Tests of private well water in Dakota County showed high levels of contaminants over the past two years.

On average, 27 percent of the wells sampled in rural Dakota County communities contained nitrate that exceeded the safe drinking standard.

“What we found there was quite disappointing in terms of water quality,” said Jill Trescott, a Dakota County groundwater protection supervisor.

High levels of nitrate can cause infants to develop a condition called blue baby syndrome, where their blood does not absorb enough oxygen. High nitrate levels also might increase the risk of non-Hodgkin’s lymphoma, but studies are not conclusive.

A county campaign called “Test the Waters” found more than half the wells tested in Coates and Marshan Township had nitrate levels above the Environmental Protection Agency’s safety standard of 10 milligrams per liter.

About 1,640 households with private wells in Dakota County have nitrate levels above the standard, according to county estimates. But only 460 households reported having a reverse osmosis system to treat nitrate. That system costs roughly $336 a year, Trescott said. Boiling water or using a water softener will not remove nitrate, according to the county.

Nitrate in groundwater can come from crop fertilizer, manure or septic systems.

Officials also tested for manganese, a naturally occurring substance. People need it — but only a small amount.

“There’s a sweet spot where human beings need a little of it, but too much of it causes nerve damage,” Trescott told county commissioners last week at a presentation of the testing results.

One in five wells tested exceeded the manganese standard for infants, and 10 percent were above the level considered safe for adults. Rosemount, Empire Township and Greenvale Township had particularly high levels of the compound, tests showed.

Officials are talking with county residents about the various test results, Trescott said.

Meanwhile, the Minnesota Department of Agriculture is updating how it manages nitrogen fertilizer.

The department will be conducting more testing and working with farmers and communities over the next few years to come up with a rule to protect groundwater, said Larry Gunderson, a supervisor at the Agriculture Department.

“This will be a team approach, and we hope to be partnering with local organizations as we move ahead,” Gunderson told commissioners last week.

One commissioner asked if she still should use fertilizer, which contains nitrate, to keep her lawn green.

People should not apply too much, but the amount of nitrate leaching into the groundwater from lawns and golf courses is fairly low, Gunderson said.

The permanent root structure of grass in yards helps prevent fertilizer from leaching, while cropland that is bare allows nitrate to seep in more easily, Trescott said.