| drainfields.

. 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 ofa 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 major 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
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, MD 21043

State of Minnesota

Lou Flynn

Minnesota Pollution Control Agency
520 Lafayette Road North .
St. Paul, MN 55155

Norther Virginia Planning District Commission
David Bulova

7535 Little River Turnpike, Suite 100
Annandale, VA 22003

Southeastern Wisconsin
Commission

Bob Biebel

916 N. East Avenue, P.O. Box 1607
Waukesha, WI 53187

Regional Planning

Washington State Department of Ecology
Stan Ciuba

Stormwater Unit

P.O . Box 47696

Olympia, WA 98504


TABLE 1 INFILTRATION DRAINFIELD DESIGN CRITERIA

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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.

Design Storm Storage Volume

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.

Excavate and grade with light equipment with tracks or oversized tires to prevent

As needed, divert stormwater runoff away from site before and during

A typical 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.

Drainage Time for Design Storm Minimum: 12 hours.
Maximum: 72 hours.
Recommended: 24 hours.
Construction
soil compaction.
construction.
Pretreatment

Dispersion Manifold

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.

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.


TABLE 2 ESTIMATED COST FOR INSTALLING AN INFILTRATION DRAINFIELD

Excavation Costs: 2,220 yd? @ $5.00/yd° $11,100
Stone Fill: (1,296 yd*)($20.00/yd*) $25,920
Sand Fill: (185 yd°)($10.00/yd°) $1,850
Filter Fabric: Top and Bottom= 10,000 ft”

Sides= 1,600 + 800= 2,400 ft? +10%= 13,640 ft? $4,550

(13,640 ft)(1 yd? /9 ft?)($3.00/yd?)
Perforated Manifold and Inlet Pipe: 75 ft + (4)(40ft)= 235 ft

+ 40 ft = 275 ft $2,750

(275)($10.00/ft)
Observation Well: 1 at $500 each $500
Pretreatment Chamber: 1 at $10,000 $10,000
Miscellaneous (back filling, overflow pipe, sodding, etc.): $1000
Subtotal: $57,670
Contingencies (engineering, administration, permits, etc.)= 25% $14,420
Total: $72,090

Note: Unit price will vary greatly depending upon local market conditions

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

Exealence in complance through opting technical sokisons
MUNICIPAL TECHNOLOGY RANCHO


United States Office of Water EPA 832-F-99-018
Environmental Protection Washington, D.C. September 1999
Agency

GEPA Storm Water

Technology Fact Sheet
Infiltration Drainfields


United States
Environmental Protection

Agency

Office of Water
Washington, D.C.

EPA 832-F-99-018
September 1999

GEPA

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.

Top Soil

Perforated Pipe Manifold

Observation Well

Washed Stone Reservoir

6” - 12" Sand Filter

Source: Metropolitan Washington Council of Governments, 1987.

FIGURE 1 TYPICAL INFILTRATION DRAINFIELD SCHEMATIC


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?

. Whatare 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.