By Brett Walton The US Environmental Protection Agency, reversing a Trump-era decision, is restarting a human health assessment of nitrate and nitrite, a move that has potentially far-reaching regulatory implications for one of the country’s most pervasive drinking water contaminants. The EPA suspended its nitrate assessment in December 2018. It was one of nine chemicals that the Trump administration deemed no longer a priority for evaluation by the agency’s health assessment division known as the Integrated Risk Information System, or IRIS. That changed in June when the EPA quietly reinserted the nitrate assessment into the IRIS agenda “to address an agency priority.” The decision to take a closer look at the chemical comes at a time of increased scrutiny and public concern over nitrate exposure and human health. Nitrate forms when nitrogen is exposed to oxygen in the air. An exponential increase in the use of synthetic, nitrogen-based fertilizers since 1950 supercharged the production of corn and soybeans. The number of large livestock feeding operations -- and the amount of manure they produce -- also increased rapidly. Nutrients are essential to crop production. But the convergence of high rates of chemical fertilizer use and immense manure production led to well-documented risks to human health and the environment. In addition to generating heat-trapping greenhouse gases, over-application of nitrogen-rich fertilizer and manure contributed to steadily rising levels of nitrate in streams and groundwater, particularly in the 11 Corn Belt states across the Great Lakes and Midwest, California’s Central Valley, North Carolina, and Washington’s Yakima Valley.