At more than 70 sites across the country, toxins like arsenic, mercury, and radium are leaching into groundwater from pond-like storage pits filled with the sludgy leftovers of coal burning. That’s the most alarming takeaway from reports that the coal industry was required to submit to the Environmental Protection Agency this month, part of the first-ever federal regulations of the waste product known as coal ash.

And yet, one day after the data was made public, EPA Administrator Scott Pruitt announced his plan to overhaul President Barack Obama’s 2015 coal ash rule, which requires companies to continuously inspect coal ash storage pits for leaks and monitor the surrounding areas for contamination. Coal companies and electric utilities had told him in May that these requirements were too expensive, and in September he said it would be “appropriate and in the public interest” to rethink them. Now, Pruitt is proposing more than a dozen changes, including giving states and power utilities more control over how they dispose of coal ash and how often they test for groundwater contamination.

Scientists and environmental watchdogs have long known that coal ash pits—of which there are more than a thousand in the United States—were leaking toxins into groundwater. “The data released is consistent with data we’ve seen before, and also measured before, that showed shallow wells underlying most everywhere are being affected by leaking coal ash,” said Avner Vengosh, a geochemist and coal ash researcher at Duke University. But this new data puts a glaring spotlight on what scientists don’t know: Where, exactly, that contamination is going, and whether it’s affecting the drinking water of nearby communities. “This data is restricted to monitoring at the site of the coal ash ponds,” Vengosh said. “We don’t know whether drinking water wells nearby have been affected.”

Vengosh’s research group has been trying to answer these questions for a decade. Communities across the country have accused coal companies of poisoning their drinking water with contaminants from leaking coal ash, which is considered the second-largest industrial waste stream in the country behind household garbage. (Approximately 130 million tons of coal ash is produced every year, according to the EPA.) But most communities with compromised water have been unable to prove that coal ash is the culprit, because pollutants like arsenic, chromium, and even radon can be naturally occurring. Vengosh’s team has found, for example, that the presence of carcinogenic hexavalent chromium in many North Carolina drinking water wells stems from volcanic rocks underground, not coal ash.

Other independent studies, however, have found coal ash–contaminated groundwater flowing into drinking water sources. In a lawsuit targeting the Tennessee Valley Authority’s coal plant in Gallatin, Tennessee, the Southern Environmental Law Center found “a hydrologic connection” between the polluted groundwater surrounding the company’s waste storage ponds and the Cumberland River, which provides drinking water to one million people living downstream of the site. A federal court later agreed, and found the Tennessee Valley Authority violated the Clean Water Act.