Nearly one-fifth of Americans relied on drinking water systems with elevated and potentially dangerous levels of nitrate in recent years, according to a new study released Thursday.

The nonprofit Environmental Working Group examined test data collected by water systems across the country between 2021 and 2023, the most recent data available. 

Water systems serving more than 3 million people exceeded the federal safety limit of 10 milligrams per liter over the three years, the research and advocacy organization found.

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The analysis also found that thousands of water systems serving more than 62 million people reported nitrate levels above 3 milligrams per liter at least once during those years, which indicates human-caused drinking-water contamination. 

Researchers are increasingly questioning whether the federal threshold should be lowered as more studies find links between even low levels of nitrate consumption and cancer and birth defects. Federal law limits nitrate levels in drinking water because of its association with blue-baby syndrome. 

Nitrate is a natural component of soil, but has become a growing problem for drinking water systems because of crop farming’s use of nitrogen fertilizers and runoff of nitrogen-rich manure from livestock operations.

States with big agricultural industries recorded more reports of elevated nitrate levels. In fact, the report found that 64% of all water systems that recorded nitrate levels at or above the legal limit were in just five states: California, Texas, Kansas, Nebraska and Oklahoma. 

But Anne Schechinger, the organization’s senior director of agriculture and climate research who authored the report, said the issue affects urban and rural areas alike.

“A lot of people have this idea that this issue is just a rural issue for small towns near farms. But we found with this analysis that that is not just the case,” she told Stateline. “Based on how watersheds work, you can live very far from a farm and still be drinking water contaminated with nitrate.”

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The analysis relies on public records obtained from public drinking water systems in every state except New Hampshire, where data was not provided, she said. In addition to its report, the Environmental Working Group created a map showing community water systems with elevated nitrate levels across the country.

Elevated nitrate levels have befuddled water providers across the country for years. Not only are they expensive to remove from drinking water supplies, but nitrate levels can fluctuate with the seasons as heavy rains can quickly push remnants of fertilizer or manure into streams and rivers. 

Iowa’s largest water provider last year asked residents to refrain from watering lawns, filling pools and washing cars as its nitrate removal system struggled to keep up with elevated levels. 

Des Moines is home to one of the largest nitrate removal systems in the world, which costs about $16,000 per day to operate, officials said. Smaller communities that rely on groundwater have been forced to dig deeper wells, Schechinger said.

Climate change is further fueling the problem: Agriculture is a major driver of greenhouse gas emission. The heavy rainfalls and prolonged droughts from more extreme weather worsen nitrate runoff into lakes, rivers and groundwater. 

“We know those climate conditions are going to make this problem worse,” Schechinger said. “And that’s likely to cost us all more and also (raise) more concerns for our health.”