United States
                     Environmental Protection
                     Off ice of Water
                     Washington, D.C.
September 1999
Storm  Water
Technology Fact Sheet
Infiltration  Drainfields

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
                                                        Washed Stone Reservoir
                                                    6" - 12" Sand Filter
 Source: Metropolitan Washington Council of Governments, 1987.


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.


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.


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

      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

      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?


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.

        Design Criteria
  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
                                Minimum depth to bedrock and seasonally high water table: 4 feet.
                                Minimum setback from building foundations:  10 feet downgradient, 100 feet
                                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
                                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
Dispersion Manifold

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

      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.


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


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.


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-

5.      Washington State Department of Ecology,
       1992.  Storm Water Management Manual
       for the Puget Sound Basin.


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
Bob Biebel
916 N. East Avenue, P.O. Box 1607

Washington State Department of Ecology
Stan Ciuba
Stormwater Unit
P.O . Box 47696
Olympia, WA 98504

   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

   Observation Well: 1 at $500 each

   Pretreatment Chamber: 1 at $10,000

   Miscellaneous (backfilling, overflow pipe, sodding, etc.):


   Contingencies (engineering, administration, permits, etc.)= 25%


   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

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