A ban on highly polluting two-stroke boat engines dramatically reduced MTBE and other fuel pollution in Lake Tahoe this past summer compared with previous years, two UC Davis scientists reported Monday. The elimination of such engines -- imposed by the Tahoe Regional Planning Agency at the beginning of the 1999 boating season -- reduced both MTBE and toluene concentrations in the Sierra Nevada lake by 90 percent or more, say Brant Allen and John Reuter, both researchers with UC Davis' Tahoe Research Group. The scientists sought to determine which action had the greater impact on MTBE pollution: service stations in the Tahoe basin selling MTBE-free gasoline or the TRPA ban on certain types of engines. "The engines had more impact. We saw a reduction in certain constituent components of engine emissions, including BTEX (benzene, toluene, ethylbenzene and xylene)," Reuter says. The study's findings are significant, Reuter says, because they show "that MTBE can be controlled through the management of boating practices." Reuter added that four-stroke or two-stroke fuel-injected engines work better at controlling MTBE and BTEX emissions than do older, two-cycle models. Previous Tahoe research showed that carbureted two-stroke engines contributed 90 percent of the MTBE found in the water, while using only 11 to 12 percent of the total fuel used for all Lake Tahoe boating. MTBE, or methyl tertiary-butyl ether, is used to oxygenate gasoline, thereby reducing polluting auto emissions. However, the chemical has been found to pollute water supplies, and gradually is being phased out in gasoline sold in California. The UC Davis Tahoe Research Group, along with researchers from the University of Nevada, Reno, and the U.S. Geological Survey, has been monitoring Lake Tahoe since 1997 for evidence of MTBE emitted from boat engines. UC Davis' research has been funded by the state of California. Reuter said Monday that UNR's monitoring of MTBE in Lake Tahoe has found similar results.