vvEPA
                      United States
                      Environmental Protection
                      Agency
                      Office of Water
                      Washington, D.C.
EPA 832-F-99-025
September 1999
Storm Water
Technology  Fact  Sheet
Storm Water  Wetlands
DESCRIPTION

Wetlands  are  those  areas that  are  typically
inundated with surface or ground water and that
support plants adapted to saturated soil conditions.
A typical shallow marsh wetland is shown in Figure
1.   Wetlands have been  described as "nature's
kidneys"  because  the physical, chemical, and
biological processes that occur in wetlands  break
down some compounds (e.g., nitrogen-containing
compounds, sulfate) and filter others  (Hammer,
1989). The natural pollutant-removal capabilities
of wetlands have brought them increased attention
as storm water best management practices (BMPs).

Wetlands  used  for storm water treatment can be
incidental, natural, or constructed.   Incidental
wetlands are those wetlands that were created as a
                     result of previous development or human activity.
                     The use of natural wetlands  for storm water
                     treatment is discouraged by many experts and/or
                     public interest groups, and may not be an option in
                     many areas. However, some states allow wetlands
                     to be used as storm water BMPs, but only in very
                     restricted circumstances. For example, the State of
                     Florida allows the use of natural wetlands that have
                     been severely  degraded or wetlands  that  are
                     intermittently connected to other waters (i.e., they
                     are connected only when groundwater rises above
                     ground level) (Livingston, 1994). Conversion of
                     natural wetlands to storm water wetlands is done on
                     a case-by-case basis and requires the appropriate
                     state and federal permits (e.g., 401 water quality
                     certification and 404 wetland permit).

                     Two types of constructed wetlands have been used
                                   maintenance bench
            25% of pond perimeter open grass             /
   25-ft wetland buffer landscaped w
     native trees/shrubs for habitat
                                                                   valves for depth control
                            /
                            v
                            wetland mulch to create diversity
Source: MWCOG, 1992a.
                         FIGURE 1 SHALLOW MARSH WETLAND

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successfully   for  wastewater  treatment:    the
subsurface flow (SF) constructed wetland and the
free water surface (FWS) constructed wetland.  In
the FWS  wetland, runoff flows through the  soil-
lined basin at shallow depths.  The wetland consists
of a shallow pool planted with emergent vegetation
(vegetation which is rooted in the sediment but with
leaves at or above the water surface).

In contrast to the FWS  wetland, the SF wetland
basin is lined with a pre-designed amount of rock or
gravel, through which the runoff is conveyed. The
water level in an SF wetland remains below the top
of the  rock or gravel bed.  Studies have indicated
that the SF wetland is well suited for the diurnal
flow pattern of wastewater; however, the peak
flows  from  storm  water or  combined  sewer
overflows  (CSOs)   may be  several  orders  of
magnitude higher than the baseflow. The cost for
a gravel bed to contain the peak storm event would
be very high,  which may preclude the use of SF
wetlands  for  storm  water  or CSO  treatment.
Therefore, the   remainder  of  this fact  sheet
addresses the FWS constructed wetland or natural
and  incidental wetlands for  use  in storm water
applications.

There  are four basic designs  of FWS constructed
wetlands:   shallow marsh,  extended detention
wetland, pond/wetland system, and pocket wetland.
As  shown in Figure 2, these wetlands store runoff
in a shallow basin vegetated with wetland plants.
The selection of one  design  over another  will
depend  on   various  factors,   including   land
availability,  level  and  reliability  of pollutant
removal, and size of the contributing drainage area.

The shallow marsh  design requires the most  land
and a sufficient baseflow to maintain water within
the wetlands.  The basic shallow marsh design can
be modified to store extra water above the normal
pool elevation.   This  wetland,  known  as an
extended detention wetland, attenuates flows and
relieves downstream flooding.

The pond/wetland system has two  separate cells: a
wet pond and a shallow marsh. The wet pond traps
sediments and reduces runoff velocities prior to
entry into  the wetland.  Less land  is required for a
pond/wetland system than for the shallow marsh
system.

Still less land is required for a pocket wetland.
Pocket  wetlands  should   be   designed  with
contributing drainage areas of 0.4 to 4 hectares (1 to
10 acres) and usually require excavation down to
the  water table  for  a  reliable water  source.
Unreliable water  sources and  fluctuating  water
levels result in low plant diversity and poor wildlife
habitat value (MWCOG, 1992b).
    A. Shallow Marsh
    normal pool elevation
      ^Jl^aJJLatfi^^
           forebay
                                  micropool
    B. Extended Detention (ED) Wetland
ED zone
V I ititrtfrCEKrtthl
/ forebay
normal pool elevation
max ED limit
*$f t HfV/V ^^^
micropool
     C. Pond/Wetland System
        normal pool elevation
    V       1
             pond
    D. Pocket Wetland
                                max storm elevation
            ifej
                                  torm
    seasonal high water table
                        normal water table
   FIGURE 2  COMPARATIVE PROFILES OF
 FOUR STORM WATER WETLAND DESJGNS

Cross-sectional profiles  of the four storm water
wetlands not drawn to scale.  In Panel A, most of the
shallow marsh is shallow, supporting emergent wetland
plants. In extended detention wetlands (Panel B), the
runoff storage  of the wetland  is  augmented by
temporary, vertical extended detention storage.  The
pond/wetland system (Panel C) is composed of a deep
and a shallow pool.  Pocket wetlands (Panel D) are
excavated to the groundwater  table  to  keep water
elevation more consistent.

Source: MWCOG, 1992b.

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APPLICABILITY

Wetlands improve the quality  of storm  water
runoff, and can also control runoff volume (e.g.,
extended detention wetland).  Wetlands are one of
the more reliable BMPs for removing pollutants and
are  adaptable to  most  locations  in the  U.S.
Locations with existing wetlands used for  storm
water  treatment   include  Alabama,  California,
Colorado,  Florida,  Illinois,  Maine,   Maryland,
Michigan, Minnesota,  Virginia, and Washington.
Wetlands have been used to treat runoff from
agricultural, commercial, industrial, and residential
areas.

In the past, the natural ability of wetlands to remove
pollutants from water has primarily been harnessed
to treat wastewater.  However, the utilization of
wetlands to treat storm water has gained attention in
recent  years,  and many  storm  water wetlands
treatment systems  are now operational.  Ongoing
evaluations are being conducted to determine the
effectiveness of wetlands in pollutant removal and
to determine the level of maintenance required to
sustain their performance, while other studies are
evaluating the potential for design modifications to
improve wetland performance.

ADVANTAGES AND DISADVANTAGES

Environmental benefits associated with storm water
wetlands  include  improvements  in downstream
water and habitat quality, enhancement of diverse
vegetation and wildlife habitat in urban areas, and
flood attenuation.  Downstream water quality is
improved by the  partial removal of  suspended
solids,  metals, nutrients, and organics from urban
runoff. Habitat quality is also improved as reduced
sediment loads are carried downstream  and the
erosion of stream banks associated  with peak storm
water flows is reduced. Wetlands can support a
diverse wildlife population, including species such
as sandpipers and herons, and can  attenuate runoff
and  alleviate downstream  flooding (particularly
extended detention wetlands).

Storm   water  wetlands   can  cause   adverse
environmental impacts upstream  of the wetland,
within  the wetland itself, and downstream of the
wetland. Storm water wetlands located in a large
watershed (larger than 40 hectares (100 acres)) may
degrade upstream headwaters, which receive no
effective hydrologic control  (MWCOG, 1992b).
The wetland designer can incorporate  upstream
modifications to relieve this negative impact.
Possible adverse effects within the wetland itself
are the potential for blocking fish passage, potential
habitation by undesirable species,  and potential
groundwater contamination. A wetland constructed
in the stream channel may block fish access to part
of the stream, thereby decreasing fish diversity in
the stream.

Geese and mallards may become  undesirable year-
round  residents  of  the  wetland  if   structural
complexity is not included in the wetland design
(i.e., features that limit deep and  open water areas
and open  grassy  areas that are favored by these
birds). These animals will increase the nutrient and
coliform loadings to the wetland and may also
become a nuisance to local residents. The takeover
of vegetation by invasive nuisance plants is also a
potential negative impact. Invasive species pose a
threat to native species and may adversely affect the
wetland's ability to treat storm water. Maintaining
and/or planting upland buffer zones can help to
reduce the introduction of nuisance plant species.
Planting emergent vegetation may  also  reduce
nuisance algal blooms (Carr, 1995).
The issue of groundwater contamination resulting
from the migration of polluted sediments to the
groundwater  has  been  considered  a  potential
negative environmental impact. However, studies
indicate that there is  little risk of groundwater
contamination (MWCOG, 1992b).
A storm water wetland can  act as a  heat sink,
especially during  the summer, and can discharge
warmer waters to downstream water bodies.  The
increased  temperatures can  affect sensitive fish
species  (such as trout and sculpins) and aquatic
insects  downstream.    Therefore,  it  is  not
recommended to construct storm wetlands upstream
of  temperature-sensitive   fish   populations.
Regardless of the sensitivity of downstream species,
the designer should always  take precautions  to
reduce the potential warming effects of wetlands
construction.

Communities may be opposed to a wetland for fear
of mosquitoes and other nuisances, or because of
wetlands' appearance.  However, wetlands can be

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designed attractively and features  (e.g., fish and
vegetation) can be adapted to control mosquitoes
and other nuisances. The use ofGambusia fish for
mosquito control has become a common practice in
warmer climates, while colder climates use  the
black striped  topminnow (Notrophus fundulus)
(U.S. EPA, 1995).  To minimize  the protection
from predators offered by taller plants, the use of
low  growing plants is recommended where pests
are a concern (U.S. EPA, 1996).

Wetlands may remove pollutants less effectively
during the  non-growing season and in localities
with lower temperatures.   Decreases in  some
pollutant-removal efficiencies have been observed
when wetlands are covered with ice and when they
receive snow melt runoff.

Finally, because of the large land requirement for
storm  water  wetlands  systems  (See  Design
Criteria), their use may be precluded in urban
settings and established communities.

Several possible  remedies to  these  impacts  are
discussed in the publication Design of Storm Water
Wetland Systems (MWCOG,  1992).

DESIGN CRITERIA

Local, state and federal permit requirements should
be determined prior to wetland design. Required
permits and certifications may include 401 water
quality  certifications,  402 storm water National
Pollutant Discharge Elimination System (NPDES)
permits, 404 wetland permits, dam safety permits,
sediment and  erosion control plans,  waterway
disturbance permits, forest-clearing permits, local
grading permits, and land use approvals.

A site  appropriate for a wetland must have an
adequate water flow and appropriate underlying
soils.  The baseflow  from the drainage area  or
groundwater must be sufficient to maintain a
shallow pool  in  the  wetland  and support the
wetlands' vegetation, including species susceptible
to damage during dry periods. Underlying soils that
are type B, C, or R (zone of accumulation, partially
altered  parent material and  unaltered parent
material, respectively)  will   have  only  small
infiltration losses. Sites with type A soils (soils rich
in organic matter) may have high infiltration rates.
These sites may require geotextile liners or a  15
centimeter (6 inch) layer of clay.  After any
necessary excavation and grading of the wetland, at
least  10 centimeters (4 inches) of soil should  be
applied to the site.  This material, which may  be
the previously-excavated soil  or  sand and other
suitable material, is needed to provide a substrate in
which the vegetation can become established and to
which it  can become  anchored.   The  substrate
should be soft so that plants can be inserted easily.

The  Metropolitan Washington  Council   of
Governments (MWCOG, 1992b) has recommended
basic sizing  criteria for  wetland design.   The
volume of the wetland is determined as the quantity
of runoff generated by 90 percent of the runoff-
producing storms.    This  volume  will  vary
throughout the U.S. due  to different  rainstorm
patterns.  In the Mid-Atlantic Region, for example,
a 1.25-inch storm is used as the sizing criterion.

Watershed imperviousness will also impact the
runoff volume generated from a  storm.   The
following equations are  used to  determine the
treatment  volume (Vt):

(1)  Rv = 0.05 + 0.009 (I)
    where:
       Rv = storm runoff coefficient
       I = % (as decimal) site imperviousness
(2)  Vt = [(1.25)(Rv)(A)/12](43,560)
    where:
       Vt = treatment volume (cubic feet)
       A = contributing area (acres)

Sizing criteria for wetlands vary, with some states
having their own methods.  For example, shallow
wetland  basins   constructed  in  Maryland are
designed to maximize  basin surface area.  The
surface area should be a minimum of 3 percent  of
the area of the watershed draining to it.  Maryland
recommends  designing for extended detention,
using 24-hour detention of the 1-year  storm for
design purposes. In contrast, the Washington State
Department of Ecology sizes wetlands using the
runoff generated from the 6-month, 24-hour rainfall
event. The minimum surface area established  by
MWCOG for shallow marshes is 2 percent of the
wetland area. The remaining three wetland designs
should have  wetland to watershed ratios greater
than 1 percent.

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                           TABLE 1  GUIDELINES FOR ALLOCATING
                    WETLAND SURFACE AREA AND TREATMENT VOLUME
Target
Allocations
Shallow
Marsh Extended Pond/Wetland
Detention
Wetland
Pocket Wetland
Percent of Wetland Surface Area
Forebay
Micropool
Deepwater
Low Marsh
High Marsh
Semi-Wet
5
5
5
40
40
5
5
5
0
40
40
10
0
5
40
25
25
5
0
0
5
50
40
5
Percent of Treatment Volume
Forebay
Micropool
Deepwater
Low Marsh
High Marsh
Semi-Wet
10
10
10
45
25
0
10
10
0
20
10
50
0
10
60
20
10
0
0
0
20
55
25
0
 Depth:
 Deepwater - 0.5 - 2 meters (1.5 to 6 feet) below normal pool level
 Low Marsh - 0.17- 0.5 meters (0.5 to 1.5 feet) below normal pool level
 High Marsh -0.5 feet below normal pool level
 Semi-Wet - 0 to 2 feet above normal pool level (includes Extended Detention)

 Source: Modified from MWCOG, 1992b.
MWCOG has also  established criteria for water
balance,  maximum flow path,  allocation  of
treatment volume, minimum surface area, allocation
of the surface area, and extended detention.  As
previously  discussed, during  dry weather,  flow
must be  adequate to provide  a baseflow  and to
maintain the vegetation. The flow path should be
maximized to increase the runoffs contact time
with plants and sediments.  The recommended
minimum length to width ratio of the wetland is
2:1. If a ratio of less than 2:1 is necessary,  the use
of baffles,  islands,  and peninsulas can minimize
short  circuiting  (allowing  runoff  to   escape
treatment) by ensuring a long distance from inlet to
outlet.
A suggestion for allocating treatment volumes is
shown in Table  1. The wetland surface area is
allocated to four different depth zones: deepwater
(0.5 to 2 meters, or 1.5 to 6 feet, below normal
pool), low marsh (0.17 to 0.5 meters, or 0.5 to 1.5
feet, below normal pool), high marsh (up to 0.17
meters,  or 0.5  feet,  below  normal  pool), and
semi-wet areas (above normal pool). The allocation
to the various depth zones will create a complex
internal  topography that will  maximize  plant
diversity and increase pollutant removal. The State
of Maryland requires that 50 percent of the shallow
marsh be less than 0.17 meters (0.5 feet) deep, that
25 percent range from 0.17 to 0.33 meters (0.5 feet
to 1 foot) deep, and that the remaining 25 percent
range from 0.67 to 1 meter (2 to 3 feet) deep.

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Extending detention within the wetland increases
the time  for sedimentation and other pollutant-
removal processes to occur and also provides for
attenuation  of flows.   Up  to 50 percent extra
treatment volume can be added into the wetland
system  for  extended  detention.   However,  to
prevent large  fluctuations in the water level that
could potentially harm the vegetation, Extended
Detention elevation should be limited to 11 meters
(33 feet) above the  normal  pool elevation.  The
Extended Detention volume should be detained
between 12 and 24 hours.

Sediment forebays are recommended to decrease
the velocity and sediment loading to the wetland.
The forebays  provide  the additional benefits of
creating sheet flow, extending the flow path, and
preventing short circuiting.   The forebay should
contain at  least 10 percent  of the  wetland's
treatment volume and should be 2 to 3 meters (4 to
6 feet) deep. The State of Maryland recommends a
depth of at least 1  meter (3 feet). The forebay is
typically separated from the wetland by gabions or
by an earthen berm (MWCOG,  1992b).

Flow from the wetland should be conveyed through
an outlet structure that is located within the deeper
areas of the wetland. Discharging from the deeper
areas using a reverse slope pipe prevents the outlet
from becoming clogged. A micropool just prior to
the outlet will also  prevent outlet clogging.  The
micropool should contain approximately 10 percent
of the treatment volume and be 2 to 3 meters (4 to
6 feet) deep. An adjustable  gate-controlled drain
capable of dewatering the wetland within 24 hours
should be located within the micropool. A typical
drain may be  constructed with an upward-facing
inverted  elbow with   its   opening above  the
accumulated sediment.   The dewatering feature
eases   planting  and   follow-up   maintenance
(MWCOG, 1992b).

Vegetation  can be  established by  any of five
methods: mulching; allowing volunteer vegetation
to become established; planting nursery vegetation;
planting underground dormant parts of a plant; and
seeding. Donor soils from existing wetlands can be
used to establish vegetation within a wetland. This
technique, known as mulching, has the advantage of
quickly establishing a diverse wetland community.
However, with mulching, the types of species that
grow within the wetland are unpredictable.

Allowing  species  transmitted  by  wind  and
waterfowl to voluntarily become established in the
wetland is also unpredictable. Volunteer species are
usually well  established  within 3  to  5  years.
Wetlands established  with volunteers are usually
characterized by low plant diversity with monotypic
stands of exotic or invasive  species. A higher-
diversity wetland can be established when nursery
plants or dormant rhizomes are planted. Vegetation
from a  nursery  should be  planted during the
growing season - not during late summer or fall - to
allow vegetation time to store food reserves for
their dormant period.  Separate underground parts
of vegetation are planted during the plants' dormant
period,  usually  October through April, but the
months will vary with local climate.  Another
planting technique, the spreading of seeds, has not
been very successful  and  therefore  is not widely
practiced as a principal planting technique.

Appropriate plant types vary with  locations and
climate. The wetland designer should select  five to
seven plants native to the area and design the depth
zones in the wetland to be appropriate for the type
of plant and its associated  maximum water  depth.
Approximately half of the  wetland should be
planted. Of the five to seven species selected, three
should be aggressive plants or those that become
established  quickly.   Examples  of aggressive
species used in the Mid-Atlantic Region include
softstem bulrush (Scirpus validus)  and  common
three-square (Scirpus americanus).  Aggressive
plants as well as other native wetland plants are
available from numerous nurseries. Most vendors
require an advance order of 3  to 6 months.

After excavation and grading the wetland should be
kept flooded until planting.  Six to  nine months
after being flooded and two weeks before planting,
the  wetland is typically drained and surveyed to
ensure that depth zones are appropriate  for plant
growth. Revisions may be necessary to account for
any changes in depth. Next,  the  site is  staked to
ensure that  the planting crew  spaces the  plants
within the correct  planting  zone.   Species are
planted in separate zones to avoid competition. The
State  of  Maryland  recommends planting  two

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aggressive or primary species in four specific areas
and planting an additional 40 clumps (one or more
individuals of a single species) per acre of each
primary species over the rest of the wetland. Three
secondary species are planted close to the edge of
the wetland at an application rate of 10 clumps of 5
individual plants per acre of wetland, for a total of
50 individuals of each secondary species per acre of
wetland.  At least 48 hours prior to planting, the
wetland should be drained; within 24 hours after
planting, it should be re-flooded.

The  wetland design should include a buffer to
separate  the  wetland from  surrounding  land.
Buffers may alleviate  some potential  wetland
nuisances, such as accumulated floatables or odors.
MWCOG recommends a buffer of 8 meters (25
feet) from the maximum water surface elevation,
plus an additional 8 meters  (25 feet) when wildlife
habitat is of concern. Leaving trees undisturbed in
the buffer zone will minimize the disruption to
wildlife and  reduce the  chance for invasion of
nuisance vegetation such as cattails and primrose
willow. If tree removal is necessary, the buffer area
should  be   reforested.     Reforestation   also
discourages the settlement  of geese, which prefer
open areas.

PERFORMANCE

Wetlands  remove pollutants  from storm  water
through   physical,   chemical,  and   biological
processes.   Chemical and physical  assimilation
mechanisms  include sedimentation,  adsorption,
filtration, and volatilization.

Sedimentation is the primary removal mechanism
for pollutants such as suspended solids, particulate
nitrogen, and heavy metals. Particulate settling is
influenced by the velocity of the runoff through the
wetland,  the  particle   size,   and  turbulence.
Sedimentation can be maximized by creating  sheet
flow conditions, slowing the velocities through the
wetland, and providing morphology and vegetation
conducive to settling. The  vegetation and its root
system  will  also decrease the resuspension of
settled particles.

Some pollutants, including metals, phosphorus, and
some hydrocarbons, are removed by adsorption- the
process whereby pollutants attach to surfaces of
suspended or settled sediments and vegetation. For
this removal process to occur, adequate  contact
time between the surface and pollutant must be
provided in the design of the system.

Wetland  plants  filter  trash,  debris,  and other
floatables. Particulates  (e.g., settleable solids and
colloidal solids) are  also filtered mechanically as
water passes through root masses. Filtration can be
enhanced by  slow  velocities, sheet  flow,  and
sufficient quantities  of  vegetation. By increasing
detention and contact time and providing a surface
for microbial growth, wetland plants also increase
the  pollutant   removal  achieved  through
sedimentation, adsorption, and microbial activity.

Volatilization plays  a  minor  role  in pollutant
removal from wetlands.  Pollutants such as oils and
hydrocarbons can be removed from the wetland via
evaporation or by aerosol formation under windy
conditions.

Biological processes that occur in wetlands result in
pollutant  uptake by  wetland plants and algae.
Emergent wetland plants absorb settled nutrients
and metals through their roots, creating new sites in
the sediment for pollutant adsorption. During the
fall the plants' above-ground parts typically die back
and the plants may potentially release the nutrients
and metals back into the water column (MWCOG,
1992b).   Recent studies, however, indicate  that
most pollutants are stored in the roots of aquatic
plants, rather  than the  stems and leaves (CWP,
1995). Additional studies are required to determine
the extent of pollutant release during the fall  die-
back.

Microbial activity helps to remove nitrogen and
organic matter from wetlands. Nitrogen is removed
by  nitrifying  and denitrifying bacteria;  aerobic
bacteria are responsible for the decomposition of
the organic matter.  Microbial processes require
oxygen  and can deplete oxygen levels in the top
layer of wetland sediments. The low oxygen levels
and  the  decomposed  organic   matter  help
immobilize metals.

Soluble forms of phosphorus, as well as ammonia,
are partially  removed by planktonic or  benthic

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algae. The algae consume the nutrients and convert
them into biomass, which settles to the bottom of
the wetland.

The  removal effectiveness of shallow marsh and
pond/wetland   systems  has  been fairly  well
documented,  while  the  amount  of  removal
efficiency data for Extended Detention wetlands
and pocket wetlands is limited. Average long-term
pollutant removal rates for constructed wetlands, as
a whole, are presented in Table 2 (CWP, 1997).

    TABLE 2  PERFORMANCE OF STORM
             WATER WETLANDS
           Pollutant
Removal Rate
     Total Suspended Solids

     Total Phosphorus

     Total Nitrogen

     Organic Carbon

     Petroleum Hydrocarbons

     Cadmium

     Copper

     Lead

     Zinc

     Bacteria
    67%

    49%

    28%

    34%

    87%

    36%

    41%

    62%

    45%

    77%
    Source: CWP, 1997.

As  shown, petroleum hydrocarbons (87%), total
suspended solids  (TSS) (67%), lead (62%), and
bacteria (77%) have the highest removal rates.
Lower removal rates have been documented for
nutrients, organic  carbon, and other heavy metals.
The removal rates  will vary with the loadings to the
wetland,  retention time in  the  BMP, and other
factors such as BMP geometry, site characteristics,
and monitoring  methodology  (CWP,   1997).
Excessive pollutant loadings   (e.g.,  suspended
solids)  may  exceed  the   wetlands'  removal
capabilities.

In general, wetlands remove pollutants about as
effectively  as do conventional pond  systems.
Constructed  storm  water  wetlands  are more
effective than natural wetlands, probably because of
their intricate design and continued monitoring and
maintenance  (MWCOG,  1992).  The wetlands'
effectiveness seems to improve after the first few
years of use as the vegetation becomes established
and organic matter accumulates.

OPERATION AND MAINTENANCE

Well-designed  and  maintained  wetlands  can
function  as  designed  for 20 years or longer.
However, wetland maintenance must actually begin
during the construction phase.  During construction
and excavation, many constructed wetlands  lose
organic matter in the soils.   The organic  matter
provides exchange  sites  for  pollutants,  and,
therefore, plays  an  important role in pollutant
removal. Replacing or adding organic matter after
construction improves performance.

After the  wetland  has  been constructed,  its
vegetation must be maintained on a regular basis.
Maintenance requirements for constructed wetlands
are particularly high while vegetation  is being
established (usually the first three years) (U.S. EPA,
1996). Monitoring during these first years is crucial
to the future success of the wetland as a storm water
BMP.  Inspections  should be conducted at least
twice per year for the first three years and annually
thereafter.  Maintenance requirements  may  also
include replacement planting, sediment removal,
and possibly plant  harvesting.   Wetland  design
should   include   access   to  facilitate  these
maintenance activities.

Vegetative  cover on embankments and spillways
should be dense and healthy. Replacement planting
may be required during the first several years if the
original plants do not flourish. First year wetland
vegetation growth at the water's edge and  on the
side slopes of the wetland can be protected from
birds by  surrounding the open water area of the
wetland with wire to limit access to the vegetation.
The embankment and maintenance bench should be
mowed twice each year. Other areas surrounding
the wetland should not require mowing.  Mowing
and fertilizing help promote vigorous growth of
plant  roots that resist  erosion.   Mowing  also
prevents the growth of unwanted woody vegetation.
Additional  routine  maintenance that  can  be
conducted on the same schedule  should include
removal  of accumulated trash from trash racks,

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outlet structures, and valves, as well as debris on
plants that could inhibit growth.

Constructed wetlands should be  inspected after
major storms during the first year of establishment.
The inspector should assess bank stability, erosion
damage,  flow  channelization,  and  sediment
accumulation within the wetland.  The inspector
shall also take note of species distribution/survival,
damage  to  embankments  and spillways  from
burrowing animals, water elevations, and outlet
condition.   Water elevations  can be  raised or
lowered  by  adjusting the outlet's gate valve if
plants  are  not receiving  an  appropriate water
supply.

Accumulated sediments will gradually decrease
wetland storage and performance.  There are two
options  to  mitigate the effects of accumulated
sediments: either the sediments should be removed
as necessary or the water level  in the wetland
should be raised (i.e., the outlet should be adjusted
to increase discharge elevation).

The construction  of  a sediment forebay  will
decrease the accumulation of sediments within the
wetland and increase the wetland's longevity.  The
forebay  will likely require sediment to be cleaned
out every three to five years. The forebay design
should allow drainage so that a  skid  loader or
backhoe can be used to remove the accumulated
deposits  (MWCOG,  1992).   Accumulation of
organic matter can be reduced by plant harvesting
or seasonal drawdown to allow organic material to
oxidize (U.S. EPA, 1996).

A number  of studies  have been performed to
determine the toxicity of  pond  sediments  and
whether they can  be  landfilled or land  applied
without   having  to   meet  hazardous  waste
requirements.  Many studies to date have found
sediments are not hazardous. However, one study
showed that toxic levels of zinc had accumulated in
sediment from the pretreatment pond  (SFWMD,
1995).    If  toxic  levels  of  metals   have  not
accumulated in the sediment,  then on-site land
application  of the sediments  away  from  the
shoreline will probably be the most cost-effective
disposal method (no transportation costs or disposal
fees are incurred). Wetlands that receive flow from
a drainage area containing commercial or industrial
land use and/or activities associated with hazardous
waste may contain toxic levels of heavy metals in
the sediments. Testing may be required for these
sediments prior to land application or disposal.

COSTS

Costs incurred for storm water wetlands  include
those for permitting,  design,  construction and
maintenance. Permitting costs vary depending on
state and local regulations, but permitting, design,
and contingency costs are estimated at 25 percent of
the construction cost. Construction costs  for an
emergent wetland with a sediment forebay range
from $65,000 to $137,500 per hectare ($26,000 to
$55,000 per acre) of wetland.  This includes costs
for clearing and grubbing,  erosion and sediment
control, excavating, grading, staking, and planting.
The  cost for constructing  the wetland depends
largely upon the amount of excavation required at
a site and plant selection.  The cost for forested
wetlands could be double that  of an emergent
wetland.   Maintenance  costs for wetlands are
estimated at 2 percent per year of the construction
costs (CWP, 1998).

REFERENCES

1.  Bowers, J. K., August 14,  1995.  Personal
    Communication.  Biohabitats, Inc., Towson,
    MD.

2.  Carr, D., and B. Rushton,  1995. Integrating A
    Herbaceous   Wetland  Into  Stormwater
    Management.   Southwest  Florida   Water
    Management District  Stormwater Research
    Program.

3.  Center for Watershed Protection (CWP), 1995.
    Pollutant  Dynamics  within Storm  Water
    Wetlands:!. Plant Uptake. Techniques, Vol.1,
    No.4. Silver Spring, MD.

4.  Center  for  Watershed   Protection,   1997.
    National  Pollutant  Removal Performance
    Database for Stormwater Best Management
    Practices.   Prepared  for the  Chesapeake
    Research Consortium.

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5.  Hammer, D.A.   (Ed),  1989.   Constructed
    Wetlands for Wastewater Treatment.  Lewis
    Publishers, Chelsea, MI.

6.  Horner, R., 1995.  Constructed Wetlands for
    Urban  Runoff  Water  Quality  Control.
    Presented at the National Conference on Urban
    Runoff Management, March 30  to April  2,
    1993. Chicago, IL.

7.  Livingston, Eric,  1994.    Water  Quality
    Considerations in the Design and Use of Wet
    Detention  and   Wetland  Storm   Water
    Management Systems.

8.  Maryland  Department of the  Environment
    (Water Management Administration), 1987.
    Guidelines for  Constructing Wetland Storm
    Water Basins.  Baltimore, MD.

9.  Maryland  Department of the  Environment
    (Water Management Administration), 1987.
    Wetland Basins for Storm Water Treatment:
    Discussion and Background. Baltimore, MD.

10.  Metropolitan   Washington  Council    of
    Governments (MWCOG), 1992a.  A Currrent
    Assessment of  Urban  Best  Management
    Practices: Techniques for Reducing Non-Point
    source Pollution   in  the  Coastal  Zone.
    Washington, DC.

11.  Metropolitan   Washington  Council    of
    Governments (MWCOG), 1992b.  Design  of
    Storm Water Wetland Systems.  Washington,
    DC.

12.  Strecker, E., 1995.  The Use of Wetlands for
    Storm Water Pollution Control.  Presented at
    the National Conference on Urban Runoff
    Management, March 30 to April 2, 1993,
    Chicago, IL.

13.  Streckler, Kersnar, Driscoll, and Horner, 1992.
    The Use of Wetlands for Controlling Storm
    Water Pollution.

14.  U.S. EPA, 1993. Subsurface Flow Conducted
    Wetlands for  Wastewater  Treatment:  A
    Technology Assessment. EPA 832-R-93-001.
    Office of Water.

15. U.S.  EPA,  1995.   Free  Water  Surface
    Constructed   Wetlands  For  Wastewater
    Treatment: A Technology Assessment. Office
    of Water.

16. U.S.  EPA,  1996.     Protecting  Natural
    Wetlands:  A  Guide  to  Stormwater  Best
    Management Practices. EPA 843-B-96-001.
    Office of Water.

17. U.S. EPA, 1998.  Preliminary Data Summary
    of Urban Storm  Water Best Management
    Practices.

ADDITIONAL INFORMATION

Buzzards Bay Project
Bernie Taber
2 Spring Street
Marion, MA 02738

CH2M Gore & Storrie Limited
John Pries
180 King Street S.,  Suite 600
Waterloo, Ontario N2J 1P8

Delaware Division of Water Resources
Mark Biddle
Watershed Assessment Section
820 Silver Lake Boulevard, Suite 220
Dover, DE  19904

City of Eugene, Oregon
Therese Walch, Team Manager
858 Pearl Street
Eugene, OR 97401

City of Orlando, Florida
Kevin McCann
400 South Orange Avenue
Orlando, FL 32801

The  mention  of trade  names  or commercial
products  does  not constitute  endorsement  or
recommendation for  the  use by  the  U.S.
Environmental Protection Agency.

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 For more information contact:

 Municipal Technology Branch
 U.S. EPA
 Mail Code 4204
 401 M St., S.W.
 Washington, D.C., 20460
IMTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY  BRM?fH

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