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. Work Plan for Additional Subsurface Assessment and Feasibility Study—Former Unocal Station#0187 September 14,2000 4,000 and 1,200 ppb, respectively MtBE by EPA Method 8020 was not detected in soil or grab groundwater samples from this boring Results of this subsurface investigation are presented in ARCADIS Geraghty&Miller's Supplemental Site Characterization Report dated June 3, 1999 In May 2000, GR installed four groundwater monitoring wells downgradient of the site Well U-10 was installed five feet north of existing nested piezometer well NP-2 to assess the reliability of chemical analytical results from well NP-2-116, and wells U-11 through U-13 were installed in a group downgradient of U-10 Each well was screened to correspond to separate water bearing zones from 20-35 feet bgs (U-11), 65-75 feet bgs (U-13), and 110 to 120 feet bgs (U-12) Soil samples were not collected from wells U-10, U-11, and U-13 Soil samples from well U-12 had detectable concentrations of benzene between 80 5 and 111 feet bgs at up to 0 0502 ppm Discrete groundwater samples collected from the screened intervals in wells U-12 and U-13 contained detectable concentrations of TPHg of 390 and 800 ppb, respectively Benzene was detected from discrete screened interval groundwater samples from wells U-10 (0 72 ppb), U-12 (58 ppb), and U-13 (430 ppb) MtBE was detected in discrete screen interval groundwater samples from wells U-10 and U-12 at concentrations of 2 3 and 3 1 ppb, respectively, by EPA Method 8260A 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, U-6, U-10, and U-11, and benzene has been detected is all wells except U-5, U-10,and U-11 MtBE has been detected is wells U-1, U-3,U-4, and U-11 During the most recent sampling event on May 30 and 31, 2000, TPHg was detected at concentrations ranging from 272 to 36,400 ppb, and benzene concentrations ranged from 0 72 to 4,590 ppb The highest concentrations of TPHg and benzene were detected in wells NP-1-65 and NP-2-60, respectively Quarterly sampling and monitoring of the wells is performed by GR Historical groundwater chemical analytical data are summarized in the most recent GR report(Second Quarter 2000 Groundwater Monitoring and Sampling Report,dated July 25,2000) SUBSURFACE ASSESSMENT In an August 30, 2000 meeting between Unocal, the San Joaquin County District Attorney's office, and SJCPHS, Unocal agreed to complete delineation of the vertical and lateral extent of petroleum hydrocarbons in soil and groundwater in the immediate vicinity of the subject site GR proposes to install ten off-site and two on-site groundwater monitoring wells (Figure 3) Field Methods and Procedures are included in Appendix A To implement this scope of work,GR proposes the following six tasks Task 1. Pre-field Activities GR will update the site-specific safety plan and obtain the necessary monitoring well installation permits from the SJCPHS, and encroachment permit from the City of Stockton Underground Service Alert (USA) will be notified at least 48 hours prior to initiating field work A private line locator will be contracted to clear the monitoring well locations Task 2. Well Installation GR will install twelve groundwater monitoring wells at the locations shown on Figure 3 Drilling and well construction will be performed by a California licensed driller The twelve groundwater monitoring 240042 08 4