DRAFT
DRAFT FINAL
INTERIM REPORT 2
SYNTHETIC CAP AND LINER SYSTEMS
• Prepared for:
U.S. Environmental Protection Agency
Office of Solid Waste
401 M Street, S.W.
Washington, D.C.
Prepared by:
Ertec Atlantic, Inc.
15 Campus Drive, Suite 100
Somerset, New Jersey 08873
July 1983
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TABLE OF CONTENTS
PAGE
LIST OF FIGURES vii
LIST OF TABLES viii
1.0 EXECUTIVE SUMMARY 1-1
1.1 INTRODUCTION 1-1
1.1.1 Objectives 1—1
1.1.2 Approach 1—2
1.2 AREAS OF INVESTIGATION 1-3
1.2.1 Chemical Resistance 1—3
1.2.2 Synthetic Liner and Cap Installation 1—4
1.2.3 Performance and Service Life 1—7
1.2.4 Failure Modes 1—9
1.3 CONCLUSIONS AND RECOMMENDATIONS 1-13
1.3.1 Overall Conclusions Regarding Synthetic Liners 1—13
1.3.2 Requirements Needed to Minimize Synthetic Liner
Leakage 1—13
1.3.3 Recommendations 1—14
1.3.3.1 Regulatory Changes 1—14
1.3.3.2 Guidance Material 1—15
1.3.3.3 Future Research 1—16
2.0 INTRODUCTION 2- ].
2.1 BACKGROUND 2-1
2.1.1 Current Material Use and Research 2—2
2.1.2 Regulated Performance Requirements 2—7
2.1.3 Need for Detailed Assessment 2—10
2.2 OBJECTIVES 2—12
2.3 APPROACH 2-13
3.0 PROPERTIES OF SYNTHETIC CAP AND LINER MATERIALS 3-1
3.1 PERFORMANCE CRITERIA 3-2
3.2 PHYSICAL PROPERTIES 3-4
3.2.1 Flexibility 3—5
3.2.2 Strength 3—6
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TABLE OF CONTENTS, continued
PAGE
3.2.2.1 Puncture Resistance 3—7
3.2.2.2 Tear Resistance 3—7
3.2.2.3 Tensile Properties 3—8
3.2.2.4 Seam Strength 3—9
3.2.2.5 Hydrostatic Resistance 3—9
3.2.3 Permeability 3—9
3.2.4 Abrasion Resistance 3—14
3.2.5 Summary 3—15
3.3 CHEMICAL PROPERTIES 3-17
3.3.1 Polymeric Material Composition 3—17
3.3.2 Analytical Properties 3—20
3.3.3 Hazardous Waste Leachates 3—22
3.3.4 Solubility Parameter Theory 3—26
3.3.5 Summary 3—28
3.4 CHEMICAL RESISTANCE TESTING 3—29
3.4.1 Exposure Test Methods 3—30
3.4.1.1 Immersion Exposure Methods 3—31
3.4.1.2 Pouch Test 3—33
3.4.1.3 Tub Test 3—33
3.4.1.4 In—service Exposure 3—35
3.4.2 Material Property Tests 3—35
3.4.2.1 Weight Gain 3—36
3.4.2.2 Dimensional Stability 3—37
3.4.2.3 Strength 3—37
3.4.2.4 Visual Inspection 3—38
3.4.3 Applicability of Test Results 3—38
3.4.4 Summary 3—40
4.0 SYNTHETIC LINER/CAP INSTALLATION 4-1
4.1 FOUNDATIONS 4—2
4.2 BEDDING MATERIALS 4-4
4.3 MATERIAL STORAGE 7,ND HANDLING 4-6
4.4 SEAMING 4—6
4.5 DRAINAGE SYSTEMS 4-9
4.6 QUALITY ASSURANCE 4-10
4.7 SUMMARY 4—12
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TABLE OF CONTENTS, continued
PAGE
5 • 0 SYNTHETIC CAP AND LINER PERFORMANCE 5-].
5.1 SERVICE LIFE 5—].
5.2 PHYSICAL AGING OF SYNTHETIC LINERS 5—2
5.3 FIELD SERVICE EXPERIENCE 5-4
5.4 PROJECTED SERVICE LIFE 5-4
5.5 SUBSIDENCE OF LEACRATE BARRIERS 5-9
5.6 SUBSIDENCE OF COVER SYSTEMS 5-11
5.7 SUMMARY 5—13
6.0 FAILURE MODES 6-1
6.1 DEFINITION 6—].
6.2 LINER HOLES 6-2
6.2.1 Sources of Potential Failure 6—3
6.2.1.1 Design 6—3
6.2.1.2 Manufacture and Fabrication 6—4
6.2.1.3 Storage and Installation 6—6
6.2.1.4 Biological Intrusion 6—6
6.2.1.5 Physical Polymer Aging 6—7
6.2.2 Significance of Holes 6—8
6.2.3 Summary 6—14
6.3 BATHTUB EFFECTS 6-15
6.3.1 Causes of the Bathtub Effect 6—17
6.3.2 Remedial Measures for Bathtub Effects 6—20
6.3.2.1 Pumping and Treatment of Leachate 6—21
6.3.2.2 Cap Reconstruction 6—21
6.3.2.3 Surface—Water/Ground—Water Diversion 6—21
6.3.2.4 Treatment of the Leachate Collection
System 6—22
6.3.3 Sensitivity Analysis for Bathtub Effect 6—22
6.3.4 Summary and Conclusions 6—26
7.0 PERFORMANCE MODELING OF SYNTHETIC LINER SYSTEMS 7-1
7.]. METHODS OF INVESTIGATION 7-1
7.2 FACILITY DESIGN CHARACTERISTICS 7-4
7.2.1 Assumptions 7—4
7.2.2 Climatic Considerations 7—6
7.2.3 Design Scenarios 7—7
7.2.4 Operating Conditions 7—15
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TABLE OF CONTENTS, continued
PAGE
7.2.4.1 Landfills 7—23
7.2.4.2 Disposal Surface Impoundments 7—23
7.2.4.3 Storage Surface Impoundments 7—24
7.2.4.4 Waste Piles 7—24
7.2.5 Scenario Identification System 7—26
7.3 RESULTS OF HELP SIMULATIONS 7—32
7.3.1 General 7—32
7.3.2 Climatic Effects 7—32
7.3.3 Effects of Facility Design and Operating
Conditions 7—37
7.3.3.1 Landfills 7—37
7.3.3.2 Disposal Surface Impoundments 7—44
7.3.3.3 Storage Surface Impoundment 7—48
7.3.3.4 Waste Piles 7—52
7.4 CONCLUSIONS 7—56
7.4.1 Landfills 7—56
7.4.2 Disposal Surface Impoundments 7—59
7.4.3 Storage Surface Impoundments 7—60
7.4.4 Waste Piles 7—61
8.0 SUMMARY AND CONCLUSIONS 8-1
8.1 INTRODUCTION 8-1
8.1.1 Study Objectives 8—2
8.1.2 Approach 8—2
8.2 CONCLUSIONS RELATIVE TO OVERALL SYNTHETIC BARRIER
PERFORMANCE 8-5
8.2.1 General Performance of Synthetic Barrier
Materials 8—6
8.2.1.1 Optimal Synthetic Liner Performance 8—6
8.2.1.2 Advantages and Disadvantages of
Synthetic Liners 8—7
8.2.1.3 Quality Assurance Requirements 8—9
8.2.1.4 Importance of Regional Factors to
Liner Design 8—10
8.2.2 Chemical Resistance of Synthetic Material to
Hazardous Waste Leachate 8—10
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TABLE OF CONTENTS, continued
PAGE
8.2.2.1 Synthetic Liner Service Life 8—12
8.2.2.2 Physical and Chemical Properties Used
in Chemical Resistance Theory 8—13
8.2.2.3 Chemical Resistance Testing Procedures 8—15
8.2.2.4 Comparability and Applicability of
Resistance Test Results 8—18
8.2.2.5 Chemical Resistance Monitoring 8—19
8.2.3 Synthetic Cap and Liner Installation 8—20
8.2.3.1 Field Preparation 8—20
8.2.3.2 Quality Assurance and Quality Control 8—21
8.2.3.3 Selection and Installation of Bedding
Materials 8—22
8.2.3.4 Storage and Handling of Synthetic
Materials 8—22
8.2.3.5 Sterilization of Liner Support and
Cover Soils 8—23
8.2.3.6 Liner Seaming Operations 8—23
8.2.3.7 Subsidence 8—25
8.2.4 Synthetic Cap and Liner Performance 8—26
8.2.4.1 Period of Performance for Synthetic
Liners 8—26
8.2.4.2 Projected Service Life of Synthetic
Liners 8—27
8.2.4.3 Synthetic Liner Permeability 8—28
8.2.4.4 Facility Design Considerations 8—29
8.2.5 Failure Modes 8—31
8.2.5.1 Liner Holes 8—31
8.2.5.2 Bathtub Effects 8—32
8.2.6 Performance Modeling 8—33
8.3 RECO 4ENDATIONS 8—34
8.3.1 Regulatory 8—34
8.3.1.1 Quality Assurance and Quality Control 8—35
8.3.1.2 Synthetic Liner Service Life 8—36
8.3.1.3 Closure Period Extension 8—36
8.3.1.4 Leachate Depth Limitation 8—37
8.3.1.5 Cap Maintenance and Leachate Control 8—37
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TABLE OF CONTENTS, continued
PAGE
8.3.2 Guidance Materials 8—38
8.3.2.1 Quality Assurance and Quality Control 8—38
8.3.2.2 Chemical Resistance and Projected
Service Life Testing 8—39
8.3.3 Future Research 8—40
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LIST OF FIGURES
FIGURE
NUMBER PAGE
2-1 BASIC STRUCTURE OF THE POLYMERIC MEMBRANE LINER
INDUSTRY 2-3
3-1 CONCEPTUAL COMPOSITION OF WASTE LIQUIDS 3-23
5-1 RETENTION OF TENSILE STRENGTH FOR SELECTED BARRIER
MATERIAL EXPOSED IN TROPICAL CLIMATES 5-7
5-2 REPRESENTATIVE SERVICE LIFE DATA 5-8
6-1 PERMEABILITY OF CAP BARRIER LAYER VS. PERCOLATION
AND HEAD 6—12
6-2 LEACRATE SYSTEM EFFICIENCY VS. PERCOLATION AND HEAD 6-13
702-1 TYPICAL CROSS-SECTION FOR LANDFILL DESIGN LFI 7-16
702-2 TYPICAL CROSS-SECTION FOR SURFACE IMPOUNDMENT
DISPOSAL DESIGN SDI 7—17
7.2-3 TYPICAL CROSS-SECTION FOR SURFACE IMPOUNDMENT
STORAGE DESIGN SSI 7-18
7.2-4 TYPICAL CROSS SECTION FOR WASTE PILE DESIGN WPII 7-19
7.2-5 TYPICAL LANDFILL LIQUID ROUTING PROFILE 7-20
7.3-1 LANDFILL CLIMATE COMPARISONS LEVEL Ii 7-34
7.3-2 COMPARISONS OF LANDFILL DESIGN MD SEEPAGE
NEW ORLEANS CLIMATE 7-38
7.3-3 COMPARISONS OF LANDFILL DESIGN AND SEEPAGE
HARTFORD CLIMATE 7-39
7.3-4 COMPARISONS OF LANDFILL DESIGN AND SEEPAGE
DENVER CLIMATE 7-40
7.3-5 COMPARISONS OF DISPOSAL SURFACE IMPOUNDMENT DESIGN
AND SEEPAGE - NEW ORLEANS CLIMATE 7-46
7 • 3-6 COMPARISONS OF STORAGE SURFACE IMPOUNDMENT DESIGN
AND SEEPAGE - NEW ORLEANS CLIMATE 7-51
7.3-7 COMPARISONS OF WASTE PILE DESIGN AND SEEPAGE
NEW ORLEANS CLIMATE 7-54
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LIST OF TABLES
TABLE
NUMBER PAGE
2-1 POLYMERIC MATERIALS USED ZN LINERS 2-6
3-1 TRANSMISSION OF UNDILUTED CHEMICALS THROUGH
SELECTED POLYMER MATERIALS 3-13
3.3.3—1 PHYSICAL CLASSES OF WASTE LIQUIDS 3-25
5-1 POLYMER FIELD SERVICE EXPERIENCE PERIOD 5-5
6-1 EQUIVALENT HYDRAULIC CONDUCTIVITY FROM HOLES IN A
HAZARDOUS WASTE LANDFILL 6-11
6-2 SUMMARY OF SENSITIVITY RUNS ON PROGRAM HELP FOR
ANALYSIS OF BATHTUB EFFECTS 6-24
7.2-1 LANDFILL DESIGN SCENARIOS 7—9
7.2—2 DISPOSAL SURFACE IMPOUNDMENT DESIGN SCENARIOS 7—11
7.2-3 STORAGE SURFACE IMPOUNDMENT DESIGN SCENARIOS 7-13
7.2—4 WASTE PILE DESIGN SCENARIOS 7-14
7.2-5 SUMMARY OF OPERATING CONDITIONS 7-22
7.2-6 HELP MODEL INPUT VALUES FOR OPERATING CONDITIONS 7-27
7.2-7 HELP MODEL INPUT VALUES FOR CLOSURE CONDITIONS 7-29
7.3-1 SUMMARY OP MEAN SEEPAGE RATES BY LANDFILL DESIGN FOR
SELECTED MODEL YEARS (NEW ORLEANS CLIMATE) 7-35
7.3-2 SUMMARY OF MEAN SEEPAGE RATES BY DISPOSAL SURFACE
IMPOUNDMENT DESIGN FOR SELECTED MODEL YEARS
(NEW ORLEANS CLIMATE) 7-45
7.3-3 SUMMARY OP MEAN SEEPAGE RATES BY STORAGE SURFACE
IMPOUNDMENT DESIGN FOR SELECTED MODEL YEARS
(NEW ORLEANS CLIMATE) 7-50
7.3-4 SUMMARY OF MEAN SEEPAGE RATES BY WASTE PILE DESIGN
FOR SELECTED MODEL YEARS (NEW ORLEANS CLIMATE) 7-53
viii
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______ DRAF”
1.0 EXECUTIVE SUMMARY
1.1 INTRODUCTION
The Environmental Protection Agency, Office of Solid Waste, ini-
tiated broad technical investigations of current land disposal technology in
response to comments received following publication of the Interim Final Land
Disposal Regulations. The project was divided into the five principal tasks
identified below:
o Clay Cap and Liner Systems
o Synthetic Cap and Liner Systems
o Leachate Quality
o - Fate and Transport Modeling
o Review of Existing Facilities Which have Failed
The following report provides a summary of available information
regarding the design, installation and operation of synthetic cap and liner
systems.
1.1.1 Objectives
The synthetic liner investigation was designed to evaluate the
advantages and disadvantages of using synthetic liner systems to contain
hazardous waste materials. Secondary objectives of the project included:
the consolidation of available information on the performance of existing
synthetic liner facilities; the development of analysis tools to aid in the
evaluation of liner designs; and, the identification of information gaps
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which will require further research. Essentially, the objective of the
project was to establish a data base defining the “state of the art” in
synthetic liner technology, and utilizing this base, provide recommendations
for further research, the need for additional technical guidance materials,
and areas requiring regulatory action.
1.1.2 A roach
The Environmental Protection Agency recognized the importance of
synthetic liner materials to the field of hazardous waste disposal and e-
signed this project to consolidate the available information in this area.
Data was obtained from existing facilities using synthetic liners, current
research efforts and from an evaluation of current facility design alter-
natives. Field data was obtained through interviews with regulatory person-
nel, facility operators and liner construction specialists. A literature
survey was conducted to identify current research efforts, and the indivi-
duals involved were contacted to provide research results and professional
perspective for incorporation into the synthetic liner data base. Finally, a
research tool, the H ILP model, was developed and used to assess the relative
merits of various facility design alternatives. Overall, the approach se], c-
ted was geared to consolidate available theoretical, field, and design infor-
mation to build a scientific data base to assess the advantages and disadvan-
tages of synthetic liners for containment of hazardous waste.
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1.2 AREAS OF INVESTIGATION DRAFT
1.2.1 Chemical_Resistance
The chemical and physical properties of synthetic liner materials
are based primarily on the type of polymer and the particular formulation
used in the liner manufacturing process. An almost unlimited number of
material variations can be achieved by changing material additives, pigments
and reinforcing materials. Similarl j, the specific components of leachate
generated from a fairly simple waste stream may exceed several hund d
individual components. Specific resistance testing of particular liner ma-
terials with each leachate component would represent a monumental task and is
impractical. The alternative, which is widely used in the synthetic liner
field, uses a representative multi-component leachate to test the resistance
of a specific liner material. Liner material resistance is inferred based on
the rate of change of the physical and chemical properties of the liner
material with exposure time. Several testing methods have been proposed and
all suffer from a basic lack of standardization, either in the parameters
used, the testing protocol or the level of parameter change which indicates a
lack of liner resistance. In general, the chemical resistance testing!is
largely a subjective evaluation at the present time depending on the specific
experience and motivation of the principal investigators.
The EPA has recently taken the first step toward meaningful stan-
dardization and quantification of chemical resistance testing with the devel-
opment of EPA Method 9090. The EPA Method 9090 defines a basic protocol for
conducting immersion testing of liner materials. The standard defines expo-
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sample leachate selection, testing temperatures and bath level maintenance
procedures. Further development of standard testing procedures is warranted
to establish chemical resistance criteria on a more quantitative basis.
At present, the existing base of chemical resistance testing re-
suits is limited. Based on the evaluation of existing facilities conducted
under this program, it appears that the resistance procedures used have
accurately assessed short—term liner performance. Extrapolation of test
results to project long-term performance cannot be verified, since the base
of actual synthetic material field data extends over the past 10 years or ly.
Bowever, the existing 10 year data base indicates that selected synthetic
liners are sufficiently resistant to specific types of leachate. Generally,
the results of resistance testing and available field data indicate that
different liner material types exhibit varying resistance to different leach-
ate types. Efforts to identify a single class of synthetic materials resis-
tant to all leachate types have not been successful. In summary, specific
liner materials can be effectively tested for resistance to particular leach-
ates with a high degree of reliability in the short—term. Extrapolation of
short—term resistance test results to long-term performance cannot be veri-
fied using current techniques. Further research in chemical resistaace
testing is warranted to resolve these problems.
1.2.2 Synthetic Liner and Cap Installation
The installation of synthetic cap and liner systems is performed
using the construction and preparation techniques developed for the instal-
lation of soil liner systems. Generally, liner installation requires comple—
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tion of a detailed geotechnical investigation of the site and the preparation
of a foundation design to accomodate anticipated loads during the life of the
facility. Due to the importance of the facility foundation design a quality
assurance program should be required to verify attainment of design specif i-
cations for proper commpaction of ala soil liners, subgrade materials, waste
materials and the final cap system.
In addition to the general problems associated with waste facility
design and construction, proper installation of synthetic materials pose
several unique problems which should be avoided. Synthetic liners are raa-
tively thin polymer barriers. Therefore they are more susceptible to punc-
ture, tearing and stress than thicker soil liners. Special precautions must
be taken in the selection of liner bedding material to avoid induced stress.
Generally, any fine sand or filtered soil can be used to provide a six inch
buffer layer above and below the liner to add the required protection. Also,
as a result of liner and cap thickness 1 penetration by vegetation and burrow-
ing animals can occur if the liner is not adequately protected. The use of a
layer of gravel or cobbles above the liner cover bed has been used success-
fully to minimize animal damage. All soils and subgrade materials must be
sterilized to prevent vegetative damage. Shallow rooted cover vegetat ion
should be selected and properly maintained to prevent cap damage. These
problems can be adequately resolved using the techniques specified. However,
a comprehensive quality assurance program is required to insure proper appli-
cation of the techniques identified during the liner and cap installation
process.
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The most significant problem encountered during the evaluation of
existing facilities was that of cap subsidence. A].]. waste materials can be
expected to consolidate to some extent as material decay progresses. Studies
have shown that most consolidation occurs during the first 5 years after
facility closure. Proper waste compaction during facility operation and the
placement of soil cover materials during all filling should reduce the over-
all subsidence problem, but some subsidence can be expected. Cap liners can
be designed to support the facility cover, if suitable reinforcing materials
are incorporated into the liner. However, the effect of the induced long—
term stress may result in the eventual weakening of the cap material. n
alternative approach would allow the facility to install a temporary soil cap
during the period of maximum subsidence, followed by the installation of a
final cap after the rate of subsidence diminished. This would require an
extension of the normal closure period and would require continued leachate
removal during this period.
The final installation problem which is peculiar to synthetic
liners is the proper seaming of liner panels. Liner seaming techniques vary
depending on the liner material selected. However, the basic procedure for
all liner types is’the same. The liner panels are cleaned, the solvent or
adhesive is applied, the panels are pressed together and allowed to cu! e.
Major problems associated with field seaming are presented by adverse weather
conditions, including wind, rain, and cold temperatures. Proper scheduling
of installation activities, and adequate storage of materials should prevent
most of these problems. The review of existing facilities indicated that
most faulty seams were attributed to the use of the wrong seaming adhesives
and cleaners. The results of this study indicate that reliable field seams
can be made if proper precautions are observed. A comprehensive quality
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ing operation it is recommended that 100 percent field testing of seams be
included in the quality assurance plan, Further, sample liner seam patches
should be prepared and subjected to laboratory testing to insure proper
strength and bonding of the seamed materials. Special precautions are re-
quired when seaming old and new liner materials. However 1 suitable techni-
ques have been developed and should produce reliable seams if correctly
applied.
1.2.3 Performance and Service Life
This investigation has identified a basic flaw in the current
regulatory concept with respect to the use of synthetic liners. Synthetic
liners cannot strictly prevent leakage of volatile leachate. Leakage occurs
through the liner via a gaseous diffusion mechanism referred to as vapor
transport. Essentially, volatized waste material is absorbed by the liner,
passed through the liner membrane due to the presence of a concentration
gradient, and is desorbed on the outer side of the liner. Proper selection
of liner material can effectively minimize the effective leakage, but minute
amounts of volatile leachate components will escape. Therefore, synthetic
liners cannot, in general, meet the performance objective of prevent!.ng
leachate escape during the active life of the facility. The regulations
should be modified to incorporate a clause allowing “de mninimus” leakage
during the facility life. The significance of the vapor transsport mechanism
is relatively minor if liner material selection is accomplished recognizing
the need to minimize vapor transport through the liner. Synthetic liner
materials can be selected using available testing techniques to minimize
leakage due to vapor transport.
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The projection of liner service life represents a more difficult prob-
lem. The regulations refer to liner “service life” and “long term” effects
without providing a working definition of these terms. Precise definitions
would provide a basis for comparison of liner performance which is presently
unavailable. The lack of clearly defined performance goals is complicated by
the extremely short experience base available for synthetic liners in hazar-
dous waste applications (10) years.
Current liner service life projections are based on exposure te ts
which provide a measure of the rate of change of liner properties as a
function of exposure time. Results available to date indicate that synthetic
liners with suitable resistance to chemical leachate attack can be expected
to meet performance objectives for a period of 40 to 45 years. This result
is based on a subjective evaluation of projected liner service life estimates
which ranged from 3 to 200 years. Existing facility data have shown that
synthetic materials are reliable, but data exists for very short periods
(10 years for hazardous waste and 25 years for other applications). Projec-
tions of 150 to 200 years using current techniques appear to be overly opti—
mistic. Similarly, projections of 3 to 15 years seem overly conservative.
The estimate of 40 to 45 years provides a resonably conservative estim’ te
based on the reliability of currently available projection techniques. Fur-
ther research is warranted to identify more reliable testing methods and to
develop a longer term field information base. Available testing methods
should be evaluated and standard techniques developed. However, based on the
results available at this time, an anticipated service life of a well de-
signed liner, which is chemically resistant to leachate attack, appears to be
in the range of 40 to 45 years.
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which is chemically resistant to leachate attack, appears to be in the range
of 40 to 45 years.
1.2.4 Failure Modes
The vapor transport mechanism is often included as a synthetic
liner failure mode. This does not appear to be justified, since vapor trans—
port is an intrinsic property of synthetic liner materials and does not
represent a material failure. Therefore, vapor transport has not been ad-
dressed as a failure mode in this report.
The evaluation of installed synthetic liners revealed the presence
of various types of holes in the liner material, Liner holes refer to
pinholes which are caused by some manufacturing processes, penetration holes
due to installation damage, and tears in the liner due to wind or equipment
damage. The significance of the presence of holes in the liner to overall
liner performance depends on the tyupe and amount of holes present. Manufac-
turers indicate that proper quality assurance procedures can effectively
minimize and even eliminate liner pinholes. The use of properly selected
liner bedding materials and soil sterilization techniques can be use to
reduce the number of installation induced holes. Any major tears should be
repaired during the liner installation phase. A comprehensive quality con-
trol and inspection procedure is required during liner manufacture, placement
and operation to minimize the number of holes in the liner system. However,
some holes can be expected in the final liner, even if proper quality assur-
ance is implemented.
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An analysis of the net effect of the presence of holes in the liner
was conducted. Assuming a 1 meter square hole or 25 million pinholes per
acre of liner material, the effective leakage rate for the synthetic liner
would be significantly less than the leakage rate through a well designed
clay liner system. Proper quality assurance could reduce the number of holes
to significantly less than the assumed case. Therefore, a properly installed
synthetic liner system can be expected to have some holes. However, the net
leakage through the system would be minimal if proper quality assurance
procedures were applied.
The second principal failure mode identified for synthetic liners
is referred to as the “bathtub effect”. The “bathtub effect” refers to the
gradual filling of the disposal facility with leachate due to increased
permeability of the facility cap and failure of the leachate collection
system. Essentially, the problem is a reflection of cap subsidence problems
or faulty facility design. As the leachate level rises, it will emerge from
the facility either through the cap or by over-topping the side walls. The
result is eventual leachate contamination of local surface water bodies and
streams in the facility area.
Several techniques are available to detect and avoid the problem.
The proper design of the cap system to prevent infiltration of precipitation
is the most important preventative measure. However, due to waste subsidence
discussed previously, failure of even well designed caps can occur. The
leachate collection system provides the necessary backup system to remove
leachate in a controlled manner to avoid buildup. A properly designed and
installed leachate collection system will allow the identification of a cap
failure due to increased leachate infiltration. A final backup system is
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o The cap and leachate control system provides a
critical role in minimizing facility leakage.
o Holding climate constant, more conservative
facility designs (double liners, leachate control,
etc.) act to reduce the total leakage of the
facility.
The results of the HELP simulation indicate that climate is the
most important variable in facility design. Facilities located in moist
climates produced more total leakage than facilities located in dry arid
climates, regardless of facility design. The results indicate the overall
importance of rainfall to the determination of facility leakage. Essentill—
ly, all water which percolates into the facility is available for ultimate
leakage.
The results of the HELP model simulation emphasize the importance
of cap design to the overall facility performance. This suggests that addi-
tional emphasis be placed on the design and installation of cap systems. It
is also recommended that leachate level monitoring be required within the
facility to provide an early warning system for the detection of cap failure.
In the event that failure occurs, the leachate level rise would be detected
before contaminant release occurred from the facility.
The HELP model provides a valuable tool for the evaluation of
design alternatives. Further research is required to verify the HELP model
before the results can be considered more than comparative.
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1.3 CONCLUSIONS AND RECOMMENDATIONS
1.3.1 Overall Conclusions Regarding Synthetic Liners
Synthetic liner systems can be used effectively to control leachate
migration from hazardous waste facilities. However, synthetic liners cannot
achieve the current facility containment goals-expressed in the Interim Final
Land Disposal regulations. Well, designed synthetic liners will be penetrated
by a number of pinholes and installa€ion holes, and will leak as a result of
vapor transport through the liner systems. Properly installed and tested
synthetic liners will reduce the effective leachate leakage rate below that
obtainable through the use of well designed soil liners. Several significant
problems remain regarding the use of synthetic liner materials in a hazardous
waste environment. The most significant problem remaining is the projection
of the anticipated service life of synthetic materials. Current experience
provides a 10 year data base for waste disposal applications and a 25 year
data base for other uses. Therefore, synthetic liner service life is cur-
rently based on laboratory testing of the rate of change of liner properties
as a result of leachate exposure. The results of laboratory projections
indicate that synthetic liners can be expected to perform in accordance 4th
design specifications for a period of 40 to 45 years.
1.3.2 Requirements Needed to Minimize Synthetic Liner Leakage
The singularly most important result of the present project is the
need for mandatory quality assurance of synthetic liner installation opera-
tion and maintenance activities. The investigation of existing facilities
has indicated a general lack of comprehensive quality control of synthetic
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liner installation at existing facilities. This problem pervades facility
design 1 liner installation and maintenance programs. The technology required
to design and construct an acceptable hazardous waste facility is available.
However, the review of existing facilities indicates a general disregard for
the critical factors of synthetic liner installation. The review of existing
facilities indicates significant problems in the following areas:
o Geotechni.cal investigation of facility areas;
o Subgrade and bedding material selection and
preparation;
o Synthetic liner seaming operations;
o Waste placement and compaction;
o Adequate facility cap design; and,
o Appropriate quality assurance control.
Further standardization of chemical resistance testing procedures
is also required. The present techniques rely on a subjective evaluation of
results. There are no established testing protocols and criteria to define
acceptable chemical resistance of selected liner materials to leachate
streams. Finally, the specification of procedures to identify representative
test leachates have not been defined.
1 • 3.3 Recommendations
1.3.3.1 Regulatory Changes
Recommendations for further action are provided in the areas of
regulation review, guidance material preparation and further research. The
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recommendations provided are based on the analysis of the data obtained
during the course of this investigation by the consultants involved.
Reg ulation Review
o The ob ective of containing all leachate emanating
from hazardous waste facilities during the active
life of the facility should be reviewed to
determine if a “de minimus” leakage level would be
acceptable.
o Leachate levels should be monitored within the
waste facility during operation and after closure.
o The closure period should be extended to allow the
installation of temporary caps in order to allow
the rate of waste subsidence to stabilize prior to
installation of the permanent cap. Leachate
collection must continue during this interim
period.
o A formal quality assurance program covering all
aspects of facility design, installation, operation
and maintenance should be established as part of
the permit process.
o Standard chemical resistance testing procedures and
criteria must be specified in the regulations.
o The definitions of “long term” and “service life”
of synthetic materials should be clarified.
o The requirement to limit leach ate rise above the
liner to 30 cm should be -modified to 60 cm for
synthetic systems in order to allow adequate
protection of the synthetic liner system.
1.3.3.2 Guidance Material
o Guidance material should be developed regarding
standard methods for resistance testing of liner
materials. Standardization of testing protocols,
sample preparation and representative leachate
should be accomplished.
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o Guidance material should be developed regarding the
use of current liner property testing methods to
project ultimate liner per forinance.
o A comprehensive quality assurance program should be
developed to support the permitting of hazardous
waste facilities. The program may be designed
using other formal EPA quality assurance programs
as guidelines.
1.3.3.3 Future Research
o Further research is needed to develop appropriate
testing methods to verify the attainment of design
specificastions of manufactured liner materials.
o A central data base of chemical resistance testing
results should be established.
o Solubility parameter theory should be developed to
provide an acceptable screening technique to narrow
the selection of acceptable synthetic materials for
specific leachate types.
o Further research is required to progress further in
the standardization of chemical resistance testing
procedures to allow direct comparison of results.
o An investigation of synthetic liner properties is
warranted to determine specific criteria for
synthetic liner chemical resistance determinations.
o Further investigation of the vapor transport
mechanism should be conducted to determine specific
leachate tyupes and the response of specific
synthetic liner materials.
o Testing methods should be developed to support
liner resistance monitoring at hazardous waste
facilities during and after the operational phase
of the facility.
o The HELP model should be tested to determine the
sensitivity of predicted results to model
assumptions and, if warranted, field verification
should follow.
o A research effort should be conducted to examine
the stability of a conservatively designed waste
facility in the presence of upward hydrostatic
forces. Such conditions could result in uplifting
and associated liner failure.
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2.0 INTRODUCTION
An evaluation of synthetic material performance in cap and liner
systems at hazardous waste storage and disposal facilities was conducted.
This evaluation, part of a more comprehensive assessment of the ability of
clay and synthetic materials to meet performance requirements established at
40 CFR Part 264, was intended to provide the U.S. Environmental Protection
Agency (EPA) with information which it may use in performing its respon-
sibilities under the Resource Conservation and Recovery Act (RCRA).
2.1 BACKGROUND
The EPA, Office of Solid Waste (0 5W), initiated a broad technical
evaluation project in response to comments received following publication of
the interim final land disposal regulations. The project was organized to
provide a summary of current information in the following technical areas:
o Clay Cap/Liner Systems
o synthetic Cap/Liner Systems
o Leachate Quality
o Fate and Transport Analysis
o Evaluation of Failures at Existing Facilities
The EPA project was designed to provide a review of the Part 264
Land Disposal Regulations to assess the need for further research, additional
technical guidance and possible regulatory reform. The following report
provides a summary of the advantages and disadvantages of the use of synthe-
tic materials for containment of hazardous wastes.
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2.1.1 Current Material Use Arid Research
Synthetic materials are currently used for barriers to waste and
liquid flow in liner and cap systems at hazardous waste storage and disposal
facilities as well as solid waste landfills, canals, and reservoirs.
These barriers, commonly referred to as synthetic liners, polymeric
membranes and liners or flexible membrane liners (FML), characteristically
have low permeability. Polymeric membrane technology for waste containment
application relies on a wide variety of types of materials which vary in
physical and chemical properties, installation methods and performance. Var-
iations also occur within a polymer type due to differences in compounding,
manufacturing, and fabrication. Polymers used in making synthetic membranes,
have been available for less than 50 years, and consist of plastics and
rubbers. Other raw materials used in manufacturing synthetic membranes
include fabrics (scrim) and other constituents (fillers/pigments, plasti-
cizers, crosslinkers, antidegradarits, and processing aids) to improve manu-
facturing or material characteristics. -. Polymer membrane manufacture and use
were described by Haxo (Haxo, 1983). They are summarized here to provide a
brief understanding of the synthetic membrane industry and the application of
membranes in hazardous waste facility cap and liner systems.
The polymeric membrane industry, represented in Figure 2-1, con-
sists of four components:
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FLEXIBLE MEMBRANE LINER INDUSTRY
Polymers Fabrics Other Ingredients
• Plastics • Square • Fillers/Pigments
RAW MATERIAL PRODUCERS — .
• Rubbers S Leno • Plasticizers
• Other S Crosslinkers
• Antidegradants
• Processing aids
Sheeting
MANUFACTURERS OF SHEETING • Thermoplastic
Compounding S Crystalline
Forming process S Crosslinked
• Fabric reinforced
Calender ing ________________________
Extrusion
Spread coating
Narrow Sheeting Wide Sheeting
(<90 in.) (21- 33feet)
in rolls
FABRICATORS Panels
Factory assembly of (<20.000 sq. ft.)
sheeting into panels
Lined Waste Containment Facilities
Types Owners
INSTALLERS
• Landfills S Cities/counttes
Assembly on site of panels • Ponds S States
or rolls into liners with field seams S Lagoons S Industrial
• Pits S Landfill operators
• Reservoirs
Figure 2— ]. Basic structure of the polymeric membrane liner industry,
Source: E.C. Jordan, Inc.
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o raw material producers;
o manufacturers of sheeting or roll goods;
o fabricators of panels or segments of barriers; and
o installers.
More than one company is typically involved in the production and
installation of synthetic liners. Design of the containment facility and
selection of the synthetic material is typically performed by engineering
firms. The polymer producers normally supply technical service to sheeting
manufacturers, including recommended formulations and manufacturing proce-
dures. Random monitoring of the sheet manufacturers by the polymer producer
is occasionally conducted.
The expertise of the sheeting manufacturer in formulating (compoun-
ding), mixing and forming sheets will control the properties of the finished
liner. Fabrication of panels by seaming of sheets is limited in size (up to
30m by 60m) by weight and the ability of a crew to place it in the field.
Factory seams are usually more reliable than field seams since they are made
under carefully controlled conditions. Recent introduction of wide (6.7 to
lOin) sheeting has reduced the need for prefabricated panels. Installation of
the synthetic material should be performed by personnel experienced in liner
installation and associated earthwork and piping installation. All field
seams require 100% inspection to assure seam integrity.
Polymers used in the manufacture of lining materials may be classi-
fied into four types:
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o Rubbers (elastomers) which are generally cross-
linked (vulcanized)
o Plastics which are generally unvulcanized, such as
PVC
o Plastics which have a relatively high crystalline
content, such as the polyolef ins
o Thermoplastic elastomers, which do not need to be
vulcanized.
Table 2—1 lists the various types of polymers that are used and
indicates whether they are used in vulcanized or nonvulcanized form and
whether they are reinforced with fabric. The polymeric materials most fre-
quently used in liners are polyvinyl chloride (PVC), chiorosulfonated poly-
ethylene (CSPE), chlorinated polyethylene (CPE), butyl rubber, ethylene pro-
pylene rubber (EPDM), neoprene, and high-density polyethylene (HDPE). The
thickness of polymeric membrane for liners range from 20 to 120 sills, with
most in the 20-60 mu range. Most polymeric lining materials are based on
single polymers. However, blends of two or more polymers, e.g. plastic-
rubber alloys, are being developed and used in liners.
Most of the membrane liners currently manufactured are based on
thermoplastic (soft and moldable when subject to heat) polymers because it is.
easier to obtain reliable seams and to make repairs in the field. Vulcanized
(heat treated) or crosslinked polymers increase the strength and elasticity
of the liner. Fabric reinforcement also increases the strength of the liner.
Synthetic materials are used at hazardous waste facilities as a component of
cap or liner systems. Proper material selection, evaluation, installation
and maintenance of each component is necessary to assure the proper function-
ing of the system. Cap and liner systems are constructed on compacted sub-
base materials and the synthetic barrier is placed between two layers of
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Table 2-1. POLYMERIC MATERIALS USED IN LINERS
Use
in
Liners
Fabric
Thermo—
Polymer plastic
Vulcanized
reinforcement
With W/O
Butyl rubber No Yes Yes Yes
Chlorinated polyethylene Yes Yes Yes Yes
Ch].orosulfonated polyethylene Yes Yes Yes Yes
Elasticized polyolefin Yes No No Yes
(partially crystalline)
Elasticized polyvinyl chloride Yes -- Yes No
Epichlorohydrin rubber Yes Yes Yes Yes
Ethylene propylene rubber Yes Yes Yes Yes
Neoprene No Yes Yes Yes
(chioroprene rubber)
Nitrile rubber Yes —- - Yes --
Polyethylene Yes No No Yes
(partially crystalline)
Polyvinyl chloride Yes No Yes Yes
*Not Available
Source: E.C. Jordan, Inc.
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bedding material. Overlying the upper bedding material is a drainage layer
for removal of leachate or infiltrating precipitation. Cap systems also
consist of an upper vegetated soil layer to control erosion.
Current research includes development of industry—wide standards,
improved formulations arid compounding, evaluation of instaliation procedures
and bedding materials, chemical resistance and testing methods, improved
seaming techniques, prediction of service life, and development of quality
assurance and quality control procedures to enhance and assure synthetic
barrier performance.
2.1.2 Regulated Performance Requirements
The EPA has promulgated interim final regulations which establish
performance requirements for barriers used in liner and cap systems at hazar-
dous waste treatment, storage and disposal facilities. The goal of these
performance requirements is to prevent migration of wastes to the subsurface
soil or to ground water and surface waters during the active life of the
facility and to minimize the leachate remaining after closure. These perfor-
mance requirements are found in: 40 CFR Part 264.
In general, the regulations require liners for all new facilities
to prevent waste migration to subsoils during the active life of the facili-
ty. The regulations further require that the liner be constructed of ma-
terials that prevent wastes from migrating into the liner during the active
life of the facility for landfills, and for waste piles and surface impound—
ments that are designed to close with wastes left in place. The liner must
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be constructed of materials that prevent failure due to the following condi-
tions:
o hydraulic pressure gradients (static head and
external hydrogeologic forces);
o physical contact when exposed to wastes or
leachate;
o climate;
o installation stresses; and
o operational stresses.
The liner must be designed to prevent failure due to settlement, compression,
or uplift by providing a foundation capable of supporting the liner.
Final cover or cap systems placed during closure of surface im-
poundment, waste pile or landfill facilities which contain wastes or contami-
nated soil must be designed and constructed to meet the following perfor-
inance requirements:
o keep migration of liquids through the facility to a
minimum over a long term;
o function with minimum maintenance;
o promote drainage and keep erosion or abrasion of
the cover to a minimum;
o accommodate settling and subsidence so that the
cover’s integrity is maintained; and
o have a permeability less than or equal to the
permeability of the liner system or natural
subsoils present.
These requirements are intended to prevent the accumulation of
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liquids inside the facility to a level which causes failure of the contain-
ment system or an increased rate of transport of wastes or hazardous consti-
tuents through the liner system to ground water or surface water.
The EPA stated the goals of the performance requirements and pro-
vided guidance in the Preamble to the published regulations (EPA, 1982).
Additional guidance has been provided in several draft RCRA Guidance Docu-
ments. Included in the Guidance Documents are discussions of the preference
for synthetic materials to meet the performance requirements for liner and
cover systems. The EPA has suggested that synthetic liners may be expected
to provide a service life of 30 years. Additionally, the Guidance Documents
encourage the use of a soil liner under a synthetic liner which serves as the
lower barrier for a double-lined facility with an operating life greater than
30 years.
The performance of the leachate collection and removal system was
also established by the Part 264 regulations. The Regional Administraator is
required to specify design and operating conditions which ensure that leach-
ate depth over the liner does not exceed 30 cm (one foot). The system must
be constructed of materials that:
o are chemically resistant to wastes and leachate;
and
o prevent collapse due to pressures of overlying
wastes, cover materials, and operating equipment.
The system must also be designed and operated to function without clogging
through the scheduled closure of the facility.
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These regulations do not prescribe liner performance. The ability
of synthetic materials to meet the liner performance requirement is influ—
enced by the conflicting requirements for adequate protective liner bedding
layers and the potentially conflicting performance requirement for a maximum
depth of liquid of 30 cm (one foot).
An implicit goal in the regulations is the desire to contain hazar-
dous wastes to the maximum extent possible. This goal is not established in
a performance regulation for individual hazardous waste facilities, but the
regulations do attempt to attain the goal through performance regulations of
individual components (e.g. liner, leachate collection and removal system,
final cover) of the surface impoundment, waste pile or landfill facility.
2.1.3 Need for Detailed Assessment
The Office of Solid Waste (0 5W) undertook a detailed assessment of
these regulations and others pertaining to the 40 CFR Part 264 Hazardous
Waste Treatment Storage and Disposal (TSD) Facilities. The need for the
assessment was caused by several factors;
o promulgation of regulations based on an assessment
that was less thorough than desired by 05W, to
allow compliance with a court ordered mandate;
o issues raised during the comment period for the
interim final standards; and
o evolving hazardous waste management technology and
practices stimulated by the Interim Standards
(40 IFR Part 265), Temporary Standards (40 CFR
Part 267) and previously enacted by incomplete
standards for treatment storage and disposal
facilities (40 CFR Part 264).
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The project was conducted to assess the technical adequacy of the Interim
Final Regulations and to identify any deficiencies which require further
regulatory action or research. A rapid evaluation was needed to respond to
comments received from the regulated community and other interested groups
following publication of the Interim Final Regulatory Program. Early resolu-
tion of the technical problems identified was required to insure the orderly
expansion of properly managed and permitted hazardous waste facilities.
Issues raised during the comment period that were related to syn-
thetic materials were grouped into the following categories:
o Liner/waste interactions;
o Installation problems;
o Serviceable life;
o Failure modes; and
o Subsidence problems;
o Short- and long—term performance
Resolution of these issues required a detailed review of the technology,
practices and experiences in the synthetic liner field.
As a result of ongoing research results and stimulation provided by
the 264, 265 arid 267 standards, the technology and practices of the hazardous
waste management field are changing. This afforded an opportunity for a
larger data base and improved understanding of the management of wastes. In
the synthetic liner area, research and experiences in chemical resistance
testing, installation practices, evaluation of leachate collection and re-
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moval systems, projection of performance, and quality assurance suggested a
more detailed assessment was warranted. -
2.2 OBJECTIVES
The primary objective of this study of synthetic materials was to
assess their performance as barriers in liner and cap systems at hazardous
waste facilites and identify appropriate regulatory reforms and/or research
needs. In order to accomplish this objective, questions raised during the
comment period for the 40 CFR Part 264 interim final standards were addres-
sed.
o Identify and evaluate the physical and chemical
properties of synthetic materials that are
important in assessing chemical resistance to
wastes, and evaluate testing methodology.
o Identify installation problems, preventive measures
and corrective actions which affect the performance
of synthetic masterials.
o Determine the service life of synthetic materials
based on existing information and projections of
changes in physical and chemical characteristics.
o identify subsidence problems, preventive measures
and corrective actions which affect synthetic
material performance.
o Identify failure modes which determine the perfor-
mance of synthetic materials as barriers in waste
facilities, and put these failure modes into a
perspective which allows a full assessment of syn-
thetic materials.
o Projected performance of synthetic materials used
in liner and cover systems were addressed based on
the results of the above findings, assumed modes of
failure, design and hydrogeologic location.
o Assess the ability of synthetic liners to comply
with existing performance standards.
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These objectives were accomplished through a cooperative investiga-
tive program involving EPA technical staff and supporting consultants. This
report presents detailed discussion of the results of this investigative
program, and provides perspective on the advantages and disadvantages of the
use of synthetic materials in liner and cap systems of hazardous waste man-
agement facilities.
2.3 APPROACH
The general approach used in assessing performance of synthetic
materials was to: 1) collect and review existing information; 2)determine
current practices and review their adequacy; 3) assess anticipated perfor—
mance; 4) identify deficiencies in the data base; and 5) compare projected
performance to the performance standard.
Many sources of information were used to develop an understanding
of the current technology, practices, and information available on the use of
synthetic materials at hazardous waste facilities. - -These included compu-
terized literature searches (including computerized data bases and refer-
rals), personal interviews, reports of site visits conducted by OSW technical
staff, reports of ongoing research, and information provided during the
comment period. Interviews were conducted with individuals of the following
groups:
o Suppliers, manufacturers, fabricators, designers,
and installers of synthetic material, clay and
other liners;
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o Owners/operators of hazardous waste management
facilities;
o State regulatory agencies;
o Researchers in academic and research organizations;
and
o Trade/professional associations and standards
setting organizations.
This report is a summary of reports and information prepared by
several contractors and EPA technical staff from OSW and the Municipal Envi-
ronznerital Research Laboratory (MERL) in Cincinnati. Major contributions to
this report included the following:
TRW, Inc. “Assessment of Technology for
Construction and Installing Cover
and Bottom Liner Systems for
Hazardous Waste Facilities.”
A.D. Little, Inc. “Analysis of Flexible Membrane
Liner Chemical Compatibility
Tests.”
OSW, MERL and “Projected Performance of Hazardous
ERTEC Waste Facilities for Selected
Design, Failure Modes and
Hydrogeologic Settings.
S.C. Jordan, Inc. “Preliminary Draft Interim
Report II Synthetic Cap and Liner
Systems.”
Observations and conclusions made by TRW in assessing construction and in-
stallation technology and A.D. Little in analyzing chemical compatability and
expected life were used to define the design and failure modes for the
projection of performance.
The purpose of this report is to summarize the significant findings
of the contributors. Specific details may be found in the respective re-
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ports. Compilation and assessment of these major contributions was conducted
by ERTEC Atlantic and E.C. 3ordan. Extensive review was provided by the team
assembled by OSW to perform the broader assessment of clay vs. synthetic
liner performance which will be described in a final project report. This
report is intended to contribute to that assessment.
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REFERENCES
Haxo, H.E., Jr. 1983. Lining of Waste Impoundment and Disposal
Facilities . U.S. Environmental Protection Agency,
Cincinnati, OH. 448 p.
EPA, 1982. Federal Register Vol. 47, No. 143, July 26, pp. 32274 .
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3.0 PROPERTIES OF SYNTHETIC CAP AND LINER MATERIALS
The performance of synthetic cap and liner materials in a hazardous
waste facility is essentially determined by the permeability of the installed
liner system and its change with time. Assuming proper installation, liner
permeability may increase with time as a result of weathering, exposure to
waste leachate or physical deformation of the liner material. Weathering and
deformation are primarily site specific problems, which require the matching
of physical liner properties to the environment and geology of the facility
location. Liner exposure to waste leachate is a more complex problem resulting
from the wide variation observed in the chemical properties of both the waste
leachate and the polymeric liner itself. The ability to predict the
performance of synthetic liners in a hazardous waste facility requires a
complete knowledge of the chemical and physical properties of the polymeric
liner material and its exposure environment. Various testing methods and
procedures are available to determine most of the required liner properties,
however, not all procedures are fully standardized. Further, there are no
commonly accepted techniques for testing long—term liner performance.
The following sections provide a brief discussion of the chemical
and physical properties of liner materials and their relationship to liner
performance criteria established in the regulations. Available testing
procedures are introduced as the principal method available for determining
the chemical resistance of liner materials to hazardous waste leachates.
The classification of liner ‘properties adopted is somewhat
arbitrary, but reflects the classification scheme used in the supporting
3-i
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consultant reports and the general literature. Under this classification
scheme, physical properties are defined as liner qualities which can be
measured externally. Properties which are intrinsically associated with
material chemistry are classified as chemical properties. The ambiguity
inherent in this classification approach does not significantly affect the
study results since the classification of properties is primarily a
construction for convenience.
3.1 PERFORMANCE CRITERIA
Liner and cap systems at hazardous waste facilities are intended to
contain hazardous constituents and prevent liquid movement. The major function
of the facility cap is to prevent the infiltration of precipitation to the
underlying waste and the subsequent formation of leachate. The function of
liner systems is the prevention of outward migration of leachate and hazardous
constituents and to prevent the inward movement of groundwater. When properly
functioning, the liner and cap systems approach the overall facility
performance objective to prevent waste migration from the facility during the
operational life of the facility and minimize migration thereafter. The cap
system essentially controls the principal leachate source, while the liner
provides the final containment barrier to prevent waste migration from the
facility.
Cap and liner barriers are subject to a variety of conditions which
result in material stress. The barrier material must be capable of resisting
or relieving this stress in order to function properly. In cover systems using
synthetic barriers, consolidation of waste can subject the barrier material to
3-2
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tensile stress, which may result in barrier failure. In liner systems,
excessive hydraulic pressures, collapse of leachate removal pipes, and
subsidence of underlying materials may also cause failure. In addition,
barrier exposure to sunlight, ozone, chemicals contained in the waste, and
other environmental hazards may affect cap and liner resistance to stress and
general performance. A properly designed synthetic barrier must be
sufficiently durable to withstand the effects of physical stress and
weathering during the life of the waste facility and beyond.
Synthetic barrier materials meeting the performance goal would be
expected to possess the following characteristics:
o Flexibility to conform to new surface contours which may result
from subsidence in a well designed facility;
o Tensile strength to resist stresses imposed;
o Low permeability to liquids and chemicals;
o Chemical resistance to waste constituents; and
o Resistance to weathering.
Flexibility, tensile strength and resistance to weathering (including wind)
are also important considerations during construction of the facility. The
duration over which barrier materials should possess these properties is
dependent on the waste characteristics. The importance of these properties is
influenced by site specific factors such as waste to be contained, soil
stability, climate and facility construction and operation practices.
Essentially, cap and liner performance depends on the ability of the material
to resist stress, weathering and chemical attack by waste leachate. Thus,
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evaluation of anticipated liner and cap performance requires a detailed
knowledge of the physical and chemical properties of the synthetic material
used and the leachate generated within the facility.
3.2 PRYSICAL PROPERTIES
Physical properties of synthetic liner materials refer to those
liner attributes which can be measured externally using various laboratory or
field techniques. The importance of these physical properties is twofold.
First, physical properties provide a suitable method of verifying liner design
specifications. Secondly, any change in physical properties resulting from
exposure of the liner material to waste leachate provides a measure of the
chemical resistance of the liner to the leachate encountered. Unfortunately,
complete standardization of liner physical properties and testing protocols
has not been achieved due to the extremely wide range of synthetic liner
materials available.
Physical properties vary between different types of materials and
among different formulations of the same type of material. The large number of
polymer formulations available for synthetic barriers do not allow a succinct
suary of material properties. Raxo (1983> has provided a summary of
suggested standard properties for the following materials:
o croaslinked and thermoplastic membranes without scrim;
o partially crystalline membranes without scrim; and
o thermoplastic coated membranes with scrim.
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These suggested standard properties are based on test results of material
specimens and are intended for use as a means of establishing the quality of
synthetic materials and not as a means of assessing chemical resistance and
durability. Such an assessment may result in barrier material specifications
which differ from those suggested by Haxo. In the absence of chemical
exposure, materials meeting the suggested properties would be expected to
perform satisfactorily if properly installed, maintained and operated.
The most significant physical properties of synthetic liner
materials are flexibility, strength, permeability and abrasion resistance.
Each of these properties is dependent on the particular polymer formulation
and the presence of reinforcing materials within the synthetic liner material.
Intrinsic liner permeability refers to the internal capacity of the liner to
transmit a specific chemical and should not be confused with the effective
permeability of an installed liner. The effective permeability of an installed
liner refers to the overall liner permeability due to the presence of
pinholes, tears and does not provide a measure of intrinsic permeability.
3.2.1 Flexibility
The flexibility of synthetic liners is controlled by the polymer
and additives used in the formulation and the fabricated material thickness.
All of the synthetic materials exhibit 1 to varying degrees, an ability to
stretch, deform or bend repeatedly without detrimental cracking. The thickness
of some materials, such as 100 mu HDPE, reduces flexibility compared to
regular 30 to 50 mil barriers, but provides greater tensile strength and
resistance to puncture and tearing.
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Flexibility ía also reduced by exposure to cold temperatures. A
variety of test procedures are available for low temperature exposure testing.
Test conditions vary, so results may not be directly comparable. High
temperatures generally increase flexibility, but may also reduce tensile
strength and affect dimensional stability of some synthetic materials.
Exposure to chemicals or other environmental conditions, such as
exposure to ozone or ultraviolet light may also reduce flexibility. These
exposures may alter the flexibility of the polymer by breaking long polymer
chains or increasing croeslinking of the polymer chains. The loss of
plasticizers from some types of materials (e.g. PVC) by volatilization or
extraction would also reduce flexibility. Synthetic materials can be
formulated to provide the flexibility required to meet specific service
conditions. A measure of flexibility is also provided by the material’s
elongation property. Elongation is discussed in the following section.
3.2.2 Strength
The strength of synthetic materials is usually characterized by
measuring the following properties:
o Puncture resistance;
o Tear resistance;
o Tensile properties; and
o Seam strength.
o Hydrostatic resistance.
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These properties have been used in evaluating the ability of synthetic
materials to meet specific service conditions and thus are used in selecting
liner materials.
3.2.2.1 Puncture Resistance
Since sythetic materals are thin, they are susceptible to puncture
from bedding materials which may contain sharp objects, or from operations
conducted above the material duriug installation or facility operations.
Several test methods have been used to measure puncture resistance caused by
the gradual exertion of force (Haxo, 1973). No method exists for measuring the
resistance to puncture of a liner by sharp objects dropped during
installation.
3.2.2.2 Tear Resistance
Tear resistance measures a material’s capability to resist stresses
encountered during installation or subsequent to puncture. Low resistance to
tear would be undesirable because a hole in the liner would expand. The use of
reinforcing fabrics in laminated materials can greatly increase the liner’s
tear resistance. Tear resistance was not included in the standards for
synthetic materials suggested by Haxo (1983). Fabric reinforced test specimens
show an unusually high tear resistance if standard samples are used. However
this may represent a testing problem since the use of larger sized samples
show a moderate increase in tear resistance.
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3.2.2.3 Tensile Properties
Measurement of tensile properties usually includes the following
determinations:
o Tensile strengths (the stresses at the yield point, fabric break
or unreinforced material break);
o Modulu8 of elasticity (measure of the rigidity of stiff
materials at a specified percent of initial specimen length);
and -
o Ultimate elongation (percent of initial specimen length at
breaking).
Tensile strength required for a particular application may be determined by an
analysis of forces to which the synthetic material may be exposed. Comparison
of specimen test results with the required stress can then be used to select a
synthetic material where tensile strength is an important consideration.
Temperature conditions of the tensile test affect measurements significantly.
Tensile stress values for a given polymeric material vary with speed of test,
specimen size, grain orientation, temperature and humidity. Consequently,
reported test results, measured under different conditions, are not directly
comparable.
The modulus of elasticity is usually determined only for the more
rigid, crystalline polymers. Elongation at yield and material and fabric break
have been used as indicators of flexibility and may be used to monitor
material quality during production, installation and chemical resistance
testing.
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3.2.2.4 Seam Strength
Factory and field seams are equally important in determining the
capability of materials to meet liner performance requirements. Because of
greater control of seaming conditions, factory seams are generally stronger.
Specimens have been tested in shear and peel and loaded statically and
dynamically. Seam strength may be used to evaluate alternative seaming methods
for specific applications. The peel teat is significantly more sensitive to
chemical and weather exposure than the shear test and should be used to insure
proper field seaming.
3.2.2.5 Hydrostatic Resistance
Synthetic materials may be subject to hydrostatic pressures where
there are voids underlying the barrier. The bursting pressure is commonly
determined for all barriers using a method developed for fabric reinforced
materials more sensitive to chemical and weather exposure than the shear test.
3.2.3 Permeability
Most synthetic liner materials have very low permeability to
liquids. However, synthetic liners do have a finite permeability to aqueous
liquids as a result of vapor transport (diffusion) through the liner. The
driving force for the vapor transport mechanism is the concentration gradient
or difference of partial pressures across the synthetic liner material. Test
procedures have been developed to determine the vapor loss across a polymeric
boundary for specific chemicals. The results show that most synthetic liner
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materials possess very low, yet finite, permeability to water. Essentially the
determination of the significance of vapor loss through a liner is complicated
by the presence of multiple chemical species in leachate as well as the liner
itself. Theoretically, vapor transport can be determined from a knowledge of
the driving gradient, the diffusion coefficient of the chemical through the
liner and the thickness of the material. Unfortunately, the diffusion
coefficient of a multi—c pOnent leachate through a complex polymeric liner
can not be determined. However, general conclusions can be developed based on
an understanding of the vapor transport process and available test results. A
brief description of the vapor transport process is provided for completeness.
A more detailed discussion is provided in the supporting technical consultants
report (ADL — 1983).
“The transmission of a chemical through a polymer film free from
cracks, pinholes, or other flaws normally occurs by a diffusion process (i.e.,
vapor transport). The chemical dissolves in the surface layers, diffuses
through the bulk material under the influence of a concentration gradient, and
evaporates or desorbs the other, low concentration surface. Diffusion in a
polymeric material is a function of the energy and molecular size of the
chemical and the freedom of movement of the molecular changes of the polymer.
In general, higher degrees of freedom are associated with rubbery materials
and less freedom is found in crystalline polymers.” (Swope, et. al 1983)
Limited data in the literature with undiluted chemicals under conditions of
maximum concentration gradients are shown in Table 3—1 and indicate that the
transport rates range from less than 1 x 1O’ kg/day/rn 2 to greater than 10
kg/day/in 2 for certain chemical and 30 mu material exposures. Lower values,
below 1 x 1O kg/day/rn 2 , are associated with water and inorganic and ionic
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constituents of aqueous systems.
Water is a volatile material and therefore is subject to vapor
transport through synthetic materials. However, the vapor transport of water
through the liner should not be confused with the permeation of water through
a soil material or a liner with holes. Permeation refers to a physical process
whereby liquid material passes through a material under hydraulic pressure.
Vapor transport is a process whereby gaseous material passes through a barrier
due to the diffusion process under the presence of a concentration gradient.
The net effect of either process is leakage, however the permeation rate is
proportional to hydrostatic head and the vapor transport rate is dependent on
the concentration gradient. Water as a volatile material can be transported or
permeated through synthetic materials if holes as present.
Transport through synthetic material requires a finite time to
reach a steady state. This time (induction time) is dependent on the
conditions which control the predominant driving forces of vapor transport,
including type of hazardous constituent, material composition and
environmental conditions. The induction time was calculated for 30 mu
synthetic materials to range from less than one hour for materials with high
diffusion coefficients to more than 80 years for materials with small
diffusion coefficients. August and Tatzky reported induction times of 4 days
to 8 weeks. These times agree with those calculated by Swope et. al. for
diffusion coefficients of i0 8 cm/sec.
The application of vapor permeation theory to hazardous waste
facility evaluation is new and not verified. It does, however, provide a
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conceptual basis for evaluating the regulatory performance standard for
hazardous waste barriers. The theory can be applied using weight change data
which is frequently determined during chemical resistance testing. A detailed
description of steady state permeation rates and time to reach steady state is
provided in a contributing report (Swope, et. al. 1983).
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TABLE 3-1 TRANSMISSION OF UNDILUTED CHEMICALS THROUGH SELECTED POLYMER MATERIALS
CHEMICALa ,b ,c ,d
Carbon
MATERIAL Tetrachioride Methanol Reptane Toluene Benzene Water
Polyethylene, low density (LDPE) 0.32 0.001
Polyethylene, high density (BDPE) 0.05 0.0001 0.03 0.06 1 .Ozl0
Styrene-Butadiene rubber (SEa) 19.3 0.04 —— ——
Acrylonitrile—butadiene rubber (HER) 4.2 0.5
Chioroprene (Neoprene) 11.2 0.09
Polyvinyl Fluoride (PvF) 0.0004 0 0003 e
Polyethylene Terephthalate (PET) 0.00005
Polyarnide (Nylon 66) 0.0003 0.02
TPE (Teflon) 0.00001 0.00003
a 2
kg/day/rn
bMaxirnuIn concentration gradients with undiluted chemicals.
CT 20—40 0 C; Thickness: 0.76 ,mn (30 mu)
i-a dNo detectable leakage — —
e hl
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The importance of this transport mode has been reported for aqueous
mixtures of hazardous constituents, August and Tatzky (1982) reported the
transmission rates for aqueous saturated solutions of toluene and xylene
ranged from 0.002 to 0.012 kg/day/rn 2 in teats of Polyethylene, Polyvinyl
Chloride and Ethylene propylene rubber materials. These rates were 10 to 50
times lower than rates measured with pure solvent. Their data further
indicated that the transmission rate was dependent but not directly
proportional to material thickness. Transmission rates for thicker materials
were less than for thinner materials. Concentrated transmission rates for
toluene were comparable to that reported by Swope, et. al. (1983).
The reported study results were conducted for purely hypothetical
situations using pure chemicals in direct contact with liners. Within a
hazardous waste facility the leachate in contact with the liner is generally
regarded as a dilute aqueous leachate. Further, the liner is covered by a
pbrous leachate collection layer which would act to avoid direct liner—waste
contact. Nevertheless, the values reported are significant for some
liner/waste combinations and should not be ignored. However further testing is
required to determine the significance of the vapor transport process in a
hazardous waste environment.
3.2.4 Abrasion Resistance
An important property of synthetic materials subjected to abrasion
is its hardness or resistance to abrasion. The property is easily measured and
can be used to monitor material quality during production and installation. It
is also a useful indicator of material quality change during chemical
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resistance testing.
3.2.5 ! !!Z
The primary importance of the physical properties of synthetic
liner. is their utility in verifying liner manufacturer specification. and in
the evaluation of chemical resistivity. Facility design should include
technical specification of acceptable ranges of these parameters using
specified testing procedure. and conditions. Of the physical properties
specified, testing procedures exist for all but liner resistance to the impact
of sharp projectiles and the evaluation of intrinsic liner permeability.
However, the procedures used are not fully standardized. The impact of sharp
projectiles on overall liner performance can be minimized by eliminating
unnecessary objects during liner installation and through proper training of
installation personnel. Permeability represents a more complex problem due to
the lack of sufficiently realistic test data for hazardous waste facility
leachate.
Where organics are present in two phases (water saturated and
organic layers), the vapor transport may be quite significant, as much as 20
kg/day/rn 2 for carbon tetrachioride (see Table 3—1). Appropriate selection of
synthetic materials may reduce transmission rates by a factor of more than 200
to a seepage rate of less than kg/day/rn 2 . Among the currently available
barrier materials sucinarized by Swope et. al. HDPE consistently exhibited the
lowest transmission rate for a limited number of pure organic chemical. .. The
rates ranged from 1 to 600 times the vapor transport rate for water.
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The reduction of transmission rates of 10 to 50 times reported by
August and Tataky (1982) for aqueous solutions of organic chemicals, compared
to pure chemicals, tentatively indicates the actual transport rate for
specific chemical constituents of an aqueous hazardous waste leachate (absence
of an organic layer) would reduce the concentration of carbon tetrachioride to
0.005 to 0.001 kgfday/m 2 . This rate of transport is 10 to 50 times greater
than the transmission rate of water through RDPE.
Current experimental data are based on test conditions which
imposed the maximum driving force of concentration differences by removing
transported chemicals from the barrier material. Reductions of the driving
force may be achieved by retaining transported materials at the barrier
material surface. The “de minimus” performance standard may be approached
using a water—saturated clay under the synthetic barrier material or possibly
a composite barrier system consisting of two types of polymer materials, each
with vapor transport characteristics that supplement the other.
The phenomenon of vapor transport through polymeric materials is
well established. There is uncertainty associated with the magnitude of the
rate which may be experienced in a hazardous waste liner service environment.
It is possible that if organic chemicals are present at concentrations less
than their water solubility, then the rate of specific chemical release may be
comparable to the transmission of water through synthetic barrier materials
(iO kg/day/m 2 for RDPE and io.2 kg/day/m 2 for Neoprene). Based on this
information leachate leakage due to vapor transport would be negligible
compared to leakage due to the presence of a moderate number of liner pinholes
(See Chapter 4.0).
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3.3 CHEMICAL PROPERTIES
The chemical properties of synthetic liner materials are determined
by the particular polymer and polymer formulation technique used in their
manufacture. Chemical properties are important in evaluating the suitability
of a liner material for hazardous waste containment. Chemical liner properties
are used to verify attainment of liner design specifications and are used as
the principal indicators of liner resistance to hazardous waste leachates. The
broad range of synthetic liner materials available and the complexity of the
chemical composition of hazardous waste leachate complicate the problem of
determining chemical resistance of cap and liner materials.
The following section provides a description of the most
significant types of liner materials currently proposed for hazardous waste
containment. A brief description of the principal types of leachate
encountered in typical waste facilities is also provided. Finally, the concept
of solute transport theory is defined. The material presented in section 3.3
provides the information required to evaluate the testing procedures and
concepts introduced in section 3.4.
3.3.1 Polymeric Material Composition
The physical and chemical properties of synthetic materials are
controlled largely by the polymer. Certain properties are improved by the
addition of selected chemicals during formulation. A brief suary of the
polymers listed in Table 2—1, the more co only used synthetic barrier
materials, was excerpted from Haxo (1983):
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o Butyl rubber is a copolymer of isobutylene (972) with small
amounts of isoprene introduced to furnish sites for
crosslinking.
It is generally compounded with fillers and 8ome oil, and
vulcanized with sulfur.
o Chlorinated polyethylene (CPE) is produced by a chemical reaction
between chlorine and a high—density polyethylene. Presently
available polymers contain 25—452 chlorine and 0-252
cryatallinity. CPE is compounded and used in both thermoplastic
and crosslinked compositions. CPE can be cross]inked with
peroxidee but, in liner compositions, it is generally used as a
thermoplastic. CPE can be compounded with other polymers, making
it a feasible base material for a broad spectrum of liners. CPE
can be alloyed with PVC, PE and numerous synthetic rubbers.
Nevertheless, at least half the polymer content of CPE liners is
CPE resin.
o Chlorosulfonated polyethylenea (CSPE) are a family of polymers
prepared by reacting polyethylene in solution with chlorine and
sulfur dioxide. Presently available CSPE polymers contain from 25
to 432 chlorine and from 1.0 to 1.42 sulfur. They are used in
both thermoplastic (uncrosslinked) and c’rasalinked (with metal
oxides) compositions. Usually, they are reinforced with a
polyester or nylon acrim and generally contain at least 45% of
CSPE polymer.
o Epichlorohydrin rubbers (Co and ECO) are epicblorohydrin—based
elastomers which are saturated, high molecular weight, aliphatic
polyethers with chioromethyl side chains. The two types include a
homopolymer and a copolymer of epichiorohydrin and ethylene
oxide. These materials are vulcanized with a variety of reagents
that react difunctionally with the chioromethyl group. Such
reagents include diamines urea, thioureas, 2-mercaptimidazoline
and ammonium salts.
o Ethylene propylene (EPOM) rubbers are a family of terpolymers of
ethylene, propylene, and a minor amount of nonconjugated diene
hydrocarbon. The diene supplies double bonds to the saturated
polymer chain to supply chemically active sites for
vulcanization, usually with sulfur. These rubbers vary in
ethylene:propylene ratio, in the type and amount of the third
monomer, and in molecular weight. Although EPDM liners are
generally based on vulcanized compounds, thermoplastic EPDM
liners are also available. Because of its excellent ozone
resistance, minor amounts of EPDM are sometimes added to butyl to
improve the weather resistance of the latter.
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o Neoprene is the generic name of synthetic rubbers based upon
chloroprene. These rubbers are vulcanizable, usually with metal
oxides, but also with sulfur.
o Nitrile rubber is a family of copolyiners of butadiene and
different amounts of acrylonitrile ranging from 18 to 50%. The
principal feature of these copolymers is their oil resistance,
which increases with increasing acrylonitrile content. Nitrile
rubber is prepared by emulsion at different temperatures. In moat
applications, nitrile rubber is compounded with plasticizers and
is vulcanized. However, it is also blended with other polymers
such as polystyrene, phenolics, and PVC to produce thermoplastic
compositions that range in flexibility from rubbery compositions
to hard impact—resistant plastics. Nitrile rubber is used by the
lining industry generally in blends of polymers to produce
thermoplastic sheetings which feature oil resistance. The nitrile
rubber is mixed with PVC in amounts less than 50% to yield
compounds in which the PVC acts as a nonmigrating and
nonextractable plasticizer.
o Polyethylene is a thermoplastic crystalline polymer based upon
ethylene. It is made in three major types: (1) low—density
polyethylene (LDPE), (2) linear low—density polyethylene (LLDPE),
and (3) high—density polyethylene (HDPE). The properties of a
polyethylene are largely dependent upon crystallinity and
density. The addition of 2 to 3% carbon black can produce
improved ultraviolet light protection. Polyethylenes, as
generally used, are free of additives such as plasticizers and
fillers.
o The polymer polyvinyl chloride is produced by any of several
polymerization processes from vinyl chloride monomer (VCM). It is
a versatile thermoplastic, which is compounded with plasticizers
and other modifiers to produce a wide range of physical
properties. PVC compounds contain 25% to 35% of one or more
plasticizers to make the sheeting flexible and rubber—like. They
also contain 1% to 52 of a chemical stabilizer and various
amounts of other additives. The use of the proper plasticizers
and an effective biocide can virtually control plasticizer loss.
o Thermoplastic elastomers (elasticized polyolefin and PVC in Table
2—1) are a relatively new class of rubbery materials. They
include a wide variety of polymeric compositions from highly
polar materials, such as the polyester elastomers, to the
nonpolar materials, such as ethylene—propylene block polymers.
These polymers are thermoplastic end nonvulcanized. They are
processed and shaped at relatively high temperatures at which
they are plastic; then they are cooled to normal ambient
temperatures. They behave like vulcanized rubbers.
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The composition of a formulation is considered proprietary and generally not
made public.
3.3.2 Analytical Properties
In addition to direct chemical analysis, several analytical
properties of synthetic materials have been found useful in identifying
materials and monitoring their quality during selection and production and
while in serwice. The following su ary of tests used to measure these
properties was adapted from Baxo (1983).
Volatitea . “Volatiles” is the percent weight lost by a specimen of liner on
drying in a desiccator at 50°C and then heating in an oven at 105°c. If the
synthetic barrier ha been exposed and has absorbed volatile liquids, the
portion of weight lost in the desiccator represents the water fraction, and
the portion lost at 105°C represents the low—boiling organic fraction that the
material absorbed. Changes in the volatile fraction can be used as a means of
monitoring a material during exposure to waste liquids. The percent of swell
can be estimated from the ratio of volatiles to the nonvolatile fraction. The
amount of plasticizer lost to the waste liquid can be calculated from an
analysis of the “extractablea” (see below) if the original plasticizer content
is known.
Ash. The ash content of a barrier material is the fraction that remains after
a sample is thoroughly burned at an elevated temperature, e.g. 550°C. The ash
content is usually made up of inorganic materials that have been used as
fillers or as curatives in the polymeric coating compound. As different
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manufactures formulate their compounds differently, determining the ash
content can be a way to “fingerprint” a polymeric barrier. The residue
obtained by ashing can be retained for further analyses, such as metals
content etc. Trace element analysis of the ash provides information regarding
the amount of contaminants absorbed by the barrier. The test method described
in ASTM D297, Paragraph 34, is generally followed in performing this analysis.
Extractablee . The extractable content of a polymeric sheeting is the fraction
that can be extracted from a sample of the barrier material with a solvent
that neither decomposes nor dissolves the barrier. “Extractables” consist of
plasticizers, oils, or other solvent—soluble constituents that impart or help
maintain specific properties, such as brittleness or processability. A
measurement of extractable content and analytical study of the extract can be
used as part of the “fingerprint” of a sheeting. An important use of this test
is monitoring of the effects of exposure to waste liquids. During exposure to
a waste liquid, constituents in the original barrier compound may be
extracted, which can result in a change in the properties of the barrier. The
measurement of extractable is particularly useful for the detection of liner
softening due to the absorption of non—volatile oils from the waste leachate.
Crystalline Structure Heat of Fusion . Differential scanning calorimetry (DSC)
is a thermal technique for measuring the melting point and the level of
crystallinity in partially crystalline polymers, such as the polyolefins, e.g.
polyethylene and polybutylene. This technique measures the heat of fusion of a
crystalline structure. It can also indicate modification of crystalline
sheeting with other polymers by alloying. Thus, this type of analysis can be
used as a means of fingerprinting crystallinepolymeric barrier materials,
3-21
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particularly high—density polyethylene, and assessing the effects of aging and
exposure to wastes. The DSC process can also be used to measure the
temperature at which a polymer converts from a brittle, glassy state to an
amorphorus, rubbery state and is thus related to its low temperature
properties.
Specific gravity . Specific gravity and density are important, easy to
determine, characteristics of a material which can give an indication of the
composition and identification of a compound. On exposed liners, specific
gravity can be measured only after the barrier material has been
devolatilized. Care must be taken to thoroughly dry the specimen before
placing it in the oven at 105°C to avoid bubbles forming in the sample. ASTM
Method D792, Method A, and D297, Paragraph 15, are generally used in
performing this test.
3.3.3 Hazardous Waste Leachates
The performance of synthetic barriers is strongly affected by the
nature of hazardous wastes to which the barriers are exposed. Exposure of
greatest concern is leachate originating from the waste and from water that
enters the facility and leaches soluble waste constituents. The following
description of waste liquids to which barrier materials are exposed and Figure
3—1 which describes components of liquids in hazardous waste facilities are
adapted from Haxo (1983).
Organic and inorganic chemicals are dissolved in the leachate of a
waste, regardless of the composition of the principal liquid of the waste.
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BASES BASES
Figure 3—1 Conceptual composition of waste Liquids. (Maxo, 1983)
ORGANIC LIQUIDS
AND/OR DISSOLVED
SOLIDS
ACIDS
ACIDS
NEUTRAL
POLAR COMPOUNDS
NEUTRAL
NON-POLAR COMPOUNDS
SALTS
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However, the relative abundance of a given dissolved component will depend on
the composition of the principal liquid. For instance, if the liquid is
neutral nonpolar organic, it will have a large carrying capacity for other
neutral nonpolar organic chemicals. If the liquid phase is predominantly
aqueous 1 its carrying capacity for nonpolar organics in its dissolved phase
will be relatively small.
Water has a relatively large carrying capacity for polar organic
chemicals (they may be miscible in each other in all proportions) and for
inorganic acids, bases, and salts. Strong inorganic acids and bases, which are
invariably water—based, may be especially aggressive to synthetic barrier
materials.
In the case of polymeric barriers, the relative solubility
parameters of the polymer and the organic solventa that are present, either
alone or in solution or dispersed in the water, can have major effects on the
barrier. When the solubility parameters of the solvent and the polymer are
close, severe swelling of the liner and even dissolution can occur.
For the purpose of assessing the effects organic liquids may have
on the integrity of a barrier materiel, the liquids may be classified into
four groups. These groups (which are specified in Table 3.3.3—1), are based on
the physical and chemical properties that govern their interactions with
barrier materials. These properties include acidity, basicity, polarity, and
solubility parameters of the organic components.
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TABLE 3.3.3—1 PHYSICAL CLASSES OF WASTE LIQUIDS
Class of waste liquid
Solvent
Solute
Aqueous-inorganic
Water
Inorganic
Aqueous—organic
Water
Organic
Organic
Organic
liquid
Organic
Sludges
Organic
liquid or water Organic and inorganic
Aqueous—inorganic liquid wastes are those in which water is the
liquid phase and the dissolved components are predominantly inorganic.
Examples of these di8solved components are inorganic salts, acids, bases, and
dissolved metals. Examples of waste liquids in this category are brines,
electroplating wastes, metals etching wastes, and caustic rinse solutions.
Aqueous—organic liquid wastes are water based solutions of
predominantly organic solute. Organic solute material can be either acidic
functional groups or polar organic molecules. Acidic groups are formed
principally through the anaerobic decay of organic waste materials. Polar,
organic molecules are miscible in water and are primarily a result of
percolation leaching within a waste facility. Examples of aqueous—organic
leachate producing waste streams are wood preserving wastes, water—based dye
wastes, pesticide container rinse water and ethylene glyco]. production wastes.
Organic liquid wastes are those that have an organic liquid phase
and the dissolved components are other organic materials. Essentially these
solutions are composed of neutral organic compounds with no net charge and
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negligible polarity. These compounds do not mix with water, although water
would be present to some extent in any leachate. Examples of this class of
wastes include oil—based paint wastes, pesticide manufacturing wastes, spent
motor oil, spent cleaning solvents, solvent refining and solvent processing
wastes.
Sludges represent the fourth class of wastes. They are generated
when a waste stream is dewatered filtered, or treated for solvent recovery.
Sludges are characterized by high solids content such as those found in
settled matter of filter cakes and consist largely of clay minerals, silt
precipitates, fine solids, and high molecular weight hydrocarbons. Examples of
this class of waste are American Petroleum Institute (API) separator sludge,
storage tank bottoms, treatment plant sludge, and filterable solids from any
pollution control process.
3.3.4 Solubility Parameter Theory
Assessment of polymer chemical resistance based on polymer and
waste constituent properties would enable rapid screening for chemical
resistance of the large number of possible liner and cap material’
combinations. The solubility parameter theory provides a quantitative basis
for making such an assessment. The theory is based on comparison of the
strength, or cohesive energy density (CED), of intermolecular forces. Similar
strength between polymer and waste indicates low chemical resistance between
materials and thus low probability of acceptable performance as a barrier
material. A solubility parameter is calculated from the CED.
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Solubility parameter theory was initially developed for comparing
liquid solutions. The CED was readily calculated from the heat of
volatilization of the liquids. Polymeric barriers have no vapor pressure, so
the ED and the solubility parameter cannot be calculated or measured
directly. Instead, the solubility parameter may be estimated by immersing the
polymer in a variety of liquid. and assigning to the polymer the solubility
parameter of the liquid which produced the greatest weight and dimension
change. This method is useful for liner and cap materials which contain one
polymer and a small percentage of additives. Alternatively, the solubility
parameter can be estimated by an empirical method referenced by Svope et. al.
(1983).
Deficiencies of the solubiUty theory limit its application to
preliminary screening of barrier materials for identification of those which
are least resistant.
The deficiencies include:
o Reliance on weight gain and dimensional stability as the only
indicators of chemical resistance;
o Interactions among components of liquids or polymers may affect’
the solubility parameter; and
o Currently the approach has been used for short term testing only.
Extension of the theory to the long—term determination of
solubility will be required before the procedure can be used for
hazardous waste applications.
Further research is required to determine the applicability of solute
transport theory to the problem of hazardous waste liner stability. Additional
investigation appears to be warranted since the technique may provide the
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capacity to rapidly screen liner materials based on anticipated Leachate
characteristics. Final testing for chemical resistance of specific liner and
leachate combinations would still be required, but the range of potential
liners could be narrowed significantly.
3.3.5 Summary
The evaluation of the long—term chemical resistance of liner
materials to leachate attack represents the most critical aspect of hazardous
waste facility design. The problem is complicated by the broad range of
polymeric formulations available for liner materials and the equally broad
range of chemical contaminants found in hazardous waste facility leachate.
Researchers have attempted to resolve the problem in alternate ways. The first
approach attempts to define the characteristics of general classes of liner
materials and waste stream leachate, to allow, prediction of effective
chemical resistance. The second approach attempts to measure the chemical
resistance of specific liner materials with a representative leachate material
or a pure chemical. Limitations exist with both approaches. The first
technique assumes that the chemical properties of specific liner or leachate
materials within a general class do not vary significantly. This has not been’
demonstrated. The second technique is impractical due to the broad range of
jnaterials and leachate available for testing. The most acceptable approach
appears to be a combination of these alternatives. The first technique would
be used to narrow the selection process by providing a initial screening of
liner materials and leachates. The second approach could then be used to
determine the most suitable liner material for a specific leachate through
direct testing. However, this approach does not completely resolve the
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problem, since chemical resistance measuring techniques are presently not
standardized (testing is discussed in detail in section 3.4).
Disregarding the lack of standard chemical resistance testing
procedures, the approach is workable if suitable screening methods can be
developed to narrow the list of available synthetic materials for testing.
Solute transport theory represents the moat promising approach toward defining
screening technique.. However, the applicability of solute transport theory to
the assessment of long term chemical resistance has not been verified. Further
research is required to assess the overall applicability of this approach to
the hazardous waste containment problem.
In conclusion, the chemical resistance of liner materials to
hazar4ous waste leachate can be conceptually determined, assuming standard
testing methods can be established. However, the broad range of synthetic
materials and leachate components render direct testing impractical. Further
research must be conducted to investigate the significance of leachate
variations to liner resistance and to assess the utility of solute transport
theory as an initial compatibility screening approach. The problems associated
-with available testing methods are discussed in section 3.4 of this report.
3.4 CHEMICAL RESISTANCE TESTING
The selection of a synthetic liner material which is resistant to
leachate attack is critically important to the design of hazardous waste
facilities. The selection requires direct testing of the potential liner
materials with representative leachate samples to evaluate anticipated long
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and short term liner performance. The following provides a discussion of two
general types of resistance testing. The first type, exposure testing,
provides a simulation of in—service liner conditions. The second testing
approach, material property testing, provides a comparative basis to establish
liner property changes after leachate exposure. The interpretation and
reliability of the various methods are discussed in the conclusion of this
section.
3.4.1 Exposure Test Methods
Exposure testing methods are designed to provide a direct measure
of liner performance during direct exposure to either pure chemicals or sample
leachates. Testing conditions may be varied to investigate particular chemical
interactions or to test broad classes of chemical types. Similarly, the period
and temperature of exposure can be varied to provide information on the short
and long term resistance of the liner material. Several specific exposure test
methods have been developed. The following methods have been defined for
hazardous waste facility liner testing:
o Immersion;
o Pouch (limited to thermoplastics and crystalline materials);
o Tub; and
o In—service
These methods are considered the most significant for hazardous
waste applications, however other methods have been proposed. The selection of
the most appropriate technique depends on the particular properties to be
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tested and the anticipated facility design. Detailed discussions of these and
other testing methods are provided by Haxo (1983).
3.4.1.1 Th.iersion Exposure Methods
Several methods exist for conducting immersion testing. The
following techniques have been used for hazardous facility testing:
o EPA Method 9090 Compatibility Testing of Membrane Liners (draft)
o National Sanitation Foundation Recommended Test Method for
Determining Long—Term Performance of Membrane Liners in a
Chemical Environment. (Proposed)
o ASTM D471—79 Rubber Property—Effect of Liquids
o ASTM D543 Resistance of Plastic to Chemical Reagents
o Matrecon Test Method 3 Iimaersion of Membrane Liner Materials for
Compatibility with Wastes
The Matrecon and EPA methods were specifically developed to
evaluate synthetic barrier materials exposed to hazardous waste. All of the
above methods use similar procedures. Samples of the specific barrier
materials are immersed in the waste and the effects of the immersion are
determined. Tested parameters include changes in sample weight, dimensions and
a selected number of physical properties as a function of immersion time. By
iimnersing the samples totally in the waste fluid, a somewhat accelerated test
is generated. Further acceleration can be effected by increasing the
temperature. However, the closer the temperature and exposure conditions are
to actual service, the more reliable the results will be. Also, the greater
the test duration, the more reliable it will be. These types of tests should
be initiated early in the design phase of the waste facility. An exposure
period of twelve months is desirable. Samples can be withdrawn at one, two,
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four, etc., months to assess the effect as a function of time. (Haxo, 1983).
limnersion tests are not suitable for direct measurement of permeabilty.
While these published methods establish protocols for conducting
immersion tests, there is sufficient variability between the methods to
prevent direct comparison of subsequent material property measurements. Method
9090 establishes a temperature at which the exposure should be conducted as
well as time periods for sample removal and testing. The Matrecon method does
not specify temperature conditions. None of the methods specify appropriate
procedures for obtaining waste liquids for use in the test. Similarly, there
is no detailed procedure for assuring maintenance of the waste liquid or
prevention of volatile losses from the i=ersion tank. Temperatures at which
material properties are to be measured are not specified. Test methods for
specific material properties are specified but no guidance is given on how to
use the measurements obtained, particularly with respect to estimating
long—term material properties and/or performance with respect to waste
containment. In general, there is a need to vigorously review the existing
methods and to develop more analytically consis.tent procedures to allow
interlaboratory material property measurements to be directly compared.
Immersion exposure testing is most frequently conducted by the
polymer barrier industry. A review of their test conditions indicated a large
variance in temperature and duration of exposure. In addition, material
properties measured subsequent to exposure are different. Tensile strength was
the only property measured by all laboratories. Weight change and elongation
were measured by most laboratories. (Swope, et. al., 1983).
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3.4.1.2 Pouch Teat
The pouch test was designed to measure the permeability of
polymeric barrier materials to water and to dissolved constituents of the
wastes. A sample of the waste is sealed in a small pouch fabricated of the
liner material which is then placed in distilled or deionized water.
Measurements are taken periodically to determine the extent of movement of
water into the membrane and/or leakage of waste into the water. A
concentration gradient is created by th’e deionized water on one aide of the
membrane and the waste on the other side. This test environment results in the
movement by osmosis of water and ions and other dissolved constituents through
the membrane due to the differences in concentrations on either side of the
membrane. Changes in liner materials are observed and subsequently physical
properties are tested. At present 1 this teat is limited to thermoplastic and
crystalline membranes. However, it can be used to assess the compatibility of
wastes with these materials. (Raxo, 1983).
The pouch test is the only exposure method which allows direct
measurement and calculation of permeability. A rigorous protocol should be
developed for this test and the scope of the test expanded to evaluate
crosslinked polymer formulations as well. Accelerated testing at higher
temperatures (comparable to EPA Method 9090) should be evaluated, to assure
reliability.
3.4.1.3 Tub Test
The tub teat consists of constructing a tub of synthetic liner
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material, filling the tub partially with waste and exposing the test cell to
the environment. The level of waste is allowed to fluctuate to simulate the
rise and fall of surface impoundment waste levels. This results in an
alternating exposure of a portion of the liner to air and waste. The effects
of exposure to the sun, temperature changes, ozone, and other weather factors
can be evaluated, as veil as the effect of a given waste on a specific
barrier. The fluctuation of the level of the waste is significant in that a
horizontal section of the barrier is subjected to the effects of both the
waste and weather. This alternating Df conditions is especially harsh on
barrier materials and is usually encountered at surface impoundments. (Haxo,
1983)
Measurements of material properties of specimens exposed by the tub
test are particularly dependent on climatic conditions. Reporting of values
measured should include the following information:
o waste liquid level variation;
o quantities of waste liquid added;
o sunlight exposure;
o temperature variations;
o formation of ice; and
o wind conditions.
The frequency of waste liquid renewal should be more vigorously established
and included in method documentation. The tub test does not provide a direct
measure of liner permeability, and is primarily useful for surface impoundment
applications.
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3.4.1.4 In—service Exposure
Coupons or test samples are occasionally used to monitor material
properties of installed barriers. Test samples at surface impoundments have
included tubs fabricated with the liner material and weighted with sand at the
closed end to provide vertical test strips for exposure to waste on one side
only, throughout the facility depth. At landfills and waste piles, coupons are
placed in leachate Bumps. No standard method or guidance exists which
specifies test sample size or configuration, number of test samples, exposure
conditions or frequency of withdrawal and testing. However, with proper
standardization, in—service testing can provide valuable information regarding
the long term resistance of synthetic materials to hazardous waste leachate.
3.4.2 Material Property Tests
Material property testing provides a technique to determine the
significance of exposure testing on the liner material under investigation.
Following liner material exposure to waste material or leachate, the liner
material properties are reanalyzed to determine if significant changes have
occurred. Currently there are no standards established regarding the specific
properties tested or the level of change which should be regarded as
significant. The following properties are most frequently tested:
o Weight change;
o Dimensional stability; and
o Strength parameters
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Other properties which are infrequently reported include analytical properties
(volatile fraction, ash content etc.), hardness, and occasionally CED
(cohesive energy density). Permeability of the liner to water may also be
reported. In addition, most laboratories report visual observations including
the presence of cracks, evidence of delamination and discoloration.
3.4.2.1 Weight Gain
The evaluation of reactivity often includes weight change.
Experience in the plastic and rubber industry supports the presumption that
materials exhibiting high weight gains or losses are less likely to serve as
functional liquid barriers. Materials exhibiting low weight gains or losses,
however, do not mean the material will definitely serve as an effective
barrier. Upper limit values of reported weight changes were reported to be 2
to 40—50 percent. A generally accepted value, but not a standard for
acceptable weight change, in the industry is 10 percent. Weight change, if it
occurs, appears to provide a good measure of poor liner resistance, however
the converse is not valid. It is an important parameter of vapor transport
theory and solubility parameter theory. Some synthetic- materials are’
formulated with compounds that may be readily extracted by certain solvents,
resulting in loss of a physical property which was provided by their presence.
Weight loss (in the absence of an off—setting weight gain) may indicate
shortened service life if the extracted material affects a property necessary
during operation, closure and post closure. A synthetic material may be
formulated with a non—migrating compound if the loss is considered
significant.
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Weight gains are usually accompanied by losses in strength and
other barrier properties. Large weight gains may result in softening of the
material which in turn may reduce resistance to abrasion, weathering, wave
action, and fluid flow.
3.4.2.2 Dimensional Stability
Changes in specimen size indicate swelling or shrinkage. There is
no general agreement regarding acceptable levels of dimensional changes. The
levels may be expected to be dependent on specific site applications and may
decrease as the rigidity of the material increases.
Swelling may produce buckling with resulting increased stress on
seams. Shrinkage may tighten the material resulting in increased stress,
reduced ability to conform to irregularities of the bedding media, and
possible tearing or puncture. Where dimensional stability is of concern, the
material may be reinforced with a scrim.
3.4.2.3 Strength
The tensile strength of synthetic materials, (the stress at the
yield point, beyond which the material will not return to its original length)
is an important design parameter that may also indicate changes in material
properties not detected by dimensional or weight changes. Increased tensile
strength may indicate a loss or mitigation of plasticizers. Lower tensile
strengths indicate a softening of the liner. Similarly, 100 percent modulus
(stress required to double the specimen length) and ultimate elongation
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(percent of initial length at breaking) also provide insight to reactivity
between the waSte and synthetic material. For design purposes, tensile
strength should be conducted with a scritu if it is planned to be included in
the final synthetic material.
Tear testing simulates the behavior of the material following the
formation of a hole due to puncture or excessive creep. Reduction in tear
strength may be indicative of material or scrim degradation and/or
delamination. Propagation of a tear in a field application would require
stresses not relieved by the formation of the hole.
3.4.2.4 Visual Inspection
Inspection of the material may reveal changes not necessarily
detected in the above quantitative teats. Reactivity may be manifested by
pitting, discoloration of waste or material, delamination and bubbling.
3.4.3 Applicability of Test Results
The testing methods discussed were essentially developed by the
liner industry to provide direct short—term liner performance data. In
general, the procedures are well suited to this purpose and have proven
effective in determining liner resistance to weathering and chemical attack.
The lack of standardization presents a significant problem since it prevents
the development of a comparable data base for a wide range of liner materials
and waste streams. The development of standard testing methods and procedures
would significantly improve the utility of these methods.
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Long term projection of liner performance is currently extrapolated
from the short term test results using the degree of change of liner
parameters as an indicator of long term performance. Since material property
testing provides a measure of the change in liner properties with time, the
results can be easily extrapolated to any length of service. However, due to
lack of long term field experience with synthetic liners, the applicability of
this procedure can not be verified.
It is assumed that the rate of change of liner properties can be
used as an indicator of long term performance. Liner property changes may
occur at different rates over the lifetime of the liner or may cease
altogether after some initial period of change. Barriers which exhibit a
continuing change in material properties are considered unacceptable.
Conversely materials which exhibit no change are considered acceptable.
While this may be true, the following should be considered when
selecting such a material:
o exposure period may have been too short
o accuracy and precision of the test method may be insufficient to
detect small changes.
Barrier materials with one or more properties which initially
change and then remain constant are suspect. But if the properties retained
after a change has ceased are sufficient to meet design strength and
permeability requirements, then the material may be considered acceptable.
However, a thorough evaluation of the potential causes and long—term effects
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of the material property changes is required.
Projection of long term retention of material property values based
on short—term exposure is common in materials testing. Selection of a barrier
material that exhibits no change in monitored property values provides better
confidence in long—term performance than one which exhibits a change.
Projection of rates of change are possible, but the relatively short history
of chemical resistance testing does not provide insight as to the long—term
performance of the material under field conditions.
3.4.4 Summary
The polymers used to fabricate synthetic liners possess unique
chemical resistance characteristics which must be evaluated by chemical
resistance testing with the hazardous wastes to be contained. A single polymer
formulation which can be used for all wastes was not identified. Analytical
properties of barrier materials can be used to identify specific formulations
and are useful in assuring quality of the selected material during
fabrication, installation and facility operation. Test methods for these
properties are currently available and considered applicable to evaluating
barrier materials.
The most widely used methods for determining chemical resistance of
liner materials with wastes are a series of immersion exposure tests followed
by measurement of selected material physical properties. Commonly measured
properties include:
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o appearance;
o weight change;
o dimensional stability; and
o tensile properties (including strength, elongation, tear, and
modulus of elasticity)
A modest chemical resistance testing data base has been developed, principally
by manufacturers and fabricators of synthetic barrier materials. This data
base provides insight on short—term reactivity. The measured results are not
correlated with long-term performance or long—term chemical resistance, since
virtually nothing is known about either. Additionally, the properties measured
are not directly related to long—term performance. This series of tests can be
effectively used to identify unsuitable synthetic materials. The tests can
also be useful in selecting materials, although the usefulness is quite
limited.
Several methods are currently available for conducting immersion
tests. A common deficiency of these methods is the absence of appropriate
specific procedures for obtaining waste liquids for use in the tests. Such
procedures should provide waste liquids which may represent worst—case as well
as typical conditions. Development of the procedures should consider the
mobility and leaching characteristics of hazardous waste constituents. A
protocol, (such as EPA Method 9090) which combines the immersion test,
material property tests, and procedures for obtaining waste liquids should be
developed. Detailed specification of test conditions such as time, rate of
stress application (tensile properties) and temperature should be included.
When developed, a test method for permeability should be included in the
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protocol. Although constrained by a lack of field data, the protocol should
also include guidance on how to use the reported measurements to estimate
long—term performance.
The pouch teat is potentially capable of providing results which
can be directly related to long—term chemical resistance of synthetic
materials. This teat places wastes inside a pouch and measures the quantity of
chemicals that have migrated from the pouch to the surrounding water. At
present, the amount of data and type of parameters measured (pH and
conductivity) is quite limited. However, this method has the potential of
providing direct measurement of vapor transport under conditions which most
closely resemble landfill exposures. This test warrants further development
and standardization.
Liner testing methods provide a subjective basis for assessing
liner resistance to waste leachate. Standardization of the testing methods and
procedures must be accomplished in order to improve the utility of the
available liner testing data base. EPA has established a testing procedure
(Method 9090) which represents an important first step in standardization.
This approach should be expanded to include the pouch test and in service
testing procedures. Finally, criteria should be established to determine the
-degree of liner property change, if any, which can be regarded as
insignificant in determining liner resistance to waste leachate.
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REFERENCES
August, H. and Tatzlcy, R. 1982. Permeation Behavior of Plastic Waterproof
Sheeting to Seal Dump Bases Against Organic Solvents . Bundesanstalt
fur Material profung. Berlin 12(1) pp. 9—14.
Svope, et. al., 1983. Analysis of Flexible Membrane Liner Chemical
Compatibility Teats . Arthur 0. Little, Inc. Cambridge, M&.
Haxo, B.E., Jr. 1983. (See References, Section 2).
Stewart, W.S., 1978. State—of—the—Art Study of Land Impoundment Techniques .
U.S. Environmental Protection Agency, EPA—60012—78—196, Cincinnati,
OR.
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4.0 SYNTHETIC LINER/CAP INSTALLATION
Liner installation represents a critical phase of the construction
of any hazardous waste facility. Problems which go undetected and unresolved
during liner or cap installation invariably result in cap or liner performance
problems. The following discussion of the principal aspects of synthetic liner
installation was obtained from a supporting report by TRW (1983).
Following the site selection and design phase of a waste facility
the liner installation phase proceeds. Prior to actual liner placement the
site is excavated, the foundation is prepared and excess material is
stockpiled for subsequent use. Berm construction and surface water drainage
systems are also installed prior to liner placement. Upon completion of the
preliminary field preparation, the liner, leachate collection and leak
detection systems are constructed. Following completion of the operating phase
of the facility, the cap and final cover systems are installed as required.
Faulty construction practices or the use of sub—design materials during these
critical installation phases will result in subsequent liner or cap
performance problems.
Due to the critical nature of the installation phase, a systematic
quality control and inspection program is needed to insure adequate liner
performance. The investigation of documented facility failures generally shows
that faulty liner or cap installation procedures lead to most of the seepage
and percolation problems reported. Regardless of the quality of the facility
design, final facility performance depends on properly installed synthetic
barriers.
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The following section provides a brief discussion of the most
significant problems associated with faulty liner and cap installation
procedures. Further details regarding the scope of adequate installation
procedures are provided in the supporting consultants report (TRW 1983) and
the companion report on clay liner systems (ERTEC 1983). Specific problems
peculiar to the installation of synthetic liner and cap systems are discussed
in further detail. The concluding section provides an assessment of the
significance of liner installation problems and the need for adequate quality
assurance monitoring of the installation process.
4.1 FOUNDATIONS
The structural stability of the subgrade or foundation must be
capable of supporting the waste facility components, including the weight of
the wastes, without damaging the liner. Problems noted which are attributable
to the structural stability of foundation include:
o Subsidence after placement of the liner;
o Excess moisture prior to placement of the liner;
o Failure to sterilize for plant control;
o Inadequate definition of subsoil conditions prior to
construction, and failure to consider these conditions in design
and installation.
Each of these problems can result in synthetic liner failure and
leakage. However, proper construction and design techniques are available to
avoid these problems. A thorough geotechnical evaluation of the site area
geology and hydrogeology is required to avoid subsidence problems. Adequate
field dewatering procedures and established methods of soil sterilization are
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also available. A proper facility design should specify the necessary field
procedures to avoid these problems. And, a formal quality assurance program
should be used to verify attainment of design specifications during
installation.
Associated with preparation of foundations is the construction of
dikes and bernie. At least two failure modes for side slopes that have been
constructed too steeply were reported:
o Failure due to equipment mobility causing liner damage; and
o Sloughing of the earthen side walls.
The use of heavy equipment on dikes should be limited, since damage to the
liner may occur as a result of maneuvering on steep slopes. A maximum slope of
3:1 (horisontal to vertical) appears to be the comaon recounnendation to
protect liners. Sloughizig of soil from side walls can stretch and possibly
tear a liner. Appropriate design would avoid using a soil susceptible to
sloughing due to high moisture content. Where high moisture conditions cannot
be avoided, the use of a drainage layer or other means of controlling soil
stability is encouraged.
Concrete or asphaltic concrete surfaces can also be used to support
liners. Preparation of these surfaces is briefly discussed in a draft manual
for AWWA on flexible tanks, covers and linings for potable water reservoirs
(AWWA, 1982). Care must be taken to assure all surface areas are smooth and
that no sharp edges contact the liner. The foundation or subbase for the
concrete pad should be carefully evaluated, designed and constructed to
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minimize settlement and the risk of liner failure. Cracks larger than 1/16
inch should be filled and leveled (TRW, 1983). At least two sites constructed
on concrete bases used a more conservative approach by providing a bedding
material of one foot of sand above the concrete.
Inadequate foundation preparation has resulted in several synthetic
liner problems at existing facilities. These problems can be avoided through
proper design and the implementation of an adequate quality assurance program.
Facility cap installation and subsidence problems are discussed in section
5.6.
4.2 BEDDING MATERIALS
Synthetic liner materials are relatively thin and can be punctured
by contact with sharp or irregularly shaped objects. The risk of puncture is
increased during installation of the leachate collection system over a liner
material placed on a bed of irregular material. Further, the induced stress on
liner materials placed over course aggregates such as gravel can lead to a
weakening of the liner material with time. This process was evidenced by a
decrease in hydrostatic resistance and some delamination of a liner placed
over 1 to 1 1/2 inch loose aggregate. Previous studies indicated that liners
are often placed without complete smoothing of the bedding material.
Bedding materials above and below the polymeric barrier are
critical in determining performance of a polymeric barrier. The bedding
material surface should be smooth to avoid indentation or distortion of the
barrier material. Depending on the smoothness of the subbase surface, only a
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few centimeters of bedding material may be needed to provide an appropriate
surface for barrier placement. To overcome deformations in the subbase surface
during construction, bedding material depths of 15 cm or greater is
reco ended.
The depth of granular material required to protect a synthetic
liner is not adequately defined. Current estimates range from 2 inches to one
foot, most researchers agree that 5 to 7 inches of protective material is
sufficient. Similarly, the grain size of the protective material is presently
unspecified. However, most experts agree that almost any clean soil or sand
which passes a No. 4 seive is suitable as a protective bedding material. More
definitive guidance would be useful in reducing the wide range of materials
and bedding depths currently employed to protect synthetic liners.
Each of the sub—liner material layers should be sterilized prior to
liner placement. Design specifications should include criteria specifying the
maximum organic content for all soils which remain on site. Soil sterilant
should be applied and moistened over the entire site area and then allowed to
dry prior to liner placement. Application rates and procedures should follow
those specified by the sterilant manufacturer. The application and
verification of herbicide placement should be subject to quality assurance
review and inspection.
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4.3 MATERIAL STORAGE AND HANDLING
Proper storage and handling of liner and cap materials is necessary
to prevent degradation due to exposure to the elements and to ensure that the
properties of the materials that are installed are the same properties
specified in the design. Specific regulatory requirements do not exist nor are
they needed for storage and handling at this time. Appropriate construction
specifications and a quality control program can assure proper handling and
storage of polymeric barriers.
Weather can affect the integrity of a stored or unfolded liner in
the following ways:
o Winds >10 mph can lift and tear the material;
o Ultraviolet light exposure accelerates aging to a limited
extent;
o Hail punctures liner; and
o Blocking, resulting from sunlight and overburden pressures,
causes the rolled up material to stick together.
These can be readily controlled by providing storage space, preferably in a,
secure place to prevent vandalism. Folding and unfolding a liner, especially
at low temperatures, should be avoided since cracking or weakening of the
liner material may result.
4.4 SEAMING
Researchers generally agree that improper seaming is responsible
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for most synthetic liner failures. Seaming represents the moat critical aspect
of liner installation, since the presence of improper seams may result in
significant leakage and expose the synthetic material to a higher risk of
chemical decay. Some seaming generally occurs at the factory as well as in the
field. Factory seams are considered more reliable, since better environmental
controls are available. Depending on the synthetic liner material selected for
the facility, one of the following three seaming techniques are used.
o Bodied solvent;
o Heat; and
o Contact adhesive.
Regardless of the process used, the technique for accomplishing the seam is as
follows:
o Clean the seaming surface;
o Apply solvent, heat or adhesive;
o Apply pressure; and
o Provide time for airing.
Facility liner seams should be oriented upward from the facility
floor to the top of the side walls in a ribbing pattern to avoid excessive
stress due to liner slippage or sloughing.
Several problems can occur during field seaming operations, due to
the lack of environmental control. The most significant problems identified
are discussed briefly in the following sections Proper scheduling and quality
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assurance reviev can be used effectively to minimize the effects of these
problems on synthetic liner performance.
Prior to field seaming, the material should be properly oriented to
avoid stretching the material around protrusions or folded seams. Bridging of
voids in the subbase by the liner material should be avoided. This often
happens around protrusions, and where the bedding or subbase was not properly
constructed. Wind damage has occurred frequently during installation and is
aggravated when the anchoring trenches re installed and backfilled before
seaming is completed.
Scheduling is an important part of successful liner installation.
Expendiency is important to keep field—time, and hence risk of rain—days, to a
minimum. However, careful attention to quality workmanship is more important.
Installation is sometimes scheduled during evening hours to avoid sunlight and
associated high temperatures and ultraviolet exposure. Such preconditions of
the facility. Documentation of a quality control program should include the
following elements:
o Polymer material;
o Seaming material;
o Compatibility of material and seaming method;
o Condition of bedding material (see Section 5.4 for specific
elements);
o Placement of panels;
o Orientation of seams;
o Seaming procedures;
o Solvent type and quantity used;
o Weather conditions;
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o Seam samples for testing and reference;
o Seam inspection (100 percent);
o Names of personnel; and
o Herbicide application, type and quantity
Implementation of a quality control program containing these
elements should identify problems which have been reported. A corrective
action program needs to be developed and implemented when problems are
encountered. When properly conducted, field seams are not considered to be a
significant factor controlling polymer barrier material performance.
4.5 DRAINAGE SYSTEMS
The installation of drainage systems over the synthetic liner, or
under it for multiple liner facilities; requires careful design and
installation. Design considerations must include an assessment of the load
bearing capacity of the porous drainage layer material and the drainage pipes
installed. Lack of adequate bearing load capacity or premature operation of
heavy construction equipment can result in damage to the underdrain system.
Drainage pipes could be crushed or broken and may puncture the synthetic’
liner, as well as reducing the effectiveness of the leachate collection
system. The problem of liner and leachate collection system vulnerability to
damage by equipment results from the requirement that a maximum of 30 cm of
leachate above the liner in an operating landfill.
The expansion of the leachate collection drainage layer and the
liner protection layer would significantly decease the risk of liner or pipe
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damage due to construction and operational activities. However, this would
require an increase in the maximum allowable leachate head. Since synthetic
liners are less susceptible to increased leakage due to hydrostatic pressure,
an increase in the allowable leachate head from 30 to 60 centimeters would be
appropriate. With this recoended leachate head increase and proper
construction and maintenance practices, the potential for liner or leachate
collection system damage would be significantly reduced.
4.6 QUALITY ASSURANCE
The installation problems identified during the course of this
project demonstrate the need for comprehensive quality assurance review of all
aspects of synthetic liner installation. A suitable quality assurance program
is required since the use of improper construction techniques or field
procedures may jeopardize the performance of the synthetic barrier system. The
following elements must be subject to quality assurance verification if design
specifications are to be attained:
o Liner bed material selection, placement, and grading;
o Foundation compaction and preparation;
o Liner material storage facilities and handling techniques;
o Seaming techniques and materials;
o Seam testing; and
o Personnel experience and qualifications.
Quality assurance procedures should insure that proper
documentation, in the form of material spec ication sheets, daily field logs
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and testing certifications, is maintained throughout the liner installation
phase. Owing to the importance of proper liner seaming, a particularly strong
material certification and field inspection program is required for the
seaming operation.
Materials and equipment used for the liner or cover installation
should be inspected to ensure they are in good condition and within
specifications. Improper materials and equipment should be removed from the
site to avoid subsequent confusion. The inventory of liner panels should be
checked against design plans and specifications before they are seamed.
Testing of field seams is veil established in the industry. The
sensitivity of liner performance to leaking seams should dictate the extent of
seam testing performed. The recommended practice is to field test 100 percent
of the seams. The associated cost, while appreciable, represents only a
fraction of the cost of the facility and potential liabilities in the event of
failure.
Available test methods include non—destructive testing such as air
lance, vacuum, ultrasonic and visual. Destructive testing requires the removal’
of samples from the liner and subsequent patching. A less acceptable
alternative would be to have extra material seamed by the work crew as they
progress throughout the day. Tests which are usually performed on such samples
include peel adhesion and shear strength. Additionally, the quality assurance
program should provide for collection and appropriate storage of record
samples of all materials in the liner system, including:
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o pipe material;
o pipe cement;
o drainage soils;
o liner material; and
o solvents/adhesives.
Additional samples of liner material should be collected and used for periodic
assessment of reactivity with wastes, sludge, and air exposure of surface
impoundments or in leachate sumps at landfills.
4.7 SUMMARY
The proper installation of synthetic liner and cap systems is
critically important to the overall performance of the facility. Several
problems documented at existing facilities have been linked to liner
installation problems. However, all of the problems identified could be
avoided using available installation techniques and construction practices.
Failure to achieve proper liner installation is attributed to the lack of
adequate quality assurance and the apparent lack of understanding on the part
of installers regarding the critical importance of properly installed liner’
systems.
Liner subsidence problems are primarily associated with poor
foundation preparation techniques. Suitable techniques are available to avoid
these problems. Cap subsidence represents a more difficult, yet manageable
problem area. Studies have shown that over 90 percent of the total waste
consolidation occurs within 5 years of facil ty closure. Further, proper waste
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stacking and filling can minimize this consolidation. It is recommended that a
cap inspection program be required for a minimum of 5 years after facility
closure.
Liner stress due to the use of sharp or irregular bedding materials
repre3ente a material selection problem. Similarly, improper grading of the
liner bed is easily avoided through quality assurance inspection prior to
liner placement.
The installation of field seams represents the most critical aspect
of the liner installation phase. However, the moat significant problems
encountered at existing facilities were the result of the use of the wrong
bonding materials and techniques. A comprehensive quality assurance program is
required to verify the use of proper bonding techniques and materials.
Further, 100 percent testing of all field seams is recommended. Testing should
be performed using non—destructive field techniques. However, peel and shear
testing should be performed on sample seam patches.
In conclusion, synthetic liner installation represents a
particularly critical phase of hazardous waste facility development. Several
problems encountered at actual facilities are attributed to installation
problems. However, current techniques are available to insure proper synthetic
liner installation. It is strongly recommended that a stringent quality
assurance program be developed to cover this critical phase of facility
development.
4—13
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REFERENCES
TRW, 1983. Assessment of Technology for Constructing and Installing Cover
and Bottom Liner Systems for Hazardous Waste Facilities . U.S.
Environmental Protection Agency, Washington. D.C.
AWWA, 1982. Manual, Flexible Tanks, Covers and Linings for Potable Water
Reservoirs . Draft. American Water Works Association, Denver,
co. 57 pp.
4—14
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5.0 SYNTHETIC CAP AND LINER PERFORMANCE
Assessment of synthetic material performance in cap and liner
systems included evaluation of the service life for installed barrier
materials, the failure from subsidence of barrier supporting materials and the
combined performance of the two systems.
5.1 SERVICE LIFE
The issue of service life of synthetic materials in waste
management facilities was addressed by reviewing data on actual field service
experience and by evaluating current methods for projecting longevity and
acceptable performance from experimental and field data. Service life of
barriers depends on chemical and physical properties of the barrier material
and how it is protected in the cap or liner system.
The regulations do not identify a period over which the performance
must be acceptable. It is expected, however, that if leachate is no longer
present, nor likely to be generated in the future, the performance life of the
liner system can terminate. A performance standard, established for cover,
systems at disposal facilities, requires long—term minimization of the
migration of liquids through the closed facility. No guidance is provided in
the regulations regarding the period referred to as the long term.
Conceptually, service life, as well as long—term, may be stated in
terms of waste degradation and release characteristics from the containment.
An analysis of risks to health and the environment might also be included to
5—1
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determine an acceptable level of release from the facility, and thereby
identify maximum required service life in terms of performance. While risk
analysis may be used as a management tool for evaluating alternatives (such as
selecting remedial alternatives for clean—up of uncontrolled hazardous waste
sites) it was found unacceptable for establishing regulations for hazardous
waste facilities under RCRA.
Alternatively, service life may be stated in terms of material
characteristics, such as permeability or- tensile properties. Unfortunately,
there is no data available which allows correlation of specific liner
properties with overall performance or finite service life. Therefore, it is
doubtful that a scientifically supportable definition of service life o r
long—term can be developed, based on current knowledge of barrier material
properties and field performance.
5.2 PHYSICAL AGING OF SYNTHETIC LINERS
The ultimate service life of a synthetic material which is properly
installed and protected from chemical attack is determined by the chemical
stability of the liner structure. Polymeric liners rely primarily on long.
chain organic molecules for stability. Long chain molecules degrade over time
through three primary mechanisms: the action of sunlight; chemical attack by
small molecules; and, biodegradation. A brief description of each mechanism is
provided in order of their potential significance to liner decay.
The sunlight mechanism, which includes photochemical and oxidation
processes, represents the most rapid deterioration mechanism. Significant
5—2
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degradation, as evidenced by discoloration and cracking has occurred for
particularly sensitive)iner inaterja-ls after 200 hours of exposure. Several
defense mechanisms are available, ranging from simple burial of the synthetic
material to the addition of polymer stabilizers and pigmentation. If properly
accounted for, liner exposure to the atmosphere does not present significant
degradation problems.
Attack of the liner by small molecules, termed hydrolytic and
solvolytic degradation, occurs on a slower scale, particularly if the liner
has been shown to be chemically resistant to the waste leachate. A
polyethylene terephthalate liner with reinforcing fabric would be expected to
hydrolyze in a few hundred years at 30 to 40°C and 100 percent relative
humidity, when exposed to aqueous leachate. The use of available chemical
resistance testing methods appears to provide sufficient information for the
evaluation of this aging mechanism.
Biodegradation is the least significant decay mechanism, since
polymer chain attack proceeds from the ends of the long chain molecules.
Except for particularly sensitive liners biodegradation would be expected to
occur over several hundred years for most polymer liners. Longer decay times,
would be expected for saturated polymeric materials. Avoidance of particularly
sensitive materials would effectively eliminate biodegradation from concern
for liner systems.
In actual field installations all three mechanisms would be
expected to contribute to liner decay simultaneously. No suitable laboratory
techniques have been developed to measure the rate of this liner weathering.
5—3
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Therefore, it is necessary to evaluate actual field performance data to assess
the anticipated life of synthetic liners in the field.
5.3 FIELD SERVICE EXPERIENCE
Most of the polymers used for waste containment have been
commercially available for less than 10 years. This places an upper limit on
actual field service experience. Based on published material and manufactures’
literature, a listing of field service experience was prepared (Table 5—1).
Information on service life is available for some materials and is reviewed in
Section 5.4. The information, however, was collected for applications which
are not specifically related to waste containment. If the barrier material
selected is chemically resistant, current data indicates that the actual
service life of polymeric barriers used in cap and liner systems ranges from 5
to 45 years, depending on type of polymer used. These projections of service
life are based on the ability of the barrier material to retain its original
chemical and physical properties.
5.4 PROJECTED SERVICE LIFE
Service life of a barrier material may be projected from measured
changes in material properties. Sufficient information on field performance
and measured material properties do not exist for any type of polymer to
determine how much change would result in failure or termination of service
life. However, it is useful to review existing information on long—term
changes of material properties.
5—4
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TABLE 5—1 POLYMER FIELD SERVICE EXPERIENCE PERIODa (LYMAN, 1983)
Years
t.n See Lyman, etal., (1983)
— polyvinyl chloride
HDPE — high density polyethylene
CSPE — chiorosulfonated polyethy. ene
N — non—linear neoprene rubber
A/B — asphalt/bitumen
LDPE — low density polyethylene
CPE — chlorinated polyethylene
EPDM — ethylene propylene rubber
B — butyl rubber
ECH — epichiorohydrin rubber
SOURCEb
M&TERIALC
PVC
LDPE
HDPE
CPE
CSPE
ECH
N
B
EPDM
A/B
Kays (1977)
12
8
17
25
NCASI (1980)
14
6
Lauritzen (1967)
19
Hercules (1981)
5
Dupont (1983)
11—17
Dupont (1983)
15
45*
Gundle (1983)
—20
—20
Schiegal (1982)
9
BF. Goodrich (1982)
3
17
Dow (1982)
12—20
Anonymous (1976)
-
13
Polysar (1982)
25
The Asphalt Inst. (1976)
423
Strong (1980)
13—22
11
8
-------
Experience with several polymer materials exposed to a minimum of
25 years to sunlight, oxygen, and rain in tropical climates indicates that the
materials were still functional. In many cases, elongation and tensile
properties retained at least 70 percent of the original values (Figure 5—1).
changes in tefl8ile strength of low density polyethylene (LDPE) were negligible
after 20 years of outdoor exposure in Canada. After 14 years of exposure,
increased tensile strength and diminished elongation were reported for a PVC
canal liner (test section) in New Mexico.
A summary of lifetime data for a number of materials is presented
in Figure 5—2, where length of time the materials have been exposed to
weathering, to soil burial, and to sea immersion is shown. The arrows indicate
that the materials were still functional at the last sampling, and therefore
material service life is at least as long as the period shown.
Also shown in Figure 5—2 are predicted service lives based on
laboratory heat aging tests, which, while generally unreliable, have some
utility for butyle rubber and chlorosulfoaated polyethylene. Based on these
laboratory measurements, it was reported that butyl rubber would retain 60
percent of its elongation characteristic in 50 years and 40 percent in 150 ’
years. This projection was the longest time period reported for any property.
However, since butyl rubber was commercialized only 41 years ago, it has yet
to be validated. Due to the uncertainty concerning the applicability of these
test results, a more conservative estimate of anticipated field service life
must be used. Based on the evaluation of the available test data, an average
service of approximately 25 years appears appropriate at this time. Further
refinement of liner life prediction techniques should be developed as the
5—6
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140 -
HYPALON 40 (Cured)
120 -
POND LINERS ( Uncred)
— —
— — — ,-- — —
0
100 .. - . - —. EPDM
U
I -
0
U.
C
0
I-
8O-
ii
C
•
— I4YPALON 20(Cured)
C
2
a-
0
60 - NEOPRENE W
40 -
20 -
U’
-J
o I I I I I
I 2 3 4 5 6 8 10 20 30 40 50 60 80 100
TIme, yeers
Figure 5-1 Retention of tensile strength for selected barrier material exposed in tropical climates.
(Lyuian et al., 1983)
-------
Actual Membrane
Service Life
(Any Service)
Actual Membrane
Service Life
Weathered,
Non-Tropical
ii *•1
I.’ i- J
Actual Membrane
Service Life
Weathered, Tropical
or Florida
Actual Membrane
Service Life
Water Immersion
Actual Membrane
Service Life
Soil Burial
Projected Service
Life Based on
Heat Aging or
Laboratory
Weathering Data
I— — — — -1
I I
L —.— --
(commercialized
for 52 years)
(commercialized
for 41 years)
Polyethylene
(commercIalIzed
for 32 years)
Polymeric Barrier Material
60
50
40
a
0
E 30
I-
20
10
0
r-- 1
I —
I r-’
I I
I I
Longer Service *
Life Expected
HDPE LDPE
r—
U i
Polychioroprene Butyl Chiorosulfonafed Ethylene Propylene Polyethylene
Rubber
(commercialized
for 20 years)
Figure 5—2 Representative service life data.
(Lyman, et al., 1983)
-------
liner experience base improves.
There is no question that a polymeric material can function as a
containment barrier to liquids for at least 25 years. Extrapolation of
material property data such as that shown in Figure 5—1 suggest retention of
tensile strength greater than 80 percent can be expected over a 100—year
period. Whether this is sufficient will depend on the anticipated stresses to
which the material is subjected.
5.5 SUBSIDENCE OF LEACRATE BARRIERS
A properly designed leachate barrier system, based on a
geotechnical analysis of soil and subsurface water conditions and loads
imposed by the facility, should not suffer from serious subsidence problems.
The geotechnical analysis should include an assessment of expansion of soils
due to relaxation of stresses from removal of overlying soils, total
subsidence, and differential subsidence. The analysis should be conducted for
conditions anticipated during development and construction of the facility and
in response to variable loading conditions during the operating and
post—closure periods of the facility. The analysis should also include
conditions to determine the effect of potential failures, such as collapse or
hydraulic overload of a leak detection system. An analysis of impact on liner
performance would evaluate the amount of subsidence, the area over which
subsidence occurs, and the rate of subsidence.
Assurance that design specifications are met during construction can be
accomplished by implementing a quality control program developed for the
5—9
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facility. Elements of such a quality control program may include the following
documentation requirements:
o Textural analysis, quantity, and source of fill material;
o Conformance with specified depth, area and volume of earth
removal;
o Groundwater conditions;
o Underlying soil conditions;
o Density, location, placement, thickness, grade and moisture
content of compacted material;
o Location, material type and quantity of emplaced equipment,
structures or piping;
o Compaction equipment and use;
o Herbicide type, quantity, and method of application;
o Presence of materials not confirming to specifications (e.g.
sticks, vegetation, rocks, etc.);
o Names of construction personnel; and
o Date, time, and weather conditions.
Current quality control practices are not well—defined. Quality control is
generally provided at all facilities, but the scope and documentation of
quality control programs vary. Available documentation indicates that,
compaction density of subgrade materials of at least 90 percent of Proctor
Density (ASTM D698) was achieved at all sites surveyed. Excavation and
recompaction of “soft” sections of subgrades and a placement of earth fill in
accordance with standard engineered earthwork construction techniques was
recommended as a result of a quality control program.
No field data were reported regarding total or differential
5—10
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subsidence or the rates of subsidence for liner systems at hazardous waste
facilities. Where subsidence does occur, the liner would be expected to
conform with the surface of the underlying soil. Based upon current
understanding of the properties of synthetic materials, the ability of these
materials to elongate and relax in response to differential subsidence may be
limited by the pressures exerted from the weight of the overlying containment
systems and the waste. This is not considered to be a serious problem provided
appropriate design and quality assurance procedures are followed.
5.6 SUBSIDENCE OF COVER SYSTEMS
Cover system sensitivity to subsidence is similar to that of the
liner system, except there is greater potential for total and differential
subsidence as waste consolidation occurs. The rate of subsidence, in response
to waste volume reduction, may exceed material elongation capacity resulting
in cover failure. The use of fabric—reinforced materials or geotextiles
underneath the barrier can reduce the significance of subsidence.
The nature of wastes, operations and closure at the facility will
greatly influence the subsidence to which the cover system is subjected.
Procedures to place wastes and cover to keep subsidence to a minimum can be
incorporated into the facility’s operational plan. Such procedures should
reduce the volume of bulk waste through compaction and reduce voids between
wastes by filling with compacted soil. No quantitative information was found
regarding subsidence of cover systems at facilities where wastes were managed
in such a controlled manner. Several feet of differential subsidence was
reported at sites which did not practice waste volume reduction.
5—11
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Municipal refuse landfill subsidence cannot be compared directly to
hazardous waste facility subsidence due to differences in waste composition
and degradation. However, relative comparisons of subsidence data are of some
use. Up to 20 percent subsidence may be expected at municipal landfills with a
significant portion of it occurring in the first year. Similar observations of
subsidence during the first year after operations stopped were made during
personal interviews vith hazardous vaste facility owners and operators. These
observations indicate that delayed placement of cover systems may reduce the
total subsidence to which a barrier may be exposed. Placement and monitoring
of settlement markers provides a quantitative basis for judging the rate of
subsidence. Placement of the cover system may then be an economic choice-
between the cost of leachate management and cost—savings incurred by delayed
cover placement. Alternatively, a temporary cap of natural materials may be
replaced after the subsidence rate has decreased.
Subsidence characteristics of uniform waste deposits can be
determined experimentally. Methods have been developed to estimate subsidence
from such wastes. Surcharging the waste with soil or installation of drainage
layers in the waste can be used to accelerate subsidence of some uniform
wastes.
Differential subsidence in cover systems occurs as a result of
nonuniform settlement, particularly in the underlying waste. Waste placement
procedures can greatly reduce potential differential subsidence. Where
differential subsidence is anticipated, the barrier material may be designed
to resist the stresses of material overlying the displacement, such as with
/
/ 5—12
L
-------
the use of fabric reinforcement. Alternatively, the barrier material
specifications may require elongation and tensile strength characteristics
which will allow the barrier to conform to the displacement. Stresses imposed
in carrying the weight of unsupported soils overlying the displacement are not
expected to be great due to the limited depth of such soils and the support
which will be provided by adjacent soils.
Experience with cover system subsidence is not quantitative.
Failures have occurred, particularly where waste placement practices result in
large void volumes which eventually collapse as the wastes degrade or
consolidate. Predictable rates and total amount of subsidence may be used to
design an appropriate cover system. Large initial rates of subsidence may
warrant delaying complete cover system placement until the rate of subsidence
slows.
/
/,
5.7 SUMMARY
The existing data of a actual service experience for hazardous
waste facilities is limited to less than 10 years, and for other applications
it is limited to 25 years. This provides a minimum service period. Exposure’
and testing of samples indicate much longer service can be expected. The EPA
guidance of 30 years is therefore considered conservative. Projections to 45
years have been made and seem reasonable. On the other hand, projections’ of
150 to 200 years on the basis of actual service appear optimistic.
The ability to project long—term integrity of physical properties
for synthetic materials is tentative due to the relatively short period of
/
5—13
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data collection, and the lack of direct correlation between these somewhat
arbitrary physical properties and the long—term projection -of failure.
Explicit determination of service life is not possible based on those
available procedures.
There is very little information relating failure of a synthetic
liner to subsidence problems. The - technology does exist for studying,
analyzing, designing, and constructing foundations suitable for supporting a
- synthetic liner at a hazardous waste facility without subsidence problems.
Many of the problems envisioned with cover systems can also be resolved
through geotechaical analysis. There is a greater uncertainty, however,
concerning the subsidence of cover systems because of the nature of the
hazardous waste operations, and the potential for significant waste
consolidation after facility closure. ( i erentiai subsidence is not expected
to be significant where waste placement practices which reduce void volumes in
the contained waste are included in the operations design plan. Where
differential settlement is anticipated, the barrier can be designed to conform
to the displacement or to support the overlying soil by the use of fabric
reinforcement. Either of these procedures would introduce stress on the cap
liner, which could affect the long—term performance of the system adversely.
V
Total and differential subsidence is not expected to affect liner
system performance when the design incorporates recommendations of a thorough
and complete geotechnical assessment and when a well—developed quality control
program, which includes regular inspection, maintenance, and repair provisions
is implemented. Construction of a temporary cap of natural materials for the
duration of the primary subsidence period is an alternative approach which
would adequately address the subsidence problem.
5—14
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REFERENCES
Lyman, et. al. 1983. Expected Life of Synthetic Liners, Draft Final Report .
U.S. Environmental Protection Agency, Washington, D.C.
TRW, 1983. Assessment for Constructing and Installing Cover and Bottom Liner
Systems for Eazardous Waste Facilities , U.S. Environmental
Protection Agency, Washington, D.C.
5—15
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6.0 FAILURE MODES
6.1 DEFINITION
The assessment of synthetic material performance requires a clear
definition of failure which specifies modes and the mechanisms and conditions
or sources that contribute to failure. A simple definition of failure is:
“the inability of a cover or liner system to meet performance standards”.
The 40 CFR Part 264 regulations establish such performance stan-
dards. The standards require that migration of wastes to the subsurface soil
or to ground water and surface water during the active life of the facility
be prevented, and that migration of liquids through the facility be kept to a
minimum over a long term after closure. More detailed requirements of cap
and liner systems specified by these regulations are presented in Section 2.
Contrary to the common perception of synthetic liners 1 some leakage
will occur through the liner by vapor transport for most leachates. There-
fore, synthetic liners in general cannot meet the criteria established in the
interim final regulations. The vapor transport mechanism was discussed in
Chapter 3 of this report. Since vapor transport is an intrinsic property of
the liner and does not represent a material “failure” per Se, vapor transport
has not been included in this section. It is recommended that the regulation
be modified to allow “de minimus” leakage through the liner to avoid this
obvious contradiction.
The following sections provide a discussion of the significant
failure modes investigated for synthetic liner systems. The information
6—1
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presented in this section was obtained from the supporting consultant re-
ports.
6.2 LINER HOLES
Holes appear in synthetic liners in the form of pinholes, tears,
faulty seams, cracks and windows (large openings). Pinholes may occur during
liner manufacture as a result of inferior product quality control procedures.
Holes may occur after liner installation due to mechanical stress and chemi-
cal decay of the liner materials. During liner installation, holes can occur
due to faulty field procedures. These problems were discussed previously and
can be repaired prior to operation of the facility.
The synthetic liner is only one component of the entire leachate
barrier system. The performance of the liner as the principal containment
barrier is dependent on the proper functioning of the entire barrier system.
Failure of other units, such as the leachate collection system, will result
in excessive hydraulic pressure on the synthetic liner. An analysis of the
consequences of potential component failures can identify problems which may
lead to failure of the synthetic membrane. Such a fault—effect analysis is
considered a key factor in the ability of synthetic materials to perform in
liner and cover systems at TSD facilities. With the single exception of
climate, all sources of liner failure can be controlled by thorough analysis
of the facility design based on an awareness of the total system and develop-
ment and implementation of thorough operation and maintenance, and quality
assurance programs. Seaming and other critical liner installation processes
should not be attempted during adverse weather conditions. An effective
6—2
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quality assurance program would include monitoring of synthetic material
coupons to obtain an early warning of the symptoms of material failure.
6.2.1 Sources of Potential Failure
The occurrence of holes in barriers may be a result of stresses
exerted by various chemical or physical problems. A brief description of
these sources and pinholes resulting from manufacturing problems is provided.
!lore detailed descriptions were presented by Lyman et al. (1983) and Haxo
(1983).
6.2.1.1 Design
A thorough and complete design includes definition of the perfor-
mance goal and provides a sound conceptual structure for the facility through
detailed analysis, and preparation of specifications, construction drawings,
procedures and schedules, provisions for material transportation and storage,
operation and maintenance plans and a quality assurance program. Each coxnpo-
nent of the facility must be designed to function as an integral part of the
total facility. Almost all of the sources of failure identified can be
controlled, if not eliminated, by a thorough and complete design. Inadequate 4
attention to design details can erthance the occurrence of failures.
Establishment of a design performance goal should consider the
characteristics of the wastes in the facility including persistence and
mobility, the duration of desired facility operation, and the hydrogeological
and environmental setting. Haxo (1983) and Lutton (1982) have described the
important design considerations for barriers.
6—3
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The design process should include analysis of the effect of speci-
fic subsystem failures and their effect on synthetic barriers. Such ‘fault—
effect’ analysis identifies elements of the facility design which are criti-
cal to the successful performance of polymeric materials (e.g. removal or
detection systems, etc.). The design should not rely on the polymer barrier
for long-term structural performance. Corrective actions during design can
then be identified, evaluated and included in the final plans.
A thorough geotechnical analysis of all dikes, berms and supporting-
soils can virtually eliminate these sources of subsidence stress on synthetic
materials. Bedding materials can be designed to provide the required protec-
tion of barriers and incorporate a leachate detection and removal system that
will not fail due to overburden pressures or facility construction and opera-
tion stresses. Close coordination between the designer and the geotechnical
engineer in preparing the operations and maintenance plan can drastically
reduce cover barrier failures due to subsidence.
6.2.1.2 Nanufacture and Fabrication
There appear to be significant differences of opinion among the
experts interviewed on the pinhole problem and its significance in liner
performance. Some indicate that it is not possible to produce pinhole-free
liners. Others believe that pinholes in liner sheets are a problem of the
past and that the present manufacturing methods, and the quality control
which is exercised in the manufacturing step, can guarantee liners without
pinholes. There appears to be a lack of technical data to support either
6—4
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assertion. Some manufacturers interviewed indicated that their material
would be “nominally” pinhole-free and that they “would be hard put” to guar-
antee that their product is completely free of pinholes. Pinholes which
penetrate the liners are considered less of a problem for multi—ply than for
single—ply liners because the chances of a pinhole in one ply matching up
with pinholes in a second ply are very small. There might be some potential
problems, however, if a pinhole exposes the scrim, such that contact with
waste fluids could cause wicking along the scrim, build-up of fluid between
plies, and eventual delamination. The probability of this happening is
greatest for tightly-woven scrims and thick yarns, but in any case is small.
Pinholes can originate during the calendaring process where air
bubbles, contaminant particles, or poorly dispersed granules (e.g. uiunelted
modules of carbon black or scrim scrap) in the mixed stock can mar the
otherwise smooth surface between rolls and result in indentations which pass
through the liner. The pinholes can vary in size, from a few microns in
dianeter to a size which can be spotted by the naked eye when the sheet is
held against light (e.g. passed over a light bar). Quality control for
pinhole prevention during manufacturing can include fine screening of the
mixed stock before calendaring or extrusion, limiting the amount of scrap
recycling, and visual inspection of the sheets on both sides to identify and
repair pinholes.
Liner designers and installers generally down-play the significance
of pinholes in liner performance and find little evidence indicating pinholes
as a cause of liner failure. One designer reports that, based on one study
with a water containment system, 2000 pinholes may be considered equivalent
to one hole of 10mm diameter or ten holes of 2xrt diameter. The amount of
6—5
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seepage through pinholes would probably be orders of magnitude less than
through a bad seam or a substantial tear. Pinhole enlargement during actual
use would be unlikely, since liner selection would provide a material with
puncture and tear resistance.
6.2.1.3 Storage and Installation
Damage to materials during storage may occur as a result of tears
from vandalism, holes from hail, and exposure to sunlight. Storing of ma-
terials securely to protect them from the elements was found to be successful
in reducing these problems.
Installation procedures are a common cause of liner failure.
more detailed description of problems encountered and remedies available was
presented in Section 4. In general, problems are attributed to the lack of
proper site preparation and adequate quality control, material handling and
seaming operations.
6.2.1.4 Biological Intrusion
Preparation of the foundation and bedding for cover and liner
systems should include the removal of vegetation and sterilization of soil to
avoid damage to the synthetic material. Use of sterilants and grubbing is an
effective means of controlling vegetation damage to liner systems.
Cover system installations must be performed with the same precau-
tions as with liner systems. In addition, the cover must be designed to
6—6
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prevent any damage from burrowing animals. Coarse grained soils which have a
low angle of repose (side slope) are more effective than clay soils in
controlling burrowing animals, Results front experiments conducted at Los
Alamos indicate a layer of cobbles may be an effective barrier to burrowing
animals.
Selection of shallow-rooted vegetation will reduce the possibility
of root penetration through the synthetic material. Grasses are the most
common species selected for final cover. Root structure of grasses is typi-
cally confined to the upper six inches of soil, although deep-rooted species
do exist. Available species and factors to consider when selecting grasses
for hazardous waste cover applications were provided by Lutton (1982). It is
desirable to avoid deep-root vegetation to protect the polymeric barrier.
Vegetation has been observed to puncture a liner when the bedding material or
subbase was not properly prepared. It is doubtful that roots will penetrate
downward through a barrier to an anaerobic, toxic environment, but control of
this potential source of failure is prudent.
6.2.1.5 Physical Polymer Aging
Polymers are materials which exhibit properties similar to elastic
solids and viscous liquids. In the short term, polymers respond much like an
elastic solid (Elasticity = stress! strain), when the material is stressed
over a period of years, there is a steady increase in strain or creep and a
decrease in rupture stress. Data obtained over one year of high (100 kilo-
2
gram force (weight) per centimeter squared (Kgf/cm )) stress with polypropy-
lene indicate a loss of tensile strength of up to 50 percent. However, long-
term tensile stresses in a hazardous waste facility application are not
6 —7
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expected to be as great. Eight years of Bervice in a pneumatic structure
indicated reinforced PVC retained more than 90 percent of its tensile
strength. The rate of long-term creep varies for synthetic materials depen-
ding on polymer composition 1 type of load, temperature, chemical reactivity,
and the material present as fillers and reinforcing scrim. Absorption of
chemicals into a synthetic material can increase the rate of creep. Repeti-
tious and dynamic (moving) loads appear to accelerate the decrease in long-
term strength.
The information collected in this assessment indicates that poly-i
meric materials lose their -ability to resist mechanical stress when under
constant loads. Liner design should not rely on polymeric barriers for long-
term structural strength.
6.2.2 Significance of Holes
The presence of holes in a liner does not constitute failure,
unless a performance requirement is not achieved. During landfill, waste
pile or disposal surface impoundment operation, the presence of a hole con-
stitutes failure to achieve the 40 CFR Part 264 requirement to prevent migra-
tion of wastes into the barrier unless the de ininimus provisions allow a
variance. After closure, holes in a liner may not violate the per—formance
requirement, particularly when the cap barrier meets the requirement for
smaller permeability than the liner barrier. Holes in cap barriers may
result in failure of the cover system. In each of these situations, an
analysis of the effect on barrier system capability to control liquid and/or
waste constituents is warranted.
6—8
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Field experience has provided several examples of the significance
of holes in synthetic barriers (Lyman, et al., 1983). The seepage rate for a
—8
45 mu synthetic liner was reported to be 2.9 z 10 cm/s., lower than most
clays. The rate was calculated to be about five times greater than could be
accounted for by moisture vapor transport. It was concluded that the liner
had holes. Similarly, a 6 mu polyethylene liner was found to have a seepage
—6
rate of 10 cm/s. Inspection revealed the presence of pinholes.
The rate of water leakage through a hole depends on hole dimen-
sions, surface tension and pressure gradients. Very small holes in non-
wetting materials such as HDPE supported by well-drained soil will not re-
lease water because of surface tension forces. When the hydraulic head
exceeds these forces, then flow will occur. Surface tension becomes less
important as hole dimensions increase.
Seepage through a larger hole is often controlled by other factors.
When seepage from a synthetic material does begin, the rate cannot exceed the
maximum that the underlying material will allow. The soil underlying a
barrier will be the rate limiting factor, and the magnitude of the leakage
rate will depend on the wetted area of soil, the soil hydraulic conductivity,
and the hydraulic gradient. Since the wetted area is likely to be a very
small fraction of the site covered by the material, the “average” seepage
through the material may be quite low. For example, in a one acre site
2 2
(4047m ), if an FNL with a hole as large as I m overlies soil with a permea-
—4
bility 10 cm/s, the total apparent permeability of the FNL plus soil could
—3 —8
be as low as 1/4047 x 10 cm/s or about 2.5 x 10 cm/s. Where large holes do
occur, an analysis of the specific conditions is necessary to assess the
seepage rate.
6—9
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The significance of holes in terms of liquid release rates in a
liner may be indicated by determining an equivalent hydraulic conductivity
for the facility. This was performed for landfill facilities constructed and
operated in accordance with the Guidance Document (EPA, 1982) and summarized
in Table 6-1.
The results presented in Table 6—i indicate synthetic liners with
hole areas less than 1 square meter per acre would be expected to leak at
similar rates to a well constructed clay liner. This represents a liner with
approximately 25 million pinholes per acre, which is unrealistic if proper
manufacturing and installation procedures are used. In general, the results
indicate that properly installed synthetic liners would be expected to leak
at a slower rate than well constructed clay liners for surface impoundments,
waste piles and landfill facilities. However, such performance would not
comply with the criteria established in the Interim Final Land Disposal
Regulations during the facility operating period.
Holes in cover system barriers may be more significant than holes
in liners. Although the equivalent hydraulic conductivity for materials
containing holes is low, the presence of holes in a cover barrier system
represents a potential failure if the liner barrier retains its designed
performance. Water, which may infiltrate through holes in the cover barrier,
may accumulate above the liner. Such accumulation may lead to unanticipated
effects on the liner barrier, resulting in failure. This failure mechanism
is discussed in Section 6.3. The probability that this type of failure will
occur is low 1 provided the cover system design accounted for subsidence of
the contained waste. Infiltrating water would not be expected to pond on the
cover barrier and thus would not be avilable to drain through holes. The
6—la
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Table 6-1. EQUIVALENT HYDRAULIC CONDUCTIVITY FROM HOLES IN A HAZARDOUS
WASTE LANDFILL 8
AREA OF HOLES
EQUIVALENT FACILITY
Equivalent
Hole
Facility
Hydraulic
Pinholes
Area
Area
Conductivity
millions
in 2
percent
cm/s
0.25
.01
.0002
10—
2.5
.1
.002
10_s
25
1
.02
10—’
250
10
.2
10—’
2500
100
2
10—’
a One acre site (4047 in 2 ) in compliance with Draft RCRA Guidance Document
Landfill Design, Liner Systems and Final Cover. U.S Environmental
Protection Agency, Washington, D.C.
6—11
-------
10
10
10-s 10-2
PERMEABILITY OF CAP BARRIER LAYER (IN/HR)
________ PERCOLATION FRON SASE
OP COVER
—PERCOLAT$ON FION SASS
OF LANDFILL
PEAK HEAD ON SASS OF
LANDFILL
10-1
FIGURE 6 -1
PERMEABILITY OF CAP BARRIER LAYER VS. PERCOLATION AND HEAD
NOTE: RESULTS SHOWN ARE BASED ON OUTPUT FROM THE HELP PROGRAM IN WHICH
FIVE YEARS OF OPERATION WERE SIMULATED USING CLIMATIC DATA FROM
EAST ST. LOUIS, ILLINOIS. THE ASSUMED PERMEABILITY OF THE BARRIER
LAYER AT THE BOTTOM OF THE LANDFILL WAS 7.IXIO-5 INCHES/HOUR .
8
z
z
0
I .-
-a
0
‘U
-I
‘C
2
2
‘ C
C
>
‘ C
4
KEY:
2
0
“I
8
0
0
2
0
‘I
r
2
4 D
I-
r
2
0
I
2 m
0
10-4
-0
6—12
-------
z
-j
-j
0
z
6.0 .100
4.0 80 0
0
o z
U.
z 0
o
3.0 •eo
o
a
z
U i 0
2.0 .40 F
.20
‘U
>
0
LEACHATE SYSTEM LEACHATE SYSTEM
COMPLETELY PLUGGED FULLY FUNCTIONING
FIGURE 6-2
LEACHATE SYSTEM EFFICIENCY VS. PERCOLATION AND HEAD
NOTE: RESULTS SHOWN ARE BASED ON OUTPUT FROM THE HELP PROaRAM,
IN WHICH FIVE YEARS OF OPERATION WERE SIMULATED USING CLIMATIC
DATA FROM EAST ST. LOUIS, ILLINOIS. THE ASSUMED PERMEABILITY OF
THE BARRIER LAYER AT THE BOTTOM OF THE LANDFILL WAS 7.1XIO-6
INCHES/HOUR. ‘1(2” REFERS TO THE VERTICAL PERMEABILITY OF THE
SECOND LAYER (THE CAP BARRIER LAYER).
0 EFFICENCY OF LEACHATE COLLECTION SYSTEM 1.0
t
KEY:
PERCOLATION FROM SASS OF
LANDFILL
PEAN HEAD ON SASS OF LANDFILL
OPENCOLATION FROM SASS OF LANDFILL
K 2 0.142/HR
•PENCOLATION FROM BASE OF LANDFILL
K 2 o.O 142 / HP
ÔPEAk HEAD ON BASE OF LANDFILL
HR
£PEAK HEAD ON BASE OF LANDFILL
K O.O142/NR
6—13
-------
estimates for liner performance in Table 6—1, therefore, represent an over-
estimate of the quantity of water which may pass through the cover barrier.
It is expected that sound design, and proper barrier construction and instal—
lation would avoid failure of a cover system.
6.2.3 Summary
Liner holes have been detected in most in-place hazardous waste
liners. The holes result from installation problems, mechanical stress,
chemical aging and in part from inferior manufacturing processes. several
techniques are available to decrease the number of holes in a liner material,
but elimination of all holes is not feasible. A comprehensive quality assur-
ance program should be implemented which covers all aspects of synthetic
liner selection, installation and monitoring, including:
o The liner manufacturing process;
o Liner foundation preparation;
o Liner placement and installation;
o Chemical resistivity testing;
o Liner seam testing; and,
o Liner maintenance and inspection.
The quality assurance program will minimize the number of holes in
the facility liner, but some holes will remain. The evaluation of hole
leakage indicates that the presence of holes does not significantly alter the
performance of synthetic liner materials. However, small amounts of leakage
will occur. While this does not present a significant problem from a practi-
cal perspective, synthetic liners cannot meet the performance objective
6—14
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stated in the Interim Final Land Disposal Regulation. It is recommended that
the regulation be modified to allow “de minimus” leakage from a disposal
facility during the period of operation of the facility.
6.3 BATHTUB EFFECTS
The phenomenon of “bathtub effect” occurs at a landfill when the
permeabililty of the cover or cap, or of the upper portion of the waste if
exposed, is significantly greater than the permeability of the underlying
barrier layer (liner). In a landfill, trenches to accoinodate the waste have
usually been below land surface. Because of the relatively high permeability
of the cap, a significant fraction of rain water infiltrates into the land-
fill and into the contained waste. The water level in the landfill trench
then tends to build up unless an efficient leachate collection system is in
operation just above the bottom barrier liner. Thus, a local perched water
table can develop at the landfill. In the extreme case, where the underlying
materials are of very low permeability and infiltration through the cap
(cover) has been relatively rapid, the perched water table can rise to the
top of the excavated trenches and even to the surface of the landfill cover
over a period of several months or years.
An effect similar to the bathtub effect is seen at landfill or
waste—pile facilities in which rainfall infiltration into the waste is signi-
ficantly greater than the downward movement through the underlying barrier
layer, but where no excavation or only minimal excavation below grade has
been performed prior to the filling or piling of the waste. We have termed
these effects as “tarmac effects”, because of the effects of storing or
6—15
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stockpiling material on tarmac aprons. In this case, although there is no
“bathtub” to visualize as filling up with rainwater/leachate, the environmen-
ta]. effects are very similar to those resulting from the “bathtub effect”.
Consequently, we have chosen to discuss in this report these so-called “tar-
mac effects” along with those that may be considered to be strictly “bathtub
effects”.
The environmental effects of the “bathtub effect” range from
nuisance-level to serious effects, depending on the nature of the waste and
the quality of the resulting leachate, and on the proximity of population
centers to the landfill. When the perched water level attains a height
greater than the top of the filled landfill trench, the leachate will begin
to move laterally and is likely to appear as seeps along the lower slopes at
the periphery of the landfill. These seeps generally result in overland flow
to the nearest stream tributary and consequently result in contamination of
stream water. In certain cases, the perched water may rise to such an extent
that it begins to pond in depressions in the landfill cover. This produces
not only an unsightly appearance but may result in the transfer of hazardous
volatiles into the atmosphere by vaporization.
Another possible serious consequence of “bathtub effects” is the
lateral sub—surface movement of the leachate into adjacent surficial aquifers
of the area. The movement is brought about by the higher head within the
landfill. Depending on local sub-surface flow conditions, deeper aquifers
may also be contaminated by the leachate that has migrated laterally away
from the landfill.
6—16
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6.3.1 Causes of the Bathtub Effect
Bathtub effects or tarmac effects may be considered to be caused by
deficiencies in landfill design or by the use of iinmnproper construction or
operation procedures. These deficiencies result in the failure of the land-
fill cap/cover or a failure of the leachate collection system. One would
hope that if the cap and the leachate collection system are properly designed
and constructed, the likelihood of failure of those two important components
during the active life of the facililty and during the post—closure care
period would be minimal. Nevertheless, cap or leachate collection system
failure can occur during this period even under the best design and construc-
tion/operation regimens.
The mechanics of cap/cover failure which serve as indicators of the
presence of, and the potential for, bathtub effects are: 1) the presence or
development of holes in the cover; 2) subsidence or settlement of the cover;
and 1 3) weathering and aging of cap liners.
Holes can develop in cap/cover systems in various ways. Pinholes
in synthetic liners can originate during material fabrication. However, data
appear to indicate that the presence of only a few pinholes would not be a
likely cause of cap failure and the onset of bathtub effects. Holes can also
result from various storage and installation factors, including: vandalism;
exposure to sunlight; failure to sterilize bedding materials; improper use of
construction vehicles; improper seaming techniques; and, penetration of the
synthetic barrier by coarse—grained materials in the overlying drainage
layer. Biological intrusion mechanisms which may cause cap/cover failure
include root penetration and small animal burrowing. An indirect method of
6—17
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failure of the synthetic barrier can result from the formation of desiccation
cracks in overlying soils. These cracks propagate from the surface down to
the synthetic barrier, thus increasing the effects of weathering agents on
the synthetic liner.
As discussed herein, cap subsidence refers to localized differen-
tial subsidence due to collapse or rapid consolidation of the waste material.
This has been referred to as the most prevalent failure mode for caps with
synthetic barriers (A.D. Little, l983) and can result in the formation of
depressions and ruptures of the cap/cover system which promote rainwater
infiltration into the landfill trench. Cap settlement ref ers to a more
uniform settlement of waste materials but which can also enhance rainwater
infiltration.
Weather can have several adverse effects on synthetic liners.
Prolonged exposure to sunlight and to ozone can effect the integrity of the
membranes. Wide fluctuations in temperature can lead to stress cracking of
the membrane resulting from repeated expansion and contraction of the ma-
terial. Synthetic liner aging considers both the physical aging process as
well as polymer degradation. The physical stress required to induce membrane.
rupture decreases with time. Polymer degradation can result from thermal 1
.mechanically—initiated, chemical, photochemical and biologically—initiated
processes which often act together. These processes can effect the tensile
strength, tear strength, impact strength and resistance to stress-cracking of
the membrane.
6—18
-------
The reader is referred to Section 4 for additional discussion of
the sources of potential synthetic liner failure and preventative measures
that can be imposed. Additional reference should be made to Sections 4 and 5
regarding installation-related issues and subsidence problems, respectively.
Failure of the leachate collection system can result from pipe
cracking during installation or during the early stages of landfilling, or by
clogging of the system by physical, chemical, biochemical or biological
mechanisms. Physical clogging mechanisms are generally failures in design,
such as insufficient drain pipe capacity. Chemical and biochemical mecha-
nisms involve pipe incrustation by precipitation of insoluble compounds.
Biological clogging occurs by organism reproduction to the point of filling
the interstices of the drainage materials.
The lack of a thorough design which provides a sound conceptual
structure and detailed provisions for materials trans—port, storage, instal-
lation, and operation and maintenance programs probably has the greatest
potential for causing bathtub effects. Some common design errors include:
o Failure to establish a performance goal;
o Failure to specify a minimum of three layers in the
cap;
o Failure to specify adequate waste compaction
procedures;
o Failure to specify, or improperly specified,
surface preparation, liner installation and cover
maintenance and operation procedures;
o Inadequate design of leachate collection system
components in an integrated manner to accomodate
the maximum expected leachate volumes;
6—19
-------
o Inadequate 1 or lack of, monitoring and maintenance
plans for leachate collection systems to provide
early warning of system failure; and,
o Failure to specify QA/QC plans.
Improper or careless construction and operating practices can
create conditions which result in bathtub effects. Some of these poor prac-
tices that can result in damage to the leachate collection system or the
liner include:
o Inadequate compaction of waste material to reduce
potential for subsidence or settlement;
o Failure to provide a suitable-constructed bedding
layer (should be smooth, free from sharp objects,
and properly sterilized) for the liner;
o Improper liner storage and handling procedures;
o improper seaxn.ing practices;
o Failure to provide interim cover to divert, collect
and remove rainwater;
o Use of heavy equi sient during the leachate
collection system installation and early stages of
landfilling; and,
o Failure to implement effective construction/opera-
tion QA/QC plans.
A more complete assessment of design installation considerations
are provided in Sections 4 and 5 of this report, and Report on Bathtub Effec-
ts in Hazardous Waste Facility Operation (Ertec, 1983).
6.3.2 Remedial Measures for Bathtub Effects
A number of corrective measures have either been instituted in the
field or recommended to bring under control rising perched water levels in
6—20
-------
landfills. In many instances, a combination of two or three of the following
remedial measures has proven to be most effective.
6.3.2.1 Pumping and Treatment of Leachate
Where a leachate-collection system exists, it may be possible to
accelerate flow and lower water levels in the trench by simply pumping from
sumps in the system on a nearly continuous basis. The leachate is then
treated at a treatment unit on site, or pumped into tank trucks for transpor-
tation to suitably licensed treatment facilities.
6.3.2.2 Cap Reconstruction
Once a bathtub or tarmac effect is identified, there is probably no
more important remedial measure than that involving sealing the cover so that
subsequent infiltration of rainwater into the waste will be negligible, or at
least extremely small. Generally, this remedial measure is combined with
some sort of drainage system designed to remove the existing accumulated
leachate.
6.3.2.3 Surface—Water/Ground-Water Diversion
The diversion of surface-water runoff from a landfill cover is
perhaps one of the first, and least expensive, remedial actions to be per-
formed at a site experiencing bathtub/tarmac effects. In most cases, infil-
trating rainfall or runoff waters generate much more leachate than do surfi-
6—21
-------
cial ground waters. In the process of regrading the site and cap reconstruc-
tion, surface-water runoff diversion is also usually provided for.
In some situations, ground-water flow toward a landfill may consti-
tute a significant component of leachate generation. In such cases, the
construction of a ground-water interceptor trench, or cutoff wall, on the up-
gradient side of the landfill may be the most efficient way to prevent ground
water from reaching the waste,
6.3,2.4 Treatment of the Leachate Collection System
Treatment of leachate collection systems can often increase the
effectiveness and longevity of these systems, provided that the problem is
identified early. Thus, monitoring is an essential component in the mainte-
nance of leachate-collection systems.
Methods of treatment of leachate—collection systems include (Bass,
et al, 1982):
o Excavation and replacement: generally expensive,
difficult and dangerous to implement
o Physical methods: use of mechanical devices and
hydraulic cleaning
o Chemical methods: generally involves the use of
acids
6.3.3 Sensitivity analysis for Bathtub Effect
A set of sensitivity runs were performed by Ertec Atlantic, Inc.,
in Report on Bathtub Effects in Hazardous Waste Facility Operation (1983)
6—22
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using program H P and clirnatological data for East St. Louis, Illinois, to
illustrate the relative importance of different factors in inducing bathtub
effects. A summary of the results of the runs is provided in Table 6.2. The
factors that were varied included: (1) vertical permeability of the cap
barrier layer; (2) evaporative zone depth; (3) the porosity and field capaci-
ty of the waste layer; and, (4) the presence or absence of a leachate collec-
tion system. Of these, the two most critical factors contributing to a rise
in the head on the base of the landfill were the vertical permeability of the
cap barrier layer (refer to the curves of Figure 6-1) and the presence or
absence of a leachate collection system (see Figure 6-2). The most dramatic
change in the head (or water level) measured from the base of the landfill
was effected by the presence or absence of a functioning leachate collection
—4
system. As the permeability of the cap barrier layer increases from 10 to
—2
10 inches/hour, the peak head on the base of the landfill, for this case,
gradually rises from less than 1 inch to more than 8 inches. In this case,
—2
increasing the permeability of the layer beyond 10 inches/hour had no
effect in increasing the head on the base of the landfill.
Table 6-2 shows that decreasing the evaporative zone depth from
8 inches to 6 arid thence to 4 inches results in no significant change in the
peak head on the base of the landfill, as long as the leachate collectiono
system is operating efficiently. The resulting increase in the average
annual percolation from the base of the cover is completely accommodated, in
this case, by lateral drainage from the base of the landfill (via the leach-
ate collection system). Increasing the porosity of the waste material from
0.52 to 0.60 and then to 0.70, with corres—ponding proportional increases in
field capacity, resulted in slight decreases in the peak head on the base of
the landfill, as is shown in Table 6—2. In general, however, a relatively
6—23
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TABLE 6-2
SUMMARY OF SENSITIVITY RUNS ON PROGRAM HELP
FOR ANALYSIS OF BATHTUB EFFECTS
RESULTS OF 5—YEAR SIMULATION, 1974—1978
USING E. ST. L 1 OUIS CLIMATOLOGICAL DATA
Avg. Annual Avg. Annual Avg. Annual
Percolation Drainage Percolation Peak Head
from Base from Base from Base from Base
2 E.Z.D. POR /FC of Cover of Landfill of Landfill of Landfill
Run (in/hr) (inches) L.C.S.? 3 3 (in/yr) (in/yr) (in/yr) (inches’
la 0.000142 10 yes 0.52/0.32 2.51 1.2]. 1.27 1.1
1 0.000142 8 yes 0.52/0.32 2.71 1.35 1.33 1.0
2 0.00142 8 yes 0.52/0.32 7.10 5.53 1.36 6.2
3 0.0142 8 yes 0.52/0.32 7.36 5.54 1.70 8.5
4 0.142 8 yes 0.52/0.32 7.42 5.45. 1.86 7.9
5 0.0142 8 yes 0.60/0.38 7.36 5.5]. 1.73 8.3
6 0.0142 8 yes 0.70/0.46 7.36 5.48 1.75 8.2
7 0.0142 6 yes 0.52/0.32 8.99 6.91 1.94 8.4
8 0.0142 4 yes 0.52/0.32 11.41 9.26 1.98 8.4
9 0.0142 8 no 0. 2/0.32 7.36 0 3.73 93.4
10 0.142 8 no 0.52/0.32 7.42 0 4.20 83.4
R 2 — vertical permeability of the cap barrier soil layer
E.Z.D. — evaporative zone depth
L.C.S.? — leachate collection system operative?
POR 3 — porosity of the 3rd (waste) layer
FC 3 — field capacity of the 3rd (waste) layer
-------
low porosity for the waste material is desired, and is produced in the field
by regular and frequent passes of heavy equipnent to effect compaction of the
waste. This helps to minimize the occurrence of localized subsidence later
in the life of the landfill.
Comparison of the peak heads in Table 6-2 for Runs 3 and 4 as well
as for Runs 9 and 10, shows that in both pairs of cases increasing the
permeability of the cap barrier layer from 0.0142 to 0.142 inches/hour ac-
tually results in a lower (7 to 12 percent) peak head on the base of the
landfill. The reason for this is unclear, but it is believed to be a func-
tion of the HELP program’s convergence routine, and does not reflect the real
world conditions.
Climatic factors also play a role in the development of bathtub or
tarmac effects. Other things being equal, areas with high rainfall and low
potential evapotranspiration will develop bathtub effects more quickly than
areas with lower rainfall and higher potential evapotranspiration. Once an
“overflowing” condition is established, the rate of flow from seeps and
outward sub-surface flow will tend to be higher in more humid areas. More-
over, the “activity” of seeps will tend to follow the incidence of rainy days
and rainy seasons. In areas having very low annual rainfall and high evapo-
rative potential, bathtub or tarmac effects may not develop during the opera-
tional and post—closure care periods, even in cases where the permeability of
the cover material is several orders of magnitude greater than that of the
bottom barrier layer.
6-25
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6.3.4 Summary and Conclusions
Bathtub or tarmac effects generally develop when rain and runoff
water percolate into a waste facility at a rate greater than the leachate
moves below the bottom of the facility. The cause of these effects has
usually lain in the absence of sound design installation and/or maintenance
procedures. The single most important design deficiency has been the lack of
an effective cap that incorporates a low-permeability barrier layer, a drain-
age layer and a vegetative cover. A proper cover design should also include
appropriate surface runoff diversions as well as erosion—control measures .
The absence of a functioning leach—ate—collection system has also contributed
to the development of bathtub/tarmac effects in many cases. Maintenance of
such collec—tion systems requires the operation of a regular trench-water
monitoring system. When monitoring indicates small but significant rises in
the trench water levels, implying potential clogging of the system, appro-
priate drain—cleaning remedies can be undertaken. The basic remedy for the
bathtub/tarmac effect generally involves: (1) collection and treatment of
the existing leachate from the waste facility; and, (2) the simultaneous
regrading and reconstruction of the cap so that infiltration of rain and
runoff water can be kept to a bare minimum.
6—26
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REFERENCES
Haxo, H.E., Jr. 1983. (See References, Section 3)
Lutton, R. 7. 1982. Evaluating Cover Systems for Solid and Hazardous Waste .
SW-867 (Revised Edition). U.S. Environmental Protection Agency,
Washington, D.C.
Lyman, W.J. 1983. Expected Life of Synthetic Liners and Caps (Draft Final
Report) . U.S. Environmental Protection Agency, Washington, D.C.
EPA, 1982. Draft RCRA Guidance Document: Landfill Design, Liner Systems
and Final Cover . U.S. Environmental Protection Agency, Washington, D.C.
6-27
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7.0 PERFORMANCE MODELING OF SYNTHETIC LINER SYSTEMS
The development of the HELP model and the basic research and analy-
ses involved in model verification, sensitivity analyses and production runs
for selected waste—facility design scenarios were conducted directly by the
EPA Office of Research and Development in Cincinnati, Ohio, with the partici-
pation of the U.S. Army Engineer Waterways Experiment Station at Vicksburg,
Mississippi. A detailed summary of th supporting technical documentation
was provided to the EPA by Ertec in memorandum form in May, 1983 (Ertec
Atlantic, Inc., l983b). This section provides a summary of the results of
this research and modeling task.
7.1 METHODS OF INVESTIGATION
The purpose of modeling the performance of caps and liners was to
compare the release rates for a range of waste—facility designs under differ-
ent climatic conditions. By comparing the computed order—of—magnitude
seepage rates through the bottom liner of waste facilities with differing
design characteristics, it was hoped that it would be possible to compare the
effectiveness of each type of design in containing the waste. For example,
one of the primary purposes of the simulations with the HELP model was to
compare the theoretical short— and long—term impacts of waste facilities with
clay liners and with synthetic flexible membrane liners (FMLs) on the upper-
most ground water. The HELP model in its present form was not designed to
produce highly accurate predictions of seepage for individual cases. It is
felt, however, that it provides a reliable basis for assessing order—of—
magnitude seepage rates, so that valid comparisons of the effectiveness of
7—1
-------
different design cases can be made. The conclusions from such comparisons
would, it was hoped, be useful with respect to reform of the Interim Final
Part 264 RCRA regulations.
Another purpose of the HELP simulations herein summarized, was to
provide release rates, as “source terms,” for generic fate and transport
modeling of leachate migration in ground water. This fate and transport
modeling was part of the locational study, which constituted another major
task under EPA’s current land—disposal study to assess the role of caps/liners
and locational factors in mitigating or promoting ground—water contamination
under different hydrologic and c].imatological conditions.
Design/operation scenarios were developed for four facility types:
landfills; disposal surface impoundments; storage surface impoundments; and
waste piles. These were evaluated for three varying climatic conditions
represented by data compiled for a 20—year period for: New Orleans,
Louisiana; Hartford, Connecticut; and Denver, Colorado. The design of the
systems and values for materials were selected to be representative of current
technology. Operating conditions were determined based on review of data from
the U.S. EPA Office of Solid Waste (OSW) site visits, review of new facility
permit applications and professional experience.
In most cases, seepage rates through the bottom liner were calcu—
lated with the use of the Hydrologic Evaluation of Landfill Performance (HELP)
model. The exception is for surface impoundment facilities during the oper-
ating period, which were analyzed using Darcy’s Equation, constant head
assumptions, and steady—state drainage calculations. The HELP model is an
7—2
-------
improved extended version of the Hydrologic Simulation model developed for
the EPA by the U.S. Army Engineers Waterways Experiment Station for estimating
percolation into the waste layer of Solid Waste Disposal Sites (HSSWDS). HELP
is a quasi—two—dimensional, deterministic model that computes the daily water
budget for a landfill design considering vertical percolation through suc-
cessive horizontal layers and lateral drainage In the coarse granular layers
overlying the barrier layers. The model was applied to each design facility
component (e.g., cell) to develop the mean monthly seepage and lateral
drainage values for that component. A 20—year daily precipitation record was
used to develop the mean monthly release rate for each climate. Thus, the
comparison of the facility—design scenarios is based on expected monthly
values rather than on time—series release rates with hydrologically—related
fluctuations. Once the HELP model was applied to determine the mean monthly
release rates for each operating condition (i.e., operation, post—closure
care, post—care), a computer program called POSTHELP was used to combine the
release rates of all the cells into the overall facility release rate.
A detailed discussion of the development history of the HELP model
is beyond the scope of this report. Interested readers are directed to the
HELP model user guide and the documentation which are included in the appendix
to the Ertec memorandum previously referenced. Similarly, the results of
verification of the HELP model using another more sophisticated model will not
be presented. For further details, the reader is referred to a report on the
University of North Carolina’s DRAINFIL model (Skaggs, 1982) and to a report
on the HELP sensitivity study included with the appendix to the referenced
Ertec memorandum.
7—3
-------
inches/year for the 20—year period used. Denver represents a dry climate
where designed drainage features may not result in a dramatically reduced
leachate generation.
Data representing the distribution of twenty monthly release rates
for a given month (one for each of the 20 years simulated) from the HELP model
runs could usually be fit (within 95% Kolmogorov—Smimov confidence intervals)
with a normal distribution with coefficients of variation (i.e., standard
deviation divided by the mean) of less than 0.5. Thus, individual monthly
values will be variable but not highly variable. For example, at least 95% of
the release rates for a given month would be less than twice the mean release
rate for the month, if the coefficient of variation is 0.5. Comparison of
different climatic conditions, designs and waste—level controls was performed
in this analysis by comparing the mean monthly rates of seepage below the
bottom of the facility, as computed by HELP. Hence, it is important to note
that extreme hydrologic conditions, both vet and dry, were not addressed by
this analysis of facility designs. The effects of extreme droughts that could
severely damage the vegetative cover and cause desiccation and cracking of
soil layers were not considered, nor were the effects of heavy storms or
floods that could severely erode the cover, damage berms and increase seepage
rates.
7.2.3 Design Scenarios
The design of the systems and specification of values for material
properties were selected to be representative of current technology. Spe-
cifically, the design of the systems was based on the following: 1) current
7—7
-------
Part 264 regu].atory requirements along with suggested guidance; 2) new fa-
cilities applications under part 264; 3) public comments on Part 264 (July
1982); and 4) existing Interim Status Standard (ISS) facilities. The values
for material properties were selected from studies and published data. Spe-
cifically, the values for the expected life of synthetic liners was derived
from review of Analysis of Flexible Membrane Liner Chemical Compatibility
Tests (A.D. Little, 1983) and Expected Life of Synthetic Liners and Caps (A.D.
Little, 1983). In—place permeability of the clay materials was derived from
review of Performance of Clay Caps and Liners for Disposal Facilities
(Research Triangle Institute, March 1983) md Assessment of Technology for
Constructing and Installing Cover and Bottom Liner Systems for Hazardous
Waste Facilities (TRW, February 1983).
A summary of the design scenarios developed for the landfill, dis-
posal surface—impoundment, storage surface impoundment and waste pile facili-
ties are shown in Tables 7.2—1 through 7.2—4, respectively. The designs are
expressed in terms of level of waste control, as characterized by the follow-
ing parameters:
o Final cover design
o Permeability of the clay barrier layers in the cap and bottom
liner
o Presence and design of a leachate collection system
o Presence and design of a leak detection system
o Liner design
7—8
-------
TABLE 7.2—1
LANDFILL DESIGN SCENARIOS
PARAMETER
I
LEVEL OF WASTE CONTROL
II
V I
Final Cover
(1)
Design
24” vegetated layer
12” drain layer
20 mu
2 ft. of clay
24” vegetated layer
12” drain layer
20 mu FML
2 ft. of clay
24” soil layer
Permeability
(3)
of Clay
(cm/sec)
Leachate
Collection
System
present (4)
present 4
not present
Leak
Detection
System
present (5)
present (5)
not present
Liner
Design
double liner
30 mu FML
A) double liner
30 mu FMI 1
2 ft. of clay
1 10 1 x 1 x
none
-------
TABLE 7.2—1, continued
NOTES: (1) Final cover design includes a final grade of 3—5%; the hydraulic conductivity of the drain layer
equals that of the drain layer in the leachate collection system.
(2) = Flexible Membrane Liner.
Refers to the permeability of the clay used in both the final cover and liner designs.
(4) Minimum design criteria are at least 12—inch thick drainage layer with hydraulic conductivity
io_2 cm/sec. Minimum slope of drainage layer 2 percent. A drainage tile system designed
to collect and remove the expected quantity of leachate that will be produced with 50—foot
spacing.
(5) Minimum design criteria are at least 12—inch thick drainage layer with hydraulic conductivity
lO cm/sec. Minimum slope of drainage layer 2 percent. A drainage tile system designed
to collect and remove the expected quantity of leachate that will be produced with 50—foot
spacing.
-------
TABLE 7.2—2
PARAMETER
DISPOSAL SURFACE IMPOUNDMENT DESIGN SCENARIOS
I
LEVEL OF WASTE CONTROL
II
Final Cover
Design
24” vegetated layer
12” drain layer
20 mu
2 ft. of clay
24” vegetated layer
12” drain layer
20 mu FML
2 ft. of clay
Permeability
(3)
of Clay
(cm/see)
Leachate
Collect ion
System
not present
not present
Leak
Detection
System
present (4)
present (4)
double liner
30 mu FML
A) double liner
30 mu FML
2 ft. of clay
1 X l0
1 X
Liner
Design
7—il
-------
TABLE 7.2—2, continued
NOTES: Final cover design includes a final grade of 3—5%; hydraulic
conductivity of drain layer is lO_2 cm/sec.
(2) FML Flexible Membrane Liner.
Refers to the permeability of the clay used in both the
final cover and liner designs.
(4) Minimum design criteria are at least 12—inch thick drainage
layer with hydraulic conductivity 1O cm/sec. Minimum
slope of drainage layer 2 percent. A drainage tile system
designed to collect and remove the expected quantity
of leachate that will be produced with 50—foot spacing.
7—12
-------
TABLE 7.2—3
STORAGE SURFACE IMPOUNDMENT
DESIGN SCENARIOS
LEVEL OF WASTE CONTROL
PARAMETER I
Final Cover Design 24’ vegetated layer
Permeability of Clay not applicable
(cm/sec)
Leachate Collection not present
System
Leak Detection not present
System
Liner Design single liner
30 mil l)
NOTES: FML Flexible Membrane Liner.
7—13
-------
TABLE 7.2—4
WASTE PILE DESIGN SCENARIOS
LEVEL OF WASTE CONTROL
PARAMETER I
Final cover Design 24” vegetated layer
12” drain layer
20 mu FML 2
2 ft. of clay
Permeability of C1ay 3 1 X 10
(cm/sec)
Leachate collection present (4)
System
Leak Detection not present
System
Liner Design single liner
30 mu FML
NOTES: Final cover design includes a final grade of 3—5%; the hydraulic
conductivity of the drain layer equals that of the drain layer in
the ].eachate collection system.
(2) = Flexible Membrane Liner.
(3) Refers to the permeability of the clay used in both the final
cover and liner designs.
(4) Minimum design criteria are at least 12—inch thick drainage layer
with hydraulic conductivity lO2 cm/sec. Minimum slope of drain-
age layer 2 percent. A drainage tile system designed to collect
and remove the expected quantity of leachate that will be pro-
duced with 50—foot spacing.
7—14
-------
The applicability of these parameters varies by facility type and
level of waste control. Refer to the above referenced tables for specifi-
cations.
The selected designs range in level of waste control from double—
and single—lined systems which meet current regulatory requirements and con-
form with suggested guidance documents, to systems which are either partially
or totally in non—compliance with said requirements and good practice. This
range in design was set by EPA to establish a suitably extensive data base for
comparative purposes.
Schematic cross sections indicating design features have been pre-
pared for each of the four facility types based upon the most conservative
design scenario for a given facility (that is, the design effecting the
highest level of waste control). Figures 7.2—1 through 7.2—4 show these cross
sections for landfills, disposal surface impoundment, storage surface
impoundment and waste piles, respectively. These figures are intended to fa-
cilitate interpretation of the design parameters in a generic sense. Extrapo-
lation is required to interpret design specifications for the less conserva-
tive levels of waste control scenarios. Also, a typical landfill facility
profile illustrating the modeled liquid routing system is shown in Figure
7.2—3.
7.2.4 Operating Conditions
Operating data for the modeling were selected by the EPA based on
site visits, review of new facility permit applications and professional
7—15
-------
TYPICAL CROSS-SECTION FOR LANDFILL DESIGN LFI
$‘ PROTECTIVE
LAYER
600
FIGURE 7.2-1
1.0’ L.EACHATE
COLLECTION SYSTE
i.o LEAK DETECTION
SYSTEM
-------
TYPICAL CROSS -SECTION FOR SURFACE iMPOUNDMENT - DISPOSAL DESIGN SD I
600’
FIGURE .7.2-2
LEAK DETECTION
t; V5 T EM
0.6’ PROTECTIVE
LAYER
-------
TYPICAL CROSS-SECTION FOR SURFACE IMPOUNDMENT - STORAGE DESIGN SSI
NOTE: AT CLOSURE THE WASTE AND BOTTOM LINER AND ANY
CONTAMINATED SUBSOIL APE REMOVED FROM TI-if “,IT fIND
DISPOSED OF AT APPROPRIATE FACILITY. THE SITE IS RECLAIMED
TO CONFORM TO ORIGINAL CONTOUR PER ACCEPTED PRACTICES.
ck5 ’
600’
FIGURE 7.2-3
-------
TYPICAL CROSS SECTION FOR WASTE PILE DESIGN WP .11
2.0’ VEGETATED LAYER
1.0. DRAINAGE
0.5’ PROTECTIVE LAYER
FIGURE 7.2-4
20 MIL PML
-------
pRECIprrATIoN EVAPOTRANSPIRATION
4
,—VEGETATION j RUNOFF
III 1t’_ II 1 l. 1r1Ii/1 ,iq’Itiii 1 ”t i irn ‘: i ‘ ji 1 i Ii i I.ll ’ It ’ r 1
INFILTRATION
I
0 VEGETATIVE LAYER
® LATERAL DRAINAGE LAYER
® BARRIER SOIL LAYER
WASTE LAYER
LATERAL ‘ —
DRAINAGE
L SLOPE
PERCOLATION
V
-.
U-
0
LATERAL DRAINAGE LAYER LATERAL DRAINAGE
OTT T T T T T LCO
RRiE .L LA$’.R M L’A S .NC
0
a.
w
z
-J
I
PERCOLATION
ii FIGURE 7.2-5
SOURCE : EPA TITLE : TYPICAL LANDFILL LIQUID
ROUTING PROFILE
:1
I>
—o
U
7—20
-------
experience. To an extent, operating conditions were Btandardized among
facility types. Each facility or unit is comprised of five components (e.g.,
five cells in the case of landfills). Each component is 350 feet wide by 600
feet in length and has an effective area of about 4.8 acres.
The unit operating period for the landfill, and the storage and
disposal surface impoundments is twenty years. In the case of the waste pile
the unit operating period is thirty years. Final closure of each unit occurs
at the end of each unit operating period. With the exception of the storage
surface impoundment, from which the-wastes are removedat closure,-the post—
closure care period is for thirty years after closure.
It should be noted that the presence of a leak detection system is
not only for monitoring purposes, but also serves to collect and remove
leachate. Unless otherwise noted, the operation of the leak detection and
leachate collection systems are assumed to cease at the end of the post—
closure care period. This is not meant to imply systems failure, but rather,
the termination of pumping activities to remove collected volumes. Thus, an
increase in head within: these systems will result.
A discussion of facility—specific operating conditions is given in
the following paragraphs. A comprehensive summary of operating conditions
for each facility type is provided in Table 7.2—5.
- 7—21
-------
TABLE 7.2-5
SUMMARY OF OPERATING CONDITIONS
LANDFILL
Cell Area — 4.8 acres
Cell Dimensions — 350 ft. X 600 ft.
Number of Cells — 5
Unit Operating Period — 20 years
Cell Operating Period — 4 years
Subcell Operating Period — 1 year with 3—month overlap on next subcell
Condition of Waste — moisture content at field capacity
Closure — AU cells closed after 20 years
Post—Closure Care Period — Runs for 30 years after closure
Leachate Collection System — stops operating 30 years after unit is closed*
Leak Detection System — stops operating 30 years after unit is closed*
SURFACE IMPOUNDMENT - DISPOSAL
Surface Impoundment Area — 4.8 acres
Surface Impoundment Dimensions — 350 ft. X 600 ft.
Number of Surface Impoundments — 5
Average Depth of Liquid — 7 ft. during operating period
Unit Operating Period — 20 years
Closure — all impoundments closed after 20 years
— waste dewatered using cement—based process
— stabilized waste has minimum strength to support final cover system
— stabilized waste has moisture content at field capacity
Post—Closure Care Period — Runs for 30 years after closure
Leak Detection System — stops operating 30 years after unit is closed
SURFACE IMPOUNDMENT - STORAGE
Surface Impoundment Area — 4.8 acres
Surface Impoundment Dimensions — 350 ft. X 600 ft.
Number of Surface Impoundments — 5 -
Average Depth of Liquid — 7 ft. during operating period
Unit Operating Period — 20 years
Closure — all waste removed
WASTE PILE
Waste Pile Area — 4.8 acres
Waste Pile Dimensions — 350 ft. X 600 ft.
Number of Waste Piles — 5
Height of Waste Piles — 15 ft.
Unit Operating Period — 30 years
Closure — all waste piles closed as landfills
Post—Closure Care Period — runs for 30 years after closure
Leachate Collection System — stops operating 30 years after unit is closed
* As discussed in Section 7.3.3.1, under certain design cases the leachate
collection and the leak detection systems were assumed to operate through-
out the modeled period.
7—22
-------
7.2.4.1 Landfills
As noted, the landfill facility is comprised of five (5) components
(i.e., cells). Each cell is in turn composed of five sub—cells. Each sub—
cell has an operating period of one year, three months of which overlap onto
the next sub—cell. This equates to a four—year operating period for each cell
and, hence, a total of twenty years for the entire unit. Placement of a final
cover on each sub—cell occurs at the end of the sub—cell operating period
(i.e., one year). The placement of the cover is assumed to occur immediately
upon reaching that milestone date. Thus, after the first year of operation
and until the end of the twenty—year unit operating period, the facility is in
a partial closure mode. At any given point in time during this period the
maximum area lacking a cover system is equivalent to about one and one—quarter
sub—cells. Final closure of the facility occurs at the end of the last sub—
cell operating period, that is, at the end of year twenty.
The post—closure care period runs for thirty years from the date of
closure of each cell. Finally, it is assumed that for the entire duration of
the model run the moisture content of the waste is at field capacity.
7.2.4.2 Disposal Surface Impoundments
Operation of the disposal surface impoundment facility begins on
day one and runs continuously at full capacity through the end of year twenty
at which time closure occurs. It is assumed that in all five lagoons, a
constant depth of seven feet of liquid is maintained throughout the operating
period. At closure, no lag time is assumed between the end of operation and
7—23
-------
the start of post—closure care. Also at closure, all five lagoons are de—
watered by means of a cement—based mixing process and a final cover is em-
placed. The stabilized waste has the minimum strength to support the cover
system, and the moisture content of the waste is assumed to be at field
capacity. The thirty—year post—closure care period runs through the end of
year fifty. Where present, the leak detection system is assumed to stop
operating at the end of the post—closure care period.
7.2.4.3 Storage Surface Impoundments
During the unit operating period, conditions are identical to those
described above for the disposal surface impoundment facility. However, at
closure all of the waste is removed from the unit. s above, closure occurs
instantaneously at the end of the twenty year operating period.
7.2.4.4 Waste Piles
The waste—pile facility operates for thirty years, in contrast to
the twenty—year operating period for the other three facility types. The
facility runs at full capacity throughout the operating period, that is, each
of the five 4.8—acre waste pile components maintains a constant pile height of
fifteen feet throughout the operating period. At closure, the wastes cannot
be removed and the facility is immediately closed as a landfill, including
placement of a final cover system. The thirty—year post—closure care period
runs through the end of year sixty at which time it is assumed that the
leachate collection system stops operating. As before, the moisture content
of the waste is at field capacity.
7—24
-------
Some of the cap/liner designs for the different facility types (as
discussed in Section 7.2.3) make use of a double barrier consisting of a
flexible membrane liner (FML) underlain by clay. The permeabilities of the
clays used in the barrier system are given (see Tables 7.2—1 through 7.2—4)
and are assumed to remain constant throughout the modeling period. The
seepage rate through the FZI I is a function of the mode of FML failure. The
following failure modes were evaluated.
FML Failure Modes for Landfills
1. No change in liner for 25 years after installation, then cata-
strophic failure
2. No change in liner for 25 years after installation, then 0 to
100% failure over next 25 years in a step function
3. No change in liner for 50 years after installation, then cata-
strophic failure
4. No change in liner for 50 years after installation, then 0 to
100% failure over next 50 years in a step function
5. No change in liner for 150 years after installation, then cata-
strophic failure
6. Fixed seepage due to installation/operational problems; 1% of
the liner area has the same vertical permeability as the under-
lying bedding layer.
7. Fixed seepage due to installation/operational problems; 0.1% of
the liner area has the same vertical permeability as the under-
lying bedding layer.
FML Failure Modes for Surface Impoundments and Waste Piles
1. No change in liner for 25 years after the start of operation,
then catastrophic failure.
2. No change in liner for 25 years after the start of operation,
then 0 to 100% failure over the next 25 years in a step function.
3. No change in liner for 50 years after the start of operation,
then catastrophic failure.
7—25
-------
4. No change in liner for 50 years after the start of operation,
then 0 to 100% failure over the next 50 years in a step function.
5. No change in liner for 150 years after the start of operation,
then catastrophic failure.
6. Fixed seepage due to installation/operational problems; 1% of
the liner area has the same vertical permeability as the under-
lying bedding layer.
7. Fixed seepage due to installation/operational problems; 0.1% of
the liner area has the same vertical permeability as the under-
lying bedding layer.
An additional FML failure mode was assumed for the disposal surface
impoundment; F? failure mode”O” assumes complete FML failure throughout the
operating period. Thereafter it is assumed to exhibit the same response as
FML failure mode 1.
The ranges in FML failure mode assumptions were based on currently
available FML lifetime data as presented in Expected Life of Synthetic Liners
and Caps (A.D. Little, 1983).
Additional HELP model input values used in the facility design
analysis are presented in Tables 7.2—6 and 7.2—7.
7.2.5 Scenario Identification System
A coded system was developed by the EPA to identify each of the
facility scenarios modeled. The codes are a function of facility type and
level of waste control. For consistency, this approach was also used to
reference scenarios for the clay cap/liner designs, addressed in Interim
7—26
-------
Report I, and with some modification, for the subsequent ground—water fate and
transport analysis.
The following format is used:
o The first two characters (i.e., letters) refer to the type of fa-
cility as follows:
LF = Landfill
SD = Surface Impoundment—Disposal
SS = Surface Impoundment—Storage
WP = Waste Pile
o The following one or two characters (i.e., Roman numerals) refer
to the level of waste control (refer to Tables 7.2—1 through
7.2—4).
o If a letter (e.g., A or B) is present after the Roman numeral,
this refers to one of the multiple cases of liner design given in
Tables 7.2—1 through 7.2—4.
o If an Arabic number is present at the end of the code, this
refers to the F failure mode (i.e., 1 through 7) as listed in
Section 7.2.4. Obviously, this applies only to those scenarios
which employ an F in addition to the clay layer in the final
cover design.
For example, Code LFIIA identifies a landfill with level of waste
control hA as described in Table 7.2—1. Addition of the number “1” to the
end of the code (i.e., LFIIA1) refers to the FML failure mode. Reference to
the Operating Conditions (Section 7.2.4) describes failure mode 1 as “no
change in liner for 25 years after installation then catastrophic failure.”
7—31
-------
7.3 RESULTS OF HELP SIMULATIONS
7.3.1 General
The following discussions summarize the results of the HELP simu-
lations with respect to climate and facility type considerations. General
trends of the seepage profiles and comparison with respect to design factors
are addressed. It should be noted that.the term “final mean seepage rate”
refers to the average rate of seepage through the bottom barrier layer
predicted to occur after the end of the post—closure care period.
7.3.2 Climatic Effects
In general, the three climatic conditions do not alter the trend of
the facility release profiles but rather the magnitude of the seepage rates.
This holds true for all of the facility types and operational conditions with
the exception of surface impoundments during the operating period. For the
latter cases, the climate has no effect on either the trend or the magnitude
of the seepage rates. The reason for this is the assumption that the liquid
level remains constant at seven feet during the operating period and, there-
fore, the driving force of seepage production is independent of rainfall
quantities. After closure, the climate effects the magnitude of seepage
production without altering the trend of the release profile.
In light of the above, discussion herein shall focus on the landfill
facility design cases to exemplify general climatic trends.
7—32
-------
Figure 7.3—1 graphically compares the release profiles for the
three climate types for landfill design case LFI].. The figure shows the
change in seepage rates over time, where seepage rates refer to the perco-
lation of leachate through the bottom liner or barrier layer. The trends
exhibited by the design cases will, of course, vary dependent upon FML failure
mode. However, for a given design case with the same F? . failure, the trends
will be similar.
Table 7.3—1 summarizes the seepage rates for each landfill design
case at select model years for the three climates.
The Denver climate generally yielded the lowest simulated seepage
rates for each design case after FML failure, when compared with the Hartford
and New Orleans climates. Hartford generally yielded the next lowest rates,
followed by New Orleans, as expected. This is shown by comparing the seepage
rates between climates for a given design and FML failure mode (see Table
7.3—1). Of course, prior to F failure the seepage rates for all of the
climates would be zero. It is interesting to note that for cases LFI and LFII
the seepage rates are always about equal under a given climate type. When
making comparisons to the above Table, the reader should note that for design
case LFIIA1 under the Hartford and Denver climates, the assumption has been
made that post—closure care activities are continued throughout the modeled
period. This is in contrast to case 1211 for all three climates, and LFIIA1
for the New Orleans climate where it is assumed that post—closure care activi-
ties end at year 50.
7—33
-------
FIGURE 7.3-1
LANDFILL CLIMATE COMPARISONS
LEVEL Ii
100
Legend
P41W O1LIAI4$
HAiTFOXD
D*KVII
I C
3
LU
2.5
1.5
LU
0 .
0,
LU
I
C.,
z
0,
LU
LU
a
IC
a.
U i
LU
a,
-J
IC
z
z
IC
z
LU
1
/
0.3
0
0 25 50 75
YEARS
SOURCE: EPA
-------
TABLE 7.3—1
YEAR
LFI1
SUMMARY OF MEAN SEEPAGE RATES
BY LANDFILL DESIGN
FOR SELECTED MODEL YEARS
(NEW ORLEANS CLIMATE)
MEAN SEEPAGE RATE
(INCHES/YEAR)
LFI 6
LFI2
LFI 3
LFI IA1
LFIIA3
LFVI
12
20
37
50
75
0.000
0.000
0.904
1.561
1.566
0.000
0.000
0.232
0.966
1.561
NEW ORLEANS CLIMATE
0.000
0.000
0.808
1.280
1.280
0.000
0.000
0.000
0.000
1.579
9.527
13.328
13.328
13.328
13.328
0.000
0.000
0.000
0.000
1.566
0.453
0.015
0.015
0.015
0.012
12
20
37
50
75
0.000
0.000
0.732
1.151
1.151
0.000
0.000
0.172
0.719
1.151
HARTFORD
CLIMATE
0.000
0.000
0.405
0.641
0.641
0.000
0.000
0.000
0.000
1.153
7.021
9.984
9.984
9.984
9.984
0.000
0.000
0.000
0.000
1.151
0.396
0.011
0.011
0.011
0.012
12
20
37
50
75
0.000
0.000
0.196
0.338
0.400
0.000
0.000
0.051
0.210
0.338
DENVER
CLIMATE
0.000
0.000
0.148
0.229
0.229
0.000
0.000
0.000
0.000
0.321
0.850
0.947
0.947
0.947
0.947
0.000
0.000
0.000
0.000
0.400
0.150
0.005
0.005
0.005
0.038
-------
With the exception of FML failure nodes 6 and 7, which assume a
fixed seepage rate (see Section 7.2.4), the final seepage rates for all LFI
and LFII scenarios were about equal for a given climate type. A brief summary
of the final seepage rates under the three climates for selected designs are
shown below:
FINAL SEEPAGE RATES (INCHES/YEAR)
New Orleans Hartford Denver
LFI3 1.56 1.15 0.40
LFIIA3 1.57 1.15 0.32
LFVI 13.32 9.98 0.97
As shown above, the variability between the design cases for a given
climate is insignificant. The release rates observed for the Hartford and New
Orleans climates were similar, with the New Orleans rate about 35 percent
higher. The release rates for the Denver climate were about 4.3 times lower
than the New Orleans rates. The lower seepage rate observed for the Denver
climate results from the lower precipitation and higher evapotranspiration
(ET) rate.
Perhaps, the scenario which best illustrates the significance of
climate in reducing seepage rates is design LFVI. This case represents “free
infiltration” and, as can be seen above, the variability between climate types
is significant. It is interesting to note that the final seepage rate of case
LFV’I for the Denver climate was lower than the final seepage rates for all
case LFI and LFIIA scenarios (assuming post—closure care activities end at
year 50 and excluding scenarios with FML failure nodes 6 and 7).
7—36
-------
7.3.3 Effects of Facility Design and Operating Conditions
7.3.3.1 Landfills
Three landfill designs were evaluated in this study. Design cases
LFI and LFIIA are identical except for the bottom liner design. Design LFI
has a double 30—mu FilL liner system; design LFIIA has a double bottom liner
consisting of an upper 30—mu FML underlain by two feet of clay. For compari-
son, as a worst—case scenario, design LFVI does not have a cap or liner and is
considered to represent a free infiltration case.
Assuming that the FML failure mode is the same, similar trends were
generally observed for the release profiles of a given design regardless of
climate type. The climate affects the magnitude of the seepage rates, as
discussed in the previous section. The timing of the release rates are a
function of the FML failure mode. Figures 7.3—2, 7.3—3 and 7.3—4 plot the
mean annual seepage rates (in inches per year) for the various design cases
for the New Orleans, Hartford and Denver climates, respectively. Table 7.3—1
summarizes the mean annual seepage rates for the design cases for select model
years. The years were selected to afford comparison of seepage rates under a
range of operating conditions, as follows:
o Year 12 — mid—way through the operating period;
o Year 20 — end of the operating period;
o Year 37 — mid—way through the post—closure care period;
o Year 50 — end of the post-closure care period; and
o Year 75 — final seepage rates.
7—37
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FIGURE 7.3-2
COMPARISONS OF LANDFILL DESIGN AND
NEW ORLEANS CLIMATE
SEEPAGE
Legend
LFII
LFI2 -
LFI3
LFI6
LFIIA1
. .-0000
LFIIA3
.—. . U
w
U i
Q.
C l )
U i
I
0
z
(1)
U i
I-
w
C.,
a-
w
Ui
U,
-J
z
z
z
U i
3
2.5
2
1.5
I
0.3
0
/4 LFI3 LFUA3
0
/
25
YEARS
50
100
SOURCE: EPA
-------
FIGURE 7.3—3
COMPARISONS OF LANDFILL DESIGN AND SEEPAGE
HARTFORD CLIMATE
3
2.5
2
1.5
I
0.5
0
YEARS
i (
LU
>.
Ui
0.
C l )
LU
I
C.)
z
U i
I-
LU
C,
0.
LU
LU
C l)
-J
z
z
z
LU
Legend
LFI I
LFI2
LFI3
LFI
LFIIA1
• S
LFIIA3
—p
SOURCE: EPA
0 25 50
-------
FIGURE 7.3—4
COMPARISONS OF LANDFILL DESIGN AND
DENVER CLIMATE
SEEPAGE
Legend
LFII
LFT3
LFI6
LFTLA1
- E) p -
LPUA3
1.50
Lu
Lu
a,
Lu
I
(3
z
w
I-
Lu
C,
Q.
Lu
Lu
a,
-I
z
z
z
Lu
1.25
I
0.75
0.50
0.25
0
0 25 50 75 100
YEARS
SOURCE: EPA
-------
The timing of the release rates. are a function of the FML failure
mode assumption employed (see Section 7.2.4). F? fl. failure modes 1, 3 and 5
undergo catastrophic failure at model years 25, 50 and 150, respectively. The
trends for the release rates are similar; only the timing varies. Similar
trends are also exhibited between FML failure modes 2 and 4 which undergo
complete failure in a step function starting at model years 25 and 50, re-
spectively. Finally, FML failure modes 6 and 7 exhibit fixed seepage rates
over the model period. Each FML failure type shall be addressed with dupli-
cation kept to a minimum.
It is assumed that seepage is not produced until FML failure occurs.
In the case of LFI1, seepage production began at year 25 and increased at a
sharp and steady rate until stabilizing at year 45. Generally, this rate
remained constant for the balance of the model run and there was no change in
the seepage rate in response to termination of post—closure care activities at
year 50.
As with case 1211, case LFIIA1 also did not show any seepage pro-
duction until year 25 when catastrophic failure occurred. However, case
LFIIA1 showed post—closure seepage rates about 1.2, 1.8 and 1.5 times lower
than comparable case LFI1 post—closure care seepage rates for the New Orleans,
Hartford and Denver climates, respectively. As all other design variables are
the same, the lower rates are believed to be related to the greater efficiency
of leachate collection resulting from the presence of a clay component in the
LFIIA bottom liner design. Under normal operating conditions (see Table
7.2—5), the operation of the leachate collection and leak detection systems is
terminated at the end of the post—closure care period. Given this assumption,
7—41
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the seepage rates for case LFIIA1 should show a sudden increase after year 50
prior to achieving a stable final seepage rate. For example, case LFIIA1
under the Hartford climate would yield a final seepage rate of about 1.2
inches/year, as exhibited by case LFIIA3. However, in contrast to the normal
operating conditions, the operation of the leachate collection and leak
detection systems is assumed to continue throughout the modeled period for the
case LFIIAI scenarios. As such, they maintain the same seepage rates as
during the post—closure care period; that is, about 1.2, 0.6 and 0.2 inches/
year for the New Orleans, Hartford and Denver climates, respectively.
The release profile for F? failure mode 2 shows a more gradual and
variable increase than for the catastrophic failure modes. Seepage pro-
duction begins at about year 27 and eventually stabilizes at about year 70.
Due to the presence of a clay component in the LFIIA2 design, a slightly lower
rate of increase would be expected during the post—closure care period as
compared with LFI2. This would be due to a more effectively operating
leachate collection system in the former case. Both scenarios would achieve
about the same final seepage rates about year 70.
FML failure modes 6 and 7 represent the only scenarios which showed
seepage production for the first 25 model years. Both failure modes cal]. for
a fixed seepage rate. FML failure modes 6 and 7 assume that 1.0 percent and
0.1 percent of the liner area has the same vertical permeability as the
underlying bedding layer. The trend exhibited by both failure modes will be
the same with the latter significantly lower than the former. Discussion
herein shall refer to FML failure mode 6. Basically, a “saw—tooth pattern
emerges over the 20—year operating period. This pattern is a result of the
7—42
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sequential manner in which the landfill sub—cells are closed (see Section
7.2.4). At final closure (year 20), the seepage rates decrease sharply to
virtually a zero rate and generally remain constant thereafter. A reduction
of about 10 percent was observed in design case LFIIA6 as compared with case
LFI6 during the operating period (see Appendix G of the aforementioned Ertec
memorandum). This is a result of the presence of the clay bottom liner
component for case LFIIA.
With the exception of FML failure modes 6 and 7 and the assumption
that post—closure care continues for any of the LFIIA scenarios after year 50,
all of the design cases eventually achieve about the same final seepage rate
for a given climate. These final rates are about 1.5 inches/year for New
Orleans, 1.1 inches/year for Hartford, and between 0.3 to 0.4 inches/year for
Denver. As noted earlier, if post—closure care activities are continued for
the duration of the modeled period for applicable cases under design LFIIA,
the seepage rates would appear to be roughly one—half of those shown above.
For design scenarios assuming FML failure modes 6 and 7, the post—operating
seepage rates are essentially zero.
Design LFVI represents a free infiltration” scenario. A detailed
evaluation of the seepage trend of this scenario is addressed in the companion
report for clay containment systems. Comparison of the final seepage rates
for the LFI and LFIIA designs with those of LFVI indicate a significant degree
of leachate containment for each climate type.
7—43
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7.3.3.2 Disposal Surface Impoundments
The surface impoundment liquid depth is the main driving force for
seepage production during the operating period, rather than precipitation.
This is the critical period for leachate production. As the effect of climate
is negated, the following discussion is limited to the New Orleans climate.
Table 7.3—2 summarizes the mean annual rates for the various design
cases for select model years. These years were chosen to allow comparison of
seepage rates by design case under the range of operating conditions,as
follows:
o Year 20 — last year of operating period;
o Year 21 — start of post—closure care period;
o Year 37 — mid—way through post—closure care period; and
o Year 51 — final seepage rates.
Figure 7.3—5 graphically displays the mean annual release rates for
the various design cases over the first 100 years modeled.
The two disposal surface impoundment designs are similar to those
discussed in the previous section for landfills. However, for the impound-
ments there is no leachate collection system. Both designs have a double
bottom liner system; Design SDI uses two 30—mil Fr s, while SDII uses a 30—mu
FML underlain by two feet of clay.
7—44
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ThBLE 7.3—2
SUMMARY OF MEAN SEEPAGE RATES
BY DISPOSAL SURFACE IMPOUNDMENT DESIGN
FOR SELECTED MODEL YEARS
(NEW ORLEANS CLIMATE)
MEAN SEEPAGE RATE
(INCHES/YEAR)
YEAR SDI1 SDI3 SDI6 SDI7 SDIIAO SDIIA1 SDIIA6
20 0.000 0.000 210.000 3.400 5.580 0.000 0.060
21 0.000 0.000 0.000 0.000 0.000 0.000 0.000
37 1.562 0.000 0.024 0.000 0.755 0.755 0.000
51 1.564 1.564 0.023 0.000 1.365 1.365 0.000
-------
FIGURE 73-5
COMPARISONS
uJ
>.
Ui
a.
a,
Ui
x
C.,
z
a)
w
I-
Ui
0
a.
U i
Ui
0)
-J
z
z
z
I C
Ui
215
210
10
5
0
OF DISPOSAL SURFACE
DESIGN AND SEEPAGE
NEW ORLEANS CLIMATE
YEARS
IMPOUNDMENT
LEGEND
soil
SDI8.
SDIT
SD tA0
.u u•.
SDIIA6
100
SOURCE: EPA
3:
0 25 50 75
-------
The timing of the release rates are a function of the FML failure
mode. As the critical period for leachate production is the 20—year operating
period, the timing of the release will also indirectly determine the magnitude
of release. With the exception of FML failure modes 6 and 7, all of the
scenarios for designs SDI and SDIIA show zero seepage production during the
operating period. This is because the FMLs did not fail until after the
operating period was over. In contrast, FML failure modes 6 and 7 were
subject to a fixed seepage rate throughout the modeled period, including the
operating period. Under the constant seven—foot liquid depth assumed during s
facility operation, design cases SDI6 and SDI7 yieldedconstant seepage rates
of about 210 and 3.4 inches/year, respectively. Design cases SDIIA6 and
SDIIA7, by comparison, only established seepage rates of about 0.06 inches/
year and essentially zero, respectively. The disparity in seepage production
between design cases Sf16 and SDI7 and those of SDIIA6 and SDIIA7 reflect the
different bottom double—liner systems. The lower values for the latter cases
result from the presence of the clay component in the bottom liner system. To
further illustrate the benefit of a clay component in the double bottom liner,
an additional FML failure mode was assumed, that is, SDIIAO. Under this
scenario, the FML is assumed to completely fail throughout the operating
period. The resulting seepage rate (about 5.6 inches/year) was still sigriifi—
cantly less than for case SDI6.
At closure, the liquid waste is solidified and the main driving
force for seepage production now becomes precipitation. In response to cata-
strophic F1I I failure, case Sf11 shows a marked increase at year 25, before
stabilizing at a final seepage rate of about 1.5 inches/year. There was no
change in the seepage rate in response to the termination of post—closure care
7—47
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activities at year 50. The same final rate was achieved for case SDI3; only
the timing of the release varies.
Case SDIIA1 also showed an increase at year 25 in response to
catastrophic F?4L failure. However, the seepage rate yielded during the post—
closure care period was about 0.75 inches/year. This is roughly one—half the
seepage rate for case SDI1. The reason for the lower seepage rate for case
SDIIA1 is the presence of the clay component in the bottom liner system. At
the end of the post—closure care period, the seepage rate for SDIIA1 increases
to a final level of about 1.4 inches/year. This is in response to the
termination of the leak detection system operation. Although not presented in
Table 7.3—2, case SDIIA3 would result in the same final seepage rate as for
SDIIA1.
At closure, the design cases exhibiting FML failure modes 6 and 7
essentially achieve zero final seepage rates. These sudden (and for case of
SDI6, dramatic) decreases in seepage rates are a result of the decrease in the
head from the constant seven—foot liquid depth during operation, and the
placement of the final cover system. For case SDIIAO, the seepage rate also
decreases at closure to zero. Thereafter, it is assumed to exhibit FML
lailure mode 1. Thus, beginning at year 25, case SDIIAO is identical to case
SDIIA1, as described above.
7.3.3.3 Storage Surface Impoundment
As with the disposal impoundment facility, the driving force for
seepage production during the operating period is the constant seven—foot
7—48
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liquid depth. The climatic conditions were not observed to alter the magni-
tude or trend of seepage during the operating period. As such, discussion
will be limited to the New Orleans climate type.
Table 7.3—3 summarizes the mean annual seepage rates for cases SS16
and SSI7. Model year 20 represents the last year of operation. Model year 21
reflects the final seepage rate. Figure 7.3—6 displays the seepage profiles
for these cases.
Only one design was selected by the EPA for evaluation herein. This
design incorporates a single 30—mu F? in the bottom liner. There are no
leachate collection or leak detection systems.
As with the disposal impoundment facility, the critical period for
leachate production is during facility operation. At closure, the waste and
liner are assumed to be removed; thus, the seepage rates shown after year 20
are not reflective of leachate rates.
Cases SSI1 through SSI5 had zero seepage production during the
operating period as the FML was functional for at least 25 years. Only cases
SSI6 and SSI7, which assume fixed seepage rates, are of concern. The
operating seepage rates for cases $5 16 and SSI7 were about 186 and 18.6
inches/year, respectively. Although not specifically addressed for this
facility type, the presence of a clay liner below the FML liner would reduce
the rate of seepage production to about 0.06 inches/year and zero, as observed
in cases SDIIA6 and SDIIA7, respectively.
7—49
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TABLE 7.3-3
SU RY OF MEAN SEEPAGE RATES
BY STORAGE SURFACE IMPOUNDMENT DESIGN
FOR SELECTED MODEL YEARS
(NEW ORLEANS CLIMATE)
MEAN SEEPAGE RATE
(INCHES/YEAR)
YEAR SSI6 5517
20 186.000 18.600
21 13.238 13.238
7—50
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FIGURE 7.3—6
COMPARISONS OF STORAGE SURFACE
IMPOUNDMENT DESIGN AND SEEPAGE
190 NEW ORLEANS CLIMATE
- S I •
16o-
0.
U,
LU
I
a
z
17O
U i
I -
‘C
U ’
0
‘ C20-
a. _________
I L ’
U i
U,
IC p * S S S S
Legend
IC SSI6
z w. S
‘C 8S17
• I
o 25 50 75 100
YEARS
SOURCE: EPA
-------
At closure, the waste and liner are removed and final seepage rates
stabilize at about 13 inches/year. These seepage rates are included to
represent free—drainage infiltration, consistent with the rates observed in
LFVI. Again, these rates are not ].eachate rates.
7.3.3.4 Waste Piles
Only one waste pile design was evaluated in this study. Design WPI
is similar to design LFI with the exception of a single FML liner and lack of a
leak detection system in WPI.
As with the landfill scenarios, climate was not found to affect the
seepage trends but, rather, the magnitude of the seepage rates. The effects
of climate were discussed in Section 7.3.2. Discussion herein shall be
limited to the New Orleans climate for illustration. Table 7.3—4 gives a
sununary of seepage rates for select model years, as follows:
o Year 30 — last year of operating period;
o Year 31 — start of post—closure care period; and
o Year 61 — post—care final seepage rates.
Figure 7.3—7 plots the mean annual seepage rates for the design
cases discussed herein over time.
In preface, FML failure modes 1 and 2 (see Section 7.2.4) were not
considered applicable for this analysis as closure does not begin until after
year 30, and PML failure modes 1 and 2 are assumed to occur at year 25. This
7—52
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TABLE 7.3—4
SUMMARY OF MEAN SEEPAGE RATES
BY WASTE PILE DESIGN
FOR SELECTED MODEL YEARS
(NEW ORLEANS CLIMATE)
MEAN SEEPAGE RATES
(INCHES/YEAR)
YEAR WPI3 WP16
30 0.000 37.295
31 0.000 0.012
61 1.565 0.012
7—53
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FIGURE 7.3—7
40
30
3
2
COMPARISONS
w
w
Q.
U)
w
U
z
I L ’
I-
w
C
0 .
w
w
U)
-J
z
z
z
w
I
0
OF WASTE PILE
NEW ORLEANS
DESIGN AND
CLIMATE
SEEPAGE
0
YEARS
LeEend
WPI3
wPI6
— — —
75 100
SOURCE: EPA
-------
evaluation shall focus on FML failure modes 3 and 6, reflective of cata-
strophic FML failure at year 50, and fixed seepage throughout the modeled
period, respectively.
During the 30—year operating period, cases WPI6 and WP17 yielded
significantly higher seepage rates than those yielded by comparable landfill
cases LFI6 and LFI7, respectively. For example, the operating seepage rate
for WP16 was about 37.3 inches/year, as compared with about 0.453 inches/year
for LFI6. This is a result of the different operating conditions assumed.
That is, the five waste pile components are assumed to operate concurrently
rather than sequentially as for the landfills. Thus, during the operating
period, all five waste piles are open and contributing seepage. At closure,
the seepage rates for WPI6 and WPI7 decreased sharply to about zero seepage as
a result of final cover placement. This is the same trend as was observed for
the comparable landfill scenarios.
Case WPI3, by definition of the FML failure mode, does not produce
seepage until year 50. At this time, in response to catastrophic F? failure,
the seepage rate increases sharply-and stabilizes with a final seepage rate of
about 1.5 inches/year. This is the same final seepage rate as yielded for I FI a
and LFIIA designs (with the exception of FML failure modes 6 and 7).
7—55
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7.4 CONCLUSIONS
7.4.1 Landfills
In general, designs LFI and LFIIA effectively contain the rate of
seepage production during the modeled period. This is shown by comparing the
seepage rates for these designs with those seepage rates reflecting free
infiltration under design LFVI. Refer to Table 7.3—1 for comparison.
The trends of the seepage-profiles for- designs-LFI and LFIIA are
generally similar for a given FML failure mode assumption. A notable
exception to this general rule regards cases LFIIA1 and LFI1 during the post—
closure care period. During this period case LFIIA1 yielded seepage rates
approximately 1.2, 1.8 and 1.5 times lower than comparable seepage rates for
case LFI1 under the New Orleans, Hartford and Denver climates, respectively.
This is a result of the more efficiently operating leachate collection and
leak detection systems for design case LFIIA1.
The efficiency -of the leachate collection and leak detection
systems to collect and remove leachate is determined by comparing the post—
closure care period seepage rates with the final seepage rates for each
design. As previously noted, it was assumed that post—closure care activities
(i.e., leachate collection system and leak detection system operation)
continued throughout the modeled period for case LFIIA1. Therefore, to
properly assess the effectiveness for leachate collection under design LFIIA,
it is necessary to compare the post—closure care period seepage rates for case
LFIIA1 with the final seepage rates for case LFIIA3 (see Table 7.31). The
7—56
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results indicate that the post—closure care period seepage rates are about 20,
45 and 30 percent lower than the final seepage rates for design LFIIA under
the New Orleans, Hartford and Denver climates, respectively. In contrast,
there was no change observed when comparing the post—closure care period
seepage rates and final seepage rates for case LFI1. This implies that the
leachate collection and leak detection systems are ineffective in reducing
seepage rates for design LFI.
The data implies that the efficiency of the leachate collection and c
leak detection systems is largely controlled by the bottom liner system. For
case LFI1 the double FML8 had failed and, as such, did not inhibit vertical
percolation. On the other hand, the clay component in the double bottom liner
for case LFIIA restricted downward percolation, thereby allowing the leachate
collection and leak detection systems to operate more effectively. This
illustrates an advantage to incorporating both synthetic and clay barriers in
the bottom liner design. This also suggests that the effectiveness of the
leachate collection and leak detection systems is interrelated to the permea-
bility of the underlying barrier liner.
The increase in seepage production after post—closure care activi-
ties are terminated underscores an additional implication. That is, the
continuation of the leachate collection system and leak detection system
operation beyond the post-closure care period for LFIIA]. will reduce the final
seepage rates. As noted above, the HELP results indicate that a reduction in
seepage rates ranging from 20 to 45 percent was achieved for LFIIA]. depending
on the climate type.
7—57
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The final seepage rates for all of the LFI and LFIIA designs which
assume complete FML failure and for which post—closure care activities end
after year 50, were about equal for a given climate type. In the case of the
New Orleans climate the final seepage rate was about 1.5 inches/year. The
con—trolling factor for the final seepage rates is the final cover system. As
complete FML failure is assumed, the final cover system essentially consists
of a 24—inch vegetated layer, a 12—inch drain layer and a two—foot clay
barrier. Thus, the presence of the clay component in the bottom liner for
design LFIIA does not appear to be a significant factor in reducing the rate
of seepage production (providing post—closure-care activities have ended).
The ability to limit leachate production is a function of the FML
failure mode assumption used. Obviously, design cases which assume a longer
FML operational life expectancy are relatively more desirable. With the
possible exception of FML failure mode 5 which assumes zero seepage through
year 150, cases which assume FML failure modes 6 and 7 probably reflect the
second and first best scenarios for seepage containment, respectively.
Whereas specific assumptions were made regarding the behavior of PMLs in this
analysis, many factors can effect an- FML’s effectiveness and life expectancy.
The reader should refer to A.D. Little (1983) for a more comprehensive assess-
ment of these factors.
The significance of the different design scenarios in terms of
limiting seepage production would appear to be greater for the New Orleans and
Hartford climates rather than for the Denver climate. The final seepage rate
produced under the free infiltration” case LFVI for Denver was less than that
yielded by the best design case for the other climates. Thus, consideration
7—58
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must be given to what level of control is needed based on locational factors.
The data suggest that a less stringent design would be required to effect an
adequate level of ]eachate control for a semi—arid region, such as Denver.
However, consideration must also be given to the relatively high value of the
ground—water resources in such a region.
7.4.2 Disposal Surface Impoundments
The results strongly demonstrate the benefit of a bottom double
liner design which incorporates an FML component underlain by a clay com-
ponent, as compared with a design which uses two FMLs. During the critical
operating period, design SDI6, which represents the former case described
above, resulted in a very high seepage rate of about 210 inches/year. In
contrast, design SDIIA6, which represents the latter case described above,
had a seepage rate of less than 0.1 inches/year. In effect, the fixed seepage
rate assumed for the FML5, in conjunction with the considerable head exerted
by the liquid level in the impoundment, essentially negated the effectiveness
of the leak detection system to collect and remove leachate. In contrast, the
presence of a low permeability (1 X l0 cut/sec) clay liner under the hhleakyu
FML liner effectively inhibits vertical percolation and allows the leak
detection system to collect and remove the great majority of the seepage
produced. To underscore the value of the bottom clay component, design case
SDIIAO was simulated to show the maximum seepage rate which would result from
complete FML failure during operation. This seepage rate was about 5.6
inches/year; that is, still significantly below the operating seepage rates
for SDI6. This suggests that surface impoundment designs should incorporate a
7—59
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combination FML/clay double bottom liner in conjunction with a leak detection
system to effectively contain leachate migration through the bottom liner.
After closure, the facility follows similar seepage trends, and
establishes similar seepage rates as described for the landfill scenarios in
the previous section. As such, post—closure seepage rates will not be ad-
dressed herein.
As the operating period is the most critical stage for leachate :
production, and as the designs appear to yield similar final seepage rates,
locational factors associated with climate do not appear to have a significant
impact on design considerations.
7.4.3 Storage Surface Impoundments
The only storage impoundment design considered by the EPA had a
single FP bottom liner and did not include leachate collection or leak
detection systems.
As shown in Table 7.3—3, the operating seepage rate for case SSI6
(about 186 inches/year) was an order—of—magnitude greater than the operating
seepage rate for case SSI7 (about 18.6 inches/year). This is directly pro-
portional to the order—of—magnitude differences assumed for the respective
FNL failure modes (see Section 7.2.4). These results suggest that a single
FML. design is inadequate to contain leachate production during the critical
operating period. In light of the negligible effect of the leak detection
system for double F design case SDI6, as discussed in the previous section,
7—60
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the presence of such a system for case SSI6 will not Improve leachate con-
tainment. The data imply that incorporation of a double bottom liner system
using a combination of FML/clay barriers and a leak detection system (as
assumed for design SDIIA) is warranted to effectively control seepage pro-
duction during operation.
The post—closure seepage rates are not ].eachate rates and, there-
fore, are not significant.
7.4.4 Waste Piles
The only waste pile design simulated in this study had a single FML
bottom liner and a leachate collection system. The high seepage rates for
case WPI6 (about 37 inches/year) during the operating period suggests that
this design may not provide effective leachate containment. As the final
cover is not placed on the facility until after year 30, the factor con-
trolling seepage production during the operating period is the single F
bottom liner. The single FML liner does not inhibit vertical percolation
sufficiently to permit effective leachate collection and removal. This
design would provide a reasonably good degree of leachate containment under
landfill operating conditions, as final cover placement is assumed to occur in
a sequential manner for the sub—cells during facility operation. flowever, the
lack of a cover during the waste—pile facility operaling period permits a much
greater infiltration rate and, in turn, a greater seepage rate.
As the presence of the clay component (with a permeability of
1 X l0 cm/sec) in design LFIIA was found to increase the effectiveness of
7—61
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leachate collection and removal, a similar effect would be expected for the
waste pile scenarios. Thus, it appears that use of a clay bottom liner (with
a permeability of 1 X lO cm/seC, or less) may be more effective in reducing
seepage rates than would the single FIlL previously noted.
At closure the seepage rate for WPI6 decreased below 0.1 inches/
year. This emphasizes the value of the final cover in reducing seepage
production.
As suggested for the landfill scenarios, the significance of the
various designs for seepage reduction would be expected to be greater for the
humid climates as opposed to the Denver climate. This would imply that a less
stringent design may be adequate to contain seepage for a semi—arid region.
7—62
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8.0 SUMMARY AND CONCLUSIONS
The Environmental Protection Agency issued Interim Final Land Dis-
posal Regulations on July 26, 1982. In response to these regulations the EPA
received several comments regarding the recommended use of synthetic liner
and cap materials. This report provides a summary of the initial EPA inves-
tigation of the anticipated performance of synthetic liners in hazardous
waste environments. The synthetic material investigation represents one
element of a broad technical investigation conducted by the EPA into the
current hazardous waste containment practices and technology. The program
was directed by the EPA Office of Solid Waste and Office of Research and
Development (Municipal Environmental Research Laboratory). The technical
investigation was performed primarily by Arthur D. Little, Incorporated and
TRW, Incorporated. Coordination and consolidation of the technical material
obtained was conducted by the Earth Technology Corporation (ERTEC) with
support from E.C. Jordan, Incorporated.
8.1 INTRODUCTION
The synthetic liner investigation was initiated by EPA to consc4i-
date available information regarding the suitability of synthetic materials
for hazardous waste containment. The resulting technical data base will be
used by EPA to support further synthetic liner studies, to develop technical
guidance for the design and installation of synthetic barrier systems and to
support further revision of the land disposal regulatory program. The proj-
ect was designed by the EPA to address significant comments received follow-
ing publication of the Interim Final Land Disposal Regulations.
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8.1.1 Study Objectives
The EPA initiated the investigation of synthetic liners and caps in
order to establish the technical adequacy of the Interim Final Land Disposal
Regulations in light of the technical comments received. The specific goals
identified to achieve the project objectives are as follows:
o Establishment of a cdinplete base of currently
avail-able information regarding synthetic liner
systems.
o The development of specific recommendations for
rregulatory reform regarding the use of synthetic
liners at hazardous waste facilities.
o The identification of further technical guidance
materials required to assist synthetic liner system
operators, designers and constructors, and synthe-
tic material manufacturers.
o Provide recommendations for further EPA research to
improve the available synthetic material and liner
system data base.
o Assess the ability of synthetic materials to con-
tain hazardous waste leachate on a short- and a
long—term basis.
o The development of a basic-modeling approach for
use in evaluating the relative performance of
synthetic-lined facility design options.
The research effort was performed under the direction of the EPA
Office of Solid Waste.
8.1.2 Approach
The EPA reviewed the various technical comments received following
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publication of the Interim Final Land Disposal Regulations to determine the
most significant areas of concern identified by the commentors. The synthe-
tic liner investigation was designed to gather the necessary information to
respond to the technical issues defined and to provide an assessment of the
need for further research and regulatory reform.
The approach selected by the EPA was designed to consolidate avail-
able information along the five principal areas of concern:
o Physical and chemical properties of synthetic liner
materials and appropriate testing methods.
o Chemical resistivity of synthetic liners with
respect to hazardous waste streams, and available
testing methods.
o Synthetic liner installation and field inspection
procedures and unique problems.
o Typical synthetic cap and liner failure modes,
which have been experienced at existing facilities.
o Synthetic liner and cap performance modeling.
The evaluation of the physical and chemical attributes of synthetic-lined
facilities was conducted using standard data search techniques and personal
interviews. The personal interview approach was employed due to the fragm n-
ted nature of the existing synthetic system data base and as a means to
overcome the reluctance of some facility operators and design firms to di-
vulge sensitive information. The evaluation of appropriate testing methods
utilized a similar approach, relying primarily on information available from
research facilities and engineering testing laboratories.
A detailed investigation of existing facilities which have ex—
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perienced synthetic liner failure was conducted in order to identify the most
common causes of liner failure and corrective actions required. Due to the
sensitive nature of this effort, most of the information was obtained through
direct communication with facility operators and their consultants as well as
interviews with government inspectors familiar with the facility under eval-
uation.
The approach used to develop design performance modeling capability
relied primarily on the extension of previously tested and verified c9mputer
modeling techniques. The principal effort was expended in the generalization
of previously developed special purpose models to facilitate their use in
evaluating typical hazardous waste disposal facility designs and liner in-
stallation options. The technique developed was designed specifically to
provide a relative assessment of design suitability.
In developing the overall study approach, the EPA recog-nized the
fragmented nature of the existing information base regarding synthetic ma-
terials used as liners and therefore emphasized the data collection effort.
The result of this additional planning ensured that the resulting information
base represented a consolidation of available data from various divergent
sources. This approach necessitated additional effort during the evaluation
phase in order to provide a common basis for subsequent data comparison. The
additional attention afforded by this approach ensured that the resulting
synthetic material data base would prove useful as a basis for the subsequent
technical evaluation of regulatory options. Technical guidance material and
further synthetic liner material research efforts.
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8.2 CONCLUSIONS RELATIVE TO OVERALL SYNTHETIC BARRIER PERFORMP NCE
Based on the results of this research project, several conclubions
were reached regarding the problems with, and anticipated performance of,
synthetic liner and cap systems at hazardous waste disposal facilities. The
study results are organized into five general areas, which correspond to the
principal concerns identified regarding synthetic liner system performance.
These areas were identified as follows: general synthetic liner performance;
chemical resistance of synthetic materials to hazardous waste leachate;
synthetic cap and liner installation; failure modes; and, alternative de gn
performance results based on the HELP analysis. Generally, the study results
indicate that properly manufactured synthetic liners will minimize leakage
from hazardous waste facilities, if the facilities are properly designed,
maintained and routinely inspected. However, small amounts of leakage will
occur, due to vapor transport through synthetic materials. Therefore, syn-
thetic liners do not meet the performance goal established in the regula-
tions. Modification of the regulatory goal to allow 11 de ininimus” leakage is
recommended. The principal issues associated with the use of synthetic
liners are the present lack of a long-term synthetic liner data base, the
lack of detailed design and installation guidance and the lack of a compre-
hensive approach toward the determination of the chemical resistance of
specific liner materials to multi—component waste leachate. Additional re-
search is recommended to determine the ignificance of these issues in terms
of synthetic liner performance at hazardous waste facilities.
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8.2.1 General Performance of Synthetic Barrier Materials
The use of synthetic materials as containment barriers can effec-
tively minimize, but not entirely prevent leakage from hazardous waste fad-
lities. In order to achieve optimum performance, synthetic barrier materials
should be specifically selected on the basis of a consideration of facility
design, regional evnironment and the type of waste material to be contained.
The design of such a facility must incorporate all aspects of leachate con-
trol within the overall facility design scenario. The facility’s regional
environment must be incorporated into the design concept to reflect intensity
of sunlight, amount of precipitation and regional hydrology. The synthetic
liner must be selected as an integral component of facility design, incor-
porating anticipated subsidence, chemical components of the waste material
and liner exposure considerations. Finally, a comprehensive quality assur-
ance program is required to insure attainment of design specifications and
proper facility maintenance. A suitably designed and installed facility
using synthetic barriers will effectively minimize leakage from a hazardous
waste facility.
8.2.1.1 Optimal Synthetic Liner Performance
The common perception that synthetic liners are impervious to
leachate is not entirely correct. Synthetic barriers can be impermeable to
hydraulically driven liquid flow, but they cannot eliminate vapor transport
of volatile material. Vapor transport is a material diffusion phenomenon.
Essentially, the volitized component of a liquid in contact with a barrier
will be absorbed by the barrier, moving through the barrier in the vapor
state, in response to the concentration gradient across the barrier. Upon
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reaching the low concentration side of the barrier (the outside) the vapor
will desorb and return to the liquid state (further discussion is provided in
Section 3). Effectively, this results in some leakage from the facility.
Thus, synthetic barriers cannot meet the regulatory performance objective of
preventing leachate penetration through the facility liner during the opera-
ting life of the facility. However, if synthetic materials are selec ted
appropriately, total vapor trans—port during the operating life of the faci-
lity can be held to very small amounts. It appears that the regulatory
performance goal should be modified to allow de minimus” effective leakage
during the operating life of the facility. -
8.2.1.2 Advantages and Disadvantages of Synthetic Liners
Synthetic cap and liner systems have several advantages over natur-
al soil barriers which should be considered in designing hazardous waste
facilities. The following provides a brief list of the most significant
advantages of synthetic liners:
o Leachate -migration .through intact synthetic bar-
riers occurs through gaseous diffusion only. Based
on the characteristics of the liner material and
the waste ].eachate, effective leakage can be mini-
mized. Non-volatile materials can be completely
contained.
o Since synthetic materials are manufactured, they
can be formulated to exhibit uniform properties
under proper quality assurance control of the
manufacturing process.
o Synthetic materials can be selected to provide
flexibility, strength and chemical resistance prop-
erties to meet specific hazardous waste facility
design requirements.
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o Leachate leakage through synthetic materials is not
hyudraulically driven and is not dependent on the
hydraulic pressure exerted by the waste material
and leachate.
o The formulation and manufacturing of synthetic
liner materials is well understood, and the tech-
nology required to produce uniform liner materials
is available. Liner installation technology is
less advanced; but if proper precautions are taken,
the present technology is capable of successful
liner installation.
o Synthetic materials can be reinforced to provide
sufficient tensile strength to prevent failure due
to minor subsidence or soil movement.
o In service testing methods are available to allow
monitoring of synthetic liner materials during the
operational life of the facility. Monitoring tests
can be defined to reasonably approximate actual
liner conditions.
Synthetic materials also have several disadvantages which must be
considered in the design of hazardous waste facilities. The most significant
disadvantages of synthetic liners with respect to their use in containment of
hazardous waste leachate are:
o Synthetic barriers are relatively thin membrane
liners which are susceptible to puncture, tearing
and other types of penetration damage.
o Synthetic materials lose tensile strength if stres-
sed over long periods of time.
o Naterial properties may be effected by exposure to
sunlight, ozone, fluctuating temperature extremes
and fluctuating loads and pressures.
o The long—term effects of liner exposure to hazar-
dous waste leachate connot be specifically deter-
mined using current technology. Laboratory testing
has shown that exposure to pure chemicals can re—
suit in alteration of the physical and chemical
properties of synthetic liner materials.
o The short experience base available for synthetic
liner materials (10 years for hazardous waste ap—
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plicatioris) does not allow verification of liner
life projections and long-term performance goals.
8.2.1.3 Quality Assurance Requirements
The single most important recommendation which was provided by all
investigators on this project is the need for a comprehensive Quality Assur-
ance program to insure attainment of design specifications. The need for a
mandatory program was expressed by the groups charged with the evaluation of
existing facility failures, installation techniques and chemical resistance
of synthetic materials. The results of this project clearly establish the
need for such a program. An acceptable quality assurance program must incor-
porate at a minimum the following broad categories of quality control to
insure the integrity of liner systems:
o Project description;
o Organization and responsibility;
o Design evaluation;
o Complete design specifications for field installa-
tion and facility operation;
o Training and minimum personnel experience require-
ments;
o Nethods and procedures;
o Equipment and materials certification and testing;
o Testing and data interpretation procedures;
o Internal quality control inspections;
o External quality assurance audits;
o Procedures for resolving identified deficiencies;
and,
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o Complete project design, installation and operation
documentation.
8.2.1.4 Importance of Regional Factors to Liner Design
Precipitation and regional hydrogeology provide the principal hy-
draulic driving mechanisms which determine the ultimate performance of hazar-
dous waste facility caps and liners. Regional precipitation essentially
determines the amount of leachate available for ultimate release from the
facility in the event that liner failure occurs. Precipitation also provides
the driving mechanism for “bathtub effect” facility failures, which have Iieen
documented at some existing facilities. Regional hydrology is important
because it determines the ultimate fate of leachate which is released from
the facility. The overall importance of regional factors lies in the fact
that if any containment system failure occurs, regional factors will deter-
mine the progression of the failure event and the ultimate distribution of
contaminants in the environment. In addition, regional hydrology and climate
essentially act to increase the significance of any design, installation or
operating flaws which are incorporated into the facility inadvertantly.
8.2.2 Chemical Resistance of Synthetic Material to Hazardous
Waste Leachate
The chemistry of synthetic liner materials is primarily based on
the specific polymer used and the particular formulation of the base polymer.
Beyond basic polymer chemistry, liner materials are amended with various
additives and reinforcing materials, to increase the desireable attributes of
the resulting liner material. As with all long chain organic chemistry, the
possible combinations are almost unlimited. This problem is complicated by
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the fact that most manufacturers regard their formulations as proprietary and
retain specific information as industrial secrets. The situation regarding
].eachate chemistry is even more complicated. Leachate from relatively simple
waste streams can contain hundreds or even thousands of chemical components
as the raw waste materials mix and decay into the leachate soup. The complex
chemistry of synthetic polymers and the cornucopia of leachate components can
combine in essentially a limit—less array of potential reactions and counter
reactions. Standard single chemical testing of liner materials cannot hope
to analyze a fraction of the potentially available reactants. Yet, field
data, although available only for the past 10 years, indicates that well
designed and installed synthetic liners are remarkably stable, even in rather
harsh environments.
In response to this apparent dilemma, researchers have developed
various laboratory and field testing methods designed to determine the resis—
tivity of classes of synthetic materials to various industrial waste streams.
The techniques monitor the change in chemical and physical properties of
liner materials which occurs with time after exposure of the material to
hazardous waste ].eachate or to pure chemicals. The results are far from
complete, and the testing procedures used are not standardized, but ie
results thus far form the framework needed to assess chemical resistance of
synthetic materials. The following provides a brief summary of the results
of chemical resistance testing and identifies the major problems which exist
regarding the interpretation of these results.
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8.2.2.1 Synthetic Liner Service Life
The procedures available for testing the chemical reactivity of
synthetic liner materials to attack by various chemicals were largely de—
veloped by liner manufacturers during product development. Recently, inde-
pendent researchers have adopted these techniques for hazardous waste facili-
ty testing. Essentially, all current methods rely on the same basic princi-
pal. The chemical and physical properties of the liner material are deter-
mined prior to leachate exposure. Changes in these parameters are recorded
as a function of time after material exposure to the leachate. The resurts
are generally accepted for determining short-term resistance, but several
researchers have ex—pressed reservations when the results are extended to the
long—term case. Part of the problem is verification of long—term results.
Short-term tests are accepted, since they have been verified in the field.
Since synthetic liners have been used in hazardous waste applications for
only 10 years, long-term resistivity has not yet been directly verified.
In general, current liner testing research is hampered by the lack
of specific guidance regarding the essential difference between the concept
of long and short-term resistance, and by the lack of any specific defirmitfon
of chemical resistance or reactivity. The Interim Final Land Disposal Regu-
lations introduced the concept of short- and long—term liner resistance, but
failed to provide any guidance regarding the definition of these periods.
Similarly, the concept of chemical resistance requires further definition.
The current procedures monitor changes in chemical and physical properties.
However, the specific properties monitored and the level of change which
should be regarded as significant has not been defined. In order for further
meaningful progress to be made in this area, further standardization of the
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reactivity concept and the definition of the interpretation of long- and
short—term chemical resistance must be established.
8.2.2.2 Physical and Chemical Properties Used in Chemical Resis-
tance Theory
The identified protocol used for testing the chemical resistance of
synthetic materials essentially utilizes the monitored change of the chemical
and physical properties of the liner material during exposure to hazardous
waste leachate. There are no standards established which specify the parti-
cular properties used in the resistance assessment or the degree of change
which should be regarded as a “significant” change. However, most facilities
which test chemical resistance of liner materials report the following ma-
terial change parameters:
o Weight change;
o Dimensional stability; arid,
o Strength properties.
Other properties which are reported sometimes include: analyti*al
properties (ash content, volatile fraction, etc.), hardness, and occasionally
CED (cohesive energy density). Permeability of the liner to water is some-
times reported, and visual observations are generally included in the labora-
tory report.
weight change appears to provide a good method of indicating liner
reactivity. Unfortunately, the lack of any weight change by itself does not
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provide conclusive evidence of chemical resistance. In general, weight gains
observed range from 2 to 50 percent of the original synthetic material
weight. Generally accepted industry practice indicates that weight change of
over 10 percent indicates poor chemical resistance.
Dimensional stability refers to the amount of swelling or contrac-
tion observed in a liner sample after exposure. There is no generally accep-
ted range of dimensional change associated with acceptable chemical resis-
tance of the liner. Acceptability at the liner material is determined on a
subjective basis only.
The strength of the synthetic material and its’ change during
exposure are viewed as important resistance monitoring parameters, since the
liner strength essentially provides a measure of its resistance to mechanical
stress in the facility environment. Liner strength may increase due to the
loss of plasticizers within the material, or strength may decrease indicating
a softening of the liner. Softening of liner materials is generally asso-
ciated with weight gain, swelling and an increase in liner permeability.
There are no standards established to indicate the level of change in liner
strength associated with chemical resistance or reactivity.
The results of a visual inspection are used to document the appear-
ance of cracks or holes in the material, and to document evidence of delanti-
nation or discoloration. Most researchers do not subject the material to
microscopic analysis, although this would appear to be warranted. The re-
suits of any visual inspection performed are subjectively treated in estab-
lishing liner acceptability, as no specific guidance for quantifying the
results exists at this time.
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8.2.2.3 Chemical Resistance Testing Procedures
Several procedures have been introduced as a means of testing
chemical resistance of synthetic liner materials. Each procedure has speci-
fic advantages and disadvantages but all suffer from three common problems.
Standard sample preparation and exposure protocols have not been established.
Liner properties are used to indicate acceptability of liner materials, but
parameters used vary by laboratory and the degree of change associated wjth
material acceptance has nbt been standardized for any parameter. Finally and
probably most important, there are no well defined procedures regarding
preparation of the test leachate used to determine the chemical resistance of
the liner material. The most commonly used laboratory test procedures and
their advantages and disadvantages are discussed in the following sections.
Pouch Test - A pouch of liner material is constructed and filled
with a test leachate solution. The pouch is im-
mersed in water and the quality of the water bath is
monitored to determine leak rates and liner deterio-
ration.
Advantages:
o Allows measurement of volume of chemical waste
liquid which permeated the barrier materials; and,
o Exposure of barrier materials to waste liquids on
one side is comparable to field exposure.
Disadvantages:
o Does not allow estimation of maximum permeability
rates because test conditions do not simulate the
driving force for vapor transport through the ma-
terial;
o Waste liquids contained in the inner pouch are not
readily rejuvenated or replaced; and,
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o Application of test is limited to thermoplastic and
crystalline materials.
Immersion Test — A sample coupon of the liner material is im-
mersed directly into a test leachate. Several
procedures exist including the EPA Z4ethod 9090
which represents a first attempt at testing
standardization.
Advantages:
o Exposes both sides of barrier material to waste
liquids, thereby providing a more aggressive envi-
ronment than field conditions;
o Applicable to all barrier materials; and,
o Waste liquids can be rejuvenated or replaced.
Disadvantages:
o Does not directly measure permeability
Tub Test - A tub of synthetic liner material is constructed,
partially filled with waste and exposed to the envi—
roninent. The level of waste is allowed to fluctuate
to simulate surface impoundment conditions.
Advantages:
o Material is subject to the effects of the air/waste
interface;
o The tub is exposed to normal climate conditions
including temperature variation, sunlight, etc.
Disadvantages:
o The method does not provide a direct measurement of
liner permeability;
o The method is most useful to simulate surface im-
poundment conditons. However, it can be used to
simulate leachate sump conditions.
In —Service Tests - Several procedures are available, including
sample coupon immersion and the construction
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of liner tubes which are partially filled with
sand and placed in the leachate suinp or
direct-ly into the surface impoundment.
Advantages:
o In—service tests provide a measure of liner resis-
tance to the actual leachate produced within the
facility;
o The methods provide a direct measure of liner decay
within a given facility, allowing the
identification of significant liner problems before
liner failure.
Disadvantages:
o The technique cannot be used to simulate facility
operation prior to facility construction.
The most advanced testing method at the present time is the immer-
sion test. Several “standard” techniques are available including the follow-
ing:
o EPA Method 9090 Compatability Testing of Membrane
Liners (draft)
o National Sanitation Foundation Recommended Test
Method for Determining Long-Term Performance of
Membrane Liners in a Chemical Environment
(proposed)
o ASTM D471-79 Rubber Property-Effect of Liquids
o ASTM D543 Resistance of Plastic to Chemical Rea-
gents
o Matrecon Test Method 3 Immersion of Membrane Liner
Materials for Compatibility with Wastes
The EPA method represents a first step toward testing procedure
standardization. However, further work is warranted. The EPA procedure does
not provide standards for leachate preparation or immersion bath level main-
tenance.
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8.2.2.4 Comparability and Applicability of Resistance Test
Results
The present lack of standardization regarding sample preparation
and analysis procedures precludes direct comparison of test results obtained
from different laboratories. At the present time, chemical resistance deter—
xnination is largely a subjective determination made by the researchers in-
volved in performing the test. Since the attitude and motivation of the
personnel involved in making this subjective evaluation may vary signi i-
cantly, the results obtained are not likely to be directly comparable.
Synthetic liner selection must be based on quantifiable evidence of
chemical resistance testing results. I owever, due to the broad range of
materials available, it is not practical to test all synthetic material
options. A definite need exists for a conceptual framework which would
narrow the range of material options for specific leachate types. Solubility
theory has been used by the chemical industry for this purpose in other
applications. Essentially, solubility theory states that materials with
similar chemical bonding energies will tend to dissolve each other. t ,a-
terials with very different bonding energies would not affect one another.
Solubility parameter theory can be used as a preliminary screening test to
identify barrier material and hazardous waste combinations which should not
be used because they are likely to dissolve each other. Additional research
is needed to determine if the theory can be used to improve the identifica-
tion of candidate materials for chemical resistance testing.
The results of the testing methods discussed are considered to
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accurately portray cheinica1 resistivity of liner materials in the short—term.
Long—term resistance is projected on the basis of these procedures. However,
there is no formal justification for this extrapolation. Further research is
required to justify this assumption through the development of a long-term
chemical resistance data base.
8.2.2.5 Chemical Resistance !4onitoring
The current chemical resistance testing procedures are not suffi-
ciently advanced to be totally reliable, although they have been successfully
compared to short-term (10 years) field experience. However, the critical
importance of liner integrity to overall facility performance suggests that
actual field monitoring of liner stability should be conducted. As men-
tioned, several in-service testing procedures have been proposed in order to
provide a suitable mechanism for liner monitoring. Based on the information
obtained during the course of this investigation, it would appear prudent to
require field monitoring of liner stability.
The results of such a program would add to the limited data base
available regarding long-term liner stability, and would provide an ea 1y
warning of impending liner failure. Should liner decay be indicated, reme-
dial action could be taken to repair the problem before contamination escaped
the facility. This represents significant advantages over the current moni-
toring requirements, which identify the problem after leachate has entered
the site environment.
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8.2.3 Synthetic Cap and Liner Installation
The results of this study indicate that current techniques are
available to adequately design and install synthetic liner systems. Unfor-
tunately, these techniques are not always applied in a uniform manner to
assure the integrity of the liner system. Field investigations revealed that
certain aspects of the design and installation were accomplished using appro-
priate techniques in most cases. Bowever, other aspects were poorly treated
or ignored. The problem may be due to the lack of awareness of the impor-
tance of long-term liner integrity on the part of facility designers, cnn-
structors and operators. Improved design and construciton monitoring along
with further technical guidance regarding acceptable synthetic cap and liner
designs and installation procedures is required to correct this situation.
8.2.3.1 Field Preparation
A number of specific field procedures and protocols have been
identified to ensure that the installed components of a facility correspond
closely with the design. Consideration should be given to the possibility of
including some of the most critical procedures in the relevant guidazice
documents. Activities for which critical procedures have been identified
include:
o Foundation preparation and bedding material
selection
o Material stockpiling and handling
o Liner placement and seaming techniques
o Synthetic material storage and handling
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o Construction of leachate collection system
o Sterilization of all soils used in providing liner
support or protection
o Cap placement and compaction
o Quality Assurance/Quality Control activities
8.2.3.2 Quality Assurance and Quality Control
The results of the investi9ation of existing facilities uniformly
indicate the need for the development of comprehensive quality assurance
procedures to ensure the integrity of designed hazardous waste facilities.
Most, if not all, of the problems encountered at existing facilities could
have been readily avoided if a properly integrated quality assurance program
was applied during facility construction. The required quality assur—ance
program must address all aspects of facility design, operation and uiainte—
nance jointly to ensure adequate performance of the facility. Based on the
investigative effort performed under this project, the development of an
integrated quality assurance program is critically important to ensure the
integrity of all waste facilities, in particular those with synthetic barrier
systems.
Quality assurance/quality control necessarily involves formulation
and approval of a comprehensive installation/construction plan, together with
specified quality control inspections and confirmatory testing. The field
testing of synthetic liner seams and the verification of the use of proper
bedding materials is particularly important to facility performance.
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8.2.3.3 Selection and Installation of Bedding Materials
Bedding materials above and below the synthetic membrane must be
smooth in order to avoid damage to the liner during subsequent construction
and operation of the facility. Coarse aggregate material should be avoided
since it may puncture or stress the liner material. Specific guidance regar-
ding the depth of bedding material or a suitable grain size is not specifi-
cally available. Most researchers agree that almost any sand or filtered
soil would be suitable as bedding material. The minimum bed depth has not
been established. However, a minimum base of approximately 6 inches of
bedding material sho ild be sufficient to protect the liner. Further techni-
cal guidance regarding the use of particular bedding materials and minimum
layer depths may be justified.
8.2.3.4 storage and Handling of Synthetic Materials
Liner materials must be properly stored and handled in order to
avoid damage prior to field installation. Materials should be stored in a
secure area to avoid damage by vandals. Synthetic materials are subject to
blocking if exposed to sunlight (liner material bonds to itself if rolls 4re
left exposed to sunlight). Therefore, materials should only be stored in
covered areas. Finally, materials should not be excessively handled prior to
placement. Repeated bending or folding, particularly in cold weather, re-
suits in the possible fracture and reduced tensile strength of the material.
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8.2.3.5 Sterilization of liner Support and Cover Soils
Organic material present in the soil layers in contact with the
synthetic liner may result in penetration of the membrane. It is essential
that soil sterilant be applied to all soil layers and subbase materials in
the facility. Sterilant application should be performed in strict accordance
with the manufacturerts specifications. Facility design specifications
should include the specification of the maximum allowable organic content for
all facility soil layers.
The facility cap should be constructed in order to minimize damage
to the liner from burrowing animals and root penetration. The use of a layer
of gravel or coarse sand above the liner and bedding material has been
effective in reducing animal damage. The vegetative cover on the facility
cap should be properly maintained to avoid in-migration of deep rooted plant
species. A properly defined quality assurance program should be developed to
insure proper design, installation and maintenance of the various liner
protection mechanisms identified. If a suitable quality assurance program is
provided, liner performance can be greatly enhanced.
8.2.3.6 Liner Seaming Operations
Liner seams represent the weakest point of the synthetic liner
material, if the seams are not constructed properly. Factory and field seams
must be subject to thorough testing arid inspection if liner integrity is to
be assured. Synthetic liner seaming techniques vary depending on the speci-
fic liner material used. However, all seaming operations have the following
components:
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o Cleaning the material to be seamed;
o Applying solvent, heat or adhesive;
o Applying pressure; and,
o Providing sufficient time for curing.
During the factory seaming process, environmental conditions can be
effectively controlled. However, field seaming is subject to the external
environment at the site. Effective field seaming cannot be accomplished
during periods of adverse weather, including windy conditions, precipitation
events and cold periods. Adverse weather conditions have been responsible
for weak seams at several locations. However, most poor field seams have
resulted from the use of the wrong solvents and adhesives for the particular
liner material. The critical nature of the seaming operation requires rigid
qulaity assurance control. A comprehensive quality assurance testing and
inspection program should include the following items:
o Material certification and testing procedures;
o 100 percent seam testing in the field;
o Documentation of weather conditions during seaming;
and,
o Complete laboratory testing of sample seam sec-
tions.
Particular attention must be afforded when seaming new material to
older liner material which has been exposed to the environment. Acceptable
seams can be accomplished, but more intensive cleaning of the old material is
required to remove any damaged surficial material prior to seaming. Similar-
ly, the seaming of two different types of liner material require special
procedures to insure compatability of 8eamiflg techniques.
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Under a proper field inspection and testing program, synthetic
liner seaming can be accomplished without jeopardizing liner performance.
However, the operation should only be conducted imder strict quality assur-
ance supervision.
8.2.3.7 Subsidence
Subsidence occurs either due to poor compaction of sub-liner ma-
terials or, in the case of cover systems, due to the decay of waste ma-
terials. In the latter case, approximately 90 percent of the total sett .e-
ment of the cover will occur within five years of site closure. Poor subsur-
face compaction and the resulting settlement was the most prevalent form of
cover system failure documented during this investigation.
Some subsidence can be expected at all waste sites due to waste
decay. However, through proper design, installation and operating proce-
dures, settlement can be minimized. The facility design must incorporate a
thorough geotechnical evalution of the facility foundation material and the
facility installation must be monitored closely to ensure attainment of
design specifications.
Some subsidence of the facility cap can be expected within any
waste disposal facility. Synthetic liners can be reinforced to support the
facility cap in order to accomodate this settlement. However, the added
stress on the synthetic liner may weaken the liner with time. It has been
suggested that the final facility cap be installed fOllow1 g the period of
significant settlement. This approach would expose the facility to precipi-
tation and weathering during the interim period between the end of facility
25
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operation and final closure. An alternative approach which appears more
reasonable, is the installation of a temporary cap which is maintained until
settlement diminishes. The temporary cap is then regraded and the final cap
can be installed.
The study results indicate that more comprehensive facility design
studies, along with more substantial quality assurance and quality control
procedures during facility installation, are required to minimize settlement
problems.
8.2.4 Synthetic Cap and Liner Performance
The overall performance of a hazardous waste facility throughout
the operational and post-closure period is largely dependent on the perfor-
mance of the synthetic barrier systems integrated within the facility. The
synthetic barriers represent only one component of the entire facility. To a
large extent, the performance of the synthetic liner system is dependent on
the performance of other facility components. Essentially, the synthetic
liner must be evaluated as a significant part of the total facility design
concept. Due to the importance of the barrier system, it is necessary o
view the other facility components in terms of their effect on the synthetic
liner. The following section provides a summary of the attributes of synthe-
tic liners and their relationship to other facility components.
8.2.4.1 Period of Performance for Synthetic Liners
Better definitions of “long-terni” (as used in the 40 CFR Part 264)
and “service life” are needed to accurately assess synthetic cap and liner
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system performance. Physical and chemical properties of synthetic barriers
change with time. Consequently, their performance must be assessed over a
specified period of time. A definition of long—term should consider the time
over which hazardous wastes retain their characteristics. A definition of
service life should consider synthetic material properties. Preliminary
definitions of these terms which should be further developed are:
“Long—term is equal to or greater than the period of time
that hazardous waste constituents migrate to the liner in
amounts which result in their presence in ground water and
surface water, at concentrations greater than background
or National Interim Primary Drinking Water Standards”; and
“Service life is that period of time over which synthetic
materials retain physical and chemical properties and
provide containment of hazardous wastes which prevent
migration of hazardous waste constituents to the ground
water and surface water at concentrations greater than
background or National Interim Primary Drinking Water
Standards.”
8.2.4.2 Projected Service Life of Synthetic Liners
Synthetic materials have been available for a relatively short
period of time. Actual synthetic liner field experience extends back over
only 25 years, and field experience with synthetic liners used for hazard us
waste containment extend over only 10 years. Therefore, the available ex-
perience base cannot be used to establish the anticipated period of perfor-
mance of synthetic liners. Liner experts and researchers have developed
various procedures to estimate liner life based on the rate of liner degrada-
tion over short test periods. The degree to which these procedures simulate
actual liner decay cannot be verified using actual field data at this time.
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The methods used to project service life vary. But all rely on the
measured rate of change of the physical and chemical properties of liner
materials over specified time periods. The procedure is similar to the
chemical resistance testing discussed previously and suffers the same lack of
standardization. Specific tests have indicated liner service life up to 150
to 200 years, which appears to be unduly optimistic. There is general agree—
sent that high quality synthetic liners should last approximately 40 to 45
years in a hazardous waste environment. A more direct determination of
synthetic liner service life is not available at present due to the lack of
longer term field data.
8.2.4.3 Synthetic Liner Permeability
The effective permeability of a synthetic liner refers to the
liner’s ability to prevent migration of materials through the barrier system.
Synthetic liners have been shown to effectively prevent specific leachate
materials from migrating, except for “de minimus” leakage amounts due pri-
marily to vapor transport through the liner. The analytical techniques
required to predict the synthetic liner’s performance with respect to speci-
fic leachate over time is presently available. However, the specification. of
the chemical components of a hypothetical leachate from a mixture of known
hazardous wastes is not possible at this time. Similarly, it is not possible
to specify with assurance which particular liner material is the optimum
choice for containment of a specific leachate. Essentially, chemical testing
procedures are far too time consuming to allow testing of a significant
fraction of synthetic material and specific leachate combinations. However,
specific testing of a suitable liner type and a class of leachate material
can be performed using cu rent techniques. Therefore, proper testing of a
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selected liner and a representative leachate can be performed to confirm
basic facility design concepts.
8.2.4.4 Facility Design Considerations
A hazardous waste facility consists of a number of systems and
subsystems. Cap and liner systems contain drainage and barrier subsystems.
The integrated performance of all subsystems determines the ability of the
facility to meet the general performance goal. A fault—effect analysis to
determine the effect on the subsystems and overall facility performance, of
failure of one subsystem, is necessary during design. Design should include
at least the following fault—effect analyses:
o Clogging of leachate removal system and effect on
barrier;
o Crushing of leachate drain pipe and effect on
barrier;
o Hole in barrier and effect on leak detection and
leachate removal system; and,
o Effect on liner of greater permeability in cap
(resulting from holes or other causes) than in
liner.
Worst-case conditions are typically used for such analyses. The design
should be adjusted where a deficiency in specific subsystems is identified.
Greater confidence in the performance of cap systems can be pro- /
vided by placing the final barrier component after subsidence has occurred.
This may be accomplished, depending on waste characteristics, by installing a
temporary cap until the rate of subsidence diminishes and then installing the
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final facility cap. An appropriate period of time for delayed placement of
the permanent barrier may be determined by monitoring the rate of consolida-
tion using settlement markers or by calculating consolidation for certain
uniform wastes. Because waste liquids would still be produced, the leachate
collection and removal system would have to operate during this extended
closure period. The extended closure period may range from several months to
several years.
Confidence in synthetic material performance may be increased by
providing thick bedding material layers, particularly where bulky or sharp
objects are placed in the facility. The current regulations require a 30 cm
maximum leachate head above the barrier. The Guidance_Documents place a
dimensional and permeability limitation on the bedding material where it is
to be used as the leachate drain layer. More cost—efficient leachate collec-
tion systems may be designed with negligible effect on performance if the
30 cm head is increased to 60 cm. This modification would not result in
substantially increased leakage through synthetic liners because synthetic
materials are not affected by minor changes in hydraulic pressure.
Synthetic barriers can be designed to resist stresses of antiQi-
pated minor differential subsidence or to elongate and conform to the dislo-
cation. The amount of stress should be minimized by proper compaction arid
filling of the waste materials. Additional strength and higher confidence in
long—term strength properties of the barrier can be accomplished using fabric
reinforced liners. The design of good waste placement procedures should
minimize subsidence. However, the increased stress on the synthetic liner
may result in the eventual weakening of the liner system over time.
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8.2.5 Failure Modes
Two primary failure modes have been identified for synthetic liner
materials, the first is referred to as liner holes and the second is the
“bathtub effect” failure mode. Some investigators refer to the vapor traris-
port mechanism as a third failure mode. However, since vapor transport is an
inherent property of synthetic materials, it should not be associated with
any material failure of the liner itself. Rather, it should be viewed as a
property of synthetic liners much as permeability is viewed as a property of
soil liners.
8.2.5.1 Liner Holes
Liner holes have been detected in most in—service liners, which
have been tested. Essentially, liner holes range from pinholes resulting
from poorly controlled manufacturing processes to holes caused by liner tears
and punctures due to poor installation techniques. Based on the information
available, it appears that liner holes can be minimized through the use of
effective quality control programs throughout the manufacturing and installa-
tion process. However, it is unlikely that holes can be entirely eliminated
from installed synthetic liners.
The significance of holes in the liner system with respect to liner
performance is generally quite small if proper precautions were taken to
minimize the number of holes present. For example, the presence of a one
square meter hole or 25 million pinholes per acre of liner material would
result in the same net leakage as an optimum clay liner syustem. This number
of holes is far greater than that which would be expected in a good quality
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liner system. The study results indicate that with the implementation of a
comprehensive quality assurance program the number of holes present in a
synthetic liner can be minimized. The required quality assurance program
must include a manufacturers’ quality control program as well as a quality
assurance evaluation of field installation techniques.
8.2.5.2 •Bathtub Effects
The “bathtub” or tarmac effect results from the gradual filling of
a landfill unit with liquid due to the presence of a cap system with a higher
permeability than that of the liner. Essentially, precipitation percolates
into the facility through the cap at a higher rate than it can move through
the bottom liner. The unit then fills with leachate which eventually escapes
over the facility sidewalls. Bathtub effect problems have been noted in
several existing landfill facilities, and are invariably the result of poor
cap design and the lack of an effective leachate removal system.
The installation of a properly designed cap system is the most
important design feature in eliminating bathtub effect problems. The second
principal design feature is the leachate collectid system. Essentially, the
leachate collection system can remove leachate before a significant leachate
buildup can occur, and thus avoid the bathtub problem. A third system, which
is recommended, is the monitoring of leachate levels in the operating or
closed landfill unit. The leachate level monitor would provide an early
warning of potential problems in the event that the leachate collection
system was not functioning properly. In the event of a rise in the leachate
level, excess leachate could be pumped from the facility to allow sufficient
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time to repair the facility cap system. Based on the results of this inves-
tigation, it appears that “bathtub effect” failures can be avoided through
proper design of the facility cap, the lechate collection system, and a
leachate level monitoring system.
8.2.6 Performance Modeling
Basically, the results of the HELP simulations verified the expec-
ted seepage trends relative to climatic and design factors. Climate is the
most important factor controlling the rate of seepage production. Low pre-
cipitation/high evapotranspiration climates, such as Denver, will result in
lower seepage production. As expected, the more conservative the engineering
design, the lower the seepage rate. Because of the limitations in the as-
sumptions and operation of the HELP model, the simulation results cannot be
considered to have absolute validity, but their utility lies in providing
comparisons of differing release rates resulting from a range of differing
design factors and climatic conditions.
The HELP simulation results imply that use of a double bottom liner
system which incorporates both an FML and an underlying clay barrier (with a.,
—7 —
permeability of 1 x 10 cm/sec or less) provides an advantage over either a
single FML or double FNL bottom liner system. The combination FML/clay
system and FML-only systems all assume that complete seepage containment is
achieved provided FML failure has not occurred. When FML failure occurs, the
clay component in the combination FNL/clay system serves as a “second
defense”. The value of the combination bottom liner system is most notable
during the operating period before final cover is emplaced.
I
,
/
8 -
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An added benefit of the clay component in the combination FML/clay
double bottom liner system is the reduction in the seepage rate f or as long /
as the leachate collection and leak detection systems are operated. In
-“
contrast, the failure of the FML—only bottom liner systems essentially ne-
gates the effectiveness for leachate collection. This implies that continued /
operation of the leachate collection and leak detection systems beyond the
post—closure care period for functioning bottom liner systems will result in
reduced final seepage rates.
8.3 RECONNENDATIONS
The following provides a summary of recommendations which resulted
from this investigation of synthetic liner and cap systems. Recommendations
are provided for regulatory reform, additional technical guidance materials
and for further research efforts. Some of the recommendations provided for
regulatory reform and additional technical guidance materials should be
viewed as long—term objectives, since further research will be required
before the changes required can be completely specified.
8.3.1 Regulatory
The results of the study indicate that regulatory action is re-
quired to correct certain deficiencies in the current regulations to reflect
an improved understanding of synthetic liner performance. The current regu-
lation requires that liners must contain all leachate throughout the active
life of the facility and minimize leakage thereafter. Synthetic liner sys—
tents cannot achieve total containment during the active life of the facility,
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due to their ability to transport vapor, and due to the presence of some
liner leaks. They can, however, achieve minimal leakage of waste leachate,
if properly designed and installed. The following regulatory recommendations
assume that the total containment restriction will be modified to allow the
use of synthetic barrier systems at hazardous waste facilities.
8.3.1.1 Quality Assurance and Quality Control
The results obtained from the investigation of failures at existing
facilities strongly indicates the need for a formal quality assurance prog am
to verify the achievement of design specifications at hazardous -waste facili-
ties. It is recommended that a comprehensive quality assurance and quality
control program be established to control all aspects of the waste facility
as part of the final land disposal regulations. The recommended program
should include installa tion verification procedures as well as operational
testing and maintenance provisions. The program envisioned should include
requirements necessary to verify the following elements of facility design
construciton and operation:
o A geotechnical evaluation of the site prior to
preparation of design specifications, including
soil samples, boring logs and soil testing results;
o Synthetic liner placement, and bedding material
selection in accordance with design specifications
for grain sizes and sterilization of subsurface
materials;
o Chemical resistivity testing of proposed liner
material and sample waste leachate;
o Inspection procedures to verify waste placement,
cover requirements and compaction;
o Final cap liner placement, bedding layer placement
and the installation of animal barrier and
7
8—35
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vegetative drainage layers according to design
specifications for grain sizes and sterilization of
subsurface materials;
o Verification of material composition for all
liners, soils and barrier materials;
o Verification of all synthetic liner seams;
o Verification of waste composition prior to
disposal;
o Inspection and maintenance of liners, caps, surface
vegetation and surface grades of- all active and
inactive waste units; and,
o Inspection and maintenance of the leachate control
system before and after waste placement.
8.3.1.2 Synthetic Liner Service Life
The present regulations do not provide a working definition for
“long—term” or “period of performance”. Consequently, determining the abi-
].ity of synthetic barriers to meet these requirements is not possible. Sug-
gested definitions for these terms are provided in Section 8.2.4. It is
necessary that the definition be sensitive to mobility characteristics of
different hazardous wastes. It is recommended that a definition of “long-
term” be included in the regulations.
8.3.1.3 Closure Period Extension
Greater confidence in performance and increased design and opera-
ting flexibility may be gained by extending the closure period. Subsidence
of cap systems can be kept to a minimum in some cases by delaying completion
of the final cap system placement (especially the barrier component). Use of
consolidation theory for high moisture soils, or observations of settlement
(:3;)
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markers can be used to determine the appropriate time to place the final
barrier layer. It is recommended that regulations be modified to provide a
longer closure period to allow the installation of a temporary soil cap
system during the period of maximum subsidence. The method of subsidence
-
monitoring should be included in the facility plan. Leachate collection and
removal should be continued until final cap placement occurs.
8.3.1.4 Leachate Depth Limitation
Chemical seepage through a synthetic barrier material is not dean-
dent on hydraulic head, unless there are holes in the barrier. The require-
ment for a maximum of 30 cm of leachate head is therefore not appropriate for
synthetic barriers. It is recommended that the limitation on leachate head
be increased to 60 cm to allow a thicker ].eachate collection layer to provide
more protection to the synthetic liner.
8.3.1.5 Cap Maintenance and Leachate Control
As previously discussed, the HELP analysis results indicate that
further attention to the design of facility caps and leachate control systems
is warranted. It is recommended that leachate level and quality monitoring
be instituted within the facility during operation and the post-closure care
period. Leachate-level monitoring provides a practical means of monitoring
cap and leachate collection system performance. Should a problem occur in
either system, a rise in leachate levels would be detected allowing correc-
tion of the problem before groundwater contamination occurred.
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8.3.2 Guidance Materials
The results of the synthetic liner study indicate that further
technical guidance is warranted in several areas related to the use of syn-
thetic liner systems at hazardous waste facilities. Recommendations regar-
ding the preparation of additional guidance material are provided in the
areas of quality assurance of hazardous waste facilities and the specifica-
tion of standard chemical resistance testing procedures.
8.3.2.1 Quality Assurance and Quality Control
An integrated quality assurance program is required to ensure that
all aspects of the hazardous waste facility are properly designed, installed
and operated. A technical guidance document specifying the minimum require-
ments of a hazardous waste facility quality assurance program should be
developed. The guidance material should provide a well documented quality
assurance approach designed to verify the proper design, installation, opera-
tion, and maintenance of the following facility components:
o Facility foundation preparation;
o Bottom liner installation and maintenance;
o Liner material specification and testing;
o Leachate collection system design, installation and
maintenance;
o Cap liner;
o Water quality monitoring system, including internal
facility water levels;
o Waste composition;
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o Waste compaction and cover; and,
o Facility unit management.
The development of such detailed technical guidance material is a
significant undertaking. The task could be reduced significantly by adopting
one of EPA’s other quality assurance programs as a model for the hazardous
waste program. The EPA has well established quality assurance programs
associated with the Prevention of Significant Deterioration (PSD) program
under the Clean Air Act and similar programs under other environmental regu-
lations. While it would appear that an air-related program could not,be
modified to suit these requirements, it should be noted that a well designed
quality assurance program provides a formal framework to verify attainment of
technical specifications. The technical specifications are not actually
incorporated into a quality assurance program per se. Further, the permit
process under the PSD program is similar to the hazardous waste facility
process.
6.3.2.2 Chemical Resistance and Projected Service Life Testing
A comprehensive chemical resistance testing’ protocol should be
developed and included as guidance material. EPA Method 9090 provides a good
starting base and should be improved by providing specific time and tempera-
ture conditions for immersion and material property testing, guidance on data
interpretation, procedures for obtaining waste liquids for use during immer-
sion, procedures for controlling waste liquid quality, and quality control
procedures. Correlation of chemical resistance test results with field
performance data should also be added to the Guidance Documents as the data
becomes available.
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Other chemical resistance testing procedures, which should be ie-
veloped further and standardized include:
o Pouch Test
o Tub Test
o In—Service Test
The standardization process should include the definition of stan-
dard liner properties used to deteripine chemical resistance and projected
service life. In addition, the degree of change which constitutes acceptable
chemical resistance should be specified foreach -testing method. Finally,
the results of all chemical resistance and service life testing should be
incorporated into a synthetic material data base, accessible to all liner
research programs.
8.3.3 Future Research
Successful performance of synthetic barriers is dependent on their
proper manufacturing and installation. Research can improve performance by
identifying and developing techniques to monitor the quality of barrier
materials during manufacture. Quality assurance procedures are currently
available f or seam testing, but more efficient methods would improve the
quality and reduce the costs for barrier installation. Priority should be
given to methods that reduce the level of expertise required to perform the
test. Protocols should be developed for different polymer types since some
testing procedures are not applicable to all polymers.
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A centralized data base of chemical resistance testing and long-
term field performance data should be developed. This data base would allow
comparison of polymer formulations and waste combinations, and would also
allow comparison of field data with chemical resistance testing data. The
data base would be a valuable resource for new facility design and barrier
material design, as well as a reference source for permit application review
purposes.
The solubility parameter theory should be developed and tested to
supplement its current capability to identify incompatible waste/barr er
material combinations, with the capability to-identify compatible barrier
material candidates. Specific areas requiring investigation include:
o Provide more sensitive indicators of chemical re-
sistance;
o Effect on solubility parameter resulting from in-
teractions of polymer constituents and interactions
of waste constituents;
o Long-term stability of solubility parameter under
exposure conditions;
Continued developnent -of the immersion test, the pouch test and in-
service tests are needed to determine appropriate temperature conditions,
methods to reduce volatile losses, and appropriate rate of stress application
and relaxation during strength tests. Integral to both tests is the use of
representative waste liquids. Procedures are needed to describe how waste
liquids are obtained for the purpose of barrier material selection. A rigor-
ous method to project performance from chemical resistance testing is needed.
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Vapor permeation theory should be investigated to determine its
significance with currently available barrier materials and materials cur-
rently under development. A consensus standard method for directly measuring
the permeability of barrier materials is needed for improved barrier material
selection and product development. Various wastes should be evaluated to
more accurately assess the barrier permeability to aqueous and pure solutions
of chemicals and to identify those chemicals which are most effectively
contained by specific barrier materials.
A comprehensive evaluation of the significance of holes in barrier
materials is needed to provide criteria for use in manufacturing and instal-
lation quality assurance programs. The evaluation should consider the size
of holes, surface tension of barrier materials, hydraulic characteristics of
underlying materials and characteristics of wastes and liquids which migrate
through the holes. The influence on barrier performance should be evaluated
on the basis of volumetric seepage rates as well as chemical mass transport
rates.
The H P model proved to be a very powerful tool in evaluating a
variety of landfill--design and failure conditions. The model should be
further developed to assess its ability to incorporate barrier specific
properties such as vapor permeation of specific compounds (e.g. hazardous
constituent and water). Sensitivity analysis and field testing of the model
to obtain correlation of predictions and actual performance are needed to
provide permit officials and designers with accuracy and precision informa-
tion needed to assess the significance of the model’s performance predic-
tions.
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