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Semivolatile Organic Compounds in Snowmobile Emissions and in the Snowpack and Surface Water in Blackwood Canyon, Lake Tahoe, CA
AuthorMcDaniel, Mark Richard
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Snowpack in alpine environments contains appreciable levels of regional pollution acquired through natural processes of wet and dry deposition, as well as from immediate sources such as snowmobile engine exhaust. Understanding the fate and transport of those pollutants in alpine environments is a crucial step towards managing emission sources in alpine terrain to mitigate their impacts on natural resources. This study presents results from detailed chemical analysis for 168 species of semivolatile organic compounds (SVOC) in exhaust from snowmobile engines and in 58 samples of snow and surface water collected in Blackwood Canyon, a tributary to the west shore of Lake Tahoe and a popular winter recreation area for snowmobile riders. Snow and water samples were collected during a six week period in the spring of 2006. SVOC analyses included highly toxic and persistent polycyclic aromatic hydrocarbons (PAH), fossil fuel biomarker compounds hopanes and steranes, and saturated alkanes commonly found in fossil fuels, all of which are emitted as products of incomplete combustion and useful for identifying fossil fuel combustion emissions in the environment. Average SVOC concentrations in background snow were as follows: total PAH was 200+/-12 ng.l-1, with 16+/-1.3 ng.l-1 PAH with three or more aromatic rings; hopanes and steranes were 56+/- 3 ng.l-1; and total alkanes were 9,900+/- 500 ng.l-1. Average background SVOC concentrations were used to estimate background SVOC surface loading (µg.m-2). Loadings in excess of the estimated background are mapped and compared with observations of snowmobile activity to determine effects of snowmobile emissions on snow water quality. Loading of PAH in snow was found to be significantly greater than background loading (α=5%) where snowmobile tracks cover more than 50% of the snow surface. Loadings were eight to 20 times greater than background levels where snowmobile traffic was heaviest over snow covered roads.SVOC were also measured in snow melt and surface water samples in Blackwood Canyon for comparison with chemical profiles in snow samples. Flux of SVOC (g.day-1) from Blackwood Creek into Lake Tahoe was calculated and compared to estimates of background flux. Initially comparable to background estimates, PAH flux was found to have increased from background levels at 74 g.day-1 to 160 g.day-1 in four days. In another sample collected eight days after the initial sample, PAH flux had increased to 480 g.day-1. Cumulative impacts to Blackwood Creek and downstream to Lake Tahoe may be inferred from the discharge hydrograph, but should be further characterized with routine sampling through the entire snowmelt period.