A buffer strip of grass and trees along the Rock River west of Edgerton is a good example of the protective strips that help filter runoff. (Star Tribune/The Minnesota Star Tribune)

Nitrate pollution in public drinking water across rural Minnesota is not only widespread but getting worse, a new report shows.

While not a surprising finding, the analysis released Wednesday is a sobering reminder that fast action is needed to control the contamination.

Using data from the Minnesota Department of Health, the Environmental Working Group studied 115 public utilities across Minnesota with elevated levels of the toxic chemical, then tracked the levels from 1995 to 2018. During that time, the nitrate levels rose in more than 60% of the affected water systems — or about 72 of 115 systems.

The average nitrate level in 1995 was 2.7 milligrams per liter of water; by 2018 the average was 4.4 milligrams. That is below the state and federal limit of 10 milligrams, but it could be high enough to pose potentially serious health risks based on newer health research, according to the report.

The worsening water systems supply tap water for more than 218,000 Minnesotans in farm country, primarily in the southern and central portions of the state.

Nitrate is a dangerous byproduct of nitrogen in farm fertilizers and manure that leaches into groundwater and fouls lakes and streams, making them toxic to fish and other aquatic life. Nitrogen is a major cause of the dead zone of depleted oxygen in the Gulf of Mexico and is particularly dangerous for infants, which prompted the federal government years ago to set a nitrate limit for drinking water of 10 milligrams per liter, or 10 parts per million.

The findings are a call to action, said Craig Cox, senior vice president for agriculture and natural resources at the Environmental Working Group.

"We don't want it to get too late for any more communities," Cox said in an interview. "It's a solvable problem if we just put our minds to it."

Cox said that recent health research, including work by the National Cancer Institute, suggests that the 10 milligram limit is out of date and should be lowered. He said the research shows "pretty troubling results" that link drinking water containing nitrate of 5 milligrams and even less to cancers and, possibly, birth defects.

The state Department of Health helps monitor nitrate in public and private drinking water wells.

Agency spokesman Scott Smith said the department hasn't had a chance to review the subset of data the Environmental Working Group used. The agency often hears that the nitrate standard is too low, or not low enough, he said.

"Our toxicologists constantly review the literature and have not found evidence to modify the current number of 10 mg/L," Smith said. "There is not scientific consensus that exposure to nitrate below 10 mg/L results in adverse health effects."

Smith said the agency constantly works with public drinking water systems to keep nitrate levels below 10, and starts when it detects levels beginning at 3 milligrams.

The new Environmental Working Group analysis follows a broader report the advocacy group released in January. That report showed that one in eight Minnesotans drink nitrate-tainted tap water, and that more than 3,000 households drink from private wells with nitrate concentrations at or above the limit.

The report comes as Minnesota begins implementing the state's new Groundwater Protection Rule, which sets fall fertilizer restrictions, among other things, to try to control nitrate in runoff. It's a first, but the nitrate rule has been widely criticized as weak. It does not contain any protections for private wells, for example, and covers only a fraction of the cropland in the state where commercial fertilizer and manure are applied.

Many communities can address high nitrate levels by blending contaminated water with water from cleaner wells. But at least 10 Minnesota communities have been forced to install nitrate-removal systems or drill new wells.

According to state health officials, the typical faucet-mounted water filter won't filter out nitrate. Removing nitrate requires installing a reverse osmosis or ion exchange system.

Jennifer Bjorhus • 612-673-4683