Factors controlling nitrate fluxes in groundwater in agricultural areas

The impact of agricultural chemicals on groundwater quality depends on the interactions of biogeochemical and hydrologic factors. To identify key processes affecting distribution of agricultural nitrate in groundwater, a parsimonious transport model was applied at 14 sites across the U.S. Simulated vertical profiles of NO₃^-, N₂ from denitrification, O₂, Cl^-, and environmental tracers of groundwater age were matched to observations by adjusting the parameters for recharge rate, unsaturated zone travel time, fractions of N and Cl^- inputs leached to groundwater, O₂ reduction rate, O₂ threshold for denitrification, and denitrification rate. Model results revealed important interactions among biogeochemical and physical factors. Chloride fluxes decreased between the land surface and water table possibly because of Cl^- exports in harvested crops (averaging 22% of land-surface Cl^- inputs). Modeled zero-order rates of O₂ reduction and denitrification were correlated. Denitrification rates at depth commonly exceeded overlying O₂ reduction rates, likely because shallow geologic sources of reactive electron donors had been depleted. Projections indicated continued downward migration of NO₃^- fronts at sites with denitrification rates