The declaration allows for direct federal assistance in dealing with the spill, says Bill Hines, of the Federal Emergency Management Agency.
West Virginia Governor Earl Ray Tomblin had previously declared a state of emergency after the spill from Freedom Industries hit the Elk River in Charleston and a nearby water treatment plant.
The chemical, a foaming agent used in the coal preparation process, leaked from a tank at Freedom Industries, overran a containment area and went into the river.
West Virginia American Water expanded its advisory to nine counties last night, up from an original five. Customers have been told to only use the water for flushing toilets.