\ 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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EPA/600/S2-89/028

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