EPA530-R-15-006
United States
Environmental Protection
Agency
Response to Peer Review
Comments on IWEM 3.0
Final
June 2015
Office of Resource Conservation and Recovery
Office of Solid Waste and Emergency Response
U.S. Environmental Protection Agency
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Response to Peer Review Comments on IWEM 3.0 Table of Contents
Table of Contents
1.0 Introduction 1
1.1 Reviewers 1
1.2 Charge Questions 2
1.3 Summary of Conclusions 2
1.4 Organization of This Document 2
2.0 Roadway Source Module 5
2.1 Complexity of Roadway Module 5
2.2 Roadway Module Formulation 5
2.2.1 Representation of the Ditch 5
2.2.2 Combining Multiple Layers and Strips 6
2.2.3 Infiltration 7
2.2.4 Pulse Source Assumption 9
2.2.5 Runoff, Leachate from Permeable Base 9
2.2.6 Roadway Module Features 10
2.2.7 Miscellaneous 10
3.0 Ground water Transport 11
3.1 Flow Velocity 11
3.2 Vertical Dispersion 12
3.3 Decay Rate 12
3.4 Water Table Boundary Condition 12
4.0 Inputs 13
4.1 General Quality and Appropriateness of Supporting Data 13
4.2 Specific Input Issues 13
4.2.1 Leachate Concentration 13
4.2.2 Material Properties 14
4.2.3 Use of Kd Values 15
4.2.4 Precipitation Data 15
5.0 Output Metrics 15
6.0 General Fitness of Software 16
6.1 Interface 16
6.2 Scope 17
6.3 Other Software Issues 17
7.0 Documentation 17
7.1 Technical Clarity and Detail 17
7.1.1 General 17
7.1.2 WMU Source 18
7.1.3 Roadway Source 18
7.1.4 Saturated Zone Modeling 19
7.1.5 Miscellaneous 20
7.2 Presentation of Example Problems 21
7.3 Editorial Comments 22
8.0 References 24
Attachment 1: Charge to Reviewers
Attachment 2: Full Text of Peer Review Comments
in
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Response to Peer Review Comments on IWEM 3.0 Table of Contents
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IV
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Response to Peer Review Comments on IWEM 3.0 Introduction
1.0 Introduction
The U.S. Environmental Protection Agency (EPA) created the Industrial Waste Management Evaluation
Model (IWEM; U.S. EPA, 2002a) as part of the Guide for Industrial Waste Management (U.S. EPA,
2002b). This model conducts screening analyses to determine the most appropriate liner design for
several types of waste management units (WMUs) that minimize or avoid adverse ground water
impacts. IWEM uses a probabilistic (Monte Carlo) approach together with a ground water fate and
transport model (EPA's Composite Model for Leachate Migration with Transformation Products
[EPACMTP], U.S. EPA, 2003a, b) to model releases from a WMU source into the underlying
unsaturated zone, subsequent subsurface transport, and expected ground water concentrations at a
downgradient receptor well. IWEM then compares the estimated 90th percentile ground water
concentrations to reference ground water concentrations (i.e., constituent- and pathway-specific human
health benchmarks) to recommend the minimum liner scenario that is protective for all constituents.
IWEM 2.0 (U.S. EPA, 2010) was created in response to a growing interest in ways to repurpose
manufacturing and power generation residuals. This version added a module that simulates an additional
source, roadways constructed with beneficially used industrial materials. After an independent external
peer review in 2008, IWEM 2.0 was finalized in 2009 to help identify the potential environmental
impacts from these beneficial uses.
IWEM 3.0 was developed in response to suggestions made by peer reviewers on IWEM 2.0, to broaden
the applicability of the model and to enhance its usability. This version introduced a more rigorous
treatment of leaching through the roadway cross section and included ditches, drainage, embankment
and surface runoff as optional elements. The beta version of IWEM 3.0 and its documentation, the
IWEM Technical Background Document (U.S. EPA, 2015 a) and the IWEM User's Guide (U.S. EPA,
2015b), were submitted for peer review in June 2014. This document summarizes and responds to the
peer review comments (U.S. EPA, 2014).
Since the peer review, EPA has developed a newer version of the model, IWEM 3.1, which addresses
peer review comments, and contains a new source module that addresses structural fills. The approaches
and methodology used for the new source term module are based on the approaches used in the existing
landfill source module, which were peer-reviewed as part of IWEM 1.0. The modification made for the
structural fills did not depart significantly from the original approaches. Therefore, the EPA determined
that another peer review was not necessary for the addition of the structural fill module. References to
"IWEM" without a version number in this response to comments document refer to the latest version,
IWEM 3.1.
1.1 Reviewers
IWEM 3.0 was reviewed by the following peer reviewers:
• Dr. Mustafa M. Aral, Professor, School of Civil and Environmental Engineering, Georgia
Institute of Technology;
• Dr. Charles Harvey, Professor, Department of Civil and Environmental Engineering,
Massachusetts Institute of Technology;
• Dr. Lin Li, Associate Professor, Department of Civil and Environmental Engineering, Jackson
State University; and
• Dr. Frank W. Schwartz, Professor and Ohio Eminent Scholar, School of Earth Sciences, the Ohio
State University.
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Response to Peer Review Comments on IWEM 3.0 Introduction
1.2 Charge Questions
Specifically, EPA asked the reviewers to address the following charge questions:
1. New Roadway Features. Comment on whether the new features (e.g., drainage system, gutter,
ditch and embankment) added to the roadway module reasonably represent a typical roadway.
Are the assumptions and parameters used to represent these components in the model appropriate
and adequate? Is there anything significant overlooked in the general road configuration?
2. Flow Equations. Are the conceptualizations and derivations of flow equations for surface
runoff, discharge from drainage and flow in ditches appropriate and adequate? Are they
represented properly in the model? Please also comment on the appropriateness of rates
developed for runoff, evaporation, and infiltration using the Hydrologic Evaluation of Landfill
Performance (HELP) model. Is the calculation of these rates for the six representative regions on
the United States adequate?
3. Contaminant Flux. Given that the incorporation of drainage system, runoff, and evaporation
can alter the contaminant flux, please provide your comments on the appropriateness of the
equations used to calculate the contaminant flux (mass flux, pulse duration, leachate
concentration) from a roadway source. Are the results derived from the use of this model
reasonably reliable?
4. Model Simplicity. With additions made to model, has the EPA appropriately kept the balance
between keeping the model simple and easy to use as compared to making it technically more
sophisticated?
5. Documentation. Does the documentation reasonably explain the assumptions/rationale behind
the modeling approach and the conceptualization of the roadway parts? Overall, is it complete
and understandable to the reader? Are there any significant omissions that need to be addressed?
The full text of the charge document, which includes background, a general charge, a list of the
materials provided for review, and the above charge questions, is included as Attachment 1.
1.3 Summary of Conclusions
The reviewers generally provided useful inputs. The two major recurring technical comments were
related to: (1) the complexity of the roadway source modeling compared to the simplicity of the WMU
source modeling, and the added value of that level of complexity for the roadway source term, given the
much more simplified subsurface fate and transport model to which it provides inputs; and (2) the age
and accuracy of the underlying databases. Most of the other comments received either reflect a
misunderstanding of IWEM or address aspects of the model that are outside the scope of this review.
The reviewers generally praised the documentation, but offered a number of specific recommendations
for enhancing the clarity of the documentation. Their overall conclusions are summarized in Table 1,
while the full text of their comments is attached as Attachment 2.
1.4 Organization of This Document
Peer reviewers raised the same issues under multiple charge questions. Therefore, this response to
comments document is not organized by charge question, but by issue, as follows:
• Section 2: Comments on the Roadway Source Module
• Section 3: Comments on Ground water Transport
• Section 4: Comments on Inputs
• Section 5: Comments on Output Metrics
• Section 6: Comments on the General Fitness of Software
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Response to Peer Review Comments on IWEM 3.0 Introduction
• Section 7: Comments on the Documentation
• Section 8: Miscellaneous Comments
• Attachment 1: Charge to Peer Reviewers
• Attachment 2: Full Text of the Letter Reviews.
The comments are summarized by topic within the above categories, followed by a response.
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Response to Peer Review Comments on IWEM 3.0
Introduction
Table 1. Summary of Peer Review Comments
Question
Dr. Aral
Dr. Harvey
Dr. Li
Dr. Schwartz
General
In general the reference documents are
well organized and well written. However,
in their current state they are not error free
for these documents and the software to
be released in public domain.
IWEM appears to be an excellent tool for
making simple but useful approximations of
the risks of ground water contamination. The
code synthesizes a remarkably wide range of
databases and predictive models. The authors
have reached a good balance of accuracy and
efficiency across the different parts.
The new version brings IWEM closer to
realistic roadway conditions. However,
there are concerns about the add-on
functions related to the pavement
material property, leaching pattern,
ditches and surface runoff. Also, the
IWEM interface and icons are too old.
Overall, the design of the code is in keeping
the vision of an easy to use package, which
is appropriate for the target audience. There
are no obvious problems in downloading and
running the code. More thought should be
given to more descriptive metrics of risk.
New Model
Features
The new roadway source model features
are a good and very detailed
characterization of a typical roadway, with
appropriate assumptions and parameters.
The new features of IWEM appear to
represent roadways adequately. One aspect
that is missing is the case where water from a
roadside ditch accumulates in a topographic
depression.
Reviewer had a number of very specific
comments on various features of the
roadway module.
The modular design of the roadway
configuration is sufficiently general that it will
be able to create common types of
roadways that will be encountered in
practice. However, it depends on several
hard-to-estimate parameters.
Flow Equations
The assumptions and limitations are all
standard for screening analysis. The
equations used to calculate the steady
state Darcy flow velocities or the pore
velocities are appropriate, and the
adjustment of the Darcy velocities in the
saturated flow domain for potential
mounding under the WMU is reasonable.
The conceptualization of flow in ditches is
nicely formulated, but the explanation can be
improved by the use of diagrams and figures.
The use of the HELP model, which has been
reviewed and described elsewhere, is smart,
but additional explanations of the fundamental
aspects of HELP would improve the IWEM
documents.
Incorporate newer references and
international climate data.
IWEM depends on generic databases to
provide parameter estimates, which is a
weakness because the approaches are
dated and without appropriate verification of
the data.
Contaminant
Flux
Accounting for the drainage, runoff, and
evaporation rates is done correctly. The
transport equations are incorrect. [In fact,
they are correctly implemented but
incompletely documented; see Sec 3.3.]
The contaminant flux through the roadway
layers is probably a reasonable model, but
needs to be better explained.
Either provide detailed documentation for
the current pulse source assumption, or
adopt a "first flush" and "lagged
response" leaching pattern instead.
The approach is generally straightforward
and well described with the exception of the
"pulse" analogy.
Model Simplicity
The level of detail of the roadway module
is inconsistent with the simplicity of the
WMU source and unsaturated/ saturated
zone model.
Not sure the roadway source module needs to
be 3-D.
The model is very complex, with many
additional input parameters. Some of the
parameters should be merged or
removed. Some of the terms are
unfamiliar even to experienced ground
water flow/transport modeler.
The level of complexity for even simple
"roadway" cases is out of proportion to the
other flow/transport process modules, e.g.,
saturated and unsaturated flow.
Documentation
The documentation covers and explains
the general assumptions/ rationale behind
the modeling approach and the
conceptualization of the roadway parts.
However, the documentation contains
some errors that should be addressed
before releasing the software and its
documentation in the public domain.
In general, better figures and diagrams would
help.
The technical manual and some parts of
technical appendix should be combined,
because all of equations are shown in
appendix. The equations are a key part to
understand the technical part.
The documentation is well organized,
reasonably well documented, and does an
excellent job in facilitating the use of the
code. However, the literature cited for the
theoretical basis of the approach is thin,
mostly of EPA reports and grey literature,
rather than primary journal articles.
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Response to Peer Review Comments on IWEM 3.0 Documentation
2.0 Roadway Source Module
2.1 Complexity of Roadway Module
Comment Summary: All four reviewers noted that the complexity of the roadway source module is
inconsistent with the simpler WMU source term modules in IWEM, and Drs. Aral and Schwartz
questioned the value of that level of complexity given the more simplified approach used to model the
unsaturated zone. Drs. Harvey, Li, and Schwartz commented that the complexity of the roadway module
is unnecessary, and Dr. Li felt that many of the terms describing the inputs would be unfamiliar to even
experienced ground water modelers. Dr. Aral felt that the uncertainty and errors introduced by
simplifying the unsaturated and saturated zones are more important than the accuracy gained by the
multi-layered roadways source term.
EPA Response: EPA acknowledges the disparity in the level of complexity between the WMU source
term modules and the roadway source module and between the roadway source module and the
approach used for the unsaturated zone modeling. However, the properties of materials within the
complex structure of a roadway are (or should be) known a priori as part of a design incorporating
beneficially reused materials that are relatively homogeneous within a particular roadway component.
The materials in a WMU such as a landfill are more heterogeneous and the properties of those materials
are not known by design, nor is it easy to ascertain them. Similarly, the properties of the unsaturated
zone are more heterogeneous and more difficult to ascertain. In addition, the added features to roadway
module are optional, and the modeler may still consider a simplified roadway design (with monolith
layers) to help reduce the level of complexity and additional data requirements. Many of the new
features added to IWEM 3.0 were in direct response to peer review comments received on IWEM 2.0
(U.S. EPA, 2009).
2.2 Roadway Module Formulation
2.2.1 Representation of the Ditch
Comment Summary: Dr. Harvey noted, with regard to the representation of the ditch in the roadway
module, that the problem is nicely formulated but better explanation is needed. The review raised the
following issues:
1. IWEM does not account for water from a roadside ditch accumulating in a topographically
depressed area and the potential for contaminants concentrating in that area, subsequently
impacting local streams and ground water resources.
2. If there is no net inflow or outflow from a ditch, then the approach is only applicable to the
segment that actively receives runoff. Equations C-38a through C-38c in Appendix C appear to
allow for different values of Q;n and Q0ut, but it is not clear how the Manning equation is applied
when Qin does not equal Qout.
3. The assumption that the depth of the water in the ditch (Hstr) is constant over time is an
acceptable approximation, but this assumption should be highlighted because the model could be
quite inaccurate in places where rain falls in shorter, more intense storms, rather than a constant
drizzle.
EPA Response:
1. The model formulation allows the water in the ditch to either move or remain stagnant. Stagnant
water can be simulated by setting the slope of the ditch to zero. The chemical concentration in
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Response to Peer Review Comments on IWEM 3.0 Documentation
areas with stagnant water is determined from the mass conservation principle expressed in
Equations C-21 through C-23.
2. The approach is based on a steady-state assumption for each segment that Q;n = Qout as the
variation of flow is expected to be small. The Manning equation is applicable when the flow is
steady and uniform. Both the velocity and flow rate in the ditch are derived based on an
assumption that the flow in each segment represents average flow within the segment. Continuity
between two adjacent segments could be estimated by interpolating between segment centroids.
However, this approach does not provide an appreciable improvement in the simulation of mass
transport for the level of complexity required to account for variations in the ditch cross-sectional
area throughout the length of the ditch.
3. The model is designed to handle long-term average steady flow. The user is advised to use a site-
specific model for sites better characterized by transient conditions, for example, where
precipitation is typically delivered through short-term, high-intensity rainstorm events.
2.2.2 Combining Multiple Layers and Strips
Comment: Drs. Harvey and Li had questions about how the multiple roadway layers and strips interact.
Specifically:
1. It is not clear whether aggregating concentrations from multiple roadway strips and layers simply
means that the concentrations from multiple strips are summed for the same constituent, or
something more complex.
2. It appears that contaminants move from the top layer sequentially downward, never overlapping
or diluting, so that all loading to the ground water is from the bottom layer, and contaminants
from different layers do not enter the ground water at once. This is a reasonable approximation,
but needs to be better explained.
3. The assumption that lateral communication between roadway source strips is insignificant and
ignores flows from sloped embankment faces.
EPA Response:
1. IWEM uses Monte Carlo analysis to develop a probability distribution of ground water exposure
concentrations for each roadway strip that contains teachable constituent mass. For each
constituent, IWEM then sums the 90th percentiles of these distributions from all strips that leach
that constituent to obtain the aggregate 90th percentile estimated ground water exposure
concentration for comparison to the reference ground water concentration(s). This is explained in
the User's Guide in Section 5.2; however, the text will be revised for clarity in other sections of
the IWEM User's Guide (U.S. EPA, 2015b) and Technical Background Document where this
approach is mentioned more fleetingly (User's Guide Section 2.2.2.3 and Technical Background
Document Sections 1.2.1 and 2.1.3).
2. The reviewer is correct in that all leachate leaves the roadway from the bottom of the lowest
layer and that releases are a contiguous series of non-overlapping pulses. The text in Section
C.2.2.1 of Appendix C of the Technical Background Document has been updated to make the
process described mathematically more clear to the reader/user.
3. The objective of IWEM is to provide a screening-level analysis for a roadway design. As such,
the decisions as to which processes to incorporate in the model were necessarily balanced against
objective, utility and relative significance. The majority of contaminant releases from the
roadway not diverted by a collection system will be vertical into underlying ground water.
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Response to Peer Review Comments on IWEM 3.0 Documentation
Comment: Dr. Li wondered how Equations C-8 (contaminant flux), C-9 (infiltration rate), C-12 (pulse
duration), and C-13 (concentration of efflux from permeable base) of Appendix C of the IWEM
Technical Background Document (U.S. EPA, 2015a) were derived.
EPA Response: The equations mentioned were derived as follows:
• Equation C-8 is derived based on the following assumptions:
- The leachate pulses from the layers in a strip move down through the layers and into the
permeable base (the bottom-most layer) without overlapping or leaving gaps between them.
As soon as the leachate pulse from one layer completely enters the permeable base, the
leachate pulse from the layer above that one begins to enter 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
that runs laterally across the roadway.
• 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.
• Equation C-13 is Equation C-8 (mass flux at any time) divided by the water flux plus the
concentration in the permeable base defined by the user.
Text has been added in Section C.2.2.4 of Appendix C of the IWEM Technical Background Document to
provide a plain language description of the assumptions and formulation of the mathematical statements.
Comment: Dr. Li noted that Equations C-10 and C-l 1 depend on the assumption of permeable road base
and filter reinforcement layer, which needs a careful justification in the typical pavement systems with
industrial materials.
EPA Response: Subsurface drainage systems (in the form of a permeable base) were originally included
in roadway cross-sections to alleviate moisture-related stress in pavements. The introduction of
industrial materials with the potential to release constituents into the environment into the design and
construction of new roadways creates additional motivation to consider including controlled diversion of
moisture away from the roadway. The rehabilitation of existing roadways designed with a permeable
drainage layer with industrial materials is also a reasonable scenario. For these reasons, and to address
peer review comments on IWEM 2.0, EPA decided to include the permeable base option. Equation C-l 1
determines the duration of the pulse of leachate from the permeable base by calculating the total mass
divided by the rate at which mass leaves the permeable base. The mass flux is the product of the per-
unit-area water flux, the leachate concentration from the permeable base material, and the area through
which the mass flux passes.
2.2.3 Infiltration
Comment: Drs. Li and Schwartz raised questions about the assumption that infiltration/runoff is steady
state and on the same order of magnitude as regional recharge. Dr. Schwartz cited the scenario of high-
intensity rainstorms, which would result in more significant runoff as compared to the hypothetical
steady-state case with continuous, low-intensity rainfall. Dr. Li noted that infiltration is time-dependent
and may be much higher in the embankment and unpaved shoulder than in the paved median. Dr Li
asked whether this assumption can be re-written as the bottom limit of infiltration from the traversing
roadway is on the order of magnitude of regional recharge.
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Response to Peer Review Comments on IWEM 3.0 Documentation
EPA Response: Regarding the magnitude of infiltration for the roadway, EPA believes the reviewers
may have misunderstood the nature of the assumptions described in Section 3.3.3 and Appendix C,
Section C.2.1, of the IWEM Technical Background Document. As noted in Appendix C, EPACMTP (not
IWEM) assumes that the general regional ground water flow pattern is not affected by the presence of a
source. This assumption implies that infiltration is on the same order of magnitude as recharge, but this
is not a condition enforced by EPACMTP or IWEM. That text has been revised to clarify that this is
only an implication of the assumption that the regional ground water flow pattern is not significantly
altered. Instead, IWEM enforces a practical design limitation that prevents the use of an infiltration rate
for each strip that would allow the water table to rise up and make a contact with the roadway. IWEM
also limits infiltration through a strip to the lowest material hydraulic conductivity value across all layers
in the strip. The model provides default values for roadway infiltration rates, based on the properties of
different construction materials and annualized environmental conditions for the selected geographic
location. Alternatively, the user may elect to supply rates that are more representative of the specific
scenario being modeled. The underlying flow models for the unsaturated and saturated zones are steady-
state, so any time-dependent infiltration rate would need to be averaged on an annual basis prior to being
entered into IWEM.
Regarding the assumption of steady-state runoff and infiltration, EPA believes this assumption to be an
appropriate approach for a screening-level analysis.
Comment: Dr. Schwartz noted that the HELP model is used extensively for parameter estimation, but the
documentation assumes that readers are well informed on how this model works and what its limitations
are. The reviewer requested a more robust description of the HELP model to support and explain the
parameter calculations, including a brief discussion of the method and any papers or reports that
describe experiences and any assessments of that material (beyond that in Section E.5). The reviewer
added that, rounding what may be highly uncertain parameter estimates (e.g., infiltration rates in Table
6-17) to three significant figures suggests foundational assumptions about the model that require careful
reconsideration.
EPA Response: The HELP model has been used exclusively to estimate water infiltration through
unlined and clay-lined WMUs, through various pavement and non-pavement surface materials, and for
regional infiltration rates (e.g., recharge rates) in the area surrounding the source of contamination. An
extensive description of HELP-derived infiltration rates through WMUs are presented in the Technical
Background Document for EPACMTP. References to that discussion have been added to Appendix D
(Infiltration Rate Data for WMUs and Structural Fills) and Appendix E (Infiltration Rates through
Pavements) of the IWEM Technical Background Document.
Comment: Dr. Harvey approved of the use of the HELP model, but felt that additional explanations of
the fundamental aspects of the HELP model would improve the documentation. Specifically, the
reviewer wondered
1. What "quasi-two-dimensional model" means?
2. How the HELP model determines evapotranspiration, and whether it is modeled as transient?
EPA Response:
These following points have been clarified in Section 6.4.1 of the IWEM Technical Background
Document:
1. The HELP model is primarily a vertical flow model (one dimensional - downward), but the
model also accounts lateral flow if a permeable drainage layers is specified as part the modeled
scenario (hence, quasi-two-dimensional).
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Response to Peer Review Comments on IWEM 3.0 Documentation
2. Potential evapotranspiration is modeled using a modified Penman method (Penman, 1963). The
HELP model can calculate transient values. However, both IWEM and EPACMTP calculate
transport based on annualized flow and neither can accept these shorter-term, transient inputs.
2.2.4 Pulse Source Assumption
Comment: Dr. Li strongly suggested that EPA either provide a detailed documentation and references
for the pulse source assumption (i.e., that leaching occurs at a constant concentration) or adopt the "first
flush" (characterized as an initial high leaching rate, followed by a gradual decrease of leachate
concentration with time) and "lagged response" (an initial low leaching rate gradually increasing until
the source is depleted) leaching patterns instead. Dr. Schwartz agreed that a better description of the
pulse analogy should be added, with conceptual description figure(s), rather than only equations, and
further explanation of the relevant assumptions.
EPA Response: EPACMTP can model the leachate source as either continuous or finite sources.
Furthermore, finite sources can be modeled as a pulse or depleting sources. Both of these modeling
options are available for organic constituents. However, for metals that generally exhibit non-linear
sorption behavior, the depleting source term modeling option cannot be used as the analytical solution
for one-dimensional transport of a solute with non-linear sorption in EPACMTP requires the
simplification of the leaching profile to a constant magnitude, finite pulse (i.e., a square pulse). These
metals transport capability provided by EPACMTP and its accompanying assumptions have governed
the development of the roadway formulation and the primary assumption that leachate profiles are
conceptualized as square pulses. More detail discussions of the pulse and depleting leaching scenarios
and the underlying assumptions can be found in Section 2.2 of the EPACTMP Technical Background
Document. In addition, the text in Section C.2.1 of Appendix C of the IWEM Technical Background
Document has also been updated to reflect how the assumptions of the transport module have influenced
the formulation of the roadway module and to improve the presentation of the pulse assumptions and
methodology.
2.2.5 Runoff, Leachate from Permeable Base
Comment: Dr. Li raised several questions about the calculation of water fluxes for runoff from pavement
and discharge from a permeable base layer (called a drain in IWEM):
1. If no drain is included as part of the roadway are Equations C-19 and C-20 in Appendix C of the
IWEM Technical Background Document valid?
2. In Equation C-19, how is ROi calculated? What do the variables Wj and L represent?
3. In Equation C-21, what does the variable CPB represent?
EPA Response:
1. IWEM is capable of accounting for surface runoff only when a ditch is included in the roadway
cross-section design, as the ditch serves as the primary destination for runoff. If the user does not
include a ditch in the roadway cross-section defined in IWEM, then the software does not ask for
or permit the entry of a runoff rate, and Equations C-19 and C-20 are not applicable. If a gutter is
included, IWEM can be directed to divert some portion of runoff away from the ditch (via the
gutter). Additional text has been added in Section C.2.2.4 of Appendix C of the IWEM Technical
Background Document to provide clarity as to when the equations are applicable or not.
2. In Equation C-19, ROi, the runoff rate per unit area of strip is an input provided by the user. The
variable Wj is the width of a particular strip of road denoted by the subscript], while L is the
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Response to Peer Review Comments on IWEM 3.0 Documentation
length of the roadway segment. These are initially defined after Equation C-4, but, for clarity, the
variable descriptions have been added to the text for Equation C-19 in Section C.2.2.4 of
Appendix C of the IWEM Technical Background Document.
3. In Equation C-21, CPB is the time-averaged leachate concentration released from the bottom of
the permeable base, as initially defined in Equations C-13a through C-13c. Again, for clarity, the
variable description has been added to the text for Equation C-21 in Section C.2.2.4 of Appendix
C of the IWEM Technical Background Document.
2.2.6 Roadway Module Features
Comment: Dr. Li made several observations and recommendations about the roadway module features:
1. The current road design options in IWEM do not allow the user to model a scenario that has two
parallel highways with a median ditch. Dr. Li also recommended increasing the number of strips
from the current maximum of 15 to a maximum of 20, increasing the number of drains and
ditches from 2 to more than 3, and adding the number of gutters per unit distance along the
segment (rather than whether a gutter is present or not).
2. Gutters should be optional.
3. There are no explicit embankment input parameters (e.g., geometry, elevation, and slope) in the
software. Dr. Li also raised the issue of surface runoff and overland transport of contaminants
from embankments constructed from industrial waste materials.
EPA Response:
1. EPA acknowledges that there are roadway design scenarios that may not be exactly captured by
IWEM. However, the number of components currently allowed in the software was selected to
support a divided highway with up to four travel lanes in each direction plus all optional features
(shoulders, embankments, and ditches). EPA believes that this is sufficient to cover the majority
of highway systems in the United States.
2. Gutters are already optional. If a ditch is specified to be present, the user may choose to specify
that there is a gutter.
3. IWEM models embankments as a type of roadway strip. Runoff from any type of roadway strip
(including an embankment) can be specified as long as a ditch to receive the runoff water has
been defined. The commenter is correct that no additional embankment-specific parameters are
included; additional parameters (such as slope) are included only for ditches. To keep the model
relatively simple and to maintain the conservative nature of the ground water screening model,
overland mass transport is not explicitly considered for embankments or other road components.
EPA recognizes that omission of overland transport of contaminants is one of the limitations of
IWEM. A modeler can still indirectly account for overland mass transport from a traveled way or
embankment and the subsequent down-gradient infiltration outside of IWEM through simple
mass balance equations. The resulting leaching information can be used as source term
information to model the subsurface migration of contaminants to a receptor well.
2.2.7 Miscellaneous
Comment: Dr. Li requested clarification on how the total concentration is used in the roadway
formulation.
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EPA Response: Total concentration (or, more properly, total teachable concentration) is used together
with the bulk density and volume of the layer to determine the total initial teachable constituent mass in
that particular layer of that roadway strip. The total teachable mass is used together with the leachate
concentration and infiltration rate, as defined in Equation C-3, to determine the how long it will take to
deplete the total teachable mass of each constituent in the layer. The text of the IWEM Technical
Background Document has been revised to use consistent terminology for this parameter and thus clarify
how total concentration is used.
Comment: Dr. Schwartz indicated that little guidance is provided for the user to aid in determining the
percentage of runoff and drainage that will reach a ditch prior to infiltration into the underlying aquifer.
Dr. Schwartz also noted that it is difficult to associate parameters with the roadway cross-section. The
reviewer requested figures that relate the input screens to the roadway geometry.
EPA Response: Text has been added to the Section 3.4.2.3 of the IWEM User's Guide (U.S. EPA,
2015b) and Section 6.3.8 of the Technical Background Document (U.S. EPA, 2015a) to provide some
guidance for specifying the percentage of runoff and drainage that reaches a ditch. Specifically:
• 100% of roadway runoff should reach the ditch if no gutter is present; if a gutter is present, the
percentage can be estimated by 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.
• The percentage of drainage reaching the ditch should be low if the drain is represented as a
continuous layer of highly permeable material; however, if drainage pipe is used only 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.
Comment: Dr. Schwartz noted that it is difficult to associate parameters with the roadway cross-section
and that figures which relate the input screens to the roadway geometry would be helpful.
EPA Response: EPA is considering adding a figure that relates the input screens to the roadway
geometry to future versions of the documentation.
3.0 Ground water Transport
3.1 Flow Velocity
Comment: Dr. Harvey noted that two basic features of ground water flow are that (1) it is slower in arid
environments and (2) it accelerates from ground water divides toward discharge, usually in rivers. The
addition of inputs for the distance from ground water divide and the distance to ground water discharge
would improve the estimates of ground water velocity. The reviewer also noted that the description of
how the saturated flow problem is formulated and velocities are calculated is confusing i.e., does
regional recharge influence the magnitude of the ground water gradient and, thus, velocity?
EPA Response: EPACMTP does have an input for a ground water discharge location that is unique to
surface impoundments (SI). Infiltration rates for Sis are determined in the unsaturated flow solution
using the height of water in the impoundment and other relevant parameters. The proximity of a
discharge point can influence the resulting infiltration rate. Incorporating the inputs for distance from
ground water divide and distance to ground water discharge into the current flow solution for WMU
sources would require modifications to the numerical solutions in EPACMTP, and the resulting better
estimates of velocity would have a greater impact on when maximum concentrations reach a well than
on the magnitude of the concentrations (and thus, exposure). Because the primary interest in a screening
tool is the magnitude of the exposure, not the timing of that exposure, this does not impact the
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usefulness of the model. However, the Agency will reconsider these comments during any future work
on the model.
3.2 Vertical Dispersion
Comment: Dr. Harvey indicated that the description of how vertical dispersion is conceptualized (e.g., is
vertical dispersion only modeled downward?) is unclear.
EPA Response: Text has been added to Section 4.1 of the IWEM Technical Background Document to
clarify the direction of vertical dispersion and to address any added conservatism resulting from the
approach in EPACMTP. EPA considers this level of conservatism appropriate for a screening model.
3.3 Decay Rate
Comment: Dr. Aral indicated that there were major problems with Equations 4-1 and 4-6 [now 4-7] in
the IWEM Technical Background Document (U.S. EPA, 2015a). Noting that these equations consider
two different types of reactions: first-order decay and interactions between the chemical and the
environment (e.g., absorption/desorption). Dr. Aral pointed out that the former is a function of the
properties of the chemical only and is independent of the ambient environment, while the latter is
dependent on the ambient environment, as well as the chemical properties. The reviewer observed that,
while this second type of reaction is correctly represented in these equations, decay is not. The reviewer
continued that interaction with the ambient environment is represented by the retardation coefficient, R,
and if these equations are divided by the retardation factor R, the result is the appropriately reduced
velocities and diffusion constants for the interaction with the environment term, as well as a reduced
decay constant. This seems to imply that the decay constant of a chemical also reduces under the
influence absorption/desorption processes, which is incorrect.
EPA Response: Dr. Aral is correct that the equations for transport in the unsaturated and saturated zone
(Equations 4-1 and 4-6 [now 4-7] in the IWEM Technical Background Document) are incorrect as
presented. The wrong versions of these equations (those reflecting degradation only in the dissolved
phase) were incorporated into the IWEM documentation, giving the false impression that IWEM
improperly ignores degradation in the sorbed phase. These equations, however, are correctly
implemented in EPACMTP (and thus IWEM). The EPACMTP Technical Background Document
presents these equations for degradation in both dissolved and sorbed phases, and both are codified in
the EPACMTP software (and thus IWEM). Equations 4-1 and 4-7 and the accompanying text have been
revised to correctly reflect the versions of the equations actually coded in the model, which include the
sorbed phase.
3.4 Water Table Boundary Condition
Comment: Dr. Aral indicated that the description of the saturated zone boundary condition at the water
table (on page 4-1, first paragraph [second paragraph in the revised text] of the IWEM Technical
Background Document) is not adequate. Specifically, the reviewer is not clear on whether time
dependence is used. The reviewer believed this is an important issue and that needs to be addressed in
detail, because the contaminant flux from the unsaturated zone is a function of time and the zone is a
boundary to the saturated zone.
Response: EPA acknowledges that details of the mathematical formulation are not presented in the
IWEM documentation. The specific paragraph referenced by the commenter is an introductory
paragraph intended to give a broad overview of the scope of EPACMTP in the context of subsurface
modeling. Thus, additional detail there is not considered appropriate. However, a reference to the
EPACMTP Technical Background Document (U.S. EPA 2003 a, b), which provides more details on the
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treatment of boundary conditions for the pseudo-3-D transport solution option(in particular the transfer
of mass at the water table) has been added to Section 4.2 (Saturated Zone Module) of the IWEM
Technical Background Document for the interested reader.
4.0 Inputs
4.1 General Quality and Appropriateness of Supporting Data
Comment: Dr. Schwartz raised concerns about the support databases used by IWEM, stating that these
data sources are old and unverified, and that site-based strategies would be preferable. In addition, the
reviewer stated that it is inappropriate to provide the user the option to model in the absence of
subsurface data (i.e., when the subsurface environment must be classified as "unknown").
EPA Response: While EPA acknowledges the comment, revising the underlying databases is outside of
the scope this project. Additionally, the EPACMTP database and the Hydrogeologic Database for
Ground water Modeling that IWEM relies on have been thoroughly peer reviewed and the model output
validated extensively using field data. EPACMTP continues to support a number of large-scale Agency
risk assessments and the Agency believes that the EPACMTP system provides the most appropriate
available tool to conduct the subsurface modeling for IWEM.
Comment: Dr. Li commented that the references cited for formulation of multiple layers with a drainage
system (Christopher and McGuffey, 1997; Apul et al., 2002) are too old, and that more recent references
are needed. Similarly, the reviewer noted that the pavement material properties in Table 6-16 are also
from Apul et al. (2002), which is not an authoritative, peer-reviewed publication. The reviewer added
that the properties data should be revised to reflect the latest peer-reviewed publication for the pavement
material properties, especially for industrial materials.
EPA Response: EPA acknowledges the age of some of the sources cited. However, this does not mean
that the information contained in these sources has become outdated. Both references provide
compilations of peer-reviewed literature and have been cited in a numerous other studies and
publications. Therefore, in the absence of more contemporary sources identified by the Agency or
provided by the commenter, EPA continues to cite to these older sources
Comment: Dr. Aral expressed concern with the random selection of data for certain parameters when the
user does not supply values. Dr. Aral felt that this approach reduces the representation of site conditions.
EPA Response: EPA agrees with the reviewer that the reference to random selection of data is
misleading and contradicts what is done in IWEM. Optional, site-specific parameters, such as aquifer
hydraulic conductivity, that are not supplied by the user are selected probabilistically from nationwide
distributions for each model iteration. Therefore, while there is some uncertainty introduced by relying
on the national distribution, which may over or under-estimate site-specific conditions in a single model
run, the overall magnitude of uncertainty of the aggregated results is likely small.
4.2 Specific Input Issues
4.2.1 Leachate Concentration
Comment: Dr. Schwartz felt that the amount of guidance provided for estimating leachate concentrations
is not sufficient in comparison to other inputs and suggested that EPA consider developing a database of
information for this input.
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EPA Response: EPA has revised Section 6.3.9 of the IWEM Technical Background Document (U.S.
EPA, 2015a) and Section 3.4.5.2 of the User's Guide (U.S. EPA, 2015b) to provide additional
references to leaching characterization methods that the Agency has recently developed (such as SW-
846 analytical methods 1313, 1314, 1315, and 1316; see
http://www.epa.gov/epawaste/hazard/testmethods/sw846/new meth.htm). These methods specifically
allow the evaluation of leaching potential over the range of leaching conditions expected to occur when
industrial materials are either disposed or beneficially used. In addition, EPA also provided in Section
6.3.9 two literature sources (Garrabrants, et al., 2013; Kosson et al., 2013) that contain leaching data for
some constituents from a fly ash concrete source using Methods 1313 and 1315.
4.2.2 Material Properties
Comment: Dr. Li noted that Table 6-16 contains low-end and high-end material properties data for the
top/base/subbase courses of Portland cement concrete and asphalt concrete pavements, which seem
correct for a single row of data, but which are incorrect and misleading for the entire pavement system
from top to bottom layers. The reviewer suggested revising the table to reflect the entire pavement
system.
EPA Response: Table 6-16 [Table 6-17 in the revised text] presents material properties used in the
HELP model for each layer in a typical roadway component (e.g., an IWEM strip) to derive practical
bounding values of infiltration appropriate for a screening model; these values are included in IWEM.
Because infiltration is a key sensitive parameter, the HELP model was run for the different layers for
which data are presented in the table (top, base, subbase courses) to enable IWEM to evaluate each layer
of a strip individually (rather than the whole pavement system at once). This approach captures the
reality that the rate of infiltration through different layers is not likely to be the same. Thus, the values
that were used to run the HELP model are presented in the table, rather than aggregated values for the
entire pavement system that are not actually used in the model.
Comment: Dr. Li asked how the values in Tables 6-17 through 6-20 (infiltration, runoff, evaporation,
and pan evaporation rates) were determined, and whether the range of values presented were verified,
noting that some appear to be outside of typical ranges. The reviewer also asked whether the presented
values represent pavement-related materials or waste materials.
EPA Response: The infiltration rates in Table 6-17 [Table 6-18 in the revised text] were generated using
the HELP model and material properties for pavement-related materials, as described in Section 6.4.3.2
of the IWEM Technical Background Document. The average precipitation rates were derived from five
years of national climate data stored within the HELP model database. The runoff rates in Table 6-18
[Table 6-19 in the revised text] and evaporation rates in Table 6-19 [Table 6-20 in the revised text] were
also estimated with the HELP model. EPA has adopted the use of the HELP model, which was subject
to both independent external and internal Agency reviews, as the source of infiltration rates for landfills
for nearly two decades. EPA acknowledges that there are limitations in using HELP; however, the
model has been tested and verified as discussed in the EPACMTP Parameter/Data Background
Document (U.S. EPA, 2003b).
Comment: Dr. Li asked how the data for pan evaporation is used in the model, and whether there is a
more recent source for these data.
EPA Response: The pan evaporation rates (and precipitation rates) are provided as potential default
values that define flow in an open ditch. The text in Section 6.4.3.2 has been updated to make that clear.
How the high and low values were determined is also described in the text of Section 6.4.3.2 and
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supplemented by the footnotes attached to the respective tables. The rates presented in the document
were taken from NOAA (1982).
4.2.3 Use of Kd Values
Comment: Dr. Aral asked why EPA does not recommend using the Monte Carlo feature of EPACMTP
on the soil-water partition coefficient (kd) value to resolve uncertainty about the effects of the
geochemical environment on the mobility of metals. More specifically, the geochemical environment at
a site is assumed to be constant and not affected by the presence of the leachate plume. In addition,
EPACMTP does not account for colloidal transport or other forms of facilitated transport that may be
significant for metals and other constituents that tend to strongly sorb to soil particles.
EPA Response: Although IWEM does not allow the user to supply a distribution for kd directly for either
organic or non-organic constituents, the probabilistic nature of kd is reflected in other ways:
• For organics, kd is calculated from Koc and fraction organic carbon. Fraction organic carbon is a
probabilistic value based on a nationwide distribution obtained from values of dissolved organic
carbon in EPA's STORET water quality database. Thus, kd calculated from Koc and values
drawn from the distribution for fraction organic carbon is also a distribution used in the Monte
Carlo analysis.
• For metals, IWEM represents sorption using empirical sorption isotherms selected based on site-
based data and national distributions of key variables. Although the user can override this with a
single value of kd, use of the isotherms is the default.
4.2.4 Precipitation Data
Comment: Dr. Li noted that only the minimum and maximum precipitations are provided for each
climatic zone (in Table 6-15), and requested the addition of mean precipitations within the climatic
zone.
EPA Response: High-end values for the environmental variables in each climatic region were chosen to
be consistent with the objective of determining a bounding range of infiltration rates through various
material configurations to support a screening level analysis. Mean precipitation can vary considerably
within the large climate zones (which encompass numerous meteorological stations), which makes the
presentation of a single mean potentially misleading for users who may assume that this value is
appropriate for site-specific evaluations anywhere within that climate zone. A high-end value for the
climate zone is more appropriately conservative. For roadways with a ditch, users are asked to enter an
annual precipitation rate, and may choose whether to enter a high-end or mean value.
5.0 Output Metrics
Comment: Dr. Schwartz recommended that IWEM rely on more descriptive metrics than the comparison
of a single high-end percentile to a risk or water quality metric. The comparison of a single percentile to
a standard is too simplistic to capture variability of system, and is not a proper procedure of risk
analysis. Dr. Aral felt that the probability of exceedance was a more computationally effective way to
calculate risk based on an exposure criteria than the use of a 90th percentile.
EPA Response: The use of a high-end (90th percentile) point estimate from a distribution to characterize
potential exposures is common and appropriate in screening-level analyses because the use of
conservative data and assumptions allows decisions to be made quickly and with greater confidence.
The roadway module in IWEM is intended to be a screening-level model for users to quickly determine
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whether a proposed use of industrial material is an appropriate beneficial use or whether additional site-
specific assessment is warranted before making that determination. The IWEM Technical Background
Document (U.S. EPA, 2015a) and User's Guide (U.S. EPA, 2015b) have been revised to clarify this
point.
6.0 General Fitness of Software
6.1 Interface
Comment: Dr. Schwartz praised IWEM for performing as expected, being easy to use, and being
appropriate for target audience. In general, this reviewer noted that screens are well described and well
organized. However, both he and Dr. Li offered several suggestions for improvement:
1. The interface and icons are old and should be modernized
2. The built-in documentation under the "Help" tab is minimal, duplicative of the written
documentation, and could be improved.
3. Some labelling could be improved (e.g., "Is Below Drain") on "Layer Properties" tab. The
acronyms on some drop menus are unintuitive.
4. On the Source Parameters screen for roadways (Screen 7), it would be helpful to number the five
data entry tabs.
5. The EPACMTP console window is distracting, and replace by a less obtrusive status line
indicating that the model is running.
EPA Response:
1. IWEM is implemented in Visual Basic 6, an older coding language that was state-of-the-art
when IWEM was originally developed in 2002, and the icons and interface are consistent with
the age of the coding language. EPA acknowledges that the suggested changes would improve
the user interface of IWEM; however, due to the considerable time and resources that would be
required to rebuild the model using a more current coding language, the Agency chose to update
the model in Visual Basic 6. EPA acknowledges that this decision resulted in a model that may
look dated, but this has minimal impact on model utility or ease of use. The Agency will
reconsider these comments during any future revisions to IWEM.
2. Unlike the documentation for many commercial software programs, the IWEM User's Guide
(U.S. EPA, 2015b) and Technical Background Document are readily available to the public.
These documents are intended to provide all the information needed to understand how to use the
model appropriately. Therefore, it is not surprising that the "Help" tab within the model would
mirror these texts. The Help is not intended to be a replacement for the documentation, but rather
a distillation of the more essential points for the convenience of the user.
3. EPA has reviewed the different input screens for places where space allows for clearer labelling
or the full spelling of acronyms and updated the model as appropriate.
4. EPA has numbered the tabbed dialogs for roadway source parameters to minimize confusion.
5. While EPA acknowledges that some users may prefer the ability to minimize the EPACMTP run
screen, this is, at most, a minor inconvenience. Thus, EPA focused available resources on areas
that would improve model utility and ease of use. The Agency will reconsider this comment
during any future revisions to IWEM.
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6.2 Scope
Comment: Dr. Li noted that the national scope of climate data contained within IWEM limits the
usefulness of the model internationally. This reviewer recommended the addition of data for global
climate stations.
EPA Response: The purview of EPA is the United States, and IWEM was initially developed to aid
decisions about disposal practices subject to EPA regulations. Even if IWEM were to be updated for a
global audience, EPA notes that climate stations are not the only type of data that would need to be
updated. Aquifer characteristics and other parameters for which distributions are built into the model
were specifically developed for the United States and would need to be recalibrated for international use.
Furthermore, the broadening of the scope raises numerous complications, such as how to make the
model simultaneously useful for multiple regions around the globe. A screening model that considers a
global high-end may become too conservative as to have limited utility for anyone. Therefore, no
changes were made in response to this comment.
6.3 Other Software Issues
Comment: Dr. Schwartz noted that he could not find the saved results after running the examples.
EPA Response: EPA has updated Section 3.2.4 (How Do I Save My Work) of the IWEM User's Guide
to clarify that the user chooses a location to save the files using the standard Windows Save As dialog.
EPA has also updated the example problem appendices (Appendices B, C, and D) in the User's Guide to
identify the default installation location of the data files that correspond to the example problems.
Comment: Dr. Aral commented that the computational times presented on page 2-4 of IWEM Technical
Background Document (U.S. EPA, 2015a) are not efficient compared to other more complex software.
EPA Response: The computation times presented on page 2-4 in Section 2.2 of the IWEM Technical
Background Document (45-60 minutes per constituent for all three liner scenarios, or 15-20 minutes per
constituent and liner) [now removed from the Technical Background Document and revised in Section
3.2.7 User's Guide} were outdated and somewhat misleading. For WMUs and structural fills, the runs
on a typical computer with a 2.5 GHz processor take about 4 minutes per liner scenario per constituent,
not the 15-20 implied by the document. Further, runtimes for all liners is a somewhat misleading metric,
given that some sources (land application units, WMUs with a user-defined liner, structural fills) have
only one liner scenario and that, for those with three, it may not be necessary to run all three (IWEM
runs the liners from least protective to most, and once it finds a liner scenario below the benchmark, no
further liners are run). Thus, a more realistic estimate would be 4-12 minutes, depending on the
necessary number of liner scenarios. Roadways can take longer, depending on the complexity of the
design and the properties of the materials, but typically take 3-12 minutes per constituent per strip. EPA
has updated the text with the more current and relevant runtimes noted here in Section 3.2.7 of the
User's Guide.
7.0 Documentation
7.1 Technical Clarity and Detail
7.1.1 General
Comment: Dr. Harvey suggested adding an upfront roadmap in the IWEM Technical Background
Document (U.S. EPA, 2015a) that traces the parallel pathways for flow and transport, along with
references to the location in the document where detailed discussion of each pathways can be found.
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EPA Response: While the suggested revisions may make the IWEM documentation easier to navigate, it
will not be a major source of confusion for the reader. After careful review of edits requested by the
commenters, EPA made the decision to focus available time and resources on those that would correct
errors and improve the clarity of the document. The Agency will reconsider these comments during any
future work on the model.
Comment: Dr. Schwartz indicated that it is not clear how concerns and suggestions from the community
of users/stakeholders are considered and influence model development.
EPA Response: Information on how to contact EPA for technical support or to provide comments is
available via the "Tools" menu (Technical Support) and in the User's Guide. Although there is no
automated feedback system, user comments are reviewed by the Agency. If the commenter identifies
potential errors within the model or points out ways to improve the utility of the model, EPA will work
to address these comments at the earliest possible date. If the comments address design or other that
topics that do not affect the model utility or ease of use, EPA will address these comments dependent on
available funds.
7.1.2 WML) Source (IWEM Technical Background Document Section 2.1.1)
Comment: Dr. Aral noted that this section of the text refers to ".. .location-specific parameters such as
precipitation." However, IWEM uses infiltration, not precipitation to model fate and transport.
EPA Response: EPA has revised the text to clarify that inputs, such as infiltration and recharge, are
influenced by local rates of precipitation and evaporation. In addition, IWEM does require the user to
input an annual precipitation rate for roadway sources with a ditch.
7.1.3 Roadway Source (IWEM Technical Background Document Section 6, Appendix C;
User's Guide Section 2)
Comment: Dr. Harvey indicated that the conceptualization of flow in and out of the ditch would be better
supported through an updated version of Figure C-9 and that such a figure would be more helpful if
presented earlier in the document.
EPA Response: The figure has been revised in Appendix C. EPA feels it is too detailed to present in the
main body of the text, as it illustrates equations presented only in the Appendix.
Comment: Dr. Li noted that the following sentence in Section 6.3.5 is confusing: "Once the mass of
teachable constituent is known, the duration of leaching from a material layer is calculated."
EPA Response: The IWEM Technical Background Document text has been reviewed and updated to
clarify.
Comment: Dr. Li stated that the structure of Table 6-16 should be modified to
1. Eliminate information that will not be used in the model and may be confusing (i.e., air void,
texture, field capacity, wilting point, and curve number);
2. Clarify the data source(s);
3. Correct typos; and
4. Include a reference to footnote (8) (not currently referenced in the table) or delete the footnote if
it is no longer relevant.
EPA Response:
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1. Table 6-16 [Table 6-17 in the revised document] is included to inform the user of the key
parameter values used in the HELP model to derive the default infiltration rates provided in
IWEM. These values are not intended to be suggested inputs for IWEM. EPA has reviewed and
updated the table title and the description of the table in the text to clarify this.
2. Data sources have been added.
3. Typos have been corrected.
4. Reference to footnote (8) has been included in the heading for Layer Thickness.
Comment: Dr. Li asked why Figure C-8 does not depict a subbase.
EPA Response: The subbase is an optional layer that can be included in the model. Figure C-8 is not
intended to show all possible layers. Therefore, no changes were made with respect to this figure.
However, EPA refers the reader to Figure 2-2 and Appendix E, Figure E-2 in the IWEM Technical
Background Document to see the depiction of the subbase as part of a roadway layer.
Comment: Dr. Li noted that Figures 2 and 3 in the IWEM User's Guide (U.S. EPA, 2015b) are low
resolution and difficult to read. The reviewer suggested a better sketch of a ditch, including a gutter, is
needed to support the definition of the related inputs.
EPA Response: The EPA assumes that the reviewer is referring to Figures 2-2 and 2-3. After review,
EPA finds the resolution of Figure 2-2 to be adequate to communicate the intended information in both
the Word and PDF files. In the absence of more specific recommendations on how to improve the
figure, EPA made no changes. EPA agrees that Figure 2-3 is low resolution and needlessly complicated.
The existing figure has been replaced with a version of Figure C-13 from Appendix C of the IWEM
Technical Background Document., modified to include gutters. This figure is simpler, higher resolution,
and better communicated the intended point.
7.1.4 Saturated Zone Modeling (IWEM Technical Background Document Section 4;
User's Guide Section 2)
Comment: Dr. Aral stated that the text on page 4-4 of the IWEM Technical Background Document
requires clarification. The last paragraph starts with a definition of steady flow, but then discusses
increases in velocity. The reviewer recommended noting that for the case of mounded ground water,
velocities are recalculated, not increased (as if it is increasing with the formation time of the mound).
EPA Response: The text on page 4-4 [4-5 in the revised text] of the IWEM Technical Background
Document has been revised to clarify that ground water mounding beneath the source is represented in
the flow system by increased head values at the top of the aquifer. In addition, while EPACMTP
calculates the degree of ground water mounding that may occur 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.
Comment: Dr. Aral stated that the discussion on page 4-1 of the IWEM Technical Background Document
is misleading because the second paragraph seems to imply that steady-state well concentrations are
only influenced by boundary conditions at the source (i.e., whether the source is finite or continuous).
Dr. Aral noted that additional factors may influence the nature of steady-state well concentrations, such
as distance between the source and the well and the exposure period.
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EPA Response: The text on page 4-1 has been revised to clarify that the steady-state well concentrations
are estimates, based on several factors not limited to the nature of the source, the distance between the
source and the receptor well, constituent fate and transport properties, and the exposure period.
Comment: Dr. Aral indicated that the presentation of the time-averaged concentration calculation in
Figure 4-1 is misleading. The reviewer noted that IWEM does not consider exposures that occur before
the peak of ground water concentration profile and, as a result, adverse effects that may be experienced
by sensitive receptors may be overlooked.
EPA Response: IWEM is designed to return a conservative, maximum time-averaged exposure
concentration using an exposure period specified by the user. In addition, the user has the option to
provide a screening benchmark that accounts for the potential variability in sensitivity or susceptibility
of different receptor groups so that the results of the screening analysis do not underestimate risks to any
sensitive receptors.
Comment: Dr. Aral indicated that the discussion at the top of page 2-9 of the IWEM User's Guide is
misleading. This text states: "These processes decrease constituent concentrations in the ground water
as the distance from the source increases.'" However, the processes described decrease the concentration
as a function of time, rather than distance. But since the contaminant is being transported over some
distance, the concentration decreases as well.
EPA Response: EPA acknowledged the comment, and revised the text on page 2-9 [bottom of the page
in the revised text] of the User's Guide.
7.1.5 Miscellaneous
Comment: Dr. Schwartz expressed concern that the report does not provide enough cited literature that
describes the theoretical basis of the modeling approach. Specifically:
• What is the basis for Equation 6-5 in the IWEM Technical Background Document^ Is this
equation from Gelhar's Electric Power Research Institute (EPRI) report or did EPA
derive this from the data presented in the report?
• What is the basis for Equation 6-14 in the IWEM Technical Background Document, and
what are the units for the conversion factor?
• No citations are provided that explain where the basis for using non-linear sorption
isotherms to characterize metal sorption (Section 6.5.2.2).
• The majority of references in the documents are from EPA. Many sources are represented by
"grey" literature, such as Gelhar's EPRI report, which was later published in Water Resources
Research, rather than primary journal articles.
EPA Response: In regards to the comment on Section 6.5.2.2, the Agency believes that the reviewer is
referring to section 6.6.2.2. EPA has included more citations to the EPACMTP Technical Background
Document (U.S. EPA, 2003a, b) where the theoretical basis for many of these equations and parameter
distributions are derived. IWEM builds on EPACMTP and does not modify these equations. Therefore,
the Agency believes it is redundant to reproduce all of the derivations presented in U.S. EPA (2003a, b).
Comment: Dr. Schwartz stated that the IWEM Technical Background Document needs a careful and
consistent description of the different types of statistical distributions used in the model. For example,
mention is made about Gaussian, lognormal and log ratio distributions in Table 6-24. The model uses
cumulative versions of these distributions, as well as other empirical or data-driven cumulative
distributions. However, no accompanying discussion of these distributions is provided. The discussion
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Response to Peer Review Comments on IWEM 3.0 Documentation
of distributions in the IWEM Technical Background Document should include a discussion of typical
distributions (e.g., uniform, normal, lognormal) and what they each looks like as cumulative
distributions. In addition, when data-driven distributions are developed, each should be explained in its
own right. Discussion of the choice of distributions is important because it has some bearing on
parameter ranges.
EPA Response: EPA has included more citations, as appropriate, to the EPACMTP Technical
Background Document (U.S. EPA, 2003 a), where the selection of parametric and empirical parameter
distributions is supported and described.
Comment: Dr. Schwartz noted that, on page 3-38 of the User's Guide (Section 3.4.4.4), it appears that if
a single Monte Carlo realization is unrealistic, then it is discarded. If some subset of realizations in a
Monte Carlo simulation is not used, there is the potential for bias in the ensemble statistics. It is not clear
what constitutes a "sufficient" number of realizations, but a more definitive cutoff should be provided.
EPA Response: EPA has revised the text in Section 3.4.4.4 of the IWEM User's Guide to add a footnote
cautioning against reducing the number of iterations from the default 10,000, and to provide a cross
reference to Section 5.2 of the IWEM Technical Background Document, where this subject is discussed
in more detail.
7.2 Presentation of Example Problems
Comment: Dr. Harvey thought that users would benefit from example problems that demonstrate how
local data may be found, show the inputs and outputs of IWEM, and contain a discussion of how to
interpret the results. These examples should span the range of IWEM's capabilities, different WMUs,
different contaminants and different geographic areas within the United States. Dr. Schwartz thought the
existing discussion of the example problems was cursory, and that including screenshots of the user
interface in the examples would be helpful. While some screenshots were provided in Appendix C, Dr.
Schwartz felt these were too small.
EPA Response: The IWEM User's Guide provides several example problems for WMUs, structural fills
and roadways. EPA is providing the IWEM project files for these examples along with the release of
IWEM 3.1. EPA will work to improve the specific examples and the resolution of smaller graphics in
the documentation.
Comment: Dr. Schwartz suggested additional discussion on parameter uncertainty related to uncertainty
in the results.
EPA Response: While EPA acknowledges that an uncertainty analysis based on input parameters may be
of interest to readers, the Agency notes that IWEM is intended to be a screening model, with most inputs
biased in a known, conservative direction (excluding user-defined inputs that may not be conservative).
Therefore, the uncertainties associated the parameters built into the model are unlikely to lead the user to
conclude incorrectly that a proposed beneficial use is appropriate.
EPA also points out that the uncertainty associated with parameter variability is accounted for in IWEM
through the use of several linked databases of the EPACMTP model, which include: (1) a nationwide
database of source term data, (2) a database climate parameters, and (3) a database of subsurface
parameters. Through the implementation of EPACMTP's Monte Carlo function, the uncertainty in local
conditions are approximated using data that characterize the variability of environmental conditions
across the United States.
Comment: Dr. Aral requested that EPA provide files that correspond to each example problems along
with the software.
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Response to Peer Review Comments on IWEM 3.0
Documentation
EPA Response: The IWEM project files associated with the example problems are now included as part
of the software installation package.
7.3 Editorial Comments
The reviewers provided a number of specific editorial comments, which are summarized in the tables
below for the IWEM Technical Background Document and the User's Guide.
Table 2. Editorial Comments on the Technical Background Document
Commenter
Dr. Schwartz
Dr. Schwartz
Dr. Harvey
Dr. Aral
Dr. Aral
Dr. Aral
Dr. Aral
Dr. Aral
Dr. Aral
Location
General
General
Figure ES-1
ES, bullet list in
right-hand column
of page ES-1
Sec 2. 1.1, First
paragraph
Figure 2-2
Page 3-4, Bottom
Figure 3-6
Section 4.2, 4.3.2
Comment
IWEM Technical Background Document
overuses acronyms
IWEM Technical Background Document mixes
metric and British units
Do not reflect the impact of clean recharge on the
elevation of the plume top
Executive Summary refers to applications that
IWEM cannot be used to analyze:
• Protecting air quality in WMU design
• Monitoring analysis or help
• Corrective action claim (only by repeated
iterative analysis, too much effort and too
uncertain)
• Post closure action claim
• Risk based approaches
Without an explicit and detailed reference to
Guide analysis system, the user may get the
impression that they can do all these application
using IWEM.
"Controlling the release" is not the proper
terminology; "management" is better.
Figure needs revision. There is vertical flow
under the roadway cross-section not the regional
flow from left to right.
"...EPACMTP does not account for fluctuations in
rainfall rate or degradation of liner systems that
may cause the rate of infiltration and release of
leachate to vary over time." This is not the correct
terminology because the depleting source
boundary condition that is used in EPACMTP is a
leachate source, which varies overtime. This is
considered as Boundary Condition in EPACMTP.
Thus, this is inconsistent wording of time
dependence, needs correction.
Figure needs revision. There is vertical flow
under the roadway cross- section not the regional
flow from left to right.
The use of regional terminology to describe
ground water flow inconsistent with inclusion of
localized mounding.
EPA Response
EPA has reviewed the use of
acronyms and limit usage to those
that are essential and used
frequently.
Revised to metric only, as used by
the model.
Corrected as suggested (also
repetitions, Figures 2-3, 6-3)
The statements regarding these
scenarios are made in reference
to the Guide for Industrial Waste
Management not IWEM. The text
has been revised to make that
distinction clearer in Section 1.1 of
the IWEM Technical Background
Document. However, the text in
the Executive Summary has been
removed because it may create
confusion without providing
appropriate level of context.
EPA has revised the text to more
appropriate language.
The figure has been revised to
eliminate the reference to regional
ground water flow direction in this
figure and all repetitions.
The text has been revised to
reduce confusion between steady
state flow assumptions and
contaminant transport boundary
conditions.
The figure has been revised to
eliminate the reference to regional
ground water flow direction in this
figure and all repetitions.
EPA has revised the language
regarding regional flow.
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Response to Peer Review Comments on IWEM 3.0
Documentation
Table 2. Editorial Comments on the Technical Background Document
Commenter
Location
Comment
EPA Response
(continued)
Dr. Li
Table 6-2 [now 6-
3]
Language describing Drain Geometry-
Configuration is confusing:
• "Ditch strip drain drains to strip number of the
ditch for this drain" is confusing
• Rename "Layer drain is above" as "layer
number underneath the drain",
• Rename "Drains what strip" as "which strip
need drains".
The table contains a column
(Section Reference) that identifies
the specific section in the
document where each term is
defined. EPA revised the labeling
of these parameters for clarity.
Dr. Li
Table 6-2 [now 6-
3]
Language describing Flow Characteristics is
confusing.
• Rename "Flow Percentages to Ditch Strips (for
relevant strips and drains)" as "Percent of
roadway runoff that reaches ditch".
• Define "Percent of flow in drain that reaches
ditch" and "Ditch strip(s) receiving overland
flows" before.
The table contains a column
(Section Reference) that identifies
the specific section in the
document where each term is
defined.
Dr. Li
Table 6-2 [now 6-
3]
Is it possible to include Flow Percentages to
Ditch Strips (for relevant strips and drains)" for
Monte Carlo Simulation?
All IWEM roadway source term
parameter are deterministic.
Dr. Li
Section 6.4, App A
Clearly define infiltration, percolation, and
evaporation and differences among them
EPA has added these terms to the
glossary (Appendix A) and
highlighted differences between
the definitions in Sec. 6.4.
Dr. Li
Appendix C
The term "permeable base" appears in Appendix
C but not in main document. The terms
"permeable base" and "filter reinforcement layer"
are not familiar to common user.
Permeable base and drainage
layer are synonymous. IWEM
calls both of these a drain, but the
formulation in Appendix C is cast
in terms of permeable base (which
will be used consistently). The
main document (Section 3) shows
more general layers, including a
base layer.
The term "filter reinforcement
layer" is specific to the appendix
and not intended to be a design
element to be considered by the
common user.
Dr. Li
Appendix C
The term "exfiltration" appears in Appendix C but
not in main document.
This term is more specific to the
appendix, but also used in the
main document to represent water
percolating from the bottom of a
ditch. The text of the appendix has
been modified so that exfiltration
is understood to be the rate of
water leaving the bottom of the
ditch and the term has been
added to the Glossary.
Dr. Li
Appendix C
The technical manual and some parts of
technical appendix [C] should be combined,
because all of equations are shown in appendix.
The equations are a key part to understand the
technical part.
The segregation of the
mathematical formulation from the
main text was based on a desire
to keep the presentations of each
source type similar in the main
body of the text.
(continued)
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Response to Peer Review Comments on IWEM 3.0
Documentation
Table 2. Editorial Comments on the Technical Background Document
Commenter
Dr. Li
Dr. Li
Dr. Harvey
Dr. Aral
Location
Figure C-7
Figure C-7
Figure 2-6
Page 2-1, above
Table 2.1 [now
Table 1.1]
Comment
The word "section" in the caption is misspelled
Should be updated using AASHTO or FHWA
publication
Does not reflect the impact of clean recharge on
the elevation of the plume top.
"No liner" is not a liner type
EPA Response
The typo in caption has been
corrected.
EPA acknowledges the comment;
however, the Agency believes the
figure is sufficient to illustrate the
point made in the preceding
paragraph.
The figure is revised as suggested
The text has been revised from
Liner Type to Liner Scenario,
which is also consistent with the
Guide.
Table 3. Editorial Comments on the User's Guide
Commenter
Dr. Aral
Dr. Schwartz
Location
page 2-3,
top [now
middle of 2-3]
Figure 3-9
Comment
What is a complex site? That definition
needs to be introduced here. There are
several possible complexities?
There are other important complexities that
are not mentioned here. Heterogeneity?
Fissures? Flow conditions Gradient?
Proximity to exposure point? The
complexities selected here are not proper.
Some graphics are difficult to see in the
document (e.g., small roadway inset in
Figure 3-9)
EPA Response
The text has been revised to state that if site
conditions do not reasonably conform to the
assumptions fundamental to the formulation
of IWEM, then the model is not appropriate
for such sites. Also, EPA provided additional
examples of site conditions that would be
deemed "too complex" for IWEM.
EPA improved figures and resolution where
possible.
8.0 References
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 03824.
Christopher, B.R., and V.C. McGuffey. 1997. National Cooperative Highway Research Program
Synthesis of Highway Practice 239. Pavement Drainage Systems. NCHRP.
Garrabrants, A.C., D.S. Kosson, R. DeLapp, 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.
Kosson, D.S., A.C. Garrabrants, R. DeLapp, H.A. van der Sloot. 2013. pH-dependent leaching of
constituents of potential concern from concrete materials containing coal combustion fly ash.
Chemosphere 103:140-147.
NOAA (National Oceanic and Atmospheric Administration). 1982. NOAA Technical Report 33,
Evaporation Atlas of the Contiguous 48 United States. National Weather Service, Washington,
DC.
Penman, H. L. 1963. Vegetation and hydrology. Technical Comment No. 53, Commonwealth Bureau of
24
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Response to Peer Review Comments on IWEM 3.0 Documentation
Soils, Harpenden, England.
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. 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). 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): Parameters/Data Background Document. Office of
Solid Waste, EPA530-R-03-003, April 2003.
U.S. EPA (Environmental Protection Agency). 2009. Response to Review Comments on IWEM Beta
Version 2.0. U.S. EPA Office of Resource Conservation and Recovery, Washington DC.
December.
U.S. EPA (Environmental Protection Agency). 2010. Industrial Waste Management Evaluation Model
(IWEM) Version 2.0 with Roadway Module: Technical Documentation and User's Guide. U.S.
EPA Office of Resource Conservation and Recovery, Washington DC. July.
U.S. EPA (Environmental Protection Agency). 2014. Peer Review Summary Report: Independent
External Peer Review of the Industrial Waste Management Evaluation Model (IWEM) Version
3.0. U.S. EPA Office of Resource Conservation and Recovery, Washington DC. June.
U.S. EPA (Environmental Protection Agency). 2015a. Industrial Waste Evaluation Model (IWEM) v3.1
Technical Documentation. Draft. Office of Resource Conservation and Recovery, Washington
DC.
U.S. EPA (Environmental Protection Agency). 2015b. Industrial Waste Evaluation Model (IWEM)
Version 3.1 User's Guide. Draft Final. Office of Resource Conservation and Recovery,
Washington DC. September.
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Charge to Reviewers
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CHARGE TO REVIEWERS
Peer Review of the Industrial Waste Evaluation Model Version 3.0
BACKGROUND
IWEM vl.O was the ground water modeling tool developed to support the U.S EPA's Guide for
Industrial Waste Management (U.S. EPA, 2002). The model simulates the subsurface migration of
chemical constituents from the bottom of a land-based waste management unit (WMU) to down-
gradient receptor well. The evaluation is based on a tiered approach analysis that consisted of
nationwide distributions (Tier 1) and a location-adjusted probabilistic analysis (Tier 2) with the
objective of determining the most appropriate liner design for WMUs that minimize or avoid adverse
ground water impacts. Both tiers are designed to assist facility manager, the public, and state regulators
a screening-level assessment tool before committing significant resources for a more complex site-
specific hydrological investigation and probabilistic modeling.
In 2006, IWEM v2.0 was developed which added a module to simulate fate and transport from a new
source type—a roadway constructed by recycling industrial materials (i.e., byproducts). This module
provides the user an easy to use tool to determine if the reuse of industrial materials in a roadway setting
is environmentally sound. The new source type was restricted to Tier 2 screening-level analyses, in
which the user can assign values to a number of key, site-specific parameters, and values for the
remaining parameters are selected from predetermined distributions for a Monte Carlo analysis Both
IWEM vl.O and v2.0 were peer reviewed by external independent scientific experts in 2002 and 2008,
respectively.
The current version of the model, IWEM v3.0, for which the EPA is seeking an external peer review,
introduces a more rigorous treatment of leaching through the roadway cross section by incorporating
ditches, drainage, gutter, and embankment/berms into the roadway design. These changes help to
simulate fate and transport of contaminants from a roadway by fully accounting flow process through
overland as well discharge through drains. Because of these features, the site conditions are better
modeled and well concentrations of contaminants are better estimated. Furthermore, the current version
restricts all evaluations for the WMUs and roadway sources to Tier 2 analysis option. The Agency opted
to remove Tier 1 analysis because the leachate concentration threshold values (LCTVs) stored in the
IWEM database and used for Tier 1 analyses were based out-of-dated human health benchmarks (e.g.,
reference doses and slope factors).
CHARGE TO REVEIWERS
The EPA is seeking an independent scientific peer review of IWEM v3.0 beta, focusing on the changes
made to the model since v2.0, which includes: the designs of drainage system, embankment/berm, and
ditches; lateral flow through overland, drain systems, and permeable bases; and the subsequent impact of
these changes on fate and transport of contaminants. The reviewers are asked to provide comments on
the modeling approaches, assumptions made, scientific rationale used to develop the model, and the
supporting documentation. In addition, during the review, the reviewers are asked to be mindful that
IWEM is designed as a screening level tool, and it is not meant to be used as a final tool in complex site-
specific evaluations.
MATERIAL FOR REVIEW
The EPA is providing the following items that include the model and its documentation for review.
1. Industrial Waste Evaluation Model Version 3 Beta (IWEM v3.0)
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Response to Peer Review Comments on IWEM 3.0 Attachment 1: Charge to Reviewers
2. Peer review draft of the Industrial Waste Evaluation Model Version 3 Beta: Technical
Document.
3. Peer review draft of the Industrial Waste Evaluation Model Version 3 Beta: User's Guide
CHARGE QUESTIONS
Based on your knowledge of hydrology and contaminant fate and transport, please provide your
comments in response to the following technical questions:
1. New Roadway Features. Please comment on whether the new features (e.g., drainage system,
gutter, ditch and embankment) added to the roadway module reasonably represent a typical
roadway? Are the assumptions and parameters used to represent these components in the model
appropriate and adequate? Is there anything significant overlooked in the general road
configuration?
2. Flow Equations. Are the conceptualizations and derivations of flow equations for surface
runoff, discharge from drainage and flow in ditches appropriate and adequate? Are they
represented properly in the model? Please also comment on the appropriateness of rates
developed for runoff, evaporation, and infiltration using the Hydrologic Evaluation of Landfill
Performance (HELP) Model. Is the calculation of these rates for the six representative regions on
the United States adequate?
3. Contaminant Flux. Given that the incorporation of drainage system, runoff, and evaporation
can alter the contaminant flux, please provide your comments on the appropriateness of the
equations used to calculate the contaminant flux (mass flux, pulse duration, leachate
concentration) from a roadway source. Are the results derived from the use of this model
reasonably reliable?
4. Model Simplicity. With additions made to model, has the EPA appropriately kept the balance
between keeping the model simple and easy to use as compared to making it technically more
sophisticated?
5. Documentation. Does the documentation reasonably explain the assumptions/rationale behind
the modeling approach and the conceptualization of the roadway parts? Overall, is it complete
and understandable to the reader? Are there any significant omissions that need to be addressed?
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Response to Peer Review Comments on IWEM 3.0 Attachment 2: Full Peer Review Comments
Attachment 2
Full Text of Peer Review Comments
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IWEM 3.0 Peer Review Letter Reports
Draft
June 30, 2014
-------
Mustafa Aral, Ph.D., P.E., p.Hy., FASCE
Consulting Engineer CPE GA 15254)
270 17th St. NW. Unit 809
Atlanta Ga 30363
e-mail: minaral@live.coin
Peer Review of the Industrial Waste
Evaluation Model (IWEM) Beta Version 3.0
Summary of Conclusions
In this review, I am providing my comments for the following documents that were submitted to
me for review:
1. TECHNICAL DOCUMENTATION: IWEM_v3B_TBD_Main_Doc_2-18-2014.pdf
2. APPENDIX for Technical Documentation: IWEM_v3B_TBD_Appendices_2-18-2014.pdf
3. USER'S GUIDE: IWEM_UGv3b_Main_2-18-2014.pdf
4. APPENDIX for User's Guide: IWEM_UGv3b_Appendices_2-18-2014.pdf
The documents listed above constitute the main reference material that were prepared for the
model IWEM - Beta Version 3.0. In general the reference documents listed above are well
organized and well written. However, in their current state they are not error free for these
documents and also the software to be released in public domain. My main points of concern
are weaknesses in the review material provided and more importantly I see technical issues
with the analysis provided on the subject matter of the model developed. Before addressing
these deficiencies I do not recommend the release of this software and its documentation in
public domain.
The strength of the application developed can be considered to be the user friendly nature of
the computational platform of the software. The GUI provided is adequate for the User to
implement the application. However, even with this user friendly platform I would think training
sessions will be necessary for the User to fully understand the software, the GUI, the database
behind the software and implement the application in their projects successfully. I presume that
the necessary support will be provided by the agencies involved. Further, it would have been
very useful for the User if the developers have provided several sample input data files of typical
projects for the User to open under the GUI of the software and see the general data structure
of a typical application and make sample runs and see the outcome. Although I have searched
for these sample input data files extensively in the installation folders of the application, I have
not found any. Inclusion of these sample input files into the software package would have been
very useful for the user. If these files exist somewhere in the installed software folders and I
could not find them in the short review time allocated for this task, I apologize.
My specific recommendations can be grouped into three topics as seen below. Before the
software and the support documentation is released in public domain the issues addressed in
these recommendations should be clarified or corrected:
1. Technical issues identified in this review needs to be addressed and the software
computations needs to be revised based on these corrections.
2. Documentation needs to be revised reflecting the recommendations on these technical
issues including the conceptual issues that are highlighted in this review.
3. Sample input data files should be included with the software installation package. After
installation, these input files should appear in some separate folder in the software
-------
Mustafa M. Aral, Ph.D., P.E., p.Hy., FASCE
Consulting Engineer CPE GA 15254)
270 17th St. NW. Unit 809
Atlanta Ga 30363
e-mail: imnaral@live.com
directory and these files should be accessible through the software by the use of an
"OPEN" command under the GUI of the software.
Charge Questions and Responses
NOTE: In the review provided below a critique of the general features of the application
and the theory used in the application is discussed first under SECTION A of my review
document. Since the documentation provided in support of the application, which I am
reviewing, include many misconceptions and misused definitions or misused
terminology or miss defined example cases, I had to also include recommended
corrections that are identified in terms of page and line positions (approximate) in
reference to a paragraph on a page in the document. These review comments are
included under SECTION B of my review document.
SECTION A: DISCUSSION OF GENERAL FEATURES OF THE APPLICATION:
1. New Model Features.
The new model features that are included in the IWEM Beta version 3.0 (e.g., drainage system,
gutter, ditch and embankment) under the roadway module represents a good and a very
detailed characterization of a typical roadway cross-section. The assumptions and parameters
used to represent the layered nature of this engineered system and its components are
appropriate and more than adequate. There is no need for further sophistication to represent
this pathway in the IWEM application.
However, the characterization detail that is included for the roadway module, i.e. representing
the layered nature of the vertical cross-section under the roadway, is incompatible with the
simplicity considered for the unsaturated/saturated vertical cross-section pathway model that is
developed and used for the WMU application for natural environments. In one case several
layers with different material properties (heterogeneity) are considered, whereas in the other
case natural (layered) heterogeneity that may exist at a typical WMU site is categorically
ignored. If a layered system analysis is doable in one application as demonstrated for the
roadway model case why is it not possible to do it for the other pathway? If complexity is the
issue than the roadway model could have also been simplified to represent the vertical cross-
section under the roadway as a single layer using average values as it is done for the
unsaturated zone model for WMU. If it is not so complex to represent a vertical cross-section as
a layered model as it is done in the roadway model, than the User should also have the option
to represent the unsaturated zone under the WMU as a layered system as well if they chose to
do so without making drastic simplifications on the unsaturated zone application.
This incompatibility between the two applications is very obvious. In one case the manual goes
through an extensive and detailed explanation of the assumptions involved to simplify the
application (unsaturated zone model). In the other case the manual again goes into so much
detail to describe the characterization and the steps necessary to represent the vertical
heterogeneity and the layered roadway application (including the angle of groundwater flow
-------
Mustafa M. Aral, Ph.D., P.E., p.Hy., FASCE
Consulting Engineer CPE GA 15254)
270 17th St. NW. Unit 809
Atlanta Ga 30363
e-mail: imnaral@live.com
direction on segments of the roadway). This is a very unbalanced analysis technique for similar
systems within the same software. It is obvious that the technical development teams of one
module was not talking to the other team to present a unified picture for both applications. Or
one of the modules was developed earlier and the other was tagged on to the software later on.
In this case one should also notice that the uncertainty and computational errors introduced to
the solution by the use of a simplified unsaturated zone analysis, which should be a zone below
the roadway, far exceeds the accuracy gained by the layered analysis of the roadway cross-
section. In this sense not much will be gained by the use of a layered roadway module in an
application.
In my opinion a balance between the sophistication levels of the two applications needs to be
considered. Unnecessarily simplifying one application while unnecessarily complicating the
other raises some doubts and concerns.
2. Flow Equations.
The IWEM model is basically an interface model which uses another EPA model (EPACMTP)
as its computational engine. In this sense the assumptions of the EPACMTP model is directly
transferred and used in IWEM software. Accordingly, in the IWEM application, the groundwater
flow is defined as steady state both in the unsaturated zone (vertically down) and also in the
saturated zone (3D) (horizontal). These assumptions and limitations are all standard for Tier 1
and Tier 2 screening applications which IWEM is another one of these screening family models.
Other than the limitations these assumptions introduce, we have to accept them as is since
these are simple screening models. In this case there is no problem with the definitions
introduced and the equations used to calculate the steady state Darcy flow velocities or the pore
velocities in the application domain. In the saturated flow domain the Darcy velocities are also
adjusted for the potential of creation of a mound under the WMU which is reasonable. However,
in that sense the use of the terminology of "regional" groundwater flow should not have been
used since the conditions on the Darcy velocity is no longer regional but local. This is a minor
point but needs some attention in the write-up.
Please also refer to recommended corrections under SECTION B for line-by-line comments
since there are several other similar conceptual errors in the text of the document.
3. Contaminant Flux.
Accounting the drainage, runoff, and evaporation rates, are done correctly but there is a major
problem with Equation (4.1) which is also repeated in Equation (4.6). I do not know where this
error is originating from. Either EPACMTP definitions is wrong which is a major problem, or the
authors of this document has misused (copied) the wrong equation from the EPACMTP
documents (I did not check). I hope it is the second case, because if the error is in EPACMTP
than both software needs to be corrected and this is a major and a significant issue.
The Equations (4.1) and (4.6) are given as:
V^-0te = 0R^ + Q (4.1)
dz 5f
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Mustafa M. Aral, Ph.D., P.E., p.Hy., FASCE
Consulting Engineer CPE GA 15254)
270 17th St. NW. Unit 809
Atlanta Ga 30363
oz
e-mail: imnaral@live.com
(4.6)
where the parameters of these two equations are appropriately defined in the documents. In the
above equations two different types of reactions are considered: First the term (#/lc) in
Equation (4.1) or the term (
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Mustafa M. Aral, Ph.D., P.E., p.Hy., FASCE
Consulting Engineer CPE GA 15254)
270 17th St. NW. Unit 809
Atlanta Ga 30363
e-mail: imnaral@live.com
generation term, an extensive layered system is considered. This does not make much sense
since the errors made in simplifying the representation of the unsaturated and saturated flow
zones is far more important than the error introduced by representing six layer source region as
a one layer region with the use of appropriate average conditions in a Tier 2 model. This is an
over kill and an unnecessary complication introduced for IWEM application which is a Tier 2
model.
5. Documentation.
The documentation covers and explains the general assumptions/rationale behind the modeling
approach and the conceptualization of the roadway parts. However, the text is full of conceptual
errors, errors in definitions and errors in example characterizations of certain cases etc. Overall
I do not consider this document to be complete. It needs to be revised significantly before it is
released in public domain.
Please also refer to corrections under SECTION B for line-by-line comments since there are
several other conceptual errors in the text of the document.
Other General Comments
This document is technically deficient per my comments on Equation (4.1) and (4.6) above. It is
also deficient in terms of definitions and descriptions used in the text of the document. In my
opinion this document is not ready for public domain release.
SECTION B: REVIEW OF IMPORTANT POINTS IN THE TEXT OF THE DOCUMENT LINE BY
LINE:
NOTE: This section includes cursory and important errors noticed while reading the
documents. This list is by no means a complete and full account of all the errors in the
text of the document since there are many repetition of similar errors in the document.
Recommended corrections on reference document:
Industrial Waste Evaluation Model (IWEM) Version 3 Beta: Technical Document
ES-1; Right Column first bullet: Protecting AIR QULITY in WMU design is mentioned. I did
not see an air quality analysis in this report? (see note below).
ES-1; Right Column second bullet: Monitoring analysis or help is mentioned. I did not see any
monitoring analysis in this report? (see note below)
ES-1; Right Column third bullet: Corrective action claim? Misleading. Can only be done by
repeated and iterative analysis. This is a lot of effort and given the uncertainty in the current
model parameters I would not use this application for corrective action study, (see my note
below)
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Mustafa M. Aral, Ph.D., P.E., p.Hy., FASCE
Consulting Engineer CPE GA 15254)
270 17th St. NW. Unit 809
Atlanta Ga 30363
e-mail: imnaral@live.com
ES-1; Right Column fifth bullet: Post closure action claim? Misleading. Same reason as
above, (see my note below)
ES-1; Right Column first paragraph: Risk based approaches? Misleading, (see my note
below)
Note: All of the above comments refer to applications that IWEM cannot be used to analyze.
Maybe the referenced GUIDE system that is mentioned in the document can be used for this
purpose, but when one sees these references in IWEM document without an explicit and
detailed reference to GUIDE analysis system, the user may get the impression that they can do
all these application using IWEM. In my opinion, without giving a clear definition of what GUIDE
is and what it does, the authors are giving the reader a misleading interpretation of the use of
IWEM.
ES-2; Right Column: Defining RISK ANALYSIS as comparing a user supplied RGC value with
model outcome is a conceptually wrong use of the RISK ANALYSIS techniques. More
importantly probability of exceedence analysis should be performed for proper evaluation of
RISK when Monte Carlo analysis is used. This is a standard procedure for risk analysis. This
would involve the computation of complementary cumulative probability density function which
is never mentioned in the document. The current definition is the wrong definition to use in RISK
ANALYSIS.
ES-6; Left Column: Statement:"If site-specific data are not entered, values are drawn
randomly." This recommendation does not make much sense. Why random data is entered for a
site specific case. At least one should recommend the selection of representative values based
on descriptive definitions of the properties of the site that is provided by the user. Otherwise
the application becomes a totally an arbitrary (random) application which would not represent
the site conditions and the MC analysis will not provide the uncertainty bounds of such a site
specific application since all the initial parameters used are random? This is a wrong
recommendation and misleading.
ES-7; Right Column: 90th percentile is not the correct definition for the risk analysis. Probability
of exceedence is a more computationally effective way to calculate risk based on an exposure
criteria. This is the wrong definition and it is misleading.
Page 1-3: 90th percentile is not the correct definition for the risk analysis. Probability of
exceedence is a more computationally effective way to calculate risk based on an exposure
criteria. This is the wrong definition and it is misleading.
Page 2-1, First paragraph under 2.1.1: "Controlling the release" is not the proper terminology
"management" is better. Obviously, we cannot control environmental systems we can only
manage them.
Page 2-1, First paragraph under 2.1.1 mid-section: "...WMUs is to evaluate the
appropriateness of a proposed liner design in the context of other location-specific parameters
such as precipitation," IWEM does not use precipitation it uses infiltration?
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Mustafa M. Aral, Ph.D., P.E., p.Hy., FASCE
Consulting Engineer CPE GA 15254)
270 17th St. NW. Unit 809
Atlanta Ga 30363
e-mail: imnaral@live.com
Page 2-3 Figure 2.2: Figure needs revision. There is vertical flow under the roadway cross-
section not the regional flow from left to right.
Page 2-4 Bottom: This is extensive computation time. Usually this process takes few minutes
for 10000 simulations on desk top computers or laptops in a similar software (ACTS) for much
more complex applications than the ones used in IWEM? This renders the current application
computationally inefficient (maybe a coding issue)?
Page 3-4, Bottom:"... EPACMTP does not account for fluctuations in rainfall rate or
degradation of liner systems that may cause the rate of infiltration and release ofleachate to
vary overtime." This is not the correct terminology because the depleting source boundary
condition that is used in EPACMTP is a leachate source which varies over time. This is
considered as Boundary Condition in EPACMTP. Thus, this is inconsistent wording of time
dependence, needs correction.
Page 3-7, Figure 3.6: Same problem with flow direction.
Page 4-1, First Paragraph mid-section: Too short a description to introduce the definition for
time dependent boundary condition used for the saturated zone model. This needs to be
extended for clarity. Or is time dependence truly used here? It is not clear? Since the
contaminant flux coming in from unsaturated zone is a function of time and that zone is a
boundary to the saturated zone this is an important issue and needs to be addresses in detail.
Page 4-1, Second Paragraph mid-section: This definition or concept is not correct. The
steady state nature of the well concentration at a distance from the source is not only a function
of Boundary Condition used but it is also a function of the distance between the well and the
source and also the exposure period that is considered in the analysis. This statement is
conceptually not correct and misleading. It should be further clarified in the document that the
well is not operating. Its location is only a reference point for the application as an exposure
point.
Page 4-1, Figure 4.1: What if the exposure averaging period belongs to another time range
(see the pink domain below?) that only corresponds to (say) the raising part of the concentration
breakthrough profile. Assume that the Trimester exposure period of a female is that period?
Than what? Here the authors have described an ideal situation which is only one of the many
possible cases. This is misleading and not correct.
-------
Mustafa M. Aral, Ph.D., P.E., p.Hy., FASCE
Consulting Engineer CPE GA 15254)
270 17th St. NW. Unit 809
Atlanta Ga 30363
e-mail: imnaral@live.com
O
~
(Q
0>
U
O
O
"ai
, Time-averaged weli\pncenlralion, C,
Peak
Concentration
Time
Exposure
Averaging Period
Page 4-2: Equation 4.1 has problems as mentioned earlier.
Page 4.4, Bottom Paragraph: The paragraph starts with steady flow definition, than it is stated
that velocity is increased. Needs better wording. Obviously both are steady state. In the mound
case velocities are recalculated not increased (as if it is increasing with the formation time of
the mound).
Page 4-5: Equation 4.6 has problems as mentioned earlier.
Page 5-2: MC section needs revisions with appropriate definition of RISK ANALYSIS.
Probability of exceedence calculations using the Complementary Cumulative Probability Density
function is the proper definition and evaluation of Risk.
Corrections on reference document:
Industrial Waste Evaluation Model (IWEM) Version 3 Beta: User's Guide
Page 2-1, above Table 2.1: No liner is not a liner type?
Page 2-3, Top: What is a complex site? That definition needs to be introduced here. There are
several possible complexities?
Page 2-3, Top: There are other more important complexities that are not mentioned here.
Heterogeneity? Fissures? Flow conditions Gradient? Proximity to exposure point? The
complexities selected here are not proper.
-------
Mustafa M. Aral, Ph.D., P.E., p.Hy., FASCE
Consulting Engineer CPE GA 15254)
270 17th St. NW. Unit 809
Atlanta Ga 30363
e-mail: imnaral@live.com
Page 2-9. Top: It is stated: "These processes decrease constituent concentrations in the
ground water as the distance from the source increases" This is misleading, the processes
described decrease the concentration as a function of TIME not by distance by their definition.
But since the contaminant is being transported over some distance its concentration decreases
by distance as well.
Page 2-11, Bottom: It is stated: "However given sufficient site-specific data, it is possible to
approximate the effect of these transport processes by using a lower value for the kd as a user-
input." Why not recommend to use the MC application to resolve the uncertainty in this issue?
General: Step-by-step instructions is good but inclusion of sample input data files will be very
beneficial for the user.
References
1. Industrial Waste Evaluation Model Version 3 Beta (IWEM v3.0)
2. U.S. EPA. 2014. Industrial Waste Evaluation Model Version 3 Beta: Technical Background
Document. Peer Review Draft. Office of Resource Conservation and Recovery, Washington,
DC. Prepared by RTI International and HydroGeoLogic.
3. U.S. EPA. 2014. Industrial Waste Evaluation Model Version 3 Beta: User's Guide. Peer
Review Draft. Office of Resource Conservation and Recovery, Washington, DC. Prepared by
RTI International and HydroGeoLogic.
4. Charbeneau, R. J. 2000. Groundwater Hydraulics and Pollutant Transport, Waveland Press
Inc., 593p.
Dr. Mustafa Aral (PhD, PE)
-------
Charles F. Harvey III' ^m Massachusetts Institute of Technology
Professor I 11 I ~~ Massachusetts Avenue, Building 48-309
1 II Cambridge, Massachusetts 02139-4307
Phone 617-258-0392
Department of Civil and Fax 617-258-8850
Environmental Engineering Email charvey@mit.edu
Peer review of: Industrial Waste Evaluation Model (IWEM)Version 3.0
Prepared for: Industrial Economics, 2076 Mass. Ave, Cambridge, MA 02140
June 29, 2014
IWEM appears to be an excellent tool for making simple but useful approximations of the
risks of groundwater contamination. The challenge in developing this tool was to find a
way to efficiently move from "soup to nuts," to link a sequence of calculations all the
way from the specific geometry of contaminant sources to finally generate Monte-Carlo
predictions of downstream concentrations. I agree that the Monte-Carlo approach is
necessary because of the large uncertainties. But, how to build this model without
creating an incomprehensible monster code? My impression is that authors have
succeeded pretty well. The code synthesizes a remarkably wide range of databases and
predictive models. I am particular impressed with how a variety of data bases of
parameters have been built into the Monte Carlo framework. I can't easily point to any
particular aspect of the model as the weakest link -1 can (and will) quibble with
individual links in the chain, but I can't name the weak link. I believe this indicates that
the authors have reached a good balance of accuracy and efficiency across the different
parts. They do not load any one step with too much detail. Below, I list questions,
concerns and suggestions and then provide specific responses to the charge questions.
(1) The authors should consider adding an early section that diagrams, with reference to
detailed future sections, the water and chemical flows that the code simulates. This
section would separate the flow calculations (water fluxes and velocities from the source
through the aquifer), from the simulations of chemical transport and transformation. This
separation would help users understand the rest of the document because the model is
coupled in only one direction. Flow drives all the chemical transport but there are no
modeled processes by which chemistry can affect flow. The sequential approach of to
chemistry is natural to follow. As it is, I find it a bit confusing that the model input and
report jump between the flow and chemical components. Such a "master diagram" would
be very helpful if flow arrows were annotated to show to show which sections of the
document describe the calculation of each component.
(2) I am confused about a number of related aspects of the groundwater flow calculations
and list these here as four separate items.
i. A set of figures (2-6 in Users Guide, ES-1, 2-3, 6-3 in the Technical Document) show a
cross section through a plume emanating from a source. All of the figures show the
contaminated water filling the area below the water table down to what appears to be a
flow line emanating from the upstream edge of the source. Is this really what the
simulated plumes look like? Does the model neglect clean recharge entering above the
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plume and displacing the plume downward in the aquifer? This appears inconsistent with
the description of recharge (section 6.3.3 in Technical Document). In the figure below, I
show what the plume should look like. Contaminant plumes can pass beneath shallow
well screens. The figures in the report appear to show that this can never happen.
LEACHATE CONCENTRATION
WASTE MANAGEMENT UNIT OR ROADWAY
WEU.
UNSATURATED
ZONE
LEAC-i4TE
Clean recharge enters
above plume
WATER TABLE
SATURATED
ZONE
LAND SURFACE
This should be
clean water
LEACHATE
Figure ES-1 Conceptual cross-section view of the subsurface system simulated by
EPACMTP.
ii. I am confused by how groundwater velocities are calculated. The program accepts
input of the saturated thickness, hydraulic conductivity, hydraulic gradient and effective
porosity. I gather that, if all of these are entered, then the velocity is calculated by
Darcy's law. If none, or only some, of these parameters are entered, then the other needed
parameters are generated randomly from the HGDB database taking into account
correlations between the parameters found in the 400 sites sampled for the data base.
With this approach, the calculated velocity is independent of the recharge (entered or
randomly generated) or the distance from a groundwater divide or discharge location (not
discussed). However, the report also talks about simulating hydraulic heads and flow. On
page 4-5, the technical document states that the "The pseudo-3-D module simulates
groundwater flow using a 1-D steady state model for predicting hydraulic head..." This
leaves the impression that Darcy fluxes are calculated after solving the differential
equation for head - such a calculation would include recharge and would need boundary
conditions.
The best approach would be to use the simple solution for the 2-D, cross-sectional, flow
field in homogeneous aquifer with constant recharge: horizontal velocity that is uniform
with depth but increases linearly with distance from the divide; downward vertical
velocity that decreases linearly from the water table, where it matches recharge, to the
aquifer base where it is zero. This simple model captures the basic flow pattern of
layered groundwater flow with flow paths entering at the surface and becoming more
horizontal with depth.
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This approach would also link to an important aspect of IWEM. IWEM considers
recharge calculated for different regions of the US to calculated the infiltration through
WMUs. This same approach could be used to determine the top boundary flux for the
groundwater model.
ill. It appears that the current model only moves solutes to depth through vertical
dispersion. This may be an adequate approximation, but it is not clearly explained. It is a
conservative "protective" approximation. In reality, the flowtube extending
downgradient from a surface source looses solute by vertical dispersion both downward
and upward into underlying and overlying flow tubes. The model here, only allows for
dispersion downward. Consequentially, it models about half of the dilution that would
really occur.
iv. Two basic features of groundwater flow are that: (1) it is slower in arid environments
and; (2) It accelerates from groundwater divides toward discharge, usually in rivers. I
believe IWEM neglects both of these features.
The first feature, faster groundwater flow in wetter regions, should relatively easily be
incorporated in IWEM through the regional recharge map - where recharge is large, the
hydraulic gradient is large and hence groundwater flow is fast. Is this part of the model?
Or is the hydraulic gradient generated without consideration of the local recharge? It is
clear that regional recharge rates are used to calculate the infiltration form WMUs, but
are they also used to estimate the groundwater velocity.
The second feature, position of the well in a flow tube, appears to be absent. I would
encourage EPA to consider adding an input for distance from groundwater divide and
distance to groundwater discharge, then using the recharge to calculate the velocity along
the flow tube. This would be an easy addition that would improve the estimate of
groundwater velocity, a key control on the ultimate Monte Carlo results.
CHARGE QUESTIONS
1. New Model Features.
The new features of IWEM appear to adequately represent roadways, although I am not a
roadway expert. One aspect that I can see missing is the case where water from a
roadside ditch accumulates in a topographic depression. In this case, contamination from
the road would be concentrated in one location, then either contaminate a stream or
infiltrate locally to produce to produce a larger groundwater contamination plume. I
don't believe the model can currently represent this situation, and in some landscapes it
may be the primary mechanism by which roadway contamination enters the environment.
2 Flow Equations.
Ditches. I only understood the modeled flow in and out of ditches after working through
the equations in appendix section C.2.2.5. In other words, the conceptualization is only
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fully explained through its mathematical formulation. The water balance and
contaminant balance for the ditch should be better explained by adding an improved
version of figures C-9 and C-16 into an earlier section of the document.
If I understand the model correctly, the crux of the approach is that the depth of water in
the ditch Hstr is constant along the ditch and is determined by the model. The depth of
water in the ditch HStr links the infiltration out the bottom of the channel to the upstream
inflow and down stream outflow because Hstr is a variable in both Darcy's law for
infiltration and in Manning's equation (C-19) for flow through the ditch (it is found in the
calculation of the hydraulic radius R which is in Manning's equation). Hstr is set by the
balance of inflow from the local road segment and outflow by infiltration, then it is used
to calculated the flow down the ditch. Inflow along the ditch from upstream of a segment
equals out flow to downstream of the segment (e.g. equation C-13), - no net inflow or
outflow through the ditch to other segments. Is this correct? If, so it is a nice approach
when only considering one segment, and I think it should be clarified earlier in the
description.
However, I am confused about how this formulation can be used for multiple ditch
segments. Equations 38a-c appear to allow for different inflow Q;n and outflow Qout.
This seems physically reasonable — steeper segments have faster flow through the ditch
allowing the water to be shallower (smaller Hstr) and diminishing infiltration so that there
would be more discharge out the downstream end of the ditch. I see how the recursive
equations C-38 and C-39 provide an elegant way to link both the flow and the
concentrations in the segments. But, I am confused about how Manning's equation is
applied to a ditch segment if Q;n does not equal Qout. (Could be as simple as using the
average Q.)
My overall feeling is that this problem is nicely formulated, but the explanation could be
better and that use of diagrams and figures could be much better. A better explaination
should address my specific questions, but also help any reader understand both the
IWEM code and the underlying physics of infiltration, flow, and contaminant transport
from roadways.
One simplification that should be highlighted is that the depth of water in the ditch is
constant over both time (steady flow assumption) and the length of the ditch (see above).
This is important because Manning's equation is nonlinear with Hstr so the steady-state
value of Hstr for the average rainfall is not the time average of varying Hstr for transient
flows driven by storms. I think this is an acceptable approximation, and trying to model
the transient effects of rainstorms would be very expensive! But this assumption should
be highlighted because the model could be quite inaccurate in places where rain falls in
few big storms instead of a constant drizzle. In big storm, most of the flow may flush
through the ditch quickly, whereas in a constant drizzle there may be much more
infiltration through the ditch.
HELP model. It was smart of the developers to employ the HELP model, a standalone
model that has been reviewed and described elsewhere. However, some additional
explanations of the fundamental aspects of HELP would improve the IWEM documents.
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What is meant by "The HELP model is a quasi-two-dimensional model" (6-16)? I would
have guessed it's one dimensional - where's the quasi-second dimension? How does
HELP calculate evapotranspiration? The best approach would be if HELP took into
account the seasonal climate and specific weather of each location to calculate transient
evapotranspiration. This would supply good estimates of the annual average to be used
in IWEM. Was HELP employed for these transient simulations?
3. Contaminant Flux.
The superposition of solute input from different roadway strips and layers should be
clarified. What is meant by "Aggregating" concentrations from multiple strips and
layers? (Top of page 2-3, "IWEM aggregates...") Does aggregate mean that the
concentrations from different strips of the same constituent are summed? Or does
aggregate mean something more complex?
Furthermore, how are inputs from the different layers handled within a strip? The
formulation in Appendix C.2.2.1 appears to model solute input to groundwater from the
layers as sequential - all of the contaminant enters from the bottom layer, followed by
transport downward of a pulse of contaminants from the next higher layer, and then
sequentially up to the top. Solutes do not mix across layers. Does this mean that different
layers in the same strip can never contribute chemical input to the aquifer at the same
time? This may be a reasonable approximation, but it should be explained. It may be a
reasonable model because, for any particular contaminant, a linear adsorption model
constrains the solutes to all move down in sequence never overlapping each other or
diluting. This is an interesting idea, but needs to be much better explained.
4 Model Simplicity
I am unconvinced that the roadway addition to IWEM must be three-dimensional. A
two-dimensional cross-section through a road segment might be sufficient and perhaps
better in certain circumstances. For a long straight road, the contaminant input may be
approximately the same along the entire road. In this case, there is no reason to include
the lateral dimensional along the direction of the road. I can see two cases where the
third dimension is needed: (1) If the distance to the receptor road is great and the road
takes a sharp curve; (2) If the transport of contaminants by flow in a ditch is significant.
(See my comments above.) If neither of these cases is true, then the model is
unnecessarily complex by including the third dimension. Unless a road has sharp bends,
it could reasonably be approximated as laterally infinite line source. So, the question is:
are there a significant number of cases where the third dimension is necessary? It seems
that the documentation should prove that there is a real advantage to the third-dimension
before adding it. The simpler 2-D approach could still work when groundwater flow is
not perpendicular to the road.
For the other WMU's, such as landfills, the lateral extend may be short relative to the
distance to the receptor well, so they could no be simplified to a line source. Roads are,
in fact, simpler in this regard.
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5. Documentation.
In general, better figures and diagrams would help. I see several categories of useful
figures that could be added:
i. Diagrams of flow and then solute transport processes. These would include variable
names as used in equations, as well as the equations themselves or references to equation
numbers. These figures should emphasize water and solute balances - inflow and
outflows and changes in storage. They could pair pictures of the true processes in the
real world, with its heterogeneity, with diagrams of the model processes. These pairings
could be used to illustrate model simplification and assumptions.
ii. Where the model shift its calculations. There are several parts of IWEM where
calculations are made differently based specific cases. For example, the code decides if a
ditch is dry or overflowing before calculating flow in the ditch. It would be particularly
useful to develop diagrams of the internal decision trees for these cases.
in. Better illustrations of databases. This would include histograms of the
hydrogeological parameters from HGDB from which realizations are drawn for the
Monte Carlo simulations and maps of the national databases.
iv. Case studies. Users would greatly benefit from example problems. Actual, real world,
case studies are a very useful mechanism for making a complex package like IWEM
accessible. These case studies would begin with data collection, giving examples of how
local data may be found, then show the input and output of IWEM, and finally contain a
discussion of the results. Users could then approach IWEM by first studying the case-
study that most resembles their site. I strongly recommend the addition of a section of
example cases that span the range of IWEM's capabilities, different WMUs different,
different contaminants and different areas of the US.
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Peer Review of the Industrial Waste Evaluation Model (IWEM) Beta Version 3.0
Dr. Lin Li
June 15, 2014
Summary of Conclusions
Compared to IWEM 2.0 version, the IWEM 3.0 (beta) adds leaching through the roadway cross
section with ditches, drainage, and surface runoff as optional elements. The additional function
can be used for the leaching impact study for the roadway system with ditch, gutter, embankment
and surface runoff consideration. The new version makes the IWEM closer to realistic roadway
condition. During the technical review, there are major concerns about the add-on functions
related to the pavement material property, leaching pattern, ditches and surface runoff.
Table 6-16 of Technical Documentation needs major revision. It contains unnecessary
information, such as "Wilting point" and "Curve Number". The low-end and high-end of
pavement materials properties are far off from the practical data range. If the user has incorrect
information from the Table 6-16, the following simulation is also incorrect. The following Table
6-17, 6-18, 6-19 and 6-20 of Technical Documentation needs much clear explanation about how
to get these data and how to verify these data, because some of these data are out of typical ranges.
The pulse source assumption is suggested to revise. The "first flush" and "lagged response"
leaching pattern should be considered.
The detailed comments are shown in the following section. A major revision is strongly
suggested.
Charge Questions and Responses
1. New Model Features.
The current version of IWEM 3.0 (Beta) setup the maximum value of 15 of roadway strips, 5
layers of per strip, 2 of drains, 2 of ditches, and 2 of gutters. The assumption may not be
validated for two parallel highways separated with median ditch (such as northbound and
southbound highway). It is suggested to increase the maximum number of roadway strip to 20,
number of drains (more than 3), number of ditches (more than 3) and number of gutters. The
number of gutters should be as a number of gutters per unit distance along the roadway.
There is no embankment input parameters in this beta version. The embankment should be
illustrated in the roadway stretch. Embankment input parameters can include the geometry of
the embankment, and elevation of the embankment. Since embankment is higher than ground
surface, the elevation of the roadway should be included in the input parameters. Another
issue about the embankment surface runoff consideration. In case the industrial materials are
used in subbase layer in the embankment which can be above ground surface, the runoff may
contain metal contaminants. Can the IWEM consider this scenario?
IWEM assumes that infiltration from the traversing roadway is on the order of regional recharge.
However, infiltration may be much higher in the unpaved shoulder than the paved median.
The infiltration may be much higher in the embankment. The regional recharge is time-
average estimation, but the infiltration is time-dependent. Can this assumption be re-written
IWEM Beta Version 3.0
Peer Review by Dr. Lin Li
-------
as the bottom limit of infiltration from the traversing roadway is on the order of regional
recharge?
IWEM assumes that lateral communication between roadway-source strips is insignificant. This
assumption has limitation for embankment. When the base layer has slope, most of infiltrated
water flows out of the roadway horizontal instead of vertical.
Including gutters in the roadway system should be optional, because a highway typically does not
have gutters.
Figure 2 and Figure 3 in the user manual are unclear to read. A better resolution should be
included for the two figures. There is a need for a better sketch of ditch to define the slope,
water depth in ditch, and thickness. The gutter shape is also needed in the sketch.
The infiltration/percolation/evaporation is used in the manual. The definition should be provided,
since the three terms are easily confused to the user.
Table 6-2, Term "Ditch strip drain drains to-^strip number of the ditch for this drain" is
confusing. Suggest rename "Layer drain is above" as "layer number underneath the drain",
suggest rename "Drains what strip "as "which strip need drains".
Table 6-2, it is difficult for user to provide "Flow Percentages to Ditch Strips (for relevant strips
and drains)". Suggest rename as "Percent of roadway runoff that reaches ditch". Is it possible
to include it for Monte Carlo Simulation?
Table 6-2, it is confusing "Percent of flow in drain that reaches ditch". Need definition first. It is
confusing for "Ditch strip(s) receiving overland flows".
Page 6-11, it is confusing "Once the mass of leachable constituent is known, the duration of
leaching from a material layer is calculated". Please revise it.
Table 6-4, how user to provide: "Infiltration rate through a strip (m/yr)", "Runoff rate (m/yr)".
"Precipitation rate (m/yr)", and "Evaporation rate (m/yr)" for roadway module. It is too
complex for common user.
Figure 6-6 divides the US into 12 climatic zones. It is acceptable for a screening model used in
US. But IWEM model is an international widely used screening model. The climatic zones in
US will limit the IWEM model as domestic code only. In my personal opinion, it will
significantly reduce the international usage of the new IWEM function. It is suggested to
consider global climatic zones. In each climatic zone, the two climate stations located within
the zone from the HELP climate database, with minimum and maximum 5-year ave
precipitations, are selected (Table 6-15). The minimum and maximum precipitations only
cover the range of the precipitations within the climatic zone. Is it possible to add mean
precipitations within the climatic zone?
Since IWEM model is screen-level model, the Table 6-16 of Technical Documentation is too
complex. It involves so many uncertainties. The user may not select correct material
properties for each layer. The Table 6-16 should be much more condensed. For example, the
"Median (unpaved) is same as shoulder (unpaved)". Most of users do not know the definition
and usage of "Air void", "Total porosity", "Field capacity", "Wilting point", and "Curve
Number". There is no "ML" or "GP" for the US Department of Agriculture soil classification
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system. It is suggested to add AASHTO soil classification system in the Table, because all of
DOT contractor/officers are familiar with AASHTO soil classification system. The "subbase
course" is one layer of "optional" layer. Sometimes it is non-existent in the pavement system.
The data source of Table 6-16 is unclear. There is no subscript of 8 in the Table. A typo of
"Eqation" is found in subscript of 4. If we only look at the low-end and high-end, the data is
far off from the practical data range. The Table 6-16 is strongly suggested to revise. If the
user has incorrect information from the Table 6-16, the following simulation is also incorrect.
Table 6-16 used Apul et al. (2002) for the pavement material properties. The Apul et al. (2002) is
a report, which is not a peer-reviewed authoritative publication. The adoption of using this
report by IWEM model (EPA) is not suitable. A more authoritative literature review
should be conducted for the latest peer-reviewed publication for the pavement material
properties, especially for industrial waste materials. I am willing to help in this part. It
takes more times, but it is definitely needed. The pavement material properties will decide the
infiltration rate through the pavement system. The Table 6-16 contains too much information
that will not be used in the model, such as "Air void", "Texture", "Field capacity", "Wilting
point", and "Curve number". All of these terms are confusing and non-familiar to ground
water flow modeler. The Table 6-16 contains low-end and high-end data for top/base/subbase
course. If we only consider the single row of data, it seems correct. But when we look at the
entire pavement from top to bottom layers, the data of this table are incorrect and misleading.
The structure of this table is suggested to modify to reflect the entire payment system.
In Table 6-17 of Technical Documentation, how to get the infiltration rates? Is it verified with
authoritative published results? It is so high of an infiltration rate for embankment at Annette,
AK. The data is out of typical ranges. In Table 6-18 of Technical Documentation, how to get
the runoff rates? Is it verified with authoritative published results? The data is out of typical
ranges. In Table 6-19 of Technical Documentation, how to get the evaporation rates? Is it
verified with authoritative published results? Is it for HIGH and LOW? In Table 6-20 of
Technical Documentation, how to get the pan evaporation rates? Is it verified with
authoritative published results? Is it for HIGH and LOW?
Table 6-17 to Table 6-20 depends on Table 6-16. How to get the data in these tables? It is unclear
in the technical documentation. Are they for general pavement materials or for industrial
waste materials? What are the reference/equations for the calculation? What is pan
evaporation used for? Is the reference NOAA (1982) too old? Can we consider the latest
reference?
2. Flow Equations.
Figure C-7 of Appendix has typo. The Figure is from Apul et al. (2002). It is suggested to update
it from the AASHTO standard or FHWA publication for the pavement section. The
"permeable base" is the first time used here, but it is not discussed in the technical
documentation. Where is the subbase in the Figure C-8? The term of "Exfiltration" is the first
time used here, but it is not discussed in the technical documentation. The permeable base is
referred from Van Sambeek (1989), and filter reinforcement layer is referred from
Christopher (1998). Both terms are not familiar to the common user. Suggest more
authoritative references to revise this part. The Equation (C-8) and (C-9) are in question. How
to derive these equations? Equation (C-10) and (C-ll) depends on the assumption of
permeable base and filter reinforcement layer, which needs more careful justification in the
IWEM Beta Version 3.0
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typical pavement systems with industrial materials. Equation (C-12) and (C-13) are in
question. How to derive these equations?
The section of "C.2.2.3 Multiple Material Layers with a Drainage System" is based on
(Christopher and McGuffey, 1997; Apul et al., 2002). It is unacceptable for these old
references for the 2014 IWEM Model. It has been updated to the current references.
The section of "C.2.2.4 Runoff from Top of Pavement and Discharge from Permeable Base" is
based on permeable base for the runoff estimation. If there is no permeable base, are Equation
(C-19) and (C-20) validated? In the Equation (C-19), how to calculate RO,, and what is WL
and Z? What is CPB in Equation (C-21) and how to get it?
Figure 6-6 divides the US into 12 climatic zones. It is acceptable for a screen model used in US.
But IWEM model is an international widely used screening model. The climatic zones in US
will limit the IWEM model as domestic code only. In my personal opinion, it will
significantly reduce the international usage of the new IWEM function. It is suggested to
consider global climatic zones.
3. Contaminant Flux.
IWEM defines contaminant flux as infiltration rate multiplied by initial leachate concentration.
The pulse source is assumed based on screen-level analysis and a pulse source is assumed
appropriate for metals. The reference for this critical assumption is missing from the technical
document. Creek and Shackelford (1992) and Sauer et al. (2012) indicate that leaching
patterns for coal combustion products (CCPs) and highway materials stabilized with CCPs
generally can be grouped into two classes referred to as "first flush" and "lagged response"
leaching. As shown in Fig. 1, the "first flush" pattern is characterized by monotonically
decreasing concentrations as water percolates through the CCPs (Bin-Shafique et al. 2006),
whereas the "lagged response" pattern is characterized by decreasing concentration followed
by increasing concentration. First-flush leaching patterns from CCPs generally correspond to
adsorption-controlled release and can be described mathematically by advection-dispersion-
reaction equation with instantaneous, linear, and reversible sorption (Bin-Shafique et al.
2006). Lagged response leaching can be attributed to a variety of geochemical processes and
generally cannot be described using a single mathematical function used in WiscLEACH (Li
et al. 2006). WiscLEACH was originally developed by Li et al. (2006) for assessing potential
groundwater impacts associated with fly ash stabilization in roadway construction. Based on
three analytical solutions for the advection-dispersion-reaction process in the subsurface, Li
et al. (2006) only evaluated the "first flush" leaching pattern in the two-dimension application
of fly ash stabilization. WiscLEACH was revised to extend the capacity of original
WiscLEACH from a two-dimensional application of fly ash stabilization to a three
dimensional application of embankment and structural fills in roadway construction. The
"lagged response "and" first flush leaching patterns are both included in this revised
WiscLEACH (Li et al. 2011). The WiscLEACH has been used in several studies of industrial
materials in embankments (Cetin et al. 2013ab, Li et al. 2014).
Since the contaminant flux is critical for the IWEM prediction, it is strongly suggested to consider
either to provide more detailed documentation for the current assumption, or to adopt the
"first flush" and "lagged response" leaching pattern instead of pulse source.
IWEM Beta Version 3.0
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35.0
1.0 2.0 3.0
Elapsed Time (yr)
4.0
5.0
e 2.0
= 1.5
0.5
1.0 1.5
Elapsed Time (yr)
2.0
Figure 1. Example of leaching pattern from the CCP application in roadways construction: (a) first flush
elution pattern measured from the fly ash subbase stabilization at STH 60 (Edil et al. 2002); and (b)
lagged response elution patterns measured from the fly ash embankment at the Colebrook, NH
(Gardner et al. 2009).
IWEM needs Constituent-specific initial leachate concentrations and total concentrations in layers
containing reused materials. How to use total concentration in the model?
4. Model Simplicity.
The IWEM 3.0 (Beta) makes the model complex, with so much additional input parameters
compared to version 2.0. Some of the parameters should be merged, or removed. Some of the
terms are unfamiliar even to experienced groundwater flow/transport modeler.
5. Documentation.
The technical manual and some parts of technical appendix should be combined, because all of
equations are shown in appendix in the beta version. The equations are a key part to
understand the technical part. After the literature are updated (see above comments), and the
tables are revised (see above comments), the technical manual can be more readable to user.
Other General Comments
The IWEM interfaces are too old. When the user opens the IWEM, the icons of first screen are
Windows 95 icons (shown in next Figure). Is it possible to change to at least Windows 7?
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The "Source Parameters" with "Flow Characteristics", "Drain Properties", "Ditch Properties",
Layer Properties" and "Geometry" are flying and confusing to the user when user inputs the
source parameters. Is it possible to fix their sequence or give labels with "1, 2, 3, 4, 5, ..." to
make a sequence for input of these source parameters. The flying interface does not help the
user to input.
File Options Help
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Peer Review by Dr. Lin Li
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When the user finishes the input and clicks the run, a DOS window is popping up and shows the
simulation in process. In the Windows 8 environment, is it possible to remove this pop up and
only in IWEM interface to show a status line "Simulation is on, please wait"?
References
Please fully cite any documents or literature that you reference in the letter review.
Bin-Shafique, S., Benson C. H., Edil T. B. and Hwang K., 2006, Leachate Concentrations from
Water Leach and Column Leach Tests on Fly-ash Stabilized Soils. Environmental
Engineering Science, 23(1): 53-67.
Cetin, B., Adyilek, A. H., and Li, L., 2013a, Leaching Behavior of Aluminum, Arsenic, and
Chromium from Highway Structural Fills Amended with High-Carbon Fly Ash, Journal of
Transportation Research Board, 2349:72-80.
Cetin, B., Adyilek, A. H., and Li, L., 2013b, Trace Metal Leaching from Embankment Soils
Amended with High Carbon Fly Ash, Journal of Geotechnical and Geoenvironmental
Engineering, ASCE, 140(1): 1-13.
Creek D.N. and Shackelford C.D., 1992, Permeability and Leaching Characteristics of Fly Ash
Liner Materials. Transportation Research Record, 1345: 74-83.
Edil, T.B. et al., 2002, Field Evaluation of Construction Alternatives for Roadways over Soft
Subgrade. Transportation Research Record, 1786: 36-48.
Gardner, K. 2009, National Green Highway Activities and Resources to Support Recycling and
Reuse. Resource Conservation Challenge (RCC) 2009 Workshop, Hyatt Regency Crystal
City, DC, March 25 - 27, 2009.
Li, L., Benson, C.H., Edil,T. B., and Hatipoglu, B., 2006, Groundwater Impact from Coal Ash in
Highways, Waste and Resource Management, 159(4): 151-162.
Li, L., Peng, B., Santos, F., Li, Y., and Amini, F., 2011, Groundwater Impacts from Leaching of
Coal Combustion Products in Roadways Embankment Constructions, Journal of ASTM
International, 8(8): 1-12.
Li, Y., Li, L., Cordero-Zayas, S., Santo, F., and Yaeger, J. H., 2014, Environmental Impact of Fly
Ash Utilization in Roadway Embankments, Journal of Material Cycles and Waste
Management, DOI 10.1007/s 10163-014-0266-6.
Sauer, J., Benson, C., Aydilek, A., and Edil, T. 2012, Trace Elements Leaching from Organic
Soils Stabilized with High Carbon Fly Ash, Journal of Geotechnical and Geoenvironmental
Engineering, ASCE, 138(8): 968-980.
IWEM Beta Version 3.0
Peer Review by Dr. Lin Li
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Peer Review of the Industrial Waste Evaluation Model (IWM) Beta Version 3.0
Dr. Frank W. Schwartz
June 6, 2014
Summary of Conclusions
(i) I reviewed earlier versions of the modeling package, specifically focused on EPACMTP. Beta
Version 3.0, including the Technical Document, User's Guide and software package has been
improved in many different ways. In particular, the Windows-based data input structure has
significantly improved the usability of the code. The modular design of the new roadway
module is sufficiently general that it will be able to facilitate the analysis of common types of
roadways that will be encountered in practice.
(ii) There are no obvious problems in downloading and running the code. I ran two test cases
from Appendix C of the User's Guide (B.I and C.2) without difficulty, yielding the answers and
reports presented in the User's Guide. My audit of the software shows that the data entry
works fine and the transport module provides correct answers. Overall, the design of the code
is in keeping the vision of an easy to use package, which is appropriate for the target user
group.
(iii) The Technical Document and User's Guide are well organized, reasonably well documented,
and overall do an excellent job in facilitating the use of the code. They are written for a
sophisticated reader with good background knowledge of groundwater flow and transport. I
think that this level of presentation is appropriate, given that the model although billed as a
screening tool does require background hydrogeological knowledge.
(iv) Beta Version 3.0 has opportunities for improvement. First, the level of complexity for even
simple "roadway" cases is out of proportion to the other flow/transport process modules, e.g.,
saturated and unsaturated flow. In particular, the multiple roadway segment is unusually
complex and relies on calculations outside of the code. I can expect that a typical user would
not be able to understand how to do such an analysis. Overall, the roadway modules are
obviously much more complicated than the other types of waste sources (landfills, ponds) and
push the "screening" paradigm that has guide the development of the package.
Second, the usefulness of IWEN depends on generic databases to provide parameter estimates.
I consider this to be a weakness because the approaches are dated and without appropriate
verification of the data. My review here suggests alternative, site-based strategies.
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(v) Following are three key recommendations. First, the simple evaporation/runoff/infiltration
model should be re-examined in the context of a road. If precipitation is commonly delivered
via high intensity rainstorms, I expect runoff to be much more significant as compared to the
steady-state case with continuous, low intensity rains.
This version of the software and report appears to have focused on new modules. Going
forward, I would secondly recommend that much more thought be given to more descriptive
metrics of risk and performance that are more than just a comparison of a probability
distribution number to some water quality metric.
Finally, I recommend improvements to the various written materials. The present Technical
Document is cumbersome and unwieldy because of the blatant overuse of unintuitive
acronyms. The mixing of metric and British units within the reports and appendices is confusing
and makes the information less transparent. The report would benefit from an assessment of
accumulated experience (concerns and suggestions) from users/stakeholders.
Charge Questions and Responses
1. New Model Features
The modular design of the roadway configuration is sufficiently general that it will be able to
create common types of roadways that will be encountered in practice. Roadways are
sufficiently complex in terms of components and design features (e.g., internal drains) that they
appear to require many parameters to describe their behavior. My opinion in reading the
reports is that routine application of the roadways module will be difficult and speculative
because so much site specific data is required for the roadway source inputs.
(a) There are many implicit assumptions in the development of the roadway module. The most
obvious is that infiltration through the roadway is steady state. This assumption of 1-D steady-
state flow has been present in all the previous versions of EPAMCT models and probably
reasonable for those applications.
Overall, the simple evaporation/runoff/infiltration model needs to be much more carefully
examined in the context of a road. If precipitation is commonly delivered via high intensity
rainstorms then runoff would be much more significant as compared to the hypothetical
steady-state case with continuous low intensity rain. The HELP model is used extensively for
parameter estimation, assuming readers are well informed as to how it works and what its
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limitation might be. The report and appendices contain tabulations, but little in the way of
material (beyond E4.2) to support and explain the parameter calculations.
(b) The usefulness of IWEN product depends in many ways on national databases to provide
parameter estimates. I consider this to be a weakness because the approaches are dated and
without no verification of the accuracy of the guidance. A following section here suggests
alternative, strategies.
Given the maturity of the product, I was expecting a more robust description of the HELP
methodology, including a brief discussion of the method, and papers or reports describing
experiences and any assessments of that material (beyond Section E.5). In many cases, reading
what are likely highly uncertain parameter estimates (e.g., infiltration Table 6-17) to three
significant figures suggests foundational assumptions about the model will require careful
reconsideration.
Along the same lines, the subsurface parameters come from an old DRASTIC-based
classification of hydrogeological settings. The DRASTIC methodology is obsolete as is for
example Dr. Newell's 1989 assessment of subsurface settings (used pg. 6-45,46). I was a
reviewer of that work back in the late 1980s but more modern approaches are available.
(c) Any estimate of concentration at a receptor well will require a good estimate of initial
leachate concentrations. The report is weak in terms of guidance provided as to how these
values will be estimated, one long paragraph on 6-13,14. The small size of this piece is out of
proportion to treatment given to other parameters, e.g., infiltration rates.
The Recycled Materials Resource Center (RMRC) website is not particularly user friendly and
would require work in extracting usable numbers for simulation purposes. I feel that it is a
stretch to assume that users can develop appropriate numbers from testing data or field data
(pg. 6-13). A suggestion would be to conduct a MINTEQ style analysis for some typical
concentrations for type materials as a future work.
My concerns in this respect stem from the problem that the state-of-technology in industry for
conducting leaching experiments is not particularly good. Examples I have seen are often
plagued by experimental errors, problems in sample handling and other things.
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2./3. Flow Equations/Contaminant Flux
Appendix C.2 to the Technical Document describes the development of equations for the
calculation and timing of loading due to a roadway. The approach generally uses mass balance
concepts to provide loading to the footprints of the various strips. The approach is generally
straightforward and well described. This Appendix also summarizes key assumptions. I did not
check the equations in detail.
Individual sections are used to describe relevant process details. For example, Appendix C 2.2.4
describes the calculation of runoff from the top of the pavement and along with discharge from
a permeable base. Again, this piece is well described along with assumptions.
(a) There are opportunities for improvement of the write up. The "pulse" analogy is poorly
described, assuming that a set of relatively complex equations provide a concept that is
understandable by stakeholders. Section C.2.2.1 (page C-7) should begin with a conceptual
model explaining the pulse modeling concept in words and with a picture. The pulse concept
comes with inherent assumptions that are not fully explained.
(b) With the roadway module, the level of complexity for even simple cases is out of proportion
to the other process modules, e.g., saturated and unsaturated flow. The details and complexity
of the hydrogeological setting greatly influence concentration distributions. Yet, this part of the
model has been simplified given the "screening" purpose for the modeling system. Moreover,
the roadway modules are noticeably more complicated than the other types of waste sources
(landfills, ponds).
Transport in the saturated zone is simulated by about 10 parameters, e.g., advective velocity, 3
dispersivity values, and sorption/decay processes. Compare this for example with the
description of the complex roadway shown in Figure 1. This kind of roadway might require 100
to 150 parameters to represent the various components. Many of the necessary parameters
will not be known for sites and end up as guestimates.
In the case of the landfill and waste rock pile etc. the model would seem to provide
recommendations about design features, liner, cover etc. The road design could be so complex
that it may not be obvious as to what parameters are driving the risk.
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Collecte
infil/leachat
Roadsi
Infiltration exfiltration
without leachat
Exfiltration with
leachat
No -recycle
material
Recycled
Highly
material
Drain/gutte
Groundwater Flow
Figure 1: Example of a complex roadway from the report Appendix.
(c) Given the comments in 2 (b), I would like to comment on the development of equations for
multiple road segments. If I interpret this short piece C.2.2.6 (Appendix pg. 20-22) correctly, the
user needs a set of standalone tools that would take output from a sequence of IWEM runs and
superimpose solutions and route mass along the ditch. I can expect that a typical user would
not understand how to do such an analysis. If such a capability is desirable, then a simple
worked example needs to be presented as a conceptual model, including steps explaining ditch
routing and superposition of mass transport calculations.
My view is that the multiple road segment analysis is far beyond the vision of screening that
guided the development of earlier modules. In addition, given the complexity and large number
of uncertain parameters, it is questionable whether doing this kind of analysis would be
credible in a regulatory sense.
(d) In terms of flow associated with the roadway, the analysis depends on several hard-to-
estimate parameters. For example, the User's Guide on pg. 3-28 explains how users should
provide a value e.g., "a percentage of Overland Flow to Each Ditch not Captured by Gutter".
This number is totally empirical with no help to help determine the value. Similarly, on pg. 3-29,
values for parameters as B. and C. are not well explained and could end up just being guesses.
The User's Guide should contain more figures that make it clear what road geometry is being
described in Figures 3-8 to 3-14. Otherwise, the parameter associations are very difficult to
follow.
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4. Model Simplicity
As part of my evaluation of the code itself, I ran two test cases from Appendix C of the User's
Guide. The first was B.I, an example of land application of foundry sand in home gardens. The
second was C.2 - Example Problem 1 for a roadway, in this case Wisconsin State Highway 60.
Running the two cases was straightforward, and the code yielded the answers provided in the
User's Guide. I printed both reports without a problem. Thus, my limited audit shows that the
data entry works fine and the transport module provides correct answers. Overall, the design
of the code is in keeping the vision of an easy to use package, which is appropriate for the
target audience.
(a) My expectation is that the model developers, as part of the model development, could have
provided an illustrative example with a detailed analysis of the output. The examples in the
Appendix to the User's Guide are helpful but cursory. One purpose would be to understand
how uncertainty in input parameters provides uncertainty in ensemble statistics for a roadway
setting. Such an analysis could serve as a starting point for a complete re-examination of what
other kinds of output could be provided to the model user.
As the report discusses, a cumulative probability distribution with the Monte Carlo module can
be interpreted to provide the probability that some standard will be exceeded. In the Technical
Document example (Figure 5-1) shows concentrations varying over 7 orders of magnitude.
Simply providing the concentration associated with a 90% probability of occurrence glosses
over the fact that this system is very uncertain and essentially unpredictable. Every realization
has an equal likelihood of occurrence and for this example concentration changes by about one
order of magnitude for every 10% in probability.
This version of the software and report appears to have focused on new modules. Going
forward, I would recommend that much more thought be given to more descriptive metrics of
risk and performance that are more than just a probability distribution number, which are not
known with much confidence.
(b) The software and databases go out of their way to provide numbers to users who have little
in the way of site specific information. I think that the authors of the report at a minimum need
to rethink their approach in dealing with sites for which the subsurface environments are
unknown. For example, the User's Guide on page 4-19 shows how "national average" values
are applied when a user selects the subsurface environment as unknown. There is no
possibility that the default values of Table 4-4 would actually describe a specific site. I think
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that it is inappropriate to provide users a capability of creating results in absence of subsurface
data -the result is meaningless.
(c) The so-called national databases need to be de-emphasized because as a practical matter
they are old assessments, minimally validated and without a transparent basis in science. This
was mentioned previously in section la.
Another possible solution would be to provide users with a series of online tools that would
take specific qualitative/easily discoverable geologic observations about the site and translate
them into model parameters. Such an approach was used in an expert system by McClymont
and Schwartz (1991a; 1991b). The notion that a three word description of a site could yield a
complete quantitative description of hydrogeologic setting is quite a stretch in my opinion. The
scientific basis for the choice of parameters would be much more transparent to the users and
evaluators alike.
(d) There is a real divergence in the roadway module with how parameters are chosen. On the
one hand for the groundwater system, one can specify "unknown" and receive a default
collection of parameters. On the other hand, as stated on pg. 3-20 of the User's Guide all of the
source parameters for the roadway are site specific and actual values must be provided. The
parameter treatment in the total package, thus, is rather inconsistent and unbalanced.
5. Documentation
The Technical Document is well organized, reasonably well documented, and does an excellent
job in facilitating the use of the code. It is written for a sophisticated reader with good
background knowledge of groundwater flow and transport. I think that this level of
presentation is appropriate, given that the model although billed as a screening tool will be
require background hydrogeological knowledge.
I have reviewed earlier versions of this software specifically focused on EPACMTP. The present
package including the Technical Document, User's Guide and software package have been
improved in many different ways. In particular, the Windows-based data input structure has
significantly improved the usability of the code.
Future versions of the Technical Document have opportunities for improvement.
(a) The report is thin in cited literature describing the theoretical basis of the modeling
approach. There is only about 1.5 pages of non-EPA references. Many sources are represented
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by "grey" literature rather than primary journal articles. A good example is Gelhar's EPRI report
which was later published in Water Resources Research.
Here are particular examples. Where did equation 6-14 come from, and what units are buried
in the so called conversion factor? Where did equation 6-5 come from? Is this Gelhar's equation
or did the authors construct this from data presented in the report?
Another example is the whole concept of sorption of metals that are treated using non-linear
sorption isotherms (Section 6.5.2.2). I have never seen this approach used and tested in
models. I couldn't find one citation that explained where this approach was developed and how
it was tested.
(b) The report omits a careful and consistent description of the different types of statistical
distributions used in the model. For example, with Table 6-24 mention is made about Gaussian,
log normal and log ratio distributions. Yet, I cannot find a discussion of these distributions. The
obvious place for such a discussion is in Section 5.1. As far as I can see, the modeling uses
cumulative versions of these as well as empirical or data-driven cumulative distributions. The
discussion of distributions in the Technical Document should include a discussion of typical
distributions - uniform, normal, lognormal etc., what they look like as cumulative distributions.
Also, when data-driven distributions are developed, they should be explained in their own right.
The choice of distribution has some bearing on parameter ranges. For example, in Table 6-24,
why is the lower limit of hydraulic conductivity 0 for lognormal distributions?
(c) The mixing of metric and British units within the reports and appendices is confusing and
makes the information less transparent. For example, climate data from Table 6-9 is in inches,
leading to infiltration rates in in/yr in Table 6-17. In the code (see Figure 3-18), apparently
infiltration rates are needed in m/yr.
Hydraulic conductivity values are sometimes in cm/s or m/yr. In the future, consistency of units
should be a priority.
(d) In my earlier reviews of versions of this material, I criticized the use of acronyms. The
present Technical Document has approximately 50 acronyms of all kind. Often the acronyms
are unintuitive. When a reader needs to constantly refer to page of acronyms, the report
becomes cumbersome and unwieldy to deal with. This feature of the report is a substantial
negative in the overall presentation.
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The User's Guide with related appendices provides an understanding of how the code is used.
As part of my evaluation, I downloaded a version of the code to my PC, and set it up according
to the introductory material in Chapter 3 of the User's Guide. The set-up went smoothly with
no problems, providing a code ready to go. The IWEM icon appeared on the desktop as
indicated on page 3-1. As an aside, the icon has a rough and amateurish look.
(e) The various screens provided to the code users are well described in the User's Guide. I
particularly liked the blue arrows added [A], [B], etc. along with the more detailed descriptions.
In the code itself, the screens are well organized and laid out. Where necessary, the units
assigned to numbers that are entered as data are indicated. There are a few places where
labelling could be improved. One example is Figure 3-11, with the notation "Is Below Drain".
The acronyms on some of the drop-down menus are also unintuitive in a few cases.
The drafted figures that sometimes turn up as part of a screen are sometimes rough looking. A
good example is Figure 3-9 on page 3-22. The colors and detail of the small roadway figure are
difficult to view.
(f) Both Appendices B and C provided a useful step-by-step description of how to do run the
test cases. My only complaint was that case B.I would have been helped by presenting actual
screens, although the example was obviously simple. For Problem 1 in Appendix C, it would be
better to associate Tables C-l to C-3 with the actual screens. The actual screen shots in
Appendix C (e.g., Figure C-5b and C-6) are too small and required a magnifying glass to read the
numbers etc. on the figures.
(g) My only problem in running the code was in saving the final output. It could be my
inexperience, but in both the examples run, I don't know where that information was saved.
(h) I examined the Built-in-Help available as part of the code. I think that what is there is
helpful, but in its present form is rather barebones, mostly repeating things in the written
manual. There is room for improvement here
Other General Comments
(a) Reading the documentation, it is not exactly clear how concerns and suggestions from the
community of users/stakeholders actually percolate down to the code developers. Is the
community happy with the model and do they use it?
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(b) A quick search of Google also indicates that this modeling approach has not gained traction
in the scientific/academic community. There are a few papers by the model developers but
otherwise just a few others. To some extent, information on actual use in site investigations
and other metrics (e.g., number of code downloads) could be interpreted in terms of its overall
usefulness. Future advances in this modeling approach could be better related to wishes and
needs of the user community. I would have liked to see this to help justify various directions
taken in the future.
(c) On page 3-38 of the User's Guide, it appears that if a single Monte Carlo realization is
unrealistic then it is discarded. If a subset of realizations in a Monte Carlo simulation is not
used, there can be issues of bias in the ensemble statistics. It is not clear what a "sufficient"
number of realizations is, but, the code developers perhaps should have a more definitive
cutoff. Or, perhaps this cutoff exists and is just not written down
References
McClymont, G.L. and F.W. Schwartz, 1991. Embedded knowledge in software: 1. Description of
a system for transport modeling. Ground Water, 29(5), 648-654.
McClymont, G.L. and F.W. Schwartz, 1991. Embedded knowledge in software: 2.
Demonstration and preliminary evaluation. Ground Water, 29(6), 878-884.
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