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during the combustion of fuel, though some PAH are also found in unburned gasoline. Further research is <br />needed to identify what impact this would have on PAH concentration in water. <br />Discharges of MTBE and PAH particularly concern scientists because of their potential to adversely <br />affect the health of people and aquatic organisms. Scientists need to conduct additional studies on PAH <br />(Allen et al. 1998) and on MTBE (N.PS 1999), as well as long -term studies on the effect of repeated <br />exposure to low levels of these pollutants (Asplund 2001). <br />At Lake Tahoe concern about the negative impact on lake water quality and aquatic life caused by the use <br />of two- stroke marine engines led to at least 10 different studies relevant to motorized watercraft in the <br />Tahoe Basin in 1997 and 1998. The results of these studies (Allen et al. 1998) confirm that (1) petroleum <br />products are in the lakes as a result of motorized watercraft operation, and (2) watercraft powered by <br />carbureted two- stroke engines discharge pollutants at an order of magnitude greater than do watercraft <br />powered by newer technology engines (TRPA 1999). <br />On June 25, 1997, the Tahoe Regional Planning Agency adopted an ordinance prohibiting the "discharge <br />of unburned fuel and oil from the operation of watercraft propelled by carbureted two -stroke engines" <br />beginning June 1, 1999. Following the release of an environmental assessment in January 1999, this <br />prohibition was made permanent. <br />PAH, as well as other hydrocarbon emissions, could potentially be reduced as new four -stroke and direct - <br />injection engines replace older carbureted two -stroke engines. The conversion of carbureted two- stroke <br />engines would be an important step toward substantially reducing petroleum related pollutants. <br />Some research shows that PAH, including those from personal watercraft emissions, adversely affect <br />water quality via harmful phototoxic effects on ecologically sensitive plankton and other small water <br />organisms (EPA 1998; Oris et al. 1998; Landrum et al. 1987; Mekenyan et al. 1994; Arfsten et al. 1996). <br />This in turn can affect aquatic life and ultimately aquatic food chains. The primary concern is in shallow <br />water ecosystems. <br />Air Pollution <br />Personal watercraft emit various compounds that pollute the air. In the two -stroke engines commonly <br />used in personal watercraft, the lubricating oil is used once and is expelled as part of the exhaust; and the <br />combustion process results in emissions of air pollutants such as volatile organic compounds (VOC), <br />NO particulate matter (PM), and carbon monoxide (CO). In areas with high PWC use, some air quality <br />degradation likely occurs (EPA 1996a). Kado et al. (2000) found that two -stroke engines had <br />considerably higher emissions of airborne particulates and PAH than four -stroke engines tested. It is <br />assumed that the 1996 EPA rule concerning marine engines will substantially reduce air emissions from <br />personal watercraft in the future (EPA 1996a). <br />PAH are released during the combustion of fuel, though some PAH are also found in unburned gasoline. <br />Kado et al. 2000 indicated that changing from two- stroke carbureted engines to two- stroke direct - <br />injection engines may result in increases of airborne particulate- associated PAH. The same study <br />indicated that four -stroke engines have considerably less PAH emissions than two -stroke engines. A <br />subsequent study of airborne emissions indicated a potential health risk from toxic pollutants in areas of <br />2. It is noted that only one engine of each type (two- stroke carbureted, two- stroke direct injection, and four - stroke) was tested. <br />L`� <br />