United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA-600/S2-84-107 Sept. 1984
&ER& Project Summary
Evaluation of Septic Tank
System Effects on Ground
Water Quality
Larry Canter and Robert C. Knox
This study summarizes literature con-
cerning the types and mechanisms of
ground-water pollution from septic
tank systems and provides information
on methodologies for evaluating the
ground-water pollution potential. The
conclusions are: (1) septic tank systems
represent a significant source of
ground water pollution in the United
States, since many systems are
exceeding their design life, the use of
synthetic organic chemicals in the
household is increasing, and larger-
scale systems are being designed and
used; (2) a key issue is related to
understanding the transport and fate of
system effluents in the subsurface
environment; (3) no specific
methodology exists for evaluating the
ground-water effects of septic tank
systems; however, two empirical
methodologies (surface impoundment
assessment (SIA) and waste-soil-site
interaction matrix), adjusted for annual
wastewater flow and analytical method
(Hantush) for determining water table
rise, and a solute-transport model
(Konikow and Bredehoeft) for ground-
water flow and pollutant concentra-
tions have been applied with some
success; (4) the empirical assessment
methodology (adjusted SIA method)
could be used in permitting or
evaluating systems serving individual
homes and subdivisions, as well as
large-scale systems; the analytical
model could be used for subdivisions
and large-scale systems; and the solute-
transport model could be used for large-
scale systems; and (5) a specific
empirical assessment methodology
should be developed for septic tank
system areas, with the methodology
using some factors from both the SIA
method and the interaction matrix, and
additional factors such as wastewater
flow, percolation rate, septic tank
density, and average life of septic tank
systems.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory. Ada, OK. to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Septic tanks were introduced in the
United States in 1884, and since then
septic tank systems have become the
most widely used method of on-site
sewage disposal, with over 70 million
people depending on them. Approximate-
ly 17 million housing units, or 1/3 of all
housing units, dispose of domestic
wastewater through these systems, and
about 25% of all new homes being
constructed are including them. The
greatest densities of usage occur in the
East, the Southeast, the northern tier,
and the northwestern portions of the
United States. A septic tank system in-
cludes both the septic tank and the
subsurface soil absorption system.
Approximately 800 billion gallons of
wastewater is discharged annually to the
soil via tile fields following the 17 million
septic tanks.
Septic tank systems that have been
properly designed, constructed, and
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maintained are efficient and economical
alternatives to public sewage disposal
systems. However, due to poor locations
for many septic tank systems, as well as
poor design, construction, and
maintenance practices, septic tank
systems have polluted or have the
potential to pollute underlying ground
waters. A major concern in many
locations is that the density of the septic
tanks is greater than the natural ability of
the subsurface environment to receive
and purify system effluents before they
move into ground water and that the
design life of many septic tank systems is
in the order of 10 to 15 years. Due to the
rapid rate of placement of septic tank
systems in the 1960's, the usable life of
many of the systems is being exceeded,
and ground-water contamination is
beginning to occur. Septic tank systems
are frequently reported sources of
localized ground-water pollution. Historic
beginning to occur.
Septic tank systems are frequently
reported sources of localized ground-
water pollution. Historic concerns have
focused on bacterial and nitrate pollution;
more recently, synthetic organic chemi-
cals from septic tank cleaners have been
identified in ground water. Regional
ground-water problems have also been
recognized in areas of high septic tank
system density. Within the United States
there are four counties with more than
100,000 housing units served by septic
tank systems and cesspools and an addi-
tional 23 counties with more than 50,000
housing units served by these systems.
Densities range from as low as 2 to
greater than 346 per square mile based
on the assumption of an even
distribution of the septic tank systems
and cesspools throughout the county. If
they are localized in segments of the
county, the actual densities could be
several times greater. An often-cited
figure is that areas with more than 40
systems per square mile can be consi-
dered to have potential contamination
problems.
Several iypes of institutional
arrangements have been developed for
regulating septic tank system design and
installation, operation and maintenance,
and failure detection and correction.
Most of the regulatory activities are con-
ducted by state and local governments.
The U.S. Environmental Protection
Agency (EPA) can become a participant in
the regulatory process based on the
provision of funding for septic tank
systems. Sections 201 (h) and (j) of the
Clean Water Act of 1977 (P.L 95-217)
authorized construction grants funding of
privately-owned treatment works serving
individual housing units or groups of
housing units (or small commercial
establishments), provided that a public
entity (which will ensure proper
operation and maintenance) apply on
behalf of a number of such individual
systems. One of the major facets of a
funding decision is the ground-water
pollution potential of the 'proposed
system or systems. This issue becomes
even more important for larger systems
serving several hundred housing units.
To serve as an illustration of possible
system size, EPA has funded a system
located in the northeastern United States
with a design flow of 100,000 gallons per
day.
Because EPA needs to evaluate the
ground-water pollution potential of septic
tank systems being considered for grant
funding and because engineering
designers and state and local regulatory
officials need similar relevant
information, this study was designed to
summarize existing literature about the
types and mechanisms of ground-water
pollution from septic tank systems and to
provide information on methodologies for
evaluating the ground-water pollution
potential of septic tank systems. The
scope of work included a survey of
published literature on the identification
and evaluation of ground-water pollution
from septic tank systems and selection
and evaluation of two empirical
assessment methodologies, one
numerical model, and one analytical
model for their applicability to septic tank
systems. The methodologies and models
were selected based on their previous or
potential use for septic tank systems; the
availability of required input data;
resource requirements in terms of gene-
ral personnel and technical specialists,
computational equipment, and time or
ease of implementation; understandabil-
ity by non-technical persons; and
previous documentation for prediction of
pollutant transport.
The basic septic tank system consists of
a buried tank, where waterborne wastes
are collected and scum, grease, and
settleable solids are removed from the
liquid by gravity separation and a
subsurface drain system, where clarified
effluent percolates into the soil. System
performance is essentially a function of
the design of the system components,
construction techniques employed,
characteristics of the wastes, rate of
hydraulic loading, climate, area! geology
and topography, physical and chemical
composition of the soil mantle, and care
given to periodic maintenance.
Septic tank design considerations
include determination of the appropriate
volume, a choice between single and
double compartments, selection of the
construction material, and placement on
the site. Placement of the septic tank on
the site basically involves consideration
of the site slope and minimum setback
distances from various natural features
or existing structures. Soil absorption is
accomplished using trenches or beds,
seepage pits, mounds, fills and artificially
drained systems. Trench and bed systems
are the most commonly used methods for
on-site wastewater treatment and
disposal. Site criteria that must be met for
septic tank system approval include a
specified percolation rate, as determined
by a percolation test, and a minimum 4 ft
(1.2m) separation between the bottom of
the seepage system and the maximum
seasonal elevation of ground water. In
addition, there must be a reasonable
thickness, again normally 4 ft, of
relatively permeable soil between the
seepage system and the top of a clay layer
or impervious rock formation.
One of the key concerns associated
with the design and usage of septic tank
systems is the potential for inadvertently
polluting ground water. This concern is
increased for systems serving multiple
housing units. Potential ground-water
pollutants from septic tank systems are
primarily those associated with domestic
wastewater, unless the systems receive
industrial wastes. Contaminants
originating from system cleaning can also
contribute to the ground-water pollution
potential of septic tank systems. The typi-
cal wastewater flow from a household
unit is about 150 to 170 liters/day/person.
Typical sources of household wastewater,
expressed on a percentage basis, are:
toilet(s) - 22 to 45%; laundry -4 to 26%;
bath(s) - 18 to 37%; kitchen -6 to 13%;
and other --0 to 14%. Ground-water
pollution is affected by the quality of the
effluent from the septic tank portion of
the system and the efficiency of
constituent removal in the soil underlying
the soil absorption system. Based on a
number of studies, the following
represent typical physical and chemical
parameter effluent concentrations from
septic tanks: suspended solids - 75 mg/l;
BOD5 - 140 mg/l; COD -- 300 mg/l;
total nitrogen -- 40 mg/l; and total
phosphorus - 15 mg/l. Studies of the
efficiency of soil absorption systems have —
indicated the following typicalfl
concentrations entering ground water:
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suspended solids --18 to 53 mg/l; BOD ~
28-84 mg/l; COD -- 57-142 mg/l;
ammonia nitrogen -- 10-78 mg/l; and
total phosphates - 6-9 mg/l. In addition,
other wastewater constituents of
concern include bacteria, viruses,
nitrates, synthetic organic contaminants
such as trichloroethylene, metals (lead,
tin, zinc, copper, iron, cadmium, and
arsenic), and inorganic contaminants
(sodium, chlorides, potassium, calcium,
magnesium, and sulfates).
Ground-water degradation, which
occurs in many areas having high
densities of septic tank systems, is
characterized by high concentrations of
nitrates and bacteria in addition to
potentially significant amounts of organic
contaminants. One common reason for
degradation is that thecapacity of the soil
to absorb effluent from the tank has been
exceeded, and the waste added to the
system moves to the soil surface above
the lateral lines. In addition, many soils
with high hydraulic absorptive capacity
(permeability) can be rapidly overloaded
with organic and inorganic chemicals and
microorganisms, thus permitting rapid
movement of contaminants from the
lateral field to the ground-water zone. In
considering ground-water contamination
from septic tank systems, attention must
be directed to the transport and fate of
pollutants from the soil absorption system
through underlying soils and into ground
water. Physical, chemical, and biological
removal mechanisms may occur in both
the soil and ground-water systems. The
transport and fate of biological (bacterial
and viral pathogens), inorganic (phos-
phorus, nitrogen and metals), and organ-
ic contaminants (synthetic organics and
pesticides) must be considered.
Biological contaminants exhibit a
variety of characteristics, including wide
ranges in size, shape, surface properties,
and die-away rates. The travel distance of
bacteria through soil is of considerable
significance, since contamination of
ground supplies may present a health
hazard. Many environmental factors can
influence the transport rate, including
rainfall, soil moisture, temperature, pH,
and the availability of organic matter. The
survival of enteric bacteria in soil is
affected by soil moisture content and
holding capacity, temperature, pH,
sunlight, organic matter, and antagonism
from soil microflora. The physical process
of straining (chance contact) and the
chemical process of adsorption (bonding
and chemical interaction) appear to be the
most significant mechanisms in bacterial
removal from water percolating through
soil. The removal efficiency of viruses by
soil is influenced by flow rate, cation
concentrations, clays, soluble organics
concentrations, pH, isoelectric point of
the viruses, and general chemical
composition of the soil. The most
important mechanism of virus removal in
soil is by adsorption of viruses onto soil
particles.
Although phosphorus can move
through soils underlying soil absorption
systems and reach ground water, this has
not been a major concern, since
phosphorus can be easily retained in the
underlying soils due to chemical changes
and adsorption. Phosphate ions become
chemisorbed on the surfaces of Fe and Al
minerals in strongly acid to neutral
systems and on Ca minerals in neutral to
alkaline systems. In the pH range
encountered in septic tank seepage
fields, hydroxyapatite is the stable
calcium phosphate precipitate. However,
at relatively high phosphorus
concentrations similar to those found in
septic tank effluents, dicalcium phos-
phate or octacalcium phosphate are
formed initially, and this is followed by a
slow conversion to hydroxyapatite.
Ammonium ions can be discharged into
the subsurface environment or they can
be generated within the upper layers of
soil from the ammonification process
(conversion of organic nitrogen to am-
monia nitrogen). The transport and fate of
ammonium ions may involve adsorption,
cation exchange, incorporation into
microbial biomass, or release to the
atmosphere in the gaseous form.
Adsorption is probably the major
mechanism of removal in the subsurface
environment. Nitrate ions can also be
discharged directly or generated within
the upper 'layers of soil. The transport
and fate of nitrate ions may involve
movement with the water phase, uptake
in plants or crops, or denitrification.
Nitrates can move with ground water
with minimal transformation.
Metals may react with soils by means
of adsorption, ion exchange, chemical
precipitation, and complexation with
organic substances. Of these four
reactions, adsorption appears to be the
most important for the fixation of heavy
metals. Ion exchange is thought to
provide only a temporary or transitory
mechanism for the retention of trace and
heavy metals. Precipitation reactions are
greatly influenced by pH and concentra-
tion, with precipitation predominately
occurring at neutral to high pH values and
in macroconcentrations. Organic
materials in soils may immobilize metals
through complexation reactions or cation
exchange. Fixation of heavy metals by
soils by either of these four mechanisms
is dependent on a number of factors
including soil composition, soil texture,
pH and the oxidation-reduction potential
of the soil and associated ions.
The transport and fate of organic
contaminants in the subsurface
environment is a relatively new area of
concern; thus, the published literature is
sparse. A variety of possibilities exist for
the movement of organics, including
transport with the water phase,
volatilization and loss from the soil
system, retention on the soil due to
adsorption, incorporation into microbial
or plant biomass, and bacterial degrada-
tion. The relative importance of these
possibilities in a given situation is depend-
ent upon the characteristics of the
organic.the soil types and characteristics,
and the subsurface environmental
conditions. This complicated topic is
being actively researched at this time.
Several studies have been conducted on
the movement and biodegradation of
large concentrations of pesticides in soils.
Technical methodologies for evaluating
the ground-water pollution potential of
septic tank systems range from empirical
index approaches to sophisticated
mathematical models. Models can range
from analytical approaches addressing
ground-water flow to numerical
approaches which aggregate both flow
and solute transport considerations.
Septic tank systems can be considered
area sources of ground-water pollution,
with the rectangular dimensions of the
drainage field representing the source
boundaries. Waste stabilization ponds
(surface impoundments), and sanitary
and chemical landfills also can be
considered potential area sources of
ground-water pollution. Empirical
assessment methodologies refer to
simple approaches for developing
numerical indices of the ground-water
pollution potential of human activities.
Several methodologies have been devel-
oped for evaluating the ground-water pol-
lution potential of wastewater ponds and
sanitary and chemical landfills.
Methodologies typically contain several
factors for evaluation, with the
number, type, and importance weighting
varying from methodology to method-
ology.
Ground-water models can be classified
as 'flow models and solute transport
models. Analytical models and numerical
models are of interest here. Analytical
models include those in which the
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behavior of an aquifer is described by
differential equations derived from basic
principles such as the laws of continuity
and conservation of energy. Numerical
models are actually analytical models
that are large enough to require the use of
digital computers, capable of multiple
iterations, to converge on a solution. The
applicability of ground-water models has
been the subject of a number of studies.
Prediction of the movement of
contaminants in ground-water systems
through the use of models has been given
increased emphasis in recent years
because of the growing trend toward
subsurface disposal of wastes.
Ground-water modeling can be useful
for evaluating specific sites for systems or
even larger geographical areas that may
be served by hundreds of systems.
Modeling could be used to include septic
tank system location on specific sites or in
larger geographical areas. In addition,
modeling can be useful in planning
ground-water monitoring programs for
specific sites or geographical areas.
Available technical methodologies for
addressing the ground-water effects of
septic tank systems, which range from
empirical assessment approaches to
ground-water flow and solute transport
models, differ in their input
requirements, output characteristics, and
general useability. Accordingly, certain
criteria were identified as basic to the
selection of technical methodologies
used in this study. The criteria were as
follows:
1. The methodologies should have
been used previously for evaluating
septic tank systems.
2. They should be adaptable for use in
evaluations of septic tank systems.
3. If they need to be calibrated before
use, the necessary data for calibra-
tion should be readily available.
4. The input data required for the
methodology should be readily
available; thus its use could be
easily implemented.
5. The resource requirements for use
of the methodology should be mini-
mal (resource requirements refer to
personnel needs and qualifications,
computer needs, and the time
necessary for calibration and usage).
6. Usage of the methodology for pre-
dicting pollutant transport in the
subsurface environment should
have been previously documented.
7. The conceptual framework of the
methodology as well as its output
should be understandable by non-
specialists.
Although no single methodology that
met all seven criteria was identified, two
empirical assessment methodologies
(Surface Impoundment Assessment (SIA)
and Waste-Soil-Site Interaction Matrix),
one analytical model (Hantush), and one
solute-transport model (Konikow and
Bredehoeft) were chosen for examina-
tion. The two empirical methodologies
were used to determine the ground-
water pollution potential of 13 septic tank
system areas in central Oklahoma. The
rank order of the ground-water pollution
potential of the 13 areas was determined
through the use of the two methodologies,
which were adjusted by considering the
annual wastewater flows in the areas.
The two adjusted methodologies provided
similar rank orderings of the 13 septic
tank system areas. Key findings from this
part of the study were as follows:
(1) The final ranking of the 13 septic
tank system areas was largely
dependent upon the annual waste-
water flow in the area, and this is
directly related to the number of
persons and septic tank systems in
the area.
(2) Both the surface impoundment
assessment method a nd the waste-
soil-site interaction matrix can be
used to develop a priority ranking of
existing or planned septic tank
system areas. Since the SIA
method has 6 items of needed
information versus 17 items in the
interaction matrix, the SIA method
is easier to use. However, neither
methodology accounts for
wastewater flow, and this is an
important consideration in the use
of either method for septic tank
system areas.
The Hantush analytical model was
developed to determine the rise and fall of
the water table under circular, rectangu-
lar, or square recharge areas, but it does
not address ground water quality. This
model was applied to a mound-type septic
tank system analogous to those used in
Wisconsin, and it was determined that
the rise of the water table only approach-
es a maximum of 8 inches; however, this
could be a significant rise in view of the
fact that mound systems are used in
areas of high water tables. Actual m
loadings from septic tank systems will be ™
intermittent and this will decrease the
actual rise of the water table, but
increases in loading rates (either by
malfunctioning or overloaded systems)
could increase the water table rise.
The Konikow-Bredehoeft (K-B) numer-
ical model was applied to a septic tank
system study area near Edmond, Okla-
homa, to determine its usefulness in
predicting nitrate concentrations in
ground water from this source type. The
K-B model is a two-dimensional solute
transport model that has been used in the
analysis of ground-water pollution from a
variety of source types. The objective of
this portion of the overall study was to
determine the feasibility of modeling the
effects of septic tank systems on ground-
water quality by direct application of the
K-B solute transport model computer
package to an existing situation. The
results of the Edmond study area analysis
by the K-B model must be classified as
disappointing and frustrating. Disappoint-
ment stems from the fact that the model
was unable to be calibrated, even for
ground-water flow (water levels).
Frustration stems from the fact that the
difficulties encountered with the model ^
were due solely to the lack or questionable jfl
validity of input data. The only conclusion
to be drawn about applying sophisticated
ground-water models to the problem of
septic tank systems is that the utility of
the models may be outweighed by their
significant data requirements. This
suggests that special field studies will be
necessary in order to gather the input
data necessary for use of solute-transport
models for evaluation of septic tank
systems or system areas.
Based on the results of this study, an
hierarchical structure for usage of the
three types of technical methodologies
has been developed. Potential usage can
be considered for three types of septic
tank systems: (1) a septic tank
systems serving an individual home; (2)
several hundred individual septic tank
systems being used in a subdivision; and
(3) a large-scale septic tank system
serving several hundred homes, with the
daily wastewater flow being upwards of
100,000 gallons. The empirical
assessment methodology (adjusted SIA
method) could be used as part of the
permitting procedure for all three .types;
however, its greatest usage should
probably be for the first two types. The
analytical model could be used for
subdivisions and large-scale systems, ^1
with the greatest usage probably ^
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associated with the former. Finally, the
solute-transport model should be used
for large-scale systems, since their
potential for ground-water pollution
could justify conducting the necessary
field studies to gather appropriate input
data.
Conclusions
Conclusions about the effects of septic
tank systems on ground-water quality are
as follows:
1. Septic tank systems represent a sig-
nificant source of ground-water pol-
lution in the United States. The
significance of this source type is
expected to increase since:
• many existing systems are be-
coming older and exceeding their
design life by several-fold;
• the use of synthetic organic chem-
icals in the household and for
system cleaning is increasing;
and
• larger-scale systems are being
designed and used, with flows up
to 100,000 gallons/day.
2. A key issue associated with septic
tank systems is related to under-
standing the transport and fate of
system effluents in the subsurface
environment. There is a consider-
able body of knowledge relative to
the transport and fate of biological
and inorganic contaminants in the
subsurface environment. However,
much research is needed relative to
the subsurface movement and
disposition of many synthetic
organic chemicals of current
concern. For example, research is
needed to:
• Develop a classification scheme
for synthetic organic chemicals in
terms of their transport and fate in
the subsurface environment
• Determine the influence of
aerobic and anaerobic conditions
on transport and fate processes
• Develop information on
intermediate products and by-
products of degradation process-
es which may be of greater
concern to ground-water pollu-
tion than the original synthetic
organic chemicals.
3. No specific technical methodology
exists for evaluating the ground-
water effects of septic tank systems
based on the seven desirable criter-
ia enumerated previously.
4. Application of two empirical assess-
ment methodologies adjusted for
annual wastewater flow, an
analytical method for determining
water table rise, and a solute-
transport model for ground-water
flow and pollutant concentrations
has met with some success. Use of
these approaches should be keyed
to the following three types of septic
tank systems: (1) a septic tank
system serving an individual home;
(2) several hundred individual septic
tank systems being used in a
subdivision; and (3) a large-scale
septic tank system serving several
hundred homes, with the daily
wastewater flow being upwards of
100,000 gallons. The empirical
assessment methodology (adjusted
SIA method) could be used as part of
the permitting or evaluation
procedure for all three types; the
analytical model could be used for
subdivisions and large-scale
systems, and the solute-transport
model could be used for large-scale
systems.
5. A usable type of methodology for
septic tank system evaluation is the
empirical assessment methodology
directed toward developing an index
of ground-water pollution potential.
A specific methodology should be
developed for septic tank sytems
areas. The methodology could use
some factors from both the SIA
method and the interaction matrix
and should include some additional
factors such as wastewater flow,
percolation rate, septic tank density,
and average life of septic tank
systems.
Larry Canter and Robert C. Knox are with the University of Oklahoma, Norman, OK
73019.
Marion P. Scalf and Ronald F. Lewis are the EPA Project Officers (see below).
The complete report, entitled "Evaluation of Septic Tank System Effects on
Ground Water Quality," (Order No. PB 84-244 441; Cost: $29.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officers can be contacted at:
Marion R. Scalf
Robert S. Kerr Environmental Research Laboratory
P.O. Box 1198
Ada, OK 74820
Ronald F. Lewis
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
US GOVERNMENT PRINTING OFFICE 1M4 - 759-102/10708
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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