Many coal sludge impoundments have weak walls, federal study says Slurry, also known as coarse coal refuse, is what is left over once companies wash coal to enable it to burn more efficiently. Coal firms have disposed of this combination of solids and water in a few different ways: damming it in large ponds, depositing it in abandoned mines and using a dry filter-press process to compact it. The Interior Department's mining agency ordered the survey - which is in draft form and has not been publicly released - after its engineers noticed that companies were using coarse refuse that will not stay compacted except "within a narrow range" of moisture conditions, according to the synopsis. The conditions were not monitored and bulldozers were used to compact the soil, a task for which they are poorly suited, it added. Chris Holmes, a spokesman for the agency, said it was examining a "potential issue" with compaction of dams but that it "has not found any indication that any coal slurry impoundment is in imminent danger of failure. Had it done so, OSM would have taken action immediately. There are 596 coal slurry impoundments in 21 states, according to the Mine Safety and Health Administration, of which the largest number, 114, are in West Virginia. From time to time, one of these slurry ponds breaks, posing a threat to both worker safety and the environment. A worker operating a bulldozer was killed in November when an embankment collapsed at Consol Energy's Robinson Run mine in West Virginia. In a much bigger spill back in 2000, slurry gushed out of holding pond owned by Massey Energy in Martin County, Ky., into an abandoned mine. That accident contaminated the water supply of more than a dozen communities and killed aquatic life in local waterways. Jack Spadaro, an engineering consultant and former director of the National Mine Health and Safety Academy, said the analysis underscores the inherent problem in how mining companies store the waste from their operations. "It's a very weak material and when it's wet, it's even weaker," Spadaro said in an interview, adding that while there are regulations aimed at preventing accidents, "the problem is the agencies in charge of enforcing them are not enforcing them." National Mining Association spokesman Luke Popovich wrote in an e-mail that the Office of Surface Mining had done a more recent report on impoundments concluding that those near underground mines posed "a minimal risk," but the agency "nevertheless urged state agencies and mines to ensure their maps of [underground] mines in the vicinity of impoundments are accurate." "We were not riled up by nor objected to anything in this report," he wrote, referring to the 2011 survey of West Virginia impoundments.