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Overview, Conclusions, and
Recommendations
OVl:RVIl:W
Mathematical models, used commonly in ground water studies,
are an attempt to represent processes by mathematical equations.
The precise language of mathematics provides a powerful mechanism
for expressing a tremendous quantity of information in an amazingly
simple and compact way. Naturally, the starting point in modeling
is a clear understanding of the processes involved. In terms of the
flow of ground water or multiphase flow (i.e., when a fluid such as
water, gasoline, or a dense nonaqueous~phase liquid is moving in the
subsurface), one mainly needs to consider two dominant processes:
flow in response to hydraulic potential gradients and the loss or gain
of water from sinks or sources (e.g., pumping or injection, or gains
and losses in storage). In the case of contaminant transport, a much
larger number of diverse and complicated processes are involved.
These processes can be divided into two groups: (~) those responsible
for fluxes and (2) those responsible for sources and sinks for the
material. Mass fluxes are prompted by processes like advection,
diffusion, and mechanical dispersion. Sources ant! sinks are provided
by a host of chemical, nuclear, and biological processes, such as
sorption, ion exchange, oxidation/reduction, radioactive decay, and
biodegradation.
In this report, Chapter 2 is devoted to explaining in a simple way
1
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2
GROUND WATER MODELS
how the differential equations for ground water flow and mass trans-
port are created to embody the various processes. To fully describe
a ground water system to be modeled, one needs in addition to the
governing equations (1) specific numerical values for parameters that
characterize the processes and for simulation parameters that are
involved with the procedure to solve the equations and (2) informa-
tion about the region, shape, and conditions along the boundaries.
Solution of the resulting modeling problem is usually carried out
analytically or numerically, depending upon the complexity of the
hydrogeologic setting and the number of processes that need to be
considered.
Few flow and transport problems can be modeled with confi-
dence. As the following discussion explains, the most satisfactory
results to date have come with models involving the flow of water
or the transport of a single nonreactive contaminant in a saturated
porous medium. As systems become more complicated through par-
tial saturation, the presence of several mobile fluids, fracturing, or the
existence of reacting contaminants, many more questions arise about
the adequacy or validity of the underlying process models. The nat-
ural reaction of researchers is to undertake long-term experimental
investigations, which in the scientific tradition will gradually improve
our understanding of these processes. Although such research is un-
deniably important, it may not provide answers in time to influence
many important national and local decisions about ground water
contamination.
Chapters 3 and 4 of this report, along with many other thought-
fu] reports, papers, and articles, reveal major areas of uncertainty
about subsurface contamination. Decisionmakers need to confront
this uncertainty realistically and not be misled by the ability of com-
puter models to always provide answers. Admitting the presence
of uncertainty, however, is not enough. There is a need to make
decisions, clean up water supplies, remove threats to public health,
and devise safer methods for disposing of our wastes. Some of the
decisions made in the short term may be inappropriate, inefficient, or
even counterproductive, but it is unacceptable to simply wait until
poorly understood environmental problems can be solved with more
confidence.
In order to examine this issue further, it is useful to briefly
review those areas where the understanding appears to be relatively
good and those areas where there is still much to learn. Each of
the major modeling categories discussed in Chapters 3 and 4 is
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OVERVIEW, OONOLUSIONS, AND RECO~NDATIONS
3
briefly examined in the following paragraphs. Then the question of
what decisionmakers can and should do now with problems requiring
immediate attention is revisited.
The processes that control saturated ground water flow are rea-
sonably well understood, and standard models of these processes are
generally believed to be able to give reliable predictions if provided
with adequate amounts of data. Nevertheless, the impacts of field-
scale heterogeneity are still widely debated, and there are few clear
guidelines on how model inputs should be estimated from limited
databases or on how hydrologic monitoring programs should be de-
signed. While saturated flow modeling is becoming more straightfor-
ward than it once was, there is much room for individual judgment,
and the experience of the modeler still makes a significant difference
in the quality of the results obtained. It is questionable whether this
experience will ever be replaced by automated techniques such as
expert systems, although such innovations may make the job of the
informed modeler easier.
Unsaturated flow is less well understood. The basic "laws" that
govern such flow are still questioned by some investigators. Much of
the conventional theory of unsaturated flow is based on small-scale,
one-dimensional laboratory experiments, which may not provide an
accurate picture of behavior at larger field scales.
There have been very few field studies of unsaturated flow that
extend over the scales of interest in most contamination applications,
and most of these have focused on one-dimensional transport in the
vertical direction. Some investigators believe that unsaturated flow
can move horizontally over significant distances, although available
evidence is insufficient to either confirm or reject this hypothesis.
Even if straightforward extrapolation from the laboratory to the
field were possible, current techniques for determining unsaturated
soil properties are too expensive and time-consuming to provide
adequate descriptions of most contaminated sites. The numerical
demands of all but the simplest unsaturated zone simulation mod-
els are formidable, and accurate three-dimensional unsaturated flow
modeling capabilities are not available to most consultants. Yet
many important contamination problems, such as leaking under-
ground storage tallks, infiltrating pesticides, and leaching mining
wastes, afl.ect the unsaturated zone. Contaminant transport in this
zone has only recently been perceived by the hydrologic community
as an important research priority. Much remains to be done.
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4
GROUND WATER MODELS
Flow through fractured media may be either saturated or un-
saturated. Both types of fracture flow are difficult to predict at a
given site unless extensive information is available about the fracture
network. In this sense, true predictive modeling is not yet a reality.
Nevertheless, recent research has provided significant advances in the
understanding of the relative importance of the fracture and matrix
systems in fractured flow. These advances have influenced some anal-
yses of candidate radioactive waste disposal sites but have not, for
the most part, reached the larger modeling community. The prevail-
ing approach is to ignore fracture flow and hope that the effects of
individual fractures will, in some sense, "average out." This can be
a misleading oversimplification in some applications, where fractures
can act as conduits for contaminant flow or can significantly modify
subsurface flow patterns. Practical modelers need better guidelines
for determining when fracture flow may be important and better
methods for incorporating such flow into their mode! predictions.
The status of contaminant transport modeling depends greatly
on the chemical species and phase of interest. In general, the pros
cesses that influence the transport of dilute, nonreactive aqueous
phase solutes are wed understood, at least in saturated media. There
is, however, still widespread disagreement about the effects of spatial
and temporal variability and about the related concept of macrodis-
Persian. Until very recently, there were very few controlled field
studies of ground water contaminant transport. Recent studies tend
to indicate that real-worId contaminant plumes have complex three-
dimensional structures. which can be difficult to Predict when soil
,
properties are very heterogeneous. It can be difficult to simply map
an existing plume, given the data typically available at a newly dis-
covered contaminated site. Prediction of plume movement over many
years is an even more difficult task.
The problems associated with transport modeling are greatly
compounded when the solutes are reactive. In this case, chemical
rather than hydrologic processes may govern the behavior of a con-
taminant plume. Ground water chemistry and ecology are relatively
new fields that have had to contend with the problems inherent in
working in an environment where processes are not readily observed
and where samples are costly and difficult to obtain. Most models
of reactive solutes are based on small-scale laboratory studies, which
may not accurately mimic conditions found in the actual subsurface
environment. This raises all of the same scale issues mentioned ear-
lier in conjunction with unsaturated flow. Despite these difficulties,
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OVERVIEW, CONCLUSIONS, AND RECO~NDATIONS
5
simple reactive transport models are in wide use and many modelers
are aware of the need to at least consider sorption, biodegradation,
and other chemical effects. It remains to be seen whether these
simple models are adequate for d~ecisionmaking purposes.
Most ground water contaminant modelers would probably agree
that multiphase contaminant transport Is the area where the basic
physical mechanisms that control contaminant movement and degra-
dation are least well understood and most difficult to model. Yet a
wide range of important contaminants probably travel as separate
liquid or gaseous phases when they move through the subsurface
environment. Field-scale experimental investigations of multiphase
transport are very limited, and existing laboratory-scale results in-
dicate that this type of transport is influenced by a number of inter-
acting factors, including viscosity and density contrasts, capilIarity,
and phase transitions. Although models of multiphase transport
are available, many of the inputs they require are, as in the re-
lated case of unsaturated flow, difficult to estimate in a field setting.
Because field data are very limited, it is practically impossible to
confirm whether or not these models accurately reflect reality. More-
over, existing multiphase modeling techniques are computationally
demanding and probably impractical to apply in situations where
dozens of different interacting species and phases coexist. Such situ-
ations occur frequently. Leaking gasoline storage tanks are just one
example.
Case studies provide a useful way to illustrate the application
of models in (1) understanding ground water systems, (2) predict-
ing contaminant rn~grations, and (3) decisionmaking by regulatory
agencies. An example of the first type of application relates to the
use of the generic vertical-horizontal spread (VHS) mode! by the
U.S. Environmental Protection Agency (EPA) to determine when
solid wastes need to be treated as hazardous wastes. In the case of
the Madison aquifer, modeling studies predicted water-level declines
due to large withdrawals by pumping.
~ ~. . ~, . .
An example of the second
type ot application, modeling In connection with contamination of
the Snake River plain, provided a prediction of the future extent of
plume development. The third type of application is illustrated by
the cases of contamination at the S-Area landfi~! in Niagara Falls and
at Tucson Airport, where modeling was an integral part of the legal
decisionmaking.
The above review of the present state of ground water contam-
inant modeling is not really as pessimistic as it may appear at first
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6
GROUND WATER MODELS
glance. In the last several years there has been substantial progress in
such fields as fracture flow modeling, modeling of reactive transport
and transformation, and multiphase modeling. The present concern
with ground water contarn~nation has stimulated a major increase in
research efforts that has resulted in advancement. Moreover, it is the
committee's opinion that the needs of decisionmakers are best served
by honest and realistic assessments of the modeling state of the art.
With such assessments we can set priorities, make difficult decisions,
and understand how to deal with pressing short-term problems.
The fact that many of the models used in practice have not been
validated to a significant extent provides an important source of
uncertainty in the predictions that come from the models. Unfortu-
nately, even more uncertainty enters the modeling process from, for
example, (1) the inability to precisely describe the natural variabil-
ity of model parameters (e.g., hydraulic conductivity) from a finite
and usually small number of measurement points, (2) the inherent
randomness of geologic and hydrogeologic processes (e.g., recharge
rates and erosion) over the long term, (3) the inability to measure or
otherwise quantify certain critical parameters (e.g., features of the
geometry of fracture networks), and (4) biases or measurement errors
that are part of common field methods. When all these sources of
uncertainty are properly considered, a single model prediction re-
alistically has to be viewed as one of a relatively large number of
possible system responses. Over the past decade, the development of
stochastic modeling techniques has been useful in quantitatively es-
tablishing the extent to which uncertainty in model input translates
to uncertainty in model prediction.
To return to the question posed earlier, what should a deci-
sionmaker do now, given existing modeling capabilities? There is
obviously no easy or comforting answer to this question. It seems
apparent, however, that it would be unwise to rely solely on any
single source of information when deciding how to formulate regula-
tions, carry out a cleanup, or protect public health. Models should
be supplemented by carefully conceived field work, which not only
provides data for estimating model inputs but also provides an inde-
pendent confirmation of conditions in the subsurface environment.
Put simply, the decisionmaker should hedge his bets and distribute
his resources, funding different types of modeling efforts and mixing
modeling with on-site monitoring. When field data are inconclu-
sive or insufficient, model results may have a significant influence
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OVERVIEW, CONCLUSIONS, AND RECOM:MENDATIONS
7
on the predicted impact of a given decision. In this case, the deci-
sionmaker should request a quantitative and defensible assessment
of the model's accuracy in order to evaluate the risk of making a
bad decision. In this regard, environmental management is no dif-
ferent from any other form of management where uncertainty and
risk are important. Modem are not going to relieve us of the burden
of mating difficult decisions. They simply provide some additional
information to consider. It is unrealistic to expect much more.
CONCLUSIONS AND RECOMMENDATIONS
Modele and Subsurface Processes
Conclusions
There is a range of capability In modeling fluid flow In geologic media.
Modeling saturated flow In porous media is straightforward with few
conceptual or numerical problems. At the present time, conceptual
issues and/or problems in obtaining data on parameter values limit
the reliability and therefore the applicability of flow models Involving
unsaturated media, fractured media, or two or more liquids.
As a group, flow processes are among the most widely charac-
terized hydrogeologic processes. The theories of flow involving either
one or more fluids in porous and/or fractured media are well es-
tablished and generally accepted. For the simplest cases involving
saturated flow in porous media, the basic theoretical models have
been validated in countless field and laboratory studies. The great-
est source of uncertainty in prediction lies in supplying values of
site-specific parameters. Flow in the unsaturated zone is less well
understood, particularly in the case of dry soils, where the transport
of water vapor can be significant. As was the case with saturated
flow, establishing values for the controlling parameters under natural
conditions is difficult, particularly for parameters like permeability
that can vary in a complex, nonlinear way with moisture content.
Flow models involving two or more liquids in porous media are
even more complicated in terms of the processes and parameters.
Nevertheless, such models have been used and applied successfully,
for example, in the petroleum industry. The greatest source of un-
certainty in prediction remains the difficulty in accurately describing
the spatial variability in controlling parameters. This problem of
data is compounded by the variety of organic liquids that can be
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8
GROUND WATER MODELS
present as contaminants and for which specific experimental data are
scarce.
In the case of fractured media, it remains to be shown through
field and laboratory experiments that existing conceptual models of
fractured systems are valid, particularly for cases involving variable
saturation and more than one liquid. In addition, there are probably
chases of fractured media that cannot be modeled with continuum
theories and for which discrete approaches are impractical. The
data problems remain. Many (controlling) parameters are difficult
to measure or estimate accurately. Thus predictions for these more
complex conditions need to be evaluated carefully and assessed in
light of possible limitations.
Mass transport is controlled by a variety of physical, chemical, and
biological processes. Quantitative descriptions of the processes con-
cerned with mass transport Erection, diffusion, and dispersion)
along with certain mass transfer processes (radioactive decay and
sorption) are wed understood. Multidimensional models of these
processes have been used succesefi~ly ~ practice. Work is still re-
quired to account for other more complicated chemical processes
(e.g., oxidation/reduction, precipitation, hydrolysis, and complexa-
tion) and biological processes (e.g., bacterial degradation) in mass
transport models. Although prototype models exist for these more
complicated systems, they are not yet developed for use ~ practice.
Contaminant transport is the outcome of mass transport pro-
cesses, such as advection, diffusion, and mechanical dispersion, that
move the mass and a multitude of mass transfer processes that
redistribute mass within or between phases through chemical and
biological reactions. Present-day understanding of mass transport
developed from early studies on laboratory columns and more re-
cent well-documented tracer studies in the field. The basic theory
of advective transport modified by diffusion and mechanical disper-
sion is embodied in the familiar advection-dispersion equation, which
provides a practical framework for modeling contaminant transport.
The main source of uncertainty in prediction lies in establishing
values of controlling parameters like velocity, effective diffusion coef-
ficient, and dispersivity, which can be difficult to measure or estimate
and vary spatially.
The complete description of mass transport usually requires that
various chemical and/or biological processes also be considered. In
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OVERVIEW, OONOLUSIONS, AND CONNATIONS
9
the case of reactions such as radioactive decay, sorption, and hydrol-
ysis, kinetic or equilibrium models describe the reactions and the
necessary rate parameters or equilibrium constants for the reactions.
These reactions can be calculated and measured with reasonable
accuracy if not tabulated and can be incorporated in contaminant
transport models in a straightforward manner. Although models
for important reactions like oxidation/reduction, precipitation, and
biodegradation exist, they are complicated to formulate and solve,
difficult to characterize in terms of kinetic parameters, and largely
unvalidated in practical applications. Thus the transport of multiple
reacting constituents such as trace metals and organic compounds
cannot be modeled with confidence.
As was the experience with flow, fracturing adds considerable
complexity to mass transport. The issue of whether fractures are
open or highly channel~zed, the importance of diffusion into the
matrix, and how mixing occurs at fracture intersections make con-
ceptualization of even mass transport processes uncertain. Coupled
with the difficulty in formulating the mode! in terms of processes is
the general lack of field and experimental data to validate models
that are available. Thus transport modeling in fractured systems
remains a highly speculative exercise.
Models and Decisionmaking
Conclusions
Properly applied models are useful tools to
assist in problem evaluation,
design remedial strategy,
conceptualize and study flow processes,
provide additional information for decisionmaking, and
recognize limitations in data and guide collection of new data.
Ground water models are valuable tools that can be used to help
understand the movement of water and chemicals in the subsurface.
The purpose of the models is to simulate subsurface conditions and
to allow prediction of chemical migration. When properly applied,
models can supply useful information about flow and transport pro-
cesses and can assist in the design of remedial programs.
The results of a mode! application are dependent on the quality
of the data used as input for the model. Generally, site-specific data
are required to develop a mode} of a site. The mode} cannot be used as
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10
GROUND WATER MODELS
a substitute for data collection. However, mode} use can help direct
a data collection program by identifying areas where additional data
are required. Closely linked data collection and mode} application
can provide an adequate representation of site conditions. Incorrect
mode! use frequently occurs when the limitations of the data used to
develop the mode! are not recognized.
When properly applied, the results of a ground water mode}
application can help in making decisions about site conditions. Mode!
results can be used to supplement knowledge of site conditions but
cannot be used to replace the decisionmaking process. The results
of the models must be evaluated with other information about site
conditions to make decisions about ground water development and
cleanup.
Generic models are useful as a too] for initial screening but can never
be used as a replacement for site-epecific models.
Geologic materials are characteristically heterogeneous. The het-
erogeneity is seen at all scales, ranging from individual laminae a few
millimeters thick to entire formations, aquifers, and drainage basins.
In contrast, ground water models commonly incorporate various sim-
plifying assumptions. Examples of some simplifications commonly
used in ground water models include the assumption that an aquifer
consists of a perfectly homogeneous, elastic material, or that the
aquifer is made up of a small number of alternating homogeneous
layers. The differences between the geologic reality of heterogeneity
and the simplifications that may be used in ground water models
make it scientifically dangerous and potentially misleading to blindly
apply generic ground water models to any specific hydrogeological
situation.
A generic mode} may be useful in offering some initial guidance
to an investigator. However, only the most naive would rely on
the predictions of a generic mode! in an attempt to understand
the details of the movement of ground water or the behavior of a
dissolved pollutant in a specific hydrogeological environment. It is
essential that an investigator gather site-specific information to use
as input to the ground water mode! of choice and, perhaps' that
the mode! itself be modified and adapted to fit the hydrogeologic
conditions at a particular site.
The rents of mathematical computer models may appear more
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OVERVIEW, CONCLUSIONS, AND RECOMMENDATIONS
11
certain than they really are; decisionma~cers must be aware of the
limitations.
Modelers must contend tenth the practical reality that mode! re-
sults, more than other expressions of professional judgment, have the
capacity to appear more certain, more precise, and more authorita-
tive than they really are. Many people who are using or relying upon
the results of contaminant transport models are not fully aware of
the assumptions and idealizations that are incorporated into them
or of the limitations of the state of the art. There is a danger that
some may infer from the smoothness of the computer graphics or the
number of decimal places that appear on the tabulation of the calcu-
lations a level of accuracy that far exceeds that of the model. There
are inherent inaccuracies in the theoretical equations, the boundary
conditions, and other conditions and in the codes. Special care there-
fore must be taken in the presentation of modeling results. Modelers
must understand the legal framework within which their work is
used. Similarly, decisionmakers, whether they operate in agencies or
in courts, must understand the limitations of models.
There are situations where government regulations require the use
of contaminant transport modeling. As a general rule, however, it is
not necessary for regulations to specify that a mode] must be used.
A few existing government regulations require that a mode} be
used in the submission to the agency. All of the examples the com-
mittee found involved situations where the law required a long-term
prediction of the migration potential of wastes. In such situations,
there is no alternative but the use of contaminant transport models.
A regulation that requires contaminant transport modeling re-
flects an implicit decision to require a given level of detail and allow
a given level of uncertainty. When regulations require the use of a
model, however, they do not imply that the solution to the problem is
susceptible to a "black-box" mode} application. Quite the contrary,
in the cases examined, the regulations seem to require contaminant
transport modeling in the most complicated site-specific problems.
Several agencies have guidelines that encourage the use of contam-
inant transport models. There are many different Apes of mod-
ele, mode} applications, modeling objectives, and legal ~ameworke.
Agencies cannot specify a list of government-approved modem. A
mode! that is appropriate for one problem may not be, and probably
is not, applicable to another problem. Such a list also tends to stifle
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12
GROUND WATER MODELS
notation and use of newer models. On the one hand, government
offlciale become reluctant to accept a nonapproved model. On the
other hand, the regulated community may use an agency-approved
mode} simply because the costs of getting go~rernmental approval will
be lese. Suth a list may also appear to be an Implied warranty of
the mode} accuracy and therefore lead to misuse of Me models.
It is impossible to specify by a generally applicable regulation a
contaminant transport mode} that would be scientifically valid in all
applications and over the typical life of a regulation.
In some circumstances, it may be appropriate to specify the use
of a particular contaminant transport model. For example, after
reviewing site-specific data from a hazardous waste site, an agency
or private company may determine that a particular mode} could be
appropriate to apply at the site and such a mode! may be specified
in a consent decree or permit for specific purposes. When a mode!
is used in such circumstances, the consent decree, permit, or other
legally enforceable procedure should require actual monitoring to
confirm the modeling results and be flexible enough to allow the
mode! to be updated and modified on the basis of new data and
recent scientific developments.
Recommendations
Models used in regulatory or legal proceedings should be available
for evaluation.
Models used in regulatory or legal proceedings are required to
undergo public comment and review by those whose interest may
be affected. The documentation associated with the mode! therefore
must enable any reviewer to
~ understand what was done;
evaluate the quality of the model, considering issues such
as the extent to which the equations describe the actual processes
(i.e., mode} validation) and the steps taken to verify that the code
correctly solves the governing equations and is fully operational (i.e.,
code verification);
evaluate the application of the mode! to a particular site; and
~ distinguish between the scientific and policy input.
A list of approved models should not be sanctioned by a regulatory
agency. Agencies should not require that specific models be used for
site-specific application by regulation, policy, or guidance. ~stead,
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OVERVIEW, CONCLUSIONS, AND RECOMMENDATIONS
13
positive attributes such as good quality assurance (QA) and doc-
umentation should be mandated, and government agencies should
continue to support and provide resources for the development of
ground water modeling codes.
The regulatory agencies should not develop a list of sanctioned
ground water models. Models are used to evaluate a wide range of
subsurface conditions for a variety of purposes. Models can be used
to gain an understanding of flow and chemical transport, to eval-
uate remedial alternatives, and to determine data collection needs.
The type of problem being evaluated and the level of understanding
required should dictate the mode! selection.
A list of government-approved models would limit the choice of
numerical codes available for problem solving. Development of a list
of government-sanctioned codes could also inhibit mode} develop-
ment and innovation. Because the process of mode} approval would
probably be lengthy, approved models are likely to lag behind the
available state of the art. As previously discussed, the quality of re-
sults is dependent on the quality of the data input and the knowledge
of the models. Sanctioning of codes would not eliminate the need for
proper mode! application and could develop a false sense of adequacy
or accuracy for mode! users.
Instead of sanctioning particular models, regulatory agencies
should provide detailed, consistent procedures for the proper devel-
opment and application of models. Detailed specifications of positive
aspects need to be developed but should include (1) good documenta-
tion of a code's characteristics, capabilities, and use; (2) verification
of the program structure and coding, including mass balance results;
(3) mode! validation, including a comparison of mode! results with
independently derived laboratory or field data and possibly other
computer codes; and (4) independent scientific and technical review.
The guidance must also be written to avoid being misconstrued
as providing a list of "approved" models. The mere approving men-
tion of a mode! in agency guidance may appear to inexperienced
and untrained agency personnel as indicating that such models are
"approved" or "sanctioned." Agency guidance therefore must stress
that the descriptions do not sanction the use of any particular model.
Instead the guidance should stress best modeling practices or princi-
ples, described above, and ensure that only experienced and properly
trained personnel are involved in the development and review of such
models.
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14
GROUND WATER MODELS
Modeling should be considered to be only one of several potable
methods of assessing liability In cases of ground water cont~;nation.
Models should not replace sound scientific and engineering judgment.
Contaminant transport models can provide one of several pos-
sible methods for identifying contarn~nant sources or apportioning
liability. However, it would be rare for modeling alone to provide an
unequivocal answer to the question of whether and to what degree a
potential source is in fact a source. Ground water models must not
be viewed as "black-box" tools that eliminate or lessen the need for
common sense and good scientific judgment.
Similarly, while models may be useful tools in regulatory deci-
sionmaking, they cannot substitute for the decisionmaking process.
Such decisions are almost always based on a wide range of factors.
Thus mode! results with attendant uncertainties should be consid-
ered along with all other information in order to make informed
regulatory decisions.
Maintaining Scientific integrity
Conclusions
Ground water models do and should vary In complexity. The com-
plexity of the mode} used to analyze a specific site should be deter-
mined by the type of problem being analyzed. While more complex
models Increase the range of situations that can be described, ~n-
creas~ng complexity requires more input data, requires a higher level
and range of skill of the modelers, and may introduce greater un-
certa~ n the output if input data are not available or of sufficient
quality to specify the parameters of the model.
Appropriate and successful models of ground water flow and
transport can range from simple analytical solutions for one-dimen-
sional flow in a homogeneous aquifer to highly complicated numerical
codes designed to simulate multiphase transport of reactive species
in heterogeneous, three-dimensional porous media. A useful mode!
need not simulate all the physical, chemical, and biological processes
that are acting in the subsurface. The mode! that is appropriate for
analyzing a particular problem should be determined primarily by
the objectives of the study. Unfortunately, there are no set rules for
determining the appropriate level of complexity. The selection of an
appropriate mode! and an appropriate level of detail and complexity
is subjective and dependent on the judgment and experience of the
analysts and on the level of prior information about the system of
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OVERVIEW, CONCLUSIONS, AND RECOMMENDATIONS
15
interest. Managers and other users of mode} results must be made
aware that these trade-offs and judgments have been made and that
they may affect the reliability of the model.
Models must be matched to the objectives of the study. Efforts
should be made to avoid using models that are more complicated
than necessary. Overly complicated modem require information that
cannot be obtained reliably from field measurements, which intro-
duces unnecessary uncertainty into the modeling output, and overly
complicated models require more time and money to operate, which
wastes resources. Because there are no set rules for selecting an ap-
propriate model, it is essential that agencies and companies employ
qualified and well-trained personnel.
One of the key requirements in successfully applying flow or contam-
inant transport models is good-quality, site-specific data. Such data
provide feasible bounds on the potable range of controlling parame-
tere or boundary conditione, thereby minimizing the impact of data
uncertainty as a major source of uncertainty associated with mode]
predictions. In cases where particular mode] parameters are not or
cannot be characterized, mode] prediction becomes much lese certain
because predicted variables like hydraulic potential or concentration
could take on a much broader range of possible values.
A variety of factors can contribute to uncertainty in mode! pre-
dictions. One of the most important is the inability to characterize
a site in terms of the boundary conditions or the key parameters
describing important flow and transport processes. This uncertainty
in data results for two basic reasons. First is the issue of the am
solute number of data points providing information about a given
parameter. Even a relatively large number of data points may not
provide a basis for estimating parameter values at locations between
them with total accuracy. As the number of data points decreases,
this uncertainty attached to a parameter estimate increases to the
point where one finally cannot describe the spatial variability in de-
tail and has to resort to a simple estimate like a mean value for a
given unit. A second issue with data is the inability in some cases to
measure or even accurately estimate values for necessary parameters.
This problem is most serious in fractured rocks for both single-phase
and multiphase flow, and for mass transport processes involving cer-
tain kinetic processes (e.g., biodegradation, redox, and precipitation)
whose rates can be extremely variable and site specific.
These two problems increase the likelihood that in many model
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16
GROUND WATER MODELS
studies there are some ciata that cannot be specified with accu-
racy. Sensitivity analyses provide one important way of establishing
the extent to which uncertainty in a given parameter contributes to
uncertainty in a prediction. Such analyses in many instances can pro-
vide the justification for carrying out additional field and laboratory
studies.
In general, data collection and mode} application should not be
viewed as sequential tasks but as tasks that should be performed
interactively, complementing each other.
Good documentation of ground water models throughout the mod-
el~ng process is necessary because of the complexities ~n~rol~red.
A hydrogeologic computer mode! may be very complex, running
to thousands of lines of code. It may include hundreds of separate
parameters and equations to mode} the movement of the water and
the transport and fate of dissolved components. For these reasons
it is essential that a mode! be accompanied by clear and thorough
documentation, and that the documentation include a set of test
problems that can be employed throughout the history of the mode!
to verify that it continues to work properly. Adequate plans for
testing and documenting a mode! should become part of any quality
assurance program. Technical review should also be included in
quality assurance plans to ensure that models have been adequately
tested.
In addition to the inherent complexity, it is common for any
given mode! to undergo repeated modifications and revisions, either
by the author or by subsequent users. Unless a record is kept of the
modifications that are made to the code, and unless the operational
accuracy of the code is periodically tested and verified, serious doubts
may develop about the validity and applicability of the code.
In addition to the original documentation, at least two types
of information should accompany the code throughout its lifetime.
First, changes in the structure of the mode! or of the database should
be documented. The documentation may be in the form of a written
record that ~ appended to the original documentation, or it may
be included as comment lines within the noncompiled code. Second,
an original set of test problems, including sample input and output,
should accompany the code so that all users can periodically verify
that the code is functioning properly, especially if changes are made.
This periodic verification of the operation and output of the code
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OVERVIEW, CONCLUSIONS, AND RECOMA~NDATIONS
17
becomes especially critical if the mode} is to be used as a part of a
regulatory or legal action.
There is no valid reason to use a mode! that is unavailable for
evaluation and testing by other qualified investigators. Similarly, new
or revised models should be accompanied by sufficient documenta-
tion, history, and test problems to allow other qualified investigators
to properly evaluate the mode} and to compare its output with that
of other models.
As ground water mode] mage has increased, a shortage of qualified
staff capable of appropriately applying modem has been identifidl.
In order to avoid mode! misuse, it is important that the mode}
user have the training and background to understand the many
processes occurring in the subsurface. Experienced staff having this
training and background are insufficient in terms of the number of
sites where modem could potentially be used.
Recornrnendations
All models must be documented so that the derivation of the mode]
can be understood and the results can be reproduced by anyone
seeking to use the model.
The documentation should include, at a minimum,
a description of the underlying problem;
~ a description of the fundamental equations that conceptualize
the solution to the problem;
~ a list of all assumptions used in the mode! and the rationale
for their use;
a description of the code used in the model;
~ a verification of mode} codes against other solutions to the
problem to verify the accuracy;
· an application of the mode} to a problem with a known so-
lution, albeit perhaps a simpler problem, and a comparison of the
results with the known results;
a sensitivity analysis;
the results of a quality assurance program;
the validation of the model;
~ a list of prior uses of the model, if any;
· a clear identification of the site-specific data used in the ap-
plication of the model;
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18
GROUND WATER MODELS
~ a characterization of the level of precision, accuracy, and
degree of uncertainty in the mode} results;
~ a description of the statutory/policy criteria, if any, used
to shape and select the assumptions and the acceptable level of
precision, accuracy, and uncertainty; and
~ any other information that is essential to understanding or
being able to replicate the results.
All models mat state quantitatively, to the extent possible, and if
not quantitatively, then qualitatively, the Tepee and direction of
uncertain n the mode] results and the time Came over which the
model's prediction can be considered acceptable.
This description of the uncertainties must be given at the be-
g~nning of the documentation of the mode} and wherever the con-
clusions of the models are used or discussed; e.g., in the conclusion
of the modeler's report, in the briefing memorandum to an agency
decisionmaker relying on the moclel, in whole or part, to make a
regulatory decision, in the preamble to an agency regulation, and in
expert testimony concerning the results of the model.
The policy assumptions used in the mode] mat be explicitly listed,
and the rationale for making each assumption mast be described In
the documentation and wherever the conclusions of the mode] are
used or discuseed; e.g., in the conclusion of the modeler's report, in
the briefing memorandum to an agency decisionmaker relying on the
mode], in the preamble to an agency regulation, in press releases and
statements to the public, In presentations to Congress, and in expert
testimony concerning the results of the model. To adroit the misuse
of ground water flow and transport models, agencies and companies
should employ qualified and well-tra~ned personnel.
Ground water flow and transport models are complex computer
codes. To ensure that the input data are appropriate, and that the
output results are properly utilized and interpreted, it is important to
employ properly trained and qualified individuals. These personnel
must be expert in both ground water science and its mathematical
representation.
A certain fascination exists among technical personnel regarding
the use of these powerful tools, and it is tempting to view them
as "black boxes" that somehow produce easy and exact answers to
previously difficult problems. This tendency may become even more
pronounced as the interfaces between the codes and the users become
more "user friendly." Indeed, it could be argued that the lack of a
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OVERYlEW, CONCLUSIONS, AND RECO~NDA~ONS
19
user-friendly interface may be a useful safety feature to help prevent
inappropriate use of the models by nonqualified personnel.
If governmental agencies or private companies make the decision
to use computer models in their work with ground water, it ~ em
sential that the personnel involved be adequately trained and fully
aware of the limitations of the code. In order to use ground water
models, an organization may have to hire new personnel or train
existing personnel. It is not acceptable, however, to assign modeling
projects to existing personnel who may simply be available for such
tasks, without intensive and appropriate training.
The best procedure to ensure competency may be to designate
one or more people as specialists in the modeling efforts within an
organization. Such specialists would then have the responsibility to
continually maintain and update their knowledge of the models being
used and to make certain that others within the organization do not
use the models inappropriately.
The problem of rapid turnover of personnel within government
regulatory agencies must also be addressed. Pressures can be very
great on regulatory personnel, without corresponding financial re-
wards. The record of high turnover rates within regulatory agencies,
especially among younger technical employees, shows that the tempo
tation to move into the private sector is very great. The Section
to leave government service seems to be made about the time the
individual achieves a relatively high level of competence and becomes
known to various private companies. To overcome this high rate of
attrition, some means of providing appropriate financial compensa-
tion must be found to properly recognize, reward, and retain highly
skilled individuals. If salaries cannot be raised, it is essential that an
active program of recruitment and training be maintained within the
agency to ensure that an adequate, high level of competency always
exists among the personnel involved in ground water modeling.
Research should be undertaken to provide the field and laboratory
data necessary to validate How and transport models.
Given that some types of models cannot be validated with ex-
isting, rather limited knowledge about some types of flow and mass
transport processes, it is recommended that research be undertaken
to fill in information gaps. The committee recognizes a need for
well-controlled field and laboratory experunents involving flow and
mass transport in fractured media, and multicomponent transport
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20
GROUND WATER MODELS
of chemically and biologically active contaminants. Such work is es-
sential to establish how well existing mathematical concepts describe
actual hydrogeological systems.
Recommendations for the Future
Governments, academic institutions, and private Industry need to
provide financial resources and substantially Increase the pool of
qualified personnel In the spectrum of fields essential to ground
water modeling.
A severe shortage of qualified personnel exists in the areas of
hydrogeology, ground water hydrology, and organic and aqueous
geochemistry. Most of the new positions are with engineering and
environmental consulting firms, and severe recruiting pressure exists
among the firms, especially for experienced people.
If the challenges posed to our ground water environment by an
ever-increasing population and continued industrialization are to be
met, significant steps to increase the supply of trained ground water
professionals must be taken. It is the strong recommendation of the
committee that additional educational resources be committed to
these fields as quickly as possible. The committee also recommends
that government and private industry join in the effort to increase
educational resources and opportunities for students entering the
spectrum of fields related to ground water modeling. In addition
to providing financial support, governmental agencies and private
industry should further help in the education of ground water profes-
sionals by developing traineeships and industrial-associates programs
to give students the opportunity to obtain practical experience in the
field.
Government agencies and priorate industry should be aware of the
need for and benefits of additional research. Research should be
pursued ~ the following areas:
~ gravitation and further development of models involved with
(~) ground water flow ~ unsaturated and Factored media, (2) mul-
tiphase flow in porous and fractured media, and (3) mass transport
coupled with chemical reaction;
role of bacteria ~ the transport end removal of contaminants;
. models in decisionmaimg, including methods for identifying
and presenting uncertainty and for eetablish~ng the reliability of
mode! results;
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OVERVIEW, CONCLUSIONS, AND RECOMMENDATIONS
21
~ process Characterization through weB-controlled field and
laboratory studies; and
~ development of new approaches for parameter estunation and
of new measurement techniques.
Although many aspects of ground water modeling have major
deficiencies in terms of scientific understanding and the availability
of field-relevant databases, research in the five areas listed here offers
especially great potential for yielding useful results. In the case of
the first area- flow and transport in fractured and cavernous media
and multiple-phase flow-the potential benefit is very high because
these types of flow situations have a relatively widespread occur-
rence, have a strong impact on the movement of large masses of
contaminants, and have not been adequately documented, resulting
in an utter lack of any reliable databases. The second area- the role
of bacteria in the transport and removal of contaminants-is critical
because of the increasing recognition that bacteria are present in the
subsurface, that most organic and some inorganic contaminants are
biotransformed, and that bioremecliation offers a potentially econom-
ical in situ cleanup technique. The third area the role of modeling
in decisionmaking, including legal and social interactions-must be
understood if the courts, enforcement agencies, industries, and the
affected public are to obtain the benefits of modeling. The last
two areas characterization through welI-controlled field and labo-
ratory studies and development of new approaches for parameter
estimation and new measurement techniques are essential if fate,
transport, and remediation are to be measured in the subsurface,
which is otherwise not easily accessible to human observation.
Representative terms from entire chapter:
water models
