HR: 14:55h
AN: H53J-06    [Abstracts]
TI: Collateral Geochemical Impacts of Agricultural N Enrichment from 1963 to 1985: A Southern Wisconsin Groundwater Depth Profile
AU: Kraft, G J
EM: gkraft@uwsp.edu
AF: College of Natural Resources, University of Wisconsin - Stevens Point, Stevens Point, WI 54481, United States
AU: Browne, B A
EM: bbrowne@uwsp.edu
AF: College of Natural Resources, University of Wisconsin - Stevens Point, Stevens Point, WI 54481, United States
AU: * Bowling, J M
EM: jbowling@uwsp.edu
AF: College of Natural Resources, University of Wisconsin - Stevens Point, Stevens Point, WI 54481, United States
AU: Devita, W M
EM: wdevita@uwsp.edu
AF: College of Natural Resources, University of Wisconsin - Stevens Point, Stevens Point, WI 54481, United States
AU: Mechenich, D J
EM: dmecheni@uwsp.edu
AF: College of Natural Resources, University of Wisconsin - Stevens Point, Stevens Point, WI 54481, United States
AB: In this study, we used CFC age-dating and biogenic N gas measurements to reconstruct the chronology of groundwater N-enrichment within a bedrock aquifer depth profile beneath a south central Wisconsin agricultural landscape. Four geochemical impacts were associated with a steady groundwater N-enrichment trajectory (40 μ mol L^-1 yr^-1, r²=0.99) between 1963 and 1985. First, NO₃ ^- became prominent as a mobile electron acceptor. Denitrified N accounted for 36% of the total N within the profile. Denitrification appeared to occur in the near water table environment, in hot spots or hot moments, rather than progressively along lengthy groundwater flowpaths. Second, as a byproduct of soil nitrification, N₂O entered groundwater via gravity drainage from the soil at a stable (r²=0.99) mole ratio of 0.24% (N₂O-N/NO₃-N). The gathering of excess N₂O in groundwater is a potential concern relative to greenhouse gas emissions and stratospheric ozone depletion after it discharges to surface water. Third, nitrification driven dissolution of dolomitic soil amendments more than doubled the concentrations of major ions (Ca, Mg, HCO₃^-) in groundwater by 1985 relative to their concentrations prior to agricultural N-enrichment. Finally, dissolved P increased 0.002 mg L^-1 yr^-1 from approximately 0.02 mg L^-1 in 1963 to 0.07 mg L^-1 in 1985, in association with rising Ca concentrations (r²=0.67). Nitrification induced mobilization of Ca may have caused a co-release of P from Ca-rich soil surfaces, promoting its loss in gravity drainage. The geochemical changes in groundwater induced by N-enrichment have implications for understanding the current and future quality of our surface waters.
DE: 0402 Agricultural systems
DE: 0432 Contaminant and organic biogeochemistry (0792)
DE: 0469 Nitrogen cycling
DE: 1065 Major and trace element geochemistry
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2007 Fall Meeting