905R90115
United States
Environmental Protection
Agency
Watershed Management Unit
Water Division, Region V
Chicago, IL
December 1990
vvEPA
Uses of Wetlands in
Stormwater Management
oth natural and manmade wetlands have many uses
and benefits in managing stormwater runoff, including:
• Improvement of water quality,
• Flood control and mitigation, and
• Low construction and maintenance costs.
Stormwater Runoff Problems
tormwater runoff carries nonpoint
source (NFS) pollutants from urban,
industrial, and commercial areas and
from highways. Uncontrolled
stormwater runoff often results in:
• high loadings of suspended
solids, nutrients, metals, and
toxics including: nitrogen,
phosphorus, calcium, potassium, sulfate,
magnesium, pesticides, and herbicides,
degradation of rivers, streams, and lakes,
rapid inflow and scouring of river beds and
stream banks, and
flooding.
Uses and Benefits
Water Quality Improvement
atural and constructed wetlands can
be effective systems for improving
water quality either alone or in
conjunction with other treatment
systems. The complex hydrologic,
biological, physical, and chemical
interactions that take place within a
wetland result in a natural reduction
and cleansing of influent pollutants.
Some of the more important wetland processes
that improve the quality of stormwater runoff
include:
• sedimentation,
• adsorption and retention,
• biological degradation and transformation,
and
• plant uptake.
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• Sedimentation—The ability of wetlands to
intercept stormwater runoff and reduce flows
results in increased sedimentation within the
wetland system. This increase in sedimentation is
an important feature and an effective mechanism
for water quality improvement. A large percentage
of suspended solids loads and concentrations in
stormwater runoff are reduced through
sedimentation. Metals, nutrients, and toxics that
are bound to and carried with the suspended
solids in stormwater runoff also are greatly
reduced through this process. Rates of
sedimentation can vary within a wetland and are
dependent on two interrelated factors:
• flows through the wetlands, and
• residence time of the water within the
wetlands.
The higher the residence time, the greater the
rate of sedimentation. Conversely, as flows
increase, sedimentation rates decrease.
• Adsorption and retention—Stormwater
detention within a wetland allows adsorption of
dissolved components within stormwater to the soil,
particularly heavy metals and phosphorus.
Adsorption is dependent on soil types or the
substrate within the wetlands. Fine soils, such as
clays and silts, or those with high organic content,
such as peats, have higher abilities to adsorb and
retain these constituents. Retention, however, is
dependent on soil and water chemistry within the
wetlands, specifically pH. Under appropriate
chemical conditions, long-term retention of these
stormwater components will
occur within the wetland.
• Biological degradation and transformation—
Wetlands have a variety of aerobic and anaerobic
biologically mediated processes that are effective
in the removal of organic and inorganic chemicals
from stormwater runoff. The most common
biological processes within wetlands include
oxidation, reduction, nitrification, and
denitrification, which reduce iron, sulfate, and
nitrogen in stormwater runoff. The rates of these
biological processes are linked to many factors
including:
• physical/chemical factors, i.e., temperature
and pH,
• number and species of microorganisms, and
• availability of food sources for the microbes.
Flood Control
Natural and manmade wetlands significantly
reduce the incidence of flooding of urban and
nonurban land by changing surface water runoff
patterns. Wetlands mitigate and control flooding
by:
• intercepting and slowing down stormwater
runoff,
• detaining stormwater runoff,
• reducing stream velocity,
• providing storage areas for stormwater
runoff,
• reducing sharp runoff peaks associated with
storms (Figure 1), and
• changing single peak discharges to slower,
smaller, and longer discharges.
• Plant uptake—Many
wetland species are able to
uptake nutrients and metals
from the system, further
improving water quality.
Plant uptake varies from
species to species. Uptake
is also dependent on the
water and soil chemistry,
soil type, and the
bioavailability of the
chemicals.
Stormwater
Runoff
^-Wetland
'»/; Discharge
time
Figure 1.—Relative flows for stormwater runoff and storm discharges from wetlands.
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Potential Impacts on Wetlands
• Sedimentation—Long-term impacts of
sedimentation on wetlands are unknown; however,
potential impacts of sedimentation on wetlands
include:
• increased siltation of the wetlands,
• loss of critical wetland habitat,
• loss of runoff storage areas, and
• loss of wetlands functions (i.e., water quality
improvement).
• Ground water contamination—Ground water
and surface water interactions in wetlands
systems are closely intertwined. As pollutants are
removed from surface water and retained within
the wetlands soils, the potential exists for these
pollutants to move into and contaminate ground
water in the wetlands system.
• Plant uptake—In many constructed wetland
systems, plants that uptake metals, phosphorous,
and nutrients are harvested to remove the
pollutants from the wetlands. In natural systems,
wetlands plant species are not harvested. Each
winter, as the wetlands plants die and decompose,
pollutants may be re-released to the system,
degrading wetlands water quality.
Example Case Studies
/. McCarrons Treatment System - Roseville, MN1
The McCarrons Treatment System is a surface
water management facility consisting of a
detention pond followed by six "chambered"
wetlands designed to improve the water quality of
Lake McCarrons in Roseville, Minnesota. The
system is located at the bottom of a 243-hectare
(ha) urban watershed. Water quality monitoring of
the McCarrons Treatment System has shown it to
be very effective in the removal of
solids-associated pollutants and moderately
effective in removing soluble nutrients. Most of the
reduction in pollutants occurs in the detention
pond.
The post-detention wetland system was
intended to "polish" outflows from the detention
pond before the water discharged to the lake. The
wetland continues the process of settling solids
begun in the pond but is less effective in removing
soluble nutrients. This situation is partially related
to additional inputs to the wetlands from another
tributary, overland runoff, and atmospheric
deposition. Even through nutrient removal in the
wetland is not high, there is a net reduction so the
wetland is performing as expected.
//. Clear Lake Treatment Marsh - Waseca, MN
Clear Lake, a 257-ha body of water located in
southcentral Minnesota, is a heavily utilized
recreational lake that has become eutrophic
because of the inflow of nutrient-rich runoff water
from the adjacent city of Waseca. In 1981, 50
percent of the hydraulic load and 55 percent of the
phosphorus load to the lake was diverted into a
21.4-ha marsh. The marsh system reduced the
annual phosphorus load to Clear Lake by 34
percent (768 kg). In 1986, construction was
completed on a second marsh system that filters
urban and agricultural runoff carrying 20 percent of
the phosphorus load into Clear Lake. The mean
total phosphorus concentration in Clear Lake has
been reduced 31 percent, from 158 ng/Lto
109 ng/L, since the diversion in 1981. The total
nitrogen:total phosphorus ratio has increased from
10:1 to 18:1 since the diversion began.
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References
1 Proceedings of the Symposium on Nonpoint Pollution: 2 John M. Barten. 1987. Stormwater Runoff Treatment in
1988 - Policy Economy, Management and a Wetland Filter: Effects on the Water Quality of Clear
Appropriate Technology, V. Novotny, ed., American Lake. Lake and Reservoir Management 3:297-305.
Water Resources Association, Bethesda, Maryland,
p. 237-247.
TERRENE
INSTITUTE
For additional information on the McCarrons Treatment System, contact Paul Wotzka and Gary
Oberts, Metropolitan Council, Mears Park Centre, 230 E. 5th Street, St. Paul, Minnesota 55101. For
additional information on the Clear Lake Treatment Marsh, contact John M. Barten, City of
Waseca, Waseca, Minnesota. This project was funded by the U.S. Environmental Protection Agen-
cy Office of Water Enforcement and Permits-Water Permits Division and managed by Region V
Watershed Management Unit-Water Division. Prepared by Dynamac Corporation, FTN Associates,
and JT&A, Inc. For copies of this publication, contact The Terrene Institute, 1000 Connecticut
Avenue, NW, Suite 300, Washington, DC 20036, (202) 833-3380.
/7TV
\A. Aj Printed on Recycled Paper
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