Scientific Investigations Report 2007–5144
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5144
The results of this study indicate that the use of motorized watercraft on Lakes Mead and Mohave has added organic compounds to the waters of those lakes. During operation and fueling of watercraft, organic compounds, such as BTEX, oxygenates, and PAHs, can be introduced to the water through direct spills into the water and unburned fuel and PAHs exhausted through the engine. The engine type plays an important role in the amount of raw fuel and PAHs that are emitted through the exhaust. Marine engines do not have the emission controls found on most automobiles and thus emit large amounts of hydrocarbons compared to an automobile. Certain marine engines (carbureted two-stroke) are especially dirty and can release as much as 30 percent of their fuel unburned out the exhaust (California Environmental Protection Agency, Air Resources Board, 1999). The manufacturers of marine engines have advanced the technology of how fuel is injected into the two-stroke engines to reduce emissions. These cleaner-burning engines are gradually replacing the older carbureted two-stroke engines and the hydrocarbon load to waters should be decreased concurrently.
Virtually all boats operated on Lakes Mead and Mohave use gasoline increasing the potential for the volatile components of gasoline (BTEX and other additives, such as oxygenates) to enter the water. BTEX compounds were detected at all sample locations in the lakes (fig. 3). Oxygenates (MTBE, ETBE, TAME, and DIPE) and their degradation products (methyl acetate, tert-amyl alcohol, and tert-butyl alcohol) were either not detected or detected at low concentrations throughout the study area (table 11). Acetone, a degradation product of oxygenates, commonly was detected at all sites.
Median BTEX concentrations in the lakes range from 0.17 µg/L at Bonelli Bay in Lake Mead to 7.52 µg/L at Katherine Landing in Lake Mohave (fig. 3). Of the BTEX compounds, toluene generally was the highest concentration at most sites with the xylene isomers being the second highest (table 11). The highest single concentration of BTEX (23.5 µg/L) sampled was at Katherine Landing on June 24, 2004. Another extremely high BTEX concentration of 22.4 µg/L was documented on June 24, 2004 at North Telephone Cove. Both of these sites are at the southern end of Lake Mohave and are very busy during the boating season. Examination of figure 3 reveals that all of the highest BTEX concentrations were detected at locations where there are launching facilities and marinas. These sites include Katherine Landing and Cottonwood Cove in Lake Mohave and Hemenway Harbor, Temple Bar, Callville Bay, and Echo Bay in Lake Mead. Interestingly, a marina is located on the Colorado River at Willow Beach and the BTEX concentrations were low (median = 0.16 µg/L; fig. 3), probably because the flow in the river moves most gasoline compounds downstream. The other anomaly seen in figure 3 is the high median BTEX concentration measured at North Telephone Cove (second highest median concentration of all sites; 5.6 µg/L), which is not a developed marina, but is a major launching area for personal watercraft.
Several mechanisms exist that can release gasoline components into the lakes at or near launching facilities. All marinas have fueling facilities where the potential for gasoline spillage into the lakes can occur. Fuel can be introduced to the lakes by overfilling boat fuel tanks by careless pump operators, leaking hoses, nozzles, or storage tanks, and pumpage from bilges. At North Telephone Cove, personal watercraft users sometimes refuel their machines right at the shoreline, which could lead to fuel spillage into the lake. Gross contamination that would be associated with a leaking fuel tank was not observed during this study.
Distributions of concentrations of volatile gasoline-related compounds were different in the sites sampled during this study. BTEX concentrations, in general, were much higher in Lake Mohave than in Lake Mead (fig. 4). Median concentrations of BTEX compounds in Lakes Mohave and Mead were 2.2 and 0.7 µg/L, respectively. Concentrations were highest in samples from Lake Mohave and the maximum value was 23.5 µg/L. BTEX concentrations were relatively low in samples collected from the Colorado River at Willow Beach with a median value of 0.2 µg/L (fig. 4). Median BTEX concentration in water samples collected from Las Vegas Wash was 0.05 µg/L.
The concentrations of BTEX compounds detected in water samples collected from Lakes Mead and Mohave show a common variation with time of year. Concentrations are low or less than the detection limit during the non-boating season as seen in the samples collected during March 2006 (table 11). 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. This typical temporal trend in BTEX concentrations can be seen clearly in figure 5 for data collected from North Telephone Cove in Lake Mohave.
Oxygenates, like MTBE, and their degradation products, with the exception of acetone, have not been detected in the lakes since summer 2004 (table 11). Prior to 2005, MTBE was the only oxygenate detected in Lakes Mead and Mohave. A likely explanation for the disappearance of MTBE is a change in the formulation of gasoline in California, which removed MTBE as an oxygen source at the end of 2004. Acetone has been routinely detected at all sampling sites, except for the Colorado River at Willow Beach. Acetone has been documented as a product of MTBE degradation in the environment (National Science and Technology Council, 1997). Apparently, acetone does not persist through the winter non-boating season as evidenced by the lack of detectable concentrations during the March 2006 sampling (table 11). The source of acetone is in Lakes Mead and Mohave is unknown. After 2004, MTBE was not present in gasoline sources and thus could no longer produce acetone by degradation processes. One possible source of acetone is Las Vegas Wash (median concentration of 2 µg/L) but that does not explain the detection of acetone farther upstream in Lake Mead at sites like Bonelli Bay (site 12, fig. 1).
Several lines of evidence indicate that boats are the major source of BTEX compounds detected in Lakes Mead and Mohave. Concentrations of BTEX are greater in areas where the number of boats is greater, such as Katherine Landing, North Telephone Cove, Callville Bay, and Echo Bay. Three of these sites have marinas with boat docking, launching, and fueling facilities (Katherine Landing, Callville Bay, and Echo Bay). The other site, North Telephone Cove, is a popular drive-to launching area for personal watercraft. 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 than in the popular areas mentioned above. Concentrations of BTEX compounds were very low after the boats are put away for the winter as seen in the results of the March 2006 sampling (table 11). Volatilization and microbial degradation most likely are the primary removal mechanisms for BTEX compounds from the lakes (Rathbun, 1998). 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 (milligram per liter concentrations) areas around fueling docks from leaking tanks or pipes. Finally, MTBE concentrations decrease to less than detection levels when MTBE was removed from gasoline although BTEX concentrations did not change much.
Several potential sources of PAH exist within the study area and include motorized watercraft, direct spills of gasoline into the lakes by leaking tanks and lines or spillage into the lakes during fueling, urban runoff from the greater Las Vegas metropolitan area and other smaller towns near the lakes, atmospheric deposition or runoff from combustion of natural vegetation in wildfires or gasoline and oil in automobiles, and industrial sources (Bertilsson and Widenfalk, 2002). PAH compounds were determined in the aqueous phase by deploying SPMDs in the lakes and in Las Vegas Wash. The PAH concentrations presented in this report are expressed as micrograms per kilogram of compound in the lipid within the SPMDs.
PAHs were detected in all SPMDs deployed on Lakes Mead and Mohave and in Las Vegas Wash (table 8). The amounts of PAHs, both concentration and number of compounds, were not the same at all sampling sites (fig. 6 and table 8). PAH concentrations in samples from two sites in Lake Mohave were much higher than sites sampled in Lake Mead and Las Vegas Wash. Concentrations of PAH were highest (about 6,400 µg/kg) in a sample from Katherine Landing in Lake Mohave, where more than 20 compounds were greater than detectable levels (table 8). PAH concentrations were lower in a duplicate sample from the same site (about 40 percent), but had the same compounds in the same relative concentrations. The highest concentration detected in Lake Mead was at Callville Bay where 930 µg/kg of PAH (15 compounds) was present in the SPMD extracts. The amounts of PAH detected in Las Vegas Wash and Las Vegas Bay were quite low (average concentrations about 100 µg/kg) compared to most other sites sampled in this study.
The most commonly detected PAH compounds were fluoranthene and pyrene, which were detected in all SPMD extracts and are indicative of emissions from internal combustion engines. Naphthalenes also were commonly detected and were quantified in all SPMD extracts from the lakes, except for the 2004 sample collected at Las Vegas Bay. Naphthalenes were not detected in a sample collected from Las Vegas Wash. Most of the PAH compounds generated in Lakes Mead and Mohave probably have a very short existence in the water column. A study of the photochemical degradation of PAH in freshwaters indicated that the half lives of anthracene, phenanthrene, and naphthalene were 1, 20.4, and greater than 100 hours, respectively, in surface waters (Bertilsson and Widenfalk, 2002). During the boating season, a daily load of PAHs enters the lakes from boating activity but these compounds generally do not reach large concentrations due to their short half lives. PAH compounds are not expected to persist throughout the winter (non-boating season) because the major source of these compounds is not present.
PAH concentrations in samples collected during 2004 from Lake Mohave were much higher than those collected during 2006 (fig. 6). The two sites on Lake Mohave where this occurred are very popular with users of personal watercraft. The reason for the decrease in PAH concentrations between 2004 and 2006 may be due to cleaner-technology (four-stroke) engines being used as carbureted two-stroke engines are retired from service.
Las Vegas Wash drains the urban area of Las Vegas and also contains treated effluent from sewage-treatment plants in the Las Vegas Valley. PAH concentrations were low in SPMD extracts at the sampling site downstream of Lake Las Vegas (fig. 6). Water flowing from the treatment plants serving the Las Vegas area apparently is not the principle source of PAH.
PAH concentrations were low in most lake- and stream-bottom sediment samples and are not a likely sink or source of PAH to the water (fig. 7). Concentrations of PAH compounds were highest in Las Vegas Wash upstream of the input of treated sewage effluent (average of about 750 µg/kg) of all sites that were sampled (table 8). The source of this water is urban runoff from the greater Las Vegas metropolitan area. The two samples collected from the Las Vegas Wash site had an abundance of the larger PAH compounds (benzo(a)pyrene, phenanthrene, pyrene, and fluoranthene), which are indicative of combustion sources, and far smaller amounts of the two-ringed PAHs (naphthalenes). Many potential sources exist for the compounds detected in the sediment samples and include runoff from asphalt areas, automobile exhaust, and other industrial sources. The smaller flow volumes upstream of the discharge of treated sewage effluent into Las Vegas Wash may allow more contact of the water containing PAHs with the sediment allowing the compounds to be sorbed more readily than occurs with higher flows downstream in the wash. The Colorado River at Willow Beach (site 21, fig. 1) had the second highest concentration of PAH compounds (about 290 µg/kg), also with a dominance of the larger compounds.
Among the lake sites where bottom-sediment samples were collected, PAH compounds were highest at two sites in Lake Mohave (Katherine Landing and Tequila Cove) (fig. 7), although not as high as the stream sites. In the lake sediment, the dominant compounds were naphthalenes and a few detectable concentrations of the larger PAH compounds (pyrene, fluoranthene, and phenanthrene). One possible explanation for the occurrence of naphthalenes in the sediment is their longer half life in water as compared to the larger PAH compounds, allowing more time for the compounds to be sorbed onto sediment before degradation. The naphthalene compounds are most likely from unburned fuel, either exhausted into the lakes by carbureted two-stroke engines or spilled directly into the lakes.
The three sites sampled at Lake Mead (Callville Bay, Hemenway Harbor, and Las Vegas Bay) and two sites at Lake Mohave (Katherine Landing and North Telephone Cove) all showed evidence of bioavailable PAH compounds in the water column (fig. 8 and table 9). As determined by the Fluoroscan method, the two Mohave sites contained about 3,000 µg/SPMD of PAH although the Lake Mead sites ranged from 750 to 1,380 µg/SPMD. A dialysate control had a PAH concentration less than the level of quantitation. The relative amounts of PAH determined by the Fluoroscan method were in general agreement with the results determined by the SPMDs and subsequent extraction with GC-MS analysis of individual PAHs (figs. 6 and 8; tables 7 and 9).
The Microtox EC50 and Toxicity Index values of all sample sites as well as the EC50 values of the dialysate and trip blank (TB) controls are shown in table 10. The Callville Bay and Lake Mohave sites showed only minor evidence that toxic compounds are present with TI values greater than 10. Dialysates from Las Vegas Bay and Hemenway Harbor sites did not indicate the presence of toxic compounds. The fluoroscan screening of SPMD sequestered samples revealed the presence of significant aromatic organic compounds not seen in the acute toxicity bioassay. The low acute toxicity and the uniform evidence of PAHs suggested that multiple-ringed-substituted PAHs, probably of petrogenic (derived from petroleum) or pyrogenic (derived from combustion) origin, were bioavailable as waterborne agents. These observations indicate that PAH compounds are present but not at concentrations high enough to cause toxicity to organisms in the lakes.
U.S. Department of the Interior | U.S. Geological Survey
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