But the study, done after the failure of a 300 million-gallon Martin County Coal Corp. dam, focused on short-term surface exposure - not long-term ingestion of slurry.
No one can declare underground injection safe without meaningful scientific study, says organic chemist Bill Orem, with the U.S. Geologic Survey in Reston, Va.
"Does coal slurry have a health impact? I don't know. And for somebody to say it has no health impact is not correct, either," Orem says.
Coal contains substances that are potentially toxic and carcinogenic, Orem says, and his agency is eager to study both the composition of slurry and the potential health hazards.
But the agency has neither the invitation nor the access it needs: Waste areas and injection sites are on private property, and coal operators usually deny access to get samples.
"If we could get access to the sites," Orem says, "we would do this for nothing."
West Virginia regulators are having water and slurry samples from six injection sites analyzed by an outside lab and expect to report their findings in May. But one state legislator has already introduced a bill to impose a moratorium on injections until more is known.
EPA hasn't studied underground injection of coal waste in a decade but said in 2002 its existing rules were adequate to protect groundwater. In response to recent AP questions, EPA pointed to the 2002 document.
While EPA does administer some injection programs, it has acknowledged many states run their own, with regulations that "vary significantly in their scope and stringency."
Allen Hershkowitz, a senior scientist at the Washington-based Natural Resources Defense Council, has studied underground injection for decades and says the regulatory framework is "fraught with loopholes."
"EPA has been particularly negligent when it comes to coal waste management," he said. "Frankly, it would not be an unfair characterization to say EPA's behavior when it comes to coal waste management borders on criminal."
Underground injection is neither ecologically nor economically defensible, Hershkowitz said, and allowing it subsidizes the industry and discourages waste-disposal innovation.
"Everything about this is wrong," he said.