RedN NAWQA
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Red River of the North Basin
National Water-Quality Assessment Program
Tornes, L.H., Puckett, L.J., Stoner, J.D., and Brigham, M.E., 1994, The Effect of Riparian Wetlands in Reducing Nitrate-Nitrogen from a Sand and Gravel Aquifer to a Stream: American Geophysical Union, EOS Transactions, Abstracts for Fall Meeting -- December 5-9, 1994, p. 261.
Abstract
An 8.3 kilometer reach of the Otter Tail River in west-central Minnesota, which receives discharge from a surficial aquifer of glacial sand and gravel, was used to study the fate of nitrogen in ground water during the summer of 1994. The study coincided with the terminus of a flow-path study site where nests of wells were installed along a transect leading to the river as part of the National Water Quality Assessment Program. The 5-kilometer flow path parallels local ground-water flow beneath adjacent agricultural land and terminates at a riparian wetland. During winter, this reach of the Otter Tail River gained 0.42 cubic meter per second, a 20 percent increase. During summer, no gain in streamflow was evident, probably because of losses to riparian evapotranspiration. Vertical hydraulic-head difference across the upper 0.5 meter (m) of sandy streambed ranged from 0.04 to 0.10 m and indicated consistently upward flow. Nitrate concentrations in ground water samples collected along the flow path and adjacent to the river have seasonally exceeded 10 milligrams per liter (mg/L) as nitrogen, possibly indicating an agricultural source. Along the ground-water flow path from the shallow well nearest the stream to sampling ports beneath the wetland and sandy streambed, dissolved oxygen concentrations were relatively small, and nitrate-nitrogen concentrations declined from about 10 mg/L to below detection near the stream. Nitrate concentrations measured during 1993-94 in the Otter Tail River downstream from the ground-water inflow ranged from below detection (0.05 mg/L) to 0.65 mg/L. These trends indicate that denitrification may be occurring in the sediment. Other redox reactions appeared to be occurring beneath the wetland including the conversion of ferric to ferrous iron and the possible reduction of sulfate to sulfide sulfur with subsequent precipitation of the sulfide. Several of these redox reactions appeared to be indicated by results from the same sample, but this may have resulted from sampling a mixture from several microenvironments where various redox reactions dominated.
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U.S. Department of the Interior |
U.S. Geological Survey