United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
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Research and Development
EPA/600'S2-85/070 Aug. 1985
Project Summary
Design and Management of
Subsurface Soil Absorption
Systems
E. Jerry Tyler, William C. Boyle, James C. Converse, Robert L. Siegrist,
David L. Hargett, and Mark R. Schoenemann
This report presents the results of
continuing research regarding onsite
wastewater disposal by the Small Scale
Waste Management Project (SSWMP)
at the University of Wisconsin-
Madison. Reported here are the results
of two separate investigations: (1) a
study of how construction practices af-
fect the hydraulic properties of soil ab-
sorption systems, and (2) a field study
of how various operational strategies
affect the soil infiltration properties of
soil absorption systems.
The investigation of construction
practices showed that construction ma-
chinery traffic can significantly reduce
the porosity and infiltration rate of a
soil absorption bed. The degree of dam-
age varies with soil texture, structure,
and moisture content. Wet soils are
particularly vulnerable. Field trials
demonstrated that a damaged absorp-
tion bed can be restored by removal of
the compacted and sheared layers.
However, the better approach is to pre-
vent unnecessary compacting of the
absorption bed area, especially that re-
sulting from construction machinery
traffic.
The study of operational strategies
for soil absorption systems consisted of
three separate experiments: (1) com-
parison of the conventional method of
applying septic tank effluent (simulated
trickle) with once-daily, uniform appli-
cations, (2) comparison of alternating
periods of effluent loading and resting
with continuous conventional applica-
tion, and (3) comparison of the in-house
segregation of greywater with the ap-
plication of greywater, combined
wastewater, and tapwater control. All
three experiments used three effluent
loading rates, including the basic de-
sign loading rate of 2 cm/day, as well as
4 cm/day and 8 cm/day. The three ex-
periments were implemented on soils
with similar hydraulic properties and
used wastewaters from typical, rural,
single-family homes. Soil absorption
cells were used in situ to simulate the
conventional application of effluent. At
each site, three replica cells were con-
structed for each treatment.
During the first year of the experi-
ment comparing conventional and
once-daily, uniform applications, soil
infiltration rates decreased significantly
for all treatments and loading rates,
suggesting the progressive develop-
ment of a biological clogging layer that
impedes flow.
In addition, the infiltration rate de-
creased most strongly for the conven-
tional treatments, with the greatest de-
crease occurring in cells with higher
loading rates. All once-daily uniform
treatments showed higher infiltration
rates than the conventional treatments,
regardless of loading rate. These pre-
liminary results imply that larger, less
frequent doses of septic tank effluent
are superior to the conventional trickle
application for maintaining hydraulic
properties that allow adequate absorp-
tion.
This Project Summary was devel-
oped by EPA's Water Engineering Re-
search Laboratory, Cincinnati, OH, 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).
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Introduction
Wastewater disposal systems com-
posed of a septic tank and a soil absorp-
tion field serve about one-fourth of U.S.
households, mostly in rural and subur-
ban areas. Failures of these systems
have frequently been traced to installa-
tions in soils with inadequate natural
drainage or hydraulic conductivity (per-
meability). Better site selection and sys-
tem design criteria are reducing the fre-
quency of these failures, but additional
factors seem to be involved in many
cases. This report examines how two
such factors—construction practices
and operating strategies—can affect the
hydraulic and infiltration properties of
soil absorption systems.
Construction Practices
Some workers have suggested that
alteration of soil structure by construc-
tion machinery during system installa-
tion may be a major cause of system
failures on otherwise suitable sites, par-
ticularly those failures that occur during
the first year of system operation. How-
ever, the link between construction
compaction and system failure has not
been documented. Cautions and recom-
mendations for system construction
have been published, but they lack sub-
stantiating evidence. Some authors
have suggested ways to avoid com-
paction and smearing during system
construction, but these suggestions are
supported only by sketchy data in the
form of approximate infiltration rates
for uncrusted and puddled soils of
sandy loam, silt loam, and clay texture.
Furthermore, the experimental proce-
dures are not detailed.
The literature was reviewed in the
areas of soil science and civil and agri-
cultural engineering to provide any
comparable information on the poten-
tial effects of construction techniques
and equipment on the performance of
soil absorption systems.
Two soils of different textures were
selected for field trials. The Piano silt
loam (a fine-silty, mixed, mesic Typic
Argiudoll) was chosen to represent soils
covering large areas of southern and
western Wisconsin. These soils are
suited for soil absorption systems but
they have enough clay to make them
susceptible to damage during construc-
tion.
The Ontonagon silty clay loam (a very
fine, mixed Glossic Eutroboralf) had a
clayey texture and represents soils oc-
curring over much of eastern Wisconsin
and some areas of the northern and
north-central parts of the state. These
soils are at best marginal in their suit-
ability of subsurface soil absorption
systems, and their high clay contents
make them very susceptible to con-
struction damage.
In each soil, investigations were car-
ried out in the Bt horizon, since the ac-
cumulated clay in this zone increased
the risk of construction damage. Each
soil was tested at a moisture content
near the plastic limit, which is close to
the water content of most efficient com-
paction for the ranges of compactive ef-
fort studied. Since dry soils should be
much less subject to compaction, each
soil was also tested at a moisture level
far below the plastic limit.
Modified Operating Strategies
The two major objectives of this
phase of the investigation were:
(1) to evaluate the effects of the efflu-
ent application method on soil in-
filtration rates, and
(2) to evaluate the effects of in-house
wastewater segregation on soil
infiltration rates.
Two distinct methods of wastewater ap-
plication were evaluated under the first
objective: (a) Once-daily uniform appli-
cation versus conventional application
(trickle flow), and (b) alternating periods
of waste application and resting versus
continuous application. The second ob-
jective was evaluated by comparing
greywater septic tank effluent with com-
bined wastewater septic tank effluent;
both were applied conventionally.
Carefully controlled experimental de-
signs and procedures were developed.
These procedures assumed that (1) col-
umn studies have not proved adequate
to establish the size of soil absorption
fields; (2) establishment of loading
rates on any soil group requires experi-
ments using a variety of loading rates,
including those that cause failure; and
(3) the same soil and wastewater re-
sources should be used to investigate a
particular management scheme so that
variations in the study can be attributed
to that management scheme. To this
end, field studies were designed using
extensively monitored soil absorption
cells at three field sites where soil condi-
tions were similar. Three wastewater
loading rates were selected, ranging
from conventional loading to four times
conventional values.
A major goal in each of these experi-
ments was to monitor the effects of the
operational strategies and application
rates on the dynamics of soil field fail-
ure as measured by soil permeability
and soil moisture regime. All cells were
therefore monitored to evaluate infiltra-
tion rates, soil moisture potential (ten-
siometry), and cell ponding.
Conclusions
Construction Practices
1. No chemical stabilization agents
can increase soil strength enough
to prevent damage during con-
struction of soil absorption sys-
tems.
2. Work should be scheduled to
avoid rainfall.
3. The type of bucket used had a pro-
nounced effect on observable
smearing. A smooth-edged bucket
created a smeared surface of 2- to
6-cm-wide plates over the whole
bed. The tooth-edged bucket left
smear marks only where the teeth
came in contact with the soil.
4. The number of large pores was re-
duced in the smooth, slick areas
left by both types of backhoe buck-
ets and wheel traffic compaction.
The pores that remained in these
areas tended to be aligned hori-
zontally.
5. Wheel traffic reduced infiltration
rates in all cases but the dry,
strongly structured, silty clay-loam
soil. In the trials where the infiltra-
tion rate was reduced, an increase
in the amount of traffic corre-
sponded with even greater de-
creases in the soil's infiltration
rate.
6. When the disturbed surface was
removed, infiltration rates re-
turned to approximately the same
rate as for undisturbed soil. The
depth of removal necessary to re-
turn to the former infiltration rate
increased with increased wheel
traffic.
7. The weaker soil structure of the dry
silt loam resulted in its being
crushed more readily by machin-
ery than the more strongly struc-
tured dry silty clay loam.
8. The following recommendations
are based on the above conclu-
sions:
a. All traffic should be kept out of
the bed;
b. A tooth-edged backhoe bucket
should be used for soil absorp-
tion system.construction;
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c. Work should be done only when
the soil is dry.
Modified Operating Strategies
Based on the experience to date with
the modified absorption system experi-
ments (particularly the extended period
of operation at Site 1), the following
conclusions can be advanced with re-
gard to alternative management strate-
gies for onsite systems. Because of the
current ongoing status of these experi-
ments, results and conclusions pre-
sented here should be regarded as pre-
liminary. The original project objectives
can be met only through continued, in-
tensive monitoring of these experi-
ments by the methods described in the
full report and by newly refined and ap-
plied techniques.
1. All experimental cells appear to be
developing biological clogging
layers with cumulative effluent ap-
plication, but the rates vary, as in-
dicated by declining infiltration
rates with time and soil moisture
tension measurements.
2. Decline of infiltration rate (or efflu-
ent absorption capacity) is very
strongly linked to the frequency
and amounts of effluent applica-
tion.
(a) Conventional (simulated trickle)
application of effluent causes a
faster and more complete de-
cline in infiltration rate than ef-
fluent applied in a once-daily
dose.
(b) Increasing the daily effluent
loading rate in conventional
systems contributes to the
steady decline of infiltration
rate over the life of the system.
(c) Increasing the effluent loading
rate in systems using the
once-daily dosing method does
not yet appear to affect the infil-
tration rate strongly over time.
3. Findings to date suggest that
once-daily dosing of the effluent
load appears to be superior to the
conventional trickle application ap-
proach in terms of maintaining the
hydraulic properties of the soil sys-
tem. However, this project has
thus far considered only the hy-
draulic loading concern and has
not addressed the issue of whether
increased hydraulic loading of ef-
fluent by dosing and higher appli-
cation rates assures an acceptable
level of effluent treatment.
4. Application of both greywater and
total waste to two parallel soil ab-
sorption fields in a silt loam soil is
under way, but it would be prema-
ture to draw any conclusions
about the relative rates of clogging
of these two systems.
The full report was submitted in fulfill-
ment of Grant No. R805531-01-2 by the
University of Wisconsin under the spon-
sorship of the U.S. Environmental Pro-
tection Agency.
E. Jerry Tyler, William C. Boyle. James C. Converse, Robert L. Siegrist, David L
Hargett, and Mark R. Schoenemann are with the University of Wisconsin,
Madison, Wl 53706.
James F. Kreissl is the EPA Project Officer (see below).
The complete report, entitled "Design and Management of Subsurface Soil
Absorption Systems." (Order No. PB 85-216 570/AS; Cost: $ 16.00, subject to
change} will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-85/070
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