LOGAN, W.Va. -- A coal treatment plant re-opened Wednesday, a day after a ruptured waste pipe spilled 100,000 gallons of coal slurry into two nearby streams, killing fish and raising concerns about the water supply.

State environmental inspectors said Bandmill Coal Co., a subsidiary of Massey Energy Co., could open as long as workers continued cleanup efforts.

By Wednesday afternoon, the spill had discolored the Guyandotte River. The slurry -- concentrated and grayish-black on Tuesday -- had broken up considerably, officials said.

"The cleanup is proceeding very well, and the operations of the plant itself were not going to impair the work," state inspection supervisor Joe Hager said.

Massey said its investigators have determined that human error caused the spill of slurry, which is a mixture of water, fine coal particles and other waste from washing coal to prepare it for market.

A mistakenly opened valve allowed slurry to enter a smaller water line, Massey said. That raised pressure inside the pipe, causing it to rupture.

Bandmill faces up to $15,000 in daily fines related to the spill and has been ordered to restore the smaller of the two creeks to its pre-spill environment. The slurry had mostly diluted by the time it reached the larger creek.

Massey is West Virginia's largest coal producer and the seventh largest in the nation. The company, which owns 18 mining complexes in central Appalachia, employs about 5,000 people in West Virginia, Kentucky and Virginia.

In July, a sediment pond overflowed at a Bandmill site near Lyburn and damaged or destroyed 10 homes. The state also ordered Massey to close a Mingo County mine after 20,000 gallons of polluted mine water was discharged into a nearby creek and lake.