By Mark Steil

MPR News, 90.1 FM

The number of southeastern Minnesota household wells that are contaminated with potentially dangerous nitrates could increase by nearly 50 percent in coming years, a University of Minnesota study shows.

The potential problem stems from high grain prices in recent years that led farmers to convert grasslands to plowed fields, said researcher Bonnie Keeler, lead scientist at the university’s Institute on the Environment. It could force homeowners and local governments to spend millions of dollars to treat their water, the report said.

Grain prices have declined sharply this year, but a few years ago they were at record levels and generating enormous profits for farmers. Spurred by especially lucrative corn and soybean crops, many southeast Minnesota farmers planted more acres between 2007 and 2012.

Keeler said many of those new farm fields had been pastureland or other unplowed areas.

“What we found were a pretty significant trend in grassland conversions to agriculture, mostly to corn agriculture,” she said.

About a quarter of the grass acres in southeastern Minnesota disappeared during the period. More land growing corn meant application of more nitrogen fertilizer. Nitrates are a by-product of nitrogen and can travel from the surface into groundwater. In great enough concentration in drinking water, they can lead to health problems, particularly for infants.

Keeler said the nitrogen fertilizer spread on the additional corn acres likely will increase groundwater pollution in the 11 Minnesota counties in the study.

“We found evidence that trends in grassland loss to agriculture were likely to increase the future number of wells that were contaminated by a pretty significant amount,” Keeler said.

The study estimated a possible 45 percent increase in wells with nitrate levels that are a threat to human health, more than 10 parts per million, the federal safe drinking water standard. That sort of increase would likely affect tens of thousands of southeast Minnesota residents.

The Minnesota Department of Agriculture is the lead state agency on combating the nitrates problem. The agency is completing work on a new way to manage nitrogen fertilizer.

As part of that effort, the agency plans to ban the fall application of nitrogen in parts of the state. It will also work with farmers to improve how they apply the fertilizer, an effort to limit the amounts going into groundwater. But the agency declined to comment on the University of Minnesota study for now, saying it needed time to review the information.

One of the counties in the grassland study was Dakota County, in the southern part of the Twin Cities metro area. Dakota County Groundwater Protection Supervisor Jill Trescott said a likely increase in nitrate contamination in the southeast part of the state means many homeowners and governments will have to pay to clean up their water.

“It’s a significant cost for a private well owner, and it can be an extremely high cost for a public water system,” Trescott said.