The vertical height of the Hoover Dam is 726 feet.
Once the impoundment reaches capacity, the remaining water will be drained, and it will be filled with coarse material, then graded and capped with soil and vegetation.
The West Virginia Department of Environmental Protection has previously ruled there was no reason to believe the impoundment was improperly built or that its contents could liquefy, triggering a failure. But the state also said it would require continued testing.
DEP spokeswoman Kathy Cosco said the agency is still reviewing Alpha's plan for the final stages, but the state needs MSHA approval before it can act. DEP has returned Marfork's application with some comments, so the timeline for state approval is uncertain.
The federal Office of Surface Mining Reclamation and Enforcement upheld the state's decision about the stability of Brushy Fork. But field director Roger Calhoun also said his agency had hired consultants to study it and six other impoundments, and the results could cause him to reopen the matter.
Rob Goodwin, who monitors slurry impoundments for Coal River Mountain Watch, sought the results of those studies under a Freedom of Information Act request.
But on March 15, the same day MSHA sent its letter to Alpha, the Office of Surface Mining denied Goodwin the documents. It cited two legal exemptions, calling the reports "interagency or intra-agency memoranda." It also cited an exemption for records "compiled for law enforcement purposes."
Companies have been doing their own impoundment tests for years, Goodwin said, "and all the regulators have just been taking their word for it."
The studies would be the first independent assessment.
Goodwin said citizens worry about Brushy Fork because the engineer long responsible for the impoundment was also involved in illegal ventilation plans at Massey's Upper Big Branch mine, where an April 2010 explosion killed 29 men.
MSHA later discovered Massey maintained two sets of safety records, one sanitized to throw off federal inspectors.
The engineering firm that Massey used on Brushy Fork, Goodwin said, was also linked to the 2000 failure of a Massey impoundment in Martin County, Ky.
Slurry burst through the bottom of a 68-acre holding pond, sending black goo through an underground mine and into 100 miles of waterways. The spill polluted the water supply of more than a dozen communities and killed aquatic life before reaching the Ohio River.
West Virginia has more slurry impoundments than any other state with 114, but it hasn't had a major failure since 1972, when the earthen dam at Buffalo Creek collapsed after heavy rain.The flood killed 125 people, injured 1,100 and left 4,000 homeless.