Nitrate Contamination of Shallow Agricultural Ground Waters at the Local and National Scale

Understanding water quality at a national scale is challenging due to the intense data requirements for even small-scale studies of patterns and trends. This thorny issue was given the name “scale-clash” by a USGS committee chaired by Jacob Rubin in 1985 to recommend a study design for what would become the USGS National Water Quality Assessment Program (NAWQA). Recent results from the NAWQA Agricultural Chemicals Team (ACT) illustrate the implementation of the committee's recommendations for intensive studies at the local scale that can be integrated and interpreted at a national scale. As a part of ACT, detailed studies of nitrogen fluxes and transformations in the unsaturated and saturated zones were conducted at five sites across the USA. Water samples were collected along well transects leading from farm fields to rivers from the saturated and unsaturated zones for analyses of N species, stable isotopes of N and O in NO3-, dissolved gases, and atmospheric age tracers. Concentrations were used to estimate stable isotope and N budgets for individual samples, and to compare NO3- attenuation with apparent ages in order to determine apparent rates of reaction. Results show that the NO3- arriving at the water table is a large fraction (0.1 to 0.6) of applied N at these sites, and that the rate of denitrification in recharging shallow ground water is too low at most of the sites to substantially reduce concentrations before arrival at discharge zones. Coupled with a broad-scale NAWQA study of redox status of the nation's waters, results indicate that nitrate contamination of shallow agricultural waters will persist for decades to come. This study illustrates how the Rubin committee's visionary study design combining local process-based studies with broader scale characterization can successfully address the challenge of “scale-clash” for national studies.