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Presented at the NABS Annual meeting, Vancouver, British Columbia, 2004
in Biogeochemistry
Inorganic nitrogen transport/transformation in hillslope groundwater and bankside hyporheic sediments in the Shingobee River, MN.
A.P. Jackman1, F.J. Triska2, J.H. Duff2, and R.J. Avanzino2. 1University of California, Department of Chemical Engineering, Davis, CA 95616, 2U.S. Geological Survey, Water Resources, 345 Middlefield Rd. Menlo Park, CA 94025
Transformation of inorganic nitrogen during subsurface transport was examined along hill slope flow paths, and within a riparian-hyporheic zone. Intersecting well transects were installed from ridge to stream (Transect 1) and along the banks (Transect 2). Near the transect's intersection (hillslope-riparian boundary) a closely-spaced well array along a hydrologically connected flow path (Transect 3)experimentally examined controls of nitrogen transport/ transformation at background and levels representing regional agriculture. Background nitrate concentration ridge to stream (Transect 1) was highest beneath the ridge (2-5 mg/L)and decreased toward the channel. Local groundwater was high in dissolved oxygen and in ammonium (<0.01 mg-N/). Nitrate in bankside wells (Transect 2) ranged two orders of magnitude indicating highly variable fates for nitrate transported in hillslope groundwater. Background nitrate-halide ratios indicate that mechanisms vary from dilution to biological transformation. In the experimental well array nitrate-halide amendment indicated nearly conservative transport. Addition of biologically available dissolved organic carbon (BDOC) as glucose stimulated microbial metabolism and nitrate retention under both background and nitrate amended conditions. Disappearance of amended nitrate was accompanied by nitrite formation under high BDOC. With acetylene present, nitrite disappearance was accompanied by nitrous oxide formation (denitrification). Nitrate-N loss approximated nitrous oxide plus nitrite gain.
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