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Mr. Kyle Christie October 9, 1990 Page 7 Similar fluctuations are seen in well ATR-2, although the degree of fluctuation has been much less. Benzene is above MCL in ATR-2. Ethylbenzene, toluene, and xylene were not detected in ATR-2 in April 1990. The benzene and toluene concentrations in ATR-3 are above the DHS MCLS. Because of the proximity of ATR-3 to the likely source of the contaminants (the underground storage tank complex and associated piping), it is not thought to be affected greatly by changes in groundwater flow direction. The April 1990 data show the benzene concentration in ATR-4 has decreased to the lowest level since sampling began in 1987. Benzene is the only BETX constituent above the DHS MCL in ATR-4. The benzene concentration continues to increase slightly in well ATR-5 in April 1990. Toluene and xylene concentrations have also increased. BETX concentrations have increased in ATR-6 in April 1990 and the benzene concentration is now above the MCL. Benzene,ethylbenzene, and xylene have decreased slightly in well ATR-7 and benzene remains above the MCL. These decreases, although relatively small, may be the result of the change in groundwater flow direction. 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 ATR-6 and ATR-7. The petroleum hydrocarbon concentration has increased in all of the other five wells with the largest increases occurring in ATR-3 and ATR-5. Purgeable Halocarbons 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 6.2 µg/1 1,2-dichloroethane are present in the groundwater in ATR-2. The DHS maximum contaminant level MCL for 1,2-dichloroethane is 0.5 µg/l. 1,2-dichloroethane is a common constituent 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 April 1990 indicate that benzene concentrations have decreased in three of the seven wells on the site since December 1989. The increases in benzene concentrations in wells ATR-1, ATR-3, and ATR-5 are not significant, considering the historical data for those wells. The increase in well ATR-6 may be an indication of plume migration to the east. Brown and Caldwell Consultants