National Water-Quality Assessment (NAWQA) Program
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By Larry J. Puckett, Timothy K. Cowdery, Peter B. McMahon, Lan H. Tornes, and Jeffrey D. Stoner
[Water Resources Research, vol. 38, no. 8, August 2002]Abstract
A combination of chemical and dissolved gas analyses,
chlorofluorocarbon age dating and hydrologic measurements were used to
determine the degree to which biogeochemical processes in a riparian
wetland were responsible for removing NO3- from ground waters
discharging to the Otter Tail River in west-central Minnesota. An
analysis of river chemistry and flow data revealed that NO3-
concentrations in the river increased in the lower half of the 8.3 km
study reach as the result of ground-water discharge to the
river. Ground-water head measurements along a study transect through
the riparian wetland revealed a zone of ground-water discharge
extending out under the river. On the basis of combined chemical,
dissolved gas, age date, and hydrologic results, it was determined
that water chemistry under the riparian wetland was controlled largely
by up-gradient ground waters that followed flow paths up to 16 m deep
and discharged under the wetland, creating a pattern of progressively
older, more chemically reduced, low NO3- water the further one
progressed from the edge of the wetland towards the river. These
findings pose challenges for researchers investigating biogeochemical
processes in riparian buffer zones because the progressively older
ground waters entered the aquifer in earlier years when less NO3-
fertilizer was being used. NO3- concentrations originally present in
the ground water had also decreased in the up-gradient aquifer as a
result of denitrification and progressively stronger reducing
conditions there. The resulting pattern of decreasing NO3-
concentrations across the riparian zone may be incorrectly interpreted
as evidence of denitrification losses there instead of in the
up-gradient aquifer. Consequently, it is important to understand the
hydrogeologic setting and age structure of the ground waters being
sampled in order to avoid misinterpreting biogeochemical processes in
riparian zones.
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