Drinking water maximum contaminant levels (MCL) are established by the U.S. EPA to protect human health. Since 1975, U.S. public water suppliers have reported MCL violations to the national Safe Drinking Water Information System (SDWIS). This study assessed temporal and geographic trends for violations of the 10 mg nitrate-N L^–1 MCL in the conterminous U.S. 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% by 2016. The number of people served by systems in violation decreased from 1.5 million in 1997 to 200 000 in 2014. Periodic spikes in people served were often driven by just one large 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 average annual number of people served by systems in violation (278 374 and 139 149 people, respectively). Even though surface water systems that serve more people have been improving over time, groundwater systems in violation and average duration of violations are increasing, indicating persistent nitrate problems in drinking water.

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全美国违反饮用水硝酸盐的趋势

饮用水最大污染物水平（MCL）是美国环境保护署（EPA）制定的，旨在保护人类健康。自1975年以来，美国公共水供应商已向国家安全饮用水信息系统（SDWIS）报告了违反MCL的行为。这项研究评估了违反10 mg硝酸盐N L ^–1的时间和地理趋势美国本土的MCL我们发现，硝酸盐违规系统的比例在1994年至2009年之间从所有系统的0.28％显着增加到0.42％，然后到2016年下降到0.32％。从1997年的150万增加到2014年的20万。服务人口的周期性峰值通常是由一个大型系统造成的。平均而言，内布拉斯加州和特拉华州的违规系统比例最高（分别为2.7％和2.4％），而俄亥俄州和加利福尼亚州的年度平均受违规系统服务的人数最多（分别为278374和139149人） ）。尽管随着时间的流逝，为更多人服务的地表水系统不断改善，但违反地下水系统和平均违法持续时间不断增加，