The Clinton administration will announce today that it plans to phase out the use of MTBE, acknowledging that the gasoline additive poses health risks because of its tendency to contaminate groundwater. The Clean Air Act calls for the use of oxygenates such as MTBE in gasoline to help fuel burn more cleanly and release fewer smog-forming emissions. The EPA said today that it will ask Congress to change the act to encourage the use of ethanol instead of MTBE. MTBE, which is now added to gasoline sold in 16 states, is considered a carcinogen.