The Wisconsin Department of Natural Resources says water contamination levels declined significantly in the days after the release of 10 million gallons of sludge from a Trempealeau County sand mine last month.
Workers at the Hi-Crush mine in Whitehall drained a holding pond on May 21 after a bulldozer slipped into the water.
The driver escaped uninjured after about 2½ hours in the air-tight cab.
Water samples taken May 23 in Pocker Coulee showed higher concentrations of metals and silt than in portions of the creek above the site of the spill, DNR spokesman James Dick said Tuesday.
Only copper was found at levels higher than state surface water toxicity standards, but Dick said the department’s analysis showed much of the copper was attached to particles and not dissolved in the water, where it could affect aquatic life.
Additional samples are still being tested at the state health lab.
Dick said water samples taken from the Trempealeau and Mississippi rivers showed no contaminants exceeding state water quality standards.
Last week the DNR released data from samples taken the day of the spill, which showed elevated levels of contaminants, including lead concentrations of more than 70 times the Environmental Protection Agency’s drinking water limit and arsenic measured at more than six times the EPA limit.
There also were elevated levels of beryllium, cadmium, chromium, copper and selenium.
The Trempealeau River was colored orange in the days after the spill as sediment from the mine made its way to the Mississippi River, but Dick said dissolved oxygen levels remained above the level needed to sustain aquatic life and that the DNR has not documented any fish kills.
The Trempealeau County Health Department last week issued a release stating water in the Trempealeau River “does not pose an immediate health risk,” although state and local officials are still evaluating the overall health concerns associated with the spill.
Public health officials say people should avoid swallowing contaminated sediments and included the following recommendations:
- Avoid areas that have orange sandy material.
- Wash after wading or swimming.
- Watch children closely to make sure they do not eat any sand or sediment from the river.
Hi-Crush officials said the pond contained mostly water, silt, clay and sand, though it could contain trace amounts of a polyacrylamide, which is used to remove silt from the water.
Jay Zambito, a geologist with the Wisconsin Geological and Natural History Survey, said the metals identified in the test results are commonly found in the rock formations being mined in Trempealeau County and could be a source of the contaminants found in the process water.
“However, the spatial variability of the trace metal-bearing sulfide minerals in these rocks suggests that the potential for water contamination at a given mine would be site specific, and further detailed study is needed,” Zambito said.
Cleanup continues on the creek and nearby land as workers collect the sludge and return it to the mine site.
Last week the federal Mine Safety and Health Administration cited Gerke Excavating for moderate negligence in connection with the accident.