The gasoline additive MTBE, a known carcinogen, has already leaked into 48 public wells that provide water to hundreds of thousands of Californians, according to a San Francisco Chronicle analysis of state data. The additive is leaking from 1,189 underground storage tanks within 1,000 feet of public wells or drinking water aquifers, threatening the water supplies of millions in the state. That’s not the full extent of the problem: No data exist on private wells, which may be more prone to contamination because they often draw water from shallow aquifers. MTBE is added to gasoline to make it burn more cleanly; oil companies have only a couple of years to phase it out.