Patterns and predictions of drinking water nitrate violations across the conterminous United States
Excess nitrate in drinking water is a human health concern, especially for young children. When a public drinking water system exceeds the 10 mg nitrate-N/L maximum contaminant level (MCL), that system is in violation of the U.S. Environmental Protection Agency’s (EPA) National Primary Drinking Water Regulations and must report this violation in the EPA’s Safe Drinking Water Information System (SDWIS). We used SDWIS data with random forest (RF) modeling to examine the drivers of nitrate violations across the conterminous U.S. and to predict where systems are most likely to exceed the nitrate MCL. As explanatory variables, we used land cover, nitrogen input, soil/hydrogeology, and climate variables. For groundwater systems, the classification model correctly classified 79% of catchments in violation and the regression model explained 43% of the variation in nitrate concentrations above the MCL. The most important variables in the groundwater classification model were percentage cropland, agricultural drainage, irrigation-to-precipitation ratio, nitrogen surplus, and surplus precipitation. For surface water (SW) systems, the classification model correctly classified 90% of catchments and the 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 percentage shrubland and canal density. Regions predicted to have greatest risk for nitrate violations in groundwater were the Central California Valley, Columbia Plateau of Washington, Snake River Plain of Idaho, Piedmont and Coastal Plains of Pennsylvania, major parts of the Great Plains in western North Dakota and northern Montana and from Nebraska to west-central Texas, and the dairy region of Wisconsin, Iowa, and Minnesota. Areas at greatest risk for surface water nitrate violations were generally in the non-mountainous west and southwest. Understanding where there is a possible future risk of violations and the potential drivers of nitrate violations across U.S., could help inform decisions on how source water protection and other management options could best protect GW and SW sources of drinking water.