Discovery of the fuel additive methyl tert-butyl ether (MTBE) in drinking water supplies is of concern to public health officials, water suppliers, and the public. Despite recent policy decisions, few published studies exist on the concentrations, sources, and fate of MTBE in surface waters. The purpose of this study was to determine (1) the relative contribution of motorized watercraft as a source of MTBE, (2) its seasonal distribution, (3) loss from the water column, (4) the extent of verticaltransport, and (5) its persistence between years; this work was done in Donner Lake, California, a multiple-use lake in the Sierra Nevada Mountains. MTBE measurements were made at 9 individual depths from surface to bottom on 16 dates. Recreationalboating was the most important source of MTBE. Statistically, 86 of the change in MTBE was explained by variation in motorized watercraft use. Neither highway runoff nor precipitation contributed significantly. MTBE concentration ranged from<0.1μgL{sub}-1 to a high of 12μgL{sub}-1. Between July 1 and 7, 1997, MTBE content rose dramatically from 115 to 365 kg. By January, levels had declined to a minimum of 15kg, suggesting little interannual persistence. The major loss of MTBE appeared tobe volatilization at the air-water interface characterized by two distinct periods. During the boating season, MTBE decline was 1.2 kg day{sup}-1 (193 day half-life). At the end of the boating season, MTBE loss increased to 8.1 kg day{sup}-1 (14 dayhalf-life). Thermal stratification acted to retard MTBE transport to deeper depths.
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