The action authorizes the Federal Emergency Management Agency to coordinate disaster relief efforts and provide help as appropriate.
The chemical spill Thursday into the Elk River affects residents in the counties of Boone, Clay, Jackson, Lincoln, Logan, Putnam, Roane and Kanawha, home of capital city of Charleston.
Gov. Earl Ray Tomblin declared a state of emergency Thursday evening for nine countries – the eight under the federal declaration, plus Cabell County – affecting West Virginia American Water Co. customers.
As a precaution, residents in the emergency area were urged not to use tap water for drinking, cooking or washing.
"Right now, our priorities are our hospitals, nursing homes and schools," Tomlin told CNN Thursday. "I've been working with our National Guard and Office of Emergency Services in an effort to provide water and supplies through the county emergency services offices as quickly as possible."
In a statement issued Friday from Charleston, W.Va., U.S. Attorney Booth Goodwin said the spill "has put hundreds of thousands of West Virginians at risk, severely disrupted our region's economy and upended people's daily lives."
Goodwin added his office, and other federal authorities, have opened an investigation.
West Virginia American Water said on its Facebook page the chemical spill occurred along the Elk River, causing contamination within the Kanawha Valley water system.
The chemical, 4-Methylcyclohexane Methanol, used to wash coal before market, is not toxic but is harmful if swallowed, Thomas Aluise, a spokesman for the state's Department of Environmental Protection, told CNN.
The leak came from a 48,000-gallon tank at Freedom Industries, a chemical storage facility about upriver from the West Virginia American Water Co. facility, officials said. The amount of the chemical leak isn't known.
Officials told CNN they weren't sure how long flushing the system would take.
A Freedom Industries toxicologist alerted the utility that "some health risk" is associated with the chemical, Laura Jordan of West Virginia American Water said.
"The safety sheet indicated there could be some skin or eye irritation if you come in contact, or possibly harmful if swallowed, but that's at full strength of the chemical," Jordan said. "The chemical was diluted in the river."