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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5144

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Conclusions

Several lines of evidence indicate that motorized watercraft are the principal source of BTEX and PAH in Lakes Mead and Mohave. All of the highest BTEX concentrations were detected at locations where there are launching facilities and marinas—areas of heaviest boat traffic. In areas where few boats travel, such as Tequila Cove, Nelson’s Landing, Bonelli Bay, and the Virgin Basin, the BTEX concentrations were much less. Concentrations are low or less than detection levels during the non-boating season as seen in the samples collected during March 2006. As the boating season progresses during the summer months, concentrations of BTEX increase and reach a maximum value in mid-summer then decrease as the boating season draws to an end. BTEX concentrations, in general, were much higher in Lake Mohave than in Lake Mead, most likely due to a larger number of boats within a smaller area.

Inputs from tributary streams to Lakes Mead and Mohave do not appear to contribute large amounts of BTEX to the lakes. BTEX concentrations were very low in Las Vegas Wash (contains urban runoff and seepage from the Las Vegas area and treated sewage effluent), the Colorado River at Willow Beach (reflects input from the Colorado River to Lake Mohave), near Overton (reflects inputs from the Muddy and Virgin Rivers), and Middle Point (near the confluence of the Virgin and Colorado River and indicative of concentrations in those two sources). The concentrations of BTEX near marinas that have fueling facilities indicates no grossly contaminated areas around fueling docks from leaking tanks or pipes. Finally, MTBE concentrations fall to below detection levels after it was taken out of gasoline in California (December 31, 2004) while BTEX concentrations changed little. Volatilization most likely is the primary removal mechanism for BTEX compounds from the lakes.

The distribution of PAHs in Lakes Mead and Mohave generally parallel that for BTEX compounds. PAHs were detected in all SPMDs deployed on Lakes Mead and Mohave and in Las Vegas Wash. The more popular boating areas had the highest concentrations and number of PAH compounds, whereas, the less trafficked parts of the lakes had low concentrations. The highest concentrations were at the south end of Lake Mohave where an abundance of personal watercraft ply the waters. The major source of PAH in the lakes is combustion of gasoline and oil in boat engines. The most commonly detected PAH compounds were fluoranthene, pyrene, and naphthalenes, which were detected in almost all SPMD extracts. Water from the Las Vegas metropolitan area sampled with SPMDs downstream of the release of treated sewage effluent in Las Vegas Wash had low concentrations of PAH and thus is not a major source of these compounds to Lake Mead.

Sediment samples from Lakes Mead and Mohave and Las Vegas Wash did not show the same distribution pattern of PAH concentrations as BTEX and PAH concentrations in water. Relatively high concentrations of PAH were detected in sediment from Las Vegas Wash upstream of the release of treated sewage effluent suggesting PAHs are generated in the Las Vegas metropolitan area, but are sorbed onto the sediment and not released into Lake Mead.

Bioavailable PAH was present at all five sites using the Fluoroscan method. PAH concentrations are relatively non-toxic as determined by EC50 and Toxicity Index calculations with samples from Katherine Landing and North Telephone Cove in Lake Mohave and Callville Bay in Lake Mead having slightly more toxicity than Hemenway Harbor and Las Vegas Bay in Lake Mead.

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