RedN NAWQA
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Red River of the North Basin
National Water-Quality Assessment Program
Puckett, L.J., Tornes, L.H., Cowdery, T.K., Brigham,
M.E., and Stoner, J.D., 1995, The role of wetlands in
reducing nitrogen loadings to the Otter Tail River in
Minnesota: American Geophysical Union, EOS
Transactions, Abstracts for Fall Meeting --
December 11-15, 1995, p. 186.
Abstract
An 8.3 km reach of the Otter Tail River in west-central Minnesota was studied
to determine the role of wetlands in reducing nitrogen loadings to the river.
The study reach receives water from a 37.6 km2 agricultural area, consisting of
about 27 km2 of cropland, underlain by a surficial aquifer of glacial sand and
gravel. The study reach gains a flow of about 0.4 m3s-1 during later summer
and winter months as the results of ground water discharge. Wells were
installed along a 5-km transect that roughly parallels the direction of
ground-water flow through the agricultural area and terminates at a 45-m wide
wetland adjacent to the Otter Tail River. Nitrate concentrations were greater
than 10 mg L-1 in shallow ground water beneath the edge of the wetland, but
NO3- concentrations in the Otter Tail River are commonly near or below
detection. Ground water sampled below the wetland revealed a trend of
decreasing NO3- and increasing N2 gas concentrations proceeding from the edge
of the wetland to the edge of the river -- most NO3- was removed within the
first 30 m. Estimates of redox state based on measurements of pairs of
dissolved constituents (NO3--NO2-, Fe+3-Fe+2, SO42--S2-) indicated that ground
water flowing under the wetland passed progressively through zones of more
negative redox potential resulting in the sequential reduction of NO3-, Fe+3,
and SO42-. Field tests for the presence of denitrifying, iron-reducing and
sulfatereducing bacteria were positive at most sites along the flow path.
These results strongly support the hypothesis that denitrification along the
ground-water flow path immediately beneath the wetland is responsible for NO3-
removal.
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