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�1 REMEDIATION CONFIRMATION SAMPUNG RESULTS AND CLOSURE RECOMMENDATION REPORT (March 1994 Revision) 909 West Hammer Lane Slocktan California • In August 1990, Applied Geosystems directed the removal of product piping from the site and subsequent soil sampling Of twelve soil samples collected from depths ranging from 2 to 3 feet, only one sample contained detectable TPH as gasoline, at 3 3 ppm This work was documented in a report included in Appendix E • EVAX operated a soil vapor extraction system on the site from March through March 1992 Soil vapors were extracted from six perforated steel probes driven into the soil and monitoring wells MW-3 and MW-4 An estimated 1,484 pounds of petroleum hydrocarbons, including 60 pounds of benzene, were removed from the subsurface The system was discontinued because less than 1 pound per day of hydrocarbons were being removed, indicating the site was remediated to the extent possible using vapor extraction Vapor extraction system design, performance, and monitoring data are included in Appendix I • From December 1986 to June 1993, depth to groundwater beneath the site has increased from a depth of approximately 47 feet to 60 feet The groundwater gradient is generally to the northeast, with occasional variations Depth to groundwater and gradient information is included to Appendix F The wells were sampled periodically from November 1986 to June 1993 All groundwater analytical results are shown in Table 3 Analytical results of groundwater samples from the wells have not indicated the presence of petroleum hydrocarbons since June 1991 • In November 1993, Aegis supervised the excavation of soil containing residual petroleum hydrocarbons found in confirmation boring SB-9,,as documented in this report Appendix J contains Aegis' soil excavation results report, and a completed "Case Closure Checklist" form from the Central Valley Regional Water Quality Control Board Aegis estimated approximately 8 6 pounds of immobile, residual hydrocarbons (180 to 240 ppm) may remain in soil from 26 to 31 feet below grade Because groundwater has been clean since 1991, and is approximately 60 feet deep, Aegis recommended case closure, with no further action 4 0 REMEDIATION CONFIRMATION SOIL BORINGS In November 1992, EA submitted a workplan, Work Plan For Soil Sampling, Former Exxon Retail Site 7-1050, 909 West Hammer Lane, Stockton, Califomia, to the San Joaquin County Environmental Health Department to drill and sample three soil borings to assess the effectiveness of the vapor extraction system The workplan was approved and Aegis was contracted by Exxon to install the borings 93-005C QMR g