Safe drinking water is essential for the health and well-being of humans and life on Earth. Previous studies have shown that groundwater and other sources of drinking water can be contaminated with nitrate above the 10 mg nitrate-N L-1 maximum contaminant level (MCL), which is known to have adverse health effects, including certain cancers. Public water systems (PWS) across the US have been required since 1979 to report violations of the nitrate MCL to the US Environmental Protection Agency’s Safe Drinking Water Information System (SDWIS). The objective of this research was to use SDWIS data to assess temporal and spatial trends for nitrate violations. We collected data from SDWIS on the number of PWS that violated the nitrate MCL at least once each year. The proportion of systems in violation each year was calculated by diving the number of systems in violation by the total number of active systems that year. The number of people served by systems in violation was calculated by summing the number of people served by each PWS in violation.
We found that the number and proportion of systems in violation for nitrate significantly increased over time, almost doubling from 280 (0.16%) to 527 (0.33%) systems between 1994 and 2010. The increase in number and proportion of systems in violation is attributed to an increase in violations for groundwater systems, whereas violations for surface water systems have decreased over time. The number of people served by systems in violation varies over time from several hundred thousand to nearly two million, but has decreased from approximately 1,000,000 to 200,000 people between 1997 to 2014. Spikes in the number of people served by systems in violation are attributed to just one or a few surface water systems, but these systems can serve over one million people. Kansas and Nebraska had the greatest proportion of systems in violation, while the greatest number of people served by systems in violation are in Ohio and California. Results show that surface water systems that serve more people generally have been improving over time. In contrast, rural groundwater systems in violation are increasing, as are the average duration of these violations, indicating persistent nitrate problems. In the future we will be investigating associations between land use or anthropogenic inputs and nitrate violations. Understanding where and when violations are most prevalent and what is causing the violations may help inform future management decisions.