Assessing and Forecasting Nitrate Fluxes in Agricultural Aquifers
U.S. Geological Survey
Research on hydrogeochemistry of agricultural aquifers is important for sustaining water supplies and drinking water quality. As a part of the NAWQA Agricultural Chemicals Team (ACT), detailed studies were conducted of chemical fluxes and transformations in the unsaturated and saturated zones at seven sites in California, Washington, Nebraska, Iowa, Mississippi, Indiana, and Maryland, USA. Unsaturated zone water and groundwater samples were collected in vertical profiles along transects leading from farm fields to rivers for analyses of N species, stable isotopes of N and O in NO3, dissolved gases, and atmospheric age tracers. Mathematical models were applied inversely to concentration profiles of age-tracers and agricultural chemicals to estimate the hydrogeological and chemical properties of the subsurface, including unsaturated zone travel time, groundwater recharge flux, fraction of applied N lost to groundwater, denitrification rates, and vertical groundwater velocities. The sums of water application + precipitation are relatively uniform among sites, yet recharge to the water table varied by a factor of five as a result of unsaturated zone properties. The NO3 flux at the water table is variable and ranges from 0.1 to 0.6 of applied N at these sites. The rates of denitrification in recharging shallow groundwater are surprisingly uniform among sites, despite reports of many orders of magnitude difference in denitrification rates in previous literature. The consistency of denitrification rates among these sites likely relates to the use of consistent methods and scales of measurement as well as control of denitrification by solid phase electron donors in aquifer materials. Vertical groundwater velocities strongly affected differences in the distribution of redox conditions among these aquifers. Zones of NO3 reduction tended to be shallower at sites with lower vertical groundwater flux. Scenario testing illustrates the effects of vertical velocities, reaction rates, and N loading on future distributions of nitrate in these aquifers. In these shallow agricultural groundwaters, denitrification rates are too low to substantially mediate nitrate contamination, which will persist for decades to come.