\ If
United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/S2-89/028 Sept. 1989
oEPA Project Summary
Groundwater Modeling: An
Overview and Status Report
Paul K.M. van der Heijde, Aly I. El-Kadi, and Stan A.Williams
This report focuses on groundwater
models and their application in the
management of water resource sys-
tems. It reviews the kinds of models
that have been developed and their
specific and general role in water
resource management
The report begins with the intro-
duction of system concepts ap-
plicable to subsurface hydrology and
presents groundwater modeling
terminology, followed by a discus-
sion of the role of modeling in
groundwater management with spe-
cial attention to the importance of
spatial and temporal scales. The
model development process is dis-
cussed together with related issues
such as model validation. A separate
section provides information on
model application procedures and is-
sues. In addition to a review of the
model application process, this chap-
ter contains discussion of model
selection and model calibration and
provides information on specific
aspects of pollution modeling. The
report also contains an extensive
overview of current model status.
Here, the availability of the models,
their specific characteristics, and the
information, data, and technical
expertise needed for their operation
and use are discussed. Also
discussed are quality assurance in
groundwater modeling and manage-
ment issues and concerns. The re-
port concludes with a review of cur-
rent limitations in modeling and of-
fers recommendations for improve-
ments in models and modeling
procedures.
This Project Summary was de-
veloped by EPA's Robert S. Kerr
Environmental Research Laboratory.
Ada, OK, to announce key findings of
the research project that is fully docu-
mented in a separate report of the
same title (see Project Report order-
ing information at back).
Background
In the mid-1970s, by request of the
Scientific Committee on Problems of the
Environment (SCOPE), part of the Inter-
national Council of Scientific Unions
(ICSU), the Holcomb Research Institute
(HRI) at Butler University, Indianapolis,
Indiana, carried out a groundwater model-
ing assessment. This international study,
funded in large part by the U.S. En-
vironmental Protection Agency (EPA)
through its R.S. Kerr Environmental Re-
search Laboratory in Oklahoma, resulted
in a report published by the American
Geophysical Union (AGU) in its series,
Wafer Resources Monographs. In 1985 a
second edition of this monograph was
published, based on information collected
at HRI through its International Ground
Water Modeling Center (IGWMC) from its
inception in 1978 until December 1983.
The Center is an international clearing-
house for groundwater models and a
technology transfer center in groundwater
modeling. Since 1983 the Center has
been linked to the TNO Institute of Ap-
plied Geosciences, Delft, The Nether-
lands, which operates the European
office of the IGWMC. Supported largely
by the EPA and in part by HRI, the
Center organizes and conducts short
courses and seminars, and carries out a
research program to advance the quality
of modeling in groundwater management,
in support of the Center's technology
transfer functions.
The report summarized herein presents
results of research and information
processing activities performed by the
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IGWMC under a research and tech-
nology transfer cooperative agreement
initiated in 1985. The report serves three
functions: (1) it provides an introduction
to groundwater modeling and related is-
sues for use as instruction material in
short courses and for self study; (2) it
provides an overview of the status of
major types of groundwater models; and
(3) it presents a discussion of problems
related to the development and use of
groundwater models.
Introduction
Groundwater modeling is a method-
ology for the analysis of mechanisms
and controls of groundwater systems and
for the evaluation of policies, actions, and
designs that may affect such systems.
Models are useful tools for under-
standing the mechanisms of groundwater
systems and the processes that in-
fluence their composition. Modeling
serves as a means to ensure orderly in-
terpretation of the data describing a
groundwater system, and to ensure that
this interpretation is a consistent repre-
sentation of the system. Modeling can
also provide a quantitative indicator for
resource evaluation where financial re-
sources for additional field data collection
are limited. Finally, models can be used
in what is often called the predictive
mode by analyzing the response a sys-
tem is expected to show when existing
stresses vary and new ones are intro-
duced. They can assist in screening al-
ternative policies, in optimizing engineer-
ing designs, and in assessing operative
actions in order to determine their im-
pacts on the groundwater system and ul-
timately on the risks of these actions to
human health and the environment.
In managing water resources to meet
long-term human and environmental
needs, groundwater models have be-
come important tools.
The field of groundwater modeling is
expanding and evolving as a result of:
• Widespread detection of contami-
nated groundwater systems
• Enhanced scientific capability in
modeling groundwater contamination
in terms of the physical, biological,
and chemical processes involved
• Rapid advancement of computer
software and hardware, and the
marked reduction in the cost associ-
ated with this technology.
The rapid growth in the use of ground-
water models has led to unforeseen
problems in project management. Some
of the projects in which these sophis-
ticated tools have been used have even
led to adversary legal procedures in
which the model application or even the
model's theoretical framework and
coding have been contested. Often, the
key issue is the validity of model-based
predictions. Other issues of concern in-
clude code availability and reliability,
model selection and acceptance criteria,
project review and procurement, data re-
quirements, information exchange, and
training.
The Groundwater System
Groundwater is a subsurface element
of the hydrosphere, which is generally
understood to encompass all the waters
beneath, on, and above the earth's
surface. Many solar-powered processes
occur in the hydrosphere, resulting in a
continuous movement of water. This dy-
namic system is referred to as the hy-
drologic cycle. Its major elements are at-
mospheric water, surface water, water in
the subsoil (shallow and deep vadose
zone), groundwater, streams, lakes and
ocean basins, and the water in the
lithosphere. (Figure 1).
Movement of water occurs both within
each element of the hydrologic cycle and
as exchanges between the elements, and
results in the dynamic character of this
relatively closed system. The exchange
processes between the surface subsys-
tem and the atmosphere include evapo-
ration, precipitation (rainfall and snowfall),
and plant transpiration. Infiltration, seep-
age, groundwater recharge from streams,
and subsurface discharge into lakes and
streams (both interflow and baseflow) are
interelement processes between the
earth's surface and subsurface. Surface
runoff forms the link between the earth's
surface and the network of streams. In
addition, interactions take place between
the subsurface hydrosphere and ele-
ments of the earth's biological environ-
ment (e.g., consumptive use of water by
plants).
A groundwater system is an aggregate
of rock in which water enters and moves,
and which is bounded by rock that does
not allow any water movement, and by
zones of interaction with the earth's surf-
ace and with surface water systems. In
such a system, the water may transport
solutes and biota; interactions of both
water and dissolved constituents with the
solid phase (rock) often occur.
Water enters the groundwater system
in recharge zones and leaves the system
in discharge areas. In a humid climate,
the major source of aquifer recharge is
the infiltration of water and its subse-
quent percolation through the soil into
the groundwater subsystem. This type of
recharge occurs in all in-stream areas
except along streams and their adjoi
floodplains, which are generally
charge areas. In arid parts of the w
recharge is often restricted to mour
ranges, to alluvial fans bordering tt
mountain ranges, and along the chan
of major streams underlain by thick
permeable alluvial deposits.
In addition to these natural rechi
processes, artificial or man-made
charge can be significant. This type 01
charge includes injection wells, indui
infiltration from surface water bodies,
irrigation.
Outflows from groundwater syst<
are normally the result of a combina
of inflows from various recharge soun
Groundwater loss appears as interflov
streams (rapid near-surface runoff);
groundwater discharge into streams
suiting in stream baseflow); as spri
and small seeps in hillsides and va
bottoms; as wetlands such as lakes •
marshes fed by groundwater; as capill
rise near the water table into a zone ft
which evaporation and transpiration i
occur; and as transpiration by phre;
phytes (plants whose roots can live in
saturated zone or can survive fluctuate
of the water table). Other outflows are
tificial or human-induced, as agricultt
drainage (tile-drains, furrows, ditch
and wells for water supply or dewater
(e.g., excavations and mining).
The unsaturated zone has a signific
smoothing influence on the tempo
characteristics of the recharge of groui
water systems. High variable (houi
precipitation and diurnal evapotranspi
tion effects are dampened and seaso
and long-term variations in flow rai
become more prominent further from I
soil surface. In this dampening t
higher-frequency fluctuations are
tered, a process that continues in I
groundwater zone. Its ultimate effect c
be observed in stream base flow, whi
is characterized by seasonal and lor
term components.
Model Development
In groundwater modeling, a distinct!
is often made between two major cai
gories of activities: model developrm
and model use in management. Moc
development consists of researching t
quantitative description of the groun
water system, a software developme
component, and model testing. Moc
development is closely related to the si
entific process of increasing knowledg
observing nature, posing hypotheses I
the observed information, verifying tl
proposed relationships, and thus esta
lishing a credible theoretical framewo
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and improving our understanding of na-
ture. Model development is often driven
by the short-term and less frequently
by the long-term needs of natural re-
sources management. The resulting,
often-generic computer codes are used
in model application as part of a larger
set of activities which included data col-
lection and interpretation, technical de-
sign, economical evaluation, and so forth.
The final report presents a complete,
detailed discussion of the model devel-
opment process, scenarios and data-
bases as well as model applications and
management issues including quality
assurance.
Groundwater Modeling and
Management
Groundwater management is con-
cerned with the efficient utilization of
groundwater resources in response to
current and future demands, while pro-
tecting the integrity of the resources to
sustain general environmental needs.
Groundwater modeling has become an
important methodology in support of the
planning and decision-making proc-
esses involved in groundwater manage-
ment.
Groundwater modeling provides an an-
alytical framework for understanding
groundwater flow systems and the proc-
esses and controls that influence their
quality, particularly those processes influ-
enced by human intervention in the hy-
drogeologic system. Models can provide
water resource managers with necessary
support for planning and screening of al-
ternative policies, making management
decisions, and reviewing technical de-
signs for groundwater remediation based
on a risk analysis of benefits and costs.
Such support is particularly advanta-
geous when applied to development of
groundwater supply, groundwater protec-
tion, and aquifer restoration.
/ / / / / / ' / / / /
\TMOSPHERE
transpiration
precip-
itation
evaporation
-//Earth/Soil
XT'/Surface
////
Infil-
tration
surface
runoll
precipitation
.
Surface Water
// Bodies //
(rivers, lakes)/'
// x /.
seepage
percolnllon
recharge
capillary
rise
Interflow
seepage
(wetlands)
discharge
(base (low)
stream
flow
evaporation
discharge
recharge
/ / / ' ' ' / / /// S7 ' ff ' ' '/ s / / ' ' ' S^^s / /
R 0 U NPW A T E R Z b N E / AQ U I ER
saltwater
Intrusion
.LITHOSPHERE
/ / / / / s / / / / / / s / /
Figure r. Elements of the hydrologic cycle and their interactions.
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Successful utilization of modeling is
possible only if the methodology is
properly integrated with data collection,
data processing, and other techniques
and approaches for evaluation of hydro-
geologic system characteristics. Further-
more, frequent communication between
managers and technical experts is es-
sential to assure that management issues
are adequately formulated and that the
technical analysis using models is well
targeted.
Where precise aquifer and contami-
nant characteristics have been reasona-
bly well established, groundwater models
may provide a viable, if not the only,
method to predict contaminant transport
and fate, locate areas of potential en-
vironmental risk, identify pollution
sources, and assess possible remedial
actions. Some examples in which mathe-
matical models have assisted in the man-
agement of groundwater protection pro-
grams are:
• Determining or evaluating the need
for regulation of specific waste dis-
posal, agricultural, and industrial
practices
• Analyzing policy impacts, as in eval-
uating the consequences of setting
regulatory standards and rules
• Assessing exposure, hazard, dam-
age, and health risks
• Evaluating reliability, technical feasi-
bility and effectiveness, cost, opera-
tion and maintenance, and other
aspects of waste disposal facility de-
signs and of alternative remedial
actions
• Providing guidance in siting new fa-
cilities and in permit issuance and
petitioning
• Developing aquifer or well head pro-
tection zones
• Assessing liabilities such as post-
closure liability for waste disposal
sites
Models generally applied to ground-
water pollution problems can be divided
into two broad categories: (1) flow mod-
els describing hydraulic behavior of sin-
gle or multiple fluids or fluid phases in
porous soils, or porous or fractured rock,
and (2) contaminant transport and fate
models for analysis of movement, trans-
formation, and degradation of chemicals
present in the subsurface. In the context
of groundwater protection programs, a
distinction is often made between site-
specific and generic modeling.
However, generic modeling ap-
proaches are being increasingly con-
tested through public comment on draft
regulations or in courtroom legal pro-
cedures. An example is the recent court
decision that EPA's VHS model (Vertical
Horizontal Spread model) cannot be
used to grant or deny a delisting petition
under the RCRA permitting program.
Site-Specific Modeling
Whether for permit issuance, investiga-
tion of potential problems, or remediation
of proven contamination, site-specific
modeling is required as a necessary in-
strument for compliance under a number
of major environmental statutes. The
National Environmental Policy Act of
1970 (NEPA) stipulates a need to show
the impact of major site-specific con-
struction activities in Environmental Im-
pact Statements; although not required
by the regulations, potential impacts are
often projected successfully by math-
ematical models.
Some of the most challenging site-
specific problems involve hazardous
waste sites falling under the purviews of
RCRA (Resource Conservation and Re-
covery Act of 1976) and CERCLA (Com-
prehensive Environmental Response,
Compensation, and Liability Act of
1980-Superfund), both administered by
the U.S. Environmental Protection Agen-
cy. Associated with most of these sites is
an intricate array of chemical wastes and
the presence of, or potential for, ground-
water contamination. Furthermore, the
hydrogeologic settings of such sites are
usually complex. Under such conditions,
groundwater models are useful instru-
ments for analyzing compliance with
RCRA and CERCLA legislation.
Generic Modeling
Where the results of environmental
analysis must be applied to many sites,
data availability is limited or other con-
straints are present. In such cases, site-
specific modeling is not feasible. As a
result, many decisions are made by ap-
plying models to generic management
issues and hydrogeologic conditions.
Models used for this type of analysis are
more often analytical than numerical m
their mathematical solutions, in contrast
to models used for detailed analysis of
site-specific conditions. Because of
their limited data requirements, analytical
models can be applied efficiently to a
larger number of simple datasets or to
statistical analyses representing a wide
variety of field conditions. The cost of
such exercises would often be prohibitive
when using numerical models.
Conclusions
An EPA Study Group has identified a
variety of new models and modeling ap-
proaches as important to groundwater
protection:
• Simulation of flow and transpo
multimedia (e.g., coupled model
surface water/groundwater ir
action)
• Representation of stochastic p
esses in predictive modeling,
improving the applicability of get
tistical models
• Improved modeling of hydrochen
speciation
• Simulation of flow and transpoi
fractured and dual-porosity me
including diffusion in dead-
pores
• Simulation of flow and transpoi
soils containing macropores
• Determination of effects of com
tration-dependent density
groundwater flow and pollu
transport
• Determination of effects of altera
of geologic media on hydrolog
and chemical characteristics (c
dehydration of clay when attac
by solvents, change in sorptive Ce
city of material when heated)
• Representation of the three-dim
sional effects of partially penetra
wells on water table aquifers
• Development of models for mane
ment of groundwater contamina
plumes
• Development of expert systems (;
ficial intelligence) for such tasks
selecting appropriate submodels
subroutines for specific problems
• Application of parameter identifi
tion models to be used with :
studies
• Further development of pre- j
post-simulation data processors
• Continued development of risk
sessment and management mode
• Modeling of volatilization, multiph;
flow, and immiscible flow
• Incorporation of economic factors
improve estimation of cleanup cos
• Development of generic and si
specific parameter databases.
Fundamental research support!
groundwater modeling is consider
necessary in such areas as:
• Transient behavior of process [
rameters (e.g., retardation, hydrai
conductivity)
• Desorption for nonhydrophot
chemicals
• Multicomponent transport and che
ical interaction
• Enhanced transport mechanist
(e.g., piggy-backing on more nr
bile chemicals)
• Transport of silt with sorbed che
icals m aquifers
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• Improved numerical accuracy, sta-
bility, and efficiency.
Modeling the transport and fate of
chemicals in groundwater is a major sub-
ject of several EPA and DOE research
programs. These programs focus on im-
miscible flow associated with organic and
oil-like liquids. Other topics currently
being studied include simulation of flow
and transport in fractured and dual-
porosity media, representation of sto-
chastic processes in predictive modeling,
multimedia risk assessment, incorpora-
tion of volatilization in multiphase trans-
port models, and simulation of density-
dependent flow.
Paul K.M. van der Heijde, Aly I. El-Kadi, and Stan A. Williams are with
International Ground Water Modeling Center, Butler University,
Indianapolis, Indiana 46208.
Joe rt. Williams is the EPA Project Officer (see below).
The complete report, entitled "Groundwater Modeling: An Overview and Status
Report," (Order No. PB 89-224 497/AS; Cost: $28.95, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada, OK 74820
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United States Center for Environmental Research BULK RATE
Environmental Protection Information POSTAGE & FEES PAII
Agency Cincinnati OH 45268 EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-89/028
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