UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
' WASHINGTON. D.C. 20460
EPA-SAB'EEC-90-009
OFFICE OF
27, 1990 THE ADMINISTRATOR
Honorable William K. ReUly
Administrator
U.S. Environmental Protection Agency
401 M Street, S.W.
Washington, D.C. 20460
Dear Mr. Reiily:
The Science Advisory Boaid has completed its review of the Office of Solid Waste's
(OSW) Combined Analytical-Numerical Saturated Zone (CANSAZ) flow and transport model
for use in the EPA Composite Model for Surface Impoundments (HPACMS), and is pleased to
submit its final report. This report resulted from a Saturated Zone Model Subcommittee review
on May 30-31.
The charge to the Subcommittee identified two major areas for SAB review. The first
relates to the appropriateness of the assumptions underlying CANSAZ when used in a
nationwide, Monte Carlo assessment, including which parameters should be estimated on a site-
specific versus a nationwide basis. The second relates to the adequacy of the code for simulating
conditions beneath surface impoundments.
The Subcommittee found that the proposed CANSAZ model represents a significant
advance over the current Vertical Horizontal Spread (VHS) model, but that a number of
difficulties remain in the model components, inputs and intended mode of application. The
following describes the major issues considered by the Subcommittee and the principal findings.
1) Proper mathematical formulation: - The numerical and solution methods are properly
formulated and solved. The technique employed is innovative and computationally efficient, and
the numerical accuracy of the method is supported by rigorous comparisons with analytical
solutions.
2) Adequacy of physical assumptions: - The CANSAZ model includes the basic
assumptions and processes incorporated in current management-oriented ground water models,
but these models neglect several important processes now known to impact contaminant
transport and fate in actual fiejd sites. A number of assumptions restrict the applicability of the
model, including:
*
a) Dimensionality Of the model (failure to include transverse flow from mounding),
b) Limitation to steady-state prediction,
c) Assumption of homogeneous aquifer media,
d) The inclusion of limited hydrodynamic, chemical and biological processes.
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Particular processes which are not incorporated, but could be important at particular sites, are
discussed in our report.
3) Adequacy of Monte Carlo Approach: - The mechanics of the Monte Carlo procedure
appear to be properly designed and implemented. However, determining the adequacy of the
input distributions is more problematic. These input data are critical to accurate model
assessment. The current input data-set is not adequately documented or supported by field data.
It is thus recommended that a panel of hydrogeologists, soil scientists, and engineers be
convened to review the proposed model input values and documentation,
4) Adequacy of field testing of the model: - The EPA has made a good start at validating
the EPACMS model by confirming the numerical accuracy and presenting a preliminary field
validation study. The field validation study should be documented, and extended in more
detailed studies at other sites, A special effort is needed to validate the EPACMS model if it is
used in a nationwide assessment. An extensive nationwide data collection and monitoring
program will be needed to accomplish this.
5) Overall modeling approach: - The Subcommittee strongly prefers site-specific
determinations. The Agency could consider the use of a provisional delisting, whereby a waste
is delisted only if it is disposed at the site which is analyzed. The Subcommittee recognizes drat
the policy criteria of the Agency may preclude this, indicating instead the use of the generic
nationwide evaluation. Should the Agency decide to utilize the nationwide Monte Carlo
approach, the assessment would be improved by incorporating regional variations in the
assessment and explicitly banning the disposal of delisted wastes in vulnerable hydrogeologic
settings so that these sites may be excluded from the model assessment.
These findings and recommendations are made for the use of the CANSAZ and EPACMS
models in the limited fashion described in the report. We are pleased to have had the
opportunity to be of service to the Agency, and look forward to your response to this report.
Sincerely,
. . _ ***>*MsL-
i- i ,ji1...M^^^^^^.i..iiiiiii-iiiiiiiiiii ii MM ~>ii^ r-JOB—1 1 «,,un,,,,im, lmm^^^^11 .•niFHiwi "Jr
Raymond C, Loehr, Chairman Richard A. Conway, Chairmaii a
Executive Committee Environmental Engineering Committee
Science Advisoty Board Science Advisory Board
Mitchell J. Small, Chairman
Saturated Zone Model Subcommittee
Science Advisory Board *
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U.S. Environmental Waahlnf ton, DC
Protection Agency EPA-SAB-EEC-90-009
Report of the Saturated Zone
Model Subcommittee
Review of the CANSAZ Flow and
Transport Model for Use in EPACMS
A SCIENCE ADVISORY BOARD REPORT March 1990
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NOTICE
This report has been written as a part of the activities of the Science Advisory Board, a public advisory
group providing extramural scientific information and advice to the Administrator and other officials of the
Environmental Protection Agency, The Board is structured to provide a balanced, expert assessment of
scientific matters related to problems facing the Agency, This report has not been reviewed for approval
by the Agency; hence, the comments of this report do not necessarily represent the views and policies of
the Environmental Protection Agency or of other Federal agencies. Any mention of trade names or
commercial products does not constitute endorsement or recommendation for use.
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ABSTRACT
The Saturated Zone Model Subcommittee of the Environmental Engineering Committee of the EPA
Science Advisory Board (SAB) has prepared a report on the Agency's CANSAZ (Combined
Analytical-Numerical Saturated Zone) flow and transport model for use in EPACMS (the EPA Composite
Mode! for Surface impoundments). The Subcommittee examined the appropriateness of the assumptions
underlying CANSAZ for use in a nationwide Monte Carlo assessment, as well as the adequacy of the
code for simulating conditions beneath surface impoundments. Specifically, the Subcommittee evaluated
five topics which were 1J proper mathematical formulation, 2) adequacy of physical assumptions. 3)
adequacy of the Monte Carlo approach, 4) adequacy of field testing of the model, and S) the overall
modeling approach.
While the mathematical equations and the numerical solution methods are property formulated and
solved, the Subcommittee suggests improvements to the physical assumptions, improvements to the
current input data-set documentation and validation, and improvements needed to validate the EPACMS
model for use in either a site-specific evaluation or a nationwide assessment. The Subcommittee highly
prefers site-specific evaluation, but recommendations are made to improve the assessment should the
Agency choose to utilize the nationwide Monte Carlo approach. These suggestions are to incorporate
regional variations in the assessment, explicitly ban the disposal of delisted wastes in extreme
hydrogeoiogtc settings, and provide a mechanism for all constituents in a waste to be evaluated in a
listing or delisting decision.
Key Words: modeling, saturated zone modeling, surface impoundment models.
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SATURATED ZONE MODEL SUBCOMMITTEE
ENVIRONMENTAL ENGINEERING COMMITTEE
of the
SCIENCE ADVISORY BOARD
Chairman
Dr. Mitchell J. Small
Department of Civil Engineering
Carnegie Mellon University
Dr. Mary P. Anderson
Department of Geology and Geophysics
University of Wisconsin - Madison
Mr. George Carpenter
Environmental Response Division
Michigan Department of Natural Resources
Dr. Keros Cartwright
Illinois State Geological Survey
Or, Ishwar P. Murarka
Electric Power Research Institute
Dr. Paul V, Roberts
Department of Civil Engineering
Stanford University
Executive Secretary:
Dr. K. Jack Kooyoomjlan
Staff Secretary
Mrs. Marie Miller
Director, Science Advisory Board:
Or, Donaid G. Barnes
Science Advisory Board (A101F)
U.S. Environmental Protection Agency
401 M Street, S.W.
Washington, D.C. 20460
Chairman, Executive Committee, Science Advisory Board:
Dr. Raymond C. Loahr
Civil Engineering Department
University df Texas
Chairman, invironmental Engineering Committee:
Mr. Richard A, Conway
Senior Corporate Fellow
Union Carbide Corporatism.
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TABLE OF CONTENTS
1.0 EXECUTIVE SUMMARY..,, ,„......,. „ .. 1
2.0 INTRODUCTION AND BACKGROUND,-.. . . B
2,1 Proposed Uses for CANSAZ and EPACMS ,,5
3.0 ISSUES ADDRESSED IN REVIEW..,,, „.. , 7
3,1 Mathematical Formulation , , .... ,7
3.2 Physical Assumptions,,,,, .,.„ ., 8
3,3 Monte Carlo Analysis , 13
3.3.1 Monte Carto Method , . 14
3,3,2 Monti Carlo Inputs 15
3.4 Model Validation ,„...... 16
3,5 Overall Approach ... ,,.......19
APPENDIX A - THE CHARGE TO THE SUBCOMMITTEE „„„.... ,.,22
APPENDIX 8 - GLOSSARY OF TERMS I... 25
APPENDIX C * RESOURCE MATERIAL AND REFERENCES CITED,.,,,,, 26
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10 EXECUTIVE SUMMARY
This report presents the EPA Science Advisory Board's (SAB) review of the Office of Solid Waste's
(OSW) CANSAZ (Combined Analytical- Numerical Saturated Zone) flow and transport model for use in
the EPACMS (EPA Composite Model for Surface Impoundments). This review is based upon two
documents, CANSAZ, published in October 1988 (Ref. 5) and EPACMS, published in April 1989 (Ref. 9}
and briefings by the OSW to the Saturated Zone Model (SZM) Subcommittee at a meeting on May 30-31,
1989.
The use of CANSAZ and EPACMS distinguishes the OSW model for surface impoundments
(EPACMS) from the OSW model for landfills (EPACML). which uses a simpler flow and transport model
for the saturated zone which is unable to account for mounding effects. The CANSAZ model is the
saturated zone component of the EPACMS, which also includes a module far ttie saturated zone,
FECTUZ (Finite Element Code for Simulating Row and Transport In the Unsaturated Zone). FECTUZ
was reviewed previously by the Environmental Engineering Committee of tie Science Advisory Board
(Ref. 20).
The primary intended use of EPACMS is in the RCRA delisting program, where it will be used to
establish ttie relationship between constituted concentrations in surface impoundment leachate and
concentrations at downgradient welt locations (i.e., the dilution attenuation factor). The model will be
used in a Monte Carlo mode to perform a nationwide assessment of dilution attenuation factors for the
delisting decision. The Subcommittee review focused on the model in this use, but because CANSAZ
and EPACMS could also be used for other, possibly site-specific applications, use in this mode was
considered as well.
To assess the adequacy of CANSAZ and its use in EPACMS, the Subcommittee considered issues
related to the technical validity of the model, and broader issues of model use and implementation in the
proposed regulatory approach. The five areas that were examined by the Subcommittee are briefly
stated as follows:
. 1) Proper Mathematical^orrnulation.
2) Adequacy of Physical Assumtions,
3) Adequacy of Monte Carlo Approach, *
4) Adequacy of Field Testing of the Model, and
5) Overall Modeling Approach.
Findings of the Subcommittee on each of the above areas are summarized as follows:
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1) Proper Mathematical Formulation:
Given the assumptions of the model, are the numerical formulation and solution methods correct?
The CANSAZ model consists of an analytical flow module coupled with a numerical solute transport
module which uses a new method known as a the Laplace Transform Galerkin (LTG) method. The
equations, boundary conditions and solution methods appear to be properly formulated given the
assumptions of the model. The technique employed is innovative and computationally efficient The
accuracy of the method is supported by rigorous comparisons with known analytical solutions and other
numerical methods. The Subcommittee thus concludes that the mathematical equations and the
numerical solution methods are properly formulated and solved,
2) Adequacy of Physical Assumptions:
Does the model include the important processes (ground water flow, pollutant transport and
transformation) which determine contaminant fate? Is it properly formulated in terms of dimensionality,
spatial and temporal aggregation? Are there omitted processes and what are their potential implications
for this assessment?
The CANSAZ model includes the basic assumptions and processes employed in current
management-oriented ground water models. It represents a significant advance over the Vertical
Horizontal Spread (VHS) model currently used for delisting. However, management-oriented models of
this type are seriously limited in their assumptions and neglect or oversimplify many important processes
now known to Impact contaminant transport and fate at actual field sites. These assumptions involve the
dimensionality of the model (failure to include transverse flow from mounding), t)e limitation to steady-
state prediction, the assumption of homogeneous aquifer media, the inclusion of limited hydrodynamic,
chemical and biological processes, and assumptions concerning ieachate and source characteristics-
Many of these limitations are noted explicitly in the CANSAZ reports and documentation.
Particular procesess which are not incorporated in the model include the transport of immiscible
organic phases, density-dependent vertical transport of the plume, vertical movement of the water table,
nonequilibrium adsorption of contaminants on the soil matrix, biodegradatfon, multispecies chemical
reactions, cosolvent effects, moWteation of chemical conditions (e.g., pH, Eh, DO) in the receiving
aquifer, and the effects of background concentrations. As a result of these limitations, tie CANSA2-
EPACMS model is not adequate for rigorous site evaluations which need to consider site-specific
processes and phenomena not incorporated in the CANSAZ-EPACMS model. The model can, however,
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be used for a broad national assessment of regulatory impacts provided adequate input data are
obtained, The development of a capability to incorporate the processes discussed above in regulatory
models will require extensive, long-term data collection and model development research,
3) Adequacy of Monte Carlo Approach:
Is the probabilistic method technically valid and supported by adequate input data? Trie purpose of
the Monte Carlo approach is to predict the distribution of the dilution attenuation factor (DAF) that would
occur between surface impoundment leaehate and downgradient well points at surface impoundment
sites throughout the United States, To perform this analysis, a distribution of inputs for the EPACMS
model is developed and sampled to represent the distribution of meteorological conditions, soil properties,
impoundment size and geometry, and welt location at existing impoundments.
The mechanics of the Monte Carlo procedure appear to be designed and implemented properly.
The Monte Carlo module includes an impressive range of available distributions and is well integrated
with the transport code. There are, however, some significant problems In the method presented- In
particular, methods are needed to incorporate correlation among parameters currently assumed to be
independent, such as the hydraulic conductivity of the soil and the hydraulic gradient at the site, the soil
porosity and bulk density, and soil properties and temperature which may covary on a regional basis.
While the mechanics of the Monte Carlo method are generally acceptable, determining the
adequacy of the input distributions is more problematic. A property formulated model can provide
accurate and meaningful predictions only if its inputs and parameters are correctly estimated. The current
input data set is not adequately documented or supported by field data. To correct this, it is
recommended that a panel of hydrogeoiogists, soil scientists, and engineers be convened to review the
proposed model input values and documentation, and consideration be given to subjecting the resulting
data set to public review and comment prior to implementing the EPACMS delisting procedure.
4) Adequacy of Field Testing of the Model:
Has the model been tested (calibrated or validated) at particular field sites? What methods are
appropriate for validating the nationwide Monte Carlo approach? The EPA has made a good start at
validating the EPACMS model by confirming the numerical accuracy and presenting a preliminary field
#
validation study. This field validation study should be documented, and extended in more detailed studies
at other sites. A special effort is needed to validate the EPACMS modal if it is used in a nationwide
assessment. An extensive nationwide data collection and monitoring program will be needed to
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accomplish this. This will provide information on the proper inputs to the model and indicate the degree of
confidence and conservatism in the predicted distribution of the dilution attenuation factors.
Improvements in tnt available data base for site characterization are needed to match the improvements
which have occurred in modeling technology.
5) Overall Modeling Approach:
Is the nationwide Monte Carlo approach appropriate for the intended uses (e.g., delating a waste,
or closure of a site}? Which inputs require site-specific data versus national distributions? Are there
alternative approaches?
The nationwide assessment using EPACMS applies a generic model to all potential impoundment
sites in the United States, with the Monte Carlo evaluation used to capture the site-to-site variability in
model parameters. Site-specific hydrogeologie parameters are not used to evaluate the deiisting petition
of a particular facility.
Site-specific determinations of waste disposal impacts are highly preferred based on scientific
criteria. The variations between sites resulting from variations in hydrogeologic conditions are known to
be so great that the particular conditions of storage or disposal must be specified to allow for scientifically
credible evaluation. The Subcommittee recognizes that the use of a site-specific model to make deiisting
decisions would require a new administrative approach to the regulation, as a waste could be delisted
only for the site that was analyzed. To implement this, a provisional deiisting approach could be
developed, whereby a waste would revert to its hazardous status if it is disposed of at any site other than
that approved. Because the EPA could decide that their administrative and policy objectives dictate that
this, or other site-specific approaches, art inappropriate or infeasible, the Agency may choose to utilize
the generic, nationwide approach. If this is the case, the Subcommittee believes the nationwide approach
could be improved by incorporating the following features:
a) Incorporating regional variations in the assessment;
b) ExpJteKty banning the disposal of delisted wastes in vulnerable hydrogeologic settings,
thereby allowing these to be excluded from the model assessment; and,
c) Providing a mechanism for all significant constituents in a waste to be evaluated in a listing
or deiisting deeisiorTShd ensuring that all significant constituents that must be
quantitatively evaluated are analyzed with the EPACMS model. -
Furthermore, the Subcommittee would encourage any evolution in the Agency programs which would
improve the capability to assess groundwater contamination on a site-specific basis.
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2.0 INTRODUCTION AND BACKGROUND
In April 1989, Devereaux Barnes, Director of the Characterization and Assessment Division of the
Office of Solid Waste (OSW), requested that the Environmental Engineering Committee (EEC) of the SAB
conduct a review of the Combined Analytical-Numerical Saturated Zone (CANSAZ) flow and transport
model for use in the EPA Composite Model for Surface Impoundments (EPACMS). Trie formal request is
included as Appendix A of this report. Trie CANSAZ code was developed to simulate the flow and
migration of contaminants beneath surface impoundments, where hydraulic mounding could occur. The
CANSAZ module for the saturated zone, together with the unsaturated zone code (FECTUZ), comprise
the OSW fate and transport module for surface impoundments (EPACMS). The unsaturated zone code
FECTUZ was reviewed previously by the SAi (SAB-6EC-88-Q3Q), The use of CANSAZ distinguishes trie
OSW model for surface impoundments (EPACMS) from trie OSW model for landfills (EPACML).
EPACML uses a simpler flow and transport model for the saturated zone that does not account for
mounding effects.
This review is based upon two documents received by the Saturated Zone Model Subcommittee:
1, CANSAZ: Combined Analytical-Numerical Code tor Simulating Flow and Contaminant
Transport in the Saturated Zone, Prepared by E.A. Sudicky, University of Waterloo and
HydroGeologic, Inc. for U.S. EPA OSW, October 1i88 (ANM/123D/FT, ID No. 89-017).
(Reference 5).
2. EPACMS: Composite Model for Simulating Leachate Migration from Surface
Impoundments and Monte Carlo Uncertainty Analysis, User's GukJe, Prepared by Hydro-
Geologic Inc. for U.S. EPA OSW, September 1988; Revised April 1989 (ANM/123D/FT, !0
No. 89-030). (Reference 9),
In addition, the Subcommittee met and was briefed by the OSW and its consultants on May 30-31,
1989. The Subcommittee review encompasses both the written reports and the oral presentation and
discussion which ensued at this meeting.
2.1 Proposed Uses tor CANSAZ and EPACMS
An important consideration in the Subcommittee review of CANSAZ ajti EPACMS is the intended
uses of the model. Considerable discussion occurred on this issue at the Subcommittee meeting, and
additional documentation was provided by OSW. The following, excerpted from this documentation,
describes the intended uses:
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OSW is considering using the coupled CANSAZ and FECTUZ modules (EPACMS) in the fiCRA Dfrlfeting
Program, In this use, the Agency intends to specify the modal input parameters, to the extent possible, as
distributions based on nationwide data. The use of the model in the Delisting Program along with other
potential uses are described below:
Dellstlng Program
Under the RCRA Delisting Program, individual waste generators cart petition th« Agency to exclude
("dslisf) their wastes from the lists of hazardous wastes in the Federal Code of Regulations (40 CFR
261.32). An integral part of the delisting evaluation is the use of fate and transport models as predictive
tools to estimate dilution/attenuation of chemical constituents leaching from waste sttes to nearby drinking
water sources.
The model in currant use in the Delisting Program (the VMS model) is a simplified one, The Agency has
stated that the simplified model will be replaced by comprehensive ones when they become available in a
form which is appropriate for use in the Delisting Program.
The CANSAZ is of interest to the Delisting Program because a large number of wastes which are the
subject of delisting petitions are managed in surface impoundments. There is no model currently in use for
delisting that directly accounts for the specific differences between landfills and surface impoundments, a
factor which has raised comments in the review of numerous petitions. The surface impoundment code (if
adopted) would provide additional flexibility for the Deiisting Program and wouW help increase th* efficiency
of the review process. Since delisting decisions are rulemakings and require Federal Register notice and
promulgation, any specific uses of the model would be proposed for public comment.
Other Uses
The OSW does not have any other specific uses planned for CANSAZ at this time. However, it is
anticipated that any other uses, if identified, would be for the development of regulations under RCRA for
the identification of hazardous wastes, The potential uses would be limited to rnisefcle flows (non-oily
wastes), and the code would be implemented with most of the input parameters as the Monte Carlo
variables.
As indicated, the primary intended use of EPACMS is in the RCRA delisfing program, where a
nationwide assessment of dilution attenuation factors will be performed to determine maximum allowable
contaminant concentrations for delisting impounded wastes. In this application, the EPACMS will replace
the current VHS model. The Subcommittee review focused on this use and evaluated the
appropriateness of CANSAZ and EPACMS for use in developing a nationwide rule. Because the uses of
CANSAZ and EPACMS could include other, possibly site-specific applications, the Subcommittee also
considered the adequacy of CANSAZ and EPACMS tor use in site-specific evaluations.
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3.0 ISSUES ADDRESSED IN REVIEW
The request from OSW identified two major areas for SAB review. The first relates to the
appropriateness of the assumptions underlying CANSAZ when used in a nationwide, Monte Carlo
assessment, including which parameters should be estimated on a site-specific vs. a nationwide basis.
The second relates to the adequacy of the code for simulating conditions beneath surface impoundments.
The Subcommittee focused on these issues by evaluating five topics:
1. Proper Mathematical Fgimujation: Given the assumptions of the model, are the numerical
formulation and solution methods correct?
2- Adequacy of Physical Assumptions: Does the model include the important processes
(ground water flow, pollutant transport and transformation) which determine contaminant
fate? Is it properly formulated in terms of dimensionality, spatial and temporal aggregation?
Are there omitted processes and what are their potential implications for the assessment?
3. Adequacy of Monte Carlo Approach: Is the probabilistic method technically valid and
supported by adequate input data?
4. Adequacy of Field Testing of the Model: Has the model been tested (calibrated or
validated) at particular field sites? What methods are appropriate for validating the
nationwide Monte Carlo approach?
5. Overall Modeling Approach: Is the nationwide, Monte Carlo approach appropriate tor the
intended uses (e.g., delisting a waste, or closure of a site)? Which inputs require site-
specific data versus national distributions? Are there alternative approaches? Can the
Subcommittee propose a data collection program to support the assessment?
The Subcommittee review thus begins with more narrow questions related to the technical validity
of the model, and moves to broader issues of model use and implementation in the regulatory setting.
Each of the above areas is now addressed.
3.1 Mathematical Formulation
The CANSAZ model consists of two major components; A flow model and a solute transport
model. The flow model is solved analytically. The solute transport model is solved with a new method
known as LTG that combines the Laplace Transform (LT) and a finite element Galerkin (G) method.
Details of the new solution techjtiflue are contained in a paper that has been published in a refereed
journal (Reference 17). *
The LTG solution approximately simulates dispersion transverse to the cross section, rather than
rigorously. Tests suggest that this quasi-3-D solution is relatively accurate when compared with a
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8
rigorous 3-D dispersion solution. Extensive comparisons of LTG solutions with analytical solutions and
conventional finite element solutions suggest that the numerical trrors associated with LTG are of the
same magnitude or smaller than other numerical solutions. CANSAZ also provides an analytical solution
option for solute transport. The analytical solution has more restrictive assumptions than the numerical
solution option, except that the analytical solution treats three-dimensional dispersion rigorously.
The equations and boundary conditions appear to be properly formulated for the assumptions of
the model. The model's developers are among the foremost practitioners of hydrogeologic transport
model development The technique employed is innovative, computationally efficient, and highly
accurate, judging from the comparisons with analytical solutions presented in the background
documentation. The LTG method appears to represent a significant advance over previous
computational methods in terms of efficiency and robustness. However, the LTG method incorporated in
CANSAZ is intrinsically limited in its ability to deal with heterogeneity and three-dimensional transport.
The implications of these and other limiting model assumptions are addressed in tie following sections.
3.2 Physical Assumptions
The CANSAZ model includes only a limited set of ground water flow and contaminant transport and
fate processes. These include the processes generally incorporated in the current generation of ground
water management models. The assumptions are internally consistent and represent a distinct
improvement over the VHS model. However, given the rapid evolution of ground water science, it is dear
that in many respects the model does not adhere rigorously to currently understood and emerging
principles of flow and transport. In most respects, the deviations represent simplifications that are
justifiable in terms of computational expediency. However, these simplifications inherently limit the
accuracy of the model's predictions, and will particularly limit or preclude its use tor detailed site-specific
applications and decisions,
The major assumptions inlRe CANSAZ model concern the dimensionality of tie model formulation,
*
the aggregation in time (i.e., dynamic vs. steady-state simulation), aggregation in space (the
consideration of spatial heterogeneity) and the inclusion of contaminant flow and transformation
processes. '
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Dimensionality: CANSAZ is restricted to two-dimensional flow in the vertical and longitudinal
directions, ignoring the horizontal transverse flow resulting from mounding. Formation of a mound at the
water table caused by seepage from the impoundment will cause radial flow away from the mound,
including a horizontal component of flow transverse to the principal axis. If attention is focused on the
centerline of the plume, the effect of neglecting the component of flow transverse to the cross section is
conservative in that concentrations will be higher in CANSAZ simulations than in a three-dimensional flow
simulation. Indeed, it is generally the case tfiat two-dimensional flow models predict higher
concentrations along the longitudinal axis than appropriate for three-dimensional ftow fields (Reference 4,
14), (Note: Although CANSAZ neglects flow and advection of contaminants transverse to the cross
section, it does simulate dispersion of contaminants transverse to the cross section.) While the two-
dimensional flow field assumption is generally conservative, ignoring transverse flow beneath
impoundments can seriously misrepresent the shape of the plume, particularly when the regional ground
water flow is low compared to the impoundment infiltration rate. The CANSAZ model is not appropriate
for use at sites where this is the case. Moreover, if the receptor well location is assumed to vary
randomly between o" and 45° relative to the plume for the Monte Carlo analysis, as described in the
EPACMS User's Guide (Ref, 9, pp 101-103), then the two-dimensional model will not be conservative.
Rather it will underestimate concentrations at observation points that are far removed from the principal
plume axis.
Steady-State .Conditions: The flow field is assumed to be steady-state. Also, infiltration from the
impoundment is assumed to continue indefinitely. High infiltration rates cause high velocities in the
saturated zone. Under the steady-state assumption, the high velocities will persist indefinitely. The
model does not allow for the possibility of dilution and attenuation of the plume if seepage from the
impoundment should cease. The assumption of steady-state flow (and steady-state input of
contaminants) wil resutt in conservative predictions of concentration.
The assumption of steady-state conditions is a major simplification that greatly facilitates
computation and communication of the results. However, such a steady-state must be considered
hypothetical as well as conservative, as it has yet to be observed in real situations, and, therefore, the
*
corresponding predictions are not amenable to field verification. The steady-state condition is acceptable
as a benchmark for Monte Carlo analysis of policy decisions, recognizing that any site-specific analysis,
including comparisons with field data for purposes of verification, should be based on transient
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10
simulations. Once a verification of this type is performed, a simpler model formulation, limited to only the
steady-state condition, could be used for regulatory application, A simpler formulation would be easier to
understand and follow by those in the EPA and the regulated community likely to use the model,
V
Homogeneous Aquifer: The flow component of CANSAZ assumes that the aquifer is relatively
homogeneous. Because the solute transport component of the model utilizes output from the flow model,
it too is necessarily restricted to homogeneous aquifers. Aquifers, of course, are never homogeneous.
While the assumption of homogeneous aquifer material has been frequently used, recent research makes
it clear that the assumption is inappropriate for simulating solute transport in ground water. Most disposal
sites are highly non-uniform, with high-permeability zones that constitute conduits of rapid transport.
Paths of high hydraulic conductivity are important avenues for contaminants in fractured rock and day,
and also exist in continuous porous media (Ref. 2,16).
There is currently much discussion in the literature over the appropriate way to incorporate
heterogeneities into solute transport models, in one approach, the heterogeneities are accounted for in
part by adjusting the dispersion coefficient, but this strategy cannot adequately represent the extreme
cases of heterogeneity such as fractured media, as these have yet to be studied adequately and deviate
significantly from the advtctjon-dispersion model. Many go beyond this viewpoint and suggest that the
advection-dispersion equation is simply not valid for application to heterogeneous aquifers of any type.
However, at present, no consensus has emerged for a practical alternative to the advection-dispersion
equation which m used in CANSAZ.. Nor is there consensus over the appropriate way to incorporate
heterogeneities. In view of these doubts about the validity of the assumption of homogenetty, it is critical
that EPA remain flexible and be willing to replace CANSAZ with another approach when there is a
consensus. Also, it would be prudent at this time to establish strict criteria to delimit the amount of
heterogeneity that can be tolerated in applying CANSAZ in site-specific applications. This criterion might
be stated in terms of a permissible range of hydraulic conductivity variation at a given site: e.g., four
orders of magnitudes or less. The framing of this criterion should be included in the scope of
deliberations of an a
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11
on the contrary, that many impoundments and landfills contain significant amounts of immiscible organic
phases, and that migrations of such phases can be an important avenue of transport. This issue was
also raised in the SAB's review of the unsaturated zone transport model (FECTUZ), Failure to
incorporate immiscible transport constitutes a serious limitation, especially in site-specific applications
where immiscible contaminant phases are believed to exist. While the OSW indicates that CANSAZ will
only be applied for misctble flows (non-oily wastes), multiphase conditions are likely to be present in many
situations, even at Subtitle D facilities.
A second hydrodynamic limitation is that CANSAZ presumes contaminants are dilute and that
density-dependent transport plays no significant role. Even slight density differences can exert a
significant influence on vertical plume movement For example, two well-documented transport studies
(at the Borden and Cape Cod sites) have revealed pronounced vertical dips in plume movement with
solute concentrations as low as 1 g/liter. Such amplification of vertical plume movement may not be of
great consequence in Monte Carlo simulations conducted with a presumed uniform distribution of
monitoring points over the depth. However, in any site-specific application with explicit positions for the
monitoring points, it is essential to include this phenomenon.
in terms of the hydraulic conditions represented by EPACMS, the linkage between the saturated
model CANSAZ and the unsaturated model FECTUZ does not allow for movement of the water table that
would shorten the length of the unsaturated column represented in FECTUZ, This assumption is not
likely to affect the proposed regulatory application, but could be an important effect in other applications
of the linked mode!.
Chemica(_ Prpcessgs_: The CANSAZ model assumes that all chemical transformations can be
represented by simulating two processes; adsorption and first-order decay due to hydrolysis. Adsorption
is represented using the linear equilibrium model. As such, adsorption has no effect on steady-state
predictions, unless the first-order decay is assumed to occur in tie adsorbed as well as the liquid phase.
Adsorption does affect transient predictions, however, delaying the arrival of the plume.
The assumption of linear equilibrium adsorption appears to be approximately valid tor most organic
contaminants at low concentration. However, there is a general consensus that linear equilibrium does
not apply to metals. CANSAZ should not be applied in its current form to simulate metal transport.
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Biodegradation is not currently included in the CANSAZ implementation, although it is recognized
as being important in many situations, and can, in principle, be incorporated by adjusting the first-order
decay rate in the model. The implications of omitting biodegradation were discussed In detail in the
SAB-EEC review of FECTUZ (SAB-EEC-8S-Q3Q). There it was noted that:
...consensus is lacking for generalized prediction of transformation rate constants, as these depend
strongly on conditions such as organism adaptations and concentrations, pH, and the presence or absence
of electron acceptors (oxygen under aerobic conditions), toxicants, essential nutrients, etc. which are
site-specific. Site-specific applications of the FECTUZ (or, in general, the EPACMS) model package can
lead to over-estimates of solute transport since site-specific biotransformation analyses generally result in
biodegradation being a primary process influencing chemical fate. Hence, estimates of chemical transport
made without considerations of biotransformation are almost always so overly conservative as to affect
regulatory decisions. Generalized chemical transport predictions will necessarily suffer dye to lack of
generally applicable biotransf or matron rate constants; however, site-specific analyses should include ail of
the fate processes for which specific data can be reasonably obtained,
Implementation of the full range of transformation possibilities, including the uncertainties In condftiorts
that influence the rate constants, would magnify enormously the uncertainty spectrum of predicted
outcomes in Monte Carlo simulation.
The report further notes that any inclusion of biological transformation must explicitly consider the
formation of possibly hazardous byproducts.
The CANSAZ and EPACMS models do not allow for chemical reactions between two or more
chemical species. As such, CANSAZ is generally restricted to the simulation of groups of contaminants in
the dilute range. The documentation, however, does not specify that range. EPA should state specific
criteria for the upper limit of concentration. At higher concentrations, a variety of phenomena not
considered in CANSAZ may play a role, including the following: facilitated transport, cosolvem effects,
and competition for sorption sites, Most of these factors would tend to increase contaminant mobility, and
thus to increase the potential health threat,
An additional chemical assumption in CANSAZ is that contaminants exert no effect on ambient
chemical conditions. The impact of the waste on the receiving environment can in fact be important
Concentrated leachates often contain sufficient acidity or alkalinity to change the pH in the plume
substantially, even where the native ground water possesses moderate buffer capacity. A change in
leachate pH, particularly acidification, would enhance the mobilization of many metal species, while
concentrated solvent exposure rgay dry and fracture clays. Many solvents are not contained by clays and
could also enhance other contaminant transport. Similar considerations .apply to dissolved oxygen
concentration and redox state (Eh), It is probabiy infeasibte to take this coupling into account on a
generalized, nationwide basis. However, these interactions should be accounted tor in any site-specific
application in which the important processes (e.g., sorption, transformation or hydraulic stability of
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13
confining layer) depend importantly on geochemical conditions (pH, Eh, DO, presence of solvents),
Leachate and Source Characteristics: The discharge rate from an impoundment Is determined by
she impoundment size (cross-sectional area) and infiltration rate. The infiltration rate can be input directly
to the model or computed from the depth of the fluid in the impoundment and the thickness and hydraulic
conductivity of the impeding layer at the base of the unit. It is unclear from the reports, however, as to
how the impoundment sizes and tht amounts of wastes are generated for the nationwide, Monte Carlo
assessment. It should be noted that the resulting leachate composition and flux could have a large effect
on the dilution attenuation factors and the resulting decisions.
The CANSAZ model evaluates the impact of the impoundment without considering other facilities or
background contamination which may be present Few impoundments are located away from other
facilities, Other landfills or lagoons, agricultural leachate, or process or potable water withdrawals may be
present and alter site conditions. In the CANSAZ simulations, the saturated zone is assumed to be free
from contaminants initially. In areas with industrial or agricultural pollution, or with naturally occurring
sources of certain constituents, contaminants may be present in background concentrations. The effects
of facilities may also limit applicability of individual replications or runs with fixed flow rates or infiltration.
Again, these limitations are most important when considering site-specific evaluations. The CANSAZ
model needs to have the ability to be initialized for existing conditions and incorporate other flow-field
modifications when used in a site-specific application.
fn summary, the CANSAZ model incorporates a number of simplifications concerning the modet
dimensionality, temporal representation, assumption of homogeneity, omission of hydrodynamic,
chemical and microbiological processes, and representation of leachate and source characteristics.
These assumptions preclude the use of CANSAZ for rigorous site-specific evaluations. However, the
model is formulated at the proper level of detail for nationwide assessments, so long as proper and
representative rnodrt inputs can be determined.
3.3 Monte Carlo Analysis .
The purpose of the Monte Carlo approach is to predict the distribution of the dilution attenuation
factor (DAF) that would occur between surface impoundment leachate and downgradient well points at
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surface impoundment sites throughout the United States, A smaller value of the DAF implies less dilution
and attenuation by the aquifer, resulting in a greater impact at the receptor well. To perform the analysis,
a distribution of inputs for the EPACMS model is developed to be representative of the distribution of
meteorological conditions, soil properties, impoundment size and geometry, and well location at existing
impoundments. The joint input distribution is sampled many times in the Monte Carlo analysis, and the
model is evaluated with a specified waste quantity for each sample, resulting in a calculated national
distribution for the DAF conditioned on the waste quantity. The DAF for which only a small percentage
(e.g., 15 percent) of the simulated sites are less than the given value is selected as the design DAF which
is assumed to occur for the given waste and waste quantity. Because the DAF is affected by chemical
properties, including hydrolysis rates and adsorption coefficients, the analysis must be performed for each
chemical,
•>S'
There are three issues that the Subcommittee addressed to determine the validity of the Monte
Carlo approach:
1, Is the nationwide, Monte Carlo approach appropriate for delisting decisions?
2, Are the mechanics of the Monte Carlo method properly formulated?
3. Are the input distributions developed for the Monte Carlo method adequately supported?
The first question is addressed in detail later in this report in Section 3.5, Overall Approach. The
general conclusion is that site-specific evaluations are highly preferred to the use of a nationwide
assessment but that if a nationwide assessment is performed because of administrative or other policy
constraints, then the second and third issues must be property addressed. These are now considered,
3.3.1 Monte Carlo Method
The mechanics of the Monte Carlo procedure appear to be designed and implemented property,
The Monte Carlo moduli includes an impressive range of available distributions and is well integrated
with the transport code. There are, however, some particular problems in the method presented.
The major problem is that covariance between model parameters is not incorporated. This is a
particular problem for soil properties such as the hydraulic conductivity, hydraulic gradient, porosity and
bulk density which are likely to exhibit a high degree of correlation (in the case of the hydraulic
conductivity and hydraulic gradient, a negative correlation is expected). Other examples include the
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aquifer geometry, soil properties, and temperature, which may covary on a regional basis,
The one area where covariance is considered is in the simulation of soil characteristic curve
parameters for the unsaturated zone component of the model (FECTUZ). Incorporating a similar
procedure for the saturated zone parameters appears to be necessary, but could result in significant
computational difficulties. Perhaps the most straightforward way to represent the covarianee which
occurs at actual field sites is to measure the soil parameters at many sites, and use the input vector for
each site as a single, joint input to the model. The Monte Carlo analysis is then equivalent to running the
model for each of tie sites sampled, assuming they provide a representative sample of the target
population. The implementation of a nationwide site sampling and characterization program, discussed
later, would thus provide the most direct solution to the problem of identifying and incorporating input
variable covariance.
3-3.2 Monte Carlo Inputs
While the mechanics of the Monte Carlo method are generally acceptable, determining the
adequacy of the input distributions is more problematic. A property formulated model can provide
accurate and meaningful predictions only if its inputs and parameters are correctly estimated.
The Subcommittee was not able to judge whether the proposed nationwide data base reported in
the documentation to EPAGMS is appropriate for the intended regulatory use. As such, the model inputs
should be carefully reviewed before EPACMS is disseminated tor use. The estimation procedures for
EPACMS are in many cases undocumented, in other cases incomplete, and in some cases inappropriate.
For example, methods proposed for calculating porosity and hydraulic conductivity are wholly
inappropriate and do not conform to standard hydrogeologic methods. Estimation of the hydraulic
conductivity from mean grain size via the Carman-Kozeny equation (Ref. 9, Eq. 8.2,14.) is invalid for
heterogeneous media, and in any case, the relevant grain size is d10, not the mean. The hydraulic
conductivity and porosity should be estimated directly from representative field data, rather than indirectly.
In other cases (e.g., for dispersivity, the ratio of horizontal to vertical hydraulic conductivity, the aquifer
thickness, length of aquifer and distribution coefficient, K^). the documentation is inadequate to judge
whether the proposed distributions are reasonable. Another example is the proposed distribution for
ground water recharge. This distribution was generated using data from 100 cities analyzed with the
HELP model. The committee was not able to review the input data, nor the HELP modtl itself. The
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resulting recharge .distribution appears to be biased toward the Great Plains and Midwest. It is unclear
how such bias, if present in the recharge distribution, and other parameters, will affect the outcome of the
Monte Carlo analysis.
Some additional concerns were noted with regards to the Monte Carlo analysis. These include:
a. More documentation is needed to support the Gelhar-distributton for longitudinal
dispersivity (Ref. 9, pp 98-99),
b. Citation and documentation is needed for the EPIC and REA data on well distances
(Ref, 9. p, 102). The report does not indicate what these acronyms represent
c. The sensitivity analysis which was presented orally to illustrate the effect of different
model parameters should be included as part of the written report. Given the assumptions of the
model on vertical mixing, and the screening depths generally used for wells, careful consideration
should be given to interpreting the effect of well depth.
d. "Monte Carlo" is not a verb and parameters are not "Monte Cartoed." They art sampled
or generated in a Monte Carlo analysis.
In summary, it appears that ranges and distributions for many of the important inputs for £PACMS
have been estimated without the support of adequate field studies and documentation, it is thus
recommended that additional work be done in this area, and that a panel of hydrogeologists. soil
scientists and engineers be convened to review the proposed or modified model input values and
documentation. Consideration should be given to subjecting the resulting data set to public review prior
to implementing the EPACMS delisting procedure. The gathering of fiild data and documentation for the
model inputs is an important part of the model validation exercise discussed in the following section.
3.4 Model Validation
There are a number of steps that can be taken to validate models for use in regulatory decisions
(e.g., Reference 3, 6). This issue was an important part of the recent SAB-EEC resolution on
mathematical models (Reference 21). The resolution states that
as a preliminary step, th« elemaatt of the basic aquations and the computational procedures employed to
solve them should toe tested to ensure that the model generates results consistent with its underlying
theory. Tha confirmed model should then be calibrated with field data and subsequently validated with
additional data collected under varying environmental conditions. After the particular regulatory program
has bean implemented, field surveys and long-term monitoring should be conducted for comparison with
model projections. The stepwise procedure of checking the numerical consistency of a model, followed by
field calibration, validation and a posteriori evaluation should bs an established protocol tor environmental
quality models in all media, recognizing that the particular implementation of this may differ for surface
water, air and ground water quality models. It is also recognized that the degree and extent to which the
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process of validation is conducted for a modal depends on the significance of the environmental issua and
the consequsfica o< an erroneous decision concerning the problem.
The OSW and its contractors have taken steps to verity and validate the EPACMS mode). The
numerical consistency and accuracy of the model have been verified by comparisons with known
analytical solutions and other numerical models. An oral presentation was provided to the Subcommittee
on the application of EPACMS to model the migration of aWiearb through the unsaturated zone and the
underlying aquifer at a field site on Long Island, New York. Site parameters were estimated based on
previous modeling studies and entered as input to the EPACMS model. Reasonable, ordeMjf-magnityde
agreement between the model and observations was demonstrated, although the model did somewhat
underestimate downgradient concentrations. The OSW personnel indicated that a better agreement
couid be obtained by adjusting model parameters, but that an order-of-magnitude agreement is what they
expect to be representative of field test conditions.
The study presented by Dr. Saleem at the Subcommittee meeting represents a good start at model
validation, and should be formally documented- It does not, however, constitute a thorough validation.
For example, the aldicarb site does not include the flow dynamics of a leaking impoundment. Stilt, it is
representative of the type of field study that the Subcommittee wishes to encourage. Because the
EPACMS is proposed for use in a nationwide assessment, the Subcommittee identified a further set of
validation studies for the Monte Carlo approach, considering both model inputs and outputs. These are
presented in a hierarchical manner, reflecting different levels of effort and resource commitment.
The first approach consists of "limited validation" for one or more actual sites (such as the aldicarb
study), tn this approach, actual field measurements from a site are utilized to conduct sensitivity and/or
site-specific Monte Carlo analysis. The statistical properties are defined by the measurements at the site.
Once the model is exercised arid the distribution of outputs is obtained, the results are compared with the
real-world observations for that site. If the model predictions are consistently higher than those observed,
an estimate of the degree of conservatism can be obtained. Use of several sites in this manner will
increase the understanding of the expected level of bias and precision in the model results.
In addition to the aldicarb site already analyzed, the EPA program staff has been provided by the
Subcommittee with EPRf data (Reference 11) for a 30-year-old site in New York State where the
migration of organic compounds in the saturated zone has been measured. The hydrogeological,
geochemical, and microbial biodegradation properties have also been investigated. The CANSAZ model
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should be applied in both a transient and steady-state mode and the results compared to measured
concentrations* "This comparison should provide an additional validation of the model.
The second approach in the hierarchy requires that a more extensive field validation effort be
launched where several sites are rigorously characterized to generate the input data for the model. The
model is then exercised to predict steady-state concentrations tor each of the validation sites. Reid
measurements of the plume concentrations at each of the sites are statistically compared ID the model
generated concentrations. Further information on the consistency, precision and degree of under- or
overpredictton are obtained from this validation effort.
The use of EPACMS in a nationwide assessment requires a consistent set of input parameters and
boundary conditions. The third level of validation requires a critical analysis of the input and output data
sets generated by the Monte Carlo scheme. To determine the actual distribution of model input at sites !n
the U.S., a broad data-gathering program is needed. Following this, the input data generated by the
Monte Carlo analysis can be examined to establish which types of the monitored sites are represented
and whether the proper geographical weighting is achieved. This type of validation should assure that trie
distribution of inputs for the model is in close proximity to the distribution of sites in the U.S.
The Subcommittee recognizes that a nationwide monitoring program represents a major effort, and
would require a special study and significant funding. If, however, a nationwide evaluation is used for
developing regulations, it is necessary to support this effort with adequate nationwide data. The first step
in such a study would be a systematic organization and cataloging of tie studies thus far conducted and
the data collected at present waste disposal sites, including land disposal and impoundment facilities.
The information already collected In site investigation studies should provide a good start in the national
characterization program, and will help Inform the selection of additional sites and monitoring efforts.
Considerable cart will be needed to ensure that the data base provides an unbiased sample of the
national distribution of geologic and hydroiogic conditions at sites, as particular locations may currently
have more information available due to special characteristics or public or political concerns. A
representative, national characterization program can provide the data necessary to estimate input
<•*•'•
parameters for the nationwide model, provide information on downgradierit well concentrations for
*
validation of the OAF distribution predicted by the national model, and provide insights on the problems
and processes which are most important it real disposal sites.
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It is clear that all three approaches will require resources and time to develop the data and conduct
the validations. In the near term, we recommend that the Agency, through the QRD, conduct a limited
validation along ttie lines of the first approach outlined above. Plans should then be developed for
long-term studies of the type necessary for the second and third levels of validation, particularly if the
generic, nationwide approach to the regulation is maintained.
3.5 Overall Approach
The nationwide assessment using EPACMS applies a generic model to all potential impoundment sites in
the United States, with the Monte Carlo evaluation used to capture the site to site variability in model
parameters. The Subcommittee believes, considering only scientific criteria, that the use of a site-specific
assessment for making delisting decisions is 'clearly preferred to the proposed generic - Monte Carlo
approach. This viewpoint is based on the belief that facility-specific decisions on delisting should be
based on a site-specific evaluation of the facility where the waste is disposed. Site-specific decisions
require site-specific data. The variation among sites resulting from variations in hydrogeotogie conditions
is known to be so great that the site-specific conditions of storage or disposal must be specified to allow
for a scientifically credible evaluation.
The Subcommittee recognizes that this perspective calls into question the entire approach to
delisting currently espoused by the Agency, The current delisting procedure applies to the waste,
regardless of the facility used for treatment or disposal, in evaluating delisting petitions for wastes, the
Agency assumes that the waste could be placed in a Subtitle D land disposal unit anywhere in the United
States. The Agency has therefore adopted a national analysts approach in determining whether or not to
reclassify a waste from hazardous to nonhazardous. If a waste is redassified as nonhazardous, its
resulting disposal location is unspecified and the waste is no longer subject to Subtitle C regulation.
The viewpoint that only site-specific evaluations are acceptable leads to administrative difficulties,
as it would require that the disposal site for the deiisted waste be specified. However, the entire purpose
of the delisting procedure is jgj-emove the burden of hazardous waste regulations from the waste in
question; requiring a specification of the waste disposal location and requiring an analysis of potential
impacts at that site would dictate a level of effort similar to that tor facility permitting. Solution of this
dilemma of an apparent conflict between the administrative and scientific objectives of the Agency would
require a new approach to the regulation. One possible suggestion is to conduct a site-specific analysis
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to implement a provisional delisting. That is, if a given waste is disposed of at a given approved site, then
it is delisted. Any disposition at other than the given site makes the waste hazardous. The burden of
gathering the site and waste data and conducting the model analysis {e.g., using a model such as
CANSAZ with appropriate modifications for the particular site), would be placed fully upon the waste
generator. The generator must demonstrate that the waste can be safely disposed of at the subject
surface impoundment for the delisting petition to be acceptable.
The Subcommittee recognizes that, based on administrative or other policy considerations {e.g.,
the desire to facilitate rapid delisting without undue administrative requirements and delay), the Agency
may elect to maintain the proposed nationwide framework. The question then arises as to how tits type
of assessment can best be performed, in particular, whether rtgionalization is appropriate, and whether a
very extreme or conservative decision threshold is necessary to be protective of the environment.
The proposal for regionalization of the ground water impact assessment has been made in a
previous SAB-EEC report on the RCRA land-ban proposals (Reference 19). Without regionaftzatton, the
same Monte Carlo data set is used to represent all sites across the country. Yet there am vast
differences in the hydrogeology and environmental sensitivity across the country. Sites in the arid west
with hundreds of feet to the water table and (Me rain, sites in the southeast karst regions, sites in the Gulf
Coastal Plain, and sites in the High Plains of the Dakotas are all represented by the same range of
national data. This places severe restrictions on facilities located in environmentally sound sites, and may
allow poorly located facilities to be dellsted. The Subcommittee thus suggests that some sort of regional
approach be considered by the Agency. Regional ranges of data inputs for the hydrotogte parameters
could reduce the uncertainty and reduce both false positives and false negatives. The regional approach
could use either geographic regions or hydrogeologic regions. The EPA has developed a ground water
flow assessment model, DRASTIC, based on hydrogeologic regions (Ref, 22). A similar approach could
be considered for the current application, however, further analysis would be required to determine the
suitability of DRASTIC, or any other framework, for this purpose. (The Subcommittee did not review the
DRASTIC model.) Once a basis for regionalization is determined, model input parameters such as
4M»*
recharge, hydraulic conductivity, porosity, hydraulic gradient distributions and ground water temperature
can be selected to be more representative of the possible range of aquifer conditions within regions.
However, as conditions still vary greatly even within regions, the use of site-specific analysis is stilt
preferable to trie regionalization approach proposed.
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Another issue related to the consideration of different hydrogeoiogic regions is whether highly
conservative delistrng thresholds should be required in the use of a nationwide model. While this is in
many respects an issue of Agency policy, it includes some scientific issues as well. If all possible
disposal sites are permitted for delisted wastes, then a nationwide demonstration must consider worst
case conditions, or at least the minimal requirements of Subtitle 0 facilities (because the waste must be
demonstrated safe for that lower level of waste management), A Monte Carte analysts of the entire range
of environmental conditions would not be appropriate because disposal in high trinsmissivity conditions,
like fractured rock or karst, will certainly lead to problems ar»d should be prohibited. If Monte Carlo
analysis is used with all sites considered, a very high level of facility protection is required to account for
disposal in these extreme settings. Certainly, an 85% threshold is unacceptable because this would allow
a 15% failure rate and result in a substantial number of contamination sites requiring remediation, A way
to address this dilemma would be to ban the disposal of delisted wastes at sites with known high
transmissivify conditions, such as fractured rock or karst. This would impose some degree of regulation
on delisted wastes, but much less than that required for a Subtitle C waste, and the simple restriction
should be straightforward to implement- Once such vulnerable hydrogeoiogic settings are excluded from
the set of potential disposal sites for delisted wastes, a decision based on Monte Carto analysis of We
remaining sites is more likely to provide adequate protection.
One final problem identified by the Subcommittee is that only a limited number of contaminants will
be simulated by the model. In the current application, unless a waste stream contains only those
simulated compounds, the waste could not be delisted. Few wastes are so simply limited to a few
compounds- The full range of compounds in a waste must be considered in a listing or delisttng decision,
Once all are identified, toxicotogtaaf and other evidence could be presented to demonstrate that certain
compounds are not hazardous, perhaps at some predetermined threshold level, and these would not be
subject to further evaluation. Then, only compounds which are hazardous would be subjected to
migration potential review before delisting. Thus, all significant constituents of the waste would be
evaluated in some manner before a delisting decision is made. It is clear, however, that the CANSAZ
model and data base will not be ready for use until they can handle ail significant constituents that must
be quantitatively addressed in a delisting petition.
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APPENDIX A - THE CHARGE TO THE SUBCOMMITTEE
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, o.c. 20410
AW d 1969
S0UO
MEMORANDUM
SUBJECT: Science Advisory Board Review of th« Surface
Impoundment Groundwat*r Code (CAHSAZV
FROM: Devereaux Barnes, Director ///£[ r*>
Characterization and Aaaeasawnt Division (OS-330)
TO: Dr. Donald Barnes, Director
Science Adviaory Board (A-101)
The purpose of this memorandum is to request your review of
the Combined-numerical Saturated 2,on* (CANSAZ) flow and transport
module for the simulation of flow nd transport of contaminants
in the saturated zone. The code was developed to better simulate
the migration of contaminants beneath those surface impoundment*
where hydraulic mounding occurs. The incorporation of CANSA1
represents the major difference between the osw fate and
transport models for landfills (EPACHL) and for surface
impoundments (EPACMS) . Both IPACMS and IPACML are applicable to
aqueous wastes and are generally implemented on a nation-wide
basis using the Monte Carlo techniques.
SPECIFIC AREAS OF REVIEW
The two major areas listed below are identified for SAB
review. However, there may be other concerns about the CAKSAZ
module. Some of these concern* may be generic to groundwater
models in general i they include the biodegradation of
contaminants, heterogeneities and fractures in the subsurface
materials, multiphase transport, and th* quality and quantity of
the data. Th* OSW is aware of these concern* and welcomes the
SAB's view* on th«»* However, at this time we are particularly
interested in comments specific to the CANSA2 module.
1} Assumptions trader! yiag the CAHSAJ
CANSAZ was developed for possible us* in th* development of
regulation* in th«Hiazmrdous waste identification progrta. The
cod* possibly could be used in th* Del is ting .Program because a
larg* portion of p*tition*d wastes ar« managed in surface
impoundnents , In thl* program, th* cod* would fa* iapl*»*nted on
a nation-wid* basis uaing th* Mont* Carlo t*chniqu**, although
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certain parameters related to th* dimensions of the surface
impoundment and the volume of the vast* may b* fixed based on
sita-specific conditions.
Two important questions concerning assumptions arei 1) Are
the assumptions made in the development of the code appropriate,
considering the intended us* and the limitations of the available
data? and 2) Which parameters should be used only as part of a
Monte Carlo Analysis and which ones could be set to site-specific
conditions?
2) Adequacy of CAMS*]
The code was developed to account for the effects of
mounding beneath surface impoundments on the transport of
contaminants. Thm mounding creates a variable velocity field
which requires that both th* horizontal and the vertical
components of the velocity be ct -sidered in si- .ting transport
of contaminants. A critical review question is whether the code
is adequate for simulating the transport of contaminants beneath
surface impoundments containing aqueous wastes, keeping in mind
the intended regulatory uses of the code.
ThanX you for your help. Please contact me, Alec McBride
(382-4761) or Dr. Zubair Saleem (3S2-4767), if we can b* of any
assistance on this project.
Attachment
cc: Matt Straus
Alec McBride
or. Jack Kooyoo»jian
Or. Zubelr Saleem
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25
APPENDIX B - ACRONYMS
ASTM - AMERICAN SOCIETY OF TESTING MATERIALS
CANSAZ - COMBINED ANALYTICAL-NUMERICAL SATURATED
ZONE FLOW AND TRANSPORT MODEL
OAF - DILUTION ATTENUATION FACTOR
DO - DISSOLVED OXYGEN
DRASTIC - DEPTH TO GROUNOWATER, NET RECHARGE, AQUIFER MEDIA,
SOIL MEDIA, TOPOGRAPHY, IMPACT OF VADOSE ZONE, HYDRAULIC
CONDUCTIVITY OF AQUIFER"
EEC - ENVIRONMENTAL ENGINEERING COMMITTEE OF THE SCIENCE ADVISORY
BOARD
Eh - REDOX STATE
EPA - U.S. ENVIRONMENTAL PROTECTION AGENCY (ALSO USEPA)
EPACML - EPA COMPOSITE MODEL FOR LANDFILLS
EPACMS - EPA COMPOSITE MODEL FOR fURFACE IMPOUNDMENTS
EPRI - ELECTRIC POWER RESEARCH INSTITUTE
FECTUZ - FINITE ELEMENT CODE FOR SIMULATING FLOW AND
TRANSPORT IN THE UNSATURATED ZONE
G - GALERKIN FINITE ELEMENT METHOD
HELP - HYDROLOGIC EVALUATION OF LANDFILL PERFORMANCE
IAHS - INTERNATIONAL ASSOCIATION OF HYDROLOGICAL SCIENCES
Kd - DISTRIBUTION COEFFICIENT
LT - LAPLACE TRANFORM
LTG - COMBINED LAPLACE TRANSFORM (LT) AND FINITE ELEMENT GALERKIN
(G) METHOD
NWWA - NATIONAL WATER-WELL ASSOCIATION
OSW - OFFICE OF SOLID WASTE OF THE U.S. ENVIRONMENTAL PROTECTION AGENCY
pH - NEGATIVE LOG OF HYDROGEN ION CONCENTRATION
RCRA - RESOURCE CONSERVATION AND RECOVERY ACT
SAB - SCIENCE ADVISORY BOARD OF THE U.S. ENVIRONMENTAL PROTECTION
AGENCY
Subtitle D
Facilities - NON-HAZARDOUS WASTE LAND TREATMENT, DISPOSAL OR STORAGE
FACILITIES AS SPECIFIED BY THE RCRA LEGISLATION AND IMPLEMENTING
REGULATIONS "
SZM - SATURATED ZONE MODEL
SZMS - SATURATED ZONE MODEL SUBCOMMITTEE
VHS - VERTICAL, HORIZONTAL SATURATED ZONE MODEL
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APPENDIX C • RESOURCE MATERIAL AND REFERENCES CITED
1) Ambrose, Robert 8., Jr. and Thomas Q, Bamweli, Jr., "Environmental Software at •
U.S. Environmental Protection Agency's Center for Exposure Assessment Modelin
(undated reprint), 18 pages.
2) Anderson, M.P., 'Hydrogeologie Fades Modes to Delineate Large-Scale Spa
Trends in Glacial and Glaciofluvial Sediments,* Geological Society of Amer
Bulletin. 101(4). 1989. PP. 501-511.
3) ASTM, "Standard Practice for Evaluating Environmental Fate Models of Chemica
American Society for Testing and Materials (ASTM), Committee E«47 on Biotogi
Effects and Environmental Fate, Designation ES78-84, Philadelphia, PA. 1984.
4) Burnett. R.D. and i.p. Frind, "Simulation of Contaminant Transport in Tni
Dimensions, 2, Dimensionality effects," Water Resources Research. 23{4), 1987.
695-705,
5) CANSA2: Combined Analytical-Numerical Code for Simulating Flow z
Contaminant Transport in the Saturated Zone, Prepared by E,A, Sudicky. J.B. K
and RS. Huyakorn, University of Waterloo and HydroGeologic, Inc. tor the U.S. E
Office of Solid Waste,(ANM/l23D/FTt ID No. S9-017). October 1988.
6) Chapra, S.L. and K.H. Reekhow, 1983, Engineering Approaches far y
Management, Volume 2: Mechanistic Modeling. Ann Arbor Science, Butterwo
Boston, 1983.
7) Carson, Alan S., U.S. EPA Office of Solid Waste Memo to Harry Tomo, Liaison to
Science Advisory Board, entitled 'Response to the Science Advisory Boar
Comments on OSWs Development of the RCRA Hazardous Waste Identification ?
Land Disposal Restrictions Groundwater Screening Procedure," February 13.1S85
8) EPACML: Background Document lor EPA's Composite Landfill Model, Woodws
Clyde Consultants, 1988.
9) EPACMS: Composite Model for Simulating Leachate Migration from Surf;
Impoundments and Monte Carlo Uncertainty Analysis, Users Guide, Prepared
HydroGeologic Inc. tor U.S. EPA Office of Solid Waste, (ANM/123D/FT, ID I
89-030) September 1988; Revised April 1989,
10} EPRI, A Review of Field Scale Physical Solute Transport Processes in Saturated «
Unsaturated Porous Media, Electric Power Research institute (EPRI), Palo Alto, (
198S.
11) IPRl, Personal Communication, I.P, Murarka.
12) FECTUZ: Finite Element and Semi-Analytical Coda tor Simulating One-Dimensional
Flow and Solute Transport in tie Unsaturateci Zone, U.S. EPA Office of Solid Waste,
Washington, D.C,, 1988.
13) Freeze, R.A. and J. Cherry, Groundwater. Prentice Hall, Englewood Cliffs, N.J,. 19'
14) Frind, E.G., E.A. Sudicky and J.W. Molson, Three-Dimensional Simulation
Organic Transport with Aerobic BiodegradattorV In: Groundwater Contaminati
IAHS PubHcation No, 185,1989, pp. 89-96.
15} Looney, Brian B. et ai, "Analysis of the Validity of Analytical Models Used
Assessjpnt of FoTty-'Five Waste Site Areas: Subsurface Flow and Chemi
1 Transport," Pioceedings of the NWWA Conference and Exposition, on Spiv
Ground Water Problems with Models. Vol. 2, Denver. Colorado, February 10-
1987, pp. 954-982.
16) Siiman, S.E. and A.L. Wright, "Stochastic Analysis of Paths of High Hydra
Conductivity in Porous Media,' Water Resources Research. 24<11). 1988.
1901-1910,
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17) Sudieky, E.A., "The Laplace Transform Gaferkin Technique: A Continuous Time-
Continuous Finite Element Theory and Application to Mass Transport in
Groundwater." Water Resources Research. 25(8), 1989, pp. 1833-1846.
18} U,S. EPA, Hazardous Waste Management System; Land Disposal Restrictions,
Proposed Rule. Federal Register. Vol. 51, No. 9, January 14,1986, pp. 1602-1766.
19) U.S. £PA/SAB, Report of the Environmental Engineering Committee of the Science
Advisory Board, "On the Review of the RCRA Hazardous Waste identification and
Land Disposal Restrictions Ground Water Screen Procedure," April 1985.
20) U.S. EPA/SAB, Report of the Unsaturated Zone Code Subcommittee of the
Environmental Engineering Committee of the Science Advisory Board, "Review of the
Office of Solid Waste's Unsaturated Zone Code (FECTUZ) for the OSW Fate and
Transport Model" SAB-EEC-88-030, July 12,1i88.
21) U.S. EPA/SAB, Report of the Environmental Engineering Committee of the Science
Advisory Board, "Resolution on the Use of Mathematical Models by EPA for
Regulatory Assessment and Decision-Making.' EPA-SAB-EEC-89-Q12, January
1989.
22) U.S. EPA, "DRASTIC: A Standardized System for Evaluating Ground Water Pollution
Potential Using Hydrogeologic Settings,' EPA/6QO/2-85/Q18.1985.
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