Six years ago, a dam in Tennessee holding back a billion gallons of coal ash from a power plant broke. More than 300 acres were covered in sludge. Homes were destroyed. Environmentalists say it was one of the largest toxic spills in history. In response, the Environmental Protection Agency pledged to pass safeguards. Yet there are still no federal regulations on coal ash disposal. And there have been more major spills, including one this year in North Carolina on the Dan River. A court ordered the E.P.A to finalize new rules this month. A discussion about regulating coal ash.