v>EPA
United States
Environmental Protection
Agency
Off ice of Water
Washington, D.C.
EPA832-F-99-018
September 1999
Storm Water
Technology Fact Sheet
Infiltration Drainfields
DESCRIPTION
Infiltration drainfields are innovative technologies
that are specially designed to promote storm water
infiltration into subsoils. These drainfields help to
control runoff and prevent the contamination of
local watersheds. The system is usually composed
of a pretreatment structure, a manifold system, and
a drainfield. Runoff is first diverted into a storm
sewer system that passes through a pretreatment
structure such as an oil and grit separator. The oil
and grit chamber effectively removes coarse
sediment, oils, and grease from the runoff. The
storm water runoff then continues through a
manifold system into the infiltration drainfield. The
manifold system consists of a perforated pipe which
distributes the runoff evenly throughout the
infiltration drainfield. The runoff then percolates
through an underlying aggregate sand filter and
filter fabric into the subsoils. An example of this
system is provided in Figure 1.
Perforated Pipe Manifold
Observation Well
Top Soil
000°0°00
0°00000°00
Washed Stone Reservoir
6" - 12" Sand Filter
Source: Metropolitan Washington Council of Governments, 1987.
FIGURE 1 TYPICAL INFILTRATION DRAINFIELD SCHEMATIC
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Common design modifications to the infiltration
drainfield best management practice (BMP) include
the installation of porous pavement surrounded by
a grass filter strip over the infiltration drainfield or
the insertion of an emergency overflow pipe in the
oil and grit pretreatment chamber. The overflow
pipe allows runoff volumes exceeding design
capacities to discharge directly to a trunk storm
sewer system.
APPLICABILITY
Infiltration drainfields are most applicable on sites
with a relatively small drainage area (less than 15
acres.) They can be used to control runoff from
parking lots, rooftops, impervious storage areas, or
other land uses. Infiltration drainfields should not
be used in locations that receive a large sediment
load that could clog the pretreatment system, which
in turn would plug the infiltration drainfield and
reduce its effectiveness.
Soils in areas where the installation of an
infiltration drainfield is being considered should
have field-verified permeability rates of greater than
0.5 inches per hour and should include a 4-foot
minimum clearance between the bottom of the
system and the bedrock or the water table.
ADVANTAGES AND DISADVANTAGES
The use of infiltration drainfields may be restricted
in regions with colder climates, arid regions,
regions with high wind erosion rates (because of
increased windblown sediment loads), and areas of
sole source aquifers. Some specific limitations of
infiltration drainfields include:
• High maintenance when sediment loads to
the drainfield are heavy.
• High costs of engineering design,
excavation, fill material, and pretreatment
systems.
• Short life span if not well maintained.
• Not suitable for use in regions with clay or
silty soils.
• Not suitable for use in regions where
groundwater is used locally for human
consumption.
• Anaerobic conditions that could clog the
soil and reduce the capacity and
performance of the system may develop in
underlying soils if there is not sufficient
time between storm events to allow the soil
to dry out.
One potential negative impact of infiltration
drainfields is the risk of groundwater
contamination. Studies to date do not indicate that
this is a major risk if site suitability guidelines are
observed. However, migration of nitrates and
chlorides from the drainfield has been documented.
Additional questions regarding infiltration
drainfields remain to be answered:
• Is the oil and grit separator the most
effective pretreatment system to protect
infiltration capacity?
• What are the pollutant removal capacities of
infiltration drainfields with various
pretreatment systems?
• Is the performance of infiltration drainfields
better than the performance of infiltration
basins and trenches during subfreezing
weather and snow melt runoff conditions?
• What level of maintenance is required to
ensure proper performance?
DESIGN CRITERIA
Infiltration drainfields, along with most other
infiltration BMP structures (infiltration trenches,
basins, etc.) have proved to have short life spans in
the past. Failure of the systems has been attributed
to poor design, inadequate construction techniques,
low permeability soils, and a lack of pretreatment.
Some design factors which could significantly
increase the longevity of infiltration drainfields and
other infiltration processes are shown in Table 1.
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TABLE 1 INFILTRATION DRAINFIELD DESIGN CRITERIA
Design Criteria
Guidelines
Site Evaluation Take soil borings to a depth of at least 4 feet below bottom of stone reservoir to
check for soil permeability, porosity, depth to seasonally high water table, and
depth to bedrock.
Not recommended on slopes greater than 5 percent and best when slopes are as
flat as possible.
Minimum infiltration rate 3 feet below bottom of stone reservoir: 0.5 inches per
hour.
Minimum depth to bedrock and seasonally high water table: 4 feet.
Minimum setback from building foundations: 10 feet downgradient, 100 feet
upgradient.
Drainage area should be less than 15 acres.
Literature values suggest this parameter is highly variable and dependent upon
regulatory requirements. One typically recommended storage volume is the
stormwater runoff volume produced in the tributary watershed by the 6- month, 24-
hour duration storm event.
Minimum: 12 hours.
Maximum: 72 hours.
Recommended: 24 hours.
Excavate and grade with light equipment with tracks or oversized tires to prevent
soil compaction.
As needed, divert stormwater runoff away from site before and during
construction.
Atypical infiltration cross-section consists of the following : 1) a stone reservoir
consisting of coarse 1.5 to 3-inch diameter stone (washed); 2) 6 to 12-inch sand
filter at the bottom of the drainfield; and 3) filter fabric.
Pretreatment is recommended to treat runoff from all contributing areas.
A dispersion manifold should be placed in the upper portions of the infiltration
drainfield. The purpose of this manifold is to evenly distribute storm water runoff
over the largest possible area. Two to four manifold extension pipes are
recommended for most typical infiltration drainfield applications.
Source: Minnesota Pollution Control Agency, 1989.
Design Storm Storage Volume
Drainage Time for Design Storm
Construction
Pretreatment
Dispersion Manifold
PERFORMANCE
The effectiveness of infiltration drainfields depends
upon their design. When runoff enters the
drainfield, 100 percent of the pollutants are
prevented from entering surface water. Any water
that bypasses the pretreatment system and drainfield
will not be treated. Pollutant removal mechanisms
include absorption and adsorption, straining,
microbial decomposition in the soil below the
drainfield, and trapping of sediment, grit, and oil in
the pretreatment chamber.
Currently there is little monitoring data on the
performance of infiltration drainfields. However,
some monitoring data is available on porous
pavements. The design criteria for porous
pavements is very similar to the design criteria of
infiltration drainfields. An estimate of porous
pavement pollutant removal efficiencies ranges
between 82 and 95 percent for sediment, 65 percent
for total phosphorus, and 80 to 85 percent for total
nitrogen. Porous pavement works most effectively
for about 6 months.
Some key factors to increase pollutant removal
efficiencies include:
• Properly maintaining the system.
• Implementing good housekeeping practices
in the tributary drainage area.
• Allowing sufficient drying time
(approximately 24 hours) between storm
events.
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• Choosing a site with highly permeable soils
and subsoils.
• Incorporating a pretreatment system.
• Ensuring that there is sufficient organic
matter in subsoils.
• Using a sand layer on top of a filter fabric at
the bottom of the drainfield.
OPERATION AND MAINTENANCE
Routine maintenance of infiltration drainfields is
extremely important. The pretreatment grit
chamber should be checked at least four times per
year and after maj or storm events. Sediment should
be cleaned out when the sediment depletes more
than 10 percent of the available infiltration capacity.
This can be done manually or by vacuum pump.
Inlet and outlet pipes should also be inspected at
this time.
The infiltration drainfield should contain an
observation well that can provide information on
how well the system is operating. It is
recommended that the observation well be
monitored daily after runoff-producing storm
events. If the infiltration drainfield does not drain
after three days, it usually means that the drainfield
is clogged. Once the performance characteristics of
the structure have been verified, the monitoring
schedule can be reduced to a monthly or quarterly
basis.
COSTS
There is little information on the cost of infiltration
drainfields. However, the construction costs for
installing an infiltration drainfield that is 30.5
meters (100 feet) long, 15 meters (50 feet) wide, 2.4
meters (8 feet) deep and with 1.2 meters (4 feet) of
cover can be estimated using the general
information in Table 2.
REFERENCES
1. Metropolitan Washington Council of
Governments, 1987. Controlling Urban
Runoff: A Practical Manual for
Planning and Designing Urban BMPs.
2. Minnesota Pollution Control Agency, 1989.
Protecting Water Quality in Urban Areas.
3. Southeastern Wisconsin Regional Planning
Commission, 1991. Costs of Urban
Nonpoint Source Water Pollution Control
Measures, Technical Report No. 31.
4. U.S. EPA, 1992. Storm water Management
for Industrial Activities: Developing
Pollution Prevention Plans and Best
Management Practices. EPA 832-R-92-
006.
5. Washington State Department of Ecology,
1992. Storm Water Management Manual
for the Puget Sound Basin.
ADDITIONAL INFORMATION
Center for Watershed Protection
Tom Schueler
8391 Main St.
Ellicott City, MD21043
State of Minnesota
Lou Flynn
Minnesota Pollution Control Agency
520 Lafayette Road North
St. Paul, MN 55155
Northern Virginia Planning District Commission
David Bulova
7535 Little River Turnpike, Suite 100
Annandale, VA 22003
Southeastern Wisconsin Regional Planning
Commission
Bob Biebel
916 N. East Avenue, P.O. Box 1607
Waukesha,WI53187
Washington State Department of Ecology
Stan Ciuba
Stormwater Unit
P.O . Box 47696
Olympia, WA 98504
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TABLE 2 ESTIMATED COST FOR INSTALLING AN INFILTRATION DRAINFIELD
Excavation Costs: 2,220 yd3 @ $5.00/yd3
Stone Fill: (1,296 yd3)($20.00/yd3)
Sand Fill: (185 yd3)($10.00/yd3)
Filter Fabric: Top and Bottom= 10,000 ft2
Sides= 1,600 + 800= 2,400 ft2 +10%= 13,640 ft2
(13,640 ft2)(1 yd2/9 ft2)($3.00/yd2)
Perforated Manifold and Inlet Pipe: 75 ft + (4)(40ft)= 235 ft
+ 40 ft = 275 ft
(275)($10.00/ft)
Observation Well: 1 at $500 each
Pretreatment Chamber: 1 at $10,000
Miscellaneous (backfilling, overflow pipe, sodding, etc.):
Subtotal:
Contingencies (engineering, administration, permits, etc.)= 25%
Total:
Note: Unit price will vary greatly depending upon local market conditions
$11,100
$25,920
$1,850
$4,550
$2,750
$500
$10,000
$1000
$57,670
$14,420
$72,090
Source: SWRPC, 1992.
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for the use by the U.S.
Environmental Protection Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
I
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY BR/TfTfff
MTB
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