Citation:

Pennino, M., S. Leibowitz, J. Compton, R. Hill, AND R. Sabo. Patterns and predictions of drinking water nitrate violations across the conterminous United States. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 722:137661, (2020). https://doi.org/10.1016/j.scitotenv.2020.137661

Impact/Purpose:

Under the Safe and Sustainable Water Resources National Program, EPA scientists are examining the spatial patterns of drinking water nitrate violations across the United States. Excess nitrate in drinking water has been a human health concern since the middle of the 20th century. Since 1979, public drinking water suppliers have been required to regularly monitor nitrate levels and EPA is notified when a public water system violates the 10 mg nitrate-N L-1 maximum contaminant level (MCL). Using this nitrate violation data, along with land cover, climate, geologic, and other variables from EPA’s StreamCat database, we performed random forest models to predict where across the conterminous U.S. regions have the highest concentrations and probability of nitrate violations. We ran separate models for groundwater (GW) and surface water (SW) public drinking water supplies. Our study found regions with the greatest risk for GW nitrate violations to be in central California; western Texas, Oklahoma, and Kansas; most of Nebraska and the Dakotas; parts of the Upper Midwest, southeast Pennsylvania, and Delaware. Areas at greatest risk for SW nitrate violations were generally in the non-mountainous west and southwest. At the national scale, the main drivers of GW nitrate violations were % cropland, agricultural drainage, irrigation-to-precipitation ratio, nitrogen surplus, and surplus precipitation. The main drivers of SW nitrate violations were largely hydroclimatic variables including surplus precipitation, mean precipitation, irrigation-to-precipitation ratio, and % shrubland and canal density. This analysis may inform decisions on how source water protection and other management options could best protect drinking water from nitrate contamination. The results of this study could be important for a number of programs within EPA’s Office of Groundwater and Drinking Water and the Regions and States.

Description:

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.

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