The impoundment is permitted to hold 3.4 billion gallons but typically operates well below that volume.Slurry is a byproduct of washing coal to help it burn more cleanly. Companies have disposed of the dirty water, silt and solids in various ways over the years, injecting it into worked-out underground mines, damming it in huge ponds like the one at Robinson Run and, less commonly, disposing of it with a dry filter-press process. West Virginia has 114 coal slurry impoundments, according the MSHA. In all, there are 596 coal slurry impoundments in 21 states. Kentucky has the second-largest number with 104, while Illinois is third with 71. Longwall mining operations at the Robinson Run mine were initially suspended after the accident, but Barletta said production resumed Wednesday. The preparation plant will also resume work soon, but the mined coal will be stockpiled until all safety precautions are in place, he said. That involves erecting a boom across the pond with a curtain that descends about 4-5 feet into the water. If the pipe dive to find the missing worker fails, Consol will resort to building what Barletta called a coffer dam. Sheet piling that weighs about 1 ton per section would be lowered into the pond around the dozer, walling it off. Consol would then pump out silt but keep water inside the walled-off area to maintain a constant pressure on both sides. Divers would then be able to enter the water and search for the missing man. The dozer is believed to be stuck 25-35 feet in the slurry, the silt and solids that settled beneath about 10-12 feet of dirty water. Consol was working to raise the elevation of the impoundment when the accident happened, Barletta said, but the investigation into what went wrong only began Tuesday, so neither he nor regulators could comment on the possible cause.