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In a new study of groundwater conditions in dairy farm-intensive Kewaunee County, researchers found higher levels of well contamination from cattle during wet weather events — when manure, rain and melting snow can seep quickly into the ground.

But the results also show that cattle in this northeastern county are not the only source of tainted drinking water. Human waste from sanitary systems is also polluting wells.

The study is the latest research on factors affecting groundwater pollution in a region where tensions over large-scale farms are the greatest in Wisconsin.

"The bottom line is that both kinds of mammals — large animals and humans on the landscape — are to blame," said Mark A. Borchardt, a microbiologist with the U.S. Department of Agriculture, whose work was funded by the Department of Natural Resources.

Kewaunee County has one of the highest concentrations of large-scale farms in the state. The farms have come under sharp criticism for having an out-sized impact.

The county also lies in a region with fractured bedrock that makes it easier for manure to pollute groundwater. The study will provide more insight into manure's potential to pollute.

Six environmental groups petitioned the U.S. Environmental Protection Agency in 2014 to investigate groundwater contamination in the county, prompting the DNR to bring together farmers, neighbors, environmentalists and government officials to find strategies to reduce the risk of groundwater pollution.

Don Niles is the owner of a large farm near Casco and is leading a farmer initiative in Kewaunee and Door counties to improve farming practices.

"This is the best piece of hard data that we've had to better understand what is going on to the groundwater; and how we, from an agricultural point of view, can have impacts," he said.

The study, he said, shows the effects of farming practices at key periods in the year.

Lee Luft, a member of the Kewaunee County Board and chairman of a county task force on groundwater, said the results will be construed as deflecting some of the criticism away from agriculture.

He said, however, the study points to the region's problems. "What we have on our hands here is a combination of problems: The ineffectiveness of some septic systems, but when there is significant groundwater recharge, what we see is a problem from bovine sources," he said.

Borchardt's study sampled water during three periods in 2016 and found polluted water was often traced to sanitary systems during relatively dry periods. But during wet conditions when groundwater was being recharged, polluted water was linked to cattle.

Borchardt's hypothesis: Septic systems can provide a steady source of potential contamination in tap water. Meanwhile, water problems tied to farming practices tend to spike after manure is applied and conditions are wet.

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