“Our goal is to find out what happened to allow a leak of such magnitude to occur and to ensure that the proper safeguards are in place to prevent a similar incident from occurring,” said Rafael Moure-Eraso, the chairman of the safety board.

At a news conference here on Saturday evening, officials said tests had begun to show concentrations of the chemical dropping below the one part per million threshold considered safe by the Centers for Disease Control and Prevention. The concentration must remain that low for 24 hours before the water system can be flushed out and the do-not-use ban can be lifted. Officials said they planned to conduct at least 100 additional tests of samples overnight and on Sunday.

The contamination level was dropping because the leak had been shut off, said Mike Dorsey, the chief of homeland security and emergency response at the State Department of Environmental Protection. Some tests showed concentrations above one part per million, which officials attributed to sludge in the river and rain breaking up contaminated ice along it.

According to Department of Environmental Protection officials, Freedom Industries, which owns the chemical tank that ruptured, is exempt from Department of Environmental Protection inspections and permitting since it stores chemicals and does not produce them, The Associated Press reported. Gov. Earl Ray Tomblin said he would look into tighter regulation of chemical storage facilities. “There are certain reporting things that companies have to do,” he said. “And I do think we have to look at them to make sure this kind of incident does not happen again.”

Mr. Tomblin also said an interagency panel was developing literature to help residents clean their plumbing systems once the ban is lifted. Customers will receive a 1,000-gallon credit on their water bills, said Mr. McIntyre, the water company president.