EPA/440/6-87/007
1 States
,_,i»i,onmental Protection
Agency
Office Of Water
(WH-550G)
EPA 440/6-87-007
June 1987
Septic Tank Siting To Minimize
The Contamination
Of Ground Water
By Microorganisms
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-------
Septic Tank Siting to Minimize the Contamination
of Ground Water by Microorganisms
Marylynn V. Yates, AAAS Fellow
jy, U.S. Environmental Protection Agency
~. Office of Ground-Water Protection
P Office of Water
Washington, D.C. 20460
y
June, 1987
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FORWARD
This report is one in a series of occasional technical
documents prepared by the U. S. Environmental Protection
Agency's (EPA) Office of Ground-Water Protection (OGWP).
These publications report on miscellaneous scientific topics
which may be of interest to Sbate ground-water program
managers. The methodologies described in these reports
reflect the views of the authors and do not represent EPA
policy. These technical documents are intended to assist
State decision-makers as well as to contribute to the
scientific literature.
This report, Septic Tank Siting to Minimize the Contam-
ination of Ground ₯ater by Microorganisms, is a reference
to be used in conjunction with, Septic Systems and Ground-
Water Protection, a publication which was directed by a
technical panel on septic system management and was organized
under the auspices of OGWP.
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ACKNOWLEDGMENTS
This report was prepared during the author's tenure as a
EPA/American Association for the Advancement of Science (AAAS)
Environmental Science and Engineering Fellow during the summer
of 1985- Mention of trade names or commercial products does
not constitute endorsement or recommendation by the author or
EPA.
The views expressed herein are entirely the author's and do
not represent official policy of either the EPA or AAAS. I would
like to express my appreciation to the staff of the Office of
Ground-Water Protection for their support. Special thanks go to
Ron Hoffer for his helpful advice and to Norbert Dee for teaching
me about environmental rating systems. The considerable efforts
of Pat Curlin and Harold Wolf to make this research effort an
enjoyable and enlightening experience are gratefully acknowledged.
I would also like to thank Scott Yates for several helpful
discussions, as well as his constant encouragement and patience.
11
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TABLE OF CONTENTS
Page
LIST OP TABLES v
LIST OF FIGURES vii
SUMMARY viii
PURPOSE ix
Objectives ix
Organization of Report x
BACKGROUND 1
Waterborne Disease Outbreaks 1
Microorganisms in the Subsurface 9
Bacteria 9
Viruses ' 12
Protozoa 13
Movement of Microorganisms from Septic Tank Systems. . . 13
Septic Tanks and Waterborne Disease 15
Evaluating the Potential for Ground-Water Contamination
by Waste Disposal Practices 16
LeGrand System 18
Surface Impoundment Assessment System 18
Environmental Impact Evaluation System 20
Waste - Soil-Site Interaction Matrix 25
DRASTIC 25
DEVELOPMENT OF A RATING SYSTEM TO EVALUATE THE POTENTIAL
FOR MICROBIOLOGICAL CONTAMINATION OF GROUND WATER 30
The Rating System 30
Depth to Water 32
Net Recharge 32
Hydraulic Conductivity 35
Temperature 37
Soil Texture 37
Aquifer Medium 44
Application Rate 44
Distance to Point of Use 47
Computation of the Rating Index 47
Interpretation of Results 52
Sources of Information 53
Use of System 53
Validation of System 54
iii
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TABLE OP CONTENTS (continued)
Page
APPENDICES AND REFERENCES 55
Appendix 1. Data used to determine temperature ranges
and ratings 56
Figure A1. Virus decay rate as a function of temperature 60
Appendix 2. Data used to determine soil texture ranges
and ratings 61
Figure A2. Vertical movement of microorganisms as a
function of soil texture 64
Appendix J>. Data used to determine aquifer medium ranges
and ratings 65
Figure A3- Horizontal movement of microorganisms as a
function of the aquifer medium 69
Appendix 4- Data used to determine application rate
ranges and ratings 70
Figure A4« Removal of microorganisms as a function of the
application rate of the effluent 76
iv
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LIST OF TABLES
Table Page
1. Causative agents of waterborne disease outbreaks
2.
3-
4-
5-
6.
7.
8.
9-
10.
11 .
12.
13-
14-
15-
16.
17.
18.
Source of contamination in waterborne disease
outbreaks caused by use of untreated ground water
in the United States, 1971-1979
Pathogenic microorganisms in domestic wastewater . . .
Factors used in LeG-rand system for evaluation of
contamination potential of waste disposal sites. . . .
Contamination potential of waste disposal site
predicted using the LeGrand system
Factors used in the surface Impoundment assessment
method
Factors used in environmental impact evaluation
system
Hazardousness of waste predicted using the system
of Pavoni et al. (1972)
Factors used in the soil-waste interaction matrix. . .
Acceptability of waste-soil-site interaction
scores as determined using the matrix of Phillips
et al. (1977)
Factors used in DRASTIC system for evaluating ground
water pollution potential
Factor and weights used in system to evaluate the
Ranges and ratings for depth to water
Ranges and ratings for net recharge
Ranges and ratings for hydraulic conductivity
Ranges and ratings for temperature
Soil hydrologic properties by soil texture
Ranges and ratings for soil texture
i
8
10
19
18
21
22
20
26
29
31
33
33
36
36
38
41
42
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LIST OP TABLES (continued)
Table Page
19. Ranges and ratings for aquifer medium 45
20. Ranges and ratings for effluent application rate ... 48
21. Ranges and ratings used for separation distance
between septic tank and point of water use 50
vi
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LIST OP FIGURES
Figure Page
1. Schematic cross-section through a conventional
septic tank-soil disposal system for on-site
disposal and treatment of domestic liquid waste. ... 2
2. Occurrence of waterborne disease outbreaks by
five-year increments 4
3. Waterborne disease outbreaks (1946-1980) and
cases of illness by type of system 5
4. Waterborne disease outbreaks (1946-1980) by
causative agent 6
5. Graph of ranges and rating for depth to water 34
6. Graph of ranges and ratings for net recharge 34
7. Graph of ranges and ratings for hydraulic
conductivity 35
8. Graph of ranges and ratings for temperature 39
9. Graph of ranges and ratings for soil texture 43
10. Graph of ranges and ratings for aquifer medium .... 46
11. Graph of ranges and ratings for application rate ... 49
12. Graph of ranges and ratings for distance between
septic tank and point of water use 51
vii
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SUMMARY
As more and more cases of ground-water contamination are
reported, the public has become increasingly aware of the impor-
tance of preserving the quality of this limited resource, especial-
ly in areas totally dependent on ground-water sources. Over one-
half of the waterborne disease outbreaks in the United States are
due to the consumption of contaminated ground water. Currently,
most of the attention is focused on pollution by organic chemicals,
although chemicals are responsible for a relatively small percent-
age of the reported ground-water-related disease outbreaks. The
majority of waterborne disease outbreaks are caused by bacteria and
viruses present in domestic sewage. Overflow or seepage of efflu-
ent from septic tanks is the most frequently reported source of
contamination in these outbreaks. Domestic sewage contains several
different types of pathogenic microorganisms, many of which are
not inactivated during treatment in the septic tank. In this way,
infective microorganisms can be released into the subsurface where
they may travel through the soil and reach ground water, posing a
potential health hazard to persons consuming the water. Indeed,
from 1971 to 1979» overflow or seepage of sewage from septic tanks
or cesspools was responsible for 43$ of the reported outbreaks and
6J>% of the reported cases of illness caused by the use of un-
treated, contaminated ground water.
In the past, minimizing the potential for ground-water contam-
ination has not always been the primary concern when siting and
installing septic tanks. Considered individually, septic tank
effluent may not seem to pose a large threat to human health. On a
nation-wide basis, however, over one trillion gallons of waste
are introduced to the subsurface by septic tanks every year,
making septic tanks the leading contributor to the total volume
of wastewater discharged directly to the subsurface. Therefore,
minimizing the potential for contaminants in septic tank effluent
to reach potable waters would make a significant contribution
towards decreasing the incidence of waterborne disease outbreaks
in this country. Currently, there is little guidance available
to aid in this effort.
The purpose of this project was to develop a rating system
using readily available data which could be used as a tool in
septic tank siting to aid in decreasing the potential for the
introduction of contaminants, specifically pathogenic microorgan-
isms, to the ground water.
The literature was reviewed to determine what factors are
important in influencing the survival and migration of micro-
organisms in the subsurface environment. Existing environmental
rating systems were reviewed to assess their applicability to
this project. Appropriate parts of these systems were adapted
for use in this system. Several of the factors were not
addressed appropriately for microorganisms by the existing systems.
Therefore, an extensive literature review was conducted to
obtain quantitative information on how these factors affect the
fate and transport of microorganisms in the subsurface. Rating
viii
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curves for these factors were developed from these empirical data.
Eight factors were used in the rating system: depth to water,
net recharge, hydraulic conductivity, temperature, soil texture,
aquifer medium, application rate, and distance to a point of
water use. The factors were ranked in terms of their importance
relative to the other factors in influencing the survival and
movement of microorganisms through the subsurface. Weights were
assigned to each factor, with a weight of 1 signifying the least
importance and a weight of 5 signifying the greatest importance.
In addition to the weights, which are constant, each factor is
assigned a rating based on the conditions found at the particular
site being considered. The ratings are determined from graphs
which have been provided for each factor. An index, which gives
an indication of the relative potential for ground-water contamina-
tion by microorganisms present in septic tank effluent, can then
be computed by multiplying each factor rating by its associated
weight and summing for all factors.
Examples were given to illustrate the use of the system.
Suggestions for the interpretation of the index and use of the
rating system were made.
PURPOSE
The purpose of this report is to develop a rating system
using readily available data which could be used as a tool in
septic tank siting to aid in decreasing the potential for the
introduction of contaminants, specifically pathogenic microorgan-
isms, to the ground water. The focus of this report will be on
evaluating the possibility of microbial contamination, by both
bacteria and viruses, because they are responsible for the major-
ity of the illness associated with ground-waterborne disease
outbreaks.
Objectives
The specific objectives of this report are:
1) Identify the important factors in the survival and
transport of pathogenic microorganisms in the subsurface
environment.
2) Assess the availability of the factors identified in 1).
5) Determine the relationship between the factors identified
in 1) and the potential of the microorganisms to contaminate
the ground water.
4) Rank and/or weight each factor identified in 1) as to its
importance in influencing the microorganisms' ability to
contaminate the ground water.
ix
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5) Using the information obtained from 3) and 4), develop a
rating system which will give the user an idea of the
relative potential for ground-water contamination
by pathogenic microorganisms present in septic tank
effluent at the site being considered.
Organization of Report
This report is divided into two main parts. The first part
contains background information on the significance of waterborne
disease in this country, the role of microorganisms in causing
the outbreaks, and the contribution of septic tanks to the prob-
lem of ground-water contamination. It also reviews several
systems which have been developed to evaluate the impact of
various waste disposal practices on the environment.
The second half of the report details the development of a
rating system which specifically addresses the potential for
microorganisms present in septic tank effluent to migrate through
the subsurface, enter the ground water, and travel to points of
use such as drinking-water wells. It provides rating curves
which can be used to calculate an index which reflects the rela-
tive potential for ground-water contamination by microorganisms
in septic tank effluent, along with the data used to develop the
curves. Examples to illustrate the use of the system are given,
and suggestions for the interpretation of results and use of the
rating system are made.
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BACKGROUND
Ground water supplies over 100 million Americans with their
drinking water. In rural areas, there is an even greater depend-
ence on ground water, as 90 to 95$ of the drinking water used is
ground water (Bitton and Gerba, 1984). The increasing dependence
on ground water as a source of potable water has spurred efforts
to protect the quality of this limited resource.
Septic tank leachate is the most frequently reported cause
of ground water contamination (U.S. Environmental Protection
Agency, 1977). In 1970, twenty-nine per cent of the United
States' population disposed of their domestic waste through
individual on-site disposal systems (U.S. Environmental Protec-
tion Agency, 1977). This represents approximately 19-5 million
single units, almost 17 million of which are septic tanks or
cesspools. About 25$ of all new homes being built in the United
States have septic tanks to dispose of domestic wastewater (U.S.
Environmental Protection Agency, 1980). While the relative
percentage of new homes using septic tanks has decreased over the
past several years, the total number of septic tanks is increas-
ing at a rate of about one-half million per year (Scalf, Dunlap,
and Kreissl, 1977).
It has been estimated that the total volume of waste dis-
posed of via septic tanks is approximately one trillion gallons
per year, virtually all of which is disposed in the subsurface
(Office of Technology Assessment, 1984). This makes septic
tanks the leading contributor to the total volume of wastewater
discharged directly to the soils overlying ground water. Figure
1 shows a typical septic tank-soil absorption system and its
relationship to the underlying ground water (Canter and Knox,
1984).
Basically, the household waste is transported to the septic
tank, where heavy materials or septage are allowed to settle out.
After a suitable retention time, which can vary from one to five
days, the effluent goes to the soil absorption system, where the
majority of the "purification" takes place. The effluent is
"purified" as it moves through the soil by attenuation and adsorp-
tion of the contaminants onto soil particles. If the system is
properly constructed and maintained, the contaminants in the
effluent should be reduced to levels that are not harmful to
human health. If, however, the system is not maintained properly
or the density of systems in an area is so great that the capacity
of the soils to treat effluent is exceeded, there is a potential
for ground-water contamination by septic tank effluent.
Waterborne Disease Outbreaks
Prom 1920 to 1983, 1517 outbreaks of waterborne disease
affecting 414,935 persons were reported in the United States
(Craun, 1986a,b). It is generally believed that the actual
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DISPOSAL
well
Figure 1. Schematic cross - section through a conventional
septic tank - soil disposal system for on-site
disposal and treatment of domestic liquid waste
(Canter and Knox, 1984).
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number of outbreaks is much higher than the number reported,
due to the fact that the outbreak may not be recognized as such,
that the disease was not serious, or that a very small number
of people was involved. This belief is supported by the fact
that an improved reporting and surveillance system in one state
accounted for 25% of the total number of reported outbreaks from
1976 through 1980 (Lippy and Waltrip, 1984). In cases with
suspected viral etiologies, outbreaks are very difficult to
document because of the wide range of symptoms which can be
manifested in people infected with the same virus.
The reported occurrence of waterborne disease outbreaks in
the United States has been increasing since 1966 (Lippy and
Waltrip, 1984)- This trend is shown in Figure 2, which illus-
trates the frequency of waterborne disease outbreaks in 5-year
increments since 1946. This figure, as well as all analyses
based on reported waterborne disease outbreaks must be inter-
preted with caution. As pointed out earlier, the intensity of
surveillance can vary greatly from state to state. In addition,
apparent increases in numbers may reflect heightened awareness
and better reporting of waterborne outbreaks, especially since
1971 when the current system of reporting was established
(Lippy and Waltrip, 1984), rather than an actual increase in the
number of outbreaks.
Almost one-half of the outbreaks occurred in non-community
systems (a public water system serving at least 25 persons or with
15 service connections but who are not year-round residents),
96$ of which use ground water, although many more cases of illness
resulted from outbreaks in community systems (those public systems
which have at least 15 service connections or provide water to
at least 25 people on a year-round basis) (Figure 3). Reported
outbreaks in individual systems, which depend primarily on ground-
water sources, accounted for only 1$ of the cases of illness.
This may not accurately reflect the actual number which occur, as
outbreaks in individual systems are usually not recognized or
reported.
Causative agents were determined in 48$ of the outbreaks
(Figure 4, Table 1). Only 7-3$ were caused by acute chemical
poisoning; the vast majority were caused by pathogenic (disease-
causing) microorganisms. The remainder (52$) were classified as
acute gastrointestinal illness of unknown etiology. It is
believed, based on recent retrospective serological studies on
the etiology of common-source outbreaks of gastroenteritis, that
many of these were caused by viruses such as the Norwalk virus or
rotaviruses (Kaplan et al., 1982), for which detection methods
have only recently become available (Gerba, 1983). It has been
suggested that the Norwalk virus is responsible for approximately
23$ of all reported waterborne outbreaks in the United States
(Keswick et al., 1985).
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200
150
to
ui
cc.
CD
D
O
100
50
WZA
Individual
Non -Community
Community
1946-
1950
1951. 1956- 1961- 1966-
1955 1960 1965 1970
1971- 1976
1975 1980
YEARS
Figure 2. Occurrence of waterborke discease outbreaks by five - year
increments (Lippy and Waltrip, 1984).
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LU
z
o
o
(0
T3
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Chemical
(7.3%)
Parasite
(7.1%)
Bacteria
(21.7%)
Virus
(11.8%)
Figure 4. Waterborne disease outbreaks (1946-1980) by
causative agent (Lippy and Waltrip, 1984).
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Table 1. Causative agents of waterborne disease outbreaks in the
United States, 1946-1980 (Lippy and Waltrlp, 1984)
Agent
Outbreaks
Cases of
Illness
Bacterial
Campylobacter
Pasteurella
Leptospira
Escherichia coli
Shigella
Salmonella
Total
Viral
Parvoviruslike
Hepatitis A
Poliovirus
Total
Parasitic
Entamoeba
Giardia
Total
Chemical
Inorganic
Organic
Unknown
Total
Grand total
2
2
1
5
61
75
146
10
68
1
79
6
42
48
29
21
49
350
672
3800
6
9
1188
13089
18590
36682
3H7
2262
16
5425
79
19734
19813
819
2725
3616
84939
150475
7
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The consumption of untreated or inadequately treated ground
water was responsible for over one-half of all the waterborne
outbreaks and 45$ of all cases of waterborne disease from 1971 to
1979 (Craun, 1984). Overflow or seepage of sewage from septic
tanks or cesspools was responsible for 43$ of the outbreaks and
63$ of the cases of illness caused by the use of untreated,
contaminated ground water from 1971-1979 (Table 2). Many septic
systems which were installed in the 1960's and designed to func-
tion for ten to fifteen years have exceeded their functional life-
span and are beginning to contaminate the ground water (Canter
and Knox, 1984)- Thus, septic tanks represent a significant
threat not only to preserving the potability of ground water, but
to human health.
Table 2. Source of contamination in waterborne disease
outbreaks caused by use of untreated ground water
in the United States, 1971-1979 (Craun, 1984)
Outbreaks Illness
overflow or seepage of sewage 33 4167
data insufficient to classify 24 934
contaminated springs 9 940
chemical contamination 9 127
contamination through limestone 5 880
or fissured rock
contamination by surface runoff 6 396
flooding 2 588
total 88 8032
8
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Microorganisms in the Subsurface
A.cute chemical poisoning accounts for a relatively small
percentage of the illness caused by the consumption of contami-
nated ground water, therefore, this report will focus on the
microbiological agents of disease. Several microorganisms which
have been isolated from domestic sewage, along with the diseases
they cause, are listed in Table 3.
Bacteria
Bacteria are microscopic organisms, ranging from approxi-
mately 0.2 to 10 u m (1 y ui = 10-6m) in length. They are distributed
ubiquitously i.i nature and have a wide variety of nutritional
requirements. Many types of harmless bacteria colonize the human
intestinal tract, and are routinely shed in the feces. One group
of intestinal bacteria, the coliform bacteria, has historically
been used as an indication that an environment has been contami-
nated by human sewage. In addition, pathogenic bacteria, such as
Salmonella and Shigella are present in the feces of infected
individuals. Thus, a wide variety of bacteria is introduced into
septic tanks. Many of these bacteria can survive and grow in
septic tanks, and are present in the liquid portion of the efflu-
ent when it moves to the soil absorption field.
As the septic tank effluent percolates through the soil, its
bacteriological quality changes depending upon the characteris-
tics of the subsurface environment. One of the most important
factors is the pore size of the soil matrix. Many bacteria are
large enough to be filtered out as the water moves through the
soil pores, thus limiting the depth of penetration. Another
limitation on the distances bacteria can travel is the moisture
content of the soil; bacteria can move greater distances in
saturated soil than in unsaturated soil (Hagedorn, 1984). Bacte-
ria are subject to biological, chemical, and physical inacti-
vation in the soil, as well as death due to lack of nutrients and
proper environmental conditions such as inadequate oxygen. These
are especially important considerations when the effluent must
move great distances to reach an aquifer.
Removal by filtration and inactivation notwithstanding,
bacteria can migrate considerable distances in the subsurface
given the proper conditions. In fractured rock and coarse-
grained soils, bacteria can move quite rapidly. For example, in
a sand and gravel aquifer, coliform bacteria have been isola-
ted 100 ft from the source just 35 hours after the sewage was
introduced (Hagedorn, 1983). Bacteria can also move considerable
distances in the soil; Kudryavtseva (1974) reports that bacteria
were transported 1000 m through a weathered limestone aquifer.
Model calculations using laboratory data indicate that coliform
bacteria can be transported for more than 1 km in loamy sand
aquifers and for several km in fissured karstic aquifers (Matthess
and Pekdeger, 1981). In addition to traveling through fractured
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Table 3« Pathogenic microorganisms in domestic wastewater
(Adapted from Kreissl, 1983, Fitzgerald, 1983 and
Sobsey, 1983a)
Microorganism
Disease(s) caused
BACTERIA:
Salmonella species
Shigella
Yersinia
Mycobacterium
Leptospira
Campylobacter jejuni
Pathogenic coliforms
Yersinia enterocolitica
Pseudomonas
Klebsiella
Serratia
VIRUSES:
polioviruses
hepatitis A
echoviruses
coxsackieviruses
Norwalk and Norwalk-like
rotaviruses
adenoviruses
typhoid, paratyphoid,
gastroenteritis
bacillary dysentery
gastroenteritis
tuberculosis
leptospirosis
gastroenteritis
gastroenteritis, urinary
tract infections
gastroenteritis
respiratory and burn
infections, diarrhea
pneumonia, bronchitis
respiratory and urinary tract
infections, summer diarrhea
poliomyelitis
infectious hepatitis
respiratory disease, aseptic
meningitis, diarrhea, fever
respiratory disease, fever,
aseptic meningitis, myocarditis
viruses gastroenteritis
gastroenteritis
respiratory disease, eye
infections
10
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Table 3* Pathogenic microorganisms in domestic wastewater
(continued)
Microorganism
Disease(s) caused
PARASITES:
Entamoeba histolytica
Giardia lamblia
Balantidium coli
Ascaris ova
Trichuris
Enterobius vermicularis
Cestode ova
Coccidia
amoebic dysentery
giardiasis ("backpacker's
diarrhea")
dysentery, gastroenteritis
pneumonitis, intestinal and
nervous system disorders
chronic gastroenteritis
enterobiasis
chronic gastroenteritis
diarrhea, toxoplasmosis
11
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rock and large pore-sized soils, bacteria have also been detected
in ground water 35 ft from the source of contamination after
moving through a sandy-clay soil (Caldwell and Parr, 1937).
Viruses
Viruses are ultramicroscopic particles, ranging from approxi-
mately 20 to 200 nm (1 nm = 10-9 m) in diameter, which are incap-
able of replication outside of a host cell. The enteric viruses,
which are of interest here, are not normally present in the gas-
trointestinal tract, and are shed only in the feces of infected
individuals. Therefore, viruses will only be present in the
septic tank system when one or more persons in the contributing
household(s) is infected and shedding virus. Most people have
at least one virus infection every year, so it is likely that a
septic tank system will receive virus-laden wastewater at some
time over the course of a year (Sobsey, 1983a). Hain and O'Brien
(1979) isolated enteric viruses from all four septic tanks they
sampled. In addition, one septic tank which was sampled periodic-
ally over a year was positive for enteric viruses on all five
occasions.
Virus concentrations may be as high as 10^ to lO^O particles
per gram of feces (Tyrrell and Kapikian, 1982). Over 100 different
types of enteric viruses capable of infecting humans are excreted
in the feces and may be present in domestic sewage (Gerba, Wallis,
and Melnick, 1975). In general, viruses are very host-specific,
that is, viruses which infect humans cannot infect any other
animals (with the exception of a few primates). Therefore the
isolation of a human enteric virus from water is pi-oof that the
contamination is from a human source.
Because viruses cannot reproduce outside of a living cell,
they behave very much like chemical particles in the soil. They
do not require any nutrients in order to survive (as bacteria
do), but they are susceptible to inactivation by physical means
such as high temperature. Due to their small size, viruses are
generally not filtered out by soil pores as the septic tank
effluent percolates through the soil unless they are aggregated
or associated with particulate matter (Sobsey, 1983b). The
major mechanism of virus removal in soil is by physical-chemical
adsorption onto soil particles, especially clays due to their
highly-charged nature. This adsorption is not permanent, however.
Viruses can be desorbed by rainfall and migrate further through
the subsurface where they can be readsorbed or remain freely
suspended in the ground water. This phenomenon was observed by
Wellings et al. (1975) at a site at which sewage effluent was
being used for irrigation purposes. After a period of heavy
rainfall, viruses were detected in ground-water samples which
had previously been free of viruses.
Viruses can travel considerable distances in the subsurface;
depths as great as 67 m (220 ft) and horizontal migrations as far
as 400 m (1310 ft) have been reported (Keswick and Gerba, 1980).
12
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Protozoa
Protozoa are single-celled organisms, generally considerably
larger in size than bacteria and viruses (as large as 100 m).
Due to their large size, they are removed fairly efficiently
during passage through sand, with a 99-3 to 99-9$ removal rate
reported for Giardia cysts (Logsdon et al., 1984). Although
there have been a few reported outbreaks of waterborne disease
caused by protozoa, especially Giardia lamblia, associated with
the consumption of contaminated ground water ("Craun, 1986a),
parasitic diseases of humans are a minor problem in the United
States (Fitzgerald, 1983). For this reason, and because very
little quantitative information on the survival and transport
of protozoa in the subsurface is available, these organisms will
not be considered in the remainder of this document.
Movement of Microorganisms from Septic Tank Systems
There have been relatively few studies which have actually
followed the movement of bacteria and viruses from septic tanks
to ground water. The most comprehensive study on this subject
was performed as a part of the Small Scale Waste Management
Project at the University of Wisconsin-Madison (Stramer, 1984).
Four septic tank systems were used for the experiments.
The first system consisted of a modified conventional septic
tank serving a family of six. Poliovirus was introduced in a
single inoculum of loH particles via the inlet baffle of the
septic tank. Twelve days after the introduction of the viruses,
220 virus particles/ml were detected in a well located 53 m (175
ft) away. It was calculated that the viruses moved at a rate of
4-5 m (14.6 ft)/day. Detection of only 220 particles/ml may seem
relatively insignificant when 1QH particles were originally
introduced; however, only one virus particle may be required to
cause disease (Westwood and Sattar, 1976), so the presence of any
viruses in water poses a potential health hazard.
The second system was a septic system located on a lakeshore
which received intermittent use, mainly on the weekends. Stools
containing poliovirus (107 particles) were introduced into the
septic tank through the inlet baffle. Eight days later, polio-
virus was isolated from ground-water wells located 12.3 ni (40-5
ft) and 20.6 m (67-5 ft) from the septic tank. One week later,
viruses were detected in greater quantities from these wells; in
addition, viruses were detected in a well 28.8 m (94-5 ft) from
the septic tank. On days 43 and 71 after the virus was intro-
duced, poliovirus was isolated from the lake water (46.2 m from
the septic tank), and from the lake sediment on day 109- It was
determined that the poliovirus moved at approximately the same
rate as the ground water.
The third system was a newly installed septic tank which
served a household of two adults. Viruses were introduced by
13
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flushing poliovirus-containing stools down the toilet in the
house. Thirteen days later, polioviruses were detected in a well
located 9-1 m (30 ft) from the vent pipe in the drainfield.
Samples from this well continued to be positive for viruses for
131 days.
In addition to monitoring the presence of viruses in the
ground-water wells, the number of indicator bacteria (total
coliforms, fecal coliforms, and fecal streptococci) was also
determined. This was done in an effort to correlate the presence
of viruses (a costly and time-consuming analysis) with the pres-
ence of indicator bacteria (a simple, quick, and relatively
inexpensive analysis) which are routinely used as an indication
of fecal contamination of water. No correlation could be found
between the presence of indicator bacteria and the detection of
viruses: on some occasions, bacteria were present when no viruses
were detected; on other occasions, viruses were isolated in the
absence of bacteria.
The fourth system was a septic tank-mound system serving a
family of four. Poliovirus was, again, introduced by flushing
virus-containing stools down the toilet in the house. Viruses
were recovered only in one well, located 1 m from the point of
wastewater application in the mound, on days 105 and 119 The
number of total coliform bacteria decreased from 1C)4 on day 56 to
less than 0.1 on days 70 to 147« Thus, again, the presence of
indicator bacteria did not correlate with the presence of viruses
in the ground-water samples. These findings corroborate those of
several other investigators that the absence of indicator bacte-
ria does not guarantee freedom from viral contamination.
Another study on the movement of viruses from a septic tank
to a ground-water well was done in New Mexico, on the floodplain
of the Rio Grande River (Hain and O'Brien, 1979). Poliovirus
was found to survive longer than 20 days in the septic tank, and
could be recovered from soil cores in the drainfields 7 days
after it was introduced. These investigators also isolated
viruses from ground-water samples taken from 4«6 m (15 ft)-deep
wells located 3-4 m (11 ft) from the septic tank discharge pipe.
In a study in Texas, enteric viruses were isolated from a
well 25 m (82 ft) distant from a septic tank system (Wang et al.,
1981). Vaughn et al. (1983) monitored the movement of viruses
from septic tanks through a sandy sole-source aquifer on Long
Island, New York. Viruses were detected in wells as far as 65 m
(213 ft) from the source of sewage.
The movement of indicator bacteria from a septic tile was
followed by Viraraghavan (1978). Although the concentration of
bacteria decreased with increasing distance from the tile, high
numbers (102 to 104) of bacteria were found 15-25 m away. Reneau
and Pettry (1975) found that total coliform bacteria were capable
of traveling at least 28 m from a septic tank drainfield located
in a fine loamy soil. Escherichia coli were isolated from samples
14
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taken 20 m from the dralnfield in a study conducted by Rahe et
al (1979).
From the results of these studies, it is apparent that
pathogenic microorganisms in domestic wastewater can survive in
septic tanks, migrate through the leach field, reach ground
water and travel towards points of use (wells). Many of these
studies were conducted in areas with shallow ground-water tables.
However, data on the movement of viruses and bacteria from sites
of land application of wastewater indicate that considerable
migration distances are possible (Keswick and Gerba, 1980). It
would seem reasonable to assume that the same could be expected
in septic tank leach fields. Indeed, reports of waterborne
disease caused by contamination of ground water with septic tank
effluent support this assumption.
Septic Tanks and Waterborne Disease
There have been several waterborne disease outbreaks attrib-
utable to the contamination of ground water with septic tank
effluent. Approximately 1200 people in a town of 6500 developed
acute gastroenteritis, probably due to Shigella sonnei, in a two-
month period (Craun, 1981). An epidemiological study showed that
illness was associated with the consumption of tap water. Fur-
ther investigation revealed that one of the community's two wells
had high levels of coliform bacteria. The source of contamina-
tion was found, using a dye tracer, to be a church septic tank
located approximately 45-7 m (150 ft) from the well. A breakdown
in the city's chlorinator had resulted in the distribution of 1
million gallons of contaminated water to the community, causing
the large outbreak.
An outbreak of 98 cases of hepatitis A (infectious hepati-
tis) in Arkansas was traced to the use of commercial pellet ice
(Craun, 1979). The water used to make the ice, as well as the
ice itself, was found to be heavily contaminated with coliform
bacteria. A dye study traced the contamination to a septic tank
leach field serving a home occupied by persons who had recently
had infectious hepatitis.
Another outbreak of hepatitis A resulted from the contami-
nation of a 3-4 m (11 ft)- to 30.5 m (100 ft)-deep aquifer over-
lain by fissured bedrock (Vogt, 1961). The drinking-water well
of the first reported case was located 1.8 m (6 ft) from the sep-
tic tank. Two other wells were located 3 m (10 ft) away. Four
weeks later, 16 individuals from the three households served by
these wells became ill within a 3-day period. The outbreak was
preceeded by a period of snowmelt and heavy rainfall, possibly
resulting in virus-contaminated effluent being carried to the
drinking-water aquifer.
In 1972, five cases of typhoid occurred in a residential
area in Washington as a result of persons consuming contaminated,
15
-------
untreated ground water from a private well (McGinnis and DeWalle,
1983)- An epidemiological investigation revealed that a typhoid
carrier lived in the area. When a dye was flushed through the
septic system of his home, it was detected 36 hours later in
several wells in the area, including the ill family's well, which
was located 64 m (210 ft) away (Craun, 1979). Salmonella typhi,
the causative agent of typhoid, was isolated from the well water,
and from stool specimens from the patients and the carrier.
A Norwalk-like agent was responsible for over 400 cases of
gastroenteritis at a resort camp in Colorado (Craun, 1984). Over
one-half of the persons visiting the camp developed diarrhea at
the camp or within one week of leaving it. It was found (using
dye tracers) that the camp tap water was contaminated with efflu-
ent from a septic tank located 15.2 m (50 ft) above the spring
supplying the camp.
An echovirus was isolated from a 12.2 m (40 ft)-deep well
during an outbreak of gastrointestinal illness in Florida
(Wellings et al., 1975). The well was located 30.5 m (100 ft)
from a solid waste field in the middle of an area bordered by
septic tanks. The virus was isolated from sewage, well water,
and stools of individuals living in the camp. In addition, six
weeks later, an outbreak of 15 cases of hepatitis A occurred in
the same camp.
A correlation between the isolation of coliforms from water
and the presence of septic tank systems has been observed in the
absence of any reported waterborne outbreaks of disease. In a
survey conducted in the State of Alaska, Hickey and Duncan (1966)
found a correlation between the rate of coliform contamination in
private wells and the use of individual septic tank systems.
Sandhu et al. (1979) made a similar observation: bacterial popula-
tions, and especially E. coli, were significantly correlated with
the distance between tb~e water supply service and the septic
tank.
Evaluating the Potential for Ground-Water Contamination
by Waste-Disposal Practices
The facts that septic tanks contribute a large volume of
waste to the subsurface every year, are the most frequently
reported sources of ground-water contamination, are responsible
for 43$ of the disease outbreaks in which consumption of contami-
nated, untreated ground water was the cause of the disease, and
that the majority of the outbreaks are caused by pathogenic
microorganisms demonstrate the magnitude of the role of septic
tanks in waterborne disease in the United States.
Several approaches could be taken to decrease ground-water
contamination, and the resulting potential public health prob-
lems, by septic tank effluent. One approach, probably the most
16
-------
simplistic, would be to prohibit the use of septic tanks altogeth-
er. This is unlikely to occur, as septic tanks are the only
economically feasible means of domestic wastewater disposal in
many rural areas. Another approach would be to require monitor-
ing of the ground water beneath a septic tank system to ensure
that the water met certain water quality standards. This option
is also unlikely because of the high costs associated with estab-
lishing and administering a program which would affect approxi-
mately 20 million homeowners. Disinfection of well water is
another approach which could be taken to protect the health of
the consumer; this may meet with some resistance due to the
associated expense.
A more feasible way to decrease the potential for microbio-
logical contamination of ground water by septic tanks would be to
carefully evaluate proposed sites for septic tank installation.
Most states have laws which regulate septic tank placement in
terms of distance to sources of potable water, minimum lot sizes,
and/or minimum soil percolation rates. However, in many cases,
these regulations have not been adequate to prevent ground-water
contamination and waterborne disease outbreaks. One reason for
this is that, many times, the laws are imposed over a state-wide
area and therefore do not allow for local variations in hydrogeo-
logic conditions. Another reason is that, in the past, septic
tank systems were designed to ensure that the effluent would
percolate efficiently through the soil, and little, if any consid-
eration was given to the possibility of ground-water contamina-
tion.
Over the past 20 years, several systems have been developed
which enable one to evaluate the potential for ground-water
contamination by various waste disposal practices. These systems
provide a methodical means of evaluating a potential site in such
a way that it can be compared directly with another site. The
end product of such a system is usually a number which reflects
the degree of the impact of the waste disposal practice on the
environment.
In many of these systems, environmental quality is defined
as a value between 0 and 1 (or any other convenient range), where
0 denotes very good quality and 1 denotes very poor quality. The
measurements of each of the variables of interest are converted
to a quality scale using a functional relationship curve (Dee et
al., 1978). For example, one variable of interest in determining
water quality is the number of total coliform bacteria in the
water. It may be decided that very good quality, a 0, should
correspond to the U.S. Environmental Protection Agency primary
drinking water standard, which in this case is less than one
total coliform per 100 ml of water. Any number greater than one
would receive a rating higher than 0. The numerical value of the
rating would reflect how much worse the measured number of coli-
forms is than the standard. The construction of the functional
relationship curve is one of the most difficult parts of devel-
oping a rating system. Generally, the relationship can be defined
17
-------
based on theory; the relationship must then "be validated using
empirical data (Dee et al., 1978).
LeGrand System
One of the earliest systems is one developed by LeG-rand
(1964)- The LeGrand sy«tem is based on characterizing a site in
terms of five environmental factors including depth to water,
distance to a point of water use, ground-water gradient, soil
permeability, and sorption. A description of these factors and
their associated values can be found in Table 4 (see next page).
In the LeGrand system, a numerical rating is obtained for each
of the five variables using a rating chart. The five ratings
are then added to produce a number which can be related to the
contamination potential of the site (Table 5).
Table 5- Contamination potential of waste disposal site
predicted using the LeGrand system (LeGrand, 1964)
Total Point Value Contamination Potential
0-4 imminent
4-8 probable or possible
8-12 possible, but not likely
12-25 very improbable
25-35 impossible
This system was intended to be used for contaminants that
attenuate or decrease in potency in time or by oxidation, chemi-
cal or physical sorption, and dilution through dispersion.
Contaminants such as sewage, detergents, viruses, and radioactive
wastes are considered to be appropriate for this system. It was
not intended to be used in the evaluation of sites for mixed
wastes (such as refuse dumps and sanitary landfills) if the
critical consideration is the movement of chemical wastes that
attenuate slowly.
Surface Impoundment Assessment System
The surface impoundment assessment (SIA) system (U.S.
Environmental Protection Agency, 1983) is a modification of a
later method developed by LeGrand (1983). The purpose of the SIA
is to provide an estimation of the ground-water contamination
potential of impoundments at a minimum cost. It is intended to
enable one to prioritize sites in order that sites with high
contamination potentials can be identified. Then, more costly
18
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-------
and time-consuming investigations into the actual contamination
at these sites can be undertaken.
The SIA system evaluates an impoundment site in terms of two
factors: the ground-water contamination potential itself, and
the relative magnitude of potential endangerment to current users
of underground drinking water sources. A description of the
factors used in the SIA system can be found in Table 6 (see next
page). The use of this system does not require precise data, as
it is only meant to be used as a first-round approximation of
the relative ground-water contamination potential of a site.
Environmental Impact Evaluation System
Another system for evaluating the environmental impact of
hazardous waste disposal in land was developed by Pavoni et al.
(1972). In addition to considering the susceptibility of ground
water to contamination, a term is included for airborne pollution.
This system involves the separate rating of the hazardous
waste of concern and the landfill site. In evaluating the waste,
the factors were weighted in the following manner:
1) Those factors which directly indicated impairment to
humans, animals, or plants were assigned a first degree
priority and a maximum value of 40 units each.
2) Those factors which directly indicated persistence in
the ecosystem were assigned a second degree priority
and a maximum value of 24 units each.
3) Those factors which directly indicated mobility in
landfill ecosystems were assigned a third degree
priority and a maximum value of 16 units each.
A description of the factors and their associated range of values
can be found in Table 7 (see page 22). The total waste ranking
is then correlated with the hazardousness of the waste (Table 8).
Table 8. Hazardousness of waste predicted using the system
of Pavoni et al. (1972)
Hazardousness
non-hazardous
slightly hazardous
moderately hazardous
hazardous
20
-------
Table 6. Factors used in the surface impoundment assessment
method (U.S. Environmental Protection Agency, 1983)
Factor Name
unsaturated zone
Description
Based on the combined
rating of the thickness
of the unsaturated zone
and the earth material
(both consolidated and
unconsolidated) in the
unsaturated zone
Value
0-9
ground-water
availability
The ability of the aquifer
to transmit ground water.
Based on the permeability
and saturated thickness
of the aquifer
0-6
ground-water
quality
A determinant of the ultimate
usefulness of the ground
water. Based on whether or
not it is a current drinking
water source and the total
dissolved solids concentration
0-5
waste hazard
potential
Potential for causing harm to
human health. Contaminant
sources are ranked using the
Standard Industrial Classi-
fication (SIC) numbers.
Contaminant types are
classified based on USEPA
publication 670-2-75-024
0-9
potential
endangerment
to a water
supply
Based on the distance from
the impoundment to a ground
or surface water source of
drinking water and the
anticipated flow direction
of the waste plume
1 - 9
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The factors used in evaluating the landfill site were weight-
ed using the following estimate:
1) The factors which would immediately affect a waste
transmission were assigned a first degree priority and
a maximum value of 20 units each.
2) The factors which would affect a waste transmission
once it was in contact with water were assigned second
degree priority and a maximum value of 15 units each.
3) The factors which represented the present condition of
receiving ground water were assigned third priority
and a maximum value of 10 units each.
4) The factors which represented factors outside the
immediate disposal site were assigned fourth priority
and a maximum value of 5 units each.
These factors and their associated values are also described in
Table 7. The landfill site ranking is found by adding the values
for the above-mentioned factors. The rank may vary between
approximately 0 and 110; the lower the rank, the better the
landfill for hazardous waste disposal.
The hazardous waste rating and the landfill site rating
provide relative indications of the potential threat to the
environment and the feasibility of disposing of hazardous waste
at the site, respectively.
Waste - Soil-Site Interaction Matrix
Phillips et al. (1977) developed a method for assessing the
environmental impact of land disposal of industrial wastes. This
system involves ranking the waste of concern, ranking the soil
site, and calculating the final score by means of a matrix which
combines the waste factor scores (rows) and soil-site scores
(columns). The rationale behind using a matrix is that soil-site
properties modify waste characteristics, therefore it is appro-
priate to multiply waste properties by soil-site properties. A
description of the factors used in this system is found in Table 9,
The total waste - soil-site score can range from a minimum of 45
(best) to a maximum of 4830 (worst). Although the authors stress
that experience in using the scores is required before they can
be applied to predicting the suitability of a waste-site combina-
tion, they do provide a means for classifying sites (Table 10).
DRASTIC
The final system to be described in this selective review is
DRASTIC (Aller et al., 1985). This system was created to enable
one to systematically evaluate the ground-water pollution poten-
tial of any hydrogeologic setting anywhere in the United States
25
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using existing information. It is intended to evaluate the
relative vulnerability of an area to ground-water contamination
from various sources.
The factors used in DRASTIC, along with their ranges of
values, can be found in Table 11. Each of the 7 factors used in
this system has been weighted to reflect its importance relative
to the other factors. The weights were derived by a committee of
experts using a Delphi (consensus) approach (Dee et al., 1973).
Separate weights were assigned to the factors for use in evaluat-
ing agricultural sites. The DRASTIC index is computed by multi-
plying each rating by its weight and summing. The index allows
one to identify areas which are more likely to be susceptible to
ground-water contamination relative to other areas. The higher the
DRASTIC index, the greater the potential for ground-water pollu-
tion.
DEVELOPMENT OF A SYSTEM TO EVALUATE THE POTENTIAL
FOR MICROBIOLOGICAL CONTAMINATION OP GROUND WATER
None of these rating systems has been developed to deal
specifically with the problem of microbial contamination of
ground water. The waste evaluation proposed by Pavoni et al.
(1972) considers the disease transmission potential as one of the
factors. This factor is also used in the soil-waste interaction
matrix of Phillips et al. (1977), although it is given less
weight. However, this is meant to be a measure of the probabili-
ty of disease transmission when pathogens come in contact with a
host, rather than a measure of the likelihood that pathogens in
the waste can be transported to the ground water where they pose
a health hazard.
This system has been developed to address the specific
problem of ground-water contamination by microorganisms present
in septic tank effluent. A large part of the system is based on
empirical data gathered from reports of experiments performed on
and field observations of the movement and persistence of micro-
organisms in the subsurface environment.
The Rating System
This rating system has been structured in the same manner as
DRASTIC. Eight factors have been identified as important in
controlling the survival and/or movement of microorganisms in the
subsurface. The factors have been ranked in terms of their
importance relative to one another using information available in
the literature and professional judgement. Weights, ranging from
1 (least important) to 5 (most important), were assigned to each
factor based on this ranking. These factors and their weights
are shown in Table 12 (see page 33) A functional relationship
curve has been provided for each of the factors. The curves
were obtained from other systems or developed from empirical
data on the effect of that factor on microorganisms in the
30
------- An error occurred while trying to OCR this image.
-------
subsurface. Each factor has been assigned a rating which varies
from 0 (or 1) to 10, with 0 signifying the least negative impact
and 10 the most negative impact on the environment. An indication
of the potential for the contamination of the ground water by
microorganisms is obtained by multiplying each factor rating by
its assigned weight and summing for all factors. A complete
explanation of how to compute the rating index, including examples,
is given in a later section. Each factor, and the rationale for
using it, will now be discussed in detail.
Depth to Water
The ratings and ranges for depth to water are the same as
those used in the DRASTIC system (Table 13). A graph of the
ranges and ratings is shown in Figure 5- Depth to water is
defined as the distance from the base of the system to the top of
the maximum seasonal elevation of the ground water (U.S.EPA, 1980).
The depth to water is important because it determines the depth
of material through which the microorganisms must travel in order
to reach the water table. This is also an indication of the
amount of material available to aid in removal of the microorganisms,
In this system, depth to water has also been considered in
determining ratings for the soil texture. This will be discussed
further in a subsequent section.
Net Recharge
The ratings and ranges for net recharge are the same as
those proposed in the DRASTIC system (Table 14, see page 36).
Figure 6 shows a graph of the ratings and ranges for net recharge.
Net recharge as defined by Aller et al. (1985) is the amount of
water (mainly from precipitation) per unit area of land which
penetrates the ground surface and reaches the water table on a
yearly basis. Recharge water is important in that it can trans-
port contaminants through the soil. Caldwell (1938a) found that
coliforms moved less than 0.3 m from a latrine under normal
conditions. However, if water was added to the soil, either
artificially or in the form of rainfall, the coliforms could be
isolated 1.8 m away.
Several investigators have shown that viruses which were
absorbed onto soil particles could be desorbed by decreasing the
ionic strength of the fluid in the matrix. This has been
observed using laboratory soil columns flooded with distilled
water to simulate rainfall (Gerba and Lance, 1978; Landry et al.,
1979, 1980; Lance et al., 1976; DuBoise et al., 1976; Sobsey et
al., 1980). However, the ability of rainfall to desorb viruses
depends on the soil type: viruses are more readily desorbed from
sandy soils than from clay soils (Gerba and Bitton, 1984)-
The ability of rainfall to desorb viruses and translocate
them to the ground water has also been observed in a field study
32
-------
Table 12. Factors and weights used in system bo evaluate
potential for microbiological contamination of
ground water
Factor Weight
Depth to water (DTW) 5
Net recharge (R) 2
Hydraulic conductivity (K) 3
Temperature (T) 2
Soil texture (S) 5
Aquifer medium (A) 3
Application rate (AR) 4
Distance to well (D) 5
Table 13- Ranges and ratings for depth to water
(adapted from Aller et al., 1985)
Depth to Water
Range (ft) Rating
0-5 10
5-10 9
10-30 7
30-50 5
50-75 3
75-100 2
> 100 1
Weight = 5
33
-------
o>
c
CO
tr
10 20 30 40 50 60 70 80 90 100
Depth to water (ft)
Figure 5. Graph of ranges and ratings for depth
to water (Aller et al., 1985).
O>
c
-------
(Wellings et al., 1974). No viruses were detected in wells at a
waste application site until after a period of heavy rainfall.
Hydraulic Conductivity
The ranges and ratings for hydraulic conductivity have been
taken from the DRASTIC system (Table 15). A graph of the ratings
as a function of the hydraulic conductivity of the aquifer is
shown in Figure 7. Hydraulic conductivity is a measure of the
ability of the aquifer medium to transmit water. This, in con-
junction with the hydraulic gradient, controls the rate at which
the ground water flows, which in turn controls the rate at which
contaminants can be transported through the aquifer.
10 -
9 -
8 -
7 -
6 -
5H
4 -
3 -
2 -
8
I
8
CO
8
8
r^
o
o
8
CM
8
HYDRAULIC CONDUCTIVITY (gpd/Ft2)
Figure 7. Graph of ranges and ratings for hydraulic
conductivity (Aller et al., 1985).
35
-------
Table 14- Ranges and ratings for net recharge
(Aller et al., 1985)
Net Recharge
Range (inches/year) Rating
0-2 1
2-4 3
4-7 6
7-10 8
>10 9
Weight = 2
Table 15- Ranges and ratings for hydraulic conductivity
(Aller et al., 1985)
Hydraulic Conductivity
Range(gpd/ft2) Rating
1-100 1
100-500 2
500-700 4
700-1000 6
1000-2000 8
>2000 10
Weight = 3
-------
Microorganisms have been found to move at the same rate as
the ground water (Strainer, 1984)- In addition, several investiga-
tors have used microorganisms as tracers to determine the rate of
ground-water flow in an area (Martin and Thomas, 1974; Pyle et
al. , 1979; Sinton, 1979, 1980; Pyle and Thorpe, 1981; Gerba,
1984; Wimpinny et al. , 1972).
Temperature
Temperature is the only environmental factor which has been
found to have a consistent effect on the length of time viruses
can remain infective in all types of water studied (Sattar,
1981), as well as in different types of soil (Hurst et al. ,
1980). As the temperature increases, the rate of virus inactiva-
tion increases. Indeed, temperature is probably the most detri-
mental factor affecting virus persistence in the subsurface (Bitton,
1978). Therefore, it was felt that temperature should be includ-
ed in the rating system.
The data used to develop the ranges and ratings for tempera-
ture (Appendix 1) were taken from experiments on poliovirus,
echovirus, and bacteriophage MS-2 persistence in ground water.
Statistical analysis has shown that there is no significant
difference in the inactivation rates of these viruses (Yates et
al. , 1985)- Data on bacteria were not used because enteric
viruses, in general, have been found to persist for longer
periods of time than bacteria in ground water under the same
conditions (Bitton et al. , 1983; Keswick et al. , 1982). In
addition, data on virus persistence in ground water were available
over a wide range of temperatures (3 - 30.5°C).
Mean virus inactivation rates L-loglO (number of viruses)/day J ,
y, were analyzed as a function of temperature, x, using linear
regression. As expected, inactivation rate was highly correlated
with temperature (r = 0.85). This relationship can be described
by the equation:
y = 0.4554x - 0.12372
and is depicted graphically in Figure A1 . These data were converted
into the ranges and ratings in Table 16, which are shown graph-
ically in Figure 8.
Soil Texture
The texture of soil through which the septic tank effluent must
percolate as it moves from the absorption system to the water
table is a very important factor in evaluating the potential
for ground-water contamination by microorganisms. There are two
major mechanisms whereby microorganisms can be removed as they
are transported through the subsurface: filtration and adsorption.
37
-------
Table 16. Ranges and ratings for temperature
Temperature
Range ("C) Rating
< 5 10
5-10 9
10 - 12.5 7
12.5 - 17 5
17-20 4
20-25 2
25-30 1
> 30 1
Weight = 2
-------
<
cr
4
3
2
1 -
5 10 15 20 25 30 35
TEMPERATURE (C)
Figure 8. Graph of ranges and ratings for temperature.
39
-------
Bacteria are removed largely by filtration, that is, they
are trapped in soil pores as the water passes through the matrix.
Therefore, bacteria are removed to a greater degree in soils with
small pores than in coarser-textured soils such as sands and
gravels. Viruses, especially if they are solids-associated, may
also be removed by filtration, although this mechanism of virus
removal is probably of minor importance (Sobsey, 1983b).
The major mechanism of virus removal in soils is by adsorp-
tion. Viruses are adsorbed more effectively by fine-textured
soils than coarser soils, due to the high sorptive capacity of
the clay fraction of the soil. Several investigators have
studied the adsorption of viruses to different textures of soils
(Burge and Enkiri, 1978; Gerba et al. , 1980, 1981; Goyal and
Gerba, 1979; Moore et al., 1981; and Sobsey et al., 1980).
Although the extent of adsorption varies among different viruses,
it is generally agreed that virus adsorption increases with
increasing clay content. Bacteria are also removed in soil by
adsorption to clay particles (Gerba and Bitton, 1984)-
In order to develop ratings based on soil texture, data on the
extent of vertical movement of microorganisms in soil were accum-
ulated. Some of the data were obtained from column studies
conducted in the laboratory, others from field studies. These
data, compiled in Appendix 2, were plotted to determine the influ-
ence of soil texture on the distance that a microorganism was ob-
served to travel in that soil (Figure A2). Soil texture is plotted
as a function of decreasing particle size from fractured rock to
fine sand, and as a function of increasing clay content from fine
sand to clay. Table 17 provides information on the clay content
of a soil as it relates to the soil texture class.
The data were analyzed using linear regression, and a high
correlation (r = -0.83) was found to exist between soil texture,
x, and the log]_o (distance) of movement, y. The relationship can
be expressed by the equation:
y = -0.28928x + 1.7967
Once the importance of soil texture in limiting microbial
movement was verified, ratings had to be developed to reflect
this. It was felt that soil texture in itself was not as important
as soil texture in relation to the depth to water. In other words,
if the site has a shallow water table, and the soil has a clayey
texture, the potential for ground-water contamination is much
less than if the soil were a coarse gravel. Also, the importance
of the depth to water in a clay soil is less than the importance
of depth to water in a sandy soil. The rating scheme developed
reflects this two-tiered approach (Table 18). To determine the
rating for soil texture from the graph (Figure 9), one first deter-
mines the soil texture, interpolating if the exact texture is not on
the x-axis, with the aid of Table 17- The rating is read by
projecting a line from the soil texture to the line closest to the
40
-------
Table 17- Soil hydrologic properties by soil texture (Dean
et al., 1984)
Texture Class
Ranges of Textural Properties
sand
loamy sand
sandy loamy
silt loam
loam
sandy clay loam
silty clay loam
clay loam
sandy clay
clay
Sand
85-100
70-90
45-85
0-50
25-50
45-80
0-20
20-45
45-65
0-45
Silt
0-15
0-30
0-50
50-100
28-50
0-28
40-73
15-55
0-20
0-40
Clay
0-10
0-15
0-20
0-28
8-28
20-35
28-40
28-50
40-60
40-100
41
-------
Table 18. Ranges and ratings for soil type
Soil Type
fractured rock
coarse gravel
coarse sand
fine sand
sandy loam
loam
sandy clay loam
clay loam
sandy clay
clay
45 m
Rating
Depth to Water
9 m
1.1m
10
9
8
7
6
5.2
4.2
3-1
2.5
1
10
10
10
10
8.6
7.4
6.1
4.4
3-6
1.4
10
10
10
10
10
10
10
7.7
6.2
2.5
Weight = 5
42
-------
TO
CJ
9 -
8 -
7 -
6
5 5
-------
depth to water at the particular site (using interpolation as
necessary), and then projecting the line from that point to the
rating axis (y-axis).
Aquifer Medium
The aquifer medium is the material through which the effluent
moves after it has traveled through the unsaturated zone (soil)
and when it enters the ground water (saturated zone or aquifer).
The same argument as was put forth in the development of ratings
for soil texture was used in determining ratings for the aquifer
medium (in terms of mechanisms of removal of microorganisms in
soil and the importance of soil texture in that process).
A graph of the horizontal distance of microbial movement as
a function of aquifer medium is shown in Figure A3- The data used
to develop this graph are contained in Appendix 3. It was felt
that the data could best be described by a shallow curve rather
than a straight line.
In an analogous fashion to the system for determining the
soil texture rating, the aquifer medium rating is also based on a
two-tiered approach (Table 19)- In this case, the horizontal
distance from the septic tank site to a point of water use (such
as a drinking-water well) was considered in rating the aquifer
material. The graph to determine the aquifer material rating
(Figure 10) is used in the same way as that for the soil texture
rating: a line is projected up from the aquifer medium to the
line corresponding to the distance (down gradient) to the nearest
drinking water well. This point is then projected across to the
y-axis to obtain the rating.
Application rate
The rate at which the effluent is applied to the soil is
important in that the lower the application rate, the longer the
time available for adsorption and retention of the microorganisms
by soil particles. The results of a study by Wang et al. (1981)
led the authors to suggest that the rate at which the water is
applied to the soil may be the most important factor in predicting
the potential for virus movement into the ground water. The
effect of changing the application rate on the degree of virus
removal has been the subject of a few studies (G-rigor'eva and
Goncharuk, 1966; Lance and Gerba, 1980; Robeck et al., 1962;
Vaughn et al., 1981; and Wang et al., 1981).
Data on the effect of application rate on the degree of
removal of microorganisms in the percolating effluent were ob-
tained by surveying the published literature (Appendix 4). The
application rate, x, was found to be highly correlated (r = 0.88)
with the degree of removal of microorganisms, y. This relationship,
shown in Figure A4, can be expressed by using the equation:
y = -0.53763X - 0.59602
44
-------
Table 19- Ranges and ratings for aquifer medium
Rating
Distance to Point of Use
Soil type
fractured rock
coarse gravel
coarse sand
fine sand
sandy loam
loam
sandy clay loam
clay loam
sandy clay
clay
Weight = 3
200 m
10
10
8.9
7.8
6-7
5-8
4.7
3-4
2.8
1 .1
20 m
10
10
10
10
8.6
7-4
6
4-4
3-6
1.4
2 m
10
10
10
10
10
10
8.4
6.2
5
2
45
-------
10
9 -
8 -
7 -
6 -
b s -
4 -
3 -
2 -
1 -
AQUIFER MEDIUM
Figure 10. Graph of ranges and ratings for aquifer medium.
46
-------
The ranges and ratings for effluent application rate are
shown in Table 20. In order to determine the rating from the
graph (Figure 11), the application rate mast first "be converted
to the log form.
Distance to Point of Use
The ranges and ratings for the separation distance between
the point of effluent introduction and a point of water use (such
as a drinking-water well) have been taken from LeGrand (1964).
His chart has been adapted to table and graph form, in order to
be consistent with this system (Table 21, Figure 12). In addi-
tion, the ratings have been reversed so that higher numbers
reflect a greater hazard than lower numbers. To use the graph,
the distance in meters must be converted to the log form before
reading the rating.
Computation of the Rating Index
After ratings have been obtained for all eight factors, the
index is then computed by using the equation:
Index = 5DT₯ + 2R + 3K + 2T + 5S + 3A + 4R + 5D
The higher the index, the higher the potential for microorganisms
to survive and be transported to the underlying ground water.
The index may range from 0 to 290. Following are two examples to
illustrate the use of the system.
Example 1:
Factor Value Rating
Depth to water table 50 ft (16.4 m) 4
Net recharge 5 inches/yr 5-5
Hydraulic conductivity 900 gpd/ft2 8
Temperature 14°C 6.1
Soil texture sandy loam 7-5
Aquifer medium sand 10
Application rate 15 cm/day (log = 1.2) 2.5
Distance 100 m (log = 2) 2.8
Index = 4(5) + 5-5(2) + 8(3) + 6.1(2) + 7-5(5) +
10(3) + 2.5(4) + 2.8(5)
= 158.7
47
-------
Table 20. Ranges and ratings for effluent application rate
Application rate
Range (cm/day) Rating
< 5 1
5-13 2
13-45 3
45-100 4
100 - 360 5
360 - 920 7
920 - 2000 9
2000 - 3300 10
Weight = 4
48
-------
10 n
9 -
8-
7-
6-
4-
3-
2-
1 -
> I
1 2 3
APPLICATION RATE [log (cm/day)]
Figure 11. Graph of ranges and ratings for application rate.
49
-------
Table 21. Ranges and ratings used for separation distance
between septic tank and point of water use
(Adapted from LeGrand, 1964)
Distance
Range
m
0 -
15 -
23 -
30 -
38 -
46 -
61 -
91 -
152 -
>305
15
23
30
38
46
61
91
152
305
0
50
75
100
125
150
200
300
500
ft
- 50
- 75
- 100
- 125
- 150
- 200
- 300
- 500
- 1000
>1000
Rating^
10
9
8
7
6
5
4
3
2
1
Weight = 5
50
-------
10 -t
9H
8-1
7H
6H
4 H
i
1.5
2 2.5
DISTANCE [log(meter)]
Figure 12. Graph of ranges and ratings for distance between septic tank and point of water use
(LeGrand, 1964).
51
-------
Example 2:
Factor
Depth to water table
Net recharge
Hydraulic conductivity
Temperature
Soil texture
Aquifer medium
Application rate
Distance
Value Rating
3 ft (1 m) 10
3 inches/yr 3.3
2000 gpd/ft2 10
10°C 9
sand 10
fractured rock 10
120 cm/day (log =2) 8". 4
50 m (log = 1.7) 4.4
Index = 10(5) + 3-3(2) + 10(3) + 9(2) + 10(5) +
10(3) + 8.4(4) + 4-4(5)
= 240.2
At many sites, there may be several soil layers underlying
the soil absorption field through which the effluent must travel
before it reaches the ground water. Under such circumstances,
it will be necessary to use professional judgement in order to
obtain the rating. For example, if the soil profile consists of
layers of coarse sands and clays, some intermediate rating will
have to be used. If the profile is primarily coarse sand with a
few thin clay layers, it may be appropriate to use the ratings for
a sandy loam, as the clay will not affect the movement of the
microorganisms to any great extent. If the clay comprises a
substantial proportion of the profile, then the rating for a sandy
clay would be more appropriate to use.
Interpretation of Results
The system is intended to be used as a guide in evaluating a
septic tank site in terms of its susceptibility to ground-water
contamination by pathogenic microorganisms. Although quantita-
tive data have been used to help derive the ranges and ratings,
professional judgement and observations were used to create the
system. Therefore, the index should not be used as a substitute
for a detailed site investigation and professional judgement.
Conditions and circumstances at the individual site as well as
common sense need to be considered in addition to the index.
-------
The index provides a relative indication of the potential
for ground-water contamination by microorganisms. A site with a
higher index is more likely to have contamination problems than
one with a lower rating. If a definitive interpretation of the
index for a particular site is desired, it is suggested that the
following scale may be used as a guide:
0-75 not very probable
75-150 possible
150-225 probable
>225 very probable
Using the scale to interpret the result of the two examples
cited, site 1 would be considered possible to probable, whereas
site 2 would be a very probable.
Sources of Information
The information needed to determine the ratings for the
eight environmental factors can be obtained from a number of
sources. The U.S. and state geological surveys can provide
information on the depth to water table, hydraulic conductivity,
aquifer medium, and net recharge. Soil texture information can be
obtained from the U.S. Department of Agriculture - Soil Conserva-
tion Service, or from a soil survey done on the site. City or
local water utilities have information on depth to water, water
temperature, and locations of nearby water supply wells. Other
sources of information include local universities and associated
agricultural extension services, state departments of natural
resources and environmental protection, and the Army Corps of
Engineers.
Use of System
It is anticipated that the rating system could be used in
two different ways:
1) The system could be used to evaluate a region in order to
delineate areas which are most susceptible to ground-
water contamination by microorganisms in septic tank
effluent as an aid in community planning. Using the
results of the rating system, a decision could be made to
allow septic tanks only in certain areas of the community
where the potential impacts on ground-water quality and
human health are minimal.
2) The second use for this system would be when someone
desires to install a septic tank at a certain location.
The calculated rating could be used as an aid in septic
tank installation. For example, if the index was above a
53
-------
certain cut-off, indicating that the potential for contamina-
tion was unacceptably high, the size of the adsorption field
could be increased to decrease the potential. Again, it
should be emphasized that this system should not be used as
a substitute for a detailed site investigation.
The USEPA's grourfd-water classification system or State
classification system could be used in conjunction with the
rating as an additional input in the siting process. For exam-
ple, a higher rating (higher contamination potential) might be
more acceptable if Class II ground water would be impacted as
opposed to Class I ground water.
Validation of the System
Once a rating system such as this has been developed, the
next step is to verify that the results will closely approximate
what happens in the environment. One way to do this would be to
perform a retrospective study on sites where waterborne disease
outbreaks have occurred and determine whether the rating system
would have predicted that ground-water contamination was likely.
It should also be tested at several field sites and be compared
with the judgement of professionals to see how well the two
correspond.
The system should also be validated using sensitivity analy-
sis. The index is calculated based on the shapes of the function-
al relationship curves as well as on the weights which have been
assigned to each of the eight environmental factors. A sensitiv-
ity analysis will indicate to which of the curves and weights the
index is most sensitive. In othe5 words, it will indicate wheth-
er the calculated index would be changed appreciably if the shape
of a functional relationship curve were altered slightly or a
weight were changed by one unit in either direction.
54
-------
APPENDICES
AND
REFERENCES
55
-------
Appendix 1. Data used to determine temperature ranges and ratings
Reference
Yates, 1985
Temperature (°C)
20
Decay Rate (-logip/dayj
it
it
M
It
II
II
II
It
II
It
II
II
It
II
II
II
M
It
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II
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II
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II
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II
23
it
it
24-5
26
it
27
0.
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20
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11
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20
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11
33
25
27
11
18
21
16
29
40
43
15
48
21
22
17
16
18
20
0.18
56
-------
Appendix 1. Data used to determine temperature ranges and ratings
(continued)
Reference
Yates, 1985
n
Temperature ("C)
27
Yates, 1984
it
28.25
30.5
IT
II
It
II
It
II
30
n
26
it
Decay Rate (,-logiQ/day)
0.23
0.21
0.46
0.17
0.29
0.58
0.25
0.34
0.22
0.19
0.26
0.16
0.22
0.19
0.64
0.21
0.22
0.29
0.46
0.23
0.28
0.71
0.31
0.36
0.29
0.23
0.39
0.58
0.64
0.21
0.42
0.28
0.23
0.28
0.25
0.27
0.29
0.36
0.27
0.29
0.32
0.32
0.40
57
-------
Appendix 1. Data used to determine temperature ranges and ratings
(continued)
Reference Temperature ("C) Decay Rate (-
Yates, 1984 26 0.44
n tt 0<;52
» " 1.10
" 30 0.42
" " 0.63
11 27 0.65
" 25 0.20
" 27 0.31
25 0.39
0.65
0.54
" " 0.13
" " 0.19
" 27 0.11
" 25 0.28
" " 0.31
30 1.87
" 27 0.14
25 0.25
" " 0.34
11 " 0.21
0.25
" 27 0.13
" 4 0.020
" 12 0.093
11 23 0.24
" 4 0.064
" 12 0.16
" 23 0.58
11 4 0.014
11 12 0.030
" 23 0.019
» 4 0.012
" 12 0.095
" 23 0.26
" 4 0.025
11 12 0.040
" 23 0.33
" 12 0.034
" " 0.037
13 0.077
" " 0.11
58
-------
Appendix 1. Data used to determine temperature ranges and ratings
(continued)
References
Yates, 1984
H
Keswick et al.,
1982
it
Bitton et al.,
1983
Temperature (°C)
18
17
5-15*
22
it
Decay Rate (-IqgiQ/day)
0.082
0.075
0.19
0.21
0.046
*Not used in regression analysis, included for completeness,
59
-------
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TEMPERATURE (°C)
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