SITE INFORMATION AND CORRESPONDENCE

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f SUMMARY CHEVRON STATION # 9-6465 5608 NORTH PERSHING AVENUE Lf STOCKTON CA In 1986 PHS/EHD responded to a report from the fire department of a massive release of petroleum from the piping from the gas station noted above. Two days later the source of the leak was found. ALL three product lines were found to be leaking. The piping was subsequently repaired. It is not known if any of the impacted soil was removed at that time. In response to the release from the piping, three monitoring wells were installed. Depth to water in 1986 was approximately 24.41 below ground surface. From 1986 to 1988, the monitoring wells showed groundwater contamination. In 1988, the underground fuel tanks, the piping and the waste oil tank were removed and upgraded. The soil samples obtained from underneath the tanks and the piping all showed soil contamination. It is unknown whether soil from the piping area was over-excavated at this time, but a limited removal of impacted soil from the fuel tank area did occur. Depth to water in 1988 was approximately 291-301 below ground surface. Between 1987 and 1990, several additional monitoring wells were installed to replace those that were currently dry and to further delineate the groundwater contaminant plume. In addition, in 1990 several of the older wells were reconstructed at the surface due to poor surface seal conditions. The condition of the wells were suspected of atlowing surface contaminants from the daily operation and cleaning of the gas station to impact the monitoring wells. Between 1990 and 1993, depth to groundwater varied from approximately 27, to 41.71 betow ground surface. During this time most the wells showed minor levels of contamination and, more often, when the water table was lower, nondetect levels of contamination. In 1993, all fuel tanks, the waste oil tank and all associated piping were removed from the site and the gasoline station was demolished. The soil samples from underneath the fuel tanks showed,llow Levels of contamination near the area where the piping entered the fuel tank pit. The soil sample from underneath the waste oil tank showed contamination. Only one area encompassed by the piping showed low levels of contamination. The waste oil tank pit area was over-excavated and the impacted soil was removed to a landfill. Confirmation soil samples indicated that the over-excavation was successful in removing the impacted soil. The piping area was over-excavated and a slightly deeper soil sample obtained which indicated all impacted soil had been removed from this area as well. An aggressive over-excavation was attempted at the fuel tank pit area. It appeared the release of petroleum in 1986 ran down the piping trenches and down into the fuel tank pit in the area where the piping entered the tank pit. During the over-excavation of this area it was apparent that the contamination went predominantly vertical to groundwater. Over the years, as the groundwater level lowered, it appeared that the groundwater contaminant plume became adsorbed onto the soil from a depth of approximately 26, below ground surface to approximately to 36' below ground surface. This was the area that visually appeared to be impacted and that was subsequently removed by over-excavation. The soil from 26' below ground surface to grade was stockpiled onsite, sampled and eventually used as backfill. The impacted soil was removed from the site to a landfill. The over-excavation of the fuel tank pit area was successful in removing the bulk of the impacted soil. Soil sample results indicated that low levels of contamination remained to the north northwest of the excavation, in the direction of groundwater ftow. Several of the monitoring wells at the site were destroyed to allow for the redevelopment of the property. Soil samples were obtained from below the more shallow monitoring wells to indicate current soil conditions in the area where groundwater once existed. These soil samples showed only a low level of xytene from below monitoring well MU#6. Groundwater sample results from immediately after the remediation attempt showed higher levels of contamination than in the recent past. This may in part be due to damage sustained by the monitoring wells 'from the heavy I equipment from the over-excavation and also from the subsequent redevelopment of the property. The two latest rounds of groundwater sampling showed low to nondetect contamination. The responsible party requested closure based on the removal of the bulk of the source of contamination to groundwater. The residual mass of hydrocarbons estimated to remain in an area of 22,750 cubic feet (843 cubic yards) is 11.5 pounds or 1.9 gallons of total petroleum hydrocarbons as gasoline. The responsible party contends that due to the limited extent of residual contamination, no significant threat to groundwater remains at the site. PNS/EHD concurs that the threat to groundwater has been abated sufficient to protect the future beneficial uses designated for groundwater in this area. t t