Journal
HYDROGEOLOGY JOURNAL
Volume 18, Issue 8, Pages 1867-1879Publisher
SPRINGER
DOI: 10.1007/s10040-010-0665-2
Keywords
General hydrogeology; Microbial processes; Nitrate; Ferrous iron; USA
Categories
Funding
- University of Wisconsin Department of Geology and Geophysics
- Brown and Caldwell Eckenfelder Scholarship
- Anna Grant Birge Memorial Award
- Sigma Xi
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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.
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