What is the largest type of trash produced in the U.S.? It's not whatever you're thinking, most likely. It's coal ash.

Burning coal produces more than 100 million metric tons of coal ash per year—the gray or black sooty aftermath of our fossil fuel habit. Even though a good chunk of it is turned into concrete, tens of millions of metric tons end up dumped back into old mines and landfills or impounded in slurry ponds (to keep it from blowing away).

Unfortunately, those slurry ponds tend to leak, as folks up and down the Dan River in North Carolina found out this week when the river turned dark gray. A more than 50-year-old slurry pond at a retired coal-fired power plant near Eden started leaking and released more than 100 million liters of water that contained as much as 74,000 metric tons of the toxic black sludge. That makes this spill the third largest coal slurry spill since 2000. The problem appears to be the collapse of a single metal pipe underneath a relatively small coal ash pond that held nearly one million tons of the hazardous residue.

Leaking coal ash is no joke. An even bigger coal ash spill in 2008 near Kingston, Tenn., killed swaths of the Emory and Clinch Rivers, and drowned a square kilometer of land.

Coal ash is not a benign form of garbage. Burning coal concentrates the heavy metals and other toxic elements (including radionuclides) found in the dirtiest of fossil fuels. Samples of Dan River water sent for testing by Waterkeeper Alliance revealed arsenic, chromium and lead at dangerously high levels. The arsenic levels of 0.349 milligrams per liter is some 35 times higher than levels considered acceptable for drinking water by the U.S. Environmental Protection Agency (EPA).

That contamination has yet to reach the town of Danville downriver, which relies on the river for drinking water. The local water authority has yet to detect heavy metals in its samples. That said, the heavy metals will remain in the river sediments and water for a long time to come, unless it is dredged out.

The EPA is still considering the larger question of what should be done nationwide about this toxic pollution that we have in copious quantities, after industry criticism stalled the first-ever proposal in 2010 to regulate the hazardous waste. But the agency has already determined that more than 580 coal-ash storage sites exist in varying states of repair across the U.S., many like the one that leaked into the Dan River. In fact, the EPA specifically warned Duke Energy about the now leaking site back in 2009—and there are 48 other sites the agency considers a risk for killing people in the event of an accident.

The Dan River spill may now be dwindling but it's another reminder that there are few good solutions for coal ash, other than not creating it in the first place by burning coal.