Nitrate retention in riparian ground water at natural and elevated nitrate levels in North Central Minnesota

Abstract

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.