v>EPA
                     United States
                     Environmental Protection
                     Agency
                    Office of Water
                    Washington, D.C.
EPA832-F-99-019
September 1999
Storm Water
Technology  Fact Sheet
Infiltration Trench
 DESCRIPTION

 Urban  development is  significantly  increasing
 surface  runoff  and  contamination  of local
 watersheds. As a result, infiltration practices, such
 as infiltration trenches, are being employed to
 remove suspended  solids, particulate  pollutants,
                   coliform bacteria, organics, and some soluble forms
                   of metals and nutrients from storm water runoff. As
                   shown in Figure 1, an infiltration trench is an
                   excavated trench, 0.9 to 3.7 meters (3 to 12 feet)
                   deep, backfilled with a stone aggregate,  and lined
                   with filter fabric.  A small portion of the runoff,
                   usually the first flush, is diverted to the infiltration
                                                     REMOVABVLE
                                                     WELL CAP
        GEOTEXTILE
        FILTER FABRIC
     UNDISTURBED SOIL

     MINIMUM INFILTRATION RATE
     OF 0.50 INCH PER HOUR
        9 INCH SQUARE STEEL FOOT PLATE
                                    4 FT. DEEP TRENCH
                                    FILLED WITH 1-3 INCH
                                      CLEAN STONE
                                6 INCH
                                DIAMETER
                                PVC PIPE
                         1/2 INCH DIAMETER REBAR ANCHOR
Source: Southeastern Wisconsin Regional Planning Commission, 1991.

                     FIGURE 1 TYPICAL INFILTRATION TRENCH

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trench, which is located either underground or at
grade.  Pollutants are filtered out of the runoff as it
infiltrates the surrounding soils. Infiltration trenches
also provide groundwater recharge and preserve
baseflow in nearby streams.

APPLICABILITY

Infiltration trenches are often used in place of other
Best Management Practices where limited land is
available.  Infiltration trenches are most widely used
in warmer, less arid regions of the U.S. However,
recent  studies conducted  in  Maryland and New
Jersey  on trench performance and operation and
maintenance have demonstrated the applicability of
infiltration trenches in colder climates if surface
icing is avoided (Lindsey, et al, 1991).

Infiltration trenches capture and treat small amounts
of runoff,  but do not control peak hydraulic flows.
Infiltration trenches may be used in conjunction with
another Best Management Practice (BMP), such as
a detention  pond, to  provide both water quality
control and peak flow control (Harrington,  1989).
Figure 2  is an  example of such  a combined
technology.  This type of infiltration trench has a
concentrated input, as opposed to dispersed input
(as  shown in  Figure  1).   This system stores  the
entire storm water volume with the water quality
(BMP) volume connected to the infiltration system.
This is commonly achieved with a slow release of
the  storm water management volume through an
orifice set at a specified level in the storage facility.
As a result the BMP water quality volume will equal
the  storm water detention area below the  orifice
level which must infiltrate to exit.

Runoff that  contains  high levels  of sediments or
hydrocarbons  (oil  and grease) that may clog  the
trench  are  often  pretreated with  other  BMPs.
Examples of some pretreatment BMPs include grit
chambers, water quality  inlets,  sediment traps,
swales, and vegetated filter strips (SEWRPC, 1991,
Harrington,  1989).
ADVANTAGES AND DISADVANTAGES

Infiltration trenches provide efficient removal of
suspended solids, paniculate pollutants, coliform
bacteria, organics and some soluble forms of metals
and  nutrients from  storm  water runoff.   The
captured runoff infiltrates the surrounding soils and
increases  groundwater recharge and baseflow in
nearby streams.

Negative   impacts  include   the  potential  for
groundwater contamination and a high likelihood of
early failure if not properly maintained.

As  with any infiltration BMP,  the  potential for
groundwater contamination  must   be  carefully
considered, especially if the groundwater is used for
human consumption or agricultural purposes.  The
infiltration trench is not suitable for sites that use or
store chemicals  or  hazardous  materials unless
hazardous and toxic materials are prevented from
entering the trench. In these areas, other BMPs that
do  not  interact with the  groundwater should be
considered.    The  potential  for  spills   can be
minimized by  aggressive  pollution prevention
measures. Many municipalities and industries have
developed comprehensive spill prevention control
and  countermeasure (SPCC) plans.  These plans
should be  modified to include the infiltration trench
and the contributing drainage area.  For example,
diversion  structures can be used to prevent spills
from entering the infiltration trench.

Because  of  the   potential   to   contaminate
groundwater, extensive site investigation must be
undertaken early in the  site  planning process to
establish site suitability for the installation of an
infiltration trench. The use of infiltration trenches
may be limited by a number of factors, including
type of native  soils,  climate,  and location of
groundwater tables.  Site characteristics,  such as
excessive  slope of the drainage area, fine-particled
soil types, and proximate location of the water table
and bedrock, may preclude the use of infiltration
trenches.  The slope of the surrounding area should
be such that the runoff is evenly distributed  in sheet
flow as it enters  the trench unless specifically
designed  for  concentrated  input.    Generally,
infiltration trenches are not suitable for areas with
relatively impermeable soils containing clay and silt

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or in areas with fill.  The trench should be located
well above the water table so that the runoff can
filter through the trench and into the surrounding
soils and eventually into  the groundwater.   In
addition, the drainage area should not convey heavy
levels of sediments or hydrocarbons to the trench.
For this reason, trenches serving parking lots must
be preceded by appropriate pretreatment such as an
oil-grit separator.  This measure will make effective
maintenance feasible. Generally, trenches that are
constructed under parking lots must provide access
for maintenance.

An additional  limitation  on use  of infiltration
trenches is the climate. In cold climates, the trench
surface  may freeze, thereby preventing the runoff
from entering the trench and allowing the untreated
runoff to enter surface water. The surrounding soils
may also freeze, reducing infiltration into the soils
and groundwater. However, recent studies indicate
that if properly designed and maintained, infiltration
trenches can operate effectively in colder climates.
By keeping the trench surface free of compacted
snow and ice, and by ensuring that part of the trench
is constructed below the frost line, the performance
of the infiltration trench during cold weather will be
greatly improved.

Finally, there have been a number of concerns raised
about the long term effectiveness of infiltration
trench systems.   In the past, infiltration trenches
have demonstrated a relatively short life span, with
over 50 percent  of  the systems checked having
partially or completely failed after 5 years. A recent
study of infiltration trenches in Maryland (Lindsey
et  al.,  1991) found that  53 percent were  not
operating as designed, 36 percent were partially or
totally clogged, and another 22  percent exhibited
slow filtration.   Longevity can be increased  by
careful geotechnical evaluation prior to construction
and by  designing and implementing an inspection
and maintenance plan. Soil infiltration rates and the
water table depth should be evaluated to ensure that
conditions are satisfactory for proper operation of
an infiltration trench. Pretreatment structures, such
as  a vegetated buffer strip or water quality inlet, can
increase  longevity   by   removing  sediments,
hydrocarbons, and other materials that may clog the
trench.    Regular  maintenance,   including  the
replacement of clogged aggregate, will also increase
the effectiveness and life of the trench.

DESIGN CRITERIA

Prior to trench construction, a review of the design
plans  may  be  required   by  state  and  local
governments.  The design  plans should include a
geotechnical  evaluation   that  determines  the
feasibility of using an infiltration trench at the site.
Soils should have a low silt and clay content and
have infiltration rates greater than  1.3 centimeters
(0.5  inches) per hour.  Acceptable soil texture
classes include sand, loamy sand, sandy loam and
loam. These soils are within the A or B hydrologic
group.   Soils in the C or D hydrologic groups
should be avoided. Soil survey reports published by
the  Soil  Conservation Service can be used  to
identify soil types and infiltration rates.  However,
sufficient soil  borings  should  always be taken to
verify site conditions. Feasible sites should have a
minimum of 1.2 meters (4 feet) to bedrock in order
to reduce excavation costs.  There should also be at
least 1.2 meters (4 feet) below the  trench to the
water table to  prevent potential ground  water
problems.  Trenches should also be located at least
30.5 meters (100 feet) upgradient from water supply
wells and 30.5  meters (100 feet) from building
foundations.    Land  availability,  the depth  to
bedrock,  and  the depth to the water table will
determine whether the infiltration trench is located
underground or at grade.   Underground trenches
receive runoff through pipes or channels, whereas
surface   trenches  collect   sheet flow  from  the
drainage area.

In general, infiltration trenches are suitable  for
drainage  areas  up to  4  hectares (10  acres)
(SEWRPC, 1991,  Harrington, 1989).  However,
when the drainage area exceeds 2 hectares (5 acres),
other BMPs should be carefully considered.  The
drainage area must be fully developed and stabilized
with vegetation before constructing an infiltration
trench. High sediment loads from unstabilized areas
will quickly clog the infiltration trench. Runoff from
unstabilized areas should be diverted away from the
trench into a construction BMP until vegetation is
established.

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                  „ „ „
                  niE=^niiiŁ=
          Stormwater
     Detention Volume
          BMP Water
       Quality Volume
                                        Two Perforated
                                        Overflow Collection
                                        Lines
                                iSediment A.$Q\ Sediment/    YSedimentJ
        Aggregate
Bottom Sand Filter
Three Corrugated
Metal Perforated
Pipes
Geotextile Filter
Fabric (Mirafi 700X
or Equivalent)
Source: Fairfax County Soils Office, 1991.

    FIGURE 2 INFILTRATION TRENCH WITH CONCENTRATED INPUT AND AUGMENTED PIPE
                                           STORAGE
The drainage area slope determines the velocity of
the runoff and  also influences  the  amount of
pollutants entrained in the runoff.   Infiltration
trenches work best  when the upgradient drainage
area slope is less than 5 percent (SEWRPC, 1991).
The downgradient slope should be no greater than
20 percent to minimize slope failure and seepage.

The  trench  surface  may  consist of  stone or
vegetation with inlets to evenly  distribute the runoff
entering the trench  (SEWRPC, 1991, Harrington,
1989). Runoff can  be captured by depressing the
trench surface or by placing a berm at the down
gradient side of the trench.

The basic infiltration trench design utilizes stone
aggregate in the top of the  trench to promote
filtration; however, this design  can be modified by
substituting pea gravel for stone aggregate in the
top 0.3 meter (1 foot) of the trench. The pea gravel
improves  sediment  filtering and maximizes the
pollutant removal in the top of the trench. When
the modified trenches become clogged,  they can
generally  be  restored to  full  performance  by
removing  and replacing only the pea gravel layer,
without replacing the lower stone aggregate layers.
          Infiltration trenches can also be modified by adding
          a layer of organic material (peat) or loam to the
          trench  subsoil.   This  modification  appears  to
          enhance the removal of metals and nutrients through
          adsorption. The trenches are then covered with an
          impermeable  geotextile membrane overlain  with
          topsoil and grass (Figure 2).

          A vegetated buffer strip (6.1 to 7.6 meters, or 20-
          25 feet, wide) should be established adjacent to the
          infiltration trench to capture large sediment particles
          in the runoff.   The buffer strip should be installed
          immediately after trench construction using sod
          instead of  hydroseeding (Schueler,  1987).  The
          buffer strip should be graded with a slope between
          0.5 and 15 percent so that runoff enters the trench
          as sheet flow. If runoff is piped or channeled to the
          trench, a level spreader must be installed to create
          sheet flow (Harrington, 1989).

          During excavation and trench construction,  only
          light equipment  such as backhoes or wheel and
          ladder type trenchers should be used to minimize
          compaction of the surrounding soils.  Filter fabric
          should be placed around the walls and bottom of the
          trench  and  0.3 meters  (1 foot)  below the trench

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surface. The filter fabric should overlap each side of
the trench in order to cover the top  of the stone
aggregate layer (see  Figure  1).   The filter fabric
prevents sediment  in the runoff and soil particles
from the sides of the trench from clogging the
aggregate. Filter fabric that is placed 0.3 meters (1
foot)  below  the  trench surface will  maximize
pollutant removal within the top layer of the trench
and decrease the pollutant loading to the trench
bottom, reducing frequency of maintenance.

The required trench volume can be determined by
several methods. One method calculates the volume
based on capture of the first flush, which is defined
as the first 1.3 centimeters (0.5 inches) of runoff
from the contributing drainage  area (SEWRPC,
1991).  The State  of Maryland  (MD., 1986) also
recommends sizing the  trench based on the first
flush, but  defines first flush  as the first  1.3
centimeters  (0.5 inches)  from  the  contributing
impervious area.   The Metropolitan  Washington
Council of Governments (MWCOG) suggests that
the trench volume  be  based  on the first  1.3
centimeters (0.5 inches) per impervious acre or the
runoff produced from a  6.4 centimeter (2.5 inch)
storm.   In Washington  D.C., the  capture of 1.3
centimeters  (0.5   inches) per  impervious  acre
accounts for 40 to  50 percent of the annual storm
runoff volume.  The runoff not captured by the
infiltration trench should be bypassed to  another
BMP (Harrington,  1989) if treatment of the entire
runoff from the site is desired.

Trench  depths are usually between  0.9 and 3.7
meters  (3  and   12 feet)  (SEWRPC,   1991,
Harrington, 1989).  However, a depth of 2.4 meters
(8 feet) is most commonly used (Schueler, 1987).
A site specific trench depth can be calculated based
on the soil infiltration rate, aggregate void space,
and the trench storage  time  (Harrington,  1989).
The stone aggregate used in the trench is normally
2.5 to 7.6 centimeters (1 to 3  inches) in diameter,
which  provides  a void  space  of  40  percent
(SEWRPC,  1991,  Harrington,  1989,  Schueler,
1987).

A minimum drainage time of 6 hours  should be
provided to ensure satisfactory pollutant removal in
the infiltration trench (Schueler,  1987, SEWRPC,
1991).  Although  trenches may  be  designed to
provide temporary  storage of  storm water,  the
trench should drain prior to the next storm event.
The drainage time will vary by precipitation zone.
In the Washington, D.C. area, infiltration trenches
are designed to drain within 72 hours.

An observation well is recommended to monitor
water levels in the trench.  The well can be a 10.2 to
15.2 centimeter (4  to 6 inch)  diameter PVC pipe,
which is anchored vertically to a foot plate at the
bottom of the trench as shown in Figure 1  above.
Inadequate drainage may indicate the need  for
maintenance.

PERFORMANCE

Infiltration  trenches  function similarly  to rapid
infiltration  systems that  are used in wastewater
treatment. Estimated pollutant removal efficiencies
from  wastewater  treatment  performance  and
modeling studies are shown in Table 1.

Based  on this data, infiltration trenches can be
expected to remove up to 90 percent of sediments,
metals, coliform bacteria and organic matter, and up
to 60 percent of phosphorus  and  nitrogen in  the
runoff (Schueler,  1992).   Biochemical  oxygen
demand (BOD) removal is estimated to be between
70 to 80 percent. Lower removal rates for nitrate,
chlorides and soluble metals should be expected,
       TABLE 1 TYPICAL POLLUTANT
           REMOVAL EFFICIENCY
        Pollutant
Typical Percent
Removal Rates
Sediment
Total Phosphorous
Total Nitrogen
Metals
Bacteria
Organics
Biochemical Oxygen
Demand
90%
60%
60%
90%
90%
90%
70-80%
  Source: Schueler, 1992.

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especially in sandy soils (Schueler, 1992).

Pollutant removal efficiencies may be improved by
using washed aggregate and adding organic matter
and  loam to the subsoil.  The stone aggregate
should be washed to remove dirt and fines before
placement in the trench.  The addition of organic
material and loam to the trench subsoil will enhance
metals and nutrient removal through adsorption.

OPERATION AND MAINTENANCE

Infiltration, as with all BMPs, must have routine
inspection and maintenance designed into the life
performance of the facility. Maintenance should be
performed as indicated by these routine inspections.
The  principal maintenance objective is to prevent
clogging,  which may  lead to  trench  failure.
Infiltration trenches and  any pretreatment BMPs
should be inspected after large storm events and any
accumulated debris or material removed.  A more
thorough  inspection  of the trench  should  be
conducted at least annually.   Annual inspection
should include monitoring of the observation well to
confirm that the trench is draining within the
specified time. Trenches with filter fabric should be
inspected for sediment deposits by removing a small
section of the top layer. If inspection indicates that
the trench  is partially  or completely clogged, it
should be restored to its design condition.

When vegetated buffer strips are used, they should
be inspected for erosion or other damage after each
major  storm  event.   The vegetated buffer  strip
should have healthy grass that is routinely mowed.
Trash,  grass clippings and other debris should be
removed from the trench perimeter and should be
disposed properly. Trees and other large vegetation
adjacent to the trench should also be removed to
prevent damage to the trench.

COSTS

Construction costs  include clearing,  excavation,
placement of the filter fabric and stone, installation
of the monitoring well,  and establishment  of a
vegetated buffer strip.  Additional  costs include
planning, geotechnical evaluation, engineering and
permitting. The  Southeastern Wisconsin Regional
Planning  Commission  (SEWRPC,  1991)   has
developed cost curves and tables  for infiltration
trenches  based  on  1989  dollars.   The 1993
construction cost for a relatively large infiltration
trench (i.e., 1.8 meters (6 feet) deep and 1.2 meters
(4 feet) wide with a 68 cubic meter (2,400 cubic
feet) volume)  ranges from $8,000 to $19,000.  A
smaller infiltration trench (i.e., 0.9 meters (3 feet)
deep and 1.2 meters (4 feet) wide with a 34 cubic
meter (1,200 cubic feet)  volume) is estimated to
cost from $3,000 to $8,500.

Maintenance costs include buffer strip maintenance
and trench inspection and rehabilitation.  SEWRPC
(1991) has also  developed  maintenance costs for
infiltration trenches. Based on the above examples,
annual operation and  maintenance costs would
average $700 for the large trench and $325 for the
small trench. Typically, annual  maintenance costs
are approximately 5 to 10 percent of the capital cost
(Schueler, 1987).  Trench rehabilitation, may  be
required every 5 to 15 years. Cost for rehabilitation
will vary  depending on  site conditions  and  the
degree of clogging. Estimated rehabilitation costs
run from 15 to  20 percent of the original capital
cost (SEWRPC,  1991).

REFERENCES

1.     Fugill,  R., 1991-1992.  Fairfax County Soil
       Science  Office. Personal  communication
       with Lauren Fillmore, Parsons Engineering
       Science, Inc.

2.     Harrington,  B.W.,  1989.   Design and
       Construction of Infiltration Trenches  in
       Design of Urban Runoff Quality Control.
       American Society  of Civil Engineers.

3.     Lindsey, G., L. Roberts, and W. Page, 1991.
       Storm  Water Management  Infiltration.
       Maryland Department of the Environment,
       Sediment and Storm Water Administration.

4.     Maryland Department of Natural Resources,
       1986. Minimum Water Quality Objectives
       and Planning Guidelines for Infiltration
       Practices. Water Resources Administration,
       Sediment and Storm Water Division.

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5.      Northern  Virginia  Planning   District
       Commission (NVPDC) and Engineers and
       Surveyors  Institute,  1992.    Northern
       Virginia BMP Handbook:  A  Guide  to
       Planning and Designing Best Management
       Practices in Northern Virginia.

6.      Schueler, T.R., 1987. Controlling Urban
       Runoff: A Practical Manual for Planning
       and Designing Urban Best Management
       Practices.     Metropolitan   Washington
       Council of Governments.

7.      Schueler,  T.R.,   1992.     A   Current
       Assessment  of Urban Best Management
       Practices.     Metropolitan   Washington
       Council of Governments.

8.      Southeastern Wisconsin Regional Planning
       Commission  (SEWRPC), 1991.  Costs  of
       Urban Nonpoint  Source  Water Pollution
       Control Measures.  Technical Report No.
       31.

9.      U. S. EPA, 1991.  Detention and Retention
       Effects on Groundwater, Region V.

10.    Washington,  State of, 1992. Storm Water
       Management Manual for the Puget Sound
       Basin (The Technical Manual), Department
       of Ecology.

ADDITIONAL INFORMATION
King County, Washington
Dave Hancock
Department of Natural Resources, Water and Land
Resources Division, Drainage Services Section
700 5th Avenue, Suite 2200
Seattle, WA 98104

Montgomery County, Maryland
Rick Brush
Department of Permitting Services, Water Resource
Section
250 Hungerford Drive, Suite 175
Rockville, MD 20850

Southeastern  Wisconsin   Regional  Planning
Commission
Bob Biebel
916 N. East Avenue, P.O. Box 1607
Waukesha, WI53187

The mention of trade names or commercial products
does not constitute endorsement or recommendation
for the use by the U.S.  Environmental Protection
Agency.
City of Alexandria, Virginia
Warren Bell
Department of Transportation and Environmental
Services
P.O. Box 178
Alexandria, VA 22313

Carroll County, Maryland
Martin Covington
Bureau of Developmental Review
225 North Center Street
Westminster, MD 21157-5194
                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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