CHANGES IN MTBE AND BTEX CONCENTRATIONS IN LAKE TAHOE, CA-NV FOLLOWING IMPLEMENTATION OF A BAN ON SELECTED 2-STROKE MARINE ENGINES Brant C. Allen & John E. Reuter Tahoe Research Group University of California, Davis Davis, CA 95616 INTRODUCTION Discovery of the fuel oxygenate methyl tert-butyl ether (MTBE) in groundwater, lakes and reservoirs used for drinking water has raised considerable concern among health officials and water suppliers. The U.S. EPA has classified MTBE as a possible human carcinogen. Recent legislation in California has established primary and secondary drinking water standards at 13 µg/L and 5 µg/L, respectively. Since 1997, the Lake Tahoe basin has received considerable state and national attention with regards to MTBE contamination of both groundwater drinking supplies and the lake itself. Protection of the lake from controllable sources of pollution is required under its designation as an Outstanding National Water Resource (ONWR) as part of the federal Clean Water Act. Lake samples collected by the University of California, Davis - Tahoe Research Group (TRG), University of Nevada Reno (UNR), and the U.S. Geological Survey during the summers of 1997 and 1998 showed detectable levels of MTBE and the BTEX fuel constituents (benzene, toluene, ethylbenzene, and xylene), lake wide (e.g. Allen et al. 1998, Boughton and Lico 1998). Concentrations were shown to vary with the level of motorized watercraft traffic. However, at specific locations, levels exceeded not only the California drinking water standards but the higher U.S. EPA advisory value of 35 µg/L. Samples from the open water in the middle of the lake, where little summer boating occurs, revealed the presence of fuel constituents to a depth of 10 m, but at concentrations near or below the analytical levels of detection (mean value of 0.3 µg/L; Allen et al. 1998). Along the shoreline of the lake where motorized watercraft activity is more common, fuel constituent concentrations were found to be about an order of magnitude higher (2.6 µg/L, mean value for MTBE). These shoreline concentrations were still below the established drinking water standards. In areas where motorized watercraft traffic is considered to be exceptionally high (marinas and fueling facilities), mean concentrations for both MTBE and benzene, during certain times of the summer boating season, exceeded primary drinking water standards. Further investigation by the California Air Resources Board (CARB) and UNR into the direct contribution of fuel constituents from various engine technologies revealed that the carbureted two stroke engines were contributing a disproportionate share of the fuel load to Lake Tahoe (Glenn Miller, University of Nevada, Reno, unpub. data). In fact, Allen et al. (1998) calculated that while using only 11-12% of the total fuel used for Lake Tahoe boating, these engines contributed 90% of the MTBE to the water. In contrast the 4-stroke engines consumed 87% of the fuel and but were responsible for only 8% of the estimated MTBE loading to the lake from all marine engines. The results of these cumulative studies resulted in regulations imposed by the Tahoe Regional Planning Agency (TRPA), banning certain two stroke engine technologies. This ban took effect on June 1, 1999. Additionally, several large oil companies began producing gasoline without MTBE and delivering it to the south end of the lake. With both programs to abate MTBE loading to the lake and groundwater in place by late spring of 1999, the summer boating season was expected to produce lower levels of in-lake fuel constituents. The TRG began sampling in August to evaluate the effectiveness of these changes, i.e. comparison of lake concentrations of MTBE and BTEX in the summer of 1999 relative to 1997 and 1998. METHODS Sampling locations were selected to describe changes in MTBE and BTEX concentration in Lake Tahoe that may have resulted from the policy decisions above. Therefore, our sampling focused on locations which had positive results during the 1997 and 1998 monitoring. Site selection was separated into three categories; 1) an open water, midlake location where boating is minimal, 2) nearshore, at 10 locations around the perimeter of the lake, where the majority of boat occurs, and 3) 10 locations on the south shore where boat traffic is concentrated ("hot spots"), often associated with launch ramps, refueling facilities, marinas or a combination of the above. Within each category, specific sites were chosen, whenever possible, to replicate those sampled in previous years. The timing of the sampling, late August and the Labor Day weekend in September, coincided with the peak of the summer boating season. Three sampling dates were chosen, mid-week (Wednesday and Thursday, 25 and 26 August, respectively). Weekend samples were collected on Monday (30 August), and the Labor Day weekend was represented by samples taken on the Tuesday after the holiday (7 September). At all locations, with the exception of mid-lake, water samples were taken by hand at a depth of 0.5 m. Our previous sampling at Lake Tahoe showed this to be a representative depth for the nearshore stations. The closed VOA vials were submerged to the sampling depth and then opened and allowed to fill completely. The cap was replaced while submerged. Samples were checked to ensure no air space remained within the VOA vial before they were placed on ice in a cooler. The mid-lake samples were collected using a 1.2 L, stainless Kemmerer well sampler with Teflon end caps. The sampler was lowered to depth and closed with a messenger. Water was then transferred to the VOA vial and filled so that no air spaces remained. All samples were kept on ice from collection through transport to Lawrence Livermore National Laboratories (LLNL). All analytical determinations were made by LLNL staff at their facilities (C. Koester, pers. comm.)   RESULTS Open water and nearshore samples showed a significant decrease in MTBE concentration when compared to data collected in 1997 and 1998. Generally, ambient concentrations decreased by a factor of 10 with samples around the north end of the lake (Homewood to Glenbrook) at or below the 0.06 µg/L level of detection. Samples collected in the vicinity of the south end of the lake (Zephyr Cove to Emerald Bay) showed a similar drop in concentration from previous years, but remained above the level of detection at a few tenths of a part per billion (µg/L). Ambient concentrations of the BTEX compounds (benzene, toluene, ethylbenzene, and xylene) at the nearshore locations were also found to be lower than levels recorded in the past two years of monitoring (Table 1). Samples from the "hot spot" locations had greater MTBE levels than the nearshore and open water areas; four concentrations approached or exceeded drinking water standards. The remaining six "hot spots" had fuel concentrations similar to nearshore areas sampled during the 1997 and 1998 monitoring. At the four "hot spots" where fuel constituent concentrations neared or exceeded drinking water standards, MTBE and BTEX concentrations were highly variable. MTBE concentrations ranged from 0.46 µg/L up to 56.5 µg/L. This high value is over four times the primary drinking water standard of 13 µg/L. The dramatic difference in results between these "hot spots" and the remainder of the lake suggests source contamination has not been completely eliminated by actions to date, but that inputs to the lake were significantly reduced in the summer of 1999. DISCUSSION The sampling dates selected during this study were at the end of the summer boating season during the month of August and after the Labor Day weekend early in September. Allen et al. (1998) showed this period representative of high MTBE and BTEX concentrations in Lake Tahoe. With the exception of a few of the "hot spots", the data collected during this study showed little variation between sampling dates. Comparisons of data collected during this study with that of previous years shows a dramatic decrease in MTBE concentration at both offshore and nearshore locations (86.7% and 95.8%, respectively)(Figure 1). This demonstrates that programs to eliminate MTBE from Lake Tahoe are having an effect. The offshore and most of the nearshore locations around the lake had MTBE concentrations at or near the analytical level of detection (LOD) throughout the sampling period The sampling of "hot spots" around the south end of Lake Tahoe resulted in highly variable results (MTBE range <0.06 to 56.5 µg/L). MTBE samples collected at Ski Run Marina exceeded the California primary drinking water standard of 13 µg/L by four-fold on two separate sampling dates. Additionally the California drinking water standard for benzene (0.1 µg/L) was surpassed on the post Labor Day sampling, 7 September. These samples stand out from the rest as being extremely high even for the "hot spot" locations. The reasons may be due to above average concentration of boats per unit area or some problem with operations at the facilities. The two other locations where measured concentrations of MTBE approached or exceeded California drinking water standards where associated with boat launch ramps. Since neither location is in the immediate proximity of fueling facilities it is expected that the fuel constituents came from the boats themselves. While it is unclear how the fuel entered the water, any number of human errors and boat malfunctions could have contributed. One distinct possibility associated with launch ramps is the draining of the bilge upon removal of the boat from the water. Either the intentional removal of boat plugs to allow draining while on the incline ramp or the automatic operation of electrical bilge pumps when water rushes to the back of the boat will cause fuel laden water to flow directly into the lake in the vicinity of the ramp. CONCLUSION On the whole, fuel constituent concentrations in Lake Tahoe are down dramatically from previous years. This could be the result of the TRPA regulation banning certain two cycle engine technologies or as a byproduct of some service stations within the Tahoe basin selling MTBE-free fuel. A comparison of the decreases in ambient MTBE and toluene concentrations was done to determine which corrective action was having the greatest impact on Tahoe water quality. If the MTBE-free fuel was having the greatest impact, the ambient MTBE concentrations would be expected to decrease while toluene concentrations in the lake remained near the levels recorded in 1997 and 1998. If the new boating regulations were having the greatest impact, both MTBE and toluene concentrations could be expected to drop. Indeed both mean MTBE and mean toluene concentrations drop significantly (95.8% and 88.3% respectively) indicating that the elimination of the highly polluting two cycle engines is having a clear impact on water quality.   REFERENCES Allen, B.C., J.E. Reuter, C.R. Goldman, M.F. Fiore and G.C. Miller. 1998. Lake Tahoe motorized watercraft - an integration of water quality, watercraft use and ecotoxicology issues. John Muir Institute for the Environment, University of California, Davis. 37 p. Contribution # 001-1998. Reuter, J.E., B.C. Allen, R.C. Richards, J.F. Pankow, C.R. Goldman, R.L. Scholl and J.S. Seyfried. 1998. Concentrations, sources, and fate of the gasoline oxygenate methyl tert-butyl ether (MTBE) in a multiple-use lake. Environ. Sci. Technol. 32: 3666-3672. Boughton, C.J. and M.S. Lico. 1998. Volatile organic compounds in Lake Tahoe, Nevada and California, July-September 1997. USGS Fact Sheet FS-055-98, June 1998.