INORGANIC NITROGEN TRANSPORT/TRANSFORMATION IN HILLSLOPE GROUNDWATER AND BANKSIDE HYPORHEIC SEDIMENTS: SHINGOBEE RIVER, MN

Inorganic nitrogen transport and transformation in groundwater was examined along a hillslope, and within a riparian, hyporheic deposit. Two intersecting transects of monitoring wells spanned from ridge to stream (Transect 1), and along the river bank, upstream and through the hyporheic, alluvial deposit (Transect 2). At the transect intersection a closely spaced well array was installed for a manipulation experiment along hydrologically-connected hyporheic flow paths (Transect 3).

Background nitrate concentrations along Transect 1 were highest beneath the ridge (2-5 mg-N/L), decreased toward the channel, and were very low in stream water (<0.10 mg-N/L) in all seasons. Groundwater was aerobic, and low in ammonium (<0.01 mg/L) and SRP (<0.01 mg-P/L). Along Transect 2 nitrate in bankside groundwater varied over two orders of magnitude (0.01-1.0 mg-N/L) indicating highly variable fates for nitrate transported in groundwater. Background nitrate:chloride ratio indicated that mechanisms for decreased nitrate varied from simple dilution to complete biotic processing.

In the field manipulation, nitrate-conservative tracer (Cl or Br) coinjections resulted in conservative nitrate transport at concentrations exceeding 10 mg-N/L (Transect 3). When organic carbon (glucose) was added to the injectate, hyporheic waters became hypoxic and nitrate disappeared relative to tracer. When acetylene was sparged into the injectate solution nitrous oxide appeared, demonstrating in situ denitrification. Nitrate mass loss nearly equalled nitrite plus nitrous oxide gain. Nitrate processing on this landscape is apparently limited by organic carbon. In similar landscapes undergoing intensive agriculture, biotic processing would be overwhelmed and excess nitrate would be carried to the channel, degrading water quality.