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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