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 unburned fuel in engine exhaust, not spills.

The study at Donner Lake is part of an overall effort to learn more about the sources, fate and transport of the gasoline oxygenate additive in California's water.

Researchers, including John Reuter and Brant Allen of UC Davis' Tahoe Research Group, found that 86% 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.

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.