A Wisconsin frac-sand mine that was "running wild" and dumping polluted wastewater into an unlined pond against regulations has been shut down by Trempealeau County.

The Guza Pit, four miles south of Independence, Wis., had been operating without a permit and was shut down Monday with a "stop-work" order from county regulators. It could face fines when the situation is sorted out, said Kevin Lien, who heads the county's zoning office.

The episode illustrates the challenges local regulators have faced as large and small mining operations, often run by absentee owners, have cropped up in rural jurisdictions across Wisconsin and Minnesota due to the Upper Midwest's frac-sand boom. The crush-resistant silica sand found in parts of the two states is an essential ingredient for the technique known as "fracking" that has set off a drilling resurgence in North Dakota and other parts of the U.S. and Canada.

Neighbors alerted the county and the Wisconsin Department of Natural Resources (DNR) to activity at the Guza site because they believed it hadn't received permission to operate after being annexed by the city of Independence early last month.

"They are just running wild, with no permit at all," Lien said.

Lien said the Guza Pit, located across the Mississippi River from Winona, was being operated by Superior Silica Sands, a Texas-based company with large frac sand plants in New Auburn and Clinton, Wis. The company did not immediately return phone calls Tuesday.

Lien said workers at the site agreed to cease activity while lawyers for the mining concern met Tuesday with Lien and his staff.

He said the operators were violating a county ordinance that requires water containing certain mining chemicals to be dumped into a lined pond. The Guza Pit is located near a stream that flows toward the Mississippi River. The chemical in question, polyacrylamide, is used to clarify frac sand wash water and contains residual amounts of acrylamide, a neurotoxin linked to cancer and infertility.

"We gave them a list of things they need to do before we would release the cease-and-desist order," Lien said.

DNR environmental enforcement specialist Deb Dix said her office sent an agent to the pit on Tuesday to gather information. She said Trempealeau County has jurisdiction over the mine because the city of Independence hasn't issued the necessary permits for operation since annexing the property. The Independence City Council is scheduled to meet Thursday to address the issue.

Lien said the 1,600-acre silica sand mining and processing site will escape the county's regulatory authority if the city of Independence follows through on its plan to provide regulatory oversight. He said several frac-sand companies operating in Trempealeau County have sought annexation to small towns to avoid county regulation. Independence, for example, agreed to annex the land in exchange for 15 cents for every ton of finished frac sand.