(NaturalNews) Two of the reservoirs supplying the city of Los Angeles with potable water were shut down when an independent laboratory test revealed them to be contaminated with high levels of the carcinogen bromate.An independent commercial customer conducted laboratory tests on the content of water from the Silver Lake and Elysian reservoirs in October, discovering bromate levels of 68 and 106 parts per billion, respectively.The maximum average bromate exposure allowed by law is 10 parts per billion over a year.According to Los Angeles Department of Water and Power (LADWP) spokesperson Joseph Ramallo, the bromate "formed unexpectedly when the water in the reservoir, combined with groundwater, was treated with chlorine and exposed to sunlight." He said that it was the first known case of such a reaction occurring.When notified of the contamination, LADWP shut down the reservoirs and alerted the Department of Public Health. The reservoirs will now be drained of their entire 600 million gallons of water. Because Southern California is currently undergoing a drought, the department says that the water will be used for irrigation, firefighting or other non-potable uses whenever possible. The contaminated water that cannot be used will be dumped into the Los Angeles River and will flow from there into the Pacific Ocean.A total of 107 tanks and reservoirs compose the water storage system for the city of Los Angeles. LADWP officials say that the city's water supplies will not be affected by the temporary unavailability of two of these. In addition to water use being reduced for the winter, LADWP says that the two reservoirs contain only one day's worth of water use for the city.Water suppliers are not required to test for bromate in drinking water, except in cases where ozone treatment is used.The Environmental Protection Agency says that it is investigation the incident to determine if similar contamination could occur in other reservoirs, posing a public safety threat.