More than a billion gallons of coal ash have spilled from a coal-burning power plant in eastern Tennessee since Dec. 22, when a retention wall at the plant burst. That’s billion with a “B,” which means the amount of gunk spilled is about 100 times larger than the mess from the Exxon Valdez disaster. Gray sludge has spread across 300 acres, wiped out three homes, oozed into a tributary of the Tennessee River, and made a lot of local residents worried about their health and water supplies. Coal ash contains mercury and traces of heavy metals like arsenic and uranium. In the wake of the spill, high levels of arsenic have been found in some rivers and wells near the spill site, though authorities insist that drinking water is still safe. Enviros are seizing the opportunity to point out that “clean coal” is an oxymoron.