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Mr. Kyle Christie May 14; 1990 Page 9 The benzene, ethylbenzene, and toluene concentrations increased slightly in well ATR-5 in December 1989, but benzene remains the only constituent above the DHS MCL. The benzene, ethylbenzene, and toluene concentrations decreased to nondetectabie in well ATR-6 in December 1989, and the xylene level decreased to a trace amount. Well ATR-7 showed slight increases in all BETX concentrations, with benzene above the DHS MCL. These increases, although I`I relatively small, may be the result of the change in groundwater flow direction. . i TPH Results. Groundwater samples from,the seven wells were analyzed for low boiling point hydrocarbons (C4-C12). Petroleum hydrocarbon concentrations were below the laboratory detection limit in ATR-1, ATR-2, and ATR-6. Only trace amounts of petroleum hydrocarbons were identified in wells ATR-4, ATR-5, and ATR-7. ATR-3 contained 27 parts per million (ppm) petroleum hydrocarbons. This is a slight increase for well ATR-3 over the September t. 1989 sampling event. Par eable Halocarbons g p { As requested by the Regional Board, the groundwater samples from the seven wells were analyzed for purgeable halocarbons, specifically 1,2-dichloroethane. Results from this analysis indicate that 7 µgh 1,2-dichloroethane are present in the groundwater in ATR-2. The DHS maximum contaminant level (MCL) for 1,2-dichloroethane is 5 µg/l. 1,2-dichloroethane had previously been detected in wells ATR-3 and ATR-4; however, the December 1989 detection limits for these wells were above the concentrations at which they were detected in September 1989 because the samples were diluted before final analysis because of high concentrations of gasoline constituents. Additionally, 2.3 µg11 tetrachloroethene were detected in ATR-6. The MCL for tetrachloroethene is 5 µg11. Tetrachloroethene and 1,2-dichloroethane are common 1 constituents of solvents, but are not found in gasoline. Therefore, these compounds are not thought to be from leaks in the gasoline storage tanks or associated piping. ARCO will continue to analyze the groundwater from the wells for purgeable halocarbons on a semi-annual basis. Conclusions Based on field observations and analytical results, the following conclusions are presented. Analytical results of groundwater samples collected during December 1989 indicate that benzene concentrations have remained constant or have decreased in three of the seven wells on the site since September 1989. The increases in benzene concentrations in wells ATR-4 and ATR-5 are not significant, considering the historical data for those wells. The increase in well ATR-7 may be an. indication oflume migration to the east, which is consistent with' the change in p � groundwater flow toward that.direction. The petroleum hydrocarbon concentration has remained constant or has decreased in three of the seven wells. The petroleum hydrocarbon concentration has increased slightly in wells ATR-3, ATR-4, ATR-5,'and ATR-7. These increases are not significant, considering the historical data for these wells, except possibly for well ATR-7, which may be an indication of plume migration, Brown and Caldwell i Consultants k