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Two Minnesota companies have agreed to pay $80,000 to settle a pollution case at a sand mine in northwestern Wisconsin where sediments from the mine infiltrated a wetland, a creek and ultimately flowed into the St. Croix River.

The settlement was announced Monday by Attorney General J.B. Van Hollen and involved Interstate Energy Partners and Tiller Corp. for their failure to stem the flow of sand and sediment from the mine in the Town of Grantsburg in Burnett County.

The companies settled with Justice Department lawyers for failing to maintain dikes and berms to control runoff from the site.

The case is the third involving a sand mine since mid-December. Altogether, the parties have agreed to pay $360,000 to settle.

The sand mining industry has grown sharply in recent years. But with the growth, some companies have run afoul with state environmental laws.

The Department of Natural Resources has referred six pollution cases targeting the industry to the Justice Department. Three referrals have come since August.

Bill Frothinger, chief executive officer of Interstate Energy, based in Plymouth, Minn., declined to comment. A call to Tiller was not returned. Tiller is headquartered in Maple Grove, Minn.

The spill into the St. Croix began with a citizen complaint on April 26, 2012.

According to the settlement, the fine-grained sediments began to flow from the property some time between April 19 and 22. Neither company reported the mishap when it occurred, according to documents.

Aerial photos revealed a trail of polluted sediment from the mine to the St. Croix, which is a federally designated national wild and scenic river. The mine is next to Governor Knowles State Park and land owned by the National Park Service.

Sand mining has grown with the surge in hydraulic fracturing or fracking —a technique where sand, chemicals and water are injected into the ground at high pressure to extract oil and natural gas that previously had been considered too difficult to tap.

Western Wisconsin sand has the characteristics that the oil producers seek for fracking.