&EPA
United States
Environmental
Protection Agency
Region 9 Ground Water
Office (WTR-9)
EPA909-F-01-001
APRIL 2001
In 1999, EPA promulgated
regulations prohibiting the use
of cesspools for the disposal
of sewage from multi-family
dwellings, and any other
buildings where cesspool
capacity was for 20 or more
persons per day, such as
schools, hospitals, and
manufacturing facilities. In that
rule, a cesspool was defined
as "a "drywell" that receives
untreated sanitary waste
containing human excreta,
and which sometimes has an
open bottom and/or perforated
sides. "Drywell"means a well,
other than an improved
sinkhole or subsurface fluid
distribution system, completed
above the water table so that
its bottom and sides are
typically dry except when
receiving fluids. These
regulations also contain a
prohibition against the use of
any seepage pit, drywell,
septic system, or other
subsurface disposal system
for the disposal of hazardous
or toxic substances (40 CFR
part 144.)
Leachfield or
Seepage Pit?
Seepage Pits May Endanger
Ground Water Quality
While the use of cesspools for sewage disposal has been prohibited in most states for
a number of years, some local ordinances still allow for the construction of drywells as
a means of dispersing effluent from septic tanks. When used in this fashion, they are
more commonly called "seepage pits." This method of effluent dispersal is deficient for
a number of reasons:
1. Seepage pits disperse effluent in anoxic, or oxygen-poor, environments, where
pathogens (especially viruses) may not be treated before they reach the water table.
They place fluids below the root zone, where there is no immediate uptake by plants of
the water and nutrients, nor is there the potential for treatment by evaporation or
evapotranspi ration.
2. If septic tanks and other treatment components are not properly sized, constructed
and maintained, seepage pits may receive sewage solids (essentially functioning like
cesspools.)
3. Water tables are not static, and may rise above the bottom of the seepage pit,
flooding it and allowing direct contact of pathogens and nitrogen species with ground
water.
4. Seepage pit construction and use may open up pathways to cracks and fissures in
rock, sending effluent directly to waterways.
5. Depending on their depth, seepage pits may allow contaminated ground water to
pollute pristine aquifers.
6. Seepage pits used for the disposal of untreated or partially treated industrial or
commercial waste may pose additional hazards to ground water quality, if the effluent
contains soluble toxics.
Seepage pits may cause other hazards not directly related to water quality. They are a
hazard for people, animals and property that may fall into them. They may also affect
slope stability and promote landslides. For all of these reasons, the Ground Water
Office at EPA, Region 9 discourages the use of seepage pits for onsite sewage (or
septic) system effluent, particularly on steep slopes, fractured rock areas, areas with
shallow ground water, and/or areas where ground water provides
the sole source of drinking water.
Exceptions should only be allowed where the seepage pit is
backfilled with cobbles or other weight-bearing material, where the
sanitary waste stream has been treated (e.g., disinfection, nitrogen
removal), and no other effluent dispersal mechanism is feasible.
Regulators should assess cumulative impacts based on the number
and types of other nearby subsurface discharges.
References are listed on the reverse of this sheet. For more
information, please call Elizabeth Janes at (415) 972-3537,
or e-mailjanes.elizabeth@epa.gov.
-------
Seepage Pit References
1. Need for Soil Treatment (see also county and state water
quality plans, and onsite sewage regulations.)
Crites, Ron andTchobanoglous, George: Small and
Decentralized Wastewater Management Systems. McGraw-
Hill, 1998, page 653: ...Removal of microorganisms,
including pathogenic bacteria, viruses, and helminths, is
accomplished by soil filtration, adsorption, desiccation,
radiation, predation, and exposure to other adverse
environmental conditions.
International Association of Plumbing and Mechanical
Officials, 1998 California Plumbing Code. Title 24. Appendix
K3: ...(4) the minimum required area of porous formation
shall be provided in one or more seepage pits. No
excavation shall extend within ten (10) feet of the water table
nor to a depth where sewage may contaminate underground
water stratum that is usable for domestic purposes.
Oakley, Stewart M., for California Wastewater Training and
Research Center, September 1999, Onsite Wastewater and
Nitrogen Removal: Within a well-designed and constructed
subsurface absorption trench, diffusion of oxygen into the
vadose zone promotes the biological oxidation of NH4+
(ammonia) to NO3- through biological nitrification. Depending
on soil moisture conditions and organic matter
concentrations within the soil column, NO3- can be reduced,
under anoxic conditions, to N2 gas through heterotrophic
biological denitrification. A carbon source is required for
denitrification to occur. In many instances there may not be
sufficient organic substrate at a depth below the "A" horizon
to promote denitrification; under these conditions N03-N can
migrate into the groundwater aquifer. The conventional
practice of constructing relatively deep subsurface soil
absorption trenches (2 to 4 feet) for septic tank effluents thus
may often have the effect of exacerbating denitrification
problems and enhancing nitrate movement into groundwater.
(page 5.)
USEPA, October 1980, Design Manual. Onsite Wastewater
Treatment and Disposal Systems: ...Travel through two to
four feet of unsaturated soil is necessary to provide adequate
removal of pathogenic organisms and other pollutants from
the wastewater before it reaches the groundwater. (p.
207)...Seepage pits are generally discouraged by many local
regulatory agencies in favor of trench or bed systems...
Maintaining sufficient separation between the bottom of the
seepage pit and the high water table is particularly important
consideration for protection of ground water quality, (p. 235)
US EPA, June 1987, Septic Tank Siting to Minimize the
Contamination of Ground Water by Microorganisms: ...As the
septic tank effluent percolates through the soil, its
bacteriological quality changes depending upon the
characteristics 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, p. 9)
2. Slope Stability
California Coastal Commission, Land Form Alteration Policy
Guidance. Attachment 2, December 1993:
...Septic systems with leach fields require fairly gently sloping
land with granular soils to be effective. Most local health
departments are familiar with the slope and soil requirements
for safe septic operation. In addition to testing that the soils
on site will percolate, it is normal for leach fields to be limited
to slopes less than 30%. In some locations this limitation can
effectively prohibit development on a lot; however, some
areas such as Los Angeles county allow seepage pits to be
used if leach fields cannot be established... In areas with
landslide potential, slides can be activated by increases in
groundwater... Although a single septic system may not be
enough to raise concern about the activation of a landslide,
the cumulative impact from ten or twenty lots, with septic
systems, irrigated landscaping and other small sources of
groundwater, may pose a serious concern.
3. Collapse Hazard: See http://www.wdailypress.com/
topstory/dp120400d.html (November 2000, Apple Valley,
California incident, reported in (High Desert) Daily Press.
of 55 counties responding
Standard drainfield 2 '-6' allowed
Shallow trenches <2' allowed
Deep trenches >6' allowed
At-grade* allowed
Imported fill* allowed
Sand-lined trenches* allowed
Gravel-less (chambers)* allowed
Seepage pits allowed
Constructed wetland* allowed
Evapotranspiration system*
allowed
Pressure drip irrigation* allowed
Absorption mound* allowed
YES
55
44
37
38
28
22
48
28
2
25
17
42
NO
0
11
18
16
27
33
7
25
51
28
36
11
EXAMPLE: Local Variation in California. From California
Onsite Status Report, CSU-Chico CWTRC/EPA (1999)
Underground
Injection Control
Regulations may
be found at Title
40 of the Code of
Regulations,
parts 144-147.
Onsite Wastewater
Online:
www.epa.gov/owm/decent/
index.htm
or www.nsfc.wvu.edu
DISCLAIMER: The statements in this document are intended solely as guidance.
This document is not intended, nor can it be relied upon, to create any rights
enforceable by any party in litigation with the United States. EPA or the program
Primacy Agency may decide to follow the guidance provided in this document, or
to act at variance with the guidance based on its analysis of the specific facts
presented. This guidance may be revised without public notice to reflect changes
in EPA's approach to implementing the authorities discussed in the document or to
clarify and update text.
------- |