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Rain or snow falling on parking
lots, farm fields, streets, industrial sites, and other areas picks
up and transports a number of pollutants such as soil, fertilizers,
pathogens, and chemicals. Wetlands may help remove nutrients such as
nitrogen and phosphorous, sediments, and toxic materials from water
and tie them up so they are no longer available to pollute the
water—thus resulting in cleaner water. Water that moves in channels
such as streams or in sheet flow across fields will slow down when
they enter wetlands due to increased friction with vegetation and
often due to a lower gradient than the stream or surrounding
landscape itself. This allows sediments and many of the chemicals
that are associated with the sediments to drop out of the water
column and settle on the bottom of the wetland. Moreover, there is
more contact between the water column and the soil in wetlands
compared to streams which facilitates the transfer of chemicals from
the water to the soil. A variety of bacteria that live in wetlands
can further change certain nutrients and harmful chemicals into less
harmful chemicals, remove the chemicals from the water, and bind
them in the sediments. The high production of some wetland
hydrophytes further help to remove many excess nutrients from the
water and allows them to be stored in the sediments once the plants
die. A number of factors
dictate the value and role of wetlands in improving water quality.
The size of the wetland area in relation to the size of the
watershed and the amount of pollution entering the wetland both play
a major role in the ability of the wetland to improve water quality.
Larger wetlands or wetland complexes are generally more effective
than smaller wetlands or wetland complexes. Wetlands that intercept
more flow are generally more effective than wetlands that only
intercept a small amount of water moving through a watershed.
Therefore, wetlands in the lower part of a watershed are usually
more effective than wetlands in the upper portions of a watershed.
However, headwater wetlands (wetlands at the head of a stream) are
extremely important for improving water quality
Because wetlands are effective at improving water quality. People
purposefully construct wetlands to aid the in the improvement of
water quality. Wetlands are created and used as treatment wetlands
for municipal wastewater, storm water runoff, dairy wastewater
treatment, fish processing waste, and a number of other activities.
Wetlands used for these purposes must be dominated by emergent plant
species that can tolerate continuous or almost continuous flooding
of between 1 and 2 feet of water. Plants typically used in
constructed treatment wetlands include cattails, common reed, and
bulrushes.
A number of constructed wetlands have been used to treat wastewater
associated with farm operations—especially dairy farm washwater and
solid manure runoff. The majority of these facilities are in warmer
climates although they can also be used in colder climates.
Treatment wetlands can remove more than 90% of the total suspended
solids and phosphorous in wastewater. The technology associated with
constructing wetlands for improving water quality of livestock
operations is improving at a rapid pace. Anyone interested in using
wetlands to improve water quality is urged to contact your local
Natural Resources Conservation Service office for help.
The photograph below depicts two
surface-flow wetlands constructed to treat a mixture of diluted
manure and milkhouse washwater in Nova Scotia, Canada. Note the
thick stands of broad-leaved cattail typical of treatment wetlands.
Photo courtesy of Erin Smith (University of New Brunswick)
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