xvEPA
EPA530-R-15-004
United States
Environmental Protection
Agency
Industrial Waste Management Evaluation
Model (IWEM) Version 3.1:
Technical Background Document
June 2015
U.S. Environmental Protection Agency
Office of Solid Waste and Emergency Response
Office of Resource Conservation and Recovery
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IWEM Technical Background Document Acknowledgments
Acknowledgments
Numerous individuals have contributed to the development of the IWEM software and
documentation since IWEM version 1. At EPA, Mr. Taetaye Shimeles served as Work
Assignment Manager for the current version of the model, providing directions and technical
assistance, and is also a contributing author. Dr. Zubair Saleem, Ms. Ann Johnson, and Mr.
David Cozzie had filled this role for the past versions. A variety of other EPA staff have
provided additional technical guidance and suggestions, including Dr. Peter Grevatt, Dr. Lee
Hofmann, Dr. Colette Hodes, Mr. Richard Kinch, Mr. Jason Mills, Mr. John Sager, Mr. Timothy
Taylor, Ms. Shen-Yi Yang, and Ms. Janvier Young. The EPA has been assisted in the
development of IWEM by several contractors: RTI International, HydroGeoLogic, and Resource
Management Concepts, Inc.
in
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IWEM Technical Background Document Software Development History and Online Resources
Software Development History
The Industrial Waste Management Evaluation Model (IWEM 3.1) is the latest version of a
ground water fate and transport developed by the U.S. EPA's Office of Resource Conservation
and Recovery (ORCR). IWEM, since its initial development in 2002, has undergone a number
changes and revisions. Some of the changes were done to expand the scope of the model from
modeling just waste management units, also to evaluate potential contaminant releases from
recycled industrial materials used in beneficial use applications. Additional revisions were also
made to increase the usability of the model, and to allow the user greater control over the input
parameters. The changes and revisions made the model more flexible and user friendly, as well
as usable by various stakeholders. Brief descriptions of the major changes made to model since
its initial release are presented below.
The original IWEM 1.0 (U.S. EPA, 2002a, b) was developed as part of the Guide for Industrial
Waste Management (U.S. EPA, 2002c) to conduct a tiered screening analysis (Tier 1 and Tier 2)
to determine the most appropriate liner design for several types of waste management units in
order to minimize or avoid adverse ground water impacts. In Tier 1, the analysis considered a
national distribution of waste management units and site conditions that affect the fate and
transport of constituents in subsurface media. On the other hand, site-specific parameters were
required for key parameters in the Tier 2 probabilistic analysis. In addition, this version was
based on Version 2.0 of the U.S. Environmental Protection Agency's (EPA's) Composite Model
for Leachate Migration and Transformation Products (EPACMTP) code (U.S. EPA, 2003a, b),
which included the vadose-zone and aquifer modules developed under the Multimedia,
Multipathway, Multireceptor Exposure and Risk Assessment (3MRA) framework (U.S. EPA,
1999).1
In 2006, building on version 1, IWEM 2.0 was developed by adding a module to simulate fate
and transport from a new source typea roadway constructed using recycled industrial materials
(i.e., byproducts). The new source type was restricted to Tier 2 analyses. In addition to the new
roadway source, IWEM 2.0 used the latest version of EPACMTP, Version 2.2. EPACMTP
Version 2.2 includes non-science related changes to the input and output streams of EPACMTP
Version 2.1 (U.S. EPA 2003c, d).
IWEM 3.0 enhanced the functionality of its predecessor, by introducing a more rigorous
treatment of leaching through the roadway cross section by including ditches, drainage, and
surface runoff as optional elements. The graphical user interface was also modified to
accommodate the improved source type. In addition, two significant revisions were made to the
model, which included the following:
Tier 1 analysis for waste management units was eliminated. The leachate concentration
threshold values stored in the IWEM database and used for Tier 1 analyses were based on
human health benchmarks (e.g., reference doses and slope factors) that were current as of
1 IWEM 1.0 and EPACMTP 2.0 were developed and tested concurrently, whereas the supporting documentation
for IWEM 1.0 was released prior to EPACMTP 2.0 documentation.
IV
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IWEM Technical Background Document Software Development History and Online Resources
2002 when IWEM 1.0 was released. To avoid generating a "protective" liner
recommendation based on an out-of-date benchmark, the Agency opted to remove the
Tier 1 analysis option from Version 3.0.
Built-in human health benchmarks, with the exception of maximum contaminant levels
(MCLs), have been removed from the database.
This decision resulted in two significant changes to the model: (1) only Tier 2 analyses are now
available in the software, so references to Tier 2 and the "tiered approach" were removed from
the software and documentation; and (2) other than MCLs, the user is now required to provide
human health benchmarks for the screening evaluation.
The current version, IWEM 3.1 replaces IWEM 3.0. IWEM 3.1 adds a new module to simulate
leaching from a structural fill site constructed with industrial materials and related byproducts.
Structural fills evaluated in IWEM 3.1 include, the use of industrial materials and related
byproducts as substitutes for the earthen materials to provide structural support for parking lots,
roads, airstrips, tanks/vaults, and buildings; construction of highway embankments and bridge
abutments; filling of borrow pits, and other landscape irregularities; and changing of landscapes
for development or reclamation projects.
Online Resources
EPA's Nonhazardous Industrial Waste Management tools web page
(http://www.epa.gov/waste/nonhaz/industrial/tools/index.htm) provides links to the Guide for
Industrial Waste Management, IWEM, and EPACMTP. The linked IWEM webpage also
provides access to the model as well as its supporting Technical Background Document and the
User's Guide.
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IWEM Technical Background Document
Acronyms
Acronyms
3-D Three-dimensional
Agency U.S. Environmental Protection Agency
EPA U.S. Environmental Protection Agency
EPACMTP EPA's Composite Model for Leachate Migration with Transformation Products
Guide The Guide for Industrial Waste Management
HBN Health-based number
HELP Hydrologic Evaluation of Landfill Performance
HGDB Hydrogeologic Database for Ground-Water Modeling
IWEM Industrial Waste Management Evaluation Model
Kd Soil-water partition coefficient
Koc Organic carbon partition coefficient
MCL Maximum Contaminant Level
MINTEQA2 EPA's geochemical equilibrium speciation model for dilute aqueous systems
RGC Reference ground-water concentration
WMU Waste management unit
Units of Measure
This Technical Background Document uses the following abbreviations for standard units of
measures; these may be found in combination. In some instances, general units (e.g., length per
time) may be used and in others, specific units (e.g., m/sec). Superscripts indicate the unit is
squared (e.g., m2) or cubed (e.g., m3).
Specific units:
|j,g microgram
cm centimeter
day day
g gram
hr hour
kg kilogram
km kilometer
L liter (if used with other specific
units, as mg/L)
m meter
mg milligram
min minutes
mL milliliter
mm millimeter
mo month
sec second
yr year
General units:
M
M/L3
M/M
General unit for length (if used with
other general units, as M/L3)
General unit for mass
General unit for mass concentration
(mass per length cubed)
General unit for mass fraction (mass
per mass)
General unit for time
VI
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IWEM Technical Background Document Table of Contents
Table of Contents
Acknowledgments iii
Software Development History iv
Online Resources v
Acronyms vi
Units of Measure vi
Executive Summary ES-1
1.0 Introduction 1-1
1.1 Guide for Industrial Waste Management and IWEM 1-1
1.2 IWEM Design 1-3
1.2.1 What Does the Software Do? 1-3
1.2.2 IWEM Components 1-4
1.3 About This Document 1-5
2.0 Overview of the Approach 2-1
2.1 Purpose of the Tool 2-1
2.1.1 WMU Evaluation 2-1
2.1.2 Structural Fill Evaluation 2-2
2.1.3 Roadway Evaluation 2-2
2.2 Approach Used to Develop the Tool 2-4
3.0 Source Modeling 3-1
3.1 WMU Source Modules 3-1
3.1.1 Releases From a WMU Source 3-1
3.1.2 Infiltration Rate for Surface Impoundments 3-3
3.1.3 Assumptions and Limitations for WMU Source Modeling 3-4
3.2 Structural Fill Module 3-5
3.2.1 Releases From a Structural Fill 3-5
3.2.2 Assumptions and Limitations for Structural Fill Source Modeling 3-6
3.3 Roadway Source Module 3-7
3.3.1 General Conceptualization 3-7
3.3.2 Approach to Integration into IWEM 3-10
3.3.3 Assumptions and Limitations of Roadway Source Module 3-11
3.4 Structural Fill or Roadway? 3-11
4.0 Unsaturated and Saturated Zone Modeling 4-1
4.1 Unsaturated Zone Module 4-2
4.2 Saturated Zone Module 4-5
4.3 Assumptions and Limitations for Unsaturated and Saturated Zone Modeling 4-8
4.3.1 Uniform Soil and Aquifer Assumption 4-8
4.3.2 Steady-State Flow Assumption 4-8
4.3.3 Constituent Fate and Transport Assumptions 4-9
5.0 Conducting Probabilistic Analyses Using EPACMTP 5-1
5.1 EPACMTP Monte Carlo Module 5-1
5.2 Implementation of Monte Carlo Analysis in IWEM 5-2
5.3 Assumptions and Limitations for Monte Carlo Module 5-3
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IWEM Technical Background Document Table of Contents
6.0 IWEM Inputs 6-1
6.1 WMU Parameters 6-1
6.1.1 WMU Types 6-1
6.1.2 WMU Parameters 6-2
6.1.3 Well Location Parameters 6-6
6.2 Structural Fill Parameters 6-8
6.2.1 The Structural Fill 6-8
6.2.2 Structural Fill Parameters 6-8
6.2.3 Well Location Parameters 6-11
6.3 Roadway Parameters 6-11
6.3.1 Well Location 6-13
6.3.2 General Roadway Description 6-14
6.3.3 Roadway Geometry 6-14
6.3.4 Drain Geometry 6-15
6.3.5 Roadway Layer Properties 6-15
6.3.6 Roadway Ditch Properties 6-15
6.3.7 Roadway Drains Properties 6-17
6.3.8 Roadway Flow Characteristics 6-17
6.3.9 Leachate Concentrations 6-18
6.4 Infiltration and Recharge Rates 6-18
6.4.1 Infiltration Rates for WMUs 6-20
6.4.2 Infiltration Rates for Structural Fills 6-34
6.4.3 Infiltration Rates for Roadways 6-34
6.4.4 Recharge Rates 6-48
6.5 Parameters Used to Describe the Unsaturated and Saturated Zones 6-48
6.5.1 Subsurface Parameters 6-50
6.5.2 Unsaturated Zone Parameters 6-51
6.5.3 Saturated Zone Parameters 6-54
6.6 Parameters Used to Characterize the Chemical Fate of Constituents 6-56
6.6.1 Constituent Transformation 6-57
6.6.2 Constituent Sorption 6-59
6.7 Screening Procedures EPA Used to Eliminate Unrealistic Parameter
Combinations in the Monte Carlo Process 6-63
7.0 Reference Ground Water Concentrations 7-1
7.1 Maximum Contaminant Levels 7-1
7.2 Health-Based Numbers 7-3
7.3 Other Standards 7-3
8.0 How Does IWEM Make Recommendations? 8-1
8.1 Making Recommendations Corresponding to a 90th Percentile Exposure
Concentration 8-1
8.2 Making Liner Recommendations for WMUs 8-3
8.3 Determining the Appropriateness of Reused Industrial Material in a Structural
Fill 8-4
8.4 Determining the Appropriateness of Reused Industrial Material in a Roadway 8-4
9.0 References 9-1
Vlll
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IWEM Technical Background Document Table of Contents
Appendix A. Glossary
Appendix B. List of IWEM Waste Constituents and Default Chemical Property Data
Appendix C. Formulation of the Roadway Module
Appendix D. Infiltration Rate Data for WMUs and Structural Fills
Appendix E. Infiltration Rates through Pavements
Appendix F. A Discussion on the Formulation of the Non-Orthogonality between the Highway
Axis and the Regional Ground water Flow Direction
Appendix G. Verification of the Roadway Module in IWEM
List of Figures
2-1. Three liner scenarios considered in IWEM 2-2
2-2. Generalized roadway scenario considered in IWEM 2-3
2-3. Conceptual cross-section view of the subsurface system simulated by EPACMTP 2-5
3-1. Leachate concentration vs. time for pulse and depleting source scenarios 3-3
3-2. Surface impoundment infiltration module 3-3
3-3. Atypical roadway with a recycled-material segment 3-8
3-4. Atypical cross section of a roadway 3-8
3-5. Modules of IWEM corresponding to multiple roadway-source strips 3-9
3-6. An example of layering in road way-source strips 3-9
4-1. Relationship between leachate concentration and well concentration 4-2
5-1. Graphical representation of the EPACMTP Monte Carlo process 5-2
6-1. WMU types modeled in IWEM 6-1
6-2. WMU with base elevation below ground surface 6-5
6-3. Position of the modeled ground water well relative to the WMU 6-6
6-4. Structural fill modeled in IWEM 6-8
6-5. Diagram used by IWEM to specify roadway geometry 6-13
6-6. Locations of HELP climate stations 6-24
6-7. Climatic zones 6-35
6-8. Ground water temperature distribution for shallow aquifers in the United States 6-54
6-9. Unsaturated zone isotherm for Cr6+ (non-carbonate environment, low LOA,
medium FeOx, high POM, pH 6.3) 6-63
6-10. Flowchart describing the infiltration screening procedure 6-66
6-11. Infiltration screening criteria 6-67
8-1. Determination of time-averaged ground water well concentration 8-1
8-2. Example cumulative distribution function of well concentrations 8-2
List of Tables
1-1. IWEM WMU and Liner Combinations 1-3
6-1. Summary of IWEM Options and Parameters for WMUs 6-3
6-2. Summary of IWEM Options and Parameters for Structural Fills 6-9
6-3. Summary of IWEM Options and Parameters for Roadways 6-11
6.4. Manning's n for Typical Roadside Channels (Cho, 1959) 6-16
6-5. Summary of IWEM Infiltration and Recharge Parameters 6-19
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IWEM Technical Background Document Table of Contents
6-6. Methodology Used to Compute Infiltration for Landfills 6-21
6-7. Methodology Used to Compute Infiltration for Surface Impoundments 6-22
6-8. Methodology Used to Compute Infiltration for Waste Piles 6-23
6-9. Methodology Used to Compute Infiltration for Land Application Units 6-23
6-10. Grouping of Climate Stations by Average Annual Precipitation and Pan
Evaporation (ABB, 1995) 6-26
6-11. Hydraulic Parameters for the Modeled Soils 6-27
6-12. Moisture Retention Parameters for the Modeled Waste Pile Materials 6-29
6-13. Cumulative Frequency Distribution of Infiltration Rate for Composite-Lined
Landfills and Waste Piles 6-32
6-14. Cumulative Frequency Distribution of Leak Density for Composite-Lined Surface
Impoundments 6-33
6-15. Cumulative Frequency Distribution of Infiltration Rate for Composite-Lined
Surface Impoundments 6-33
6-16. Climatic Zones and Corresponding 5-year Average Annual Precipitations 6-36
6-17. Material Properties Used in the HELP Model to Estimate Default Infiltration
Rates for Roadway Module 6-38
6-18. Infiltration Rates (m/yr) for Common Pavement Types: Pavements and
Embankments 6-40
6-19. Infiltration Rates (m/yr) for Common Pavement Types: Shoulders 6-41
6-20. Infiltration Rates (m/yr) for Common Pavement Types: Medians 6-42
6-21. Runoff Rates (m/yr) for Common Pavement Types: Pavements and Embankments .... 6-43
6-22. Runoff Rates (m/yr) for Common Pavement Types: Shoulders 6-44
6-23. Runoff Rates (m/yr) for Common Pavement Types: Medians 6-45
6-24. Climatic Zones and Corresponding Embankment Evaporation Rates (m/yr) 6-46
6-25. Climatic Zones and Corresponding Pan Evaporation Rates (m/yr) 6-47
6-26. Summary of IWEM Subsurface Parameters 6-49
6-27. HGDB Subsurface Environments (from Newell, 1989) 6-51
6-28. Nationwide Distribution of Soil Types Represented in IWEM 6-51
6-29. Statistical Parameters for Soil Properties for Three Soil Types Used in IWEM
Development (Carsel and Parrish, 1988) 6-52
6-30. Probability Distribution of Soil and Aquifer pH 6-53
6-31. Empirical Distribution of Mean Aquifer Particle Diameter (from Shea, 1974) 6-55
6-32. Ratio Between Effective and Total Porosity as a Function of Particle Diameter
(after McWorther and Sunada, 1977) 6-55
6-33. Cumulative Probability Distribution of Longitudinal Dispersivity at Reference
Distance of 152.4m 6-56
6-34. Summary of IWEM Chemical Fate Parameters 6-56
7-1. MCLs Included in IWEM 7-1
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IWEM Technical Background Document
Executive Summary
Executive Summary
ES.1 Objectives and Background
This technical background document
provides the assumptions, methodologies,
and data used by the U.S. Environmental
Protection Agency (EPA) to develop a
ground water impact evaluation tool for
waste management units, structural fills, and
roadways in the Industrial Waste
Management Evaluation Model (IWEM).
IWEM was originally introduced as part of
the Agency's Guide for Industrial Waste
Management (U.S. EPA, 2002c; hereafter,
the Guide) in 2002. This voluntary guide
was developed by EPA and representatives
from 12 state environmental agencies to
recommend a baseline of protective design
and operating practices for managing non-
hazardous industrial waste nationwide. The
guidance was intended for facility
managers, regulatory agencies , and
interested members of the public.
The Guide recommended best management
practices and key factors to consider in all
phases of the waste management unit
lifecycle. Among the recommendations, the
Guide called for risk-based approaches to
designing waste management units, and
determining waste application rates for the
protection of ambient air quality and ground
water resources. IWEM 1.0, was the ground
water risk evaluation tool developed to
support the Guide. Over the years, several
newer versions of the model were developed
expanding the scope and usability of the
model.
IWEM 1.0 provided a tiered approach that
consisted of a national screening-level
analysis (Tier 1) and a location-specific
probabilistic analysis (Tier 2) for waste
management units (e.g., landfills, waste
piles, surface impoundments, and land
application unites). Both tiers of the model
considered all aspects of the Agency's risk
assessment paradigm (i.e., problem
formulation, exposure assessment, toxicity
assessment, and risk characterization) to
generate results that vary from a national-
level screening evaluation to a site-specific
assessment.
Building on IWEM 1.0, IWEM 2.0 added a
new evaluation tool to evaluate contaminant
releases from roadways constructed with
beneficially reused industrial materials or
byproducts. This evaluation tool was limited
to Tier 2 analysis.
IWEM 3.0 provided the same location-
specific analysis tools for waste
management units and roadways, as its
predecessors. However, Tier 1 analysis was
eliminated. In addition, it provided some
refinements to the roadway source, the
benchmarks, and the user interface.
The current version, IWEM 3.1, adds
structural fills as source. A companion
document, IWEMv3.1 User's Guide (U.S.
EPA, 2015a; hereafter, the User's Guide),
describes how to use the software.
ES.2 Overview
IWEM is a screening level ground water
model developed to evaluate ground water
impacts from industrial waste management
practices or the beneficial reuse of industrial
materials. The software helps determine the
most appropriate liner design for a waste
management unit that is protective of human
health as well as ground water resources. In
addition, the model helps determine the
appropriateness of reusing industrial
materials in beneficial use applications such
as structural fills or roadways. IWEM does
this by considering several factors that
determine the fate and transport of
contaminant in ground water, which include:
one or more types of liners (for waste
ES-1
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IWEM Technical Background Document
Executive Summary
management units) or the material properties
and structure (of a structural fill or
roadway), the expected leachate
concentrations of the anticipated waste or
reused industrial material constituents, and
the hydrogeologic conditions of the site. The
ultimate goal of the model is to minimize, or
avoid adverse ground water impacts.
IWEM models four types of waste
management units:
Landfills,
Waste piles,
Surface impoundments, and
Land application units.
For landfills, waste piles, and surface
impoundments, IWEM evaluates three liner
scenarios: no liner, single clay liner, and
composite liner. For land application units,
only the "no liner" scenario is evaluated,
because liners are not typically used for this
type of unit. The user specifies the
dimensions and other properties of the waste
management unit.
For structural fills, IWEM evaluates a single
emplacement scenario where a permanent,
unlined monofill containing a specified
mixture of reused industrial materials and
other native or non-native soils. The user
specifies the dimensions and other
properties of the structural fill.
For roadways, IWEM evaluates a variety of
roadway components, including paved areas,
medians, shoulders, ditches, embankments
and drains. The design of the roadway (and
the inclusion of various components) is user-
specified.
For waste management units, structural fills,
and roadways, IWEM uses leachate
concentrations entered by the user to model
the fate and transport of the specified
constituents from the source (the waste
management unit, structural fill or roadway)
through subsurface soils and ground water to
calculate a distribution of estimated ground
water concentrations at a downgradient
well.1 For roadways, IWEM compiles the
results for all roadway components. A
representative value from the distribution of
estimated ground water well concentrations
is compared with the reference ground water
concentrations (to user-specified
constituent- and exposure-route-specific
human health or regulatory benchmarks), to
determine whether a modeled scenario is
protective of human health (i.e., the
representative ground water well
concentration is less than or equal to the
reference ground water concentration) or
not.
Finally, IWEM compiles the results for all
constituents expected in the leachate and
then reports:
For waste management units, the
minimum liner scenario at which the
resulting ground water well
concentrations of all constituents are at
or below their respective reference
ground water concentrations. In the case
of land application units, since IWEM
only models the "no liner" scenario, the
outcome of the analysis reflects whether
resulting ground water well
concentrations from land application are
In IWEM, the term "well" is used to represent an actual or hypothetical ground water monitoring well or drinking-water well,
located downgradient from a source.
ES-2
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IWEM Technical Background Document
Executive Summary
above or below reference ground water
concentrations;
For structural fills, whether the reuse of
industrial materials in the fill application
generates ground water well
concentrations above or below a user-
specified reference ground water
concentrations; or
For roadways, whether the reuse of
industrial materials in the specified
roadway design is appropriate or not,
given the reference ground water
concentrations specified.
ES.3 Source Modeling
As noted in the Overview, IWEM models
three types of sources: waste management
units, structural fills, and roadways
constructed of reused industrial materials.
ES.3.7 Waste Management Units
The four types of waste management unit
represented in IWEM have the following
key characteristics:
Landfills. IWEM considers closed
landfills with an earthen cover and either
no liner; a single clay liner; or a
composite, clay-geomembrane liner. The
release of waste constituents into the soil
and ground water underneath the landfill
is caused by dissolution and leaching of
the constituents due to precipitation that
percolates through the landfill. The type
of liner controls the amount of leachate
that is released from the unit. Because
the landfill is closed, the concentration
of the waste constituents will diminish
with time due to depletion of landfill
wastes. The leachate concentration
value, an IWEM input, is the expected
initial leachate concentration, when the
waste is "fresh."
Waste Piles. Waste piles are typically
used as temporary storage units for solid
wastes. Due to their temporary nature,
they are typically not covered. However,
IWEM does allow use of liners similar
to landfills. In IWEM, the user specifies
the fixed operational life in years, after
which the waste pile is removed. IWEM,
therefore, models waste piles as a
temporary source.
Surface Impoundments. In IWEM,
surface impoundments are modeled as
flow-through units situated at or below
ground level. Surface impoundments
may have the same liner types as
landfills and waste piles. Release of
leachate is driven by the ponding of
water in the impoundment, creating a
hydraulic head gradient with the ground
water underneath the unit.
Land Application Units. Land
application units (or land treatment
units) are areas of land that receive
regular applications of waste that can be
either tilled into the soil or sprayed
directly onto the soil and subsequently
mixed with the soil. IWEM models the
leaching of wastes after tilling with soil
and does not account for the losses due
to volatilization during or after waste
application. Only the no-liner scenario is
evaluated for land application units,
because liners typically are not used for
this type of unit.
Releases from waste management unit
sources are modeled using EPA's
Composite Model for Leachate Migration
with Transformation Products (EPACMTP).
A small number of site-specific inputs are
required parameters for waste management
units:
Area,
Depth (landfill and surface
impoundment),
Geographic location (to select the
appropriate climate parameters).
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IWEM Technical Background Document
Executive Summary
However, the user can also enter site-
specific data for up to 20 of the most
sensitive waste management unit and
hydrogeologic characteristics to assess
whether a particular liner design will be
protective with respect to the user-specified
reference ground water concentration. In
addition, some default constituent fate
parameters can be modified, including
adding biodegradation.
ES.3.2 Structural Fills
IWEM considers structural fills to be any
one of the following permanent
constructions that include the use of
industrial wastes and related byproducts as
substitutes for earthen materials for the
support of parking lots, roads, airstrips,
tanks/vaults, and buildings; construction of
highway embankments and bridge
abutments; filling of borrow pits, and other
landscape irregularities; and changing the
landscape for development or reclamation
projects. The release of waste constituents
into the soil and ground water underneath
the structural fill is caused by dissolution
and leaching of the constituents due to
precipitation that percolates through the
structural fill.
Releases from structural fill sources are
modeled in the same way as waste
management units. However, the structural
fill source term requires additional input
parameters to calculate the amount of
teachable mass in the fill; the time it takes to
deplete the teachable mass; and to cap the
rate of percolation if material properties
limit the vertical flow of water. The
additional parameters include:
Material properties of the structural fill
(bulk density and hydraulic
conductivity), and
The fractional volume of the structural
fill occupied by teachable materials.
The user can also enter site-specific data for
sensitive hydrogeologic characteristics to
assess whether a particular use of industrial
materials in a structural fill design will be
protective with respect to a user-specified
reference ground water concentration. In
addition, some default constituent fate
parameters can be modified, including
adding biodegradation.
ES.3.3 Roadways
IWEM provides a flexible framework for
defining a roadway cross-section consisting
of one or more idealized columns to
represent the various components of a
roadway (e.g., a travel lane, median,
shoulder, or embankment). Each column
may be composed of one or more material
layers, which are assumed to be uniform
along the length of interest. Industrial
materials containing teachable components
used as structural elements in any of the
layers are treated as individual finite sources
where the leachate is released from the
bottom of the column. Much like a landfill,
the release of constituents from a layer
containing industrial materials into the soil
and ground water underneath the roadway
column is caused by dissolution and
leaching of the constituents due to
precipitation which percolates through the
column. IWEM will cap the rate of
percolation if material properties limit the
vertical flow of water. IWEM determines
the time to deplete the teachable mass in
each layer. Releases from a roadway source
are modeled with a model developed
specifically for IWEM.
Site-specific data to define the geometry and
material properties for all columns and
layers of a roadway are required inputs:
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IWEM Technical Background Document
Executive Summary
Number of roadway strips,
Roadway segment length,
Roadway geometry parameters,
Layer properties,
Ditch properties (required if a ditch is
included in the roadway scenario),
Drain properties (required if a drain is
included in the roadway scenario), and
Location of nearest down-gradient well.
The user can also enter site-specific data for
sensitive hydrogeologic characteristics to
assess whether a particular use of industrial
materials in a roadway design will be
appropriate with respect to a user-specified
saturated zones). Figure ES-1 shows a
conceptual, cross-sectional view of the
subsurface system modeled by EPACMTP
(and hence, IWEM).
EPACMTP simulates fate and transport in
both the unsaturated zone and the saturated
zone (ground water) using the advection-
dispersion equation with terms to account
for equilibrium sorption and first-order
transformation. The source of constituents
(i.e., the waste management unit, structural
fill, or roadway) is assumed to be overlying
an unconfined aquifer. The base of a waste
management unit or structural fill can be
LEACHATE CONCENTRATION
WASTE MANAGEMENT UNIT OR ROADWAY
UNSATURATED
ZONE
SATURATED
ZONE
LEACHATE PLUM
Figure ES-1 Conceptual cross-section view of the subsurface system simulated by
EPACMTP.
reference ground water concentration. In
addition, some default constituent fate
parameters can be modified, including
adding biodegradation.
ES.4 Unsaturated and Saturated
Zone Modeling
IWEM uses EPACMTP to model the fate
and transport of constituents in the
subsurface (i.e., the unsaturated and
below the actual ground surface, whereas
the base of a roadway is always assumed to
be built upon the actual ground surface.
Waste constituents leach from the base of a
source into the underlying soil. They
migrate vertically downward until they
reach the water table. As the leachate enters
the ground water, it will mix with ambient
ground water (which is assumed to be free
of pollutants) and a ground water plume will
ES-5
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IWEM Technical Background Document
Executive Summary
develop that extends in the direction of
downgradient ground water flow.
EPACMTP accounts for the spreading of the
plume in all three dimensions.
Leachate generation is driven by the
infiltration of precipitation that has
percolated through the source, from the base
of either the waste management unit,
structural fill, or roadway, into the soil.
Different waste management unit liner
designs control the rate of infiltration that
can occur. Similarly, the properties of
structural fill and roadway materials will
govern the rate of infiltration through those
sources. EPACMTP models flow in the
unsaturated zone and in the saturated zone
as steady-state processes (that is,
representing long-term average conditions).
In addition to dilution of the constituent
concentration caused by the mixing of the
leachate with ground water, EPACMTP
accounts for attenuation due to sorption of
waste constituents in the leachate onto soil
and aquifer solids, and for bio-chemical
transformation (degradation) processes in
the unsaturated and saturated zone. By
default, IWEM instructs EPACMTP to
account for chemical transformations caused
only by hydrolysis reactions. However, the
user can enter site-specific degradation rates
that include other degradation processes.
EPACMTP simulates all transformation
processes as first-order reactions (i.e.,
processes that can be characterized with a
half-life).
For organic constituents, EPACMTP models
sorption between the constituents and the
organic matter in the soil or aquifer, based
on constituent-specific organic carbon
partition coefficients, and a site-specific
organic carbon fraction in the soil and
aquifer. For metals, EPACMTP accounts for
more complex geochemical reactions by
using effective sorption isotherms for a
range of aquifer geochemical conditions.
These isotherms were generated using
MINTEQA2, EPA's geochemical
equilibrium speciation model for dilute
aqueous systems.
EPACMTP outputs the predicted maximum
ground water exposure concentration,
measured at a well situated down-gradient
from a waste management unit, structural
fill, or roadway. In IWEM, the well is
restricted to be on the plume centerline for a
waste management unit or structural fill, but
the distance (up to one mile) can be entered
as a site-specific value. For roadways,
IWEM permits more flexibility when
specifying the well location, as well as the
orientation of the roadway to the direction of
ground water flow.
ES.5 Monte Carlo
Implementation
IWEM uses Monte Carlo simulation to
determine the probability distribution of
predicted ground water concentrations as a
function of the variability of modeling input
parameters. The Monte Carlo technique is
based on the repeated random sampling of
input parameters from their respective
frequency distributions, and executing the
EPACMTP fate and transport model for
each combination of input parameter values.
At the conclusion of the Monte Carlo
analysis, it is then possible to construct a
probability distribution of ground water
concentration values and associated ground
water dilution and attenuation factors.
IWEM suggests that results are based on
Monte Carlo analyses of 10,000 realizations;
however, the number of iterations can be
changed.
IWEM selects the 90th percentile of the
predicted distribution of ground water
concentration values for comparison to user-
entered reference ground water
concentrations to determine if a waste
management unit liner scenario, structural
ES-6
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IWEM Technical Background Document
Executive Summary
fill design, or roadway design is protective
or not. The resulting location-specific
estimated ground water concentrations,
therefore, represent a 90th percentile
protection level for the specified site
conditions.
ES.6 Inputs
An IWEM evaluation uses site-specific data
for key, sensitive parameters related to
waste management units, structural fills, or
roadways, supplemented by nationwide
parameter distributions for less sensitive
parameters. The user must enter site-specific
values for some parameters, and may for
additional parameters if site-specific data are
available. If site-specific data are not
entered, values are selected probabilistically
from nationwide distributions for each
model iteration. The underlying assumption
is that the uncertainty in the value of the
parameter is captured by the nationwide
range in values of that parameter. However,
the use of site-specific data, when available,
may help to avoid unnecessarily costly
waste management unit designs or allow a
larger fraction of industrial materials to be
incorporated into a structural fill or roadway
structure if appropriate for certain kinds of
site conditions (e.g., an arid climate where
little infiltration and leaching will occur).
Site-specific data may also provide an
additional level of certainty that liner
designs are protective of sites in vulnerable
settings, such as sites with high rainfall and
shallow ground water.
ES.7 Reference Ground Water
Concentrations
Reference ground water concentrations are
maximum allowable concentrations of
constituents in ground water, based on a
specified exposure pathway (e.g.,
consumption of drinking water) and
exposure route (i.e., oral, inhalation,
dermal). IWEM evaluations incorporate
three types of reference ground water
concentrations:
Maximum Contaminant Levels
(MCLs) MCLs are available in IWEM
for some IWEM constituents. MCLs are
maximum permissible constituent
concentrations allowed in public
drinking water and are established under
the Safe Drinking Water Act. In
developing MCLs, EPA considers not
only a constituent's health effects via
consumption of drinking water, but also
additional factors, including the cost of
treatment.
Health-based numbers. IWEM allows
health-based numbers for residential
exposures to ground water to be entered
for three pathways/routes of exposure:
ingestion (of drinking water), inhalation
(of constituents that volatilize from
drinking water during household water
use), and dermal (exposure to ground
water during bathing or showering).
Health-based numbers are the maximum
constituent concentrations in ground
water that are not expected to cause
adverse noncancer health effects in the
general population (including sensitive
subgroups), or that will not result in an
additional incidence of cancer in more
than approximately one person out of a
specified number of individuals exposed
to the constituent (usually 100,000 or 1
million). Although MCLs are provided
in IWEM, health-based numbers must be
supplied by the user. One example of a
health-based numbers is the "Regional
Screening Levels for Chemical
Contaminants at Superfund Sites"
(http://www.epa. gov/reg3 hwmd/ri sk/hu
man/rb-concentration_table/index.htm).
Other Drinking Water Standards. These
are comparable to MCLs, but state
standards may be more stringent than
federal MCLs. These must be entered by
the user.
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IWEM Technical Background Document
Executive Summary
IWEM evaluations do not consider
combined exposure from ground water
ingestion (from drinking water), ground
water inhalation (from showering or other
household uses), or dermal exposure to
ground water (while showering or bathing),
nor do they consider the potential synergistic
effect of exposure to multiple constituents.
ES.8 IWEM Recommendations
The IWEM tool provides recommendations
for waste management units and beneficial
reuse of industrial materials in structural fills
and roadways in terms of how the estimated
90th percentile ground water concentration
compares to reference ground water
concentrations. IWEM uses ground water
modeling to predict expected waste- and
site-specific ground water exposure
concentrations for all waste constituents
evaluated. IWEM then compares the
estimated exposure concentrations to
reference ground water concentrations to
determine whether or not a liner is
protective for waste management units, or
the reuse of industrial materials for
structural fill or roadway design is
appropriate for each constituent evaluated.
For waste management units that can have
liners, the final IWEM liner
recommendations are based on the minimum
liner design that is protective for all
constituents.
ES-8
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IWEM Technical Background Document Introduction
1.0 Introduction
This document provides technical information on the Industrial Waste Management Evaluation
Model version 3.1 (IWEM 3.1). A companion document, Industrial Waste Management
Evaluation Model (IWEM) v3.1: User's Guide (hereafter, IWEM 3.1 User's Guide; U.S. EPA,
2015a) provides detailed information on installation and use of the IWEM software.
Section 1.1 describes how IWEM was developed. Section 1.2 provides an overview of IWEM's
design (i.e., what IWEM does and key components of IWEM). Section 1.3 provides a guide to
the organization of the rest of this document. Appendix A is a glossary that defines many of the
technical terms used in this document.
1.1 The Guide for Industrial Waste Management and IWEM
The U.S. Environmental Protection Agency (EPA or the Agency) and representatives from 12
state environmental agencies developed a voluntary Guide for Industrial Waste Management
(hereafter, the Guide) to recommend a baseline of protective design and operating practices to
manage non-hazardous industrial waste throughout the country. The guidance was designed for
facility managers, regulatory agency staff, and the public, and it reflects four underlying
objectives:
Adopt a multimedia approach to protect human health and the environment;
Tailor management practices to risk using the innovative, user-friendly modeling tools
provided in the Guide;
Affirm state and tribal leadership in ensuring protective industrial waste management,
and use the Guide to complement state and tribal programs; and
Foster partnerships among facility managers, the public, and regulatory agencies.
The Guide recommended best management practices and key factors to consider in all phases of
the lifecycle of a waste management unit (WMU):
Protecting ground water, surface water, and ambient air quality during the design, siting,
and operation of WMUs;
Monitoring the impact of WMUs on the environment;
Determining necessary corrective action;
Closing WMUs; and
Providing post-closure care.
In particular, the Guide recommended risk-based approaches to designing liner systems,
determining waste application rates for ground water protection, and evaluating the need for air
controls. The original version of the Guide included user-friendly air and ground water models to
conduct these risk evaluations. The IWEM software distributed with the Guide (on a CD), IWEM
1.0, was the ground water tool developed to support the Guide. IWEM 1.0 provided a tiered
approach that consisted of a national screening-level analysis (Tier 1) and a location-adjusted
probabilistic analysis (Tier 2).
Over the years, the Agency made progress in regulating the proper management of industrial
wastes under the Resource Conservation and Recovery Act. The current challenge for EPA is to
1-1
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IWEM Technical Background Document Introduction
find ways to repurpose manufacturing and power generation byproducts, historically viewed as
"wastes," in beneficial ways while minimizing human and environmental impacts. To address
this need, the IWEM software has been expanded and enhanced to provide a risk-based approach
for evaluating the use of materials generated from industrial processes in structural fills and
roadways. The current version of the IWEM software, IWEM 3.1, is described in this Technical
Background Document and provides the same location-adjusted analysis tools as previous
versions for WMUs, and structural fills and roadways containing beneficially reused industrial
byproducts. However, the national screening-level analysis is not available in this version. A
complete history of the software development from version 1.0 to the current version 3.1 is
provided at the beginning of this document under the heading Software Development History.
IWEM helps determine the most appropriate design for a WMU or evaluates the design of a
structural fill or roadway built with reused industrial materials to minimize or avoid adverse
ground water impacts by evaluating
One or more liner scenarios for WMUs or the material properties and structure of a
structural fill or roadway;
The hydrogeologic conditions of the site; and
The expected leachate concentrations of the anticipated waste or reused industrial
material constituents.
The evaluation is completed by comparing the estimated ground water concentrations to user-
specified constituent- and exposure-route-specific human health benchmarks, called reference
ground water concentrations (RGCs) in IWEM.
The anticipated users of the original IWEM software were managers of proposed or existing
units, state regulators, interested private citizens, and community groups. For example:
Managers of a proposed unit may use the software to determine what type of liner
would be appropriate for the particular type of waste that is expected at the WMU and the
particular hydrogeologic characteristics of the site.
Managers of an existing unit may use the software to determine whether to accept a
particular waste at that WMU by evaluating the performance of the existing liner design.
State regulators may use the software to develop permit conditions for a WMU.
Interested members of the public or community groups may use the software to
evaluate a particular WMU and to participate during the permitting process.
In addition, the incorporation of the structural fill and roadway modules provide state and local
regulators, engineers, and other stakeholders (e.g., generators, beneficial users, and the public) a
tool that can be used to determine if the reuse of industrial materials is environmentally sound.
The unique aspect of the IWEM software is that it allows the user to perform location-adjusted
analyses and obtain either liner recommendations or beneficial reuse evaluations in structural
fills or roadways with minimal data requirements. Users interested in a comprehensive and
detailed site assessment are directed to the Guide for information regarding the selection of an
appropriate site-specific ground water fate and transport model.
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IWEM Technical Background Document
Introduction
1.2 IWEM Design
1.2.1
What Does the Software Do?
IWEM uses a probabilistic (Monte Carlo) approach and a ground water fate and transport model
to calculate a distribution of estimated ground water concentrations at a well resulting from the
release of leachate containing dissolved constituents at concentrations entered by the user. IWEM
then compares the 90th percentile of the distribution of estimated ground water concentrations to
RGCs (constituent- and pathway-specific human health benchmarks provided by the user) to
determine if the modeled liner scenario is protective, or whether the beneficial use of industrial
material is appropriate (i.e., the representative ground water well concentration is less than or
equal to the RGC).
In the case of WMUs, IWEM helps the user determine a recommended liner design that will
minimize the potential for adverse ground water impacts caused by the leaching of waste
constituents. The IWEM tool compares the estimated exposure concentration calculated by a
ground water fate and transport model for three standard liner types. The IWEM software
compiles the results for all constituents expected in the leachate and then reports the minimum
liner scenario that is protective for all constituents. Table 1-1 shows the WMU types and liner
types that are evaluated in IWEM. For land application units only the "No Liner" scenario is
evaluated, because liners are not typically used at this type of facility.
Table 1-1. IWEM WMU and Liner Combinations
WMU Type
Landfill
Waste Pile
Surface Impoundment
Land Application Unit
Liner Type
No Liner (in-situ soil)
S
S
S
S
Single Clay Liner
S
S
S
X
Composite Liner
S
S
S
X
S= applies to WMU * = does not apply to WMU
For structural fills, similar to waste management units, the IWEM software calculates
distributions of estimated ground water well concentrations for all teachable constituents present
in the reused industrial materials included in the structural fill.
For roadways, which can include multiple structural components (strips and layers), the IWEM
software calculates distributions of estimated ground water well concentrations for all teachable
constituents present in the reused industrial materials for each roadway component containing
teachable constituent mass. For each constituent, IWEM then sums the 90th percentiles of these
distributions across all strips leaching that constituent to obtain the aggregate 90th percentile
ground water estimated exposure concentration for comparison to the RGC(s) for that
constituent.
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IWEM Technical Background Document Introduction
1.2.2 IWEM Components
The IWEM software consists of four main components:
Graphical User Interface: The IWEM user interface consists of a series of user-friendly
data input and display screens guide the user through defining all aspects of an IWEM
evaluation. The user interface provides a tailored front-end to the EPACMTP
computational engine and built-in databases that support IWEM. The user interface
module is described in detail in the companion to this Technical Documentation, the
IWEMv3 User's Guide (U.S. EPA, 2015a).
Source Term Modules: Source term modules represent the key characteristics and
processes that simulate the release of dissolved constituents in leachate from a source (the
WMU, structural fill, or roadway) to the subsurface. For WMUs and structural fills,
IWEM uses EPACMTP to model the source, by passing information to EPACMTP to
represent the source. For roadways, a special module determines constituent
concentrations in leachate over time and prepares an additional input file for EPACMTP
containing the release information for fate and transport modeling.
Fate and Transport Model: IWEM uses EPACMTP to simulate the migration of
chemical waste constituents in leachate through the subsurface (i.e., soil and ground
water). In an IWEM evaluation, the fate and transport simulation is performed directly
inside the IWEM tool. EPACMTP is described in detail in the EPACMTP Technical
Background Document and its Draft Addendum (U.S. EPA, 2003 a, c). This IWEM
technical documentation discusses the application of EPACMTP as part of IWEM.
Databases: IWEM contains an integrated set of databases that include waste constituent
properties and default ground water modeling parameters for IWEM evaluations.
- The waste constituent database includes 206 organic chemicals and 25 metals. The
constituent databases include physical and chemical data needed for ground water
transport modeling, as well as maximum constituent levels (MCLs) (which can be
selected by the user as RGCs). Appendix B provides a complete list of all
constituents and constituent property data.
- The ground water modeling parameters database includes infiltration rates for
different WMU types and liner designs and roadway materials for a range of locations
and climatic conditions throughout the United States, and soil and hydrogeological
data for different soil types and aquifer conditions.
Details of the databases are provided in this background document, and in the EPACMTP
Parameters/Data Background Document and its Draft Addendum (U.S. EPA, 2003b, d).
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IWEM Technical Background Document Introduction
1.3 About This Document
The remainder of this document is organized as follows:
Section 2.0 (Overview of the Approach) presents the purpose and methodology of the
IWEM analysis tools;
Section 3.0 (Source Modeling) describes how IWEM represents the releases of
constituents in leachate to the subsurface fate and transport model;
Section 4.0 (Unsaturated and Saturated Zone Modeling Using EPACMTP ) provides an
overview of the EPACMTP ground water simulation model;
Section 5.0 (Conducting Probabilistic Analyses Using EPACMTP) shows how the Monte
Carlo module in EPACMTP generates distributions of estimated ground water exposure
concentrations;
Section 6.0 (How EPA Developed the IWEM Evaluation) describes the application of
EPACMTP for the development of the IWEM tools, highlighting the input parameters
used for WMU, structural fill, and roadway analyses;
Section 7.0 (Reference Ground Water Concentrations) describes the types of RGCs
IWEM can use, and lists the MCLs included in IWEM;
Section 8 .0 (How IWEM Makes Recommendations) describes how IWEM uses the
results of EPACMTP and user-supplied RGCs to provide protective liner
recommendations for a WMU, or determine the appropriateness of using industrial
materials in a structural fill or roadway application;
Section 9.0 (References) lists literature references that are cited in the document;
Appendix A (Glossary) presents descriptions of the technical terms used in this
document;
Appendix B (List of IWEM Waste Constituents and Default Physical and Chemical
Property Data) presents the list of chemicals included in IWEM and the default values
for the chemical-specific inputs;
Appendix C (Formulation of the Roadway Module) presents the technical details on the
development of the roadway source term module;
Appendix D (Infiltration Rate Data for WMUs and Structural Fills) presents infiltration
rate data for WMUs and structural fills;
Appendix E (Infiltration Rates Through Pavements) presents the development of
infiltration rate data for roadway materials available in IWEM;
Appendix F (Formulation of Non-Orthogonality between the Highway Axis and the
Regional Ground Water Flow Direction) presents the methodology used in IWEM to
represent roadways overlying ground water flow systems that are not perpendicular to the
travel direction; and
Appendix G (Verification of the Roadway Module in IWEM) describes the verification of
the roadway module.
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IWEM Technical Background Document Introduction
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IWEM Technical Background Document Overview of the Approach
2.0 Overview of the Approach
This section provides an overview of the methodology used to develop IWEM. Section 2.1
discusses the purpose of the tools included in IWEM: WMUs, structural fills, and roadways.
Section 2.2 describes the approach used to develop IWEM and the input parameters required to
run the model for all three cases.
2.1 Purpose of the Tool
IWEM analyzes the potential ground water impacts of managing a waste in four types of WMU
(landfills, surface impoundments, waste piles, and land application units); three WMU liner
scenarios (no liner, single clay liner, and composite liner); and a structural fill or roadway
containing beneficially reused industrial materials. The purpose of an IWEM evaluation for
WMUs is to determine the minimum recommended liner design generates an estimated 90th
percentile ground water concentration that is equal to or less than a user-specified RGC for all
constituents in the waste of concern. For structural fills and roadways, an IWEM evaluation
determines if a specific fill or roadway design containing beneficial used industrial materials,
generate an estimated 90th percentile ground water concentration equal to or less than a user-
specified RGC for all constituents of concern in the materials.
IWEM chooses a high-end (i.e., 90th percentile) point estimate from a distribution of potential
ground water exposure concentrations, because the use of conservative data and assumptions
allows protective decisions to be made quickly and with greater confidence. This is a common
approach used by EPA for screening-level analyses.
2.1.1 WMU Evaluation
The most effective approach to managing the release of waste constituents from a WMU to the
subsurface is to install a low permeability liner at the base of the WMU. A liner generally
consists of a layer of clay or other materials (e.g., geomembrane and geotextiles) with a low
hydraulic conductivity that is used to prevent or mitigate the flow of liquids from a WMU. The
amount of liquid that migrates into the subsurface from a WMU has been shown to be a highly
sensitive parameter in predicting the release of constituents to ground water (and hence, human
health risks). However, the type of liner necessary to protect human health for a specific WMU
depends heavily on location-specific conditions such as climate and hydrogeology. Therefore,
one of the main objectives of the IWEM modeling approach for WMUs is to evaluate the
appropriateness of a proposed liner design in the context of other location-specific parameters
such as the long-term recharge rate of water influenced by local precipitation and evaporation,
and the hydrogeologic characteristics of the soil and aquifer beneath a facility.
EPA chose to evaluate three liner scenarios, which are shown in Figure 2-1: no-liner, single-
liner, and composite-liner. The no-liner design (Figure 2-la) represents a WMU that relies on
location-specific conditions such as low permeability native soils beneath the unit or low annual
precipitation rates to mitigate the release of constituents to ground water. The single-liner design
(Figure 2-lb) represents a 3-foot-thick (0.91 m) clay liner with a low hydraulic conductivity
(IxlO"7 cm/sec). The composite liner design (Figure 2-lc) consists of a 1.5 mm high-density
polyethylene layer underlain by either a geosynthetic clay liner with a maximum hydraulic
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IWEM Technical Background Document
Overview of the Approach
conductivity of 5x 10~9 cm/sec or a three-foot (0.91 m) compacted clay liner with a maximum
hydraulic conductivity of IxlO"7 cm/sec.
Was*
'"::.:.Native'!Soil:::'.-.;J Compacted Clay
'**..' i ** -*!
Waste
HOPE
Waste
Compacted Clay
Geosynthetic Liner'
a) No-Liner Scenario b) Single Liner Scenario c) Composite Liner Scenario
Figure 2-1. Three liner scenarios considered in IWEM.
For a given waste management scenario and waste leachate concentration, IWEM uses ground
water modeling to estimate the ground water concentration at a well located downgradient from
the WMU. IWEM then compares the 90th percentile estimated ground water concentration to
established regulatory RGCs (i.e., an MCL) or to user-supplied, RGCs (see Section 7 for a
discussion of the types of RGCs that can be entered in IWEM). The recommended liner design is
the minimum liner for which the estimated ground water concentration of all constituents is less
than their selected RGC. For land application, the model evaluates whether waste can be applied
on land without adverse impacts to ground water; only the "no liner" scenario is considered,
because land application units do not typically have liners.
2.1.2
Structural Fill Evaluation
Structural fills evaluated in IWEM include the use of industrial materials and related byproducts
as substitutes for earthen materials for support of parking lots, roads, airstrips, tanks/vaults, and
buildings; construction of highway embankments and bridge abutments; filling of borrow pits,
and other landscape irregularities; and changing the landscape for development or reclamation
projects. IWEM can evaluate structural fills using both flowable fill and compacted installation
methods. The general conceptual model for a structural fill is very similar to that of the unlined
landfill scenario, graphically depicted by Figure 2-la with two fundamental exceptions. First, the
waste region shown in Figure 2-la is assumed to consist of either all reused industrial materials
or a mixture of reused materials and other inert earthen materials. Second, the duration of
leaching for structural fills is calculated and completely dependent upon user-supplied
information about material properties and concentrations of constituents in the materials.
For a given structural fill geometry and material and leachate concentrations, IWEM uses ground
water modeling to predict the groundwater concentration at a well located down gradient from
the source. IWEM compares the 90th percentile predicted exposure concentration to established
regulatory RGCs (e.g., the MCL) or to user-supplied benchmark RGCs. If the estimated 90th
percentile ground water concentrations of all constituents are at or below their respective
benchmarks, then the structural fill application is deemed appropriate. The user should consider
the conclusions in the context of the underlying assumptions of the model and the input data.
2.1.3 Roadway Evaluation
IWEM provides a flexible framework for defining a roadway cross-section consisting of one or
more idealized columns to represent the various components of a roadway (e.g., lanes, median,
shoulder, or embankment). Figure 2-2 shows an example of a roadway cross section comprising
three roadway-source strips representing, respectively, a median, a lane, and a ditch. Note that a
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IWEM Technical Background Document
Overview of the Approach
more typical roadway may consist of up to 15 roadway-source strips: for example, left shoulder,
left-lane, median, right-lane, and right shoulder. More strips are possible to account for drainage
ditches, berms, and different configurations of layers; the IWEM roadway module limits the total
number of roadway-source strips to 15. An example of only three roadway source strips is used
here as a basis for further discussion.
Each roadway source strip may consist of several layers, depending on how a given roadway was
constructed. A traffic lane strip may be composed of a pavement layer (such as portland cement
concrete or asphalt concrete), a base-course layer, a subbase layer, and a subgrade layer. A
median may comprise a base layer, a subbase layer, and a subgrade layer. An unpaved road
shoulder may have only one layera subgrade layer. Industrial materials containing teachable
components used as structural elements in any of the layers are treated as individual finite
sources where the leachate is released from the bottom of the column. Much like a landfill, the
release of constituents from a layer containing industrial materials into the soil and ground water
underneath the roadway column is caused by dissolution and leaching of the constituents due to
precipitation that percolates through the column, a key sensitive input. IWEM will cap the rate of
percolation if material properties limit the vertical flow of water. IWEM determines the time to
deplete the teachable mass in each layer.
For a given roadway scenario and material and leachate concentrations, IWEM uses ground water
modeling to develop a probability distribution of the groundwater concentration at a well located
downgradient from the roadway for each of the column strips containing teachable constituent
materials. If only one strip is modeled, IWEM compares the 90th percentile predicted exposure
concentration to established regulatory RGCs (e.g., the MCL) or to user-supplied benchmark
RGCs. If all of the estimated 90th percentile ground water concentrations of all constituents are
at or below their respective benchmarks, then the roadway design is deemed appropriate based on
user-supplied information and assumptions. For roadways with multiple strips with teachable
content, IWEM develops a distribution of exposure concentrations for each constituent for each
roadway strip containing teachable constituent mass. IWEM then sums the 90th percentiles of
these distributions across all strips leaching that constituent to obtain the aggregate 90th
percentile ground water exposure concentration for comparison to the RGC(s) for that
constituent. Aggregated ground water concentrations for any constituent are not allowed to
exceed the maximum expected leachate concentration for that constituent.
Travel Lane
Road Shoulder,
'Embankment, and Ditch
Layer 3
Layer 2
Layer 1
Pavement
Layer 4
Base
Layeft
Subbase
Layer2
Subgrade
LayeM
Layer 1
Source Strip 3 Source Strip 2 Source Strip 1
Figure 2-2. Generalized roadway scenario considered in IWEM
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Overview of the Approach
EPACMTP consists of four major components:
A source module that simulates the rate and
concentration of leachate exiting from
beneath a WMU, structural fill, or accepts
pre-simulated releases for a roadway and
entering the unsaturated zone. The source
model aspects of EPACMTP are described in
Section 3.
An unsaturated zone module which
simulates 1-D vertical flow of water and
dissolved constituent transport in the
unsaturated zone. The unsaturated zone
aspects of EPACMTP are described in
Section 4.
A saturated zone module which simulates
ground water flow and dissolved constituent
transport in the saturated zone. The
saturated zone aspects of EPACMTP are
described in Section 4.
A Monte Carlo module for randomly
selecting input values to account for the
effect of variations in model parameters on
estimated ground water well concentrations,
and determining the probability distribution of
estimated ground water concentrations. The
Monte Carlo aspects of EPACMTP are
described in Section 5.
2.2 Approach Used to Develop the Tool
IWEM uses EPACMTP to model the WMU and
structural fill sources, and an external roadway
source module to model the roadway source (see
Section 3). IWEM also relies on EPACMTP to
model subsurface fate and transport of
contaminants leaching from WMU, structural fill,
or roadway sources (see Section 4). In addition,
IWEM utilizes EPACMTP's Monte Carlo
capabilities (see Section 5) to account for
variability that occur in climate conditions, soil
types, and subsurface characteristics.
IWEM links EPACMTP and the roadway source
module to a series of databases behind a user-
friendly interface. The databases describe source
characteristics, hydrogeological characteristics, and
constituent fate and transport data (see Section 6).
An IWEM evaluation for a WMU, structural fill, or
roadway consists of a comparison of an estimated
ground water well concentration to a health-based
number (user-specified benchmark) or an MCL
(provided in IWEM). The representative ground water concentration is derived from a
probabilistic ground water fate and transport simulation using EPACMTP. Based on the
comparisons, IWEM recommends the minimum protective liner design for WMUs, or
determines the appropriateness of reusing of industrial waste materials in structural fills or
roadways.
Figure 2-3 depicts a cross-sectional view of the subsurface system simulated by EPACMTP. For
the purposes of simplicity, the WMU, structural fill, and roadway are considered as just a
"source." EPACMTP treats the subsurface aquifer system as a composite domain, consisting of
an unsaturated (vadose) zone and an underlying saturated zone. The two zones are separated by
the water table.
IWEM uses EPACMTP in a probabilistic (Monte Carlo) mode to generate a probability
distribution of well concentrations that reflects the variability in the various modeling
parameters, for instance the variation of rainfall rate across the United States. IWEM uses the
90th percentile exposure concentration to represent the estimated constituent concentration at a
well for a given leachate concentration. The 90th percentile exposure concentration is determined
by running EPACMTP in a Monte Carlo mode for 10,000 realizations (the user also has the
option of adjusting the number of simulations). For each realization, EPACMTP calculates a
peak and one or more maximum time-averaged concentrations at a well, depending on the
exposure duration of the RGC of interest. For example, if a 30-year exposure duration for
carcinogens is specified, the maximum time-averaged concentration is the highest 30-year
average across the modeling horizon. To enable the IWEM software to perform the Monte Carlo
analyses required for an evaluation on common desktop computer systems, the implementation
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Overview of the Approach
of EPACMTP in IWEM uses a computationally efficient pseudo-3-D approximation for
modeling saturated zone plume transport (see Section 4.2).
LEACHATE CONCENTRATION
WASTE MANAGEMENT UNIT OR ROADWAY
Figure 2-3. Conceptual cross-section view of the subsurface system simulated by EPACMTP.
After calculating the maximum time-averaged concentration for each of the 10,000 realizations,
the concentrations are arrayed from lowest to highest and the 90th percentile of this distribution
is selected as the constituent exposure concentration. IWEM then directly compares the estimated
90th percentile ground water concentrations generated by EPACMTP to the respective RGCs for
those constituents to determine whether a particular liner scenario is protective or not; similarly,
following the structural fill and roadway evaluation, IWEM makes a determination on the
appropriateness of using industrial materials for these beneficial use applications. If the estimated
ground water concentration is less than the RGC for every constituent, then the modeled source
scenario being evaluated is deemed protective (for WMUs) or appropriate (for beneficial use). If
the estimated exposure ground water concentration of any waste constituent exceeds its RGC,
then the scenario is not protective.
For WMU analyses, up to 20 of the most sensitive site-specific, waste-specific and hydrogeologic
characteristics can be entered to assess whether a particular design will be protective with respect
to a user-specified benchmark. In addition, some default constituent fate parameters can be
modified, including adding biodegradation.
A small number of site-specific parameters are required inputs for WMUs:
WMU area
WMU depth (landfill and surface impoundment)
WMU geographic location (to select the appropriate climate parameters).
Structural fills are treated very similarly to the unlined landfill type WMU with the addition of
three new required parameters for the reused industrial materials or the material mixture that
includes the industrial byproduct:
Hydraulic conductivity
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IWEM Technical Background Document Overview of the Approach
Dry bulk density
The ratio of the volume of industrial materials to the volume of the structural fill.
However, the user can also enter site-specific data for up to 20 of the most sensitive source
(WMU or structural fill) and hydrogeologic characteristics to assess whether a particular liner
design or beneficial reuse will be protective or appropriate (for structural fills) with respect to the
user-specified RGC. In addition, some default constituent fate parameters can be modified,
including adding biodegradation. See Section 6 for more details on specific inputs.
For roadway analyses, site-specific data to define the geometry and material properties for all
columns and layers are required inputs:
Number of roadway strips
Roadway segment length
Roadway geometry parameters:
- Strip type
- Width
- Numb er of 1 ay ers
Layer properties:
- Layer type
- Thickness
- Hydraulic conductivity of layer material
- Dry bulk density of layer material
Ditch properties (required if a ditch is included in the scenario):
- Manning's n coefficient
- Slope of the ditch
- Maximum water depth in the ditch
- Location of gutter(s)
Drain properties (required if a drain is included in the scenario):
- Thickness
- Hydraulic conductivity
- Bulk density of layer material
Location to the nearest down-gradient well
- Distance along roadway edge from midpoint to location of previous measurement
- The angle between the ground water flow direction and the edge of the roadway
- The location setting of the receptor well with respect to the roadway and the ground
water flow direction
However, the user can also enter site-specific data for sensitive hydrogeologic characteristics to
assess whether a particular use of industrial materials in a roadway design will be appropriate
with respect to a user-specified RGC. In addition, some default constituent fate parameters can
be modified, including adding biodegradation. See Section 6 for more details on specific IWEM
inputs.
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Source Modeling
IWEM's Source Term Modules
Integrated into EPACMTP:
- Landfill
- Waste Pile
- Surface Impoundment
- Land Application Unit
- Structural Fill (modeled like WMUs)
External:
Roadway (executes prior to
EPACMTP and passes magnitude
and duration of leaching to
EPACMTP)
3.0 Source Modeling
IWEM uses EPACMTP's built-in source term
modules to simulate the leachate flux from WMUs
(landfill, waste pile, surface impoundment, and land
application unit) to the underlying unsaturated zone.
Although the structural fill source term is not directly
integrated into EPACMTP, IWEM utilizes the
WMUs source term module in EPACTMP to
estimate the leachate flux from structural fills.
Improvements to EPAMCTP have made it possible to
couple external source term modules to EPACMTP
to expand the applicability of the ground water model. The roadway source term is an external
module developed specifically for IWEM to address the beneficial reuse of industrial materials as
structural components of a roadway design.
The purpose of these sources is to provide EPACMTP with a leachate concentration and a long-
term infiltration rate for a period of time to simulate the vertical migration of the leached
constituents and water through the unsaturated zone to the water table and into the saturated
zone. The period of time that constituents leach from the source may be predetermined (e.g., the
operating life of the WMU equals the leaching duration); nearly infinite, as is the case for
landfills; or derived from user inputs (e.g., structural fills and roadways). The leachate
concentration value might change over time, as well.
This section will review the internal and external source modules available in IWEM: Section
3.1 describes the WMU source modules built into EPACMTP; Section 3.2 describes the
structural fill source module; and Section 3.3 describes the external roadway source module
briefly. Additional details on the roadway source-term module are presented in Appendix C. For
some modeling scenarios, either the structural fill or roadway module may be used; Section 3.4
provides guidance on deciding which is more appropriate for a particular scenario.
3.1 WMU Source Modules
This section describes how EPACMTP models the release of constituents from a WMU:
Section 3.1.1 provides a general overview of the EPACMTP source module; Section 3.1.2
presents a discussion of how EPACMTP handles infiltration from surface impoundment units;
and Section 3.1.3 identifies the assumptions and limitations of the WMU source terms.
3.1.1
Releases From a WMU Source
The purpose of the WMU source module in EPACMTP is to provide a leachate flux and
concentration to the unsaturated zone. The source module relies on the design and operational
characteristics of the WMU and the waste stream characteristics (quantity and concentrations);
specifically, it uses four primary parameters provided by IWEM user:
Area of the waste unit;
Leachate flux rate emanating from the waste unit (infiltration rate);
Constituent-specific leachate concentration; and
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Leaching duration.
Based on these parameters, EPACMTP generates a rate of leaching and the constituent
concentration in the leachate as a function of time from the bottom of the WMU.
Mathematically, EPACMTP regards the source as a rectangular planar area located between the
bottom of the WMU and the top of the unsaturated zone column, through which leachate passes.
The WMU source module estimate the magnitude of the rate of water infiltration and constituent
concentration crossing this plane. The model does not attempt to account explicitly for the
multitude of physical and biochemical processes inside the waste unit that may control the
release of waste constituents to the subsurface. Instead, the net results of these processes are used
as inputs to the model. For instance, for landfills, waste piles, and land application units, the
Hydrologic Evaluation of Landfill Performance (HELP) model (Schroeder et al, 1994) was used
to estimate infiltration rates for unlined and single clay lined units outside of EPACMTP, and
those infiltration rates are then supplied as inputs to EPACMTP (see Appendix D for the
infiltration rates and supporting data). Likewise, the model does not explicitly account for the
complex physical, biological, and geochemical processes that may influence leachate
concentration. These processes are typically estimated outside the EPACMTP model using
geochemical modeling software, equilibrium partitioning models, or analytical procedures such
as the Toxicity Characteristic Leaching Procedure or Synthetic Precipitation Leaching Procedure
test; the resulting leachate concentration is then used as an EPACMTP input.
EPACMTP can model sources as continuous or finite. Continuous sources are characterized by a
fixed leachate concentration for an infinite time. Under these conditions, the ground water
concentration at the modeled receptor well location will eventually reach a constant value. For
finite source conditions, the leachate concentration is a function of time, and the constituent
presents in the leachate for a finite time. EPACMTP models the duration of finite source leachate
concentration releases in one of two ways:
Depleting source: the WMU is considered permanent, and leaching continues (at a
concentration that varies over time) until all waste that was originally present has been
depleted; or
Pulse source: the WMU is considered temporary, and leaching occurs at a constant
leachate concentration for a specified fixed time, after which clean closure occurs and
leaching stops.1 Usually the leaching period represents the operational life of the unit.
In IWEM, waste piles, surface impoundments, and LAUs are modeled as pulse (finite) sources.
Landfills are modeled as depleting finite sources; however, in practice, the duration of leaching is
long enough for landfills that the landfill source behaves like a continuous source.
Figure 3-1 graphically presents the leachate concentration under the depleting source (dashed
line) and pulse source (solid line) scenarios. In the depleting source scenario, the leachate
concentration gradually decreases over time. The user must provide a value for the initial
leachate concentration (for example, a measured value from a leaching test), and EPACMTP will
calculate the rate of depletion as a function of the infiltration rate through the unit. The
EPACMTP Technical Background Document (U.S. EPA, 2003a) provides a detailed discussion
of the depleting source scenario. In the pulse source scenario, the user must provide the value of
If the leaching period is set to a very large value, EPACMTP will simulate continuous source conditions.
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Source Modeling
the leachate concentration (for example, a measured value from a leaching test) and the duration
of the leaching period. Based on these values, EPACMTP will calculate the leachate pulse.
. Initial Leachate Concentration
, Pulse Source
, Depleting Source
Time *
Figure 3-1. Leachate concentration vs. time for pulse and depleting source scenarios.
3.1.2 Infiltration Rate for Surface Impoundments
Because the infiltration rate from surface impoundments is controlled primarily by the unit's
engineering and operational characteristics rather than external climate factors, the EPACMTP
source module includes the capability to calculate surface impoundment infiltration rates as a
function of impoundment depth and other surface impoundment parameters. In particular, the
surface impoundment module calculates the infiltration rate through a zone of reduced
permeability materials (which may or may not include engineered liners) at the base of the
impoundment. The various reduced permeability layers represented in the surface impoundment
infiltration module are depicted graphically in Figure 3-2.
Ground
Surface
Elevation
Top of Liquid Compartment
Ground
Surface
Elevation
Unaffected Native Material
,, Infiltration
Water
Table
Figure 3-2. Surface impoundment infiltration module.
EPACMTP assumes that while the impoundment is in operation, a layer of fine-grained sediment
("sludge") naturally accumulates at the bottom of the impoundment as the result of the settling of
suspended solids in the waste liquid. The upper half of this layer consists of unconsolidated
material; the lower half is consolidated (compacted) due to the weight of the sediment and
wastewater above it. EPACMTP calculates the effective hydraulic conductivity of the
consolidated sediment layer as a function of its porosity, using an empirical relationship based on
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IWEM Technical Background Document Source Modeling
work of Lambe and Whitman (1969), which results in a calculated hydraulic conductivity on the
order of 1 x 10"7 to 6x 10"7 cm/s. The module also takes into account the hydraulic properties of a
clay liner (if present), as well as the properties of the native soil underlying the impoundment. If
no liner is present, EPACMTP assumes that over time, the upper soil layer becomes clogged due
to deposition of solids from the impoundment. The thickness of this clogged layer is always
assigned a value of 0.5 m, and the hydraulic conductivity of this clogged layer is assigned a value
that is 10% of the hydraulic conductivity of the native soil material.
If a clay liner is present, the liner replaces the clogged native material layer that is depicted in
Figure 3-3. If EPACMTP is used to model a lined surface impoundment, the thickness and
hydraulic conductivity of the clay liner are model inputs.
The EPACMTP surface impoundment module calculates the steady state infiltration rate through
the multi-layer system of sediment-clogged native soil/clay liner-native soil by applying the one-
dimensional Richards equation (Jury et al., 1991) with a constant head boundary condition that is
given by the ponding depth of the impoundment. EPACMTP uses the Richards equation to
accommodate partially saturated conditions that may exist in the multi-layer system. For a
detailed description of the solution of the Richards equation for the system, see the EPACMTP
Technical Background Document (U.S. EPA, 2003 a).
3.1.3 Assumptions and Limitations for WMU Source Modeling
EPA designed the EPACMTP fate and transport model to be used for regulatory assessments in a
probabilistic framework. The simulation algorithms that are incorporated into the model are
intended to meet the following requirements:
Account for the primary physical and chemical processes that affect constituent fate and
transport in the unsaturated and saturated zone;
Run (and produce useful results) with relatively little site input data; and
Be computationally efficient for Monte Carlo analyses.
This section discusses the primary assumptions that EPA made in developing the EPACMTP
WMU source module to balance these competing requirements, and the resulting limitations.
EPACMTP may not be suitable for all sites, and the user should understand the
capabilities and limitations of the model to ensure that it is used appropriately.
The EPACMTP source module provides a relatively simple representation of different types of
WMUs. WMUs are represented in terms of a source area and a defined rate and duration of
leaching. EPACMTP only accounts for the release of leachate through the base of the WMU and
assumes that the only mechanism of constituent release is through dissolution of waste
constituents in the water that percolates through the WMU. In the case of surface impoundments,
EPACMTP assumes that the leachate concentration is the same as the constituent concentration
in the wastewater in the surface impoundment. EPACMTP does not account for the presence of
non-aqueous free-phase liquids, such as an oily phase that might provide an additional release
mechanism into the subsurface. EPACMTP does not account for releases from the WMU via
other environmental pathways, such volatilization or surface run-off EPACMTP assumes that
the rate of infiltration through the WMU is constant, representing long-term average conditions.
EPACMTP does not account for fluctuations in rainfall rate or degradation of liner systems that
may cause the rate of infiltration to vary over time. It is important to note that while the
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concentration of constituents dissolved in infiltrating water may change over time, the rate at
which water infiltrates to the subsurface must remain constant to satisfy steady state flow
assumptions EPACMTP cannot simulate stored solid wastes managed in landfills, waste
piles, or land application units that are in direct contact with the water table
Three of the four WMUs in IWEM and EPACMTP (surface impoundment, waste pile, and land
application unit) are considered finite sources, and their leaching durations are determined by the
user-supplied value for operational life. Although these WMUs might be described as temporary,
default values for their operational lives range from 20 to 50 years. EPACMTP was designed to
estimate peak and average ground water concentrations at a well assuming that leaching
durations would be long enough to capture the changing ground water concentrations in terms of
years rather than days. As a result, EPACMTP may not adequately represent well
concentrations for extremely short leaching durations of 1 year or less.
A detailed discussion of assumptions and limitations and the numerical formulation for
EPACMTP can be found in Section 2 of the EPACMTP Technical Background Document (U.S.
EPA, 2003a).
3.2 Structural Fill Module
This section describes how EPACMTP models the release of constituents from a structural fill.
Section 3.2.1 provides a general overview of the EPACMTP structural fill source module; and
Section 3.2.2 identifies the assumptions and limitations of the structural fill source terms.
3.2.1 Releases From a Structural Fill
The purpose of the structural fill source module is the same as the WMU source modules: to
provide a leachate flux and concentration to the unsaturated zone. The module relies on the
design characteristics of the structural fill and the waste stream characteristics (material
properties, quantity, and concentrations). The release of constituent mass from the structural fill
is modeled as a finite pulse source (see Figure 3-1 above). The main difference between a
structural fill and a WMU is in how the duration of leaching is controlled. As mentioned above,
WMUs other than the landfill have a specified operational life, which determines how long
leachate is released from the unit. The duration of leaching from a structural fill is controlled by a
combination of the leachate flux rate and the amount of available constituent mass to be leached;
the pulse will persist until all available mass is depleted. To define this behavior, the structural
fill source module uses the following equation to calculate the length of the pulse, tp, in years
(units conversion factors are omitted):
1 ^ Total 'x ^ " rb '^J /"> l\
*>= ^7 (3'1}
where
tp = length of pulse (years)
C^otal = constituent-specific total teachable materials concentration (mg/kg)
d = depth of the structural fill (m)
PB = bulk density of the structural fill material (g/cm3)
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f = fractional volume of the structural fill occupied by materials with teachable
components of interest (unitless)
C°L = constituent-specific leachate concentration (mg/L)
I = leachate flux rate emanating from the structural fill (m/yr).
Mathematically, EPACMTP treats the structural fill the same way as it treats WMUs. A
rectangular footprint is assumed between the bottom of the structural fill and the top of the
unsaturated zone column, through which leachate passes. The structural fill source module
estimates the magnitude of the rate of water infiltration and constituent concentration crossing
this plane. Input leachate and total teachable material concentrations are assumed to represent the
net result of internal chemical and physical processes within the modeled unit.
Typically, structural fills are unlined and when not covered by an impermeable or nearly
impermeable surface (e.g., pavement or a building), they are often designed to have the same or
greater permeability as the surrounding soils (NRMCA, nd). Therefore, IWEM provides the
regional infiltration rates developed with the HELP model (Schroeder et al, 1994) as available
default values based on the geographic location. IWEM requires the user to provide a value of
the effective hydraulic conductivity of the structural fill material as a potential physical limiter on
the HELP-derived infiltration rates or user-specified infiltration rate.
3.2.2 Assumptions and Limitations for Structural Fill Source Modeling
As mentioned in Section 3.1.3, EPA designed the EPACMTP fate and transport model to be
used for regulatory assessments in a probabilistic framework. The simulation algorithms that are
incorporated into the model are intended to meet the following requirements:
Account for the primary physical and chemical processes that affect constituent fate and
transport in the unsaturated and saturated zone;
Run (and produce useful results) with relatively little site input data; and
Be computationally efficient for Monte Carlo analyses.
As also mentioned in Section 3.1.3, the same assumptions and caveats that were applicable to the
WMU source term apply to the structural fill source term.
The EPACMTP source module provides a relatively simple representation of the structural fill:
structural fills are represented in terms of a source area and a defined rate and duration of
leaching. EPACMTP only accounts for the release of leachate through the base of the fill and
assumes that the only mechanism of constituent release is through dissolution of waste
constituents in the water that percolates through the fill. EPACMTP does not account for releases
from the structural fill via other environmental pathways, such volatilization or surface run-off.
EPACMTP assumes that the rate of infiltration through the structural fill is constant, representing
long-term average conditions. EPACMTP does not account for fluctuations in rainfall rate that
may cause the rate of infiltration and release of leachate to vary over time. Like landfills, waste
piles, and land application units, EPACMTP cannot simulate a structural fill that is in direct
contact with the water table
Structural fills are considered permanent constructions, and thus, the leaching duration is
calculated using the site-specific, user-supplied inputs that define the geometry, material and
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constituent composition, and infiltration rate. EPACMTP was designed to determine peak and
average ground water concentrations at a well assuming that leaching durations would be long
enough to capture the changing ground water concentrations in terms of years rather than days.
As a result, EPACMTP does not adequately represent well concentrations for extremely
short leaching durations of 1 year or less.
3.3 Roadway Source Module
Unlike the WMU source modules, the roadway source-term module is outside of EPACMTP, but
incorporated in IWEM. Therefore, prior to the execution of EPACMTP subsurface modeling, the
external roadways source module provides the magnitude and duration of leaching to
EPACMTP. This section describes how IWEM determines the release of constituents from a
roadway constructed with industrial materials. Section 3.3.1 provides a general overview of the
roadway source module; Section 3.3.2 presents a discussion of how the roadway source module
was integrated into IWEM; and Section 3.3.3 identifies the assumptions and limitations of the
roadway source module. The details of the roadway source-module design and implementation
are provided in Appendix C. Appendix E addresses the derivation of infiltration rates through
pavements and other roadway design elements available in IWEM. Appendix F describes the
mathematical approach of accommodating modeling scenarios where the direction of ground
water flow is not perpendicular to the axis of travel on the roadway. Appendix G describes the
verification of the roadway module.
3.3.1 General Conceptualization
The purpose of the roadway source module is to provide a leachate fluxes and concentrations to
the unsaturated zone. Whereas the WMU source modules provide a uniform, finite source
leachate concentration over the entire footprint of the WMU, the structure of a roadway cross-
section and the source module make it possible to have different leachate concentrations and
infiltration rates across the roadway cross-section. Leachate releases from the roadway source
module are a function of the design of the roadway, the material properties of the unique
structural elements in the roadway, and the characteristics (quantity and concentrations) of reused
materials having teachable components incorporated in the structural elements.
Figure 3-3 depicts a typical roadway with a segment constructed with industrial materials. For the
purposes of model simplicity, that segment is assumed to be nearly linear and thus can be
approximated by the straight line segment. If the segment to be modeled is long and meandering,
it must be subdivided into several nearly linear segments that can each be represented by a
straight line, each segment requiring a separate evaluation (see Example Problem 4 in Appendix
C of the IWEM v. 3.1 User's Guide for an example of how to evaluate a multi-segment roadway).
Figure 3-4 shows a typical cross section of a roadway, which may comprise several components
(e.g., lanes, shoulder, and ditch). For the roadway module, each component was idealized as a
column, referred to henceforth as the roadway-source column. In the vertical direction, as shown
in Figure 3-5, each roadway-source column includes materials starting vertically upward from a
reference datum (which could be the top of subgrade), to the surface of a pavement or a road
shoulder or an embankment or a ditch. As shown in Figure 3-5, each roadway-source column is
underlain by a corresponding vadose-zone column.
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Highway
Regional Flow Direction
/ Linear Approximation of the
Highway Segment of Interest
Highway Segment
of Interest
Receptor
Location
Figure 3-3. A typical roadway with a recycled-material segment.
Traveled Lanes Traveled Lanes
I Pitch!
T
Ditch
~ f Drain
Gutter Subgrade
Permeable Drain
Base
Figure 3-4. Atypical cross section of a roadway.
Gutter
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Source Strip i = 1 Source Strip i = 2 Source Strip i =3 Source Strip i = 4
Ditch & Left Shoulder Left Lane Right Lane Right Shoulder
n
1;
Vadose Zone 1
_T\_
^J^
Vadose Zone 2
n
^^
Vadose Zone 3
_n_
±j^
Vadose Zone 4
Figure 3-5. Modules of IWEM corresponding to multiple roadway-source strips.
A roadway-source column was assumed to be uniform in terms of parameters and properties
along the length of interest (i.e., the modeled segment shown in Figure 3-4). Therefore, a
roadway-source column becomes a road way-source strip in three dimensions. Figure 3-6 shows
an example of a roadway cross section comprising three roadway-source strips representing,
respectively, a median, a lane, and a ditch. Note that a more typical roadway may consist of up to
15 roadway-source strips: for example, left shoulder, left lane, median, right lane, and right
shoulder. More strips are possible to account for drainage ditches and berms and different
configurations of layers; the IWEM roadway module limits the total number of roadway-source
strips to 15.
Median
Travel Lane
Road Shoulder,
Embankment, and Ditch
V
V
/
\
S*
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IWEM Technical Background Document Source Modeling
An example of only three roadway-source strips is used here as a basis for further discussion.
Each roadway-source strip may consist of several layers, depending on how a given roadway was
constructed. A lane strip may be composed of a pavement layer (Portland cement concrete or
asphalt concrete), a base-course layer, a subbase layer, and a subgrade layer. A median may
comprise a base layer, a subbase layer, and a subgrade layer. An unpaved road shoulder may have
only one layera subgrade layer. With this type of conceptualization, one can easily see that
each roadway-source strip is nearly equivalent to the existing landfill source module that is
available within EPACMTP. However, the EPACMTP landfill module integrated into IWEM
can accommodate only sources with a square footprint and one layer.
As illustrated in Figure 3-4, the ground water flow direction may not be perpendicular to the
segment of interest. The roadway module can accommodate a scenario where the ground water
flow direction is not perpendicular to the model. In addition, the location of the ground water
well is not restricted. These two features are unique to the roadway module (and are discussed in
more detail in Appendix F).
As mentioned above, the roadway source module is based on the design, material properties, and
constituent characteristics in the materials; specifically, it uses six primary parameters:
Area of each roadway source strip (specified as a width for each strip and the length of
the segment being modeled);
The thickness of each layer in each strip;
The material properties of each layer (bulk density, hydraulic conductivity)
Leachate flux rate emanating from the strip (infiltration rate subject hydraulic
conductivity);
Constituent-specific leachate and total teachable material concentrations in layers
containing reused materials; and
Leaching duration (derived from material properties and dimensions, leachate flux rate
and constituent concentrations).
Based on these parameters, the roadway source module generates a rate of leaching and the
constituent concentration in the leachate as a function of time from the bottom of each strip
containing teachable constituent mass.
3.3.2 Approach to Integration into IWEM
Based on the general conceptualization described above, a number of modifications were made
to the IWEM interface to simulate a roadway, including changes to
Accommodate multiple layers;
Handle rectangular sources;
Account for multiple roadway-source strips, drainage systems and ditches and their
material properties and flow characteristics;
Account for a general regional flow field that may not be perpendicular to the roadway
axis; and
Include default pavement infiltration rates (Appendix E addresses the derivation of
infiltration rates through pavements and other roadway design elements available in
IWEM).
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Additional parameters were added to IWEM to describe the pavement geometry, receptor
location, material properties, and the concentrations of constituents present in the industrial
materials. Also, the IWEM database was updated to store these new parameters associated with
the roadway, as well as intermediate values calculated for the module.
The roadway module was verified to ensure that these modifications were correctly implemented
and to ascertain the degree of accuracy of the transformation of the transport equation for non-
perpendicular regional flow. Appendix G details the verification process.
3.3.3 Assumptions and Limitations of Roadway Source Module
The following assumptions are used in the formulation of the roadway source term:
In the region of interest, the general regional ground water flow pattern is assumed not to
be affected by the presence of a traversing roadway. It follows from this assumption that
infiltration from the traversing roadway is on the same order of magnitude as regional
recharge. Furthermore, the areal coverage of the roadway contributing infiltration is
assumed to be very small compared to the total regional area contributing recharge, so
that any difference between the infiltration and recharge rates does not significantly
influence the regional flow field.
Lateral communication between roadway-source strips is assumed to be insignificant.
A single, long-term average infiltration rate is assumed to represent percolation through
each roadway-source strip.
Leaching begins at the end of pavement construction and is modeled as a depleting finite
source.
Material properties of each road way-source strip do not vary in time.
3.4 Structural Fill or Roadway?
Structural fills evaluated in IWEM include the use of industrial materials and related byproducts
as substitutes for the earthen materials for supporting parking lots, roads, airstrips, tanks/vaults,
and buildings; construction of highway embankments and bridge abutments; filling of borrow
pits, and other landscape irregularities; and changing the landscape for development or
reclamation projects. IWEM can evaluate structural fills using both flowable fill and compacted
installation methods. Applications that include reused materials can range from the conceptually
simple (e.g., filling a borrow pit) to the very complex (e.g., support for a multi-lane roadway and
component layers in an adjoining embankment). The complexity of the specific application will
govern the choice between a structural fill and a roadway source term.
If the application to be modeled can be conceptualized as containing a single layer of reused
material having the same material (i.e., hydraulic conductivity and bulk density) and constituent
(i.e., leachate and total teachable material concentrations) characteristics, then the structural fill
source term would be most applicable. Even in the case of a multi-layer structure where the same
reused material is employed, these layers can be collapsed into a single layer - the time it takes
for leachate to travel through "clean" layers between layers containing reused materials is not
modeled in either the structural fill module or in the roadway module; only the net leaching
profile is released to the soil column beneath the source area (See Appendix C, Section C.2.2.1
for more details).
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The structural fill source-term would be applicable to a parking lot or a simple roadway
presuming that (1) the one or more layers of the structure with reused materials have same
material and constituent properties; (2) the layering configuration is the same everywhere in the
structure; and (3) an infiltration rate that represents the flux of water through the paved surface is
available. If, on the other hand, there are multiple layers and the layering structure within the
source varies from one end to the other, the roadway source term would be more appropriate.
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4.0 Unsaturated and Saturated Zone Modeling Using EPACMTP
This section describes the EPACMTP's subsurface modeling modules used in IWEM to simulate
the migration of waste constituents through the ground water pathway from land-based sources to
wells. The section provides information about EPACMTP that is relevant to IWEM, including
important fate and transport equations and some of the key assumptions and limitations of the
model. However, the section does not attempt to provide detailed derivations of the fate and
transport equations. For complete documentation on the EPACMTP model, the reader should
refer to EPACMTP Technical Background Document (U.S. EPA, 2003a).
EPACTMP contains two modular components to model the subsurface migration of constituents:
the unsaturated and saturated zone modules. The unsaturated zone module simulates one-
dimensional, vertically downward flow and transport of constituents in the vadose zone
underneath a source. On the other hand, the saturated zone module simulates the ground water
flow and three-dimensional constituent transport within the saturated zone. These modules are
computationally linked through continuity of flow and constituent concentration across the water
table directly underneath the source. The modules account for several processes affecting
constituent fate and transport including: advection, hydrodynamic dispersion and molecular
diffusion; linear or nonlinear equilibrium sorption; first-order decay and zero-order production
reactions (to account for transformation breakdown products); and dilution from recharge in the
saturated zone.
The main inputs to the subsurface component of EPACMTP are the rate of constituent release
(leaching) from a source, source design, and site hydrogeological characteristics. The output from
EPACMTP, as it is employed in IWEM, is a time-dependent estimation of the constituent
concentrations arriving at a down gradient well. The timing and magnitude of the prediction can
vary, depending on a number of factors not limited to the nature of the source,1 the distance
between the source and the receptor well, constituent fate and transport properties, and the
exposure period. EPACMTP can calculate the peak concentration arriving at the well or a time-
averaged concentration corresponding to a specified exposure duration (for example, a 30-year
average exposure time).
The relationship between the constituent concentration leaching from a source and the resulting
ground water exposure at a well located down-gradient from the source is depicted in Figure 4-1.
Figure 4-la shows how the leachate concentration emanating from the source gradually
diminishes over time as a result of depletion of the contaminant mass remaining in the unit. As
seen in Figure 4-lb, the constituent does not arrive at the well until sometime after the leaching
begins. Eventually the ground water concentration will reach a peak value and then begin to
diminish because the leaching from the source occurs only over a finite period of time. This
curve is also called the breakthrough curve. The maximum constituent concentration at the well
will generally be lower than the original leachate concentration as a result of various dilution and
attenuation processes, which occur during the transport through the unsaturated and saturated
zones. EPACMTP has the capability to calculate the maximum average ground water
1 See the discussion of source scenarios in Section 3.1.1; the IWEM sources are all modeled as finite sources,
although landfills behave more like continuous sources due to the long time to deplete.
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IWEM Technical Background Document
Unsaturated and Saturated Zone Modeling
concentration over a specified exposure period, as depicted by the horizontal dashed line in
Figure 4-lb.
o
0)
o
c
o
o
c
I
o
o
PeaK
Concentration
Time
Time
Exposure
Averaging Psrigtl
(a) Leachate concentration vs time (b) Ground water well concentration vs time
Figure 4-1. Relationship between leachate concentration and well concentration.
The following sections provide more detailed discussions of the unsaturated and saturated zone
modules of EPACMTP and the role of each in simulating constituent fate and transport. Section
4.1 describes the unsaturated zone module and the mathematical equations used to model
constituent fate and transport; Section 4.2 describes the saturated zone module and mathematical
equations related to this environment; and, Section 4.3 describes the important modeling
assumptions and limitation considered in developing the modules.
4.1 Unsaturated Zone Module
IWEM uses the unsaturated zone module in EPACMTP to model water flow and solute transport
in the unsaturated zone - between the base of the source and the water table. EPACMTP assumes
that constituent migration through this media is entirely one-dimensional (vertically downward).
EPACMTP also assumes the flow rate is steady state, that is, it does not change in time. The soil
underneath the source is assumed to be uniform with hydraulic properties described by the
Mualem-Van Genuchten model (Jury et al., 1991). The flow rate is determined by the long-term
average infiltration rate through the source. Inputs to the unsaturated zone module are the rate of
water and constituent leaching from the source, as well as soil hydraulic properties. EPACMTP
solves the governing one-dimensional steady-state Richards flow equation (Jury et al., 1991)
using a semi-numerical technique described in the EPACMTP Technical Background Document
(U.S. EPA, 2003a).
Constituent transport in the unsaturated zone is assumed to occur by advection and dispersion.2 It
is also assumed that the unsaturated zone is initially constituent-free and that constituents migrate
vertically downward from the source. EPACMTP can simulate both steady state and transient
In the case of metals, which are subject to nonlinear sorption, EPACMTP uses a method-of-characteristics solution
method that does not include dispersion. In this case, transport is dominated by the nonlinear sorption behavior,
and dispersion effects are minor.
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transport in the unsaturated zone, with single-species or multiple-species chain decay reactions.
The transport module can also simulate the effects of both linear and nonlinear sorption
reactions. When decay reactions involve the formation of daughter products, EPACMTP has the
capability to perform a multi-species transport simulation of a decay chain consisting of up to
seven members.
Mathematically, the transport process is represented by the advection-dispersion equation:
8R^ + Q (4-1)
dz dzj dz
where (using general units for L[ength], M[ass], and T[ime])
z = Soil depth coordinate (L)
c = Constituent concentration (M/L3)
t = Time(T)
D = Dispersion coefficient, (L2/T)
V = Darcy velocity (L/T)
6 = Volumetric water content (dimensionless)
B = Phase distribution coefficient (dimensionless)
X = Lumped first-order decay constant (1/T)
R = Retardation factor (dimensionless)
Q = Zero-order production term to account for transformation of parent constituents
(M/L3-T)
EPACMTP uses units of meters for L(ength), years for T(ime), and kilograms for M(ass).
Consistent with common practice, EPACMTP uses units of mg/L for constituent concentration.
Numerically, this is the same as kg/m3.
The dispersion coefficient, D, in the above transport equation accounts for the effects of
hydrodynamic dispersion and molecular diffusion in the vertical direction and is defined as
D = aV + Dm (4-2)
where
D = Dispersion coefficient (m2/yr)
a = Dispersivity (m)
V = Darcy velocity (m/yr)
Dm = Molecular diffusion coefficient (m2/yr)
Accounting for dispersion only in the vertical direction (downward only) is consistent with the
one-dimensional unsaturated flow formulation, and also provides some additional, but relatively
small, conservatism for a screening model. The effective molecular diffusion coefficient, Dm, is
calculated using the Millington-Quirk relationship (Jury et al., 1991) as
Dm=D*Pltf- (4-3)
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where
Dm = Effective molecular diffusion coefficient (m2/yr)
Dw = Free-water diffusion coefficient (m2/yr)
6 = Volumetric water content (dimensionless)
The retardation factor, R, in the transport equation (Equation 4-1) accounts for the effects of
equilibrium sorption of dissolved constituents onto the solid phase and is calculated as
R=\+(pbkd)/e (4-4)
where
R = Retardation factor (dimensionless)
pb = Bulk density (kg/L)
kd = Constituent-specific soil-water partition coefficient (L/kg)
6 = Volumetric water content (dimensionless)
EPACMTP's unsaturated zone module includes options for both linear and nonlinear sorption
isotherms. In the first case, the partition coefficient, kd, is independent of the constituent
concentration. In the second case, the value of the partition coefficient is a function of
concentration. For linear sorption isotherms, the partition coefficient can be entered as a single
EPACMTP parameter, or the model can calculate its value from the fraction organic carbon in
the soil and a constituent-specific organic carbon partition coefficient as
kd=foc*Koc (4-5)
where
kd = Partition coefficient (L/kg)
foe = Fraction organic carbon in the soil (dimensionless)
Koc = Constituent-specific organic carbon partition coefficient (L/kg)
The phase distribution coefficient, B, is identical to the retardation factor and accounts for
degradation in both the dissolved and sorbed phases when multiplied by the lumped degradation
coefficient, A,. B is calculated as
B = l + (pb kd) /6 (4-6)
where
B = Phase distribution coefficient (dimensionless)
pb = Bulk density (kg/L)
kd = Constituent-specific soil-water partition coefficient (L/kg)
6 = Volumetric water content (dimensionless)
When modeling constituents with non-linear sorption isotherms, the partition coefficient data are
read in by EPACMTP as a table of paired concentration-kd values. In principle, the user can
employ a variety of methods for generating the concentration-kd values, including using
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IWEM Technical Background Document Unsaturated and Saturated Zone Modeling
measured data. In practice, EPACMTP applications typically use data generated using the
MINTEQA2 geochemical speciation model (see Section 6.5.2).
The parameter X in the transport equation (Equation 4-1) accounts for first-order transformation
processes. Finally, the term Q in the transport equation is a source term that represents the
production of a constituent species due to the transformation of parent constituents. This term is
zero for parent constituents that are at the beginning of a decay chain, but non-zero for any
transformation daughter products.
The output from the unsaturated zone transport solution is a time history (breakthrough curve) of
the constituent concentration arriving at the water table, which provides the input for the
saturated zone transport simulation.
4.2 Saturated Zone Module
The saturated zone module of EPACMTP used in IWEM is designed to simulate flow and
transport in an unconfined aquifer with constant saturated thickness. The model simulates
regional flow in a horizontal direction with recharge and infiltration from the overlying
unsaturated zone and source entering at the water table. Localized water table mound effects are
possible when infiltration through the source area is greater than the regional recharge. The lower
boundary of the aquifer is assumed to be impermeable.
EPACMTP assumes that flow in the saturated zone is steady state. In other words, EPACMTP
models long-term average flow conditions. The steady-state ground water flow solution provided
in EPACMTP accounts for different recharge rates beneath and outside the source area. Ground
water mounding beneath the source is represented in the flow system by increased head values at
the top of the aquifer. It is important to realize that while EPACMTP calculates the degree of
ground water mounding that may occur underneath a source due to high infiltration rates, the
actual saturated flow and transport modules in EPACMTP are based on the assumption of a
constant saturated thickness. The only direct effect of ground water mounding in EPACMTP is to
increase localized, simulated ground water velocities where the water table has been elevated.
EPACMTP incorporates a number of different mathematical solutions for saturated zone flow
and transport. The EPACMTP Technical Background Document (U.S. EPA, 2003a) discusses
these in detail. Because of the high premium on computational efficiency in the IWEM Monte
Carlo tool, a pseudo-3-dimensional modeling approach was used in IWEM. The pseudo-3 -
dimensional module simulates ground water flow using a one-dimensional steady-state solution
for predicting hydraulic head and Darcy velocities. The flow solution is formulated based on the
Dupuit-Forchheimer's assumption of hydrostatic pressure distribution (de Marsily, 1986). The
hydraulic head is also horizontally averaged in the cross-gradient direction.
EPACMTP models transport of dissolved constituents in the saturated zone using the advection-
dispersion equation. The aquifer is assumed to be initially constituent-free, and constituents enter
the saturated zone only from the unsaturated zone directly beneath the source. In the pseudo-3 -
dimensional option of EPACMTP used for IWEM, it is assumed that advection is predominantly
along the longitudinal direction (direction along the ambient ground water gradient), while
dispersion occurs in three dimensions.
The pseudo-3-dimensional transport option is based on the concept that when ground water flow
is predominantly in one direction, the movement of a dissolved constituent plume can be
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IWEM Technical Background Document Unsaturated and Saturated Zone Modeling
approximated as the product of three terms: the first term describes the movement by advection
and dispersion along the direction of ground water flow (the x-direction); and the second and
third terms account for the effect of dispersion in the horizontal transverse (y-) direction, and the
vertical (z-) direction, respectively. The effects of constituent sorption and transformation are
incorporated into the first term of the mathematical solution. The second (y-direction) and third
(z-direction) terms in the solution can be regarded as adjustment factors that account for the
reduction in concentration along the x-direction, due to dispersion into the y- and z-directions.
The y- and z- solution terms are given by straight-forward error-functions that can be computed
very quickly. From a computational point, the pseudo-3 -dimensional solution option therefore
requires about the same effort as a one-dimensional solution. The treatment of boundary
conditions for the pseudo-3 -dimensional transport solution option, especially the transfer of mass
at the water table, is discussed in much greater detail in the EPACMTP Technical Background
Document (U.S. EPA, 2003a).
The governing equation for transport in the saturated zone can be written as:
etc
(pR + Q (4-7)
where (using general units for L[ength], M[ass], and T[ime])
i,j = Indices to represent different spatial directions; i,j = 1, 2, or 3
Xi = Spatial coordinate (L)
c = Constituent concentration (M/L3)
t = Time(T)
Dy = Dispersion coefficient (L2/T),
Vx = Ground water flow rate in the x-direction (L/T)
(p = Porosity (dimensionless)
B = Phase distribution coefficient (dimensionless)
X = First-order transformation coefficient (1/T)
R = Retardation coefficient (dimensionless)
Q = Zero-order production term to account for transformation of parent constituents
(M/L3-T)
EPACMTP uses units of meters for L(ength), years for T(ime), and kilograms for M(ass).
Consistent with common practice, EPACMTP uses units of mg/L for constituent concentration,
which numerically is the same as kg/m3.
The transport processes modeled in the saturated zone module of EPACMTP are analogous to
those in the unsaturated zone, but they are extended to three dimensions instead of just one. The
spatial coordinate, Xi, in Equation 4-7 represents the three dimensions. The coordinate xi (or just
x), represents the horizontal coordinate along the direction of ground water flow. The coordinate
X2 (or;/) represents the horizontal coordinate perpendicular to the flow direction; and the
coordinate xs (orz) represents the vertical direction. The dispersion coefficient Z)y (where /' andy
can be 1, 2, or 3) is subscripted to indicate that this coefficient has components in all three
directions. Conversely, the ground water flow term, Vx, has only a single subscript to indicate the
assumption in the pseudo-3 -dimensional option of EPACMTP, that ground water flow is a one-
dimensional process. The other terms in Equation 4-7 are defined in the same way as in Equation
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IWEM Technical Background Document Unsaturated and Saturated Zone Modeling
4-1, except that the porosity,
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IWEM Technical Background Document Unsaturated and Saturated Zone Modeling
Likewise, the retardation and transformation terms are modeled in the same way in the saturated
zone module of EPACMTP as they are in the unsaturated zone module.
A key distinction between the way the saturated zone module handles constituent fate and
transport, as compared to the unsaturated zone module, is the approach for constituents with
nonlinear sorption isotherms. The saturated zone module only simulates linearized isotherms. For
constituents with nonlinear sorption isotherms, the unsaturated zone module simulates
partitioning by using concentration-dependent partitioning coefficient; the saturated zone module
uses a linearized isotherm, based upon the maximum constituent concentration at the water table
(see EPACMTP Technical Background Document; U.S. EPA, 2003a). The reason is that upon
dilution of the leachate in the ambient ground water as the leachate enters the saturated zone,
concentrations will be reduced to a range in which constituent isotherms generally are linear.
4.3 Assumptions and Limitations for Unsaturated and Saturated Zone
Modeling
EPA designed the EPACMTP fate and transport model to be used for regulatory assessments in a
probabilistic framework. The simulation algorithms that are incorporated into the model are
intended to meet the following requirements:
Account for the primary physical and chemical processes that affect constituent fate and
transport in the unsaturated and saturated zone;
Be able to be used with relatively little site input data; and
Be computationally efficient for Monte Carlo analyses.
This section discusses the primary assumptions that EPA made in developing the model to
balance these competing requirements, and the resulting limitations. EPACMTP may not be
suitable for all sites conditions. As such, the IWEM user should understand the capabilities and
limitations of EPACMTP to ensure the use of IWEM is appropriate.
4.3.1 Uniform Soil and Aquifer Assumption
EPACMTP simulates the unsaturated zone and saturated zone as separate domains that are
connected at the water table. Both zones are assumed to be uniform porous media. EPACMTP
does not explicitly account for the presence of macro-pores, fractures, solution features, faults or
other heterogeneities in the soil or aquifer that may provide pathways for rapid movement of
constituents. A certain amount of heterogeneity always exists at actual sites and it is not
uncommon in ground water modeling to use average parameter values. This means that
parameters such as hydraulic conductivity and dispersivity represent effective site-wide average
values. However, EPACMTP may not be appropriate for sites overlying fractured or very
heterogeneous aquifers.
4.3.2 Steady-State Flow Assumption
Flow in the unsaturated zone and saturated zone is assumed to be driven by long-term average
infiltration and recharge; EPACMTP treats flow in the unsaturated zone as steady state and does
not account for fluctuations in the infiltration or recharge rate, either over time or over area. The
use of EPACMTP may not be appropriate at sites with large seasonal fluctuations in
rainfall conditions or at sites where the recharge rate varies locally Examples of the latter
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include the presence of surface water bodies such as rivers, lakes, ponds, or man-made recharge
sources near the source.
EPACMTP models ground water flow based on the assumption that the contribution of recharge
and infiltration from the unsaturated zone are small relative to the regional ground water flow,
and that the saturated aquifer thickness is large relative to the head difference that establishes the
regional gradient. While horizontal flow conditions and recharge represent regional flow
conditions, infiltration through the source area can result in localized mounding of the water
table when infiltration is larger than recharge. The implication is that the saturated zone can be
modeled as having a uniform thickness, with mounding underneath the source represented by an
increased head distribution along the water table. The mathematical ground water flow solutions
incorporated in EPACMTP are based on confined aquifer conditions. While EPACMTP accounts
for ground water mounding underneath a source, the saturated zone module of EPACMTP only
accounts for the effect of mounding on ground water flow velocities; it does not simulate the
actual physical increase in the thickness of the saturated zone. The assumption of constant and
uniform saturated zone thickness means that EPACMTP may not be suitable at sites with a non-
uniform thickness of the water-bearing zone, or sites with significant seasonal variations in water
table elevation. EPACMTP is designed for relatively simple ground water flow systems in which
flow is dominated by a regional gradient. EPACMTP does not account for the presence of ground
water sources or sinks such as pumping or injection wells. The presence of such man-made or
natural features may cause a more complicated flow field than EPACMTP can handle.
EPACMTP does not account for free-phase flow conditions of an oily or non-aqueous phase
liquid.
4.3.3 Constituent Fate and Transport Assumptions
The unsaturated zone and saturated zone modules of EPACMTP account for constituent fate and
transport by advection, hydrodynamic dispersion, molecular diffusion, sorption and first-order
transformation. Advection refers to transport along with ground water flow. Hydrodynamic
dispersion and molecular diffusion both act as mixing processes. Hydrodynamic dispersion is
caused by local variations in ground water flow rate and is usually a significant plume-spreading
mechanism. Molecular diffusion, on the other hand, is usually a minor mechanism, except when
ground water flow rates are very low. EPACMTP does not account for matrix-diffusion
processes, which may occur when the aquifer formation is comprised of zones with widely
varying permeabilities. In these situations, transport occurs primarily in the more permeable
zones, but constituents can move into and out of the low permeability zones by diffusion.
Leachate constituents can be subject to complex geochemical interactions in soil and ground
water. EPACMTP treats these interactions as equilibrium sorption processes. The equilibrium
assumption means that the sorption process occurs instantaneously or at least very quickly
relative to the time-scale of constituent transport. Although sorption, or the attachment of
leachate constituents to solid soil or aquifer particles, may result from multiple chemical
processes, EPACMTP lumps these processes together into an effective soil-water partition
coefficient.
For organic constituents, EPACMTP assumes that the partition coefficient is constant and equal
to the product of the mass fraction of organic carbon in the soil or aquifer and a constituent-
specific organic carbon partition coefficient (see Equation 4-5). In the case of metals, EPACMTP
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allows the partition coefficient to vary as a function of a number of primary geochemical
parameters, including pH, leachate organic matter, soil organic matter, and the fraction of iron-
oxide in the subsurface.
For metals, EPACMTP uses a set of effective sorption isotherms, which were developed by EPA
by running the MINTEQA2 geochemical speciation model (U.S. EPA, 1991) for each metal and
each combination of geochemical parameters. In modeling metals transport in the unsaturated
zone, EPACMTP uses the complete, nonlinear sorption isotherms. In modeling metals transport
in the saturated zone, EPACMTP uses linearized MINTEQA2 isotherms, based on the
assumption that after dilution of the leachate plume in ground water, concentration values of
metals will typically be in a range where the isotherm is approximately linear. This assumption
may not be valid when the metal concentrations in the leachate are high. Although EPACMTP is
able to account for the effect of the geochemical environment at a site on the mobility of metals,
the model assumes that the geochemical environment at a site is constant and not affected by the
presence of the leachate plume. In reality, the presence of a leachate plume may alter the ambient
geochemical environment.
EPACMTP does not account for colloidal transport or other forms of facilitated transport. For
metals and other constituents that tend to strongly sorb to soil particles, and which EPACMTP
will simulate as relatively immobile, movement as colloidal particles can be a significant
transport mechanism. It is possible to approximate the effect of these transport processes by
using a lower value of the partition coefficient as a user-input. In the IWEM application of
EPACMTP, the model uses the same partition coefficient for the unsaturated and saturated zone
if this parameter is provided as a user-input.
EPACMTP accounts for biological and chemical transformation processes as first-order
degradation reactions. That is, it assumes that the transformation process can be described in
terms of a constituent-specific half-life. EPACMTP allows the degradation rate to have different
values in the unsaturated zone and the saturated zone, but the model assumes that the value is
uniform throughout the unsaturated zone and uniform throughout the saturated zone for each
constituent. EPA's ground water modeling database includes constituent-specific hydrolysis rate
coefficients for constituents that are subject to hydrolysis transformation reactions; for these
constituents, EPACMTP simulates transformation reactions subject to site-specific values of pH
and soil and ground water temperature, but other types of transformation processes are not
explicitly simulated in EPACMTP.
For many organic constituents, biodegradation can be an important fate mechanism, but
EPACMTP has only limited ability to account for this process. The user must provide an
appropriate value for the effective first-order degradation rate. In the IWEM application of
EPACMTP, the model uses the same degradation rate coefficient for the unsaturated and
saturated zone if this parameter is provided as a user-input. In an actual leachate plume,
biodegradation rates may be different in different regions in the plume; for instance in portions of
the plume that are anaerobic some constituents may biodegrade more readily, while other
constituents will biodegrade only in the aerobic fringe of the plume. EPACMTP does not account
for these or other processes that may cause a constituent's rate of transformation to vary in space
and time.
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5.0 Conducting Probabilistic Analyses
The final component of EPACMTP is the Monte Carlo module, and the integration of
EPACMTP in IWEM, provides IWEM the capability to simulate constituent fate and transport
probabilistically. Monte Carlo simulation is a statistical technique by which a quantity is
calculated repeatedly, using randomly selected parameter values for each calculation. The results
approximate the full range of possible outcomes and the likelihood of each. The Monte Carlo
module in EPACMTP makes it possible to incorporate variability into the subsurface pathway
modeling analysis, and to quantify the impact of parameter variability on ground water
concentrations. In particular, Monte Carlo simulation is used to determine the likelihood, or
probability, that the ground water concentration of a constituent at a well, and hence exposure
and risk, will be either above or below a certain regulatory or health-based value.
In a Monte Carlo simulation, the values of the various source-specific, chemical-specific,
unsaturated zone-specific, and saturated zone-specific model parameters are represented as
probability distributions. Precisely, Monte Carlo analysis can account only parameter variability,
not uncertainty. Variability describes parameters whose values are not constant, but which can be
measured and characterized with relative precision in terms of a frequency distribution. An
example is annual rainfall in different parts of the country. Uncertainty, on the other hand,
pertains to parameters whose values are known only approximately, such as the hydraulic
conductivity of an aquifer. In practice, probability distributions are used to describe both
variability and uncertainty, and for the purpose of the EPACMTP Monte Carlo module, are
treated as more or less equivalent. Thus, the probability distributions used in IWEM reflect both
the range of variation that may be encountered at different waste sites, as well as uncertainty
about the specific conditions at each site.
5.1 EPACMTP Monte Carlo Module
The Monte Carlo module in EPACMTP is described in detail in the EPACMTP Technical
Background Document (U.S. EPA, 2003a), and $\Q EPACMTP Parameters/Data Background
Document (U.S. EPA, 2003b). A general overview of the methodology is presented in the
following paragraphs.
Figure 5-1 presents a graphical illustration of the Monte Carlo simulation process. The Monte
Carlo method requires that for each input parameter, except constant parameters, a probability
distribution be provided (Figure 5-la). The method involves the repeated generation of random
values of the input variables (drawn from the known distribution and within the range of any
imposed bounds). The EPACMTP model is executed (Figure 5-lb) for each set of randomly
generated model parameters and the corresponding ground water well exposure concentration is
calculated and stored. Each set of input values and corresponding well concentration is termed a
realization.
At the conclusion of the Monte Carlo simulation, the realizations are statistically analyzed to
yield a cumulative probability density function of the ground water exposure concentration
(Figure 5-lc). The construction of the cumulative probability density function simply involves
sorting the ground water well concentrations calculated in each of the individual Monte Carlo
realizations from low to high. In the example used to construct Figure 5-1, an EPACMTP input
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IWEM Technical Background Document
Conducting Probabilistic Analyses
leachate concentration value of 10 mg/L was assumed and a Monte Carlo simulation of 10,000
iterations was performed. The well concentration values simulated in the EPACMTP Monte
Carlo process range from very low values to values that approach the leachate concentration. By
examining how many of the 10,000 Monte Carlo realizations resulted in a high value of the
estimated ground water concentration, it is possible to assign a probability to these high-end
events, or conversely determine what is the estimated ground water concentration corresponding
to a specific probability of occurrence.
Distribution of values
for Input Parameter "X|"
(A)
Distribution of values
tor Input Parameter "X2°
Distribution ot values
tor Input Parameter "X
Distribution of values
for Input Parameter "Xn"
Input parameter values
randomly selected for
7,594th realization of
EPACMTP
EPACMTP
(B) ( Contaminant Fate and
Transport Equations
100%
I
(C)
o%
Result of 7,594th
realization of
EPACMTP
577th realization
11th realization
* 2,965th realization
* 64th realization
etc...
10 10 10 10" 10
Groundwater Well Concentration
10
Figure 5-1. Graphical representation of the EPACMTP Monte Carlo process.
5.2 Implementation of Monte Carlo Analysis in IWEM
To conduct the Monte Carlo analysis in IWEM, the user is required to input a small set of site-
specific source parameters; the user may also set values for additional parameters if site-specific
data are available. For optional inputs for which site-specific data that are not available, and for
additional input parameters that cannot be modified by the user, EPACMTP draws values
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IWEM Technical Background Document
Conducting Probabilistic Analyses
randomly from national or regional distributions stored in the databases. The underlying
assumption is that if a site-specific parameter value is not available, the uncertainty in the value
of the parameter is captured by the nationwide range in values of that parameter.
IWEM can reduce the uncertainty associated with some of the modeling parameters even if the
actual value of a parameter is not known, by using supporting site characterization data. For
instance, if the actual value of hydraulic conductivity in the saturated zone is unknown, but
information is available about the type of subsurface environment at the site (for example,
alluvial versus sedimentary rock), IWEM will
use this information to reduce the uncertainty
in the hydraulic conductivity by selecting only
hydraulic conductivity values in the Monte
Carlo process that are representative of alluvial
aquifers. This methodology is discussed in
detail in Section 6.4.1.
In using a Monte Carlo modeling approach,
more iterations usually leads to a more stable
and more accurate result. However, it is
generally not possible to determine beforehand
how many iterations are needed to achieve a
specified degree of convergence (that is,
stability), because the value can be highly
dependent on parameter distributions. EPA has
used an empirical technique called bootstrap
analysis to determine that an appropriate
number of iterations for EPACMTP Monte
Carlo analyses is about 10,000 (see side bar
box). Consequently, IWEM defaults to 10,000
iterations. The user can change this value, but
the Agency cautions that significantly fewer
iterations will affect the repeatability of the
results.
EPACMTP Monte Carlo Bootstrap Analysis
In a Monte Carlo analysis, the output percentile values
depend on the number of iterations. For instance, if a
Monte Carlo analysis consisting of 10 iterations of
randomly selected model input values is performed, the
90th percentile of the model output can be determined by
ordering the output values from low to high and then
picking the ninth highest value. This 90th percentile
value is likely to be different if another Monte Carlo
simulation of 10 iterations with randomly selected inputs
is performed, and different still if 1,000 iterations are
simulated to calculate the 90th percentile output value.
Bootstrap analysis is a technique of replicated
resampling of a large data set for estimating standard
errors, biases, confidence intervals, or other measures of
statistical accuracy. It can produce accuracy estimates
in almost any situation without requiring subjective
statistical assumptions about the original distribution.
As part of the background for EPA's proposed 1995
Hazardous Waste Identification Rule (HWIR), a bootstrap
analysis was conducted for the EPACMTP model to
evaluate how Monte Carlo convergence improves with
increasing numbers of realizations. The analysis was
based on a continuous source, LF disposal scenario in
which the 90th percentile dilution and attenuation factor
(DAF) was 10. The bootstrap analysis results suggested
that, with 10,000 iterations, the expected value of the
90th percentile DAF was 10 with a 95 percent
confidence interval of 10 ± 0.7. Decreasing the number
of iterations to 5,000 increased the confidence interval to
10± 1.0.
5.3 Assumptions and Limitations for Monte Carlo Module
The Monte Carlo module used in IWEM allows to account for the effect of parameter variability
on estimated ground water concentrations. The resulting probability distribution of outcomes is
valid only to the extent that EPACMTP can accurately simulate actual constituent fate and
transport processes; it does not account for the uncertainty arising from the omission of some
processes from EPACMTP, or the simplification of other processes that are modeled in
EPACMTP. For instance, the Monte Carlo modeling process can account for the site-to-site
variability in the average hydraulic conductivity in the aquifer, but it cannot account for the
uncertainty associated with treating each site as uniform and ignoring aquifer heterogeneity.
Thus, the IWEM user should interpret the results of the Monte Carlo outputs in the context of the
capabilities and limitations of EPACMTP.
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IWEM Technical Background Document Conducting Probabilistic Analyses
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IWEM Technical Background Document
IWEMInputs
6.0 IWEM Inputs
This section describes the various parameters used in IWEM, including data sources,
methodologies, and values. Sections 6.1, 6.2, and 6.3 describe WMU, structural fill, and roadway
parameters, respectively. Section 6.4 describes the infiltration and recharge parameters. Section
6.5 describes the unsaturated zone and saturated zone parameters. Section 6.6 describes
constituent-specific chemical fate parameters. Finally, Section 6.7 describes the screening
procedures implemented in the Monte Carlo analysis to eliminate physically unrealistic
parameter combinations.
6.1 WMU Parameters
This section provides details about the four types of WMUs and the specific parameters in
IWEM used to define intrinsic and operational WMU characteristics.
6.1.1 WMU Types
IWEM simulates four different types of WMUs. Each of the four IWEM units reflects waste
management practices that are likely to occur at industrial Subtitle D facilities. The WMU can be
a landfill, a waste pile, a surface impoundment, or a land application unit. The four WMU types
are represented graphically in Figure 6-1. All units are assumed to contain only one type of
waste, so that the entire capacity of
the WMU is devoted to a single
waste.
Landfills. IWEM only considers
closed landfills. A closed landfill
is assumed to have a 2-ft (0.6 m)
soil cover and one of three liner
types: no liner; a single clay liner;
or a composite liner. The landfill
is filled with waste during the
unit's operational life. Upon
closure of the landfill, the waste is
left in place, and a final soil cover
is installed. The starting point for
the simulation is when the landfill
is closed, i.e., the unit is at
maximum capacity. The release of
waste constituents into the soil and
ground water underneath the
landfill is caused by dissolution
and leaching of the constituents
due to precipitation that percolates
through the unit. The type of liner
that is present controls, to a large
extent, the amount of leachate that
Cov«f
unuturaud rant
s.v._ \r ce lone
unsaluralad zone
saturated zone
(A) UWOFILL
(B) SURFACE IMPOUNDMENT
saluraiod zon*
(C1 WASIE "*
"»«»w» «"
Figure 6-1. WMU types modeled in IWEM.
6-1
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IWEM Technical Background Document IWEM Inputs
is released from the unit. Landfills are modeled as a permanent WMU, with a rectangular
footprint and a uniform depth. IWEM does not simulate any loss processes that may occur during
the unit's active life (for example, due to leaching, volatilization, runoff or erosion, or
biochemical degradation). Landfills are modeled as a depleting source: the WMU is considered
permanent and leaching continues until all teachable mass present has been depleted. In IWEM,
the magnitude of the initial leachate concentration, when the waste is "fresh," is a model input;
the rate of depletion is calculated internally by EPACMTP (see EPACMTP Technical
Background Document, U.S. EPA, 2003 a/1
Waste Piles. IWEM models waste piles as temporary sources used for storage of solid wastes.
Due to their temporary nature, they typically will not be covered. IWEM allows liners to be
present, similar to landfills. In IWEM, waste piles are modeled as a pulse-type source, with pulse
duration equal to the unit's operating life.
Surface Impoundments. In IWEM, surface impoundments are ground level or below-ground
level, flow-through units, which may be unlined, have a single clay liner, or have a composite
clay-geomembrane liner. Release of leachate is driven by the ponding of water in the
impoundment, which creates a hydraulic head gradient with the ground water underneath the
unit. At the end of the unit's operational life, IWEM assumes there is no further release of waste
constituents to the ground water (i.e., clean closure assumed). Surface impoundments are
modeled as pulse-type sources; leaching occurs at a constant leachate concentration over a fixed
time equal to the unit's operating life. IWEM also assumes a constant ponding depth (depth of
wastewater in surface impoundment) during the operational life.
Land Application Units. Land application units (or land treatment units) are areas of land that
receive regular applications of waste that can be either tilled or sprayed directly onto the soil and
subsequently mixed with the soil. IWEM models the leaching of wastes after tilling with soil.
IWEM does not account for the losses due to volatilization during or after waste application.
Land application units are modeled in IWEM as a constant pulse-type leachate source, with a
leaching duration equal to the unit's operational life. Only the no-liner scenario is evaluated for
land application units, because liners are not typically used at this type of unit.
6.1.2 WMU Parameters
Table 6-1 summarizes the modeling options and parameters used to develop WMU analyses in
IWEM. The required site-specific parameters are shown in bold italics in Table 6-1. Also, the
last column in Table 6-1 provides the user section references for detailed discussions of each
parameter. The user may refer to thelWEMvS.l User'sGuide (U.S. EPA, 2015a) for additional
guidance in selecting site-specific values for these parameters.
1 In EPACMTP' s finite source module for landfills, the rate of depletion is a function of the ratio between the waste
concentration (Cw) and the leachate concentration (CL). In IWEM, this ratio is set to a constant, protective value
of CW/CL= 10,000.
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IWEM Technical Background Document
IWEMInputs
Table 6-1. Summary of IWEM Options and Parameters for WMUs
Modeling Element
Description or Value
Required or
Default Value
Section
Reference
WMU Parameters
WMU Area (m2)
Depth of Waste in WMU (m)
WMU Location (Nearest
Climate Station)
Waste leachate
concentration (mg/L)
Operational Life/Leaching
Duration (yrs)
WMU Base Elevation below
Ground Surface (m)
Distance to Nearest Surface
Water Body (m)
SI sediment layer thickness
(m)
Waste type permeability
(cm/sec)
Required site-specific user input
Required site-specific user input for LF and
SI (equivalent to ponding depth for Sis)
Not applicable in case of WP or LAU.
Required site-specific user input
Required constituent-specific user input
LF: leaching duration calculated inside
EPACMTP; continues until all waste
depleted.
SI, WP & LAU: Operational life is an optional
user input with defaults as shown; leaching
duration is set equal to operational life
Optional user input
Used to evaluate water table mounding for
SI units
Thickness of accumulated sediment (sludge)
layer in SI. Optional user input
Used for WPs only; not applicable to other
WMUs. Optional user input
Required
Required
Required
Required
WP =20 yrs
LAU =40 yrs
SI = 50 yrs
0 m
360 m
0.2m
low, medium,
high selected
with equal
probability
6.1.2
6.1.2
6.1.2
6.1.2
6.1.2
6.1.2
6.1.2
6.1.2
6.1.2
Well Location Parameters
Downgradient Distance from
WMU (m)
Transverse Distance from
Plume Centerline (m)
Vertical distance below the
water table (m)
Optional user input (maximum of 1,600 m)
Well always on centerline of plume (user
cannot change)
Depth of the well intake below water table
(user cannot change)
150m
0 m
Uniform
distribution from
0-10m
6.1.3
6.1.3
6.1.3
LAU = land application unit LF = landfill
SI = surface impoundment WP = waste pile
6.1.2.1 Required User Inputs
WMU Area (m2). This parameter reflects the footprint area of the WMU (i.e., length by width).
This parameter represents the total surface area over which infiltration and leachate enter the
subsurface.
WMU Waste Depth (m). The WMU waste depth is used for landfill and surface impoundment
simulations only. For landfills, this parameter represents the average waste thickness in the
landfill at closure. EPACMTP uses the waste depth as one of the parameters to calculate the
landfill source depletion rate (see EPACMTP Technical Background Document; U.S. EPA,
2003a). For surface impoundments, the waste depth is equal to the ponding depth, or average
depth of free liquid in the impoundment. The surface impoundment ponding depth represents the
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IWEM Technical Background Document IWEM Inputs
hydraulic head that drives leakage of water from the surface impoundment; EPACMTP uses this
parameter to calculate surface impoundment infiltration rates (see Section 3.1.2).
WMU Location. Location is needed by IWEM to assign the appropriate climate-related
parameter values and is represented by selecting one of the 102 climate stations for which the
HELP model database provides climatological data. Location-specific climate data from these
climate stations were used to develop infiltration and recharge rates using the HELP model for
unlined and single-lined WMUs (see Section 6.4.2 and 6.4.3), and to determine soil and aquifer
temperature in order to calculate hydrolysis transformation rates (see Section 6.5.2).
Waste Leachate Concentration (mg/L). Values of leachate concentration for all constituents of
concern are required input parameters. This parameter can be an actual measured value or an
expected or estimated value. The user-provided leachate concentration values are the basis for
IWEM's estimation of a well concentration and recommendation of the minimum protective liner
design. IWEM compares the entered leachate concentration values against each constituent's
aqueous solubility from the IWEM database. If the entered value exceeds the solubility, IWEM
will display a warning message. A leachate concentration value above the aqueous solubility
value may indicate a measurement error, in which case, the value should be corrected. It may also
reflect a modeling scenario that is outside the range of validity of the EPACMTP fate and
transport model. EPACMTP is designed to simulate transport of dissolved aqueous phase
constituents, and therefore, the solubility is the theoretical maximum concentration value for
which EPACMTP is valid. Despite this, IWEM will not reject user-entered leachate
concentration values that exceed the solubility; however, such scenarios are inappropriate for
modeling with IWEM and may indicate that more detailed site-specific evaluation is needed.
6.1.2.2 Optional Parameters
Operational Life (Duration of Leaching Period) (yr). For waste piles, surface impoundments,
and land application units, operational life is used to establish the duration of leach in the finite
pulse source modeling. Default values for this parameter are as follows:
Waste pile = 20 years
Surface impoundment =50 years
Land application unit = 40 years
For landfills, which are modeled as a finite depleting source, IWEM does not use an operational
life, but estimates the duration of the leaching period internally, as a function of the amount of
waste in the unit at closure and IWEM.
WMU Base Elevation Below Ground Surface (m). This parameter represents the depth of the
base of the unit below the ground surface, as schematically depicted in Figure 6-2. Constituents
leaching from a unit with a base located below the ground surface will experience reduced travel
distances through the unsaturated zone before reaching the ground water. This parameter is an
optional site-specific user input parameter, with a default value of zero. If a non-zero value is
entered, IWEM will verify that the entered value, in combination with the depth to the water
table and magnitude of the unit's infiltration rate, does not lead to a physically infeasible
condition (e.g., water table mound height above the ground surface or above the level of the
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IWEM Technical Background Document IWEM Inputs
waste liquid in an impoundment), in accordance with the infiltration screening methodology
presented in Section 6.7.
. WASTE MANAGEMENT UNIT
DEPTH OF THE WMU BASE
BELOW GROUND SURFACE
WATER TABLE y
GROUND SURFACE
DEPTH TO WATER TABLE
SATURATED ZONE
THICKNESS
Figure 6-2. WMU with base elevation below ground surface.
Distance to Nearest Surface Water Body (m). For surface impoundments only, IWEM uses
information on the distance to the nearest permanent surface water (that is, a river, pond or lake)
in the infiltration screening procedure presented in Section 6.7. This parameter is an optional
site-specific user input. Because the exact distance may not be known in many cases, the input is
framed in terms of whether or not there is surface water body within 2,000 m of the unit. If a
surface water body is present within 2,000 m, IWEM uses the median value of 360 m as a
default. If there is no water body within 2,000 m, IWEM will use a value of 5,000 m in its
calculations.
Surface impoundment Sediment Layer Thickness (m). This parameter is applicable to surface
impoundments only and represents the average thickness of accumulated sediment (sludge)
deposits on the bottom of the impoundment. This layer of accumulated sediment is different from
an engineered liner underneath the impoundment, but its presence will serve to retard the leakage
of water from an impoundment, especially in unlined units. EPACMTP uses this parameter to
calculate the rate of infiltration from unlined and single lined surface impoundments. The
EPACMTP surface impoundment infiltration module is described in Section 3.1.2, with a
detailed description in the EPACMTP Technical Background Document (U.S. EPA, 2003 a). The
accumulated sediment is divided into two equally thick layers, an upper unconsolidated layer and
a lower consolidated layer that has been compacted due to the weight of the sediment above it,
and therefore has a reduced porosity and permeability. This is an optional site-specific user input
parameter, with a default value of 0.2 m (for total thickness; 0.1 m unconsolidated and 0.1 m
consolidated).
Waste Permeability.This parameter is used only for waste piles. Waste piles are not typically
covered and the permeability of the waste itself is a factor in determining the rate of leachate
released due to water percolating through the WMU. For waste piles, IWEM recognizes three
categories of waste permeability and their associated infiltration rate: high permeability (0.041
cm/sec); moderate permeability (0.0041 cm/sec); and low permeability (0.00005 cm/sec). The
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IWEM Technical Background Document
IWEMInputs
waste permeability is correlated with the grain size of the waste material, ranging from coarse to
five-grained materials. If a waste type is not specified for waste piles, IWEM will default to
randomly selecting between the infiltration rates for each of the three waste types in the Monte
Carlo process, with each type having equal probability. That is, IWEM will use a uniform
probability distribution.
6.1.3
Well Location Parameters
In IWEM, the ground water exposure location is modeled as the intake point of a ground water
well located down gradient from the source. The location of the well in IWEM is described by
three parameters depicted schematically in Figure 6-3, which shows the location of the well
relative to the WMU in plan view (top) and cross-section view (bottom).
PLAN VIEW
CONTAMINANT
PLUME
CENTERLINE
SECTIONAL VIEW
WMU
WELL
LOCATION
DOWNGRADIENT DISTANCE (X)
LAND SURFACE
Figure 6-3. Position of the modeled ground water well relative to the WMU.
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IWEM Technical Background Document IWEM Inputs
Downgradient Distance from WMU, x (m). This parameter represents the distance between the
downgradient edge of the WMU and the position of the well, measured along the direction of
ground water flow. This direction represents the x-coordinate as depicted in Figure 6-3. In
IWEM, this parameter is an optional site-specific user input value, with a maximum allowed
value of 1,600 m (1 mile). The default value is 150 m.
A cautionary warning is necessary when specifying a down gradient distance of a ground water
well that is very close to the WMU (for example 5 m or less). The random nature of the well
intake depth (described below) for a well that is close to the WMU can produce unreliable
estimates. As shown in Figure 6-3, the penetration depth of the leachate plume increases with
increasing distance from the WMU. If the well is placed very close to the WMU, there is an
increased likelihood that the well intake point will be below the penetration depth of the plume.
These types of configurations will likely lead to underestimating the 90th percentile ground water
concentration. For such type of configurations, IWEM is not recommended.
If the objective is to determine the maximum possible ground water impact, a recommended
approach would be to experiment with the distance from the WMU, gradually increasing the
distance from 1 meter until the 90th percentile concentration reaches a definitive maximum
value. The distance that generates the maximum value will be sensitive to the initial penetration
depth of the leachate plume at the down-gradient edge of the WMU. Higher values of infiltration
and source area will result in deeper penetration depths.
Well Transverse Distance from the Plume Centerline, y (m). This parameter represents the
horizontal distance between the well and the modeled centerline of the plume, or they y
coordinate depicted in Figure 6-3. This parameter is always set to zero for IWEM (i.e., the
ground water well is always located on the centerline of the plume) and cannot be changed by the
user. This is a conservative assumption because the ground water concentrations predicted by the
model will be highest along the centerline of the plume, and decrease with distance away from
the centerline.
Well Intake Depth Below the Water Table, z (m). This parameter represents the vertical
distance of the well intake point below the water table. In calculating the position of the well
intake, the model uses the water table elevation before any mounding effects are taken into
consideration. In IWEM, the well depth parameter has a uniform probability distribution with a
range of 0 to 10m. This means that all depth values are between 0 to 10m below the water table
are equally likely. For each Monte Carlo simulation in which the modeled saturated zone
thickness is less than 10m, the maximum well depth of 10 m is replaced with the actual saturated
zone thickness used in the iteration. This parameter cannot be changed by the user.
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IWEMInputs
6.2 Structural Fill Parameters
This section provides details about the structural fill source term and the specific parameters in
IWEM used to define intrinsic and operational structural fill characteristics.
6.2.1
The Structural Fill
The structural fill is conceptualized in much the same way as
the landfill WMU type, as represented graphically in Figure
6-4. A structural fill is assumed to contain only one type of
reused material, but the entire volume of the structural fill is
not required to consist of only that materialother non-
reused industrial materials may be included. As a result,
IWEM requires that the user provide the ratio of the volume
of reused materials to the volume of the structural fill, as a
fraction. IWEM assumes that the construction of the
structural fill is complete and that the fill will have a cover
material of some type (e.g., soil, pavement), which may or
may not limit the infiltration of water. The selection of an
infiltration rate implies what type of cover material is
present. For example, if the regional infiltration rates
developed with the HELP model are used, then that implies
that regional soils (or the equivalent) are used to cover the
structural fill.
unsaturated zone
v
saturated zone
STRUCTURAL FILL
Figure 6-4. Structural fill modeled
in IWEM.
The starting point for the simulation is when construction of
the structural fill is completed. The release of waste constituents into the soil and ground water
underneath the structural fill is caused by dissolution and leaching of the constituents due to
precipitation that percolates through the unit. The cover material or the hydraulic conductivity of
the reused materials in the structural fill controls, to a large extent, the amount of leachate that is
released from the structural fill. Structural fills are modeled as a permanent construction with a
rectangular footprint and a uniform depth. IWEM does not simulate any loss processes that may
occur during construction (for example, due to leaching, volatilization, runoff or erosion, or
biochemical degradation). Structural fills are modeled as a pulse-type source: leaching occurs at a
constant leachate concentration until the mass in the structural fill is depleted. EPACMTP
determines the pulse duration from required and optional inputs described in Section 3.2.1 and
below, in Section 6.6.2.
6.2.2
Structural Fill Parameters
Table 6-2 summarizes the modeling options and parameters used to develop structural fill
analyses in IWEM. The required site-specific parameters are shown in bold italics in Table 6-2.
The last column in Table 6-2 provides the user section references for detailed discussions of each
parameter. The IWEMv3.1 User's Guide (U.S. EPA, 2015a) provides additional guidance in
selecting site-specific values for these parameters.
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IWEMInputs
Table 6-2. Summary of IWEM Options and Parameters for Structural Fills
Modeling Element
Description or Value
Required or
Default Value
Section
Reference
Structural Fills Parameters
Structural Fill Area (m2)
Depth of Structural Fill (m)
Effective Bulk Density
(g/cm3)
Effective Hydraulic
Conductivity (m/yr)
Volume Fraction Occupied
by Leachable Material
Structural Fill Location
(Nearest Climate Station)
Waste Leachate
Concentration (mg/L)
Total Waste Concentration
(mg/kg)
Structural Fill Base Elevation
below Ground Surface (m)
Required site-specific user input
Required site-specific user input
Required site-specific user input
Required site-specific user input
Required site-specific user input
Required site-specific user input
Required constituent-specific user input
Required constituent-specific user input
Optional user input
Required
Required
Required
Required
Required
Required
Required
Required
0 m
6.2.2
6.2.2
6.2.2
6.2.2
6.2.2
6.2.2
6.2.2
6.2.2
6.2.2
Well Location Parameters
Downgradient Distance from
SF(m)
Transverse Distance from
Plume Centerline (m)
Vertical distance below the
water table (m)
Optional user input (maximum of 1,600 m)
Well always on centerline of plume (user
cannot change)
Depth of the well intake below water table
(user cannot change)
150m
0 m
Uniform
distribution from
0-10m
6.1.3
6.1.3
6.1.3
6.2.2.1 Required User Inputs
Structural Fill Area (m2). This parameter reflects the footprint area of the structural fill (that is,
length by width). This parameter represents the total surface area over which infiltration and
leachate enter the subsurface.
Structural Fill Depth (m). The structural fill depth represents the average thickness of all
materials in structural fill when construction is complete. EPACMTP uses the depth as one of the
parameters to calculate the mass of teachable constituent in the source and the time it takes to
deplete that mass (see EPACMTP Technical Background Document; U.S. EPA, 2003a).
Structural Fill Location. Location is needed by IWEM to assign the appropriate climate-related
parameter values and is represented by selecting one of the 102 climate stations for which the
HELP model database provides climatological data. Location-specific climate data from these
climate stations were used to develop recharge rates using the HELP model, and to determine
soil and aquifer temperature in order to calculate hydrolysis transformation rates (see Section
6.5.2).
Effective Bulk Density (g/cm3). The dry bulk density is one of the parameters used to calculate
the mass of teachable constituent present in structural fill. Once the mass of teachable constituent
is known, the duration of leaching from the structural fill is calculated in EPACMTP.
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IWEM Technical Background Document IWEM Inputs
Effective Hydraulic Conductivity (m/yr). The material hydraulic conductivity is a required
parameter for determining the limiting value of infiltration through the structural fill.
Volume Fraction Occupied by Leachable Material (unitless). IWEM does not assume that the
entire structural fill is comprised of reused industrial materials. In practice, reused materials are
only one of several components or layers in the structure. Therefore, IWEM requires the user to
provide a number greater than 0 and less than or equal to 1 to represent the fractional volume of
the structural fill occupied by reused materials with teachable components. A value less than 1
would indicate that only part of the structural fill contains teachable materials; for example, a
value of 0.5 would reflect that half of the structural fill by volume contains reused materials.
Waste Leachate Concentration (mg/L). Values of leachate concentration for all constituents of
concern are required input parameters. This parameter can be an actual measured value or an
expected or estimated value. Input values for reused industrial materials can be obtained from
empirical testing data or field data. In practice, the producer of an industrial material would be
the most likely resource for obtaining this data through engineering and environmental testing,
both in the laboratory and in the field. The user-provided leachate concentration values are the
basis for IWEM's determination of whether the predicted ground water exposure concentration is
below or exceeds user-specified benchmarks. IWEM screens structural fill leachate
concentrations against aqueous solubility data in the same manner as for WMUs. The leachate
concentration is also used to determine the time it takes to deplete teachable mass from the
structural fill (e.g., pulse duration).
Total Leachable Waste Concentration (mg/kg). Values of total teachable concentration in
reused material for all constituents of concern are required input parameters. This parameter can
be an actual measured value or an expected or estimated value. Input values for reused industrial
materials can be obtained from empirical testing data or field data. In practice, the producer of an
industrial material would be the most likely resource for obtaining this data through engineering
and environmental testing, both in the laboratory and in the field. User-provided total
concentration is the basis for computing the time it takes to deplete the teachable mass from the
structural fill.
6.2.2.2 Optional Parameters
Structural Base Elevation Below Ground Surface (m). This parameter represents the depth of
the base of the fill below the ground surface, as schematically depicted in Figure 6-2 for WMUs
(the principle is the same for structural fills). Constituents leaching from a unit with a base
located below the ground surface will experience reduced travel distance through the unsaturated
zone before reaching the ground water. This parameter is an optional site-specific user input
parameter, with a default value of zero. If a non-zero value is entered, IWEM will verify that the
entered value, in combination with the depth to the water table and magnitude of the unit's
infiltration rate, does not lead to a physically infeasible condition (e.g., water table mound height
above the ground surface or above the level of the waste liquid in an impoundment), in
accordance with the infiltration screening methodology presented in Section 6.7.
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6.2.3
Well Location Parameters
IWEM treats the ground water exposure location due to leaching from structural fills in the same
way as for WMUs, as described in Section 6.1.3.
6.3 Roadway Parameters
Few of the parameters for the IWEM roadway source module correspond to those of the existing
EPACMTP parameters. For roadway analyses, site-specific data are required to define the
geometry and material properties for all strips and layers.
Table 6-3 summarizes the modeling options and parameters used to develop roadway analyses in
IWEM. IWEM parameters for roadways module can be grouped into seven categories: well
location, general parameters, roadway geometry, layer properties, ditch properties, drain
properties, and flow characteristics. The required site-specific parameters are shown in bold
italics in Table 6-3. The last column in Table 6-3 indicates where the user can find a detailed
discussion of each parameter in this document. The IWEMv3.1 User's Guide (U.S. EPA, 2015a)
provides additional guidance in selecting site-specific values for these parameters.
Table 6-3. Summary of IWEM Options and Parameters for Roadways
Modeling Element
Description
Required
or Default
Value
Section
Reference
Well Location Parameters
Angle between roadway and
ground water flow (degrees)
Location of receptor well relative
to 90° line from roadway edge
Distance from edge of roadway
(m)
Distance from middle of roadway
(m)
Required site-specific user input
Required site-specific user input
Shortest distance between roadway edge
and monitoring well. Required site-specific
user input
Distance along roadway from point at which
distance measurement was made to
midpoint of roadway segment. Required site-
specific user input
Required
Required
Required
Required
6.3.1
6.3.1
6.3.1
6.3.1
General Parameters
Number of roadway strips
(including ditches)
Roadway segment length (m)
Number of drains
Required site-specific user input
Required site-specific user input
Optional user input if ditches are defined as
a strip type; maximum = 2
Required
Required
0
6.3.2
6.3.2
6.3.2
Geometry Parameters
Roadway Geometry Parameters
Strip type
Strip width (m)
Number of layer in a strip
Required site-specific user input
Required site-specific user input
Required site-specific user input
Required
Required
Required
6.3.3
6.3.3
6.3.3
(continued)
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Table 6-3. Summary of IWEM Options and Parameters for Roadways
Modeling Element
Description
Required
or Default
Value
Section
Reference
Geometry Parameters (continued)
Drain Geometry - Configuration
Drained strip(s) (can specify more
than one)
Ditch strip that the drain discharges
into
Layer that the drain lies over
Required user input IF ditches are defined
AND number of drains > 0. Identifies which
strips contain a particular drain
Required user input IF ditches are defined
AND number of drains > 0. Connects a drain
to a ditch
Required user input IF ditches are defined
AND number of drains > 0. Specifies where
the drain is located in the strip layer
structure
6.3.4
6.3.4
6.3.4
Layer Properties
Layer type
Layer thickness (m)
Layer hydraulic conductivity
(m/yr)
Layer dry bulk density (g/cm3)
Required site-specific user input
Required site-specific user input
Required site-specific user input
Required site-specific user input
6.3.5
6.3.5
6.3.5
6.3.5
Ditch Properties
Manning's n coefficient
Slope of the ditch (m/m)
Maximum water depth in the
ditch
Is there a gutter?
Location of gutter(s) (between what
strips)
Required site-specific user input if a ditch is
defined
Required site-specific user input if a ditch is
defined
Required site-specific user input if a ditch is
defined
Optional user input (default is no gutter)
Required user input IF a gutter is present
Required
Required
Required
6.3.6
6.3.6
6.3.6
6.3.6
6.3.6
Drain Properties
Layer thickness (m)
Layer hydraulic conductivity
(m/yr)
Layer dry bulk density (g/cm3)
Required site-specific user input if a drain is
defined
Required site-specific user input if a drain is
defined
Required site-specific user input if a drain is
defined
Required
Required
Required
6.3.7
6.3.7
6.3.7
Flow Characteristics
Percent of Runoff or Flow That Reaches Ditch Strips (for relevant strips and drains)
Percent of roadway runoff that
reaches ditch
Percent of flow in drain that
reaches ditch
Required user input if a ditch is defined
Required user input if a drain is defined
Required
Required
6.3.8
6.3.8
Flow Paths to Ditches
Ditch strip(s) receiving overland
flows
Required user input if a ditch is defined
Required
6.3.8
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6.3.1
Well Location
EPACMTP can only simulate a receptor well that is down-gradient of the leachate source where
ground water flow is perpendicular to the source of leachate (as shown in the top of Figure 6-3).
In order to accommodate non-perpendicular ground water flow directions, IWEM applies a
geometric transformation to the conceptual model that allows IWEM and EPACMTP to
represent non-perpendicular flow as perpendicular. The details of the transformation are
presented in Appendix F.
IWEM uses the angle between the roadway edge, the ground water flow direction away from the
roadway, and the general location of the well to help determine the exact location of the well,
first in general terms and then in more refined terms. Figure 6-5 helps illustrate the inputs
described below.
Figure 6-5. Diagram used by IWEM to specify roadway geometry.
The angle between roadway and ground water flow (degrees). This is labeled "Angle" in red
in Figure 6-5 and is specified first as a range (0 - 90° or 90 - 180°) and then as the actual angle
between the ground water flow and roadway. In Figure 6-5, the angle is 45°.
Location of receptor well relative to 90° line from roadway edge. The well can be in either
Region I (above the 90° line from the center point of the roadway segment length, shown in
green in Figure 6-5) or Region II (below the 90° line). In the example in Figure 6-5, the well is in
Region n.
Shortest distance between roadway edge and monitoring well (m). This is the distance from
the well to the roadway along a line perpendicular to the roadway length. It is labeled D in Figure
6-5.
Distance along roadway from point at which distance measurement was made to midpoint
of roadway segment (m). This is the distance from the midpoint of the roadway segment length
to the location where the distance between the roadway edge and well was measured. It is labeled
L in Figure 6-5.
As mentioned above in Section 6.1.3, IWEM may not generate reliable results for a well that is
very close (less than 5 m) to the source. The maximum ground water exposure concentration will
likely be found at a distance of 5 m or greater due to the combination of a random well intake
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depth and the penetration depth of the leachate plume. See Section 6.1.3 for additional
discussion.
6.3.2 General Roadway Description
Number of Strips. The number of roadways strips represents the number of major designs
element in a roadway cross-section such as paved or unpaved driving surfaces, embankments,
shoulders, medians, or ditches. The number of strips is generally equal to the number of surface
material types encountered as one traverses the roadway cross-section. If there are changes in
material types within a layer below a single surface material, the user may want to consider
dividing that strip into as many different materials that comprise that subsurface layer. The
number of roadway strips is a required input parameter used to define the problem size. IWEM
allows for a roadway to be composed of a maximum of 15 strips, 5 layers per strip, 2 drains, 2
ditches, and 2 gutters.
Number of Drains. A drain moves water from underneath the roadway to a ditch. Thus, the user
must define at least one strip as a ditch before n adding the number of drains. A maximum of
two drains is allowed.
Roadway Length (m). The length of the modeled roadway section is measured in meters (m)
and this parameter represents the idealized, straight line length of the roadway, as depicted in
Figure 3-3. Roadway length is a required input parameter used to determine the areal extent of
any potential leaching through components of the modeled roadway.
6.3.3 Roadway Geometry
Strip Type. Roadways strips represent the major designs elements of a roadway cross-section
used in IWEM. IWEM provides the following five strip types:
Paved Areas are typically used for the traveled surface;
Median is usually an unpaved or vegetated region between traveled surfaces;
Shoulders are found on the sides of traveled surfaces;
Embankments are raised structures (as of earth or gravel) used especially to carry a
roadway or provide separation between a roadway and the surrounding area; and,
Ditches are used to receive runoff from the roadway and diverted flows from drainage
layers.
Strip Width (m). The width of a roadway strip is measured in meters. The strip width is a
required input parameter used to determine the areal extent of any potential leaching through
components of the modeled roadway. If ditches are defined in the roadway cross-section, the
width is also used to determine the volume of runoff water that may flow to the ditch.
Number of Layers in a Strip. The number of roadway layers in a strip represents the number of
distinct material layers in the cross section of a roadway strip. The number of layers in a strip is a
required parameter used to define the problem size. Examples of layers are pavement, base, sub-
base, and sub-grade. At least one layer of at least one roadway strip is required to contain a
material with teachable constituents to perform the analysis.
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6.3.4 Drain Geometry
Drained strip(s). A drain can drain more than one strip, but the user must specify which strips
are drained by each drain. The strips drained by a drain must be located above the drain.
Ditch strip that the drain discharges into. This is the strip number of the ditch that the drain
discharges into. It does not have to be on the same side of the roadway as the drain.
Layer that the drain lies over. This is the layer number for the layer below the drain.
6.3.5 Roadway Layer Properties
Layer Type. Layer types are provided as descriptive labels for the convenience of the user and
are as follows
Base Grade Fill
Sub-base Sub-grade Pavement
Layer Thickness (m). Layer thickness is a required input for strip layers. The thickness is used
along with bulk density of the layer material to calculate the mass of teachable constituent
present in a layer. The mass of teachable constituent is used in conjunction with the infiltration
rate through a layer to calculate the time required to exhaust the mass from that layer given a
leachate concentration.
Layer Hydraulic Conductivity (m/yr). The material hydraulic conductivity is a required
parameter for determining the limiting value of infiltration through strip layers.
Bulk Density (g/cm3). The dry bulk density is a required input for strip layers. As mentioned
above, the bulk density and layer thickness are used to calculate the mass of teachable constituent
present in a layer. Once the mass of teachable constituent is known, IWEM uses the constituent
mass, infiltration rate, and leachate concentration to determine the time required to deplete all of
the constituent mass from a material layer.
6.3.6 Roadway Ditch Properties
Manning's Roughness Coefficient, n. If ditches are defined in the roadway cross-section, the
user is required to provide a value for Manning's roughness coefficient, n, for each ditch. An
estimate of the average water velocity in a ditch is estimated using Manning's equation which
requires a non-dimensional coefficient, n, that reflects the hydraulic resistance induced from the
roughness of the channel surface. A smooth channel generally has less hydraulic resistance and is
represented by a lower coefficient value, resulting in higher velocity estimates. A rough channel
is generally more hydraulically resistant and has correspondingly higher coefficient values. The
best source for this parameter would be engineering design drawings or a design report.
Appendix C (Section C.2.2.5), describes how IWEM treats flow in roadside drainage areas,
ditches, or streams. An estimate of the average water velocity in an open-channel cross-section in
a water-filled or wet ditch is determined using Equation C-31. The average velocity is estimated
using Manning's equation (Equation C-29), which requires a non-dimensional coefficient, n, that
reflects the hydraulic resistance induced from the roughness of the channel surface. A smooth
channel generally has less hydraulic resistance and is represented by a lower coefficient value,
resulting in higher velocity estimates. A rough channel is generally more hydraulically resistant
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and has correspondingly higher coefficient values. Chow (1959) compiled many values for
Manning's n for a wide range of channel conditions. Table 6-4 presents values for n
corresponding to typical roadside drainage conditions.
Table 6-4. Manning's n for Typical Roadside Channels (Chow, 1959)
Type of Channel and Description
Minimum
Normal
Maximum
Excavated or Dredged Channels
Earth, Straight and Uniform
Clean, recently completed
Clean, after weathering
Gravel, uniform section, clean
With short grass, few weeds
0.016
0.018
0.022
0.022
0.018
0.022
0.025
0.027
0.02
0.025
0.03
0.033
Earth Winding and Sluggish
No vegetation
Grass, some weeds
Dense weeds or aquatic plants in deep channels
Earth bottom and rubble sides
Stony bottom and weedy banks
Cobble bottom and clean sides
0.023
0.025
0.03
0.028
0.025
0.03
0.025
0.03
0.035
0.03
0.035
0.04
0.03
0.033
0.04
0.035
0.04
0.05
Dragline-Excavated or Dredged
No vegetation
Light brush on banks
0.025
0.035
0.028
0.05
0.033
0.06
Rock Cuts
Smooth and uniform
Jagged and irregular
0.025
0.035
0.035
0.04
0.04
0.05
Channels Not Maintained, Weeds and Brush Uncut
Dense weeds, high as flow depth
Clean bottom, brush on sides
Same as above, highest stage of flow
Dense brush, high stage
0.05
0.04
0.045
0.08
0.08
0.05
0.07
0.1
0.12
0.08
0.11
0.14
Constructed Channel with Vegetal Lining
Constructed channel with vegetal lining
0.03
0.5
Slope (m/m). For each ditch, slope of the ditch bed must be provided. The slope can be
calculated as the change in elevation of the ditch bed over its length divided by the length of the
ditch. The slope should be set to zero if there is stagnant water in the ditch (no flow).
Maximum Depth (m). To safeguard against possible unrealistic values of water depth in a ditch,
the estimated water depth is limited to this maximum water depth. The maximum water depth
corresponds to the height from the ditch bed to the lowest cresting side.
Gutter. IWEM allows the user to define a gutter between two adjacent roadway strips for each
ditch in the roadway cross-section (however, a gutter is not required, and the default if the user
does not specify a gutter is not to include a gutter). A gutter is a structure on the surface of the
roadway that can intercept and divert runoff from roadway strips uphill of a gutter. The user can
define the percentage of runoff not diverted by a gutter. Runoff diverted by a ditch is assumed to
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leave the modeled system. All runoff from roadway strips downhill of the gutter will flow to their
assigned ditch.
6.3.7 Roadway Drains Properties
Drains are an optional design element that can be used in the roadway cross-section. The purpose
of the drain is to divert a portion of vertically infiltrating water, and any dissolved constituent in
that water, to a ditch and prevent the constituents from leaching into the environment. IWEM
allows the used to add up to two drains, as long as there is at least one ditch in the cross-section.
A drain consists of a highly permeable material layer placed between layers of a single strip or
between the same layers of multiple contiguous strips. To define a drain, the user must provide
the following properties:
Thickness (m). Drain thickness is a required input for drains.
Hydraulic Conductivity (m/yr). The hydraulic conductivity is a required parameter for
determining the limiting value of infiltration through strip layers and for drains.
Bulk Density (g/cm3). The dry bulk density is a required input for strip layers and for drains. The
thickness and bulk density are used to calculate the mass of teachable constituent present in a
layer. Once the mass of teachable constituent is known, the duration of leaching from a material
layer is calculated.
6.3.8 Roadway Flow Characteristics
Percent of roadway runoff that reaches ditch beyond gutter (%). A gutter is used to divert
some or all of the runoff water from strips above or uphill of the gutter, away from the associated
ditch and out of the modeled system. Including a gutter is optional. When a gutter has been
defined for a ditch, the user is required to provide a value for the percentage of runoff from those
strips that flows to the ditch. In other words, the percentage of runoff NOT diverted by the
gutter. If a gutter is not present, then 100% of the runoff should reach the ditch. If a gutter is
present, the percentage should be equal to the ratio of the width of all strips between the gutter
and the ditch to the width of all strips that are associated with the ditch (See Section 3.4.2.3 in
the IWEM User's Guide under Roadway Source Parameters, Ditch Properties and Flow
Characteristics and the section below, Ditch strip(s) receiving overland flows).
Percent of flow in drain that reaches ditch (%). This parameter accounts for the possibility
that not all infiltrating water, and the constituents dissolved in that water, is diverted by the
permeable layer or drain to its associated ditch. A value must be provided for each defined drain,
a percentage ranging from 0 to 100%, to indicate how much of the infiltrate entering the drain is
diverted to the ditch. A value of 0 indicates that no drainage flow will reach the ditch. A value of
100% indicates that all infiltrate entering the drain will be diverted to the ditch. Selecting a value
for a drain will depend on the continuity of the drain in the direction of travel. If the drain is
represented as a continuous layer of highly permeable material, then the value would tend to be
low. If, however, drainage pipe is used at intervals, then the value could be estimated as a ratio of
the area drained by the drainage pipe to the entire area of the roadway underlain by the drain.
Ditch strip(s) receiving overland flows. When a ditch has been defined for a roadway cross-
section, the user is required to associate every non-ditch strip with a ditch. The association directs
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the model to apply any runoff from that strip to the specified ditch. For roadway cross-sections
where two ditches are defined, the association of strip runoff to ditches cannot create a scenario
where runoff flows cross each other - IWEM will prevent that scenario.
6.3.9 Leachate Concentrations
Input values for source constituent parameters (i.e., initial leachate concentration, initial total
teachable mass concentration) can be obtained from empirical testing data or field data. In
practice, the producer of an industrial material would be the most likely resource for obtaining
this data through engineering and environmental testing, both in the laboratory and in the field.
The Recycled Materials Resource Center (RMRC; http://rmrc.wisc.edu/), a federal university-
partnered research and outreach facility for the highway community, has developed the User
Guidelines for Byproducts and Secondary Use Materials in Pavement Construction., available
online2. The online guidance document provides detailed information on many industrial
materials commonly used in roadway construction.
Recently, new leaching test methods, EPA SW-846 Methods 1313, 1314, 1315, and 13163, were
developed to support the evaluation of coal combustion residual materials. These methods were
developed by a collaborative research effort between the U.S. EPA, Vanderbilt University, and
Dutch and Danish partners (Kosson et al, 2002; U.S. EPA, 2010; Garrabrants et al., 2013;
Kosson et al., 2013). Leaching test results acquired from these new methods were recently used
in probabilistic fate and transport modeling of managed coal combustion wastes (U.S. EPA,
2014a). In addition, data acquired from these methods were also used by EPA to evaluate the
beneficial use of coal combustion residuals in concrete (U.S. EPA, 2014b).
6.4 Infiltration and Recharge Rates
IWEM requires the input of the rate of downward percolation of water and leachate through the
unsaturated zone to the water table. The model distinguishes between two types of percolation,
which differ in where they occur relative to the source:
Infiltration is defined as water percolating through the source (i.e., WMU, structural fill,
or roadway) to the underlying soil.
Recharge is water percolating through the soil outside the footprint of the source to the
aquifer.
Infiltration is one of the key parameters affecting the leaching of waste constituents into the
subsurface. For a given leachate concentration, the mass of constituents leached is directly
proportional to the infiltration rate. For WMUs, the different liner types correlate directly to
changing the infiltration rate; more protective liner designs reduce leaching by decreasing the rate
of infiltration. The user can select either a liner type, or an infiltration rate (which will be
evaluated as a user-specified liner, in place of the three predefined liner types). For structural fills
and roadways, the type of reused materials, the nature of compaction, and the sized, number and
orientation of cracks present on the road surface can influence the amount of water infiltration.
2 http://rmrc.wisc.edu/user-guidelines-2/
3 http://www.epa.gov/epawaste/hazard/testmethods/sw846/new metkhtm
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In contrast, recharge introduces pristine water into the aquifer, from the area outside the source.
Increasing recharge therefore tends to result in a greater degree of plume dilution and lower
constituent concentrations. High recharge rates may also affect the extent of ground water
mounding and ground water velocity. The recharge rate is independent of the type and design of
the source; rather it is a function of the climatic and hydrogeological conditions at the source
location, such as precipitation, evapotranspiration, surface run-off, and regional soil type.
Table 6-5 summarizes the parameters used to characterize infiltration and recharge. The required
site-specific parameters are shown in bold italics. The last column guides the user where to find
a detailed discussion of each parameter in this document. The IWEMv3.1 User's Guide (U.S.
EPA, 2015a) provides additional guidance in selecting values for these parameters.
Table 6-5. Summary of IWEM Infiltration and Recharge Parameters
Modeling Element
Description or Value
Section
Reference
Infiltration Rates
WMUs
Unlined Infiltration (m/yr)
Single Liner Infiltration
(m/yr)
Composite Liner
Infiltration (m/yr)
LF, WP, LAU: Optional user input; default generated using HELP model
based on site location
SI: Optional user input; default calculated by EPACMTP based on site-
specific ponding depth
LF, WP: Optional user input; default generated using HELP model
based on site location and 3-ft (0.9-m) clay liner
SI: Optional user input; default calculated by EPACMTP based on site-
specific ponding depth and 3-ft (0.9-m) clay liner
LAU: Not Applicable
LF, WP: Optional user input; nationwide distribution of reported leak
detection system flow rates for composite lined units
SI: Optional user input; calculated using Bonaparte (1989) equation for
geomembrane liner using nationwide distribution of leak densities and
unit-specific ponding depths
LAU: Not Applicable
6.4.1.2
6.4.1.3
6.4.1.4
Structural Fills
Infiltration rate (m/yr)
Optional user input; default values are assumed to be the same as no
liner infiltration rates for LFs, based on climate center and cover soil
type; user-specified value can also be provided
6.4.2
Roadways
Infiltration rate through
a strip (m/yr)
Runoff rate (m/yr)
Precipitation rate (m/yr)
Evaporation rate (m/yr)
Required site-specific user input for each strip; default values are based
on climate center and surface material type; user-specified value can
also be provided
Required user input if a ditch is defined; defaulted
Required user input if a ditch is defined; defaulted
Required user input if a ditch is defined; defaulted
6.4.3
6.4.3
6.4.3
6.4.3
Recharge Rate
Recharge Rate (m/yr)
All source types (WMU, structural fills and roadways): Monte Carlo
based on distribution of soil types and location-specific climate
conditions
6.4.4
LAU = land application unit LF = landfill
SI = surface impoundment WP = waste pile
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6.4.1 Infiltration Rates for WMUs
Several methodologies were used to estimate infiltration:
Landfills, waste piles, and land application units (no-liner, single-liner [landfill,
waste pile only]). The HELP model (Schroeder et al, 1994) was used to compute
infiltration rates. A complete description of how the HELP model was used to develop
infiltration rates is presented in Appendix A of the EPACMTP Parameters/Data
Background Document (U.S. EPA, 2003b).
Landfills and waste piles (composite liner). Infiltration rates were compiled from leak-
detection-system flow rates reported for actual composite-lined waste units (TetraTech,
2001).
Surface impoundments (no liner, single liner). Infiltration through the bottom of the
impoundment is calculated internally by EPACMTP, as described in Section 3.1.2.
Surface impoundments (composite liner). The Bonaparte equation (Bonaparte et al.,
1989) was used to calculate the infiltration rate assuming circular (pin-hole) leaks with a
uniform leak size of 6 mm2, and using the distribution of leak densities (number of leaks
per hectare) assembled from the survey of composite-lined units (TetraTech, 2001).
Tables 6-6 through 6-9 summarize the liner assumptions and infiltration rate calculations for
landfills, waste piles, surface impoundments, and land application units, respectively. The
remainder of Section 6.4.1 provides background on how the HELP model was used in
conjunction with data from climate stations across the United States to develop nationwide
recharge and infiltration rate distributions and provides detailed discussion of how infiltration
rates for different liner designs were developed for each type of WMU.
The HELP model is a quasi-two-dimensional hydrologic model for computing water balances of
landfills, cover systems, and other solid waste management facilities (Schroeder et al., 1994).
The HELP model is primarily a vertical flow model with some lateral flow in permeable drainage
layers. Potential evapotranspiration is modeled by a modified Penman method. Transient values
are calculated and may be able to be extracted; however, IWEM and EPACMTP are based on
steady-state flow, and thus, long-term infiltration rates are generated with HELP. The primary
purpose of the model is to assist in the comparison of design alternatives. The HELP model uses
weather, soil, and design data to compute a water balance for landfill systems accounting for the
effects of surface storage, snowmelt, runoff, infiltration, evapotranspiration, vegetative growth,
soil moisture storage, lateral subsurface drainage, leachate recirculation, unsaturated vertical
drainage, and leakage through soil, geomembrane or composite liners. The HELP model can
simulate landfill systems consisting of various combinations of vegetation, cover soils, waste
cells, lateral drain layers, low permeability barrier soils, and synthetic geomembrane liners.
For IWEM evaluations, HELP Versions 3.03 and 3.07 were used. An existing database of no-
liner infiltration for landfills, waste piles and land application units, and recharge rates for 97
climate stations in the lower 48 contiguous states (ABB, 1995), representing 25 climatic regions,
that was developed with HELP version 3.03, was used as a starting point. To develop the IWEM
evaluations, five climate stations (located in Alaska, Hawaii, and Puerto Rico) were added to
ensure coverage throughout all of the United States. Figure 6-6 shows the locations of the 102
climate stations.
6-20
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IWEM Technical Background Document
IWEMInputs
Table 6-6. Methodology Used to Compute Infiltration for Landfills
Method
Final Cover
Liner Design
IWEM Infiltration
Rate
No Liner
HELP model simulations to compute an
empirical distribution of infiltration rates for a
2-ft (0.6-m) thick cover of three native soil
cover types using nationwide coverage of
climate stations. Soil-type specific infiltration
rates for a specific site are assigned by
using the infiltration rates for respective soil
types at the nearest climate station.
Monte Carlo selection from distribution of soil
cover types: 2-ft (0.6-m) thick native soil (one
of three soil types: silty clay loam, silt loam,
or sandy loam) with a range of mean
hydraulic conductivities (4.2x10"5 cm/s to
7.2x1 0-4cm/s).
No liner
Monte Carlo selection from HELP generated
location- specific values.
Single Liner
HELP model simulations to compute an
empirical distribution of infiltration rates
through a single clay liner using nationwide
coverage of climate stations. Infiltration rates
for a specific site were obtained by using the
infiltration rate for the nearest climate
station.
3-ft (0.9-m) thick clay cover with a hydraulic
conductivity of 1x1 0~7 cm/sec and a 10-ft (3-
m) thick waste layer. On top of the cover, a
1-ft (0.3-m) layer of loam to support
vegetation and drainage and a 1-ft (0.3-m)
percolation layer.
3-ft (0.9-m) thick clay liner with a hydraulic
conductivity of 1x1 0~7 cm/sec. No leachate
collection system. Assumes constant
infiltration rate (assumes no increase in
hydraulic conductivity of liner) over modeling
period.
Monte Carlo selection from HELP generated
location-specific values.
Composite Liner
Compiled from literature sources
(TetraTech, 2001) for composite liners
No cover modeled; the composite liner
is the limiting factor in determining
infiltration
1.5-mm high-density polyethylene layer
with either an underlying geosynthetic
clay liner with maximum hydraulic
conductivity of 5x1 0'9 cm/sec, or a 3-ft
(0.9-m) compacted clay liner with
maximum hydraulic conductivity of
1x10~7 cm/sec.
Assumes same infiltration rate (i.e., no
increase in hydraulic conductivity of
liner) over modeling period.
Monte Carlo selection from distribution
of leak detection system flow rates.
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IWEM Technical Background Document
IWEMInputs
Table 6-7. Methodology Used to Compute Infiltration for Surface Impoundments
Method
Ponding Depth
Liner Design
IWEM Infiltration
Rate
No Liner
EPACMTP SI module for infiltration through
consolidated sludge and native soil layers
with a unit-specific ponding depth from
EPA's SI Study (U.S. EPA, 2001).
Unit-specific based on EPA's SI study.
None. However, barrier to infiltration is
provided by layer of consolidated sludge at
the bottom of the impoundment, and a layer
of clogged native soil below the
consolidated sludge. The sludge thickness
is assumed to be constant over the
modeling period. The hydraulic conductivity
of the consolidated sludge is between
1.3x10-7and 1.8x1Q-7 cm/sec. The hydraulic
conductivity of the clogged native material is
assumed to be 0.1 of the unaffected native
material in the vadose zone.
Calculated by EPACMTP based on Monte
Carlo selection of unit-specific ponding
depth.
Single Liner
EPACMTP module for infiltration
through a layer of consolidated sludge
and a single clay liner with unit-specific
ponding depth from EPA's SI study.
Unit-specific based on EPA's SI study.
3-ft (0.9-m) thick clay liner with a
hydraulic conductivity of 1x1 0~7 cm/sec.
No leachate collection system. Assumes
no increase in hydraulic conductivity of
liner over modeling period. Additional
barrier is provided by a layer of
consolidated sludge at the bottom of the
impoundment, see no-liner column.
Calculated based on Monte Carlo
selection of unit-specific ponding depth
Composite Liner
Bonaparte equation (Bonaparte et al., 1989)
for pin-hole leaks using distribution of leak
densities for units installed with formal
construction quality assurance programs
Unit-specific based on EPA's SI study.
1.5-mm high-density polyethylene layer with
either an underlying geosynthetic clay liner
with maximum hydraulic conductivity of
5x1 0"9 cm/sec, or a 3-ft (0.9-m) compacted
clay liner with maximum hydraulic
conductivity of 1x10"7 cm/sec.
Assumptions:
Constant infiltration rate (i.e., no increase
in hydraulic conductivity of liner) over
modeling period;
Geomembrane liner is limiting factor that
determines infiltration rate.
Calculated based on Monte Carlo selection
of unit-specific ponding depth and
distribution of leak densities
6-22
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IWEM Technical Background Document
IWEMInputs
Table 6-8. Methodology Used to Compute Infiltration for Waste Piles
Method
Cover
Liner Design
IWEM
Infiltration Rate
No Liner
HELP model simulations to compute
distribution of infiltration rates for a 10-ft
(3-m) thick layer of waste, using three
waste permeabilities (copper slag, coal
bottom ash, coal fly ash) and nationwide
coverage of climate stations. Waste-type-
specific infiltration rates for a specific site
are obtained by using the infiltration rates
for respective waste types at the nearest
climate station.
None
No liner
Monte Carlo selection from HELP
generated location-specific values.
Single Liner
HELP model simulations to compute
distribution of infiltration rates through 10-ft
(3-m) waste layer using three waste
permeabilities and nationwide coverage of
climate stations. Infiltration rates for a
specific site were obtained by using the
infiltration rate for the nearest climate
station.
None
3-ft (0.9-m) thick clay liner with a hydraulic
conductivity of 1x10"7 cm/sec, no leachate
collection system, and a 10-ft (3-m) thick
waste layer. Assumes no increase in
hydraulic conductivity of liner over unit's
operational life.
Monte Carlo selection from HELP
generated location- specific values.
Composite Liner
Compiled from literature sources
(TetraTech, 2001) for composite liners
None
1.5-mm high-density polyethylene layer
with either an underlying geosynthetic clay
liner with maximum hydraulic conductivity
of 5x1 0'9 cm/sec, or a 3-ft (0.9-m)
compacted clay liner with maximum
hydraulic conductivity of 1x1 0~7 cm/sec.
Assumptions:
Same infiltration rate (i.e., no increase
in hydraulic conductivity of liner) over
unit's operational life;
Geomembrane is limiting factor in
determining infiltration rate.
Monte Carlo selection from distribution of
leak detection system flow rates
Table 6-9. Methodology Used to Compute Infiltration for Land Application Units
Method
Liner Design
IWEM Infiltration
Rate
No Liner
HELP model simulations to compute an empirical distribution of infiltration rates for a 0.5-ft (15-cm)
thick sludge layer, underlain by a 3-ft (0.9-m) layer of three types of native soil using nationwide
coverage of climate stations. Soil-type specific infiltration rates for a specific site are assigned by
using the infiltration rates for respective soil types at the nearest climate station.
No liner
Monte Carlo selection from HELP generated location specific values.
Single Liner
N/A
N/A
N/A
Composite Liner
N/A
N/A
N/A
6-23
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IWEM Technical Background Document
IWEMInputs
Alaska
Hawdi
Puerto Rico
Figure 6-6. Locations of HELP climate stations.
6-24
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IWEM Technical Background Document IWEM Inputs
The current version of HELP (version 3.07) was used for the additional modeling for the no-liner
scenario. The results of Version 3.07 were compared to those of Version 3.03, and the
differences in calculated infiltration rates were insignificant. This comparison was also used to
verify a number of counter-intuitive infiltration rates that were generated with HELP Version
3.03. For some climate stations located in areas of the country with low precipitation rates, the
net infiltration for unlined landfills did not always correlate with the relative permeability of the
landfill cover. In some cases, a less permeable cover resulted in a higher modeled infiltration rate
as compared to a more permeable cover. Examples can be seen in the detailed listing of
infiltration data in Appendix D. Table D-l shows that for a number of climate stations, including
Albuquerque, Denver, and Las Vegas, the modeled infiltration rate for landfills with a silty clay
loam cover is higher than the values corresponding to silt loam and sandy loam soil covers. In all
these cases, the HELP modeling results for unlined landfills were determined to be correct and
could be explained in terms of other water balance components, including surface run-off and
evapotranspiration.
The first 97 climate stations were grouped into 25 climate regions based on ranges of average
annual precipitation and pan evaporation, as shown in Table 6-10. For each modeled climate
station, HELP provides a database of five years of climatic data. This climatic data was used,
along with data on the regional soil type and WMU design characteristics, to calculate a water
balance for each applicable liner design as a function of the amount of precipitation that reaches
the top surface of the unit, minus the amount of runoff and evapotranspiration. The HELP model
then computed the net amount of water that infiltrates through the surface, waste, and liner
layers, based on the initial moisture content and the hydraulic conductivity of each layer.
In addition to climate factors and liner designs, the infiltration rates calculated by HELP are
affected by landfill cover design, permeability of the waste material in waste pile, and land
application unit soil type. For every climate station and WMU type, three HELP infiltration rates
were calculated. The WMU location is a required user input, and the climate factors used in
HELP are therefore also fixed; however, IWEM still accounts for local variability in landfill soil
cover type and waste pile waste permeability.
The permeability of the soil used in the landfill cover affects the HELP-generated infiltration
rates. A consistent set of soil properties were used in the infiltration (and recharge) rate
calculations, as was done in the unsaturated zone fate and transport simulations (see Section
6.5.2). HELP was used to calculate infiltration for sandy loam, silty loam, and silty clay loam
soils.
In the case of waste piles, which do not have a cover, the permeability of the waste material itself
plays a role similar to that of a landfill cover in regulating infiltration rate. Waste piles were
modeled with three different waste types, having different waste permeabilities, and each having
equal likelihood of occurrence. The permeabilities for the three different waste types are
discussed in Section 6.1.2.
6-25
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IWEM Technical Background Document
IWEMInputs
Table 6-10. Grouping of Climate Stations by Average Annual Precipitation and Pan Evaporation
(ABB, 1995)
City
Boise
Fresno
Bismarck
Denver
Grand Junction
Pocatello
Glasgow
Pullman
Yakima
Cheyenne
Lander
Rapid City
Los Angeles
Sacramento
San Diego
Santa Maria
Ely
Cedar City
Albuquerque
Las Vegas
Phoenix
Tucson
El Paso
Medford
Great Falls
Salt Lake City
Grand Island
Flagstaff
Dodge City
Midland
St. Cloud
E. Lansing
North Omaha
Dallas
Tulsa
Brownsville
State
ID
CA
ND
CO
CO
ID
MT
WA
WA
WY
WY
SD
CA
CA
CA
CA
NV
UT
NM
NV
AZ
AZ
TX
OR
MT
UT
NE
AZ
KS
TX
MN
Ml
NE
TX
OK
TX
Climate Region
Precipitation
(m/yr)
<0.40
<0.40
<0.40
<0.40
<0.40
0.40-0.61
0.40-0.61
0.40-0.61
n A.r\ n RI
0.61-0.81
0.61-0.81
0.61-0.81
0.61-0.81
Evaporation
(m/yr)
<0.76
0.76-1.0
1.0-1.3
1.3-1.5
> 1.5
0.76-1.0
1.0-1.3
1.3-1.5
>1.5
<0.76
0.76-1.0
1.0-1.3
1.3-1.5
City
Columbia
Put-in-Bay
Madison
Columbus
Cleveland
Des Moines
E. St. Louis
Topeka
Tampa
San Antonio
Portland
Hartford
Syracuse
Worchester
Augusta
Providence
Nashua
Ithaca
Boston
Schenectady
NY City
Lynchburg
Philadelphia
Seabrook
Indianapolis
Cincinnati
Bridgeport
Jacksonville
Orlando
Greensboro
Watkinsville
Norfolk
Shreveport
Astoria
New Haven
Plainfield
State
MO
OH
Wl
OH
OH
IA
IL
KS
FL
TX
ME
CT
NY
MA
ME
RI
NH
NY
MA
NY
NY
VA
PA
NJ
IN
OH
CT
FL
FL
NC
GA
VA
LA
OR
CT
MA
Climate Region
Precipitation
(m/yr)
0.81-1.0
0.81-1.0
n si 1 n
1.0 1.2
1.0-1.2
1 n 19
>1.2
Evaporation
(m/yr)
0.76-1.0
1.0-1.3
1 ^ 1 ^
<0.76
0.76-1.0
1 n 1 ^
<0.76
(continues)
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IWEM Technical Background Document
IWEMInputs
Table 6-10. Grouping of Climate Stations (continued)
City
Oklahoma City
Bangor
Concord
Pittsburgh
Portland
Caribou
Chicago
Burlington
Rutland
Seattle
Montpelier
Sault St. Marie
State
OK
ME
NH
PA
OR
ME
IL
VT
VT
WA
VT
Ml
Climate Region
Precipitation
(m/yr)
0.61-0.81
0.81-1.0
Evaporation
(m/yr)
>61.5
<0.76
City
Nashville
Knoxville
Central Park
Lexington
Edison
Atlanta
Little Rock
Tallahassee
New Orleans
Charleston
W. Palm
Beach
Lake Charles
Miami
State
TN
TN
NY
KY
NJ
GA
AK
FL
LA
SC
FL
LA
FL
Climate Region
Precipitation
(m/yr)
>1.2
>1.2
>1.2
Evaporation
(m/yr)
0.76-1.0
1.0-1.3
1 ^ 1 ^
6.4.1.2 Infiltration Rates for Unlined Units
Landfill. The HELP model was used to simulate infiltration through closed landfills for each of
the 102 climate station locations shown in Figure 6-6. A 2-ft (0.6-m) cover was included as the
minimum Subtitle D requirement. Three different soil cover types were modeled: sandy loam,
silty loam, and silty clay loam soils. Table 6-11 presents the hydraulic parameters used in the
HELP modeling for these three soil types.
Table 6-11. Hydraulic Parameters for the Modeled Soils
Soil Type
Sandy Loam
Silt Loam
Silty Clay Loam
HELP Soil
Number
6
9
12
Total
Porosity
(vol/vol)
0.453
0.501
0.471
Field
Capacity
(vol/vol)
0.190
0.284
0.342
Wilting
Point
(vol/vol)
0.085
0.135
0.210
Saturated Hydraulic
Conductivity (cm/sec)
0.000720
0.000190
0.000042
Other landfill design criteria included:
A vegetation cover of "fair" grass this is the quality of grass cover suggested by the
HELP model for landfills where limitations to root zone penetration and poor irrigation
techniques may limit grass quality.
The evaporation zone thickness selected for each location was generally the depth
suggested by the model for that location for a fair grass crop; however, the evaporation
zone thickness was not allowed to exceed the soil thickness (2 ft, or 0.6 m).
6-27
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IWEM Technical Background Document IWEM Inputs
The leaf area index (LAI)4 selected for each location was that of fair grass (2.0) unless the
model indicated a lower maximum for that location.
The landfill configuration was based on a facility with an area of 4,047 m2 (1 acre) with a
2% top slope and a drainage length of 61 m (represents one side of a 4,047 m2 square).
Runoff was assumed to be possible from 100% of the cover.
Appendix D, Table D-l, presents the infiltration rate data for the 102 climate stations. The
unlined landfill infiltration rate for each soil type at each of the 102 climate centers was used as
the ambient regional recharge rate for that climatic center and soil type.
Surface Impoundment. Surface impoundment infiltration rates were calculated using the built-
in surface impoundment module in EPACMTP (see Section 3.1.2). This means that for
EPACMTP, the surface impoundment infiltration rate is not really an input parameter, rather the
model calculates infiltration rates "on the fly" during the simulation, as a function of
impoundment ponding depth and other surface impoundment characteristics. For unlined surface
impoundments, the primary parameters that control the infiltration rate are the ponding depth in
the impoundment, the thickness and permeability of any accumulated sediment layer at the base
of the impoundment, and the presence of a "clogged" (i.e., reduced permeability) layer of native
soil underneath the impoundment caused by the migration of solids from the impoundment. In
addition, IWEM checks that the calculated infiltration rate does not result in an unrealistic degree
of ground water mounding (see Section 6.7).
For IWEM, unit-specific data on surface impoundment ponding depths from EPA's Surface
Impoundment Study (U.S. EPA, 2001) were used, along with an assumed fixed sediment layer
thickness of 20 cm at the base of the impoundment. The resulting sediment layer permeability
has a relatively narrow range of variation between 1.26xlO"7 and 1.77xlO"7 cm/s. The depth of
clogging underneath the impoundment was assumed to be 0.5 m in all cases, and the saturated
hydraulic conductivity of the clogged layer was assumed to be 10% of that of the native soil
underlying the impoundment.
In the event that the surface impoundment is reported to have its base below the water table, the
infiltration was calculated using Darcy's law based on the hydraulic gradient across and the
hydraulic conductivity of the consolidated sediment at the bottom of the impoundment unit.
Waste Pile. For the purpose of estimating leaching rates, waste piles were considered to be
similar to non-covered landfills with a total waste thickness of 3 m (10 ft). Therefore, the
infiltration rates for unlined waste piles were generated with the HELP model using the same
general procedures as for landfills, but with the following modifications:
No cover. The leachate flux was modeled through active, uncovered piles, with a surface
having no vegetation. The evaporative zone depth was taken as the suggested HELP
model value for the "bare" condition at each climate center. The leaf area index was set to
zero to eliminate transpiration.
Variable waste permeability. For uncovered waste piles, the infiltration rates predicted
by HELP model were sensitive to the permeability of the waste material itself. Based on
these results, waste pile infiltration rates were simulated for three different waste pile
4 HELP defines LAI as a dimensionless ratio of the leaf area of actively transpiring vegetation to the normal surface
area of the land on which the vegetation is growing
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IWEM Technical Background Document
IWEMInputs
materials: relatively high permeability, moderate permeability, and relatively low
permeability. Parameters for the three waste types are presented in Table 6-12.
Table 6-12. Moisture Retention Parameters for the Modeled Waste Pile Materials
Waste Type
Low Permeability
Moderate Permeability
High Permeability
HELP Soil
Number
30
31
33
Total
Porosity
(vol/vol)
0.541
0.578
0.375
Field
Capacity
(vol/vol)
0.187
0.076
0.055
Wilting
Point
(vol/vol)
0.047
0.025
0.020
Saturated
Hydraulic
Conductivity
(cm/sec)
0.00005
0.00410
0.04100
Waste pile infiltration rates were calculated for all 102 climate stations and waste material
permeabilities. Appendix D, Table D-2, presents the waste pile infiltration rate values for all
climate stations and waste types.
Land Application Unit. Land application units were modeled with HELP using two soil layers.
The top layer was taken to be 0.5 ft (15 cm) thick and represented the layer into which the waste
was applied. The bottom layer was of the same material type as the top layer and was set at a
thickness of 3 ft (0.9 m). Both of these layers were modeled as vertical percolation layers. The
same three soil types used for landfills were also used for land application units.
The waste applied to the land application unit was assumed to be a sludge-type material with a
high water content. A waste application rate of 18.4 cm/yr was assumed, with the waste having a
solids content of 20% and a unit weight of 1,200 kg/m3. Assuming that 100% of the water in the
waste was available as free water, an excess water amount of 15 cm/yr, in addition to
precipitation, would be available for percolation. HELP model analyses showed that the
additional water available for percolation generally would have little effect on the simulated
water balance and net infiltration, except for sites located in arid regions of the United States
with very little natural precipitation. For more representative waste application rates, the effect
disappeared because introducing additional moisture in the simulated water balance results in a
commensurate increase in runoff and removal by evapotranspiration. The land application unit
infiltration values are presented in Appendix D, Table D-3.
6.4.1.3 Infiltration Rates for Single-Lined Waste Units
IWEM includes infiltration rates for lined landfills, waste piles, and surface impoundments. In
the case of land application units, only unlined units are considered.
Landfill. Infiltration rates were calculated for single-lined landfills using the HELP model and
modeling the landfill as a four-layer system, consisting, from top to bottom of:
1-ft (0.3-m) percolation cover layer;
3-ft (0.9-m) compacted clay cover with hydraulic conductivity of IxlO"7 cm/s ;
10-ft (3-m) thick waste layer; and
3-ft (0.9-m) thick compacted clay liner with a hydraulic conductivity of IxlO"7 cm/sec.
6-29
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IWEM Technical Background Document
IWEMInputs
Climate Stations with Location-Specific
HELP Infiltration Rates for Clay-Lined Units
The cover layer was simulated as a loam drainage layer supporting a "fair" cover crop with an
evaporative zone depth equal to that associated with a fair cover crop at the climate center. The
remaining conditions were identical to those described in Section 6.4.1.2 for unlined landfills.
The grouping of climate stations into 25 regions of similar climatic conditions depicted in
Table 6-10 were used to reduce the number of required HELP simulations. Infiltration rates were
calculated for the 25 climate regions, and then the same value was assigned to each climate
station in one region, rather than calculating rates for all of the 102 individual climate stations.
To ensure a conservative result, the climate center with the highest average precipitation in each
climate region was chosen to represent that region. Appendix D, Table D-4, shows the
infiltration rate values for clay-lined landfills. The actual climate stations that were used in the
HELP simulations for each climate region are shown in bold face in the table. Individual
infiltration rates were calculated for the five new climate centers in Alaska, Hawaii, and Puerto
Rico that were not assigned to a climate region.
The database of HELP-generated infiltration rates is used to provide estimates of landfill
infiltration rates in IWEM when a user does not have site-specific data. The grouping of climate
centers into regions for clay-lined units resulted in a number of apparent anomalies in which the
suggested infiltration rate for a lined unit was higher than the unlined infiltration rate at the same
climate station. This resulted from using the infiltration rate for the climate center with the
highest annual precipitation in each region for clay-lined units, but then comparing it with a
location-specific infiltration value for unlined units.
These anomalies occurred only for climate stations
in arid parts of the United States, and were
noticeable only when the absolute magnitude of
infiltration was low. To eliminate these counter-
intuitive results, location-specific HELP infiltration
rates for clay-lined units were calculated for 17
climate stations (listed at right). These location-
specific infiltration rates for these 17 climate
stations were then incorporated into the IWEM
software, replacing the regional values developed for
these stations.
Waste Pile. Infiltration rates for single-lined waste piles were calculated using the HELP model
and modeling the waste pile as a two-layer system, consisting, from top to bottom, of:
10-ft (3-m) thick, uncovered, waste layer; and
3-ft (0.9-m) thick compacted clay liner with a hydraulic conductivity of IxlO"7 cm/sec.
Other parameters were set to the same values as in the unlined waste pile case. The same three
waste material types were used. A bare surface was modeled for the evaporative zone depth.
The same grouping of climate stations in 25 climate regions as was previously discussed for
landfills was used. Appendix D, Table D-4, shows the infiltration rate values for clay-lined
waste piles. The actual climate centers that were used in the HELP simulations for each climate
region are shown in bold face in the table. Individual infiltration rates were calculated for the five
new climate centers, in Alaska, Hawaii, and Puerto Rico, which were not assigned to a climate
region.
Phoenix, AZ
Tucson, AZ
Denver, CO
Grand Junction, CO
Pocatello, ID
Great Falls, MT
Glasgow, MT
Ely, NV
Las Vegas, NV
Rapid City, SD
El Paso, TX
Cedar City, UT
Salt Lake City, UT
Pullman, WA
Yakima, WA
Cheyenne, WY
Lander, WY
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IWEM Technical Background Document IWEM Inputs
Analogous to the situation encountered for landfills, we found a number of apparent anomalies
between waste pile infiltration rates for unlined as compared to clay-lined waste piles occurred
with the regional infiltration values for clay-lined units. The occurrence of these anomalies for
waste piles was also restricted to climate centers in arid parts of the United States, for which the
absolute magnitude of infiltration was low. These were corrected in the same way as described
above for landfills.
During the process of verifying the HELP-generated infiltration rates for clay-lined units,
incorrect values for clay-lined waste piles assigned to the Lake Charles, LA and Miami, FL,
climate stations were replaced. These two climate stations have high precipitation (Table 6-10),
but were assigned low infiltration rates (see Appendix D, Table D-4). The HELP model was
rerun for the clay-lined waste pile scenario for the three clay-lined waste pile scenarios: low,
medium, and high waste permeability. The re-calculated infiltration rate values averaged 0.066
m/yr, as compared to the previously generated rate of 0.019 m/yr. The re-calculated values were
incorporated in the IWEM software tool.
Surface Impoundment. For single-lined surface impoundments, infiltration rates were
calculated by EPACMTP in the same manner as described in Section 6.4.1.2 for unlined units,
with the exception of that a 3-ft (0.9-m) compacted clay liner with a hydraulic conductivity of
IxlO"7 cm/s was modeled at the bottom of the WMU. In addition, the effect of clogged native
material was not included due to the filtering effects of the liner.
6.4.1.4 Infiltration Rates for Composite-Lined Units
For composite liners, data on liner integrity and leachate infiltration through composite liners
were collected and compiled from the available literature (TetraTech, 2001). This section
describes how those data were applied to develop the IWEM analyses.
Landfill and Waste Pile. Composite-lined landfills and waste piles were treated the same for the
purpose of determining infiltration rates. For these WMUs, an infiltration rate distribution was
developed from actual leak detection system flow rates reported for clay composite-lined landfill
cells. The distribution of composite-lined landfill and waste pile infiltration rates was based on
available monthly average leak detection system flow rates from 27 landfill cells reported by
TetraTech (2001). The data and additional detail for the 27 landfill cells are provided in
Appendix D, Table D-5. The data included monthly average leak detection system flow rates for
22 operating landfill cells and 5 closed landfill cells. The 27 landfill cells are located in eastern
United States: 23 in the northeastern region, one in the mid-Atlantic region, and 3 in the
southeastern region. Each of the landfill cells is underlain by a geomembrane/geosynthetic clay
liner which consists of a geomembrane of thickness between 1 and 1.5 mm (with the majority, 22
of 27, being 1.5 mm thick), overlying a geosynthetic clay layer of reported thickness of 6 mm.
The geomembrane is a flexible membrane layer made from high-density polyethylene. The
geosynthetic clay liner is a composite barrier consisting of two geotextile outer layers with a
uniform core of bentonite clay to form a hydraulic barrier. The liner system is underlain by a leak
detection system.
A subset of the reported flow rates compiled by TetraTech (2001) was used in developing the
composite liner infiltration rates for IWEM. Leak detection system flow rates for
geomembrane/compacted clay composite-lined landfill cells were not included. For compacted
clay liners (including composite geomembrane), there is the potential for water to be released
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IWEMInputs
during the consolidation of the clay liner and yield an unknown contribution of water to leak
detection system flow. Thus, it is very difficult to determine how much of the leak detection
system flow is due to liner leakage, versus due to clay consolidation. Leak detection system flow
rates from three geomembrane/geosynthetic clay lined-cells were also omitted. For one cell, flow
rate data were available for the cell's operating period and the cell's post-closure period. The
average flow rate for the cell was 26 L/ha/day when the cell was operating and 59 L/ha/day when
the cell was closed. These flow rates, which were among the highest reported, are difficult to
interpret because the flow rate from the closed cell was over twice the flow rate from the open
cell, a pattern inconsistent with the other open cell/closed cell data pairs we reviewed. For the
two other cells, additional verification of the data may be needed in order to fully understand the
reported flow rates.
The resulting cumulative probability distribution of infiltration rates for composite-lined landfills
and waste piles for use in this application is based on the 27 remaining data points is presented in
Table 6-13. Note that over 50% of the values are zero, that is, they have no measurable
infiltration.
Table 6-13. Cumulative Frequency Distribution of Infiltration Rate for
Composite-Lined Landfills and Waste Piles
Percentile
Infiltration Rate (m/yr)
0
0.0
10
0.0
25
0.0
50
0.0
75
7.30x10-5
90
1.78x10-4
100
4.01x10-4
Surface Impoundment
Leakage through circular defects (pinholes) in a composite liner were calculated using the
following equation developed by Bonaparte et al. (1989):
O =
0.9 jr^O.74
(6-1)
where:
Q
a
h
Ks
steady-state rate of leakage through a single hole in the liner (m3/s)
area of hole in the geomembrane (m2)
head of liquid on top of geomembrane (m)
hydraulic conductivity of the low-permeability soil underlying the geomembrane
(m/s).
This equation is applicable to cases where there is good contact between the geomembrane and
the underlying compacted clay liner. For each surface impoundment unit, the infiltration rate was
determined using the above equation based on the unit-specific ponding depth data
(corresponding to h in the above equation) from the Surface Impoundment Study (U.S. EPA,
2001) in combination with a distribution of leak densities (expressed as number of leaks per
hectare) compiled from 26 leak density values reported in TetraTech (2001). The leak densities
are based on liners installed with formal Construction Quality Assurance programs.
The 26 sites with leak density data are mostly located outside the United States: 3 in Canada, 7 in
France, 14 in United Kingdom, and 2 in unknown locations. The WMUs at these sites (8
landfills, 4 surface impoundments, and 14 unknown) are underlain by a layer of geomembrane of
thickness varying from 1.14 to 3 mm. The majority of the geomembranes are made from high-
density polyethylene (23 of 26) with the remaining 3 made from prefabricated bituminous
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IWEMInputs
geomembrane or polypropylene. One of the sites has a layer of compacted clay liner beneath the
geomembrane, however, for the majority of the sites (25 of 26) material types below the
geomembrane layer are not reported. The leak density data above were used for surface
impoundments. The leak density distribution is shown in Table 6-14. Table D-6, Appendix D,
provides additional detail.
Table 6-14. Cumulative Frequency Distribution of Leak Density for
Composite-Lined Surface Impoundments
Percentile
Leak density
(Num. leaks/ha)
0
0
10
0
20
0
30
0
40
0.7
50
0.915
60
1.36
70
2.65
80
4.02
90
4.77
100
12.5
To use the Bonaparte equation, a uniform leak size of 6 mm2 was assumed. The leak size is the
middle of a range of hole sizes reported by Rollin et al. (1999), who found that 25% of holes
were less than 2 mm2, 50% of holes were 2 to 10 mm2, and 25% of holes were greater than 10
mm2. The geomembrane was assumed to be underlain by a compacted clay liner whose hydraulic
conductivity is 1 x 10"7 cm/s.
An infiltration rate calculation to estimate the range of infiltration resulting from the leaks in
geomembrane was conducted to ascertain the plausibility of the leak density data. Because of the
absence of documented infiltration data for surface impoundments, the infiltration data for
landfills, described above for landfills and waste piles, were used as a surrogate infiltration data
set for comparison purposes. Because the comparison was made on the basis of landfill data, the
head of liquid above the geomembrane was set to 1 ft (0.3 m), which is a typical maximum
design head for landfills. Calculation results are shown in Table D-6, Appendix D. The results
indicate that the calculated leakage rates, based on the assumptions of above-geomembrane head,
hole dimension, hydraulic conductivity of the barrier underneath the geomembrane, and good
contact between the geomembrane and the barrier, agree favorably with the observed landfill
flow rates reported in Table D-5, Appendix D. This result provided confidence that the leak
density data could be used as a reasonable basis for calculating infiltration rates using actual
impoundment ponding depths.
The resulting frequency distribution of calculated infiltration rates for composite- lined surface
impoundments is presented in Table 6-15. In IWEM, the user is required to specify the unit's
ponding depth. IWEM will then determine the unit's infiltration distribution using the Bonaparte
equation and the leak density distribution in Table 6-13.
Table 6-15. Cumulative Frequency Distribution of Infiltration Rate for
Composite-Lined Surface Impoundments
Percentile
Infiltration Rate (m/yr)
0
0.0
10
0.0
25
0.0
50
1.34x10-5
75
1.34x10-4
90
3.08x10-4
100
4.01xio-3
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IWEM Technical Background Document IWEM Inputs
6.4.2 Infiltration Rates for Structural Fills
The HELP model (Schroeder et al, 1994) rates computed for unlined landfills were assumed to
be representative of infiltration rates through structural fills. The hydraulic performance of a
structural fill is often designed to behave similarly to the surrounding soils (NRMCA, n.d.).
Therefore, in IWEM, recharge rates are assumed to be similar to the infiltration rates through
unlined landfills (see Section 6.4.4). In the event that the surface material (or compaction of the
structural fill materials) results in a less permeable medium, IWEM will use the effective
hydraulic conductivity (a required input) to limit the HELP-derived or user-specified infiltration
rate to one no greater than the hydraulic conductivity of the structural fill materials.
6.4.3 Infiltration Rates for Roadways
Subgrade infiltration is an input parameter for the IWEM roadway source module, required to
define the mass flux emanating from the highway source area. Subgrade infiltration refers to
water exiting from the subbase layer into the subgrade below. Subgrade infiltration is governed
by pavement configuration, pavement hydraulic properties, climatic conditions, and drainage
system. Nationwide, a multitude of combinations of the above four factors are possible. At
present, there are very little subgrade infiltration data. To assist the IWEM user in estimating
subgrade infiltration for different configurations, conditions, and settings, a procedure for
estimating subgrade infiltration is presented in this section. The procedure involves dividing the
United States into 12 climatic zones. For each zone, pre-determined infiltration rates for major
types of pavement configuration with a range of material properties and climatic conditions are
given. The 12 climatic zones are described in Section 6.4.3.1. Tabulated subgrade infiltration
rates are discussed and presented in tables described in Section 6.4.3.2. A procedure to estimate
subgrade infiltration rates for specific zones, configurations, and material properties is presented
in Section 6.4.3.3. Finally, a procedure for estimating runoff and evaporation in climatic zones is
presented in Section 6.4.3.4.
6.4.3.1 Climatic Zones
Following the report by Jackson and Puccinelli (2006), the environmental regions of interest in
the United States may be defined based on three temperature ranges (i.e., deep freeze, moderate
freeze, and no freeze) and precipitation ranges. According to Jackson and Puccinelli (2006),
deep-freeze, moderate-freeze, and no-freeze geographical regions, defined in terms of freezing
index, are shown in Figure 6-7. The freezing index is used as a measure of the combined
duration and magnitude of below freezing temperatures occurring during any given freezing
season (Tuhkanen, 1980). As defined by the U.S. Army Corps of Engineers, freezing index is the
number of Celsius degree-days (above and below 0°C) between the highest and lowest points on
the cumulative degree-days time curve for one freezing season. According to Jackson and
Puccinelli (2006), the no-freeze region defines areas with a freezing index less than 50 °C-days,
while the moderate-freeze is defined with a freezing index between 50 and 400 °C-days. The
deep-freeze region consists of locations exhibiting a freezing index greater than 400 °C-days.
Each region is further subdivided into four zones based on similarity in climate. Note that Alaska
is in Zone A4, and Hawaii is in Zone C4.
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Legend:
A Deep Freeze (4 subzones; Subzone A4, which includes only Alaska is not shown)
B Moderate-Freeze (4 subzones)
C No-Freeze (4 subzones with C4, including Hawaii, not shown)
Climate station location
Figure 6-7. Climatic zones.
The EPACMTP in IWEM uses climate data from the HELP climate database, which includes
102 climate stations in 45 states and Puerto Rico (U.S. EPA, 2003b). For each zone in Figure 6-
7, two climate stations located within the zone from the FtELP climate database, with minimum
and maximum precipitations, are selected. The selected 24 climate stations are listed in Table 6-
16
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Table 6-16. Climatic Zones and Corresponding 5-year Average Annual Precipitations
Freezing Index
Deep-freeze
Moderate-freeze
No-freeze
Zone3
A1
A2
A3
A4d
B1
B2
B3
B4
C1
C2
C3
C44
Minimum 5-Year Average
Precipitation for Zone
Station13
Montpelier, VT
Rapid City, SD
Cheyenne, WY
Fairbanks, AK
Philadelphia, PA
Dodge City, KS
Grand Junction, CO
Las Vegas, NV
Tampa, FL
Midland, TX
Phoenix, AZ
Fresno, CA
(m/yr)
0.88
0.39
0.28
0.24
1.1
0.5
0.18
0.13
0.97
0.41
0.2
0.26
Maximum 5-Year Average
Precipitation for Zone
Station13
New Haven, CT
Syracuse, NY
Great Falls, MT
Annette, AK
Edison, NJ
Nashville, TN
Flagstaff, AZ
Salt Lake City, UT
Tallahassee, FL
New Orleans, LA
Tucson, AZ
Astoria, OR
(m/yr)
1.3
1.2
0.46
2.6
1.3
1.4
0.54
0.41
1.7
1.8
0.26
1.7
Notes:
a Zone geographical coverage is given in Figure 6-7.
b Two climate stations in each zone are selected: one with the lowest precipitation, and the other with the
highest precipitation.
c Precipitation data are obtained from HELP model.
d Zone A4 comprises Alaska only. Hawaii is incorporated into Zone C4 (not shown in Figure 6-7).
6.4.3.2 Zone-Specific Subgrade Infiltration Rates
The default zone-specific subgrade infiltration rates included in IWEM were estimated for
various pavement components using the HELP model (Schroeder et al, 1994). Table 6-17
presents the material properties used as inputs to the HELP model to estimate default infiltration
rates for IWEM for the various pavement components. (Note most of these variables are not
inputs to IWEM.) The pavement components for various pavement types in Table 6-17 were
represented by vertical-percolation layers as described by Schroeder et al (1994). However, for a
special case of low-end portland cement concrete pavement top-course layers with hydraulic
conductivity less than 10~6 cm/sec, the pavement layers were represented in the infiltration
analysis by soil-liner layers (Schroeder et al, 1994). In addition, to avoid unrealistic ponding
above the pavement represented by the soil-liner layers, a very thin vertical-percolation layer
(0.25 cm) was placed on top of the uppermost soil-liner layer to limit the ponding elevation to the
road surface elevation. In all cases, the pavement system was assumed to rest on top of a vertical-
percolation layer with the hydraulic conductivity of at least 1.7xlO~3 cm/sec. In each zone, the
climate stations with the highest and lowest 5-year average precipitations were selected to
provide an estimate of the range of infiltration variation within that zone. A comprehensive
literature survey of hydraulic properties of asphalt concrete, portland cement concrete, base,
subbase, and embankment has been undertaken by Apul et al. (2002). Ranges of hydraulic
conductivity values for these pavement components, based on the values reported by Apul et al
(2002) and references therein, are provided in Table 6-17 as a guide. The HELP modelestimated
default subgrade infiltration rates using the zone-specific maximum and minimum precipitations
(presented in Table 6-16) and the highest and lowest values of material properties (presented in
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IWEM Technical Background Document IWEM Inputs
Table 6-17) are listed in Tables 6-18, 6-19, and 6-20 for the following pavement types and
roadway components :
Table 6-18: Asphaltic concrete pavement, portland cement concrete pavement, and
Embankment
Table 6-19: Unpaved shoulder, shoulder paved with asphaltic concrete, and shoulder
paved with portland cement concrete
Table 6-20: Unpaved median, median paved with asphaltic concrete, and median paved
with portland cement concrete.
The selection of extreme values for these inputs and environmental variables in each climatic
region is consistent with the objective of determining a bounding range of infiltration rates
through various material configurations to support a screening level analysis.
For given precipitations in some zones, the differences between the respective high and low
infiltration values are relatively small. This is due to the fact that, especially in zones with low
precipitation, the amount of water available after runoff and evaporation can easily percolate into
the pavement because the low values of hydraulic properties are comparable to or greater than the
rate of infiltration. An inspection of Table 6-17 reveals that the infiltration values are dependent
on precipitation, although the relationship may not be linear. For a given zone, if the precipitation
at the user's site is different from the zone-specific values used in Table 6-17, the user may
obtain preliminary estimates of the location-specific minimum and maximum subgrade
infiltration rates by linearly interpolating between respective values associated with minimum
and maximum precipitations in the table. For the user who wishes to further refine the infiltration
values for specific sites and/or states, it is recommended that location-specific information be
utilized to run the HELP model. Parameters suggested in Table 6-17 may be used in the case that
pavement-specific data are not available. For the estimation of infiltration rates in embankments
and ditches, it may be necessary to account for runoff that emanates from the pavements to the
ditches and evaporation over the ditch surfaces, as discussed in Section C.2.2.4 of Appendix C.
Runoff rates (meters/year) estimated by the HELP model are given in Tables 6-21, 6-22, and 6-
23 for the same pavement types and roadway components listed above for infiltration rates. Ditch
evaporation data are not available. However, a range of evaporation rates based on evaporation
from a non-vegetated soil surface (estimated using the HELP with an embankment as a surrogate
soil area) and pan evaporation data from NOAA (1982) is given in Tables 6-24 and 6-25,
respectively, and can be used for estimating evaporation rates for ditches.
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Table 6-17. Material Properties Used in the HELP Model to Estimate Default Infiltration Rates for Roadway Module
Layer
Description
Hydraulic
Conductivity
(cm/sec)
Air void
(%)
Thickness
(cm)a
Textureb
Total
Porosity0
(vol/vol)
Field
Capacity0
(vol/vol)
Wilting
Point0
(vol/vol)
Curve
Numberd
Flexible Pavement (asphaltic concrete pavement)
Low-end8
High-end'
L-1
L-2
L-3
H-1
H-2
H-3
Top course
Base course
Subbase course
Top course
Base course
Subbase course
1.00E-05
4.30E-05
4.30E-05
48'
35
35
29
50
50
24
39
39
22
37
46
7.6
5.1
31
_h
ML
ML
j
GP
GP
0.02
0.50
0.50
0.24
0.39
0.39
0.011
0.280
0.280
0.020
0.032
0.032
0.005
0.130
0.130
0.007
0.013
0.013
99
N/A
N/A
97
N/A
N/A
Rigid Pavement (portland cement concrete pavement)
Low-end
High-end
L-1
L-2
H-1
H-2
Top course
Subbase course
Top course
Subbase course
2.00E-10
4.30E-05
48
35
1
50
35
39
33
41
20
10
_h
ML
j
GP
0.01
0.50
0.35
0.39
0.006
0.280
0.028
0.032
0.003
0.130
0.011
0.013
99
N/A
97
N/A
Shoulder (unpaved)
Low-end
High-end
L-1
L-2
H-1
H-2
Base course
Subbase course
Base course
Subbase course
4.30E-05
4.30E-05
35
35
50
50
39
39
28
46
5.1
25
ML
ML
GP
GP
0.50
0.50
0.39
0.39
0.280
0.280
0.032
0.032
0.130
0.130
0.013
0.013
72
N/A
72
N/A
Shoulder (paved with asphaltic concrete)
Low-end
High-end
L-1
L-2
L-3
H-1
H-2
H-3
Top course
Base course
Subbase course
Top course
Base course
Subbase course
1.00E-05
4.30E-05
4.30E-05
48
35
35
2
50
50
24
39
39
15
28
46
7.6
5.1
25
_h
ML
ML
j
GP
GP
0.02
0.50
0.50
0.24
0.39
0.39
0.011
0.280
0.280
0.020
0.032
0.032
0.005
0.130
0.130
0.007
0.013
0.013
99
N/A
N/A
97
N/A
N/A
Shoulder (paved with portland cement concrete)
Low-end
High-end
L-1
L-2
H-1
H-2
Top course
Subbase course
Top course
Subbase course
2.00E-10
4.30E-05
48
35
1
50
1
39
29
47
15
11
_h
ML
j
GP
0.01
0.50
0.35
0.39
0.006
0.280
0.028
0.032
0.003
0.130
0.011
0.013
99
N/A
97
N/A
Median (unpaved)
Low-end
High-end
L-1
L-2
H-1
H-2
Base course
Subbase course
Base course
Subbase course
4.30E-05
4.30E-05
35
35
50
50
39
39
28
46
5.1
25
ML
ML
GP
GP
0.50
0.50
0.39
0.39
0.280
0.280
0.032
0.032
0.130
0.130
0.013
0.013
72
N/A
72
N/A
(continued)
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Table 6-17. Material Properties Used in the HELP Model to Estimate Default Infiltration Rates for Roadway Module
Median (paved
Low-end
High-end
Layer
Description
Hydraulic
Conductivity
(cm/sec)
Air void
(%)
Thickness
(cm)a
Textureb
Total
Porosity0
(vol/vol)
Field
Capacity0
(vol/vol)
Wilting
Point0
(vol/vol)
Curve
Numberd
with asphaltic concrete)
L-1
L-2
L-3
H-1
H-2
H-3
Top course
Base course
Subbase course
Top course
Base course
Subbase course
1.00E-05
4.30E-05
4.30E-05
489
35
35
2e
50
50
24
39
39
22
28
46
7.6
5.1
25
j
ML
ML
_h
GP
GP
0.02
0.50
0.50
0.24
0.39
0.39
0.011
0.280
0.280
0.020
0.032
0.032
0.005
0.130
0.130
0.007
0.013
0.013
99
N/A
N/A
97
N/A
N/A
Median (paved with port/and cement concrete)
Low-end
High-end
L-1
L-2
L-3
H-1
H-2
H-3
Top course
Base course
Subbase course
Top course
Base course
Subbase course
2.00E-10
4.30E-05
4.30E-05
48
35
35
1
50
50
35
39
39
33
28
46
20
5.1
25
j
ML
ML
_h
GP
GP
0.01
0.50
0.50
0.35
0.39
0.39
0.006
0.280
0.280
0.028
0.032
0.032
0.003
0.130
0.130
0.011
0.013
0.013
99
N/A
N/A
97
N/A
N/A
Embankment
Low-end
High-end
L-1
H-1
Base course
Base course
4.30E-05
35
50
39
56
56
ML
GP
0.50
0.39
0.280
0.032
0.130
0.013
72
72
Layer thicknesses were obtained from Jackson and Puccinelli (2006).
Texture: the soil texture types are classified according to two standard systems-U.S. Department of Agriculture and Unified Soil Classification System. According to the latter, ML
denotes silt; and GP denotes gravel.
Source: Schroeder et al. (1996).
Source: USDA(1986).
Low-end: parameter set expected to yield minimum infiltration.
High-end: parameter set expected to yield maximum infiltration.
Air voids for top courses were based on Tables 6.1 and 7.1 of Apul et al. (2002) for asphaltic concrete and portland cement concrete, respectively. Air voids for base/subbase
courses were based on HELP parameters for ML and GP (Table 1, HELP User's Guide).
Soil texture type unavailable. Field capacity and permanent wilting point were determined from the total porosity using the ratios of field capacity/total porosity, and permanent
wilting point/total porosity from ML.
High hydraulic conductivity value for the top course is based on an assumption that each square meter of the pavement is traversed by three 5-cm wide fractures. A similar value
may be obtained using Ridgway (1976) infiltration data and Equation 9.1 in Apul et al. (2002). High hydraulic conductivity for base and subbase courses were obtained from pg. 90
of Apul et al. (2002). Low hydraulic conductivity values were obtained from Apul et al. (2002).
Soil texture type unavailable. Field capacity and permanent wilting point were determined from the total porosity using the ratios of field capacity/total porosity, and permanent
wilting point/total porosity from GP.
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IWEMInputs
Table 6-18. Infiltration Rates (m/yr) for Common Pavement Types: Pavements and Embankments
Region
Deep-freeze
Moderate -
freeze
No-freeze
Zone
A1
A2
A3
A4
B1
B2
B3
B4
C1
C2
C3
C4
Selected Station3
Montpelier, VT
New Haven, CT
Rapid City, SD
Syracuse, NY
Cheyenne, WY
Great Falls, MT
Fairbanks, AK
Annette, AK
Philadelphia, PA
Edison, NJ
Dodge City, KS
Nashville, TN
Grand Junction, CO
Flagstaff, AZ
Las Vegas, NV
Salt Lake City, UT
Tampa, FL
Tallahassee, FL
Midland, TX
New Orleans, LA
Phoenix, AZ
Tucson, AZ
Fresno, CA
Astoria, OR
Within-Zone
Precipitation
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
5-Year Average
Precipitation
(m/yr)a
0.88
1.3
0.39
1.2
0.28
0.46
0.24
2.6
1.1
1.3
0.5
1.4
0.18
0.54
0.13
0.41
0.97
1.7
0.41
1.8
0.2
0.26
0.26
1.7
Infiltration Rate for
Asphaltic Concrete
HIGHb
0.25
0.39
0.11
0.34
0.062
0.13
0.090
1.0
0.35
0.39
0.15
0.45
0.041
0.15
0.047
0.14
0.27
0.47
0.13
0.47
0.060
0.068
0.10
0.79
LOWb
0.12
0.17
0.010
0.14
0.011
0.0058
0.052
0.33
0.16
0.17
0.056
0.12
4.3E-05
0.0015
0.0015
0.0010
0.046
0.067
0.012
0.060
0.0015
0.0015
0.00025
0.29
Infiltration Rate for
Portland Cement
Concrete
HIGHb
0.27
0.42
0.11
0.35
0.068
0.13
0.092
1.1
0.37
0.41
0.16
0.47
0.017
0.092
0.025
0.10
0.30
0.50
0.12
0.50
0.030
0.028
0.056
0.81
LOWb
3.0E-05
4.1E-05
1.9E-05
3.6E-05
2.0E-05
2.3E-05
2.3E-05
6.1E-05
3.8E-05
3.8E-05
2.1E-05
4.6E-05
2.3E-05
2.5E-05
1.2E-05
2.3E-05
3.3E-05
3.6E-05
2.1E-05
3.8E-05
1.2E-05
1.9E-05
1.5E-05
5.1E-05
Infiltration Rate for
Embankment
HIGHb
0.41
0.86
0.19
0.67
0.11
0.21
0.11
2.1
0.72
0.84
0.30
1.0
0.055
0.25
0.069
0.20
0.61
1.3
0.25
1.3
0.096
0.11
0.15
1.4
LOWb
0.16
0.41
0.018
0.31
0.0043
0.031
0.02
1.6
0.27
0.37
0.064
0.49
7.4E-05
0.069
0.00076
0.053
0.16
0.65
0.050
0.66
0.00025
0.00025
0.038
1.1
a Two climate stations in each subzone are selected: one with the lowest precipitation and one with the highest precipitation. Precipitation data are from the HELP model database.
b Material properties are those of the high- and low-end values of the respective ranges given in Table 6-16.
6-40
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IWEM Technical Background Document
IWEMInputs
Table 6-19. Infiltration Rates (m/yr) for Common Pavement Types: Shoulders
Region
Deep-freeze
Moderate -
freeze
No-freeze
Zone
A1
A2
A3
A4
B1
B2
B3
B4
C1
C2
C3
C4
Selected Station3
Montpelier, VT
New Haven, CT
Rapid City, SD
Syracuse, NY
Cheyenne, WY
Great Falls, MT
Fairbanks, AK
Annette, AK
Philadelphia, PA
Edison, NJ
Dodge City, KS
Nashville, TN
Grand Junction, CO
Flagstaff, AZ
Las Vegas, NV
Salt Lake City, UT
Tampa, FL
Tallahassee, FL
Midland, TX
New Orleans, LA
Phoenix, AZ
Tucson, AZ
Fresno, CA
Astoria, OR
Within-Zone
Precipitation
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
5-Year Average
Precipitation
(m/yr)a
0.88
1.3
0.39
1.2
0.28
0.46
0.24
2.6
1.1
1.3
0.5
1.4
0.18
0.54
0.13
0.41
0.97
1.7
0.41
1.8
0.2
0.26
0.26
1.7
Infiltration Rate for
Shoulder
(un paved)
HIGHb
0.41
0.86
0.19
0.67
0.11
0.21
0.11
2.1
0.72
0.84
0.30
1.01
0.024
0.18
0.041
0.16
0.61
1.3
0.25
1.3
0.056
0.052
0.10
1.4
LOWb
0.16
0.41
0.017
0.31
0.004
0.030
0.024
1.6
0.27
0.37
0.065
0.49
6.1E-05
0.068
0.00076
0.053
0.16
0.65
0.050
0.66
5.6E-05
0.00025
0.037
1.1
Infiltration Rate for
Shoulder
(paved with asphaltic
concrete)
HIGHb
0.25
0.39
0.11
0.34
0.062
0.13
0.090
1.0
0.35
0.39
0.15
0.45
0.026
0.11
0.035
0.14
0.27
0.47
0.13
0.47
0.041
0.042
0.077
0.79
LOWb
0.03
0.056
0.0010
0.047
0.0036
0.00076
0.053
0.23
0.044
0.053
0.020
0.034
7.6E-06
0.00127
0.00025
0.00051
0.014
0.017
0.007
0.015
8.9E-05
0.00025
0.00025
0.22
Infiltration Rate for
Shoulder
(paved with portland
cement concrete)
HIGHb
0.26
0.41
0.10
0.35
0.065
0.12
0.092
1.1
0.37
0.41
0.15
0.47
0.016
0.090
0.024
0.082
0.29
0.49
0.12
0.50
0.026
0.025
0.058
0.81
LOWb
6.6E-05
7.6E-05
5.3E-05
7.1E-05
4.8E-05
5.8E-05
4.8E-05
1.2E-04
9.1E-05
1.0E-04
6.4E-05
9.9E-05
6.6E-05
5.8E-05
3.0E-05
5.6E-05
7.4E-05
9.1E-05
4.8E-05
9.7E-05
3.3E-05
5.1E-05
3.8E-05
0.00011
a Two climate stations in each subzone are selected: one with the lowest precipitation and one with the highest precipitation. Precipitation data are from the HELP model database.
b Material properties are those of the high- and low-end values of the respective ranges given in Table 6-16.
6-41
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IWEM Technical Background Document
IWEMInputs
Table 6-20. Infiltration Rates (m/yr) for Common Pavement Types: Medians
Region
Deep-freeze
Moderate -
freeze
No-freeze
Zone
A1
A2
A3
A4
B1
B2
B3
B4
C1
C2
C3
C4
Selected Station3
Montpelier, VT
New Haven, CT
Rapid City, SD
Syracuse, NY
Cheyenne, WY
Great Falls, MT
Fairbanks, AK
Annette, AK
Philadelphia, PA
Edison, NJ
Dodge City, KS
Nashville, TN
Grand Junction, CO
Flagstaff, AZ
Las Vegas, NV
Salt Lake City, UT
Tampa, FL
Tallahassee, FL
Midland, TX
New Orleans, LA
Phoenix, AZ
Tucson, AZ
Fresno, CA
Astoria, OR
Within-Zone
Precipitation
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
5-Year Average
Precipitation
(m/yr)
0.88
1.3
0.39
1.2
0.28
0.46
0.24
2.6
1.1
1.3
0.5
1.4
0.18
0.54
0.13
0.41
0.97
1.7
0.41
1.8
0.2
0.26
0.26
1.7
Infiltration Rate for
Median
(un paved)
HIGHb
0.41
0.86
0.19
0.67
0.11
0.21
0.11
2.1
0.72
0.84
0.30
1.0
0.024
0.18
0.041
0.16
0.61
1.3
0.25
1.3
0.056
0.052
0.10
1.4
LOWb
0.16
0.41
0.017
0.31
0.0041
0.030
0.02
1.6
0.27
0.37
0.065
0.49
6.1E-05
0.068
0.00076
0.053
0.16
0.65
0.050
0.66
5.6E-05
0.00025
0.037
1.1
Infiltration Rate for
Median
(paved with asphaltic
concrete)
HIGHb
0.25
0.39
0.11
0.34
0.062
0.13
0.090
1.0
0.35
0.39
0.15
0.45
0.026
0.11
0.035
0.14
0.27
0.47
0.13
0.47
0.041
0.042
0.077
0.79
LOWb
0.12
0.17
0.0097
0.14
0.011
0.0056
0.052
0.33
0.16
0.17
0.056
0.12
4.3E-05
0.0018
0.0015
0.0013
0.047
0.069
0.012
0.060
0.0015
0.00152
0.00025
0.29
Infiltration Rate for
Median
(paved with
Portland cement
concrete)
HIGHb
0.27
0.42
0.11
0.35
0.068
0.13
0.092
1.1
0.37
0.41
0.16
0.47
0.037
0.15
0.043
0.14
0.30
0.50
0.12
0.50
0.057
0.067
0.096
0.82
LOWb
2.5E-05
3.3E-05
1.9E-05
2.8E-05
1.7E-05
2.1E-05
1.7E-05
4.6E-05
3.6E-05
3.6E-05
2.2E-05
3.6E-05
2.0E-05
2.2E-05
1.1E-05
2.1E-05
2.5E-05
3.3E-05
1.6E-05
2.8E-05
1.0E-05
1.7E-05
1.3E-05
3.8E-05
a Two climate stations in each subzone are selected: one with the lowest precipitation and one with the highest precipitation. Precipitation data are from the HELP model database.
b Material properties are those of the high- and low-end values of the respective ranges given in Table 6-16.
6-42
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IWEM Technical Background Document
IWEMInputs
Table 6-21. Runoff Rates (m/yr) for Common Pavement Types: Pavements and Embankments
Region
Deep-freeze
Moderate -
freeze
No-freeze
Zone
A1
A2
A3
A4
B1
B2
B3
B4
C1
C2
C3
C4
Selected Station3
Montpelier, VT
New Haven, CT
Rapid City, SD
Syracuse, NY
Cheyenne, WY
Great Falls, MT
Fairbanks, AK
Annette, AK
Philadelphia, PA
Edison, NJ
Dodge City, KS
Nashville, TN
Grand Junction, CO
Flagstaff, AZ
Las Vegas, NV
Salt Lake City, UT
Tampa, FL
Tallahassee, FL
Midland, TX
New Orleans, LA
Phoenix, AZ
Tucson, AZ
Fresno, CA
Astoria, OR
Within-Zone
Precipitation
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
5-Year Average
Precipitation
(m/yr)a
0.88
1.3
0.39
1.2
0.28
0.46
0.24
2.6
1.1
1.3
0.5
1.4
0.18
0.54
0.13
0.41
0.97
1.7
0.41
1.8
0.2
0.26
0.26
1.7
Runoff Rate for
Asphaltic Concrete
HIGHb
0.39
0.58
0.10
0.58
0.055
0.11
0.039
1.2
0.46
0.61
0.17
0.61
0.019
0.17
0.028
0.082
0.37
0.87
0.15
0.92
0.042
0.055
0.059
0.62
LOWb
0.60
0.94
0.21
0.89
0.13
0.25
0.097
2.0
0.78
0.97
0.32
1.0
0.062
0.31
0.073
0.21
0.68
1.4
0.28
1.4
0.11
0.14
0.15
1.2
Runoff Rate for
Portland Cement
Concrete
HIGHb
0.38
0.57
0.10
0.57
0.055
0.11
0.036
1.2
0.45
0.60
0.17
0.60
0.019
0.17
0.029
0.081
0.37
0.86
0.14
0.91
0.042
0.055
0.060
0.60
LOWb
0.79
1.2
0.33
1.1
0.23
0.39
0.18
2.5
1.1
1.3
0.47
1.3
0.15
0.46
0.13
0.36
0.95
1.7
0.41
1.7
0.19
0.25
0.25
1.6
Runoff Rate for
Embankment
HIGHb
0.21
0.10
0.0086
0.23
0.00076
0.022
0.010
0.19
0.073
0.14
0.0033
0.026
0
0.053
0
0.007
0.010
0.039
0.00025
0.056
0
0
0
0.0066
LOWb
0.25
0.17
0.013
0.31
0.0013
0.026
0.020
0.46
0.12
0.23
0.015
0.11
0.00025
0.083
0
0.014
0.039
0.16
0.0023
0.19
0
0
0.00076
0.12
a Two climate stations in each subzone are selected: one with the lowest precipitation and one with the highest precipitation. Precipitation data are from the HELP model database.
b Material properties are those of the high- and low-end values of the respective ranges given in Table 6-16.
6-43
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IWEM Technical Background Document
IWEMInputs
Table 6-22. Runoff Rates (m/yr) for Common Pavement Types: Shoulders
Region
Deep-freeze
Moderate -
freeze
No-freeze
Zone
A1
A2
A3
A4
B1
B2
B3
B4
C1
C2
C3
C4
Selected Station3
Montpelier, VT
New Haven, CT
Rapid City, SD
Syracuse, NY
Cheyenne, WY
Great Falls, MT
Fairbanks, AK
Annette, AK
Philadelphia, PA
Edison, NJ
Dodge City, KS
Nashville, TN
Grand Junction, CO
Flagstaff, AZ
Las Vegas, NV
Salt Lake City, UT
Tampa, FL
Tallahassee, FL
Midland, TX
New Orleans, LA
Phoenix, AZ
Tucson, AZ
Fresno, CA
Astoria, OR
Within-Zone
Precipitation
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
5-Year Average
Precipitation
(m/yr)a
0.88
1.3
0.39
1.2
0.28
0.46
0.24
2.6
1.1
1.3
0.5
1.4
0.18
0.54
0.13
0.41
0.97
1.7
0.41
1.8
0.2
0.26
0.26
1.7
Runoff Rate for
Shoulder
(un paved)
HIGHb
0.21
0.10
0.0086
0.23
0.00076
0.022
0.010
0.19
0.073
0.14
0.0033
0.026
0
0.054
0
0.007
0.010
0.039
0.00025
0.056
0
0
0
0.0066
LOWb
0.25
0.17
0.013
0.31
0.0013
0.026
0.020
0.46
0.12
0.23
0.015
0.11
0.00025
0.083
0
0.014
0.039
0.16
0.0023
0.18
0
0
0.00076
0.12
Runoff Rate for
Shoulder
(paved with asphaltic
concrete)
HIGHb
0.39
0.58
0.10
0.58
0.055
0.11
0.039
1.2
0.46
0.61
0.17
0.61
0.019
0.17
0.029
0.082
0.37
0.87
0.15
0.92
0.042
0.055
0.060
0.62
LOWb
0.60
0.93
0.20
0.88
0.13
0.24
0.097
2.0
0.78
0.97
0.32
1.0
0.061
0.30
0.072
0.20
0.68
1.4
0.28
1.4
0.11
0.14
0.15
1.2
Runoff Rate for
Shoulder
(paved with portland
cement concrete)
HIGHb
0.38
0.57
0.10
0.57
0.055
0.11
0.036
1.2
0.45
0.60
0.17
0.60
0.019
0.17
0.029
0.082
0.37
0.86
0.14
0.91
0.042
0.055
0.060
0.60
LOWb
0.79
1.2
0.33
1.1
0.23
0.39
0.18
2.5
1.1
1.3
0.47
1.3
0.15
0.46
0.13
0.36
0.95
1.7
0.41
1.7
0.19
0.25
0.25
1.6
a Two climate stations in each subzone are selected: one with the lowest precipitation and one with the highest precipitation. Precipitation data are from the HELP model database.
b Material properties are those of the high- and low-end values of the respective ranges given in Table 6-16.
6-44
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IWEM Technical Background Document
IWEMInputs
Table 6-23. Runoff Rates (m/yr) for Common Pavement Types: Medians
Region
Deep-freeze
Moderate -
freeze
No-freeze
Zone
A1
A2
A3
A4
B1
B2
B3
B4
C1
C2
C3
C4
Selected Station3
Montpelier, VT
New Haven, CT
Rapid City, SD
Syracuse, NY
Cheyenne, WY
Great Falls, MT
Fairbanks, AK
Annette, AK
Philadelphia, PA
Edison, NJ
Dodge City, KS
Nashville, TN
Grand Junction, CO
Flagstaff, AZ
Las Vegas, NV
Salt Lake City, UT
Tampa, FL
Tallahassee, FL
Midland, TX
New Orleans, LA
Phoenix, AZ
Tucson, AZ
Fresno, CA
Astoria, OR
Within-Zone
Precipitation
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
Lowest
Highest
5-Year Average
Precipitation
(m/yr)a
0.88
1.3
0.39
1.2
0.28
0.46
0.24
2.6
1.1
1.3
0.5
1.4
0.18
0.54
0.13
0.41
0.97
1.7
0.41
1.8
0.2
0.26
0.26
1.7
Runoff Rate for
Median
(un paved)
HIGHb
0.21
0.10
0.0086
0.23
0.00076
0.022
0.010
0.19
0.073
0.14
0.0033
0.026
0
0.054
0
0.007
0.010
0.039
0.00025
0.056
0
0
0
0.0066
LOWb
0.25
0.17
0.013
0.31
0.0013
0.026
0.020
0.46
0.12
0.23
0.015
0.11
0.00025
0.083
0
0.014
0.039
0.16
0.0023
0.18
0
0
0.00076
0.12
Runoff Rate for
Median
(paved with asphaltic
concrete)
HIGHb
0.39
0.58
0.10
0.58
0.055
0.11
0.039
1.2
0.46
0.61
0.17
0.61
0.019
0.17
0.029
0.082
0.37
0.87
0.15
0.92
0.042
0.055
0.060
0.62
LOWb
0.60
0.94
0.21
0.89
0.13
0.25
0.097
2.0
0.78
0.97
0.32
1.0
0.062
0.31
0.073
0.21
0.68
1.4
0.28
1.4
0.11
0.14
0.15
1.2
Runoff Rate for
Median
(paved with
Portland cement
concrete)
HIGHb
0.38
0.57
0.10
0.57
0.055
0.11
0.036
1.2
0.45
0.60
0.17
0.60
0.019
0.17
0.028
0.081
0.37
0.86
0.14
0.91
0.042
0.055
0.059
0.60
LOWb
0.79
1.2
0.33
1.1
0.23
0.39
0.18
2.5
1.1
1.3
0.47
1.3
0.15
0.46
0.13
0.36
0.95
1.7
0.41
1.7
0.19
0.25
0.25
1.6
a Two climate stations in each subzone are selected: one with the lowest precipitation and one with the highest precipitation. Precipitation data are from the HELP model database.
b Material properties are those of the high- and low-end values of the respective ranges given in Table 6-16.
6-45
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IWEM Technical Background Document
IWEMInputs
Table 6-24. Climatic Zones and Corresponding Embankment Evaporation Rates from HELP (m/yr)
Freezing Index
Deep-freeze
Moderate-freeze
No-freeze
Zone3
A1
A2
A3
A4d
B1
B2
B3
B4
C1
C2
C3
C4d
Station13
Montpelier, VT
New Haven, CT
Rapid City, SD
Syracuse, NY
Cheyenne, WY
Great Falls, MT
Fairbanks, AK
Annette, AK
Philadelphia, PA
Edison, NJ
Dodge City, KS
Nashville, TN
Grand Junction, CO
Flagstaff, AZ
Las Vegas, NV
Salt Lake City, UT
Tampa, FL
Tallahassee, FL
Midland, TX
New Orleans, LA
Phoenix, AZ
Tucson, AZ
Fresno, CA
Astoria, OR
5-Year Ave Evaporation0
Low-end Inputs
0.26
0.30
0.20
0.31
0.17
0.22
0.11
0.36
0.31
0.35
0.20
0.36
0.13
0.22
0.066
0.21
0.35
0.42
0.16
0.41
0.10
0.14
0.11
0.27
High-end Inputs
0.48
0.68
0.35
0.60
0.27
0.40
0.18
0.57
0.71
0.74
0.42
0.79
0.18
0.37
0.13
0.35
0.76
0.92
0.36
0.94
0.19
0.25
0.22
0.47
Notes:
a Zone geographical coverage is given in Figure 6-7.
b Two climate stations in each subzone are selected: one with the lowest precipitation, and the other with the highest precipitation.
c Evaporation data are obtained from HELP model with embankment.
d Zone A4 comprises Alaska only. Hawaii is incorporated into Zone C4 (not shown in Figure 6-7).
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Table 6-25. Climatic Zones and Corresponding Pan Evaporation Rate Ranges from NOAA (m/yr)
Freezing Index
Deep-freeze
Moderate-freeze
No-freeze
Zone3
A1
A2
A3
A4d
B1
B2
B3
B4
C1
C2
C3
C4d
Station13
Montpelier, VT
New Haven, CT
Rapid City, SD
Syracuse, NY
Cheyenne, WY
Great Falls, MT
Fairbanks, AK
Annette, AK
Philadelphia, PA
Edison, NJ
Dodge City, KS
Nashville, TN
Grand Junction, CO
Flagstaff, AZ
Las Vegas, NV
Salt Lake City, UT
Tampa, FL
Tallahassee, FL
Midland, TX
New Orleans, LA
Phoenix, AZ
Tucson, AZ
Fresno, CA
Astoria, OR
Pan Evaporation0
0.57
1.0
1.2
0.57
1.0
1.1
1.8
1.3
1.1
1.8
1.3
1.3
1.2
1.3
1.1
1.4
1.2
2.8
1.0
1.2
1.3
1.1
1.4
1.2
2.8
2.4
3.1
2.8
1.6
2.2
3.0
3.1
2.4
3.1
2.8
1.6
2.2
3.0
Notes:
a Zone geographical coverage is given in Figure 6-7.
b Two climate stations in each zone are selected: one with the lowest precipitation, and the other with the highest precipitation.
c Source of Pan Evaporation data: NOAA (1982).
d Zone A4 comprises Alaska only. Hawaii is incorporated into Zone C4 (not shown in Figure 6-7).
6.4.3.3 Procedure for Estimating Subgrade Infiltration in Climatic Zones
Using the information presented in the above sections, the IWEM user can estimate generic
subgrade infiltration rates by following the steps below.
Step 1 Determine the appropriate climatic zone using Figure 6-7.
Step 2 Consists of two sub-steps:
- Step 2a Determine the range (highlow) of subgrade infiltration from Tables 6-18 to
6-20 for a given pavement type (i.e., asphaltic concrete pavement, portland cement
concrete pavement, unpaved shoulder, asphaltic concrete shoulder, portland cement
concrete shoulder, unpaved median, asphaltic concrete median, portland cement
concrete median, and embankment). For a given zone, if the precipitation at the user's
site is between the zone-specific maximum and minimum values used in Tables 6-18
to 6-20, the user may obtain preliminary estimates of the location-specific minimum
and maximum subgrade infiltration rates by linearly interpolating between respective
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subgrade infiltration rates associated with minimum and maximum precipitations in
the table. In the event that the high and low subgrade infiltration rates are not
significantly different, the user may use the mean value instead of both the high and
low values. Both high and low infiltration rates are recommended to bracket the range
of uncertainty.
- Step 2b In the event that the climatic conditions, pavement configurations, and
drainage systems at the user's site are different from those given in Tables 6-17 to 6-
23 and that the user wishes refine the range of subgrade infiltration rate obtained from
Step 2a to closely reflect the climatic conditions at the user's site, it is recommended
that the user run the HELP model with site-specific climatic conditions and drainage
configurations.
It should be noted that the pavement infiltration rate should not exceed lowest value of the
saturated hydraulic conductivity of the underlying pavement layers. This principle is applicable
to both the with-drainage and without-drainage pavement systems. The IWEM module performs
this check.
6.4.3.4 Procedure for Estimating Runoff and Evaporation in Climatic Zones
If a ditch is present, it will be necessary to estimate runoff from the nearby pavement and
evaporation from the ditch in order to estimate the exfiltration rate5 from the ditch. Similar to the
procedure outlined above in Section 6.4.3.3, based on a known climatic zone, the corresponding
rates of runoff, soil evaporation, and pan evaporation can be obtained from Tables 6-21 to 6-23
(runoff), Table 6-24 (evaporation), and Table 6-25 (pan evaporation). However, as always, site-
specific values for these parameters are preferred. If the ditch evaporation rate is not known, it is
recommended that the user perform screening analyses with the soil evaporation rate
(Table 6-24), pan evaporation rate (Table 6-25), and a mean value of the two rates.
6.4.4 Recharge Rates
The HELP model (Schroeder et al, 1994) was used to compute recharge rates for all sources. The
factors related to soil type that affect the HELP-generated recharge rates are the permeability of
the soil used in the landfill cover, and - in the case of recharge or for land application units - the
permeability of the soil type in the vicinity of the source. HELP was used to calculate recharge
for the three primary soil types across the United States (sandy loam, silty loam, and silty clay
loam soils) and ambient climate conditions at 102 climate stations through the use of the HELP
water-balance model as summarized in Section 6.4.1. We assumed the ambient regional recharge
rate for a given climate center and soil type (for all source types) is the same as the corresponding
unlined landfill infiltration rate.
6.5 Parameters Used to Describe the Unsaturated and Saturated Zones
Parameter values for the unsaturated and saturated zone modeling in IWEM were obtained from
a number of data sources. A primary data source was the Hydrogeologic Database for Ground-
water Modeling (HGDB), assembled by Rice University on behalf of the American Petroleum
Institute (Newell, 1989). This database provides probability distributions of a number of key
ground water modeling parameters for various types of subsurface environments.
' Exfiltration is defined as the process of water percolating down to the unsaturated zone from the bottom of a ditch.
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For unsaturated zone modeling, a database of soil hydraulic properties for various soil types,
assembled by Carsel and Parrish (1988), was used in combination with information from the Soil
Conservation Service on the nationwide prevalence of different soil types across the United
States.
Table 6-26 summarizes the parameters used to characterize subsurface parameters. The last
column indicates where the user can find a detailed discussion of each parameter in this
document. The IWEMv3.1 User's Guide (U.S. EPA, 2015a) provides additional guidance in
selecting values for these parameters.
Table 6-26. Summary of IWEM Subsurface Parameters
Modeling Element
Description or Value
Section
Reference
General Subsurface Parameters
Subsurface environment
Depth to ground water (m)
Saturated Zone Hydraulic
Conductivity (m/yr)
Saturated Zone Hydraulic Gradient
Saturated Zone Thickness (m)
Optional user input; default is unknown subsurface
environment
Optional user input; default derived from subsurface
environment if known, otherwise national average value
(5.18m)
Optional user input; default derived from subsurface
environment if known, otherwise national average (1890
m/y)
Optional user input; default derived from subsurface
environment if known, otherwise national average (0.0057
m/m)
Optional user input; default derived from subsurface
environment if known, otherwise national average (10.1 m)
6.5.1
6.5.1
6.5.1
6.5.1
6.5.1
Unsaturated Zone Parameters
Soil Hydraulic Parameters:
(Hydraulic conductivity; saturated
water content; residual water
content; moisture retention curve
parameters)
Soil Bulk density (kg/L)
Soil Percent Organic Matter (%)
Soil Temperature (°C)
Unsaturated Zone pH
Distribution of values corresponding to three major soil types
(sandy loam, silt loam, and silty clay loam). Probability of
occurrence of each soil type based on nationwide
distribution
Assigned based on selected soil type (sandy loam, silt loam,
or silty clay loam)
Distribution of values corresponding to three major soil types
(sandy loam, silt loam, and silty clay loam). Probability of
occurrence of each soil type based on nationwide
distribution
Assigned based on Source location
Assumed to be same as saturated zone pH; nationwide
distribution derived from STORET ground water quality
database
6.5.2
6.5.2
6.5.2
6.5.2
6.5.2
Saturated Zone Parameters
Saturated Zone Porosity
Saturated Zone Bulk Density (kg/L)
Saturated Zone Fraction Organic
Carbon
Saturated Zone Temperature (°C)
Saturated Zone pH
Derived from nationwide distribution of mean aquifer particle
diameter
Derived from saturated zone porosity
Nationwide distribution derived from STORET water quality
database
Assigned based on Source location
Nationwide distribution derived from STORET water quality
database
6.5.3
6.5.3
6.5.3
6.5.3
6.5.3
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6.5.1 Subsurface Parameters
The database, HGDB, provides site-specific data on four key subsurface parameters:6
Depth to ground water;
Saturated zone thickness;
Saturated zone hydraulic conductivity; and
Saturated zone hydraulic gradient.
The data in this hydrogeological database were collected by independent investigators for
approximately 400 hazardous waste sites throughout the United States.
In HGDB, the data are grouped into 12 subsurface environments, which are based on EPA's
DRASTIC classification of hydrogeologic settings (U.S. EPA, 1987). Table 6-27 lists the
subsurface environments. The table includes a total of 13 categories; 12 are distinct subsurface
environments, while the 13th category, which is labeled "other" or "unknown", was used for
waste sites that could not be classified into one of the first 12 environments. The subsurface
parameter values in this 13th category are simply averages of the parameter values in the 12
actual subsurface environments. Details on the individual parameter distributions for each
subsurface environment are provided in the EPACMTP Parameters/Data Background Document
(U.S. EPA, 2003b).
The key feature of this database is that it provides a set of correlated values of the four
parameters for each of the 400 sites in the database. That is, the value of each parameter is
associated with the three other subsurface parameters reported for the same site. These
correlations were preserved, because having information on some parameters allows the
development of more accurate estimates for missing parameter values.
In IWEM, the type of subsurface environment, as well as each of the four individual subsurface
parameters (depth to ground water, saturated thickness, saturated hydraulic conductivity, and
hydraulic gradient) are optional, site-specific user inputs. Depending on the extent of available
site data, IWEM will use statistical correlations developed from the HGDB to estimate missing
or unknown parameters. If site-specific values for all four parameters are known, then IWEM
will use these values and in this case, information on the type of subsurface environment is not
needed. If one or more of the four subsurface parameters are unknown, but the type of subsurface
environment at the site is known, IWEM will use the known parameters to generate a probability
distribution for the unknown parameters, using the statistical correlations that correspond to the
type of environment at the site. If no site-specific hydrogeologic information is known, IWEM
will treat the site as being in subsurface environment number 13 and assign values that are
national averages.
6 The database also provides data on ground water seepage velocity and on "vertical penetration depth" of a waste
plume below the water table. These data were not used. EPACMTP calculates the ground water velocity directly,
and the vertical penetration depth is not used in EPACMTP.
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Table 6-27. HGDB Subsurface Environments (from Newell, 1989)
Region
1
2
3
4
5
6
7
8
9
10
11
12
13
Description
Metamorphic and Igneous
Bedded Sedimentary Rock
Till Over Sedimentary Rock
Sand and Gravel
Alluvial Basins Valleys and Fans
River Valleys and Floodplains with Overbank Deposit
River Valleys and Floodplains without Overbank Deposits
Outwash
Till and Till Over Outwash
Unconsolidated and Semi-consolidated Shallow Aquifers
Coastal Beaches
Solution Limestone
Other (Not classifiable)
6.5.2
Unsaturated Zone Parameters
Soil Hydraulic Parameters. Data on unsaturated hydraulic properties assembled by Carsel and
Parrish (1988) were used in conjunction with information from the Soil Conservation Service on
the nationwide prevalence of different soil types across the United States to model flow of
infiltration water through the unsaturated zone. First, Soil Conservation Service soil mapping
data were used to estimate the relative prevalence of light- (sandy loam), medium- (silt loam),
and heavy-textured (silty clay loam) soils across the United States. The estimated percentages are
shown in Table 6-28. The soil types used in the unsaturated zone modeling were also used in the
HELP model to derive infiltration and recharge rates (see Section 6.4) in order to have a
consistent set of soil modeling parameters. The Carsel and Parrish (1988) soil property data were
used to determine the probability distributions of individual soil parameters for each soil type,
and these distributions were used in the Monte Carlo modeling for IWEM. Table 6-29 presents
the unsaturated zone parameter values used in IWEM. The development of the distributions and
use of the parameters presented in Table 6-29 is described in detail in Section 5.2.3 of the
EPACMTP Parameters/Data Background Document (U.S. EPA, 2003b).
Table 6-28. Nationwide Distribution of Soil Types Represented in IWEM
Texture Category
Light textured
Medium textured
Heavy textured
Soil Conservation Service Soil
Type
Sandy Loam
Silt Loam
Silty Clay Loam
Relative Frequency
(%)
15.4
56.6
28.0
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Table 6-29. Statistical Parameters for Soil Properties for Three Soil Types Used in IWEM
Development (Carsel and Parrish, 1988)
Parameter1
Distribution Type2
Limits of Variation
Minimum
Maximum
Mean
Standard
Deviation
So;7 Type-Silty Clay Loam
Ksat (cm/hr)
0r
a (cm-1 )
P
%OM
pb
0s
SB
NO
SB
NO
SB
Constant
Constant
0
0
0
1.0
0
-
-
3.5
0.115
0.15
1.5
8.35
-
-
0.017
0.089
.009
1.236
0.11
1.67
0.43
2.921
0.0094
.097
0.061
5.91
-
-
So;7 Type-Silt Loam
Ksat (cm/hr)
0r
a (cm'1 )
P
%OM
pb
0s
LN
SB
LN
SB
SB
Constant
Constant
0
0
0
1.0
0
-
-
15.0
0.11
0.15
2.0
8.51
-
-
0.343
0.068
0.019
1.409
0.105
1.65
0.45
0.989
0.071
0.012
1.629
5.88
-
-
So;7 Type-Sandy Loam
Ksat (cm/hr)
0r
a (cm-1 )
P
%OM
pb
0S
SB
SB
SB
LN
SB
Constant
Constant
0
0
0
1.35
0
-
-
30.0
0.11
0.25
3.00
11.0
-
-
2.296
0.065
0.070
1.891
0.074
1.60
0.41
24.65
0.074
0.171
0.155
7.86
-
-
1 Ksat is saturated hydraulic conductivity; 0r is residual water content; a, P are retention curve parameters;
% OM is percent Organic Matter, pb is bulk density; 0S is saturated water content.
2 NO is Normal (Gaussian) distribution; SB is Log ratio distribution where Y = In [(x-A)/(B-x)], A < x < B;
LN is Log normal distribution, Y = In [x], where Y = normal distributed parameter
The parameters a, p, and 9r in Table 6-29 are specific to the Mualem-Van Genuchten model that
is employed in the EPACMTP unsaturated zone flow module described in Section 4.1 (see the
EPACMTP Technical Background Document, U.S. EPA, 2003a, for details).
Soil Bulk Density and Percent Organic Matter. These soil transport parameters are used to
calculate the constituent-specific retardation coefficients, the unsaturated zone dispersivity, and
the soil pH and temperature. The latter two parameters are used to calculate hydrolysis
transformation rates; pH is also a key parameter for modeling transport of metals. Soil bulk
density and percent organic matter were obtained from the Carsel and Parrish (1988) database
and are presented in Table 6-29. These parameters are used to calculate the retardation factor in
the constituent transport equation (see Equation 4-1 in Section 4.1). We used the data on the
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percent organic matter to calculate the fraction organic carbon, assuming that 58% of soil organic
matter is organic carbon:7
%OM
foc =
1.74x100
(6-2)
where:
foe = Mass fraction organic carbon in the soil (kg/kg)
% OM = Percent organic matter (%)
1.74 = Conversion factor (=1/0.58) (dimensionless)
100 = Conversion factor (% to mass fraction).
Dispersivity in the unsaturated zone, auz, is calculated as a function of the travel distance (Du,)
between the base of the source and the water table, according to the following relationship:
auz =0.02 + (0.022 xDu)
(6-3)
where:
auz = longitudinal dispersivity in the unsaturated zone (m)
Du = Depth of the unsaturated zone, from the base of the source to the water table (m).
This relationship is based on a regression analysis of field scale transport data presented by
Gelhar et al. (1985). The maximum allowed value of dispersivity is capped at 1 m in IWEM. The
development of Equation 6-3 is described in detail in Section 3.9 of the EPACMTP Technical
Background Document (U.S. EPA, 2003 a).
Soil temperature and pH were obtained from nationwide distributions. The same distributions
were used for the entire subsurface, that is, both for the unsaturated zone and for the saturated
zone. In IWEM, a nationwide aquifer pH distribution, derived from EPA's STORET database,
was used. The pH distribution is an empirical distribution with a median value of 6.8 and lower
and upper bounds of 3.2 and 9.7, respectively, as shown in Table 6-30. The development of the
pH distribution in Table 6-30 is described in detail in Section 5.3.10 of the EPACMTP
Parameters/Data Background Document (U.S. EPA, 2003b).
Table 6-30. Probability Distribution of Soil and Aquifer pH
Percentile
pH Value
0
3.20
1
3.60
5
4.50
10
5.20
25
6.07
50
6.80
75
7.40
90
7.90
95
8.2
99
8.95
100
9.7
As modeled in IWEM, soil and aquifer temperature affects the transformation rate of constituents
that are subject to hydrolysis, through the effect of temperature on reaction rates (see Section
6.6.1). In the IWEM development, average annual temperatures in shallow ground water systems
(Todd, 1980) were used to assign a temperature value to each climate center (Figure 6-6) in the
modeling database, based on the climate center's geographical location. For each climate center,
the assigned temperature was an average of the upper and lower values for that temperature
region, as shown in Figure 6-8. In other words, all climate centers located in the band between
10°C and 15°C were assigned a temperature value of 12.5 °C.
This is a typical value; see, for example, http://soilquality.org/indicators/total_organic_carbon.html.
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Figure 6-8. Ground water temperature distribution for shallow aquifers in the United States
(from Todd, 1980).
IWEM Monte Carlo Methodology for Soil Parameters. In IWEM, soil properties are assumed
to be uniform at each site. A new set of soil parameters is selected for each iteration in the Monte
Carlo modeling process, but the soil properties were assumed uniform for a given simulation.
However, the methodology for assigning soil types differed. In IWEM, the soil type is an
optional site-specific user input parameter. Because the site location must always be entered by
the user, the selection of the soil type determines the recharge rate, as well as the HELP-derived
infiltration rates which the IWEM tool will use in the evaluation. Based on the selected soil type,
the IWEM tool will randomly select values for the parameters in Table 6-29 from the probability
distributions corresponding to the soil type. If the soil type is entered as "unknown," the Monte
Carlo process for the unsaturated zone parameters will randomly select one of the three possible
soil types in accordance with their nationwide frequency of occurrence.
6.5.3
Saturated Zone Parameters
In addition to the four site-related subsurface parameters discussed in Section 6.5.1, IWEM
requires a number of additional saturated zone transport parameters. They are: saturated zone
porosity; saturated zone bulk density; longitudinal, transverse and vertical dispersivities; fraction
organic carbon; aquifer temperature; and aquifer pH.
Saturated zone porosity is used in the calculation of the ground water seepage velocity;
saturated zone porosity and bulk density are used in the calculation of constituent-specific
retardation coefficients. IWEM uses default, nationwide distributions for aquifer porosity and
bulk density, that is, they are not user inputs. Both were derived from a distribution of aquifer
particle diameter presented by Shea (1974). This distribution is presented in Table 6-31. Using
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the data in Table 6-30 as an input distribution, IWEM calculates porosity, N, from particle
diameter using an empirical relationship based on data reported by Davis (1969) as:
N= 0.261 - 0.0385 x \n(d) (6-4)
where
N = Porosity (dimensionless)
d = Mean particle diameter (cm)
In = Natural logarithm.
Additionally, relationships presented in McWorther and Sunada (1977) were used to establish
relationships between total (N) and effective porosity (Ne) as a function of mean particle
diameter, see Table 6-32. The development of Equation 6-4 and the distributions in Table 6-31
and Table 6-32 are described in detail in Section 5.3.2 of the EPACMTP Parameters/Data
Background Document (U.S. EPA, 2003b).
Table 6-31. Empirical Distribution of Mean Aquifer Particle Diameter (from Shea, 1974)
Percentile
Particle
Diameter
(cm)
0.0
3.9x1 0-4
3.8
7.8x10-4
10.4
0.0016
17.1
0.0031
26.2
0.0063
37.1
0.0125
56.0
0.025
79.2
0.05
90.4
0.1
94.4
0.2
97.6
0.4
100
0.8
Table 6-32. Ratio Between Effective and Total Porosity as a Function of Particle Diameter
(after McWorther and Sunada, 1977)
Mean Particle Diameter (cm)
<6.25 xlQ-3
6.25x10-3-2.5x10-2
2.5x10-2- 5.0x10-2
5.0x1 0-2 -10-1
>io-1
Ne/N Range
0.03-0.77
0.04-0.87
0.31-0.91
0.58-0.94
0.52-0.95
Dispersivity. IWEM calculates apparent saturated zone dispersivities as a function of the
distance between the waste unit and the modeled ground water well, using regression
relationships based on a compilation of field-scale dispersivity data in Gelhar et al. (1985). These
relationships are:
where
a
OLL =
x =
REF _
L
av =
av =
longitudinal dispersivity (m)
downgradient ground water travel distance (m)
reference dispersivity value (m)
horizontal transverse dispersivity (m)
vertical transverse dispersivity (m).
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A longitudinal dispersivity corresponding to a distance of 152.4 m was used as a reference to
calculate dispersivity at different well distances, according to the probability distribution
presented in Table 6-33. The development of Equation 6-5 and the distribution in Table 6-33 are
described in detail in Section 5.3.8 of ihzEPACMTP Parameters/Data Background Document
(U.S. EPA, 2003b).
Table 6-33. Cumulative Probability Distribution of Longitudinal Dispersivity
at Reference Distance of 152.4 m
Percentile
Dispersivity, af£F (m)
0.0
0.1
1.00
1.0
70.0
10.0
100.0
100.0
Bulk Density and Fraction Organic Carbon. Sorption of organic constituents was modeled
using data such as the fraction organic carbon (foc), as discussed in Section 4.1 (Equation 4-5). In
the development of IWEM, a nationwide distribution obtained from values of dissolved organic
carbon in EPA's STORET water quality database was used. The distribution was modeled as a
Johnson SB frequency distribution (see EPACMTP Parameters/Data Background Document,
U.S. EPA, 2003b) with a mean of 4.32xlO'4, a standard deviation of 0.0456, and lower and upper
limits of 0.0 and 0.064, respectively.
Temperature and pH. Values of the ground water temperature and pH were determined in the
same way as soil pH and temperature (see Section 6.5.2).
6.6 Parameters Used to Characterize the Chemical Fate of Constituents
For IWEM evaluations, the chemical fate of constituents as they are transported through the
subsurface is presented in terms of an overall first-order decay coefficient, a retardation
coefficient which reflects equilibrium sorption reactions, and for transformation daughter-
products, a production term that represents the formation of daughter compounds due to the
transformation of parent constituents. This section describes how constituent-specific parameter
values were developed for these chemical fate processes. Section 6.6.1 describes constituent
transformation processes, while Section 6.6.2 discusses all constituent degradation processes.
Section 6.6.3 describes how we modeled sorption processes.
Table 6-34 summarizes the parameters used to characterize the chemical fate of constituents.
The last column indicates where the user can find a detailed discussion of each parameter in this
document. The IWEMv3.1 User's Guide (U.S. EPA, 2015a) provides additional guidance in
selecting values for these parameters.
Table 6-34. Summary of IWEM Chemical Fate Parameters
Modeling Element
Description or Value
Section
Reference
Constituent Transformation Parameters
Hydrolysis Rate (yr1)
Overall (Bio-)
degradation (yr1)
IWEM accounts for hydrolysis transformation reactions using constituent-specific
hydrolysis rate constants.
Other types of (bio-) degradation processes can be entered as optional constituent-
specific parameters.
6.6.1
6.6.1
Constituent Sorption Parameters
Soil-Water Partition
Coefficient (Kd) (kg/L)
For organic constituents, equilibrium sorption is taken into account via constituent-
specific organic carbon partition coefficients; for metals, effective equilibrium partition
coefficients are generated using the MINTEQA2 geochemical speciation model.
6.6.2.1,
6.6.2.2
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6.6.1 Constituent Transformation
For organic constituents, IWEM accounts for chemical and biological transformations by
considering a first-order overall degradation coefficient in the transport analysis (see
Section 4.1). The default hydrolysis rate coefficients in the IWEM constituent database can be
replaced with a user-specified overall degradation rate that can account for any type of
transformation process, including biodegradation.
Hydrolysis Rate. Hydrolysis refers to the transformation of chemical constituents through
reactions with water. For organic constituents, hydrolysis can be one of the main degradation
processes that occur in soil and ground water and is represented in the EPACMTP model by
means of an overall first-order chemical decay coefficient. For modeling hydrolysis in IWEM, we
used constituent-specific hydrolysis rate constants compiled at the EPA's Environmental
Research Laboratory in Athens, GA (Kollig, 1993). These are listed in Appendix B.
The hydrolysis process as modeled in IWEM is affected by aquifer pH, aquifer temperature, and
constituent sorption through the following equations. The tendency of each constituent to
hydrolyze is expressed through constituent-specific acid-catalyzed, neutral, and base-catalyzed
rate constants. The values of the rate constants are modified to account for the effect of aquifer
temperature through the Arrhenius equation:
(6-6)
K \^lr +2/3 1 +2/3J
where:
R(T+273 T + 273
Kj = Hydrolysis rate constant for reaction process J at temperature T (1/mole/yr for
acid or base catalyzed, and 1/yr for neutral)
K]' = Hydrolysis rate constant for reaction process J at reference temperature, Tr
(1/mole/yr for acid or base catalyzed, and 1/yr for neutral)
J = a for acid-catalyzed, b for base-catalyzed, and n for neutral
T = Temperature of the subsurface (°C)
Tr = Reference temperature (°C)
273 = Conversion factor from °C to K
R = Universal gas constant (1.98?x 10'3 Kcal/K-mole)
Ea = Arrhenius activation energy (Kcal/mole).
Next, the effect of pH on hydrolysis rates is incorporated via:
^=KTa[H+] + KTn+KTb[OH-] (6-7)
where
Ai = First-order decay rate for dissolved phase (1/yr)
KTa = Acid-catalyzed hydrolysis rate constant (1/mole/yr)
[H+] = Hydrogen ion concentration (mole/L)
KTn = Neutral hydrolysis rate constant (1/yr)
KTb = Based-catalyzed hydrolysis rate constant (1/mole/yr)
[OH ] = Hydroxyl ion concentration (mole/L).
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[H+] and [OH ] are computed from the pH of the soil or aquifer using:
[H+] = 10-pH (6-8)
[OH-] = 10-(14-pH) (6-9)
The sorbed phase hydrolysis rate is calculated as:
A2=WKTa[H+] + KTn (6-10)
where:
A,2 = First-order hydrolysis rate for sorbed phase (1/yr)
KTa = Acid-catalyzed hydrolysis rate constant (1/mole/yr)
KTn = Neutral hydrolysis rate constant (1/yr)
10 = Acid-catalyzed hydrolysis enhancement factor.
Finally, the overall first-order transformation rate for hydrolysis is calculated as:
+ Pbd
where:
A = Overall first-order hydrolysis transformation rate (1/yr)
hi = Dissolved phase hydrolysis transformation rate (1/yr)
fa = Sorbed phase hydrolysis transformation rate (1/yr)
N = Porosity (water content in the unsaturated zone) (dimensionless)
pb = Bulk density (L/kg)
Kd = Partition coefficient (kg/L).
Toxic hydrolysis daughter products were identified using the information on hydrolysis
transformation pathways presented in Kollig (1993).
Biodegradation and Overall Degradation Rate. Many organic constituents may be subject to
biodegradation in the subsurface, and the IWEM tool allows the user to provide a constituent-
specific overall degradation coefficient, which can include either aerobic or anaerobic
biodegradation. However, IWEM does not specifically simulate biodegradation reactions. The
IWEM user must, therefore, ensure that the value entered is representative of actual site
conditions, and that the transformation reactions can be adequately characterized as a first-order
rate process (i.e., process that can be represented in terms of a characteristic half-life). The
overall degradation rate parameter that is used as an IWEM input is related to the constituent's
subsurface half-life and is expressed as:
(6-12)
^1/2
where
2 = IWEM degradation rate input value (1/yr)
0.693 = Natural log of 2 (dimensionless)
ti/2 = Constituent half-life (yr).
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6.6.2 Constituent Sorption
In addition to physical and biological transformation processes, the transport of constituents can
be affected by a wide range of complex geochemical reactions. From a practical view, the
important aspect of these reactions is the removal of solute from solution, irrespective of the
process. For this reason, IWEM lumps the cumulative effects of the geochemical processes into a
single term (i.e., solid-water partition coefficient), which is one of several parameters needed to
describe the degree a constituent's mobility is retarded relative to ground water. In the
EPACMTP fate and transport model upon which IWEM is based, this process is defined by the
retardation factor defined in Section 4.1 (Equation 4-5). The remainder of this section describes
the procedures that IWEM uses to model sorption for organic constituents and inorganic
constituents, specifically, metals.
6.6.2.1 Sorption Modeling for Organic Constituents
For organic constituents, Kd values are calculated as the product of the constituent-specific Koc
and the fraction organic carbon in the soil or ground water:
Kd = Koc*foc (6-13)
where
Kd = partition coefficient (L/kg)
Koc = normalized organic carbon distribution coefficient (kg/L)
foe = fractional organic carbon content (dimensionless).
Koc values for IWEM constituents are listed in Appendix B. For IWEM, the fraction organic
carbon in the unsaturated zone was calculated from the percent organic matter in the soil as
shown in Equation 6-2 (see Section 6.5.2).
In the saturated zone modeling, direct values for foc were based on the nationwide data on the
fraction organic carbon in ground water (see Section 6.5.3).
6.6.2.2 Sorption Modeling for Inorganic Constituents
Partition coefficients (Kd) for inorganics in IWEM are selected from non-linear sorption
isotherms estimated using the geochemical speciation model, MINTEQA2. For a particular
inorganic species, Kd values in a soil or aquifer are dependent upon the species concentration and
various geochemical characteristics of the soil or aquifer and the associated porewater. The
approach and development of non-linear sorption isotherms and their use in EPACMTP are
described in detail in Appendix G of the EPACMTP Technical Background Document (U.S.
EPA, 2003a) and in Appendix B of the EPACMTP Parameters/Data Background Document
(U.S. EPA, 2003b).
Geochemical parameters that have the greatest influence on the magnitude of Kd include the pH
of the system and the nature and concentration of sorbents associated with the soil or aquifer
matrix. In the subsurface beneath a disposal facility, the concentration of leachate constituents
may also influence Kd. Although the dependence of metal partitioning on the total metal
concentration and on pH and other geochemical characteristics is apparent from partitioning
studies reported in the scientific literature, the reported Kd values for individual metals do not
cover the range of metal concentrations or geochemical conditions relevant in the IWEM
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Aluminum (AI3+)
Antimony (Sb5+)
Arsenic (As3+, As5+)
Barium (Ba)
Beryllium (Be)
Boron (B)
Cadmium (Cd)
Chromium (Cr3+, Cr6+
Cobalt (Co)
Copper(Cu)
Fluoride (F)
Iron (Fe2+)
Manganese (Mn2+)
Mercury (Hg)
Lead (Pb)
Molybdenum (Mo5+)
Nickel (Ni)
Selenium (Se4+> Se6+
Silver (Ag)
Thallium (TI1+)
Vanadium (V5+)
Zinc (Zn)
scenarios. For this reason, the Agency chose to use an equilibrium speciation model,
MINTEQA2, to estimate inorganic metal partition coefficients for the IWEM development.
From input data consisting of total concentrations of the inorganic
chemicals, the model calculates the fraction of a constituent that is
dissolved, adsorbed, and precipitated at equilibrium. The ratio of the
adsorbed fraction to the dissolved fraction is the dimensionless partition
coefficient. The dimensionless partition coefficient for each inorganic
species was converted to Kd with units of L/kg by normalizing the mass of
soil (in kg) with one liter of porewater in which it is equilibrated (U.S.
EPA, 2003a,b). Isotherms are generated when the equilibrium metal
distribution between sorbed and dissolved fraction are estimated for a
series of input total concentrations.
Using MINTEQA2, the list of inorganic species for which adsorption
isotherms were developed are listed on the right side. For these inorganic
species, two sets of isotherms are provided in IWEM based on the
characterization of leachate data used for MINTEQ2 modeling (discussed
below).
MINTEQA2 Input Parameters. The expected natural variability in Kd
for a particular metal was accounted for in the MINTEQA2 modeling by including variability in
important input parameters upon which Kd depends. The input parameters for which variability
was incorporated include ground water compositional type, pH, concentration of sorbents, and
concentration of metal (U.S. EPA, 2003 a,b). In addition, the concentration of representative
anthropogenic organic acids that may be present in leachate from a waste site were varied.
Two ground water compositional types were modeled- one with composition representative of a
carbonate-terrain system and one representative of a non-carbonate system. The two ground
water compositional types are correlated with the subsurface environment (see Section 6.5.1,
Table 6-27). The carbonate type corresponds to the "solution limestone" subsurface environment
setting. The other 11 subsurface environments in IWEM are represented by the non-carbonate
ground water type. If the subsurface environment is "unknown," then IWEM will also assume it
is a non-carbonate type. For both ground water types, a representative, charge-balanced ground
water chemistry specified in terms of major ion concentrations and natural pH was selected from
the literature. The carbonate system was represented by a sample reported in a limestone aquifer.
This ground water had a natural pH of 7.5 and was saturated with respect to calcite. The non-
carbonate system was represented by a sample reported from an unconsolidated sand and gravel
aquifer with a natural pH of 7.4, selected because it is the most frequently occurring of the 12
subsurface environments in HGDB.
Two types of adsorbents were used in modeling the Kd values: ferric oxide and paniculate
organic matter (U.S. EPA, 2003a, b). Mineralogically, the ferric oxide was assumed to be
goethite (FeOOH). To represent the interactions of protons and metals with the goethite surface,
a database of sorption reactions for goethite reported by Mathur (1995) was used with the
diffuse-layer sorption model in MINTEQA2. The concentration of sorption sites used in the
model runs was based on a measurement of ferric iron extractable from soil samples using
hydroxylamine hydrochloride as reported in EPRI (1986). This method of Fe extraction is
intended to provide a measure of the exposed amorphous hydrous oxide of Fe present as mineral
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coatings and discrete particles and available for surface reaction with pore water. The variability
in ferric oxide content represented by the variability in extractable Fe from these samples was
included in the modeling by selecting low, medium and high ferric oxide concentrations
corresponding to the 17th, 50th, and 83rd percentiles of the sample measurements. The specific
surface area and site density used in the diffuse-layer model were as prescribed by Mathur.
Although the same distribution of extractable ferric oxide sorbent was used in the saturated and
unsaturated zones, the actual concentration of sorbing sites corresponding to the low, medium,
and high ferric oxide settings in MTNTEQA2 was different in the two zones because the phase
ratio was different (4.57 kg/L in the unsaturated zone; 3.56 kg/L in the saturated zone).
The concentration of the second adsorbent, percent organic matter was obtained from organic
matter distributions already present in the IWEM modeling database. In the unsaturated zone,
low, medium, and high concentrations for components representing percent organic matter in the
MTNTEQA2 model runs were based on the distribution of solid organic matter for the silt loam
soil type. (The silt loam soil type is intermediate in weight percent organic matter in comparison
with the sandy loam and silty clay loam soil types and is also the most frequently occurring soil
type among the three.) The low, medium, and high percent organic matter concentrations used in
the saturated zone MTNTEQA2 model runs were obtained from the organic matter distribution
for the saturated zone. For both the ferric oxide and percent organic matter adsorbents, the
amount of sorbent included in the MINTEQA2 modeling was scaled to correspond with the
phase ratio in the unsaturated and saturated zones.
A dissolved organic matter distribution for the saturated zone was obtained from the EPA's
STORET database. This distribution was used to provide low, medium, and high dissolved
organic matter concentrations for the MTNTEQA2 model runs. The low, medium, and high
dissolved organic matter values were used exclusively with the low, medium, and high values,
respectively, of percent organic matter. In the unsaturated zone, there was no direct measurement
of dissolved organic matter available. The ratio of percent organic matter to dissolved organic
matter for the three concentration levels (low, medium, and high) in the unsaturated zone was
assumed to be the same as for the saturated zone. In MINTEQA2, the percent organic matter and
dissolved organic matter components were modeled using the Gaussian distribution model. This
model includes a database of metal- dissolved organic matter reactions (Susetyo et al., 1991).
Metal reactions with percent organic matter were assumed to be identical in their mean binding
constants with the dissolved organic matter reactions.
As mentioned above, two sets of isotherms are provided in IWEM based on the characteristic of
the leachate data used for MINTEQ2 modeling. For the first case, sorption isotherms were
developed using leachate data that represent acid conditions at the base of a landfill resulting
from decaying organic matter (U.S EPA, 2003a, b). Many organic acids found in landfill leachate
have significant metal-complexing capacity that may influence metal mobility. Representative
carboxylic acids for leachate from industrial WMUs were included in the MINTEQA2 modeling.
An analysis of total organic carbon in landfill leachate by Gintautas et al. (1993) was used to
select and quantify the organic acids. The low, medium, and high values for the representative
acids in the modeling were assigned based on the lowest, the average, and the highest measured
total organic carbon among the six landfill leachates analyzed. Because leachate from industrial
WMUs is expected to be lower in organic matter than in municipal landfills, only the low and
medium leachate organic acids values were included in IWEM. The isotherms developed using
this leachate are available for all WMUs, structural fill, and roadway modules.
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For the second set, the agency developed nonlinear sorption isotherms based on leachate data
specific to coal combustion residuals (CCRs) disposal sites, which is known to span a broad pH
range - from acidic pH =2 to highly alkaline pH =13 (U.S.EPA, 2014b). This dataset contains
Kds for CCR waste types that include ash, ash and coal, flue gas desulfurization (FGD), and
fluidized bed combustion (FBC). The user is referred to Appendix H of the (U.S. EPA, 2014b)
for detailed discussion of the development of CCR-specific isotherms using MINTEQA2. These
isotherms8 are available for the roadway module only in this version of IWEM.
MINTEQA2 Modeling and Results. The MINTEQA2 modeling was conducted separately for
each metal in three steps for the unsaturated zone, and these were repeated for the saturated zone:
Sorbents were pre-equilibrated with ground waters: Each of nine possible combinations
of the two ferric oxide and percent organic matter sorbent concentrations (low ferric
oxide, low percent organic matter; low ferric oxide, medium percent organic matter; etc.)
were equilibrated with each of the two ground water types (carbonate and non-carbonate).
Because the sorbents adsorb some ground water constituents (calcium, magnesium,
sulfate, fluoride), the input total concentrations of these constituents were adjusted so that
their equilibrium dissolved concentrations in the model were equal to their original
(reported) ground- water dissolved concentrations. This step was conducted at the natural
pH of each ground water, and calcite was imposed as an equilibrium mineral for the
carbonate ground water type. Small additions of inert ions were added to maintain charge
balance.
The pre-equilibrated systems were titrated to new target pHs. Each of the nine pre-
equilibrated systems for each ground water type were titrated with sodium hydroxide to
raise the pH or with nitric acid to lower the pH. Nine target pHs spanning the range 4.5 to
8.2 were used for the non-carbonate ground water. Three target pHs spanning the range
7.0 to 8.0 were used for the carbonate ground water. Titration with acid or base to adjust
the pH allowed charge balance to be maintained.
Leachable organic acids and the constituent metal were added. Each of the 81 pre-
equilibrated, pH-adjusted systems of the non-carbonate ground water and the 27 pre-
equilibrated, pH-adjusted systems of the carbonate ground water were equilibrated with
two concentrations (low and medium) of teachable organic acids. The equilibrium pH
was not imposed in MTNTEQA2; pH was calculated and reflected the acid and metal
additions. The constituent metal was added as a metal salt (e.g., PbNOs) at a series of 44
total concentrations spanning the range 0.001 mg/L to 10,000 mg/L of metal. Equilibrium
composition and Kd were calculated at each of the forty-four total metal concentrations to
produce an isotherm of sorbed metal versus metal concentration. The isotherm can also
be expressed as Kd versus metal concentration.
This modeling resulted in 81 isotherms for the non-carbonate environment and 27 isotherms for
the carbonate environment for the unsaturated zone. A like number of isotherms for each
environment was produced for the saturated zone. Each isotherm corresponds to a particular
setting of ferric oxide sorbent concentration, percent organic matter sorbent (and associated
dissolved organic matter) concentration, leachate acid concentration, and pH. An example
isotherm for chromium (VI) is shown in Figure 6-9.
! Only CCR-specific sorption isotherms are provide for aluminum, boron, and iron ions, thus these constituents are
not available for WMUs and structural fill modules.
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IWEMInputs
For chromium, arsenic, and selenium, isotherms were calculated for two environmentally
relevant oxidation states. The different oxidation states of these metals have different
geochemical behavior, and in the case of chromium also distinctly different toxicological
behavior. Chromium (HI) exhibits behavior typical of a cation, but chromium (VI) behaves as an
anion (chromate). Chromium (ID) and chromate are most strongly sorbed at opposite ends of the
pH spectrum: sorption of Chromium (HI) tends to increase with pH over the pH range 4 to 8,
whereas sorption of chromate tends to decrease with pH over this range.
The two oxidation states of arsenic and selenium also exhibit differences in sorption behavior.
Therefore, both forms of arsenic and selenium are incorporated in IWEM. The user may select
the more mobile species for a more conservative evaluation. The more mobile oxidation state
was determined by running EPACMTP with both sets of isotherms for these metals. The results
indicate that arsenic (in) and selenium (VI) are the more mobile forms.
120
100 -
80 -
60 -
40 -
20 -
0
6.7
-3-2-101234
log Total Cr(VI) (mg/L)
Figure 6-9. Unsaturated zone isotherm for chromium (VI) (non-carbonate environment, low
leachate organic acids, medium ferric oxide, high percent organic matter, pH 6.3).
Screening Procedures EPA Used to Eliminate Unrealistic Parameter
Combinations in the Monte Carlo Process
Inherent to the Monte Carlo process is that parameter values are drawn from multiple data
sources and then combined in each iteration of the modeling process. Because the parameter
values are drawn randomly from their individual probability distributions, it is possible that
parameters are combined in ways that are physically infeasible and that violate the validity of the
EPACMTP flow and transport model. Therefore, a number of checks were implemented to
eliminate or reduce these occurrences as much as possible.
As a relatively simple measure, upper and lower limits are specified on individual parameter
values to ensure that their randomly generated values are within physically realistic limits. Where
possible, data sources were used that contained multiple parameters, and the Monte Carlo
process was implemented in a way that preserved the existing correlations among the parameters.
Upper and lower limits were implemented on secondary parameters whose values are calculated
(derived) internally in the Monte Carlo module as functions of the primary EPACMTP input
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parameters (see the EPACMTP Parameters/Data Background Document, U.S. EPA, 2003b). A
set of screening procedures was also implemented to ensure that infiltration rates and the
resulting predicted ground water mounding would remain physically plausible. Specifically,
parameter values generated in each Monte Carlo iteration were screened for the following
conditions:
Infiltration and recharge so high that they cause the water table to come into contact with
the bottom of landfills, waste piles, structural fills, roadways or rise above the ground
surface;
Water level in a surface impoundment unit below the water table, causing flow into the
surface impoundment;
Infiltration rate from a surface impoundment exceeds the saturated hydraulic conductivity
of the soil underneath.
These screening procedures are discussed in more detail below. Mathematical details of the
screening algorithms are presented in the EPACMTP Technical Background Document (U.S.
EPA, 2003a).
The logic diagram for the infiltration screening procedure is presented in Figure 6-10, and
Figure 6-11 provides a graphical illustration of the screening criteria. The numbered criteria
checks in Figure 6-10 correspond to the numbered diagrams in Figure 6-11. Note that high
infiltration rates are most likely with (unlined) surface impoundments. Therefore, the screening
procedure is the most involved for surface impoundment WMUs.
Figure 6-10(a) depicts the screening procedures for landfills, waste piles, land application units,
structural fills and average, effective infiltration rate through roadways. For these source types,
after user-supplied default and randomly generated parameters are selected for each Monte Carlo
simulation, IWEM calculates the estimated water table mounding that would result from the
selected combination of parameter values. The combination of parameters is accepted if the
calculated maximum water table elevation (the ground water "mound") remains below the
bottom of these source types or the ground surface elevation at the site, whichever is lower. If the
criterion is not satisfied, the randomly selected parameters for the simulation are rejected and a
new data set is selected.
For surface impoundments, there are two additional screening steps, as depicted in Figure 6-
10(b). At each Monte Carlo iteration, the user-supplied and default parameters are used to
determine whether the surface impoundment unit is hydraulically connected to the water table. If
the base of the surface impoundment is below the water table, the surface impoundment unit is
said to be hydraulically connected to the water table (see Figure 6-11, Criterion 1). This scenario
is rejected and a new set of random parameters is generated if the hydraulically connected surface
impoundment is an inseeping type, that is, the water surface in the surface impoundment is below
the water table (see Figure 6-11, Criterion l(b)). As long as the elevation of the waste water
surface in the impoundment is above the water table, the first criterion is passed (Figure 6-11,
Criterion l(a)).
If the base of the unit is located above the ambient water table, that is, before any adjustment to
the water table elevation to account for mounding is made, the unit is said to be hydraulically
separated from the water table (see Figure 6-11, Criterion 2). However, in this case, it is
necessary to ensure that the calculated infiltration rate does not exceed the maximum feasible
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infiltration rate. The maximum feasible infiltration rate is the maximum infiltration that allows
the water table to be hydraulically separated from the surface impoundment. In other words, the
infiltration rate does not allow the crest of the local ground water mound to be higher than the
base of the surface impoundment. This limitation allows IWEM to determine a conservative
infiltration rate that is based on the free-drainage condition at the base of the surface
impoundment. The infiltration rate is no longer conservative if the water table is allowed to be in
hydraulic contact with the base of the surface impoundment. If the maximum feasible infiltration
rate (Imax) is exceeded, IWEM will set the infiltration rate to this maximum value.
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IWEMInputs
(A)
Landfill/Structural fill
Waste pile
Land application unit
Roadway
Next 'N
realization )
(B)
Surface
impoundment
Figure 6-10. Flowchart describing the infiltration screening procedure.
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1.a OLtseepmg SI Unit
Ground surface
\ si /
w . j^. \ / \ / = ? Accepted
Tab!e The unsaturated zone is bypassed.
1.b Inseeping SI Unit
Ground eurfaca |
r
Rejected
Water
Table
; \
(1) Surface impoundment initially hydraulically connected with the saturated zone
Ground surface
\"~7
\ .).. '
«" *«.
».
Groundwater mound
due to infiltration
I,,.,; = maximum feasible infiltration rate
Initial Water Table
2 Surface impoundment initially hydraulically separated from the saturated zone.
Rtcharyt
SI / ------^ NewWatorTable
Initial Water Toblc
3 Water table beiow ground surface criterion for all WMU lypes.
Figure 6-11. Infiltration screening criteria.
IWEM handles the screening in this order to accommodate the internal software logic in
EPACMTP. If the surface impoundment is a hydraulically connected type based on the user-
supplied information on the impoundment and water table positions, EPACMTP will simulate
this system by bypassing the unsaturated zone module. On the other hand, if the hydraulic
connection results from water table mounding, i.e., the original water table elevation is below the
impoundment, EPACMTP cannot easily handle this situation, and the scenario is therefore
rejected.
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Once the infiltration limit has been imposed, the third criterion is checked to ensure that any
ground water mounding does not result in a rise of the water table mound above the ground
surface, in the same manner as done for other types of sources.
In the IWEM software, the parameter constraints are checked after all inputs have been specified,
but before the actual EPACMTP Monte Carlo simulations are initiated. The first check applies
when the user provides all input parameters as site-specific values. In this case, the software
checks that the combination of input values does not violate the infiltration and water table
elevation constraints. The second check applies when some inputs are set to site-specific values,
while default probability distributions are used for other inputs. In this case, it is possible that the
combination of fixed, site-specific values with national or regional distributions, results in a high
frequency of rejections in the EPACMTP simulations. An example would be simulating an
unlined surface impoundment at a site where the depth to ground water is set to a very small
value. This combination is likely to lead to a large number of rejections in the EPACMTP Monte
Carlo simulation due to violation of the ground water mounding constraint. This, in turn, may
result in very long EPACMTP run times. It also indicates that IWEM may not be appropriate for
that site.
IWEM therefore checks the user inputs through a probabilistic screening routine that generates
random combinations of EPACMTP parameter values in accordance with the specified inputs
and measures the number of rejections. This routine will check that 20,000 acceptable parameter
combinations can be generated in 100,000 or less random iterations. If the inputs fail this test, the
software will report the most frequently violated constraint and suggest potential remedies in the
user inputs.
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Reference Ground Water Concentrations
7.0 Reference Ground Water Concentrations
This section presents the reference ground water concentrations (RGCs) that IWEM uses for the
screening evaluation. An IWEM evaluation can accommodate three types of RGCs:
Maximum Contaminant Levels (MCLs)
Health-based numbers (HBNs)
Other standards (e.g., state standards).
7.1 Maximum Contaminant Levels
MCLs are included in IWEM for 59 constituents for which values are available. MCLs are the
highest level of contaminants allowed in public drinking water and are established under the Safe
Drinking Water Act. In developing MCLs, EPA considers not only a constituent's health effects,
but also additional factors, such as the cost of treatment. The constituent-specific MCL values
included in IWEM are provided in Table 7-1, and were obtained from the Regional Screening
Level Generic Tables (U.S. EPA, 2015b). The values in Table 7-1 are current as of January 2015;
however, the IWEM user is urged to check for the latest values.
Table 7-1. MCLs Included in IWEM (Current as of January 30, 2015)2
CAS No.
7440-36-0
22569-72-8
15584-04-0
7440-39-3
71-43-2
50-32-8
7440-41-7
117-81-7
75-27-4
88-85-7
7440-43-9
56-23-5
57-74-9
108-90-7
124-48-1
67-66-3
16065-83-1
18540-29-9
7440-50-8
106-46-7
96-12-8
Chemical Name
Antimony
Arsenic (III)3
Arsenic (V)a
Barium
Benzene
Benzo{a}pyrene
Beryllium
Bis(2-ethylhexyl)phthalate
Bromodichloromethane
Butyl-4,6-dinitrophenol,2-sec-
Cadmium
Carbon tetrachloride
Chlordaneb
Chlorobenzene
Chlorodibromomethane
Chloroform
Chromium (III)
Chromium (VI)
Copper
Dichlorobenzene 1,4-
Dibromo-3-chloropropane 1,2-
MCL
(mg/L)
0.006
0.01
0.01
2
0.005
0.0002
0.004
0.006
0.08
0.007
0.005
0.005
0.002
0.1
0.08
0.08
0.1
0.1
1.3
0.075
0.0002
(continued)
2 The latest MCLs can be found at http://water.epa.gov/drink/contaminants/#List
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IWEM Technical Background Document
Reference Ground Water Concentrations
CAS No.
95-50-1
107-06-2
75-35-4
156-59-2
156-60-5
94-75-7
78-87-5
72-20-8
100-41-4
106-93-4
6984-48-8
58-89-9
76-44-8
1024-57-3
118-74-1
77-47-4
7439-92-1
7439-97-6
72-43-5
75-09-2
87-86-5
1336-36-3
10026-03-6
7782-49-2
100-42-5
1746-01-6
127-18-4
7440-28-0
108-88-3
8001-35-2
75-25-2
120-82-1
71-55-6
79-00-5
79-01-6
93-72-1
75-01-4
1330-20-7
Chemical Name
Dichlorobenzene 1,2-
Dichloroethane 1,2-
Dichloroethylene 1,1-
Dichloroethylene cis-1,2-
Dichloroethylene trans-1,2-
Dichlorophenoxyacetic acid 2,4-
Dichloropropane 1,2-
Endrin
Ethylbenzene
Ethylene dibromide
Fluoride
HCH (Lindane) gamma-
Heptachlor
Heptachlor epoxide
Hexachlorobenzene
Hexachlorocyclopentadiene
Lead
Mercury
Methoxychlor
Methylene Chloride
Pentachlorophenol
Polychlorinated biphenyls (Aroclors)
Selenium (IV)C
Selenium (VI)
Styrene
TCDD 2,3,7,8-
Tetrachloroethylene
Thallium
Toluene
Toxaphene (chlorinated camphenes)
Tribromomethane
Trichlorobenzene 1,2,4-
Trichloroethane 1,1,1-
Trichloroethane 1,1,2-
Trichloroethylene, 1,1,2-
Trichlorophenoxy)propionic acid 2-(2,4,5- (Silvex)
Vinyl chloride
Xylenes (total)
MCL
(mg/L)
0.6
0.005
0.007
0.07
0.1
0.07
0.005
0.002
0.7
5E-05
4
0.0002
0.0004
0.0002
0.001
0.05
0.015
0.002
0.04
0.005
0.001
0.0005
0.05
0.05
0.1
3E-08
0.005
0.002
1
0.003
0.08
0.07
0.2
0.005
0.005
0.05
0.002
10
a Not in Regional Screening Level tables, used value for 7440-38-2 Arsenic, Inorganic
as surrogate.
b Not in Regional Screening Level tables, used value for 12789-03-6 Chlordane as
surrogate.
c Not in Regional Screening Level tables, used value for 7782-49-2 Selenium as
surrogate.
7-2
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IWEM Technical Background Document Reference Ground Water Concentrations
7.2 Health-Based Numbers
HBNs are the constituent concentrations in ground water that would generally be expected not to
cause adverse noncancer health effects in the general population (including sensitive subgroups),
or not to result in an additional incidence of cancer in more than some specified fraction of
individuals exposed to the constituent (e.g., one in one million) via ingestion, inhalation, or
dermal exposure. Calculated HBNs are no longer included in IWEM, but the reader can obtain or
calculate their own HBNs and enter them for use in an IWEM evaluation. Not all IWEM
constituents have an MCL; thus, for those chemicals, a user-specified HBN or other standard (see
Section 7.3) is the only evaluation option.
A good online source for HBNs is the "Regional Screening Levels for Chemical Contaminants at
Superfund Sites" (http://www.epa.gov/reg3hwmd/risk/human/rb-concentration table/index.htm).
The Regional Screening Levels site is the product of an interagency effort between the U.S.
Department of Energy's Oak Ridge National Laboratory and EPA Regions 3, 6, and 9. The site
provides screening levels for more than 700 chemicals for various exposure pathways, as well as
a link to the Regional Screening Levels User's Guide, which documents all input assumptions
and equations used to develop the health-based screening levels.
In addition to health-based screening levels, the Regional Screening Levels website also provides
a link to a screening level calculator, the latest toxicity values (e.g., Reference Doses and Cancer
Slope Factors), exposure factors, and physical and chemical properties. The reader can use the
calculator to develop site-specific HBNs using different assumptions for toxicity values,
chemical properties, and exposure factors. Using the Regional Screening Levels Calculator, the
reader can also calculate screening levels for chemicals not included in the Regional Screening
Levels generic tables and database, provided that the reader can identify and justify input data for
toxicity, exposure factors, and chemical properties data.
When the IWEM user enters an HBN, the user will also be required to enter the associated
exposure duration. This enables IWEM to average results over the appropriate exposure duration
to estimate the RGC. All user-specified cancer HBNs must have the same value for exposure
duration for all pathways for a particular chemical (but it can vary from chemical to chemical).
Likewise, all user-specified non-cancer HBNs must have the same value for exposure duration
for all pathways for a particular chemical. However, the exposure durations for cancer and non-
cancer HBNs do not have to be the same.
7.3 Other Standards
The IWEM user can also enter a different standard (such as a state standard) or other user-
defined RGCs and associated exposure duration. This allows the user to enter a different standard
than the MCL (for example, a California EPA standard or other state standard) if state standards
are more stringent than the MCL. The reader can usually find state drinking water standards by
searching "[state] drinking water standards" online. For example, California standards can be
found at http://www.cdph.ca.gov/certlic/drinkingwater/Pages/MCLsandPHGs.aspx. Not all states
have their own standards, preferring to use the federal MCLs.
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IWEM Technical Background Document Reference Ground Water Concentrations
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7-4
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IWEM Technical Background Document
How Does IWEM Make Recommendations?
8.0 How Does IWEM Make Recommendations?
The objective of the ground water fate and transport model is to determine the extent of dilution
and attenuation a constituent may undergo as it migrates from a source to a ground water well
and thereby estimate the constituent concentration at the well. The level of dilution and
attenuation helps determine the magnitude of exposure concentration that can be compared to
RGCs. This section describes the methods used to develop the basis for the recommendations in
IWEM.
8.1 Making Recommendations Corresponding to a 90th Percentile Exposure
Concentration
Every single simulation of EPACMTP in the Monte Carlo process results in an estimate
concentration at the modeled ground water well. Because the estimated ground water
concentrations are compared to health-based RGCs, which reflect specific exposure duration
assumptions, the ground water concentrations calculated in IWEM represent maximum time-
averaged values, as depicted conceptually in Figure 8-1
01
E,
o
0)
o
o
o
1
Time
Exposure
Averaging Period
Figure 8-1. Determination of time-averaged ground water well concentration.
Depending on the type of RGC, the IWEM tool uses different averaging times in calculating
ground water well concentrations, as follows:
MCL: Peak ground water well concentration
User-specified HBNs: Specified exposure duration
Other standards: Specified exposure duration.
The EPACMTP simulation runs until the observed ground water concentration of a constituent at
the well peaks and falls below a model specified concentration (10~16 mg/1). The maximum time
averaged concentration is calculated around this peak based on a user-specified exposure
duration as depicted in Figure 8-1. However, in certain cases (e.g., low infiltration rate, deep
unsaturated zone, strongly sorbing constituents), the peak ground water concentration would not
occur up to a maximum of 10,000 years after the simulation started. For such cases, EPACMTP
8-1
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IWEM Technical Background Document
How Does IWEM Make Recommendations?
would stop the simulation and returns a maximum time-averaged concentration up to 10,000
years.
All user-specified cancer HBNs must have the same value for exposure duration for all pathways
for a particular chemical (but it can vary from chemical to chemical). For instance, if cancer
HBNs are provided for both inhalation and ingestion exposure pathways, the exposure duration
for both HBNs must be the same, for example 30 years. Likewise, all user-supplied non-cancer
HBNs must have the same value for exposure duration for all pathways for a particular chemical.
However, the exposure durations for cancer and non-cancer HBNs do not have to be the same.
At the conclusion of a Monte Carlo simulation consisting of 10,000 iterations, the 10,000 values
of estimated ground water concentration for each specific time-averaging period are sorted from
low to high into a cumulative distribution function (CDF), see Figure 8-2. The CDF represents
the range in the expected location-specific ground water concentration due to uncertainty and
variability in the local conditions.
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
1
Monte Carlo result
of individual
realization
E-06 l.E-05 l.E-04 l.E-03 l.E-02 l.E-01
Ground-water Well Concentration
l.E+00 l.E+01
Figure 8-2. Example cumulative distribution function of well concentrations.
For the development of the IWEM tool, EPA selected the 90th percentile of the estimated ground
water concentration cumulative distribution function as the point of comparison. This was done
to allow conservative decisions to be made quickly with large degree of confidence that the
results of the evaluation are adequately protective of human health and the environment, given
the selected RGC and the degree of uncertainty inherent in the data and the analyses. In addition,
this approach was also consistent with the recommendation of the Guidance for Risk
Characterization (U.S. EPA, 1995). Therefore, IWEM evaluations are based on a high-end
exposure assessment that is used to describe the risk or hazard for individuals in small, but
definable segments of the population.
EPA's Guidance for Risk Characterization advises that "conceptually, high-end exposure means
exposure above about the 90th percentile of the population distribution, but not higher than the
individual in the population who has the highest exposure." Use of the 90th percentile protection
level in IWEM implies that, of the modeled scenarios, 90% result in well concentrations that are
lower than the specified RGC, and thus, are considered protective for at least 90% of the cases.
8-2
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IWEM Technical Background Document How Does IWEMMake Recommendations ?
8.2 Making Liner Recommendations for WMUs
The recommended liner design is the minimum liner (i.e., unlined is being the first, followed by a
clay liner, then a composite liner) for which the estimated ground water concentrations of all
constituents is less than their specified RGC. For land application, only the "no liner" scenario is
considered (because land application units do not typically have liners). Therefore, the model
evaluates whether wastes can be protectively land applied, based on leachate constituent
concentrations.
After conducting an IWEM evaluation, the user can choose to implement the recommendation by
designing the unit based on the liner recommendations given by IWEM, or to continue to a more
detailed site-specific analysis.
When interpreting IWEM liner recommendations, the following key risk assessment issues
should be kept in mind:
All HBNs correspond to a specified target risk (for carcinogens) or a target hazard
quotient (for noncarcinogens). Thus, the recommendations will only be protective
relative to those target risks or hazard quotients (and the other assumptions underlying
the HBNs).
IWEM evaluations do not consider combined exposure from the different pathways
evaluated (ingestion of drinking water, inhalation of constituents volatilized from ground
water during household use, or dermal exposure while showering). Nor do they consider
the potential for additive exposure to multiple constituents. Therefore, use caution when
evaluating multiple constituents that have similar fate and transport characteristics (e.g.,
similar KdS and hydrolysis rates), as well as constituents with non-cancer health effects
associated with the same target organ. The additive exposures could result in risks or
hazard quotients above the targets of the selected HBNs.
Usually, exposures below a noncancer RGC (i.e., hazard quotient <1) are not likely to be
associated with adverse health effects, and are therefore less likely to be of regulatory
concern. As the frequency and/or magnitude of the exposures exceeding the noncancer
RGC (hazard quotient >1), the probability of adverse effects in a human population
increases. However, it should not be categorically concluded that all exposures below a
noncancer RGC are "acceptable" (or will be risk-free) and that all exposures in excess of
a noncancer RGC are "unacceptable" (or will result in adverse effects).
As with all modeling, the model output, interpretation of the results, and the recommendations
should be taken with the consideration of the assumptions underlying the model and the
adequacy of the input data. In addition, IWEM liner recommendations should be implemented in
consultation with state authorities to ensure compliance with state regulations, which may require
more protective measures than the IWEM results recommend. Alternatively, if the waste has only
one, or very few "problem" constituents that call for a more stringent and costly liner system (or
which make land application inappropriate), it may make sense to evaluate pollution prevention,
recycling, and treatment efforts for those specific constituents. If site-specific conditions seem
likely to support the use of a liner design different from the one recommended (or suggest a
different conclusion regarding the appropriateness of land application of a waste), a full site-
specific ground water fate and transport analysis may be needed.
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IWEM Technical Background Document How Does IWEMMake Recommendations ?
8.3 Determining the Appropriateness of Reused Industrial Materials in a
Structural Fill
For structural fills, IWEM estimates a 90th percentile well concentration from the Monte Carlo
simulation results, as described in Section 8.1. Like WMUs, IWEM compares that well
concentration to the specified RGCs. If the estimated 90th percentile well concentration is lower
than the specified RGC for all modeled constituents, IWEM considers the reuse of industrial
material in a structural fill design may be appropriate, given the RGCs. However, if the 90th
percentile well concentration of any of the modeled constituents is greater than its corresponding
RGC, then IWEM determines that the reuse of industrial materials in a structural fill may not be
appropriate. As with all modeling, the model output, interpretation of the results, and the
recommendations should be taken with the consideration of the assumptions underlying the
model and the adequacy of the input data. It is recommended that the user consult with the state
authorities on the appropriateness of the IWEM design scenario, results, and recommendation
based on state requirements.
8.4 Determining the Appropriateness of Reused Industrial Materials in a
Roadway
For roadways, IWEM calculates a 90th percentile well concentration for each strip of the
roadway (as described in Section 8.1), and then sums those concentrations across all strips to
estimate an overall 90th percentile well concentration. If the sum of 90th percentile
concentrations exceeds the maximum leachate concentration specified across all strips, the sum
is set equal to the maximum leachate concentration. IWEM then compares that overall well
concentration to the specified RGCs. If the overall estimated 90th percentile well concentration is
lower than the specified RGC for all modeled constituents, IWEM considers the reuse of
industrial materials in the modeled roadway design may be appropriate. As with all modeling, the
model output, interpretation of the results, and the recommendations should be taken with the
consideration of the assumptions underlying the model and the adequacy of the input data.
IWEM can model only one segment of roadway at a time. If multiple segments are needed to
fully evaluate a section of road, the segments must be run separately and a combined result
calculated outside of IWEM. Briefly, this involves obtaining the 90th percentile exposure level
for each constituent for each segment from the detailed results screen. Those values are then
summed across segments for each constituent, and the resulting overall exposure level can then
be compared to the RGC for each constituent. If the overall exposure concentration is less than
the RGC for all modeled constituents, the reuse of industrial materials in the roadway design is
appropriate, based on the specified RGCs. If any exposure concentrations exceed the specified
RGC, then such application of industrial materials is not appropriate. Example 4 in Appendix C
of the IWEMv3.1 User's Guide deals with a multi-segment problem and demonstrates this
summary procedure.
8-4
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IWEM Technical Documentation References
9.0 References
ABB Environmental Services. 1995. Estimation ofLeachate Rates from Industrial Waste
Management Facilities. August.
Apul, D.S., K. Gardner, T. Eighmy, J. Benoit, and L. Brannaka. 2002. A Review of Water
Movement in the Highway Environment: Implications for Recycled Materials Use.
Recycled Resource Center, University of New Hampshire, Durham, NH.
Bonaparte, R., J. P. Giroud, and B.A. Cross. 1989. Rates of leakage through landfill liners.
Geosynthetics 1989 Conference, San Diego, California.
Burnett, R.D., andE.O. Frind. 1987. Simulation of contaminant transport in three dimensions.
2. Dimensionality effects. Water Resources Research 23: 695-705.
Carsel, R.F., and R.S. Parrish. 1988. Developing joint probability distributions of soil water
retention characteristics. Water Resources Research 24:755-770.
Chow, V.T. 1959. Open-Channel Hydraulics. New York: McGraw-Hill.
Davis, S. N. 1969. Porosity and permeability of natural materials. In Flow Through Porous
Media., R. J. M. de Wiest, Editor, Academic Press, NY.
deMarsily, G. 1986. Quantitative HydrogeologyGroundwater Hydrology for Engineers.
Academic Press, 44 pp.
EPRI (Electric Power Research Institute). 1986. PhysiochemicalMeasurements of Soils at Solid
Waste Disposal Sites. EPRI EA-4417. Prepared by Battelle Pacific Northwest
Laboratories, Richland, WA, for EPRI.
Garrabrants, A.C., D.S. Kosson, R. DeLapp, and H.A. van der Sloot. 2013. Effects of coal fly ash
use in concrete on the mass transport-based leaching of potential concern. Chemosphere
703:131-139.
Gelhar, L.W., A. Mantoglou, C. Welty, and K.R. Rehfeldt. 1985. A Review of Field Scale
Physical Solute Transport Processes in Saturated and UnsaturatedPorous Media. Report
EPRI-EA-4190. Electric Power Research Institute, Palo Alto, CA.
Gintautas, P.A., K.A. Huyck, S.R. Daniel, and D.L. Macalady. 1993. Metal-organic interactions
in Subtitle D landfill leachates and associated groundwaters. In Metals in Groundwaters,
H.E. Allen, E.M. Perdue, and D.S. Brown, eds. Lewis Publishers, Ann Arbor, MI.
Jackson, N., and J. Puccinelli. 2006. Long-Term Pavement Performance (LTPP) Data Analysis
Support. National Pooled Fund Study TPF-5(013): Effects of Multiple Freeze Cycles and
Deep Frost Penetration on Pavement Performance and Cost. FHWA-HRT-06-121.
Jury, W.A., W.R. Gardner, and W.H. Gardner. 1991. Soil Physics. J. Wiley and Sons, 327 pp.
Kollig, H. 1993. Environmental Fate Constants for Organic Chemicals Under Consideration for
EPAs Hazardous Waste Identification Projects. Report No. EPA/600/R-93/132.
Environmental Research Laboratory, Athens, GA.
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IWEM Technical Documentation References
Kosson, D.S., H.A. van der Sloot, F. Sanchez, and A.C. Garrabrants. 2002. An Integrated
Framework for Evaluating Leaching in Waste Management and Utilization of Secondary
Materials. Environmental Engineering Science 79(3): 159-204. Available online at
www.niehs.nih. gov/news/assets/docs_a_e/an_integrated_framework_for_evaluating_leac
hing_in_waste_management_and_utilization_of_secondary_materials_508.pdf
Kosson, D.S., A.C. Garrabrants, R. DeLapp, and H.A. van der Sloot. 2013. pH-dependent
leaching of constituents of potential concern from concrete materials containing coal
combustion fly ash. Chemosphere 703:140-147.
Lambe, T.W., and R.V. Whitman. 1969. Soil Mechanics . John Wiley and Sons.
Mathur, S.S. 1995. Development of a Database for Ion Sorption on Goethite Using Surface
Complexation Modeling. Master's Thesis, Department of Civil and Environmental
Engineering, Carnegie Mellon University, Pittsburgh, PA.
McWorther, D. B., andD. K. Sunada. 1977. Groundwater Hydrology and Hydraulics. Water
Resources Publications, Fort Collins, CO.
Newell, C. J. 1989. Hydrogeologic Database for Ground Water Modeling. API Publication No.
4476. American Petroleum Institute, Washington, DC.
NOAA (National Oceanic and Atmospheric Administration). 1982. NOAA Technical Report 33,
Evaporation Atlas of the Contiguous 48 United States. National Weather Service,
Washington, DC.
Ridgeway, H. 1976. Infiltration of water through the pavement surface. Transportation Research
Record 6 J 6:98-
Rollin, A.L., M. Marcotte, T. Jacquelin, and L. Chaput. 1999. Leak location in exposed
geomembrane liners using an electrical leak detection technique. Geosynthetics '99:
Specifying Geosynthetics and Developing Design Details 2:61 5-626 .
Schroeder, P.R., T.S., Dozier, P.A. Zappi, B.M. McEnroe, J. W. Sjostrom, and R.L. Peton. 1994.
The Hydrologic Evaluation of Landfill Performance Model (HELP): Engineering
Documentation for Version 3. EPA/600/R-94/168b. U.S. EPA, Office of Research and
Development, Washington, DC.
Schroeder, P. R., C.M. Lloyd, P.A. Zappi, andN.M. Aziz. 1996. The Hydrologic Evaluation of
Landfill Performance (HELP) Model, User 's Guide For Version 3. Risk Reduction
Engineering Laboratory, Office of Research and Development, U.S. Environmental
Protection Agency, Cincinnati, Ohio.
Shea, J.H. 1974. Deficiencies of elastic particles of certain sizes. Journal of Sedimentary
Petrology ₯₯:985-1003.
Susetyo, W., L.A. Carreira, L.V. Azarraga, and D.M. Grimm. 1991. Fluorescence techniques for
metal-humic interactions. Fresenius J Anal Chem 339:624-635.
TetraTech, Inc., 2001 . Characterization of Infiltration Rate Data to Support Groundwater
Modeling Efforts (Draft). Prepared for the U.S. Environmental Protection Agency, Office
of Solid Waste, Contract No. 68-W6-0061, May.
9-2
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IWEM Technical Documentation References
Todd, D.K. 1980. Groundwater Hydrology (2nd edition), John Wiley & Sons, 535 pp.
Tuhkanen, S. 1980. Climatic parameters and indices in plant geography. Ada Phytogeographica
Suecica 67:1-105.
USDA (U.S. Department of Agriculture). 1986. Urban Hydrology for Small Watersheds.
Technical Release 55, Conservation Engineering Division, Natural Resources
Conservation Service, USDA, Washington, DC.
U.S. EPA (Environmental Protection Agency). 1987. DRASTIC: A Standardized System for
Evaluating Ground Water Pollution Potential Using Hydrogeologic Settings. EPA/600/2-
87/035. Office of Research and Development, Robert S. Kerr Environmental Research
Laboratory, Ada, OK. June.
U.S. EPA (Environmental Protection Agency). 1991.MINTEQA2/PRODEFA2, A Geochemical
Assessment Model for Environmental Systems: Version 3.0 User's Manual. EPA/600/3 -
91/021, Office of Research and Development, Athens, Georgia 30605.
U.S. EPA (Environmental Protection Agency). 1995. Guidance for Risk Characterization.
Science Policy Council, Washington, DC, February.
U.S. EPA (Environmental Protection Agency). 1999. A Framework for Finite-Source
Multimedia, Multipathway, andMultireceptor Risk Assessment3MRA. Office of Solid
Waste, Washington, DC.
U.S. EPA (Environmental Protection Agency). 2001. Industrial Surface Impoundments in the
United States. EPA530-R-01-005. Office of Solid Waste, Washington, DC.
U.S. EPA (Environmental Protection Agency). 2002a. Industrial Waste Management Evaluation
Model (IWEM) Technical Background Document. EPA530-R-02-012. Office of Solid
Waste and Emergency Response, Washington, DC. August. Available at
http://www.epa.gov/epawaste/nonhaz/industrial/tools/iwem/index.htm.
U.S. EPA (Environmental Protection Agency). 2002b. Industrial Waste Management Evaluation
Model (IWEM) User's Guide. EPA530-R-02-013. Office of Solid Waste and Emergency
Response, Washington, DC. August. Available at http://www.epa.gov/epawaste/
nonhaz/industrial/tools/iwem/index.htm.
U.S. EPA (Environmental Protection Agency). 2002c. Guide for Industrial Waste Management.
Office of Solid Waste, Washington, DC. Available at http://www.epa.gov/epawaste/
nonhaz/industrial/guide/index.htm.
U.S. EPA (Environmental Protection Agency). 2003 a. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Technical Background
Document. Office of Solid Waste, EPA530-R-03-002, April.
U.S. EPA (Environmental Protection Agency). 2003b. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Parameters/Data Background
Document. Office of Solid Waste, EPA530-R-03-003, April.
U.S. EPA (Environmental Protection Agency). 2003 c. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Draft Addendum to the Technical
Background Document. Office of Solid Waste. September.
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IWEM Technical Documentation References
U.S. EPA (Environmental Protection Agency). 2003d. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Draft Addendum to the
Parameter/Data Background Document. Office of Solid Waste. April.
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Environmental Assessment Framework (LEAF) Test Methods. EPA/600/R-10/170. U.S.
EPA, Washington, DC.
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(IWEM) Version 3.1 User's Guide. Final. Office of Resource Conservation and Recovery,
Washington DC. September.
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Contaminants at Superfund Sites: Regional Screening Levels Generic Tables. Developed
in cooperation with Oak Ridge National Laboratory. Available at http://www.epa.gov/
reg3hwmd/risk/human/rb-concentration_table/Generic_Tables/index.htm. Accessed
November 2014.
U.S. EPA (Environmental Protection Agency). 2014a. Human and Ecological Risk Assessment of
Coal Combustion Wastes. Final. Office of Resource Conservation and Recovery,
Washington DC. December.
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Conservation and Recovery, Washington DC. February.
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IWEM Technical Background Document Appendix A: Glossary
Appendix A: Glossary
B
Adsorption isotherm: The relationship between the concentration of constituent in
solution and the amount adsorbed at constant temperature.
Adsorption: Adherence of molecules in solution to the surface of solids.
Advection: The process whereby solutes are transported by the bulk mass of flowing
fluid.
Alluvium: The general name for all sediments, including clay, silt, sand, gravel or similar
unconsolidated material deposited in a sorted or semi-sorted condition by a stream or
other body of running water, in a streambed, floodplain, delta, or at the base of a
mountain slope as a fan.
Anisotropy: The condition of having different properties in different directions.
Aquifer system: A body of permeable material that functions regionally as a water-
yielding unit; it comprises two or more permeable beds separated at least locally by
confining beds that impede ground water movement but do not greatly affect the regional
hydraulic continuity of the system; includes both saturated and unsaturated parts of
permeable material.
Aquifer: A geologic formation, group of formations, or part of a formation that contains
sufficient saturated permeable material to yield significant quantities of water to wells
and springs.
Area of influence of a well: The area surrounding a pumping or recharging well within
which the potentiometric surface has been changed.
Base: A layer of material in an asphalt roadway that is located directly under the surface
or paved layer. Typically, from bottom to top, the layers of a roadway are subgrade,
grade, subbase, base, and pavement.
Breakthrough curve: A graph of concentration versus time at a fixed location.
Beneficial Reuse: The reuse of industrial waste or byproducts in a product or application
that provides functional benefits, thus conserving natural resources that would otherwise
be used.
Cation exchange capacity: The sum total of exchangeable cations that a porous medium
can absorb. Expressed in moles of ion charge per kilogram of soil.
Confined aquifer: An aquifer bounded above and below by impermeable beds or by beds
of distinctly lower permeability than that of the aquifer itself; an aquifer containing
confined ground water.
Confined: A modifier that describes a condition in which the potentiometric surface is
above the top of the aquifer.
Confining unit: A body of impermeable or distinctly less permeable material which
separates water-bearing layers.
A-l
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IWEM Technical Background Document Appendix A: Glossary
Darcian velocity: The rate of ground water flow per unit area of porous or fractured media
measured perpendicular to the direction of flow. See specific discharge.
Darcy's law: An empirical law which states that the velocity of flow through porous
medium is directly proportional to the hydraulic gradient.
Desorption: Removal of a substance adsorbed to the surface of an adsorbent. Also, the
reverse process of sorption.
Diffusion coefficient: The rate at which solutes are transported at the microscopic level
due to variations in the solute concentrations within the fluid phases.
Diffusion: Spreading of solutes from regions of higher concentration to regions of lower
concentration caused by the concentration gradient. In slow-moving ground water, this
can be a significant mixing process.
Dispersion coefficient: A measure of the tendency of a plume of dissolved constituents in
ground water to spread. Equal to the sum of the coefficients of mechanical dispersion and
molecular diffusion in a porous medium.
Dispersion, longitudinal: Process whereby some of the water molecules and solute
molecules travel more rapidly than the average linear velocity and some travel more
slowly. Results in the spreading of the solute in the direction of the bulk flow.
Dispersion, transverse: Process whereby some of the water molecules and solute
molecules spread in directions perpendicular to the bulk flow.
Dispersivity: A geometric property of a porous medium that determines the dispersion
characteristics of the medium by relating the components of pore velocity to the
dispersion coefficient.
Distribution coefficient: The quantity of a constituent sorbed by a solid per unit weight of
solid divided by the quantity dissolved in water per unit volume of water.
Ditch: Part of a roadway that receives drainage and runoff.
Drain: A special type of roadway layer that moves water from underneath the roadway to
a ditch. See also permeable base.
Embankment: A raised area at the edge of a road. An embankment is a type of strip in
IWEM.
Evapotranspiration: The combined loss of water from a given area by evaporation from
the land and transpiration from plants.
Exfiltration: In IWEM, the rate of water leaving the bottom of any component of the
roadway, including the ditch.
Exposure pathway: The course a chemical or physical agent takes from a source to an
exposed organism. An exposure pathway describes a unique mechanism by which an
individual or population is exposed to chemicals or physical agents at, or originating
from, a site. Each exposure pathway includes a source or release from a source, an
exposure point, and an exposure route. If the exposure point differs from the source,
transport/exposure medium (e.g., water) or media (in case of intermedia transfer) also is
included.
-------
IWEM Technical Background Document Appendix A: Glossary
Exposure point concentration: An estimate of the arithmetic average concentration of a
contaminant at exposure point.
Exposure point: A location of potential contact between an organism and a chemical or
physical agent.
FFill: A screened earthen material used to create a strong and stable base. In roadway
applications, fill is often used for abutments or slabs, backfill for retaining structures, or
filling of trenches and other excavations that will support roadways or other structures
when completed.
Flow velocity: The rate of ground water flow per unit area of porous or fractured media
measured perpendicular to the direction of flow. See specific discharge.
Flow, steady: A characteristic of a flow system where the magnitude and direction of
specific discharge are constant in time at any point. See also flow, unsteady.
Flow, uniform: A characteristic of a flow system where specific discharge has the same
magnitude and direction at any point.
Flow, unsteady: A characteristic of a flow system where the magnitude and/or direction
of the flow rate changes with time.
Flowable fill: A liquid-like material that is self-compacting and self-leveling, and is used
as a substitute for conventional compacted fill material.
Flux: The rate of ground water flow per unit area of porous or fractured media measured
perpendicular to the direction of flow. See specific discharge.
Fracture: A break or crack in the bedrock.
Freezing degree-day: A measure of the departure of the mean daily temperature above
and below 32°F, positive if above and negative if below.
Freezing season: The period of time between the highest point and the succeeding lowest
point on the time curve of cumulative degree-days above and below 32°F; the opposite of
thawing season.
Geohydrologic system: The geohydrologic units within a geologic setting, including any
recharge, discharge, interconnections between units, and any natural or human-induced
processes or events that could affect ground water flow within or among those units. See
ground water system.
Geohydrologic unit: An aquifer, a confining unit, or a combination of aquifers and
confining units comprising a framework for a reasonably distinct geohydrologic system.
See hydrogeologic unit.
Grade: A capping layer added to the subgrade in a roadway to protect it in new
construction. Typically, from bottom to top, the layers of a roadway are subgrade, grade,
subbase, base, and pavement.
Ground water, confined: Ground water under pressure significantly greater than
atmospheric and whose upper limit is the bottom of a confining unit.
Ground water: Water present below the land surface in a zone of saturation. Ground
water is the water contained within an aquifer.
-------
IWEM Technical Background Document Appendix A: Glossary
Ground water discharge: Flow of water out of the zone of saturation.
Ground water flow: The movement of water in the zone of saturation.
Ground water flux: The rate of ground water flow per unit area of porous or fractured
media measured perpendicular to the direction of flow. See specific discharge.
Ground water mound: A raised area in a water table or potentiometric surface created by
ground water recharge.
Ground water recharge: The process of water addition to the saturated zone or the volume
of water added by this process.
Ground water system: A ground water reservoir and its contained water. Also, the
collective hydrodynamic and geochemical processes at work in the reservoir.
Ground water table: That surface below which rock, gravel, sand or other material is
saturated. It is the surface of a body of unconfined ground water at which the pressure is
atmospheric. Also called water table; synonymous with phreatic surface.
Ground water travel time: The time required for a unit volume of ground water or solute
to travel between two locations. The travel time is the length of the flow path divided by
the pore water velocity. If discrete segments of the flow path have different hydrologic
properties, the total travel time will be the sum of the travel times for each discrete
segment.
Ground water, unconfined: Water in an aquifer that has a water table. See also ground
water, confined.
H
Gutter: A channel that captures runoff (overland flow) from a roadway, preventing some
or all of it from reaching the ditch
Health-based number: The maximum constituent concentration in ground water that is
expected to not usually cause adverse noncancer health effects in the general population
(including sensitive subgroups), or that will not result in an additional incidence of cancer
in more than approximately one in one million individuals exposed to the contaminant.
Heterogeneity: A characteristic of a medium in which material properties vary throughout
the medium.
Homogeneity: A characteristic of a medium in which material properties are identical
throughout the medium.
Hydraulic conductivity: A coefficient of proportionality describing the rate at which water
can move through an aquifer or other permeable medium. Synonymous with
permeability.
Hydraulic gradient: Slope of the water table or potentiometric surface.
Hydraulic head: The level to which water rises in a well with reference to a datum such as
sea level.
Hydrodynamic dispersion: The spreading of the solute front during ground water plume
transport resulting from both mechanical dispersion and molecular diffusion.
Synonymous with mechanical dispersion.
A-4
-------
IWEM Technical Background Document Appendix A: Glossary
Hydrogeologic unit: Any soil or rock unit or zone that by virtue of its porosity or
permeability, or lack thereof, has a distinct influence on the storage or movement of
ground water.
Hydrologic properties: Those properties of a rock that govern the entrance of water and
the capacity to hold, transmit, and deliver water. Hydrologic properties include porosity,
effective porosity, and permeability.
Hydrolysis: The splitting (lysis) of a compound by a reaction with water. Example are the
reaction of salts with water to produce solutions that are not neutral, and the reaction of
an ester with water.
Hydrostratigraphic unit: See hydrogeologic unit.
I Igneous rocks: Rocks that solidified from molten or partly molten materials, that is from
a magma or lava.
Immiscible: The chemical property of two or more phases that, at mutual equilibrium,
cannot dissolve completely in one another, for example, oil and water.
Impermeable: A characteristic of some geologic material that limits its ability to transmit
significant quantities of water under the head differences ordinarily found in the
subsurface.
Infiltration: The downward entry of water into the soil or rock (i.e., percolation),
specifically from a waste management unit. See also percolation and recharge.
Isotropy: The condition in which the property or properties of interest are the same in all
directions.
L Layer: A portion of the depth of a roadway that corresponds to a separate material; a
material layer.
Leachate: A liquid that has percolated through waste and has extracted dissolved or
suspended materials.
Leaching: Separation or dissolving out of soluble constituents from a waste by
percolation of water.
Leaching duration: The period of time that leachate is released from a source.
Matrix diffusion: The tendency of solutes to diffuse from the larger pores in the system
into small pores inside the solid matrix from where they can be removed only very
slowly.
Matrix: The solid particles in a porous system and their spatial arrangement. Often used in
contrast to the pore space in a porous system.
Maximum Contaminant Level (MCL): Legally enforceable standards regulating the
maximum allowed amount of certain chemicals in drinking water.
Mechanical dispersion: The process whereby solutes are mechanically mixed during
advective transport caused by the velocity variations at the microscopic level.
Synonymous with hydrodynamic dispersion.
A-5
M
-------
IWEM Technical Background Document Appendix A: Glossary
Median: The part of a roadway that separates the travel lanes in one direction from those
in the other. A median is a type of strip in IWEM.
Metamorphic rocks: Any rock derived from pre-existing rocks by mineralogical,
chemical, and/or structural changes, essentially in the solid state, in response to marked
changes in temperature, pressure, shearing stress, and chemical environment, generally at
depth in the Earth's crust.
Miscible: The chemical property of two or more fluid phases that, when brought together,
have the ability to mix and form one phase.
Model: A simplified representation of a physical system obeying certain specified
conditions, whose behavior is used to understand the real world system. Often, the model
is a mathematical representation, programmed into a computer.
Moisture content: The ratio of either (a) the weight of water to the weight of solid
particles expressed as moisture weight percentage or (b) the volume of water to the
volume of solid particles expressed as moisture volume percentage in a given volume of
porous medium. See water content.
Molecular diffusion: The process in which solutes are transported at the microscopic level
due to variations in the solute concentrations within the fluid phases. See diffusion.
Monte Carlo simulation: A method that produces a statistical estimate of a quantity by
taking many random samples from an assumed probability distribution, such as a normal
distribution. The method is typically used when experimentation is infeasible or when the
actual input values are difficult or impossible to obtain.
Mounding: Commonly, an outward and upward expansion of the free water table caused
by surface infiltration or recharge.
Outwash deposits: Stratified drift deposited by meltwater streams flowing away from
melting ice.
Overburden: The layer of fragmental and uncon soli dated material including loose soil,
silt, sand and gravel overlying bedrock, which has been either transported from elsewhere
or formed in place.
Paved Area: The travel lanes in a roadway; the part vehicles drive on. The paved area is a
type of strip in IWEM.
Pavement: A type of roadway layer that consists of paving material such as asphalt.
Typically, from bottom to top, the layers of a roadway are subgrade, grade, subbase, base,
and pavement.
Percolation: The downward entry of water into the soil or rock and ultimately the
saturated zone. In IWEM, there are two types: infiltration (through the waste
management unit) and recharge (through the soil outside the waste management unit
footprint).
Permeability: The property of a porous medium to transmit fluids under a hydraulic
gradient.
A-6
-------
IWEM Technical Background Document Appendix A: Glossary
Permeable: The property of a porous medium to allow the easy passage of a fluid through
it.
Permeable Base: The permeable base is a layer of high permeability materials that serve
to divert water and any dissolved constituents in the water from further downward
migration. In the IWEM software, the permeable base is called a drain. A ditch must be
included in the design before a drain can be included, as the ditch serves as the
destination for diverted waters.
pH: A numerical measure of the acidity or alkalinity of water ranging from 0 to 14.
Neutral waters have pH near 7. Acidic waters have pH less than 7 and alkaline waters
have pH greater than 7.
Pore-water velocity: Average velocity of water particles. Equals the Darcian velocity
divided by the effective porosity. Synonymous with seepage velocity.
Porosity, effective: The ratio, usually expressed as a percentage, of the total volume of
voids (or pores) available for fluid transmission to the total volume of the porous
medium.
Porosity: The ratio, usually expressed as a percentage, of the total volume of voids (or
pores) of a given porous medium to the total volume of the porous medium.
Portland Cement Concrete: Hydraulic cement (cement that not only hardens by reacting
with water but also forms a water-resistant product) produced by pulverizing clinkers
consisting essentially of hydraulic calcium silicates, usually containing one or more of
the forms of calcium sulfate as an inter ground addition.
Receptor: The potentially exposed individual for the exposure pathway considered.
Recharge: The downward entry of water to the saturated zone; also the water added. In
IWEM, recharge is the result of natural precipitation around a waste management unit.
Retardation factor: The ratio of the average linear velocity of ground water to the velocity
of a dissolved constituent. A value greater than one indicates that the constituent moves
more slowly than water, usually caused by sorption.
Risk assessment: The process used to determine the risk posed by contaminants released
into the environment. Elements include identification of the contaminants present in the
environmental media, assessment of exposure and exposure pathways, assessment of the
toxicity of the contaminants present at the site, characterization of human health risks,
and characterization of the impacts or risks to the environment.
Risk: The probability that a constituent will cause an adverse effect in exposed humans or
to the environment.
Road segment: A length of roadway being modeled in IWEM.
Roadway: A road, including not just the paved road surface, but other structures such as a
median, road shoulders, embankments, and ditches.
R
A-7
-------
IWEM Technical Background Document Appendix A: Glossary
S Saturated Zone: The part of the water bearing layer of rock or soil in which all spaces,
large or small, are filled with water.
Sedimentary rocks: Rocks formed from consolidation of loose sediments such as clay,
silt, sand, and gravel.
Seepage velocity: See pore-water velocity.
Shoulder: Part of a roadway that is adjacent to the travel lane(s) but may not be paved. A
shoulder is a type of strip in IWEM.
Slope: The ratio of the change in elevation to the distance over which the elevation
change is measured. For a roadway, slope is measured along the direction of travel; for a
ditch, along the direction of flow.
Soil bulk density: The mass of dry soil per unit bulk soil.
Soil moisture: Subsurface liquid water in the unsaturated zone expressed as a fraction of
the total porous medium volume occupied by water. It is less than or equal to the
porosity.
Solubility: The total amount of solute species that will remain indefinitely in a solution
maintained at constant temperature and pressure in contact with the solid crystals from
which the solutes were derived.
Solute transport: The net flux of solute (dissolved constituent) through a hydrogeologic
unit controlled by the flow of subsurface water and transport mechanisms.
Sorption: A general term used to encompass the process of adsorption.
Source term: The kinds and amounts of constituents that make up the source of a
potential release.
Specific discharge: The rate of discharge of ground water per unit area of a porous
medium measured at right angle to the direction of flow. Synonymous with Darcian
velocity, or (specific) flux.
Strip: A portion of the width of a roadway; include both the actual road and strips along
the side or down the middle that are not actual driving surface (such as shoulder or
median).
Structural Fill: The use of industrial wastes and related byproducts as substitutes for
earthen materials to support parking lots, roads, airstrips, tanks/vaults, and buildings; to
construct highway embankments and bridge abutments; to fill borrow pits, mines, and
other landscape irregularities; and to change the landscape for development or
reclamation projects. Structural fills may be either flowable or compacted; IWEM can
model both.
Subbase: The layer of aggregate material laid on top of the subgrade or grade, on which
the base course layer is laid. Typically, from bottom to top, the layers of a roadway are
subgrade, grade, subbase, base, and pavement.
Subgrade: The layer of naturally occurring material a road is built upon. Typically, from
bottom to top, the layers of a roadway are subgrade, grade, subbase, base, and pavement.
A-8
-------
IWEM Technical Background Document Appendix A: Glossary
TTill: Till consists of a generally unconsolidated, unsorted, unstratified heterogeneous
mixture of clay, silt, sand, gravel and boulders of different sizes and shapes. Till is
deposited directly by and underneath glacial ice without subsequent reworking by
meltwater.
Toxicity: The degree to which a chemical substance elicits a deleterious or adverse effect
on a biological system of an organism exposed to the substance over a designated time
period.
Transient flow: See flow, unsteady.
Transmissivity: The rate at which water is transmitted through a unit width of the aquifer
under a unit hydraulic gradient. It is equal to an integration of the hydraulic
conductivities across the saturated part of the aquifer perpendicular to the flow paths.
Transport: Conveyance of dissolved constituents and particulates in flow systems. See
also solute transport.
Unconfined aquifer: An aquifer that has a water table.
Unconfined: A condition in which the upper surface of the zone of saturation forms a
water table under atmospheric pressure.
Unconsolidated deposits: Deposits overlying bedrock and consisting of soil, silt, sand,
gravel and other material which have either been formed in place or have been
transported in from elsewhere.
Unsaturated flow: The movement of water in a porous medium in which the pore spaces
are not filled to capacity with water.
Unsaturated zone: The subsurface zone between the water table and the land surface
where some of the spaces between the soil particles are filled with air.
Vadose zone: See unsaturated zone.
u
V
w
Volatiles: Substances with relatively large vapor pressures that easily volatilize when in
contact with air.
Water content: The amount of water lost from the soil after drying it to constant weight at
105 °C, expressed either as the weight of water per unit weight of dry soil or as the
volume of water per unit bulk volume of soil. See also moisture content.
Water table aquifer: See unconfined aquifer.
Water table: The upper surface of a zone of saturation except where that surface is formed
by a confining unit. The water pressure at the water table equals atmospheric pressure.
Well: A bored, drilled or driven shaft, or a dug hole extending from the ground surface
into the ground water, that is used to inject (injection well) or extract ground water. Well
screen. A cylindrical filter used to prevent sediment from entering a water well. There are
several types of well screens, which can be ordered in various slot widths, selected on the
basis of the grain size of the aquifer material where the well screen is to be located. In
very fine grained aquifers, a zone of fine gravel or coarse sand may be required to act as a
filter between the screen and the aquifer.
A-9
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IWEM Technical Background Document Appendix A: Glossary
This page intentionally left blank.
A-10
-------
IWEM Technical Background Document Appendix B: Chemical Properties
Appendix B: List of IWEM Waste Constituents and Default Physical
and Chemical Property Data
Table B-l lists the 231 chemicals in IWEM and their default physical and chemical properties.
References
deMarsily, G. 1986. Quantitative Hydrogeology. Academic Press
Kollig, H. P. (ed.). 1993. Environmental fate consultants for organic chemicals under
consideration for EPA's hazardous waste identification projects. Environmental Research
Laboratory, Office of R&D, U.S. EPA, Athens, GA.
Lyman, W.J., W.F. Reehl, and D.H. Rosenblatt. 1990. Handbook of Chemical Property
Estimation Methods: Environmental Behavior of Organic Compounds. Washington, DC:
American Chemical Society.
MI DEQ (Michigan Department of Environmental Quality), nd. Environmental response Division
Operational Memorandum #18 (Opmemo 18): Part 201 Generic Cleanup Criteria Tables,
Revision 1, State of Michigan, Department of Environmental Quality.
http://www.deq.state.mi.us/erd/opmemol8/index.html.
NLM (U.S. National Library of Medicine). 2001. Hazardous Substances Data Bank (HSDB).
http://toxnet.nlm.nih.gov/cgi-bin/sis/htmlgen/HSDB. Accessed July 2001.
SRC (Syracuse Research Corporation). 1999. CHEMFATE Chemical Search, Environmental
Science Center, Syracuse, NY. http://esc.syrres.com/efdb/Chemfate.htm. Accessed July
2001.
U.S. EPA (Environmental Protection Agency). 1987. Process Coefficients and Models for
Simulating Toxic Organics and Heavy Metals in Surface Waters. Office of Research and
Development, Washington, DC: US Government Printing Office (GPO).
U.S. EPA (Environmental Protection Agency). 1997. Superfund Chemical Data Matrix (SCDM).
SCDMWIN 1.0 (SCDM Windows User's Version), Version 1. Office of Solid Waste and
Emergency Response Washington DC: GPO.
http://www.epa.gov/superfund/resources/scdm/index.htm. Accessed July 2001
U.S. EPA (Environmental Protection Agency). 1999. Region in Soil-to-Groundwater SSLs.
Region HI, Philadelphia, PA. http://www.epa.gov/reg3hwmd/risk/ssl.pdf
U.S. EPA (Environmental Protection Agency). 2000a. Exposure and Human Health
Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds,
Part 1, Vol. 3. Office of Research and Development, Washington, DC: GPO.
U.S. EPA (Environmental Protection Agency). 2000b. Physical-chemical Data.
http://www.epa.gov/Rgeion9/waste/sfund/prg/index.htm
U.S. EPA (Environmental Protection Agency). 2001. Water 9. Office of Air Quality Planning
and Standards, Research Triangle Park, NC.
http://www.epa.gov/ttn/chief/software/water/ind Accessed July 2001
B-l
-------
IWEM Technical Background Document
Appendix B: Chemical Properties
Table B-1. Constituent Physical and Chemical Properties
CAS
83-32-9
75-07-0
67-64-1
75-05-8
98-86-2
107-02-8
79-06-1
79-10-7
107-13-1
309-00-2
107-18-6
7429-90-5
62-53-3
120-12-7
7440-36-0
22569-72-8
15584-04-0
7440-39-3
56-55-3
71-43-2
92-87-5
50-32-8
205-99-2
100-51-6
100-44-7
7440-41-7
111-44-4
39638-32-9
117-81-7
7440-42-8
Constituent Name
Acenaphthene
Acetaldehyde [Ethanal]
Acetone (2-propanone)
Acetonitrile (methyl cyanide)
Acetophenone
Acrolein
Acrylamide
Acrylic acid [propenoic acid]
Acrylonitrile
Aldrin
Allyl alcohol
Aluminum (CCR waste only)
Aniline (benzeneamine)
Anthracene
Antimony
Arsenic (III)
Arsenic (V)
Barium
Benz{a}anthracene
Benzene
Benzidine
Benzo{a}pyrene
Benzo{b}fluoranthene
Benzyl alcohol
Benzyl chloride
Beryllium
Bis(2-chloroethyl)ether
Bis(2-chloroisopropyl)ether
Bis(2-ethylhexyl)phthalate
Boron (CCR waste only)
Molecular
Weight3
(g/mol)
154.21
44.10
58.08
41.05
120.15
56.06
71.08
72.10
53.06
364.91
58.10
26.98
93.13
178.23
121.76
74.92
74.92
137.33
228.29
78.11
184.24
252.31
252.31
108.14
126.59
9.01
143.01
171.07
390.56
10.81
Solubilityb
(mg/L)
4.24
1E+06
1E+06
1E+06
6,130
213,000
640,000
1E+06
74,000
0.18
1E+06
1E+06
36,000
0.043
1E+06
1E+06
1E+06
1E+06
0.0094
1,750
500
0.00162
0.0015
40,000
525
1E+06
17,200
1,310
0.34
1E+06
Log Koc°
(log[mL/g])
3.75
-0.21
-0.59
-0.71
1.26
-0.22
-0.99
-1.84
-0.09
6.18
1.47
0.60
4.21
5.34
1.80
1.26
5.80
5.80
0.78
2.84
0.80
2.39
7.13
Hydrolysis Rate Constants0
Acid
Catalyzed (Ka)
(1/mol/yr)
0
0
0
0
0
0
31.50
0
500
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Neutral
(Kn)
0
0
0
0
0
6.68E+8
0.018
0
0
0
0
0
0
0
0
0
0
0
0
410
0.23
0
0
Base
Catalyzed (Kb)
(1/mol/yr)
0
0
0
45
0
0
0
0
5,200
0
0
0
0
0
0
0
0
0
0
0
0
0
1,400
Diffusion
Coefficient
in Air (Da)
(m2/yr)
404
334
423
353
337
325
360
71.9
262
161
282
112
80.4
150
200
179
126
54.6
Diffusion
Coefficient in
Water" (Dw)
(m2/yr)
0.0426
0.0363
0.0445
0.0385
0.0397
0.0378
0.0388
0.0184
0.0319
0.0186
0.0325
0.0239
0.0208
0.0174
0.0278
0.0275
0.0233
0.0132
Henry's Law
Coefficent
(HLC)
(atm-m3/mol)
7.89E-05
3.88E-05
3.46E-05
1.22E-04
1.00E-09
1.17E-07
1.03E-04
1.70E-04
1.90E-06
3.35E-06
5.55E-03
3.88E-11
1.13E-06
1.11E-04
4.15E-04
1.80E-05
1.34E-04
1.02E-07
B-2
-------
IWEM Technical Background Document
Appendix B: Chemical Properties
CAS
75-27-4
74-83-9
106-99-0
71-36-3
85-68-7
88-85-7
7440-43-9
75-15-0
56-23-5
57-74-9
126-99-8
106-47-8
108-90-7
510-15-6
124-48-1
75-00-3
67-66-3
74-87-3
95-57-8
107-05-1
16065-83-1
18540-29-9
218-01-9
7440-48-4
7440-50-8
108-39-4
95-48-7
106-44-5
1319-77-3
Constituent Name
Bromodichloromethane
Bromomethane
Butadiene 1 ,3-
Butanol n-
Butyl benzyl phthalate
Butyl-4,6-dinitrophenol,2-sec-
(Dinoseb)
Cadmium
Carbon disulfide
Carbon tetrachloride
Chlordane
Chloro-1 ,3-butadiene 2-
(Chloroprene)
Chloroaniline p-
Chlorobenzene
Chlorobenzilate
Chlorodibromomethane
Chloroethane [Ethyl chloride]
Chloroform
Chloromethane
Chlorophenol 2-
Chloropropene 3- (Allyl
Chloride)
Chromium (III) (Chromic Ion)
Chromium (VI)
Chrysene
Cobalt
Copper
Cresol m-
Cresol o-
Cresol p-
Cresols
Molecular
Weight3
(g/mol)
163.83
94.94
54.09
74.12
312.36
240.22
112.41
76.13
153.82
409.78
88.54
127.57
112.56
325.19
208.28
64.50
119.38
50.49
128.56
76.53
52.00
52.00
228.29
58.93
63.55
108.14
108.14
108.14
324.42
Solubilityb
(mg/L)
6,740
15,200
735
74,000
2.69
52
1E+06
1,190
793
0.056
1,740
5,300
472
11.1
2,600
5,680
7,920
5,330
22,000
3,370
1E+06
1E+06
0.0016
1E+06
1E+06
22,700
26,000
21,500
23,400
Log Kocc
(log[mL/g])
1.77
0.76
2.06
0.50
4.23
2.02
1.84
2.41
5.89
1.74
1.61
2.58
4.04
1.91
0.51
1.58
0.91
1.82
1.13
5.34
1.76
1.76
1.76
2.12
Hydrolysis Rate Constants0
Acid
Catalyzed (Ka)
(1/mol/yr)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Neutral
(Kn)
0
9.46
0
0
0
0
0.017
0
0
0
0
0
0
0
0.0001
0
0
40
0
0
0
0
0
Base
Catalyzed (Kb)
(1/mol/yr)
50,000
0
0
120,000
0
31,500
0
37.7
0
0
0
2.80E+06
25,000
0
2,740
0
0
0
0
0
0
0
0
Diffusion
Coefficient
in Air (Da)
(m2/yr)
178
315
315
334
180
67.8
265
227
68.8
115
328
243
391
208
295
82.3
230
239
228
232
Diffusion
Coefficient in
Water" (Dw)
(m2/yr)
0.0337
0.0426
0.0325
0.0410
0.0308
0.0172
0.0315
0.0299
0.0173
0.0334
0.0366
0.0344
0.0429
0.0299
0.0341
0.0213
0.0294
0.0311
0.0291
0.0299
Henry's Law
Coefficent
(HLC)
(atm-m3/mol)
1.60E-03
6.24E-03
7.36E-02
3.03E-02
3.04E-02
4.86E-05
1.19E-02
3.70E-03
7.24E-08
7.83E-04
8.82E-03
3.67E-03
8.82E-03
3.91 E-04
1.10E-02
9.46E-05
8.65E-07
1.20E-06
7.92E-07
9.52E-07
B-3
-------
IWEM Technical Background Document
Appendix B: Chemical Properties
CAS
98-82-8
108-93-0
108-94-1
72-54-8
72-55-9
50-29-3
2303-16-4
53-70-3
96-12-8
95-50-1
106-46-7
91-94-1
75-71-8
75-34-3
107-06-2
75-35-4
156-59-2
156-60-5
120-83-2
94-75-7
78-87-5
542-75-6
10061-01-5
10061-02-6
60-57-1
84-66-2
56-53-1
60-51-5
119-90-4
Constituent Name
Cumene
Cyclohexanol
Cyclohexanone
ODD
DDE
DDT p,p'-
Diallate
Dibenz{a,h}anthracene
Dibromo-3-chloropropane 1 ,2-
Dichlorobenzene 1 ,2-
Dichlorobenzene 1 ,4-
Dichlorobenzidine 3,3'-
Dichlorodifluoromethane
(Freon12)
Dichloroethane1,1-
Dichloroethane 1 ,2-
Dichloroethylene 1,1-
Dichloroethylene cis-1 ,2-
Dichloroethylene trans-1 ,2-
Dichlorophenol 2,4-
Dichlorophenoxyacetic acid
2,4-(2,4-D)
Dichloropropane 1 ,2-
Dichloropropene 1 ,3-(mixture
of isomers)
Dichloropropene cis-1 ,3-
Dichloropropene trans-1 ,3-
Dieldrin
Diethyl phthalate
Diethylstilbestrol
Dimethoate
Dimethoxybenzidine 3,3'-
Molecular
Weight3
(g/mol)
120.19
100.16
98.14
320.05
318.03
354.49
270.22
278.35
236.33
147.00
147.00
253.13
120.91
98.96
98.96
96.94
96.94
96.94
163.00
221.04
112.99
110.97
110.97
110.97
380.91
222.24
268.35
229.25
0.00
Solubilityb
(mg/L)
61.3
43,000
5,000
0.09
0.12
0.025
40
0.00249
1,230
156
73.8
3.11
280
5,060
8,520
2,250
3,500
6,300
4,500
677
2,800
2,800
2,720
2,720
0.195
1,080
0.0956
25,000
60
Log Kocc
(loglmL/g])
3.40
1.11
1.82
5.89
6.64
6.59
4.17
6.52
1.94
3.08
3.05
3.32
2.16
1.46
1.13
1.79
1.70
1.60
2.49
0.68
1.67
1.43
1.80
1.80
5.08
1.99
4.09
0.13
1.49
Hydrolysis Rate Constants0
Acid
Catalyzed (Ka)
(1/mol/yr)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Neutral
(Kn)
0
0
0
0.025
0
0.060
0.1
0
0.004
0
0
0
0
0.0113
0.0096
0
0
0
0
0
0
0
40
40
0.063
0
0
1.68
0
Base
Catalyzed (Kb)
(1/mol/yr)
0
0
0
22,000
0
310,000
8,000
0
120,000
0
0
0
0
0.378
54.7
0
0
0
0
0
0
0
0
0
0
310,000
0
4.48E+06
0
Diffusion
Coefficient
in Air (Da)
(m2/yr)
190
239
57.7
74.4
101
177
173
150
240
264
269
272
231
241
241
241
73.5
Diffusion
Coefficient in
Water" (Dw)
(m2/yr)
0.0248
0.0295
0.014
0.0190
0.0281
0.0281
0.0274
0.0173
0.0341
0.0334
0.0344
0.0347
0.0307
0.0319
0.0322
0.0319
0.0190
Henry's Law
Coefficent
(HLC)
(atm-m3/mol)
1.16E+00
1.02E-04
8.10E-06
1.47E-08
1.47E-04
1.90E-03
2.40E-03
4.00E-09
3.43E-01
5.62E-03
9.79E-04
2.61 E-02
2.80E-03
1.77E-02
2.40E-03
1.80E-03
1.51E-05
B-4
-------
IWEM Technical Background Document
Appendix B: Chemical Properties
CAS
68-12-2
57-97-6
119-93-7
105-67-9
84-74-2
99-65-0
51-28-5
121-14-2
606-20-2
117-84-0
123-91-1
122-39-4
122-66-7
298-04-4
115-29-7
72-20-8
106-89-8
106-88-7
110-80-5
111-15-9
141-78-6
60-29-7
97-63-2
62-50-0
100-41-4
106-93-4
107-21-1
75-21-8
Constituent Name
Dimethyl formamide N,N-
[DMF]
Dimethylbenz{a}anthracene
7,12-
Dimethylbenzidine 3,3'-
Dimethylphenol 2,4-
Di-n-butyl phthalate
Dinitrobenzene 1 ,3-
Dinitrophenol 2,4-
Dinitrotoluene 2,4-
Dinitrotoluene 2,6-
Di-n-octyl phthalate
Dioxane 1 ,4-
Diphenylamine
Diphenylhydrazine 1 ,2-
Disulfoton
Endosulfan (Endosulfan I & II
mixture)
Endrin
Epichlorohydrin
Epoxybutane 1 ,2-
Ethoxyethanol 2-
Ethoxyethanol acetate 2-
Ethyl acetate
Ethyl ether
Ethyl methacrylate
Ethyl methanesulfonate
Ethylbenzene
Ethylene dibromide (1 ,2-
Dibromoethane)
Ethylene glycol
Ethylene oxide
Molecular
Weight3
(g/mol)
73.10
256.35
212.29
122.17
278.35
168.11
184.11
182.14
182.14
390.56
88.11
169.23
184.24
274.39
406.92
380.91
92.52
72.11
90.12
132.16
88.11
74.12
114.14
124.15
106.17
187.86
62.10
44.10
Solubilityb
(mg/L)
1E+06
0.025
1,300
7,870
11.2
861
2,787
270
182
0.02
1E+06
35.7
68
16.3
0.51
0.25
65,900
95,000
1E+06
229,000
80,300
56,800
3,671
6,300
169
4,180
1E+06
1E+06
Log Kocc
(loglmL/g])
-0.99
6.64
2.55
2.29
4.37
1.31
-0.09
1.68
1.40
7.60
-0.81
3.30
2.82
2.94
3.55
4.60
-0.53
0.90
-0.54
0.70
0.35
0.55
1.27
-0.27
3.00
1.42
-1.50
-1.10
Hydrolysis Rate Constants0
Acid
Catalyzed (Ka)
(1/mol/yr)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
25,000
0
0
3,500
0
0
0
0
0
0
290,000
Neutral
(Kn)
0
0
0
0
0
0
0
0
0
0
0
0
0
2.3
0
0.055
30.9
0
0
0.0048
0
0
1,250
0
0.63
0
21
Base
Catalyzed (Kb)
(1/mol/yr)
0
0
0
0
1.80E+06
0
0
0
0
520,000
0
0
0
54,000
0
0
0
0
0
3.40E+06
0
1.10E+06
0
0
0
0
0
Diffusion
Coefficient
in Air (Da)
(m2/yr)
307
149
118
276
108
280
294
258
180
216
136
369
423
Diffusion
Coefficient in
Water" (Dw)
(m2/yr)
0.0353
0.0172
0.0249
0.0331
0.0229
0.0350
0.0331
0.0308
0.0252
0.0267
0.0331
0.0429
0.0460
Henry's Law
Coefficent
(HLC)
(atm-m3/mol)
7.39E-08
3.11E-08
9.26E-08
4.80E-06
1.53E-06
3.04E-05
1.80E-04
1.23E-07
1.80E-06
7.88E-03
7.43E-04
6.00E-08
1.48E-04
B-5
-------
IWEM Technical Background Document
Appendix B: Chemical Properties
CAS
96-45-7
206-44-0
16984-48-8
50-00-0
64-18-6
98-01-1
319-85-7
58-89-9
319-84-6
76-44-8
1024-57-3
87-68-3
118-74-1
77-47-4
55684-94-1
34465-46-8
67-72-1
70-30-4
110-54-3
7783-06-4
193-39-5
7439-89-6
78-83-1
78-59-1
143-50-0
7439-92-1
7439-96-5
7439-97-6
126-98-7
Constituent Name
Ethylene thiourea
Fluoranthene
Fluoride
Formaldehyde
Formic acid
Furfural
HCH beta-
HCH (Lindane) gamma-
HCH alpha-
Heptachlor
Heptachlor epoxide
Hexachloro-1 ,3-butadiene
Hexachlorobenzene
Hexachlorocyclopentadiene
Hexachlorodibenzofurans
[HxCDFs]
Hexachlorodibenzo-p-dioxins
[HxCDDs]
Hexachloroethane
Hexachlorophene
Hexane n-
Hydrogen Sulfide
lndeno{1,2,3-cd}pyrene
Iron (CCR waste only)
Isobutyl alcohol
Isophorone
Kepone
Lead
Manganese
Mercury
Methacrylonitrile
Molecular
Weight3
(g/mol)
102.15
202.26
19.00
30.03
46.03
96.10
290.83
290.83
290.83
373.32
389.32
260.76
284.78
272.77
374.87
390.86
236.74
406.91
86.20
34.08
276.34
55.85
74.12
138.21
490.64
207.20
54.90
200.59
67.09
Solubilityb
(mg/L)
62,000
0.206
550,000
1E+06
110,000
0.24
6.8
2
0.18
0.2
3.23
0.005
1.8
8.25E-06
4E-06
50
140
12.4
437
2.2E-05
1E+06
85,000
12,000
7.6
1E+06
1E+06
0.0562
25,400
Log Kocc
(log[mL/g])
0.00
4.63
-1.30
-2.70
0.80
3.43
3.40
3.43
5.21
4.90
4.46
5.41
4.72
7.00
6.38
3.61
5.00
2.95
6.26
0.44
1.90
4.15
0.22
Hydrolysis Rate Constants0
Acid
Catalyzed (Ka)
(1/mol/yr)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
500
Neutral
(Kn)
0
0
0
0
0
0
1.05
0
61
0.063
0
0
24.8
0
0
0
0
0
0
0
0
0
0
0
Base
Catalyzed (Kb)
(1/mol/yr)
0
0
0
0
0
0
1.73E+06
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
5,200
Diffusion
Coefficient
in Air (Da)
(m2/yr)
274
527
269
87.4
86.4
86.7
70.3
69.1
84.2
91.5
85.8
138
135
101
230
141
166
225
304
Diffusion
Coefficient in
Water" (Dw)
(m2/yr)
0.0319
0.0549
0.0337
0.0233
0.023
0.0232
0.018
0.0176
0.0222
0.0248
0.0228
0.0133
0.013
0.0280
0.0256
0.0164
0.0238
0.0949
0.0334
Henry's Law
Coefficent
(HLC)
(atm-m3/mol)
3.08E-10
3.36E-07
4.00E-06
7.43E-07
1.40E-05
1.06E-05
1.10E-03
9.50E-06
8.15E-03
1.32E-03
2.70E-02
1.43E-05
1.10E-05
3.89E-03
1.43E-02
1.60E-06
6.64E-06
1.14E-02
2.47E-04
B-6
-------
IWEM Technical Background Document
Appendix B: Chemical Properties
CAS
67-56-1
72-43-5
109-86-4
110-49-6
78-93-3
108-10-1
80-62-6
298-00-0
1634-04-4
56-49-5
74-95-3
75-09-2
7439-98-7
91-20-3
7440-02-0
98-95-3
79-46-9
55-18-5
62-75-9
924-16-3
621-64-7
86-30-6
10595-95-6
100-75-4
930-55-2
152-16-9
56-38-2
608-93-5
Constituent Name
Methanol
Methoxychlor
Methoxyethanol 2-
Methoxyethanol acetate 2-
Methyl ethyl ketone
Methyl isobutyl ketone
Methyl methacrylate
Methyl parathion
Methyl tert-butyl ether [MTBE]
Methylcholanthrene 3-
Methylene bromide
(Dibromomethane)
Methylene Chloride
(Dichloromethane)
Molybdenum
Naphthalene
Nickel
Nitrobenzene
Nitropropane 2-
Nitrosodiethylamine N-
Nitrosodimethylamine N-
Nitroso-di-n-butylamine N-
Nitroso-di-n-propylamine N-
Nitrosodiphenylamine N-
Nitrosomethylethylamine N-
Nitrosopiperidine N-
Nitrosopyrrolidine N-
Octamethyl
pyrophosphoramide
Parathion (ethyl)
Pentachlorobenzene
Molecular
Weight3
(g/mol)
32.04
345.65
76.10
118.13
72.11
100.16
100.12
263.20
88.15
268.36
173.83
84.93
95.90
128.17
58.69
123.11
89.09
102.14
74.08
158.24
130.19
198.22
88.11
114.15
100.12
286.25
291.26
250.34
Solubilityb
(mg/L)
1E+06
0.045
1E+06
1E+06
223,000
19,000
15,000
55
51,300
0.00323
11,930
13,000
1E+06
31
1E+06
2,090
17,000
93,000
1E+06
1,270
9,890
35.1
19,700
76,500
1E+06
1E+06
6.54
1.33
Log Kocc
(loglmL/g])
-1.08
4.90
0.95
0.00
-0.03
0.87
0.74
2.47
1.05
7.00
1.21
0.93
3.11
1.51
0.23
-0.03
0.45
2.09
1.03
2.84
1.03
-0.02
-0.57
-0.51
3.15
5.39
Hydrolysis Rate Constants0
Acid
Catalyzed (Ka)
(1/mol/yr)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1,900
0
0
Neutral
(Kn)
0
0.69
0
0
0
0
0
2.8
0
0.017
0
0.001
0
0
0
0
0
0
0
0
0
0
0
0
2.4
0
Base
Catalyzed (Kb)
(1/mol/yr)
0
12,000
0
0
0
0
0
0
0
0
0
0.6
0
0
0
0
0
0
0
0
0
0
0
0
3.70E+06
0
Diffusion
Coefficient
in Air (Da)
(m2/yr)
498
300
208
289
220
237
238
76
315
191
215
267
233
312
133
178
89.6
265
220
252
Diffusion
Coefficient in
Water" (Dw)
(m2/yr)
0.0520
0.0347
0.0275
0.0322
0.0264
0.0292
0.0272
0.0194
0.0394
0.0264
0.0298
0.0322
0.0288
0.0363
0.0215
0.0245
0.0227
0.0315
0.0290
0.0319
Henry's Law
Coefficent
(HLC)
(atm-m3/mol)
4.55E-06
8.10E-08
3.11E-07
5.59E-05
1.38E-04
3.37E-04
5.87E-04
9.40E-07
2.19E-03
4.83E-04
2.40E-05
1.23E-04
3.63E-06
1.20E-06
3.16E-04
2.25E-06
5.00E-06
1.40E-06
2.80E-07
1.20E-08
B-7
-------
IWEM Technical Background Document
Appendix B: Chemical Properties
CAS
30402-15-4
36088-22-9
82-68-8
87-86-5
108-95-2
62-38-4
108-45-2
298-02-2
85-44-9
1336-36-3
23950-58-5
75-56-9
129-00-0
110-86-1
94-59-7
10026-03-6
7782-49-2
7440-22-4
57-24-9
100-42-5
95-94-3
51207-31-9
1746-01-6
630-20-6
79-34-5
Constituent Name
Pentachlorodibenzofurans
[PeCDFs]
Pentachlorodibenzo-p-dioxins
[PeCDDs]
Pentachloronitrobenzene
(PCNB)
Pentachlorophenol
Phenol
Phenyl mercuric acetate
Phenylenediamine 1 ,3-
Phorate
Phthalic anhydride
Polychlorinated biphenyls
(Aroclors)
Pronamide
Propylene oxide [1 ,2-
Epoxypropane]
Pyrene
Pyridine
Safrole
Selenium (IV)
Selenium (VI)
Silver
Strychnine and salts
Styrene
Tetrachlorobenzene 1 ,2,4,5-
Tetrachlorodibenzofuran
2,3,7,8-
Tetrachlorodibenzo-p-dioxin
2,3,7,8-
Tetrachloroethane 1,1,1,2-
Tetrachloroethane1,1,2,2-
Molecular
Weight3
(g/mol)
340.42
356.42
295.34
266.34
94.11
336.74
108.14
260.36
148.12
256.13
58.10
202.26
79.10
162.19
78.96
78.96
107.87
334.42
104.15
215.89
305.98
321.97
167.85
167.85
Solubilityb
(mg/L)
0.00024
0.000118
0.55
1,950
82,800
2,000
2.55E+06
50
6,200
0.07
32.8
405,000
0.135
1E+06
810.67
1E+06
1E+06
1E+06
160
310
0.595
0.000692
7.91 E-06
1,100
2,970
Log Kocc
(log[mL/g])
4.93
6.30
4.57
3.06
1.23
0.00
-0.30
2.64
1.56
6.19
2.63
1.40
4.92
0.34
2.34
1.90
2.84
4.28
6.62
6.10
2.71
2.07
Hydrolysis Rate Constants0
Acid
Catalyzed (Ka)
(1/mol/yr)
0
0
0
0
0
0
0
0
0
0
59
0
0
0
0
0
0
0
0
0
0
0
Neutral
(Kn)
0
0
0
0
0
0
0
62
490,000
0
0
0
0
0
0
0
0
0
0
0
0.0137
0.0051
Base
Catalyzed (Kb)
(1/mol/yr)
0
0
0
0
0
0
0
0
0
0
610
0
0
0
0
0
0
0
0
0
11,300
15,900,000
Diffusion
Coefficient
in Air (Da)
(m2/yr)
144
141
93
263
188
73.5
347
294
225
152
148
152
154
Diffusion
Coefficient in
Water" (Dw)
(m2/yr)
0.0142
0.0138
0.0253
0.0325
0.0308
0.0189
0.0382
0.0344
0.0278
0.0153
0.0148
0.0287
0.0293
Henry's Law
Coefficent
(HLC)
(atm-m3/mol)
5.00E-06
2.60E-06
2.44E-08
3.97E-07
1.63E-08
2.60E-03
1.23E-04
8.88E-06
2.75E-03
1.54E-05
7.92E-05
2.42E-03
3.45E-04
B-8
-------
IWEM Technical Background Document
Appendix B: Chemical Properties
CAS
127-18-4
58-90-2
3689-24-5
7440-28-0
137-26-8
108-88-3
95-80-7
95-53-4
106-49-0
8001-35-2
75-25-2
76-13-1
120-82-1
71-55-6
79-00-5
79-01-6
75-69-4
95-95-4
88-06-2
93-72-1
93-76-5
96-18-4
121-44-8
99-35-4
Constituent Name
Tetrachloroethylene
Tetrachlorophenol 2,3,4,6-
Tetraethyl dithiopyrophosphate
(Sulfotep)
Thallium
Thiram [Thiuram]
Toluene
Toluenediamine 2,4-
Toluidine o-
Toluidine p-
Toxaphene (chlorinated
camphenes)
Tribromomethane (Bromoform)
Trichloro-1 ,2,2-trifluoro-ethane
1,1,2-
Trichlorobenzene 1,2,4-
Trichloroethane 1,1,1-
Trichloroethane 1,1,2-
Trichloroethylene
(Trichloroethylene 1,1,2-)
Trichlorofluoromethane (Freon
11)
Trichlorophenol 2,4,5-
Trichlorophenol 2,4,6-
Trichlorophenoxyjpropionic
acid 2-(2,4,5- (Silvex)
Trichlorophenoxyacetic acid
2,4,5-
Trichloropropane 1,2,3-
Triethylamine
Trinitrobenzene
(Trinitrobenzene 1 ,3,5-) sym-
Molecular
Weight3
(g/mol)
165.83
231.89
322.31
204.38
240.40
92.14
122.17
107.15
107.15
252.73
187.38
181.45
133.40
133.40
131.39
137.37
197.45
197.45
269.51
255.48
147.43
101.20
213.11
Solubilityb
(mg/L)
200
100
25
1E+06
30
526
33,700
16,600
782
0.74
3,100
170
34.6
1,330
4,420
1,100
1,100
1,200
800
140
268
1,750
55,000
350
Log Kocc
(loglmL/g])
2.21
2.32
3.51
2.83
2.43
0.02
1.24
1.24
4.31
2.05
2.97
3.96
2.16
1.73
2.10
2.11
2.93
2.25
1.74
1.43
1.66
1.31
1.05
Hydrolysis Rate Constants0
Acid
Catalyzed (Ka)
(1/mol/yr)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Neutral
(Kn)
0
0
84
0
0
0
0
0
0.070
0
0
0
0.64
2.73E-05
0
0
0
0
0
0
0.0170
0
0
Base
Catalyzed (Kb)
(1/mol/yr)
0
0
9,000,000
0
0
0
0
0
28,000
10,000
0
0
2.40E+06
49,500
0
0
0
0
0
0
3,600
0
0
Diffusion
Coefficient
in Air (Da)
(m2/yr)
159
246
243
228
68.1
113
119
125
204
211
217
207
99
181
209
Diffusion
Coefficient in
Water" (Dw)
(m2/yr)
0.0298
0.0291
0.0282
0.0290
0.0173
0.0328
0.0271
0.0265
0.0303
0.0315
0.0322
0.0319
0.0255
0.0291
0.0247
Henry's Law
Coefficent
(HLC)
(atm-m3/mol)
1.84E-02
6.64E-03
7.92E-10
2.72E-06
6.00E-06
5.35E-04
4.81 E-01
1.42E-03
1.72E-02
9.13E-04
1.03E-02
9.70E-02
7.79E-06
4.09E-04
1.38E-04
B-9
-------
IWEM Technical Background Document
Appendix B: Chemical Properties
CAS
126-72-7
7440-62-2
108-05-4
75-01-4
108-38-3
95-47-6
106-42-3
1330-20-7
7440-66-6
Constituent Name
Tris(2,3-
dibromopropyl)phosphate
Vanadium
Vinyl acetate
Vinyl chloride
Xylene m-
Xylene o-
Xylene p-
Xylenes (total)
Zinc
Molecular
Weight3
(g/mol)
697.61
50.94
86.10
62.50
106.17
106.17
106.17
318.50
65.39
Solubilityb
(mg/L)
8
1E+06
20,000
2,760
161
178
185
175
1E+06
Log Kocc
(loglmL/g])
3.19
0.45
1.04
3.09
3.02
3.12
3.08
Hydrolysis Rate Constants0
Acid
Catalyzed (Ka)
(1/mol/yr)
0
0
0
0
0
0
0
Neutral
(Kn)
0.088
0
0
0
0
0
0
Base
Catalyzed (Kb)
(1/mol/yr)
300,000
0
0
0
0
0
0
Diffusion
Coefficient
in Air (Da)
(m2/yr)
268
337
216
218
216
217
Diffusion
Coefficient in
Water" (Dw)
(m2/yr)
0.0315
0.0378
0.0267
0.0270
0.0267
0.0268
Henry's Law
Coefficent
(HLC)
(atm-m3/mol)
5.11E-04
2.70E-02
7.34E-03
5.19E-03
7.66E-03
6.73E-03
Note: Data sources for chemical property values are indicated in the column headings; exceptions are noted in parentheses for individual chemical values.
a http://chemfinder.cambridgesoft.com (CambridgeSoft)
b U.S. EPA (1997)
c Kollig(1993)
d Calculated based on Water 9 (U.S. EPA, 2001)
e SRC (1999)
f Calculated based on U.S. EPA (2000a)
9 HSDB(NLM,2001)
h Ml DEQ (nd)
' Calculated based on U.S. EPA (1987)
i U.S. EPA (1999)
k U.S. EPA (2000b)
1 Calculated from I using regression equation log[Koc] = 1.029x log[Kow] - 0.18; presented in Table 10.2 of deMarsily (1986)
m Lymanetal. (1990)
B-10
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
Appendix C: Formulation of the Roadway Module
C.1 General Conceptualization
The roadway in the Industrial Waste Management Evaluation Model (IWEM) is conceptualized
as illustrated in Figure C-l, which depicts a typical roadway with a segment constructed with
industrial materials. For the purposes of model simplicity, that segment is assumed to be nearly
linear and thus can be approximated by the straight line segment AB. If the segment to be
modeled is long and meandering, it must be subdivided into several nearly linear segments that
can each be represented by a straight line.
Regional Flow Direction
Receptor
Well
Highway Segment of Interest
____ Linear Approximation of Highway
Segment of Interest
Figure C-1. Atypical roadway with a recycled-material segment.
C-l
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
Figure C-2 shows a typical cross section of a roadway, that may comprise several components
(e.g., lane, shoulder, ditch). For the model, each component was idealized as a column, referred
to henceforth as the roadway-source column. In the vertical direction, as shown in Figure C-3,
each roadway-source column included materials starting vertically upward from a reference
datum (which could be the top of subgrade), to the surface of a pavement(or a road shoulder, an
embankment, a ditch). As shown in Figure C-3, each roadway-source column was underlain by a
corresponding vadose-zone column.
A roadway-source column was assumed to be uniform in terms of parameters and properties
along the length of interest (i.e., the modeled segment shown in Figure C-l). Therefore, a
road way-source column becomes a road way-source strip in three dimensions. Figure C-4 shows
an example of a roadway cross section comprising three roadway-source strips representing,
respectively, a median, a travel lane, and a ditch. Note that a more typical roadway may consist of
up to fifteen roadway-source strips: for example, left shoulder, left-travel lane, median, right-
travel lane, and right shoulder in Figure C-3. More strips are possible to account for drainage
ditches and berms and different configurations of layers; the IWEM roadway module limits the
total number of roadway-source strips to 15. An example of only three roadway-source strips is
used here as a basis for further discussion. Each roadway-source strip may consist of several
layers, depending on how a given roadway was constructed. A travel lane strip may be composed
of a pavement layer (portland cement concrete or asphalt concrete), a base-course layer, a
subbase layer, and a subgrade layer. A median may comprise a base layer, a subbase layer, and a
subgrade layer. An unpaved road shoulder may have only one layera subgrade layer. With this
type of conceptualization, one can easily see that each roadway-source strip was equivalent to the
existing waste management units (WMUs) source modules that are available within EPA's
Composite Model for Leachate Migration with Transformation Products (EPACMTP). However,
the WMUs module in IWEM can accommodate only sources with a square footprint and one
layer.
Traveled Lanes
Traveled Lanes
Ditch
Gutter Drain subgrade
Permeable
Base
Figure C-2. A typical cross section of a roadway.
C-2
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
Source Strip i = 1 Source Strip i = 2 Source Strip i =3 Source Strip i = 4
Ditch & Left Shoulder Left Lane Right Lane Right Shoulder
n
JJ
Vadose Zone 1
r\
<>
Vadose Zone 2
n
^u^
Vadose Zone 3
_n_
^u^
Vadose Zone 4
Figure C-3. Modules of IWEM corresponding to multiple roadway-source strips.
Median
Travel Lane
Road Shoulder,
'Embankment, and Ditch
^
\
/
\
/
Sou
SSS V>V> V
Layers
Layer 2
Layer 1
W3
Pavement Layer 4
Base .-X'X'Xd^yer^-'-
Subbase Layer 2
Subgrade LayeM
W2
^^^]
Layer 1
"
w'
rce Strip 3 Source Strip 2 Source Strip 1
Figure C-4. An example of layering in roadway-source strips.
The roadway axis may not be normal to the regional flow direction. Figure C-5 shows an
idealized (straight line) roadway segment with several laterally contiguous rectangular roadway-
source strips oriented at a positive angle a with respect to a line orthogonal to the regional flow
direction. The current aquifer transport module can handle only the case where a = 0°. To handle
a general case with |n > 0°, the result of the existing aquifer transport module must be modified
after the simulation. A general approach, which is discussed in Section C.2.2.2, is that the
reference x-y coordinate system is transformed into the x'-y' coordinate system, shown in
Figure C-6, which aligns with both the roadway axis and the regional flow direction. A further
transformation to the x"-y" coordinate system (see Figure C-6) results in an orthogonal system
consistent with the existing aquifer transport module for describing the fate and transport of
contaminants in the transformed domain. It should be noted that the IWEM software asks the
user to provide a positive angle 0, which represents the angle between the down-gradient
C-3
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
roadway edge and the regional flow direction and is defined by the relationship 6 = 90° - a. The
equations that define the transformation from the x-y coordinate system to the x"-y" coordinate
system encoded into the IWEM software account for this change in angle representation.
Regional Flow Direction
\
W
Figure C-5. Non-orthogonal source.
Y:
(a) Strip Source i prior to transformation
Ft -
(b) Strip Source! after transformation
X'
x:
-. X"
Figure C-6. Roadway-source strip /(a) in its original form; and (b) after transformation, where Ft is
the plume front position at time t.
C-4
-------
IWEM Technical Background Document Appendix C: Formulation of Roadway Model
C.2 Formulation
A detailed mathematical formulation for the general conceptualization of the roadway, as
described in Section C.I, is presented here. The formulation is based on a number of simplifying
assumptions, which are listed in Section C.2.1. The formulation is presented in Section C.2.2.
C.2.7 List of Assumptions
The general assumptions for EPACMTP (U.S. EPA 2003a, b, c, d) relevant to the IWEM's
roadway module are listed below:
The rate of percolation through the unsaturated zone is constant over time.
Flow and transport through the unsaturated zone is in the vertical, downward direction
and assumed to be one-dimensional; flow is assumed to be steady, and transport is
transient.
Flow through the homogeneous saturated zone is assumed to be uniform, steady, and one-
dimensional; transport is assumed to be three dimensional and transient.
Material properties do not vary in time.
As the primary sources of industrial materials used in roadway construction are combustion
byproducts, the constituents of concern are essentially metals, which generally exhibit non-linear
sorption behavior. EPACMTP provides an analytical solution for one-dimensional transport of a
solute with non-linear sorption (See Appendix B.2 in U.S. EPA [2003a]), which requires the
simplification of the leaching profile to a constant magnitude, finite pulse (i.e., a square pulse).
In addition, a database of empirical non-linear sorption isotherms (See Appendix G in U.S. EPA
[2003a]) developed specifically for combustion byproducts is incorporated into the IWEM
software and available to the user. The metals transport capability provided by EPACMTP and
its accompanying assumptions have governed the development of the roadway module
formulation and the primary assumption that leachate profiles are conceptualized as square
pulses.
For more details on the assumptions incorporated into the EPACMTP, see Section 4.3. In
addition to these general assumptions for EPACMTP, the following IWEM specific assumptions
are used in the formulation:
In the region of interest, the general regional ground water flow pattern is assumed not to
be affected by the presence of a traversing roadway. It follows from this assumption that
infiltration from the traversing roadway is on the same order of magnitude as regional
recharge. Furthermore, the areal coverage of the roadway contributing infiltration is
assumed very small compared to the total regional area contributing recharge, so that any
difference between the infiltration and recharge rates does not significantly influence the
regional flow field.
For a screening-level analysis, lateral communication between roadway-source strips is
assumed to be insignificant.
A single, long-term average infiltration rate is assumed to percolate through each
roadway-source strip.
Leaching begins at the end of pavement construction.
C-5
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
Material properties of each roadway-source strip do not vary in time.
C.2.2 Mathematical Formulation
This section describes the mathematical formulation for multiple
material layers (Section C.2.2.1); source geometry and non-
orthogonality of regional flow field (Section C.2.2.2); multiple
material layers with a drainage system (Section C.2.2.3); runoff
from top of pavement and discharge from a permeable base that
constitutes the drainage system, or drain (Section C.2.2.4); roadside
drainage areas, ditches, or streams (Section C.2.2.5); single and
multiple road segments (Section C.2.2.6); single and dual drainage
systems (Section C.2.2.7); and a discussion on the versatility of the
roadway module (Section C.2.2.8).
The formulation presented
here refers to drainage
systems that are integrated
into the roadway cross-
section as "permeable
bases." The IWEM software
refers to these drainage
systems as "drains." Please
be mindful of these
synonyms when reading
through this section.
C.2.2.1
Multiple Material Layers
For the IWEM roadway source module, a pulse source scenario was assumed based on two
reasons. First, IWEM is a screening-level analysis, therefore the simplicity and conservatism of a
pulse source type is an appropriate assumption. Second, a pulse source is appropriate for metals,
which are anticipated to be the predominant constituents of concern in recycled industrial
materials.
Using Figure C-4 as reference and assuming that leachate is generated from layer k of roadway-
source strip / in a pulse-like manner, the leachate concentration in layer k of roadway source strip
/' can be calculated as follows:
C/"fO
= C^
cr . = o
C°
where:
CL,ki
°L,kl
to,ki
t
tp,ki
where tw tpki+t
(C-l)
Leachate concentration in layer k of roadway-source strip /' (M/L3)
Initial leachate concentration in layer k of roadway-source strip / (M/L3)
Time at which leachate leaves the bottom surface of roadway-source strip /' (T)
Time (T)
Pulse duration for layer k of roadway-source strip /' (T)
Note that in Equation (C-l) and the ensuing equations, constituent indices are dropped for the
sake of generality.
C-6
-------
IWEM Technical Background Document Appendix C: Formulation of Roadway Model
Leachate from a given layer is assumed to leave the bottom surface of roadway- source strip /'
immediately after the pulse from the layer below has completely left the strip. Each layer with
teachable material contributes to the final release, with leachate from the lowest layer leaving the
bottom of the roadway cross-section first.
In this manner, leaching occurs downward in a series of sequential pulses that never overlap or
dilute as dissolved constituent mass migrates to and through the bottom layer. There are no gaps
between pulses. Numbering the layers beginning with the bottom-most layer, it can be stated that
where:
to,ki = Time at which leachate leaves the bottom surface of roadway-source strip /' (T)
t = Time(T)
tpji = Pulse duration for layer y of roadway-source strip /' (T)
With infiltration rate L, the pulse duration for pavement layer k of roadway- source strip /' is given
by
where:
tp,ki = Pulse duration for layer k of roadway-source strip /' (T)
Mo, ki = Initial total mass in pavement layer k of roadway- source strip /' (M)
Wi = Width of roadway- source strip / (L)
L = Length of roadway-source strip /' (L)
I; = Infiltration rate for roadway-source strip / (L/T)
C°L,ki = Initial leachate concentration in layer k of roadway- source strip /' (M/L3)
For a source with multiple layers, the duration of leaching may be derived from the mass balance
principle. The initial total mass of a constituent at the time leaching begins is given by
M0,fo = W, Ldk,, Cltal.k,, Plulk,k,, (C-4)
where:
Mo, ki = Initial total mass in pavement layer k of roadway- source strip /' (M)
w; = Width of roadway- source strip / (L)
L = Length of roadway-source strip /' (L)
dk,i = Thickness of layer k and roadway-source strip / (L)
C° Total, k, i = Initial total constituent concentration (M/M) in layer k and roadway- source strip /'
p°Buik, k, i = Initial bulk density (M/L3) in layer k and roadway-source strip /'
C-7
-------
IWEM Technical Background Document Appendix C: Formulation of Roadway Model
Leachate from layer 1 is assumed to leave the bottom surface of the strip instantaneously at time
= 0, and the pulse duration of layer k is assumed not to exist if the constituent of interest is absent
from layer k. Hence
'- = 0
where:
to.ii = Time at which leachate leaves the bottom surface of roadway-source strip /' (T)
tp,ki = Pulse duration for layer k of roadway-source strip / (T)
C°L,ki = Initial leachate concentration in layer k of roadway- source strip /' (M/L3)
C.2.2.2 Source Geometry and Non-orthogonality of Regional Flow Field
Regional Flow Field
Based on the assumptions that
in the region of interest, the general regional ground water flow pattern is not affected by
the presence of a traversing roadway;
infiltration from the traversing roadway is on the order of regional recharge, and;
the areal coverage of the roadway is very small compared to the total regional area so that
the difference between the infiltration and recharge does not cause significant impact on
the regional flow field; the regional flow field may be approximated by a solution with
infiltration equal to recharge.
As a result of these assumptions, there is no distortion in the flow field in the vicinity of the
roadway.
Treatment of a Rectangular and Non-orthogonal Source
Source Geometry
In the case of roadway-source module, the source is rectangular. It is necessary to re-specify the
source geometry in IWEM, from rectangular to square. Re-specification of the source geometry is
relatively straightforward and is handled as a pre-processing step prior the fate and transport
simulations.
Non-Orthogonality between the Source Orientation and the Regional Flow Field
In general, a roadway may be oriented in such a way that the roadway axis is not orthogonal to
the regional ground water flow direction. To accommodate the non-orthogonality of the roadway
source, it is necessary to transform the transport domain in such a way that the roadway becomes
orthogonal to the regional ground water flow direction and is approximately rectangular in the
transformed domain. Transformation details are given in Appendix F. A summary of the
transformation is given below.
-------
IWEM Technical Background Document Appendix C: Formulation of Roadway Model
For the current aquifer module, the reference frame for transport in a horizontal plane is the
system of x-y axes shown in Figure C-4. The inclination angle, 0, which is equal to 90° - a (a
being the conjugate inclination angle), is incorporated into the analysis via the following
transformation:
C\ (x,y,z,t) = CV (x',y',z,t) = C\ (x",y",z,t) (C-6)
x' = x-ytana,x' > 0; a <
y' = y/cosa (C-7}
x" = x'
y" = y'cosa
where:
C'i = Concentration in the transport domains emanating from roadway-source strip /'
(M/L3)
x = Distance along the flow direction measured from the midpoint of the down
gradient face of the strip of interest (L)
y = Distance normal to the flow direction measured from the midpoint of the down
gradient face of the strip of interest (L)
z = Depth measured from the water table (L)
t = Time(T)
x' = Distance along the flow direction measured from the down gradient face of the
strip of interest in the transformed domain (L)
y' = Distance measured from the x' along the direction parallel to the axis of the
roadway (L)
x" = Distance along the flow direction measured from the down gradient face of the
strip of interest in the transformed domain (L)
y" = Distance normal to the flow direction measured from the midpoint of the down
gradient face of the strip of interest in the transformed domain (L)
a = conjugate inclination angle = 90°- 0 (the inclination angle)
With the transformation in Equations (C-6) and (C-7), the source in Figure C-5 is transformed
into the one shown in Figure C-6. Note also that in the transformed domain, the dimensions of
the roadway source are also accordingly transformed (compare Figures C-5 and C-6). The above
transformation has two limitations: (i) it is an approximate transformation and does not account
for source end effects, and (ii) the angle of inclination must remain below 90° and therefore, the
regional flow direction is not allowed to be completely parallel to the roadway axis. Suggested
analytical procedures to overcome the end effects, and the case with the angle of inclination, 6,
equal to 0°, are given in Appendix F.
The rationale for the above transformations is given diagrammatically in Figures C.5 and C.6.
The objective of these transformations is to take advantage of the existing IWEM flow and
transport modules developed for a rectangular source in a rectangular coordinate system, where
the width of the source is aligned along the flow direction and its length normal to the flow
direction. In Figure C-5, the transformation from the x-y to the x'-y' coordinate systems is to
render the y' axis parallel to the roadway axis. In Figure C-6, a front location at time t in the x"-y"
C-9
-------
IWEM Technical Background Document Appendix C: Formulation of Roadway Model
coordinate system and the corresponding front in the x'-y' coordinate system are shown. Based on
the corresponding front locations, it can be stated without proof that the transformation is valid
for advection-dominated systems. When a roadway of interest is very long (length » width),
lateral dispersion across stream tubes is expected to be relatively small as the lateral
concentration gradient approaches zero. In addition, when the roadway is very long, the end
effects (in Figure C-6, the ends of the roadway segment are not parallel to the x, x', and x" axes)
are relatively small and, as a result, errors arising from the orientation of the roadway ends are
relatively small compared to the amount of mass released by the entire length of the roadway.
It should be noted also that contaminant fluxes from each roadway-source strip are determined
using strip-specific infiltration. Modifications to IWEM will manage roadway-source strip-
specific fluxes and coordinate the presentation of these data to EPACMTP.
In Figure C-6, one can recognize that the configuration conforms to the existing aquifer module.
However, it will be necessary to modify the inputs to EPACMTP and the simulated results of
EPACMTP to include coordinate and dimensional transformations.
The enhanced transport module was verified against a numerical model. Verification results are
given in Appendix G
C.2.2.3 Multiple Material Layers with a Drainage System
Subsurface drainage systems were originally included in roadway cross-sections to alleviate
moisture-related stress in pavements such as rutting, stripping,
cracking, and pumping. As a result, the roadway pavement life
could be extended threefold by proper installation and maintenance
of subsurface pavement drainage systems (Christopher and
McGuffey, 1997; Apul et al., 2002). Many states adopted the use of
such drain systems such as permeable bases to remove water that
percolated through pavement.
In IWEM, a ditch is a required
element in a roadway design if
a drainage system is included.
The ditch serves as the
destination of any waters
diverted through the drainage
system, as well as any runoff
not captured by a gutter.
Ditches are discussed in
Section C.2.2.4.
Recently, the introduction of industrial materials, with the potential
to release constituents into the environment, into the design and construction of new roadways
created additional motivation to consider including controlled diversion of moisture away from
the roadway. In addition, permeable bases could also serve as a leachate capturing system to
divert water percolating through the cracks of a pavement made with industrial materials. This
will help prevent leachate from infiltrating further into the unsaturated zone. Captured leachate
can be channeled from permeable base into localized bioretention facilities, where it can be
treated before the water allowed to infiltrate into the soil. The roadway module in IWEM
provides the option to integrate the permeable drainage layers as part of the roadway design
scenario, while accounting lateral transport of contaminants.
A typical permeable base pavement section, shown in Figure C-7, is provided to help visualize
pavement drainage components. A common design approach in subsurface drainage systems is
the installation of a permeable base, which serves to remove infiltration water. This highly
permeable layer is at least 7 to 10 centimeters thick and extends under the full width of the
roadway exposed to traffic loads. Permeable bases are used in both Portland cement concrete and
asphalt concrete pavements (see Figure C-8). The permeable base may be located just above the
C-10
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IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
subgrade or above the base (Van Sambeek, 1989). The permeable layer may also be used without
another base. A filter reinforcement layer is typically placed between the permeable base and
natural soils to prevent infiltration of fines into the subbase and the migration of subbase into the
permeable base. In the field, a filter layer may consist of a dense-graded subbase, or geotextile
may be used as the filter layer (Christopher, 1998).
A Base is used as the drainage layer
Drainage layer
as a base
B Drainage layer is part of or below the subbase.
Drainage layer
as part of or
b*low the subbase
Figure C-7. Typical permeable base pavement sections: (a) base used as drainage layer (b)
drainage layer is part of or below the subbase (from AASHTO, 1993).
Filter
Median
Base
Pavement
Base
Permeable Base
Subgrade
«*
CollectorPipe
Exfiltration from Subgrade
Runoff
QRO
Collected leachate
Exfiltration from Embankment
Figure C-8. General configuration of the highway module with a drainage system.
C-ll
-------
IWEM Technical Background Document Appendix C: Formulation of Roadway Model
In Figure C-8, a pavement underlain by a drainage system, as in Figure C-4, is represented by
contiguous columns. However, in this case, the base course is underlain by a drainage system
comprising a permeable base on top of a filter layer, which rests above a subgrade layer of the
same material. Although it is not depicted in Figure C-8, a roadside ditch is required to receive
runoff and leachate collected by the permeable base.
At any given time, t, the contaminant flux that enters the permeable base layer, is described by
FPB = LWt ^Clk\H(t-tOM}-H\t-\tOM+tp^} (C-8)
i=l k=\
and the average infiltration rate from all columns is given by
(C-9)
where:
FPB = Mass flux from pavement layers above the permeable base (M/T)
Nc = Number of columns above the permeable base (dimensionless)
L = Length of roadway-source strip /' (L)
Wi = Width of roadway- source strip /' (L)
Ii = Infiltration rate for roadway-source strip /' (L/T)
NL = Number of layers in respective column above the permeable base (dimensionless)
C°L,ki = Initial leachate concentration in layer k of roadway- source strip /' (M/L3)
H(t) = Heaviside's unit function, = 0 when t < 0, = 1 when t > 0 (dimensionless)
IPB = Average infiltration rate pavement columns above the permeable base (L/T)
Equation C-8 is derived based on the following in assumptions:
Leachate from a given layer is assumed to leave the bottom of the layer just above the
permeable base immediately after the pulse from the layer below has completely left the
same layer just above the permeable base;
Any layers that do not contribute to leachate concentrations are ignored; and
Water and mass flux are constant for each individual pulse from a layer.
Equation C-9 represents a weighted average of infiltration rates through each strip above a drain.
In the event that the layer beneath the permeable base has relatively low hydraulic conductivity,
the mass in the permeable base will be transported laterally to a collector pipe. In this case, the
infiltration from the permeable base to the layer below is limited by the hydraulic conductivity of
the underlying layer. The water flow rate diverted to the collector pipe is given by
QPB =lPB-l'PBLl (C-10)
!=1
where:
QPB = Water flux from collector pipe in the permeable base (L3/T)
IPB = Infiltration rate from the layer above permeable base to the permeable base (L/T)
I'PB = Infiltration rate from the permeable base to the subgrade layer below (L/T)
C-12
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IWEM Technical Background Document Appendix C: Formulation of Roadway Model
= KSG, if IPB> KSG
L = Length of roadway-source strip / (L)
Wi = Width of roadway-source strip /' (L)
With infiltration rate IPB from the layer above, the pulse duration for the permeable base is given
by
P'PB~
Nc
\ /t T ^ ' J f~* 0 0
"^0,PB ~ ^2-lW' PB ^-Total.PB PsulkfB
i=\
where:
tp PB = Time required for the contaminant to completely leach from the permeable base
(T)
Mo, PB = Initial total mass in permeable base (M)
IPB = Infiltration rate from the layer above the permeable base to the permeable base
(L/T)
L = Length of roadway-source strip /' (L)
Wi = Width of roadway-source strip / (L)
C°L,PB = Initial leachate concentration in the permeable base (M/L3)
dPB = Thickness of permeable base (L)
C°Total, PB = Initial total constituent concentration in permeable base (M/M)
p°Buik, PB = Initial bulk density in permeable base (M/L3)
The pulse duration, Equation C-l 1, is derived by dividing the total mass of constituent in the
permeable base by the rate at which mass leaves the permeable base. The mass flux is equal to
the product of the per-unit-area infiltration rate, the area through which infiltration passes and the
leachate concentration from the permeable base.
Equation C-12 defines the total mass of constituent in the permeable base as the product of the
volume of material used in the permeable base and the initial total concentration of teachable
constituent in the material.
The concentration of contaminant that leaves the permeable base via the collector pipe or to the
layer below is estimated from the mass flux from the pavement (Equation C-8) divided by the
water flux into the permeable base from the layers above, and the leachate flux from the
materials in the permeable base, thus
t-PB ~
£>,./,. i=1 k=l (C-l3a)
z=l
+
The time-average concentration for CPB is
C-13
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IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
1PB
P,PB
(C-13b)
CPB = 0, t > AtPB
(C-13c)
where:
CPB = Concentration of efflux from the permeable base (M/L3)
tpB = Concentration averaging period (T)
CPB = Time-average concentration of efflux from the permeable base (M/L3)
Similar to the derivation in the previous section, with NLB layers below the permeable base, the
time taken to leach out contaminant from all the layers below the permeable base is given by
La
ISG ~ / Jn.SG.i
j ^
= I<
p,SG,m
t
M,
O.SG.j
P,SG,j
Oj ^Total,SG,jPBulk,SG,j
(C-14)
(C-15)
(C-16)
(C-17)
where:
tso =
to,soj
Mo, SGJ
C° Total, SGJ =
p°Buik, SGJ =
Time required for the contaminant immediately below the permeable base to
travel to the bottommost extent of the subgrade layer (T)
Time required for the contaminant to completely leach from subgrade layer y' (T)
Time at which leachate from subgrade layer y' begins to leave the bottom surface
of the subgrade (T)
Initial total mass in subgrade layer y' (M)
Thickness of subgrade layer y' (L)
Initial total constituent concentration (M/M) in subgrade layer y'
Initial bulk density (M/L3) in subgrade layery
C-14
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IWEM Technical Background Document Appendix C: Formulation of Roadway Model
With NLB subgrade layers below the permeable base, the concentration that leaves to bottom of
the subgrade layer is given by
CSG = -
so
+ c° (ti(t }-n(t-t
T ^L,raV VSG/ JJ\
+ ICM- w)- 4 -tw, + w
>=i
where:
= Concentration that leaves to the bottom of the subgrade layer (M/L3)
Nc = Number of columns above the permeable base (dimensionless)
Wi = Width of roadway- source strip / (L)
Ii = Infiltration rate for roadway-source strip / (L/T)
NL = Number of layers in respective column above the permeable base (dimensionless)
C°L,ki = Initial leachate concentration in layer k of roadway- source strip /' (M/L3)
H(t) = Heaviside's unit function, = 0 when t < 0, = 1 when t > 0 (dimensionless)
tso = Time required for the contaminant immediately below the permeable base to
travel to the bottommost extent of the subgrade layer (T)
tP,pB = Time required for the contaminant to completely leach from permeable base (T)
to,h = Time at which leachate leaves layer k of roadway-source strip / (T)
tp,h = Pulse duration for layer k of roadway-source strip / (T)
C°L,PB = Initial leachate concentration in the permeable base (M/L3)
NLB = Number of layers in respective column below the permeable base (dimensionless)
C°LJ = Initial leachate concentration in subgrade layer y (M/L3)
to,SGj = Time at which leachate from subgrade layer y' begins to leave the bottom surface
of the subgrade (T)
tp.SGj = Time required for the contaminant to completely leach from subgrade layer y' (T)
Note that in Equation (C-18), the travel time of all pulses above the filter layer (immediately
below the permeable base) is delayed by tso + tp, PB and the travel time of the pulse from the
permeable base is delayed by tp, PB.
C-15
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IWEM Technical Background Document Appendix C: Formulation of Roadway Model
C. 2.2.4 Runoff from Top of Pavement and Discharge from Permeable Base
As shown in Figure C-7, runoff from the pavement surface and collected leachate from the
permeable base may drain into a roadside ditch. The runoff is divided into two categories:
divertible and indivertible. The divertible runoff flux from the pavement is estimated from
where:
QDRO = Divertible runoff water flux before a gutter (L3/T)
NCBG = Number of strips with divertible runoff to gutter
ROi = Runoff rate per unit area of strip /' (L/T)
w; = Width of roadway- source strip /' (L)
L = Length of roadway-source strip /' (L)
Parts of the surface runoff and permeable base fluxes may also be diverted or lost. For the surface
runoff, the total flux that reaches the roadside ditch consists of the remainder of divertible runoff
and indivertible runoff. The effective runoff flux and collected leachate flux from the permeable
base that may reach the roadside ditch, as shown in Figure C-9, may be described by:
& DRO
QRO = QDRO +Sr RO, xW]xL (C-20)
Q PB = ^PB X QpB
where:
Q 'DRO = Remaining divertible runoff water flux (L3/T) that reaches the ditch
KDRO = Divertible runoff coefficient (dimensionless), varying from 0 to 1 .
QDRO = Divertible runoff water flux before a gutter (L3/T)
Q'RO = Total effective runoff water flux (L3/T) that reaches the ditch
NCAG = Number of strips with indivertible runoff
ROi = Runoff rate per unit area of strip /' (L/T)
w; = Width of roadway- source strip /' (L)
L = Length of roadway-source strip / (L)
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
KPB = Water flux coefficient (dimensionless), varying from 0 to 1
QPB = Water flux from collector pipe in the permeable base (L3/T)
Note that equations C-19 and C-20 are not used if no drain is specified in the roadway design.
C-16
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
Flow into ditch
Diverted
runoff
Divertable runoff K
1
QDRO
>
V
i
Water flux from
collector pipe in
permeable base
1
QPB
l\
>
V
Gutter
DRO ~ Q DRO
Remaining
divertable runoff K
Q DRO
Indivertable runoff.
iv
\y
Q R0~ Q DRO
Water flux from
collector pipe in
permeable base that
reaches ditch K
i |N
Q'PB
Diverted leachate
from collector pipe in
permeable base
. PB
C.2.2.5
Flow out of ditch
Figure C-9. Paths for runoff and collected leachate from permeable base.
Roadside Drainage Areas, Ditches, or Streams
A typical roadway may be flanked by a drainage ditch or a stream with flowing/stagnant water on
either side. In this document, the term ditch will be used throughout. A typical ditch cross section
is shown in Figure C-10, along with mass influxes and effluxes. The ditch is assumed to be
adjacent to the embankment of a road segment, which consists of an assemblage of travel lane
columns and an embankment column (Figure C-ll).
C-17
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
Runoff
Precipitation Evaporation
L
LeachateFrom
drainage
Exfiltration
Figure C-10. Fluxes to and from a ditch and the ditch cross section.
, '
An
assemblai
of paverm
columnsv
a permea
base
Xv2
t
\
i
?e
ant
vith
ble E
V
\
\
.n
WDitch
.s Segment
^
\
\
\
\\
"ibankment
Figure C-11. A plan view showing highway source term components.
The ditch area may or may not be water-filled. In arid areas, the roadside ditch may be mostly
dry. For the roadway analysis, it is assumed that there are two possible roadside sources that
derive contaminant mass from the pavement assemblage: water-filled ditches and dry ditches.
These two source types are discussed below.
C-18
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IWEM Technical Background Document Appendix C: Formulation of Roadway Model
Water-Filled Ditches
Water-filled ditches are assumed to be water-filled on a long-term basis. The water may be
flowing or remain stagnant. Water that flows into the ditches is derived from discharge from the
permeable base and runoff. In this analysis, no contaminant mass is assumed to be transported by
runoff. Water leaves the ditch by outflow to the downstream ditch segment (if the water is
flowing) and by exfiltration. Exfiltration is defined as the process of water percolating down to
the unsaturated zone from the bottom of the ditch.
Based on the principle of mass conservation, it can be stated that
In Out Out oExfil DExfil
where:
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
CPB = Time-average concentration of efflux from the permeable base (M/L3)
Qin = Ditch inflow rate from upstream (L3/T)
Cin = Contaminant concentration of ditch inflow (M/L3)
Qout = Ditch outflow rate to downstream (L3/T)
Cout = Contaminant concentration of ditch outflow (M/L3)
QoExfii = Exfiltration rate from the ditch (L3/T)
fii = Contaminant concentration of ditch exfiltration (M/L3)
Based on an assumption that
(-out = ^DExfll (C-22)
Contaminant concentration in the exfiltration from a given ditch can be described as
r _ QPBcPB+Qm cm
DE*fi' 0+0 (C-23)
\iout ^ ^DExfil ^ '
where:
CDExfu = Contaminant concentration of ditch exfiltration (M/L3)
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
CPB = Time-average concentration of efflux from the permeable base (M/L3)
Qin = Ditch inflow rate from upstream (L3/T)
Cin = Contaminant concentration of ditch inflow (M/L3)
Qout = Ditch outflow rate to downstream (L3/T)
QnExfii = Exfiltration rate from the ditch (L3/T)
Note that in the ditch associated with the upstream-most segment of the ditch, C/« = 0. Qout from
the current segment becomes Qin for the downstream ditch segment.
C-19
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IWEM Technical Background Document Appendix C: Formulation of Roadway Model
Assuming steady-state flow, from the principle of mass conservation, Qout is given by
Qout = Q,n + QRO + QPB + Qprectp ~ QEvap ~ QoExfil (C-24)
where:
Qout = Ditch outflow rate to downstream (L3/T)
Qin = Ditch inflow rate from upstream (L3/T)
Q'RO = Total effective runoff water flux (L3/T) that reaches the ditch
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
Qpreap = Precipitation flux (L3/T)
= Evaporation flux (L3/T)
fu = Exfiltration rate from the ditch (L3/T)
The evaporation, precipitation, and exfiltration fluxes are determined from
(C-25)
(C-26)
where:
QEvap = Evaporation flux (L3/T)
Qpredp = Precipitation flux (L3/T)
w Ditch = Ditch width (L)
L = Length of ditch (L)
Ep = Evaporation rate over the ditch (L/T)
P = Precipitation rate over the ditch (L/T)
Assuming that the water table is located below the bottom layer underlying the ditch, QDEX/H is
approximated by
J-f -4-T
n -M, T v str Bed
^DExfil W Ditch ^ ^Bed (C-27)
*Bed
where:
QnExfii = Exfiltration rate from the ditch (L3/T)
w Ditch = Ditch width (L)
L = Length of ditch (L)
Ksed = Vertical hydraulic conductivity of the ditch bed (L/T)
Hstr = Depth of water in the ditch (L)
Tsed = Thickness of the ditch bed (L)
C-20
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IWEM Technical Background Document Appendix C: Formulation of Roadway Model
Of interest is the case in which Qin = Qout, using Equations (C-24) and (C-27), one obtains
HStr = (QRO + QPB + Qprec,p ~ Qsvap } ^ TBed (C-28a)
WDitch L^Bed
where:
Hstr = Depth of water in the ditch (L)
Q'RO = Total effective runoff water flux (L3/T) that reaches the ditch
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
= Precipitation flux (L3/T)
= Evaporation flux (L3/T)
TBed = Thickness of the ditch bed (L)
wmtch = Ditch width (L)
L = Length of ditch (L)
Ksed = Vertical hydraulic conductivity of the ditch bed (L/T)
To safeguard against possibly unrealistic values ofHstr, the estimated water depth is limited to
HstrLimit. In the event thatHstr is negative, the ditch is regarded as dry (see subsection below on
dry ditches), and the following equations are not applicable. In the event thatHstr, is limited to
HStrUmt, QDE&I is given by:
QDW = QRO +QPB +QPrecip ~ QEmp (C-28b)
where:
QoExfii = Exfiltration rate from the ditch (L3/T)
Q'RO = Total effective runoff water flux (L3/T) that reaches the ditch
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
Qpredp = Precipitation flux (L3/T)
Qsvap = Evaporation flux (L3/T)
Qin and Qout may be may be calculated from cross-sectional average velocity along the ditch,
which, in turn, may be estimated using Manning's equation, thus
V =-R°-661 S°-5 (C-29)
n
where:
V = Cross-section average water velocity (L/T)
k = Conversion factor = 1 if Fis in m/s, = 1.486 for Fin ft/s
n = Manning's coefficient
R = Hydraulic radius (area/wetted perimeter) (L)
S = Slope of the water surface (dimensionless)
S is assumed to be equal to streambed slope and should be set to zero in the case of stagnant
water.
C-21
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IWEM Technical Background Document Appendix C: Formulation of Roadway Model
Hydraulic radius R is given by
where:
R = Hydraulic radius (area/wetted perimeter) (L)
w Ditch = Ditch width (L)
Hstr = Depth of water in the ditch (L)
R = WD,tch HStr (C-30)
Based on the assumption that Qin = Qout, the water flux along the ditch is determined from
(C-31)
Qln = Qout = V W Ditch H Str
where:
Qin = Ditch inflow rate from upstream (L3/T)
Qout = Ditch outflow rate to downstream (L3/T)
V = Cross-section average water velocity (L/T)
w mtch = Ditch width (L)
Hstr = Depth of water in the ditch (L)
Dry Ditches
In arid areas where the roadside ditch is mostly dry, it is likely that QDEX/H is negative. In the
event that QDEX/II is negative, it is assumed that the some or all Q 'RO and Q 'PB are discharged to
the roadside without forming a surface water body. It is also assumed that the infiltration can be
estimated using the following relationship:
RO ' ^PB ' ^Preap = ^Prec,p (C-32)
where:
Q'RO = Total effective runoff water flux (L3/T) that reaches the ditch
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
= Precipitation flux (L3/T)
= Exfiltration rate from the ditch (L3/T)
= Recharge flux (L3/T)
QRech is calculated as
(C-
where:
QRech = Recharge flux (L3/T)
RRech = Recharge flux per unit area (L/T)
w Ditch = Ditch width (L)
L = Length of ditch (L)
C-22
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IWEM Technical Background Document Appendix C: Formulation of Roadway Model
Note that RRech is available in the EPACMTP database.
From Equation (C-32), one obtains the following:
O -(O' +O< +O } Rech -O (C-34}
^DExfil ~ \^ RO^V- PB^^-Precip) ^Rech \^J^J
\precip
where:
QnExfii = Exfiltration rate from the ditch (L3/T)
Q'RO = Total effective runoff water flux (L3/T) that reaches the ditch
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
Qpreap = Precipitation flux (L3/T)
Q,Rech = Recharge flux (L3/T)
However, QDE^U is limited to
QDE&I ^WD,tchLKTopSal (C-35)
where:
QnExfii = Exfiltration rate from the ditch (L3/T)
w Ditch = Ditch width (L)
L = Length of ditch (L)
KropSoii = Vertical hydraulic conductivity of the top soil (L/T)
In the case that QDEX/II from Equation (C-33) is greater than the right-hand side of Equation (C-
35), it is limited to the right-hand side of Equation (C-34), and the excess flux is assumed lost to
evaporation. The resultant contaminant concentration in the ditch area is estimated using QDEX/H
from Equation (C-35), thus
n" (C-36)
\2oExfil ^^Rech
where:
Contaminant concentration of ditch exfiltration (M/L3) the ditch
CPB = Time-average concentration of efflux from the permeable base (M/L3)
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
= Exfiltration rate from the ditch (L3/T)
= Recharge flux (L3/T)
C.2.2.6 Single and Multiple Road Segments
Single Road Segment
Each roadway- source strip is treated as an individual source, and a number of individual
EPACMTP simulations for all the strips and layers must be performed. The fate and transport
effects observed at receptor locations are estimated by summing the 90th percentile exposure
concentrations for all layers for each constituent. These strips and layers constitute a composite
C-23
-------
IWEM Technical Background Document Appendix C: Formulation of Roadway Model
source or a single roadway segment source. The summing of contributions from all strips and
layers for a single road segment source is managed by IWEM. The maximum value of the
resulting aggregate exposure concentration for each constituent is limited by the maximum
leachate concentration provided by the user for that constituent.
Multiple Road Segments
If several road segments are modeled, each segment must be treated as a composite source
consisting of strips and layers, as described above. Contaminant concentrations at receptor wells
resulting from contributions from all composite sources are determined by the method of
superposition. In other words, concentration of a contaminant at a receptor well location is the
sum of concentration contributions from all segments. Aggregation of concentrations from all
segments must be performed by the user outside IWEM.
The road constructed with recycled materials (with contaminants) may be longitudinally divided
into several contiguous segments, as shown in Figure C-12. To account for contaminants carried
by the flowing water in the roadside ditch, additional ditch segments farther downstream than the
road segments may be required. Between two connecting ditch segments, Cout of the upstream
segment becomes Cin for the downstream segment. In other words,
CM = Cl CC-37")
^Out ^In \^ J ' )
where:
C1'1 out = Concentration in the outflow of the /-1st ditch segment (M/L3)
C1 in = Concentration in the inflow of the /-th ditch segment (M/L3)
Assuming that there is no influx of contaminant at the upstream-most segment of the ditch, using
Equation (C-23) recursively, it can be shown that:
For the first segment:
n
i -
t-PB ^ PB
Qout + QL
For the second segment:
^ QlnQPBCPB+QPBCPB(Qout
For the w-th segment:
QPB 'CpB X \Q"nl+l (Qout + GDBX i Yl 1
(C-38c)
\^out ii
where:
C^ut = Concentration in the outflow of the 1st ditch segment (M/L3)
C2out = Concentration in the outflow of the 2nd ditch segment (M/L3)
C"out = Concentration in the outflow of the nth ditch segment (M/L3)
C-24
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IWEM Technical Background Document Appendix C: Formulation of Roadway Model
Q 'PB = Effective water flux from collector pipe in the permeable base (L3/T) that reaches
the ditch
CPB = Time-average concentration of efflux from the permeable base (M/L3)
= Ditch outflow rate to downstream (L3/T)
= Exfiltration rate from the ditch (L3/T)
Qin = Ditch inflow rate from upstream (L3/T)
The required number of ditch segments beyond the road segments may be user-defined or
dependent on pre-specified criteria. All the segments are assumed to be subject to the same water
fluxes (runoff and permeable base discharge) from all road segments. As shown in Figure C-12,
in each segment, the effective runoff and permeable base fluxes are the same for all segments;
however, the concentration from the permeable base, CPB, is zero in road segments constructed
by conventional materials (without contaminants).
Let Cnout(t) be the outflow concentration in the ditch segment corresponding to the last road
segment (w-th segment). As the contaminant moves through the ditch, its concentration
continually and gradually diminishes through the loss to the ditch exfiltration and through the
addition of contaminant-free water from runoff, precipitation, and permeable base. With the
addition of Lsegment ditch segments beyond the road segments, using the mass conservation
equation (Equation C-23) with CPB = 0 recursively, the exfiltration concentration in this segment
may be shown to be
( O
/"* ^Segment Z-> In
where:
DExfli = Exfiltration concentration in the ditch segment beyond the road segment (M/L3)
Qin = Ditch inflow rate from upstream (L3/T)
Qout = Ditch outflow rate to downstream (L3/T)
= Exfiltration rate from the ditch (L3/T)
= Number of ditch segments beyond the road segments (unitless)
Cnout(t) = Outflow concentration in the ditch segment corresponding to the last (wth) road
segment (M/L3)
In order that the following condition is satisfied:
where:
DExfli = Exfiltration concentration in the ditch segment beyond the road segment (M/L3)
Cent = Criterion for exfiltration concentration (M/L3)
C-25
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
The minimum number of ditch segments beyond the road segments must be at least
L
Segment
log
log
rn
^Out
ccnt_
Qln
^2out + ^DExfil
(C-41)
where:
= Number of ditch segments beyond the road segments (unitless)
C"out = Outflow concentration in the ditch segment corresponding to the last (nth) road
segment (M/L3)
Cent = Criterion for exfiltration concentration (M/L3)
Qin = Ditch inflow rate from upstream (L3/T)
Qout = Ditch outflow rate to downstream (L3/T)
fn = Exfiltration rate from the ditch (L3/T)
Roadway
Segment
Lsegment segments of ditch
beyond the roadway
segments constructed
with recycled materials
Ditch
Segment
Flow
Direction
Legends
Q'PB with CPB = 0
Q'PB with CPB ...0
Pavement
Embankment
Figure C-12. Concatenated roadway segments and ditch segments (with and without roadway
segments).
C-26
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
C.2.2.7
Single and Dual Drainage Systems
The derivation in the foregoing subsection assumes that there is only one drainage ditch along the
roadway. In the event that the roadway is flanked by two drainage ditches, one on either side, it is
assumed that the two ditches receive drainage from two different permeable bases, as shown in
Figure C-13. It should be noted that the number of permeable bases is restricted to two. Runoff
and drainage in two different systems is directed to respective drainage ditches.
C.2.2.8
Versatility of the IWEM Roadway Module
The IWEM Roadway module may be utilized to simulate leachate from a major berm or
embankment constructed using recycled materials, as shown in Figure C-14. In the case shown
in the figure, the berm may be represented by a single or multiple columns. Ditch-side berms, as
shown in Figure C-15, may also be included in the roadway analyses. Other similar structural
fills, such as backfills of retaining structures, and landfill caps may also be analyzed using the
IWEM module.
Collected
leachate Subgrade
Permeable
Base
Collected
leachate
Figure C-13. A symmetric roadway segment with two symmetric assemblages with identical
properties and drainage configurations.
C-27
-------
IWEM Technical Background Document
Appendix C: Formulation of Roadway Model
Stabilized Soil/
Berm/Embankment
Native Soil
ltration
Leachate^^^'
Exfiltration
with
Figure C-14. A major berm/embankment.
Infiltration/exfiltration
without leachate
Exfiltration with
leachate
Non-recycled
material
Recycled material
Highly permeable
material
Drain/gutter
Groundwater Flow
Strip numbering conforms to IWEM specifications - beginning numbering at the down-gradient edge.
IWEM limits the maximum number of strips and layers to 15 and 5, respectively.
Figure C-15. An asymmetric roadway segment with two drainage configurations and a ditch-side
berm (represented by Column 1).
C-28
-------
IWEM Technical Background Document Appendix C: Formulation of Roadway Model
C.3 References
AASHTO (American Association of State Highway and Transportation Officials).
1993. AASHTO Guide for Design of Pavement Structures. American Association of State
Highway and Transportation Officials, Washington, D.C.
Christopher, B.R. 1998. Performance of drainable pavement systems: A U.S. perspective.
Pp. 205-215 in E. J. Hoppe, editor. International Symposium on Subdrainage in
Roadway Pavements and Subgrades. Grafistaff, Granada, Spain.
Christopher, B.R., and V.C. McGuffey. 1997. National Cooperative Highway Research Program
Synthesis of Highway Practice 239. Pavement Drainage Systems. NCHRP.
Van Sambeek, RJ. 1989. Synthesis on Subsurface Drainage of Water Infiltrating a Pavement
Structure. St. Paul: Braun Pavement Technologies.
U.S. EPA (Environmental Protection Agency). 2003a. EPA's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Technical Background Document.
Office of Solid Waste, EPA530-R-03-002, April 2003.
U.S. EPA (Environmental Protection Agency). 2003b. EPA's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Parameter/Data Background
Document. Office of Solid Waste, EPA530-R-03-003, April 2003.
U.S. EPA (Environmental Protection Agency). 2003c. EPA's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Draft Addendum to the Technical
Background Document. Office of Solid Waste. September 2003.
U.S. EPA (Environmental Protection Agency). 2003d. EPA's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Draft Addendum to the
Parameter/Data Background Document. Office of Solid Waste. April 2003.
C-29
-------
IWEM Technical Background Document Appendix C: Formulation of Roadway Model
[This page intentionally left blank.]
C-30
-------
IWEM Technical Background Document Appendix D: Infiltration Rate Data for WMUs
Appendix D: Infiltration Rate Data for WMUs and Structural Fills
This appendix provides the infiltration rates derived with the Hydrologic Evaluation of Landfill
Performance (HELP) model (Schroeder et al, 1994) for landfills (unlined landfill rates are also
used for structural fills) (Table D-l), waste piles (Table D-2), and land application units (Table
D-3). In addition, Tables D-4 and D-5 provide, respectively, the flow rate data from (TetraTech,
2001) used to develop infiltration rates for composite liners for landfills and waste piles and the
leak density data (TetraTech, 2001) used to develop infiltration rates for composite liners for
surface impoundments. Table D-6 presents a comparison of composite liner infiltration rates
using different methods for landfills. Figure D-l shows that comparison graphically. For the
interested reader, a detailed description of how the HELP model was used to develop infiltration
and recharge rates is provided in Appendix A of the EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Parameter/Data Background Document
(U.S. EPA, 2003).
References
Eithe, A. W., and G.R. Koerner. 1997. Assessment of HOPE Geomembrane Performance in
Municipal Waste Landfill Double Liner System after Eight Years of Service. Geotextiles
and Geomembranes, Vol. 15, pp. 277-287.
Laine, D.L. 1991. Analysis of Pinhole Seam Leaks Located in Geomembrane Liners Using the
Electrical Leak Location Method. Proceedings, Geosynthetics '91, pp.239-253.
McQuade, S.J., and A.D. Needham. 1999. Geomembrane Liner Defects - Causes, Frequency and
Avoidance. GeotechnicalEngineering., Vol., 137. No. 4, pp. 203-213.
Rollin, A.L., M. Marcotte, T. Jacquelin, and L. Chaput. 1999. Leak Location in Exposed
Geomembrane Liners Using an Electrical Leak Detection Technique. Geosynthetics '99:
Specifying Geosynthetics and Developing Design Details, Vol. 2, pp 615-626.
Schroeder, P.R., T.S., Dozier, P.A. Zappi, B.M. McEnroe, J. W. Sjostrom, and R.L. Peton. 1994.
The Hydrologic Evaluation of Landfill Performance Model (HELP): Engineering
Documentation for Version 3. EPA/600/R-94/168b. U.S. EPA, Office of Research and
Development, Washington, DC.
TetraTech, Inc. 2001. Characterization of Infiltration Rate Data to Support Groundwater
Modeling Efforts (Draft). Prepared for the U.S. Environmental Protection Agency, Office
of Solid Waste, Contract No. 68-W6-0061, May, 2001.
U.S. EPA (Environmental Protection Agency). 1998. Assessment and Recommendations for
Optimal Performance of Waste Containment Systems. Office of Research and
Development, Cincinnati, Ohio.
U.S. EPA (Environmental Protection Agency). 2003. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Parameter/Data Background
Document. Office of Solid Waste, EPA530-R-03-003, April 2003.
D-l
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
Table D-1. HELP-Derived Infiltration Rates for Landfills and Structural Fills (m/yr)
ID
19
98
82
95
62
44
99
7
2
67
75
35
43
41
18
86
93
10
42
74
52
55
51
38
36
3
53
29
32
54
88
23
16
100
28
City
Albuquerque
Annette
Astoria
Atlanta
Augusta
Bangor
Bethel
Bismarck
Boise
Boston
Bridgeport
Brownsville
Burlington
Caribou
Cedar City
Central Park
Charleston
Cheyenne
Chicago
Cincinnati
Cleveland
Columbia
Columbus
Concord
Dallas
Denver
Des Moines
Dodge City
E. Lansing
E. St. Louis
Edison
El Paso
Ely
Fairbanks
Flagstaff
State
NM
AK
OR
GA
ME
ME
AK
ND
ID
MA
CT
TX
VT
ME
UT
NY
SC
WY
IL
OH
OH
MO
OH
NH
TX
CO
IA
KS
Ml
IL
NJ
TX
NV
AK
AZ
No Liner (landfills, structural fills)
Silt loam
O.OOE+00
1.68E+00
1.08E+00
3.42E-01
2.12E-01
1.47E-01
5.64E-02
2.39E-02
8.00E-04
2.33E-01
1.95E-01
5.49E-02
1.36E-01
1.08E-01
O.OOE+00
3.36E-01
2.61 E-01
5.00E-04
7.98E-02
1.55E-01
7.80E-02
1.53E-01
7.65E-02
1.59E-01
5.99E-02
8.00E-04
1.14E-01
1.35E-02
1.09E-01
1.44E-01
3.12E-01
7.60E-03
O.OOE+00
1.04E-02
2.39E-02
Sandy
loam
O.OOE+00
1.84E+00
1.15E+00
3.99E-01
2.70E-01
2.05E-01
7.21 E-02
3.00E-02
9.40E-03
2.38E-01
2.46E-01
1.05E-01
1.78E-01
1.49E-01
8.00E-04
4.17E-01
3.29E-01
1.30E-03
1.14E-01
2.21 E-01
1.21 E-01
1.99E-01
1.16E-01
2.06E-01
1.07E-01
8.00E-04
1.64E-01
3.45E-02
1.45E-01
1.68E-01
3.91 E-01
1.30E-02
O.OOE+00
2.34E-02
6.30E-02
Silty clay
loam
3.00E-04
1.46E+00
9.65E-01
2.82E-01
1.67E-01
1.23E-01
5.54E-02
1.96E-02
3.80E-03
1.54E-01
1.62E-01
3.84E-02
1.17E-01
8.86E-02
O.OOE+00
2.74E-01
2.12E-01
8.60E-03
6.20E-02
1.54E-01
8.23E-02
1.22E-01
6.63E-02
1.37E-01
5.31 E-02
3.60E-03
1.16E-01
2.26E-02
1.10E-01
7.04E-02
2.49E-01
8.10E-03
3.00E-04
1.17E-02
2.26E-02
Clay
Liner
(landfills)
O.OOE+00
3.38E-02
5.26E-02
4.77E-02
4.45E-02
4.32E-02
2.95E-02
1.88E-02
4.61 E-03
4.45E-02
4.44E-02
2.41 E-02
4.32E-02
4.32E-02
6.69E-05
4.86E-02
4.77E-02
2.38E-05
4.32E-02
4.44E-02
4.09E-02
4.09E-02
4.09E-02
4.32E-02
2.41 E-02
1.83E-05
4.09E-02
9.44E-03
3.74E-02
4.09E-02
4.86E-02
1.03E-04
3.54E-05
9.40E-03
2.41 E-02
D-2
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
ID
1
6
27
4
25
77
59
101
73
66
78
85
96
11
20
87
90
12
69
50
24
97
30
47
65
89
83
92
70
80
33
37
76
71
21
39
City
Fresno
Glasgow
Grand Island
Grand Junction
Great Falls
Greensboro
Hartford
Honolulu
Indianapolis
Ithaca
Jacksonville
Knoxville
Lake Charles
Lander
Las Vegas
Lexington
Little Rock
Los Angeles
Lynchburg
Madison
Medford
Miami
Midland
Montpelier
Nashua
Nashville
New Haven
New Orleans
New York City
Norfolk
North Omaha
Oklahoma City
Orlando
Philadelphia
Phoenix
Pitts burg
State
CA
MT
NE
CO
MT
NC
CT
HI
IN
NY
FL
TN
LA
WY
NV
KY
AK
CA
VA
Wl
OR
FL
TX
VT
NH
TN
CT
LA
NY
VA
NE
OK
FL
PA
AZ
PA
No Liner (landfills, structural fills)
Silt loam
3.07E-02
9.90E-03
4.42E-02
O.OOE+00
3.60E-03
3.26E-01
1.71E-01
5.23E-02
1.30E-01
1.68E-01
1.51E-01
4.11E-01
3.65E-01
3.30E-03
O.OOE+00
3.29E-01
3.53E-01
7.87E-02
3.08E-01
9.12E-02
2.07E-01
1.45E-01
1.80E-02
1.06E-01
2.27E-01
4.67E-01
3.52E-01
5.89E-01
2.44E-01
3.12E-01
6.71 E-02
6.12E-02
1.02E-01
2.01 E-01
O.OOE+00
8.94E-02
Sandy
loam
3.68E-02
7.40E-03
6.27E-02
O.OOE+00
6.90E-03
3.90E-01
2.23E-01
9.45E-02
1.86E-01
2.14E-01
2.11 E-01
4.46E-01
4.64E-01
5.30E-03
O.OOE+00
3.97E-01
4.34E-01
9.50E-02
3.61 E-01
1.40E-01
2.31 E-01
2.20E-01
2.54E-02
1.48E-01
2.81 E-01
5.40E-01
4.63E-01
7.45E-01
2.94E-01
O.OOE+00
7.95E-02
9.42E-02
1.70E-01
2.61 E-01
3.00E-04
1.31 E-01
Silty clay
loam
3.81 E-02
9.90E-03
3.23E-02
3.00E-04
7.40E-03
2.71 E-01
1.41 E-01
3.66E-02
1.06E-01
1.39E-01
1.10E-01
3.54E-01
2.82E-01
9.40E-03
1.80E-03
2.70E-01
2.82E-01
6.99E-02
2.57E-01
6.86E-02
2.10E-01
1.02E-01
1.35E-02
8.79E-02
1.94E-01
3.77E-01
2.86E-01
4.50E-01
1.97E-01
2.69E-01
5.36E-02
3.89E-02
8.05E-02
1.64E-01
3.00E-04
7.92E-02
Clay
Liner
(landfills)
4.61 E-03
6.69E-05
1.96E-02
2.70E-05
1.02E-04
3.62E-02
4.45E-02
4.83E-03
4.44E-02
4.45E-02
3.62E-02
4.86E-02
4.92E-02
1.28E-04
6.89E-05
4.86E-02
4.77E-02
1.26E-03
4.44E-02
4.09E-02
4.32E-02
4.92E-02
9.44E-03
4.32E-02
4.45E-02
4.86E-02
5.26E-02
4.77E-02
4.44E-02
3.62E-02
2.91 E-02
2.46E-02
3.62E-02
4.44E-02
1.69E-05
4.32E-02
D-3
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
ID
84
5
40
64
63
8
49
17
45
13
26
58
14
102
15
48
68
72
46
81
31
60
91
57
56
22
34
94
79
61
9
City
Plainfield
Pocatello
Portland
Portland
Providence
Pullman
Put-in-Bay
Rapid City
Rutland
Sacramento
Salt Lake City
San Antonio
San Diego
San Juan
Santa Maria
Sault St. Marie
Schenectady
Seabrook
Seattle
Shreveport
St. Cloud
Syracuse
Tallahassee
Tampa
Topeka
Tucson
Tulsa
W. Palm Beach
Watkinsville
Worchester
Yakima
State
MA
ID
OR
ME
Rl
WA
OH
SD
VT
CA
UT
TX
CA
PR
CA
Ml
NY
NJ
WA
LA
MN
NY
FL
FL
KS
AZ
OK
FL
GA
MA
WA
No Liner (landfills, structural fills)
Silt loam
1.90E-01
O.OOE+00
4.17E-01
2.29E-01
2.13E-01
6.90E-03
5.08E-02
5.00E-04
1.21E-01
1.02E-01
1.30E-02
1.10E-01
2.21 E-02
1.27E-01
9.47E-02
1.65E-01
1.47E-01
1.81E-01
4.38E-01
2.30E-01
6.02E-02
2.55E-01
5.91 E-01
6.58E-02
1.05E-01
O.OOE+00
6.86E-02
2.61 E-01
2.89E-01
2.02E-01
O.OOE+00
Sandy
loam
2.54E-01
O.OOE+00
4.39E-01
2.84E-01
2.86E-01
1.32E-02
1.00E-01
7.10E-03
1.60E-01
8.76E-02
2.69E-02
1.65E-01
3.40E-02
1.92E-01
1.15E-01
2.10E-01
1.93E-01
2.43E-01
4.58E-01
2.94E-01
8.31 E-02
3.25E-01
7.31 E-01
1.03E-01
1.48E-01
3.00E-04
1.01 E-01
3.49E-01
3.56E-01
2.59E-01
2.30E-03
Silty clay
loam
1.52E-01
O.OOE+00
3.93E-01
1.87E-01
1.75E-01
8.40E-03
4.95E-02
3.30E-03
1.01 E-01
9.45E-02
1.85E-02
8.20E-02
2.41 E-02
9.45E-02
8.41 E-02
1.44E-01
1.22E-01
1.43E-01
4.08E-01
1.84E-01
5.54E-02
2.12E-01
4.56E-01
4.75E-02
7.62E-02
5.00E-04
4.65E-02
1.78E-01
2.33E-01
1.70E-01
3.00E-04
Clay
Liner
(landfills)
5.26E-02
5.50E-04
4.32E-02
4.45E-02
4.45E-02
2.27E-04
4.09E-02
6.40E-05
4.32E-02
1.26E-03
5.10E-04
2.53E-02
1.26E-03
1.93E-02
1.26E-03
4.32E-02
4.45E-02
4.44E-02
4.32E-02
3.62E-02
3.42E-02
4.45E-02
4.77E-02
2.53E-02
3.50E-02
2.23E-05
2.41 E-02
4.77E-02
3.62E-02
4.45E-02
1.15E-04
D-4
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
Table D-2. HELP-Derived Infiltration Rates for Waste Piles (m/yr)
ID
19
98
82
95
62
44
99
7
2
67
75
35
43
41
18
86
93
10
42
74
52
55
51
38
36
3
53
29
32
54
88
23
16
100
City
Albuquerque
Annette
Astoria
Atlanta
Augusta
Bangor
Bethel
Bismarck
Boise
Boston
Bridgeport
Brownsville
Burlington
Caribou
Cedar City
Central Park
Charleston
Cheyenne
Chicago
Cincinnati
Cleveland
Columbia
Columbus
Concord
Dallas
Denver
Des Moines
Dodge City
E. Lansing
E. St. Louis
Edison
El Paso
Ely
Fairbanks
State
NM
AK
OR
GA
ME
ME
AK
ND
ID
MA
CT
TX
VT
ME
UT
NY
SC
WY
IL
OH
OH
MO
OH
NH
TX
CO
IA
KS
Ml
IL
NJ
TX
NV
AK
No Liner3
Low
Permea-
bility
Waste
2.54E-04
1.54E+00
1.21E+00
5.16E-01
3.14E-01
2.57E-01
5.02E-02
2.59E-02
2.54E-04
3.22E-01
3.69E-01
2.27E-01
2.13E-01
1.88E-01
2.54E-04
5.23E-01
4.83E-01
4.32E-03
1.68E-01
3.10E-01
1.82E-01
3.10E-01
1.72E-01
2.35E-01
2.58E-01
7.62E-04
2.51E-01
1.01E-01
1.36E-01
2.63E-01
4.90E-01
2.31 E-02
2.54E-04
7.67E-03
Medium
Permea-
bility
Waste
2.54E-04
1.81E+00
1.21E+00
5.16E-01
3.14E-01
2.57E-01
7.25E-02
2.59E-02
2.54E-04
3.22E-01
3.69E-01
2.27E-01
2.13E-01
1.88E-01
2.54E-04
5.23E-01
4.83E-01
4.32E-03
1.68E-01
3.10E-01
1.82E-01
3.10E-01
1.72E-01
2.35E-01
2.58E-01
7.62E-04
2.51 E-01
1.01E-01
1.36E-01
2.63E-01
4.90E-01
2. 31 E-02
2.54E-04
1 .67 E-02
High
Permea-
bility
Waste
2.54E-04
1.88E+00
1.21E+00
5.16E-01
3.14E-01
2.57E-01
1.23E-01
2.59E-02
2.54E-04
3.22E-01
3.69E-01
2.27E-01
2.13E-01
1.88E-01
2.54E-04
5.23E-01
4.83E-01
4.32E-03
1.68E-01
3.10E-01
1.82E-01
3.10E-01
1.72E-01
2.35E-01
2.58E-01
7.62E-04
2.51 E-01
1.01E-01
1.36E-01
2.63E-01
4.90E-01
2. 31 E-02
2.54E-04
7.77E-02
Clay Liner3
Low
Permea-
bility
Waste
1.60E-03
1.35E-01
1.32E-01
1.18E-01
1.19E-01
1.13E-01
3.52E-02
1 .24 E-02
1.36E-02
1.19E-01
1.06E-01
4.97E-03
1.13E-01
1.13E-01
4.82E-03
1.26E-01
1.18E-01
1.37E-03
1.13E-01
1.06E-01
6.88E-02
6.88E-02
6.88E-02
1.13E-01
4.97E-03
1.97E-03
6.88E-02
3.26E-03
4.81 E-02
6.88E-02
1.26E-01
5.81E-03
5.89E-03
9.80E-03
Medium
Perma-
bility
Waste
1.51 E-02
1.36E-01
1.35E-01
1.35E-01
1.29E-01
1.27E-01
3.64E-02
6.89E-02
4.34E-02
1.29E-01
1.34E-01
1.33E-01
1.27E-01
1.27E-01
8.26E-04
1.35E-01
1.35E-01
2.92E-04
1.27E-01
1.34E-01
1.32E-01
1.32E-01
1.32E-01
1.27E-01
1.33E-01
1.28E-03
1.32E-01
1.06E-01
1.15E-01
1.32E-01
1.35E-01
2.63E-03
1.12E-03
1.18E-02
High
Permea-
bility
Waste
7.43E-03
1.35E-01
1.35E-01
1.35E-01
1.28E-01
1.27E-01
6.60E-02
9.50E-02
6.06E-02
1.28E-01
1.33E-01
1.32E-01
1.27E-01
1.27E-01
5.32E-03
1.35E-01
1.35E-01
7.07E-03
1.27E-01
1.33E-01
1.32E-01
1.32E-01
1.32E-01
1.27E-01
1.32E-01
3.66E-03
1.32E-01
1.19E-01
1.11E-01
1.32E-01
1.35E-01
6.74E-03
3.61 E-03
4.07E-02
D-5
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
ID
28
1
6
27
4
25
77
59
101
73
66
78
85
96
11
20
87
90
12
69
50
24
97
30
47
65
89
83
92
70
80
33
37
76
71
City
Flagstaff
Fresno
Glasgow
Grand Island
Grand Junction
Great Falls
Greensboro
Hartford
Honolulu
Indianapolis
Ithaca
Jacksonville
Knoxville
Lake Charles
Lander
Las Vegas
Lexington
Little Rock
Los Angeles
Lynch burg
Madison
Medford
Miami
Midland
Montpelier
Nashua
Nashville
New Haven
New Orleans
New York City
Norfolk
North Omaha
Oklahoma City
Orlando
Philadelphia
State
AZ
CA
MT
NE
CO
MT
NC
CT
HI
IN
NY
FL
TN
LA
WY
NV
KY
AK
CA
VA
Wl
OR
FL
TX
VT
NH
TN
CT
LA
NY
VA
NE
OK
FL
PA
No Liner3
Low
Permea-
bility
Waste
4.04E-02
4.22E-02
3.66E-02
9.63E-02
2.54E-04
2.59E-02
4.84E-01
2.79E-01
5.01 E-02
2.69E-01
2.61 E-01
4.09E-01
5.42E-01
6.07E-01
2.03E-03
2.54E-04
4.52E-01
5.38E-01
1.33E-01
2.69E-01
2.02E-01
2.50E-01
4.23E-01
7.57E-02
1.76E-01
3.34E-01
6.14E-01
5.42E-01
8.49E-01
3.99E-01
4.54E-01
1.62E-01
2.42E-01
3.84E-01
3.53E-01
Medium
Permea-
bility
Waste
4. 04 E-02
4.22E-02
3. 66 E-02
9.63E-02
2.54E-04
2.59E-02
4.84E-01
2.79E-01
1.08E-01
2.69E-01
2. 61 E-01
4.09E-01
5.42E-01
6.07E-01
2.03E-03
2.54E-04
4.52E-01
5.38E-01
1.33E-01
2.69E-01
2.02E-01
2.50E-01
4.23E-01
7.57E-02
1.76E-01
3.34E-01
6.14E-01
5.42E-01
8.49E-01
3.99E-01
4.54E-01
1.62E-01
2.42E-01
3.84E-01
3.53E-01
High
Permea-
bility
Waste
4.04E-02
4.22E-02
3.66E-02
9.63E-02
2.54E-04
2.59E-02
4.84E-01
2.79E-01
1.98E-01
2.69E-01
2.61 E-01
4.09E-01
5.42E-01
6.07E-01
2.03E-03
2.54E-04
4.52E-01
5.38E-01
1.33E-01
2.69E-01
2.02E-01
2.50E-01
4.23E-01
7.57E-02
1.76E-01
3.34E-01
6.14E-01
5.42E-01
8.49E-01
3.99E-01
4.54E-01
1.62E-01
2.42E-01
3.84E-01
3.53E-01
Clay Liner3
Low
Permea-
bility
Waste
1.05E-02
1.36E-02
5.35E-04
4.22E-02
4.59E-03
1.94E-03
8. 04 E-02
1.19E-01
3.23E-02
1.06E-01
1.19E-01
8.04E-02
1.26E-01
4.89E-02
4.19E-03
5.15E-03
1.26E-01
1.18E-01
O.OOE+00
1.06E-01
6.88E-02
1.26E-01
4.89E-02
3.26E-03
1.13E-01
1.19E-01
1.26E-01
1.32E-01
1.18E-01
1.06E-01
8. 04 E-02
2.02E-02
7.47E-03
8.04E-02
1.06E-01
Medium
Perma-
bility
Waste
1.23E-01
4.34E-02
2.25E-04
1.35E-01
1.66E-03
4.66E-03
1.27E-01
1.29E-01
4. 94 E-02
1.34E-01
1.29E-01
1.27E-01
1.35E-01
5.58E-02
1.25E-03
1.79E-03
1.35E-01
1.35E-01
5. 56 E-02
1.34E-01
1.32E-01
1.33E-01
5.58E-02
1.06E-01
1.27E-01
1.29E-01
1.35E-01
1.35E-01
1.35E-01
1.34E-01
1.27E-01
1.26E-01
1.31 E-01
1.27E-01
1.34E-01
High
Permea-
bility
Waste
1.23E-01
6.06E-02
2.34E-02
1.34E-01
1.98E-03
3.34E-02
1.27E-01
1.28E-01
8.71 E-02
1.33E-01
1.28E-01
1.27E-01
1.35E-01
9.27E-02
2.00E-02
7.97E-03
1.35E-01
1.35E-01
7.18E-02
1.33E-01
1.32E-01
1.31E-01
9.27E-02
1.19E-01
1.27E-01
1.28E-01
1.35E-01
1.35E-01
1.35E-01
1.33E-01
1.27E-01
1.27E-01
1.30E-01
1.27E-01
1.33E-01
D-6
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
ID
21
39
84
5
40
64
63
8
49
17
45
13
26
58
14
102
15
48
68
72
46
81
31
60
91
57
56
22
34
94
79
61
9
City
Phoenix
Pittsburg
Plainfield
Pocatello
Portland
Portland
Providence
Pullman
Put-in-Bay
Rapid City
Rutland
Sacramento
Salt Lake City
San Antonio
San Diego
San Juan
Santa Maria
Sault St. Marie
Schenectady
Seabrook
Seattle
Shreveport
St. Cloud
Syracuse
Tallahassee
Tampa
Topeka
Tucson
Tulsa
W. Palm Beach
Watkinsville
Worchester
Yakima
State
AZ
PA
MA
ID
OR
ME
Rl
WA
OH
SD
VT
CA
UT
TX
CA
PR
CA
Ml
NY
NJ
WA
LA
MN
NY
FL
FL
KS
AZ
OK
FL
GA
MA
WA
No Liner3
Low
Permea-
bility
Waste
2.54E-04
1.72E-01
3.03E-01
2.54E-04
5.06E-01
3.25E-01
3.48E-01
2.54E-04
1.48E-01
1.35E-02
2.13E-01
1.23E-01
1.93E-02
2.95E-01
6.58E-02
1.50E-01
1.51E-01
2.37E-01
2.75E-01
3.41 E-01
5.31E-01
4.46E-01
1.52E-01
4.10E-01
8.22E-01
2.72E-01
2.47E-01
2.54E-04
2.49E-01
5.64E-01
4.67E-01
3.31E-01
2.54E-04
Medium
Permea-
bility
Waste
2.54E-04
1.72E-01
3.03E-01
2.54E-04
5.06E-01
3.25E-01
3.48E-01
2.54E-04
1.48E-01
1.35E-02
2.13E-01
1.23E-01
1.93E-02
2.95E-01
6.58E-02
2.88E-01
1.51 E-01
2.37E-01
2.75E-01
3.41 E-01
5. 31 E-01
4.46E-01
1.52E-01
4.10E-01
8.22E-01
2.72E-01
2.47E-01
2.54E-04
2.49E-01
5.64E-01
4.67E-01
3.31 E-01
2.54E-04
High
Permea-
bility
Waste
2.54E-04
1.72E-01
3.03E-01
2.54E-04
5.06E-01
3.25E-01
3.48E-01
2.54E-04
1.48E-01
1.35E-02
2.13E-01
1.23E-01
1.93E-02
2.95E-01
6.58E-02
4.44E-01
1.51E-01
2.37E-01
2.75E-01
3.41 E-01
5.31 E-01
4.46E-01
1.52E-01
4.10E-01
8.22E-01
2.72E-01
2.47E-01
2.54E-04
2.49E-01
5.64E-01
4.67E-01
3.31 E-01
2.54E-04
Clay Liner3
Low
Permea-
bility
Waste
4.73E-03
1.13E-01
1.32E-01
5.86E-03
1.13E-01
1.19E-01
1.19E-01
9.27E-03
6.88E-02
9.92E-04
1.13E-01
O.OOE+00
9.11E-03
2.00E-02
O.OOE+00
6.37E-02
O.OOE+00
1.13E-01
1.19E-01
1.06E-01
1.13E-01
8.04E-02
2.64E-02
1.19E-01
1.18E-01
2.00E-02
1 .74E-02
6.41 E-03
4.97E-03
1.18E-01
8.04E-02
1.19E-01
4.86E-03
Medium
Perma-
bility
Waste
2. 01 E-03
1.27E-01
1.35E-01
1.50E-03
1.27E-01
1.29E-01
1.29E-01
1.43E-02
1.32E-01
1.14E-03
1.27E-01
5.56E-02
1.05E-02
1.34E-01
5.56E-02
7.93E-02
5.56E-02
1.27E-01
1.29E-01
1.34E-01
1.27E-01
1.27E-01
1.26E-01
1.29E-01
1.35E-01
1.34E-01
1.31 E-01
7.53E-03
1.33E-01
1.35E-01
1.27E-01
1.29E-01
4.74E-03
High
Permea-
bility
Waste
7.62E-04
1.27E-01
1.35E-01
3.19E-02
1.27E-01
1.28E-01
1.28E-01
3.44E-02
1.32E-01
1.92E-02
1.27E-01
7.18E-02
3.68E-02
1.33E-01
7.18E-02
1.11E-01
7.18E-02
1.27E-01
1.28E-01
1.33E-01
1.27E-01
1.27E-01
1.26E-01
1.28E-01
1.35E-01
1.33E-01
1.30E-01
1.69E-03
1.32E-01
1.35E-01
1.27E-01
1.28E-01
2.84E-02
a Low, Medium, and High denote representative waste types with different hydraulic conductivities:
Low = Fine-grained waste (e.g., fly ash), Hydraulic conductivity is 5x105 cm/sec
Medium = Medium-grained waste (e.g., bottom ash), Hydraulic conductivity is 0.0041 cm/sec
High = Coarse-grained waste (e.g., slag), Hydraulic conductivity is 0.041 cm/sec
D-7
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
Table D-3. HELP-Derived Infiltration Rates for Land Application Units (m/yr)
ID
19
98
82
95
62
44
99
7
2
67
75
35
43
41
18
86
93
10
42
74
52
55
51
38
36
3
53
29
32
54
88
23
16
100
28
City
Albuquerque
Annette
Astoria
Atlanta
Augusta
Bangor
Bethel
Bismarck
Boise
Boston
Bridgeport
Brownsville
Burlington
Caribou
Cedar City
Central Park
Charleston
Cheyenne
Chicago
Cincinnati
Cleveland
Columbia
Columbus
Concord
Dallas
Denver
Des Moines
Dodge City
E. Lansing
E. St. Louis
Edison
El Paso
Ely
Fairbanks
Flagstaff
State
NM
AK
OR
GA
ME
ME
AK
ND
ID
MA
CT
TX
VT
ME
UT
NY
SC
WY
IL
OH
OH
MO
OH
NH
TX
CO
IA
KS
Ml
IL
NJ
TX
NV
AK
AZ
No Liner
Silty loam
O.OOE+00
1.80E+00
1.08E+00
3.42E-01
2.12E-01
1.47E-01
1.85E-01
2.39E-02
8.00E-04
2.33E-01
1.95E-01
5.49E-02
1.36E-01
1.08E-01
O.OOE+00
3.36E-01
2.61 E-01
5.00E-04
7.98E-02
1.55E-01
7.80E-02
1.53E-01
7.65E-02
1.59E-01
5.99E-02
8.00E-04
1.14E-01
1.35E-02
1.09E-01
1.44E-01
3.12E-01
7.60E-03
O.OOE+00
1.46E-01
2.39E-02
Sandy
loam
O.OOE+00
1.98E+00
1.15E+00
3.99E-01
2.70E-01
2.05E-01
1.98E-01
3.00E-02
9.40E-03
2.38E-01
2.46E-01
1.05E-01
1.78E-01
1.49E-01
8.00E-04
4.17E-01
3.29E-01
1.30E-03
1.14E-01
2.21 E-01
1.21 E-01
1.99E-01
1.16E-01
2.06E-01
1.07E-01
8.00E-04
1.64E-01
3.45E-02
1.45E-01
1.68E-01
3.91 E-01
1.30E-02
O.OOE+00
1.48E-01
6.30E-02
Silty clay
loam
3.00E-04
1.52E+00
9.65E-01
2.82E-01
1.67E-01
1.23E-01
1.78E-01
1.96E-02
3.80E-03
1.54E-01
1.62E-01
3.84E-02
1.17E-01
8.86E-02
O.OOE+00
2.74E-01
2.12E-01
8.60E-03
6.20E-02
1.54E-01
8.23E-02
1.22E-01
6.63E-02
1.37E-01
5.31 E-02
3.60E-03
1.16E-01
2.26E-02
1.10E-01
7.04E-02
2.49E-01
8.10E-03
3.00E-04
1.45E-01
2.26E-02
D-8
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
ID
1
6
27
4
25
77
59
101
73
66
78
85
96
11
20
87
90
12
69
50
24
97
30
47
65
89
83
92
70
80
33
37
76
71
21
39
City
Fresno
Glasgow
Grand Island
Grand Junction
Great Falls
Greensboro
Hartford
Honolulu
Indianapolis
Ithaca
Jacksonville
Knoxville
Lake Charles
Lander
Las Vegas
Lexington
Little Rock
Los Angeles
Lynchburg
Madison
Medford
Miami
Midland
Montpelier
Nashua
Nashville
New Haven
New Orleans
New York City
Norfolk
North Omaha
Oklahoma City
Orlando
Philadelphia
Phoenix
Pitts burg
State
CA
MT
NE
CO
MT
NC
CT
HI
IN
NY
FL
TN
LA
WY
NV
KY
AK
CA
VA
Wl
OR
FL
TX
VT
NH
TN
CT
LA
NY
VA
NE
OK
FL
PA
AZ
PA
No Liner
Silty loam
3.07E-02
9.90E-03
4.42E-02
O.OOE+00
3.60E-03
3.26E-01
1.71E-01
5.41 E-02
1.30E-01
1.68E-01
1.51E-01
4.11E-01
3.65E-01
3.30E-03
O.OOE+00
3.29E-01
3.53E-01
7.87E-02
3.08E-01
9.12E-02
2.07E-01
1.45E-01
1.80E-02
1.06E-01
2.27E-01
4.67E-01
3.52E-01
5.89E-01
2.44E-01
3.12E-01
6.71 E-02
6.12E-02
1.02E-01
2.01 E-01
O.OOE+00
8.94E-02
Sandy
loam
3.68E-02
7.40E-03
6.27E-02
O.OOE+00
6.90E-03
3.90E-01
2.23E-01
9.83E-02
1.86E-01
2.14E-01
2. 11 E-01
4.46E-01
4.64E-01
5.30E-03
O.OOE+00
3.97E-01
4.34E-01
9.50E-02
3.61 E-01
1.40E-01
2.31 E-01
2.20E-01
2.54E-02
1.48E-01
2.81 E-01
5.40E-01
4.63E-01
7.45E-01
2.94E-01
O.OOE+00
7.95E-02
9.42E-02
1.70E-01
2.61 E-01
3.00E-04
1.31 E-01
Silty clay
loam
3.81 E-02
9.90E-03
3.23E-02
3.00E-04
7.40E-03
2.71 E-01
1.41 E-01
3.63E-02
1.06E-01
1.39E-01
1.10E-01
3.54E-01
2.82E-01
9.40E-03
1.80E-03
2.70E-01
2.82E-01
6.99E-02
2.57E-01
6.86E-02
2.10E-01
1.02E-01
1.35E-02
8.79E-02
1.94E-01
3.77E-01
2.86E-01
4.50E-01
1.97E-01
2.69E-01
5.36E-02
3.89E-02
8.05E-02
1.64E-01
3.00E-04
7.92E-02
D-9
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
ID
84
5
40
64
63
8
49
17
45
13
26
58
14
102
15
48
68
72
46
81
31
60
91
57
56
22
34
94
79
61
9
City
Plainfield
Pocatello
Portland
Portland
Providence
Pullman
Put-in-Bay
Rapid City
Rutland
Sacramento
Salt Lake City
San Antonio
San Diego
San Juan
Santa Maria
Sault St. Marie
Schenectady
Seabrook
Seattle
Shreveport
St. Cloud
Syracuse
Tallahassee
Tampa
Topeka
Tucson
Tulsa
W. Palm Beach
Watkinsville
Worchester
Yakima
State
MA
ID
OR
ME
Rl
WA
OH
SD
VT
CA
UT
TX
CA
PR
CA
Ml
NY
NJ
WA
LA
MN
NY
FL
FL
KS
AZ
OK
FL
GA
MA
WA
No Liner
Silty loam
1.90E-01
O.OOE+00
4.17E-01
2.29E-01
2.13E-01
6.90E-03
5.08E-02
5.00E-04
1.21E-01
1.02E-01
1.30E-02
1.10E-01
2.21 E-02
1.49E-01
9.47E-02
1.65E-01
1.47E-01
1.81E-01
4.38E-01
2.30E-01
6.02E-02
2.55E-01
5.91 E-01
6.58E-02
1.05E-01
O.OOE+00
6.86E-02
2.61 E-01
2.89E-01
2.02E-01
O.OOE+00
Sandy
loam
2.54E-01
O.OOE+00
4.39E-01
2.84E-01
2.86E-01
1.32E-02
1.00E-01
7.10E-03
1.60E-01
8.76E-02
2.69E-02
1.65E-01
3.40E-02
2.16E-01
1.15E-01
2.10E-01
1.93E-01
2.43E-01
4.58E-01
2.94E-01
8.31 E-02
3.25E-01
7.31 E-01
1.03E-01
1.48E-01
3.00E-04
1.01 E-01
3.49E-01
3.56E-01
2.59E-01
2.30E-03
Silty clay
loam
1.52E-01
O.OOE+00
3.93E-01
1.87E-01
1.75E-01
8.40E-03
4.95E-02
3.30E-03
1.01 E-01
9.45E-02
1.85E-02
8.20E-02
2.41 E-02
1.05E-01
8.41 E-02
1.44E-01
1.22E-01
1.43E-01
4.08E-01
1.84E-01
5.54E-02
2.12E-01
4.56E-01
4.75E-02
7.62E-02
5.00E-04
4.65E-02
1.78E-01
2.33E-01
1.70E-01
3.00E-04
D-10
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
Table D-4. Flow Rate Data Used to Develop Composite Liner Infiltration Rates for Landfills and Waste Piles (from TetraTech, 2001)
All data are for high density polyethylene geomembrane/geosynthetic clay liner and municipal solid waste.
Landfill Cell IDa
G228
G232
G233
G234
G235
G236
G237
G238
G239
G240
G241
G242
G243
G244
G245
G246
G247
G248
<
"o
o
51
4.7
2
2
1.7
1.7
2.8
3.9
2.6
3.8
3.3
3.9
3
4
3
2.8
2.8
4.5
Geomembrane
Liner Thickness
(mm)
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
Geosynthetic Clay
or Compacted Clay
Liner Thickness
(mm)
NA
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
Maximum Height of
Waste (m)
NA
NA
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
End Construction
Date
1988
May-92
Jun-88
Jun-88
Aug-88
Aug-88
Sep-88
Dec-88
Jan-89
Jul-89
Dec-89
Feb-90
Feb-90
Oct-90
Jan-91
Apr-92
May-92
Jan-93
Waste Placement
Start Date
1989
May-92
Jul-88
Jul-88
Sep-88
Sep-88
Oct-88
Dec-88
Feb-89
Jul-89
Dec-89
Jul-90
Feb-90
Oct-90
Jan-91
Apr-92
May-92
Jan-93
Final Closure Date
NA
Jul-94
Feb-91
Feb-91
Apr-93
Apr-93
-
-
-
-
-
-
-
-
-
-
-
-
Source of Datab
Eithe&Koerner(1997)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
EPA (1998)
D-ll
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
ro
ns
< v.
760
1090
1090
1090
990
1040
1040
1040
1040
'o
w
-
S^^l
0 o 3 B
250
6
6
6
6
6
6
6
6
0
^
O)
'
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
Table D-5. Leak Density Data Used to Develop Composite Liner Infiltration Rates for Surface Impoundments (from TetraTech, 2001)
Q
0)
(75
L1
L2
L3
L4
L5
L6
17
L8
L9
L10
L86
L103
L110
L114
L136
L144
cu
s
1995
1996
1994
1995
1997
1998
1995
1995
1997
1998
Apr-96
Oct-96
Jan-97
Jan-97
Oct-97
May-98
j=.
1
18500
14926
13480
11652
8200
9284
67100
66150
11460
18135
9416
4980
11720
7000
13526
5608
Location
France
France
France
France
France
France
Canada
Canada
Canada
France
UK
UK
UK
UK
UK
UK
Waste Type
domestic
domestic
HW
HW
HW
HW
waste water
treatment
waste water
treatment
black liqueur
domestic
NA
NA
NA
NA
NA
NA
cu
Q.
i
landfill
landfill
landfill
landfill
landfill
landfill
pond
pond
pond
landfill
NA
NA
NA
NA
NA
NA
Type of
Geomembrane
Linerb
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
PBGM
PBGM
PP
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
cu
M- C
O CO
2
2
2
2
2
2
3
3
1.14
2
NA
NA
NA
NA
NA
NA
Quality of
Material Beneath
Geomembrane
high
high
high
high
high
high
high
high
high
high
NA
NA
NA
NA
NA
NA
o
0
4
1
1
0
0
3
1
2
0
0
0
0
0
0
0
Knife Cuts/Tears
0
0
1
2
0
1
0
1
2
3
0
0
2
3
1
0
Seam or Weld
Defects
5
2
1
2
0
0
2
7
2
3
0
0
1
1
0
0
CO
cu
o
h-
5
6
3
5
0
1
5
9
6
6
0
0
3
4
1
0
Range of Hole
Size (mm)
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
-
-
-
-
30x50
-
>!
II
SCO
CD
| ^3,
2.7
4.02
2.23
4.29
0
1.08
0.75
1.36
5.24
3.31
0
0
2.6
5.7
0.7
0
Source0
Rollinetal. (1999)
Rollinetal. (1999)
Rollinetal. (1999)
Rollinetal. (1999)
Rollinetal. (1999)
Rollinetal. (1999)
Rollinetal. (1999)
Rollinetal. (1999)
Rollinetal. (1999)
Rollinetal. (1999)
McQuade and
Needham(1999)
McQuade and
Needham(1999)
McQuade and
Needham(1999)
McQuade and
Needham(1999)
McQuade and
Needham(1999)
McQuade and
Needham(1999)
D-13
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
Q
0)
(75
L152
L159
L160
L176
L177
L178
L179
L180
L181
L182
0)
CO
Q
Aug-98
NA
NA
May-98
Sep-96
Apr-97
Sep-98
Sep-98
NA
NA
fsl "'
E
CO
1
3742
15000
10000
13500
15000
7500
5000
13200
48600
8000
c
o
CO
o
3
UK
UK
UK
UK
UK
UK
UK
UK
NA
NA
|
CD
I
NA
NA
NA
NA
NA
NA
NA
NA
waste water
containment
HW
Q.
S?
s
NA
NA
NA
NA
NA
NA
NA
NA
pond
landfill
CU
2
.Q
"5 2 ^
S.| |
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
HDPE/C
CL
CU
"5 2
(ft .Q
If: f=
C &
ill
NA
NA
NA
NA
NA
NA
NA
NA
1.5
2
-C
CO CU
C CO
CU i;
O £
>, CO CU
is s o
3 CO CU
O S CD
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
CU
o
0
0
0
1
0
0
0
0
NA
NA
<£.
CO
=
o
'E
is:
0
0
0
0
0
1
0
0
NA
NA
-a
j_
0
-------
IWEM Technical Background Document
Appendix D: Infiltration Rate Data for WMUs
Table D-6. Comparison of Composite Liner Infiltration Rates Calculated Using Bonaparte
Equation and Infiltration Rates for Composite-Lined Landfill Cells
Percent! le
0
10
20
30
40
50
60
70-
80
90
100
Calculated
Infiltration (m/yr)
0
0
0
0
1.05E-05
1.37E-05
2.03E-05
3.96E-05
6.01 E-05
7.13E-05
1.87E-04
Observed Infiltration
(m/yr)
0
0
0
0
0
0
2.19E-05
7.30E-05
7.30E-05
1.73E-04
4.02E-04
0.00045
0.0004
0.00035
0.0003
0.00025
5 0.0002
0.00015
0.0001
0.00005
Calculated Infiltration
Actual Infiltration values
20 30 40 50
70 80 90 100
Figure D-1. Infiltration rate comparison (Head =0.3 m, Hole Area = 6 mm2).
D-15
-------
IWEM Technical Background Document Appendix D: Infiltration Rate Data for WMUs
This page intentionally left blank.
D-16
-------
IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
Appendix E: Infiltration Rates through Pavements
E.1 Introduction
The Industrial Waste Management Evaluation Model (IWEM) simulates the migration of
contaminants from a roadway source to a down-gradient receptor well. To use the roadway
source module, IWEM requires that the user provide infiltration rates through a roadway
pavement; Infiltration rate is one of the key parameters influencing the downward migration of
contaminants of concern. Recognizing the importance of this parameter, the EPA has developed
infiltration rates for various types of roadway materials and provided them in IWEM as default
values. The user may either consider using these values or provide the model their own site-
specific infiltration rates. The main objective of this section is to discuss the approach used to
develop long-term infiltration rates through various types of roadway material in a number of
climatic zones.
A description of the approach used is presented in Section E.2. The data available in the
literature are reviewed in Section E.3. The models used and the verification results are presented
in Sections E.4 and E.5, respectively.
E.2 General Approach
Infiltration rates through the roadway pavements are essential to predict the transport of
contaminants from the roadways to the down-gradient receptor wells. However, sources of
empirical data on infiltration rates through roadway pavements are limited. Therefore,
mathematical models, as described in Sections E.4 and E.5, were used to help generate the
required infiltration rates included in IWEM. To ensure that the models could be reliably used to
estimate infiltration rates, the models were first verified against available infiltration data from
actual field observations. The verified models were then utilized to generate infiltration rates for
various types of pavements in different climatic zones.
E.3 Available Infiltration Data for Pavements
E.3.1 Pavement Overview
A typical highway cross-section is presented in Figure E-l. As shown in the figure, a typical
cross section consists of two or more travel lanes, two road shoulders/embankments, and a
median in the middle between the travel lanes. It may also include ditches and gutters. Pavement
is a major component of the travel lane that acts as a means to dissipate vehicular loads from its
traffic surface (the surface course) to the subgrade. The subgrade could be either native soil,
modified native soil (through densification or other treatments), or fill/embankment materials.
There are two major types of pavement (Apul et al., 2002): flexible pavement and rigid
pavement.
E-l
-------
IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
Traveled Lanes
Traveled Lanes
Ditch
Embank-
ment
~ J Drain
Gutter Subgrade
Ditch
Permeable Drain
Base
Figure E-1. A typical cross section of a roadway.
Gutter
Flexible pavements consist of a combination of layers that includes an asphalt surface (wearing
surface) constructed over a granular or asphalt base and a subbase. The entire pavement structure
is constructed over the subgrade. Pavements can be constructed using hot mix or cold mix
asphalt. Rigid pavements or portland cement concrete pavements consist of a portland cement
concrete slab that is usually supported by a granular or stabilized base and a subbase. In some
cases, the portland cement concrete slab may be overlaid with a layer of asphalt concrete. A
typical pavement structure is shown in Figure E-2. A pavement is typically attached to an
engineered drainage system because pavement failures are attributable to elevated moisture
conditions. Figure E-3 shows examples of the many types of subsurface highway drainage.
Road Shoulder,
Embankment, and Ditch
Base
Subbase
Subgrade
Figure E-2. Typical pavement layers.
E-2
-------
IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
A Base is used as the drainage layer
Drainage layer
as a base
I^M *l «
B Drainage layer is part of or below the subbase.
Drainage layer
as part of or
b*bw the subbase
Figure E-3. Typical permeable base pavement sections: (a) base used as drainage layer;
(b) drainage layer is part of or below the subbase (from AASHTO, 1993).
E.3.2 Infiltration Data
Infiltration-related data are relatively limited and available in two general categories: indirect
short-term data and direct long-term data. Indirect short-term data normally involve water
collected at drain outlets during rainfall or storm events. The duration of data collection varies
from a few hours to a day depending on the respective duration of precipitation or storm event.
For drains with surface inlets, the difference between precipitation and the amount of water that
passes through the drain system accounts for the sum of evaporation and surface runoff not
captured by the surface drainage system, and downward percolation into the pavement. Subgrade
infiltration (exfiltration or downward percolation at the base of pavement structure) may not
begin until late in the rainfall event as the saturation condition in the pavement structure may not
exceed field capacity until that time. Based on indirect measurements in a highway litter
management pilot study conducted by Caltrans (2000) in Los Angeles County, CA, the amount
of collected drained water varies from 62 to 89 percent of precipitation. In a controlled pilot
study, the amount of 85 and 94 percent at two sites in Austin, Texas, was obtained by Irish et al.
(1995).
For drains with subsurface inlets that may be connected to the base and/or subbase layers, the
drainage data obtained by Ahmed et al. (1997) in Indiana indicate that the collected drained water
varied between 0.1 to 70 percent of precipitation depending on the conditions of the pavement
surface. Data in Indiana obtained by Feng et al. (1999) from three types of pavements indicate a
relatively uniform value of 8 percent. Recently, indirectly measured short-term drainage data
from a number of road sections in Minnesota indicate a variation between 2 to 35 percent of
precipitation (Minnesota DOT, 2007; Apul, 2007). For data collected from subsurface drain
outlets, the difference between precipitation and the amount of collected drained water accounts
for evaporation, surface runoff, and subgrade infiltration at the bottom of pavement structure.
E-3
-------
IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
Therefore, the above limited data sets cannot be used to explicitly determine subgrade infiltration
from the bottom of the pavement structure because it will be necessary to estimate runoff and
evaporation in order to estimate short-term subgrade infiltration. In the IWEM document, the
term "infiltration" refers to water exiting from the subbase layer (the bottom of the source) into
the subgrade material (the top of the vadose zone) below. This infiltration is referred to as
"subgrade infiltration" in this document to distinguish it from top-of-pavement infiltration, which
refers to water entering the pavement surface from precipitation and runoff.
Long-term data normally involve multiple-year monitoring of subgrade infiltration from
pavement structures above subgrade. In the literature, long-term subgrade infiltration monitoring
data are very limited. Rainwater et al. (2001) provide subgrade infiltration data for three sites
with asphaltic concrete pavements in Tennessee over a monitoring period of 3 years. In this case,
one-square meter free-tension lysimeters were placed below an asphalt-stabilized base layer and
above an unbound aggregate base layer. The experiment resulted in subgrade infiltration rates
ranging from 0 to 1 percent of the total rainfall. These small subgrade infiltration rates may be
due to the relatively new pavement surfaces and the presence of subsurface drainage systems
above the lysimeters. A relatively comprehensive experiment was conducted at various road
sections constructed with recycled materials along Wisconsin State Highway 60 (Li et al., 2005).
Data on subgrade infiltration were provided by Li et al. (2005). In this experiment, lysimeters
were placed below the subbase layer at each section. Both subgrade infiltration rate and
contaminant concentrations leaching from the recycled materials were monitored. The subgrade
infiltration rates were observed to vary from approximately 5 to 7 percent of total rainfall over a
5-year period. This experiment is described in more detail in Section E-5.
E.4 Descriptions of Models
Contaminant release and transport is directly affected by the presence and flow of water in
pavements (Apul et al., 2007). While pavements are often considered impervious structures,
roads constructed with portland cement concrete or asphalt concrete surface courses can
experience water entry to the base layer through cracks (Ridgeway, 1976; Ahmed et al., 1997).
The extent and rate of infiltration into the pavement structure also depends on rain intensity. If
the infiltration capacity of the cracks is exceeded, then some of the rain becomes runoff and does
not influence the mobility of the contaminants in pavements.
Two models were considered to simulate flow through pavements and estimate default
infiltration rates for IWEM's roadway module. The movement of fluid through pavements may
be simulated using a variably saturated flow model. Or, another approach, runoff and infiltration
through the pavement surface may also be simulated by a water-budget model. The latter may
also be used to approximate fluid fluxes through pavement structures that eventually exit through
the bottom. These two types of models are described below.
E.4.1 Variability-Saturated Flow Model
The environment within pavement structures is variably saturated because of the temporal
variability of meteoric fluid that enters the pavements through cracks and fractures. Simulation of
flow and transport in pavements has been based on an implicit assumption that the hydraulic
E-4
-------
IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
behavior of a fractured pavement may be approximated by that of an equivalent porous medium.
Based on this assumption, the flow may be described by the following equations (Bear, 1972).
d (, dh] .dS dh
^T kr»Kv ^T ~ W = 0^T + S-S* 17 (E-l)
dxi \ dXj I dt dt ^ *>
where
Xi, Xj = Cartesian coordinates (/', j = 1, 2, 3)
Ky = Intrinsic permeability tensor
krw = Relative permeability, which is a function of water saturation
W = Volumetric flux per unit volume and represents sources and/or sinks
N = Drainable porosity taken to be equal to the specific yield
Sw = Degree of saturation of water, which is a function of the pressure head
Ss = Specific storage of the porous material
t = Time
Equation (E-l) may be solved by many flow and transport codes. The computer code used in this
study, MODFLOW-SURF ACT (HydroGeoLogic, 2011), is an enhanced version of the U.S.
Geological Survey's modular three-dimensional ground water flow code (McDonald and
Harbaugh, 1988). The code has been selected for the following reasons:
The Menu Bar allows the user to perform common file operations;
It can handle variably saturated flow and transport, including equilibrium sorption, and
first-order degradation;
It has been extensively verified and documented (HydroGeoLogic, 2011); and,
It has been implemented in many different settings (e.g., Guvanasen et al., 2004; Tu et al.,
2006).
E.4.2 Water Budget Model
This type of model is based solely on a water budget and does not address the physical aspects of
flow and transport in a variably saturated environment. In 1994, EPA developed a quasi-two-
dimensional hydrologic model for conducting water balance analyses of landfills, cover systems,
and other solid waste containment facilities (Schroeder et al., 1994 a, b). The model is referred to
as the Hydrologic Evaluation of Landfill Performance (HELP) model. The model accepts
weather, soil, and design data and uses solution techniques that account for the effects of surface
storage, snowmelt, runoff, infiltration, evapotranspiration, vegetative growth, soil moisture
storage, lateral subsurface drainage, leachate recirculation, unsaturated vertical drainage, and
leakage through soil, geomembrane, or composite liners. Landfill systems, including various
combinations of vegetation, cover soils, waste cells, lateral drain layers, low-permeability barrier
soils, and synthetic geomembrane liners, may be modeled. The model facilitates rapid estimation
of the amounts of runoff, evapotranspiration, drainage, leachate collection, and liner leakage that
may be expected to result from the operation of a wide variety of landfill designs. The HELP
model was used to develop infiltration and recharge rates for landfills and other waste
management units used in conjunction with the EPA's Composite Model for Leachate Migration
with Transformation Products (EPACMTP) (U.S. EPA, 2003a-b), the ground water flow and
E-5
-------
IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
transport engine used by IWEM. A detailed description of how the HELP model was used to
develop infiltration and recharge rates is provided in Appendix A of the EPACMTP
Parameter/Data Background Document (U.S. EPA 2003b).
Because the layering of landfill cover, waste materials, and liner is similar to the layering in
pavement structures, HELP is also applicable to water-balance analyses of roadways.
E.5 Verification Results
In order to demonstrate that the MODFLOW-SURFACT and HELP models can be used to
simulate flow in pavements, the two models were verified against infiltration data collected from
pavement sections in Wisconsin, described by Li et al. (2005).
The test sections were constructed along a 1.4-km stretch of Wisconsin State Highway 60 near
Lodi, WI, on soft subgrade. One of the test sections was constructed with bottom ash from a dry-
bottom furnace at Alliant Energy's Columbia Power Station that burns sub-bituminous coal from
the Wyoming Powder River Basin. The bottom ash is a coarse-grained material that is classified
as well-graded sand in the Unified Soil Classification System and A-3 in the American
Association of State Highway and Transportation Officials system (see Figure E-4).
Legend:
| | Lysimeter
O Collection
Tank
Note: Not to Scale
Bottom Ash
Inner
Outer
115 mm Crushed
Aggregate Base
140 mm Salvaged
Asphalt Base
(i) Plan View (Not to Scale)
125 mm AC
600 mm
Bottom Ash
Subbase
- Lysimeter
Subgrade
(Hi) Layer Thicknesses
(Not to Scale)
(ii) Side View (Not to Scale)
Figure E-4. Wisconsin State Highway 60 test section, experimental set up
(from Li et al., 2005, and Sauer et al., 2005).
E-6
-------
IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
Two pan lysimeters (3.57 m x 4.75 m) were placed beneath the bottom subbase, immediately
above the subgrade, to monitor the quality and quantity of water discharged from the base of the
pavement. One lysimeter was located directly under the centerline of the highway, and the other
was located underneath the edge of the pavement, with one-half of the lysimeter under the
highway shoulder. Water collected by the lysimeters drains to 120-L high-density polyethylene
(HDPE) drums located below ground surface adjacent to the highway (see Figure E-4 (ii)).
Samples were collected from these lysimeters over a 5-year period. During each sampling event,
water contained in each drum was removed with a pump, the total volume of the water in the
drum was recorded, and samples were collected for chemical analysis for the concentrations of
cadmium, chromium, selenium, and silver. Infiltration data at the bottom of the subbase layer
measured by lysimeters between September 2000 and September 2005 are presented in
Figure E-5. The median of infiltration rate over the 5-year observation period is approximately
0.15 mm/day (averaged from two lysimetes) or approximately 6.5 percent of total precipitation
(2.3 mm/day or 865 mm/year). A summary of the volumetric leachate flux at the STH 60 Field
Site is presented in Table E-l.
0.0
9/1/00
9/1/01
9/1 /02
9/1/03
9/1/04
9/1/05
Figure E-5. Subgrade infiltration data (from Li et al., 2005).
Table E-1. Volumetric Leachate Flux at the STH60 Field Site
Flux Condition
Q50, 50th Percentile
Q90, 90th Percentile
Infiltration rate (mm/day)
Infiltration rate (mm/day)
QAp, Annual infiltration Rate (mm/day)
Q50/QAp
Q90/QAp
Lysimeters
Inside
(centerline)
0.13
0.43
2.35
0.06
0.18
Outside
(shoulder)
0.16
0.53
2.35
0.07
0.23
Source: Li et al. (2005)
E-7
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IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
E.5.1 Variability-Saturated Flow Model
A cross-section of State Highway 60 simulated by the model is depicted in Figure E-6. Because
of symmetry, only one half of the cross-section was modeled. The cross section is 12.4 m wide. It
consists of a 5.0-m wide asphalt concrete pavement, and a 1.6-m wide shoulder. The
embankment was assumed to extend for 5.8 m from the outer edge of the shoulder. In the
document by Li et al. (2005), slope values of the various components of the test section were not
available. In the simulation, slopes consistent with highway engineering practice were used. The
slopes of the pavement, shoulder, and embankment were assumed to be 2, 4, and 10 percent,
respectively. These are typical slopes used in highway engineering practice (Apul et al., 2007).
The model was discretized into 62 layers and 62 columns (Figure E-7). At this test section, there
is no subsurface drainage system installed.
Unit: meter
Figure E-6. Model cross-section of State Highway 60 (not to scale, all dimensions in meters).
Layers
50-62
Inner
Lysi meter
Outer
Lysi meter
Constant Head BC
Figure E-7. Model grid corresponding to the cross-section in Figure E-6.
E-8
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IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
The pavement included four layers with different material types: asphalt concrete (pavement
surface), crushed aggregate (top base course), salvaged asphalt (bottom base course), and bottom
ash (subbase). The thicknesses of the four pavement layers are shown in Figure E-4(iii). The
material type for subgrade was not reported and was assumed to be sand for simulation purposes.
The relative permeability and saturation terms (krw and Sw, respectively) in Equation (4-1) were
described using van Genuchten's characteristic functions (van Genuchten, 1980), which are
dependent on two material-dependent parameters, a and p. These two parameters, along with
other material properties used in the simulation, are given in Table E-2.
E-9
-------
IWEM Technical Background Document
Appendix E: Infiltration Rates Through Pavements
Table E-2. Material and Properties Used in Simulation: Flow Parameters
Material
Asphalt concrete b
Crushed aggregate base c
Salvaged asphalt base c
Bottom ash subbase d
Subgrade below lysimeters e
Thickness
(Inches [mm])
4.9 [125]
4.5 [115]
5.5 [140]
23.6 [600]
Remaining
Layer
Top
1
7
13
20
50
Bottom
6
12
19
49
62
Hydraulic
Conductivity
(cm/sec)
1.00E+01
1.505E-03
1.505E-03
4.63E-01
8.25E-02
Total
Porosity
(vol/vol)
0.03
0.3
0.3
0.3
0.3
aa<1/cm)
1.000
0.063
0.063
0.0431
0. 145
Pa
(unitless)
2.19
1.3
1.3
3.1
2.68
Residual
Saturation
(Vol/vol)
0.02
0.06
0.06
0.02
0.05
a van Genuchten parameters (van Genuchten, 1980)
b Hydraulic properties of fractured asphalt concrete from Stormont and Zhou (2001) - gravel
c Hydraulic properties of base materials from Apul et al. (2007) - asphalt aggregate base
d Hydraulic properties of bottom ash subbase from Stormont and Zhou (2001) - medium sand
e Hydraulic properties of embankment and subgrade from Carsel and Parrish (1988) - sand
E-10
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IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
Infiltration into the pavement was assumed to be uniform over the entire asphalt concrete surface.
This assumption is based on a premise that the pavement hydraulic behavior may be
approximated by that of an equivalent porous medium. The pavement hydraulic properties were
assumed to be uniform throughout the pavement cross-section. These properties were obtained
from the literature. References are provided at the bottom of Table E-2.
Because runoff and evaporative loss information was not available, it was not possible to
estimate the amount of water infiltrating into the pavement. For the simulation, the monthly top-
of-pavement infiltration rate was adjusted until the simulated subgrade infiltration (exfiltration at
the bottom of the subbase layer) was in reasonable agreement with the observed fluxes at the two
lysimeters (Figure E-8).
1.6
1.4 -
I 1H
X
IT 0.8
0.6 -
0.4 -
0.2 -
Observed Outer Lysimeter
Simulated Outer Lysimeter
o Observed Inner Lysimeter
H Simulated Inner Lysimeter
- -*- - Long-term Average
Aug-OO
Apr-01
Aug-01 Oct-01
Date
Apr-02
Aug-02
Figure E-8. Comparison between simulated and observed subgrade infiltrations (exfiltration from
subbase) at the bottom of the subbase layer.
Precipitation data are compared against top-of-pavement infiltration and subgrade infiltration
(subbase exfiltration) in Figure E-9. In the figure, it can be seen that, during the same month,
top-of-pavement infiltration tends to be slightly larger than subgrade infiltration. The difference
between the two rates could be due to possible lateral flow around the lysimeters to the
embankment. In addition, it can also be seen that the two rates are generally much smaller than
precipitation.
E-ll
-------
IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
10
9 -
> 8 -
re
3
ul 5
u
I 4
I 3
> 2 -
1 -
II
LiU
Sep- Oct- NONA Dec- Jan- Feb- Mar- Apr- May- Jun- Jul- Aug- Sep- Oct- NONA Dec- Jan- Feb- Mar- Apr- May- Jun- Jul- Aug-
00 00 00 00 01 01 01 01 01 01 01 01 01 01 01 01 02 02 02 02 02 02 02 02
Date
D Rainfall D Surface Infiltration Bottom Exfiltration
Figure E-9. Surface infiltration and bottom exfiltration
(subgrade infiltration) vs. precipitation.
The results shown in Figure E-9 also suggest that storage effects within the STH 60 experimental
section are relatively small and, as a consequence, infiltration tends to occur soon after
precipitation events. Short-term data from pavements with subsurface drains indicate that water
in the drains generally appears soon after the onset of rainstorm events, thereby suggesting that
the lag time between the start of top-of-pavement infiltration and the onset of subgrade
infiltration is very small, on the order of hours.
E.5.2 Water Budget Model
The water budget HELP model can be used to estimate the infiltration rate through pavement.
The HELP model utilizes pavement properties, surface conditions, and climatic conditions (see
Section E.4.2). The HELP model was evaluated by applying it to the setting, similar to that
described in Section E.5.1. Using the input parameters in Table E-3 and the material properties
contained in Table E-2, the HELP-model-based subgrade infiltration rate is approximately 2.42
inches/year (6.1 cm/yr), which is a close approximation of the measured subgrade infiltration rate
of approximately 2.35 inches/year (6.0 cm/yr or 0.16 mm/day).2
Note that the HELP model uses English system units, not metric. For clarity, therefore, units here are given in the
English units used, and metric equivalents are shown in parentheses.
E-12
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IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
Table E-3. HELP Model Parameters and Results
Input Parameters
Maximum Leaf Index
Curve Number (CN)
Root Zone Depth
Precipitation
0
99.75
0.10 in (0.25 cm)
32.07 in/yr(81 cm/yr)
Water Budget of Average Annual Totals for Years 1974 through 1978
Evapotranspiration
Runoff
Change in Water Storage
Subgrade Infiltration
2.01 in/yr(5.1 cm/yr)
27.58 in/yr(70 cm/yr)
0.05 in/yr(0.13 cm/yr)
2.42 in/yr(6.1 cm/yr)
It was found that appropriate parameters were required to obtain simulated subgrade infiltration
close to the observed subgrade infiltration. To determine appropriate ranges of parameters,
material properties and surface conditions were varied to examine the HELP model sensitivity to
input parameters. Among the three key parameters (curve number [Chow et al., 1988], saturated
hydraulic conductivity, and evaporation depth), the curve number was found to be most
influential to the subgrade infiltration rate. The curve number describes the imperviousness of a
surface; the higher the number, the more impervious the surface. Although curve numbers up to
98 are recommended in the literature for pavement surface, a curve number value greater than 99
was found to generate more realistic subgrade infiltration rates (see Figure E-10, graph a).
Figure E-10, graph b, shows that saturated hydraulic conductivity in the range shown is not a
sensitive factor for subgrade infiltration. In the figure, it can be seen that saturated hydraulic
conductivity begins to decrease subgrade infiltration when it is below a certain threshold value.
Above this threshold value, infiltration remains constant and impact on runoff is very small.
Evaporative zone depth is an important factor to evaporation as it dictates the amount of
evaporative loss that may occur (see Figure E-10, graph c)the more evaporative loss, the
smaller the subgrade infiltration. As a result, a small value (less than 0.5 inches[about 1 cm]) of
evaporative zone depth should be used. It was found that infiltration is not sensitive to layer
thickness. The insensitivity to pavement thickness implies that the storage effects in pavements
are small. This finding is consistent with results obtained from the variably saturated flow model
reported in Section E.5.1. All the analyses reported here are based on HELP-determined, long-
term average steady-state moisture profiles at the beginning of the simulations because they are
likely to be the average conditions found in the field.
E-13
-------
IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
c
o
10
£ F 6 -
_c >,
Si
w
98 98.5 99
Curve Number
99.5
100
a) Effect of Curve Number (Evaporative Zone Depth=0.1 in, Hydraulic Conductivity = 10~3 cm/sec)
_ fc o
^25
c
O 9
5
U-
c
-1
o I
o
re
- n ^ -
O) u-;-'
^2
= n .
^
(/) U -i i i
1.00E-07 1.00E-05 1.00E-03 1.00E-01 1.00E+01
Hydraulic Conductivity (cm/sec)
b) Effect of Hydraulic Conductivity (Curve Number = 99.75, Evaporative Zone Depth=0.1 in)
- * o J^n
-C
c
~- o nn
c
g
s 1 c;n
+j
"? 1 nn
o
n
m n ^n
u>
^j
3 n nn
w °-°° ^
0.
*"^-^_^_
*
30 0.20 0.40 0.60 0.80 1.00 1.20
Evaporative Zone Depth (in)
c) Effect of Evaporative Zone Depth (Curve Number= 99.75, Hydraulic Conductivity = 10~3 cm/sec)
Figure E-10. Sensitivity of HELP-determined bottom exfiltration to: a) curve number; b) hydraulic
conductivity; and c) evaporation zone depth.
E-14
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IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
E.6 Summary
To use the roadway source term in IWEM, the user is required to provide infiltration rate through
roadway pavements. Recognizing the importance of this parameter and coupled with the
difficulty for the user to obtain reliable values from literature, the EPA provided in IWEM
estimated infiltration rates in various roadway materials. The main objective of this section was
to present the general approach used to determine physics-based, long-term subgrade infiltration
rates at the bottom of various types of pavements in a number of climatic zones. Subgrade
infiltration refers to water that exits the subbase layer (the bottom of the source) and percolates
into the subgrade material (the top of the vadose zone), below.
Because the infiltration data through the roadway pavements are limited, mathematical models,
described in Sections E-4 and E-5, were used to help generate the required infiltration rates. In
order to ensure that the models could be reliably used to estimate infiltration rates, the models
were first verified against available infiltration data from actual field observations. The verified
models were then utilized to generate infiltration rates for various types of pavements in different
climatic zones.
For the purposes of subgrade infiltration estimation, six climatic zones were defined using the
freezing index and precipitation as demarcation criteria. For each of these zones infiltration rates
for several types of pavements and respective ranges of material properties were estimated using
the water-budget HELP model. A procedure to utilize the estimated subgrade infiltration rates
corresponding to given climatic conditions is presented in Section 6.4.3.
E.7 References
Ahmed, Z., T.D. White, and T. Kuczek. 1997. Comparative Field Performance of Subdrainage
Systems. Journal of Irrigation and Drainage Engineering 723(3): 194-201.
Apul, D., K. Gardner, T. Eighmy, J. Benoit, and L. Brannaka. 2002. A Review of Water
Movement in the Highway Environment. Recycled Materials Resource Center, University
of New Hampshire, Durham, NH.
Apul, D.S., K.H. Gardner, and T.T. Eighmy. 2007.Modeling Hydrology and Reactive Transport
in Roads: The Effect of Cracks, The Edge, and Contaminant Properties. Waste
Management 2 7( 10).
Apul, D.S. 2007. E-mail from Dr. D.S. Apul (University of Toledo, OH) to Dr. Z. Saleem of
USEPA on 5/30/2007.
Bear, J., 1972. Dynamics of Fluids in Porous Media. American Elsevier, New York, NY.
Caltrans. 2000. California Department of Transportation District 7 Litter Management Pilot
Study -Final Report. 26 June 2000, Department of Transportation, Sacramento, CA.
Carsel, R.F., and R.S. Parrish. 1988. Developing Joint Probability Distributions of Soil Water
Retention Characteristics. Water Resources Research 2₯(5):755-769.
E-15
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IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
Chow, V.T., D.R. Maidment, and L.R. Mays. 1988. Applied Hydrology. New York: McGraw-
Hill.
Feng, A., J. Hua, and T.D. White. 1999. Flexible Pavement Drainage Monitoring Performance
and Stability. Purdue University, West Lafayette, IN.
Guvanasen, V., S. Frost, C-T. Huang, and P. Burger. 2004. Development of a Three-Dimensional
Density-Dependent Saltwater Intrusion Model for an East-Central Florida Region.
Proceedings of the 6th Conference on Groundwater Resources and Water Quality
Protection, Feng-Chia University, Tai-Chung, R.O.C. (Taiwan). April 8-9.
HydroGeoLogic, 2011. MODHMS/MODFLOW-SURFACT: A Comprehensive MODFLOW-
basedHydrologic Modeling System., HydroGeoLogic, Reston, VA.
Irish, L.B., Jr., W.G. Lesso, M.E. Barrett, J.F. Malina, Jr., R.J. Charbeneau, and G.H. Ward.
1995. An Evaluation of the Factors Affecting the Quality of Highway Runoff in the
Austin, Texas Area. Center for Research in Water Resources Technical Report 264, The
University of Texas at Austin, TX.
Li, L., C.H. Benson, and T.B. Edil. 2005. Assessing Groundwater Impacts from Coal.
Combustion Products Used In Highways. Geo Engineering Report No. 05-22, University
of Wisconsin at Madison, WI.
McDonald, M.G., and A.W. Harbaugh, 1988. A Modular Three-dimensional Finite Difference
Groundwater Flow Model: U.S. Geological Survey Techniques of Water-Resources
Investigations Book 6, chapter Al.
Minnesota DOT. 2007. E-mail from A. Eller (MnDOT) to Dr. Z. Saleem of USEPA on
10/17/2007.
Rainwater, N.R., G. Zuo, E.C. Drumm, W.C. Wright, and R.E. Yoder, 2001. In Situ
Measurement and Empirical Modeling of Base Infiltration in Highway Pavement
Systems, Transportation Research Record, Paper No.01-2868, Vol. 1772, pp: 143-150.
Ridgeway, H. 1976. Infiltration of Water Through the Pavement Surface. Transportation
Research Record (57(5:98-101.
Sauer, J.J., C.H. Benson, and T.B. Edil. 2005. Metals Leachingfrom Highway Test Sections.
Constructed with Industrial Byproducts. Geo Engineering Report No. 05-21, University
of Wisconsin at Madison, WI.
Schroeder, P.R., T.S. Dozier, P.A. Zappi, B.M. McEnroe, J.W. Sjostrom, andR.L. Peton. 1994a.
The Hydrologic Evaluation of Landfill Performance Model (HELP): User's Guide for
Version 3. EPA/600/R-94/168a. U.S. EPA Risk Reduction Engineering Laboratory,
Cincinnati, OH. Available at http://nepis.epa.gov/EPA/html/Pubs/pubtitleORD.htm.
E-16
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IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
Schroeder, P.R., T.S. Dozier, P.A. Zappi, B.M. McEnroe, J.W. Sjostrom, andR.L. Peton. 1994b.
The Hydrologic Evaluation of Landfill Performance (HELP) Model: Engineering
Documentation for Version 3. EPA/600/R-94/168b. U.S. EPA Risk Reduction
Engineering Laboratory, Cincinnati, OH. Available at http://nepis.epa.gov/EPA/html/
Pub s/pubtitl eORD. htm.
Stormont, J.C. and S. Zhou. 2001. Improving Pavement Sub-surface Drainage Systems by
Considering Unsaturated Water Flow. Cooperative agreement DTFH61-00-X-00099,
report to the U.S. Department of Transportation, Federal Highway Administration.
Tu, K., J. Kool, V. Guvanasen, and M-S. Tsou. 2006. Assessment ofLNAPL Source Zone and
Impacts on Longevity of Dissolved Phase Benzene Plume. MODFLOW and More 2006,
Colorado School of Mines, Golden, CO, May 22-24.
U.S. EPA (Environmental Protection Agency). 2003a. EPA 's Composite Modelfor Leachate
Migration with Transformation Products (EPACMTP): Technical Background
Document. Office of Solid Waste, EPA530-R-03-002, April 2003.
U.S. EPA (Environmental Protection Agency). 2003b. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Parameter/Data Background
Document. Office of Solid Waste, EPA530-R-03-003, April 2003.
Van Genuchten, M.Th. 1980. A Closed-form Equation for Predicting the Hydraulic Conductivity
of Unsaturated Soils, Soil Sci. Soc. J. 44: 892-898.
E-17
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IWEM Technical Background Document Appendix E: Infiltration Rates Through Pavements
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E-18
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IWEM Technical Background Document
Appendix F: Non-Orthogonality
Appendix F:
A Discussion on the Formulation of the Non-
Orthogonality between the Highway Axis and the
Regional Ground Water Flow Direction
F.1 Introduction
Given the general settings of roadways, it is possible that the centerline axis of a roadway may
not be directly perpendicular to the regional ground water flow direction. Figure F-l shows an
idealized (straight line) highway segment with several laterally contiguous rectangular highway-
source strips oriented at an angle a with respect to the normal of the regional ground water flow
direction. However, the current aquifer transport module in the EPA's Composite Model for
Leachate Migration and Transformation Products (EPACMTP) code (U.S. EPA, 2003a, b, c, d)
adopted in the Industrial Waste Management Evaluation Model (IWEM) can handle only the case
with a = 0°. In order to handle a general case with a > 0°, the results of the existing aquifer
transport module must be modified post simulation. A general approach, which is discussed in
Appendix C , is that the reference x-y coordinate system is transformed into the x'-y' coordinate
system, which aligns with both the highway axis and the regional flow direction (see Figure F-l).
The latter is, in turn, transformed into the x"-y" coordinate system, which is rectilinear, as shown
in Figure F-2. Once transformed, the existing aquifer transport module can be used to describe
fate and transport of contaminants in the transformed domain.
Regional Flow Direction
Y'
\ 9
\
(x1, y')
w
/\ I
A ' ?
X'
Figure F-1. Non-orthogonal source.
F-l
-------
IWEM Technical Background Document
Appendix F: Non-Orthogonality
Y'
X:
(a) Strip Source i prior to transformation
Ft
-+ X"
(b) Strip Source i after transformation
Figure F-2. Transformed orthogonal source for strip /.
With the approximation of the source subject to a non-orthogonal flow direction, as shown in
Figures F-l and F-2, it is necessary to address the issue of the accuracy of the transport module.
In this note, the factors that may affect the accuracy of the transport module using the
transformation are discussed in detail in Section F.2. Recommended analysis procedures to
obtain conservatively accurate simulation results are given in Section F.3.
F.2 Formulation
F.2.1 Source Geometry and Non-Orthogonality of Regional Flow Field
F.2.1.1
Regional Flow Field
Based on the assumptions listed below, the regional flow field may be approximated by a
solution with infiltration equal to recharge.
in the region of interest, the general regional ground water flow pattern is not affected by
the presence of a traversing highway;
infiltration from the traversing highway is on the order of regional recharge; and,
the areal coverage of the highway is very small compared to the total regional area so that
the difference between the infiltration and recharge does not cause significant impact on
the regional flow field.
F-2
-------
IWEM Technical Background Document Appendix F: Non-Orthogonality
As a result of these assumptions, there is no distortion in the flow field in the vicinity of the
highway.
F.2.1.2 Treatment of a Rectangular and Non-orthogonal Source
In general, a highway may be oriented in such a way that the highway axis is not orthogonal to
the regional ground water flow direction. To accommodate the non-orthogonality of the highway
source, it is necessary to transform the transport domain in the following manner.
For the current aquifer module, the reference frame for transport in a horizontal plane is the
system of x-y axes shown in Figure F-l. The complementary inclination angle a (= 90° - 9,
where 9 is the inclination angle) is incorporated into the analysis via the following two
successive transformations:
C'1(x,y,z,t)=C'1(x,y,z,t)=C'1(x,y\z\t) (F-l)
where:
x = x-ytana; x > 0,a <
y = _yseca
x" = x (F-2)
y" = y cosa = y
C'i = Concentration in the transport domains emanating from highway-source strip /'
(M/L3)
x = Distance along the flow direction measured from the midpoint of the down
gradient face of the strip of interest (L)
y = Distance normal to the flow direction measured from the midpoint of the down
gradient face of the strip of interest (L)
x' = Distance along the flow direction measured from the down gradient face of the
strip of interest in the transformed domain (L)
y' = Distance measured from the x' axis along the direction parallel to the axis of the
highway (L)
x" = Distance along the flow direction measured from the down gradient face of the
strip of interest in the transformed domain (L)
y" = Distance normal to the flow direction measured from the midpoint of the down
gradient face of the strip of interest in the transformed domain (L)
z, z', z" = Depth measured from the water table (L)
With the transformation in Equations (F-l) and (F-2), the source in Figure F-l is approximately
transformed into the one shown in Figure F-2. Note also that in the transformed domain, the
dimensions of the highway source are also accordingly transformed (compare Figures F-l and F-
2).
F-3
-------
IWEM Technical Background Document Appendix F: Non-Orthogonality
The above transformation has two limitations:
It is an approximate transformation and does not account for source end effects (see
Section F.2.4 for a detailed discussion on the end effects), and
The complementary angle of inclination must remain below 90°; therefore, the regional
flow direction is not allowed to be parallel to the highway axis.
The objective of these transformations is to take advantage of the existing IWEM flow and
transport modules developed for rectangular coordinate systems in which the source is always
rectangular , with its width along to the flow direction and its length normal to the flow direction.
In Figure F-l, the transformation from the x-y to the x'-y' coordinate systems is to render the y'
axis parallel to the highway axis. As an illustration of the transformation on the advective
transport process, in Figure F-2, a front location at time t of a plume emanating from the leading
(downgradient) edge of the source in the x'-y' coordinate system and the corresponding front of
the corresponding plume in the x"-y" coordinate system are shown. The transformation enables
the plume in the x'-y' system to be mapped onto the x"-y" system. A detailed derivation in
Section F.2.2, below, indicates that the transformation is valid for all transport processes
(retardation, advection, dispersion, and decay) in the existing transport module.
F.2.2 Transport Equation in the Transformed Domains
In the original frame of reference x-y-z, the transport equation may be written as follows:
c _K,
, , >,
ot dx dx dy
In the transformed frame of reference x'-y'-z', Equation (F-3) becomes
dx cos a dy' dz (F-4)
In the transformed frame of reference x"-y"-z", Equation (F-4) becomes
dRdC dC d2C ._ d2C ._ d2C
, -AC'
St dx dx" " dy" dz"* (p_5)
Where:
Rd = Retardation factor (unitless)
Dxx = Horizontal longitudinal dispersion coefficient along the x direction (L2/T)
Dyy = Horizontal transverse dispersion coefficient along the x direction (L2/T)
Dzz = Vertical transverse dispersion coefficient along the x direction (L2/T)
A, = Decay constant (1/T)
F-4
-------
IWEM Technical Background Document Appendix F: Non-Orthogonality
Based on the transformations given in Equation (2.2), it can be stated that:
dC' dC dC
dx dx' dx"
dC 1 8CdC
dy cos a dy' dy"
(F-6)
dz dz' dz"
assuming that the longitudinal concentration gradient is much smaller than the lateral
concentration gradient. The terms in Equation (F-6) are used in transforming Equation (F-3) to
Equations (F-4) and (F-5).
Note that Equations (F-3), (F-4), and (F-5) are identically bounded at infinity in the x and y
directions. In other words, concentration becomes zero at infinity. In the z direction, identical
flux conditions (input flux at the water table, and no advective and dispersive fluxes at the base
of the aquifer) are identically applied to all equations. Note also that the retardation factor,
dispersion coefficients, and decay constant are assumed to be uniform throughout the flow and
transport domain.
A comparison between Equations (F-3) and (F-5) reveals that the transformed transport equation
is identical in form to the original transport equation (Equation F-3). The identicality between the
two equations implies that Equation (F-l) is correct. The two equations will give identical
solutions at a homologous position, provided that all other constraints and boundary conditions
are identical.
F.2.3 Transformation of Source and Permissible Complementary Inclination
Angle
The rectangular source shown in Figure F-l is reproduced in Figure F-3 as Rectangle ABCD to
illustrate the transformation process and to describe the end effects. The rectangle after
transformation from x-y to x"-y" becomes a parallelogram, as shown in Figure F-4. The
parallelogram in Figure F-4 is further simplified and approximated by a rectangular source, as
shown in Figure F-2. Note that the area of the source remains invariant and equals to LW.
F-5
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IWEM Technical Background Document
Appendix F: Non-Orthogonality
Leading corner
x
"Trailing corner
Figure F-3. Source before transformation in x-y coordinate system.
Y" Leading corner
B
P
n_
C
^
\
\,
A
^\D
r
W/cos a
/
/
t 0.5 I
0.5 L cos a + W
.
-------
IWEM Technical Background Document
Appendix F: Non-Orthogonality
In comparing Figures F-2 and F-4, one can see that the parallelogram in Figure F-4 is
approximated by the rectangular source shown in Figure F-2. For this approximation to be valid,
it is necessary that
0.5L cosa + Wsina , W _
= 1+2 tana =! + £
0.5L cosa L
(F-7)
Proviso (F-7) implies that A and D are of approximately the same respective ordinates as B and
C on the y" axis so that ABCD is approximately rectangular as shown in Figure F-2.
For the second term in Proviso (F-7) to be very small, with tolerance = s, the following condition
must be satisfied.
a < tan
(W2,
(F-8)
Proviso (F-8) gives the maximum value of the complementary inclination angle that Proviso (F-
7) is not violated as a function of the source aspect ratio (LAV) and tolerance (s). Examples of
limits on the conjugate inclination angle as a function of aspect ratio and tolerance are given in
Table F-l
Table F-1. Limits of Complementary Inclination Angle a
Aspect Ratio (L/W)
500a
1,000b
10,000C
a (Degrees)
e = 0.01
68
79
89
e = 0.02
79
84
89
Notes
3 A typical example: a mile long 10-ft lane
b A typical example: two-mile long 10-ft lane
c A typical example: twenty-mile long 10-ft lane
From Table F-l, one can see that the larger the aspect ratio of the source or tolerance, the larger
the permissible complementary inclination angle for Proviso (F-7) to be valid.
F.2.4 End Effects
A typical concentration distribution along line A*B* (in Figure F-4) due to a typical rectangular
source is given in Figure F-5. In the figure, a reference concentration profile without lateral
dispersion, with a well-defined profile width based on advection alone (L cos a), is shown.
Shown along with the reference profile is a typical corresponding plume with lateral dispersion.
F-7
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IWEM Technical Background Document
Appendix F: Non-Orthogonality
Trailing
Profile due to
parallelogram source
Leading
c/a
Reference profile
V
\
>
.'
*
//
/
f
\/
*
^ ^
\ >
0.5
0
Profile due to rectangular
\ source
* /
'\/
: ?
X \ y"
Lcos a
Figure F-5. Concentration profiles along line A*B* (in Figure F-4) due to rectangular source and
parallelogram source. The black solid line represents a reference profile without lateral dispersion.
In the previous subsection, it has been shown that if the complementary inclination criterion is
violated, the approximation by a rectangular source may incur error because the actual geometry
of the transformed source is that of a parallelogram. Plotted in Figure F-5 is a typical
concentration profile along Line A*B* due to a parallelogram source. In the figure, it can be seen
that the mass is shifted laterally towards the positive y". The reason for this shift can be inferred
from Figure F-4. In the figure, it can be seen that in order to approximate parallelogram ABCD
by rectangle AEFD, the mass in triangle ABE has to be shifted to triangle CDF. For this reason,
the solution based on the rectangular source may tend to overestimate concentration in the
vicinity of the plume emanating from the leading corner (see Figures F-3 and F-4) and
underestimate concentration in the vicinity of the plume emanating from the trailing corner (see
Figures F-3 and F-4). It should be noted also that, because of the geometry of the source near the
parallelogram's top and bottom apices (B and D), the profile of the parallelogram source shows
more apparent lateral dispersion.
To overcome the problem of end effects, a recommendation is given in Figure F-6. In the figure,
it can be seen that source is artificially extended from either end by a length of W tan a. This
extension makes the transport solution conservative as it will tend to overestimate concentration
in the area of the plume that emanates from the leading corner of the source.
F-8
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IWEM Technical Background Document
Appendix F: Non-Orthogonality
O Receptor
Well
Figure F-6. Artificial extension of the source length to overcome the end effects.
F.2.5 Special Case with Complementary Angle of Inclination = 90°
In the case that the complementary angle of inclination is 90°, it is recommended that the length
L be treated as the width W and vice versa as shown in Figure F-7.
t y
W
B
A
X
Figure F-7. Special case with conjugate angle of inclination (a) = 90 degrees.
F-9
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IWEM Technical Background Document Appendix F: Non-Orthogonality
F.3 Summary
Based on the derivation shown in Section F-2, the following can be stated:
The transport equation remains unchanged in form after two successive transformations
from x-y-z to x'-y'-z' to x"-y"-z" coordinate systems. The resulting equation in the x"-y"-
z" system is identical to the one in the original x-y-z system.
End effects are minimal if Proviso (F-8) is not violated.
End effects may be circumvented by extending the source artificially from either end by a
length of W tan a, W being the width of the source and a, the complementary angle of
inclination.
In a special case where a = 90°, it is recommended that the length L be treated as the
width W and vice versa.
F.4 References
U.S. EPA (Environmental Protection Agency). 2003 a. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Technical Background
Document. U.S. EPA, Office of Solid Waste, EPA530-R-03-002, April.
U.S. EPA (Environmental Protection Agency). 2003b. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Parameter/Data Background
Document. U.S. EPA, Office of Solid Waste, EPA530-R-03-003, April.
U.S. EPA (Environmental Protection Agency). 2003c. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Draft Addendum to the Technical
Background Document. USEPA, Office of Solid Waste. September.
U.S. EPA (Environmental Protection Agency). 2003d. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Draft Addendum to the
Parameter/Data Background Document. U.S. EPA, Office of Solid Waste. April.
F-10
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Appendix G: Verification of the Roadway Module in IWEM
G.1 Introduction
This appendix describes the verification performed on the roadway module. The verification was
conducted using the roadway module in IWEM 3.0. IWEM 3.0 is a descendent of IWEM 2.0,
which was revised to enhance the functionality and usability of the module. The enhancement
allow for a more rigorous treatment of leaching through the roadway cross-section by accounting
surface runoff and flow through ditches and drains; these processes were omitted in IWEM 2.0.
G.1.1 Background
A typical highway is depicted in Figure G-l. It was assumed that only a segment of the highway
shown in the figure is constructed with industrial materials. For the sake of simplicity, it was
assumed that the segment is sublinear and can be approximated by a straight line. In the event
that the segment is long and meandering, it must be subdivided into several sublinear segments
so that each sublinear segment can be represented by a straight line.
Highway
Regional Flow Direction
Linear Approximation of the
Highway Segment of Interest
»** Highway Segment
of Interest
Figure G-1. A Typical Highway with a Recycled-Material Segment
Figure G-2 shows a typical cross section of a highway, indicating that a highway may comprise
several components (e.g., travel lane, shoulder, and ditch). Each component was idealized as a
column, referred to henceforth as the "highway-source column." In the vertical direction, as
shown in Figure G-3, each highway-source column included materials starting vertically upward
from a reference datum (which could be the top of subgrade) to the surface of a pavement or a
G-l
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
road shoulder or an embankment or a ditch. As shown in Figure G-3, each highway-source
column was underlain by a corresponding vadose-zone column.
Traveled Lanes
Traveled Lanes
Ditch
Gutter Drains4rade
Permeable Drain
Gutter
Base
Figure G-2. A typical cross section of a roadway.
Source Strip i = 1 Source Strip i = 2 Source Strip i =3 Source Strip i = 4
Ditch & Left Shoulder Left Lane Right Lane Right Shoulder
Vadose Zone 1
Vadose Zone 2
Vadose Zone 3
Vadose Zone 4
Figure G-2. Modules in IWEM Corresponding to Multiple Highway-Source Strips
A highway-source column was assumed to be uniform in terms of parameters and properties
along the length of interest. Therefore, a highway-source column becomes a highway-source strip
in three dimensions. Figure G-4 shows an example of a highway cross section comprising three
highway-source strips representing, respectively, a median, a travel lane, and a ditch. Note that a
typical highway may consist of at least five highway-source strips: left shoulder, left travel lane,
median, right travel lane, and right shoulder. An example of only three highway-source strips is
used here as a basis for further discussion. Each highway-source strip may consist of several
layers, depending on how a given highway was constructed. A travel lane way strip may be
composed of a pavement layer (pPortland-cement concrete or asphalt concrete), a base-course
layer, a subbase layer, and a subgrade layer. A median may comprise a base layer, a subbase
layer, and a subgrade layer. An unpaved road shoulder may have only one a layer: a subgrade
layer. With this type of conceptualization, one can see that each highway-source strip was
G-2
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
equivalent to the existing landfill source module that is available within EPACMTP. However,
the existing landfill module could accommodate only sources with square footprint and one layer.
Road Shoulder,
Embankment, and Ditch
*-*
Source Strip i = 1 Source Strip i = 2 Source Strip i = 3
Figure G-3. An Example of Layering in Highway-Source Strips
Furthermore, given a highway's general settings, it is possible that the highway axis may not be
normal to the regional flow direction. Figure G-5 shows an idealized (straight line) highway
segment with several laterally contiguous rectangular highway-source strips oriented at an angle
a with respect to the normal of the regional flow direction. The existing aquifer transport module
could handle only the case with a = 0°, in which the flow direction is normal to the highway axis.
Regional Flow Direction
Y'
X1
Figure G-5. Non-Orthogonal Source
G-3
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
G.I.2 Verification Objectives
In order to improve the highway source term, the following enhancements were implemented to
the IWEM 3.0..
Non-aligned flow direction. It is no longer necessary for the regional flow direction to
coincide with the normal of the highway axis.
Multiple-layer and multiple-strip source. The source module can now comprise
multiple layers and multiple strips.
The main objective of the verification was to ensure that the above two new features have been
correctly incorporated into the IWEM software. In addition, the non-aligned flow direction
feature involves an approximate transformation of the transport equation. The objective of the
verification was also to ascertain the degree of accuracy of the transformation.
G.1.3 Verification Approach
The verification was carried out by comparing the results obtained from a number of verification
scenarios using the IWEM software and U.S. Geological Survey (USGS) MODHMS (HGL,
2006). MODHMS can solve the flow-and-transport equations without the approximate
transformation used in the non-aligned flow direction feature (RTI and HGL, 2006). The
transport-equation solutions provided by the MODHMS code were considered accurate and
treated as a standard with which the IWEM software was compared.
MODHMS is an enhanced version of the USGS MODFLOW modular three-dimensional
groundwater flow code (McDonald and Harbaugh, 1988). The MODHMS code has been selected
for the following reasons:
It can handle variably saturated flow and transport.
It has been extensive verified and documented.
It has been implemented by a number of government agencies in different settings.
The verification scenarios are described in Section G.2. Verification results are presented in
Section G.3.
G.2 Verification Scenario Description
Contaminant transport simulations using the new IWEM source module and MODHMS under
the following three verification scenarios were performed:
1. Transport with a single-strip single-layer source;
2. Transport with a single-strip and multiple-layer source; and
3. Transport with a multiple-strip and single-layer source.
In all cases, it was assumed that the vadose zone did not exist so that the infiltration with
contaminant entered the water table immediately after leaving the bottom of the roadway source.
This assumption makes the leachate influx at the top of the water table to be well defined,
thereby allowing the influx conditions for the MODHMS model and the IWEM saturated zone
module to be identical. The vadose-zone module in EPACMTP (U.S. EPA, 2003) was not part of
G-4
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
the verification exercise because it is not impacted by the non-aligned flow direction feature and
it has been previously verified as part of the EPACMTP code. In all cases, it was assumed that
neither adsorption nor degradation occurred in the groundwater. The regional groundwater
gradient in the MODHMS-based model was imposed by constant heads along the boundaries.
The above three scenarios are described in the following subsections. Results are given in
Section G-3
G.2.1 Verification Scenario 1: Single-Strip and Single-Layer Roadway Source
with Flow at Different Angles to the Axis of the Roadway
Verification Scenario 1 represents a single-strip and single-layer highway source with flow at two
different angles to the axis of the roadway:
(a) Orthogonal: Flow orthogonal to the roadway (a = 0°, 9 = 90°); and
(b) Sub-Orthogonal: Flow at 45E to the roadway (a = 45°, 9 = 45°).
The above two scenarios are diagrammatically summarized in Figure G-6. A continuous
infiltration source with a constant concentration (Co = 1 mg/L) was assumed. For each case,
MODHMS and IWEM models were constructed.
Case (b) 9 = 45°
\
Case (a) Orthogonal Flow, 9 =
Figure G-6. Flow Angles
It has been shown that the conjugate inclination angle must satisfy the following proviso:
G-5
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
(G-l)
Proviso (G-l) gives the maximum value of the conjugate inclination angle as a function of the
source aspect ratio (LAV) and tolerance (unitless) that the approximation of the source as a
rectangle in the x"-y" coordinate system remains valid.
Based on the aspect ratio value of 19 (47.5 ft/ 2.5 ft, Figure G-6), maximum permissible
conjugate inclination angle values as a function of tolerance are given below
E
0.05
0.1
a (Degrees)
25
45
According to the table above, for the tolerance value = 0.1 (which is considered relatively large),
the conjugate inclination angle is maximum permissible based on Proviso (G.I) is 45°.
Simulation results from these models are summarized in Section G.3.
G.2.2 Verification Scenario 2: Single-Strip and Multiple-Layer Roadway Source
with Flow at Different Angles to the Axis of the Roadway
Verification Scenario 2 consists of a three-layer single-strip roadway source. The
hydrogeological properties of the sources were assumed to be identical. The only differences
among the sources were the thickness and initial concentration. As shown in Figure G-7, the
thicknesses of the top layer to the bottom layer are 1 foot, 1 foot, and 2 feet, respectively. Only
the middle layer contains a contaminant at an initial concentration of 1 unit mass/unit volume.
The other two layers are contaminant-free. This configuration of the source layers results in a
square concentration pulse at the bottom of the bottommost layer, as shown in Figure G-7. Note
that the infiltration contains no mass when entering the top layer. It was also assumed that no
adsorption occurred in any of the three layers. The shape of the pulse in Figure G-7 is based on
the pulse formulation given in (RTI and HGL, 2006). In Figure G-7, one can see that the pulse
first appears at 300 days because it takes 300 days for the mass from the middle layer to traverse
the bottom layer before it emerges as leachate. The length of the pulse is 100 days.
The simulation of the MODHMS model was divided into three stress periods to account for the
square pulse. In all stress periods, the infiltration was kept constant at 0.01-ft/day. As shown in
Figure G-7, the first stress period is 300 days and contaminant free (concentration = 0). The
second stress period is 100 days with constant leachate concentration (1 mg/L). The last stress
period is 36,500 days with leachate concentration = 0. Two test cases were simulated under this
scenario: (a) the orthogonal flow case; and (b) the sub-orthogonal flow case (45° inclination).
These two cases correspond to Cases (a) and (b), respectively, in Section G.2.1. Simulation
results are summarized in Section G.3.
G-6
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Infiltration =0.01 ft/day
Thickness = 1ft, Initial Concentration = 0
Thickness=l ft, Initial Concentration^ unit
mass/ unit volume
Thickness=3 ft, Initial Concentration = 0
Concentration (unit mass/ unit volume)
100 days
300 days
36,500 days
Time (day)
Figure G-7. Single-strip, Multiple-layer Source.
G.2.3 Verification Scenario 3: Multiple-Strip and Single-Layer Roadway Source
with Flow at Different Angles to the Axis of the Roadway
Verification Scenario 3 consists of a single-layer three-strip roadway source. The strips have
identical dimensions of 500-ft by 25-ft, and are placed side-by-side as shown in Figure G-8. An
upgradient and a downgradient strips were added to the original strip in Verification Cases 1 and
2. The infiltration rate was assumed to be 0.01-ft/day for all three strips, but the initial
concentrations of individual strips were different. The strip closest to the upstream boundary was
assigned with the highest concentration of lunit mass/unit volume. The second strip had a
concentration of 0.5 mg/L. The downstream-most strip had the lowest concentration of 0.1 mg/L.
Two test cases were simulated under this scenario: (a) the orthogonal flow case and (b) the sub-
orthogonal flow case (45° inclination). These two cases correspond to Cases (a) and (b),
respectively, in Section G.2.1. Simulation results are summarized in Section G.3.
G-7
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Case (b) 9 = 45C
Case (a) Orthogonal flow, 9 = 90C
4
\
\
, 250 I
^
»
\
; \
X 4
^^^oc^c
;\\\\\\\\\X
Obsl '
iA
\
^5 J
J^^^^C
;\\\\\\\\V
\ -<
^
I Obs2
00
[
4
* *
cs
P-J*
)
N
> ^i
Kj
CS
C5
f
J
4
Figure G-8. Multiple-strip, Single-layer Source Scenario
G.2.4 Verification Scenario 4: Multiple-Strip Roadway Source with Drainage
Systems and Ditches
Figure G-9 depicts the settings of Verification Scenario 4 in which the flow is perpendicular to
the highway axis. The size of the model domain, as shown in Figure G-9a, is 1,200 ft by 1,600 ft.
Flow and transport properties in the model domain are homogeneous with a hydraulic
conductivity of 10 ft/day and an effective porosity of 0.3. A hydraulic gradient of 0.0075 along
the direction perpendicular to the highway axis was assigned across the model domain. A 400 ft
by 90 ft strip source is located 110 ft from the up-stream boundary and 600 ft from the upper
boundary. The 90 ft wide source is composed of two 10 ft wide roadside ditches and seven strips
with the following widths: 10, 10, 20, 10, 20, 10, and 10 ft. An infiltration rate of 7.3xlO'3 ft/day
was imposed on the seven roadway-source strips. Two infiltration rates of 1.6634 and 1.9644
ft/day were applied to the left-hand-side and right-hand-side ditches, respectively. As shown in
Figure G-9a, four observation wells (Obsl to Obs4) are located down-gradient from the source.
Obsl is 100 ft from the down-gradient edge of the source and on the centerline of the domain.
Obs2 is 100 ft from the down-gradient edge of the source and 180 ft from the centerline of the
domain. Obs3 is 800 ft from the down-gradient edge of the source and 180 ft from the centerline
of the domain. Obs4 is 800 ft from the down-gradient edge of the source and on the centerline of
G-8
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
the domain. All four observation wells are screened in the top layer. The longitudinal and
transverse dispersivities of 100 and 10 ft, respectively, were used in the simulation. A uniform
saturated thickness of 100 ft exists throughout the model domain. In the vertical direction, the
entire saturated thickness was represented by ten uniform layers. In the horizontal direction, the
domain was discretized into uniform 25 ft by 25 ft grid cells. The source was discretized into
four grid intervals along the flow direction (20 ft, 25 ft, 25 ft, and 20 ft) and 16 uniform grid
intervals (25 ft) perpendicular to the flow direction. Based on the current configuration, the
aspect ratio of the source is equal to 400 ft/ 90 ft or 4.4.
1200
110
n
700
9.
;Obs2
Obs3
"Obs1
ro
o
00
o
Flow Direction
Figure G-9. Multiple-strip (7 strips) with Drainage Systems and Ditches Scenario
Figure G-10 shows the roadway cross section comprising two ditches and seven roadway-source
strips: one left embankment; two left travel lane; one median; two right travel lane; and one right
embankment. Two symmetrical drainage systems with a permeable base and an edge drain
leading to a ditch were adopted for both sides of the roadway in this example. Each travel lane
strip is composed of two pavement layers, one permeable base layer, and one subgrade layer.
Each permeable base layer diverts part of infiltration water to a receiving ditch. The median
comprises two layers: a base layer and a subgrade layer. The dimensions are shown in Figure G-
9b.
Contaminant pulses entering the vadose zone are based on the parameters given in Table G-l
and are shown in Figure G-ll. All the pulses were verified manually. The pulses exiting the base
of the pavement systems from different strips are shown in the figure.
G-9
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Runoff
Runoff
Traveled Lanes Traveled Lanes
Embankment
Embankme
Figure G-10. Cross section of roadway segment in Case 4 with two symmetric drainage systems.
Table G-1 Parameter Values for IWEM Roadway Source Module Example
Parameter
Description
Remarks
Source Geometry
Q
dk,:
Wi
L
n Strips
nLayers(i)
nSubLayers (ten)
nSubLayers (right)
dpB(left)
woitch (ten)
dsG.i (left)
dpB(right)
woitch (right)
dsc,: (right)
Angle of inclination (see Figure 1 .5, Chapter 1 )
Thickness of layer k of the pavement structure
in roadway-source strip / (see Figure 1.4,
Chapter 1, and Figure G-10, this appendix)
Width of roadway-source strip / (see Figure 1.5)
Length of given roadway segment (see Figure
1.5)
Number of roadway-source strips
Number of material layers for roadway-source
strip /
Number of the left-hand side subgrade layers
Number of the right-hand side subgrade layers
Thickness of the left-hand side permeable base
Width of the left-hand side ditch
Thickness of the left-hand side subgrade layer)
Thickness of the right-hand side permeable
base
Width of the right-hand side ditch
Thickness of the right-hand side subgrade layer
I
90
dkj=1 ft (0.3 m) for the top 2 layers above
permeable base; dk,:=2.5 ft (0.75 m) for
roadway-source strip without permeable
base.
10 ft (3m)
400 ft (120m)
7
2 layers in each strip. Note that the Strips
1 and 7 represent embankments, and
Strip 4 represents a median. These strips
are is not underlain by a permeable base
1
1
0.5 ft (0.15m)
10 ft (3m)
2 ft (0.6m)
0.5 ft (0.15m)
10 ft (3 m)
2 ft (0.6m)
(continued)
G-10
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Table G-1 Parameter Values for IWEM Roadway Source Module Example
Parameter
Description
Remarks
Source Constituent Information
cVi
C°Total,k,i
C°L,PB (left)
C°Total,PB (left)
C°L,sG,j(left)
C°Total,SG, j(left)
C°L,PB (right)
C°Totai,pB (right)
C°L,SG,J (right)
C°Totai, SGJ (right)
Cin(left)
Cm (right)
Ccrit
Initial leachate concentration for layer k in
roadway-source strip i
Initial total constituent concentration for
material layer k in roadway-source strip i
Initial leachate concentration for the left-hand
side permeable base
Initial total constituent concentration for the left-
hand side permeable base
Initial leachate concentration for the left-hand
side subgrade layerj
Initial total constituent concentration for the left-
hand side subgrade layerj
Initial leachate concentration for the right-hand
side permeable base
Initial total constituent concentration for the
right-hand side permeable base
Initial leachate concentration for the right-hand
side subgrade layerj
Initial total constituent concentration for the
right-hand side subgrade layerj
Contaminant concentration of the left -hand
side ditch inflow;
Contaminant concentration of the right -hand
side ditch inflow;
minimum concentration level in the ditch
Top layer: Strips 1 to 7 (1, 0, 0, 1, 0, 0, 1
mg/L)
Below-top layer: Strips 1 to 7 (1, 1, 1, 1,
1,1,1 mg/L)
Top layer: Strips 1 to 7 (0.05, 0, 0, 0.05,
0, 0, 0.05 mg/kg)b
Below-top layer: Strips 1 to 7 (0.05, 0.05,
0.05, 0.05, 0.05, 0.05, 0.05 mg/kg)b
1 mg/L
0.05 mg/kg
1 mg/L
0.05 mg/kg2
1 mg/L
0.05 mg/kg
1 mg/L
0.05 mg/kg
0.05 mg/L
0.05 mg/L
0.05 mg/L (Assumed)
Note that the above information must be provided for all constituents
P°Bulk,k,i
P°Bulk,PB (left)
p°Buik, SGJ (left)
p°Buik,pB (right)
p°Buik, SGJ (right)
Initial bulk density for material layer k in
roadway-source strip i
Initial bulk density in the left-hand side
permeable base
Initial bulk density for the left -hand side
subgrade layerj
Initial bulk density in the right-hand side
permeable base
Initial bulk density for the right-hand side
subgrade layerj
2 g/cm3 (all layers and strips)
2 g/cm3 (all layers and strips)
2 g/cm3 (all layers and strips)
2 g/cm3 (all layers and strips)
2 g/cm3 (all layers and strips)
Hydrologic Parameters
li
Infiltration rate for roadway-source strip i
4.62 inches/year(11.7 cm/yr)
(Site: Montepelier.VT;
From: AC-Low in Table 1.4)
(continued)
G-ll
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Table G-1 Parameter Values for IWEM Roadway Source Module Example
Parameter
Description
Remarks
Hydrologic Parameters (continued)
ROi
EP
P
RRech
KDRO (ten)
KPB (left)
KDRO (right)
KPB (right)
n(left)
n (right)
S (left)
S (right)
Runoff rate per unit area of strip i
Evaporation rate over the ditch
Precipitation rate over the ditch
Recharge flux per unit area
Divertible runoff coefficient for the left-hand
side roadway-source strips (dimensionless),
varying from 0 to 1
Water flux coefficient for the left-hand side
roadway-source strips, varying from 0 to 1.
Divertible runoff coefficient for the right-hand
side roadway-source strips (dimensionless),
varying from 0 to 1
Water flux coefficient for the right-hand side
roadway-source strips, varying from 0 to 1.
Manning's coefficient of the left-hand side ditch
Manning's coefficient of the right-hand side
ditch
Slope of the left-hand side water surface and
stream bed
Slope of the right-hand side water surface and
stream bed
23.69 inches/year(60.2 cm/yr)
(Site: Montepelier.VT;
From: AC-Low in Table 1.4)
22.25 inches/year (56.5 cm/yr)
36 inches/year (91.4 cm/yr)
9.18 inches/year (23. 3 cm/yr)
0.5 (Assumed)
0.5 (Assumed)
0.5 (Assumed)
0.5 (Assumed)
0.016a
0.016a
-1 0-8
-I O-s
Flow and transport properties for the vadose and saturated zones
KM
KKPB (left)
KSG (left)
Koitch (left)
Teed (left)
HstrLimit (left)
KKPB (right)
Hydraulic conductivity for material layer k in
roadway-source strip i
Saturated hydraulic conductivity of the left-hand
side permeable base
Saturated hydraulic conductivity of the left-hand
side subgrade layers underlying the permeable
base
Vertical hydraulic conductivity of the left-hand
side ditch bed
Thickness of the left-hand side ditch bed
The maximum possible depth of water in the
left-hand side ditch
Saturated hydraulic conductivity of the right-
hand side permeable base
Top layer: Strips 1 to 7 (all equals to
3.139m/yr)
Below-top layer: Strips 1 to 7 (0.0017,
3.139, 3.139, 0.0017, 3.139, 3.139,
0.001 7 m/d)
Note that Strips 1, 4, and 7 of the below-
top layer represent subgrade.
105ft/d(1.095x107m/yr)
6.6 x10-5ft/d (0.0073 m/yr)
0.01 ft/d (1.095 m/yr) (Silt)
1.5 ft (0.45m)
3.28 ft (1m)
105 ft/d (1.095E07 m/yr)
(continued)
G-12
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IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Table G-1 Parameter Values for IWEM Roadway Source Module Example
Parameter
KSG (right)
Koitch (right)
Teed (right)
HstrLimit (right)
Description
Saturated hydraulic conductivity of the right-
hand side subgrade layers underlying the
permeable base
Vertical hydraulic conductivity of the right-hand
side ditch bed
Thickness of the right-hand side ditch bed
The maximum possible depth of water in the
right-hand side ditch(L)
Remarks
6.6 x10-5ft/d (0.0073 m/yr)
0.01 ft/d (1.095 m/yr) (Silt)
1.5 ft (0.45m)
3. 28 ft ( 1m)
a Source: http://docs.bentley.com/en/HMSewerCAD/SewerCAD_Help-14-116.html
b Fictitiously low values are used to control the pulse length for model verification purposes.
(1) Left-hand-side ditch
0.049378^
Concentrati
36,500 days
(2) Right-hand-side ditch
Time
Concentration (unit mass/unit volume)
(3) Travel lanes (left and right hand sides)
n n49fi?n ,
36,500 days
(4) Embankments (left and right hand sides), median
Time (day)
Figure G-11. Contaminant Pulses for Verification Case 4
G-13
-------
IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
G.3 Verification Results
G.3.1 Verification Scenario 1: Single-Strip and Single-Layer Roadway Source
with Flow at Different Angles to the Axis of the Roadway
Normalized breakthrough curves for the two cases are shown in Figures G-12 to G-15. In each
figure, an IWEM-generated and a corresponding MODHMS-generated normalized breakthrough
curves are compared. Results are summarized in Table G-2.
Breakthrough Curves
Breakthrough Curves
i.uut+uu -
1.00E-02 -
O) 1.UUb-U4 -
g i.OUb-Ob -
£ 1.00E-08 -
c
ID
O * r\r\c -1 r\
C I.UUb-IU -
O
1 nnF.19
f
1.00E-02 <
U
"en 1 OOE-04
£
c i nnF-OR
& 1 flDF-Oft
c
o
" 1 OOE-10 -
0
° 1.00E-12 -
ff~
2000 4000 6000 8000
Time (days)
2000 4000 6000 8000
Time (days)
IWEM
-MODHMS
IWEM
MODHMS
Figure G-12. Concentration at observation
location 1 under single-strip, single-layer
scenario with orthogonal flow.
Breakthrough Curves
Figure G-13. Concentration at observation
location 2 under single-strip, single-layer
scenario with orthogonal flow.
Breakthrough Curves
Concentration (mg/L)
1 .UUCTUU
1 nnF n9
^ * -4 * » » * » »
^ nnp r\A \i
I .UUt-LW-
'i nnd n£
1 .UUt-UO
1 nnF no
I .UUt-UO
1 nnF in
I .UUt- IU
1 nnF 10 -
Concentration (mg/L)
1 .UUIZTUU
1 nnF n?
1 nnF n/i *
i .UUC-UH
1 ODF HR
1 nnF OR
1 nnF m -
1 nnF.19 -
/T" ' '
/
0 2000 4000 6000 8000
Time (days)
Figure G-14. Concentration at observation
location 1 under single-strip, single-layer
0 2000 4000 6000 8000
Time (days)
Figure G-15. Concentration at observation
location 2 under single-strip, single-layer
G-14
-------
IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
scenario with 45° flow. scenario with 45° flow.
Table G-2. Summary of Results for Verification Scenario 1
Case
(a) Orthogonal
(b) Sub-orthogonal
a(°)
0
45
en
90
45
In Main Plume (Obs 1)
Figure G-11a
Good match overall
Figure G-11c
Good match, IWEM is slightly
more conservative
Edge of/Off Main Plume (Obs 2)
Figure G-11b
Good match overall
Figure G-11d
Good match overall
For comparison purposes, the normalized breakthrough curves are divided into two categories: in
main plume, and off main or edge of plume. In the former category, the observation location is
located well within the swath of paths of fluid particles from the source. The latter refers to the
observation location that is near the edge of the particle path swath or just outside the swath. The
results summarized in Table G-2 indicate that, for in-main-plume observation wells, the IWEM
results generally agree well with the more accurate numerical results. This is true even when the
conjugate inclination angle approaches the maximum permissible value of 45°, which is based a
relatively large tolerance of 0.1. This observation is thought to be due to the smoothing effect of
lateral dispersion near the fringe of the plume originating from near the edge of the source.
The difference between the numerical solution based on the actual source and the IWEM solution
based on the approximate source is expected to increasing more evident as a exceeds 45° and
approaches 90°.
G.3.2 Verification Scenario 2: Single-Strip and Multiple-Layer Roadway Source
with Flow at Different Angles to the Axis of the Roadway
Normalized breakthrough curves for the two cases are shown in Figures G-16toG-19. In each
figure, the IWEM-generated and the corresponding MODHMS-generated normalized
breakthrough curves are compared. Results are summarized in Table G-3.
In this verification scenario, the multiple-layer IWEM model agrees reasonably well with the
more accurate numerical MODHMS-based model. It can be seen that when the distortion is kept
within the sub-orthogonal range, the IWEM-generated breakthrough curves agree reasonably well
with the MODHMS-generated.
G-15
-------
IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Breakthrough Curves
Breakthrough Curves
1 .OOE+00
1 .OOE-02
=5, 1.00E-04
E,
r 1.00E-06
o
+j
| 1 .OOE-08
o
o
1.00E-10
1.00E-12
1000 2000 3000 4000
Time (days)
1000 2000 3000 4000
Time (days)
IWEM
-MODHMS
IWEM
MODHMS
Figure G-16. Concentration at observation
location 1 under single-strip,
multiple-layer scenario with orthogonal flow.
Breakthrough Curves
Figure G-17. Concentration at observation
location 2 under single-strip,
multiple-layer scenario with orthogonal flow.
Breakthrough Curves
1 .UUCTUU
1 nnF n9
T 1 nnF n.4
o
E1 nnF nfi
01 nnF ns
I
i 1 nnF m
o
0 ^ QQ^ -|2
0
0 1.00E-14 -
<
*=»^»^
r^^*^*-^*^.
r ^^t**-M^^
1. OOE+00 -,
1 nnF n9 -
_i
p -i nnF nft
0)
o 1 nnF m -
o
° 1 nnp.19 -
^
fi?
-------
IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Table G-3. Summary of Results for Verification Scenario 2
Case
(a) Orthogonal
(b) sub -orthogonal
a(°)
0
45
en
90
45
In Main Plume (Obs 1)
Figure G-11e
Relatively good match near
peak where IWEM is more
conservative
Figure G-11g
Relatively good match near
peak where IWEM is more
conservative
Edge of/Off Main Plume (Obs 2)
Figure G-11f
Relatively good match near peak where
IWEM is more conservative
Figure G-11h
Relatively good match near peak where
IWEM is more conservative
G.3.3 Verification Scenario 3: Multiple-Strip and Single-Layer Roadway Source
with Flow at Different Angles to the Axis of the Roadway
Normalized breakthrough curves for the two cases are shown in Figures G-20 to G-23. In each
figure, the IWEM-generated and the corresponding MODHMS-generated normalized
breakthrough curves are compared. Results are summarized in Table G-4.
Multiple Strip, Single Layer
Multiple Strip, Single Layer
l.UUh+UU -.
1 OOE 10 -
(
S^*^
r
I 2000 4000 6000 8000
Time (days)
-IWEM -MODHMS
1 ftflF fi9 t
=d 1 OOF 04 -
§
o>
o
c
C
^^
x^
2000 4000 6000 8000
Time (days)
-IWEM -MODHMS
Figure G-20. Concentration at observation
location 1 under multiple-strip, single-layer
scenario with orthogonal flow.
Figure G-21. Concentration at observation
location 2 under multiple-strip, single-layer
scenario with orthogonal flow.
G-17
-------
IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
Multiple Strip, Single Layer
2000
4000 6000
Time (days)
8000
Figure G-22. Concentration at observation
location 1 under multiple-strip, single-layer
scenario with 45° flow.
1.00E-KH)
1. OOE-02
1. OOE-04
~ 1. OOE-06
c
o
"£» 1. OOE-08
1
c 1.00E-10
o
O
1.00E-12
Multiple Strip, Single Layer
2000
4000 6000
Time (days)
8000
Figure G-23. Concentration at observation
location 2 under multiple-strip, single-layer
scenario with 45° flow.
Table G-4. Summary of Results for Verification Scenario 3
Case
(a) Orthogonal
(b) sub -orthogonal
a(E)
0
45
2(E)
90
45
In Main Plume (Obs 1)
Figure G-11i
Good match overall
Figure G-11k
Good match overall, IWEM
is slightly more conservative
Edge of/Off Main Plume (Obs 2)
Figure G-11J
Good match overall
Figure G-111
Good match overall
In this verification scenario, the multiple-strip IWEM model agrees reasonably well with the
more accurate numerical MODHMS-based model. It can be seen that when the distortion is kept
within the sub-orthogonal range, the IWEM-generated breakthrough curves agree reasonably well
with the MODHMS-generated.
G.3.4 Verification Scenario 4: Multiple-Strip Roadway Source with Drainage
Systems and Ditches
Normalized breakthrough curves for this case are shown in Figures G-24 to G-27. In each figure,
the IWEM-generated and the corresponding MODHMS-generated normalized breakthrough
curves are compared. In this verification scenario, the IWEM-based multiple-strip model agrees
reasonably well with the more accurate numerical MODHMS-based model. However, the
IWEM-based model tends to be more conservative than the MODHMS-based model.
G-18
-------
IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
i
2- o.oi
a
go. 0001
o
'S 1E-06
i
g IE-OS
o
u IE-10
1E-12
5000 10000 15000
Time(days)
20000
-MODHMS
IWEM
l
3 a01
a
§0.0001
'§ 1E-06
g IE-OS
u 1E-10
IE-12
5000 10000 15000
Time(days)
20000
-MODHMS
-IWEM
Figure G-24. Concentration at observation
location 1.
Figure G-25. Concentration at observation
location 2.
1E-12
5000 10000 15000
Time(days)
20000
Figure G-26. Concentration at observation
location 3.
1
2- o.oi
§0.0001
1 1E-06
| IE-OS
o
0 1E-10
IE-12
5000 10000 15000
Time(days)
20000
Figure G-27. Concentration at observation
location 4.
G.4 Summary
The results reported in this appendix show that, in general, effects due to distortion (the use of
non-orthogonal reference frame) is not significant if the conjugate angle of inclination is smaller
than the theoretical maximum permissible value. The maximum permissible value is defined by
the length and the width of the source, as well as a user-defined tolerance value. For the
verification case, the maximum permissible angle based on the tolerance value of 0.1 is 45°. In
many cases, the IWEM solutions tend to be slightly more conservative than the fully numerical
counterpart (MODHMS). As the deviation from conjugate angle of inclination exceeds the
maximum permissible value to sub-parallelism (2 approaches but is smaller than 90°), the
difference between the IWEM and MODHMS solutions is expected to become increasingly more
evident.
G-19
-------
IWEM Technical Background Document Appendix G: Verification of the Enhanced Transport Module
In order to ensure reasonably conservative estimates of the contaminant concentrations at
receptor wells, in the event that maximum permissible conjugate inclination angle # a < 90° or a
= 90°, the reader is referred to Appendix F.
G.5 References
McDonald, M.G., and A.W. Harbaugh, 1988. A modular three-dimensional finite difference
groundwater flow model: U.S. Geological Survey Techniques of Water-Resources
Investigations Book 6, chapter Al.
HGL (HydroGeoLogic). 2006. MODEMS V 3: Documentation and Users Guide.
RTI International and HGL (HydroGeoLogic). 2006. IWEM with Highway Source Module IWEM
Version 2.0. Memorandum submitted to U.S. EPA, Office of Solid Waste, Washington,
DC, October, 2006.
U.S. EPA (Environmental Protection Agency). 2003. EPA 's Composite Model for Leachate
Migration with Transformation Products (EPACMTP): Technical Background
Document. U.S. EPA, Office of Solid Waste, EPA530-R-03-002, April 2003.
G-20
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