planning. In addition, these re-
cords can be valuable if hazardous wastes are found in
a particular hauler's waste load. Knowledge of the con-
tent of previous loads that this hauler has brought to the
site and the sources of these loads might help identify
where problem wastes might be located in the landfill.
The information also can be used with data from other
facilities to track wastes from sources to disposal areas.
4.3 Daily Cover Material
4.3.1 Purpose of Daily Cover
Daily cover is placed each day over the waste received
in a landfill to control disease vectors such as rodents,
insects, and birds and to control odors, litter, and scav-
engers. A daily cover placed between individual landfill
cells also helps create a firebreak, preventing fire from
spreading throughout the landfill. If a landfill fire starts,
it is almost impossible to stop. Water might successfully
douse the fire but could create additional leachate. Ex-
cavating the burning area also might work for small fires
or small landfills, but excavation can introduce oxygen
into the landfill waste, possibly increasing the intensity
of the fire.
Daily cover has other benefits as well. Leachate genera-
tion and gas migration can be controlled (by controlling
infiltration) with an appropriate cover. Vehicle access
also is improved if daily cover is provided, although the
required 6 inches of daily cover might not be sufficient
to support loaded trucks. Also, a tidy landfill with well-
covered waste helps improve public perception of the
landfill.
4.3.2 So/7 Covers
Six inches of soil compacted on the waste generally
is sufficient to control vectors, litter, and other potential
problems (see Figure 4-2). The daily cover soil typically
is spread with a bulldozer and compacted. Waste should
be compacted before covering. If the waste is not
compacted or is poorly compacted, the cover soil will fill
26
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6-inch thick daily soil cover
Working
face
Figure 4-2. Daily soil cover for landfill operations (provided by AB3 Environmental Services).
in the voids in the waste material and more cover soil
will be required. Excessive cover soil is not only an unnec-
essary expense, but also uses up landfill space. Two types
of soil covers are typically used: coarse, permeable soils
(sands), and fine-grained, low-permeability covers (silty
clays). Each type has distinct advantages, discussed
below. For any type of soil cover, a small working face
minimizes the amount of cover material needed.
4.3.2.1 Sand Covers
Sandy soils have been used for years as daily cover.
They are easy to use and are relatively inexpensive,
making them an ideal cover material. Unlike some other
cover types, sandy soils do not create erosion problems
on the site and provide a good traveling surface for
vehicles. Sand is not difficult to handle when it gets wet,
as is often the case with other materials. Gas movement
in the landfill is not restricted by sandy soils. Sand
covers do have some disadvantages, however. Sand
allows percolation of rainfall into the landfill, thereby
increasing the amount of leachate.
4.3.2.2 Silty Clay
Another type of soil cover is a silty clay material. This
material has the advantage of restricting water infiltra-
tion into the landfill, a key factor in minimizing leachate
production, which is desirable at some landfills. Working
with a clay cover, however, is more difficult than working
with sand. Silty clay is very difficult to obtain and use
during the winter in northern climates. Also, because this
type of cover restricts vertical leachate percolation,
leachate within the landfill can migrate laterally and
possibly break out on the side slopes of the landfill.
The selection of a cover is a compromise and is highly
facility-specific. Some landfill owners use sand as a
daily cover but use an intermediate cover consisting of
a more silty soil for areas that have reached final slope
or grade limits (as specified in the landfill permit). This
silty soil is intended to remain as a cover for an extended
period of time and is designed to inhibit water infiltration
into the landfill. A silty soil cover should be seeded to
protect against erosion.
4.3.2.3 Costs of Soil Covers
Soil cover costs can vary significantly depending on
whether the soil material can be obtained onsite or
needs to be brought in from another location. The real
cost of using soil for daily cover involves two compo-
nents: (1) the cost of obtaining and applying the soil
cover, and (2) the revenue lost by filling potential landfill
capacity with cover soil. The impact of 6 inches of daily
cover soil on landfill capacity is shown in Figure 4-3. The
graph presented is based on a specific working face size
and waste density, but the general shape of the curve
reflects the impact of cover soil on all landfills. (It does
not reflect additional soil that might be necessary to fill
waste voids). As the graph shows, a considerable amount
of landfill space can be lost to daily cover. At a small
landfill, where a relatively small amount of waste is
handled each day (perhaps 20 or 50 cubic yards), as
much as 25 percent of the landfill capacity might be filled
by daily cover. Even at larger landfill sites, as much as
14 to 16 percent of the waste volume can be occupied
by daily cover.
Approximately 0.2 cubic yards of soil are required to
cover 1 square yard of waste to a depth of 6 inches. At
an estimated cost of cover soil between $3 and $6 (1992
dollars) per cubic yard, the direct costs of daily cover soil
for a landfill are approximately $0.60 to $1.20 per square
yard. In addition to this direct cost, however, the 0.2
cubic yards per square yard of cover soil is also a loss
of landfill capacity. The value of landfill space varies
between landfills, but assuming a tipping fee of between
$10 and $20 per cubic yard, the potential value of the
lost landfill airspace would be between $2.00 and $4.00
per square yard of waste covered. Therefore, the actual
cost of daily soil coverthe cost of the cover and the
cost of lost landfill spacewould be between $2.60 and
$5.20 per square yard covered. An evaluation of the cost
27
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Daily 26
Cover 24
Volume
50 100 200 400
Disposal Rate in Cubic Yards per Day
800
Figure 4-3. Impacts of daily soil cover on landfill capacity (U.S.
EPA, 1992c).
of daily soil cover that includes the value of lost airspace
has increased the attractiveness of alternate cover ma-
terials.
4.3.3 Alternative Cover Materials
Alternative cover materials could reduce the expense of
daily cover. As long as the alternative material meets the
intent of the 6-inch soil cover requirement for controlling
disease vectors, fires, odors, litter, and scavengers, an
alternative material can be used, subject to approval by
the director of an approved state program, and based
on a performance demonstration. Whether it is advan-
tageous to use alternative covers will depend on a num-
ber of factors: the cost of materials and labor, whether
the cover meets the particular landfill's requirements,
and whether it is functionally equivalent to sand or other
soil materials.
4.3.3.1 Geosynthetics
One possible alternative to soil covers is geosynthetic
sheets, which are usually geotextiles and are sometimes
geomembranes. Geosynthetics are relatively impermeable,
do not take up any landfill volume, and inhibit vectors, litter,
and scavengers. The sheets often are placed over the
waste at the end of a day's operation and removed the
next day prior to operation. Geosynthetic sheets have
limited effectiveness in controlling odors, and concentrated
odors can be released when the sheet is removed from
the working face. The sheets also have minimal effective-
ness in controlling landfill fires.
Before a geosynthetic is placed over the landfill, the
waste should be well compacted. The geosynthetic can
be dragged over the top of the working face by attaching
the ends of the sheet to bulldozer blades or other suit-
able equipment. If mechanical equipment is not avail-
able, a group of workers can place the sheet, but this
approach generally would be effective only on small
working faces. After placement, the sheet should be
anchored with sandbags, tires, or other means to keep
it from being lifted by wind. The number of times a
geosynthetic can be reused will depend on its continued
integrity and its ability to remain relatively clean. A sheet
left on the landfill and subsequently filled over could
create a barrier to both the downward migration of water
(leachate) and the upward migration of gas. Therefore,
when a sheet is to be left in place it should be shredded
sufficiently to avoid these barrier problems. The cost of
using geosynthetics for daily cover has been reported to
be between $1.50 and $3.00 (1992 dollars) per square
yard. The range in cost to a large degree reflects the
number of times a geosynthetic can be reused.
4.3.3.2 Foams
Different types of foam material are available at varying
costs. Some foams are a plastic-like material that will
become relatively rigid when placed. These foams can
be placed at a depth of 1 to 2 inches and left for
extended periods. Other foams do not become rigid and
are suitable for shorter periods. Foams can be placed
either with specialized vehicles or by hand. Some types
of foams are difficult to place in wet weather or just
before rain is expected because the rain will break apart
the foam, although some foam manufacturers claim
their foam will remain intact under these conditions. One
advantage of a foam cover is that when waste is placed
on top, the foam collapses, minimizing lost landfill
space. Foams also do not inhibit the movement of gas
or leachate. The cost of foam covers is reported to be
about $1.50 to $2.00 (1992 dollars) per square yard.
4.3.3.3 Other Types of Alternative Daily Covers
Other materials, including some waste materials, have
been used for covering MSWLFs. Various sludges have
been used as covers, including industrial sludges. Some
facilities have used pulp and paper mill sludge for cover,
in part because of the relatively low permeability of these
materials. Alternative cover materials often are tried be-
cause they can cost much less than soil covers.
4.3.4 Temporary Waivers for Daily Covers
Temporary waivers for daily covers can be granted by
directors of approved state programs for extreme seasonal
climatic conditions. Under these conditions, cover material
might be frozen during parts of the year, or other conditions
might exist that make daily covering impractical.
4.4 Run-on and Run-off Control
4.4.7 Run-on Control
Run-on water from outside the landfill that runs toward
the landfill should be prevented from entering the con-
tainment area. Subtitle D regulations require a control
system for run-on to prevent flow onto the active portion
of the landfill during the peak discharge from a 25-year
storm. The run-on requirement determines the size of
28
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ditches, dikes, culverts, etc. If run-on is not prevented
from entering the landfill, it can percolate into the landfill
and increase the amount of water and leachate that
must be managed. Uncontrolled run-on also can cause
potentially expensive erosion problems.
4.4.2 Run-off Control
Run-off from precipitation falling within the landfill itself
must be managed to prevent the escape of contamina-
tion from the containment area and avoid erosion of the
cover system. Subtitle D regulations require a system to
collect and control the accumulated flow of water result-
ing from a 24-hour, 25-year storm at a minimum. This
system must not discharge pollutants into surface-water
bodies in violation of the Clean Water Act. Run-off must
be controlled in two ways. First, run-off from active
portions of the landfill, where it could contact waste or
leachate, must be managed as leachate. The size of the
leachate collection, transport, storage, and treatment
systems must be sized to handle this run-off as well as
the daily leachate generated. Keeping active landfill
cells small and controlling grading to divert run-off from
working areas also helps minimize the amount of run-off
collected. Second, on inactive portions of the landfill,
any rainfall that does not percolate into the ground or
through the cover can be discharged as stormwater
without having to be collected as leachate, thus reduc-
ing leachate collection costs (see Section 3.4). This
uncontaminated run-off must be managed to control
erosion using perimeter ditches, berms, siltation fences,
hay bales, sedimentation basins, or other mechanisms,
described below. An example of run-off control formed
by waste slope and containment sideslope is presented
in Figure 4-4.
4.4.2.1 Perimeter Ditches
Perimeter ditches commonly are used to control run-off
and consist of a ditch upgradient of the landfill to keep
run-on from entering the landfill site. Ditches downgradi-
ent of the site also are used to collect clean run-off from
covered portions of the landfill. If a ditch has a relatively
steep slope, riprap, pavement, or other surface might
have to be placed on the slope to prevent erosion. On
very steep slopes, gabion steps can be used to control
the run-off velocity. Ditches should be oversized where
possible to avoid overflowing. Ditch overflows can cause
numerous problems. For example, landfill dikes can be
eroded by ditch overflows, allowing water to flow into
the landfill.
4.4.2.2 Berms
Temporary berms (dikes) can be used within a landfill
for run-off control. These berms are small earthen struc-
tures, constructed of one or two feet of soil, that direct
run-off away from the operating face, where the run-off
could become contaminated. The berms should redirect
the run-off at a shallow slope and slow velocity so that
the berms themselves do not create an erosion problem.
4.4.2.3 Siltation Fences and Hay Bales
Siltation fences consist of a 2- to 3-ft wide geotextile fabric
that is placed horizontally and supported by wooden posts.
These fences slow the flow of water and retain sediment
as the water filters through the geotextile. Hay bales
perform a similar function. These fences or bales can be
used temporarily for erosion control on the landfill cover or
around the perimeter of the landfill before permanent grass
growth is established. Siltation fence posts located within
the landfill containment area should be installed carefully
to avoid puncturing the landfill liner.
4.4.2.4 Sedimentation Basins
A sedimentation basin is an area that allows water to
stand long enough to allow sediment in the water to
settle out and accumulate in the bottom of the basin. The
size of the basin depends on the drainage area upgradi-
ent of the basin. Periodically, the basin must be dredged
to remove the sediment. Sedimentation basins (and
ditches) quickly tend to become overgrown with aquatic
plants if they are not maintained, reducing their effec-
tiveness.
Run-off control formed
by waste slope
and containment
sideslope
IP
Ground surface
Run-on control
ditch (grass
or stone lined)
Figure 4-4. Example of run-on/run-off control structures (provided by ABB Environmental Services}.
29
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4.4.3 Factors To Consider in Selecting
Run-on-Run-off Control Methods
Structures to control run-on and run-off generally must
be designed for a storm of a particular intensity and
duration. Because it is possible that the intensity and
duration of the design storm might be exceeded during
the active life of the facility, the ramifications of a larger
storm on the environmental integrity of the landfill should
be considered during the design process. Where pos-
sible, ditches and storage basins should be oversized if
the overflow of these structures is likely to cause serious
environmental damage. For example, if a perimeter
ditch were to overflow and allow water to enter the
landfill, leachate volumes could increase, or possible
erosion of the cover soil could occur. Overflowing of a
ditch also could erode the ditch banks, resulting in a
diversion of water from the ditch into the landfill. Excess
storm flow within a landfill and consequent increased
leachate generation could cause an overflow of the
leachate storage pond. The risks of these occurrences
should be weighed against the costs of increasing the
capacities of the control structures.
Some general suggestions regarding run-on and run-off
control include: minimize the area from which run-off
needs to be collected; use temporary berms to divert clean
run-off to areas from which run-off does not require collec-
tion; and divert clean run-off from the leachate collection
area to avoid collecting this water. The method used for
controlling run-on and run-off depends on the options
available; if run-on/run-off is being collected with the
leachate and is pumped to a large-capacity treatment
plant, then disposal is rapid and perhaps inexpensive;
installing extensive run-on/ run-off control structures might
not be necessary. If leachate and run-off are trucked to
a disposal facility, berms or other onsite run-on/run-off
collection methods could significantly minimize costs.
Design and operating considerations also should be
investigated when determining run-on/run-off control
methods. For example, the effect of run-off diversion on
leachate generation from other areas of the landfill, now
or in the future, should be considered. Also, if leachate
is seeping through the cover from closed portions of the
landfill, run-off from these portions must be considered
contaminated and handled appropriately.
4.4.4 Leachate Storage
Treatment options for leachate include onsite treatment
and discharge, and treatment at offsite facilities, with
leachate transported by truck or pipeline. The onsite
leachate storage requirements depend on the quantity
and rate fluctuation of leachate generation, the limita-
tions of the treatment system, and the rate at which
leachate can be withdrawn from the storage facility. The
size of a storage facility needed often depends on the
amount of anticipated precipitation and the size of the
portion of the landfill from which precipitation will be
collected. A landfill that relies on a small, onsite treat-
ment facility or trucking to an offsite treatment plant
might require a greater storage capacity than a facility
that can pump large quantities of leachate directly to a
large treatment facility. The landfill should have sufficient
storage capacity to handle the expected run-off from a
25-year, 24-hour storm plus the volume of leachate
estimated to be generated during the storm and draw-
down period, as required by Subtitle D regulations.
Oversizing a storage basin provides additional security
against overflowing, but the additional surface area can
increase the amount of rainfall that is collected. Minimiz-
ing run-on and run-off can help reduce storage capacity
needs.
4.5 Safety
4.5.1 General Operations
Operator personnel should be trained in workplace
safety, including Occupational Safety and Health Act
(OSHA) training programs, first aid, and emergency re-
sponse. A health and safety plan also should be devel-
oped for the landfill operation. This plan should include
risks and associated symptoms of exposure to types of
wastes that are commonly brought to the landfill. It also
should address other, less common wastes that may
come to the site, such as municipal and industrial
sludges or other industrial wastes. Employees should be
aware that this plan is available to help them determine
proper response should they suspect a certain sub-
stance is present. The health and safety plan also should
include an evacuation plan and telephone numbers and
names of contact persons at hospitals, first aid opera-
tions, and the local fire department.
A contingency plan should be developed to address
potential landfill operation problems. This plan should
identify who is in charge during emergencies, who should
be contacted, and under what circumstances, and when
the site should be closed. If the operators cannot handle
a problem, the contingency plan should indicate which
experts can be called. Having protocols for these issues
ahead of time will reduce problems if an emergency
occurs.
If an operational activity requires a person to be at a
remote part of the landfill, he or she should have some
kind of communication device, such as a two-way radio.
Using the "buddy system" is also recommended; at
least two people should be assigned to potentially
dangerous tasks so that if someone is injured, for exam-
ple, the other person can offer them assistance or sum-
mon help.
30
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4.5.2 Access Restrictions
One of the Subtitle D regulatory requirements is restriction
of public access to the landfill. This requirement pertains
to restriction of access to the site as well as restrictions
on the site. To restrict access to the site, perimeter
fencing, gates, or other devices should be installed. How
far a fence needs to extend depends on how easily the
site can be accessed. Natural barriers such as a thick
grove of trees, slopes, or banks can be used to prevent
access to the landfill site or operating equipment. Sepa-
rate dumping areas at the entrance to the landfill, such
as boxes or dumping platforms, can be used to keep the
public away from active landfill operations. If recycled
materials are collected at the landfill, separate dumping
areas could be incorporated into the recycling area.
4.5.3 Traffic Control
Other access control devices, including barriers, gates,
and signs, should be used to direct the public to their
destination once they are on the landfill site. Consider-
able traffic moves around the landfill, and the routes of
traffic and people should be controlled. Trucks usually
have back-up alarms, but people generally become de-
sensitized to frequently sounded alarms, so alarms
alone should not be relied on exclusively for traffic
safety. Public access and traffic restrictions not only help
control public exposure to potential hazards but also
help prevent illegal dumping during operating and non-
operating hours.
4.5.4 Personnel Equipment
Protective face masks and outer suits, chemical-resistant
gloves and boots, and other safety equipment should be
readily available if any chemicals are handled on site. Face
masks should be fit-tested to ensure proper sealing. Other
types of safety equipment include safety glasses and
various types of face shields. Air packs might be needed
if maintenance is performed in areas that could contain
contaminated air or have oxygen-deficient atmospheres.
4.5.5 Hazardous Waste Inspections
The operator conducting hazardous waste inspections
at the landfill site must understand the materials he or
she might encounter and must be able to handle unex-
pected situations or emergencies. The Subtitle D regu-
lations require that facility personnel be trained to
recognize hazardous wastes and PCBs. This training
should include the proper ways to conduct inspections
to identify these wastes and provide an overview of
Subtitle D, state regulations, OSHA safety regulations,
and any applicable local regulations.
An important part of operator training is to identify the
common types of MSW that are disposed of in the
landfill. Many MSWLF sludges are nonhazardous, and
operators should be able to recognize them. If an un-
usual waste (i.e., one with which the operator is not
familiar) is brought to the landfill, an inspection is prob-
ably warranted.
Hazardous wastes and PCBs must be handled safely.
Health and safety procedures for handling these wastes
are published and maintained by OSHA. OSHA also
provides a 40-hour course on health and safety for
hazardous waste site workers that provides training for
people conducting hazardous waste investigations. This
course might provide more information than that
required for municipal landfill personnel, but it does
have some applicable information. Also, the Red Cross
provides good first aid and CPR courses. Local fire
departments and safety crews also will have helpful
information.
The National Institute for Occupational Safety and
Health (NIOSH) Pocket Guide to Chemical Hazards
(Department of Health and Human Services, no date) is
a valuable document that should be kept at a landfill site.
This document includes characteristics of many sub-
stances as well as chemical names, synonyms for the
chemical name, exposure limits, physical descriptions, in-
compatibility with other substances, and personnel protec-
tion that should be worn when handling different types of
materials.
4.5.6 Gaseous Conditions
Potential safety problems, such as explosion or asphyxi-
ation risks, exist when landfill gases such as methane
and other toxic materials are generated. The landfill area
is susceptible to gas accumulation. Gas enters leachate
pipes and then can infiltrate manholes, pump stations,
garages, operators' quarters, or other spaces near the
landfill.
The Subtitle D regulations require onsite methane moni-
toring. (A more detailed discussion of methane and other
landfill gases is presented in Section 4.6.) Monitoring for
oxygen also should be conducted when anyone enters
a confined space to ensure enough oxygen is present.
Also, some monitoring devices, such as draeger tubes,
can identify the specific gases and approximate concen-
trations present.
All structures within the landfill site should be vented,
especially if they are anywhere near potential routes of
gas movement. Belowground structures (e.g., leachate
pump stations) that are likely to pick up gas should be
vented before entry. Entry procedures should be devel-
oped for confined spaces, which pose a risk to workers
if not vented properly. A number of such spaces, includ-
ing manholes, will need to be accessed periodically for
sampling, maintenance, or cleaning. For example, when a
worker enters a leachate manhole, air can be pumped in
to ensure that the manhole has an adequate atmosphere.
31
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The worker probably should enter the manhole with an
air pack, and another worker should have a hoist avail-
able to help remove the person entering the manhole in
an emergency. If a worker collapses while in the man-
hole, a second worker should not enter the manhole to
rescue the victim, because the rescuer might collapse,
too. The victim should be removed from the manhole
using the hoist, or a worker with a clean air supply
should descend to retrieve the person. Rescue proce-
dures should be established before a confined space is
entered.
4.6 Landfill Gas Monitoring and
Management
4.6.1 Gas Generation
Gas generation is a biological process in which micro-
organisms decompose organic wastes to produce
carbon dioxide, methane, and other gases. Gases of
concern are generated mostly by the anaerobic process,
in which microorganisms generate gas in an environ-
ment deficient in oxygen. The ability of a landfill to
generate gas depends on many factors, including waste
composition, moisture content, pH, and nutrient avail-
ability. If the landfill is very dry, little gas will be gener-
ated. When older, well-covered landfills have been
excavated, little waste decomposition (and therefore
gas production) has been noted.
In some landfills, seasonal temperature changes also
might influence gas generation. In the colder months,
the rate of gas generation can decrease significantly in
shallow landfills. Gas generation also can vary signifi-
cantly throughout a landfill because of the distribution of
different wastes types in the facility. Pockets of high
microbial activity can be interspersed with other areas
where little decomposition and gas generation occur.
Initially, when waste is placed in a landfill, conditions are
aerobic. Atmospheric oxygen is present, and the organic
materials in the waste begin to break down, producing
mainly carbon dioxide and water. These aerobic proc-
esses are exothermic (heat-producing), and the landfill
temperature begins to rise. As a landfill ages and more
waste is placed in it, the oxygen is depleted, and anaero-
bic (oxygen-deficient) conditions eventually predomi-
nate. The organic materials in the waste break down into
organic acids, which then further break down into meth-
ane and carbon dioxide.
Generation rates for gases vary among landfills and
within each individual landfill. Theoretically, between 7
to 9 cubic feet of gas can be generated per pound of dry
organic matter. In general, from 0.05 to 0.2 cubic feet of
gas per year can be generated for every pound of
refuse. Pressure in the landfill builds up as the gas is
generated, and pressures of 1 to 3 inches of water (and
as high as 6 inches in some cases) have been measured.
It is this pressure that drives the gas from the landfill
into the atmosphere, into the soil, and, potentially, into
surrounding structures.
Figure 4-5 presents a graph showing landfill gas com-
position over time. Initially, the distribution of gases in
the landfill is representative of the distribution of gases in
the atmosphereabout 80 percent nitrogen, 20 percent
oxygen, with some carbon dioxide and other compounds.
Then the microorganisms present in the landfill begin to
break down the organic material. Oxygen becomes de-
pleted, the carbon dioxide level increases, and aerobic
conditions begin changing to anaerobic conditions. Initially,
the methane content is very low in the landfill, but be-
comes higher over time, whereas the carbon dioxide
content tends to fall slightly. These gases tend to migrate
as one mixed gas, rather than as segregated methane
and carbon dioxide. Over longer periods, as the organic
matter in the landfill is consumed, the rate of all gas
generation ceases.
4.6.2 Characteristics and Potential Hazards
of Landfill Gases
Potential hazards posed by landfill gas include explo-
sions, asphyxiation, offsite gas migration, and disrup-
tions of cover systems. Major constituents of landfill gas
include methane, carbon dioxide, and hydrogen sulfide.
Other compounds also can be present, including trace
amounts of volatile organic compounds. Methane is the
constituent of most concern. It is colorless and odorless,
and therefore cannot be detected by smell. It is lighter
than air and highly combustible. Methane will combust
Aerobic
Anaerobic
100
Time after placement
Figure 4-5. Changes in landfill gas composition over time (U.S.
EPA, 1993d).
32
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if its presence in air is between 5 and 15 percent. At
lower than 5 percent methane content, not enough
methane is present to allow combustion. At greater than
15-percent, not enough oxygen is available. Five per-
cent is defined as the lower explosive limit (LEL) for
methane. The 15 percent limit is the upper explosion
limit (UEL). Subtitle D requires that landfill gas (meth-
ane) must not exceed 25 percent of the LEL in facility
structures, such as operators' quarters, garages, pump
stations, and any other facilities, and must not exceed the
LEL, or 5 percent methane, in soil at the facility boundary.
Methane monitoring at the facility boundary should include
monitoring with soil gas monitoring probes.
Carbon dioxide is a very common gas, colorless and
odorless, heavier than air, and noncombustible. Although
carbon dioxide is heavier than air and methane is lighter
than air, they are generated together, tend to travel
together, and generally are found in a relatively uniform
mixture throughout the landfill. Carbon dioxide is a concern
because it can displace air in structures, creating an
unbreathable and potentially asphyxiating atmosphere.
Hydrogen sulfide is a colorless gas but has a relatively
strong odor, like that of rotten eggs. The odor can begin
to be observed at about 5 parts per billion. The gas is
an immediate danger to life and health at about 300
parts per million. Unfortunately, high concentrations of
hydrogen sulfide cannot be differentiated from low con-
centrations by smell. OSHA/NIOSH recommends an
action level of 10 to 20 parts per million for hydrogen
sulfide, the level at which the owner/operator must take
measures to protect worker safety.
In addition, because of its sulfur content, hydrogen sul-
fide can be transformed into sulfuric acid when com-
bined with oxygen. Sulfuric acid is extremely corrosive
and has caused many problems with pump stations and
metallic devices in the leachate collection system.
4.6.3 Landfill Gas Migration
Landfill gas migrates in response to pressure, concen-
tration, and possibly temperature gradients. Because
gas generation causes gas pressure to build, a gradient
is established that seeks to equalize itself. Gas migra-
tion in the landfill follows the path of least resistance.
The degree to which gas migrates vertically or horizon-
tally depends on many factors, including the nature of
the landfill design, surrounding soils, type of waste,
degree of waste segregation in the landfill, and the type
of daily or final cover used at the facility. With a sand
and gravel soil cover of relatively high permeability, gas
tends to vent equally and vertically, perhaps through the
cover onto the surface, and at a relatively uniform rate.
With a low-permeability cover, gas tends to migrate
horizontally. If a low-permeability cover is placed on the
landfill, the gas no longer has a pathway for upward
release. If lateral resistance to migration is less than
vertical resistance, the gas tends to move laterally and
might collect in low spots, such as basements, man-
holes, and pump stations adjacent to the landfill, resulting
in oxygen-depleted and potentially explosive environ-
ments. Any structures on or near the landfill must be
monitored to ensure that gas is not accumulating in that
area. These migration concepts are shown in Figures
4-6 and 4-7.
Gas generally is transported through the unsaturated
portion of the soil (i.e., that portion not filled with water)
or fractures in rock. But because gas is soluble, it can
migrate through saturated soil under sufficient pressure.
For example, in some large, old landfills, relatively im-
permeable waste often was placed directly on the
ground with no leachate collection system or drainage
beneath the waste. These landfills can generate signifi-
cant amounts of gas. At one facility, the pressure was so
high that it drove the gas into the ground water. The gas
then exited from the ground water into unsaturated soil
at the site perimeter, where pressures were lower.
EXTENSIVE VERTICAL MIGRATION
Clay or synthetic liner
(low permeability)
Sand and gravel cap
(high permeability)
Figure 4-6. Landfill conditions that result in vertical gas migra-
tion (EMCON, 1981).
EXTENSIVE LATERAL MIGRATION
Clay or synthetic cap
^(low permeability)
Clay soil, frozen or
saturated soil, or pavement J
(low permeability).
Sand and gravel soil * »>
(high permeability) », ;j"|
Figure 4-7. Landfill conditions that result in lateral gas migra-
tion (EMCON, 1981).
33
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4.6.4 Landfill Gas Monitoring
Subtitle D requires a routine monitoring program for
methane. This program should be based on facility-
specific soil conditions. A landfill built in relatively per-
meable soils (sands and gravels) probably should be
monitored for methane more frequently than one con-
structed in relatively impermeable (clay) soils. Also,
nearby pipelines, sewer lines, water lines, and other
utilities, even those not associated with the landfill, can
become primary pathways for gas migration and thus
might require monitoring. These structures are condu-
cive to gas migration because they frequently are sur-
rounded by special pipe-bedding soil, which is often
much more permeable than native soil. Utilities associ-
ated with the landfill, such as leachate manholes and
pump stations, also might contain methane. Elevated
methane concentrations found in these locations might
not trigger reporting or remediation activities, however.
If methane limits are exceeded, the landfill owner/operator
is required to take several steps to protect human health
and the environment. First, protective procedures must
be undertaken immediately, and the appropriate state
authorities must be notified. If a high methane gas level
is detected in a structure, such as operators' quarters,
the structure should be evacuated and the gas vented.
If methane levels are exceeded in a pump station or
elsewhere where personnel access might be required,
proper confined entry procedures must be taken before
the structure is entered. These procedures would in-
clude checking for oxygen as well as explosive condi-
tions, using protocols for ventilating the space and
emergency rescue, when necessary.
The owner/operator also must implement a remediation
plan when methane limits are exceeded. The plan
should address the nature and extent of the migration,
where it was found, and what levels were monitored. It
is useful to record soil characteristics (e.g., sandy soils)
and note if the water table has dropped below normal
during a dry season (which could increase pathways for
gas migration). If soil conditions vary within the landfill,
gas could be migrating in the permeable soils but not in
other soils. A review of operations might help the owner/
operator identify the potential causes of gas migration.
For example, has there been a change in the landfill
operation, such as a different area being worked or failure
of the leachate or gas removal system? Closure activities
also could alter available pathways for gas migration.
In addition to monitoring for gas in facility structures,
owners/operators should install gas monitoring wells to
measure belowground gas. Facility structures could in-
clude basements, manholes, and pump stations. Some
of these structures, such as manholes and pump sta-
tions in which leachate is exposed to the atmosphere,
would be expected to have some methane present.
Elevated levels of methane found in these locations
might not trigger remedial action, however.
Locations for gas monitoring wells, or probes, should be
selected based on a review of the landfill's design, site
geology and hydrology, and other features relevant to
gas migration such as subsurface sewer or utility pipe
locations. Wells should be placed in transmissive geo-
logic strata at locations that allow gas to be measured
near both the solid waste boundary (to identify gas
migration) and the facility's property boundary (for com-
pliance monitoring).
Homogeneous soils are perhaps best monitored with
continuously screened wells. Heterogeneous soils, how-
ever, might require well clusters to be installed with dis-
crete screened intervals to monitor different geologic
strata. Well construction details will vary according to the
geologic stratigraphy to be monitored at the well loca-
tion. When multilevel monitoring well clusters are used,
the screened intervals must be sealed from one another
so that specific strata can be monitored discretely. The
screened intervals can be constructed from slotted PVC
well screen or from pipe into which holes have been
drilled. Because these monitoring points are intended to
be somewhat permanent, they must be protected from
damage by vehicular traffic, either using wells with sur-
face casing and protective posts or installing flush-
mounted (e.g., roadbox) wells.
Typical gas monitoring wells are shown in Figures 4-8
and 4-9. These devices are similar in construction to
ground-water monitoring wells, except that the screened
area of the gas probe is located in the soil above the
ground-water table, rather than in the ground water
itself. Gas probes should be purged before sampling
similarly to ground-water monitoring wells (see Chapter
5). At least one or two pore volumes of air should be
purged from the well before sampling. Alternatively, the
well can be purged until a constant gas concentration is
evident in the monitoring equipment. Ambient tempera-
ture, barometric pressure, and gas pressure in the probe
should be measured before a gas sample is taken for
analysis.
Portable methane meters and explosimeters are used
most commonly to monitor gas concentrations. They are
relatively simple, durable devices that can be hand-held
while gas measurements are taken. Methane meters
indicate the percentage of methane present, whereas
explosimelers measure the combustibility of the gas as
the percentage of the LEL (see Section 4.6.2). Either
device can be used for compliance monitoring. An oxy-
gen meter also is commonly used with these devices.
For more sophisticated analysis, a portable gas chroma-
tograph can be used, or samples can be collected and
sent to a laboratory to determine the gas constituents by
gas chromatography analytical methods. All instruments
must be properly calibrated. Other gases, such as
34
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Protective cover
\
\
I
Natural '
ground "
^
>
/
1
't\';
&
"'.''
^
^r_
'1
'.'V
L ,
'i-
''.''.
'."''.
/V
~f-':'
<£%&
\
s
/
C
Grout or clay plug
W-1J4"
Perforated PVC pipe
Natural
ground
Washed pea gravel
4" Minimum bores
Figure 4-8. Typical single screen gas monitoring probe
(EMCON, 1980).
carbon dioxide, can interfere with the accuracy of read-
ings by some meters. Consultation with the manufacturer
of the equipment to determine its limitations is recom-
mended.
Other indicators of gas migration, besides actual sam-
pling, include the odor of hydrogen sulfide, the presence
of stressed vegetation on closed portions of the landfill,
or a grass kill where the grass is otherwise growing well.
Dead or dying vegetation is a good indication that meth-
ane is seeping through the soil and replacing the oxy-
gen in the soil, because such conditions interfere with
vegetation growth.
4.6.5 Gas Collection
On May 30, 1991, EPA published proposed regulations
(40 CFR Parts 51,52, and 60) which included standards
of performance for new MSWLFs and emissions guide-
lines for existing MSWLFs pursuant to sections 111 (b)
and 111 (d) of the Clean Air Act. Under the proposed
regulations, MSWLFs emitting greater than 150 mega-
grams per year (167 tons per year) of nonmethane
Brass keyed _
alike padlock
Sand pack
'"
'8 "I.D.
Cement polyethylene
grout to tubing
top of
r
2'-0" Stick up
on \* tubing
L
Ground
surface
" I.D. PVC
slotted screen section
4' or less
depending on
freeze-thaw
conditions
Backfill soil
; ^ Cement grout
' 2'-0" minimum
;; " Sand pack
Cement grout
2'-0" minimum
Sand pack typical
Figure 4-9. Typical multiple screen gas monitoring probe (B.C.
Jordan Co., 1986).
organic compounds would be required to design and
install gas collection systems. The final regulations are
expected in late 1994. These regulations could apply to
hundreds of new and existing MSWLFs across the
country and will likely result in subsequent state regula-
tions that will also establish limitations on emissions
from MSWLFs.
During landfill operations, and more frequently during
and after landfill closure, operators might need to control
gas movement. Two different systems can be used to
collect vented gaspassive and active collection sys-
tems. With either type, redundancy in the gas collection
system is important for ensuring continued operation of
the system. Redundancy protects against the loss of
system components caused by settlement and failure of
the entire system from a single malfunctioning compo-
nent. Redundancy can include additional gas extraction
wells and header pipes (see Section 4.6.5.2).
4.6.5.1 Passive Collection Systems
Passive gas collection systems allow gas to be released
without using mechanical devices such as blowers or
pumps. The systems can be used outside or within the
landfill. Perimeter trenches and pipes vented to the
atmosphere can act as a passive system by intercepting
lateral migration of gas through the soil. A trench is dug
35
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around the landfill to the depth of the water table (if
shallow), and is backfilled with pervious stone and
pipes, which act as a passive barrier.
Depending on the types of soil at the facility, a more
solid and less permeable barrier might be needed on
the trench side away from the landfill to improve pas-
sive venting within the trench. If the soil is sandy with
a permeability similar to that of the trench, a flexible
membrane liner placed on the outside of the trench will
help stop gas migration and allow the gas to pass up
through the vent. For facilities with a deeper water table,
a slurry wall can be used as a remedial measure to stop
gas migration.
Figure 4-10 presents a typical passive vent used at
landfills with both intermediate and final cover systems.
A perforated collection pipe is placed in a granular vent
layer above the waste. Typically, coarse sand is used for
the vent layer, but geotextiles and geonets can be com-
bined as an alternative. This pervious pipe is connected
to a vertical riser pipe, which is connected to a 90-degree
elbow (gooseneck) through which gas is vented. A barrier
layer placed above the vent layer causes gases to stop
at the geomembrane or the clay surface and migrate
laterally to the pipes and up to the atmosphere. Vents
can be independent or connected in a system of lateral
header pipes. Piping should be buried deep enough to
prevent frost-heaving. Care must be taken to protect
these vents; if they are broken the piping will provide a
conduit for surface water into the waste.
The advantage of passive gas collection systems is that
they are relatively inexpensive and require little mainte-
nance. If a passive system is not working properly and
vents are connected with a header pipe in a portion of
the landfill, the system can be converted to an active gas
collection system.
4.6.5.2 Active Collection Systems
If a passive system is insufficient to manage landfill gas
problems, an active collection system might be neces-
sary. In an active system, power is applied to create a
vacuum or positive pressure, forcing gas from the land-
fill. Most active gas collection systems use negative
pressure and apply a vacuum to pull gas out of the soil
in the landfill via extraction wells, extraction trenches,
or a venting layer.
Although positive pressure systems are not commonly
used, a positive pressure system could be used in a
trench around the landfill perimeter to create a higher
pressure zone. This higher pressure zone would tend to
force air back toward the landfill, thereby redirecting any
gas migrating through the ground upwards into the at-
mosphere.
Typically, active gas collection systems are designed
based on pilot test results that are used to determine
spacing and operating flow parameters for gas extrac-
tion wells or trenches. As a rule of thumb, extraction
wells should be approximately 200 feet apart. A better
way to determine gas well locations is to conduct a pilot
pumping test, similar to a pumping test for ground water.
For a pilot extraction test, a pilot well is installed, along
with vacuum pressure monitoring wells at about 25,50,
and 100 feet away from the extraction well. As gas is
pulled out of the extraction well, vacuum pressures in
piezometers are monitored to determine how far away
from the extraction well gas movement occurs. From
these data, the permeability of air flow in the waste, the
radius of influence of the extraction wells, and maximum
flow rates per unit length of well or extraction trench
length can be determined.
Gas vent
Top layer
Drain layer
- Low-permeability
geomembrane/soil layer
Vent layer
Figure 4-10. Typical passive gas collection system for venting of landfill gas (U.S. EPA, 1992c).
36
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Figure 4-11 illustrates a gas extraction well that can be
used within the landfill or around the perimeter. Gas
wells or header systems should be equipped with a
valve that regulates flow and serves as a sampling port.
Such a valve is important because gas generation can
vary throughout different parts of the landfill. Addition-
ally, over time, the flow from certain areas might need to
be adjusted. By monitoring gas quality (e.g., methane
content) and measuring gas pressures, the operator can
assess more readily the seasonal and long-term
changes in gas production and distribution within the
landfill and make appropriate adjustments.
Gas extraction wells should be sealed to minimize at-
mospheric releases. Depending on the age of the landfill
and the location of the wells, differential settlement can
occur, leading to well damage. Efforts to design the
extraction system with flexible connections and materi-
als capable of withstanding strain will help maintain sys-
tem integrity. Also with differential settlement, low spots in
the collection or header pipes can develop, and the pipe
can fill with gas condensate, which effectively plugs the
pipe. Condensate traps, spaced 300 to 500 feet apart,
should be included in the design of the gas collection
system. These traps will allow the condensate, which mi-
grates with the gas, to drop out of the gas collection pipe,
thus preventing pipe plugging. For every million cubic
feet of gas that is generated, about 50 to 600 gallons of
condensate might be generated, depending on the vac-
uum pressure of the system and the moisture content of
the waste. Condensate is one of the few liquids that can
be disposed in Subtitle D landfills.
Blowers used to pull gas from the landfill generally
operate from 300 to 2,000 cubic feet per minute and
apply 10 to 100 inches of negative water pressure at the
well heads. The size of the blower and the amount of
head are design parameters that should be based on
actual pilot field data to ensure proper system design
and operation. Blower capacity should be matched
against future needs for gas management as a facility
expands over time or as needs change, such as when
landfill cells are added to or disconnected from the gas
recovery system.
4.6.6 Gas Treatment
If landfill gas is collected in an active system, the gas
must be treated. Gas treatment usually involves either
thermal destruction of organic compounds by flaring or
gas processing and energy recovery.
4.6.6.1 Flaring
A flare is a controlled combustion unit. Flaring is a
common treatment method when enough methane
(e.g., greater than approximately 20 percent by volume)
is present in the gas. Flaring reduces odors and often is
a much more effective method for odor control than
Valve box
and cover
Gas collection
header
r'pvc
monitoring
port with cap
Cap
Figure 4-11. Schematic of gas extraction well (SCS, 1980).
passive venting. Most flares designed today are en-
closed flares, which allow longer residence times, ele-
vated combustion temperatures, and greater thermal
destruction efficiency than open flares.
Generally, gas enters the flare system from the landfill
through a valve located upstream of the blower (Fig-
ure 4-12). The blower outlet exits through a pipe to the
flare stack, which contains instruments to verify tem-
perature and flame presence and to prevent burnback
of gas into the blower. These instruments use passive
safety mechanisms, such as flame arresters and liquid-
filled flashback units, or active protection systems, such
as thermocouples (to detect combustion flashback),
self-actuated valves (to shut off gas entry), and auto-
shutdown sensors. If for any reason the flame goes out,
a flame detector will immediately sense that no flame is
present and will shut down the self-actuated valve, thus
preventing uncombusted gas from escaping into the
atmosphere. A flare should include both passive and
active safety systems; if one of these systems malfunc-
tions, the other system can take over. The stack is
generally purged before flare startup, and, typically, pro-
pane is used for ignition and pilot fuel. The flare stack
also can include equipment for monitoring air quality
exiting the system, which might be required by some
state permits.
37
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f Propane J
Concrete base
Gas from
landfill
Figure 4-12. Schematic of a landfill flare system with blower (ABB Environmental Services, 1990).
4.6.6.2 Gas Processing and Energy Recovery
Gas also can be processed by removing water and
impurities, including carbon dioxide. The heat value of
unprocessed landfill gas is about 500 Btu per standard
cubic foot. This heat value is about half that of natural
gas, primarily because only about half of landfill gas is
methane. Processing increases the heat value of the
gas to approximately 1,000 Btu per cubic foot. At this
level, gas can be directed into a pipeline and sold to a
utility as natural gas.
Energy recovery is being used at some landfills, particu-
larly larger landfills where the magnitude of the opera-
tion and the potential life of the project make energy
recovery economically viable. Whether energy can be
recovered at a reasonable cost depends on the quality
and volume of the gas. At a small landfill, gas with a heat
value of 500 Btu per pound can be used to run a
modified internal combustion engine or a generator
to convert gas to electrical energy. At a larger landfill,
moisture and carbon dioxide removal (through scrub-
bing and gas polishing with carbon or polymer adsorp-
tion) enables the gas to be used to run boilers and
turbine generators for energy recovery.
Generally, landfills closed for fewer than 5 years are the
best candidates for energy recovery because over time,
even with proper conditions, the ability of a landfill to
generate gas decreases. With optimum conditions, how-
ever, an area of a landfill might produce gas for 15 years
or more, depending on the rate of gas generation, the
water content of the waste, and the manner in which the
landfill was closed. Modern closure requirements for
landfills are intended to limit moisture infiltrating the
landfill. To what degree these requirements will affect
long-term gas generation is unknown, but they should
lead to a reduced period of gas generation after closure.
4.7 Special Wastes
By definition, a number of wastes are classified between
what is commonly considered municipal solid waste and
what constitutes potentially hazardous waste. Although
these wastes do not, in general, pose a public health or
environmental problem if managed properly, they might
require special handling procedures. These wastes
include materials such as medical wastes, sewage
sludge, and municipal solid waste incinerator ash.
4.7.1 Medical Wastes
Most medical wastes, such as disposable clothing, ban-
dages, syringes (sharps), and other disposable instru-
ments, come from hospitals and clinics. Medical waste
is not directly regulated under Subtitle D. Certain RCRA
Subtitle C listed hazardous wastes may be generated
by medical facilities, or certain medical wastes may
exhibit hazardous characteristics which would require
special packaging, storage, labeling, transport, and dis-
posal procedures. Many state regulations affect medical
waste handling and disposal. It is strongly recom-
mended that landfill owners and operators contact their
state regulators regarding disposal requirements and
restrictions if medical wastes are delivered to the facility.
Human tissues generally cannot be disposed of as
medical waste and require special treatment, such as
incineration. Because medical waste regulations vary
38
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from state to state, the degree to which different waste
types require sterilization before disposal also varies.
Most waste materials that have come in contact with
human fluids are required to have been treated for
pathogens by either high temperature steam, autoclave,
or microwave sterilization procedures. Before medical
wastes are transported from a hospital, they must be
clearly marked, packaged, labeled, and contained within
special medical waste disposal bags. Sharps must be
contained within crush-proof plastic containers to pro-
tect workers from incidental contact. The greatest con-
cern in handling medical wastes after they arrive at a
landfill is incidental infection, thus operators must be
careful when handling these wastes.
Segregation of medical wastes in dedicated disposal
areas is usually appropriate, because most people per-
ceive these wastes as health risks, even if they have
been disinfected and treated. Protection of public health
and worker safety is paramount. Operators should not
drive over bags of medical wastes with landfill equip-
ment, which can tear the bags apart and scatter the
contents over the landfill. Medical wastes should be
covered carefully and immediately after disposal. Dis-
posal areas should be recorded in the facility's operation
records so these areas can be located later, especially
if work, such as large-scale removal of wastes, will be
occurring.
The owner/operator should contact all local waste haul-
ing firms involved in medical waste transportation, as
well as hospitals and the state, to develop sound con-
tractual and operational procedures to ensure proper
management of these wastes.
4.7.2 Sewage Sludge and Industrial Sludge
Sewage sludge comes from two primary sources
publicly owned treatment works and other wastewater
treatment facilities. This byproduct of wastewater treat-
ment is composed of organic and inorganic solids and
water. Usually water and organic material constitute 90
percent, and inorganic material 10 percent, of sewage
sludge. The sludge is very high in nutrients and can be
biologically active and odorous if not stabilized. Sewage
sludge can have an industrial waste component if the
treatment plant services industrial facilities.
Sewage sludge cannot be disposed at a MSWLF until it
passes the paint filter liquids test (PFLT). Generally, it
must be mixed with soil or in some way dewatered to
approximately 20 percent solids content to be consid-
ered a solid for disposal at a MSWLF. Stabilized sewage
sludge, such as compost, which has been treated to kill
pathogens, might be used in a landfill as a cover mate-
rial. Composted sewage sludge also might be used as
a soil conditioner to promote grass growth on the landfill
if the sludge has been properly treated to destroy
pathogens according to 40 CFR Part 503 regulations.
Domestic septagematerial from septic tanksis a
similar type of material that generally is very wet. There-
fore, domestic septage will not readily pass the paint
filter liquids test and cannot be disposed at a MSWLF
until it has a sufficient solids content to pass the test.
Disposal of sewage sludge can be problematic for two
reasons. First, unstabilized sewage sludge can create
odor problems. Second, sewage sludge lacks good
compaction characteristics. Large volumes of sewage
sludge with poor compaction and strength charac-
teristics should not be concentrated near the sideslope
of most landfills because of potential for waste slope
stability problems. Sludge with pronounced odor prob-
lems might require additional daily cover to control odors
or the use of a different cover material that more com-
pletely contains the migration of odors (e.g., a silly or
clay-rich cover soil). In most cases, sewage sludge
disposal in a landfill can be accommodated readily with
minor modifications to operating procedures.
Industrial sludges can be disposed in landfills provided
they are determined to be neither characteristic nor
listed hazardous wastes. Some industrial sludges are
relatively inert, can be dewatered to a large extent, and
potentially can be used as interim or daily cover material,
which saves the expense of using soil materials. For
example, paper mill primary sludges, which are high in
fiber content and clays, can be dewatered to 40 to 50
percent solids, possess good material handling proper-
ties, and have been used as daily cover.
4.7.3 Incinerator Ash
Municipal solid waste incinerator ash is the residual
product of a variety of incinerator types, including modem
waste-to-energy conversion plants, such as large mass bum
facilities, and older incinerators, such as small, modular
incinerators. In the last half decade, waste-to-energy
technology has reemerged as a practical, although at
times socially unpopular, means of managing MSW.
During incineration, metals contained in the waste ash are
partitioned between the bottom and fly ash and the flue
gas. (Flue gas treatment residues also might be present in
incinerator ash.)
Concerns over ash management at landfills has focused
on leachate quality at ash monofills and the potential
effects of MSW leachates on the environmental mobility
of certain components, such as heavy metals and dioxin
and furan isomers potentially formed during the com-
bustion process and contained in the ash. The U.S.
Supreme Court ruled on May 2, 1994, that municipal
waste combustion ash must be managed under federal
hazardous waste rules. This ruling will result in municipal
solid waste incinerators and waste-to-energy facilities be-
ing required to conduct regular Toxicity Characteristic
Leaching Procedure (TCLP) testing on their combustion
ash. Ash that exhibits toxicity characteristics must then
39
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be managed as a hazardous waste in compliance with
RCRA Subtitle C.
From the perspective of the landfill operator, ash man-
agement issues can be separated into two areas: poten-
tial hot loads and blowing ash. In most incinerators, the
fly ash is mixed with the quenched bottom ash and
delivered to the landfill as a wet mixture with limited free
liquid. This material, if the ash was created during proper
combustion conditions, has a consistency of semiwet
concrete with large fragments of metallic objects. If the
fly ash has been treated with calcium-based flue gas
scrubbing agents (lime, CaO), the ash probably will
have moderate to high pozzuolanic characteristics; that
is, under proper moisture and compaction control, the
ash will harden into a low-strength concrete-like material.
Therefore, dedicated disposal of the ash might be war-
ranted if water is added as appropriate and the ash is
compacted. Under these conditions of proper moisture
and compaction, the ash solidifies within several weeks
of disposal.
By controlling water content, either at the source or at the
landfill, the operator can avoid problems associated with
blowing ash and fires. MSW incinerator ash can be a
desirable material in landfills because of its mechanical
properties, such as strength, compactability, etc. Ash
leachates do not appear to present difficulties for modem
Subtitle D facilities because of the long-term leachate
quality of the ash and stringent waste containment and
leachate collection systems required for modern landfills.
40
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Chapters
Ground-Water Monitoring
5.1 Introduction to Subtitle D
Ground-Water Monitoring
Requirements
Subtitle D ground-water monitoring requirements apply
to new and existing MSWLF units, as well as to lateral
expansions of units. Subtitle D includes limited waivers
if a MSWLF owner/operator can demonstrate that the
landfill is located in a hydrologic setting that will prevent
hazardous constituents from migrating into ground
water. This demonstration must be certified by a quali-
fied ground-water scientist and approved by the director
of an approved state program. Limited waivers require
that no ground-water contamination occur during the
active life of the unit, at facility closure, and during
post-closure. For all other MSWLF units, ground-water
monitoring must be performed.
The time frame for implementing the ground-water
monitoring requirements varies depending on the type
of landfill unit. New units must have an adequate
ground-water monitoring system in place at the time the
new unit begins accepting waste. The compliance date
for lateral expansions and for existing units depends on
their location relative to municipal drinking water intakes.
The regulation specifies a phased approach, dependent
on the relative location of the units, that requires compli-
ance by the dates listed in the regulation or according to
an alternative schedule set by the director of an approved
state program. All existing or laterally expanding
MSWLFs must have a ground-water monitoring system
in place by October 9,1996, at the latest.
The regulation also states that a MSWLF must have a
sufficient number of appropriately located ground-water
monitoring wells. This seemingly vague wording has a
very exacting purposeto account for site-specificity.
Well locations and sampling must be based on the
distinctive hydrologic circumstances at each site. A
"spandex hydrology" approach, that is, a one-size-fits-all
strategy, which assumes that monitoring systems at
different landfills will be the same, is unacceptable.
Ground-water flow and hydrogeologic conditions might
be similar at some sites, but the variety of circumstances
that affect the potential movement of pollutants from a
MSWLF must be assessed individually. Unsupported
assumptions, such as presupposing that ground-water
flow at a site is parallel to the topographic gradient, can
be erroneous and costly.
Ground-water monitoring systems must be capable of
yielding samples from the uppermost aquifer, that is, the
highest water-bearing strata that can release water in
usable amounts (usually, to the water-table aquifer).
Samples must be representative of ground-water quality,
particularly at the point of compliance, which is usually a
hydrologically downgradient point that any potential pollut-
ant plume is expected to intersect. Determination of the
point of compliance can be particularly important de-
pending on site conditions (see Sections 5.3 and 5.3.2)
and is specified by the director of an approved state
program. In addition, representative background water
quality must be able to be ascertained from the wells in
the monitoring system. The effort to delineate the natu-
rally occurring, ambient ground water has potential bene-
fits for the landfill owner/operator, particularly in
instances where naturally high background concentra-
tions of chemical pollutants can be identified.
Subtitle D states that the appropriate number, location,
and depth of monitoring wells should be based on site-
specific data, including ground-water elevation meas-
urements, stratigraphy, and measurements of aquifer
parameters (such as hydraulic conductivity, transmissiv-
ity, and storage capacity). The regulation also states that
each landfill unit should have a separate ground-water
monitoring system, although multiunit systems may be
used instead of separate monitoring systems at each
MSWLF unit if approved by the director of an approved
state program. Each monitoring system must be certi-
fied by either a qualified ground-water scientist or the
director of an approved state program.
Sections 5.2 and 5.3 discuss Subtitle D ground-water
monitoring requirements in the context of ground-water
movement, the types of pollutants commonly found at
landfills, and potential pollutant transport. These factors
determine to a large degree where ground-water moni-
toring wells should be located.
Construction of ground-water monitoring wells, including
selection of well locations (Section 5.4), installation of
wells (Section 5.5), well development and maintenance
41
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(Section 5.6), and abandonment of wells (Section 5.7),
is then described. Section 5.8 discusses documentation
requirements for ground-water monitoring. Section 5.9
presents ground-water sampling techniques, as well as
monitoring of the vadose zone, which is the predomi-
nantly unsaturated zone between the ground surface
and the ground-water zone. Also included in this section
are methods for ground-water elevation and aquifer pa-
rameter measurements. Section 5.11 provides an over-
view of sample analysis, with an emphasis on statistical
significance. Finally, Sections 5.10 and 5.12 discuss de-
tection and assessment monitoring, two types of monitor-
ing specified in Subtitle D. For more information on
ground-water monitoring methods, see U.S. EPA (1993c.)
5.2 Overview of Ground-Water
Movement
The following discussion covers selected concepts con-
cerning ground-water flow, focusing on the relationship
between ground-water movement and the design of
MSWLF ground-water monitoring systems. Ground-water
flow concepts underlie the fundamental principles of
contaminant plume migration, the location of the point
of compliance, and release characterization and reme-
diation. For more comprehensive reviews of the princi-
ples of ground-water hydrology, refer to Freeze and
Cherry (1979).
Historically, ground-water flow has been not well under-
stood, as evidenced by early environmental laws, which
described all ground-water flow in terms of underground
rivers and streams. In reality, there are few underground
rivers and streams where ground water moves through
channels or conduits. (Such constrained flow paths do
exist, such as fractured rock or cave systems in karst
regions, but these ground-water environments are not
prevalent at most landfills near the ground surface.)
More commonly, ground-water movement is laminar
(occurs through small, interstitial spaces [intercon-
nected voids] between solid, granular particles) and is
of low velocity. Turbulent, high-velocity ground-water
movement can exist in fractured rock, karst terrain, or
certain gravel systems, or near pumping wells. More
often, however, ground-water flux is nonturbulent.
5.2.1 Hydraulic Head, Hydraulic Gradient,
and the Water Table
Ground water flows from regions of high hydraulic head
to low hydraulic head. Total hydraulic head is the sum
of elevation head (potential energy expressed as a dis-
tance above a reference plane) and pressure head (ex-
pressed as a depth below a free-water surface). In
regions with little vertical ground-water movement, the
slope of a water table, which is the interface between
the vadose zone and the ground-water zone, is a meas-
ure of the change in total hydraulic head with distance.
This change in hydraulic head with distance is also
called the hydraulic gradient. The hydraulic gradient can
be thought of as the driving force for ground-water flow.
Therefore, in the absence of vertical ground-water flow,
water will move horizontally from regions of high water-
table (or piezometric) elevations to low elevations. With
respect to landfills, the uppermost aquifer is of primary
concern; thus the slope of a water table will provide a
first approximation of the direction of ground-water flow.
Because the slope of the water table may change with
time, temporal considerations can be of particular con-
sequence to the final spatial design of a ground-water
monitoring system at a MSWLF. If vertical ground-water
flow also is present at a site, both horizontal and vertical
hydraulic gradients are crucial for interpreting ground-
water flow under a landfill.
In the aqueous phase, the water-soluble components of
leachate from a landfill move similarly to any infiltrating
water. The leachate will percolate down to the ground-
water zone and be transported according to the hydrau-
lic gradient. As the leachate plume moves with the
ground water, mechanical dispersion will occur, spread-
ing the plume longitudinally (in the direction the plume
is moving) and transversely (perpendicular to the plume
movement). Typically, longitudinal dispersion is greater
than transverse dispersion; this difference is normally
accentuated in highly permeable zones.
5.2.2 The Ground-Water/Surface' Water Link
Ground-water flow and surface-water drainage sur-
rounding landfills are closely linked. The ground-water
monitoring system designer needs to understand the
links between surface water and ground water to design
a sensible ground-water monitoring system. Key to this
understanding is a knowledge of gaining and losing
streams. In gaining streams or perennial streams, the
stream gains water from the adjoining aquifer because
the water table in the aquifer is higher than the water
level in the stream itself. Ephemeral streams, or losing
streams, exist where the water table is lower than the
water level in a stream (or bottom of the streambed if
the channel is dry). Water can infiltrate from a surface-
water channel to the ground water or can be supplied
from the subsurface to a gaining stream. Consequently,
pollution from a surface discharge can become a ground-
water problem or vice versa. For example, any water in
surface channels under losing-stream conditions will
tend to move downward. Surface discharge of liquid
pollutants adjacent to landfills with deep water tables will
create downward migration of contaminants to ground
water, and the resultant pollutant plume could be mis-
taken for leachate originating from the landfill.
Figure 5-1 shows a gaining stream with adjacent ground
water flowing from high hydraulic head to lower hydrau-
lic head in the stream. Note that water is also flowing
42
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upward under the stream; although topographically up-
ward, the flow is moving hydraulically from a higher
hydraulic head to a lower hydraulic head. Piezometers,
which are tubes or small-diameter wells used to meas-
ure pressure, can be placed at different depths to ascer-
tain vertical head differences and vertical direction of
flow. Head differences between two vertically displaced
piezometers could indicate upwelling water or downward-
moving water. Knowledge of these types of movements
near landfill sites is profoundly important.
Siting landfills near losing streams can result in several
problems. For example, many regions' landfills often
have been sited in the cavities left by sand and gravel
operations or other excavation activities. In arid regions
of the United States, sand and gravel operations often
are conducted in dry, ephemeral streambeds. Stream-
beds typically have potentially high infiltration rates and
serve as ground-water recharge areas when stormwater
flow occurs. In addition to the obvious drawback of pos-
sible intermittent flooding of a landfill near the streambed,
the water tables in recharging areas become elevated,
or mounded, during storms. When the water table
mounds, the distance from the ground water to the
bottom of the landfill decreases, and ground water can
even flood a landfill from below. In addition, in regions
of the United States where glaciation has occurred,
sand and gravel companies have often mined in eskers,
which are remnant fluvial features possessing high hy-
draulic conductivity. These mined areas, when aban-
doned, often have been used as landfills. Because
eskers are primary aquifers in glaciated terrain, landfills,
with their potential pollutants, and aquifer systems might
be in close contact.
5.2.3 Factors Affecting Point-of-Compliance
Selection
Subtitle D requires that MSWLFs be designed in one of
two ways. Either (1) specifications detailed in the regula-
tion for a particular type of liner and leachate collection
system must be met, or (2) the design must ensure that
the concentrations of certain chemicals listed in the regu-
lation do not exceed specified maximum contaminant lev-
els at the relevant point of compliance. (The second design
option may be used in states with approved programs or
by petition in states without approved programs.) Design
criteria are discussed in more detail in Chapter 3. The point
of compliance for a MSWLF, specified by the director of an
approved state program or set at the waste management
unit boundary in states without an approved program, is
based on hydrogeological characteristics of the area, physi-
cal and chemical characteristics of the landfill leachate,
ground-water quality and use, and other relevant factors
specified in the regulation.
Ground-water flow
Figure 5-1. Example of a gaining stream (U.S. EPA, 1987a).
The purpose of establishing a point of compliance is to
locate a measurement point where any ground-water
pollutant from the landfill will be sure to pass, and where
a representative sample can be obtained. Although de-
termining a point that is hydraulically downgradient
seems to be straightforward, many complicating factors
can make selection of a point of compliance difficult. For
example, if an area is located near a losing stream or
recharge area, the hydraulic gradient and direction of
flow could vary over time.
Other factors that can affect the direction, velocity, and
water level of ground water include well use and tidal
cycles. For example, seasonal fluctuations in ground-
water pumping, such as those associated with agricul-
tural water use, can alter and even reverse the direction
of flow. Ground-water and landfill leachate flow can be
slowed, stopped, diverted, or sped up.
Thus, ground water near a landfill cannot be assumed
to be a static system that is only affected by leachate or
water moving through the landfill. If head conditions vary
over time, the optimal point of compliance also will vary.
Therefore, in situations where temporal changes in hy-
draulic head occur, a greater density of monitoring wells
will be needed at and around the landfill facility to account
for ground-water movement in several directions.
5.2.4 Subsurface Heterogeneity
So far, this discussion has assumed that a homogene-
ous, isotropic aquifer surrounds a landfill siteone in
which hydraulic conductivity at any point is of the same
magnitude in any direction, and hydraulic properties are
uniform at different points in the subsurface. Actual land-
fill sites can be quite heterogeneous in the subsurface
environment and may include clay lenses and other
barriers to ground-water flow. Faults, fractures, and sec-
ondary porosities, such as animal burrows, worm holes,
43
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or plant root perforations, can redirect leachate move-
ment and change the rate of vertical migration.
Heterogeneous hydraulic properties in an aquifer can be
the result of spatial variation in geologic structure and
materials. This variability can include aquifers on top of
other aquifers, as shown in Figure 5-2. The top, uncon-
fined water-table aquifer must be monitored according
to the Subtitle D regulations. An underlying confined
aquifer also is shown in the figure, sandwiched between
confining layers of soil or other geologic materials.
A confined aquifer is also known as an artesian aquifer,-
and a well placed in this type of aquifer is referred to as an
artesian well, named after a town in France where this type
of well was first used. If the pressure is high enough in an
artesian aquifer, the water will flow out freely at the top of
the well. Artesian aquifers are rarely uppermost aquifers
because they are sandwiched between confining layers
(aquicludes) or semiconfining layers (aquitards). Although
the Subtitle D regulation states that monitoring is required
in the uppermost aquifer (typically an unconfined or water-
table aquifer), an underlying, confined aquifer also can
become contaminated.
A confined aquifer, bounded by aquitards that can trans-
mit water from overlying or underlying aquifers, is
termed a leaky aquifer. If municipal water supply wells
twere drawing water from a leaky confined aquifer, the
reduction in hydraulic head would encourage flow into
the leaky aquifer from overlying and underlying water-
bearing strata. Any landfill leachate present in a nearby
shallow, unconfined aquifer would be influenced to
move vertically downward at these types of sites. In this
situation, monitoring may be required in an underlying
aquifer as well as in an uppermost aquifer.
'.'?.'£ Aquifer A -."?
-'SB-------
;: Aquifer B '?:
^IS&S
;.>VS" Aquifer C;.'A;
Figure 5-2. Example of geologic heterogeneity, with one
aquifer above another (U.S. EPA, 1987a).
5.3 Pollutants at Landfills
5.3.1 Overview of Types of Pollutants
Pollutant migration depends not only on the complexi-
ties of ground-water movement, but also on the physical
and chemical characteristics of the pollutants them-
selves. These characteristics can determine pollutant
transport to a large extent. Historically, many different
pollutants have been dumped into landfills (e.g., liquids,
caustic materials, pesticides, and sludges, many of
which are hazardous). In new and future landfill units,
hazardous pollutants probably will be less of a problem
because of better landfill construction and control of the
types of wastes disposed at landfill sites.
Several classes of chemicals typically found in landfill
leachate might be detected in a monitoring well located
on the leading edge of a contaminant plume. One im-
portant chemical parameter that might be an early warn-
ing of the potential migration of other landfill pollutants
is nitrate, which is found at high levels in the breakdown
products of organic material. Nitrates (NO3) typically are
very mobile in the subsurface system and often are
generated readily; thus, the presence of nitrates might
be a good indicator of landfill leachate pollution at some
sites if other nitrate sources, such as fertilizers, are not
located nearby. Increased total dissolved solids (TDS)
in ground water can be another indicator of impending
leachate movement from landfills. In older landfills, a
variety of wastes often can be found in the subsurface.
Many of these wastes are soluble in ground water and
can contribute to increased TDS levels.'Such wastes
found in landfills might include pesticides, solvents, pe-
troleum products, and metals.
5.3.1.1 Aqueous-Phase Pollutants
Aqueous-phase pollutants, which are dissolved con-
taminants that can move in ground water, are important
considerations at all landfill sites. These dissolved chemi-
cal compounds generally move according to the same
principles as water (see Section 5.2): from high to low
hydraulic head and in a laminar fashion, unless the flow
is altered by pumping, fractured rock media, confining
layers, or other factors. Their movement, however, can
be retarded by their sorption onto soil particles.
5.3.1.2 Nonaqueous-Phase Pollutants
Some liquid landfill pollutants generally do not dissolve
in water. These nonaqueous-phase liquids (known as
NAPLs) exist in a separate liquid phase that is immis-
cible with water; that is, NAPLs do not mix freely with
water. NAPLs include petroleum products, such as oils,
diesel fuel, and gasoline; industrial solvents, such as
degreasing agents; PCBs; and other related com-
44
-------
pounds. Historically, NAPLs have been disposed at
many municipal landfills, creating a legacy of tainted
soils and contaminated ground water.
In spite of the inability of these liquids to mix with water,
the compounds that constitute NAPLs can dissolve slowly
in water. Many NAPLs are hydrocarbon mixtures, which
contain hundreds of component compounds; for exam-
ple, gasoline contains 200 to 300 compounds. These
individual compounds have varying aqueous solubilities.
Compounds that dissolve readily in water can be swept
from a subsurface spill, for example, leaving a residual
of less-soluble products in the soil. The movement of'
NAPLs, which differs from general ground-water flow, is
described in Section 5.3.2.2.
5.3.2 Pollutant Transport
5.3.2.1 General Principles
Fortunately, many pollutants in landfill leachate do not
migrate rapidly. Many chemicals bind to soil and other
geologic material, which inhibits their movement. For
example, many metals and pesticides have a tendency
to adsorb onto soil and can be retained in one area for
long periods. But chemicals that tend to mobilize pollut-
ants also can exist in landfill wastes. Mobilizing pollutants
include chelating agents (e.g., soaps), complexation
agents, or solvating agents. These agents restrict the
ability of certain pollutants to sorb onto porous material by
wrapping themselves around a chemical, thus inhibiting
adsorption, allowing mobility, and possibly increasing mi-
gration of landfill contaminants.
An important factor in the mobility of metals is specia-
tion. Speciation is the ability of an element to assume a
certain chemical configuration similar to that of closely
related compounds. Metals can exist in more than one
form in subsurface environments. Different species of
metals have different mobilities, sorption coefficients
(which describe the ability of a compound to adsorb onto
soils), and partitioning coefficients (which describe the
ability of a compound to volatilize, dissolve, or otherwise
change phase). Chromium, for example, can take the
form of hexavalent chromium (including chromate,
bichromate, and dichromate) and trivalent chromium.
The hexavalent chromium species are very mobile in the
subsurface, whereas the trivalent species are relatively
immobile and will not readily move in the subsurface.
Speciation is determined by subsurface oxygen and pH
conditions. Basic solutions are represented by pH
greater than 7, and acidic solutions by pH less than 7.
The term eH is used to represent oxygen conditions.
Zero eH represents a neutral oxygen condition, high eH
represents high oxygen conditions, and low eH repre-
sents lower oxygen (o; reducing) conditions. Shifts in eH
or pH can change the species of chromium and there-
fore its mobility because each metal in the landfill will
move according to its speciation. Other ambient condi-
tions can affect the pH or eH and therefore speciation;
for example, microbiological conditions in the geologic
environment surrounding a landfill can affect oxygen
concentrations and, thus, transport.
In addition to the chemical characteristics of pollutants, other
processes also are important to the transport of pollutants
from landfills. These processes are discussed below.
Advection
Advection involves the mass flux of a fluid or gas from
one region to another according to pressure or head
gradients. Movement by advection in ground water re-
flects pressure/elevation potential (e.g., from high hy-
draulic head to low hydraulic head). For gaseous
migration, advection involves vapor flux from high gase-
ous pressure to low gaseous pressure.
Diffusion
Diffusion is the process by which elements equilibrate.
It results from the Brownian (random) motion of ener-
getic molecules in a fluid undergoing constant collision
with other particles. Diffusive processes are driven by
concentration gradients rather than pressure or head
gradients (as in advection). Diffusion does not result in
the large-scale lateral movement of a fluid body, as does
advection. Rather, diffusive processes are typically
slower than advective processes and generally proceed
four or five orders of magnitude slower in a liquid than
in a vapor. Diffusive processes dominate in media with
low permeability, such as clay landfill liners.
Dispersion
Gas or liquid cannot move in a straight line in geologic
media. Solid particles block flow paths, and the migrating
fluid moves around these particles and spreads out. This
spreading process is called mechanical dispersion.
Spreading occurs transversely (perpendicular to the flow
direction) and also longitudinally (in the direction of flow)
because pore space size varies. Flow velocity will vary as
well. Longitudinal dispersion typically is several times
greater than transverse dispersion.
Other Transport Considerations
Advection, diffusion, and dispersion are the basic
mass transport mechanisms, but other processes also
allow mass transfer, partitioning, and transformation.
These processes include secondary porosity, cosolvent
effects, and particle transport, which are discussed below.
Additional transport mechanisms include sorption, com-
plexation, acid-base reactions, dissolution, and biological
transformation. Discussion of these latter mechanisms is
beyond the scope of this document.
Secondary porosity can be created by naturally occur-
ring phenomena, such as animal burrows, root holes,
45
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worm holes, and shrinking clays, which enhance down-
ward movement of liquids. Acosolvent effect is the change
in aqueous solubility of a compound caused by the intro-
duction of another compound. This effect can create sig-
nificant changes in the rate of a pollutant's migration.
Because a landfill can contain many solvent mixtures, it is
possible that cosolvent effects could occur in MSWLFs.
Particle transport is associated with ground water flow-
ing through fractured rock. Normally, metals or pesti-
cides in a fractured rock media will adsorb onto the sides
of the walls and not move (with some exceptions). If
colloidal material is carried with the ground water, how-
ever, a pollutant may sorb onto the suspended particles
that are flowing with the water and become mobile.
Particles that can serve as vehicles for pollutants in-
clude clay material, asbestos, bacteria, viruses, and
yeast particles.
5.3.2.2 A Special CaseTransport of NAPLs
NAPLs, which are characterized by their inability to dis-
solve in water, were introduced in Section 5.3.1.2. These
compounds pose environmental problems not only be-
cause of their individual toxicity, carcinogenicity, or tera-
togenicity, but also because they can serve as preferred
solvents for other pollutants. Many pesticides, for ex-
ample, have low aqueous solubility but will readily dissolve
in an organic liquid. The preferential partitioning of some
pollutants into any existing fuel, solvent, or oil near a landfill
can dramatically affect the ability of the pollutants to mi-
grate. Movement of certain landfill pollutants is dictated
primarily by whether NAPLs exist in or near the facility,
such as from a service station or other potential source.
Aromatic hydrocarbons, such as benzene, toluene, ethyl
benzene, the xylene compounds, and naphthalene, are
the most water-soluble components of NAPLs and often
will undergo dissolution from a landfill environment. Ap-
pearance of these compounds in ground water could be
a precursor to more widespread petroleum contamina-
tion at a site.
Liquid movement in the vadose zone is dominated by
competing gravity and capillary forces. In regions where
only a small amount of liquid is present, or in fine-grained
material, liquids are held in tension (under negative-
gage pressure) and flow occurs because of capillary
forces. If that same liquid is allowed to build up and the
liquid assumes a positive gage pressure, gravity flow
can begin. The two types of flow are notably different
under capillary flow conditions, a liquid will remain in
small pore spaces; under gravity flow conditions, liquids
will flow readily into large pore spaces.
Aqueous-phase and nonaqueous-phase liquids compete
for pore space in strikingly different ways. Many soils and
other porous geologic materials are hydrophilic; that is,
they have greater adhesive forces with water than with
NAPLs. Because of the hydrophilic environment and
because water has over twice the cohesive force be-
tween its molecules than that of most other liquids, water
will often "win the battle" for small pore spaces in a
NAPL-soaked vadose zone or NAPL-contaminated
aquifer.
As a result of these physical properties, wet, porous clay
layers can block downward-moving NAPLs by occluding
the pore space with water. The blocking action in these
fine pore spaces, often referred to as a capillary barrier,
can deflect NAPLs in seemingly unexpected directions.
For example, if a NAPL is allowed to accumulate above
a capillary boundary, it will build up its own fluid pressure
and eventually will be able to penetrate and move
through the underlying barrier. Such penetration is nor-
mally along narrow bands or fingers that represent paths
of least resistance for the NAPL. If the source of a NAPL
is discontinuous over time and if a leak slows down,
water could reinvade regions previously saturated with
NAPL, leaving isolated NAPL globules in the larger pore
spaces. Generally, movement of these globules would
be inhibited by buoyant or gravity forces because of the
water-NAPL interfacial tension that holds them in place.
Soaps or surfactants, however, can break down inter-
facial tension and mobilize NAPL globules.
When NAPLs are present at a landfill site, their individ-
ual compounds can volatilize to gases in the vadose
zone, dissolve in water, or adsorb onto solids. The ability
of these compounds to partition in these different ways
underlines the importance of calculating a mass balance
at landfill sites. For example, if a pollutant has a high
volatility, it may direct remedial efforts toward the gas-
eous phase rather than the liquid phases. Mass balance
estimates can be useful in quantifying and more accu-
rately representing the distribution of contaminant con-
centrations (i.e., aqueous or nonaqueous), validating
sampling results, and designing remedial alternatives.
Light Nonaqueous-Phase Liquids
An important consideration in understanding the move-
ment of NAPLs at a landfill site is whether they are light
(i.e., lighter than water) or dense (i.e., heavier than water).
The movement of light nonaqueous-phase liquids
(LNAPLs) differs significantly from dense nonaqueous-
phase liquids (DNAPLs) in subsurface environments.
LNAPLs have a specific gravity of less than one, and thus
will float on top of a free surface of water and on top of a
water table. Conversely, DNAPLs, such as solvents,
PCBs, or creosote, will move downward relative to the
ground water around them.
Figure 5-3 illustrates the different ways that light
nonaqueous-phase liquids can exist in the subsurface.
LNAPLs can float on top of the water table if a sufficient
quantity has leaked into the subsurface. In many landfill
environments, however, where relatively little NAPL has
46
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been spilled, a pure product will not reach the water
table. Instead, as LNAPLs leak through the vadose
zone, residual globules will be left and held in this pre-
dominantly unsaturated medium.
A dissolved or aqueous phase also can occur either
when percolating rainwater contacts residual LNAPL in
the vadose zone or, as illustrated in Figure 5-3, when
ground water contacts a floating LNAPL pool in the
saturated zone. In addition, when a water table overlaid
with floating LNAPL rises or lowers, LNAPL globules can
be smeared. This smearing can Increase the contact
area between the water and the LNAPL globules, lead-
ing to Increased dissolution rates.
Dense Nonaqueous-Phase Liquids
DNAPLs have a propensity to move downward relative
to the surrounding water. For example, trlchloroethylene
(TCE) has a specific gravity of 1.46 at room tempera-
ture, which Is about one and a half times the density of
water. Downward-moving DNAPLs that reach the water
table will tend to move toward the bottom of an aquifer.
Figure 5-4 illustrates this downward migration pattern.
The migration of DNAPLs is a function of the physical
and chemical properties of the liquid, the site geology,
and the size of the release. The important physical and
chemical properties affecting migration include the liq-
uid's density, viscosity, solubility, ability to partition into
an organic liquid, volatility, interfacial tension, wetability
to solid surfaces, and ability to have its chemical com-
ponents absorbed. Perhaps the most important geologic
site factor at landfills (aside from clay liners) affecting
the downward gravity movement of DNAPLs is the site
stratigraphy, particularly the presence of any capillary
barriers where small, water-filled pore spaces block mi-
gration. The size of the liquid leak is also important. A
larger leak will have more potential to retain positive fluid
pressure and move downward. Figure 5-5 shows a
schematic in which enough DNAPL has leaked to cause
full vertical penetration through an aquifer. Note that
geologic barriers can cause downward-moving DNAPLs
to deflect in unusual directions; in the schematic,
DNAPLs deflecting off a bedrock surface are moving
opposite from the direction of ground-water flow. Such
unusual movement profoundly changes the way that
point of compliance is defined at a site.
5.4 Selecting Monitoring Well Locations
Two major considerations in siting a ground-water
monitoring system around a landfill are (1) the number
of wells appropriate to the particular site and (2) geo-
logic and stratigraphic conditions that might affect
choice of locations.
Product source
tMM
Product^
entering""^
subsurface !
Top of
capillary
^fringe
Ground-water
I=^>
flow
Ground-water Water table
flow
Product source
Httt
Product
entering
subsurface
Top Of
capillary
fringe
..a .^
Jt
Ground-water
flow
Ground-water
flow
Water table
Product
source
Inactive
Product
at residual
saturation
Pradu
Ground-water Product
c^t> at residual
flow saturation
Water table
Ground-water
=t>
flow
Figure 5-3. Movement of LNAPLs In the subsurface (Palmer
and Johnson, 1989).
5.4.1 Number of Monitoring Wells
An appropriate question to ask at all landfill sites is, "How
many monitoring wells should be put in, and where?" The
spacing and depth of monitoring wells are crucial design
factors and should be based on site-specific charac-
teristics. The geology at different landfills is often pro-
foundly different, and the optimal point of compliance at
even a single landfill can vary with time and water
movement. Therefore, no one rule for the number and
configuration of monitoring wells is appropriate at all
landfills at all times. Likewise, there is no predetermined
number of wells at a site that is appropriate for all
monitoring needs.
Monitoring wells have several purposes. They are used
primarily to extract samples for water quality purposes,
but also can be used to ascertain hydraulic conductivity
of the porous medium and to measure hydraulic head,
47
-------
DNAPL source
tmtt
Ground surface
T , Residual DNAPL
Top of
capillary
fringe «. 3. _...
Water table
Ground-
water flow
Lower *
permeability
strata ^
Dissolved /
chemical^
plume
DNAPL source
ttttt*
To Qf Residual DNAPL
capillary
fringe .». .v.
Water table s_
Ground- c^> DNAPL layer
water flow
_ Lower
Ground- c=> permeability
water flow strata*
DNAPL source
tttttt
Top of
capillary
fringe *
Water table
Residual DNAPL-*
i
' Dense vapors
Ground- cc> DNAPL layer]
water flow ^ -I
* ,-
Lower
Ground- ct> permeability
water flow strata.
'., , '- {
Ois$o!ve
-------
zone, particularly if that zone is thick and horizontally
extensive. Distinctions between an upper water-bearing
aquifer and perched water are made on a site-by-site
basis, usually with the assistance of and/or agreement
with the state.
Perched layers in the vadose zone are important for other
reasons as well. Landfill leachate moving downward can
be held in perched layers under a landfill. Leachate pollut-
ants can build up on this subsurface impoundment, and
horizontal movement of these pollutants will probably in-
crease. As liquid builds up on top of a perched layer,
positive fluid pressure also will build up. A poorly con-
structed well that is drilled through this perched layer can
allow leachate and contaminants to cascade down the
bore hole. Years of benign contamination can be exacer-
bated rapidly when a perched layer is pierced.
Application of proper well construction techniques can
prevent downward flow when a monitoring well pene-
trates a perched water layer. If the local stratigraphy is
known, for example, a well telescoping method can be
used. In this method, a well is drilled in steps. First, a
bore hole is drilled to the low-permeability unit, casing
is installed, and then grout is added between the casing
and the bore-hole wall. Downward drilling is then con-
tinued with a smaller-diameter drill; the upper grout seal
prevents downward leakage from the perched system.
Alternatively, horizontal drilling or slant drilling tech-
niques, discussed in Section 5.5.2, can be used.
5.4.2.2 Presence of DNAPLs
In the vadose zone, NAPLs, if present in sufficient quan-
tities to move downward, can build up on or deflect off
of low-permeability units. Because DNAPLs can, and
almost certainly will, continue their downward move-
ment below the water table, they also can pool up on
low-permeability units in the ground-water zone, such
as a bedrock surface or a clay lens. As with perched
systems, improperly constructed wells that penetrate
through DNAPL pools can allow DNAPL movement
down the bore hole. If enough DNAPL exists to provide
sufficiently positive fluid pressure, DNAPLs will flow
down an open bore hole and can be found at the deep-
est parts of a well.
At many landfill sites, environmental professionals often
are pressured to drill immediately in the "hot spof of
known pollution leakage. As discussed above, however,
drilling in zones of extensive contamination without
clear knowledge of the site stratigraphy can be very
risky. When contamination, particularly DNAPL con-
tamination, is known to exist at a site where the geol-
ogy is not well characterized, drilling operations might
be more safely begun near the suspected outer limits
of a plume. Drilling operations can then be moved to-
ward the region where higher contamination is sus-
pected.
5.4.2.3 Vadose-Zone Monitoring
As already indicated, the vadose-zone (or zone of aera-
tion) is the mostly unsaturated region between the
ground surface and the water table. Although vadose-
zone monitoring is not required by Subtitle D, it is often
extremely useful and is certainly not restricted by the
regulation. In a basic monitoring system located only in the
aquifer, downgradient monitoring wells are placed in the
uppermost aquifer around landfills to identify a leaking
landfill. By the time pollution is identified in a monitoring
well, however, the ground water will have already been
contaminated and remedial costs will be very high.
Vadose-zone monitoring can allow early detection of land-
fill leaks, before ground water becomes contaminated,
thus allowing much more cost-effective remediation.
Vadose-zone monitoring is particularly useful where the
water table is deep and extensive subsurface contami-
nation could occur before a ground-water monitoring
well could indicate a problem. In regions with a very
shallow water table, vadose-zone monitoring might not
be as effective because ground-water contamination
might quickly follow leak detection in the vadose zone.
5.5 Installation of Monitoring Wells
Several handbooks provide detailed information on the
design and installation of ground-water monitoring wells,
including U.S. EPA, 1989a, and the Illinois State Water
Survey (1983.) The following sections provide a brief
description of monitoring wells and their installation.
5.5.1 Basic Components of Monitoring Wells
Subtitle D requires that a rendering of the design and
installation of monitoring wells be placed in the operat-
ing record and that the state director be notified. Atypical
monitoring well is shown in Figure 5-6. The well casing
is a pipe with openings, known as screened intervals,
located near the bottom of the well that allow water to
enter. The screened intervals are either slots in the
casing itself or commercially available, perforated at-
tachments. The well casing includes a plug at the bottom
to prevent sediment from entering the well. Between the
screened area of the well and the bore-hole wall, sand
or pea-gravel, known as filter pack, is added to prevent
fine material from being drawn into the well while per-
mitting water to pass easily into the well. The filter pack
increases the effective radius of a well. Above the filter
pack, in the space between the unperforated casing and
the bore-hole wall, an annular seal of grout is placed to
prevent vertical movement of water (and any potential
pollutants). The grout material is usually either cement
or bentonite clay.
The upper portion of a monitoring well is designed to
provent liquid from entering and contaminating, diluting,
or changing the nature of the water in the well. A surface
49
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Locking casing cap
Vent hole
Protective casing
Ground surface
Well casing
Annular seal
Filter pack
Inner casing cap
Completion depth
Well intake
Plug
Figure 5-6. Components of a typical ground-water monitoring
well (U.S. EPA, 1989a).
capping system is crucial, not only where water other-
wise would penetrate into a well from above but also in
artesian systems, in which water in the well is under
pressure and could escape. The surface construction of
a monitoring well includes inner and outer casing and
caps, surface grout, and installation security measures.
The inner casing that appears at the surface is an exten-
sion of the same casing that runs the length of the well.
Locking, watertight caps for the inner casing are com-
mercially available. Surrounding the inner casing is an
outer or protective casing, often of anodized steel, for
which locking caps also are available. To prevent accu-
mulation of water between the inner and outer casing
(and subsequent freeze-thaw problems), some installers
drill a vent hole in the outer casing. A surface grout or
seal prevents surface water from flowing down the well
and holds surface casings in place. In heavily trafficked
areas, monitoring wells must be built at or below grade.
5.5.2 Drilling
Many well drilling methods are used for installing
ground-water monitoring wells. These methods include
traditional vertical drilling, slant drilling, horizontal drill-
ing, and innovative techniques. This discussion focuses
primarily on vertical drilling. The three basic types of
vertical drilling techniqueshollow-stem auger, direct
rotary, and cable-toolare discussed below and are
shown in Figure 5-7.
5.5.2.1 Hollow-Stem Auger Drilling
Hollow-stem auger drilling is a reliable method for drill-
ing monitoring wells down to 150 feet deep in many
types of unconsolidated material. Indeed, the most ap-
propriate auger method for environmental work is usu-
ally a hollow-stem auger, which generally is attached in
a series of 5-foot sections. A hollow-stem auger has a
bit at the bottom that rotates and brings cuttings up to
the ground surface on the auger flighting (helical metal
strips). The hollow auger allows coring tools to be low-
ered inside the auger flights so that soil core samples
can be taken in advance of the drill bit. Such samples
can be taken through the center of a hollow-stem auger
using devices known as split-spoon samplers, shelby
tubes, or thin-walled samplers.
Typically, the 5-foot sections of hollow auger drills are
connected with bolts. In the past, threaded bolt connec-
tions were heavily greased throughout the system for
lubrication. This practice produced oil and petroleum
residuals in the hole. In current practice, modern com-
mercial lubricants that contain no metals and no petroleum
products are used to ensure clean drill-rig operation.
5.5.2.2 Direct Rotary Drilling
In rotary drilling, a fluid is circulated in the subsurface.
Traditionally, in water production wells, the circulating
fluid was typically a mixture of water and bentonite clay,
known as mud. Mud was used because it could cool and
lubricate the bit, hold up the cuttings, coat the bore-hole
walls, and prevent fluid loss from the bore hole. But mud
can contaminate bore holes and generally is not used
for monitoring wells. Other circulating agents can be
used, including water, foam, and, more commonly in the
monitoring well industry, air. Rotary drills include a tri-
cone bit that grinds the rock; the cuttings are then pulled
up the bore hole by entrainment in the upflowing fluid.
In reverse rotary drilling, fluids circulate down the annu-
lar space and up the central tube of a drilling system, which
is the opposite of straight rotary methods. Higher upward
velocities can be achieved with reverse circulation, allow-
ing heavier, larger cuttings to be brought to the surface.
As mentioned above, air is a common circulating agent
used for environmental work. The air must be well filtered
before it is circulated downhole, so that oil and petro-
leum products do not contaminate the inside of the well
bore. Because circulating air can readily escape into a
geologic formation, an outside casing typically is driven
down the bore-hole wall to reduce the amount of escaping
air into the subsurface. Even in a well-designed system,
however, approximately one-third of the circulating air
50
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Air, water, or
drilling fluid
Auger
'flight
Hollow-stem auger
Direct rotary
Cable tool
Figure 5-7. Schematics of the three basic types of vertical well drilling methods (U.S. EPA, 1992c).
can be lost into the subsurface. Air is circulated usually
at a rate of about 750 cubic feet per minute. If one-third
of that is lost on average into the subsurface system,
250 cubic feet of air per minute is blown into the subsur-
face. Soil gas surveys, which are run concurrently with
air-rotary drilling, will be strongly affected by this distur-
bance in the natural gaseous system.
5.5.2.3 Cable-Tool Drilling
In cable-tool drilling, a cable on a rig alternately raises
and drops a heavy bit. As the bit is lifted and dropped,
it rotates, chopping material at the bottom of the hole
and creating cuttings. The driller concurrently drives a
casing down the hole, preventing loose material on the
bore-hole walls from collapsing. To remove cuttings, the
driller stops the drilling, pulls the bit out of the hole, and
lowers a bucket called a bailer to collect the cuttings.
Cable-tool drilling is a laborious process, particularly as
a hole becomes deeper. This technique, therefore, is
more useful for shallow bore holes drilled in soft, uncon-
solidated materials.
5.5.3 Casings and Screens
Monitoring wells must be cased to maintain bore-hole
integrity and meet design specifications. Just as there is
no one perfect well design, there is no single, perfect
well material for all sites. Screens and casings can be
made of many different materials, with PVC being the
most commonly used at landfill sites. Stainless steel
often is used, although in many corrosive environments
(e.g., acidic, clayey soils) even stainless steel will disin-
tegrate. Another popular type of well casing material is
polytetrafluoroethylene (PTFE), often referred to by the
brand name, Teflon. Although Teflon is thought to be
relatively chemically inert, it is porous and will flow under
compressive pressures. Conse^'ently, slots in Teflon cas-
ing can close under compressive forces, restricting water
movement into older well installations.
Two or three primary monitoring well screen designs
typically are used at landfills. The most prevalent type
used in PVC casings is constructed with a series of
horizontal slots that are directly cut into the casing.
Various aperture widths are available for slotted casings.
Examples of common slot aperture widths include 10-
slot, 28-slot, or 64-slot designs. These designations re-
fer to the thousands-of-an-inch spacing in the slot. The
slotting typically is cut with a circular saw, which means
that the outside slot is longer than the inside slot. Slot
size is usually dependent on the grain size of the sur-
rounding filter-pack material or the naturally occurring
grain sizes in the geologic formation. With finer material,
such as clays, a smaller slot size is used; with sand or
gravel filter-pack material (which can filter out fine ma-
terial), a larger slot size can be used. Slot size is often
just smaller than the grain size of the filter pack. This
sizing allows bulk purchases of slotted screen and filter-
pack material. Other slot configurations are available.
Slots also can be vertical, although this configuration is
less common than horizontal slotting. Another perfora-
tion type is a louvered or shutter-type screen, schemati-
cally presented in Figure 5-8a.
A popular type of monitoring well screen is a continuous
slot, wire-wound design shown in Figure 5-8b. Continu-
ous wire-wound well screen consists of a bevelled wire
51
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en
en
C2 £=1
en c=i
en ca
en en
en en
Shutter-type
screen
(a)
Continuous slot
wire-wound
screen
(b)
Figure 5-8. Examples of a shutter-type screen and a wire-
wound screen (U.S. EPA, 1989a).
that is helically wrapped around small vertical rods.
Designs such as continuous wire-wrapped screen have
a large perforated or open area relative to screen length.
The advantage of this design is that the added opening
per screen length allows lower ground-water entrance
velocities. At lower velocities, sediment is less likely to
be carried into the well, and the dissolved gaseous
content and aquatic chemistry of a sample taken during
ground-water withdrawal will be more representative of
the ground water from which it was removed.
5.5.4 Joints
Casings or well screens usually are available in 5-foot
sections that must be connected through threaded or
glued joints. These connections come in several con-
figurations, shown in Figure 5-9. Several nonthreaded
joint types are less appropriate for environmental
monitoring-well applications because they involve
cementing agents that have the potential to contaminate
water samples. Nonflush joints, which extend radially
outward from the casing, are not recommended for
monitoring wells because they restrict the available an-
nular space for filter-pack material, grout, and/or tremie
tubing (pipe used to direct materials such as filter pack
down the bore hole). With a nonflush joint, the joint
sleeve, which extends into the annular space, could
potentially intercept falling filter-pack material or grout
and cause bridging during emplacement. Bridging oc-
curs when falling material forms a span between the
casing and the bore-hole wall, leaving an underlying
cavity.
Threaded joints are now fairly standard for casing con-
nections and are available in PVC or metal with a small
O-ring for better sealing. A threaded joint is the preferred
way to join casing and screen together, but different size
threads are available; mismatching thread size must be
avoided. A standard-size thread meeting American So-
ciety for Testing and Materials (ASTM) criteria is now
available for well casing and screen joints.
coupling
Threaded casing
(joined by threaded
couplings)
Bell-end casing Plain square-end casing
(joined by solvent welding) (joined by heat welding)
coupling
Flush-joint casing Threaded flush-joint casing Plain square-end casing
(joined by (joined by threading (joined by solvent
solvent welding) casing together) welding with
couplings)
Figure 5-9. Various types of casing Joints (U.S. EPA, 1989a).
5.5.5 Filter Packs
The particle size of filter-pack material depends on the
aperture size of the slots or perforations in the screen.
Typically, clean graded, kiln-dried sand is used as filter-
pack material to avoid introducing contaminated mate-
rial into the well bore. Sand should be added to the bore
hole through a tremie tube, which directs the material to
a proper depth, inhibits the gravity separation of granular
material according to particle size, and reduces bridging
of the material. Filter-pack material can be prepackaged
and enclosed between an inner and outer well screen.
Prepack filters/screens that are configured in this way
are attached to the bottom of the casing and lowered
down the bore hole in the same way as a normal well
screen. During installation, the bore-hole environment
must not be contaminated; any prepack, screen, or cas-
ing must be clean before placement and handled with
clean gloves during placement.
5.5.6 Grouting
Grouting in the annular space outside the casing is
necessary to prevent vertical movement of water or
other fluids along the bore-hole wall. Two generic types
of grout are available: bentonite clay or cement grout.
Pelletized bentonite grout is used to seal annular space
below the water table, whereas a bentonite clay slurry
is used to seal regions in the vadose zone. Adequate
52
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time must be allowed for clay hydration, and pellets or
slurry should be tremied down the hole when possible.
Cement grout can be used as an alternative to clay
grout. A cement-grout mixture should not be too lean
or too rich; otherwise, fractures, cavities, or other void
spaces might be produced. Figure 5-10 shows cavi-
ties, cracks, and fractures in improperly installed an-
nular seals. Grout shrinkage or improperly prepared
grout also will create cavities through which liquids
can flow vertically along the well bore. In addition,
because the hydration of clay or cement can produce
heat, caution should be exercised if any of the well
material is thermally sensitive.
5.5.7 Well Surface Considerations
5.5.7.1 Surface Cap and Protective Covering
The top of a monitoring well must be clearly marked and
accessible, must protect the well from impact and van-
dalism, and must prevent surface water from draining
down the well bore. The components of the surface
protection system include surface protective grout, inner
casing and an inner casing cap, outer protective casing
with a locking cap, and bumper guards or other forms of
protection from vehicular traffic.
Surface grout, typically cement, is mounded slightly
above grade to discourage ponding of surface water at
the wellhead. In a cross-sectional view, Figure 5-11
shows that the surface grout is slightly wedge-shaped
(although slanted sides should not be pronounced) to
avoid frost heaving in colder climates. The cement sur-
face grout is typically 8 inches to 1 foot deep and ap-
proximately 1-1/2 to 2 feet in diameter around the inner
casing.
The inner surface casing is an extension of the casing
that runs to the bottom of the well and must be capped
at the top with a watertight seal. Commercially available
caps have gaskets that expand to block and seal the top
of the inner casing when a butterfly nut is tightened on
the top of the cap. When the well can be built above
grade, an outer surface casing that surrounds and pro-
tects the inner casing is built. This outer protective cov-
ering is usually a short, wide, anodized pipe with a cover
that can be locked to restrict access. Some wells must
be built to allow traffic to pass over the well. In this case,
the well is constructed below grade, with the inner cas-
ing protected by a flush-mount outer well casing (a small
watertight manhole) or a heavy-duty utility box. Opti-
mally, the area immediately surrounding the well instal-
lation should be mounded to discourage ponding of
surface water near the well. The cover of the manhole,
box, or outer casing should be watertight, as should the
cap of the inner well casing.
In a landfill environment, potential subsidence problems
can destroy the integrity of a monitoring well. Wells
should not be located in areas susceptible to subsidence.
Otherwise, cracks and fissures can form along a well
casing, allowing pollutants to enter in or near the well.
Pollutants entering the well environment in this way not
only defeat the purpose of monitoring, but also exacer-
bate pollution.
Tampering also can be a problem. Locking covers can
prevent tampering, but locks can corrode. Plastic mate-
rials that cover locks are available commercially. The
outer casing can be anodized metal, which also pre-
vents corrosion. It is usually a good idea to purchase a
set of locks that all use the same key.
Proper labeling of monitoring wells is important for sev-
eral reasons. Monitoring wells must be distinguished
from underground storage tank fill lines, for example.
Annular seal
Filter pack> ~i =
a) Between casing
and seal material
b) Through seal c) By bridging
material
Figure 5-10. Void spaces produced by Improperly installed
annular seals (U.S. EPA, 1989a).
Figure 5-11. Correct wedge shape for surface grouting (U.S.
EPA, 1989a).
53
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Also, different monitoring wells must be distinguished
from each other; therefore, labeling only the cap can
create problems if the well caps are shuffled. Monitoring
wells should be labeled on immovable parts of the well.
Documentation also is important for surveying and lo-
cating the well, particularly for vertical elevation of a well.
A key element in assessing ground-water flow and di-
rection is the relative water-level difference in several
wells at a site. Because ground-water gradients can be
somewhat flat under many landfills, relative water levels
must be measured correctly to within at least a tenth of
an Inch, necessitating accurate vertical surveys.
5.5.7.2 Well Construction and Site Selection
Safety
In traffic areas, bumper guards around monitoring wells will
help protect aboveground installations from damage.
Bumper guards come in various sizes and strengths and
are typically constructed for high visibility and trimmed with
reflective tape or highly visible paint containing reflective
material.
Drilling operations should give overhead powerlines
wide berth, and, to prevent electrocution, rigs should
move only when the mast has been lowered. Precau-
tions also must be taken when drilling near subsurface
utilities, such as water, power, and sewer lines.
Special dangers are associated with drilling in the mid-
dle of landfills. Because many drill rigs are very heavy,
caution is required at certain sites to avoid subsidence.
Also, drilling through material in an old landfill can be
dangerous; some municipal landfills historically had
hazardous material deposited, and serious drilling risks
are possible. If it is necessary to drill under a landfill,
slant or horizontal drilling techniques often are war-
ranted to avoid drilling through old refuse. Occasionally
explosive material has been deposited in landfills. If
explosive material is suspected of being mixed with soils
at a site, special techniques should be used to test the
soils to determine whether an explosion is likely to occur.
During all types of drilling activities, material brought out
of the ground might be contaminated and therefore might
require special handling and disposal.
5.6 Well Development and Maintenance
After a well is installed, well development and mainte-
nance activities ensue. Plans for well development must,
under Subtitle D regulations, be placed in the operating
record, and the state director must be notified. Several
types of development and maintenance activities are
likely to be required. Residuals from the drilling process,
such as fine, suspended particles, can be present in
bore-hole water and eventually inhibit water movement
into the well. The well development process is designed
to remove these particles. Particles are removed by
creating a surging action of water in and out of the well
screen and filter pack. Also, over time, encrustation can
build up in some wells; for example, calcium carbonate
can be deposited from "hard" water systems. Biological
clogging also can occur in the form of algal or microbial
mats in well screens or well bores. Physical scraping or
swabbing can remove encrustation or biological clog-
ging. Another well maintenance problem, particularly
significant at landfills because of the potential for ground
subsidence and settling, is casing failure and collapse.
The following sections describe in detail these and other
development and maintenance activities.
5.6.1 Techniques To Clean Wells and
Control Problems
The purpose of withdrawing water from a monitoring well
is to obtain a representative sample. Water that is rep-
resentative of an aquifer is never assured, but definite
steps can be taken to secure the best sample possible.
Before a water sample is withdrawn from a well, any
stagnant water must be purged. Fine material in the well
also can lead to unrepresentative water samples. Like-
wise, encrustation or clogging that occludes portions of
the well screen can cause incoming water to have
greater velocity and thus a greater potential to change
pH, carbon dioxide levels, and chemical concentrations.
Therefore, well development and maintenance goes be-
yond aesthetic considerations; procedures are designed
to enhance the chances of gathering samples that are truly
representative of ground water near the well.
5.6.1.1 Physical Methods
Surging is mentioned above as a technique to remove
fine sediments during the well development process.
Often, wells are cleaned using a surge block, which is a
metal disk that acts somewhat like a plunger. Surge
blocks for production wells historically were constructed
of wood; for monitoring wells, however, wooden surge
blocks are not used because porous material can trap
and hold contaminants.
The surge block is alternately pulled up and pushed
down the well. When pushed down, it propels water out
of the well through the screen; when pulled up, it conveys
water in. The in-and-out motion of water caused by
surging dislodges fine material. This method of well
development also can break up encrustation.
Another well development method is jetting, in which
water is shot at high velocity through well perforations.
Commercially available jetting tools discharge an out-
ward stream of water through a well screen; return flows
come back into the well above and below the outward
jet. The tool is lowered and raised in the well bore, with
nozzles on the jetting tool typically pointing outward in
several directions. The outward and inward flow create
54
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the necessary in-and-out surge to break up and extricate
clogging particles.
Sonar jetting also can be used to reduce encrustation
and particulate buildup problems by cleaning out perfo-
rations in the well. Sonar jetting uses very small explo-
sive charges to set up shock waves in the water. The
shock waves find weaknesses in the casing; because
the encrusted perforations are weaker than steel pipe
casing and many other casing materials, the shock
waves pass through them and knock the buildup out of
the perforations. The sonar jet charges can be accu-
rately positioned to clean only the portion of the well that
needs to have perforations opened.
Another technique is air development or air eduction,
which is a two-step process. In step one, air is blown
down a pipe (eductor pipe) placed in the well. The
bubbles flow back up and entrain water, producing a
slight pumping effect and drawing water out of the for-
mation. Step two involves shutting off the air flow, allow-
ing air pressure to build up, and lowering the eductor
pipe down the well. The air is then suddenly turned back
on, and a large slug of pressurized air forces water from
the well bore into the formation, creating a surging ac-
tion. As the air pressure decreases, the eductor pipe is
raised and the process is repeated.
For encrustation problems, swabbing techniques, such
as brushing and scraping, often are used as a quick
technique for breaking up material that clogs a well.
5.6.1.2 Chemical Methods
Certain chemicals also can be used to control encrusta-
tion and other problems. Yet some of the chemicals used
to control these problems in production wells should not
be used for monitoring wells because if these particular
chemicals enter a monitoring well, they can change the
acidity or the chemical constituents in the well water or
introduce pollutants.
Chelating agents (such as soaps), wetting agents, sur-
factants, or inhibitors have been used to unblock ob-
structed well screens. Acids could be used to degrade
calcium carbonate encrustation, but the change in pH
caused by acids might mobilize some contaminants in
the subsurface.
5.6.2 Decontamination
When a well is drilled, a pathway is opened in the earth.
While samples of subsurface fluids can be extracted
through a well and remedial substances such as nutri-
ents can be introduced, unfortunately, contamination can
be released into the subsurface from an unclean well.
Pollution problems must not be exacerbated by im-
proper decontamination of drilling and sampling equip-
ment.
Wherever possible, drilling or sampling operations
should begin outside of the hot spot of recognized pol-
lution and proceed toward the hot spot; equipment
should be decontaminated after each hole is drilled or
each sample taken. If the regions of highest pollution are
left until last, contamination is less likely to be carried
outward from the hot spot.
Decontamination involves specific procedures. The proc-
ess of washing materials can generate contaminated rin-
sate, which becomes a pollutant. Minimizing the rinsate
generated usually is a cost-effective measure during the
cleaning of drilling and sampling equipment. A decon-
tamination area usually is established at a drilling or
sampling site, which often is fenced or gated and locked,
or otherwise secured. An impermeable ground cover
such as plastic should be spread on the ground to catch
any run-off at these areas. A three-bucket method of
washing and rinsing often is used to minimize the water
generated from the cleaning process.
Anything put into a well or bore hole, such as bits, auger
flights, bailers, pumps, samplers, clamps, or tremie
pipes, should be decontaminated. Heavy equipment,
such as drill rigs, also should be decontaminated. Work-
ers should use clean, protective gloves during drilling
and sampling operations. Porous gloves and ropes and
other porous materials cannot be reused; they should
be thrown away after use. Drilling equipment normally
is decontaminated after each hole is drilled, although
equipment can be washed more often as needed. Every
time a new hole is drilled, everything used in the drilling
process must be washed. For sampling equipment,
every time a new sample is taken, the sampling imple-
ments must be washed. A dedicated pump that remains in
a well is particularly advantageous for sampling because
repetitive cleaning is avoided. Disposable bailers are also
available that should be discarded after a single use.
Quality assurance procedures for decontamination in-
volve checks to ensure complete cleaning. To check the
effectiveness of decontamination, the final rinse water
can be tested periodically. In this procedure, a sample
of the final rinse water is collected and sent to an ana-
lytical laboratory to determine its cleanliness and chemi-
cal composition. This procedure is an "after-the-fact"
determination, however, because there often is a lag
time between sampling and receipt of analytical results.
Another type of decontamination testing is wipe testing,
in which a piece of gauze or a cotton ball is used to wipe
the equipment. The cloth is then put in a container and
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sent to a laboratory for analysis. An emergency shower
for human use also is requisite at sites where the pres-
ence of hazardous material is suspected.
5.7 Well Abandonment
The design for decommissioning any monitoring wells
must, under Subtitle D regulations, be placed in the
operating record, and the state director must be notified.
If a well must be abandoned, certain procedures are
necessary to ensure that the well does not become a
conduit for the downward flow of pollutants. Most impor-
tantly, the well must be sealed throughout its length to
prevent vertical migration of water. The decision to either
perform maintenance on a failing well or abandon it
entirely often is difficult.
The procedures for sealing a well to prevent vertical flow
sometimes are dictated by individual states, but all in-
volve grouting the well bore. In some states the entire
well length must be grouted, whereas in others, selected
layers in the well can be sealed. Because both the
outside and inside of a well casing are potential conduits
for flow, both areas must be grouted. Grouting of both
areas can be achieved by removing the well casing, if
bore-hole collapse is not anticipated. Casing removal
can be difficult, but for shallow wells with bentonite clay
grout, a large-diameter hollow-stem auger might be able
to over-drill the entire monitoring well, simplifying re-
moval. Grout must be placed while the casing is being
removed to help prevent bore-hole collapse.
If the casing is not removed, grout must be injected into
both the well and the annular space between the casing
and bore-hole wall. To inject grout between the casing
and the bore-hole wall, the casing might need to be
perforated. Perforating tools cut the casing by either
shooting pellets or burning holes sideways through the
well and into the formation. Cement grout is then
pumped into the annular space.
5.8 Documentation
Careful documentation is required by Subtitle D during
all stages of well drilling, completion, operation, and
abandonment. Procedures and information important to
record include: drill-hole logging and core sampling,
which indicate lithology and the stratigraphy of a site;
geophysical testing and data; soil sampling methodology;
water sampling methodology; sampling results; and well
design details. Chain-of-custody procedures for sam-
ples are required to ensure that the source of informa-
tion gathered is verifiable. It is important to regulatory
agencies that the locations of abandoned wells be
known. Abandonment notification should be considered
and may be required in some states.
5.9 Ground-Water and Vadose-Zone
Sampling
Subtitle D requires that ground-water samples be taken
from the saturated zone, specifically from "the upper-
most aquifer," and describes appropriate procedures
for sampling monitoring wells for specific hazardous
constituents. The rule includes requirements for deter-
mining background ground-water quality, ground-water
elevations, and number of samples to be collected.
Methods for sampling these parameters in the saturated
zone are presented in Section 5.9.2, along with specific
issues that should be considered when monitoring in the
saturated zone. Samples also can be taken from the
vadose, or unsaturated, zone, as discussed in Section
5.9.1. Screening techniques, designed to optimize sam-
pling, are beneficial at most sites.
5.9.7 Vadose-Zone Sampling Techniques
Vadose-zone sampling is associated with three phases:
a solid phase (soil), a liquid phase, and a gaseous or
vapor phase. Samples from all three phases can be
taken, as discussed below. The basic advantage of
vadose-zone sampling is that it can provide advance
warning of ground-water pollution, and thus may reduce
or eliminate the need for remediating ground water.
5.9.1.1 Soil Samples in the Vadose Zone
Soil samples can be taken from a range of locations in
the soil profile. Shallow sampling methods are available for
soil material, and deeper methods for aquifer solids, which
are usually carried out with drill rigs. Shallow samples can
be taken with hand augers, a brace and bit, a post hole
digger, or coring devices. For deeper samples, split-spoon
samplers, thin-walled samplers (sometimes referred to as
shelby tubes), California ring samplers, and other coring-
type devices that can be driven down the center of a
hollow-stem auger can be used.
5.9.1.2 Liquid Samples in the Vadose Zone
Liquid samples in the vadose zone typically are withdrawn
through a ceramic cup lysimeter, which draws pore water
into a porous cup under negative gage pressure and
collects the water. Extracting water samples from the
vadose zone is very difficult because volatile constituent
concentrations are perturbed by the partial vacuum ex-
erted on the water, leading to nonrepresentative samples.
There are PTFE (Teflon) cup lysimeters available, but
these cups have a larger pore size and maintain lower
vacuum pressures than ceramic cups; thus, Teflon
cups are less effective than ceramic cups in very dry
conditions. Typically the lysimeter is placed in a hole
56
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surrounded with fine silica flour so that any water in the
soil is drawn into the flour. When negative pressure is
created inside the lysimeter with a pump, water is pulled
out of the ground into the cup.
Another type of lysimeter is the pan or glass-block
lysimeter, which uses the downward, free gravity flow of
water to fill a flat container or collection pan. Because
free drainage is required for the successful operation of
pan lysimeters, these devices are used only in ex-
tremely wet vadose-zone environments. At some land-
fills natural geologic or landfill design features can aid in
collecting escaping leachate.
The same type of ceramic cup used in a lysimeter can
be used in a tensiometer to measure the matrix potential
(negative water pressure) in the vadose zone. The ma-
trix potential is an indicator of moisture content. The
tensiometer is a water-filled tube with a ceramic cup on
one end hooked to a pressure gage. The instrument is
placed in the ground, and water in the tube is drawn out
of the ceramic cup into the surrounding soil. As soil
moisture and material size decrease, the water volume
and negative gage pressure in the tube increase. The
negative pressure in the tube essentially equilibrates
with the negative pressure of water held in the soil. A
tensiometer can therefore give a quick estimate of soil
moisture variation at a site.
5.9.1.3 Soil Vapor Samples
Analysis of soil vapor is a quick screening method that
can help identify onsite and offsite contaminant plumes
in the landfill environment. Vapor samples can be col-
lected with a probe driven into the ground and hooked
up to a pump or with a passive device containing mate-
rial to adsorb target vapors. In the former method, the
inside of the driven probe is purged of vapor and a
syringe or evacuated container is used to collect a gase-
ous sample. Typically, this vapor sample then is injected
into a gas chromatograph and analyzed. Elevated con-
centrations of contaminant vapors or reduction in oxy-
gen conditions can be indicators of pollution. Passive
samplers usually are buried in the soil to adsorb vapors.
The adsorbent is then exhumed and taken to a labora-
tory where adsorbed vapors are released (normally by
heating) and analyzed.
Soil vapor sampling can be a quick, relatively inexpen-
sive method to screen a site. Because natural biodegra-
dation of some contaminants can occur, carbon dioxide
(CO2) vapor can be used as an indicator of pollution
even for nonvolatile contaminants. The CO2 method is
more effective farther away from a landfill, where offsite
vapors will not be influenced by landfill gas generation.
Some vapor monitoring system designs contain slotted
collection pipes beneath a new landfill. An advantage of
this strategy is that a liquid leak could be dried by air
circulation (evaporation) beneath a landfill, reducing
fluid potential, downward gravity movement, and, con-
sequently, remediation costs. This design is in an experi-
mental stage. The prospects of promoting landfill fires
with the introduction of oxygen or introducing a conduit
for surface-water flow downward if the top of a slotted
pipe were to become damaged are important problems
to be resolved in the development of this technology.
Another gaseous monitoring technique that is very use-
ful in landfills, particularly to identify methane migration,
is a flux chamber, which measures the flux of gases
across the ground surface. If a site has methane prob-
lems, flux chambers can help determine the potential for
migration into structures and the potential for explosion.
The device, with a small, closed dome driven into the
ground, periodically samples the air space under the
dome. To keep the fluxing gases from building up under
the dome, which would inhibit upward movement of
gases, air is constantly circulated under the dome.
5.9.2 Saturated-Zone Sampling Techniques
Sampling in the saturated zone, as required by Subtitle
D, involves measuring water quality, ground-water ele-
vation, and the aquifer parameters of transmissivity and
storage coefficients, as discussed below. These ground-
water measurements are critical to site investigations
and should be considered in the construction of moni-
toring wells. Other sampling considerations, also dis-
cussed below, include sample filtering, sampling at
different depths, and frequency of sampling.
5.9.2.1 Sample Collection Methods for Water
Quality Measurements
Devices for withdrawing water from a well for the pur-
pose of water quality measurements include bailers,
submersible pumps, bladder pumps, and driven wells.
Bailers are similar to buckets or ampules with either
double- or single-ball check valves. As the bailer is
lowered down a well, water is propelled upward through
a valved cylinder. When the bailer is hauled upward, the
ball valve at the bottom of the bailer moves down,
sealing the bottom and permitting a sample to be raised
to the surface. Another type of bailer consists of an open
cylinder with spring-loaded closures on either end. After
this sampler is lowered to the desired sampling depth,
a weighted messenger slides down the haul line, strikes
the trigger for the spring-loaded closure, and closes up
the sampling tube. Bailers are inexpensive, can be made
of a variety of material, and are easy to repair in the field.
Bailers, however, cannot quickly purge a well (particu-
larly a large well), and they require time-consuming
decontamination between samples.
Submersible pumps are particularly useful for purging
stagnant water from a well bore because of their high
pumping rates. They generally are made of stainless
57
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steel and have low operating costs if one pump is dedi-
cated to each well. Submersible pumps are very effective,
are sized to fit small-diameter wells, and can operate at
variable speeds. In addition, some models can operate at
low pumping rates, making them appropriate for sampling
volatile compounds. Submersible pumps, although prob-
ably more versatile, are more expensive than bailers.
The bladder pump is a diaphragm pump. Bladder pumps
operate by injecting and releasing air in and out of a
flexible diaphragm, gently squeezing water to the sur-
face. This method is probably one of the most accurate
for sampling volatile organic chemicals because of its
ability to retain sample integrity. Bladder pumps can be
operated at variable speeds, can be made of different
materials, and are easy to repair in the field. Although
bladder pumps can be effective, they can be expensive
if a source of compressed gas is not readily available.
Driven samplers are specialized, removable drive points
that act as temporary wells. Driven samplers, such as
the cone penetrometer, can allow rapid samples of shal-
low ground water to be extracted (in areas with shallow
water tables and loose, unconsolidated geologic mate-
rial). Driven samplers are particularly useful because
they can take preliminary ground-water samples. These
preliminary samples can serve as a guide to placing more
expensive monitoring wells. Driven samplers are not ac-
ceptable as permanent ground-water monitoring wells be-
cause they are not grouted, and surface water can move
down the side of the probe. This short-circuiting precludes
the long-term effectiveness of driven samplers.
5.9.2.2 Sampling Methods for Ground-Water
Elevation Measurements
Subtitle D requires ground-water elevation monitoring to
facilitate accurate ground-water flow and direction de-
termination. Ground-water elevation measurements must
be made every time a well is sampled, immediately
before well purging. If a sufficient density of water-level
elevations is known, a series of water-level contour
lines, called equipotential lines, can be mapped. Flow
lines can then be drawn perpendicular to these equipo-
tential lines. These two sets of lines are commonly
referred to as a flow net. Water-level contour maps,
which show the elevations of either a water table or a
piezometric surface, indicate the pathways of ground-
water flow at any point in time. The direction of ground-
water flow can change with time at a landfill site;
nonetheless, water contour maps are useful tools. Addi-
tionally, water levels in two adjacent wells screened at
different depths can indicate vertical ground-water flow,
which is crucial information at many sites.
Several monitoring devices are available to measure
ground-water elevations in wells. One of the simplest
methods is an incremented steel tape chalked on its
downhole end. When the lowered tape strikes the water
in the well, the chalk at that particular depth is rinsed off.
When withdrawn, the demarcation of the water level is
visible, and the depth to water can be measured. The
water-level elevation from the top of the well then can
be calculated.
Other devices to measure water level include electric
probes, bubble tubes, and pressure transducers. Electric
probes set off an alarm when water comes in contact with
the probe. The probe is lowered into a well at the end of
an incremented cable, which usually is unwound from a
reel. The water acts as an electrical conductor, complet-
ing a circuit on the probe. Bubble tubes and transducers
measure the pressure at a known distance below the top
of a well. The pressure measurement then can be used
to calculate the depth below a water surface and infer
the water-level elevation.
5.9.2.3 Sampling Methods for Aquifer
Parameters
To predict ground-water velocity, an estimate of subsur-
face hydraulic conductivity must be made. Standard
hydrologic field tests for hydraulic conductivity and other
hydrologic parameters include slug (rate-of-rise) tech-
niques and aquifer (pumping) tests. A common labora-
tory procedure to determine hydraulic conductivity is a
permeameter test; this test, however, is rarely used
because it requires an undisturbed sample of soil or
aquifer material, which is very difficult to obtain.
A slug test allows the hydraulic conductivity in the area
of the well to be estimated. With this type of test many
wells can be analyzed quickly for differences in hydraulic
conductivity at a site. This localized test is performed by
raising or lowering the water level in a well or piezometer
and noting the rate at which the water level recovers to
its previous level. If the screen in the well being tested
extends through and above the water table, the water
level should be lowered, not raised. To avoid physically
removing or adding water during a slug test, a solid,
heavy cylinder can be placed in the well below the water
level and, after equilibration, removed. The water-level
recovery is then measured. This method eliminates the
need for removing and disposing contaminated water.
A pumping or aquifer test examines the properties of a
bigger area than a slug test. Typical pumping tests
measure hydraulic conductivity and coefficient of stor-
age over areas from approximately 5 to 200 meters in
diameter. A well is pumped, and the drawdown of the
water level in nearby wells is observed. Standard equa-
tions for unconfined aquifers are used to determine
transmissivity and storage coefficients, such as Bolton's
equation. The storage coefficient is a measure of the
58
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amount of water stored in the aquifer, and the transmis-
sivity is the hydraulic conductivity times the aquifer thick-
ness.
5.9.2.4 Filtering Water Quality Samples
Subtitle D requires that samples must not be filtered
prior to chemical analyses. Colloidal material (e.g., clay,
asbestos fibers) might be present in some ground water;
if the ground water is moving through fractured rock,
these fine, suspended particles can facilitate the trans-
port of adsorbed pollutants. Nonfiltered sampling provides
information on the presence of these types of materials.
Some states might interpret the need for filtering differ-
ently. In individual cases, there can be strong scientific
arguments for why one might or might not want to filter
a sample. Subtitle D regulations do not preclude doing
both. Analyzing both filtered and nonfiltered samples is
more costly, but having both sets of data available might
be important at some sites.
5.9.2.5 Sampling at Different Depths and
Distances
Because contaminant plumes can move vertically as
well as horizontally, water quality and hydraulic head
often should be measured in both directions. Many own-
ers/operators, to save money, equip a site with only a
minimum number of shallow wells. In many cases, this
supposed cost-saving measure results in higher costs
to the owner/operator because an inadequate number
of wells could miss a contamination event, particularly
one with a strong vertical flow. Remedial costs are al-
ways profoundly higher if a contaminant plume is not
detected early. One deep well with a long screen that
fully penetrates the aquifer is an unsatisfactory solution
to the problem of identifying vertical pollution movement,
because if water enters the well bore from clean portions
of the aquifer, samples will become greatly diluted. The
optimal arrangement at many sites is to install wells that
allow sampling at different depths.
Two types of systems are available for sampling at
different depths, as shown in Figure 5-12. The first is a
multiport sampler. The second is a nested sampler,
either in a single bore hole or in multiple bore holes. A
multiport sampler has a hollow tube that is lowered
through the center of a well. This sampler has multiple
windows or ports vertically distributed along the well
length. An ampule is sent down the center tube, and
when it arrives at the desired port, it is stopped. Activa-
tion of the port and ampule from the surface opens up
the system and permits water to flow and fill the ampule
from that particular interval. Thus, a discrete interval
sample is obtained and hauled to the surface. Multiport
sampling systems are fairly expensive.
When a nested sampler in a single bore hole is used,
several wells, screened at different intervals with grout
between the layers, are installed. Alternatively, depth-
specific wells can be nested in individual bore holes.
This latter method is highly recommended at many sites
because it captures water quality at different depths as
well as vertical water pressures (hydraulic head). If
water is moving upward under a landfill, as indicated by
greater hydraulic head at different depths, the leachate
might not spread quickly. If water is moving downward,
however, the leachate probably will be less constrained.
Vertical head measurement is a very useful tool for
predicting the direction of ground-water flow.
The distance between monitoring wells also is impor-
tant. Several pollutants released at the same time might
move at different rates. This difference in transport
speed is important because samples taken from one
well might indicate the existence of only a single pollut-
ant. Several pollutants, however, could be present in a
plume but be separated because of differences in trans-
port rates. The proper density of monitoring wells is not
easily anticipated; adjustments are necessary in most
monitoring designs. The number, spacing, and depths
of monitoring systems must, under Subtitle D, take into
account site-specific geology and be certified by a quali-
fied ground-water scientist, as defined in the regulation,
or the director of an approved state program.
5.9.2.6 Frequency of Sampling
The Subtitle D regulations state that ground-water moni-
toring must be performed "at least semiannually." The
object is to understand the subsurface system, the hy-
drology, and the spatial and temporal distribution of
contaminants. If sampling frequency is inadequate, it is
possible to sample and not understand the system at all,
to not know the best and most cost-effective remediation
approach, and to be misled by the periodic data col-
lected. For example, if a cave or limestone system
underlies a landfill, it might be better to measure subsur-
face parameters before it rains, while it rains, and after
it rains, rather than just once every 3 or 6 months
because significant changes in water quality and hy-
draulic head often are associated with storm events in
these karst areas. Also, periodic sampling undertaken at
a different frequency than that of a natural periodic
change can make the process appear to be going back-
wards, and the resulting information can be confusing.
Choosing a frequency for sampling is a crucial decision.
It is often best to determine first what frequency is
required to understand the system and then examine
regulatory requirements. Some types of frequent meas-
urements can be relatively inexpensive; for example,
transducer-type water-level measuring devices can be
59
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(a)
Multiport
samplers
(b)
Nested sampler:
multiple wells
single borehole
(c)
Nested sampler:
multiple wells
multiple boreholes
Open
borehole
or filter ^
pack
^M
,
*
*-
Packer or
^
annular seal
- Sampling
ports
u«.
LT-
T
f
*
ll
^
f
' s
.JC.
f
/
'/
=
,
^
»
«H!
/*'
V
Borehole
*"~ well "^
Annula
» "- Scre<
_
_
r seals
3ns
packs
Figure 5-12. Examples of a multlport sampler and two types of nested samplers (Johnson, 1983).
placed in a well and linked to a data logger to assemble
essentially continuous records.
5.10 Detection Monitoring
The monitoring requirements for Subtitle D are divided
between detection monitoring and assessment monitor-
ing. The flow path for required actions is diagrammed in
Figure 5-13, and described below. Detection monitoring
is required by Subtitle D to establish initial background
levels and potential migration of contaminants. The ele-
ments and compounds that must be analyzed include
47 volatile compounds and 15 metals listed in the regu-
lation (see Table 5-1). Detection monitoring must be
performed at all MSWLFs at least semiannually. The
director of an approved state may: (1) specify an alter-
nate sampling frequency, with a minimum of annual
sampling; (2) delete constituents from the list, based on
what is reasonably expected from conditions at the site;
and (3) establish an alternative list of inorganic constitu-
ents that provides a reliable indication of inorganic re-
leases at the site. If a statistically significant increase over
background levels is found for one or more of the constitu-
ents, the owner/operator must establish an assessment
monitoring program (see Section 5.12) and notify the state.
Monitoring programs should be continually reviewed and
modified, if necessary, based on results obtained.
5.11 Statistical Data Analysis
The owner/operator must specify a statistical method in
the operating record, to be chosen from a list of methods
in Subtitle D, to evaluate ground-water monitoring data
for each contaminant. The statistical test chosen must
be conducted separately for each contaminant in each
well. The choices of tests to identify statistically signifi-
cant evidence of contamination are: (1) a parametric
analysis of variance, followed by multiple comparisons
procedures; (2) an analysis of variance based on ranks,
followed by multiple comparisons procedures; (3) a toler-
ance or prediction interval procedure; (4) a control chart
approach that gives control limits for each constituent; or
(5) another statistical test that meets performance stand-
ards.
What is statistically significant? A whole range of an-
swers to this question exist. Subtitle D allows for flexi-
bility regarding analytical methods if proper justification
is given. Steps can be taken to obtain a better under-
standing of water quality data, such as plotting pollutant
concentrations over time. Plots will provide visual infor-
mation on data distribution and variability and will show
outliers from average values.
What sort of distribution does the data have? If there is
variance, is there homogeneity in that variance? A nor-
mal distribution (i.e., bell curve) can be determined by
constructing a simple "box and whisker" diagram or
probability plots; if the median of the values is within an
intercortile range, then there is relative homogeneity of
variance, and the data are normally distributed. A
straight line on a probability plot is another indicator of
normal data distribution. With scant data, it is best not
to assume that the data are normally distributed. A log
normal distribution might, for example, be a better as-
sumption in the absence of sufficient data.
60
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EPA has developed a statistical analysis tool designed
to facilitate the storage, analysis, and reporting of
ground-water data. The Ground Water Information
Tracking System with Statistical Analysis Capability
(GRITS/STAT) can be used to assist MSWLF own-
ers/operators in evaluating ground-water monitoring re-
sults (U.S. EPA, 1992d).
5.12 Assessment Monitoring
If detection monitoring at a MSWLF shows evidence of
a statistically significant increase in an Appendix I pa-
rameter over background levels, assessment monitoring
is required.
Ground-Water
Monitoring Program
> Install monitoring system
(258.51)
> Establish sampling and
analysis program (258.53)
Detection
Monitoring (258.54)
Begin semiannual
detection monitoring for
Appendix I constituents
Assessment Monitoring (258.55)
Sample for all Appendix II constituents
Set ground-water protection standard for detected
Appendix II constituents
Resample for detected Appendix II constituents and all
Appendix I constituents semi-annually
Repeat annual monitoring for all Appendix II constituents
Characterize nature and extent of release
Is
there a
statistically
significant increase
in Appendix I
constituents?
Continue/return to detection
monitoring
Is
there a
statistically
significant increase
in Appendix II
constituents over
ground-water
protection
standard?
Are all
Appendix II
constituents
below
background?
Continue assessment
monitoring
Corrective
Action
Assess corrective
measures (258.56)
Evaluate corrective
measures and select
remedy (258.57)
Implement remedy
(258.58)
Figure 5-13. Subtitle D ground-water detection and assessment monitoring (40 CFR, Part 258, July 1, 1992).
61
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Table 5-1. Constituents for Detection Monitoring (40 CFR Part 258, Appendix I)1
Common Name2
CAS RN3 Common Name2
CAS RN3
Inorganic Constituents
(1) Antimony (Total)
(2) Arsenic (Total)
(3) Barium (Total)
(4) Beryllium (Total)
(5) Cadmium (Total)
(6) Chromium (Total)
(7) Cobalt (Total)
(8) Copper (Total)
(9) Lead (Total)
(10) Nickel (Total)
(11) Selenium (Total)
(12) Silver (Total)
(13) Thallium (Total)
(14) Vanadium (Total)
(15) Zinc (Total)
Organic Constituents
(16) Acetone 67-64-1
(17) Acrylonitrile 107-13-1
(18) Benzene 71-43-2
(19) Bromochloromethane 74-97-5
(20) Bromodichloromethane 75-27-4
(21) Bromoform; Trlbromomethane 75-25-2
(22) Carbon disulfide 75-15-0
(23) Carbon tetrachloride 56-23-5
(24) Chlorobenzene 108-90-7
(25) Chloroethane; Ethyl chloride 75-00-3
(26) Chloroform; Trichloromethane 67-66-3
(27) Dibromochioromethane; 124-48-1
Chlorodibromomethane
(28) 1,2-Dibromo-3-chloropropane; DBCP 96-12-8
(29) 1,2-Dibromoethane; Ethylene dibromide; EDB 106-93-4
(30) o-Dichlorobenzene; 1,2-Dichlorobenzene 95-50-1
(31) p-Dichlorobenzene; 1,4-Dichlorobenzene 106-46-7
(32) trans-1,4-Dichloro-2-butene 110-57-6
Organic Constrltuents
(33) 1,1-Dichloroethane; Ethylidene chloride 75-34-3
(34) 1,2-Dichloroethane; Ethylene dichloride 107-06-2
(35) 1,1-Dtehloroethylene; 1,1-Dichloroethene; 75-35-4
Vinylidene chloride
(36) cis-1,2-Dichloroethylene; 156-59-2
cis-1,2-Dichloroethene
(37) trans-1,2-Dichloroethylene; 156-60-5
trans-1,2-Dichloroethene
(38) 1,2-Dichloropropane; Prapylene dichloride 78-87-5
(39) cis-1,2-Dichloropropene 10061-01-5
(40) trans-1,3-Dichloropropene 1006-02-6
(41) Ethyibenzene 100-41-4
(42) 2-Hexanone; Methyl butyl ketone 591-78-6
(43) Methyl bromide; Bromomethane 74-83-9
(44) Methyl chloride; chloromethane 74-87-3
(45) Methylene bromide; Dibromomethane 74-95-3
(46) Methylene chloride; Dichloromethane 75-09-2
(47) Methyl ethyl ketone; MEK; 2-Butanone 78-93-3
(48) Methyl iodide; lodomethane 74-88-4
(49) 4-Metrtyl-2-pentanone; Methyl isobutyl ketone 108-10-1
(50) Styrene 100-42-5
(51) 1,1,1,2-Tetrachloroethane 630-20-6
(52) 1,1,2,2-Tetrachloroethane 79-34-5
(53) Tetrachloroethyiene;Tetrachloroethene; 127-18-4
Perchloroethylene
(54) Toluene 108-88-3
(55) 1,1.1-Trichloroethane;Methylchloroform 71-55-6
(56) 1,1,2-Trichloroethane 79-00-5
(57) Trfchloroethylene; Trichloroethene 79-01 -6
(58) Trichlorofluoromethane; CFC-11 75-69-4
(59) 1,2,3-Trichloropropane 96-18-4
(60) Vinyl acetate 108-05-4
(61) Vinyl chloride 75-01-4
(62) Xylenes 1330-20-7
This list contains 47 volatile organics for which possible analytical procedures provided in EPA Report SW-846 Test Methods for Evaluating
Solid Waste," third edition, November 1986, as revised December 1987, includes Method 8260 and 15 metals for which SW-846 provides
either Method 6010 or a method from the 7000 series of methods.
^Common names are those widely used in government regulations, scientific publications, and commerce; synonyms exist for many chemicals.
Chemical Abstracts Service registry number. Where Total" is entered, all species in the ground water that contain this element are included.
62
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5.12.1 When Assessment Monitoring Is Not
Required
The owner/operator does not have to proceed to assess-
ment monitoring if: (1) contamination from the site is shown
to be from another source; (2) there has been an error in
sampling, analysis, or statistical evaluation of data; or (3)
there is a natural variation in the ground-water quality at
the site. The decision not to proceed to assessment moni-
toring must be based on certification by a qualified ground-
water scientist, as defined in Subtitle D.
5.12.2 Elements of an Assessment
Monitoring Program
If a statistically significant increase in an Appendix I
parameter over background levels exists, the owner/
operator must initiate assessment monitoring. At a mini-
mum, assessment monitoring requires annual sampling
for many more parameters, called Appendix II parameters.
Some flexibility in developing an assessment monitoring
program is allowed. Sampling a subset of wells, for ex-
ample, is acceptable if the plume definition and hot spots
already have been determined, which in turn determines
which wells are the most important to sample. Also,
chemical parameters other than those listed in Appendix II
could be sampled, or an alternative sampling frequency
could be used, with the approval of the state director.
If any Appendix II constituents are detected in assess-
ment monitoring, the landfill owner/operator must notify
the state director and continue sampling at least semi-
annually for the Appendix II parameters. Also, if any Ap-
pendix II constituents are detected, the owner/operator
must establish the background concentration, and a
ground-water protection standard (GWPS) must be set
for each detected parameter. A GWPS is defined in
Subtitle D as either the MCL for that parameter, if one
exists, or the background concentration level for that con-
stituent. A GWPS is established for each detected con-
taminant.
If, during subsequent assessment monitoring, the contami-
nant previously detected is no longer found above back-
ground levels, the owner/operator can return to detection
monitoring. To return to detection monitoring, however, the
owner/operator must have at least two consecutive sam-
ples that are at or below background concentrations. If this
situation occurs, then the owner/operator must notify the
state before returning to detection monitoring.
If, however, the level of a contaminant listed in Appendix
II remains at a statistically significant level above the
GWPS in subsequent monitoring, the owner/operator
has to notify state and local officials and clean up the
contamination. The owner/operator must make a best
effort to characterize the nature and extent of pollution,
particularly the delineation of any plume. Additional
monitoring wells might be required, but at least one is
required at the facility boundary in the direction of
ground-water flow, or, more precisely, contaminant mi-
gration (because LNAPLs and DNAPLs can move dif-
ferently than ground water). If the plume is offsite, Subtitle
D requires that the owner/operator notify the downgradi-
ent individuals whose land overlies the plume.
Also, if the GWPS is exceeded, it is necessary to evalu-
ate alternative corrective measures and select an appro-
priate remedy. A description of the selected remedy
must be placed in the operating record and the state
director must be notified. Remediation might not be
necessary if certain conditions are met, as discussed in
Chapter 6.
63
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Chapter 6
Release Characterization and Remediation
6.1 Introduction
During operation or post-closure, ground-water monitor-
ing might detect pollutants from leachate entering
ground water at concentrations that exceed applicable
standards (see Chapters 1 and 5). In this situation, the
owner/operator of the facility is required to clean up and
control the contamination as required in Subtitle D regu-
lations. Some exceptions to this remediation require-
ment are allowed if:
The ground water is contaminated by multiple sources,
and cleanup of the MSWLF plume will not reduce
risk.
The ground water is not and will not be used as a
drinking water source.
Remediation is not technically feasible.
Unacceptable cross-media impact would result from
remediation.
If remediation is required, two major steps should be
undertaken: (1) release characterization, which encom-
passes delineating the contaminant plume, describing
hydrologic processes pertinent to remediation, and com-
piling other site information; and (2) cleanup, which
includes selecting and implementing remedies.
6.2 Release Characterization
6.2.1 Site Assessment
Site assessment is the basic strategy for evaluating the
extent of released leachate contaminants and develop-
ing other information pertinent to remediation. Prelimi-
nary site assessment includes assembling all historical
information on the site, analyzing photographic archives,
interviewing operators, reviewing landfill design blue-
prints, compiling facts from utility company records,
checking well logs on nearby wells, and collecting exist-
ing geologic and hydrologic information. A detailed site
assessment often includes installing sampling wells, col-
lecting water and soil samples, conducting a geophysi-
cal investigation, analyzing data, and assessing feasible
remediation technologies.
During the site assessment, factors that might affect
contaminant migration must be evaluated. Factors that
speed up contaminant migration include hydrological
transport, facilitated transport, and dispersion. Factors
that slow down contaminant movement include soil ad-
sorption, chemical precipitation, biotransformation, and
other considerations (see Chapter 5 for descriptions of
major transport mechanisms). A mass balance can help
to estimate how much contaminant has been released,
has been volatilized into the gaseous phase, has gone
into the nonaqueous phase, has been adsorbed, or has
the potential to migrate in various phases. Even if the
overall contaminant mass at a site is unknown, the
knowledge of the area! distribution of pollutants and a
quantitative understanding of which phases a pollutant
resides in and how easily it can change phase are
crucial information when selecting a remedial strategy.
A review of the basic physical and chemical properties of
known or potential contaminants is a first step in estimat-
ing mass distribution at a site. For example, reference
values for aqueous solubility can give a first approxima-
tion of the amount of leachate that is generated at a site.
These estimates can then be refined as more information
is added, such as the pH, oxygen content, and dissolved
salt content of ground water in the area. Batch tests,
column tests, and other laboratory bench-scale experi-
ments can further define the site-specific partitioning co-
efficients in a simulated landfill environment.
A geological investigation can help define the relation-
ship between geology, hydrology, and site remediation.
Site geology delineation is crucial for determining which
cleanup options are optimal choices and the effective-
ness of the remedial alternatives. Questions that should
be answered during the investigation include: what geo-
logical factors are significant to remediation, how will geo-
logical data be collected, and how will the data be
interpreted? Information on stratigraphy, lithology, struc-
ture geology, and hydrogeology of the site also must be
obtained. Such hydrogeological information gathered
during the siting and drilling of monitoring wells can help
relate site conditions to remedial efforts.
Stratigraphy is one of the most important factors that
must be investigated. Stratigraphic studies can define
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the structure of the contaminated soil so that proper
remediation methods are selected. For example, if a
sand layer occurs naturally in the contaminated media,
it will act as a conduit for either liquid pollution in the
saturated zone or gaseous pollution in the vadose zone.
Consider a situation where a pumped air method is used
to remove gaseous vadose-zone contamination at a site
containing a single sand layer surrounded by a finer,
wetter material. Because the sand is naturally drier,
circulating air will move preferentially through the sand.
The sand layer, as a result, will become drier and an
even better conduit for flow. In this situation, air will
begin to completely circumvent the finer material. The
air, therefore, strips volatile organic compounds (VOCs)
out of the sand layer only and has hardly any removal
effect on VOCs in the clay layer. Pumping air intermit-
tently so that the moisture can be redistributed from the
clay into the sand might solve this problem.
Aerial photography can be used to identify past land-use
patterns at a landfill and can help define site geology.
Particularly in hard rock systems, surface features such
as depressions and lineaments can indicate subsurface
fracturing and flow conduits. Because landfill operations
involve a tremendous amount of shallow excavation, large
areas of near-surface geology are exposed for inspection.
These excavations normally provide a good initial picture
of the stratigraphy in the shallow, unconsolidated mate-
rial and reveal the degree of local heterogeneity.
6.2.2 Characterization Methods
Certain field techniques can be used for release char-
acterization, including mapping surface features, col-
lecting and analyzing ground water, surveying soil gas,
and analyzing soil cores. Other characterization meth-
ods include surface and bore-hole geophysics. More
than one geophysical technique typically is used to help
define a site. A more complete discussion of site char-
acterization methods is available in EPA guidance docu-
ments, such as U.S. EPA (1991 c).
6.2.2.1 Surface Geophysics and Other Surface
Measurements
Noninvasive surface geophysics can be beneficially em-
ployed to help delineate the extent of a contaminant
plume. Electrical geophysical methods, particularly re-
sistivity and time-domain reflectometry techniques, can
be useful when the salinity of the contaminant plume is
different from that of ambient ground water. Leachate
plumes from landfills typically have high total dissolved
solids (TDS) compared to that of ambient ground water,
and shallow leachate plumes often can be identified by
surface resistivity measurements. Conversely, in a salty
seawater environment, a freshwater leachate release
also might be located using electrical methods; these
methods might be less useful, however, if generalized
freshwater recharge and saltwater/freshwater mixing
occurs near the plume. These methods are most suc-
cessful at sites where the salinity of leachate liquids and
ambient ground water is sufficiently different.
Other surface geophysical techniques can be useful for
characterizing potential pollutant migration and identifying
remedial alternatives at a site. For example, stratigraphy
can be well defined through seismic surveys, and there is
presently interest in developing 3-dimensional seismic sur-
veys to help define NAPL contamination in the subsurface.
Electromagnetic techniques and ground-penetrating radar
can provide information on buried waste drums, clay
lenses, and water-table depths.
Soil sampling and analysis can be conducted at the
surface to estimate the areal extent of contaminated
soil. Specific protocols are available for collecting,
documenting, showing chain-of-custody for, and analyzing
samples of solid material at a site. Screening techniques
can assist in selecting the best samples for laboratory
analysis. For example, a fairly new technique called an
immunological survey, currently used at hazardous
waste sites, can be adapted for release characterization
of contamination near MSWLFs. In this method, a quick,
colorimetric test is conducted on soil or water samples
using pollutant-specific, polyclonal antibodies. At certain
landfill sites, nearby surface water also might require
sampling and analysis. Again, rigorous sampling and
analysis protocols are required.
6.2.2.2 Downhole Techniques
Downhole logging can provide important clues to the
geologic structure surrounding a landfill. Whereas sur-
face geophysical techniques are considered noninva-
sive, bore-hole logging requires drilling at a site. Several
bore-hole logging techniques are available for site char-
acterization, including self-potential, electrical resistivity,
temperature, caliper, neutron, natural gamma, gamma-
gamma, flow-meter, and television methods.
Many of these techniques involve the use of a special-
ized probe called a sonde. Several logging devices can
be attached to the sonde, which then can be sent down
a drilled hole. Types of logging devices that can be
attached to the sonde include caliper loggers, a neutron
source and detector, and gamma instruments. A caliper
logger measures bore-hole diameter, which is an indica-
tor of the degree of consolidation and cohesiveness of
porous material. Neutron techniques measure porosity
below the water table and regions of saturation in the
vadose zone. A neutron logger emits fast neutrons from
a radioactive source. When a neutron hits a water mole-
cule or hydronium ion, it is reflected back as a thermal-
ized neutron or a slow neutron, which can be detected
by the instrument. The more moisture in the soil, the
more neutrons are reflected back to the instrument.
Because gamma radiation is naturally emitted by some
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geologic materials (shales, some clays), gamma de-
vices can be helpful in identifying stratigraphy.
Self-potential and downhole resistivity are important
electrical methods for defining stratigraphy. A flow meter
is another device that can be used downhole. This de-
vice measures flow rate and direction of ground water
at different subsurface elevations. Flow-meter logs and
vertical temperature profiles of ground water can be
used to identify variations in hydraulic conductivity with
depth. In fractured rock media, television logs can es-
tablish the location and orientation of some fractures
when the drilling process itself has not created numer-
ous secondary fractures.
Hydrologic testing of the site provides estimates of hy-
draulic conductivity and subsurface flow velocity, which
are critical in predicting plume migration. Storage coef-
ficients also can be assessed by some methods, provid-
ing an approximation of the water stored in the medium.
Such field techniques include rate-of-rise (slug) tests,
aquifer (pumping) tests, and laboratory estimates such
as permeameter tests on "undisturbed" soil samples
collected from the site.
The collection of ground-water and aquifer solids for
chemical analysis often forms the basis for an evalu-
ation of a site. As in surface soil sample collection,
screening techniques such as soil gas surveys can as-
sist in optimizing placement of monitoring wells and in
obtaining deeper soil collection. Ground-water charac-
terization methods are described in Chapter 5.
6.3 Remedy Selection and
Implementation
This section discusses requirements that must be met
during remedy selection and implementation and briefly
presents some of the major remediation technologies
that are used to clean up contaminated sites. A more
complete discussion can be found in U.S. EPA (1991 d).
6.3.1 Regulatory Requirements
Based on the results of the corrective measures assess-
ment required by Subtitle D, the owner/operator must
select a remedy that, at a minimum, meets the require-
ments listed below. Within 14 days of selecting a rem-
edy, the owner/operator must place a report in the
operating record describing the selected remedy and
how it meets the requirements and notify the director of
an approved state program. The regulation states that
the remedies must:
Be protective of human health and the environment.
Attain the ground-water protection standard as speci-
fied pursuant to 40 CFR 258.55 (h) or (i).
Control the source(s) of releases to reduce or elimi-
nate, to the maximum, further release into the envi-
ronment of the constituents listed in 40 CFR 258
Appendix II.
Comply with standards for management of wastes as
specified in 40 CFR 258.58 (d).
In selecting a remedy that meets the standards, the
owner/operator must consider the following evaluation
factors:
The long- and short-term effectiveness and protec-
tiveness of the potential remedy.
The effectiveness of the remedy in controlling the
source to reduce further releases.
The ease or difficulty of implementing a potential
remedy.
The practicable capability of the owner/operator, in-
cluding a consideration of technical and economic
capability.
The degree to which community concerns are ad-
dressed by a potential remedy.
Once a remedy is selected and implemented, a correc-
tive action program (including ground-water monitoring)
must be established. Any necessary interim measures
also must be taken during either the site characterization
process or the major remedial effort.
If, during remedy implementation, unexpected difficulties
arise and a requirement for the remedy cannot be met,
the owner/operator must:
Obtain certification from a qualified ground-water sci-
entist that remediation is not effective.
Notify the director of an approved state program.
Implement an alternative measure.
Continue the alternative corrective action, once effec-
tive remedial actions are implemented, until compliance
with the ground-water protection standards are met for
3 years (after which it is assumed that the release has
been cleaned up).
6.3.2 Remediation Alternatives
After careful release characterization, remediation of the
contaminated site should proceed based on the results
of the characterization. Methods to achieve objectives of
a remedial action can include several, sometimes concur-
rent, activities to protect human health and the environ-
ment. Preventing direct human or animal contact with
contamination can be facilitated by institutional controls
such as deed or access restrictions, by physical barriers
(e.g., fences), and by covering waste. Migration of large
masses of contaminants can be controlled by treating
principal threats ("hot spots"), installing barriers to protect
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surrounding ground water, reducing contaminant leach-
ing (often by capping), and by controlling surface run-off
and erosion with grading and revegetation. It may be
necessary to collect and treat leachate. In some cases
where treatment of the waste source is impractical,
hydraulic barriers must be maintained for very long pe-
riods of time. Wherever practical, remedial efforts should
attempt to return ground water to beneficial use, clean
up surface water and sediments, and protect wetlands.
Collection and treatment of landfill gas also is a common
remedial goal, particularly where there are severe odors,
nearby homes, and/or when the final disposition of the
landfill property will involve public access.
Remediation procedures can include:
Focused feasibility study (FS)
Interim remedial measures
Bench- and pilot-scale studies
Formal FS
Selection and design of final remediation
Implementation
Monitoring
Closure (if appropriate)
The following sections briefly describe several common
remediation technologies.
6.3.2.1 Excavation
In remedial excavation, equipment is used to dig up the
polluted area and transport the soil to another location
for treatment or cleanup. This technique is simple and
readily available because most landfills have excavation
equipment on site. It is especially effective for pollutants
that disperse slowly (i.e., pollutants that linger in the
vadose zone) or for removal of specific waste drums or
canisters. One of the major concerns associated with
this method is the amount of the soil that must be
excavated. Removing and transporting a large volume
of contaminated soil is very expensive. Thus, excavation
might not be feasible at landfills with extensive soil
contamination or where the primary concern is a
leachate release. Removal of contaminated soil ("hot
spots"), however, often is an important factor in reducing
the source of leachate generation. Removal of such
highly contaminated material by excavation could re-
duce future leachate production. Although excavation
will not clean up the leachate plume, it can be an effec-
tive tool in reducing risk. Major concerns of using exca-
vation include proper treatment and disposal of the
excavated soil and operational safety. For small spills of
low-mobility chemicals, however, excavation is a par-
ticularly cost-effective cleanup procedure.
6.3.2.2 Fixation and Stabilization
Fixation is the process of adding reagents or hardening
agents that absorb, encapsulate, or chemically bond
with contaminants, thereby preventing them from mov-
ing into the ground water. This process changes the
physical characteristics of the waste (e.g., it becomes
less water-soluble and sometimes less toxic) and de-
creases the surface area of pollutants available for
leaching. Waste solidification, one type of fixation proc-
ess, is rarely cost-effective as a pollution prevention
measure at Subtitle D facilities; it can, however, be a
practical remedial method for reducing the leaching po-
tential of contaminated material removed from landfills
during cleanup efforts. In situ stabilization involves the
mixing of solidifying reagents or substances (pozzuo-
lanic material) with contaminated soils, typically using
standard earthmoving equipment such as backhoes, large
diameter augers, and draglines. Mixtures vary depending
on what is locally available; a mixture might contain port-
land cement, fly ash, kiln dust, and/or hydrated lime.
Extraneous materials or impurities can strengthen or
weaken the solidified mass; therefore, careful evaluation
and occasionally pretreatment should accompany any
stabilization effort. To date, stabilization has been rarely
used at municipal waste sites.
6.3.2.3 Physical and Hydrologic Barriers
The installation of physical barriers to contain ground-
water flow is an effective remedial method used in con-
cert with hydrologic barriers and with ground-water
cleanup methods, such as pump-and-treat systems (de-
scribed below). If shallow, unfractured bedrock underlies
a site, a slurry trench, grout curtain, or cutoff wall con-
structed to the depth of bedrock can functionally seal the
unconfined aquifer.
A slurry wall is constructed by trenching to bedrock with
the trench filled with a mixture of water and clay (e.g.,
bentonite slurry). The dried slurry in the trench becomes
a low-permeability zone that blocks the movement of
leachate into downgradient ground water. In regions of
topographic variability, bentonite-cement slurries can be
used to prevent flow of trench fill to the low topographic
point. Cutoff walls or driven pilings also can be placed
to block subsurface water flow.
Physical barriers have several limitations, however. En-
suring that the barrier's level of permeability is suffi-
ciently low to prevent the movement of contaminants is
difficult. Although low permeability is achieved with a
well-designed wall, the barrier also acts as a ground-
water dam, producing buildup of hydraulic head on the
upgradient side and lowering hydraulic head on the
downgradient side. The increase in hydraulic gradient
can lead to loss of containment. Also, underflow can
occur where the enclosing wall is not well keyed into
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bedrock. If these problems are not solved, the barrier
will lose its ability to contain contaminants.
Where bedrock is very deep, shallow collection trenches
or interception wells can contain a pollutant plume hy-
draulically. Any contaminated water pumped from such
a system must be properly treated and disposed.
6.3.2.4 Soil Flushing
Soil flushing is another method for removing subsurface
contaminants. If relatively immobile contaminants are
located in the vadose zone or the shallow saturated
zone, they can be removed by passing specialized
washing liquids through the contaminated soil and col-
lecting those liquids downgradient. Because this method
mobilizes previously immobile contaminants, the collec-
tion system must be particularly efficient. The use of this
method at municipal landfills is infrequent.
6.3.2.5 Pump and Treat
Pump-and-treat systems often are used in landfill reme-
diations. In these operations, contaminated ground
water is pumped from a collection well, subsurface
drain, or trench to an aboveground treatment facility for
cleanup. This method requires that all contaminated
water be treated to reduce concentrations of target com-
pounds to an acceptable level. Generally, pump-and-
treat remediations are time-consuming. Also, even when
ground water flowing to a well is apparently clean, ad-
sorbed contaminants in low-permeability areas or resid-
ual NAPLs (see Chapter 5) can bleed off once the
pumping wells are shut down. Because high contami-
nant concentrations in ground water can reappear at
significant and unacceptable levels after pumps are
turned off, it becomes particularly important to define the
nature and distribution of contamination during site char-
acterization.
6.3.2.6 In Situ Heating
Heating of subsurface materials can provide several
remediation benefits. Warmer subsurface environments
can increase evaporation of volatile contaminants, en-
hance biodegradation rates, and reduce the viscosity of
liquids, thus increasing their ability to flow through a
porous medium. The subsurface can be heated by
steam injection or radio-frequency energy (see Figure
6-1), but caution must be exercised whenever energy is
added to the subsurface near a landfill because signifi-
cant subsurface methane at a site could create poten-
tially explosive conditions.
6.3.2.7 Vapor Extraction and Air Sparging
In vapor extraction, also known as enhanced volatilization,
volatile contaminants are stripped out of contaminated soil
using forced subsurface ventilation (see Figure 6-2). As
.Transition section
RF power feed point
Vapor barrier
Concrete pad
Pea gravel
Vapor collection
manifold
Electrodes
Figure 6-1. In-situ heating device (U.S. EPA, 1992c).
Air/vapor
manifold
Blower or
vacuum pump
Vapor treatment
system (where
uired)
Grout seal
Screen
Sand pack
Water table
'Contaminated soils
Figure 6-2. Soil vapor extraction (U.S. EPA, 1993b.)
circulating air passes through residual leachate in the
vadose zone, it will evaporate the leachate, slowing
down leachate movement into the ground water. This
method is effective for removal of volatile contaminants
when applied to a vadose zone with reasonably high air
permeability and moderate- to low-moisture content.
Air sparging techniques, as shown in Figure 6-3, in
which air is injected below the water table, can be used
to compensate for ineffective aeration of ground water.
Unfortunately, rising air in saturated media typically fol-
lows certain constrained pathways and does not typi-
cally spread out and produce large regions of aeration.
In addition, changes in the oxygen content of the sub-
surface can cause speciation of metals into more mobile
fractions. Thus, remedial actions utilizing air sparging
techniques should be approached very carefully. These
somewhat innovative technologies are acceptable when
they represent a low-cost alternative to effectively treat
ground water.
6.3.2.8 Bioremediation
Bioremediation is the use of microbial degradation proc-
esses in a relatively controlled environment to remove
a variety of pollutants from a contaminated site. The
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Air compressor
ON API.
Vapor
treatment
Tiny
bubbles
Figure 6-3. Air sparging (U.S. EPA, 1993b).
microbial ecology of the subsurface has the following
general characteristics: 1 x 106 to 1 x 103 microbes/g
soil (lower in pristine environments); less than 90 percent
of the microbes attached to soils; metabolically active, but
slow-growing organisms; metabolically versatile organ-
isms; specific microbes that can live in oxic and/or anoxic
conditions; and biofilms (polysaccharide exudate) pro-
duced by subsurface microbes, which can provide nutri-
ents at later times. These characteristics can be useful in
demonstrating the feasibility of bioremediation at a site.
To ensure efficient biodegradation of contaminants,
proper microbial growth conditions must be maintained,
including the availability of proper amounts and ratios of
nutrients (e.g., carbon, nitrogen, phosphorus, and other
inorganic substances). A critical factor is the presence
of the proper electron acceptor for different types of
degradation (e.g., oxygen for aerobic respiration, sulfate
and nitrate for reduction, or carbon dioxide and organics
for fermentation). Although some research has been
conducted to create engineered microbes suitable for deg-
radation, naturally occurring, indigenous microorganisms
have been the most successful in contaminant removal.
The designer of a bioremediation system, such as the
system shown in Figure 6-4, must demonstrate the fea-
sibility of applying this technology to a specific site. As
a part of the feasibility study, the ability of the microor-
ganisms to degrade the contaminants present, as well
as limitations in the availability of any nutrients or elec-
tron acceptors, should be quantified. The rate of ex-
pected degradation relative to the rate of subsurface
contaminant migration must be established. This is usually
done by the careful measurement of available nutrients
and calculation of flow rates. The degradation rate of an
analogous compound (e.g., a radioactively tagged com-
pound) that has been added to the site can provide a
controlled experiment to delineate degradation rate. A
feasibility study also should determine the number of
microbes present. Microbial enumeration can be carried
out by plating techniques, most-probable-number tech-
niques, staining methods, phospholipid characterization,
or other methods.
Claims of biodegradation can be supported by a number
of tests that show the biological removal of contami-
nants. The following characteristics are indicative of
microbial degradation:
Reduction of contaminant concentration in the sub-
strate over time, supported by proper mass balance
determinations. Data must show substrate distribu-
tion of volatiles in the gaseous phase, adsorbed ma-
terial on soils, and dissolved-pnase contaminants in
liquids, so that the consumption of contaminants by
microorganisms can be quantified.
Increase in biomass activity. This information can be
obtained through microbial enumeration such as plate
methods, staining techniques, phospholipid charac-
terization, or DNA counts.
Production of daughter products. When microorgan-
isms degrade contaminants, intermediate products
(daughters) indicate the first level of biodegradation.
Adaptation/acclimation phenomenon. In general, mi-
croorganisms need some (relatively brief) time to ad-
just themselves to a new environment before they
start to degrade contaminants effectively. When this
lag period is demonstrated, the stable and healthy
growth of microorganisms is indicated.
Consumption of terminal electron acceptors.
Ability to describe the degradation processes mathe-
matically using biodegradation-rate kinetics.
Abiotic controls. If claims of biodegradation are to be
fully supported, data must show that the contaminant
transformation occurring is not caused by chemical
degradation.
6.3.2.9 Bioventing
Bioventing is a method that combines soil venting and
biological degradation for enhanced contaminant re-
moval. The pumped air not only volatilizes the contami-
nants in the subsurface, but also supplies oxygen to
microorganisms for biodegradation of contaminants.
6.3.3 Sources for Further Information on
Remediation Techniques
U.S. EPA. 1985. Handbook of Remedial Action of Waste
Disposal Sites (Revised). U.S. Environmental Protec-
tion Agency. EPA/625/6-85/006.
U.S. EPA. 1987. Technology Briefs, Data Requirements
for Selecting Remedial Action Technology. U.S. Envi-
ronmental Protection Agency. EPA/600/2-87/001.
U.S. EPA. 1989. Evaluation of Ground-Water Extraction
Remedies. Vol. I, Summary Report. U.S. Environmental
Protection Agency. EPA/540/2-89/054.
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Oxygen
addition
Nutrient
addition
To:
Treatment
Treatment/recycle
Recycle
In situ biodegradation
zone
DNAPL
Figure 6-4. Bioremediation system (U.S. EPA, 1993b).
U.S. EPA. 1989. Stabilization/Solidification of CERCLA
and RCRA Wastes. U.S. Environmental Protection
Agency. EPA/625/6-89/022.
U.S. EPA. 1989. Technology Evaluation Report, Vac-
uum Extraction System. Groveland, MA. U.S. Environ-
mental Protection Agency, Office of Research and
Development, Risk Reduction Engineering Laboratory.
Authored by Michaels, P.A. and M.K. Stinson. EA68-03-
3255.
U.S. EPA. 1990. Basics of Pump and Treat Ground-
Water Remediation Technologies. U.S. Environmental
Protection Agency. EPA/600/8-90/003.
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Chapter 7
Closure and Post-Closure
7.1 Introduction
Subtitle D requires owners/operators of all MSWLF units
to install, at closure, a final cover system designed to
minimize infiltration and erosion. The final cover system
must be designed and constructed to:
Have a permeability less than or equal to the perme-
ability of any bottom liner system or natural subsoils
present, or a permeability no greater than 1 x 10"5
centimeters per second, whichever is less.
Minimize infiltration through the closed MSWLF using
an infiltration layer that contains a minimum of 18
inches of earthen material.
Minimize erosion of the final cover using an erosion
layer that contains a minimum of 6 inches of earthen
material capable of sustaining native plant growth.
The owners/operators of all MSWLFs also must prepare
written closure plans that describe the steps necessary
to close all MSWLF units at any point during their active
life. After the closure of each MSWLF unit, the owner/op-
erator must conduct post-closure care for at least 30
years and at a minimum:
Maintain the integrity and effectiveness of any final
cover.
Maintain and operate the leachate collection system
in accordance with the requirements specified in 40
CFR 258.40.
Monitor the ground water in accordance with the re-
quirements of Subpart E of 40 CFR 258 and maintain
the ground-water monitoring system.
Maintain and operate the gas monitoring system in
accordance with the requirements of 40 CFR 258.23.
More detailed regulatory requirements are presented in
Chapter 1.
Subtitle D provides little guidance on the design of final
covers and specific elements that might be required in
the cover. This section reviews design considerations for
both the Subtitle D design objectives and for objectives not
directly addressed by Subtitle D. Design considerations
discussed include those for the required infiltration and
erosion control layer. Also discussed are supplementary
layers, which commonly are used in final covers. The
supplementary layers reviewed here include a drainage
layer used to maintain the stability of the erosion control
layer on sideslopes and the gas venting system used to
reduce the buildup of gas pressure within the MSWLF.
7.2 Closure Design Considerations
The design components and considerations for MSWLF
closure include:
Profile of the cover
Infiltration (barrier) layer or an alternative barrier system
Drainage layer
Erosion control layer
Gas venting system
Landfill cover slope stability
Subsidence effects
Weather effects
Documentation of closure
These components and considerations are discussed below.
7.2.1 Profile of the Cover
The profile of the minimal landfill cover required by Sub-
title D is shown in Figure 7-1. Usually, however, the cover
also includes supplemental layers to accommodate non-
regulatory design criteria. Regulatory and supplemental
layers include:
Initial layerAn interim cover installed above the waste.
Gas venting layerA porous, highly permeable system
to collect gases produced during waste stabilization.
Low permeability layerA soil and/or geomembrane layer
with low permeability installed above the gas venting sys-
tem to limit infiltration of surface waters into the MSWLF.
Drainage layerA layer located above the low-
permeability layer that maintains the stability of
cover slopes by eliminating pore water pressures
above the low-permeability layer.
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6"
18'
Erosion (vegetative) layer
Infiltration layer (hydraulic conductivity
must be less than or equal to any
bottom liner system or natural soils but not
greater than 1 x 10'5 cm/s)
Waste
Figure 7-1. Minimum requirement for final cover design (U.S. EPA, 19924).
Erosion control layerThe top cover layer consisting
of soil covered with vegetation to protect the landfill
cover from erosion caused by rain, wind, or animals.
7.2.2 Infiltration (Barrier) Layer
The infiltration (barrier) layer for MSWLFs having only a
soil liner consists of a compacted soil layer with a mini-
mum thickness of 18 inches and a maximum permeabil-
ity of 1 x 10"5 centimeters per second. For MSWLFs that
use a composite liner system, a geomembrane must be
added above the compacted soil layer. Both infiltration
layer systems are designed to reduce the rate at which
surface waters infiltrate the MSWLF to below the rate at
which leachate moves through the liner system. An
alternative barrier system with infiltration equivalent to
or less than the system described in Subtitle D may be
used if approved by the director of an approved state
program.
The geomembrane material used for the final cover
must be long-lasting and must tolerate anticipated
subsidence-induced strains. As an alternative to HOPE,
polymers with more suitable biaxial stress-strain capac-
ity should be considered. Typical biaxial stress-strain
curves for HOPE and alternative geomembrane poly-
mers are shown in Figure 7-2. Materials with high biaxial
strength more easily withstand the differential settling
that can occur after closure, thereby resisting failure.
7.2.3 Drainage Layer
Subtitle D does not require a drainage layer in landfill
cover systems. Many owners/operators of large landfills,
however, usually design a drainage layer in portions of
the cover system that exceed a 5H:1 V slope. The cover
drainage layer prevents the moisture that infiltrates the
erosion control layer from accumulating above the bar-
rier layer. Such accumulated water can generate excess
7,000
6,000
5,000
-«_ PVC
-- CSPEFVgeotextfle composite
-- VLDPE 40 smooth
-*- VLDPE 40 textured
-*- HOPE 60 textured
30 40 50
Strain (%)
Figure 7-2. Multlaxial stress vs. strain for five geomembrane
materials (Frobel, 1991).
pore water pressure above the geomembrane and cause
the erosion control layer to slide off the cover sideslopes.
The sideslope drainage layer commonly is drained to
a large capacity toe drain, as shown in Figure 7-3.
7.2.4 Erosion Control Layer
The minimum thickness of the erosion layer required by
Subtitle D is 6 inches. Establishing a healthy growth of
vegetation in 6 inches of soil can be difficult, however.
The minimum practical thickness of the erosion layer
should be evaluated using a water-balance analysis,
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/Drainage layers
Flexible membrane liner (FML)
Top layer
Toe drain
FML anchors
(separate anchor trench for each geosynthetic)
Low-permeability soil
FML
Waste
Figure 7-3. Schematic of a sideslope drainage layer (U.S. EPA, 1989c).
such as that performed by EPA's HELP Model (U.S.
EPA, 1984). The minimum thickness of the erosion con-
trol layer should provide available moisture to plants
even during prolonged periods of drought.
Soil loss (erosion) caused by rainfall can be calculated
by the universal soil loss equation:
X = RKSLCP
where:
X = Soil loss
R = Rainfall erosion index
K = Soil erodability factor
S = Slope gradient factor
L = Slope length factor
C = Crop management factor
P = Erosion control practice
These parameters can be evaluated using data avail-
able in soil erosion textbooks and EPA technical re-
source documents. Erosion-related soil loss should not
exceed 2 tons per acre per year to minimize long-term
maintenance. Meeting this level of erosion control com-
monly requires the use of slopes less than 4H:1V and
drainage swales placed at 20-foot vertical increments.
Water-related erosion can be controlled not only by
vegetation, but also by hardening the cover surface
using stones or riprap. Such hardened covers allow
more water to infiltrate than vegetative covers because
no vegetative evapotranspiration occurs. Hardened
covers increase the need for a barrier layer but reduce
long-term maintenance.
7.2.5 Gas Collection System
A minimum of one passive gas vent per acre of cover
should be installed to prevent the buildup of gas pres-
sure beneath the cover. The gas venting system can
use vertical gravel wells, blanket collectors (beneath the
barrier layer), or gravel trench drains (also beneath the
barrier layer) to collect landfill gases. The collected
gases are routed through the cover using vent pipes, as
shown in Figure 7-4.
Methane is generated from MSW only when the mois-
ture content of the waste exceeds 40 percent under
anaerobic conditions. For example, if a landfill facility
contains wastes at 15 percent moisture, the waste will
be fossilized; that is, it will not decay and therefore will
produce very little methane.
7.2.6 Landfill Cover Slope Stability
The landfill cover slope must be stable enough to sus-
tain infiltration and run-off from a 24-hour, 25-year storm.
For slopes steeper than 5H:1V, the designer should
ensure that a drainage layer is provided, if needed, and
that the interface friction between adjacent layers form-
ing the cover is sufficient to prevent a sliding failure. If
sliding occurs, cover integrity can be affected, and other
liner systems also might be damaged.
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Vent to atmosphere
or collect and utilize
Steel damp
Welds
Boot seal (flexible)
Gasket
Geomembrane
Operational cover
Flange seal (fixed)
I Waste
Figure 7-4. Landfill gas vents passing through geomembrane covers (U.S. EPA, 1987c).
Interface friction tests should be conducted to help deter-
mine an acceptable maximum slope for a landfill cover.
Two types of testsdry and soakedshould be con-
ducted on interfaces between different cover layers using
a direct shear device or a tilt-table. The lowest interface
friction slope obtained during the tests then can be desig-
nated as the maximum cover slope.
7.2.7 Subsidence Effects
Landfill subsidence can be global (e.g., because of uniform
settlement of waste) or localized (e.g., because of the
collapse of a large void immediately below a portion of the
cover). In general, global subsidence does not result in
excessive tensile strains on the cover and improves the
stability of the cover by reducing sliding. Therefore, even
dramatic global subsidence of the landfill will not harm
the final cover.
Localized subsidence, however, can produce small de-
pressions on the cover that can produce excessive ten-
sile strains in cover layers and can lead to ponding of
water on the cover. The impact of tensile strains can be
minimized using a geomembrane with large ultimate
biaxial strain characteristics. These geomembranes are
composed of PVC, very low density polyetylene, and
polypropylene. Ponding of water must be avoided be-
cause it can kill or distress cover vegetation, and the
weight of the water can accelerate expansion of a pond
on the cover.
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7.2.6 Weather Effects
7.3.1 Required Post-Closure Care
The cover also must be able to withstand extreme
weather conditions and remain functional with minimal
maintenance. The two extreme weather conditions for
which a final cover should be designed are extreme
drought conditions and ground freezing. Extreme
drought was discussed previously (see Section 7.2.4)
and should be considered during the design of the ero-
sion control layer. Freezing of the cover is a concern
because of the impact of freezing on clay permeability.
Repeated cycles of freezing and thawing can dramati-
cally increase the permeability of compacted clays.
7.2.9 Documentation of Closure
MSWLFs are commonly designed so that new cells are
added as contiguous lateral expansions to currently ac-
tive cells. The final cover for such MhWLF complexes is
constructed incrementally, with the final cover being
constructed as final cover grades are achieved. Landfill
closure is therefore a lengthy process that can extend
beyond a single designer's career. For design continuity,
as-built drawings and material samples must be main-
tained for all final cover sections. In this way, the com-
patibility of abutting geomembranes that have been
placed in various years and the continuity of drainage
and gas collection systems can be ensured as place-
ment of the final cover progresses.
7.3 Post-Closure Care
After a landfill is closed and the final cover is installed,
monitoring and maintenance are necessary to ensure
that the landfill remains secured and stable. Subtitle D
requires that post-closure care and monitoring be per-
formed for at least 30 years. The owner or operator must
prepare a written post-closure care and monitoring plan
for review by the director of an approved state program.
This plan must include:
The start and completion dates of the post-closure period
The monitoring plan description
The maintenance program description
The facility's personnel list of contacts for emergencies
A description of the end-use plan for the site
Post-closure care activities must include but are not
limited to:
Maintaining the integrity and effectiveness of erosion
controls.
Maintaining and operating the leachate collection
system.
Maintaining and operating the gas venting system.
Monitoring ground water for any contamination.
Erosion control maintenance includes routine vegetation
management (such as mowing and planting), subsidence
repair, and run-on/run-off control. Sedimentation basins
and drainage swales must be inspected after every major
rainstorm and repaired or cleaned if required.
After a final cover is placed on the MSWLF, the leachate
collection system will have a very small leachate load
and should be easy to maintain. Leachate generation
should drop to less than 1,000 gallons per acre per day,
which should not tax a system designed to handle
stormwaters. During the post-closure period, leachate
production rates should be monitored to identify drops
in production rates. If leachate production drops dra-
matically, then the primary leachate pipes should be
inspected for biological clogging. Such inspections cart
be performed using television cameras commonly used
to inspect sewers. The leachate line should be hydro-
flushed if clogging is found.
The vent pipes in a passive gas venting system must be
inspected frequently for damage that can be caused by
mowing or other traffic. A damaged vent pipe can allow
surface water to enter the gas venting system and
quickly bypass the cover. Damaged vent pipes must be
repaired promptly.
Ground-water monitoring has been discussed extensively
in Chapter Five. During the post-closure period,
ground-water monitoring must continue to be conducted
on a routine basis. The owner/operator must be alert to
any possible sign of contamination and must take nec-
essary remedial action if contamination occurs. See
Chapter 6 for a discussion of remedial action.
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Chapter 8
Financial Assurance Criteria
8.1 Introduction
To prove financial assurance, owners/operators of all
MSWLFs (except state or federal facilities, which are
exempt from the financial assurance requirements) must
demonstrate that they have access to sufficient funds to
cover the applicable costs of (1) final landfill closure, (2)
30-year post-closure care, and (3) corrective action for
known releases of hazardous constituents. The first two
requirements are mandatory for all MSWLFs. The third
financial assurance requirement is triggered only if a
leachate release is detected. Cost estimates must reflect
the costs, in current dollars, of hiring a third party to
conduct the activity. For closure and post-closure care,
cost estimates must be based on the highest costs that
could be incurred at the site (e.g., the largest area that
might need to have a cover placed on it). For corrective
action, cost estimates must reflect the total cost of com-
pleting the activity.
For closure, post-closure, or corrective action, the owner/
operator can increase or decrease the cost estimates
and the amount of financial assurance provided if physi-
cal changes in these activities warrant cost modifica-
tions. Decreases in cost estimates must be justified and
reported to the director of an approved state program.
Annual adjustments in cost estimates must be made for
inflation. In addition, the owner/operator must provide
continuous financial assurance coverage until all Subti-
tle D requirements for closure, post-closure, and/or cor-
rective action have been met. Completion of required
activities must be certified in writing with the approval of
either an independent professional engineer or the di-
rector of an approved state program.
The Subtitle D financial assurance criteria was due to
become effective April 9, 1994. A 12-month extension,
however, was given (Federal Register, July 28,1993) to
allow EPA to better define a mechanism for local gov-
ernment financial tests (see Section 8.5). The financial
assurance requirements for closure, post-closure, and
corrective action are described below. Allowable finan-
cial mechanisms also are discussed.
8.2 Financial Assurance for Closure
Financial assurance for closure of an MSWLF ensures
that the owner or operator will have the necessary funds
available to complete construction of the final cover. The
owner/operator must provide a detailed cost estimate,
in current dollars, for a third party to close the largest
open area of the MSWLF. The third-party requirement
does not preclude facility personnel from performing the
actual work, but it does prevent reliance on such cost-
saving measures (e.g., using internal staff rather than
contract labor) in cost estimates for financial assurance.
For many facilities, financial assurance for closure will
change over time because the placement of final cover
and the opening of new disposal cells are ongoing proc-
esses; closure costs probably will be updated annually
to accommodate these adjustments. Subtitle D requires
annual adjustment in closure cost estimates to account
for inflation and for physical changes during closure that
deviate from the closure plan developed before closure
(see Chapter 7).
8.2.1 Estimating Final Cover Costs
The cost of constructing a final cover for an MSWLF will
depend on the complexity of the cover profile, final slope
contours of the cover, and other site-specific factors.
This section reviews the costs of individual layers within
the final cover and presents current (1993) construction
cost guidelines.
8.2.1.1 Infiltration Layer
As discussed in Chapter 7, the infiltration layer can
range from an 18-inch layer of soil to a composite barrier
composed of a geomembrane overlying a 2-foot layer of
soil (in either case, the soil layer must have a permeabil-
ity equal to or less than 1 x 10~5 centimeters per second).
Guidance issued by EPA (Federal Register, June 26,
1992) has eliminated the need for compacted clay infil-
tration layers with permeabilities less than 1 x 10~7 cen-
timeters per second in the cover. This interpretation can
provide a cost savings to landfill owners of up to $60,000
per acre.
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The cost of the geomembrane component of an infiltra-
tion layer ranges from $0.20 to $0.80 per square foot.
The less expensive geomembranes can be used on final
covers having slopes less than 4H:1 V. As the maximum
slope of the final cover increases, the geomembrane
surface must be roughened to improve the slope stability
of the cover system. This roughening is accomplished
by either texturizing the surface of the geomembrane or
laminating a nonwoven geotextile to both faces of the
geomembrane. The cost of such enhanced stability
geomembranes is at the upper end of the range.
In many regions of the country, the required soil layer
can be constructed using onsite soils. Placing and com-
pacting onsite soils costs $4 to $6 per cubic yard. The
recent interpretation of the permeability requirement dis-
cussed above is easily achieved with typical soil compac-
tion equipment. If onsite soils are not available, then the
cost estimate must include monies for transporting soil
to the site. Such transportation costs are typically $0.15
to $0.25 per ton per mile.
MSWLF sites consisting of granular soils might require
amendment of available soils to meet the 1 x 10"5 cen-
timeters per second criteria. Amendment might include
blending the soils with a local source of soil fines (e.g.,
quarry fines) or using commercially available bentonite.
Soil amendment costs using commercial bentonite are
approximately $5 per ton for blending in a pug mill and
$2.50 per ton for each percent of bentonite in the mixture.
For example, a 3-percent bentonite-amended infiltration
layer using onsite soils would cost $12.50 per ton for the
bentonite amendment and $6 a ton for placement and
compaction.
8.2.1.2 Drainage Layer
A drainage layer in the final cover is required only when
the slope of the cover is so steep that water percolating
down through the cover will build up excess pore water
pressures as it moves down the slopes of the infiltration
layer. Such water pressures reduce the stability of the
overlying erosion control layer and can lead to cover
slope failures. The drainage layer can be constructed
using a 6-inch sand layer (at $12 to $20 per ton) or a
bonded geonet (at $0.55 to $0.70 per square foot).
8.2.1.3 Erosion Control Layer
Subtitle D requires a minimal erosion control layer con-
sisting of a vegetated 6-inch layer of topsoil. In reality,
however, if a geomembrane is incorporated in the final
cover, this layer typically must be significantly thicker to
maintain vegetation during droughts. The required thick-
ness should be determined by a water-balance analysis.
Erosion control layers are commonly 18 to 30 inches
thick. Suitable soils to build the erosion control layer
typically cost $8 to $14 per ton (including costs of
transportation and soil placement). Additional costs for
fertilizing, seeding, and hydromulching the erosion con-
trol layer range from $1,200 to $1,800 per acre.
Final MSWLF covers also commonly include swales on
the sideslopes to control run-off velocities and to convey
run-off water off the cover. Swales and associated con-
veyance devices add approximately $1,100 to $2,000
per acre to the cover cost.
8.2.1.4 Passive Gas Venting Layer
Typically, a minimum of one passive gas vent per acre
is incorporated in a final cover. Such vents include a
perforated pipe, a gravel collector (both located beneath
the infiltration barrier), and a plastic gas vent pipe, which
passes through the cover. Gas collectors include both
vertical well systems and surface trench drain-type sys-
tems. The wells are drilled to the zone of saturation and
cost $3,000 to $8,000 to complete. Surface trench col-
lectors are simpler to install and typically cost less than
$2,000 each to install.
8.2.2 Annual Updating of Closure Costs
Each year, the estimated cost for constructing a final
cover must be updated to account for final cover place-
ment in certain areas of the landfill (resulting in a de-
crease in the cost estimate) and increased costs of new
cell construction during the previous year. Such yearly
cost updates also allow changing regulatory require-
ments or financial assurance mechanisms to be incor-
porated. Most financial assurance mechanisms (see
Section 8.5) will require closure construction costs to be
updated annually.
8.3 Financial Assurance for
Post-Closure Care
The owner/operator of an MSWLF must demonstrate
financial assurance for providing long-term maintenance
and monitoring over the 30-year post-closure period.
Long-term maintenance can include repair of damaged
or stressed vegetation, cleanout of sedimentation ba-
sins, maintenance and cleanout of the leachate collection
system, and general facility maintenance. Long-term moni-
toring includes sampling and analysis of ground water, gas
emissions testing, and any additional state-required testing.
8.3.1 Estimating Post-Closure Care Costs
Post-closure care costs should be updated annually as a
record of actual facility costs is developed. Some costs,
such as erosion control and ground-water sampling, might
be reduced over time as the cover matures and a mean-
ingful amount of monitoring data is accumulated.
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8.3.1.1 Long-Term Maintenance
Erosion-related damage to the final cover increases with
increases in the area of the cover and the steepness of
its slopes. For typical MSWLF covers with slopes less
than 4H:1V, the owner/operator should assume that
5 percent of the final cover will require maintenance (i.e.,
rebuilding) each year. Such maintenance commonly is
performed by facility staff on a monthly basis, but Sub-
title D requires that the estimate must be based on hiring
a third party to do this work. For this reason, a unit cost
ranging from $1,500 to $3,000 per acre should be used.
If swales on the sideslopes are used and a design
providing less than 2 tons per acre per year of soil loss
is developed, annual erosion control costs can be re-
duced (perhaps to a maintenance cost of 5 percent of
the cover). With good erosion control procedures, main-
tenance to prevent erosion damage will involve repairing
the damage caused by mowing equipment; on wet days,
a mower can create ruts and can tear up part of the
vegetative erosion control layer.
8.3.1.2 Leachate-Related Costs
The leachate collection system also must be maintained
and operated throughout the post-closure period, involv-
ing an annual inspection of primary leachate collection
lines and possibly hydroflushing to remove sediments
and biological growth. Such inspection and cleaning can
cost $10,000 to $25,000 annually, depending on the
number and length of leachate lines to be cleaned.
Operational costs for leachate treatment, repair of lift
stations, or hauling leachate to treatment also will be
incurred. Costs of maintaining and operating the
leachate collection systems during the post-closure pe-
riod will vary significantly from site to site. A conservative
estimate of annual leachate treatment costs can be
made by assuming a long-term leachate generation rate
of 1,000 gallons per acre per day and a range of
leachate treatment costs of $0.15 to $0.25 per gallon.
8.3.1.3 Ground-Water Monitoring
Ground-water monitoring programs will need to be ad-
justed as a facility increases in size, and such physical
changes will need to be incorporated into the cost esti-
mate. Ground-water monitoring wells must be installed
in the uppermost aquifer. Typical monitoring well costs
can range from $50 to $100 per foot, including ground
pad and locking cap. The number of wells required to
monitor a given MSWLF is influenced by the site hydro-
geology and facility layout. Typically, the number of
ground-water monitoring wells is negotiated with the ap-
propriate state regulators and is known before the landfill
begins operation. Such negotiations can be long term
and can require modification as new MSWLF cells are
opened.
Annual ground-water monitoring analysis costs are in-
fluenced by the number of wells monitored and the
number of contaminants being tested. Full biannual test-
ing for contaminants listed in Appendix I of 40 CFR Part
258 costs from $2,500 to $3,200 per well. Directors of
authorized state programs might approve a reduced
ground-water monitoring program that focuses on site-
specific contaminants.
8.3.1.4 Gas Monitoring System
The gas monitoring system also will need to be main-
tained and monitored quarterly during the post-closure
period. Gas monitoring is relatively inexpensive during
post-closure, requiring only a technician to check gas
levels in perimeter gas monitoring wells with a handheld
explosimeter. Annual gas monitoring costs range from
$1,000 to $1,600.
Passive gas venting pipes must be protected from dam-
age by traffic (such as mowing equipment). Damaged
vent pipes must be repaired quickly to prevent surface
water from entering the gas venting system, and, sub-
sequently, the landfill. Such repairs are inexpensive,
costing less than $200 per damaged well. An annual
budget of $1,000 for gas vent repair is appropriate.
8.4 Financial Assurance for Corrective
Action
The third financial assurance component requires the
MSWLF owner/operator to demonstrate that funds are
available to complete remediation if corrective action
has been deemed necessary at the site (see Chapter 6).
The financial assurance requirement for corrective ac-
tion is not needed unless ground-water contamination is
detected in a monitoring well. After the initial detection,
the MSWLF owner must develop and implement a correc-
tive action plan that includes identification of actual or
potential exposures to the contaminants. The owner/op-
erator of the landfill must notify the director of an approved
state program that a corrective action plan exists and
also must provide financial assurance for implementing the
plan. The amount of money designated for financial assur-
ance can be adjusted annually as remediation progresses.
Financial assurance must be provided until the remedia-
tion is completed, as certified by a qualified ground-water
scientist or the director of an approved state program.
8.5 Financial Assurance Mechanisms
Eleven financial assurance mechanisms are presented
as options in Subtitle D, including trust funds, surety
bonds, letters of credit, insurance, corporate financial
tests, local government financial tests, corporate guar-
antees, local government guarantees, state-approved
mschanisms, state assumption of responsibility, and
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use of multiple financial mechanisms. These mecha-
nisms are discussed below.
8.5.1 Trust Funds
The owner/operator of a MSWLF may establish a trust
fund to demonstrate financial assurance by providing
money to a reputable third party, a trustee, who holds
the funds until they are needed for closure, post-closure,
and/or corrective action. Payments must be made an-
nually into the trust fund. The initial payment must be
made before initial receipt of waste or before the effec-
tive dates in Subtitle D for closure or post-closure. For
corrective action, payments must be made no later than
120 days after a remedy has been selected. The trust
fund can be terminated if the owner/operator substitutes
another form of financial assurance or is no longer
required to demonstrate financial assurance.
8.5.2 Surety Bonds
An MSWLF owner/operator also may demonstrate fi-
nancial assurance by obtaining a surety bond for closure,
post-closure, and/or corrective action. Surety bonds are
issued by private firms, which typically require full collat-
eral for the bond. Such collateral usually involves assets
independent of the MSWLF. Both payment and perform-
ance surety bonds are acceptable to show financial
assurance for closure or post-closure. For corrective
action, only performance surety bonds are acceptable.
The surety company must be listed on an approved U.S.
Department of Treasury list referred to in Subtitle D. The
bond must be effective before initial receipt of waste or
before the effective dates in Subtitle D for closure or
post-closure, or for corrective action, no later than 120
days after a remedy has been selected. The owner/
operator also must establish a standby trust fund if a
surety bond is used as the primary financial assurance
mechanism. The owner/operator may cancel the bond if
he or she substitutes another form of financial assur-
ance or is no longer required to demonstrate financial
assurance.
8.5.3 Letter of Credit
The owner/operator also may use a letter of credit to
demonstrate financial assurance. The letter of credit
must be irrevocable and issued for at least one year. If
the letter of credit is canceled, the owner/operator must
obtain another form of financial assurance. The letter of
credit must be effective before the initial receipt of waste
or before the effective dates in Subtitle D for closure or
post-closure. For corrective action, the letter of credit
must be effective no later than 120 days after a remedy
has been selected. The owner/operator may cancel the
letter of credit if he or she substitutes another form of
financial assurance or is no longer required to demonstrate
financial assurance.
6.5.4 Insurance
The owner/operator may obtain an insurance policy to
demonstrate financial assurance. The policy must be
issued for a face amount at least equal to the current
cost estimate for closure, post-closure, and/or corrective
action, whichever is applicable. The face amount refers
to the total amount the insurer is obligated to pay; actual
payments do not change the face amount (although
future liability will be decreased by the amount of the
payments). For post-closure care, the insurer must in-
crease the face amount annually, as specified in the
Subtitle D regulation.
The insurance policy must include a provision assigning
the policy to a succeeding owner/operator. If the insur-
ance policy is canceled, the owner/operator must obtain
another form of financial assurance. The insurance policy
must be effective before initial receipt of waste or before
the effective dates in Subtitle D for closure or post-closure.
For corrective action, the policy must be effective no
later than 120 days after the remedy is selected. The
owner/operator may cancel the insurance policy if he or
she substitutes another form of financial assurance or is
no longer required to demonstrate financial assurance.
At least one insurance company has begun marketing
financial assurance as required in Subtitle D.
0.5.5 Corporate and Local Government
Financial Tests and Guarantees
Criteria for financial assurance mechanisms for corpo-
rate and local government financial tests and for corpo-
rate and local government guarantees currently are being
developed by EPA. Local financial assurance probably will
be an important financial assurance mechanism when
EPA determines criteria for local governments. Although
local governments probably will not be allowed to use
ad valorem (general revenue) taxes as a mechanism for
guaranteeing closure, the current bond rating and in-
debtedness of the local government most likely will be
important factors.
8.5.6 State-Approved Mechanisms
The MSWLF owner/operator may use any other finan-
cial assurance mechanism that meets the financial as-
surance requirements of Subtitle D and is approved by
the director of an approved state program.
8.5.7 State Assumption of Responsibility
Financial assurance requirements also may be met if the
director of an approved state program assumes legal
responsibility for an MSWLF's closure, post-closure,
and/or corrective action as required in Subtitle D or en-
sures that state funds will be available to meet these
requirements. Subtitle D is the first federal regulation that
explicitly treats counties and municipalities as transient,
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nonpermanent forms of government by requiring finan-
cial assurance for landfills. Where populations are de-
creasing, cities and counties are facing increased
financial hardships. In some states, landfills have been
abandoned by financially constrained local governments.
Financial assurance by states is a vehicle to prevent future
abandonment of MSWLFs. State, county, and municipal
fiscal responsibility can vary from state to state, depend-
ing on government organization. In Tennessee and
South Carolina, for example, counties (but not munici-
palities) are an extension of the state government and
therefore covered by state guarantees of financial sol-
vency; this may not be the case in other states.
8.5.8 Use of Multiple Financial Assurance
Mechanisms
An owner/operator may use a combination of the finan-
cial assurance mechanisms discussed above to demon-
strate financial assurance. Subtitle D includes restrictions
on using more than one financial mechanism, however,
if the mechanisms are not truly independent. For exam-
ple, the financial test and guarantee provided by a cor-
porate parent may not be combined with the guarantee
of a subsidiary if the financial statements of the two
firms are consolidated.
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Chapter 9
References
When an NTIS numebr is cited in a reference, that
document is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
703-487-4650
ABB Environmental Services. 1990. As cited in U.S. EPA.
1992, Seminars: Design, Operation, and Closure of Mu-
nicipal Solid Waste Landfills. U.S. Environmental Pro-
tection Agency, Office of Research and Development,
Washington, DC. EPA/600/K-92/002.
Algermissen, ST., et al. 1982. Probabilistic Estimates of
Maximum Acceleration and Velocity in Rock in the Con-
tiguous United States. U.S. Geological Survey, Open-
File Report 82-1033.
Algermissen, ST., et al. 1990. Probabilistic Earthquake
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86
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