-fctl
EPA/600/2-90/010 March 1990
The Use of Models for Granting Variances
from Mandatory Disinfection of Ground
Water Used as a Public Water Supply
Marylynn V. Yates
Department of Soil and Environmental Sciences
University of California
Riverside, California 92521
Interagency Agreement DW1293380
USDA/ARS U.S. Salinity Laboratory
Riverside, California 92521
Project Officer
David M. Walters
Processes and Systems Research Division
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
Robert S. Kerr Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Ada, Oklahoma 74820
-------
Foreword
EPA Is charged by Congress to protect the nation's land, air and water systems. Under a mandate of national
environmental laws focused on air and water quality, solid waste management and the control of toxic substances,
pesticides, noise and radiation, the Agency strives to formulate and Implement actions which lead to a compatible
balance between human activities and the ability of natural systems to support and nurture life.
The Robert S. Kerr Environmental Research Laboratory Is the Agency's center of expertise for Investigation of the
soil and subsurface environment. Personnel at the Laboratory are responsible for management of research
programs to: (a) determine the fate, transport and transformation rates of pollutants in the soil, the unsaturated and
the saturated zones of the subsurface environment; (b) define the processes to be used in characterizing the soil and
the subsurface environment as a receptor of pollutants; (c) develop techniques for predicting the effect of pollutants
on ground water, soil, and indigenous organisms; and (d) define and demonstrate the applicability and limitations of
using natural processes, Indigenous to soil and subsurface environment, for the protection of this resource.
In November 1985, a Maximum Contaminant Level Goal of zero viruses in drinking water was published. By 1991,
the U.S. Environmental Protection Agency Office of Drinking Water expects to promulgate regulations requiring that
all ground water used for potable purposes be disinfected prior to distribution instead of requiring monitoring for
viruses. This document discusses the possibility of using a virus transport model for granting variances from this
requirement. The current state of knowledge in the area of virus transport Is reviewed, and the information needed to
model virus transport is examined. Two different approaches to modeling virus transport are described, including
data requirements, model outputs, and limitations of the model. Several areas in which research needs to be
performed In order to use models of virus transport for granting variances from the disinfection requirement are
presented.
Clinton W. Hall
Director
Robert S. Kerr Environmental Research Laboratory
-------
Contents
FOREWORD ii
INTRODUCTION 1
Importance of the Problem 1
Properties of Viruses 1
Sources of Viruses In the Environment 2
PREVENTING VIRUS CONTAMINATION OF DRINKING WATER 3
Alternatives to the Disinfection Requirement 3
THE USE OF MODELS IN GRANTING VARIANCES FROM A MANDATORY
GROUND-WATER DISINFECTION REQUIREMENT 4
Factors Controlling the Fate of Viruses in the Subsurface 5
Examples of Virus Transport Models 5
Geostatistical Model 5
Background 5
Data Input Requirements 5
Model Output 5
Limitations 9
Advection-Dispersion Contaminant Transport Model 9
Background 9
Data Input Requirements 10
Model Output 10
Limitations 10
Discussion 10
Questions to be Considered 11
CONCLUSIONS 11
REFERENCES 12
-------
-------
INTRODUCTION
In November, 1985, the United States Environmental
Protection Agency (U.S.EPA) proposed a Maximum
Contaminant Level Goal (MCLG) for viruses in drinking
water, setting a level of zero viruses (Federal Register,
1985). For various technical and economic reasons,
monitoring the water for the presence of viruses most likely
will not be required; instead, various treatment techniques
to eliminate or reduce virus contamination of drinking water
were discussed. At that time, mandatory filtration and
disinfection of surface water and disinfection of ground
water were discussed as possible treatment requirements.
Since then, treatment techniques for surface water have
been proposed, and in June 1989, the final rule requiring
surface water sources to be filtered and disinfected was
published.
To this date, treatment techniques for ground water have
not been proposed. It is expected that mandatory
disinfection of ground-water sources of drinking water will
be proposed in the near future. The purpose of this
document is to discuss some of the conditions under which
it might be possible to use models to grant a variance from
this requirement.
Importance of the Problem
Ground water supplies over 100 million Americans with their
drinking water; in rural areas there is an even greater
reliance on ground water as it comprises up to 95% of the
water used (Bitton and Gerba, 1984). It has been assumed
traditionally that ground water is safe for consumption
without treatment because the soil acts as a filter to remove
pollutants. As a result, private wells generally do not
receive treatment (DiNovo and Jaffe, 1984), nor do a large
number of public water supply systems that use ground
water (U.S. Public Health Service, 1965). However, the use
of contaminated, untreated or inadequately treated ground
water has been the major cause of waterborne disease
outbreaks in this country since 1920 (Craun, 1986a,b).
Between 1920 and 1980,1405 waterborne outbreaks were
reported in the United States, involving over 386,000 people
and resulting in 1083 deaths (Craun, 1986a). In 1981,
1982, and 1983; there were 112 reported waterborne
outbreaks and 28,791 cases of illness associated with
drinking water (Craun, 1986a). Since 1971, the average
annual number of reported outbreaks has increased: during
1971-1975, an average of 25 outbreaks was reported; from
1976 to 1983, this number increased to 40. The increase in
reported numbers of outbreaks may be due to an improved
system for reporting implemented in 1971 (Craun, 1985),
however, it is still believed that only a fraction of the total
number of outbreaks is reported (Lippy and Waltrip, 1984).
When considering outbreaks that have occurred due to the
consumption of contaminated, untreated or inadequately
treated ground water from 1971-1982, the most commonly
identified disease-causing agents were Shigellae (a group
of bacteria) and hepatitis A virus (See Table 1). Hepatitis A
virus was responsible for 7.8% of the reported ground-
waterborne disease outbreaks; in all, viruses were identified
as the disease-causing agents in 11.2% of the outbreaks.
In almost two-thirds (64.7%) of the outbreaks, no causative
agent could be identified, and the illness was simply listed
as gastroenteritis of unknown cause.
The difficulty in the detection and isolation of many human
enteric viruses from clinical and environmental samples
probably accounts for the limited number of viruses
identified as causes of waterborne disease. As methods for
the detection of enteric viruses have improved, so has the
percentage of waterborne disease identified as having a
viral origin (Gerba, 1984). Using newer identification
methods, studies of outbreaks that occurred from 1976
through 1980 for which no cause was identified at the time
of the outbreak indicated that 42% of these outbreaks (i.e.,
the 64.7% for which no causative agent was identified) were
caused by the Norwalk virus (Kaplan et al., 1982). Thus, it
has been suggested that the Norwalk virus is responsible
for approximately 23% of all reported waterborne outbreaks
in the United States (Keswick et al., 1985). Adding this
23% to the 11.2% in which viruses were identified as the
causative agents reveals that viruses may be responsible
for one-third of all the waterborne disease outbreaks
that occur In this country.
Currently, drinking water is monitored for the presence of
total coliform bacteria, which are used as indicators of the
presence of pathogenic (disease-causing) microorganisms.
However, several studies have shown that the absence of
coliforms does not guarantee that the water is free from
viruses (Gerba et al., 1985). There have also been virus-
caused waterborne disease outbreaks in which the water
involved met the coliform standards (Federal Register,
1985). There are several reasons why coliforms are not
good indicators for the presence or absence of viruses.
One is that viruses are generally more resistant to
inactivation by various treatment processes (chlorination,
heat, etc.) than are coliform bacteria. In addition, viruses
are much smaller than bacteria and thus are able to travel
greater distances through soil than most bacteria. For
these reasons, it is necessary to study the viruses
themselves or find a better indicator of their presence than
coliforms.
Properties of Viruses
Some of the important properties of viruses are:
• They are very small, ranging in size from approximately
20 to 200 nm (1 nm = 10'9 m) in diameter.
• They are obligate intracellular parasites; that Is, they
are incapable of replication outside of a host organism.
This means that, once in the environment, they cannot
normally increase in number. This is different from
bacteria, which can grow and multiply if the proper
nutrients and environmental conditions are present.
-------
Tabla 1. Causative Agents of Waterborne Disease Outbreaks In Untreated or Inadequately Treated Ground-Water Systems,
1971-1982
Outbreaks
Disease
Gastroenteritis, unknown
cause
Shfgellosls
Hepatitis A
Chemical poisoning
Viral gastroenteritis
Glardlasls
Typhoid fever
Salmonellosis
Ysrslntosls
E.coll diarrhea
TOTAL
Number
132
20
16
12
7
7
4
4
1
1
204
% of total
64.7
9.8
7.8
5.9
3.4
3.4
2
2
0.5
0.5
100
Illnesses
Number
25700
4938
493
157
1363
96
222
352
16
1000
34337
% of total
74.85
14.38
1.44
0.46
3.97
0.28
0.65
1.03
0.05
2.91
100
• They are very host specific. In other words, a virus that
infects humans cannot, generally speaking, infect any
other animals. This means that if a human virus is
found in soil or water, there is conclusive proof that a
source of human waste has contaminated the environ-
ment.
• Viruses that replicate In the intestinal tract of man are
referred to as human enteric viruses. These viruses
are shed In the fecal material of Individuals who are
infected either purposely (i.e., by vaccination) or
Inadvertently by consumption of contaminated food or
water, swimming In contaminated water, or person to
person contact with an infected individual.
• More than one hundred different enteric viruses may be
excreted In human fecal material (Melnlck and Gerba,
1980); as many as one million Infectious units of
enteroviruses (a subgroup of the enteric viruses) per
gram and 10 billion rotavlruses per gram may be
present In the feces of an Infected individual (Tyrrell
and Kapiklan, 1982).
• Because they are shed In the fecal material of infected
Individuals, viruses are present in domestic sewage
and, depending on the type of treatment process(es)
used, between 50 and 99.999% of the viruses are
inactivated during sewage treatment (Gerba, 1981).
• Most of the viruses involved in waterborne disease
outbreaks cause gastroenteritis. The symptoms of
gastroenteritis include diarrhea, nausea, vomiting,
fever, and general malaise. Probably the most serious
disease caused by a waterborne virus is hepatitis,
which Is caused by hepatitis A virus.
• Viruses are generally more resistant to inactivation by
various disinfection techniques than are bacteria.
• The number of viruses required to cause disease is
very low. It has been estimated that exposure to only
one virus particle may be sufficient to result in infection.
Sources of Viruses in the Environment
Viruses may be introduced into the subsurface environment
in a variety of ways. Goyal et al. (1984) isolated viruses
from the ground water beneath cropland being irrigated with
sewage effluent. Viruses have been detected in the ground
water at several sites practicing land treatment of
wastewater; these cases were reviewed by Keswick and
-------
Gerba (1980). The burial of disposable diapers in sanitary
landfills is a means by which disease-causing micro-
organisms in untreated human waste may be introduced
into the subsurface. Vaughn et al. (1978) detected viruses
as far as 408 m downgradient of a landfill site in New York.
Land application of treated sewage effluent for the purposes
of ground-water recharge has also resulted in the intro-
duction of viruses to the underlying ground water (Vaughn
andLandry,1977;1978).
Septic tank effluent may be the most significant source of
pathogenic bacteria and viruses in the subsurface
environment. Septic tanks are the source of approximately
one trillion gallons of waste disposed to the subsurface
every year (Office of Technology Assessment, 1984) and
are the most frequently reported sources of ground-water
contamination in waterborne disease outbreaks (U.S. EPA,
1977). The overflow or seepage of sewage, primarily from
septic tanks and cesspools, was responsible for 43% of the
reported outbreaks and 63% of the reported cases of illness
caused by the use of untreated water (Craun, 1985).
PREVENTING VIRUS
CONTAMINATION OF
DRINKING WATER
The importance of viruses as agents of waterborne disease
in this country led to the establishment of an MCLG for
viruses in drinking water. As stated previously, it is
expected that the EPA will propose a mandatory disinfection
requirement for all ground waters used for potable purposes
to inactivate any viruses that may be present in that water.
This requirement will place a heavy economic burden on the
numerous small communities as well as many large
communities that rely heavily on ground water for drinking
purposes but do not routinely treat that water prior to
distribution.
Chlorination is probably the most commonly practiced
means of disinfecting water. However, there are potential
health hazards associated with the chlorinatipn of both
surface and ground waters. Naturally occurring humic
substances such as humic and fulvic acids may react with
free chlorine, producing compounds known as trihalo-
methanes (THMs). The predominant THMs produced in
drinking water are chloroform and bromodichloro-methane,
although dibromochloromethane and bromoform may also
be produced (Craun, 1986c). Chloroform, which is the most
commonly produced compound, has been shown to be a
carcinogen in mice and rats at high dose levels. It has been
estimated that an incremental risk of 3 to 4 cancers per
10,000 population may be associated with the consumption
of 2 liters of water containing 0.10 mg/liter chloroform daily
for 70 years (Craun, 1986c). There have been studies
conducted to try to determine whether consumption of
chlorinated water is linked with the incidence of various
types of cancer. Craun (1986c) reviewed two such studies
which suggested an association between bladder and colon
cancer and the consumption of chlorinated water over a
long period of time for nonsmokers and for a moderate
period of time in an elderly population. He cautions,
however, that more studies are necessary before these
conclusions can be extended to other population groups.
There are many other drinking water disinfection techniques
available, including ozonation and reverse osmosis. These
methods are generally more expensive than Chlorination,
and the by-products of these processes are, in many cases,
unknown. The risks associated with the consumption of
virus-contaminated water must be weighed against the risks
of consuming water containing cancer-causing trihalo-
methanes or the costs of other treatment methods and the
presence of by-products. In view of the risks posed by
Chlorination and the expense associated with other
treatment processes, consideration should be given to
granting variances to the mandatory disinfection
requirement for ground-water sources of drinking water.
Alternatives to the Disinfection
Requirement
There are several alternatives to disinfection which could be
considered as criteria for granting variances. Five possible
alternatives will be discussed in detail below.
1) Monitor the water for the presence of viruses.
The techniques currently used to detect viruses in
drinking water are time-consuming, expensive, and
require highly trained personnel. Large volumes of
water, up to 10,000 liters, are passed through a filter.
The filter is then taken to the laboratory where it is
processed to remove any viruses adhering to the filter
medium. The concentrated sample is then placed on
live monkey kidney cells, which are incubated for up to
six weeks to allow adequate time for any viruses
present to infect the cells and produce visible signs of
infection. At the end of six weeks, if no signs of
infection have appeared, it is unlikely that there are
viruses of a certain group in that water sample.
One problem with this technique is that several
different types of cells would have to be used to detect
the common waterborne viruses, as not all viruses can
grow in one cell type. Another problem is that there is
currently no known cell type in which the Norwalk and
related viruses can be grown. This group of viruses is
thought to be responsible for a large proportion of the
waterborne disease outbreaks that occur in this
country.
In addition to not being able to detect some of the
important viruses, the length of time required to obtain
a result makes this technique of little use as far as
warning the public that their water is contaminated. By
the time the viruses have been detected, it may be too
-------
late to prevent Illness In persons who have consumed
the water.
The development of gene probes may make It feasible
to monitor for viruses In a timely and relatively
Inexpensive manner In the foreseeable future. The
results of tests using gene probes can be obtained In
48 hours, which Is similar to the time required for
standard coliform tests. If the use of gene probes to
detect viruses In water were allowed, decisions about
the volume of water to be sampled, the detection
capabilities of the assay procedure used (in terms of
whether It detected all viruses, Infective and non-
Infective, or just Infective viruses; the minimum number
of virus particles it could detect), and the number and
types of viruses detected to meet the criteria would
have to be made.
2) Monitor the water for the presence of Indicator viruses.
As discussed above, coliform bacteria are not always
good Indicators of the presence of viruses. There has
been a considerable amount of work done to try to find
a microorganism which would be a good indicator for
human viruses, and be much simpler and less
expensive to detect. To this date, no one micro-
organism has been found that can be used as an
Indicator of the presence of all of the human viruses of
concern. One group of microorganisms, the male-
specific bacteriophages are promising candidates to
use as Indicators of the presence of human viruses in
water and soils. These viruses are relatively easy to
work with In the laboratory, and their presence In water
can be detected In about 24 hours. Several more
studies will be required before It can be determined
how accurate these bacterial viruses are as indicators
of human viruses.
3) A variance could be granted if the nearest potential
source of human waste to the well was far enough
away If viruses were Introduced Into the soil, their
numbers would be reduced to some predetermined
cutoff level before the water reach a well.
In establishing criteria for such a variance,
consideration would have to be given not only to
horizontal separation between the waste source and
the well, but the vertical distance between the waste
source and the ground water. Based on literature
reports on the movement of viruses In soils, Gerba
(1984) suggested that 1500 m be required as a
minimum separation distance between a source of
contamination and a drinking water well. In karst
terrains or fractured media, this distance would have to
be much greater; or possibly no variance could be
granted under these conditions. Gerba also suggested
that there be at least 1 to 2 m unsaturated zone at the
site of the contamination source. These recommen-
dations are very general and would not necessarily be
applicable to all sites. In some areas, 1500 m
horizontal separation would be unnecessarily large and
impractical. A1 - to 2- m vertical separation might be
adequate in some areas but would probably not provide
adequate protection in sandy soils. It would be very
difficult to establish criteria which would be practical for
all situations.
4) A variance could be granted based on a DRASTIC -
type evaluation of the site (Alleret al., 1985).
DRASTIC is a numerical rating system that was
developed to evaluate the ground-water pollution
potential of a region based on its hydrogeologlc
characteristics. It considers several factors including
the depth to ground water, the type of soil material, the
rate of ground-water flow, and the distance between
the contamination source and the well. Each of these
factors has a weight associated with it, and by
multiplying the weight by the numerical rating for each
factor, a numerical index Is obtained. The index is then
used to describe the relative vulnerability of a region to
ground-water contamination. DRASTIC was developed
to be used as a regional screening model; therefore, it
may not be appropriate to use it on a site-specific
basis.
5) A variance could be granted if it could be shown, using
a virus transport model, that the number of viruses
would be reduced to acceptably low numbers in the
time required for the water to travel from the waste
source to a drinking water well.
The remainder of this document will be devoted to a
discussion of the potential for the use of models in granting
variances from a mandatory ground-water disinfection
requirement. This discussion is also pertinent to the
development of wellhead protection zones with regards to
protection from viral contamination.
THE USE OF MODELS IN
GRANTING VARIANCES
FROM A MANDATORY
GROUND-WATER
DISINFECTION
REQUIREMENT
The facts that viruses remain infective long enough and can
travel far enough in the subsurface to contaminate drinking
water and cause waterborne disease outbreaks have led to
attempts to develop predictive models of virus fate in the
subsurface. In order to model the survival and transport of
viruses in the subsurface, it is necessary to determine the
factors which influence them. Over the past several years,
a great deal of research has been done to determine the
-------
factors that influence how long viruses can survive in the
environment as well as how far they can be transported In
soils and ground water. One of the provisions of the Safe
Drinking Water Act states that "the Administrator shall cany
out a study of virus contamination of drinking water sources
and means of control of such contamination" (section
1442). Research on virus contamination of drinking water
has been ongoing in this country for over 20 years. A large
amount of data has been collected by various researchers
funded by local, state, and federal agencies, including the
U.S. EPA. The results of this research will be summarized
here.
Factors Controlling the Fate of Viruses in
the Subsurface
Once in the subsurface, there are two major factors which
control virus fate: survival and movement. Both factors
must be considered when determining whether there is a
hazard to human health associated with the contamination
of ground water by viruses. If a virus can survive for a long
time in the subsurface, but cannot move through the soil
very easily, it is not likely that it will pose a large threat to
the ground water. Similarly, if the virus is easily transported
through the soil, but it does not survive for a very long
period of time, it Is probably not of much concern.
However, if the virus can survive in an infective form long
enough to be transported through the soil and into the
ground water, this may be cause for concern if the water is
used for potable purposes.
In general, both the survival and movement are controlled
by the specific type of virus, the physical and chemical
properties of the soil, and the climate of the environment.
The susceptibility of viruses to different environmental
factors varies considerably among different species as well
as strains. The size and chemical composition of different
viruses influence the extent to which they can travel in the
subsurface. The soil properties play a major role in the
survival and migration of bacteria and viruses. The texture
of the soil, its pH, organic matter content, and moisture
content all influence how long viruses can survive and how
far they can travel in the subsurface. Two aspects of
climate are particularly important in determining microbial
fate: temperature and rainfall. Viruses can survive for
extended periods of time at low temperatures. Rainfall is
important in that it can mobilize adsorbed viruses and
promote their migration to the ground water. A list of the
factors Important in controlling virus survival and movement
in the subsurface is contained in Table 2.
Examples of Virus Transport Models
There have been a few models developed to describe virus
transport with the goal of calculating safe distances
between contaminant sources and drinking water wells.
Two very different models will be described to illustrate the
types of modeling approaches that can be used. The data
requirements and limitations of each of the models will also
be discussed.
Geostatistical Model
One type of model which could be used to grant variances
from a disinfection requirement is a regional screening
model. This type of model is useful for regional planning
purposes in that areas in a community with relatively higher
vulnerability to ground-water contamination can be
distinguished from areas where contamination is less likely
to be a problem.
A model of this type has been developed and used to
predict setback distances between sources of viruses (in
this case, septic tanks) and drinking water wells for a 200
km2 area in the city of Tucson, Arizona (Yates and Yates,
1989). Although septic tank setback distances are used
here for illustrative purposes, this model could be used for
determining separation distances between any potential
source of contamination and a drinking water well.
Background
Septic tanks are the most frequently reported causes of
contamination in ground-water disease outbreaks
associated with the consumption of untreated ground water
in the United States. The placement of septic tanks is
generally controlled by county-wide or state-wide
regulations, with little consideration given to the local
hydrogeologic, climatic, and land use conditions. This
model illustrates the effects of including local variation in
subsurface conditions using geostatistics in the calculation
of septic tank setback distances in a part of the city of
Tucson, Arizona.
Data Input Requirements for Regional Screening
Model
9 Virus inactivation rates in the ground water (or ground
water temperatures) at 71 locations in the city. Figure 1
shows the relative locations of the samples used.
• Hydraulic conductivity of the aquifer at those 71
locations (at least).
• Hydraulic gradients at those 71 locations (at least).
Model Output
The output from this model is in the form of contour maps.
Figure 2 shows the distances between contamination
sources (e.g., septic tanks) and drinking water wells that
would be required to achieve a 7-order-of-magnitude re-
duction in virus number (e.g., the removal of 10 million
viruses) in the time necessary for the water to move that
distance. To Interpret this and following contour maps: if a
septic tank is placed on a contour marked 30, this means
that a well would have to be 30 m away for there to be a
removal of 10 million virus particles. (The model can be run
for any amount of virus reduction desired.) A wide range of
septic tank setback distances (from less than 15 m to
greater than 75 m) was calculated for a part of the city of
Tucson.
-------
Table 2. Factors Influencing Virus Fate In the Subsurface (Yates and Yates, 1988)
Factor
Influence on
Survival
Movement
Temperature
MforoWal activity
Moisture content
pH
Salt species and concentration
Virus association with soil
Vims aggregation
SoR properties
Virus typo
Organic matter
Hydraulic conditions
Viruses survive longer at lower
temperatures.
Some viruses are inactivated more readily in
the presence of certain microorganisms;
however, adsorption of the surface of
bacteria can be protective.
Some viruses persist longer in moist soils
than dry soils.
Most enteric viruses are stable over a pH
range of 3 to 9; survival may be prolonged
at near-neutral pH values.
Some viruses are protected from Inactivatlon
by certain cations; the reverse is also true.
In many cases, survival is prolonged by
adsorption to soil; however, the opposite
has also been observed.
Enhances survival.
Effects on survival are probably related to
the degree of virus adsorption.
Different virus types vary in their
susceptibility to inactivation by physical,
chemical and biological factors.
Presence of organic matter may protect
viruses from inactivation; others have found
that it may reversibly retard virus infectivity.
Unknown.
Unknown.
Unknown.
Generally, virus migration increases under
saturated flow conditions.
Generally, low pH favors virus adsorption
and high pH results in virus desorption from
soil particles.
Generally, increasing the concentration of
ionic salts and increasing cation valences
enhance virus adsorption.
Virus movement through the soil is slowed or
prevented by association with soil.
Retards movement.
Greater virus migration in coarse-textured
soils; there is a high degree of virus retention
by the clay fraction of soil.
Virus adsorption to soils is probably related
to physico-chemical differences in virus
capsid surfaces.
Soluble organic matter competes with
viruses for adsorption sites on soil particles.
Generally, virus migration increases with
increasing hydraulic loads and flow rates.
Using this model one can also calculate the conditional
probabilities associated with the estimated separation
distances. In other words, we can use this model to answer
the following questions.
1) Given a setback distance (e.g., specified by
regulation), what Is the probability that this would be
adequate to protect the ground water from viral
contamination at different locations in the city?
2) Given a desired probability level, what setback distance
would be necessary to be confident that the ground
water would be protected from contamination by
viruses?
Case 1: Probabilities Associated with Specified
Setback Distances.
Probability maps were calculated for two setback distances
for comparative purposes. Suppose that the local
ordinance requires a minimum of 15 m separation distance
between a septic tank and a drinking water well. Figure 3
shows the probability that there would be a 7-order-of-
magnitude reduction in virus numbers in the time required
for the water to travel 15 m. For the contour marked 0.85,
we are 85% sure that a 15-m separation distance will be
adequate to meet our criterion of protection of the well
water from virus contamination.
-------
10
DC
o
0
0
o o
O D O
O O
O O
5 10 15
EAST (km)
20
10
8
'e 6
.*
^ 4
2
V » f .•^.\\-. \ .-• ..... v . .
£^)1V\ fs&h '-
:^^Nte>: \(«<^(o}-
=^^ { ^:)\ \^-^^>v\V./ -
0
;r;\*,:>-:xr
\.-..--. ••. f ^- /1 \. »
^ffiAM
; :v V( • •• \ \ / • • Wv •i':
'•••' ; * / \ IN^.";!^.' /
v,.'N70 - -.; M\--;
1 i v. i /i V^/ V /
8 12
X (km)
20
Figure 1. Relative Location of the Sample Collection sites
Figure 2. Distances Between Contamination Sources and
Drinking Water Wells Required to Achieve a 7-log
Reduction In Virus Number
figure 3. Probability of a 7-Iog Reduction In Virus Levels with a
Well 15 Meters from a Septic Tank
Figure 4 shows the probability contour map calculated using
a 30-m separation distance between a septic tank and a
well. Comparing this figure with Figure 3, it can be seen
that the contour which had a 70% probability in Figure 3
now has an 85% probability of meeting our criterion. This Is
because of the fact that we have now set 30 m as the
separation distance, which means that it will take longer for
the viruses to travel to the well. The longer the travel time,
the more inactivation of virus that will occur. Thus, it follows
that the probability that a 30-m separation distance would
be adequate is higher (85%) as compared with the
probability estimated for a 15-m separation distance(70%).
Case 2: Setback Distances Associated with Specified
Probabilities
In this case, rather than specifying a setback distance and
calculating the associated probabilities, the desired
probability level is specified and the associated setback
distances are calculated. In the first example, a probability
level of 0.9 was specified. In other words, what setback
distance is necessary to be 90% certain that the actual
setback distance required to achieve our criterion of 7-
orders-of-magnitude reduction in virus number is less than
or equal to that distance? In Figure 5, it can be seen that
the required setback distances range from 20 to over 100
m. If one wanted to be 99% certain that the setback
distances were adequate to prevent viral contamination,
much larger separation distances are calculated (Figure 6).
-------
10
8
6
4
2
0
.•'"•.. ..J-r.--.\ \' ' y ' ^'
- ! ' ff+-\ ' \ S*"^ ^^^N.
^H3p}\ \ / .X-, ^
. ^m- L/ ^5,--;:\ ., .
n c\ \°-x^U v.; _
8 ,12
X (km)
20
Figure 4, Probability of a 7-Iog Reduction In Virus Levels with a
Well 30 Meters from a Septic Tank
10
8
-s 6
4
2,
0
0
8 12
X (km)
16
20
Figure 6. Required Setback Distances (meters) to Achieve a 7-
log Reduction In Virus Number for a Conditional
Probability Level of 99%
10
8
9 6
JK
\ j"iooV\ \ /•'
-•-:r:4c'J i ! /
0
.• /
8 12 16- 20
X (km) '
Figure 5. Required Setback Distances (meters) to Achieve a 7-
log Reduction In Virus Number for a Conditional
Probability Level of 90%
For example, in some areas a 100-m setback distance
would be required rather than the 60 m calculated when a
90% probability of achieving our criterion was required.
To demonstrate the effect of adding pumping wells to the
regional ground-water flow in the model calculations, a
simple one-well case was used. The well chosen is
pumped at a rate of 150 gpm. In the former calculation, in
which only regional ground-water flow was used In the
setback distance calculation, this well was located on a 60-
m contour (Figure 2). When the 150 gpm pumping rate is
added to the travel time calculation, a setback distance of
156 m Is required to achieve a 7-order-of-magnitude
reduction in virus number (Figure 7). If only four orders of
magnitude of virus inactivation are required, the setback
distance would be 93 m, which is still 1.5 times greater than
that calculated without adding the effects of pumping. The
actual calculations would be more complicated than
described here, as the effects of all of the wells' pumping
would have to be included to get an accurate picture of the
flow field in the Basin. This simple example does show,
however, that pumping has a large impact on the travel
time, and thus setback distance calculations, and must be
considered if the method is to be used for municipal
planning purposes.
With the appropriate modifications to model the specific
situation of interest, the methods could be used for
community planning purposes. The first case described,
namely calculating the conditional probabilities given a
-------
250
UJ
O
CO
0 -
-250
-100
100
DISTANCE (m)
Figure 7. Required Setback Distances (meters) Calculated for a
Pumping Well in a Regional Flow Field for 4,7, and 10
Orders of Magnitude Reduction in Virus Number
specified setback distance, would be useful in a situation
where the minimum setback distance was specified by
regulation. For example, a certain community has a
regulation stating that 30 m is the minimum separation be-
tween a well and a septic tank. This model could be used
to generate a conditional probability contour map. A
decision to allow a septic tank to be placed in a certain
location could then be based on the calculated probabilities.
For example, it might be decided that if the probability was
75% or greater, a septic tank would be permitted on any
lot, provided that soil percolation test requirements were
met. If the probability were between 50% and 75%, soil
percolation test requirements could be made more stringent
or the minimum lot size could be increased in order for a
septic tank permit to be issued. If the probability were less
than 50%, it might be decided that septic tanks would not
be allowed at all.
The approach described in the second case could also be
used for community planning purposes, in that a desired
probability level could be specified (e.g., in a regulation),
and the setback distances necessary to achieve that level
would be calculated. One advantage of using this method
is that the implicit assumption that the hydrogeologic
characteristics of the area are constant would be avoided.
The regulations would only have to specify a probability
level to be met in order to allow a septic tank permit.
Limitations
There are several limitations in this model in its current form
which must be recognized when using it. These include:
• Only saturated zone transport is considered. There Is
no allowance for reduction in virus number as the water
moves vertically through the unsaturated soil. This is a
serious limitation because the greatest percentage of
virus loss during subsurface transport is most likely to
occur in the unsaturated zone. A model of unsaturated
zone transport is currently being developed.
• The influence of multiple pumping wells on the pattern
and rate of ground-water flow has not been considered.
Pumping wells act to increase the flow rate of water in
some parts of the aquifer, slow it in others, and may
cause the direction of ground-water flow to be reversed
in certain areas. This will have a profound effect on the
calculated setback distances, as was illustrated above.
The effects of multiple pumping wells will change the
flow field even more, and must be considered in actual
practice
• Inactivation of the viruses was the only removal
mechanism included. From the previous discussion, it
is obvious that adsorption to soil particles is an
important removal mechanism, especially in the
unsaturated zone.
• This model used a bacterial virus, MS-2 coliphage, as a
model for the behavior of human viruses. The viruses
of concern may or may not behave in the same
manner.
Advection-Dispersion Contaminant Transport
Model
Background
A second type of model which could be used to grant
variances from a disinfectant requirement is a site-specific
contaminant transport model using the advection-dispersion
equation. There are four main processes involved in
contaminant fate which must be characterized quantitatively
and input into the model: decay, advection, dispersion, and
adsorption. Brief definitions of these processes are:
• Decay - the irreversible reduction in the concentration
of the contaminant due to chemical, physical, and/or
biological processes.
• Advection - movement of the contaminant with the bulk
flow of the water.
• Dispersion - the spreading out of the contaminant as it
travels around soil particles in the subsurface.
• Adsorption - binding of the contaminant to a solid
surface. The binding may be either reversible or
irreversible.
-------
The use of the advectlon-dlsperslon equation In a
contaminant transport model to predict the number of
viruses at various locations downstream of the point of
Introduction towards a pumping well will be Illustrated using
an example by Grosser (1984). This Is a site-specific
model, in contrast to the regional model presented
previously. In other words, use of a model such as this
enables one to predict the extent of viral movement from a
particular site of interest In one horizontal direction. This
model tells the user how many viruses will be present in the
waer at various distances from a source.
Data Input Requirements for Advectlon-
Dlsperslon Contaminant Model
• aquifer permeability
• storage coefficient of the aquifer
• soil porosity
• radius of the well
• maximum radius of calculation
• location of top of aquifer
• location of base of aquifer
• location of water level
• amount of recharge to aquifer
• boundary conditions
• nodes of observation wells
• pumping rate of well
• time of cessation of pumping
• virus Inactivatlon (decay) rate
• longitudinal dispersivity
• virus adsorption coefficient
• Initial virus concentration
• ground-water gradient
• distance from contamination source to well
Model Output
Figure 8 shows output results of this model using "typical
data for Long Island" (Grosser, 1984). Assuming an initial
concentration of 2300 viruses per ml of water, the figure
shows the concentration of viruses that would be present at
various points downstream in the direction a pumping well.
The approximate distance requied for a 7-order-of-
magnlture reduction In virus number in this model was 10
m. The virus concentration at this distance from the source
was 82.8 viruses per 100 gallons of water. This Is well
above the limits of zero viruses per 1000 liters (264
gallons) suggested by an EPA virus monitoring workshop
(Karaganls et al., 1983).
Limitations
• The number of required Input parameters is very high.
• Input values for many of the parameters were
unknown, therefore estimates were made. For
example, data on the virus Inactivation rate and
adsorption coefficient were taken from the literature
and not from experiments conducted under the site
conditions used in the model. Obtaining actual values
for many of the Input parameters would be very costly.
> Only transport In the saturated zone was considered,
so any removal of viruses In the unsaturated zone Is
not taken Into account.
> This is a site-specific application of the advection-
dlspersion equation which would not be applicable for
regional screening purposes in its current form.
UETERS DOWNSTREAM
Figure 8. Modeled Steady State Virus Concentrations
Downstream from a Contaminant Site
Discussion
Both of the models discussed above have several
limitations, as noted. For example, in both cases, a model
virus was used to predict the behavior of all viruses of
concern, and it has been well documented that there is a
large degree of variation In the behavior of different viruses
in the environment. Ideally, one would want to input data
about how the particular virus(es) of concern survive and
are transported in the particular soil and aquifer of interest
under the environmental conditions present at the site. The
more accurate the input data, the more accurate the
predictions made by the model will be. In most cases,
however, data are not available on the particular virus(es) of
concern under the environmental conditions of interest. In
such cases, there are several things that can be done.
10
-------
• Obtain data (by performing experiments) on the
behavior of all of the different viruses of concern under
the specific environmental conditions of interest.
Several different things can be done at this point.
• run the model for each of the viruses
• pick the virus which survives the longest and can
be transported the most easily and run the model
using these data. The resulting prediction will
represent a worst-case scenario given the
modeling assumptions.
• choose data from an average virus
• Choose an indicator virus, that is, one which behaves
in a general way like most of the viruses of concern.
Obtain data on the behavior of this virus for the specific
circumstances to use as model Input.
• Obtain values from the literature on the behavior of
similar or indicator viruses that have been obtained in
conditions resembling those of interest as closely as
possible. Use these data in the model.
Another point to consider when using virus transport models
is that in many situations, our knowledge of the physical
and hydraulic properties of the soil and ground-water
systems used as input is as uncertain as that of the
behavior of viruses. Thus, while there are differences in the
length of time that various viruses remain infective and In
the distances they can travel in soil, these differences are
relatively small when considering the large variation In soil
and aquifer properties that can occur in a relatively small
area. In other words, the predictions made by a transport
model will be affected much more by uncertainties in the
soil and aquifer properties than by differences among
viruses.
Questions to be Considered
There are several decisions that must be made if the use of
a model to show that the ground water is "free" of viruses
will be allowed as a means of granting a variance from the
disinfection requirement.
• Are models available in a form for general use? The
models discussed above are both research models
which would require extensive documentation and
modification to make them "user-friendly".
• What model(s) will be acceptable for use? Will the
model have to be approved by someone? If so,
whom?
• Will a regional model or a site-specific one (or both) be
used?
• What type of data will be required to be used for
input?
o Can one use laboratory data obtained using a
"model" virus?
o Can values from the literature for a "model" virus
obtained under experimental conditions similar to
the situation being modeled be used?
o Must experiments be performed to obtain data on
the behavior of viruses under the specific conditions
at the site?
o If so,
1) What viruses should be used in the
experiments?
- a model virus
- the actual viruses of concern
2) What procedure(s) should be followed In
conducting experiments of virus fate?
3) What quality assurance/quality control
criteria should the experimenter meet?
• What concentration of viruses will be allowed to reach
the well, or how many orders-of-magnltude reduction
will be required for a variance to be granted?
CONCLUSIONS
There has been a large increase In our knowledge of the
factors that influence the fate of viruses in the environment
during the past several years. Based on the risks
associated with the production of carcinogens by
chlorination and the costs associated with other treatment
technologies, granting variances from mandatory ground-
water disinfection will have to be seriouly considered. There
are several alternatives to disinfection which could be used
in granting variances, each of which has its limitations.
1) Monitoring for the presence of viruses at this time Is
too time-consuming and expensive. It will be several
more years before the gene-probe technology will be
available for general use, and even then, not all
viruses will be detectable. The use of gene probes
also requires trained personnel.
2) Monitoring for indicator bacterial viruses has promise,
but several more studies need to be done to
determine whether these viruses behave in a similar
enough manner to the human viruses to be used as
indicators. These studies will require several years to
conduct.
3) Setting standard, predetermined cutoff levels for re-
quired separation distances between potential sources
of contamination would not be able to address the
variability that is present in actual field settings.
4) DRASTIC was developed for use on a regional basis,
and it is questionable whether it would work on a
small-scale or site specific setting.
5) The virus transport models currently in use are in a
format suitable for research purposes. They would
have to be modified and made user-friendly before
they could be distributed for general use.
11
-------
As stated previously, with the information that is available
about the length of time viruses remain infective and the
factors Influencing the distances they can travel in the
subsurface, model predictions can be made that would
provide useful Information In terms of protecting ground
water from contamination. Uncertainties In the soil and
hydraulic properties have the potential to cause greater
uncertainties In model predictions than do our knowledge of
virus behavior In the subsurface.
If, however, the use of models to show that drinking water is
likely to be free from viruses Is advocated as a means
whereby variances from a disinfection requirement is
granted, there are several areas which need research to
refine our capabilities to make accurate predictions of virus
transport. These Include:
• Experiments of virus behavior In the unsaturated zone.
This Is the portion of the subsurface where the greatest
potential for removal exists.
• Development of methods for the isolation and
Identification of Norwalk virus and other viruses which
are Important causes of waterbome disease. Once we
can work with these viruses in the laboratory,
experiments need to be done to determine how they
behave In the environment relative to the other viruses
that have been studied and to any model virus.
• Risk assessment studies to determine the risks
associated with the consumption of water containing
low numbers of viruses.
• Development of predictive models of virus transport in
unsaturated and saturated soils.
• Field studies to tell us whether the predictive models
that have been developed really tell us what happens in
the environment.
REFERENCES
Aller, L, T. Bennett, J. H. Lehr, and R. J. Petty. 1985.
DRASTIC: A standardized system for evaluating ground
water pollution potential using hydrogeologic settings.
EPA/600/2-85/018, Ada, OK.
Bitton, G. and C. P. Gerba. 1984. Groundwater pollution
microbiology: the emerging issue, in Groundwater Pollution
Microbiology, Bitton, G. and Gerba, C. P., Eds., John Wiley
aSons, New York, p. 1.
Craun, G. F. 1985. A summary of waterborne illness
transmitted through contaminated groundwater, J. Environ.
HKh., 48:122.
Craun, G. F. 1986a. Statistics of waterborne outbreaks in
the U.S. (1920-1980), in Waterborne Diseases in the
United States, Craun, G. F., Ed., CRC Press, Boca Raton,
Florida, Chap. 5.
Craun, G. F. 19865. Recent statistics of waterborne
disease outbreaks (1981-1983), in Waterborne Diseases in
the United States, Craun, G. F., Ed., CRC Press, Boca
Raton, Florida, Chap. 6.
Craun, G. F. 1986c. Chemical drinking water contaminants
and disease, in Waterborne Diseases in the United States,
Craun, G. F., Ed., CRC Press, Boca Raton, Florida,
Chap. 4.
DiNovo, F. and M. Jaffe. 1984. Local Groundwater
Protection, Midwest Region, American Planning
Association, Chicago, p. 18.
Federal Register. 1985. vol. 50, no. 219, pp. 46936-47022.
Gerba, C. P. 1981. Virus survival in wastewater treatment,
in Viruses and Wastewater Treatment, Goddard, M. and
Butler, M., Eds., Pergamon Press, Inc., Elmsford, New
York, p. 39.
Gerba, C. P. 1984. Strategies for the Control of Viruses in
Drinking Water, Report to Amer. Assoc. Adv. Sci.,
Washington, D.C.
Gerba, C.P., J.B. Rose, G.A. Toranzos, S.N. Singh, L.M.
Kelley, B.H. Keswick, and H.L DuPont. 1985. Virus
removal during conventional drinking water treatment.
EPA/600/1-85/017.
Goyal, S. M., B. H. Keswick, and C. P. Gerba. 1984.
Viruses in groundwater beneath sewage irrigated cropland,
Water Res., 18:299.
Grosser, P. W. 1984. A one-dimensional mathematical
model of virus transport. In: Proceedings of the Second
International Conference on Groundwater Quality
Research, Tulsa, OK, p. 105.
Kaplan, J. E., G. W. Gary, R. C. Baron, W. Singh, L. B.
Schonberger, R. Feldman, and H. Greenberg. 1982.
Epidemiology of Norwalk gastroenteritis and the role of
Norwalk virus in outbreaks of acute nonbacterial
gastroenteritis, Ann. Intern. Med., 96:756.
Karaganis, J. V., E. P. Larkin, J. L. Melnick, P. V. Scarpino,
S. A. Schaub, C. A. Sorber, R. Sullivan, and F. M. Wellings.
1983. Research Priorities for Monitoring Viruses in the
Environment. EPA/600/9-83/010, Cincinnati, OH.
Keswick, B. H. and C. P. Gerba. 1980. Viruses in
groundwater, Environ. Sci. Techno!., 14:1290.
Keswick, B. H., T. K. Satterwhite, P. C. Johnson, H. L.
DuPont, S. L. Secor, J. A. Bitsura, G. W. Gary, and J. C.
Hoff. 1985. Inactivation of Norwalk virus in drinking water
by chlorine, Appl. Environ. Mlcroblol., 50:261.
12
-------
Lippy, E. C. and S. C. Waltrip. 1984. Waterborne disease
outbreaks--1946-1980:a thirty-five-year perspective, J.
Aimer. Water Works Assoc., 76:60.
Melnick, J. L. and C. P. Gerba. 1980. The ecology of
enteroviruses in natural waters, CRC Grit. Rev. Environ.
Contr., 10:65.
Office of Technology Assessment. 1984. Protecting the
Nation's Groundwaterfrom Contamination - Volume 1, U.S.
Congress, Office of Technology Assessment, Washington,
D.C., OTA-0-233.
Tyrrell, D. A. and A. Z. Kapikian. 1982. Virus Infections of
the Gastrointestinal Tract, Marcel Dekker, Inc., New York.
U.S. Environmental Protection Agency. 1977. The Report
to Congress, Waste Disposal Practices and Their Effects
on Ground Water, Washington, D.C.
U.S. Public Health Service. 1965. Statistical Summary of
Municipal Water Facilities in the United States. Jan. 1,
1963, Publ. 1039, Washington, D.C..
Vaughn, J. M. and E. F. Landry. 1977. Data Report: an
Assessment of the Occurrence of Human Viruses in Long
Island Aquatic Systems, Department of Energy and
Environment, Brookhaven National Laboratory, Upton, New
York.
Vaughn, J. M. and E. F. Landry. 1978. The occurrence of
human enteroviruses in a Long Island groundwater aquifer
recharged with tertiary wastewater effluents, in State of
Knowledge in Land Treatment of Wastewater, Vol. 2, U.S.
Government Printing Office, Washington, D.C., 233.
Vaughn, J. M., E. F. Landry, L. J. Baranosky, C. A.
Beckwith, M. C. Dahl, and N. C. Delihas. 1978. Survey of
human virus occurrence in wastewater-recharged
groundwater on Long Island, Appl. Environ. Microbiol.,
36:47.
Yates, M.V. and S.R. Yates. 1988. Modeling microblal fate
in the subsurface environment. CRC Grit. Rev. Environ.
Contr., 17:307-344.
Yates, M. V. and S. R. Yates. 1989. Septic tank setback
distances: a way to minimize virus contamination of drinking
water. Ground Water, 27:202.
13
. S. GOVERNMENT PRINTING OFFICE: 1990/748-159/00417
-------
------- |