Schools are closed and stores have been stripped of bottled water in eastern China after a pair of incidents contaminated drinking water used by hundreds of thousands of people.

In the first incident, a traffic accident late Saturday caused 21 tons of carbolic acid to spill into the Xin'an River, the main source of drinking water for cities including Hangzhou, the capital of Zhejiang province. Authorities temporarily shut down five water plants serving more than 500,000 people and released extra water from nearby dams to dilute the chemical. Carbolic acid can damage internal organs if ingested.

Separately, officials discovered benzene and nine other chemicals in the Shaozi River, which also provides drinking water for Hangzhou and other parts of Zhejiang. The pollution was traced to an industrial park, and factories in the park were ordered to stop discharging the chemical.

Authorities said the chemicals were not highly enough concentrated to endanger health, but residents complained of a bad taste, and schools in two communities were closed for three days beginning Tuesday. Ingesting benzene can cause vomiting, dizziness and convulsions.

The official Xinhua news agency quoted an official with Hangzhou's environmental protection bureau as saying the agency has begun round-the-clock surveillance of the city's water quality and will keep residents informed. He said the city hopes the pollution will have cleared by Thursday.

China has long been troubled by severe pollution problems linked to its rapid industrial expansion.