United States
Environmental Protection
Agency
Robert S. Kerr
Environmental Research Laboratory
Ada, OK 74820
Research and Development
EPA/600/SR-93/118 August 1993
EPA Project Summary
Compilation of Ground-Water
Models
Paul K.M. van der Heijde and Osman A. Elnawawy
The full report presents an overview
of currently available computer-based
simulation models for ground-water
flow, solute and heat transport, and
hydrogeochemistry in both porous me-
dia and fractured rock. Separate sec-
tions address multiphase flow and
related chemical species transport, and
ground-water management models. The
study reflects the on-going ground-wa-
ter modeling information collection and
processing activities at the International
Ground Water Modeling Center
(IGWMC). The full report includes a
section that defines ground-water mod-
eling, presents the classification ap-
proach taken by the IGWMC and
discusses different types of models and
the mathematical approaches invoked
for developing the models. Separate
sections discuss and review the differ-
ent categories of ground-water mod-
els: flow models, transport models,
chemical reaction models, stochastic
models, models for fractured rock and
ground-water management models. The
appendices include a listing and de-
scription from the IGWM Model Anno-
tation Search and Retrieval System
(MARS) database of selected models
from each category.
This Project Summary was devel-
oped by EPA's Robert S. Kerr Environ-
mental Research Laboratory, Ada, OK,
to announce key findings of the research
report that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The full report contains the results of
research and information processing ac-
tivities performed by the IGWMC under a
research and technology transfer coop-
erative agreement with the U.S. Environ-
mental Protection Agency, initiated in 1988.
The report, together with the reports on
management models and multiphase flow
and transport, provides an overview of the
status of major types of ground-water mod-
els. These reports present an update of
Chapter 5 and the appendices of the re-
port, "Groundwater Modeling: An Over-
view and Status Report," (EPA/600/2-89/
028) prepared in 1988 under a previous
cooperative agreement with the US EPA.
The review of models has been based
on information gathered by the IGWMC
through research and interviews on an
on-going basis since 1978. To manage
the rapidly growing amount of information,
IGWMC maintains a descriptive model in-
formation system, MARS. Currently, this
database is installed on a microcomputer
operating under MS-DOS. Detailed infor-
mation on the reviewed models is pre-
sented in a series of tables, preceded by
an introduction on model classification and
principal characteristics of the described
models.
Discussion
Ground-water modeling is a computer-
based methodology for mathematical
analysis of the mechanisms and controls
of ground-water systems, and for the
evaluation of policies, actions, and de-
signs that may affect such systems (van
der Heijde et al., 1988). In addition to
satisfying scientific interest in the work-
ings of subsurface fluid flow and fluid-
related mass-transfer and transformation
processes, models assist in analyzing the
responses of subsurface systems to varia-
tions in both existing and potential
stresses. Models play an increasingly
dominant role in the determination of the
physical and economical effects of pro-
posed ground-water protection policy al-
ternatives, and thus in the protection of
human and ecological health. Computer
models are essential tools in the screen-
ing of alternative remediation technolo-
gies and strategies in cleaning up
ground-water systems polluted in the (re-
cent) past, in the sound design of ground-
water resource development schemes for
Printed on Recycled Paper
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water supply, and for other land use modi-
fications affecting ground-water systems.
Although a consensus may exist as to
what ground-water modeling entails, the
definition of a "model" per se is somewhat
nebulous. In hydrogeology, the term
"ground-water model" has become syn-
onymous with conceptual ground-water
models, mathematical ground-water mod-
els (including analytical and numerical
models), computer models, and simula-
tion models. Furthermore, the term
"ground-water model" may apply to either
a generic model or computer code (with-
out she-specific data) or the representa-
tion of a site-specific system using such a
generic code. The IGWMC defines a model
as a non-unique, simplified, mathematical
description of an existing ground-water
system, coded in a programming language,
together with a quantification of the ground-
water system the code simulates in the
form of boundary conditions, system pa-
rameters, and system stresses. The ge-
neric computer code used in this
problem-specific system simulation is
sometimes referred to as a (ground-wa-
ter) simulation code or a generic ground-
water model. This use of the term
"ground-water model" includes both the
saturated and unsaturated zones.
Ground-water models are generally in-
tended to perform as practical, descrip-
tive, and predictive problem-solving tools.
Most ground-water models are mathemati-
cal models in which the causal relation-
ships among various components of the
ground-water system and between the sys-
tem and its environment are quantified
and expressed in terms of mathematics
and uncertainty of information. Mathemati-
cal models range from rather simple, em-
pirical expressions to complex mechanistic,
mufti-equation formulations. As the prob-
lems encountered in protecting and reme-
diating ground-water resources are highly
complex in nature, their study requires
cooperation between many disciplines.
Routinely, simulations of the complex
ground-water systems involved require
characterization of hydrology, physical
transport processes, geochemistry, con-
taminant chemistry and biochemistry in
the near-surface and deep underground.
Therefore, contemporary ground-water
modeling is highly multidisciplinary in na-
ture.
Models are useful tools for understand-
ing the structure and dynamics of ground-
water systems and the processes that
influence their composition. Modeling
serves as a means to ensure orderly in-
terpretation of the data describing a
ground-water system and to ensure that
this interpretation is a consistent repre-
sentation of the system. It can also pro-
vide a quantitative indicator for efficient
resource utilization when additional field
data collection is required and financial
resources are limited. Finally, models can
be used in what is often called the predic-
tive mode by analyzing the response a
system is expected to show when existing
stresses vary and new ones are intro-
duced, or by optimizing the response of
the system by varying the stresses in a
systematic way. Increasingly, the objec-
tives behind the ground-water modeling
efforts are protection and improvement of
human health through providing good qual-
ity drinking water and reducing the risks
resulting from exposure to contaminated
ground water
Where precise aquifer and contaminant
characteristics have been reasonably well
established, ground-water models may pro-
vide a viable, if not the only, method to
.predict contaminant transport and fate, lo-
cate areas of potential environmental risk,
identify pollution sources, and assess pos-
sible remedial actions. Some examples in
which mathematical models have assisted
in the management of ground-water pro-
tection programs are the following(van der
Heijde et al., 1988):
Determining or evaluating the need
for regulation of specific waste dis-
posal, agricultural, and industrial
practices
Analyzing policy impacts, as in
evaluating the consequences of set-
ting regulatory standards and rules
Assessing exposure, hazard, dam-
age, and health risks
Evaluating reliability, technical fea-
sibility and effectiveness, cost, op-
eration and maintenance, and other
aspects of waste disposal facility
designs and of alternative remedial
actions
Providing guidance in siting new
facilities 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.
Computer-based ground-water model-
ing began in the mid 1960's and has gradu-
ally grown into a widely accepted and
applied decision-support tool. In the last
few years, modeling has been made
easier, faster, and "flashier" by rapidly
evolving computer hardware and software
technologies. The widespread availability
of powerful desktop microcomputers and
user-oriented software interfaces has made
running ground-water computer codes a
rather mundane task in hydrogeological
assessments. The mechanics of entering
data, running simulations, and creating
high-quality graphics have become less
time-consuming and less complex due to
the availability of various, extensively sup-
ported window environments and ex-
panded functionality, easy-to-debug,
programming languages. These high-pow-
ered software development systems inte-
grate editing, compiling, and debugging
functions with additional programming tools
and libraries allowing efficient development
of flexible, menu-driven software while fa-
cilitating achievement of high quality-as-
surance goals Increased portability of the
software due to the development of multi-
platform operating systems such as UNIX,
and standardization of high-level program-
ming languages (e.g., FORTRAN 90) and
the subsequent release of new compilers,
makes it possible for software develop-
ment groups to continue to improve the
simulation components of the software.
Furthermore, the use of object-oriented
programming holds the promise of more
flexibility regarding post-development ex-
pansion and maintenance, and in overall
reliability and portability of the software.
Recently, geographical information sys-
tems (GIS) have become prominent tools
in model preparation and evaluation of
modeling results. Automatic allocation of
model parameters is facilitated by over-
laying the spatially organized geological
and hydrological parameters with a model-
defined computational mesh. The signifi-
cance of the results of model simulations
in the final decision-making process can
be further enhanced by importing the ras-
ter- or vector-based simulation results back
into a GIS and combining these with back-
ground maps of the area under study and
other spatial information important for de-
cision-making.
However, the reduction in time and ef-
fort in modeling due to new software de-
velopments does not mean that modeling
has become a simple task; in fact, model-
ing is becoming more challenging as
ground-water specialists are able to deal
with increasingly complex mathematical
descriptions of natural systems and re-
source management problems. In addi-
tion, these problems can be studied in
much more detail by using high-order
spatial and temporal resolution. In-depth
treatment of the theoretical basis of
ground-water models can be found in
NRC (1990), among others. Extensive
discussion on modeling methodology is
given in van der Heijde et al. (1988) and
NRC (1990).
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Classification of the Ground-
Water System
The nature of the ground-water system
is characterized by the system's hydro-
geological characteristics (i.e., hydrogeo-
logic schematization and geometry,
parameter variability in space and time,
boundary locations and conditions, and
system stresses) and the physical, chemi-
cal, and biological processes that take
place (type of processes, their spatial and
temporal characteristics, and their relative
importance). Accordingly, we distinguish
between two classification types in de-
scribing the ability of models to represent
the nature of the ground-water (or soil-
water) system: (1) hydrogeology-based
model types, and (2) process-based model
types.
One way to distinguish between differ-
ent types of ground-water models is based
on the kind of hydrogeologic features they
can simulate as shown in the full report.
Among others, distinction may be made
among various kinds of hydrogeologic con-
ceptualizations or zonings, e.g., saturated
zone versus unsaturated zone, a single
aquifer system versus a multilayered sys-
tem of aquifers and aquitards. Another
distinction is based on scale, e.g., site,
local, or regional scale.
A classification based on processes dis-
tinguishes between flow, transport (solute
and heat), fate of chemical compounds,
phase transfers and other processes
(Table 1 lists important processes encoun-
tered in ground-water systems). Flow mod-
els simulate the movement of one or more
fluids in porous or fractured rock. One
such fluid is water; the others, if present,
can be air, methane, or other vapors (in
soil) or immiscible nonaqueous phase liq-
uids (NAPLs) sometimes having a density
distinct from water (LNAPLs, DNAPLs). A
special case of mufti-fluid flow occurs when
layers of water of distinct density are sepa-
rated by a relatively small transition zone,
a situation often encountered when sea
water intrusion occurs. Most flow models
are based on a mathematical formulation
which considers the hydraulic system pa-
rameters as independent field information,
and hydraulic head and flux as dependent
variables. They are used to calculate
steady-state distribution or changes in time
in the distribution of hydraulic head or
fluid pressure, drawdown, rate and direc-
tion of flow (e.g., determination of stream-
lines, particle pathways, velocities, and
fluxes), travel times, and the position of
interfaces between immiscible fluids. In-
verse flow models simulate the flow field
to calculate the spatial distribution of un-
known system parameters using field in-
Tablo 1. Important Processes in Ground-Water Modeling
Flow:
Fate:
single fluid flow
multifluid flow
- multicomponent
- multiphase
laminar flow
linear/Darcian
- nonlinear/non-Darcian
turbulent flow
Transport
advection/con vection
conduction (heat)
mechanical dispersion
molecular diffusion
radiation (heat)
formation on the dependent variables such
as hydraulic head and flux.
Two types of models can be used to
evaluate the chemical quality of ground-
water: (1) pollutant transformatbn and deg-
radation models, where the chemical and
microbia! oiocesses are posed indepen-
dent of the movement of the pollutants;
and (2) solute transport models simulating
displacement of the pollutants only (con-
servative transport), or including the ef-
fects of phase transfers, (bio-chemical
transformation and degradation processes
(transport and fate; non-conservative trans-
port) In fact, one may argue a third type
exists, where a conservative solute trans-
port model is coupled with a hydrogeo-
chemical speciation model.
Hydrogeochemical speciation models
represent the first type, as they consist
solely of a mathematical description of
equilibrium reactions or reaction kinetics.
These models, which are general in na-
ture and are used for both ground water
and surface water, simulate chemical pro-
cesses in the liquid phase and sometimes
between the liquid and solid phase (pre-
cipitation-dissolution; sorption) that regu-
late the concentration of dissolved
constituents. They can be used to identify
the effects of temperature, speciation, sorp-
tion, and solubility on the concentrations
of dissolved constituent.
Solute transport models are used to
predict movement and concentration of
water-soluble constituents and radionu-
clides. A solute transport model requires
hydrolysis/substitution
dissolution/precipitation
reduction/oxidation
complexation
radioactive decay
microbial decay/biotransformation
Phase Transfers:
solid<->gas:
solids-liquid:
liquid<->gas:
- (vapor) sorption
- sorption
- ion exchange
- volatilization
- condensation
- sublimation
Phase Changes:
• freezing/thawing
• vaporization
(evaporation/condensation
velocities for the calculation of advective
displacement and spreading by dispersion.
If the velocity field is constant, it may be
either calculated once using a program
module or read into the program as data.
If the velocity field is dependent on time
or concentration, calculation of velocities
at each time step is required, either through
an internal flow simulation module or an
external, coupled flow module.
The nonconservative solute transport
models include some type of solute trans-
formation, primarily adsorption, radioac-
tive decay, and simple (bio-)chemical
transformations and decay.
The inclusion of geochemistry in solute
transport models is often based on the
assumption that the reaction proceeds in-
stantaneously to equilibrium. Recently,
various researchers have become inter-
ested in the kinetic approach that incorpo-
rates chemical reactions in transport
model. This inclusion of geochemistry has
focused on a single reaction such as ion-
exchange or sorption for a small number
of reacting solutes.
Conclusions
Systematically analyzing, evaluating,
and characterizing the capabilities and per-
formance of mathematical models for
studying such a complex system as en-
countered in managing and protecting
ground-water resources is a highly chal-
lenging activity. In recent years the amount
of information resulting from research in
the many different disciplines involved has
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grown rapidly. Moreover, the character of
the information available and sought by
the users of this information has changed
and expanded. The IGWMC has re-
sponded to this challenge by focusing re-
search on code performance evaluation
and the improvement of information sys-
tems. Currently, a microcomputer-based
database containing descriptions of more
than 450 ground-water modeling codes is
being tested. Plans are under develop-
ment to bring this information system to
the graphic microcomputer-based environ-
ment.
In the meantime, ground-water model-
ing continues to evolve. A wide range of
flow characterizations are now possible.
Many of the flow models include options
for various types of time-varying boundary
conditions and have the ability to handle a
wide variety of hydrologic processes such
as evapotranspiration, stream-aquifer ex-
changes, spatial and temporal variations
in areal recharge and pumping or recharg-
ing wells. Some models have options to
change the field parameters during the
simulation runs, thus recognizing the po-
tential influence of contaminants on the
hydraulic parameters. Due to significant
improvements in the mathematical formu-
lation of the soil hydraulic characteristics,
the treatment of boundary conditions, and
the numerical solution methods employed,
models for simulating flow in the unsatur-
ated zone have become more accurate,
realistic, and reliable.
However, il may be argued that the
progress in understanding the transport
and fate of contaminants has not yet re-
sulted in a significant increase in the ap-
plicability of models to contamination
problems. As the complexity of the phys-
ics and chemistry involved in the interac-
tion between water, soil/rock matrix and
the multi-component (sometimes immis-
cible) contaminant mixtures has not yet
been resolved, models are lacking to ad-
equately simulate many of the contami-
nant problems encountered in the field.
The same conclusion might be drawn
for modeling flow and transport in frac-
tured rock systems. Improved site charac-
terization and stochastic analysis of
fracture geometry, together with an im-
proved capability to describe the interac-
tions of chemicals between the active and
passive fluid phases and the rock matrix,
have facilitated increasingly realistic simu-
lation of real-world fractured rock systems.
However, lack of practical field charac-
terization methods still impedes the rou-
tine use of such models in support of
management's decision-making.
Finally, developments most promising
for practical application may be found in
the area of parameter estimation. Various
geostatistic and stochastic approaches
have become available together with new
or updated parameter estimation models
and are increasingly used in the field,
specifically in determining the distribution
of hydraulic parameters.
References
National Research Council (NRC). 1990.
Ground Water Models—Scientific
and Regulatory Applications. Na-
tional Academy Press, Washington,
D.C.
van der Heijde, P.K.M., A.I. El-Kadi, and
Stan A. Williams. 1988. Groundwa-
ter Modeling: An Overview and Sta-
tus Report. EPA/600/2-89/028, U.S.
EPA, R.S. Kerr Environmental Re-
search Laboratory, Ada, Oklahoma.
P. K. M. van der Heijde is with the Colorado School of Mines. Golden, CO 80401, and
Osman A. Elnawawy is with the Indiana University/Purdue University at India-
napolis, Indianapolis, IN 46204.
Joseph R. Williams is the EPA Project Officer (see below).
The complete report, entitled "Compilation of Ground-Water Models," (Order No.
PB93-209401; Cost: $44.50, 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
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
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