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Request for Site Closure Page 4 Chevron U.S.A. Products Company,3246 East Fremont Street, Stockton, CA January 3, 1995 sample #318 to the north, and MW-4 and EA-6 and MW-1 (no benzene) to the east. The extrapolated boundaries of hydrocarbon extent from 15-30 feet BGS is defined by pit sample #1 to the south, non-existent to slight hydrocarbon odors noted in boring logs from 26-30 feet BGS in MW-3 (GTI, 1987) to the west, low benzene concentrations in pit sample #3, and benzene concentrations below laboratory detection limits in MW-4 and MW-1. Although laboratory data does not exist for each sampling point at the 15-30 ft. BGS interval, declining concentrations in each direction allow for extrapolation of hydrocarbon-impacted soil boundaries (Figure 4). In fact the presence of low and non-detectable concentrations in soil samples collected from MW-4 and SB-1 indicate that significant volatilization and/or biologic degradation of hydrocarbons has occurred throughout the time the UST pit was left open for approximately one year. Based on a estimated lateral extent of approximately 5nn squares faet and an estimated vertical extent of 25 feet (15 - 40 feet BGS), total volume of hydrocarbon impacted soil is approximately 465 cubic yards. Using a conservatively high estimate of 130 ppm asap average concentration of hydrocarbon impacted soil in the calculated soil volume, hydrocarbon mass to be left in place is approximately 200 pounds (approximately 30 gallons gasoline equivalent) (Attachment 3). On June 2, 1994 quarterly groundwater sampling event, no wells on site showed measurable concentrations of hydrocarbons (Figure 7, Table 2). Hydraulic gradient at the site has varied with the season as shown In Figures 8 and 9 and by others (EA, 1990). Dissolved hydrocarbons have not been detected in groundwater since the February 1988 groundwater sampling event when groundwater elevations were an average of 19.65 feet below mean sea level (MSL) approximately 26 feet higher than in June 1994. In February 1988, wells MWs-1 through 3 and EA-1, 2 and 4 contained detectable concentrations of dissolved hydrocarbons up to 250 parts per billion (ppb) TPH-G. Groundwater samples obtained from EA-1, 3 and 4 in April and .lune 1989 were below laboratory detection limits for BTEX and TPH-G when average groundwater elevations were 21.06 and 28.06 below MSL, respectively. Dissolved hydrocarbon concentrations and groundwater elevations over time are plotted on Figure 10. The graph shows the absence of dissolved hydrocarbons in groundwater when elevations fell below 20 feet below MSL. Groundwater sampled from MWs-1, 2 and 3 in 1987 and 1988 was likely sampled from a perched groundwater zone of limited volume and areal extent. Wells EA-1 through 4 are screened at similar depths as MWs-1 through 3 and have been dry since September 1989, indicating the limited areal extent and limited lifespan of the previously perched groundwater zone. In fact the existence of the perched zone is coeval with the exposed tank cavity, which may have caused an artificial recharge basin for wells MW-1 through 3 between 1987 and 1988, prior to the backfilling of the UST pit. Groundwater elevations are currently an average of 46.87 feet below MSL, approximately 26 feet lower than levels measured in February 1988. Because of the lack of measurable concentrations of hydrocarbons in groundwater and the vertical distance between the impacted soil and the current water-bearing zone 0072clos.rpt ❑01W GROUNDWATER ❑D❑ TECHNOLOGY ,