Agricultural Chemical Movement through a Field-Size Watershed in Iowa:  Subsurface Hydrology and Distribution of Nitrate in Groundwater

A 40-ha field is under study in the loess hills of southwestern Iowa to determine the impact of corn production in ridge-tilled soils on the nitrate-nitrogen loading in groundwater. Within the vadose zone, nitrate concentration between June 1989 and December 1991 ranged from <10 to >80 mg/L. Well water concentrations increased from <5 mg/L in 1972 to >60 mg/L in 1994. In both hydrogeologic compart ments, time of sampling and landscape position are important factors influencing concentrations. The unsaturated zone groundwater system has a high potential for storage of unutilized nitrogen as nitrate. Leaching resulted in the drinking water MCL being exceeded for several wells screened within the saturated loess, which is characterized by relatively high hydraulic conductivity. Concentrations within and below the loess−glacial till interface did not exceed the standard. A conservative solute transport model was used to predict the concentration of nitrate exiting the field in basin drainage. Denitrification in which nitrate is reduced to nitrite by autotrophic bacteria and then further reduced geochemically to nitric oxide, nitrous oxide, or nitrogen may be an important mechanism for reducing the nitrate concentration within selected landscape positions, especially those in near proximity to the water table. Due to its relatively rapid conductance of both water and applied agchemicals, the loess hills represent a vulnerable agricultural landscape on which nitrogen fertilization impacts groundwater quality.