Dozens of households in rural Dakota County recently received sobering news about their drinking water: A county study of 66 privately owned wells has found at least trace amounts of pesticides in more than half. In about a quarter of the wells, the contaminants exceeded state health standards.

The tainted wells are heavily clustered around Vermillion, Marshan and other rural, southeastern townships with a history of farming.

"These private wells are owned by residents of rural Dakota County -- it's a mixture of farmers and other homeowners," said Jill Trescott, a county groundwater specialist, who plans to retest the wells this month to confirm the results. "I think that they should be concerned, and they should be attentive to the information that we will be providing later this year."

The study examined levels of pesticides, nitrates, iron and other contaminants in 66 wells, including 42 bedrock wells and 24 sand-and-gravel wells. Letters went out to the study participants in April with detailed descriptions of results, along with an analysis of each well's health risks from the Minnesota Department of Health.

The study has surprised and alarmed environmental officials, who say many of the private wells show elements of a banned pesticide -- Cyanazine -- that hasn't been sold commercially in years. The pesticide, which has been linked to birth defects in animals, was found with evidence of other agricultural chemicals in 17 of 24 sand-and-gravel wells.

"Six of those (wells) exceeded health standards," said David Swenson, the county supervisor for groundwater protection. "That's the most surprising part of the study. The other surprising part is that so many of these wells exhibited multiple pesticides."

Environmental advocates said the study's results underscore the need to keep closer tabs on agricultural practices in areas like Dakota County, where farming takes place on porous, sandy soil over a groundwater aquifer especially vulnerable to contamination.

"No one would have known that there were pesticides in people's wells if Dakota County had not done this study," said Daniel Huff, watershed program director for the Friends of the Mississippi River, based in St. Paul.

"That's one of my biggest concerns. If we have other counties that don't have the wherewithal or the staff or the resources to do that, the state needs to do it."

Water samples also were tested for contamination associated with fuel spills, leaking underground storage tanks and other organic compounds, but none was found. Municipal wells, which are more closely monitored by the cities they serve, were not included in the study. County commissioners asked staff to retest the wells this month, and results are expected in August. If the results are similar, the county Department of Environmental Management will create a list of recommendations for well owners.

Among the options, private-well owners may have to use bottled water or install advanced filtering systems on their water taps.

"It will be basically up to those particular individuals as to what they wish to do," said Dakota County Board Chairman Joseph Harris, who represents rural Dakota County. "We're going to continue to monitor, and we're going to actually widen our monitoring of this."

Harris said surveys show farmers in Dakota County have taken pains to voluntarily adhere to farming standards issued by the Department of Agriculture and the University of Minnesota. The guidelines suggest how to save money by using more limited amounts of pesticides at key times.

"The problem is (that) 20 years ago when that product was being used, the knowledge wasn't there," said Kevin Chamberlain, a farmer who keeps a dairy herd just west of Hastings. "Now, unfortunately, we have to deal with it after the fact."

Pat Sajevic, who heads the Marshan Board of Supervisors, said she was pleasantly surprised to see at a board meeting last year that many residents said they had their wells tested every two or three years.

"When you have a private well, you have a private responsibility to yourself and your family," Sajevic said.