Using a Sierra Nevada lake as a laboratory, UC Davis scientists have found that summertime recreational boating is the primary source of MTBE in the lake's water, and that the contamination most likely stems from engine exhaust, not spills. The scientists' work at Donner Lake is part of an overall UC Davis research effort to learn more about the sources, fate and transport of the gasoline oxygenate additive in California's water. MTBE (methyl tert-butyl ether) is classified by the U.S. Environmental Protection Agency as a possible human carcinogen. The Donner Lake findings are among the first to show conclusively the impact of motorized boating on MTBE surface-water concentrations. The results have been reviewed by the U.S. Geological Survey and are being used in scientific discussions of MTBE nationwide. Researchers, including John Reuter and Brant Allen of UC Davis' Tahoe Research Group, found that 86 percent of the variation in the seasonal trend of total lake MTBE levels was explained by recreational boating. Neither urban runoff nor precipitation contributed significantly, the researchers concluded, based on low concentrations of MTBE in the lake during spring months. "Direct exhaust of unburned fuel into surface waters during the operation of marine gasoline combustion engines appears to be a more likely source of increased MTBE. The greatest releases probably occur from two-cycle engines, which have exhaust ports at or below the water surface," the researchers report. MTBE concentrations began to increase slightly in Donner Lake by mid-May of 1997, and rose dramatically during early July. Following the Labor day weekend, when boat use declines dramatically, MTBE levels dropped almost as dramatically as they had risen earlier in the summer. "The relationship between calculated values for whole-lake MTBE and boat use data obtained from day-use boating permits records taken at the sole public boat ramp on the lake shows the direct and strong relationship between MTBE and watercraft use during the study period," Reuter says. MTBE levels retreat from the water through volatilization into the air. The research also showed that MTBE did not mix to the deeper waters of the lake during the summertime when a difference in water temperature between top and bottom layers prevents complete mixing.