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Trends in Drinking Water Nitrate Violations Across the United States
Pennino, M., J. Compton, AND S. Leibowitz. Trends in Drinking Water Nitrate Violations Across the United States. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 51(22):13450-13460, (2017).
Under the Safe and Sustainable Water Resources National Program, EPA scientists are examining the temporal and 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). Our study found an increase in the proportion of systems in violation for nitrate between 1994 and 2016. However, we found a decrease in the number of people served by systems in violation, when excluding several spikes in the people served caused by large surface water systems. Surface water systems that serve more people have generally been improving over time. Smaller groundwater systems in violation are increasing, as is the average duration of these violations, indicating persistent nitrate problems. This analysis also identifies specific systems or locations that consistently experience violations of the nitrate MCL or where there are higher populations served by systems in violations. Currently, it is not fully known what is causing this increase in nitrate violations in drinking water supplies. This analysis, along with future research on the underlying factors driving the violations, may inform decisions on how treatment, 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. This paper contributes to SSWR 3.01 and 4.03.
Drinking water maximum contaminant levels (MCL) are established by the U.S. EPA in order to protect human health. Since 1975, public water suppliers across the U.S. have reported violations of the MCL to the national Safe Drinking Water Information System (SDWIS). Nitrate is one of the more prevalent drinking water contaminants causing violations, and this study assessed temporal and geographic trends for violations of the 10 mg nitrate-N L-1 MCL in the US. We found that the proportion of systems in violation for nitrate significantly increased from 0.28% to 0.42% of all systems between 1994 and 2009 and then decreased to 0.32% between 2009 to 2016. The number of people served by systems in violation decreased from 1.5 million in 1997 to 200,000 in 2014. However, periodic spikes in these numbers were often driven by just one large surface water system in violation. On average, Nebraska and Delaware had the greatest proportion of systems in violation (2.7% and 2.4%, respectively), while Ohio and California had the greatest number of people served by systems in violation (278,374 and 139,149 people, respectively). Even though surface water systems that serve more people generally have been improving over time, smaller groundwater systems in violation are increasing, as are the average duration of these violations, indicating persistent nitrate problems.