Patterns and Predictions of Drinking Water Nitrate Violations across the Contiguous U.S.
On this page:
Excess nitrate in drinking water is a human health concern, especially for young children. The EPA has set a maximum contaminant level (MCL) for nitrate at 10 mg nitrate-N/L. Systems in violation of the nitrate MCL must be reported in EPA’s Safe Drinking Water Information System (SDWIS). We used SDWIS data with random forest modeling to examine the drivers of nitrate violations across the conterminous U.S. and to predict where systems are at highest risk of exceeding the nitrate MCL. We were not able to include treatment type as a factor due to incomplete characterization of treatment in SDWIS. For groundwater (GW) systems, a classification model correctly classified 79% of catchments in violation and a regression model explained 43% of the variation in nitrate concentrations above the MCL. The most important variables in the GW classification model were % cropland, agricultural drainage, irrigation-to-precipitation ratio, nitrogen surplus, and surplus precipitation. Regions predicted to have greater risk for nitrate violations in GW were the Central California Valley, Columbia Plateau of Washington, Snake River Plain of Idaho, Piedmont of Pennsylvania and Coastal Plains of Delaware, major parts of the Great Plains, and regions of Wisconsin, Iowa, and Minnesota. For surface water (SW) systems, a classification model correctly classified 90% of catchments and a regression model explained 52% of the variation in SW nitrate concentration. The variables most important for the classification SW model were largely hydroclimatic variables including surplus precipitation, mean precipitation, irrigation-to-precipitation ratio, and % shrubland. Areas at greatest risk for SW nitrate violations were generally in the non-mountainous west and southwest. The proportion of catchments predicted to be in risk of violation for GW and SW systems (14.6% and 9.3%, respectively) is much higher than the observed proportion of catchments that have violations (0.8 and 0.4%), suggesting that treatment or other factors may be reducing the violation risk. Identifying the areas with possible risk for future violations and the potential drivers of nitrate violations across U.S. can inform decisions related to how source water protection and other management options could best protect drinking water.