Federal regulators have approved Alpha Natural Resources' plan to expand a West Virginia coal slurry impoundment that is one of the nation's biggest to a height taller than the Hoover Dam.

The plan will also increase the volume of waste it holds to 8.5 billion gallons.

The Mine Safety and Health Administration confirmed it's given Virginia-based Alpha permission to expand the Brushy Fork impoundment near Whitesville.

Alpha calls it an incremental development in what has long been part of the construction plan.

Brushy Fork now holds 6.5 billion gallons and hasn't failed, but some people worry it will. They question whether it was properly built.

Activist Joe Stanley believes waste has never properly compacted and dried, meaning Alpha could be building on an unstable base.

Alpha says past state and federal reviews found no deficiencies.