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5• CONCLUSIONS f During the excavation of underground storage tanks at former Chevron SS 9-7780• petroleum hydrocarbons were identified in the :,oils. The highest concentration of hydrocarbons{5,300 mg/kg at a depth of 14 feet(Table 1)in soil was found directly beneath the Vis• The concen- tration dropped to 1.2 mg/kg at 36 :eet (MW I), 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 tarik pit or deeper beneath it did not have concentrations of petro- leum hydrocarbons greater than method iatection limits.surface. Fivest of the soil months after excavatio thntaining e levels leum hydrocarbons was excavated and aerates' on the of hydrocarbons in the aerated soil hpd dropped to below detection limits. The hydrocarbon levels in any unexcavated soils have also;1robably decreased significantly,since the pit has been open to the atmosphere. The decrease to hydrocarbm.concentrations in the soil was reflected in significant decreases in hydrocarbons in groundwater (wells MW1, MW2, MW3), where, for example,concentrations in MW2 dropped from 2,400µgtL of TPH in March 1997 to 250 gg/L in February 1988. No hydrocarbons were found in the domestic wells in the vicinity of the site (Table 4). In the peripheral wells(EAI through EA4),concentrations of petroleum hydrocarbons greater than detection limits were fowid only in EA2, and these did not exceed the MCL for drinidng weer in California. The results of previous and present investigations indicate that the presence of hydrocarbons in 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 presence of oxygen. All these alternating processes,-the—limited extent of hydrocarbons, and the drought during the last four years have probably 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,petroleum hydrocarbons have not migrated off the site to nearby water supply wells, and petroleum hydrocarbons that might remain in the soil are not actively contributing petroleum hydrocarbons. to the groundwater. c67/97780/tx 8