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TECHNICAL REPORTS

Ground Water Quality

Nitrate Retention in Riparian Ground Water at Natural and Elevated Nitrate Levels in North Central Minnesota

^a U.S. Geological Survey, Water Resources Division, 345 Middlefield Road, MS 439, Menlo Park, CA 94025
^b Dep. of Chemical Engineering and Materials Science, Univ. of California, Davis, CA 95616

^* Corresponding author (jhduff{at}usgs.gov)

The relationship between local ground water flows and NO₃^– transport to the channel was examined in three well transects from a natural, wooded riparian zone adjacent to the Shingobee River, MN. The hillslope ground water originated as recharge from intermittently grazed pasture up slope of the site. In the hillslope transect perpendicular to the stream, ground water NO₃^– concentrations decreased from 3 mg N L^–1 beneath the ridge (80 m from the channel) to 0.01 to 1.0 mg N L^–1 at wells 1 to 3 m from the channel. The Cl^– concentrations and NO₃/Cl ratios decreased toward the channel indicating NO₃^– dilution and biotic retention. In the bankside well transect parallel to the stream, two distinct ground water environments were observed: an alluvial environment upstream of a relict beaver dam influenced by stream water and a hillslope environment downstream of the relict beaver dam. Nitrate was elevated to levels representative of agricultural runoff in a third well transect located 5 m from the stream to assess the effectiveness of the riparian zone as a NO₃^– sink. Subsurface NO₃^– injections revealed transport of up to 15 mg N L^–1 was nearly conservative in the alluvial riparian environment. Addition of glucose stimulated dissolved oxygen uptake and promoted NO₃^– retention under both background and elevated NO₃^– levels in summer and winter. Disappearance of added NO₃^– was followed by transient NO₂^– formation and, in the presence of C₂H₂, by N₂O formation, demonstrating potential denitrification. Under current land use, most NO₃^– associated with local ground water is biotically retained or diluted before reaching the channel. However, elevating NO₃^– levels through agricultural cultivation would likely result in increased NO₃^– transport to the channel.

Abbreviations: BDOC, biologically available dissolved organic carbon • DOC, dissolved organic carbon • DO, dissolved oxygen • SRP, soluble reactive phosphorus • NO₃/Cl, nitrate N/chloride ratio • NO₃/Br, nitrate N/bromide ratio