Polycyclic aromatic hydrocarbons (PAH) are toxic products of incomplete combustion and are released into the Lake Tahoe Basin from marine engines, as well as from urban and highway runoff. Most marinas receive majority of PAH loading from direct release from marine engine exhaust. Since marinas are generally less subject to lake currents and wind-driven turbulence, fine sediment particles tend to settle, and show relatively high accumulations of PAH. Although extensive efforts have been made to reduce urban runoff into the Lake, primarily to limit phosphorus loading, PAH loading from vehicles also constitutes a large source of PAH into the Lake. Fluoranthene and pyrene are generally observed in highest concentrations in both marina and road runoff sediment samples, and the PAH profiles were similar, regardless of whether the PAH came from runoff or from marine engines. Samples collected from southeast Lake Tahoe have shown that PAH from road runoff is lower (<2000 ng/g dry weight basis) compared to PAH found in marina sediments (nearly 6000 ng/g dry weight basis). Also, marinas had generally much higher loadings than the coarse grain sediments near the lake-shore area. Sediment samples collected at several catchment basins at different depths showed that concentration decreased with depth and indicated that these catchment basins are effective in trapping organic contaminants from surface runoff. Based on sorption characteristics and water solubility, the catchment basins contain enhanced concentrations of the higher molecular weight PAH compounds (fluoranthene, pyrene, benzo(a)pyrene, etc.,) compared to the relative abundance of lower molecular weight PAH compounds in watercraft emissions (acenaphthylene and acenaphthene)