Millions of people in the United States drink water contaminated with nitrates from agricultural runoff, which can have adverse effects on human health. For decades, cities and towns in California and the Midwest, where much of this pollution is concentrated, have tried to clean up their water—for a high price. Now, an Environmental Working Group report has found that the brunt of this cost falls on small, rural communities, where a disproportionate amount of residents are living in poverty.