Localized zones of denitrification in a floodplain aquifer in southern Wisconsin, USA

Abstract

A floodplain aquifer within an agricultural watershed near Madison, Wisconsin (USA), was studied to determine whether denitrification was occurring below the surface organic layer. Groundwater levels and concentrations of O2, Cl−, NO3−, SO42−, dissolved organic carbon (DOC), and major cations were monitored over a 1-year period along a 230-m transect between an agricultural field and a stream discharge point. Seventeen groundwater samples were analyzed for δ15NNO3 and δ18ONO3 composition. Samples in which NO3− was too low for stable isotope analysis were analyzed for excess dissolved N2. Groundwater NO3− concentrations declined between the agricultural field and the discharge point. Chloride and δ15NNO3/δ18ONO3 data indicated that the drop in NO3− was caused primarily by dilution of shallow NO3−-rich water with deeper, NO3−-depleted groundwater. Two localized zones of denitrification were identified in the upland-wetland transition by their δ15NNO3 and δ18ONO3 signatures, and two in the stream hyporheic zone by the presence of excess dissolved N2. The combined stratigraphic, hydrologic, and geochemical data in these locations correspond to groundwater mixing zones where NO3− is delivered to subsurface layers that support denitrification fueled by dissolved (e.g. DOC or dissolved Fe(II)) and/or solid-phase (e.g. particulate organic carbon, solid-associated Fe(II), or pyrite) electron donors.