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MINNEAPOLIS (AP) — Wisconsin officials were testing for environmental and health hazards Thursday after an accident at a frack sand mine sent millions of gallons of sludge into waterways, tinting them orange as the thick plume traveled downstream into the Mississippi River.

A contractor’s bulldozer slid into a pond earlier this week at the Hi-Crush mine in the western Wisconsin town of Whitehall, leading to an hours-long effort to rescue the man from a 15-foot-deep (5-meter-deep) pond. Rescuers freed the driver after emptying an estimated 10 million gallons (37.8 million liters) of water from the pond into a Trempealeau River tributary.

It’s left questions for residents and county officials as they await the results of tests to determine what’s in the water and if it’s dangerous to human health and the environment.

“We’re worried about the possibility of this happening again,” said Tim Zeglin, chairman of the Trempealeau County Board. He added that no homes were in the path of the spill.

The pond spillage near the border of the 1,447-acre (586-hectare) mine was mostly water, sand, clay and silt, according to company officials who’ve maintained the spill has no health or environmental impacts.

Hi-Crush, which operates three other mines in Wisconsin, said the spill might have also released trace elements of polyacrylamide, a chemical used to remove silt from the water. It’s considered nontoxic but contains small amounts of a neurotoxin known as acrylamide, which is left over from the manufacturing process.

The Wisconsin Department of Natural Resources is testing the water for metals and other chemicals used in sand mining. Department spokesman Jim Dick said in an email those samples could take up to a week to get back from a state water lab.

Several state and federal agencies, including the Minnesota Pollution Control Agency and U.S. Environmental Protection Agency, are monitoring the spill after the sludge reached the Mississippi River Wednesday, as well as a national wildlife refuge earlier in the week.

U.S. Fish and Wildlife supervisor Sabrina Chandler said her agency is keeping an eye on potential environmental impacts. So far, she said she hasn’t noticed any fish or plants deaths in the refuge.

“We do have concerns for the environment every time foreign chemicals get in the water that shouldn’t be there,” Chandler said.

Zeglin said the orange stain left by the spill had cleared up Thursday, leaving crusty orange residue near the river’s shorelines.