Drinking water for more than 5 million Americans contains nitrate levels that may be high enough to increase the risk of cancer and birth defects, a study published earlier this month said.

The problem is especially acute with community water systems that serve Hispanic residents, the report in Environmental Health said. In the wake of the 2015 water crisis in Flint, Michigan, the authors said that results raise new concerns about the connection between socioeconomic factors and water contaminants.

The study, carried out under auspices of the nonprofit Silent Spring Institute, was first reported by Yale Environment 360.

The U.S. Environmental Protection Agency sets a maximum acceptable nitrate level of 10 milligrams per liter (mg/L), but evidence suggests there is an elevated risk of several types of cancer and birth defects at lower levels.

Researchers looked at data from 39,466 community water systems in the U.S. covering the years 2010 through 2014. They concluded that 5.6 million Americans get their water from systems with nitrate concentrations equal to or greater than 5 mg/L. Those levels were highest in agricultural areas that relied on groundwater for drinking water.

Community water systems located in areas with the highest percentage of Hispanic residents exceeded the 5 mg/L level three times as often as water systems with the lowest quartile of Hispanic residents, researchers said.

“Low-income and minority communities face disproportionately high pollutant exposures,” the report said. “The lead crisis in Flint, Michigan, has sparked concern about broader socioeconomic disparities in exposures to drinking water contaminants.”

In the Flint case, more than 100,000 city residents were exposed to elevated lead levels in their drinking water when the heavy metal leached out of old supply pipes.

Nitrate is often found in drinking water. Low concentrations occur naturally, but fertilizers, animal manure, and wastewater discharges can lead to higher levels of the chemical in water supplies. And when nitrate levels are high, other contaminants also are more likely to be found.

The researchers theorized that community water systems in areas with a higher proportion of Hispanic residents would have higher nitrate levels because 80% of U.S. farm workers are Hispanic, and because synthetic fertilizers used in agriculture are the biggest source of nitrate contamination in the U.S.

Researchers also said that communities with more minority residents, especially those with fewer native English speakers, may have less political clout, making it harder to force cleanups. Results  indicate that towns and cities with more renters also tend to have higher nitrate levels in the water.

The study didn’t consider the 44 million Americans who rely on private wells for their drinking water.