ST. PAUL — A spill at a sand mining facility in Wisconsin has dumped an unknown amount of sand and other sediment into the St. Croix River and wetlands near the Minnesota border, the Wisconsin Department of Natural Resources said.

The sand-laden water may have harmed aquatic life, but DNR officials said on Thursday that it’s too soon to know how much damage was done. The DNR plans an investigation with help from the U.S. Army Corps of Engineers, the National Park Service and Burnett County Conservation officials.

“I’m sure there were things living there that are going to have difficulty living there now that they’re covered with sand,” said Tom Woletz, a senior manager at the Wisconsin DNR who specializes in sand mining.

Conservation officials first learned of the leak on April 22, when a hiker reported seeing cream-colored water in a creek flowing to the St. Croix River. DNR investigators used a plane to survey the area four days later. They traced the murky water back to a 72-acre sand mining facility operated by Maple Grove-based Tiller Corp., where they located a leak in one of the facility’s holding ponds.

The company was notified and agreed to repair the pond immediately, according to DNR officials and Tiller Corporation’s land use affairs director Mike Caron.

Caron said he doesn’t know how mine employees failed to notice the leak.

“I don’t have an answer for that question,” he told MPR Thursday. “We’ve asked our people to do their inspections. Somehow this one got missed by them.”

The sand is mined for use by the petroleum industry to help extract underground natural gas and oil supplies in a controversial practice called hydraulic fracturing, or fracking. The technique works by blasting water, sand and chemicals into the ground to break up shale rock formations and release oil and natural gas deposits. Wisconsin is home to one of the largest known deposits of the kind of sand most prized by the petroleum industry.

County officials are responsible for approving new mine sites but are not required to regularly inspect the facilities, Burnett County Conservationist Dave Ferris said.

“We go out on an occasional basis,” Ferris said. “We did not have any particular inspection regime in our permit.”

Ferris said he plans to increase inspections at the site, which is the western Wisconsin county’s only frac sand mine.

Jill Medland, environmental coordinator at the St. Croix National Scenic Riverway, said the spill shows the danger of allowing a sand mining facility so close to a pristine waterway.

“It seems that there’s such a demand for frac sand that things are getting permitted quickly without full environmental review, and then things happen that should or could have been avoided,” she said.