Gasoline-Related Compounds in Lakes Mead and Mohave, Nevada, 2004-06

Abstract

The distribution of man-made organic compounds, specifically gasoline-derived compounds, was investigated from 2004 to 2006 in Lakes Mead and Mohave and one of its tributary streams, Las Vegas Wash. Compounds contained in raw gasoline (benzene, toluene, ethylbenzene, xylenes; also known as BTEX compounds) and those produced during combustion of gasoline (polycyclic aromatic hydrocarbon compounds; also known as PAH compounds) were detected at every site sampled in Lakes Mead and Mohave. Water-quality analyses of samples collected during 2004-06 indicate that motorized watercraft are the major source of these organic compounds to the lakes. Concentrations of BTEX increase as the boating season progresses and decrease to less than detectable levels during the winter when few boats are on the water. Volatilization and microbial degradation most likely are the primary removal mechanisms for BTEX compounds in the lakes. Concentrations of BTEX compounds were highest at sampling points near marinas or popular launching areas. Methyl tert-butyl ether (MTBE) was detected during 2004 but concentrations decreased to less than the detection level during the latter part of the study; most likely due to the removal of MTBE from gasoline purchased in California. Distribution of PAH compounds was similar to that of BTEX compounds, in that, concentrations were highest at popular boating areas and lowest in areas where fewer boats traveled. PAH concentrations were highest at Katherine Landing and North Telephone Cove in Lake Mohave where many personal watercraft with carbureted two-stroke engines ply the waters. Lake-bottom sediment is not a sink for PAH as indicated by the low concentrations detected in sediment samples from both lakes. PAH compounds most likely are removed from the lakes by photochemical degradation. PAH compounds in Las Vegas Wash, which drains the greater Las Vegas metropolitan area, were present in relatively high concentrations in sediment from the upstream reaches. Concentrations of PAH compounds were low in water and sediment samples collected farther downstream, thus the bottom sediment in the upstream part of the wash may be an effective trap for these compounds. Bioavailable PAH compounds were present in all samples as determined using the Fluoroscan method. Microtox acute toxicity profiles indicated that Callville Bay in Lake Mead and the two Lake Mohave sites had only minor evidence that toxic compounds are present.

Sources and Distribution of Organic Compounds Using Passive Samplers in Lake Mead National Recreation Area, Nevada and Arizona, and Their Implications for Potential Effects on Aquatic Biota

“…Th e PAHs likely come from in-lake sources, such as boats, where higher concentrations should be concentrated at the surface of the lake. Lico and Johnson (2007) have shown that boat traffi c is the major source of PAHs in Lake Mead, and our study shows that the highest concentrations in LVB occur near the surface of the lake, indicating boats as the likely source. Concentrations of PAHs are high at the surface of the lake even though volatilization and photolytic degradation of PAHs lowers the concentration of many PAHs near the surface of the lake (Lico and Johnson, 2007).…”

“…Th e reason for this is not clear but may be due to discharge of unburned petroleum products from boat exhaust from boats with two-stroke engines near the site. It is unlikely that the high values of petrogenic PAHs were caused by the boat used to deploy and retrieve the samples because the same boat was used at each site and it has a four-stroke engine, which would be less likely to discharge unburned fuel into the lake than a two-stroke engine (Lico and Johnson, 2007). Th e LVB site at 8 m depth showed the same SOCs (if detected) as the Outlet site (Tables 3 and 4) but at lower concentrations.…”

Lake water quality: Chapter 4 in <i>A synthesis of aquatic science for management of Lakes Mead and Mohave</i>

“…From 2004 through 2006, SMPDs were used to collect samples to investigate the distribution of gasoline-derived VOCs in Lakes Mead and Mohave (Lico and Johnson, 2007). Most of these compounds are toxic to varying degrees.…”

“…Studies by Lico and Johnson (2007) at Lakes Mead and Mohave, and by Lico (2004) at Lake Tahoe (Calif. and Nev.), showed that two-stroke gasoline engines can release up to 40 percent of their fuel into water bodies and are a major source of gasoline-derived organic compounds. In response to these and other studies, the National Park Service is phasing out the use of two-stroke engines that do not use direct injection within LMNRA by 2013 (http://www.nps.gov/lake/ parkmgmt/twostroke.htm).…”

“…Past studies suggested that inorganic and organic chemicals are major environmental stressors at LMRNA (for example, Bevans and others, 1996;Hamilton and others, 2002;Goodbred and others, 2007). Sources of these chemicals include inputs from the urbanized tributary Las Vegas Wash, transport of naturally occurring selenium from rocks and soil in Las Vegas Valley to Lake Mead (Hamilton and others, 2002;Cizdziel and Zhou, 2005), or organic compounds from fuels and oils used in recreational watercraft on Lakes Mead and Mohave (Lico and Johnson, 2007). Other past or potential stressors on the aquatic ecosystem include algal blooms, invasive species [such as quagga mussels (Dreissena rostriformis bugensis), New Zealand mudsnails (Potamopyrgus antipodarum), and Asian clams (Corbicula fluminea)], pathogens, viruses, and parasites, effects of population growth, and changes in climate.…”