SITE INFORMATION AND CORRESPONDENCE

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6. CONCLUSIONS During the excavation of underground storage tanks at former Chevron SS 9-7780, petroleum 3 hydrocarbons were identified in the soils.,,The highest concentration of hydrocarbons (5,300 mg/kg at a depth of 1'4 feet (Table 1) in soil was found directly beneath the tanks. The concen- "i tration dropped,to 1.2 mg/kg at 36 feet (MWl), and no concentrations of'petroleum hydrocar- bons greater than detection limits were detected in soils'away from the tank-field (MW2, MW3). The extent of petroleum hydrocarbons in the soil cannot be extensive, since soil samples taken from the area surrounding the tank pit or deeper bedeath it did not have concentrations of petro- leum hydrocarbons greater than method detection limits. Most of the soil containing petroleum hydrocarbons was excavated and aerated ori the surface . Five months after excavation the levels of hydrocarbons in the aerated soil had dropped to below detection limits. The hydrocarbon levels in any unexcavated soils have also probably decreased significantly, Since the pit has been open to the atmosphere. The decrease in hydrocarbon concentrations in the il soil was reflected in . significant decreases in hydrocarbons in groundwater (wells MW 1, MW2, MW3), where, for example, concentrations in MW2 dropped from 2,400'µg/L of TPH in March 1987 to 250 µglL in February 1988.1 No hydrocarbons were found'in the domestic wells in the vicinity of the site (Table 4). In the peripheral wells (EA I,through EA4), concentrations of petroleum hydrocarbons greater than detection limits were found only in E_A2, and these did not'exceed the MCL for drinking water in California. The results of previous and resent investigations indicate that the presence of hydrocarbons in P P g the soil and groundwater was very limited. It is reasonable to suppose that it was caused by small surface spills or by tank overfilling during operation of the station. A severe drought and the related 30-foot drop of water level under the site have created condi- tions favoring decreases of hydrocarbon concentrations in soil and groundwater. Relatively high subsurface temperatures, reflected in a groundwater temperature under the site of 21 C--70 F, increase the rate of evaporation of volatile hydrocarbons from the water as the water table moves down. Hydrocarbons absorbed by clayey soils, which extend up to 70 feet beneath the site, are undergoing further evaporation, biodegradation, and decay in the presenceoF oxygen. All these alternating processes, the limited extent of hydrocarbons, and the drought during the last four years have probabiy,:diminished the concentrations of hydrocarbons in the groundwater and soil to at or below detection limits.' The shifting direction of groundwater flow,'influenced by nearby domestic wells, has also, to a certain degree,prevented the plume from migrating off the site. On the basis of the current investigation, etroleurri hydrocarbons have not migrated off the site - g ., P �- to nearby water supply wells, and petroleum hydrocarbons that might rerziain in the soil are not actively contributing'petroleum hydrocarbons to thel groundwater. _ c67/97780/tx a 8