Four environmental groups reviewed data submitted by power companies in March in response to a federal coal-ash rule that required them to install groundwater monitoring wells and publicly disclose leak data under a citizens enforcement provision that could be used in legal challenges. Coal ash contains pollutants that include arsenic, boron, cadmium, chromium, lead and radium. In Illinois, groups wonder if seepage from coal ash stored in unlined retention ponds has reached groundwater. “Once [it] is polluted, it is extremely difficult to stop contamination unless the sources of pollution is removed,” the data report said.

The data shows arsenic exceeds safe levels in groundwater monitoring wells by a factor of more than 2,000, and boron by 11 times more at one Lake Michigan shore site. Other sites exceed safe levels by similar amounts, the report said. Vistra and NRG Energy, which own a number of sites, plan to close some ponds by leaving ash in place and capping unlined pits. But with ponds close to waterways, contamination plumes will still endanger drinking-water sources, the report said. “While the report compares the results to drinking water standards, the water is not used for drinking-water purposes,” said NRG spokesman Dave Knox. Drinking-water wells near the sites have not been tested, the report said. Vistra did not return a request for comment.