chapter 4 subpart d design criteria - epa archives€¦ · physical processes controlling...

CHAPTER 4

SUBPART DDESIGN CRITERIA

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CHAPTER 4SUBPART D

TABLE OF CONTENTS

4.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

4.2 PERFORMANCE-BASED DESIGN 40 CFR §258.40 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1224.2.1 Statement of Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

4.2.2 Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1234.2.3 Technical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

Demonstration Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123Leachate Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124Assessment of Leakage Through Liners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Leachate Migration in the Subsurface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Physical Processes Controlling Contaminant Transport in the Subsurface . . 126Chemical Processes Controlling Contaminant Transport in the Subsurface

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128Biological Processes Controlling Contaminant Transport in the Subsurface

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129Leachate Migration Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130Overview of the Modeling Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130Model Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

Analytical Versus Numerical Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135Spatial Characteristics of the System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136Steady-State Versus Transient Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136Boundary and Initial Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137Homogeneous Versus Heterogeneous Aquifer/Soil Properties . . . . . . . . . . . 137Availability of Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

Summary of Available Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138The EPA Multimedia Exposure Assessment Model (MULTIMED) . . . . . . . . . . . . . 139

Overview of the Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147Application of MULTIMED to MSWLF Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

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4.3 COMPOSITE LINER AND LEACHATE COLLECTION SYSTEM 40 CFR §258.40 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1494.3.1 Statement of Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1494.3.2 Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1504.3.3 Technical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

Standard Composite Liner Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150Soil Liner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151Lift Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151Bonding Between Lifts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152Placement of Soil Liners on Slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152Hydraulic Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152Soil Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153Amended Soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154Soil Liner Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

Geomembranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160Material Types and Thicknesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160Chemical and Physical Stress Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . 160Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

Leachate Collection Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165Grading of Low-Permeability Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166High-Permeability Drainage Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167Soil Drainage Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167Geosynthetic Drainage Nets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

Leachate Collection Pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171Protection of Leachate Collection Pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173Protection of the High-Permeability Drainage Layer . . . . . . . . . . . . . . . . . . 178Soil Filter Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178Geotextile Filter Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

Leachate Removal System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181Other Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

Construction Quality Assurance and Quality Control . . . . . . . . . . . . . . . . . . . . . . . . 182CQA/CQC Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

Soil Liner Quality Assurance/Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183Soil Liner Pilot Construction (Test Fill) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185Geomembrane Quality Assurance/Quality Control Testing . . . . . . . . . . . . . . . . . . . . 185

Destructive Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185Non-Destructive Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

Geomembrane Construction Quality Assurance Activities . . . . . . . . . . . . . . . . . . . . 186Leachate Collection System Construction Quality Assurance . . . . . . . . . . . . . . . . . . 187

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4.4 RELEVANT POINT OF COMPLIANCE 40 CFR §258.40(d) . . . . . . . . . . . . . . . . . . . . . . . 1884.4.1 Statement of Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1884.4.2 Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1894.4.3 Technical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

Site Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189Leachate Volume and Physical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189Quality, Quantity and Direction of Ground-Water Flow . . . . . . . . . . . . . . . . . . . . . . 189Ground-Water Receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190Alternative Drinking Water Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190Existing Ground-Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190Public Health, Welfare, Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190Practicable Capability of the Owner or Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

4.5 PETITION PROCESS 40 CFR §258.40(e) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1914.5.1 Statement of Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1914.5.2 Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

4.6 FURTHER INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1934.6.1 REFERENCES (Specific to Performance-Based Design Assessment and Solute

Transport Modeling) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1934.6.2 REFERENCES (Specific to Design Criteria) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1994.6.3 Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

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CHAPTER 4SUBPART D

DESIGN CRITERIA

4.1 INTRODUCTION

New MSWLF facilities and lateral expansions of existing units must comply with either a designstandard or a performance standard for landfill design. The Federal Criteria do not require existingunits to be retrofitted with liners. The design standard requires a composite liner composed of twofeet of soil with a hydraulic conductivity of no more than 1 x 10 cm/sec, overlain by a flexible-7

membrane liner (FML) and a leachate collection system. A performance-based design mustdemonstrate the capability of maintaining contaminant concentrations below maximum contaminantlevels (MCLs) at the unit's relevant point of compliance. The performance standard has beenestablished to allow design innovation and consideration of site-specific conditions; approved Statesmay have adopted alternative design standards. Owners/operators are advised to work closely withState permitting agencies to determine the applicable design standard. Owners/operators inunapproved States may use the petition process (§258.40(c)) to allow for use of a performance-based design. This process is discussed in Section 4.5.

The technical considerations discussed in this chapter are intended to identify the key design featuresand system components for the composite liner and leachate collection system standards, and forthe performance standard. The technical considerations include 1) design concepts, 2) designcalculations, 3) physical properties, and 4) construction methods for the following:

1) Designs Based on the Performance Standard

! Leachate characterization and leakage assessment;

! Leachate migration in the subsurface;

! Leachate migration models; and

! Relevant point of compliance assessment.

2) Composite Liners and Leachate Collection Systems

! Soil liner component (soil properties lab testing, design, construction, and qualityassurance/quality control testing);

! Flexible membrane liners (FML properties, design installation, and qualityassurance/quality control testing);

! Leachate collection systems (strength and compatibility, grading and drainage, cloggingpotential, and filtration);

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! Leachate removal systems (pumps, sumps, and standpipes); and

! Inspections (field observations and field and laboratory testing).

Designs based on the performance standard are described in Section 4.2. Requirements forcomposite liners are discussed in Section 4.3. These sections address the minimum regulatoryrequirements that should be considered during the design, construction, and operation of MSWLFunits to ensure that they perform in a manner protective of human health and the environment.Additional features or procedures may be used to demonstrate conformance with the regulations orto control leachate release and subsequent effects. For example, during construction of a newMSWLF unit, or a lateral expansion of an existing MSWLF unit, quality control and qualityassurance procedures and documentation may be used to ensure that material properties andconstruction methods meet the design specifications that are intended to achieve the expected levelof performance. Section 4.4 presents methods to assess ground-water quality at the relevant pointof compliance for performance-based designs. Section 4.5 describes the applicability of the petitionprocess for States wishing to petition to use the performance standard.

4.2 PERFORMANCE-BASED DESIGN the regulatory language for requirements40 CFR §258.40(a)(1)

4.2.1 Statement of Regulation

(a) New MSWLF units and lateralexpansions shall be constructed:

(1) In accordance with a designapproved by the Director of an approvedState or as specified in §258.40(e) forunapproved States. The design must ensurethat the concentration values listed inTable 1 will not be exceeded in theuppermost aquifer at the relevant point ofcompliance as specified by the Director ofan approved State under paragraph (d) ofthis section, or

(2) (See Statement of Regulation inSection 4.3.1 of this guidance document forthe regulatory language for composite linerrequirements).

(b) (See Statement of Regulation inSection 4.3.1 of this guidance document for

pertaining to composite liner and leachatecollection systems).

(c) When approving a design thatcomplies with paragraph (a)(1) of thissection, the Director of an approved Stateshall consider at least the following factors:

(1) The hydrogeologic characteristicsof the facility and surrounding land;

(2) The climatic factors of the area;and

(3) The volume and physical andchemical characteristics of the leachate.

(d) (See Statement of Regulation inSection 4.4.1 of this guidance document for adiscussion of the determination of the relevantpoint of compliance.)

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TABLE 1(40 CFR 258.40; 56 FR 51022;

October 9, 1991)

Chemical MCL(mg/l)

Arsenic 0.05Barium 1.0Benzene 0.005Cadmium 0.01Carbon tetrachloride 0.005Chromium (hexavalent) 0.052,4-Dichlorophenoxy acetic acid 0.11,4-Dichlorobenzene 0.0751,2-Dichloroethane 0.0051,1-Dichloroethylene 0.007Endrin 0.0002Fluoride 4.0Lindane 0.004Lead 0.05Mercury 0.002Methoxychlor 0.1Nitrate 10.0Selenium 0.01Silver 0.05Toxaphene 0.0051,1,1-Trichloroethane 0.2Trichloroethylene 0.0052,4,5-Trichlorophenoxy acetic acid 0.01Vinyl Chloride 0.002

4.2.2 Applicability

The Director of an approved State mayapprove a performance-based design for newMSWLF units and lateral expansions ofexisting units (see Section 4.3.2), if it meetsthe requirements specified in 40 CFR258.40(a)(1). A performance-based design isan alternative to the design standard

(composite liner with a leachate collectionsystem). The composite design is required inunapproved States; however, if EPA does notpromulgate procedures for State approval byOctober 9, 1993, the performance-baseddesign may be available through the petitionprocess (see Section 4.5).

4.2.3 Technical Considerations

Demonstration Requirements

For approval of landfill designs notconforming to the uniform design standard ofa composite liner system and a leachatecollection system (40 CFR §258.40(a)(2)), theowner or operator of the proposed MSWLFunit must demonstrate to the Director of anapproved State that the design will not allowthe compounds listed in Table 1 of §258.40 toexceed the MCLs in ground water at therelevant point of compliance. Thedemonstration should consider an assessmentof leachate quality and quantity, leachateleakage to the subsurface, and subsurfacetransport to the relevant point of compliance.These factors are governed by sitehydrogeology, waste characteristics, andclimatic conditions.

The nature of the demonstration is essentiallyan assessment of the potential for leachateproduction and leakage from the landfill toground water, and the anticipated fate andtransport of constituents listed in Table 1 tothe proposed relevant point of compliance atthe facility. Inherent in this approach is theneed to evaluate whether contaminants inground water at the relevant point ofcompliance will exceed the concentrationvalues listed in Table 1. If so, then the owneror operator needs to obtain sufficient site-specific data to adequately characterize theexisting ground-

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water quality and the existing ground-water Assuming leachate is produced, theflow regime (e.g., flow direction, horizontal demonstration should evaluate whetherand vertical gradients, hydraulic conductivity, constituents listed in Table 1 can be expectedstratigraphy, and aquifer thickness). to be present at concentrations greater than the

An assessment should be made of the effect must address the hydrogeologic characteristicsMSWLF facility construction will have on of the facility and the surrounding land tosite hydrogeology. The assessment should comply with §258.40(d). The followingfocus on the reduced infiltration over the sections describe the various parts of alandfill area and altered surface water run-off demonstration in greater detail.patterns. Reduction of ground-water rechargeand changes in surface water patterns Leachate Characterizationresulting from landfill construction may affectground-water gradients in some cases and Leachate characterization should include anmay result in changes in lateral flow assessment and demonstration of the quantitydirections. One example of a hypothetical and composition of leachate anticipated at theperformance-based demonstration follows. proposed facility. Discussion of this

It is possible that a MSWLF unit located in anarid climatic zone would not produce leachate Estimates of volumetric production rates offrom sources of water (e.g., precipitation) leachate are important in evaluating the fateother than that existing within the waste at the and transport of the constituents listed intime of disposal. In such an environment, an Table 1. Leachate production rates depend onowner or operator may demonstrate that rainfall, run-on, run-off, evapotranspiration,significant quantities of leachate would not be water table elevation relative to the bottom ofproduced. The demonstration should be the landfill unit, in-place moisture content ofsupported by evaluating historic precipitation waste, and the prevention of liquid disposal atand evaporation data and the likelihood that the site. Run-on, run-off, and water tablethe unit could be flooded as the result of factors can be managed traditionally throughheavy rains, surface run-off, or high water design and operational controls. The MSWLFtables. It may be possible, through Criteria prohibit bulk or containerized liquidoperational controls, to avoid exposing waste disposal. Incident precipitation andto precipitation or infiltration of water evapotranspiration can be evaluated usingthrough overlying materials. If significant models (e.g., HELP) or other methods ofleachate production would not be expected, estimating site-specific leachate productionthe regulatory authority, when reviewing the (e.g., local historical meteorologic data). demonstration, should consider thehydrogeologic characteristics of the facility If leachate composition data that areand the surrounding area, in addition to the representative of the proposed facility are notexpected volume of leachate and climatic available, then leachate data with a similarfactors. expected composition should be presented.

MCLs. If such a demonstration is possible, it

assessment follows.

Landfill leachate composition is influencedby:

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(1) The annual infiltration of precipitation In lieu of the existence or availability of suchand rate of leaching; information, conservative analytical

(2) The type and relative amounts of anticipated leakage rates.materials in the waste stream; and

(3) The age and the biological maturity of through geomembranes differs in principlethe landfill unit, which may affect the from transport through soil liner materials.types of organic and inorganic acids The dominant mode of leachate transportgenerated, oxidation/reduction potential through liner components is flow through(Eh), and pH conditions. holes and penetrations of the geomembrane,

An existing landfill unit in the same region, Transport through geomembranes where tears,with similar waste stream characteristics, may punctures, imperfections, or seam failures areprovide information that will allow the owner not involved is dominated by molecularor operator to anticipate leachate composition diffusion. Diffusion occurs in response to aof the proposed landfill unit. A review of concentration gradient and is governed byexisting literature also may be required to Fick's first law. Diffusion rates throughassess anticipated leachate composition if geomembranes are very low in comparison toactual data are unavailable (see U.S. EPA, hydraulic flow rates in soil liners, including1987b). A wide range of leachate compacted clays. For synthetic liners, theconcentrations are reported in the literature most significant factor influencing linerwith higher concentrations of specific performance is penetration of the liner,constituents typically reported for the initial including imperfect seams or pinholes causedleachate from laboratory or field experimental by construction defects in the geomembranetest fills or test cells. These "batch" one-day (U.S. EPA, 1989).landfill tests do not account for the long-termclimatic and meteorological influences on a A relatively new product now being used infull-scale landfill operation. Such high initial liner systems is the geosynthetic clay linerconcentrations are not typical of full-scale (GCL). GCLs consist of a layer of pureoperations (which are subject to the dilution bentonite clay backed by one or twoeffects of incidental rainfall on unused geotextiles. GCLs exhibit properties of bothportions of the unit). soil liners and geomembranes, and have

Assessment of Leakage Through Liners component in composite liner designs. GCLs

An assessment of leakage (the volumetric through diffusion according to their lowrelease of leachate from the proposed hydraulic conductivities (i.e., 1 x 10 cm/secperformance-based design) should be based reported by manufacturers). Applications foron analytical approaches supported by GCLs are discussed further in the sections thatempirical data from other existing operational follow.facilities of similar design, particularly thosethat have leak detection monitoring systems Several researchers have studied the flow of(see U.S. EPA, 1990b). fluids through imperfections in single

assumptions may be used to estimate

The transport of fluids and waste constituents

and Darcian flow through soil components.

successfully substituted for the soil

are believed to transport fluids primarily

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geomembrane and composite liner systems. degradation of contaminants. The degree ofFurther discussion of liner leakage rates canbe found in Section 4.3.3 below. Forempirical data and analytical methods thereader is referred to Jayawickrama et al.(1988), Kastman (1984), Haxo (1983), Haxoet al. (1984), Radian (1987), Giroud andBonaparte (1989, Parts I and II), and Giroudet al. (1989). Leakage assessments also maybe conducted with the use of the HELP model(U.S. EPA, 1988). Version 3.0 of the modelis under revision and will include an updatedmethod to assess leakage that is based onrecent research and data compiled by Giroudand Bonaparte.

Leachate Migration in the Subsurface

Leachate that escapes from a landfill unit maymigrate through the unsaturated zone andeventually reach the uppermost aquifer. Insome instances, however, the water table maybe located above the base of the landfill unit,so that only saturated flow and transport fromthe landfill unit need to be considered. Onceleachate reaches the water table, contaminantsmay be transported through the saturated zoneto a point of discharge (i.e., a pumping well,a stream, a lake, etc.).

The migration of leachate in the subsurfacedepends on factors such as the volume of theliquid component of the waste, the chemicaland physical properties of the leachateconstituents, the loading rate, climate, and thechemical and physical properties of thesubsurface (saturated and unsaturated zones).A number of physical, chemical, andbiological processes also may influencemigration. Complex interactions betweenthese processes may result in specificcontaminants being transported through thesubsurface at different rates. Certainprocesses result in the attenuation and

attenuation is dependent on the time that thecontaminant is in contact with the subsurfacematerial, the physical and chemicalcharacteristics of the subsurface material, thedistance that the contaminant has traveled,and the volume and characteristics of theleachate. Some of the key processes affectingleachate migration are discussed briefly here.The information is based on a summary inTravers and Sharp-Hansen (1991), who inturn relied largely on Aller et al. (1987),Keely (1987), Keely (1989), Lu et al. (1985),and U.S. EPA (1988a).

Physical Processes ControllingContaminant Transport in the Subsurface

Physical processes that control the transport ofcontaminants in the subsurface includeadvection, mixing and dilution as a result ofdispersion and diffusion, mechanicalfiltration, physical sorption, multi-phase fluidflow, and fracture flow. These processes, inturn, are affected by hydrogeologiccharacteristics, such as hydraulic conductivityand porosity, and by chemical processes.

Advection is the process by which solutecontaminants are transported by the overallmotion of flowing ground water. A non-reactive solute will be transported at the samerate and in the same direction as ground waterflow (Freeze and Cherry, 1979). Advectivetransport is chiefly a function of thesubsurface hydraulic conductivity distribution,porosity, and hydraulic gradients.

Hydrodynamic dispersion is a non-steady,irreversible mixing process by which acontaminant plume spreads as it is transportedthrough the subsurface. Dispersion resultsfrom the effects of two

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components operating at the microscopic and alter its flow direction to conform to thelevel: mechanical dispersion and moleculardiffusion. Mechanical dispersion results fromvariations in pore velocities within the soil oraquifer and may be more significant thanmolecular diffusion in environments wherethe flow rates are moderate to high.Molecular diffusion occurs as a result ofcontaminant concentration gradients;chemicals move from high concentrations tolow concentrations. At very slow ground-water velocities, as occur in clays and silts,diffusion can be an important transportmechanism.

Mechanical filtration removes from groundwater contaminants that are larger than thepore spaces of the soil. Thus, the effects ofmechanical filtration increase with decreasingpore size within a medium. Filtration occursover a wide range of particle sizes. Theretention of larger particles may effectivelyreduce the permeability of the soil or aquifer.

Physical sorption is a function of Van derWaals forces, and the hydrodynamic andelectrokinetic properties of soil particles.Sorption is the process by which contaminantsare removed from solution in ground waterand adhere or cling to a solid surface. Thedistribution of a contaminant between thesolution and the solid phase is calledpartitioning.

Multiphase fluid flow occurs because manysolvents and oils are highly insoluble in waterand may migrate in the subsurface as aseparate liquid phase. If the viscosity anddensity of a fluid differ from that of water, thefluid may flow at a different rate and directionthan the ground water. If the fluid is moredense than water it may reach the bottom ofthe aquifer (top of an aquitard)

shape and slope of the aquitard surface.

Hydraulic conductivity is a measure of theability of geologic media to transmit fluids(USGS, 1987). It is a function of the size andarrangement of water-transmitting openings(pores and fractures) in the media and of thecharacteristics of the fluids (density, viscosity,etc.). Spatial variations in hydraulicconductivity are referred to as heterogeneities.A variation in hydraulic conductivity with thedirection of measurement is referred to asanisotropy.

Variable hydraulic conductivity of thegeologic formation may cause ground-waterflow velocities to vary spatially. Variations inthe rate of advection may result in non-uniform plume spreading. The changes inaquifer properties that lead to this variabilityin hydraulic conductivity may be three-dimensional. If the geologic medium isrelatively homogeneous, it may beappropriate, in some instances, to assume thatthe aquifer properties also are homogeneous.

Secondary porosity in rock may be caused bythe dissolution of rock or by regionalfracturing; in soils, secondary porosity may becaused by desiccation cracks or fissures.Fractures or macropores respond quickly torainfall events and other fluid inputs and cantransmit water rapidly along unexpectedpathways. Secondary porosity can result inlocalized high concentrations of contaminantsat significant distances from the facility. Therelative importance of secondary porosity tohydraulic conductivity of the subsurfacedepends on the ratio of fracture hydraulicconductivity to intergranular hydraulicconductivity (Kincaid et al., 1984a). Forscenarios in which fracture flow is dominant,the relationships

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used to describe porous flow (Darcy's Law) generally occurs at a relatively rapid ratedo not apply. compared to precipitation reactions.

Chemical Processes Controlling The dominant mechanism of organic sorptionContaminant Transport in the Subsurface is the hydrophobic attraction between a

Chemical processes that are important in in some aquifers. The organic carbon contentcontrolling subsurface transport include of the porous medium, and the solubility ofprecipitation/dissolution, chemical sorption, the contaminant, are important factors for thisredox reactions, hydrolysis, ion exchange, and type of sorption.complexation. In general, these processes,except for hydrolysis, are reversible. The There is a direct relationship between thereversible processes tend to retard transport, quantity of a substance sorbed on a particlebut do not permanently remove a contaminant surface and the quantity of the substancefrom the system. Sorption and precipitation suspended in solution. Predictions about theare generally the dominant mechanisms sorption of contaminants often make use ofretarding contaminant transport in the sorption isotherms, which relate the amount ofsaturated zone. contaminant in solution to the amount

Precipitation/dissolution reactions can control contaminants, these isotherms are usuallycontaminant concentration levels. The assumed to be linear and the reaction issolubility of a solid controls the equilibrium assumed to be instantaneous and reversible.state of a chemical. When the soluble The linear equilibrium approach to sorptionconcentration of a contaminant in leachate is may not be adequate for all situations.higher than that of the equilibrium state,precipitation occurs. When the soluble Oxidation and reduction (redox) reactionsconcentration is lower than the equilibrium involve the transfer of electrons and occurvalue, the contaminant exists in solution. The when the redox potential in leachate isprecipitation of a dissolved substance may be different from that of the soil or aquiferinitiated by changes in pressure, temperature, environment. Redox reactions are importantpH, concentration, or redox potential (Aller et processes for inorganic compounds andal., 1987). Precipitation of contaminants in metallic elements. Together with pH, redoxthe pore space of an aquifer can decrease reactions affect the solubility, complexingaquifer porosity. Precipitation and dissolution capacity, and sorptive behavior ofreactions are especially important processes constituents, and thus control the presence andfor trace metal migration in soils. mobility of many substances in water.

Chemical adsorption/desorption is the most proportion of redox reactions that occur incommon mechanism affecting contaminant ground water. The redox state of an aquifer,migration in soils. Solutes become attached and the identity and quantity of redox-activeto the solid phase by means of adsorption. reactants, are difficult to determine. L i k e p rec ip i t a t ion /d i s so lu t ion ,adsorption/desorption is a reversible process.However, adsorption/desorption

chemical and natural organic matter that exists

adsorbed to the solids. For organic

Microorganisms are responsible for a large

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Hydrolysis is the chemical breakdown of Therefore, these processes are usuallycarbon bonds in organic substances by water grouped together as one mechanism.and its ionic species H and OH . Hydrolysis+ -

is dependent on pH and Eh and is most Biological Processes Controllingsignificant at high temperatures, low pH, and Contaminant Transport in the Subsurfacelow redox potential. For many biodegradablecontaminants, hydrolysis is slow compared to Biodegradation of contaminants may resultbiodegradation. from the enzyme-catalyzed transformation of

Ion exchange originates primarily from Contaminants can be degraded to harmlessexchange sites on layered silicate clays and byproducts or to more mobile and/or toxicorganic matter that have a permanent negative products through one or more of severalcharge. Cation exchange balances negative biological processes. Biodegradation of acharges in order to maintain neutrality. The compound depends on environmental factorscapacity of soils to exchange cations is called such as redox potential, dissolved oxygenthe cation exchange capacity (CEC). CEC is concentration, pH, temperature, presence ofaffected by the type and quantity of clay other compounds and nutrients, salinity, depthmineral present, the amount of organic matter below land surface, competition amongpresent, and the pH of the soil. Major cations different types of organisms, andin leachate (Ca, Mg, K, Na) usually dominate concentrations of compounds and organisms.the CEC sites, resulting in little attenuation in The transformations that occur in a subsurfacesoils of trace metals in the leachate. system are difficult to predict because of the

A smaller ion exchange effect for anions is reactions that may occur. Quantitativeassociated with hydrous oxides. Soils predictions of the fate of biologically reactivetypically have more negatively charged clay substances are subject to a high degree ofparticles than positively charged hydrous uncertainty, in part, because little informationoxides. Therefore, the transport of cations is is available on biodegradation rates in soilattenuated more than the transport of anions. systems or ground water. First-order decay

Complexation involves reactions of metal ionswith inorganic anions or organic ligands. The The operation of Subtitle D facilities canmetal and the ligand bind together to form a introduce bacteria and viruses into thenew soluble species called a complex. subsurface. The fate and transport of bacteriaComplexation can either increase the and viruses in the subsurface is an importantconcentration of a constituent in solution by consideration in the evaluation of the effectsforming soluble complex ions or decrease the of MSWLF units on human health and theconcentration by forming a soluble ion environment. A large number of biological,complex with a solid. It is often difficult to chemical, and physical processes are known todistinguish among sorption, solid-liquid influence virus and bacterial survival andcomplexation, and ion exchange. transport in the subsurface. Unfortunately,

organic compounds by microbes.

complexity of the chemical and biological

constants are often used instead.

knowledge of the processes and the availabledata are insufficient to develop models thatcan

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simulate a wide variety of site-specific evaluated by qualified investigators, shouldconditions. adequately simulate the significant processes

Leachate Migration Models consistent with the complexity of the study

After reviewing the hydrogeologic of the study. characteristics of the site, the nature of linerleakage, and the leachate characteristics, it First, an evaluation of the need for modelingmay be appropriate to use a mathematical should be made (Figure 4-1). When selectingmodel to simulate the expected fate and a model to evaluate the potential for soil andtransport of the constituents listed in Table 1 ground-water contamination (Boutwell et al.,to the relevant point of compliance. Solute 1986), three basic determinations must betransport and ground-water modeling efforts made (Figure 4-2). Not all studies require theshould be conducted by a qualified ground- use of a mathematical model. This decisionwater scientist (see Section 5.5). It is should be made at the beginning of the study,necessary to consider several factors when since modeling may require a substantialselecting and applying a model to a site. amount of resources and effort. Next, theTravers and Sharp-Hansen (1991) provide a level of model complexity required for athorough review of these issues. The text specific study should be determined (Figureprovided below is a summary of their review. 4-3). Boutwell et al. (1986) classify models

Overview of the Modeling Process (complex/numerical) models. A flowchart for

A number of factors can influence leachate required is shown in Figure 4-3. Finally, themigration from MSWLF units. These model capabilities necessary to represent ainclude, but are not limited to, climatic particular system should be consideredeffects, the hydrogeologic setting, and the (Figure 4-4). Several models may be equallynature of the disposed waste. Each facility is suitable for a particular study. In some cases,different, and no one generic model will be it may be necessary to link or couple two orappropriate in all situations. To develop a more computer models to accurately representmodel for a site, the modeling needs and the the processes at the site. In the section thatobjectives of the study should be determined follows, specific issues that should befirst. Next, it will be necessary to collect data considered when developing a scenario andto characterize the hydrological, geological, selecting a model are described.chemical, and biological conditions of thesystem. These data are used to assist in the Models are a simplified representation of thedevelopment of a conceptual model of the real system, and as such, cannot fullysystem, including spatial and temporal reproduce or predict all site characteristics.characteristics and boundary conditions. The Errors are introduced as a result of: 1)conceptual model and data are then used to simplifying assumptions; 2) a lack of data; 3)select a mathematical model that accurately uncertainty in existing data; 4) a poorrepresents the conceptual model. The model understanding of the processes influencing theselected should have been tested and fate and transport of contaminants; and

present in the actual system, and should be

area, amount of available data, and objectives

as Level I (simple/analytical) and Level II

determining the level of model complexity

131

Figure 4-1Three Basic Decisions in Model Selection

(Boutwell et. al., 1986)

132

Figure 4-2Flow Chart to Determine if Modeling is Required


133

Figure 4-3Flow Chart to Determine the Level of Modeling Required for

Soil and Groundwater Systems(Boutwell et. al., 1986)

134

Figure 4-4Flow Chart for Required Model Capabilities for Soil and Groundwater Systems


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5) limitations of the model itself. Therefore, Model Selectionmodel results should be interpreted asestimates of ground-water flow andcontaminant transport. Bond and Hwang(1988) recommend that models be used forcomparing various scenarios, since allscenarios would be subject to the samelimitations and simplifications.

The quality of model results can depend to alarge extent on the experience and judgementof the modeler, and on the quality of the dataused to develop model input. The process ofapplying the model may highlight datadeficiencies that may require additional datacollection. The model results should becalibrated to obtain the best fit to the observeddata. The accuracy of the results obtainedfrom modeling efforts should then bevalidated. Model validation, which is thecomparison of model results withexperimental data or environmental data, is acritical aspect of model application, and isparticularly important for site-specificevaluations.

Several recent reports present detaileddiscussions of the issues associated withmodel selection, application, and validation.Donigian and Rao (1990) address each ofthese issues, and present several options fordeveloping a framework for model validation.EPA's Exposure Assessment Group hasdeveloped suggested recommendations andguidance on model validation (Versar Inc.,1987). A recent report by the NationalResearch Council (1990) discusses the issuesrelated to model application and validation,and provides recommendations for the properuse of ground-water models. Weaver et al. limited data available in most field situations(1989) discuss options for selection and fieldvalidation of mathematical models.

Ground-water flow and solute transportmodels range from simple, analyticalcalculations to sophisticated computerprograms that use numerical solutions to solvemathematical equations describing flow andtransport. A sophisticated model may notyield an exact estimate of water quality at therelevant point of compliance for a given set ofsite conditions, but it may allow an estimateof the effects of complex physical andchemical processes. Depending on thecomplexity of site conditions and theappropriateness of the simplifyingassumptions, a fairly sophisticated numericalmodel may provide useful estimates of waterquality at the relevant point of compliance.

The following considerations should beaddressed when selecting a model.

Analytical Versus Numerical Models

Mathematical models use either analytical,semi-analytical, or numerical solutions forground-water flow and transport equations.Each technique has advantages anddisadvantages. Analytical solutions arecomputationally more efficient than numericalsimulations and are more conducive touncertainty analysis (i.e., Monte Carlotechniques). Typically, input data foranalytical models are simple and do notrequire detailed familiarity with the computermodel or extensive modeling experience.Analytical solutions are typically used whendata necessary for characterization of the siteare sparse and simplifying assumptions areappropriate (Javandel et al., 1984). The

may not justify the use of a detailed numericalmodel; in some cases, results from simpleanalytical models may be appropriate

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(Huyakorn et al., 1986). Analytical models "screening level" model to define the range ofrequire simplifying assumptions about the possible values, and to use a numerical modelsystem. Therefore, complex interactions when there are sufficient data. involving several fate and transport processescannot be addressed in detail. Analytical A highly complex hydrogeologic systemmodels generally require a limited number of cannot be accurately represented with aparameters that are often assumed to be simple analytical model. Heterogeneous orconstant in space and time (van der Heijde anisotropic aquifer properties, multipleand Beljin, 1988). aquifers, and complicated boundary

Semi-analytical models approximate complex models. In addition, sophisticated numericalanalytical solutions using numerical models are available that can simulatetechniques (van der Heijde and Beljin, 1988). processes such as fracture flow. Because eachSemi-analytical methods allow for more site is unique, the modeler should determinecomplex site conditions than those that can be which conditions and processes are importantsimulated with a purely analytical solution. at a specific site, and then select a suitableSemi-analytical solution methods can consider model. multiple sources or recharging anddischarging wells. However, they still require Spatial Characteristics of the Systemsimplifying assumptions about thedimensionality and homogeneity of the Although actual landfill units andsystem. hydrogeologic systems are three-dimensional,

Numerical models are able to evaluate more dimensions simulated in a mathematicalcomplex site conditions than either analytical model to one or two. Two- and three-or semi-analytical models. Numerical models dimensional models are generally moreprovide the user with a large amount of complex and computationally expensive thanflexibility; irregular boundaries and spatial one-dimensional models, and thereforeand temporal variations in the system can be require more data. In some instances, a one-considered. Numerical models require dimensional model may adequately representsignificantly more data than analytical the system; the available data may not warrantmodels, and are typically more the use of a multi-dimensional model.computationally intensive. Use of a However, modeling a truly three-dimensionalnumerical model requires an experienced system using a two-dimensional model maymodeler, and can involve a larger amount of produce results without adequate spatialcomputer time than simulations using an detail. The choice of the number ofanalytical or semi-analytical method. dimensions in the model should be made for

To select an appropriate model, the site and the availability of data.complexity of the site hydrology and theavailability of data should be considered. If Steady-State Versus Transient Modelsdata are insufficient, a highly sophisticatedand complex model should not be used. In Models can simulate either steady-state orsome situations, it is beneficial to use ananalytical or semi-analytical model as a

conditions can be simulated using numerical

it is often desirable to reduce the number of

a specific site, based on the complexity of the

transient flow conditions. It may be

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appropriate to assume that some ground-water One of the most significant boundaryflow systems have reached approximate"steady-state" conditions, which implies thatthe system has reached equilibrium and nosignificant changes are occurring over time.The assumption of steady-state conditionsgenerally simplifies the mathematicalequations used to describe flow processes, andreduces the amount of input data required.

However, assuming steady-state conditions ina system that exhibits transient behavior mayproduce inaccurate results. For example,climatic variables, such as precipitation, varyover time and may have strong seasonalcomponents. In such settings, the assumptionof constant recharge of the ground-watersystem would be incorrect. Steady-statemodels also may not be appropriate forevaluating the transport of chemicals whichsorb or transform significantly (Mulkey et al.,1989). The choice of simulating steady-stateor transient conditions should be based on thedegree of temporal variability in the system.

Boundary and Initial Conditions

The solution of differential equationsdescribing flow and transport processesrequires that initial and boundary conditionsbe specified. The initial conditions describethe conditions present in the system at thebeginning of the simulation. In many ground-water flow and transport models, theseconditions are related to the initial hydraulicconditions in the aquifer and the initialconcentration of contaminants. Boundaryconditions define the conditions present on theborders of the system, which may be steady-state or temporally variable. The initial andboundary conditions chosen to represent a sitecan significantly affect the results of thesimulation.

conditions in solute transport models is theintroduction of a contaminant to the system.A source of ground-water contaminationshould be described in terms of its spatial,chemical, and physical characteristics, and itstemporal behavior. Spatially, a source may beclassified as a point source, line source, adistributed source of limited areal or three-dimensional extent, or as a non-point sourceof unlimited extent (van der Hjeide et al.,1988). Typically, temporal descriptions of thesource term boundary conditions for modelswith analytical solutions are constant, constantpulse, and/or exponential decay (Mulkey etal., 1989). Numerical models typically handlea much wider range of source boundaryconditions, allowing for a wide range ofcontaminant loading scenarios.

Homogeneous Versus HeterogeneousAquifer/Soil Properties

The extent of the spatial variability of theproperties of each aquifer will significantlyaffect the selection of a mathematical model.Many models assume uniform aquiferproperties, which simplifies the governingequations and improves computationalefficiency. For example, a constant value ofhydraulic conductivity may be assumed atevery point in the aquifer. However, thisassumption may ignore the heterogeneity inthe hydrogeologic system. Bond and Hwang(1988) present guidelines for determiningwhether the assumption of uniform aquiferproperties is justified at a particular site.They state that the error associated with usingan average value versus a spatial distributionis site-specific and extremely difficult todetermine.

When site-specific data are limited, it iscommon to assume homogeneous and

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isotropic aquifer properties, and to develop a Uncertainty in model predictions results from"reasonable worst-case" scenario for the inability to characterize a site in terms ofcontaminant migration in the subsurface. the boundary conditions or the key parametersHowever, as Auerbach (1984) points out, the describing the significant flow and transportassumption of homogeneous and isotropic processes (National Research Council, 1990).aquifers often will not provide a "worst-case" The application of a mathematical model to ascenario. For example, a continuous zone of site typically requires a large amount of data.higher hydraulic conductivity in the direction Inexperienced modelers may attempt to applyof ground-water flow can result in much a model with insufficient data and, as a result,higher rates of contaminant movement than produce model results that are inconclusive. would be predicted in a completelyhomogeneous aquifer. To develop a true To obtain accurate model results, it is"worst-case" model, information on the essential to use data that are appropriate forprobable heterogeneity and anisotropy of the the particular site being modeled. Models thatsite should be collected. include generic parameters, based on average

The number of aquifers in the hydrogeologic initial guidance and general information aboutsystem also will affect the selection of a the behavior of a system, but it ismathematical model. Some systems include inappropriate to apply generic parameters to aonly a single unconfined or confined aquifer, specific hydrogeologic system. An excellentwhich is hydraulically isolated from the summary of the data required to modelsurrounding layers. Some mathematical saturated and unsaturated flow, surface watermodels, and in particular those with analytical flow, and solute transport is presented insolutions, can simulate only single layers. In Mercer et al. (1983). This report providesother cases, the upper aquifer may be definitions and possible ranges of values forhydraulically connected to underlying source terms, dependent variables, boundaryaquifers. The MSWLF Criteria specify that conditions, and initial conditions.MCLs not be exceeded at the relevant point ofcompliance within the uppermost aquifer. Summary of Available ModelsThe uppermost aquifer includes not only theaquifer that is nearest the ground surface, but Several detailed reviews of ground-wateralso all lower aquifers that are hydraulically models are available in the literature. Aconnected to the uppermost aquifer within the number of ground-water models, includingvicinity of the facility. saturated flow, solute transport, heat transport,

Availability of Data are summarized in van der Heijde et al.

Although computer models can be used to (1988) provides detailed descriptions of 64make predictions about leachate generation ground-water flow and solute transportand migration, these predictions are highly models that were selected for use independent on the quantity and quality of the determining wellhead protection areas. Aavailable data. One of the most common review of ground-water flow and limitations to modeling is insufficient data.

values for similar sites, can be used to provide

fracture flow, and multiphase flow models,

(1988). A report by van der Heijde and Beljin

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transport models for the unsaturated zone is contaminant source terms and the releases ofpresented in Oster (1982). A large number of contaminants to the subsurface. Flow andground-water flow and transport models are transport models simulate the transport ofsummarized by Bond and Hwang (1988). contaminants released from the source to theFinally, Travers and Sharp-Hansen (1991) unsaturated and saturated zones.summarize models that may be applicable to Geochemical models are available thatproblems of leachate generation and migration consider chemical processes that may befrom MSWLF units. (See References active in the subsurface such as adsorption,supplied in Section 4.6.) precipitation, oxidation/reduction, aqueous

Table 4-1 (adapted from Travers and Sharp-Hansen (1991)) provides information on Complex flow models have been developed toselect leachate generation models. Tables 4- simulate the effects of nearby pumping and1a, b, and c list some of the available models discharging wells, fracture flow, conduit flowthat can be used to predict contaminant in karst terrane, and multiphase flow for fluidstransport. The factors used to select these that are less dense or more dense than water.models include availability, documentation, However, the use of the more complexuniqueness, and the size of the user models requires additional data based on acommunity. These models are categorized by thorough investigation of the subsurfacethe techniques used to solve flow and characteristics at a site as well as well-trainedtransport equations. Table 4-1a lists users to apply the model correctly.analytical and semi-analytical models, andTables 4-1b and 4-1c list numerical models Most of the ground-water flow and solutethat are solved by the finite-difference and the transport models are deterministic. However,finite-element method, respectively. the use of stochastic models, which allow for

The types of models that are available for variability in systems, is increasing. A few ofapplication to the evaluation of MSWLF the models include a Monte Carlo capabilitydesigns include leachate generation models for addressing the uncertainty inherent in theand saturated and unsaturated zone flow and input parameters. transport models. The level of sophisticationof each of these types of models is based on The EPA Multimedia Exposurethe complexity of the processes being Assessment Model (MULTIMED)modeled. The majority of the modelsconsider flow and transport based on EPA has developed a modeling package toadvection dispersion equations. More meet the needs of a large percentage ofcomplex models consider physical and MSWLF unit owners and operators who willchemical transformation processes, fracture require fate and transport modeling as part offlow, and multiphase fluid flow. the performance-based design demonstration.

Leachate generation models predict the Assessment Model (MULTIMED), isquantity and characteristics of leachate that is intended for use at sites where certainreleased from the bottom of a landfill. These simplifying assumptions can be made.models are used to estimate MULTIMED can be used in

speciation, and kinetics.

characterization of spatial and temporal

This model, the Multimedia Exposure

Table 4-1. Models for Application to Leachate Generation Problems (adapted from Travers and Sharp-Hansen, 1991)

Table 4-1a. Analytical and Semi-Analytical Models for Application to Leachate Migration Problems(adapted from Travers and Sharp-Hansen, 1991)

Table 4-1a. Analytical and Semi-Analytical Models for Application to Leachate Migration Problems(adapted from Travers and Sharp-Hansen, 1991) (continued)

Table 4-1b. Finite-Difference Models for Application to Leachate Migration Problems(adapted from Travers and Sharp-Hansen, 1991)

Table 4-1b. Finite-Difference Models for Application to Leachate Migration Problems(adapted from Travers and Sharp-Hansen, 1991) (continued)

Table 4-1c. Finite-Element Models for Application to Leachate Migration Problems(adapted from Travers and Sharp-Hansen, 1991)

Table 4-1c. Finite-Element Models for Application to Leachate Migration Problems(adapted from Travers and Sharp-Hansen, 1991) (continued)

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conjunction with a separate leachate source because the effects of lateral or verticalmodel, such as HELP (Schroeder et al., 1984). dispersion may significantly affect the modelOutput from HELP is then used in results. MULTIMED to demonstrate that either alandfill design or the specific hydrogeologic Therefore, reducing the dimensions to one inconditions present at a site will prevent this module would produce inaccurate results.contaminant concentrations in ground water The saturated zone transport module alsofrom exceeding the concentrations listed in considers linear adsorption, first-order decay,Table 1 of §258.40. (Refer to pp. 4-53 and 6- and dilution as a result of ground-water8 for further discussion of HELP.) A recharge. In addition, MULTIMED has thedescription of MULTIMED follows with capability to assess the impact of uncertaintyguidance for determining if its use is in the model inputs on the model outputappropriate for a given site. (contaminant concentration at a specified

[NOTE: Version 3.0 of the HELP model will technique.be available during the fall of 1993. Toobtain a copy, call EPA's Office of Research The simplifying assumptions required toand Development (ORD) in Cincinnati at obtain the analytical solutions limit the(513) 569-7871.] complexity of the systems that can be

Overview of the Model not account for site-specific spatial variability

The MULTIMED model consists of modules land disposal facility, site-specific boundarythat estimate contaminant releases to air, soil, conditions, or multiple aquifers and pumpingground water, or surface water. General wells. Nor can MULTIMED simulateinformation about the model and its theory is processes, such as flow in fractures andprovided in Salhotra et al. (1990). chemical reactions between contaminants, thatAdditionally, information about the may have a significant effect on theapplication of MULTIMED to MSWLF units concentration of contaminants at a site. In(developed by Sharp-Hansen et al. [1990]) is more complex systems, it may be beneficial tosummarized here. In MULTIMED, a steady- use MULTIMED as a "screening level" modelstate, one-dimensional, semi-analytical to allow the user to obtain an understanding ofmodule simulates flow in the unsaturated the system. A more complex model couldzone. The output from this module, which is then be used if there are sufficient data.water saturation as a function of depth, is usedas input to the unsaturated zone transport Application of MULTIMED to MSWLFmodule. The latter simulates transient, one- Unitsdimensional (vertical) transport in theunsaturated zone and includes the effects of Procedures have been developed for thedispersion, linear adsorption, and first-order application of MULTIMED to the design ofdecay. Output from the unsaturated zone MSWLF units. They are explained in Sharp-modules is used as input to the semi-analytical Hansen et al. (1990) and are brieflysaturated zone transport module. The latter summarized here. The procedures are: considers three-dimensional flow

point), using the Monte Carlo simulation

evaluated with MULTIMED. The model does

(e.g., aquifer heterogeneities), the shape of the

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! Collect site-specific hydrogeologic data, 4-2. A "no" answer to any of the questions inincluding amount of leachate generated Table 4-2 may indicate that MULTIMED is(see Section 4.3.3); not the most appropriate model to use. As

! Identify the contaminant(s) to be and semi-analytical solution techniques tosimulated and the point of compliance; solve the mathematical equations describing

! Propose a landfill design and determinethe corresponding infiltration rate; then

! Run MULTIMED and calculate thedilution attenuation factor (DAF) (i.e.,the factor by which the concentration isexpected to decrease between thelandfill unit and the point ofcompliance); and

! Multiply the initial contaminantconcentration by the DAF and comparethe resulting concentration to the MCLsto determine if the design will meet thestandard.

At this time, only contaminant transport in theunsaturated and/or saturated zones can bemodeled, because the other options (i.e.,surface water, air) have not yet beenthoroughly tested. In addition, only steady-state transport simulations are allowed. Nodecay of the contaminant source term ispermitted; the concentration of contaminantsentering the aquifer system is assumed to beconstant over time. The receptor (e.g., adrinking water well) is located directlydowngradient of the facility and intercepts thecontaminant plume; also, the contaminantconcentration is calculated at the top of theaquifer.

The user should bear in mind thatMULTIMED may not be an appropriatemodel for some sites. Some of the issues thatshould be considered before modeling effortsproceed are summarized in Table

stated above, MULTIMED utilizes analytical

flow and transport. As a result, therepresentation of a system simulated by themodel is simple, and little or no spatial ortemporal variability is allowed for theparameters in the system. Thus, a highlycomplex hydrogeologic system cannot beaccurately represented with MULTIMED.

The spatial characteristics assumed inMULTIMED should be considered whenapplying MULTIMED to a site. Theassumption of vertical, one-dimensionalunsaturated flow may be valid for facilitiesthat receive uniform areal recharge.However, this assumption may not be validfor facilities where surface soils (covers ordaily backfill) or surface slopes result in anincrease of run-off in certain areas of thefacility, and ponding of precipitation inothers. In addition, the simulation of one-dimensional, horizontal flow in the saturatedzone requires several simplifyingassumptions. The saturated zone is treated asa single, horizontal aquifer with uniformproperties (e.g., hydraulic conductivity). Theeffects of pumping or discharging wells on theground-water flow system cannot beaddressed with the MULTIMED model.

The MULTIMED model assumes steady-stateflow in all applications. Some ground-waterflow systems are in an approximate "steady-state," in which the amount of water enteringthe flow system equals the amount of waterleaving the system. However, assumingsteady-state conditions in a system thatexhibits transient behavior may produceinaccurate results.

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TABLE 4-2ISSUES TO BE CONSIDERED

BEFORE APPLYING MULTIMED(from Sharp-Hansen et al., 1990)

________________________________

Objectives of the Study

! Is a "screening level" approachappropriate?

! Is modeling a "worst-case scenario"acceptable?

Significant Processes Affecting ContaminantTransport

! Does MULTIMED simulate all thesignificant processes occurring at the site?

! Is the contaminant soluble in water and ofthe same density as water?

Accuracy and Availability of the Data

! Have sufficient data been collected toobtain reliable results?

! What is the level of uncertainty associatedwith the data?

! Would a Monte Carlo simulation beuseful? If so, are the cumulativeprobability distributions for the parameterswith uncertain values known?

Complexity of the Hydrogeologic System

! Are the hydrogeologic properties of thesystem uniform?

! Is the flow in the aquifer uniform andsteady?

! Is the site geometry regular?! Does the source boundary condition

require a transient or steady-state solution?

MULTIMED may be run in either adeterministic or a Monte Carlo mode. TheMonte Carlo method provides a means ofestimating the uncertainty in the results of amodel, if the uncertainty of the input variablesis known or can be estimated. However, itmay be difficult to determine the cumulativeprobability distribution for a given parameter.Assuming a parameter probability distributionwhen the distribution is unknown does nothelp reduce uncertainty. Furthermore, toobtain a valid estimate of the uncertainty inthe output, the model must be run numeroustimes (typically several hundred times), whichcan be time-consuming. These issues shouldbe considered before utilizing the MonteCarlo technique.

4.3 COMPOSITE LINER ANDLEACHATE COLLECTIONSYSTEM 40 CFR §258.40

4.3.1 Statement of Regulation

(a) New MSWLF units and lateralexpansions shall be constructed:

(1) See Statement of Regulation inSection 4.2.1 of this guidance document forperformance-based design requirements.

(2) With a composite liner, as definedin paragraph (b) of this section and aleachate collection system that is designedand constructed to maintain less than a 30-cm depth of leachate over the liner,

(b) For purposes of this section,composite liner means a system consistingof two components; the upper componentmust consist of a minimum 30-mil flexible

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membrane liner (FML), and the lower Standard Composite Liner Systemscomponent must consist of at least a two-foot layer of compacted soil with ahydraulic conductivity of no more than 1 x10 cm/sec. FML components consisting of-7

high density polyethylene (HDPE) shall beat least 60-mil thick. The FML componentmust be installed in direct and uniformcontact with the compacted soilcomponent.

4.3.2 Applicability

New MSWLF units and expansions ofexisting MSWLF units in States withoutapproved programs must be constructed witha composite liner and a leachate collectionsystem (LCS) that is designed to maintain adepth of leachate less than 30 cm (12 in.)above the liner. A composite liner consists ofa flexible membrane liner (FML) installed ontop of, and in direct and uniform contact with,two feet of compacted soil. The FML must beat least 30-mil thick unless the FML is madeof HDPE, which must be 60-mil thick. Thecompacted soil liner must be at least two feetthick and must have a hydraulic conductivityof no more than 1 x 10 cm/sec.-7

Owners and operators of MSWLF unitslocated in approved States have the option ofproposing a performance-based designprovided that certain criteria can be met (seeSection 4.2.2).


This section provides information on thecomponents of composite liner systemsincluding soils, geomembranes, and leachatecollection systems.

The composite liner system is an effectivehydraulic barrier because it combines thecomplementary properties of two differentmaterials into one system: 1) compacted soilwith a low hydraulic conductivity; and2) a FML (FMLs are also referred to asgeomembranes). Geomembranes may containdefects including tears, improperly bondedseams, and pinholes. In the absence of anunderlying low-permeability soil liner, flowthrough a defect in a geomembrane isessentially unrestrained. The presence of alow-permeability soil liner beneath a defect inthe geomembrane reduces leakage by limitingthe flow rate through the defect.

Flow through the soil component of the lineris controlled by the size of the defect in thegeomembrane, the available air space betweenthe two liners into which leachate can flow,the hydraulic conductivity of the soilcomponent, and the hydraulic head. Fluidflow through soil liners is calculated byDarcy's Law, where discharge (Q) isproportional to the head loss through the soil(dh/dl) for a given cross-sectional flow area(A) and hydraulic conductivity (K) where:

Q = KA(dh/dl)

Leakage through a geomembrane withoutdefects is controlled by Fick's first law, whichdescribes the process of liquid diffusionthrough the membrane liner. The diffusionprocess is similar to flow governed by Darcy'slaw for soil liners except that diffusion isdriven by concentration gradients and not byhydraulic head. Although diffusion rates ingeomembranes are several orders ofmagnitude lower than comparable hydraulicflow rates in low-permeability soil liners,construction of a completely impermeablegeomembrane is

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difficult. The factor that most strongly Soil Linerinfluences geomembrane performance is thepresence of imperfections such as improperly The following subsections discuss soil linerbonded seams, punctures and pinholes. A construction practices including thicknessdetailed discussion of leakage through requirements, lift placement, bonding of lifts,geomembranes and composite liners can be test methods, prerequisite soil properties,found in Giroud and Bonaparte (1989 (Part I quality control, and quality assuranceand Part II)). A geomembrane installed with activities.excellent control over defects may yield theequivalent of a one-centimeter-diameter hole Thicknessper acre of liner installed (Giroud andBonaparte, 1989 (Part I and Part II)). If the Two feet of soil is generally considered thegeomembrane were to be placed over sand, minimum thickness needed to obtain adequatethis size imperfection under one foot of compaction to meet the hydraulic conductivityconstant hydraulic head could be expected to requirement. This thickness is consideredaccount for as much as 3,300 gal/acre/day necessary to minimize the number of cracks(31,000 liters/hectare/ day) of leakage. Based or imperfections through the entire linerupon measurements of actual leakage through thickness that could allow leachate migration.liners at facilities that have been built under Both lateral and vertical imperfections mayrigorous control, Bonaparte and Gross (1990) exist in a compacted soil. The two-foothave estimated an actual leakage rate, under minimum thickness is believed to be sufficientone foot of constant head, of 200 to inhibit hydraulic short-circuiting of theliters/hectare/day or about 21 gallons/acre/day entire layer.for landfill units.

The uniformity of the contact between thegeomembrane and the soil liner is extremely Soil liners should be constructed in a series ofimportant in controlling the effective flow compacted lifts. Determination of appropriatearea of leachate through the soil liner. Porous lift thickness is dependent on the soilmaterial, such as drainage sand, filter fabric, characteristics, compaction equipment,or other geofabric, should not be placed firmness of the foundation materials, and thebetween the geomembrane and the low anticipated compactive effort needed topermeability soil liner. Porous materials will achieve the required soil hydrauliccreate a layer of higher hydraulic conductivity. Soil liner lifts should be thinconductivity, which will increase the amount enough to allow adequate compactive effort toof leakage below an imperfection in the reach the lower portions of the lift. Thinnergeomembrane. Construction practices during lifts also provide greater assurance thatthe installation of the soil and the sufficient compaction can be achieved togeomembrane affect the uniformity of the provide good, homogeneous bonding betweengeomembrane/soil interface, and strongly subsequent lifts. Adequate compaction of liftinfluence the performance of the composite thickness between five and ten inches isliner system. possible if appropriate equipment is used

Lift Thickness

(USEPA, 1988). Nine-inch loose liftthicknesses that will yield a 6-

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inch soil layer also have been recommended includes scarifying (roughening), and possiblyprior to compaction (USEPA, 1990a). wetting, the top inch or so of the last lift

Soil liners usually are designed to be of equipment before placing the next lift.uniform thickness with smooth slopes over theentire facility. Thicker areas may be Placement of Soil Liners on Slopesconsidered wherever recessed areas forleachate collection pipes or collection sumps The method used to place the soil liner on sideare located. Extra thickness and compactive slopes depends on the angle and length of theefforts near edges of the side slopes may slope. Gradual inclines from the toe of theenhance bonding between the side slopes and slope enable continuous placement of the liftsthe bottom liner. In smaller facilities, a soil up the slopes and provide better continuityliner may be designed for installation over the between the bottom and sidewalls of the soilentire area, but in larger or multi-cell liner. When steep slopes are encountered,facilities, liners may be designed in segments. however, lifts may need to be placed andIf this is the case, the design should address compacted horizontally due to the difficultieshow the old and new liner segments will be of operating heavy compaction equipment onbonded together (U.S. EPA, 1988). steeper slopes.

Bonding Between Lifts When sidewalls are compacted horizontally,

It is not possible to construct soil liners bottom of the soil liner to reduce thewithout some microscopic and/or macroscopic probability of seepage planes (USEPA, 1988).zones of higher and lower hydraulic A significant amount of additional soil linerconductivity. Within individual lifts, these material will be required to construct thepreferential pathways for fluid migration are horizontal lifts since the width of the lifts hastruncated by the bonded zone between the to be wide enough to accommodate thelifts. If good bonding between the lifts is not compaction equipment. After the soil liner isachieved during construction, the vertical constructed on the side slopes using thispathways may become connected by method, it can be trimmed back to thehorizontal pathways at the lift interface, required thickness. The trimmed surface ofthereby diminishing the performance of the the soil liner should be sealed by a smooth-hydraulic barrier. drum roller. The trimmed excess materials

Two methods may be used to ensure proper specified moisture-density requirements.bonding between lifts. Kneading or blendinga thinner, new lift with the previously Hydraulic Conductivitycompacted lift may be achieved by using afooted roller with long feet that can fully Achieving the hydraulic conductivity standardpenetrate a loose lift of soil. If the protruding depends on the degree of compaction,rods or feet of a sheepsfoot roller are compaction method, type of clay, soilsufficient in length to penetrate the top lift and moisture content, and density of the soilknead the previous lift, good bonding may be during liner construction. Hydraulic achieved. Another method

placed with a disc harrow or other similar

it is important to tie in the edges with the

can be reused provided that they meet the

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conductivity is the key design parameter when characterize proposed liner soils shouldevaluating the acceptability of the constructed include grain size distribution (ASTM D-soil liner. The hydraulic conductivity of a soil 422), Atterberg limits (ASTM D-4318), anddepends, in part, on the viscosity and density compaction curves depicting moisture andof the fluid flowing through it. While water density relationships using the standard orand leachate can cause different test results, modified Proctor (ASTM D-698 or ASTM D-water is an acceptable fluid for testing the 1557), whichever is appropriate for thecompacted soil liner and source materials. compaction equipment used and the degree ofThe effective porosity of the soil is a function firmness of the foundation materials.of size, shape, and area of the conduitsthrough which the liquid flows. The Liner soils usually have at least 30 percenthydraulic conductivity of a partially saturated fines (fine silt- and clay-sized particles).soil is less than the hydraulic conductivity of Some soils with less than 30 percent finesthe same soil when saturated. Because may be worked to obtain hydraulicinvading water only flows through water- conductivities below 1 x 10 cm/sec, but usefilled voids (and not air-filled voids), the of these soils requires greater control ofdryness of a soil tends to lower permeability. construction practices and conditions.Hydraulic conductivity testing should beconducted on samples that are fully saturated The soil plasticity index (PI), which isto attempt to measure the highest possible determined from the Atterberg limits (definedhydraulic conductivity. by the liquid limit minus the plastic limit),

EPA has published Method 9100 in However, soils with very high PI, (greaterpublication SW-846 (Test Methods for than 30 percent), are cohesive and sticky andEvaluating Solid Waste) to measure the become difficult to work with in the field.hydraulic conductivity of soil samples. Other When high PI soils are too dry duringmethods appear in the U.S. Army Corps of placement, they tend to form hard clumpsEngineers Engineering Manual 1110-2-1906 (clods) that are difficult to break down during(COE, 1970) and the newly published compaction. Preferential flow paths may be"Measurement of Hydraulic Conductivity of created around the clods allowing leachate toSaturated Porous Materials Using a Flexible migrate at a relatively high rate.Wall Permeameter" (ASTM D-5084). Toverify full saturation of the sample, this latter Soil particles or rock fragments also canmethod may be performed with back pressure create preferential flow paths. For thissaturation and electronic pore pressure reason, soil particles or rock fragments shouldmeasurement. be less than 3 inches in diameter so as not to

Soil Properties the soil liner (USEPA, 1989).

Soils typically possess a range of physical The maximum density of a soil will becharacteristics, including particle size, achieved at the optimum water content, butgradation, and plasticity, that affect their this point generally does not correspond to theability to achieve a hydraulic conductivity of point at which minimum hydraulic 1 x 10 cm/sec. Testing methods used to-7

-7

should generally be greater than 10 percent.

affect the overall hydraulic performance of

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conductivity is achieved. Wet soils, however, is difficult and may lead to inconsistent resultshave low shear strength and high potential for with respect to complying with the hydraulicdesiccation cracking. Care should be taken conductivity criterion.not to compromise other engineeringproperties such as shear strengths of the soil The most common additive used to amendliner by excessively wetting the soil liner. soils is sodium bentonite. The disadvantageDepending on the specific soil characteristics, of using sodium bentonite includes itscompaction equipment and compactive effort, vulnerability to degradation as a result ofthe hydraulic conductivity criterion may be contact with chemicals and waste leachatesachieved at moisture values of 1 to 7 percent (U.S. EPA, 1989).above the optimum moisture content.

Although the soil may possess the required than sodium bentonite, also is used as a soilproperties for successful liner construction, amendment. Approximately twice as muchthe soil liner may not meet the hydraulic calcium bentonite typically is needed toconductivity criterion if the construction achieve a hydraulic conductivity comparablepractices used to install the liner are not to that of sodium bentonite.appropriate and carefully controlled.Construction quality control and quality Soil/bentonite mixtures generally requireassurance will be discussed in a later section. central plant mixing by means of a pugmill,

Amended Soils where water can be added during the process.

If locally available soils do not possess distribution must be controlled during mixingproperties to achieve the specified hydraulic and placement. Spreading of theconductivity, soil additives can be used. Soil soil/bentonite mixture may be accomplishedadditives, such as bentonite or other clay in the same manner as the spreading of naturalmaterials, can decrease the hydraulic soil liners, by using scrapers, graders,conductivity of the native soil (USEPA, bulldozers, or a continuous asphalt paving1988b). machine (U.S. EPA, 1988).

Bentonite may be obtained in a dry, powdered Materials other than bentonite, including lime,form that is relatively easy to blend with on- cement, and other clay minerals such assite soils. Bentonite is a clay mineral atapulgite, may be used as soil additives (U.S.(sodium-montmorillonite) that expands when EPA, 1989). For more informationit comes into contact with water (hydration), concerning soil admixtures, the reader isby absorbing the water within the mineral referred to the technical resource document onmatrix. This property allows relatively small the design and construction of clay linersamounts of bentonite (5 to 10 percent) to be (U.S. EPA, 1988).added to a noncohesive soil (sand) to make itmore cohesive (U.S. EPA, 1988b). Thorough Testingmixing of additives to cohesive soils (clay)

Calcium bentonite, although more permeable

cement mixer, or other mixing equipment

Water, bentonite content, and particle size

Prior to construction of a soil liner, therelationship between water content, density,

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and hydraulic conductivity for a particular soil construction contractor during soil placement.should be established in the laboratory. Figure 4-6 presents compaction data as aFigure 4-5 shows the influence of molding function of dry unit weight and molding waterwater content (moisture content of the soil at content for the construction of clay liners.the time of compaction) on hydraulic The amount of soil testing required toconductivity of the soil. The lower half of the determine these construction parameters isdiagram is a compaction curve and shows the dependent on the degree of natural variabilityrelationship between dry unit weight, or dry of the source material. density of the soil, and water content of thesoil. The optimum moisture content of the Quality assurance and quality control of soilsoil is related to a peak value of dry density liner materials involve both laboratory andknown as maximum dry density. Maximum field testing. Quality control tests aredry density is achieved at the optimum performed to ascertain compactionmoisture content. requirements and the moisture content of

The lowest hydraulic conductivity of quality assurance provide an opportunity tocompacted clay soil is achieved when the soil check representative areas of the liner foris compacted at a moisture content slightly conformance to compaction specifications,higher than the optimum moisture content, including density and moisture content.generally in the range of 1 to 7 percent (U.S. Quality assurance laboratory testing is usuallyEPA, 1989). When compacting clay, water conducted on field samples for determinationcontent and compactive effort are the two of hydraulic conductivity of the in-place liner.factors that should be controlled to meet the Laboratory testing allows full saturation of themaximum hydraulic conductivity criterion. soil samples and simulates the effects of large

It is impractical to specify and construct a done conveniently in the field (U.S. EPA,clay liner to a specific moisture content and a 1989).specific compaction (e.g., 5 percent wet ofoptimum and 95 percent modified Proctor Differences between laboratory and fielddensity). Moisture content can be difficult to conditions (e.g., uniformity of material,control in the field during construction; control of water content, compactive effort,therefore, it may be more appropriate to compaction equipment) may make it unlikelyspecify a range of moisture contents and that minimum hydraulic conductivity valuescorresponding soil densities (percent measured in the laboratory on remolded, pre-compaction) that are considered appropriate to construction borrow source samples are theachieve the required hydraulic conductivity. same as the values achieved during actualBenson and Daniel (U.S. EPA, 1990) propose liner construction. Laboratory testing onwater content and density criteria for the remolded soil specimens does not account forconstruction of clay liners in which the operational problems that may result inmoisture-density criteria ranges are desiccation, cracking, poor bonding of lifts,established based on hydraulic conductivity and inconsistent degree of compaction ontest results. This type of approach is sidewalls (U.S. EPA, 1988b). Therecommended because of the flexibility and relationship between field and laboratoryguidance it provides to the hydraulic conductivity testing has been

material delivered to the site. Field tests for

overburden stress on the soil, which cannot be

investigated by the U.S. Environmental

156

Figure 4-5Hydraulic Conductivity and Dry Unit Weight as a

Function of Molding Water Content

Figure 4-6. Compaction Data for Silty Clay

157

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Protection Agency using field case studies susceptible to the effects of temperature(U.S. EPA, 1990c). variation; as the water temperature increases,

In situ, or field, hydraulic conductivity testing the system contracts. This situation couldoperates on the assumption that by testing lead to erroneous measurements when the ratelarger masses of soil in the field, one can of flow is small.obtain more realistic results. Four types of insitu hydraulic conductivity tests generally are The sealed double-ring infiltrometer hasused: borehole tests, porous probes, proven to be the most successful method andinfiltrometer tests, and underdrain tests. A is the one currently used. The outer ringborehole test is conducted by drilling a hole, forces infiltration from the inner ring to bethen filling the hole with water, and more or less one-dimensional. Covering themeasuring the rate at which water percolates inner ring with water insulates it substantiallyinto the borehole. In the borehole test, water from temperature variation.also can percolate through the sidewalls of theborehole. As a result, the measured hydraulic Underdrains, the fourth type of in situ test, areconductivity is usually higher than that the most accurate in situ permeability testingmeasured by other one-dimensional field device because they measure exactly whattestings. migrates from the bottom of the liner.

The second type of test involves driving or data for low permeability liners, because ofpushing a porous probe into the soil and the length of time required to accumulatepouring water through the probe into the soil. measurable flow. Also, underdrains must beWith this method, however, the advantage of installed during construction, so fewertesting directly in the field is somewhat offset underdrains are used than other kinds ofby the limitations of testing such a small testing devices.volume of soil.

A third method of testing involves a device usually performed on the completed linercalled an infiltrometer. This device is because the tests may take several weeks toembedded into the surface of the soil liner complete (during which time the liner may besuch that the rate of flow of a liquid into the damaged by desiccation or freezingliner can be measured. The two types of temperatures) and because large penetrationsinfiltrometers most widely used are open and must be made into the liner. If fieldsealed. Open rings are less desirable because, conductivity tests are performed, they arewith a hydraulic conductivity of 10 cm/sec, usually conducted on a test pad. The test pad-7

it is difficult to detect a 0.002 inch per day should be constructed using the materials anddrop in water level of the pond from methods to be used for the actual soil liner.evaporation and other losses. The width of a test pad is usually the width of

With sealed rings, very low rates of flow can length is one to two times the width.be measured. However, single-ring Thickness is usually two to three feet. Testinfiltrometers allow lateral flow beneath the pads can be used as a means for verifying thatring, which can complicate the interpretation the proposedof test results. Single rings are also

the entire system expands. As it cools down,

However, under-drains are slow to generate

Field hydraulic conductivity tests are not

three to four construction vehicles, and the

Design Criteria

159

materials and construction procedures will ! Soil water content; andmeet performance objectives. If a test pad isconstructed, if tests verify that performanceobjectives have been met, and if the actualsoil liner is constructed to standards that equalor exceed those used in building the test pad(as verified through quality assurance), thenthe actual soil liner should meet or exceedperformance objectives.

Other than the four types of field hydraulicconductivity tests described earlier, ASTM D2937 "Standard Test Method for Density ofSoil in Place by the Drive-Cylinder Method"may be used to obtain in-place hydraulicconductivity of the soil liner. This testmethod uses a U.S. Army Corps of Engineerssurface soil sampler to drive a thin-walledcylinder (typically 3-inch by 3-inch) into acompleted lift of the soil liner to obtainrelatively undisturbed samples for laboratorydensity and hydraulic conductivity testings.This test can provide useful correlation toother field and quality assurance testingresults (e.g, Atterberg limits, gradation, in-place moisture and density of the soil liner) toevaluate the in-place hydraulic conductivity ofthe soil liner.

Soil Liner Construction

Standard compaction procedures are usuallyemployed when constructing soil liners. Thefollowing factors influence the degree andquality of compaction:

! Lift thickness;

! Full scale or segmented lift placement;

! Number of equipment passes;

! Scarification between lifts;

! The type of equipment and compactiveeffort.

The method used to compact the soil liner isan important factor in achieving the requiredminimum hydraulic conductivity. Higherdegrees of compactive effort increase soildensity and lower the soil hydraulicconductivity for a given water content. Theresults of laboratory compaction tests do notnecessarily correlate directly with the amountof compaction that can be achieved duringconstruction.

Heavy compaction equipment (greater than25,000 lbs or 11,300 kg) is typically usedwhen building the soil liner to maximizecompactive effort (U.S. EPA, 1989). Thepreferred field compaction equipment is asheepsfoot roller with long feet that fullypenetrates loose lifts of soil and provideshigher compaction while kneading the clayparticles together. The shape and depth of thefeet are important; narrow, rod-like feet witha minimum length of about seven inchesprovide the best results. A progressive changefrom the rod-like feet to a broader foot maybe necessary in some soils after initialcompaction, to allow the roller to walk out ofthe compacted soil. The sheepsfoot feet alsoaid in breaking up dry clods (see SoilProperties in this section). Mechanical roadreclaimers, which are typically used to stripand re-pave asphalt, can be extremelyeffective in reducing soil clod size prior tocompaction and in scarifying soil surfacesbetween lifts. Other equipment that has beenused to compact soil includes discs andrototillers.

To achieve adequate compaction, the liftthickness (usually five to nine inches) may bedecreased or the number of passes over

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160

the lift may be increased. Generally, fillers, plasticizers, processing aids,compaction equipment should pass over the crosslinking chemicals, anti-degradants, andsoil liner five to twenty times to attain the biocides. The polymers used to manufacturecompaction needed to comply with the geomembranes include a wide range ofminimum hydraulic conductivity criterion plastics and rubbers differing in properties(U.S. EPA, 1989). such as chemical resistance and basic

Efforts made to reduce clod size during 1988e). The polymeric materials may beexcavation and placement of the soil for the categorized as follows:liner should improve the chances forachieving low hydraulic conductivity in ! Thermoplastics such as polyvinylseveral ways. Keeping clods in the soil liner chloride (PVC);material small will facilitate a more uniformwater content. Macropores between clodremnants can result in unacceptably high fieldhydraulic conductivity.

Opinions differ on acceptable clod sizes in theuncompacted soil. Some suggest a maximumof one to three inches in diameter, or no largerthan one-half the lift thickness. The mainobjective is to remold all clods in thecompaction process to keep hydraulicconductivity values consistent throughout thesoil liner (U.S. EPA, 1988).

Geomembranes

Geomembranes are relatively thin sheets offlexible thermoplastic or thermoset polymericmaterials that are manufactured andprefabricated at a factory and transported tothe site. Because of their inherentimpermeability, use of geomembranes inlandfill unit construction has increased. Thedesign of the side slope, specifically thefriction between natural soils andgeosynthetics, is critical and requires carefulreview.

Material Types and Thicknesses

Geomembranes are made of one or morepolymers along with a variety of otheringredients such as carbon black, pigments,

composition (U.S. EPA, 1983 and U.S. EPA,

! Crystalline thermoplastics such as highdensity polyethylene (HDPE), very lowdensity polyethylene (VLDPE), andlinear low density polyethylene(LLDPE); and

! Thermoplastic elastomers such aschlorinated polyethylene (CPE) andchlorosulfonated polyethylene (CSPE).

The polymeric materials used most frequentlyas geomembranes are HDPE, PVC, CSPE,and CPE. The thicknesses of geomembranesrange from 20 to 120 mil (1 mil = 0.001 inch)(U.S. EPA, 1983 and U.S. EPA, 1988e). Therecommended minimum thickness for allgeomembranes is 30 mil, with the exceptionof HDPE, which must be at least 60 mil toallow for proper seam welding. Somegeomembranes can be manufactured by acalendering process with fabric reinforcement,called scrim, to provide additional tensilestrength and dimensional stability.

Chemical and Physical Stress Resistance

The design of the landfill unit should considerstresses imposed on the liner by the designconfiguration. These stresses include thefollowing:

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! Differential settlement in foundation of the original material every thirty days. Asoils; software system entitled Flexible Liner

! Strain requirements at the anchor trench; in the hazardous waste permitting process,and may aid in interpreting EPA Method 9090 test

! Strain requirements over long, steep side of both Method 9090 and FLEX is availableslopes. from EPA.

An extensive body of literature has been It is imperative that a geomembrane linerdeveloped by manufacturers and independent maintain its integrity during exposure toresearchers on the physical properties of short-term and long-term mechanical stresses.liners. Geosynthetic design equations are Short-term mechanical stresses includepresented in several publications including equipment traffic during the installation of aKastman (1984), Koerner (1990), and U.S. liner system, as well as thermal expansion andEPA (1988e). shrinkage of the geomembrane during the

The chemical resistance of a geomembrane to unit. Long-term mechanical stresses resultleachate has traditionally been considered a from the placement of waste on top of thecritical issue for Subtitle C (hazardous waste) liner system and from subsequent differentialfacilities where highly concentrated solvents settlement of the subgrade (U.S. EPA, 1988a).may be encountered. Chemical resistancetesting of geomembranes may not be required Long-term success of the liner requiresfor MSWLF units containing only municipal adequate friction between the components ofsolid waste; EPA's data base has shown that a liner system, particularly the soil subgradeleachate from MSWLF units is not aggressive and the geomembrane, and betweento these types of materials. Testing for geosynthetic components, so that slippage orchemical resistance may be warranted sloughing does not occur on the slopes of theconsidering the waste type, volumes, unit. Specifically, the foundation slopes andcharacteristics, and amounts of small quantity the subgrade materials must be considered ingenerator waste or other industrial waste design equations to evaluate:present in the waste stream. The followingguidance is provided in the event such testing • The ability of a geomembrane tois of interest to the owner or operator. support its own weight on the side

EPA's Method 9090 in SW-846 is theestablished test procedure used to evaluate • The ability of a geomembrane todegradation of geomembranes when exposed withstand down-dragging during andto hazardous waste leachate. In the after waste placement;procedure, the geomembrane is immersed inthe site-specific chemical environment for at • The best anchorage configuration for theleast 120 days at two different temperatures. geomembrane;Physical and mechanical properties of thetested material are then compared to those

Evaluation Expert (FLEX), designed to assist

data (U.S. EPA, 1989). A detailed discussion

construction and operation of the MSWLF

slopes;

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• The stability of a soil cover on top of a Manufactured Sheet Specificationsgeomembrane; and

• The stability of other geosyntheticcomponents such as geotextile or geoneton top of a geomembrane.

These requirements may affect the choice ofgeomembrane material, including polymertype, fabric reinforcement, thickness, andtexture (e.g., smooth or textured for HDPE)(U.S. EPA, 1988). PVC also can be obtainedin a roughened or file finish to increase thefriction angle.

Design specifications should indicate the typeof raw polymer and manufactured sheet to beused as well as the requirements for thedelivery, storage, installation, and sampling ofthe geomembrane. Material properties can beobtained from the manufacturer-suppliedaverage physical property values, which arepublished in the Geotechnical Fabrics Report'sSpecifier's Guide and updated annually. Theminimum tensile properties of thegeomembrane must be sufficient to satisfy thestresses anticipated during the service life ofthe geomembrane. Specific raw polymer andmanufactured sheet specifications and testprocedures include (U.S. EPA, 1988e, andKoerner, 1990):

Raw Polymer Specifications

! Density (ASTM D-1505);

! Melt index (ASTM D-1238);

! Carbon black (ASTM D-1603); and

! Thermogravimetric analysis (TGA)or differential scanning calorimetry(DSC).

! Thickness (ASTM D-1593);

! Tensile properties (ASTM D-638);

! Tear resistance (ASTM D-1004);

! Carbon black content (ASTM D-1603);

! Carbon black dispersion (ASTM D-3015);

! Dimensional stability (ASTM D-1204); and

! Stress crack resistance (ASTM D-1693).

Geomembranes may have different physicalcharacteristics, depending on the type ofpolymer and the manufacturing process used,that can affect the design of a liner system.When reviewing manufacturers' literature, itis important to remember that eachmanufacturer may use more than one polymeror resin type for each grade of geomembraneand that the material specifications may begeneralized to represent several grades ofmaterial.

Installation

Installation specifications should addressinstallation procedures specific to theproperties of the liner installed. Thecoefficient of thermal expansion of thegeomembrane sheet can affect its installationand its service performance. Thegeomembrane should lie flat on theunderlying soil. However, shrinkage andexpansion of the sheeting, due to changes intemperature during installation, may result inexcessive wrinkling or tension in the

Design Criteria

163

geomembrane. Wrinkles on the in an EPA guidance document (USEPA,geomembrane surface will affect theuniformity of the soil-geomembrane interfaceand may result in leakage throughimperfections. Excessive tautness of thegeomembrane may affect its ability to resistrupture from localized stresses on the seamsor at the toe of slopes where bridging over thesubgrade may occur during installation. Inaddition to thermal expansion and contractionof the geomembrane, residual stresses frommanufacturing remain in some geomembranesand can cause non-uniform expansion andcontraction during construction. Someflexibility is needed in the specifications forgeomembrane selection to allow foranticipated dimensional changes resultingfrom thermal expansion and contraction (U.S.EPA, 1988).

Technical specifications for geomembranesalso should include: information forprotection of the material during shipping,storage and handling; quality controlcertifications provided by the manufacturer orfabricator (if panels are constructed); andquality control testing by the contractor,installer, or a construction quality assurance(CQA) agent. Installation proceduresaddressed by the technical specificationsinclude a geomembrane layout plan,deployment of the geomembrane at theconstruction site, seam preparation, seamingmethods, seaming temperature constraints,detailed procedures for repairing anddocumenting construction defects, and sealingof the geomembrane to appurtenances, bothadjoining and penetrating the liner. Theperformance of inspection activities, includingboth non-destructive and destructive qualitycontrol field testing of the sheets and seamsduring installation of the geomembrane,should be addressed in the technicalspecifications. Construction quality assuranceis addressed

1992).

The geomembrane sheeting is shipped in rollsor panels from the supplier, manufacturer, orfabricator to the construction site. Each rollor panel may be labeled according to itsposition on the geomembrane layout plan tofacilitate installation. Upon delivery, thegeomembrane sheeting should be inspected tocheck for damage that may have occurredduring shipping. (U.S. EPA, 1992).

Proper storage of the rolls or panels prior toinstallation is essential to the finalperformance of the geomembrane. Somegeomembrane materials are sensitive toultraviolet exposure and should not be storedin direct sunlight prior to installation. Others,such as CSPE and CPE, are sensitive tomoisture and heat and can partially crosslinkor block (stick together) under improperstorage conditions. Adhesives or weldingmaterials, which are used to joingeomembrane panels, also should be storedappropriately (U.S. EPA, 1992).

Visual inspection and acceptance of the soilliner subgrade should be conducted prior toinstalling the geomembrane. The surface ofthe subgrade should meet designspecifications with regard to lack ofprotruding objects, grades, and thickness.Once these inspections are conducted andcomplete, the geomembrane may be installedon top of the soil liner. If necessary, othermeans should be employed to protect thesubgrade from precipitation and erosion, andto prevent desiccation, moisture loss, anderosion from the soil liner prior togeomembrane placement. Such methods mayinclude placing a plastic tarp on top ofcompleted portions of the soil liner

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(USEPA, 1992). In addition, scheduling soil tested non-destructively (U.S. EPA, 1988).liner construction slightly ahead of the Destructive testing should be done at regulargeomembrane and drainage layer placement intervals along the seam (see page 4-66).can reduce the exposure of the soil liner to theelements. Consistent quality in fabricating field seams is

Deployment, or placement, of the that may affect seaming should be monitoredgeomembrane panels or rolls should be and controlled during installation. Andescribed in the geomembrane layout plan. inspection should be conducted in accordanceRolls of sheeting, such as HDPE, generally with a construction quality assurance plan tocan be deployed by placing a shaft through document the integrity of field seams. Factorsthe core of the roll, which is supported and affecting the seaming process include (U.S.deployed using a front-end loader or a winch. EPA, 1988):Panels composed of extremely flexible linermaterial such as PVC are usually folded on ! Ambient temperature at which the seamspallets, requiring workers to manually unfold are made;and place the geomembrane. Placement ofthe geomembrane goes hand-in-hand with the ! Relative humidity;seaming process; no more than the amount ofsheeting that can be seamed during a shift or ! Control of panel lift-up by wind;work day should be deployed at any one time(USEPA, 1988). Panels should be weighted ! The effect of clouds on thewith sand bags if wind uplift of the membrane geomembrane temperature;or excessive movement from thermalexpansion is a potential problem. Proper ! Water content of the subsurface beneathstormwater control measurements should be the geomembrane;employed during construction to preventerosion of the soil liner underneath the ! The supporting surface on which thegeomembrane and the washing away of the seaming is bonded;geomembrane.

Once deployment of a section of thegeomembrane is complete and each section ! Quality and consistency of the chemicalhas been visually inspected for imperfections or welding material;and tested to ensure that it is the specifiedthickness, seaming of the geomembrane may ! Proper preparation of the liner surfacesbegin. Quality control/quality assurance to be joined;monitoring of the seaming process should beimplemented to detect inferior seams. ! Moisture on the seam interface; andSeaming can be conducted either in thefactory or in the field. Factory seams are ! Cleanliness of the seam interface (e.g.,made in a controlled environment and are the amount of airborne dust and debrisgenerally of high quality, but the entire seam present).length (100 percent) still should be

critical to liner performance, and conditions

! The skill of the seaming crew;

Design Criteria

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Depending on the type of geomembrane, design plans and specifications (USEPA,several bonding systems are available for the 1988).construction of both factory and field seams.Bonding methods include solvents, heat seals, Geomembranes that are subject to damageheat guns, dielectric seaming, extrusion from exposure to weather and work activitieswelding, and hot wedge techniques. To should be covered with a layer of soil as soonensure the integrity of the seams, a as possible after quality assurance activitiesgeomembrane should be seamed using the associated with geomembrane testing arebonding system recommended by the completed. Soil should be placed withoutmanufacturer (U.S. EPA, 1988). EPA has driving construction vehicles directly on thedeveloped a field seaming manual for all geomembrane. Light ground pressuretypes of geomembranes (U.S. EPA, 1991a). bulldozers may be used to push material out

Thermal methods of seaming require not attempt to push a large pile of soil forwardcleanliness of the bonding surfaces, heat, in a continuous manner over the membrane.pressure, and dwell time to produce high Such methods can cause localized wrinkles toquality seams. The requirements for adhesive develop and overturn in the direction ofsystems are the same as those for thermal movement. Overturned wrinkles create sharpsystems, except that the adhesive takes the creases and localized stresses in theplace of the heat. Sealing the geomembrane geomembrane that could lead to prematureto appurtenances and penetrating structures failure. Instead, the operator shouldshould be performed in accordance with continually place smaller amounts of soil ordetailed drawings included in the design plans drainage material working outward over theand approved specifications. toe of the previously placed material.

An anchor trench along the perimeter of the place soil over the geomembrane that can latercell generally is used to secure the be spread with a bulldozer or similargeomembrane during construction (to prevent equipment. Although such methods maysloughing or slipping down the interior side sound tedious and slow, in the long run theyslopes). Run out calculations (Koerner, 1990) will be faster and more cost-effective thanare available to determine the depth of burial placing too much material too fast and havingat a trench necessary to hold a specified length to remobilize the liner installer to repairof membrane, or combination of membrane damaged sections of the geomembrane. Theand geofabric or geotextile. If forces larger QA activities conducted during constructionthan the tensile strength of the membrane are also should include monitoring theinadvertently developed, then the membrane contractor's activities on top of the liner tocould tear. For this reason, the geomembrane avoid damage to installed and acceptedshould be allowed to slip or give in the trench geomembranes.after construction to prevent such tearing.However, during construction, the Leachate Collection Systemsgeomembrane should be anchored accordingto the detailed drawings provided in the Leachate refers to liquid that has passed

in front over the liner, but the operator must

Alternatively, large backhoes can be used to

through or emerged from solid waste andcontains dissolved, suspended, or immiscible

Subpart D

166

materials removed from the solid waste. At layer to collect leachate and carry itMSWLF units, leachate is typically aqueous rapidly to a sump or collection headerwith limited, if any, immiscible fluids or pipe;dissolved solvents. The primary function ofthe leachate collection system is to collect and ! A protective filter layer over the highconvey leachate out of the landfill unit and to permeability drainage material, ifcontrol the depth of the leachate above the necessary, to prevent physical cloggingliner. The leachate collection system (LCS) of the material by fine-grained material;should be designed to meet the regulatory andperformance standard of maintaining less than30 cm (12 inches) depth of leachate, or ! Leachate collection sumps or header"head," above the liner. The 30-cm head pipe system where leachate can beallowance is a design standard and the Agency removed.recognizes that this design standard may beexceeded for relatively short periods of time The design, construction, and operation of theduring the active life of the unit. Flow ofleachate through imperfections in the linersystem increases with an increase in leachatehead above the liner. Maintaining a lowleachate level above the liner helps to improvethe performance of the composite liner.

Leachate is generally collected from thelandfill through sand drainage layers,synthetic drainage nets, or granular drainagelayers with perforated plastic collection pipes,and is then removed through sumps or gravitydrain carrier pipes. LCS's should consist ofthe following components (U.S. EPA, 1988):

! A low-permeability base (in this case acomposite liner);

! A high-permeability drainage layer,constructed of either natural granularmaterials (sand and gravel) or syntheticdrainage material (e.g., geonet) placeddirectly on the FML, or on a protectivebedding layer (e.g., geofabric) directlyoverlying the liner;

! Perforated leachate collection pipeswithin the high-permeability drainage

LCS should maintain a maximum height ofleachate above the composite liner of 30 cm(12 in). Design guidance for calculating themaximum leachate depth over a liner forgranular drainage systems materials isprovided in the reference U.S. EPA (1989).The leachate head in the layer is a function ofthe liquid impingement rate, bottom slope,pipe spacing, and drainage layer hydraulicconductivity. The impingement rate isestimated using a complex liquid routingprocedure. If the maximum leachate depthexceeds 30 cm for the system, except forshort-term occurrences, the design should bemodified to improve its efficiency byincreasing grade, decreasing pipe spacing, orincreasing the hydraulic conductivity(transmissivity) of the drainage layer (U.S.EPA, 1988).

Grading of Low-Permeability Base

The typical bottom liner slope is a minimumof two percent after allowances for settlementat all points in each system. A slope isnecessary for effective gravity drainagethrough the entire operating and post-closureperiod. Settlement estimates of thefoundation soils should set this two-

Design Criteria

167

percent grade as a post-settlement design friction angle between the geomembrane andobjective (U.S. EPA, 1991b). soil, and if possible, supported by laboratory

High-Permeability Drainage Layer

The high-permeability drainage layer is designation of GW or GP on the Unified Soilsplaced directly over the liner or its protective Classification Chart can be expected to havebedding layer at a slope of at least two percent a hydraulic conductivity of greater than 0.01(the same slope necessary for the composite cm/sec, while sands identified as SW or SPliner). Often the selection of a drainage can be expected to have a coefficient ofmaterial is based on the on-site availability of permeability greater than 0.001 cm/sec. Thenatural granular materials. In some regions of sand or gravel drains leachate that enters thethe country, hauling costs may be very high drainage layer to prevent 30 cm (12 in) orfor sand and gravel, or appropriate materials more accumulation on top of the liner duringmay be unavailable; therefore, the designer the active life of the MSWLF unit LCS. Themay elect to use geosynthetic drainage nets design of a LCS frequently uses a drainage(geonets) or synthetic drainage materials as an material with a hydraulic conductivity of 1 xalternative. Frequently, geonets are 10 cm/sec or higher. Drainage materialssubstituted for granular materials on steep with hydraulic conductivities in this order ofsidewalls because maintaining sand on the magnitude should be evaluated for biologicalslope during construction and operation of the and particulate clogging (USEPA, 1988).landfill unit is more difficult (U.S. EPA, Alternatively, if a geonet is used, the design is1988). based on the transmissivity of the geonet.

Soil Drainage Layers If a filter layer (soil or geosynthetic) is

If the drainage layer of the leachate collection protect it from clogging, and the LCS issystem is constructed of granular soil designed and operated to avoid drasticmaterials (e.g., sand and gravel), then it changes in the oxidation reduction potential ofshould be demonstrated that this granular the leachate (thereby avoiding formation ofdrainage layer has sufficient bearing strength precipitates within the LCS), then there is noto support expected loads. This conceptual basis to anticipate thatdemonstration will be similar to that required conductivity will decrease over time. Wherefor the foundations and soil liner (U.S. EPA, conductivity is expected to decrease over1988). time, the change in impingement rate also

If the landfill unit is designed on moderate-to- because the reduced impingement rate andsteep (15 percent) grades, the landfill design hydraulic conductivity may still comply withshould include calculations demonstrating that the 30 cm criterion.the selected granular drainage materials willbe stable on the most critical slopes (e.g., Unless alternative provisions are made tousually the steepest slope) in the design. Thecalculations and assumptions should beshown, especially the

and/or field testing (USEPA, 1988).

Generally, gravel soil with a group

-2

constructed on top of a drainage layer to

should be evaluated over the same time period

control incident precipitation and resultingsurface run-off, the impingement rate duringthe operating period of the MSWLF unit is

Subpart D

168

usually at least an order of magnitude greater Granular materials are generally placed usingthan the impingement rate after final closure.The critical design condition for meeting the30 cm (12 in) criterion can therefore beexpected during the operating life. Thedesigner may evaluate the sensitivity of adesign to meet the 30 cm (12 in) criterion asa result of changes in impingement rates,hydraulic conductivity, pipe spacing, andgrades. Such sensitivity analysis may indicatewhich element of the design should beemphasized during construction qualitymonitoring or whether the design can bealtered to comply with the 30 cm (12 in)criterion in a more cost-effective manner.

The soil material used for the drainage layershould be investigated at the borrow pit priorto use at the landfill. Typical borrow pitcharacterization testing would includelaboratory hydraulic conductivity and grainsize distribution. If grain size distributioninformation from the borrow pitcharacterization program can be correlated tothe hydraulic conductivity data, then the grainsize test, which can be conducted in a shorttime in the field, may be a useful constructionquality control parameter. Compliance withthis parameter would then be indicative thatthe hydraulic conductivity design criterionwas achieved in the constructed drainagelayer. This information could be incorporatedinto construction documents after the borrowpit has been characterized. If a correlationcannot be made between hydraulicconductivity and grain size distribution, thenconstruction documents may rely on directfield or laboratory measurements todemonstrate that the hydraulic conductivitydesign criterion was met in the drainage layer.

conventional earthmoving equipment,including trucks, scrapers, bulldozers, andfront-end loaders. Vehicles should not bedriven directly over the geosyntheticmembrane when it is being covered. (U.S.EPA, 1988a).

Coarse granular drainage materials, unlikelow-permeability soils, can be placed dry anddo not need to be heavily compacted.Compacting granular soils tends to grind thesoil particles together, which increases thefine material and reduces hydraulicconductivity. To minimize settlementfollowing material placement, the granularmaterial may be compacted with a vibratoryroller. The final thickness of the drainagelayer should be checked by optical surveymeasurements or by direct test pitmeasurements (U.S. EPA, 1988).

Geosynthetic Drainage Nets

Geosynthetic drainage nets (geonets) may besubstituted for the granular layers of the LCRson the bottom and sidewalls of the landfillcells. Geonets require less space thanperforated pipe or gravel and also promoterapid transmission of liquids. They do,however, require geotextile filters above themand can experience problems with creep andintrusion. Long-term operating andperformance experience of geonets is limitedbecause the material and its application arerelatively new (U.S. EPA, 1989).

If a geonet is used in place of a granulardrainage layer, it must provide the same levelof performance (maintaining less than 30 cmof leachate head above the liner). Anexplanation of the calculation used to computethe capacity of a geonet may be found in U.S.EPA (1987a). The

Design Criteria

169

transmissivity of a geonet can be reduced critical specification is the ability to transmitsignificantly by intrusion of the soil or a fluids under load. The specifications alsogeotextile. A protective geotextile between should include a minimum transmissivitythe soil and geonet will help alleviate this under expected landfill operating (dynamic)concern. If laboratory transmissivity tests are or completion (static) loads. Theperformed, they should be done under specifications for thickness and types ofconditions, loads, and configurations that material should be identified on the drawingsclosely replicate the actual field conditions. It or in the materials section of theis important that the transmissivity value used specifications, and should be consistent within the leachate collection system design the design calculations (U.S. EPA, 1988).calculations be selected based upon thoseloaded conditions (U.S. EPA, 1988). It is also Geonets are often used on the sidewalls ofimportant to ensure that appropriate factors of landfills because of their ease of installation.safety are used (Koerner, 1990). They should be placed with the top ends in a

The flow rate or transmissivity of geonets longitudinal length extending down the slope.may be evaluated by ASTM D-4716. This The geonets need not be seamed to each otherflow rate may then be compared to design-by- on the slopes, only tied at the edges, butted, orfunction equations presented in U.S. EPA overlapped. They should be placed in a loose(1989). In the ASTM D-4716 flow test, the condition, not stretched or placed in aproposed collector cross section should be configuration where they are bearing theirmodeled as closely as possible to actual field own weight in tension. The constructionconditions (U.S. EPA, 1989). specifications should contain appropriate

Figure 4-7 shows the flow rate "signatures" of or the requirements of the geoneta geonet between two geomembranes (upper manufacturer. All geonets need to becurves) and the same geonet between a layer protected by a filter layer or geotextile toof clay soil and a geomembrane (lower prevent clogging (U.S. EPA, 1988).curves). The differences between the two setsof curves represent intrusion of the The friction factors against sliding forgeotextile/clay into the apertures of the geotextiles, geonets, and geomembranes oftengeonet. The curves are used to obtain a flow can be estimated using manufacturers datarate for the particular geonet being designed because these materials do not exhibit the(U.S. EPA, 1989). Equations to determine the range of characteristics as seen in soildesign flow rate or transmissivity are also materials. However, it is important that thepresented in U.S. EPA (1989), Giroud (1982), designer perform the actual tests using siteCarroll (1987), Koerner (1990), and FHWA materials and that the sliding stability(1987). calculations accurately represent the actual

Generally, geonets perform well and result in specified material characteristics (U.S. EPA,high factors of safety or performance design 1988).ratios, unless creep (elongation under constantstress) becomes a problem or adjacentmaterials intrude into apertures (U.S. EPA,1989). For geonets, the most

secure anchor trench with the strongest

installation requirements as described above

design configuration, site conditions, and the

Figure 4-7. Flow Rate Curves for Geonets in Two Composite Liner Configurations

170

hmax'L c

2[

tan2"c

%1&tan"

ctan2"%c]

Design Criteria

171

Leachate Collection Pipes ! The required flow using known

All components of the leachate collectionsystem must have sufficient strength tosupport the weight of the overlying waste,cover system, and post-closure loadings, aswell as the stresses from operatingequipment. The component that is mostvulnerable to compressive strength failure isthe drainage layer piping. Leachatecollection system piping can fail byexcessive deflection, which may lead tobuckling or collapse (USEPA, 1988). Pipestrength calculations should includeresistance to wall crushing, pipe deflection,and critical buckling pressure. Designequations and information for most pipetypes can be obtained from the major pipemanufacturers. For more informationregarding pipe structural strength, refer toU.S. EPA (1988).

Perforated drainage pipes can provide goodlong-term performance. These pipes havebeen shown to transmit fluids rapidly and tomaintain good service lives. The depth ofthe drainage layer around the pipe should bedeeper than the diameter of the pipe. Thepipes can be placed in trenches to providethe extra depth. In addition, the trenchserves as a sump (low point) for leachatecollection. Pipes can be susceptible toparticulate and biological clogging similarto the drainage layer material. Furthermore,pipes also can be susceptible to deflection.Proper maintenance and design of pipesystems can mitigate these effects andprovide systems that function properly.Acceptable pipe deflections should beevaluated for the pipe material to be used(USEPA, 1989).

The design of perforated collection pipesshould consider the following factors:

percolation impingement rates and pipespacing;

! Pipe size using required flow andmaximum slope; and

! The structural strength of the pipe.

The pipe spacing may be determined by theMound Model. In the Mound Model (seeFigure 4-8), the maximum height of fluidbetween two parallel perforated drainagepipes is equal to (U.S. EPA, 1989):

where c = q/k k = permeability q = inflow rate " = slope.

The two unknowns in the equation are:

L = distance between the pipes; and c = amount of leachate.

Using a maximum allowable head, h , of 30max

cm (12 in), the equation is usually solved for"L" (U.S. EPA, 1989).

The amount of leachate, "c", can be estimatedin a variety of ways including the WaterBalance Method (U.S. EPA, 1989) and thecomputer model Hydrologic Evaluation ofLandfill Performance (HELP). The HELPModel is a quasi-two-dimensional hydrologicmodel of water movement across, into,through, and out of landfills. The model usesclimatologic, soil, and landfill design data andincorporates a solution technique thataccounts for the effects of surface storage,run-off, infiltration, percolation, soil-moisture

Subpart D

172

Figure 4-8. Definition of Terms for Mound ModelFlow Rate Calculations

Design Criteria

173

storage, evapotranspiration, and lateral ! Diameter and wall thickness;drainage. The program estimates run-offdrainage and leachate that are expected to ! Size and distribution of slots andresult from a wide variety of landfill perforations;conditions, including open, partially open, andclosed landfill cells. The model also may be ! Type of coatings (if any) used in theused to estimate the depth of leachate above pipe manufacturing; andthe bottom liner of the landfill unit. Theresults may be used to compare designs or to ! Type of pipe bedding material andaid in the design of leachate collection required compaction used to support thesystems (U.S. EPA, 1988). pipes.

Once the percolation and pipe spacing are The construction drawings and specificationsknown, the design flow rate can be obtained should clearly indicate the type of bedding tousing the curve in Figure 4-9. The amount of be used under the pipes and the dimensions ofleachate percolation at the particular site is any trenches. The specifications shouldlocated on the x-axis. indicate how the pipe lengths are joined. The

The required flow rate is the point at which placed with respect to the perforations. Tothis value intersects with the pipe spacing maintain the lowest possible leachate head,value determined from the Mound Model. there should be perforations near the pipeUsing this value of flow rate and the bottom invert, but not directly at the invert. The pipeslope of the site, the required diameter for the invert itself should be solid to allow forpipe can be determined (see Figure 4-10). efficient pipe flow at low volumes (U.S. EPA,Finally, the graphs in Figures 4-11 and 4-12 1988).show two ways to determine whether thestrength of the pipe is adequate for the landfill When drainage pipe systems are embedded indesign. In Figure 4-11, the vertical soil filter and drainage layers, no unplugged endspressure is located on the y-axis. The density should be allowed. The filter materials inof the backfill material around the pipe is not contact with the pipes should be appropriatelygoverned by strength, so it will deform under sized to prevent migration of the material intopressure rather than break. Ten percent is the the pipe. The filter media, drainage layer, andabsolute limiting deflection value for plastic pipe network should be compatible and shouldpipe. Using Figure 4-11, the applied pressure represent an integrated design.on the pipe is located and traced to the trenchgeometry, and then the pipe deflection value Protection of Leachate Collection Pipesis checked for its adequacy (U.S. EPA, 1989).

The LCS specifications should include (U.S. depends on the design used to protect pipesEPA, 1988): from physical clogging (sedimentation) by the

! Type of piping material; material around the pipes is most effective if

drawings should show how the pipes are

The long-term performance of the LCS

granular drainage materials. Use of a graded

accompanied by proper sizing of pipeperforations. The Army Corps of

Subpart D

174

Figure 4-9. Required Capacity of Leachate Collection Pipe

Figure 4-10. Leachate Collection Pipe Sizing Chart

Subpart D

176

Figure 4-11. Vertical Ring Deflection Versus Vertical Soil Pressure for 18-inch Corrugated Polyethylene in High Pressure Soil Cell

Design Criteria

177

Figure 4-12. Example of the Effect of Trench Geometryand Pipe Sizing on Ring Deflection

Subpart D

178

Engineers (GCA Corporation, 1983) hasestablished design criteria using graded filtersto prevent physical clogging of leachatedrainage layers and piping by soil sedimentdeposits. When installing graded filters,caution should be taken to prevent segregationof the material (USEPA, 1991a).

Clogging of the pipes and drainage layers ofthe leachate collection system can occurthrough several other mechanisms, includingchemical and biological fouling (USEPA,1988). The LCS should be designed with acleanout access capable of reaching all partsof the collection system with standard pipecleaning equipment.

Chemical clogging can occur when dissolvedspecies in the leachate precipitate in thepiping. Clogging can be minimized byperiodically flushing pipes or by providing asufficiently steep slope in the system to allowfor high flow velocities for self-cleansing.These velocities are dependent on thediameter of the precipitate particles and ontheir specific gravity. ASCE (1969) discussesthese relationships. Generally, flow velocitiesshould be in the range of one or two feet persecond to allow for self-cleansing of thepiping (U.S. EPA, 1988).

Biological clogging due to algae and bacterialgrowth can be a serious problem in MSWLF There are three parts to an analysis of a sandunits. There are no universally effective filter that is placed above drainage material.methods of preventing such biological The first determines whether or not the filtergrowth. Since organic materials will be allows adequate flow of liquids. The secondpresent in the landfill unit, there will be a evaluates whether the void spaces are smallpotential for biological clogging. The system enough to prevent solids from being lost fromdesign should include features that allow for the upstream materials. The third estimatespipe system cleanings. The components of the long-term clogging behavior of the filterthe cleaning system should include (U.S. (U.S. EPA, 1989).EPA, 1991b):

! A minimum of six-inch diameter pipesto facilitate cleaning;

! Access located at major pipeintersections or bends to allow forinspections and cleaning; and

! Valves, ports, or other appurtenances tointroduce biocides and/or cleaningsolutions.

In its discussion of drainage layer protection,the following section includes furtherinformation concerning protection of pipesusing filter layers.

Protection of the High-PermeabilityDrainage Layer

The openings in drainage materials, whetherholes in pipes, voids in gravel, or apertures ingeonets, must be protected against cloggingby accumulation of fine (silt-sized) materials.An intermediate material that has smalleropenings than those of the drainage materialcan be used as a filter between the waste anddrainage layer. Sand may be used as filtermaterial, but has the disadvantage of takingup vertical space (USEPA, 1989). Geotextilesdo not use up air space and can be used asfilter materials.

Soil Filter Layers

The particle-size distribution of the drainagesystem and the particle-size distribution of theinvading (or upstream) soils are required

Design Criteria

179

in the design of granular soil (sand filter) specifications should indicate the extent of thematerials. The filter material should have its envelope. The construction quality controllarge and small size particles intermediate program should document that the envelopebetween the two extremes. Equations for was installed according to the plans andadequate flow and retention are: specifications (U.S. EPA, 1988).

! Adequate Flow: A granular filter layer is generally placedd > (3 to 5)d using the same earthmoving equipment as the85 15f d.s.

! Adequate Retention: should be checked by optical survey or byd < (3 to 5)d direct test pit measurement (U.S. EPA, 1988).15 85f w.f.

Where f = required filter soil; This filter layer is the uppermost layer in the d.s. = drainage stone; and leachate collection system. A landfill design

w.f. = water fines. option includes a buffer layer, 12 inches thick

There are no quantitative methods to assess drainage layer from damage due to traffic.soil filter clogging, although empirical This final layer can be general fill, as long asguidelines are found in geotechnical it is no finer than the soil used in the filterengineering references. layer (U.S. EPA, 1988). However, if the

The specifications for granular filter layers leachate recirculation attempts.that surround perforated pipes and that protectthe drainage layer from clogging are based on Geotextile Filter Layersa well-defined particle size distribution. Theorientation and configuration of filter layersrelative to other LCS components should beshown on all drawings and should bedescribed, with ranges of particle sizes, in thematerials section of the specifications (U.S.EPA, 1988a).

Thickness is an important placement criterionfor granular filter material. Generally, thegranular filter materials will be placed aroundperforated pipes by hand, forming an"envelope." The dimensions of the envelopeshould be clearly stated on the drawings or inthe specifications. This envelope can beplaced at the same time as the granulardrainage layer, but it is important that thefilter envelope protect all areas of the pipewhere the clogging potential exists. The plansand

granular drainage layer. The final thickness

(30 cm) or more, to protect the filter layer and

layer has a low permeability, it will affect

Geotextile filter fabrics are often used. Theopen spaces in the fabric allow liquid flowwhile simultaneously preventing upstreamfine particles from fouling the drain.Geotextiles save vertical space, are easy toinstall, and have the added advantage ofremaining stationary under load. Geotextilesalso can be used as cushioning materialsabove geomembranes (USEPA, 1989).Because geotextile filters are susceptible tobiological clogging, their use in areasinundated by leachate (e.g., sumps, aroundleachate collection pipes, and trenches) shouldbe avoided.

Geotextile filter design parallels sand filterdesign with some modifications (U.S. EPA,1989). Adequate flow is assessed by

Subpart D

180

comparing the material (allowable) to the inside so that the glass beads used in thepermittivity to the design imposed test cannot escape around the edges of thepermittivity. Permittivity is measured by the geotextile filter. The particle-size distributionASTM D-4491 test method. The design of retained glass beads is compared to thepermittivity utilizes an adapted form of allowable value using any of a number ofDarcy's law. The resulting comparison yields existing formulas (U.S. EPA, 1989).a design ratio, or factor of safety, that is thefocus of the design (U.S. EPA, 1989): The third consideration in geotextile design is

DR = ø /ø problem that may be adopted by ASTM isallow reqd

called the Gradient Ratio Test. In this test,where: the hydraulic gradient of 1 inch of soil plus

ø = permittivity from ASTM the underlying geotextile is compared with theallow

D-4491 hydraulic gradient of 2 inches of soil. The ø = (q/a) (1/h ) higher the gradient ratio, the more likely thatreqd max

q/a = inflow rate per unit area a clog will occur. The final ASTM gradient h = 12 inches ratio test will include failure criteria. Anmax

The second part of the geotextile filter design flow test that also is performed in ais determining the opening size necessary for laboratory. The test models a soil-to-fabricretaining the upstream soil or particulates in system at the anticipated hydraulic gradient.the leachate. It is well established that the 95 The flow rate through the system ispercent opening size is related to particles to monitored. A long-term flow rate willbe retained in the following type of gradually decrease until it stops altogetherrelationship: (U.S. EPA, 1989).

O < fct. (d , CU, DR) The primary function of a geotextile is to95 50

where: leachate pipes while allowing the passage ofO = 95% opening size of leachate. The most important specifications95

geotextile; are those for hydraulic conductivity andd = 50% size of upstream particles; retention. The hydraulic conductivity of the50

CU = Uniformity of the upstream geotextile generally should be at least ten particle size; and times the soil it is retaining. An evaluation of

DR = Relative density of the the retention ability for loose soils is based on upstream particles. the average particle size of the soil and the

The O size of a geotextile in the equation is The maximum apparent opening size,95

the opening size at which 5 percent of a given sometimes called equivalent opening size, isvalue should be less than the particle size determined by the size of the soil that will becharacteristics of the invading materials. In retained; a geotextile is then selected to meetthe test for the O size of the geotextile, a that specification. The material specifications95

sieve with a very coarse mesh in the bottom is should contain a range of AOS values for theused as a support. The geotextile is placed on geotextile, and top of the mesh and is bonded

long-term clogging. A test method for this

alternative to this test method is a long-term

prevent the migration of fines into the

apparent opening size (AOS) of the geotextile.

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these AOS values should match those used in operational problems. Because they may runthe design calculations (U.S. EPA, 1988).

One of the advantages of geotextiles is theirlight weight and ease of placement. Thegeotextiles are brought to the site, unrolled,and held down with sandbags until they arecovered with a protective layer. They areusually overlapped, not seamed; however, onslopes or in other configurations, they may besewn (U.S. EPA, 1988).

As with granular filter layers, it is importantthat the design drawings be clear in theirdesignation of geotextile placement so that nopotential route of pipe or drainage layerclogging is left unprotected. If geotextiles areused on a slope, they should be secured in ananchor trench similar to those forgeomembranes or geonets (U.S. EPA, 1988).

Leachate Removal System

Sumps, located in a recess at the low point(s)within the leachate collection drainage layer,provide one method for leachate removalfrom the MSWLF unit. In the past, lowvolume sumps have been constructedsuccessfully from reinforced concrete pipe ona concrete footing, and supported above thegeomembrane on a steel plate to protect thegeomembrane from puncture. Recently,however, prefabricated polyethylenestructures have become available. Thesestructures may be suitable for replacing theconcrete components of the sump and havethe advantage of being lighter in weight.

These sumps typically house a submersiblepump, which is positioned close to the sumpfloor to pump the leachate and to maintain a30 cm (12 in) maximum leachate depth.Low-volume sumps, however, can present

dry frequently, there is an increasedprobability of the submersible pumps burningout. For this reason, some landfill operatorsprefer to have sumps placed at depths between1.0 and 1.5 meters. While head levels of 30cm or less are to be maintained on the liner,higher levels are acceptable in sumps.Alternatively, the sump may be designed withlevel controls and with a backup pump tocontrol initiation and shut-off of the pumpingsequence and to have the capability ofalternating between the two pumps. Thesecond pump also may be used in conjunctionwith the primary pump during periods of highflow (e.g., following storm events) and as abackup if the primary pump fails to function.A visible alarm warning light to indicatepump failure to the operator also may beinstalled.

Pumps used to remove leachate from thesumps should be sized to ensure removal ofleachate at the maximum rate of generation.These pumps also should have a sufficientoperating head to lift the leachate to therequired height from the sump to the accessport. Portable vacuum pumps can be used ifthe required lift height is within the limit ofthe pump. They can be moved in sequencefrom one leachate sump to another. The typeof pump specified and the leachate sumpaccess pipes should be compatible and shouldconsider performance needs under operatingand closure conditions (U.S. EPA, 1988).

Alternative methods of leachate removalinclude internal standpipes and pipepenetrations through the geomembrane, bothof which allow leachate removal by gravityflow to either a leachate pond or exteriorpump station. If a leachate removal standpipeis used, it should be extended through theentire landfill from liner to

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cover and then through the cover itself. If a sliding material to tear the geomembranegravity drainage pipe that requiresgeomembrane penetration is used, a highdegree of care should be exercised in both thedesign and construction of the penetration.The penetration should be designed andconstructed in a manner that allowsnondestructive quality control testing of 100percent of the seal between the pipe and thegeomembrane. If not properly constructedand fabricated, geomembrane penetrations canbecome a source of leakage through thegeomembrane.

Other Design Considerations

The stability of the individual leachatecollection system components placed ongeomembrane-covered slopes should beconsidered. A method for calculating thefactor of safety (FS) against sliding for soilsplaced on a sloped geomembrane surface isprovided in Koerner (1990). This methodconsiders the factors affecting the system,including the slope length, the slope angle,and the friction angle between thegeomembrane and its cover soil. Generally,the slope angle is known and is specified onthe design drawings. A minimum FS is thenselected. From the slope angle and the FS, aminimum allowable friction angle isdetermined, and the various components ofthe liner system are selected based on thisminimum friction angle. If the designevaluation results in an unacceptably low FS,then either the sidewall slope or the materialsshould be changed to produce an adequatedesign (U.S. EPA, 1988). For short slopes ina landfill unit, the FS can be as low as 1.1 to1.2 if the slope will be unsupported (i.e., nowaste will be filled against it) for only a shorttime, and if any failures that do occur can berepaired fairly easily. Longer slopes mayrequire higher factors of safety due to thepotential of

along the slope or near the toe of the slope.

Construction Quality Assurance andQuality Control

The following section is excerpted from U.S.EPA (1992). This section discusses qualityassurance and quality control (QA/QC)objectives. For a more detailed discussion onQA/QC and specific considerations, refer toU.S. EPA (1992).

CQA/CQC Objectives

Construction quality assurance (CQA)consists of a planned series of observationsand tests to ensure that the final product meetsproject specifications. CQA plans,specifications, observations, and tests are usedto provide quantitative criteria with which toaccept the final product.

On routine construction projects, CQA isnormally the concern of the owner and isobtained using an independent third-partytesting firm. The independence of the third-party inspection firm is important, particularlywhen the owner is a corporation or other legalentity that has under its corporate "umbrella"the capacity to perform the CQA activities.Although "in-house" CQA personnel may beregistered professional engineers, a perceptionof misrepresentation may exist if CQA is notperformed by an independent third party.

The CQA officer should fully disclose anyactivities or relationships with the ownerthat may impact his impartiality orobjectivity. If such activities orrelationships exist, the CQA officer shoulddescribe actions that have been or can betaken to avoid, mitigate, or neutralize thepossibility they might affect the CQA

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officer's objectivity. Regulatory project plans and specifications,representatives can then evaluate whether thereby allowing the contractor tothese mechanisms are sufficient to ensure an correct the construction process if theacceptable CQA product. quality of the product is not meeting

Construction quality control (CQC) is anon-going process of measuring and • Construction Quality Assurancecontrolling the characteristics of the product (CQA) Testing by the Ownerin order to meet manufacturer's or project (Acceptance Inspection) performed byspecifications. CQC is a production tool the owner usually through the third-that is employed by the manufacturer of party testing firm, provides a measurematerials and by the contractor installing the of the final product quality and itsmaterials at the site. CQA, by contrast, is a conformance with project plans andverification tool employed by the facility specifications. Due to the size andowner or regulatory agency to ensure that costs of a typical MSWLF unitthe materials and installations meet project construction project, rejection of thespecifications. CQC is performed project at completion would be costlyindependently of the CQA Plan. For to all parties. Acceptance Inspectionsexample, while a geomembrane liner as portions of the project becomeinstaller will perform CQC testing of field complete allow deficiencies to beseams, the CQA program will require found and corrected before theyindependent CQA testing of those same become too large and costly.seams by a third-party inspector.

The CQA/CQC plans are implemented performed by a regulatory agency tothrough inspection activities that include ensure that the final product conformsvisual observations, field testing and with all applicable codes andmeasurements, laboratory testing, and regulations. In some cases, theevaluation of the test data. Inspection regulatory agency will use CQAactivities typically are concerned with four documentation and the as-built plansseparate functions: or "record drawings" to confirm

• Quality Control (QC) Inspection bythe Manufacturer provides an in- Soil Liner Quality Assurance/Qualityprocess measure of the product quality Controland its conformance with the projectplans and specifications. Typically, Quality control testing performed onthe manufacturer will QC test results materials used in construction of the landfillto certify that the product conforms to unit includes source testing and constructionproject plans and specifications. testing. Source testing defines material

• Construction Quality Control (CQC) Source testing commonly includes moistureInspection by the Contractor provides content, soil density, Atterberg limits, grainan in-process measure of construction size, and laboratory hydraulic conductivity.quality and conformance with the Construction testing ensures that landfill

the specifications and plans.

• Regulatory Inspection often is

compliance with the regulations.

properties that govern material placement.

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construction has been performed in Quality assurance testing for soil linersaccordance with the plans and technical includes the same testing requirements asspecifications. Construction testing specified above for control testing.generally includes tests of soil moisture Generally, the tests are performed lesscontent, density, lift thickness, and frequently and are performed by anhydraulic conductivity. individual or an entity independent of the

The method of determining compliance with quality assurance (CQA) officer arethe maximum hydraulic conductivity essential to document quality ofcriterion should be specified in the QA/QC construction. The CQA officer'splan. Some methods have included the use responsibilities and those of the CQAof the criterion as a maximum value that officer's staff members may include:never should be exceeded, while othermethods have used statistical techniques to ! Communicating with the contractor;estimate the true mean. The samplecollection program should be designed to ! Interpreting and clarifying projectwork with the method of compliance drawings and specifications with thedetermination. Selection of sample designer, owner, and contractor;collection points should be made on arandom basis. ! Recommending acceptance or

Thin wall sampling tubes generally are used work completed by the constructionto collect compacted clay samples for contractor;laboratory hydraulic conductivity testing. Itis important to minimize disturbance of the ! Submitting blind samples (e.g.,sample being collected. Tubes pushed into duplicates and blanks) for analysis bythe soil by a backhoe may yield disturbed the contractor's testing staff or one orsamples. A recommended procedure (when more independent laboratories, asa backhoe is available during sample applicable;collection) is to use the backhoe bucket as astationary support and push the tube into theclay with a jack positioned between the clayand the tube. The sample hole should befilled with bentonite or a bentonite claymixture, and compacted using short lifts ofmaterial.

If geophysical methods are used formoisture and density measurements, it isrecommended that alternative methods beused less frequently to verify the accuracyof the faster geophysical methods.Additional information on testingprocedures can be found in U.S. EPA(1988b) and U.S. EPA (1990a).

contractor. Activities of the construction

rejection by the owner/operator of

! Notifying owner or operator ofconstruction quality problems notresolved on-site in a timely manner;

! Observing the testing equipment,personnel, and procedures used by theconstruction contractor to check fordetrimentally significant changes overtime;

! Reviewing the constructioncontractor's quality control recording,maintenance, summary, andinterpretations of test data foraccuracy and appropriateness; and

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! Reporting to the owner/operator on between lifts or between placements inmonitoring results.

Soil Liner Pilot Construction (Test Fill)

A pilot construction or test fill is a small-scale test pad that can be used to verify thatthe soil, equipment, and constructionprocedures can produce a liner that performsaccording to the construction drawings andspecifications. An owner or operator maywant to consider the option of constructinga test fill prior to the construction of theliner. A test pad is useful not only inteaching people how to build a soil liner, italso can function as a construction qualityassurance tool. If the variables used to builda test pad that achieves a 1x10 cm/sec-7

hydraulic conductivity are followed exactly,then the completed full-size liner shouldmeet the regulatory requirements (U.S.EPA, 1989). A test fill may be a cost-effective method for the contractor toevaluate the construction methods andborrow source. Specific factors that can beexamined/tested during construction of atest fill include (U.S. EPA, 1988b):

! Preparation and compaction offoundation material to the requiredbearing strength;

! Methods of controlling uniformity ofthe soil material;

! Compactive effort (e.g., type ofequipment, number of passes) toachieve required soil density andhydraulic conductivity;

! Lift thickness and placementprocedures to achieve uniformity ofdensity throughout a lift and theabsence of apparent boundary effects

the same lift;

! Procedures for protecting againstdesiccation cracking or other site- andseason-specific failure mechanisms forthe finished liner or intermediate lifts;

! Measuring the hydraulic conductivityon the test fill in the field andcollecting samples of field-compactedsoil for laboratory testing;

! Test procedures for controlling thequality of construction;

! Ability of different types of soil tomeet hydraulic conductivityrequirements in the field; and

! Skill and competence of theconstruction team, includingequipment operators and qualitycontrol specialists.

Geomembrane Quality Assurance/Quality Control Testing

As with the construction of soil liners,installation of geomembrane liners shouldbe in conformance with a qualityassurance/quality control plan. Testsperformed to evaluate the integrity ofgeomembrane seams are generallyconsidered to be either "destructive" or"non-destructive."

Destructive Testing

Quality control testing of geomembranesgenerally includes peel and shear testing ofscrap test weld sections prior tocommencing seaming activities and atperiodic intervals throughout the day.Additionally, destructive peel and shearfield

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tests are performed on samples from the seam or reseaming the affected area (U.S.installed seams. EPA, 1988). In situations where the seams

Quality assurance testing generally requires may have to be retrained.that an independent laboratory perform peeland shear tests of samples from installed Non-Destructive Testingseams. The samples may be collectedrandomly or in areas of suspect quality. Non-destructive test methods are conductedHDPE seams are generally tested at in the field on an in-place geomembrane.intervals equivalent to one sample per every These test methods determine the integrity300 to 400 feet of installed seam for of the geomembrane field seams. Non-extrusion welds, and every 500 feet for destructive test methods include the probefusion-welded seams. Extrusion seams on test, air lance, vacuum box, ultrasonicHDPE require grinding prior to welding, methods (pulse echo, shadow andwhich can greatly diminish parent material impedance plane), electrical spark test,strengths if excessive grinding occurs. pressurized dual seam, electrical resistivity,Detailed discussion of polyethylene welding and hydrostatic tests. Detailed discussion ofprotocol can be found in U.S. EPA (1991a). these test methods may be found in U.S.For dual hot wedge seams in HDPE, both EPA (1991a). Seam sections that failthe inner and outer seam may be subjected appropriate, non-destructive tests must beto destructive shear tests at the independent carefully delineated, patched or reseamed,laboratory. Destructive samples of installed and retested. Large patches or reseamedseam welds are generally cut into several areas should be subjected to destructive testpieces and distributed to: procedures for quality assurance purposes.

! The installer to perform construction the degree to which non-destructive andquality control field testing; destructive test methods will be used in

! The owner/operator to retain andappropriately catalog or archive; and

! An independent laboratory for peeland shear testing.

If the test results for a seam sample do notpass the acceptance/rejection criteria, thensamples are cut from the same field seam onboth sides of the rejected sample location.Samples are collected and tested until theareal limits of the low quality seam aredefined. Corrective measures should beundertaken to repair the length of seam thathas not passed the acceptance/rejectioncriteria. In many cases, this involvesseaming a cap over the length of the rejected

continually fail testing, the seaming crews

The specifications should clearly describe

evaluating failed portions of non-destructiveseam tests.

Geomembrane Construction QualityAssurance Activities

The responsibilities of the constructionquality assurance (CQA) personnel for theinstallation of the geomembrane aregenerally the same as the responsibilities forthe construction of a soil liner with thefollowing additions:

! Observation of liner storage area andliners in storage, and handling of theliner as the panels are positioned in thecell;

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! Observation of seam overlap, seam they meet the construction plans andpreparation prior to seaming, and specifications. These include (U.S. EPA,material underlying the liner; 1988):

! Observation of destructive testing ! Geonets;conducted on scrap test welds prior toseaming; ! Geotextiles;

! Observation of destructive seam ! Pipe size, materials, and perforations;sampling, submission of the samplesto an independent testing laboratory, ! Granular material gradation andand review of results for conformance prefabricated structures (sumps,to specifications; manholes, etc.);

! Observation of all seams and panels ! Mechanical, electrical, and monitoringfor defects due to manufacturing equipment; andand/or handling and placement;

! Observation of all pipe penetrationboots and welds in the liner; The leachate collection system foundation

! Preparation of reports indicating liner) should be inspected and surveyedsampling conducted and sampling upon its completion to ensure that it hasresults, locations of destructive proper grading and is free of debris andsamples, locations of patches, liquids (U.S. EPA, 1988).locations of seams constructed, andany problems encountered; and, During construction, the following

! Preparation of record drawings of the observed and documented (U.S. EPA,liner installation, in some cases. 1988):

The last responsibility is frequently assigned ! Pipe bedding placement includingto the contractor, the owner's representative, quality, thickness, and areal coverage;or the engineer.

Leachate Collection System including material quality andConstruction Quality Assurance thickness;

The purpose of leachate collection systemCQA is to document that the systemconstruction is in accordance with thedesign specifications. Prior to construction,all materials should be inspected to confirmthat

! Concrete forms and reinforcement.

(geomembrane or low permeability soil

activities, as appropriate, should be

! Granular filter layer placement

! Pipe installation including location,configuration, grades, joints, filterlayer placement, and final flushing;

! Granular drainage layer placementincluding protection of underlyingliners, thickness, overlap with filter

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fabrics and geonets if applicable, and 4.4 RELEVANT POINT OFweather conditions; COMPLIANCE

! Geonet placement including layout,overlap, and protection from clogging 4.4.1 Statement of Regulationby granular material carried by windor run-off during construction; (a) (See Statement of Regulation in

! Geotextile/geofabric placement the regulatory language for performance-including coverage and overlap; based design requirements.)

! Sumps and structure installation; and (b) (See Statement of Regulation in

! Mechanical and electrical equipmentinstallation including testing.

In addition to field observations, actual fieldand laboratory testing may be performed todocument that the materials meet the designspecifications. These activities should bedocumented and should include thefollowing (U.S. EPA, 1988):

! Geonet and geotextile sampling andtesting;

! Granular drainage and filter layersampling and testing for grain sizedistribution; and

! Testing of pipes for leaks,obstructions, and alignments.

Upon completion of construction, eachcomponent should be inspected to identifyany damage that may have occurred duringits installation, or during construction ofanother component (e.g., pipe crushingduring placement of granular drainagelayer). Any damage that does occur shouldbe repaired, and these corrective measuresshould be documented in the CQA records(U.S. EPA, 1988).

40 CFR §258.40(d)

Section 4.2.1 of this guidance document for

Section 4.3.1 of this guidance document forthe regulatory language for requirementspertaining to composite liner and leachatecollection systems.)

(c) (See Statement of Regulation inSection 4.2.1 of this guidance document forthe regulatory language for performance-based design requirements.)

(d) The relevant point of compliancespecified by the Director of an approvedState shall be no more than 150 metersfrom the waste management unitboundary and shall be located on land owned by the owner of the MSWLF unit.

In determining the relevant point ofcompliance, the State Director shallconsider at least the following factors:

( 1 ) The hydrogeo log iccharacteristics of the facility andsurrounding land;

(2) The volume and physical andchemical characteristics of the leachate;

(3) The quantity, quality, anddirection of flow of ground water;

(4) The proximity and withdrawalrate of the ground-water users;

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(5) The availability of alternativedrinking water supplies;

(6) The existing quality of theground water, including other sources ofcontamination and their cumulativeimpacts on the ground water and whetherthe ground water is currently used orreasonably expected to be used fordrinking water;

(7) Public health, safety, and welfareeffects; and

(8) Practicable capability of theowner or operator.

4.4.2 Applicability

In States with approved permit programs,owners/operators may have the opportunityto employ an alternative liner design, as per§258.40(a)(1). In these situations, someflexibility is allowed in terms ofestablishing a relevant point of compliance.The relevant point of compliance may belocated a maximum of 150 meters from thewaste management unit boundary; however,the location must be on property owned bythe MSWLF unit owner or operator.

In unapproved States the relevant point ofcompliance is set at the waste managementunit boundary. The waste management unitboundary is defined as the vertical surfacelocated at the hydraulically downgradientlimit of the unit. This vertical surfaceextends down into and through the entirethickness of the uppermost aquifer.


At least eight factors should be consideredin establishing the relevant point of

compliance for any design under §258.40.The factors provide information needed todetermine if the alternative boundary issufficiently protective of human health andthe environment and if the relevant point ofcompliance is adequate to measure theperformance of the disposal unit.

Site Hydrogeology

The first factor to be considered whendetermining the relevant point ofcompliance is site hydrogeology. Sitehydrogeologic characteristics should beused to identify additional informationrequired to set the relevant point ofcompliance. The site data should besufficient to determine the lateral well-spacing required to detect contaminantreleases to the uppermost aquifer.Hydrogeologic information required to fullycharacterize a site is presented in greaterdetail in Section 5.6.3.

Leachate Volume and PhysicalCharacteristics

Data on leachate volume and quality areneeded to make a determination of the"detectability" of leakage from the facilityat the relevant point of compliance. The netconcentration at any given point resultingfrom the transport of contaminants from thelandfill is a function of contaminant type,initial contaminant concentration, andleakage rate. Assessment of leachatevolume is discussed in Sections 4.2 and 4.3.The assessment of contaminant fate andtransport was discussed in Section 4.3.

Quality, Quantity and Direction ofGround-Water Flow

The hydrogeologic data collected shouldprovide information to assess the ground-water flow rate, ground-water flow

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190

direction, and the volume of ground-water unit, should be determined prior toflow. Background ground-water quality establishing the relevant point ofdata should be used to establish baseline compliance (see Section 5.6.3). Theconcentrations of the monitoring performance standard for landfill designconstituents. This information will be requires that landfill units be designed sorequired as input to determine if that the concentrations listed in Table 1 arecontaminants from the landfill unit have not exceeded at a relevant point ofbeen released and have migrated to the compliance. Issues for approved States torelevant point of compliance. consider are whether the ground water is

Ground-Water Receptors be used as a drinking water source when

The goal of establishing the relevant point the ground water is not currently orof compliance is to ensure early detection of reasonably expected to be used for drinkingcontamination of the uppermost aquifer. water, the State may allow the relevantThe distance to the relevant point of point of compliance to be set near the 150-compliance should allow sufficient time for meter limit.corrective measures to be implemented priorto the migration of contaminants to private Public Health, Welfare, Safetyor public water supply wells.

Existing users of ground water immediately potential overall effect on public health,downgradient from the facility should be welfare, and safety of the proposed relevantidentified on a map. Users located at a point of compliance. Issues that should bedowngradient point where contaminants considered include:might be expected to migrate during theactive life and post-closure care period ofthe facility should be identified.

Alternative Drinking Water Supplies

Consideration should be given to theavailability of alternate drinking watersupplies in the event of a ground-watercontamination problem. If the uppermostaquifer is the sole water supply sourceavailable, all reasonable efforts should bemade to locate the relevant point ofcompliance as close as possible to the actualwaste management unit boundary.

Existing Ground-Water Quality

The existing ground-water quality, bothupgradient and downgradient of theMSWLF

currently used or is reasonably expected to

setting a relevant point of compliance. If

Consideration should be given to the

! Distance to the nearest ground-wateruser or potentially affected surfacewater;

! The response time (based on thedistance to the proposed relevant pointof compliance) required to identifyand remediate or otherwise containground water that may becomeimpacted and potentially affectdowngradient water supplies; and

! The risk that detection monitoring datamay not be representative of a worstcase release of contaminants to groundwater.

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Practicable Capability of the Owner or 4.5 PETITION PROCESSOperator 40 CFR §258.40(e)

If the relevant point of compliance is placed 4.5.1 Statement of Regulationfarther from the waste management unitboundary, the volume of water requiring (a) - (d) (See Statement of Regulationtreatment, should the ground water become in Sections 4.2.1, 4.3.1, and 4.4.1 of thiscontaminated, will increase. One or more of guidance document for regulatorythe following conditions could affect the language.)owner's or operator's practicable capability(technical and financial) to remediate (e) If EPA does not promulgate acontaminant releases: rule establishing the procedures and

! Area of impact, remedial costs, scope RCRA Section 4005(c)(1)(B) by Octoberof remedial investigation, and site 9, 1993, owners and operators incharacterization; unapproved States may utilize a design

! Increased response time due to higher §258.40(a)(1) if the following conditionscosts and increased technical scope of are met:selected remedial method;

! A reduction of the removal efficiency meets the performance standard inof treatment technologies; and §258.40(a)(1);

! Increased difficulty in ground-water (2) The State petitions EPA toextraction or containment if these review its determination; andtechnologies are chosen.

The Director may require some indication of determination or does not disapprove thefinancial capability of the owner or operator determination within 30 days.to maintain a longer and more costlyremedial program due to the longer [Note to Subpart D: 40 CFR Part 239 isdetection time frame associated with a reserved to establish the procedures andrelevant point of compliance located at a requirements for State compliance withgreater distance from the waste management RCRA Section 4005(c)(1)(B).]unit boundary. Additional information onremedial actions for ground water is 4.5.2 Applicabilityprovided in this document in Chapter 5.

requirements for State compliance with

meeting the performance standard in

(1) The State determines the design

(3) EPA approves the State

If EPA does not promulgate procedures andrequirements for state approval by October9, 1993, owners and operators of MSWLFunits located in unapproved States may beable to use an alternative design (incompliance with §258.40(a)(1)) undercertain circumstances.

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Owners or operators of MSWLF unitsshould contact the municipal solid wasteregulatory department in their State todetermine if their State has been approvedby the U.S. EPA.

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4.6 FURTHER INFORMATION

4.6.1 REFERENCES

(Specific to Performance-Based Design Assessment and Solute Transport Modeling)

Abriola, L.M., and G.F. Pinder, (1985a). A Multiphase Approach to the Modeling of PorousMedia Contamination by Organic Compounds 1. Equation Development. Water ResourcesResearch 21(1):11-18.

Abriola, L.M., and G.F. Pinder, (1985b). A Multiphase Approach to the Modeling of PorousMedia Contamination by Organic Compounds 2. Numerical Simulation. Water ResourcesResearch 21(1):19-26.

Aller, L., T. Bennett, J.H. Lehr, R.J. Petty, and G. Hackett, (1987). DRASTIC: A StandardizedSystem for Evaluation Ground Water Pollution Potential Using Hydrogeologic Settings.EPA-600/2-87-035, Kerr Environmental Research Lab, U.S. Environmental ProtectionAgency, Ada, Oklahoma. 455 pp.

Auerbach, S.I., C. Andrews, D. Eyman, D.D. Huff, P.A. Palmer, and W.R. Uhte, (1984).Report of the Panel on Land Disposal. In: Disposal of Industrial and Domestic Wastes:Land and Sea Alternatives. National Research Council. National Academy Press.Washington, DC. pp. 73-100.

Beljin, M.S., (1985). A Program Package of Analytical Models for Solute Transport inGroundwater "SOLUTE". BAS15, International Groundwater Modeling Center, HolcombResearch Institute, Butler University, Indianapolis, Indiana. 163 pp.

Bond, F., and S. Hwang, (1988). Selection Criteria for Mathematical Models Used in ExposureAssessments: Groundwater Models. EPA/600/8-88/075, U.S. Environmental ProtectionAgency, Washington, DC.

Boutwell, S.H., S.M. Brown, B.R. Roberts, and D.F. Atwood, (1986). Modeling RemedialActions at Uncontrolled Hazardous Waste Sites. EPA/540/2-85/001, U.S. EnvironmentalProtection Agency, Athens, Georgia.

Cederberg, G.A., R.L. Street, and J.O. Leckie, (1985). A Groundwater Mass Transport andEquilibrium Chemistry Model for Multicomponent Systems. Water Resources Research,21(8):1095-1104.

Dean, J.D., P.S. Huyakorn, A.S. Donigian, Jr., K.S. Voos, R.W. Schanz, Y.J.Meeks, and R.F.Carsel, (1989). Risk of Unsaturated/Saturated Transport and Transformation of ChemicalConcentrations (RUSTIC). EPA/600/3-89/048a, U.S. EPA, Athens, Georgia.

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de Marsily, G., (1986). Quantitative Hydrogeology: Groundwater Hydrology for Engineers.Academic Press, San Diego, California. 440 pp.

Dillon, R.T., R.M. Cranwell, R.B. Lantz, S.B. Pahwa, and D.S. Ward, (1978). RiskMethodology for Geologic Disposal of Radioactive Waste: The Sandia Waste IsolationFlow and Transport (SWIFT) model. Sand 78-1267/NUREG-CR-0424, Sandia nationalLaboratories, Albuquerque, New Mexico.

Domenico, P.A., and V.V. Palciauskas, (1982). Alternative Boundaries in Solid WasteManagement. Ground Water, 20(3):303-311.

Domenico, P.A., and G.A. Robbins, (1985). A New Method for Contaminant Plume Analysis.Ground Water, 23(4):476-485.

Donigian, A.S., and P.S.C. Rao, (1990). Selection, Application, and Validation ofEnvironmental Models. In: Proceedings of the International Symposium on Water QualityModeling of Agricultural Non-Point Sources, Part 2. D.G. DeCoursey (ed.). ARS-81,U.S. Department of Agriculture Agricultural Research Services. pp. 577-600.

Erdogen, H., and R.D. Heufeld, (1983). Modeling Leachates at Landfill Boundaries. Journalof Environmental Engineering, 109(5):1181-1194.

Faust, C.R., J.H. Guswa, and J.W. Mercer, (1989). Simulation of Three-Dimensional Flow ofImmiscible Fluids Within and Below the Unsaturated Zone. Water Resources Research,25(12):2449-2464.

Freeze, R.A., and J.A. Cherry, (1979). Ground Water. Prentice-Hall, Englewood Cliffs, NewJersey. 604 pp.

GeoTrans, Inc., (1985). SWANFLOW: Simultaneous Water, Air and Non-Aqueous PhaseFlow, Version 1.0-Code Documentation. Herndon, Virginia. 97 pp.

Grove, D.B., and K.G. Stollenwerk, (1987). Chemical Reactions Simulated by GroundwaterQuality Models. Water Resources Bulletin, 23(4):601-615.

Gupta, S.K., C.R. Cole, C.T. Kincaid, and F.E. Kaszeta, (1982). Description and Applicationsof the FE3DGW and CFEST Three-dimensional Finite Element Models, Battelle PacificNW Laboratories, Richland, Washington.

Gupta, S.K., C.T. Kincaid, P. Meyer, C. Newbill, and C.R. Cole, (1982). CFEST:Multidimensional Finite Element Code for the Analysis of Coupled Fluid, Energy andSolute Transport. PNL-4260, Battelle Pacific NW Laboratories, Richland, Washington.

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Gureghian, A.B., D.S. Ward, and R.W. Cleary, (1980). A Finite Element Model for theMigration of Leachate from a Sanitary Landfill in Long Island, New York - Part I:Theory. Water Resources Bulletin, 16(5):900-906.

Guvanasen, V., (1984). Development of A Finite Element Code and Its Application toGeoscience Research. Proceedings 17th Information Meeting of the Nuclear Fuel WasteManagement Program, Atomic Energy of Canada, Ltd. Technical Record TR-199. pp.554-566.

Haji-Djafari, S., (1983). User's Manual GEOFLOW Groundwater Flow and Mass TransportComputer Program. D'Appolonia, Pittsburgh, Pennsylvania.

Huyakorn, P.S. et al., (1984). Testing and Validation of Models for Simulating SoluteTransport in Groundwater: Development, Evaluation and Comparison of BenchmarkTechniques. GWMI 84-13, International Groundwater Modeling Center, HolcombResearch Institute, Indianapolis, Indiana.

Huyakorn, P.S., M.J. Ungs, L.A. Mulkey, and E.A. Sudicky, (1987). A Three-DimensionalAnalytical Method for Predicting Leachate Migration. Ground Water, 25(5):588-598.

Huyakorn, P.S., H.O. White, Jr., V.M. Guvanasen, and B.H. Lester, (1986). TRAFRAP: ATwo-dimensional Finite Element Code for Simulating Fluid Flow and Transport ofRadionuclides in Fractured Porous Media. FOS-33, International Groundwater ModelingCenter, Holcomb Research Institute, Butler University, Indianapolis, Indiana.

Javandel, I., C. Doughty, and C.F. Tsang, (1984). Groundwater Transport: Handbook ofMathematical Models. Water Resources Monogram 10, American Geophysical Union,Washington, DC 228 pp.

Keely, J.F., (1987). The Use of Models in Managing Ground-Water Protection Programs. U.S.Environmental Protection Agency. EPA/600/8-87/003, Ada, Oklahoma. 72 pp.

Keely, J.F., (1989). Performance Evaluations of Pump-and-Treat Remediations. EPA/540/4-89/005, U.S. Environmental Protection Agency, Ada, Oklahoma. 19 pp.

Kincaid, C.T., J.R. Morrey, and J.E. Rogers, (1984a). Geohydrochemical Models for SoluteMigration- Volume 1: Process Description and Computer Code Selection. EA-3417,Electric Power Research Institute, Palo Alto, California.

Kincaid, C.T., J.R. Morrey, S.B. Yabusaki, A.R. Felmy, and J.E. Rogers, (1984b).Geohydrochemical Models for Solute Migration- Volume 2: Preliminary Evaluation ofSelected Computer Codes. EA-3417, Electric Power Research Institute, Palo Alto,California.

Subpart D

196

Kipp, K.L., Jr., (1987). HST3D: A Computer Code for Simulation of Heat and Solute Transportin Three-Dimensional Groundwater Flow Systems. WRI 86-4095, U.S. GeologicalSurvey, Lakewood, Colorado.

Konikow, L.F., and J.D. Bredehoeft, (1985). Method-of-Characteristics Model for SoluteTransport (1985 revision). U.S. Geological Survey.

Lindstrom, F.T., and L. Boersma, (1989). Analytical Solutions for Convective-DispersiveTransport in Confined Aquifers with Different Initial and Boundary Conditions. WaterResources Research, 25(2):241-256.

Lu, J.C.S., B. Eichenberger, and R.J. Stearns, (1985). Leachate Migration from MunicipalLandfills. Pollution Technology Review No. 19. Noyes Publications, Park Ridge, NewJersey. 453 pp.

Mercer, J.W., S.D. Thomas, and B. Ross, (1983). Parameters and Variables Appearing inRepository Siting Models. NUREG/CR-3066. Prepared for U.S. Nuclear RegulatoryCommission, Washington, DC. 244 pp.

Mulkey, L.A., A.S. Donigian, Jr., T.L. Allison, and C.S. Raju, (1989). Evaluation of SourceTerm Initial Conditions for Modeling Leachate Migration from Landfills. U.S. EPA,Athens, Georgia.

Narasimhan, T.N., A.F. White, and T. Tokunaga. 1986. Groundwater Contamination From anInactive Uranium Mill Tailings Pile 2. Application of a Dynamic Mixing Model. WaterResources Research, 22(13):1820-1834.

National Research Council, (1990). Ground Water Models: Scientific and RegulatoryApplications. National Academy Press, Washington, DC. 320 pp.

Nelson, R.W., and J.A. Schur, (1980). PATHS Groundwater Hydrologic Model. PNL-3162,Battelle Pacific NW Laboratories, Richland, Washington.

Osborne, M., and J. Sykes, (1986). Numerical Modeling of Immiscible Organic Transport atthe Hyde Park Landfill. Water Resources Research, 22(1):25-33.

Ostendorf, D.W., R.R. Noss, and D.O. Lederer, (1984). Landfill Leachate Migration throughShallow Unconfined Aquifers, Water Resources Research, 20(2):291-296.

Oster, P.A. Review of Ground-Water Flow and Transport Models in the Unsaturated Zone,(1982). NUREG/CR-2917, PNL-4427. Pacific Northwest Laboratory, Richland,Washington.

Prakash, A., (1984). Groundwater Contamination Due to Transient Sources of Pollution. J. of

Hydraulic Engineering, 110(11):1642-1658.

Design Criteria

197

Prickett, T.A., T.G. Naymik, and C.G. Lonnquist, (1981). A Random-Walk Solute TransportModel for Selected Groundwater Quality Evaluations. Bulletin 65, Illinois State WaterSurvey, Champaign, Illinois.

Runchal, A.K., (1985). PORFLOW: A General Purpose Model for Fluid Flow, Heat Transferand Mass Transport in Anisotropic, Inhomogeneous, Equivalent Porous Media, VolumeI: Theory, Volume II: User's Manual. ACRI/TN-O11. Analytic and ComputationalResearch, Inc. West Los Angeles, California.

Runchal, A.K., (1985). Theory and Application of the PORFLOW Model for Analysis ofCoupled Flow, Heat and Radionuclide Transport in Porous Media. Proceedings,international Symposium on Coupled Processes Affecting the Performance of a NuclearWaste Repository, Berkeley, California.

Salhotra, A.M., P. Mineart, S. Sharp-Hansen, and T. Allison, (1990). Multimedia ExposureAssessment Model (MULTIMED) for Evaluating the Land Disposal of Wastes--ModelTheory. Prepared for U.S. Environmental Protection Agency, Environmental ResearchLaboratory, Athens, Georgia.

Schroeder, A.C., A.C. Gibson, and M.D. Smolen, (1984). The Hydrologic Evaluation ofLandfill Performance (HELP) Model, Volumes I and II. EPA/530/SW-009 andEPA/530/SW-010, U.S. Environmental Protection Agency, Cincinnati, Ohio.

Sharp-Hansen, S., C. Travers, P. Hummel, and T. Allison, (1990). A Subtitle D LandfillApplication Manual for the Multimedia Exposure Assessment Model (MULTIMED).Prepared for the U.S. EPA, Environmental Research Laboratory, Athens, Georgia.

Summers, K.V., S.A. Gherini, M.M. Lang, M.J. Ungs, and K.J. Wilkinson, (1989). MYGRTCode Version 2.0: An IBM Code for Simulating Migration of Organic and InorganicChemicals in Groundwater. EN-6531. Electric Power Research Institute, Palo Alto,California.

Temple, Barker and Sloane, Inc., (1988). Draft Regulatory Impact Analysis of ProposedRevisions to Subtitle D Criteria for Municipal Solid Waste Landfills. Prepared for Officeof Solid Waste, U.S. Environmental Protection Agency.

Theis, T.L., D.J. Kirkner and A.A. Jennings, (1982). Multi-Solute Subsurface TransportModeling for Energy Solid Wastes. Technical Progress Report for the Period September1, 1981-August 31, 1982, COO-10253-3, Prepared for Ecological Research Division,Office of Health and Environmental Research, U.S. Department of Energy.

Travers, C.L., and S. Sharp-Hansen, (1991). Leachate Generation and Migration at Subtitle DFacilities: A Summary and Review of Processes and Mathematical Models. Prepared forU.S. EPA, Environmental Research Laboratory, Athens, Georgia.

Subpart D

198

Travis, B., (1984). TRACR3D: A Model of Flow and Transport in Porous/Fractured Media.LA-9667-MS. Los Alamos National Laboratory, Los Alamos, New Mexico.

Unge, M.J., K.V. Summers, and S.A. Gherini, (1986). MYGRT: An IBM Personal ComputerCode for Simulation Solute Migration in Groundwater, User's Manual. EA-4545-CCM.Electric Power Research Institute, Palo Alto, California.

U.S. EPA, (1988). Superfund Exposure Assessment Manual. EPA/540/1-88/001, Washington,DC. NTIS No. PB89-135859. 157 pp.

U.S. EPA, (1993). Compilation of Ground-Water Models; PB93-209401; U.S. EPA; Office ofSolid Waste; Washington, D.C.

van der Heijde, P.K., Y. Bachmat, J. Bredehoeft, B. Andrews, D. Holtz, and S. Sebastian,(1985). Groundwater Management: The Use of Numerical Models. AmericanGeophysical Union, Washington, D.C.

van der Heijde, P.K., and M.S. Beljin, (1988a). Model Assessment for Delineating WellheadProtection Areas. EPA-440/6-88-002, U.S. EPA, Washington, DC.

van der Heijde, P.K., El-Kadi, A.I., and S.A. Williams, (1988b). Groundwater Modeling: AnOverview and Status Report. EPA/600/2-89/028, U.S. EPA, Ada, Oklahoma.

van Genuchten, M.T., (1978). Simulation Models and Their Application to Landfill DisposalSiting; A Review of Current Technology. In Land Disposal of Hazardous Wastes. EPA-660/9-78-016, U.S. EPA, Cincinnati, Ohio.

van Genuchten, M.T., and W.J. Alves, (1982). Analytical Solutions of the One-DimensionalConvective-Dispersive Solute Transport Equation. USDA, Technique Bulletin No. 1661.U.S. Department of Agriculture, Washington, DC.

Versar, Inc., (1987). Current and Suggested Practices in the Validation of Exposure AssessmentModels, Draft Report. Prepared for U.S. EPA Office of Health and EnvironmentalAssessment, Exposure Assessment Group, Washington, DC. EPA Contract No. 69-02-4254, Work Assignment No. 55.

Voss, C.I., (1984). SUTRA: A Finite Element Simulation Model for Saturated-UnsaturatedFluid Density-Dependent Groundwater Flow with Energy Transport or ChemicallyReactive Single Species Solute Transport. Water Resources Investigations 84-4369, U.S.Geological Survey.

Walton, W.C., (1984). 35 Basic Groundwater Model Programs for Desktop Microcomputers.GWMI 84-06/4, International Groundwater Modeling Center, Holcomb Research Institute,Butler University, Indianapolis, Indiana.

Design Criteria

199

Weaver, J., C.G. Enfield, S. Yates, D. Kreamer, and D. White, (1989). Predicting SubsurfaceContaminant Transport and Transformation: Considerations for Model Selection and FieldValidation. U.S. EPA, Ada, Oklahoma.

Yeh, G.T., (1981). AT123D: Analytical, Transient, One-, Two-, Three-DimensionalSimulation of Waste Transport in the Aquifer System. Publication No. 1439. Oak RidgeNational Laboratory, Oak Ridge, Tennessee.

Yeh, G.T., (1990). Users' Manual of a Three-Dimensional Hybrid Lagrangian-Eulerian FiniteElement Model of WASTE Transport through Saturated-Unsaturated Media. PennsylvaniaState University, University Park, PA.

Yeh, G.T., and D.S. Ward, (1981). FEMWASTE: A Finite-Element Model of Waste Transportthrough Saturated-Unsaturated Porous Media. ORNL-5601. Oak Ridge NationalLaboratory, Oak Ridge, Tennessee.

Yeh, G.T., and D.S. Ward, (1987). FEMWATER: A Finite-Element Model of Water Flowthrough Saturated-Unsaturated Porous Media. ORNL-5567/R1. Oak Ridge NationalLaboratory, Oak Ridge, Tennessee.

4.6.2 REFERENCES

(Specific to Design Criteria)

ASCE, (1969). "Design and Construction of Sanitary and Storm Sewers"; ASCE Manual onEngineering Practice; No. 37.

Bear, Jacob and Arnold Veruijt, (1987). "Modeling Groundwater Flow and Pollution"; D.Reidel Publishing Company; Dordracht, Holland.

Benson, Craig H. and David E. Daniel, (1990). "Influence of Clods on Hydraulic Conductivityof Compacted Clay"; Journal of Geotechnical Engineering; Volume 116, No. 8; August,1990.

Bonaparte, R. and Gross, B.A., (1990). "Field Behavior of Double-Liner Systems. In WasteContainment Systems: Construction, Regulation, and Performance, Edited by R.Bonaparte. Geotechnical Pub 1.26, ASCE.

Carroll, Jr., R.G., (1987). "Hydraulic Properties of Geotextiles." Geotextile Testing and theDesign Engineers, ASTEM 952, American Society for Testing and Materials, Philadelphia,PA, pp 7-20.

COE, (1970). "Laboratory Soils Testing"; EM1110-2-1906; Headquarters, Department of theArmy; Office of the Chief of Engineers; Washington, DC 20314.

Subpart D

200

FHWA, (1985). "FHWA Geotextile Engineering Manual." Contract No.l DTF H61-83-C-00150.

FHWA, (1990). "Geotextile Design & Construction Guidelines. Contract No. FHWADTFH61-86-C-00102.

GCA Corporation, (1983). "Draft Permit Writers' Guidance Manual for Hazardous WasteTreatment, Storage, and Disposal Facility"; 1983.

Giroud, J.P., (1982). "Filter Criteria for Geotextiles," Proceeding 2nd International Conferenceon Geotextiles, Las Vegas, Nevada.

Giroud, J.P. and R. Bonaparte, (1989). "Leakage Through Liners Constructed withGeomembranes - Part I: Geomembrane Liners"; Geotextiles and Geomembranes 8(2);0266-1144/89; pp. 27-67; Elsevier Science Publishers Ltd., England, Great Britain.

Giroud, J.P. and R. Bonaparte, (1989). "Leakage Through Liners Constructed withGeomembranes - Part II: Composite Liners"; Geotextiles and Geomembranes 8(2); 0266-1144/89; pp. 71-111; Elsevier Science Publishers Ltd., England, Great Britain.

Giroud, J.P., A. Khatami and K. Badu-Tweneboah, (1989). "Technical Note - Evaluation of theRate of Leakage Through Composite Liners"; Geotextiles and Geomembranes 8; 0266-1144/89; pp. 337-340; Elsevier Science Publishers Ltd., England, Great Britain.

Haxo, H.E., Jr., (1983). "Analysis and Fingerprinting of Unexposed and Exposed PolymericMembrane Liners"; Proceedings of Ninth Annual Research Symposium: Land Disposal,Incineration, and Treatment of Hazardous Waste; EPA/600/9-83/018; U.S. EPA;Cincinnati, Ohio.

Haxo, H.E., Jr., J.A. Miedema and H.A. Nelson, (1984). "Permeability of Polymeric MembraneLining Materials"; Matrecon, Inc.; Oakland, California; International Conference onGeomembranes; Denver, Colorado.

Industrial Fabrics Association International (1990). "1991 Specifiers Guide;" GeotechnicalFabrics Report, Volume 8, No. 7, 1990.

Javendale, I., C. Doughty and C.F. Tsang, (1984). "Groundwater Transport; Handbook ofMathematical Models"; American Geophysical Union; Washington, DC 20009.

Jayawickrama, P.W., K.W. Brown, J.C. Thomas and R.L. Lytton, (1988). "Leakage RatesThrough Flaws in Membrane Liners"; Journal of Environmental Engineering; Vol. 114,No. 6; December, 1988.

Design Criteria

201

Kastman, Kenneth A., (1984). "Hazardous Waste Landfill Geomembrane: Design, Installationand Monitoring"; Woodward-Clyde Consultants, Chicago, Illinois; InternationalConference on Geomembranes; Denver, Colorado.

Koerner, Robert M., (1990). "Designing with Geosynthetics"; 2nd Edition; Prentice Hall;Englewood-Cliffs, New Jersey 07632.

Radian Corporation, (1987). "Technical Data Summary: Hydraulic Performance of MinimumTechnology Double Liner Systems"; Radian Corporation; Austin, Texas 78766 for U.S.EPA; Contract No. 68-01-7310; Task 7-4.

U.S. EPA, (1982). "Landfill and Surface Impoundment Performance Evaluation"; SW-869;Charles A. Moore; U.S. EPA. NTIS PB-81-166357.

U.S. EPA, (1983). "Lining of Waste Impoundment and Disposal Facilities"; SW-870; U.S.EPA; Office of Solid Waste and Emergency Response; Washington, DC 20460. NTIS PB-81-166365 Revised: PB-86-192796.

U.S. EPA, (1987a). "Background Document on Bottom Liner Performance in Double-LinedLandfills and Surface Impoundments"; EPA/530/SW-87/013; U.S. EPA; Washington, DC.NTIS PB-87-182291.

U.S. EPA, (1987b) " Characterization of MWC Ashes and Leachates from MSW Landfills,Monofills and Co-Disposal Sites: Volume VI of VII ; Characterization of Leachates fromMunicipal Solid Waste Disposal Sites and Co-Disposal Sites"; EPA/530/SW-87/028F,Washington, D.C. NTIS PB-88-127998.

U.S. EPA, (1988). "Guide to Technical Resources for the Design of Land Disposal Facilities";EPA/625/6-88/018; U.S. EPA; Risk Reduction Engineering Laboratory; Center forEnvironmental Research Information; Cincinnati, Ohio 45268.

U.S. EPA, (1988a). Superfund Exposure Assessment Manual. EPA/540/1-88/001, Washington,D.C., NTIS No. PB89-135.859, 157 pp.

U.S. EPA, (1988b). "Design, Construction and Evaluation of Clay Liners for WasteManagement Facilities; EPA/530/SW-86/007F; U.S. EPA; Office of Solid Waste andEmergency Response; Washington, DC 20460. NTIS PB-86-184496.

U.S. EPA, (1988c). "Groundwater Modeling: An Overview and Status Report"; EPA/600/2-89/028; U.S. EPA; Environmental Research Laboratory; Ada, Oklahoma74820. NTIS PB-89-224497.

Subpart D

202

U.S. EPA, (1988d). "Multimedia Exposure Assessment Model for Evaluating the LandDisposal of Hazardous Wastes, Volume I."; Woodward-Clyde Consultants, Oakland, CA94607-4014 for U.S. EPA; Environmental Research Laboratory; Office of Research andDevelopment; Athens, Georgia 30613.

U.S. EPA, (1988e). "Lining of Waste Containment and Other Impoundment Facilities";EPA/600/2-88/052; U.S. EPA; Risk Reduction Engineering Laboratories; Cincinnati, Ohio45268. NTIS PB-89-129670.

U.S. EPA, (1989). "Seminar Publication - Requirements for Hazardous Waste Landfill Design,Construction and Closure"; EPA/625/4-89/022; U.S. EPA; Center for EnvironmentalResearch Information; Office of Research and Development; Cincinnati, Ohio 45268.

U.S. EPA, (1990a). "Seminars - Design and Construction of RCRA/CERCLA Final Covers",CERI 90-50; U.S. EPA; Office of Research and Development; Washington, DC 20460.

U.S. EPA, (1990b). "Draft - LDCRS Flow Data from Operating Units - Technical Support forProposed Liner/Leak Detection System Rule"; Geoservices, Inc. Consulting Engineers;Norcross, Georgia 30093.

U.S. EPA, (1990c). "Relationship of Laboratory- and Field-Determined HydraulicConductivity in Compacted Clay Layer"; EPA/600/2-90/025; U.S. EPA; Risk ReductionEngineering Laboratory; Cincinnati, Ohio 45268. NTIS PB-90-257775.

U.S. EPA, (1991a). "Technical Guidance Document: Inspection Techniques for the Fabricationof Geomembrane Field Seams"; EPA 530/SW-91/051, May 1991, Cincinnati, Ohio.

U.S. EPA, (1991b). "Landfill Leachate Clogging of Geotextiles (and Soil) Filters";EPA/600/2-91/025, August 1991; Risk Reduction Engineering Laboratory; Cincinnati,Ohio, 45268. NTIS PB-91-213660.

U.S. EPA, (1992). "Technical Guidance Document: Construction Quality Management forRemedial Action and Remedial Design Waste Containment Systems"; EPA/540/R-92/073,October 1992; Risk Reduction Engineering Laboratory, Cincinnati, Ohio 45268 andTechnology Innovation Office, Washington, D.C. 20460.

4.6.3 Models

List of Contacts for Obtaining Leachate Generation and Leachate Migration Models

Center for Exposure Assessment Modeling (CEAM), U.S. EPA, Office of Research andDevelopment, Environmental Research Laboratory, Athens, Georgia 30605-2720, ModelDistribution Coordinator (706) 546-3549, Electronic Bulletin Board System (706) 546-3402:MULTIMED, PRZM, FEMWATER/FEMWASTE, LEWASTE/3DLEWASTE

Design Criteria

203

Electric Power Research Institute, Palo Alto, California, (214) 655-8883: MYGRT,FASTCHEM

Geo-Trans Inc., 46050 Manekin Plaza, Suite 100, Sterling, VA 20166, (703) 444-7000:SWANFLOW, SWIFT, SWIFT II, SWIFT III, SWIFT/386.

Geraghty & Miller, Inc., Modeling Group, 10700 Parkridge Boulevard, Suite 600 Reston,VA 22091: MODFLOW , MODPATH , MOC , SUTRA , Quickflow, 386 386 386 386

International Groundwater Modeling Center, Colorado School of Mines, Golden, Colorado(303) 273-3103: SOLUTE, Walton35, SEFTRAN, TRAFRAP,

National Technical Information Services (NTIS), 5285 Port Royal Road, Springfield, VA22161, (703) 487-4650: HELP

Dr. Zubair Saleem, U.S. EPA, 401 M Street SW, Washington, DC, 20460, (202) 260-4767:EPACML, VHS

Scientific Software Group, P.O. Box 23041, Washington, DC 20026-3041 (703) 620-9214:HST3D, MODFLOW, MOC, SUTRA, AQUA, SWIMEV.

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