Use of classes based on redox and groundwater age to characterize the susceptibility of principal aquifers to changes in nitrate concentrations, 1991 to 2010

The National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey is using multiple approaches to measure and explain trends in concentrations of nitrate in principal aquifers of the United States. Near decadal sampling of selected well networks is providing information on where long-term changes in nitrate concentrations have occurred. Because those studies do not include all the NAWQA well networks, a determination has yet to be made as to what might be expected in networks from which timeseries data have not been collected. Characterizing aquifer susceptibility to changes in nitrate concentrations on the basis of data collected from all the NAWQA well networks would be a step toward extrapolating findings from those studies to broader regions.