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

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Introduction

Emissions from watercraft engines represent a potential chronic or acute toxic threat to aquatic wildlife at Lake Mead National Recreation Area (fig. 1), which includes Lakes Mead and Mohave and the Colorado River between the two reservoirs. Lakes Mead and Mohave are used intensively by many types of watercraft. Watercraft engines release uncombusted gasoline and oil to the water and air. Two-stroke carbureted engines, typically used on many older boats and on larger numbers of personal watercraft, are estimated to discharge as much as 30 percent of their fuel and oil during operation (California Environmental Protection Agency, Air Resources Board, 1999).

Gasoline contains such compounds as benzene, toluene, ethylbenzene, and xylene (BTEX) and oxygenated additives such as methyl tert-butyl ether (MTBE) that are known to have adverse effects on human health and aquatic life. Watercraft engines also can emit polycyclic aromatic hydrocarbon (PAHs) compounds. These compounds are formed by combustion of oil and are composed of two or more fused benzene rings and can have any combination of alkyl or aromatic functional groups attached to the molecule. PAHs can occur as low molecular weight compounds, such as naphthalene, or as high molecular weight compounds, such as coronene.

The toxicity of PAHs generally increases as the molecular weight increases (Agency for Toxic Substances and Disease Registry, 1994; Moring, 1996). High molecular weight PAHs, however, are not acutely toxic in concentrations generally detected in the environment partly because of their low solubility in water. Many PAHs may have adverse effects on aquatic organisms because of their high lipid solubilities. High molecular weight PAHs or their metabolites can be carcinogenic. High molecular weight PAHs can be produced by the incomplete combustion of fuels, during forest fires or incineration of waste materials, and as part of industrial manufacturing (Mill and others, 1981; Kochany and Maguire, 1994).

Exposure to ultraviolet solar radiation, termed photoactivation, can cause increased toxicity, but only to certain compounds (Kochany and Maguire, 1994). Compounds with one benzene ring, such as the BTEX compounds, commonly are acutely toxic when exposed to ultraviolet radiation, whereas those compounds with many rings (PAHs) are chronically toxic when exposed to ultraviolet radiation. The intensity of ultraviolet solar radiation and the depth of penetration into the water also are factors effecting the photoactivation and PAH toxicity.

Only minor reliable scientific data are available on the effects of watercraft emissions on major use recreational desert lakes or impoundments such as Lakes Mead and Mohave. In June 1999, five water samples were collected at 1-meter depths from both lakes. Analysis of these samples indicates that BTEX compounds and MTBE were detected at all sampling sites on both lakes (Jones and others, 1999). Bevans and others (1996) detected numerous PAHs in bottom-sediment samples and in the water column at several sites in Boulder Basin of Lake Mead with semipermeable membrane devices (SPMDs). The detection of these compounds was associated with discharge of wastewater from Las Vegas Valley. Covay and Beck (2001) detected numerous PAHs and other semivolatile compounds in intervals from lake-sediment cores collected at several sites in Las Vegas Bay and Overton Arm. Studies at Lake Tahoe evaluated the occurrence and distribution of gasoline-related compounds (Lico and Pennington, 1999; Lico, 2003 and 2004) and the toxicity of PAHs to fish and zooplankton (Oris and others, 1998).

Hydrologic Setting

Lake Mead was created in 1935 by construction of Hoover Dam on the Colorado River. Lake Mead is the largest reservoir by volume in the United States (LaBounty and Burns, 2005). The major surface-water inflows to Lake Mead are the Colorado, Virgin, and Muddy Rivers, and Las Vegas Wash with the Colorado River providing about 97 percent of the annual inflow (LaBounty and Burns, 2005). Las Vegas Wash has the most potential for delivering contaminants to Lake Mead because of intensive urban development and discharge of treated sewage effluent.

Davis Dam, 67 mi downstream of Hoover Dam, was completed in 1953 (Bureau of Reclamation website: http://www.usbr.gov/lc/region/pao/brochures/davis.html, last accessed Jan. 22, 2007) forming Lake Mohave. Aside from ephemeral streams, some perennial springs, and scant rainfall, releases of water from Hoover Dam are the only source of water for Lake Mohave.

Use of Resource

Lakes Mead and Mohave are popular recreation areas used by many people from Las Vegas, southern California, Arizona, and elsewhere. Activities generally are concentrated near developed marinas and popular beaches on the lakes. Lake Mead National Recreation Area (includes both Lakes Mead and Mohave) was the fifth most visited National Park in the country during 2005. Some of the more common activities people come to Lakes Mead and Mohave for are boating, fishing, scuba diving, hiking, and relaxing.

Another major asset of Lakes Mead and Mohave is the important habitat they provide for populations of several endangered species; including the Razorback sucker (Xyrauchen texanus), Bony tailed chub (Gila elegans), Virgin River chub (Gila seminuda), and the Woundfin (Plagopterus argentissimus).

Purpose and Scope

The purpose of this report is to document concentrations of organic compounds related to the use of gasoline-powered engines on Lakes Mead and Mohave. Organic compounds are BTEX, oxygenates, degradation products of oxygenates, and PAH. In all, water, SPMD, and bottom-sediment samples were collected from 22 sites (table 1) in the study area (water, 20 sites; SPMDs, 9 sites; and bottom sediment, 10 sites). Samples were collected in 2004, 2005, and 2006, generally during the boating season (May–September) and once during early spring 2006.

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