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Mr. Kyle Christie February 26, 1990 Page 7 Purgeable Halocarbons As requested by the Regional Board, the groundwater samples from the seven wells is being analyzed for purgeable halocarbons, specifically 1,2-dichloroethane. Results from this analysis indicate that 1,2-dichloroethane is present in the groundwater above detectible levels in ATR-2, ATR-3, and ATR-4. The Department of Health Services maximum contaminant level (MCL) for 1,2-dichloroethane is 5 parts per billion (ppb). Only the concentration (19 ppb) in ATR-3 exceeds the MCL. The concentration in ATR-2 and ATR-4 is 1.6 ppb and 0.9 ppb. ARCO will continue to analyze the groundwater from the wells for purgeable halocarbons. Groundwater Flow Water-level elevations at the site are measured in each well on a quarterly basis. The water levels are measured relative to the top of well casing with an electronic water/oil interface probe. The groundwater flow direction in June and September 1989 was approximately north, under a hydraulic gradient of 12 feet per mile or 0.2 percent. Table 3 presents water level data collected during the September sampling event. The water levels have dropped 2 to 6 inches since June 1989. %Ww, Conclusions Based on field observations and analytical results, the following conclusions are presented. Analytical results of groundwater samples collected during September 1989 indicate that benzene concentrations have remained constant or have decreased in five of the seven wells on the site since June 1989. The benzene concentration in well ATR-6 has increased from non-detectable to 0.5 ppb. The benzene concentration has increased slightly from 890 ppb to 1,000 ppb in well ATR-3. The petroleum hydrocarbon concentration has remained constant or has decreased in five of the seven wells. The petroleum hydrocarbon concentration has increased slightly in well ATR-3 and has increased significantly in well ATR-1. As stated above, the reason for the increase in ATR-1 is unknown at this time. Well's ATR-1 through ATR-4 were installed in March 1987. Historically, petroleum hydrocarbons have not been detected in ATR-1. ATR-3 (85 feet to the southeast) has had detectible amounts of petroleum hydrocarbons in all sampling events (as high as 110 ppm, February 1989). Perhaps the contamination plume is just now reaching ATR-1. If so, this would indicate low hydraulic conductivity of the shallow aquifer materials at this site as well as slow groundwater and pollutant movement. It is unlikely that the contamination has moved very far off-site, if at all. 1,2-Dichloroethane was detected in ATR-2, ATR-3, and ATR-4 but only exceeded the MCL for this constituent in ATR-3. ARCO will continue to monitor this constituent during each quarterly sampling event. The source of the 1,2-dichloroethane is unknown. Brown and Caldwell Consultants