UNITED STATES ENVIRONMENTAL PRO~ISa<9"KBHHBES
WASHINGTON. DC. 20460
SAB-EEC-88-030
1QRR OFFICE OF
J-.7OO THE ADMINISTRATOR
Honorable Lee M. Thomas
Administrator
U.S. Environmental Protection Agency
401 M Street, S.w.
Washington, D.C. 20460
Dear Mr. Thomas:
The Science Advisory Board has reviewed the Unsaturated Zone
Code (FECTUZ) for the Office of Solid Waste's Fate and Transport
Model. The Office of Solid Waste sought to develop a code which
would account for the attenuation of chemical constituents in
the unsaturated zone and have potential uses in the Toxicity
Characteristic and Concentration-Based Listing rulemakings.
FECTUZ is a model package consisting of an analytical code and a
finite element numerical code; each code contains a flow module
and a transport module. In requesting the review, the Director
of the Characterization and Assessment Division asked the Board
to address three issues: the dimensionality of FECTUZ, the
appropriateness of the assumptions underlying the code, and code
implementation.
The Unsaturated Zone Code Subcommittee of the Science
Advisory Board's Environmental Engineering Committee reviewed the
documentation for the code at open meetings December "10, 1987 in
Denver and January 19, 1988 in Washington.
The Subcommittee concluded that the dimensionality of the
code (one-dimensional transport in the vertical dimension) is
probably adequate for situations in which the porous medium can
be considered relatively homogeneous, without substantial
stratification and that the one-dimensional limitation may not be
a serious one from the standpoint of asymptotic, steady state
analysis of groundwater protection, since the primary effect of
lateral spreading would be to retard downward mobility and hence
to increase the time available for transformation of the
contaminant, thus reducing the amount that reaches the water
table.
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•" The Subconunitte'e believes that there are no serious problems
associated with treating the fluid as incompressible, isothermal,
and homogeneous,. The acceptability of all the other assumptions
hinges on the application of the model. Certain classes of
phenomena are not well enough understood to be incorporated into
a management model of the kind reviewed here. Because of the
importance of these phenomena to site-specific applications,
there is an urgent need for scientific research to clarify the
scientific principles underlying these phenomena. Such research
would have benefits beyond this model. The FECTUZ model package
is also subject to limitations imposed by its simplifying
assumptions and the scarcity of data necessary for parameter
estimation. Both versions of the model are incomplete in the
sense that several potentially important governing processes are
neglected altogether. The Subcommittee believes these
limitations are not so debilitating as to preclude its employment
for generalized regulatory development applications, but believes
that the inability to take into account several potentially
relevant processes casts serious doubt on the advisability of
site-specific applications. It is recommended that the Agency
mount directed and continuing efforts (a) to improve the
knowledge base concerning relevant processes which have been
neglected on grounds of inadequate understanding,and (b) to seek
expert consensus in these areas, especially biotransformation,
immiscible transport, and fracture transport.
The Subcommittee finds that for the intended tasks in
regulatory development, the composite model consisting of FECTUZ-
A and EPA-SMOD is acceptable from the standpoint of simplicity
and computational ease. Where Monte Carlo replication of the
model is planned, the overall uncertainty of the transport model
should be addressed because the Monte Carlo methodology is not
able to account for uncertainties arising from incompleteness or
deficiencies in the underlying model. However, for site-specific
decisions where the accuracy and completeness of representation
of transport and transformation processes is of paramount
importance, the FECTUZ model seems bound to be inadequate,
especially the analytical version owing to its implicit
simplifications and its inability to take into account temporal
variations, site-specific conditions and heterogeneity. The
Office of Solid Waste should take special care to warn potential
users against site-specific applications of the composite model,
because there is a substantial danger that such usage could be
misleading and detrimental to the protection of groundwater
quality.
Additionally, the Subcommittee has expressed concerns
and. made suggestions relating to how the Agency establishes the
need for the development of a new model, how existing models are
evaluated, and on the existence and use of adequate in-house
capability for evaluating issues related to transport model
development and application.
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The attached report contains more detailed responses to
these issues.
We are pleased to have had the opportunity to be of service
to the Agency and look forward to a written response to our
report.
Sincerely,
Norton Nelson, Chairman
Executive Committee
Science Advisory Board
Raymond Loehr, Chairman
Environmental Engineering Committee
Science Advisory Board
Paul Roberts, Chairman
Unsaturated Zone Code Subcommittee
Environmental Engineering Committee
cc: W. Porter
S. Lowrance
J. Denit
S. Weil
M. Strauss
Z. Saleem
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SAB-EEC-88-030
REPORT OF THE
UNSATURATED ZONE CODE SUBCOMMITTEE
ENVIRONMENTAL ENGINEERING COMMITTEE
SCIENCE ADVISORY BOARD
U.S. ENVIRONMENTAL PROTECTION AGENCY
ON THEIR REVIEW OF THE
OFFICE OF SOLID WASTE'S
UNSATURATED ZONE CODE FOR THE OSW FATE
AND
TRANSPORT MODEL
June, 1988
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TABLE OF CONTENTS
Background 1
Procedural 1
Model Structure 2
Intended Uses of the Model 2
Responses to Questions Posed by the
Office of Solid Waste 3
Dimensionality of the FECTUZ Code 3
Appropriateness of FECTUZ Assumptions 3
Compatability with Comprehensive Transport Model 7
Validation. 9
Uncertainty Analysis with FECTUZ 9
Additional Issues. . 10
References 11
Appendix: Request for Review
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U.S. Environmental Protection Agency
Science Advisory Board
Environmental Engineering Committee
Unsaturated Zone Code Subcommittee
Chairman, Dr. Paul Roberts, Department of Civil Engineering,
Stanford University, Stanford, CA 94305-4020
Dr. Keros Cartwright, Illinois State Geological Survey, 615 East
Peabody, Champaign, IL 61820
Dr. Raymond C. Loehr, Civil Engineering Department, 8.614 ECJ
Hall, University of Texas, Austin, TX 78712
Dr. Mitchell Small, Department of Civil Engineering, Carnegie-
Mellon University, Schenley Park, Pittsburgh, PA 15213
Dr. Calvin Herb Ward, Chairman Department Environmental Science
and Engineering, Rice University, P.O. Box 1892, Houston, TX
77251
Dr. M. Th. van Genuchten, U.S. Salinity Laboratory, 4500
Glenwood Drive, Riverside, CA 92501
Executive Secretary
Mrs. Kathleen Conway, Science Advisory Board, A-101F, U.S.
Environmental Protection Agency, Washington, D.C. 20460
Staff Secretary
Mrs. Marie Miller, Science Advisory Board, A-101-F, U.S.
Environmental Protection Agency, Washington, D.C. 20460
Director. Science Advisory Baord
Dr. Donald G. Barnes, Acting Director, Science Advisory Board, A-101,
U.S. Environmental Protection Agency, Washington, D.C. 20460
Date: June 1988
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NOTICE
This report has been written as part of the activities of
the Science Advisory Board, a public group providing extramural
advice on 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, and
hence, the contents of this report do not necessarily represent
the views and policies of the Environmental Protection Agency,
nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
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BACKGROUND
Procedural
On October 22, 1987 the Office of Solid Waste (OSW) made a
presentation to the Environmental Engineering Committee on its
unsaturated zone code (FECTUZ) for the Office of Solid Waste's
Fate and Transport Model. In developing FECTUZ, the Office of
Solid Waste sought to create a code which would account for the
attenuation of chemical constituents in the unsaturated zone and
have potential uses in the Toxicity Characteristic and
Concentration-Based Listing rulemakings. The Committee formed the
Unsaturated Zone Subcommittee to conduct the review of FECTUZ.
The review was formally requested in an October 16, 1987
memorandum from the Director of OSW's Characterization and
Assessment Division (Appendix). The memorandum asked the Board
to address three issues: the dimensionality of FECTUZ, the
appropriateness of the assumptions underlying the code, and code
implementation.
The Subcommittee held open meetings December 10, 1987 in
Denver, Colorado and January 19, 1988 in Washington, D.C. The
Environmental Engineering Committee heard the preliminary report
of the Subcommittee January 20, 1988 and approved this report on
April 15, 1988.
Model Structure
To summarize briefly, FECTUZ is a model package that has
been developed in two versions: an analytical code and a finite
element numerical code. For clarity we will refer to the two
versions as FECTUZ-A (analytical) and FECTUZ-N (numerical),
respectively. FECTUZ-A and FECTUZ-N each consists of two
modules: flow and transport. An acronym, FECTUZ stands for Finite
Element Code for Transport in the Unsaturated Zone. In fact,
this acronym is a misnomer if applied to the analytical version,
as it is not a finite element code. This inconsistency was
confusing to the Subcommittee, is likely to mislead users, and
therefore should be corrected by renaming the analytical code.
In developing the model, OSW and their consultant placed a
high priority on two objectives: 1) computational efficiency, and
2) ability to deal with a wide range of soil characteristics,
including highly nonlinear relationships between soil moisture
and permeability. The justification for these priorities is to
permit use of the model in Monte Carlo evaluations of expected
concentration levels at water supply wells under a wide variety
of soil conditions.
The flow module is based on the assumption of one-
dimensional, isothermal flow of an incompressible, homogeneous
fluid according to Darcy's Law. Vapor flow, transport through
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secondary porosity (fractures and fissures), and migration of
immiscible liquids are neglected. A flux-type boundary condition
is imposed at the top of the unsaturated zone.
Likewise, solute transport is assumed to be one-dimensional,
incorporating advection and Fickian dispersion, i.e. a diffusion-
like model of solute front spreading. Sorption is assumed to
follow a linear equilibrium relation. First-order
transformation is incorporated, but at present only hydrolysis
is considered in the selection of the rate coefficient, owing to
a perceived lack of knowledge concerning biotransformation
processes. The documents provided are ambiguous about the nature
of the transformation submodel, as they refer to "first-order
decay", implying that biotransformation and chemical reactions
other than hydrolysis are included. These other reactions can be
incorporated within the current structure of the model, but they
are not included in the current implementation.
The model is capable of performing either transient (FECTUZ-
N) or steady-state (FECTUZ-A) simulations of flow and transport.
However, in the proclaimed uses for regulatory development,
namely Toxicity Characteristics and Concentration-Based Listings,
it apparently will be used only in the steady-state mode, i.e.
a constant water flux is assumed. The procedure consists of
calculating the steady-state concentration at a monitoring well
ouside the landfill site, with the distance to the monitoring well
treated as a distributed parameter estimated from OSW's data
base.
Intended Uses of the Model
The questions posed to the Science Advisory Board cannot be
addressed properly without a clear idea of the prospective uses
of the model. In. delineating the intended applications, OSW
informs us that they envision using FECTUZ in the immediate
future mainly as a tool in developing regulations for disposal to
municipal landfills: i) for use by the regulated community to
assess whether a waste is hazardous (Toxicity Characteristic);
and ii) for use by OSW to reassess previous listings, to set
concentration criteria, and to delist substances if appropriate
(Concentration-Based Listing). It is implicit that, in these
applications, FECTUZ will be used in combination with the
saturated zone groundwater flow and transport model, EPA-SMOD,
which is a two-dimensional, steady-state model. A previous
version of the model was reviewed by the Environmental
Engineering Committee of the SAB. As such, the steady-state
version of FECTUZ (what we refer to above as FECTUZ-A) will te
used.
Concerning other uses of the FECTUZ model, OSW states that
it "does not have any other specific uses planned for the FECT'JZ
Code at this time. However, two site-specific uses are possible.
These are use in decisions on ACLs (alternate concentration
limits) and in Clean Closure decisions." It is implicit that, :r.
these site-specification applications, the intended use of FECTV:
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will require the more sophisticated dynamic mode (what we refer
to above as FECTUZ-N) in conjunction with a dynamic numerical
simulation model for the saturated zone. The use of the more
detailed models is intended to allow incorporation of site-
specific soil and geological properties in these applications.
If EPA changes its assumption to include a liner in its
assumed landfill, the liner effect should be incorporated in
FECTUZ, as it will control flow.
RESPONSES TO QUESTIONS POSED BY OFFICE OF SOLID WASTE
Dimensionality of FECTUZ Code
The dimensionality of the code (one-dimensional transport in
the vertical dimension) is probably adequate for situations in
which the porous medium can be considered relatively homogeneous,
without substantial stratification. However, in soil layers or
zones with low permeability relative to the surrounding media,
considerable lateral spreading may occur, particularly if free-
phase hydrocarbons are present (Mull, 1971; Schwille, 1984). The
one-dimensional limitation may not be very serious from the
standpoint of an asymptotic, steady state analysis of groundwater
protection, since the primary effect of lateral spreading would
be to retard downward mobility and hence to increase the time
available for transformation of the contaminant, thus reducing
the amount that reaches the water table. On the other hand,
this same phenomenon could lead to circumstances in which the
unsaturated zone accumulates contamination to a greater degree
prior to detection in groundwater monitoring wells, compared to
the case without stratification.
Appropriateness of FECTUZ Assumptions
The Subcommittee believes that there are no serious problems
associated with treating the fluid as incompressible, isothermal,
and homogeneous. The reliance on Darcy's Law is a widely used
expedient that is justifiable at the level of regulatory
development applications, but may be unsatisfactory in site-
specific applications if transport through secondary porosity or
migration of immiscible liquids plays a role. These factors are
discussed below under the category of processes neglected in the
model.
Neglecting hysteresis effects (physical effects which lag
their cause) in the relative permeability relationships is
permissible under asymptotic (long-term, steady state)
conditions. However, the omission of the differences in relative
permeability relations between imbibition (saturation with
liquid) and drainage will result in a gross misrepresentation of
transport under nonsteady conditions, especially for soils having
strongly nonlinear characteristics. Hence, this simplification
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will hinder accurate simulation of nonsteady-flow situations with
FECTUZ-N, even though the numerical model is otherwise capable of
handling nonsteady inputs.
The formulation of the dispersion term according to Fick's
Law is a representation that is commonly practiced in transport
modeling, and is acceptable in most situations. This
representation is certainly adequate for simulations at the level
of regulatory development. Substantial deviations from Fickian
behavior must be anticipated in strongly heterogeneous or
stratified media, however, and should be taken into account in
site-specific applications under such circumstances. Even more
serious inadequacies will arise if transport in secondary porosity
is significant, as discussed below.
The treatment of sorption as conforming to the linear
equlibrium assumption is also adequate for dealing with most
situations of trace organic contaminant transport in a
homogeneous medium. However, in concentrated leachates,
cosolvent effects and facilitated transport may be significant.
Cosolvent effects occur when chemicals are more readily dissolved
in the fluid phase due to the presence of other chemicals in the
contaminant stream. Facilitated transport occurs when contaminant
mobility is increased due to interactions with other dissolved
organic substances such as naturally occuring humic acids.
These processes tend to increase the mobility of the
contaminants through the unsaturated zone and along the ground
water flow path. The linear equilibrium assumption is usually
inadequate for contaminants such as trace metals, for which
selective cation exchange is the dominant sorption mechanism.
Moreover, in fractured or strongly stratified media, it is
questionable whether equilibrium will be obtained for contaminants
of any kind, as diffusional limitations govern the exchange between
the mobile fluid and the matrix or immobile zones. Ignoring
these complexities is justifiable in regulatory development
applications, but certainly not in site-specific applications.
Solute transformation according to a first-order rate
expression likewise can be justified as an expedient in
simulations supporting regulatory development. .The first-order
rate formulation is an acceptable approximation for many chemical
reactions, and in some instances for biotransformation processes.
On the other hand, the transformation submodel and its associated
data base are inadequate in some respects, and the FECTUZ
documentation does not clearly recognize those inadequacies. At
present, only a single transformation mechanism is incorporated:
chemical hydrolysis. Although hydrolysis rates of many compounds
are strongly dependent on pH, the model" is not capable of
predicting the pH in the leachate plume, relying instead on
estimates of the pH of native groundwater. The data-base for this
estimate currently relies on sampling data from saturated zor.e
measurements. This is inappropriate for use in an unsaturated zor.e
model. However, even if native unsaturated zone data were
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collected, this would not account for the effect of the leachate
on the pore-water pH. This deficiency could lead to inaccuracies
of several orders of magnitude in estimating hydrolysis rate
constants. Further, the analysis could be seriously biased in the
direction of overestimating transformation rates for compounds for
which base hydrolysis is the dominant mechanism, if the majority
of leachate plumes are acidic.
Similarly, biotransformation could be included as a simple
first-order rate process. However, 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 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 due to lack of generally
applicable biotransformation rate constants; however, site-
specific analyses should include all of the fate processes for
which specific data can be reasonably obtained.
Implementation of the full range of transformation
possibilities, including the uncertainties in conditions that
influence the rate constants, would magnify enormously the
uncertainty spectrum of predicted outcomes in Monte Carlo
simulation.
The transformation submodel considers neither chain
reactions nor the formation of hazardous daughter products.
Including transformation reactions without considering the possible
formation of hazardous intermediate products can bias the results
of simulations toward underestimating the risk of serious
contamination of groundwater resources. In regulatory
uses, including both toxicity characteristic assessment and
concentration-based listings, it would be dangerous to use the
transformation submodel without considering byproduct formation
where relevant. It is recommended that the transformation
submodel be decoupled in the analysis of any compound known to
form hazardous intermediate products, unless the byproduct
formation is incorporated explicity.
Important mechanisms of contaminant transport or
transformation that are neglected by the FECTUZ-A model include
vapor phase transport (volatilization), abiotic tranformation
other than hydrolysis, biotransformation, free-phase flow of
organic liquids, and fracture flow. Further, FECTUZ does not
permit nonlinear equilibrium relations for sorbing solutes, nor
does it allow for competition or similar interactions that may
arise in multisolute systems, especially cosolvent effects.
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Neglecting such potentially important mechanisms raises questions
as to whether the model can provide accurate estimates of the
central tendency, whether the overall effect of the omissions may
tend to underestimate the threat to groundwater quality, and
whether the range of uncertainty may be substantially
underestimated.
For organic contaminants, the first three mechanisms —
vapor phase transport, abiotic transformation, and
biotransformation -- serve mainly to reduce the amount of
contaminant which would otherwise reach the water table. Hence,
neglecting these processes in the FECTUZ model does not increase
the risk of protecting groundwater resources inadequately if the
model is used as a basis for regulatiory decisions. However,
transport in fractures or macropores, free-phase flow of
immiscible organic liquids, and cosolvent effects' would allow
contaminants to migrate more rapidly than would be predicted by
the model, and thereby reduce the contaminant's residence time in
the unsaturated zone, thus reducing the opportunity for
transformation.
Transport in fractures is a phenomenon that has only
recently attracted attention as an important mechanism of
contaminant migration. In a recent review, Jury (1987) states
that "in any natural setting, interaggregate structural features
such as soil macropores, cracks, plant root holes, or animal
burrows will be interspersed with the bulk structural
characteristics" and that "these geometric features, although
very small in volume, can have a significant effect on the
transport of chemicals and particularly on those which are highly
absorbed." In general, it can be expected that, in systems that
possess substantial secondary porosity in the form of fractures
or macropores, transport will be characterized by early
breakthrough and extended tailing of contaminants (van Genuchten,
1985). Fracture transport is of special significance in media of
low permeability such as clays, which furthermore are prone to
fracturing owing to their tendency to swell or shrink in response
to changing chemical conditions or moisture content. Hence, this
mechanism is likely to be of great importance precisely in the
kind of media that we tend to rely on to isolate waste disposal
sites. Despite this recognition of the potential significance of
fracture transport, there is a dearth of conclusive confirmatory
evidence, because of the extreme difficulty of conducting the
necessary investigations, which would need to be conducted at
field scale. At present, various modeling approaches are being
explored to deal with this problem, including deterministic
models based on advection in fractures with diffusion into the
matrix and stochastic models that rely on probabilstic
representation of transport, but experimental evidence suitable
for model verification is meager. A recent review by
van Genuchten and Jury (1987) summarizes the promise and limitations
of modeling approaches, concluding that rigorous, structure-
oriented models are presently too complex for routine use in
management. In summary, our inability to formulate an adequate
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model for this transport mechanism is a severe hindrance to
modeling transport at sites where secondary porosity exists in
substantial measure. The importance for transport
notwithstanding, it is understandable that fracture flow has not
been corporated into FECTUZ.
The migration of free-phase organic liquids in the
subsurface has been incorporated in several models (Corapcioglu
and Baehr, 1987; Hochmuth and Sunada, 1986; Abriola and Finder,
1985; Faust, 1985). These models could be used in conjunction
with or in place of FECTUZ if the presence of free-phase organics
is suspected. Leakage of free-phase organics into the subsurface
might occur if the low-permeability clay liners used at landfill
sites are attacked by the waste. Brutsaert (1987) showed that
some solvents can cause these clay liners to shrink and develop
cracks that could allow breakthrough of the solvents and
associated contaminents. Another aspect of contamination by
free-phase organics that cannot be modeled with FECTUZ is the
long-term contamination of groundwater caused by dissolution of
residual hyrocarbons trapped in the capillary fringe (Pfannkuch,
1983) .
In summary, the FECTUZ model package is subject to
limitations imposed by its simplifying assumptions, our lack of
understanding of important phenomena, and the scarcity of data
necessary for parameter estimation. Both versions of the model are
incomplete in the sense that several potentially important
governing processes are neglected altogether. The Subcommittee
believes that these limitations are not so debilitating as to
preclude its employment for generalized regulatory development
applications, but believes that the inability to take into
account several potentially relevant processes casts serious doubt
on the advisability of site-specific applications.
Compatability With Comprehensive Transport Model
This question is conceptually difficult to answer, because
the scope of modeling objectives is subject to change. The
Office of Solid Waste should recognize that the requirements for
a model to be used in regulatory development are different frcr.
those for other more site-specific applications for which the
unsaturated zone transport code is intended to be used as a tool.
Further, the Subcommittee has been informed by OSW that the existing
model for groundwater transport (EPA-SMOD) is an analytical code
that is compatible only with the analytical version of FECTUZ.
Hence the comprehensive transport model is subject to the same
limitations as FECTUZ-A, namely it is suitable only for steady-
state simulations of transport in a homogeneous medium. The
Subcommittee is hindered from answering the question of compatabil;ty
in detail, because OSW has not provided updated documentation on
the current version of EPA-SMOD.
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In order to derive any benefit from the non-steady state
capabilities of the FECTUZ model, the Agency would need to couple
it with an appropriate peer-reviewed dynamic saturated zone
model. Full use of non-steady state capabilities will be
particularly important if EPA modifies its assumption of an
infinite contaminant source.
For the intended tasks in regulatory development, the
composite model consisting of FECTUZ-A and EPA-SMOD is acceptable
from the standpoint of its simplicity and computational ease.
Indeed, for the kinds of tasks contemplated in regulatory develop-
ment applications, an even simpler, steady-state model relying
on analytical solutions, such as the RITZ Model described at the
December 1987 meeting of the Subcommittee, would probably be a
suitable compromise between adequacy of representation and
computational efficiency. If the situation is to be simplified
by assuming steady, one-dimensional flow and considering only the
asymptotic condition at very long times, it is not necessary to
consider dispersion in the unsaturated zone, and the nonlinearity
of the soil's permeability relation is of minor importance.
Hence, if the transport in the unsaturated zone is
restricted to a one-dimensional, steady-state situation, some of
the complexities of FECTUZ could be avoided in the interest of
computational efficiency. Rather, in such an asymptotic analysis,
predicting the extent of transformation is the crux of the matter,
because in the long-term steady state, all of the solute that is
not transformed ultimately will reach the water table. Assuming
a first-order reaction, the extent of transformation depends on
the product, kt, of the transformation rate constant times the
solute's residence time. The transformation rate constant k
depends on the reactivity of the solute as well as on chemical
conditions. The solute residence time is influenced mainly by
advection and sorption. Dispersion influences the residence time
distribution but not the average residence time, and might be
safely neglected as a factor of secondary importance in a
steady-state analysis of the kind contemplated by OSW for
regulatory development purposes, in the sense that its influence
is much smaller than the potential impact of transformation
processes.
For site-specific decisions where the accuracy and
completeness of representation of transport and transformation
processes is of paramount importance, the FECTUZ model seems
bound to be inadequate, especially the analytical version,
FECTUZ-A, owing to its implicit simplifications and its inability
to take into account temporal variations,. site-specific
conditions and heterogeneity. The Subcommittee feels strongly
that OSW should take special care to warn potential users
against site-specific applications of the composite model.
8
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There is a substantial danger that such usage could be
misleading and detrimental to the protection of groundwater
quality.
Validation
The flow module of the FECTUZ model package has been
validated for extreme conditions of soil characteristic
nonlinearity, and can be considered to represent state-of-the-art
modeling capability. However, the Subcommittee has seen no
evidence that the combined flow and transport model, including
the transformation module, has been confirmed by comparing to
observations in a real system. Neither has the comprehensive
model, combining transport in the unsaturated and saturated
zones, been validated by comparing to observed field behavior.
Given this lack of validation, OSW should be cautious in
authorizing general use of these models for any purpose. The
Agency should initiate a concerted effort to gather or generate
the needed field data to allow validation of the proposed
groundwater models.
Uncertainty Analysis with FECTUZ
The FECTUZ model is to be applied in conjunction with EPA-
SMOD to determine the degree of uncertainty associated with
Toxicity Characteristic and deListing decisions. The intent of
the uncertainty analysis in the rulemaking procedure is to
identify the fraction of potential land disposal sites across the
United States where a given leachate concentration will result in
exceedances of a health-based standard. So long as this fraction
is low, eg., below 5 or 1 percent (the actual value used is a
policy decision), then the allowable leachate concentration is
considered to be protective for hazardous waste classification.
Monte Carlo replication of the model is thus intended to
characterize the range and distribution of soil transport
conditions encountered throughout the United States.
A particular problem with the Monte Carlo methodology
described above is that it is limited to evaluations of parameter
uncertainty: it is not able to account for uncertainties which
arise due to incompleteness or deficiencies in. the underlying
transport models such as those described previously for the
FECTUZ model. Unfortunately, this is the case for virtually all
model uncertainty analyses performed today, as effective methods
for representing and evaluating the uncertainty in model
structure are not currently available. Implicit in the use of
parameter uncertainty techniques is the assumption that the
variations in parameter values used in the Monte Carlo analysis
somehow capture the overall uncertainty in. the model itself.
This may not always be the case, depending on the potential
impact and importance of omitted or misrepresented processes in
the model.
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The overall uncertainty of the transport model should
be considered. First, there needs to be a more detailed discussion
of omitted processes and their potential impact in the EPA
reports. Second, it should be recognized that in the current
application, the Monte Carlo distributions are formulated to
represent only the spatial variability of parameters across the
United States. The distributions do not reflect uncertainties,
either in the parameters themselves or in the overall output of
the model. To obtain a reasonable, conservative level of
regulation, it may thus be necessary to superimpose overall
judgments of model uncertainty on the model results. For
example, the distribution of unsaturated zone travel times may be
modified to account for the occurrence of direct macropore or
fracture transport. The consideration of model uncertainty
is certainly a new and difficult area, however, some initial steps
incorporating basic engineering judgments are appropriate if
uncertainty analysis techniques are to be used and trusted for
regulatory development.
ADDITIONAL ISSUES
The Subcommittee is concerned about several other issues
related to the development and use of FECTUZ that arose in the
course of its deliberations:
1. How does OSW proceed in establishing the need for the
development of a new model? How are existing models
evaluated, including those recently developed within
EPA and at other Federal agencies such as USGS and ARS?
2. Does OSW have adequate in-house capability for evaluating
issues related to transport model development and
application, and does OSW capitalize on the expertise
available elsewhere within EPA?
These and other issues related to the use of models by the
EPA have been of concern to the Environmental Engineering
Committee during reviews of other programs within the Agency.
The problems appear to be common to many of the modeling studies
we have reviewed. The EEC is in the process of developing a
generic resolution on the proper selection and use of models by
the Agency, where these issues are addressed.
10
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REFERENCES
1. Abriola and Finder, "A Multiphase Approach to the Modeling of
Porous Media Contamination by Organic Compounds, I.
Equation Development, and II. Numerical Simulation", Water
Resources Research, 21(1), 11-18 and 19-26 (1985).
2. Brutsaert, W.F., "Suitability of Marine Clay as Hazardous
Waste Site Liners", J. of Environ. Eng., 113(5), 1141-1148
(1987) .
3. Corapcioglu, M.Y., and A.L. Baehr, "A Compositional
Multiphase Model for Groundwater by Petroleum Products, I.
Theoretical Considerations, and II. Numerical Solution",
Water Resources Research, 23(1), 191-200 and 201-213
(1987).
4. Faust, C.R., "Transport of Immiscible fluids within and
Below the Unsaturated Zone: A Numerical Model", Water
Resources Research, 21(4), 587-596 - 1985.
5. Hochmuth, D.P., and O.K. Sunada,. "Ground-Water Model of Two-
Phase Immiscible Flow in Coarse Material", Ground Water,
23 (5), 617-626 (1985).
6. Jury, W.A., "Chemical Movement Through Soil", Chapter 6 in
"Vadose Zone Modeling of Organic Pollutants", S.C. Hern and
S.M. Melancon, editors, Lewis Publishers, 1987.
7. Mull, R., "The Migration of Oil Products in the Subsoil with
Regard to Ground-Water Pollution by Oil, Paper HA-7A in
Advances in Water Pollution Research, Proceedings of the Fifth
International Conference of the Intrnational Association for
Water Pollution Research, 1971.
8. Pfannkuch, H.O., "Hydrocarbon Spills, Their Retention in the
Subsurface, and Propagation into Shallow Aquifers", Office of
Water Research and Technology, April 1983, available through
NTIS as PBB3-196477.
9. Schwille, F., "Migration of Organic Fluids Immiscible with
Water in the Unsaturated Zone", in Pollutants in Porous Media •
The Unsaturated Zone Between Soil Surface and Groundwater, B.
Yaron, G. Dagan, and J. Goldschmid, eds., Springer Verlag,
1984.
11
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10. van Genuchten, M. Th., "Solute Transport in Structured Soils",
in Proceedings of the 5th Annual AGU (Front Range Branch)
Hydrology Days and 14th Annual Groundwater Conference, H.J.
Morel-Seytoux and D.O. Doehring, editors, Hydrology Days
Publications, Fort Collins, CO, 1985.
In addition, the following two Proceedings volumes provide a
good overview of recently gained insights into transport in the
unsaturated zone.
Hern, S.C., and S.M. Melancon, editors, Vadose Zone Modeling of
Organic Pollutants, Lewis Publishers, 1987.
Yaron, B., G. Dagan, and J. Goldschmid, editors, Pollutants in
Porous Media: The Unsaturated Zone Between Soil Surface and
Groundwater, Springer Verlag, 1984.
12
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•••?
>SR
\«***
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
V WASHINGTON, D.C. 20460
OCT I 6 1987
OFFICE OF
SOLID WASTE AND EMERGENCY RESPONSE
MEMORANDUM
SUBJECT: Science Advisory Board Review of the Unsaturated 7one
Code for the OSW Fate and Transport Model
FROM: Sylvia K. Lowrance, Qirector^/X-A- r-- j-&*
Characterization and Assessment Division (WH-562B)
TO: Or. Terry F. Yosie, Director
Science Advisory Board (A-101)
The purpose of this memo is to transmit for your review a code
for the simulation of flow and transport of chemical constituents
through an unsaturated zone. The code accounts for the attenuation
of chemical constituents in the unsaturated zone and has ootential
uses in the Toxicity Characteristic (TC) and the Concentration-
Based Listing rulemakings. The code was developed to fully explore
the comments received on OSW's TC proposal (SI FP 21648, June 13,
1986). As we have indicated to you earlier, the TC proposal would
modify the existing toxicity characteristic by: (1) introducing a
new leach procedure -- Toxicity Characteristic Leaching Procedure
(TCLP); (2) adding 38 organic compounds to the list of TC consti-
tuents; and (3) using a ground-water fate and transport model to
establish dilution/attenuation factors for the specific organic
compounds. The ground-water fate and transport model used in
the TC proposal was reviewed by SAB as part of the land disposal
restrictions ground-water screening procedure in 1985.
In the TC proposal, the ground-water model did not incoroorate
an unsaturated zone. The model assumed that the bottom of the
landfill was directly in contact with the wat^r table. Conseouentlv,
any attenuation of chemicals in the unsaturated zone was ignored
in the model. OSW made this assumption at that time because of
a lack of adequate data to characterize the unsaturated zone for
the implementation of the model on a generic 'oasis. Because of
this, OSW opted to be conservatively protective of human health
and the environment.
The Agency received numerous comments on the absence of
unsaturated zone in the model. The commenters stated that the
assumption was unrealistic and landfills are not directly connected
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to the saturated zone. In addition, since the proposal date,
the Agency has obtained data, as part of OSW's Solid Waste Landfill
Survey, on the distribution of unsaturated zones at the municipal
landfill sites in the U.S. These data indicate that at least 95
percent of these sites have unsaturated zones. Therefore, in
order to fully explore the comments on the TC proposal, OSW has
developed an unsaturated zone code (FECTUZ).
FECTUZ is a finite-element code for the simulation of water
flow and solute transport in the unsaturated zone (variably
saturated porous media). The code allows for a wide range of
nonlinear flow conditions and handles various transport processes,
which are considered in the saturated zone code, including hydro-
dynamic dispersion, advection, sorption, and first order decay.
The code allows for consideration of heterogeneities in the
vertical dimension (layering) in the unsaturated zone.
SPECIFIC AREAS FOR SAB REVIEW
We are interested in SAB's evaluation of the FECTUZ code and
have identified the following specific areas for Board's review: •
1) DIMENSIONALITY OF FECTUZ CODE
FECTUZ code is one-dimensional in the vertical dimension for
the following reasons:
(a) The flow in the unsaturated zone for all practical purposes
can be assumed to be in the vertical direction; and
(b) OSW needs a code which can handle nonlinearites in the
unsaturated, zone and at the same time is computationally
efficient because of the need to perform Monte Carlo
analyses. Therefore, FECTUZ was developed as a one-
dimensional numerical code (unlike the saturated
zone code, which is an analytical code). Two- or
three-dimensional codes will not satisfy this criteria.
(c) In addition, data to characterize the unsaturated zone
for modeling in two or three dimensions are not available
at this time.
Is the one-dimensional code (FECTUZ) an appropriate tool
for the intended use under the circumstances characterized by the
limitations of available data?
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2) ASSUMPTIONS UNDERLYIMG THE CODE
FECTUZ code was developed based on several simplifying
assumptions to represent the physical system to be modeled. The
assumptions include:
(a) Flow of the fluid phase is considered isothermal and
governed by Darcy's law.
(b) The fluid considered is slightly compressible and
homogeneous.
(c) Hysteresis effects in the constitutive relationships of
relative permeability versus water saturation, and *ater
saturation versus capillary pressure head, are assumed to
be negligible.
(d) Diffusive/dispersive transport in the porous media is
governed by Pick's law. The hydrodynamic dispersion
coefficient is defined as the sum of the coefficients of
mechanical dispersion and molecular diffusion.
(e) Adsorption and decay of the solute may be described by
a linear equilibrium isotherm and a first order decay
constant.
(f) The code handles only single phase Flow (water) and
ignores the flow of second phase (air).
(g) The code does not consider chain reactions. The porous
media is considered as single-porosity soil media
(non-fractured media). The kinematic sorption effects
are not considered.
Are the assumptions made in the development of the code
appropriate, considering its intended use and the limitations of
the available data?
3) CODE IMPLEMENTATION
If we decided to incorporate an unsaturahsd zone into the
ground water model, PFCTUZ is to be implemented as an integral
component of the ground water flow and transport code. The
implementation of the code is described in the attachment.
We would appreciate your review of the procedure for
incorporation of FECTUZ in the ground-water f.ow and transport
model, and any specific suggestions the Board .nay have for
improvements considering the limited data available.
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Thank you for your help on this project. The code alona with
the necessary support materials are attached for your consideration
Please contact me (382-4637), or Dr. Zubair Saleem (382-4767), if
we can be any assistance during the review process.
Attachment
cc: Matt Straus
Stephen R. Weil
R. Scarberry
Zubair Saleem
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ATTACHMENTS
1. Finite Element Code for Simulating One-Dimensional
Flow and Solute Transport in the Unsaturated Zone.
2. Methodology for Simulating Flow and Transport in the
Unsaturated Zone.
3. Developing Joint Probability Distributions of Soil-Water
Retention Characteristics — Robert F. Carsel and
Rudolf S. Parrish. Paper under publication in
Water Resources Research.
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