Public water systems serving more than 5.6 million Americans contain concentrations of nitrate at levels found to cause health problems, including cancer and birth defects, according to a new study published in the journal Environmental Health. Systems serving Hispanic populations tended to have the highest levels of nitrate in drinking water.

Nitrate in water originates from various sources, including fertilizers, sewage treatment systems, and animal manure. The research, conducted by scientists at the Silent Spring Institute, a non-profit research organization, examined drinking water data from 39,466 public water systems that serve 70 percent of the U.S. population. Nearly 1,650 of these systems had average nitrate concentrations above 5 parts per million (ppm). Communities in the West and Midwest had the highest concentrations of nitrate.

U.S. Environmental Protection Agency guidelines set a safe drinking water standard for nitrate of 10 ppm. But several recent studies have found adverse health effects from concentrations as low as 5 ppm, including elevated risks for bladder, thyroid, colon, and kidney cancers and birth defects. Scientists then analyzed their water data with U.S. Census Bureau information on race, ethnicity, poverty, home ownership, and population density, as well as land-use data on agricultural activity. They found that as the proportion of Hispanic residents increased, so did the likelihood of finding nitrate levels over 5 ppm.

“Since the lead crisis in Flint, Michigan, there’s been a real push to document other types of disparities in drinking water quality in the U.S. and understand the factors that drive them,” Laurel Schaider, lead author of the new study and an environmental chemist at the Silent Spring Institute, said in a statement.