ST. PAUL — Hundreds of thousands of Minnesotans drink from tap water sources that contain potentially unsafe amounts of nitrate, according to a new report by an environmental nonprofit.

Levels of nitrate observed in roughly 1,100 groundwater systems and more than 13,000 private wells are still low enough to meet federal guidelines. But because drinking tap water with even small amounts of nitrate in it may be a risk factor for cancer and birth defects, the Environmental Working Group called in its report for an "aggressive policy and programmatic approach" to address the situation in Minnesota.

Under the federal Clean Water Act, drinking water that contains less than 10 milligrams of nitrate per liter is considered to be safe for human consumption. In past research, however, the Environmental Work Group found thresholds of half that amount and less may increase the risk of illness.

"It is clear that protecting public health requires keeping the contamination level far below the legal limit," the report reads.

Nitrate, a compound found in fertilizer and manure, can be washed away from crop fields and seep into groundwater sources by rain and irrigation. Many of the contaminated water sources identified in the Working Group's report are located in rural Minnesota.

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Based of state and federal data, the Working Group estimates that 55 public water systems serving approximately 259,000 Minnesotans tested for nitrate levels greater than or equal to the 5 milligram threshold on at least one occasion between 2009 and 2018. A further 358 non-community systems — which provide on-site water service to facilities like hotels, churches and apartment buildings — tested similarly.

A total of 727 public and non-community systems that serve a combined 470,000 people contain nitrate levels greater than or equal to 3 milligrams per liter, according to the report. According to a state Department of Health web page, that threshold suggests that man-made nitrate sources "have contaminated the water and the level could increase over time."

The situation for households reliant on private wells may be more severe. Approximately 13,400 households tested positive for nitrate levels between 3 and 5 milligrams per liter, according to the report.

Because upgrading a water system to treat for nitrate levels can be costly, Working Group economic analyst Anne Schechinger said that many private well users do not have the option to do so.

"There really has not been a lot of work done on private wells in Minnesota or in the rest of the country," she said in an interview.

Researchers from the group said that states can adopt stricter water safety standards than the federal government's. At the federal level, the EPA in April announced it would scuttle plans to review its legal nitrate limit.

According to the report, approximately 124 public and non-community water systems meet or exceed the level nitrate considered unsafe. Those systems serve approximately 154,000 people.

The same can be said of private wells serving more than 3,000 households.

Adding to the issue is that utilities Minnesota are required to periodically test their public water systems but well owners are not. Researchers from the Working Group suggested that state agencies also subject private wells to periodic testing and perhaps offer to help pay for it, too.

Calling efforts to prevent further nitrate leaching through Minnesota's Groundwater Protection Rule a "welcome first step," the Working Group added that that measure does not go far enough. Beginning this year, the rule will prohibit agricultural producers from spreading nitrogen fertilizer and manure in the fall or on frozen soil in parts of the state prone to leaching.

The group said that does little for other parts of the state where fertilizer use is still common. And because the program is intended only to prevent nitrate levels from exceeding the EPA's legal limit, the group said that it may do "too little, too late."

More than 800 crop producers in Minnesota have been certified through a separate state program that aims to reduce water contamination incidents through water conservation.