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TECHNICAL REPORTS

Ground Water Quality

Collateral Geochemical Impacts of Agricultural Nitrogen Enrichment from 1963 to 1985: A Southern Wisconsin Ground Water Depth Profile

College of Natural Resources, Univ. of Wisconsin, Stevens Point, WI 54481

^* Corresponding author (bbrowne{at}uwsp.edu).

Received for publication February 7, 2007. In this study, we used chlorofluorocarbon (CFC) age-dating to investigate the geochemistry of N enrichment within a bedrock aquifer depth profile beneath a south central Wisconsin agricultural landscape. Measurement of N₂O and excess N₂ allowed us to reconstruct the total NO₃^– and total nitrogen (TN) leached to ground water and was essential for tracing the separate influences of soil nitrification and ground water denitrification in the collateral geochemical chronology. We identify four geochemical impacts due to a steady ground water N enrichment trajectory (39 ± 2.2 µmol L^–1 yr^–1, r² = 0.96) over two decades (1963–1985) of rapidly escalating N use. First, as a by-product of soil nitrification, N₂O entered ground water at a stable (r² = 0.99) mole ratio of 0.24 ± 0.007 mole% (N₂O–N/NO₃–N). The gathering of excess N₂O in ground water is a potential concern relative to greenhouse gas emissions and stratospheric ozone depletion after it discharges to surface water. Second, excess N₂ measurements revealed that NO₃^– was a prominent, mobile, labile electron acceptor comparable in importance to O_2. Denitrification transformed 36 ± 15 mole% (mol mol^–1 x 100) of the total N within the profile to N₂ gas, delaying exceedance of the NO₃^– drinking water standard by approximately 6 yr. Third, soil acids produced from nitrification substantially increased the concentrations of major, dolomitic ions (Ca, Mg, HCO₃^–) in ground water relative to pre-enrichment conditions. By 1985, concentrations approximately doubled; by 2006, CFC age-date projections suggest concentrations may have tripled. Finally, the nitrification induced mobilization of Ca may have caused a co-release of P from Ca-rich soil surfaces. Dissolved P increased from an approximate background value of 0.02 mg L^–1 in 1963 to 0.07 mg L^–1 in 1985. The CFC age-date projections suggest the concentration could have reached 0.11 mg L^–1 in ground water recharge by 2006. These results highlight an intersection of the N and P cycles potentially important for managing the quality of ground water discharged to surface water.

Abbreviations: ANC, acid neutralizing capacity • ASW, air-saturated water • CFC(s), chlorofluorocarbon(s) • DOC, dissolved organic carbon • ECD, electron capture detector • FID, flame ionization detector • IPCC, Intergovernmental Panel on Climate Change • N, nitrogen • PDD, pulse discharge detector • PIE, pumping-induced ebullition • SRP, soluble reactive phosphorus • TKN, total Kjeldahl nitrogen • TN, total nitrogen • X, dry gas mole fraction