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

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 O-2, Cl-, NO (3) (-) , SO (4) (2-) , 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 delta N-15(NO3) and delta O-18(NO3) composition. Samples in which NO (3) (-) was too low for stable isotope analysis were analyzed for excess dissolved N-2. Groundwater NO (3) (-) concentrations declined between the agricultural field and the discharge point. Chloride and delta N-15(NO3)/delta O-18(NO3) data indicated that the drop in NO (3) (-) was caused primarily by dilution of shallow NO (3) (-) -rich water with deeper, NO (3) (-) -depleted groundwater. Two localized zones of denitrification were identified in the upland-wetland transition by their delta N-15(NO3) and delta O-18(NO3) signatures, and two in the stream hyporheic zone by the presence of excess dissolved N-2. The combined stratigraphic, hydrologic, and geochemical data in these locations correspond to groundwater mixing zones where NO (3) (-) 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.