LOS ANGELES - 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 Elysian and Silver Lake reservoirs early next year, leaving them out of use for three to four months amid drought conditions, DWP spokesman Joseph Ramallo said.

High levels of the carcinogen bromate were found in early October by a commercial customer who conducted a lab test, officials said.

The utility confirmed the finding, immediately removed the reservoirs from service and notified the state Department of Public Health.

"Every drop is precious to us, but letting it go is the more advisable option," DWP General Manager David Nahai said.

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

"It's a lot of water, but not compared to the total amount we use in this city," Nahai said.

Some water from the reservoirs may be used for irrigation or firefighting, but not for human consumption. The rest will be dumped into the Los Angeles River, which drains into the Pacific Ocean.

Bromate "formed unexpectedly when the water in the reservoir, combined with groundwater, was treated with chlorine and exposed to sunlight," Ramallo said, adding it was "the first time an occurrence like this has ever been observed."

Bromate is normally a byproduct of purifying and disinfecting water with ozone, a process not used in the two reservoirs, so testing was less frequent. That regulation will be changed, state health officials said.

The elevated levels of the chemical did not put the DWP in violation of regulations, which are based on an annual average.

DWP could have mixed the water back in with the general supply, diluting the amount of bromate to acceptable levels.

"We do have other options," Nahai said. "But in the equation between blending this water and serving it to the public, and having high public confidence in the water supply, we believe having public confidence wins."

Meanwhile, officials emphasized the chemical is only dangerous after long-term consumption.

The state health department's Web site says drinking water containing more than the allowable level of 10 parts per billion of bromate "over many years" increases the risk of cancer.

"There is no immediate health risk," Ramallo's statement said.

Four other open-air reservoirs in Los Angeles, including one adjacent to the Silverlake Reservoir, tested negative for the chemical.

The closures haven't left the city short of water because demand decreases in the fall and winter, Ramallo said.

Utility workers will attempt to drain, clean and refill the reservoirs by June, when demand increases significantly, Nahai said.