United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/SR-94/028 May 1994
& EPA Project Su m mary
Identification and Compilation of
Unsaturated/Vadose Zone Models
Paul K.M. van der Heijde
Many ground-water contamination
problems are derived from sources at or
near the soil surface. Consequently, the
physical and (bio-)chemical behavior of
contaminants in the shallow subsurface is
of critical importance to the development
of protection and remediation strategies.
Mathematical models, representing our
understanding of such behavior, provide
tools useful in assessing the extent of
pollution problems and evaluating means
to prevent and remediate them.
In classifying models generally applied
to soil- and ground-water pollution
problems, a distinction can be made
between the transport of the contaminants
from the point of their introduction into the
subsurface (i.e., contaminant source) to
the location of concern (i.e., point of
exposure), and the (bio-)chemical
transformations that may occur in the
subsurface. Models specifically simulating
fluid flow are referred to as flow models.
Models describing the movement of
dissolved chemicals and their interaction
with the soil or rock matrix in terms of
concentrations and mass fluxes are often
referred to as contaminant transport
models or solute transport models.
Furthermore, somevadose modelsfocus
on the resulting fate of contaminants, in
particular, simulating the (bic-)chemical
changes and transformations that occur in
the subsurface. Increasingly, combinations
of these three model types are employed to
adequately simulate site-specific pollution
problems and their remediation.
To identify existing models foir simulation
of flow and contaminant transport in the
unsaturated subsurface, a database search
and literatu re reviewwere conducted. From
the review a catalogue was developed
consisting of approximately 100 flow and
transport models that may be used for the
simulation of flow and transportprocesses
in the unsaturated zone to determine the
effectiveness of soil remediation
schemes among other uses. The models
considered range from simple mass
balance calculations to sophisticated,
multi-dimensional numerical simulators.
There are six categories of models listed,
including models for single-fluid flow,
coupled and uncoupled flow and solute
and/or heat transport, and solute transport
for given pressure head distribution.
Finally, models are listed that provide soil
parameters from column experiments on
soil samples.
In the full report each model is described
in an uniform way by a set of annotations
describing its purpose, majorhydrological,
mathematicalandoperational characteristics,
input requirements, simulative capabilities,
level of documentation, availability, and
applicability. In some cases, the model
description includes comments made by
the model author and the investigator
concerning development, testing, quality
assurance and use, as well as references
of studies using the model and references
that are part of the documentation or
considered pertinent to the model.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory, Ada, OK, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Many contamination problems find their
cause at or nearthe soil surface. Consequently,
the physical and (bio-)chemical. behavior of
Printed on Recycled Paper
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these contaminants in the shallow subsurface
is of critical importance to the development of
protection and remediation strategies.
Mathematical models, representing our
understanding of such behavior, provide tools
useful in assessing the extent of pollution
problems and evaluating means to prevent
and remediate them.
Increasingly, detailed understanding and
subsequent modeling of the near-surface
zone are crucial in designing effective
remediation approaches. At many sites, this
near-surface zone is only partially saturated
with water, requiring specially designed
mathematical models. Thefull reportfocuses
on models that might prove useful in
simulating contaminant levels in such
partially saturated systems.
Modeling contaminant behavior in the
unsaturated zone is generally aimed to
address such Issues as [NRC, 1990]:
• Determining the arrival time of a
contaminant at a certain depth; this
requires a prediction of the travel time
forthecontaminant. Examples of depths
of interest are the bottom of the root
zone, the bottom of the treatment zone
of a hazardous waste land treatment
system facility, or the water table.
• Predicting the amount of the surface-
applied (or spilled) contaminant that
might arrive at the depth of interest
within a certain time (or mass flux
passing this depth); this requires
assessment of the transport,
(temporary) retention, transformation
and degradation (fate) of the
contaminant.
• Predicting the concentration distribution
and/or the contaminant mass flux in the
unsaturated zone (in both the aqueous
and solid phases) at a particulartime or
their changes over time.
The latter purpose is of specific interest to
this study as it relates to predicting the
amountof hazardous constituents remaining
in the soil following a soil remediation or due
to natural processes.
Contaminating chemicals may leave the
soil zone by leaching downwards to the
water table, by volatilization and escape to
the atmosphere, by (bio-)chemical
transformation or degradation, and by plant
uptake [Jury and Valentine, 1986]. Leaching
constitutes mass flow of a chemical
constituent and is the product of water flux
and dissolved chemical concentration. Mass
flow is dependent on the amount of applied
water, the water application intensity, the
saturated hydraulic conductivity of the soil,
the chemical concentration, the adsorption
sitedensity, and, indirectly, temperature [Jury
and Valentine, 1986]. Soils provide a strong
capacity for adsorbing chemicals and thus a
factor in determining the amount of chemical
available for mass flux. This is due to the
presence of electrically charged clay
minerals and organic matter and the large
surface areas of minerals and humus.
Volatilization of chemical to the atmosphere
takes place in the vapor phase of the soil
and is controlled by chemical, soil, and
atmospheric conditions. Volatilization is
dependent on Henry's constant, chemical
concentration, adsorption site density,
temperature, water content, wind speed,
and water evaporation. Other potentially
importanttransport processes include vapor
and liquid diffusion. Transformation and
degradation processes determine the "fate"
of the chemical of concern in the soil. The
most important processes include chemical
hydrolysis, (bio-)chemical transformations,
and oxidation-reduction.
In classifying models generally applied to
soil-and ground-water pollution problems, a
distinction can be made between the
transport of the contaminants from the point
of their introduction into the subsurface (i.e.,
contaminantsource) to the location of concern
(i.e., pointof exposure), and the (bio-) chemical
transformations that may occur in the
subsurface. A major transport mechanism
results from the hydrodynamic behavior of
contaminant carrying fluids or fluid phases in
porousorfractured media. Models specifically
simulating fluid flow are referred to as flow
models. Models describing the movement
of dissolved chemicals and their interaction
with the soil or rock matrix in terms of
concentrations and mass fluxes are often
referred to as contaminant transport models
or solute transport models. Furthermore,
some vadose models focus on the resulting
fate of contaminants, in particular, simulating
the (bio-)chemical changes and
transformations that occur in the subsurface.
The latter type of models may be based on
a simple mass balance approach for the
chemical of concern, lumping spatial
variations in a single valueforthe parameters
of interest (e.g., SUMMERS model; U.S.
EPA, 1989, pp. 28-29), or it may constitute a
set of complex equations describing the
(bio-)chemical reactions of interest including
a reaction constant database. Increasingly,
combinations of these three model types are
employed to adequately simulate site-specific
pollution problems and their remediation (e.g.,
Yehetal., 1993).
The success of a given model depends
on the accuracy and efficiency with which
the physical and (bio-)chemical processes
controlling the behavior of water and
introduced nonaqueous liquids and the
chemical and biological species they
transport are simulated. The accuracy and
efficiency of the simulation, in turn, depend
heavily on the applicability of the
assumptions and simplifications adopted
in the model, the availability and accuracy
of process information and site
characterization data, and subjective
judgments made by the modeler and
management.
As stated, flow models 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 or vapors such
as methane (in soil) or immiscible
nonaqueous phase liquids (NAPLs, in both
fully and partially saturated systems) such
as certain solvents, which may have a density
distinct from water (LNAPLs, DNAPLs). In
the context of the full report, only the flow of
water (under unsaturated conditions) is
considered. Most flow models are based on
a mathematical formulation that considers
the hydraulic system parameters as
independent field information and hydraulic
head, fluid pressure or water content and
fluid flux as dependent variables. They are
used to calculate the steady-state spatial
distribution, the changes in time in the spatial
distribution, or the temporal distribution at a
particular location of such variables as:
• hydraulic head, pressure head (or matric
head), and suction head;
• saturation or moisture content;
• magnitude and direction of flow in terms
of flow velocities or water mass fluxes;
• flowlines and travel times; and
• position of infiltration fronts.
Inverse flow models simulate the flow
field to calculate the spatial distribution of
unknown system parameters using field or
experimental observations on the state
variables such as hydraulic head, fluid
pressure, water content and fluid flux. Due
to the complexity of the relationships between
pressure head, saturation and hydraulic
conductivity, there are no truly inverse
models availableforflowin partially saturated
porous media.
The dominant parameter affecting flow
and contaminant transport in the unsaturated
zone is hydraulic conductivity. Since
hydraulic conductivity varies with water
content, accurate measurements of this
parameter are difficult to make and very
time-consuming. Therefore, theoretical
methods have been developed to calculate
the hydraulic conductivity from more easily
measured soil water retention data based
on statistical pore-size distribution models
[van Genuchten etal., 1991]. The resulting
functional relationship between pressure
head and volumetric water content (i.e., soil-
water retention function) is presented in
tabular form or as closed-form analytical
solutions that contain functional parameters
that are fitted to observed data. With the soil
water retention function known, the
unsaturated hydraulic conductivity can be
calculated using the model of Mualem [1976].
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Models have been developed to fit
mathematical functions to water retention
with known hydraulic conductivity or to water
retention and hydraulic conductivity
simultaneously [van Genuchten etal., 1991].
These models may also be used to predict
hydraulic conductivity forgiven soil retention
data.
Solute transport models are used to predict
movement or displacement, concentrations,
and mass balance components of water-
soluble constituents and to calculate
concentrations or radiological doses of
soluble radionuclides [van derHeijde etal.,
1988]. To do so, solute transport models
incorporate various relevant physical and
chemical processes. Flow is represented in
the governing convective(-dispersive)
equation by thefiow velocity in theadvective
transport term. The velocities are also used
for the calculation of the spreading by
dispersion. If the velocity field is stationary,
it may be either calculated once using an
external flow program or read into the
program as observed or interpreted data. If
the velocity field (i.e., spatial distribution of
velocities in terms of direction and
magnitude) is dependent on time and/or
concentration, then calculation of velocities
at each time step is required, either through
an internal flow simulation module or an
external flow model linked by means of input
and output files. If a dissolved contaminant
is present in relative high concentrations,
changes in its distribution during the
simulations might affect the flow behavior
through changes in the fluid density. In that
case, coupling of thefiow and solute transport
equations occurs through an equation of
state, resulting in a system of equations that
needs to be solved simultaneously (i.e.,
iteratively-sequentially [Huyakorn and
Finder, 1983]).
Generally, modeling the transformation
and fate of chemical constituents is
conducted in one of three possible ways
[van derHeijde etal., 1988]: (1) incorporating
simplified transformation orfateformulations
in the equation describing solute transport;
(2) formulating a mass-balance approach
to (bio-)chemical transformation and fate;
and (3) by coupling separate equations
describing the (bio-)chemical processes with
the advective-dispersive transport equation.
Including transformation processes in solute
transport models results in so-called
nonconservative (i.e., with respect to mass
in solution) transport and fate models. The
more complex of these nonconservative
transport models may include advective
and dispersive transport, molecular
diffusion, adsorption (equilibrium and
kinetics based), ion-exchange, radioactive
decay, and (bio-)chemical decay.
Mathematical Solutions in
Unsaturated/Vadose Models
Most mathematical models for the
simulation of flow and solute transport in the
unsaturated zone are distributecl-parameter
models, either deterministic or stochastic
[van der Heijde et al., 1988]. Their
mathematical framework consists of one or
more partial differential equations describing
the flow and/or transport and fate processes,
as well as initial and boundary conditions
and solution algorithms. Some of these
models assume that the processes active in
the system are stochastic in nature or, at
least, that the process variables may be
described by probability distributions. In such
stochastic models, system responses are
characterized by statistical distributions
estimated by solving a deterministic
governing equation.
The governing equations for flow and
transport in the unsaturated zone are usually
solved either analytically or numerically.
Analytical models contain a closed-form or
analytical solution of the field equations
subject to specified initial and boundary
conditions. To obtain these analytical
solutions, simplifying assumptions have to
be made regarding the nature of the soil-
water-solute system, geometry, and external
stresses, often limiting their application
potential. Because of the complex nature of
single- and multi-phase flow in the
unsaturated zone and the resulting
nonlinearity of the governing equation(s),
very few analytical flow solutions have been
published [Bear, 1979]. With respect to
transport and fate the situation is somewhat
different. Many one-, two-, and three-
dimensional analytical solutions for the
classical convection-dispersion equation
exist, often requiring a uniform flow field.
Some of these solutions, specifically one-
dimensional solutions, can be used in the
unsaturated zone assuming a uniform
vertical soil water flux.
In semi-analytical models, complex
analytical solutions are approximated, often
using numerical techniques. In the case of
unsaturated flow, semi-analytical solutions
may be derived by using analytical
expressions for the relationships between
the dependent variables and the hydraulic
parameters and involving numerical
integration [Bear, 1979]. Models based on a
closed-form solution for either the space or
time domain, and which contain additional
numerical approximations for the other
domain, are also considered semi-analytical
models. Various quasi-analytical techniques
and approximate (analytical) equations have
been developed for simulating infiltration of
water in soils [El-Kadi, 1983]. These
techniques are also used for the one-
dimensional transport of solutes [van
Genuchten and Alves, 1982].
In numerical models, a discrete solution is
obtained in both the space and time domains
by using numerical approximations of the
governing partial differential equation. As a
result of these approximations, the
conservation of mass and accuracy in the
prediction variable are not always assured
(becauseof truncation and round-off errors),
and thus, these need to be verified for each
application. Spatial and temporal resolution
in applying such models is user-defined. If
the governing equations are nonlinear, as is
thecase in simulating flow in the unsaturated
zone, linearization often precedes the matrix
solution [Remson et al., 1971; Huyakorn
and Finder, 1983]. Usually, solution of
nonlinear equations is achieved employing
nonlinear matrix methods such as the Picard,
Newton-Raphson, and Chord-Slope
methods [Huyakorn and Finder, 1983].
The numerical solution techniques used
for approximating the spatial components of
the governing flow equations in the
unsaturated zone are primarily the finite-
difference methods (FD), the integral finite-
differencemethods(IFDM),andtheGalerkin
finite-element method (FE). In most cases,
time is approximated by finite-difference
techniques resulting in an explicit, (weighted)
implicit or fully implicit solution scheme. A
finite-difference solution is obtained by
approximating the derivatives of the
governing equation. In the finite-element
approach an integral equation is formulated
first, followed by the numerical evaluation of
the integrals over the discretized flow or
transport domain. The formulation of the
solution in each approach results in a set of
algebraic equations that are then solved
using direct or iterative matrix methods.
Specific schemes may be required for the
constitutive relationships, specifically in the
presence of hysteresis.
There are many numerical considerations
in selecting a model for simulation of a
particular soil-water-solute system.
Simulating flow in relatively wet soils (e.g.,
nearly saturated conditions and ponding)
requires expression of the Richard's equation
in terms of hydraulic head, matric head or
suction head, especially when parts of the
modeled soil system becomefully saturated.
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However, application of this form of the
Richard's equation causes significant
convergence problems when simulating an
infiltration front in extremely dry soil
conditions; in the latter case, formulation of
Richard's equation should be based on
saturation or mixed pressure-saturation
[Huyakorn and Finder, 1983; Celia et al.,
1990], An advantage of the mixed form is
that it allows the transition from unsaturated
to saturated conditions while maintaining
numerical mass conservation [Celia et al.,
1990]. Also, significant mass balance
problems- might occur when site-specific
conditions result in highly nonlinear model
relationships [Celia et al., 1990].
Other issues that should be addressed in
selecting a model for simulating flow in the
unsaturated zone are the possible needs for
double precision versus single precision
variables, the time-stepping approach
incorporated, the definition used for intercell
conductance (e.g., harmonic mean versus
geometric mean), and, if present, the way
steady-state simulation is achieved (most
models do not provide steady-state flow
solutions). Some problems encountered with
specific models (or modeling techniques)
Include code limitations on gridding flexibility,
numerical problems in zones with high-
contrast soil or rock properties, and
Inaccuracy and instability inareas where the
flow field changes significantly in magnitude
and direction. In some cases, avoiding
inaccuracy and instability problems require
very small spatial and temporal increments,
making multi-dimensional simulations
expensive or even unfeasible. Sometimes,
an adaptive time-stepping scheme is
implemented in the computer program to
optimize time-step requirements.
Typical numerical techniques encountered
In solving the convective-dispersive solute
transport equation in the unsaturated zone
are comparable to those employed in
simulating solute transport in the saturated
zone and include various finite-difference
methods, the integral finite-difference
method, various Galerkin finite-element
formulations, and variants of the method of
characteristics [Yeh et al., 1993]. As with
flow, time is generally approximated by finite-
difference techniques resulting in an explicit,
(weighted) implicit or fully implicit solution
scheme.
Typical problems found in applying
traditional finite-difference and finite-element
techniques to simulatecontaminanttransport
in both the saturated and unsaturated zones
Include numerical dispersion and
oscillations. Numerical dispersion is referred
to when the actual physical dispersion
mechanism of the contaminant transport
cannot be distinguished from the front-
smearing effects of the computational
scheme [Huyakorn and Finder, 1983]. For
the finite-difference method, this problem
can be reduced by using the central
difference approximation. Spatial
concentration oscillations (and related
overshoot and undershoot) may occur near
a sharp concentration front in an advection-
dominated transport system. Remedies for
these problems are found to some extent in
the reduction of grid increments ortime-step
size, or by using upstream weighting for
spatial derivatives. The use of weighted
differences (combined upstream and central
differences) ortheselection of other methods
(e.g., the method of characteristics, and the
Laplace transform Galerkin method)
significantly reduces the occurrence of these
numerical problems.
Unsaturated/Vadose Model Data
Requirements
The number and type of parameters
required for modeling flow and transport
processes in soils depend on the type of
model chosen. These parameters can be
categorized as control parameters
(controlling the operation of the computer
code), discretization data (grid and time
stepping), and material parameters. The
material parameters can be grouped in six
sets [Jury and Valentine, 1986]: static soil
properties, water transport and retention
functions, basic chemical properties, time-
dependent parameters, soil adsorption
parameters, and tortuosity functions. Table
1 lists many of the relevant material model
parameters.
Selection of Models
To identify existing models for simulation
of flow and contaminant transport in the
unsaturated subsurface, a database search
and literature review have been conducted.
The database search was focused on the
MARS model annotation database of the
IGWMC, which contains about 650
descriptions of soil- and ground-water
simulation models. Information for the
Table 1. Selected Material Parameters for Flow and Transport Parameters in Soils
(After Jury and Valentine, 1986)
Static Soil Properties
porosity
bulk density
particle size
specific surface area
organic carbon content
cation exchange capacity
pH
soil temperature
Flow and Transport Variables and Properties
saturated hydraulic conductivity
saturated water content
matric head-water content function
hydraulic conductivity function
dispersion coefficient or dispersivity
Basic Chemical Properties
molecular weight
vapor pressure
water solubility
Henry's constant
vapor diffusion coefficient in air
liquid diffusion coefficient in water
octanol-water or oil-water partition
coefficient
half-life or decay rate of compound
hydrolysis rate(s)
Contaminant Source Characteristics
solute concentration of source
solute flux of source
source decay rate
Time Dependent Parameters
water content
water flux
infiltration rate
evaporation rate
solute concentration
solute flux
solute velocity
air entry pressure head
volatilization flux
Soil Adsorption Parameters
distribution coefficient
isotherm parameters
organic carbon partition coefficient
Tortuosity Functions
vapor diffusion tortuosity
liquid diffusion tortuosity
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literature review has been obtained from
various sources, including the IGWMC
literature collection of more than 3000 titles
and about 20 serials, and through interlibrary
loan. Additional information was received
from the U.S. EPA Center for Subsurface
Modeling Support (CSMoS) located at
RSKERL, Ada, Oklahoma. After reviewing
the model's documentation and other
pertinent literature, additional information
was obtained from model authors and code
custodians when necessary.
From the review a catalogue was
developed consisting of approximately 100
flow and transport models that may be used
for the simulation of flow and transport
processes in the unsaturated zone to
determine the effectiveness of soil
remediation schemes among other uses.
The models considered range from simple
mass balance calculations to sophisticated,
multi-dimensional numerical simulators.
There are six categories of models listed,
including models for single-fluid flow, coupled
and uncoupled flow and solute and/or heat
transport, and solute transport for given
pressure head distribution. Finally, models
are listed that provide soil parameters from
column experiments on soil samples.
In the full report each model is described in
a uniform way by a set of annotations defining
the purpose and major hydrologic,
mathematical and operational characteristics,
input requirements, simulative capabilities,
level of documentation, availability, and
applicability. In some cases, the model
description includes comments made by the
model author and IGWMC staff concerning
development, testing, quality assurance and
use, as well as references of studies using
the model and references that are part of the
documentation or considered pertinent to
the model.
It should be noted that the full report does
not pretend to be complete in its listing of
appropriate models. Moreover, many codes
have been developed primarily for research
purposes and are not readily available. Also,
there are many simple models based on
mass balance evaluation or analytical
solution of highly simplified systems not
presented in this catalogue. An effort has
been made to select those 'simple' models
that either are known for their use in a
regulatory or enforcement mode or that are
considered representative for a certain type
of model. The report does not discuss multi-
fluid flow and associated transport of
contaminants since a considerable amount
of research is currently focussed on
understanding and mathematically
describing the physics and chemistry of these
systems.
References
Bear, J. 1979. Hydraulics of Groundwater.
McGraw-Hill, New York, NY.
Celia, M.A., E.T. Bouloutas, and R.L Zarba.
1990. A Genera! Mass-Conservative
Numerical Solution for the Unsaturated
F'low Equation. Water Resources Res.,
Vol. 26(7), pp. 1483-1496.
El-Kadi, A.I. 1983. Modeling Infiltration for
Water Systems. GWMI 83-09,
International Ground Water Modeling
Center, Holcomb Research Institute,
Indianapolis, IN.
Huyakorn, P.S., and G.F. Pinder. 1983.
Computational Methods in Subsurface
Flow. Academic Press, New York, NY.
Jury, W.A., and R.L. Valentine. 1986.
Transport Mechanisms and Loss
Pathways for Chemicals in Soil. In: S.C.
Hern and S.M. Melancon (eds.), Vadose
Zone Modeling of Organic Pollutants,
Lewis Publishers, Inc., Chelsea, Ml, pp.
37-60.
Mualem, Y. 1976. A New Model for Predicting
the Hydraulic Conductivity of Unsaturated
Porous Media. Water Resources Res.,
Vol. 12(3), pp. 513-522.
National Research Council (NRC). 1990.
Ground Water Models—Scientific and
Regulatory Applications. National
Academy Press, Washington, DC.
Remson, I., G.M. Hornberger, and F.J. Molz.
1981. Numerical Methods in Subsurface
Hydrology. Wiley Interscience, New York,
NY.
U.S. Environmental Protection Agency.
1989. Determining Soil Response Action
Levels Based on Potential Contaminant
Migration to Ground Water: A
Compendium of Examples. EPA/540/2-
89/057, Office of Emergency and
Remedial Response, Washington, DC.
van der Heijde, P.K.M., A.I. El-Kadi, and
S.A. Williams. 1988. Groundwater
Modeling: An Overview and Status Report.
EPA/600/2-89/028, U.S. Environmental
Protection Agency, R.S. Kerr
Environmental Research Lab., Ada, OK.
van Genuchten, M.Th., and W.J. Alves. 1982.
Analytical Solutions of the One-
Dimensional Convective-Dispersive
Solute Transport Equation. Techn. Bull.
1661, U.S. Dept. of Agriculture, Riverside,
CA.
van Genuchten, M.Th., F.J. Leij, and S.R.
Yates. 1991. The RETC Code for
Quantifying the Hydraulic Functions of
Unsaturated Soils. EPA/600/2-91/065,
U.S. Environmental Protection Agency,
R.S. Kerr Environmental Research Lab.,
Ada, OK.
Yeh, T.-C., R. Srivastava, A. Guzman, and
T. Harter. 1993. A Numerical Model for
Water Flow and Chemical Transport in
Variably Saturated Porous Media. Ground
Water, Vol. 31 (4), pp. 634-644.
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Paul K.M. van der Heijde is with the international Ground Water Modeling Center,
Institute for Ground-Water Research and Education, Colorado School of Mines,
Golden, CO 80401-1887.
Joseph Williams is the EPA Project Officer (see below).
The complete report, entitled/'Identification and Compilation of UnsaturatedA/adose
Zone Models," (Order No. PB 94-157773; Cost: $27.00, 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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EPA
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EPA/600/SR-94/028
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