The state of Wisconsin is suing two Twin Cities companies over allegations they violated state law, contributing to a serious sand spill in the St. Croix River this spring.

The Wisconsin Department of Natural Resources notified Plymouth-based Interstate Energy Partners and Maple Grove-based Tiller Corp. last week that a civil case was referred to the state Department of Justice to prosecute.

For an estimated five days, ultra-fine sand being prepared for oil-drilling operations, known as "fracking," leaked from a berm at the Soderbeck mine, near Grantsburg, Wis., into nearby wetlands and a creek that flows into the river. A hiker discovered the cream-colored spill in April.

"It can smother the small aquatics and damage habitats," said Deb Dix, DNR environmental enforcement specialist. "That mine is in a pretty environmentally critical area."

Mike Caron, spokesman for Tiller, told the Star Tribune last month that the company regretted the leaking berm. "This is the last thing we'd ever want to happen at any of our facilities," he said. He could not be reached Saturday.

The case and another in Trempealeau County against Preferred Sands of Wisconsin are the first ever "frac sand" cases the DNR has referred to the state, DNR spokesman Ed Culhane said.

Western Wisconsin is seeing a surge in frac sand companies because of oil industry demand. The DNR estimates the industry has grown from five mines and five sand-processing plants in 2010 to 63 mines and 36 processing plants in 2012.

"It's been a kind of gold rush," Culhane said. "There are very large profits to be made because the sand is highly sought after. ... It's an exponential growth we've never seen before."

In Grantsburg, the mine is about 100 feet outside the St. Croix National Scenic Riverway and is owned by Interstate Energy Partners and operated by Tiller. In letters dated June 4, the DNR alleged the companies illegally discharged water without a permit, failed to maintain berms and failed to notify the DNR of facility expansion. The companies since have fixed the berm, Dix said. "They've been very cooperative," she said.

The DNR refers serious cases to the state to determine fines and corrective action, Dix said. In the meantime, the DNR, U.S. Army Corps of Engineers, the National Park Service and Burnett County conservation officials are analyzing damage to the federally protected river from the sand, which isn't native to the St. Croix. Results should be released this month.

Staff writer Kevin Giles contributed to this report. Kelly Smith • 612-673-4141 Twitter: @kellystrib