The chemical, a foaming agent used in the coal preparation process, leaked yesterday from a tank at Freedom Industries, overran a containment area and went into the Elk River.
The spill shut down much of the rural state's capital city and surrounding counties, even as the cause and extent of the incident remained unclear.
Schools and restaurants closed, and grocery stores sold out of bottled water.
Officials said they were not sure what hazard the spill posed to residents. It was not immediately clear how much of the chemical spilled into the river and at what concentration.
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Kanawha County emergency officials said the chemical is called 4-methylcyclohexane methanol. McIntyre said the chemical isn't lethal in its strongest form.
Officials from Freedom, a manufacturer of chemicals for the mining, steel, and cement industries, haven't commented since the spill, but a woman who answered the phone at the company said it would issue a statement today.
Bill Hines with the Federal Emergency Management Agency said the emergency declaration allows for direct federal assistance in dealing with the spill.
The West Virginia National Guard planned to distribute bottled drinking water to emergency services agencies in the nine affected counties. About 100,000 water customers, or 300,000 people total, were affected, state officials said.