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Mr. Kyle Christie <br /> October 9, 1990 <br /> Page 7 <br /> Similar fluctuations are seen in well ATR-2, although the degree of fluctuation has been much <br /> less. Benzene is above MCL in ATR-2. Ethylbenzene, toluene, and xylene were not detected <br /> in ATR-2 in April 1990. The benzene and toluene concentrations in ATR-3 are above the DHS <br /> MCLS. Because of the proximity of ATR-3 to the likely source of the contaminants (the <br /> underground storage tank complex and associated piping), it is not thought to be affected greatly <br /> by changes in groundwater flow direction. The April 1990 data show the benzene concentration <br /> in ATR-4 has decreased to the lowest level since sampling began in 1987. Benzene is the only <br /> BETX constituent above the DHS MCL in ATR-4. <br /> The benzene concentration continues to increase slightly in well ATR-5 in April 1990. Toluene <br /> and xylene concentrations have also increased. BETX concentrations have increased in ATR-6 <br /> in April 1990 and the benzene concentration is now above the MCL. Benzene,ethylbenzene, and <br /> xylene have decreased slightly in well ATR-7 and benzene remains above the MCL. These <br /> decreases, although relatively small, may be the result of the change in groundwater flow <br /> direction. <br /> TPH Results. Groundwater samples from the seven wells were analyzed for low boiling-point <br /> hydrocarbons (C4-C12). Petroleum hydrocarbon concentrations were below the laboratory <br /> detection limit ATR-6 and ATR-7. The petroleum hydrocarbon concentration has increased in <br /> all of the other five wells with the largest increases occurring in ATR-3 and ATR-5. <br /> Purgeable Halocarbons <br /> As requested by the Regional Board, the groundwater samples from the seven wells were <br /> analyzed for purgeable halocarbons, specifically 1,2-dichloroethane. Results from this analysis <br /> indicate that 6.2 µg/1 1,2-dichloroethane are present in the groundwater in ATR-2. The DHS <br /> maximum contaminant level MCL for 1,2-dichloroethane is 0.5 µg/l. 1,2-dichloroethane is a <br /> common constituent of solvents, but are not found in gasoline. Therefore, these compounds are <br /> not thought to be from leaks in the gasoline storage tanks or associated piping. ARCO will <br /> continue to analyze the groundwater from the wells for purgeable halocarbons on a semi-annual <br /> basis. <br /> Conclusions <br /> Based on field observations and analytical results, the following conclusions are presented. <br /> Analytical results of groundwater samples collected during April 1990 indicate that benzene <br /> concentrations have decreased in three of the seven wells on the site since December 1989. The <br /> increases in benzene concentrations in wells ATR-1, ATR-3, and ATR-5 are not significant, <br /> considering the historical data for those wells. The increase in well ATR-6 may be an indication <br /> of plume migration to the east. <br /> Brown and Caldwell <br /> Consultants <br />