United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada, OK 74820
Research and Development
EPA/600/S2-91/062 Mar. 1992
EPA Project Summary
A Model of Virus Transport in
Unsaturated Soil
M.V. Yates, S. R. Yates, and Y. Ouyang
As a result of the recently-proposed
mandatory ground-water disinfection
requirements to inactivate viruses in
potable water supplies, there has been
increasing interest in virus fate and
transport in the subsurface. Several
models have been' developed to pre-
dict the fate of viruses in groundwater,
but few include transport in the unsat-
urated zone, and all require a constant
virus inactivation rate. These are seri-
ous limitations in the models, as it has
been well documented that consider-
able virus removal occurs in the unsat-
urated zone, and that the inactivation
rate of viruses is dependent on envi-
ronmental conditions. The purpose of
this research was to develop a predic-
tive model of virus fate and transport
in Unsaturated soils that allows the vi-
rus inactivation rate to vary based on
changes in soil temperature. The model
was developed based on the law of
mass conservation of a contaminant in
porous media and couples the flow of
water, viruses, and heat through the
soil. Model predictions were compared
to measured data of virus transport in
laboratory column studies, and were
within the 95% confidence limits of the
measured concentrations. The model
should be a useful tool for anyone wish-
ing to estimate the number of viruses
entering ground water after traveling
through the soil from a contamination
source. In addition, model simulations
were performed to identify variables
that have a large effect on the results.
This information can be used to help
design experiments so that important
variables are measured accurately.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory, Ada, OK, to an-
nounce 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
The significance of viruses as agents of
ground-waterborne disease in the United
States has been well documented. The
increasing interest in preventing ground-
water contamination by viruses and other
disease-causing microorganisms has led
to new U.S. Environmental Protection
Agency proposed regulations regarding
ground-water disinfection, the development
of wellhead protection zones, and stricter
standards for the microbiological quality of
municipal sludge and treated effluent that
is applied to land. For many of the new
regulations, a predictive model of virus (or
bacterial) transport would be helpful in the
implementation process. For example,
such a model could be used to determine
where septic tanks could be placed or
where land application of sludge or efflu-
ent could be practiced relative to drinking
water wells to minimize negative impacts
on the ground-water quality. Another ap-
plication of microbial transport models is
related to the ground-water disinfection
rule. Water utilities wishing to avoid
ground-water disinfection may use a patho-
gen transport model to demonstrate that
adequate removal of viruses in the source
Printed on Recycled Paper
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water occurs during transport to the well-
head.
Several models of microbial transport
have been developed during the past 15
to 20 years . The models range from the
very simple, requiring few input param-
eters, to the very complex, requiring nu-
merous Input parameters. For many of
the more complex models, the data re-
quired for input are not available except
for very limited environmental conditions.
They may be useful for research purposes,
but would be impractical for widespread
use. The potential applications of these
models also range considerably, from
being useful,only for screening purposes
on a regional scale, to predicting virus
behavior at one specific location.
One limitation of almost all of these
models is that they have been developed
to describe virus transport in saturated
soils (i.e., ground water). However, it has
been demonstrated many times that the
potential for virus removal is greater in the
unsaturated zone than in the ground wa-
ter. Neglecting the unsaturated zone in
any model of virus transport could lead to
inaccurately high predictions of virus con-
centrations at the site of interest. This
omission would be especially significant in
areas with thick unsaturated zones, such
as those in many western states. The
one transport model that has reportedly
been developed for predicting virus trans-
port in variably saturated media is not
specific for viruses, but can be used for
any contaminant. In addition, it has not
been tested using data of virus transport
in unsaturated soil.
Another, and more important, limitation
of published models of virus transport is
that none of them has been validated us-
ing actual data of virus transport in unsat-
urated soils. Most models are developed
based on theory, and are fitted to data
obtained from one or two experiments.
Rarely are they tested by applying the
model to data collected under a variety of
conditions and then determining how well
the model predicts what has been ob-
served in the laboratory or field without
any fitting or calibration of the model.
Transport Processes
The transport of viruses through a po-
rous medium such as soil is affected pri-
marily by the following mechanisms and
processes: Advection; Hydrodynamic dis-
persion; Adsorption (and desorption); Fil-
tration; and Inactivation.
Factors Affecting Transport
Processes
The transport of viruses through soil is
controlled by climatic conditions such as
the rate of rainfall (or water application)
and evaporation and by soil properties
such as soil water content, soil tempera-
ture, adsorption and desorption, filtration,
soil pH, and salt concentration. The prop-
erties of the specific virus of interest are
also important in determining its behavior
in the subsurface. Some of the most
important factors that affect the transport
of viruses through soil include soil water
content, soil temperature, the rate of wa-
ter application and evaporation , and soil
heterogeneity.
Objectives
The purpose of this research was to
develop a model that can be used to pre-
dict virus movement from a contamination
source through unsaturated soil to the
ground water. "Several mocfel simulations
were performed to determine the effects
of different input variables on model pre-
dictions. The model was tested by com-
paring model prediction to results of labo-
ratory studies.
The specific objectives of this project
were:
1. To develop a mathematical model to
describe the transport of viruses in
unsaturated soil that includes fac-
tors specific to viruses, and
2. To test model predictions with ex-
perimental data of virus transport in
soil.
Transport equations were derived to
describe the simultaneous transport of
water, viruses, and heat for a soil profile.
VIRTUS: A Model of VIRUS
Transport in Unsaturated Soil
The mathematical model developed is
entitled VIRTUS (VIRus Transport in Un-
saturated Soil), and programmed in FOR-
TRAN for use on IBM and IBM-compatible
PCs. In the Project Report there is a
document describing the use of the pro-
gram which is located in Appendix III.
Sample input and output data that can be.
used to test the model are listed in Ap-
pendix IV. The mathematical model and
corresponding computer program,
VIRTUS, are demonstrated in a variety of
situations in the Project Report. The po-
tential applications of this model and its
limitations are also discussed.
Model Applications and
Limitations
Some of the features of this model in-
clude its ability to simulate:
1. unsteady flow in variably-saturated
media
2. transport in layered soils
2
3. variable virus inactivation rate (e.g.,
function of temperature)
4. different virus inactivation rates for
adsorbed versus freely suspended
virus particles
5. the flow of heat through soil (which
affects water flow, virus inactivation
rate, etc.)
Discussion
The ultimate measure of a model's use-
fulness as a predictive tool is its ability to
accurately predict field observations of vi-
rus transport under a variety of environ-
mental conditions. However, most mod-
els that have been developed to predict
microbial transport have not been tested
using field or laboratory data. There are a
few exceptions to this (e.g., Teutsch et
al., 1991; Harvey and Garabedian, 1991).
However, both of these models were de-
veloped for use by the investigators in
order to simulate their own data. In the
case of the colloid filtration model of
Harvey and Garabedian, extensive fitting
of the required input parameters was per-
formed by calibrating different solutions of
the transport equation to the observed
bacterial breakthrough curves. Thus, while
these models may be able to simulate the
investigator's data reasonably well, they
may not be able to predict the results of
other investigator's transport experiments.
If a model is to be used for purposes
other than research, such as for commu-
nity planning or for making regulatory de-
cisions, it must be able to predict micro-
bial transport using data obtained by any-
one under a wide range of environmental
conditions.
In this research a model to describe
virus transport was developed based on
the factors known to affect virus fate in
the subsurface. A survey of the literature
was conducted to locate data sets in which
the investigators made measurements of
not only virus properties, but also soil and
hydraulic properties. Two data sets were
located and used to test VIRTUS. No
fitting or calibration of the model was per-
formed; the data and measurements as
reported by the respective investigators
were used as model input. Model predic-
tions compared favorabry to measured ex-
perimental data as predictions were within
the 95% confidence limits of the mea-
sured data. However, only one compari-
son to one laboratory transport study in
unsaturated soil using a single soil type
and a single virus type was performed.
In addition, the temperature-dependent
inactivation rate capabilities of the model
could not ba tested by comparison to ex-
perimental data. This is due to the fact
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that the experiments were conducted un-
der constant temperature conditions in the
laboratory, thus the virus inactivation rate
remained constant (theoretically) through-
out the course of the experiment. In
order to test the model's capacity to cal-
culate new virus inactivation rates as a
function of the changing soil temperature,
data from a laboratory study in which the
temperature, is allowed to change (and is
closely monitored) or from a field study in
which the temperature is monitored will be
required. This will allow an assessment
of the model's capability to accurately cal-
culate heat flow through the soil, which
affects water flow (and thus virus trans-
port) as well as the rate of virus inactiva-
tion during transport. More testing of the
model is required before using it for any
purposes other than research.
Conclusions
This research project has resulted in
the development of a mathematical model
that can be used to predict virus (or bac-
terial) transport in unsaturated soils. The
model allows the user to specify the virus
inactivation rate as a function of soil tem-
perature or any other input parameters. It
will also allow the user to specify different
inactivation rates for adsorbed versus
freely suspended virus particles, if that
information is available.
A sensitivity analysis of the model indi-
cated that the inactivation rate of the virus
has a large effect on model predictions.
The adsorption coefficient and dispersivity
also affect model predictions, although to
a smaller extent.
Model predictions compared favorably
to two data sets against which the model
was tested. However, there is a lack of
data available for extensive model testing.
No complete data sets from field transport
experiments were found that could be used
to test VIRTUS. Before the model can be
used for any purposes other than research,
it should be extensively tested using ac-
tual field data.
In its present condition, the model re-
quires the user to input several pieces of
information related to climatic conditions.
It also requires a large amount of informa-
tion characterizing the physical properties
of the soil, as do most models of contami-
nant transport. Before VIRTUS could be
used for purposes other than research, a
user interface, extensive help facilities, and
a library of soil and virus properties would
have to be added to the model.
Harvey, R.W., and S.P. Garabedian. 1991.
Use of colloid filtration theory in model-
ing movement of bacteria through a con-
taminated sandy aquifer. Environ. Sci.
Techno!. 25:178-185.
Teutsch, G., K. Herbold-Paschke, D.
Tougianidou, T. Hahn, and K.
Botzenhart. 1991. Transport of micro-
organisms in the underground - pro-
cesses, experiments, and simulation
models. Wat. Sci. Tech. 24:309-314.
•U.S. Government Printing Office: 1992 — 648-060/60045
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M.V. Yates and Y. Ouyang are with the University of California, Riverside, CA
92501; and S. R. Yates is with the USDA/ARS, U.S. Salinity Laboratory,
Riverside, CA 92501.
David M. Walters is the EPA Project Officer (see below).
The complete report, entitled "A Model of Virus Transport in Unsaturated Soil;"
(OrderNo.PB92-119 957/AS; Cost: $26.00; 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
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/062
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