CHARLESTON, W.Va.—The state will require stricter monitoring of storage tanks and water systems under a bill Gov. Earl Ray Tomblin is expected to sign in response to a chemical spill that tainted the water supply for 300,000 people here.

The legislation passed unanimously in its final vote on Saturday, two months after the Jan. 9 spill. About 10,000 gallons of a coal-processing substance called Crude MCHM leaked from a tank into the Elk River, contaminating a water-treatment plant about a mile downstream.

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