The site of a West Virginia chemical spill that contaminated the water supply for 300,000 people operated largely outside government oversight, highlighting gaps in regulations and prompting questions on whether local communities have a firm grasp on potential threats to drinking water.

The storage facility owned by Freedom Industries Inc. on the banks of the Elk River was subject to almost no state and local monitoring, interviews and records show. The industrial chemical that leaked into the river, 4-methylcyclohexane methanol,...