CHARLESTON, W.Va.—Officials in Kentucky and Ohio were preparing Monday for the arrival of a slow-moving, 60-mile-long chemical plume drifting toward them down the Ohio River, after its accidental release prompted a five-day water ban here.

While many said they didn't foresee a major problem, they also weren't taking chances after the release of the chemical, called 4-methylcyclohexane methanol, or MCHM. Cincinnati Mayor John Cranley said his city is going to shut down its water valves for 48 hours beginning Tuesday night, but has enough stored supply to continue providing water.

Water sanitation officials had found the leading edge of the plume near Portsmouth, Ohio, by 11:30 a.m. Monday.

"They could smell it in the air," said Jerry Schulte, manager of source water protection and emergency response for the Ohio River Valley Water Sanitation Commission. MCHM has a distinct smell, like black licorice, he said.

The water ban in West Virginia, which affected 300,000 people, began being partially lifted Monday as tests showed the chemical that infiltrated the supply had largely faded. Authorities said more tests overnight Sunday and early Monday showed the water supply had widely tested at below one part per million of the coal-processing chemical that spilled into the Elk River. The Elk ultimately feeds into the Ohio River.