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Monitoring Well Installation Report—Former Unocal Service Station 110187, Stockton,CA . August 2,2000 Quarterly monitoring and sampling of the groundwater monitoring wells began in August 1994 Dissolved concentrations of TPHg have been detected in all wells except U-5 and U-6, and benzene has been detected in all wells except U-5 MtBE has been detected in wells U-1, U-3, and U-4 During the most recent sampling event on May 30 and 31, 2000, TPHg were detected at concentrations ranging from 272 to 36,400 ppb, and benzene concentrations ranged from 0 625 to 4,590 ppb The highest concentrations of TPHg were detected in well NP-1, and the highest concentration of benzene was detected in well NP-2 FIELD ACTIVITIES To evaluate the extent of petroleum hydrocarbons in soil and groundwater downgradient of the subject site, GR installed four off-site groundwater monitoring wells U-10 through U-13 at the locations shown on Figure 3 Field work was performed in accordance with GR's Site Safety Plan 4240042 05, dated February 25, 2000 GR Field Methods and Procedures are included in Appendix A Underground Service Alert was notified prior to beginning site activities The wells were installed by Woodward Drilling (C57 9710079) under SJCPHS-EHD well installation permits #0022189 and 40022190, and CSDPW encroachment permit 465697 Copies of the permits are included in Appendix B The four groundwater monitoring wells were installed on May 8 through 10, 2000 Well borings U-10 through U-13 were drilled to depths of 120, 35, 120, and 75 feet below ground surface (bgs), respectively, using a truck-mounted drill rig equipped with 8-inch-diameter hollow-stem augers A GR geologist observed the drilling activities Soil samples were collected from well boring U-12 at five-foot intervals for description and preparation of a log, and for possible chemical analysis Samples were not collected from well borings U-11 and U-13 because they «ere drilled within 5 feet of well boring U-12, and well boring U-10 was drilled within 5 feet of existing well NP-2 A boring log for well U-12 is included in Appendix C Copies of the Well Driller's Reports for wells U-10 through U-13 are included in Appendix C Soil cuttings generated during drilling activities were placed on and covered with plastic A composite disposal confirmation sample SP-1(A-D) was collected from the stockpiled soil cuttings Stockpile sampling procedures are presented in Appendix A Well Installation Groundwater monitoring wells U-10 through U-13 v�ere constructed using 2-inch diameter Schedule 40 polyvinyl chloride (PVC) casing and 0 020-inch machine-slotted screen material Wells U-14 through U-13 are screened from 110 to 120, 20 to 35, 110 to 120, and 65 to 75 feet bgs, respectively The annular space around each screened interval is packed with Lonestar +3 graded sand The sandpack is followed by a seal of bentonite chips hydrated with clean water in well U-11, and coated bentonite pellets in wells U-10, U-12, and U-13,then neat cement to approximately 1 foot bgs The top of each well is protected by a vault box, locking well cap, and lock The vault boxes are installed flush with the ground surface, and are set in concrete Well construction details are included in Appendix C Well Monitoring Develo ment and Sam lin An initial attempt to develop the wells occurred on May 30, 2000, but excessive silt in wells U-10, U-12, and U-13 prevented development On June 2, 2000, Woodward Drilling removed the silt from the wells 24004205-1 4