LOS ANGELES (AP)  Two reservoirs that supply drinking water to parts of the city have been shut down and will be drained after a rare sunlight and chlorine reaction tainted the water with a cancer-causing chemical, utility officials said Friday.

The Los Angeles Department of Water and Power plans to drain 600 million gallons from the reservoirs, the Elysian and the Silver Lake, early next year, said a water department spokesman, Joseph Ramallo. The reservoirs will be out of use for three to four months amid drought conditions.

High levels of the carcinogen bromate were found in early October by a commercial customer who ran a laboratory test, officials said. The utility confirmed the finding, immediately removed the reservoirs from service and notified the Department of Public Health.

Officials emphasized that the chemical is dangerous only after long-term consumption.

The two reservoirs supply the water for about one-third of 1 percent of the city’s annual consumption, or about the amount the entire city consumes in a day, Mr. Ramallo said.