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4 November 21, 2011 Cardno ERI 2789C.R02 Former Mobil Service Station 99CAS, Stockton, California test, approximately 115.74 pounds of TPHg and 2.8 pounds of benzene were removed from soil vapor and approximately 0.279 pound of TPHg and less than 0.1 pound of benzene were removed from groundwater. Based on the results of the test, ETIC concluded that DPE would be an effective remedial method for the shallow water -bearing zone at the site and proposed the installation of a DPE/SVE (co -extraction system) (ETIC, 2011). In April 2011, Cardno ERI evaluated potential hydrocarbon migration and determined that dissolved -phase hydrocarbon concentrations in the wells were stable or decreasing. Based on this finding, Cardno ERI concluded that dissolved -phase hydrocarbon concentration migration control was not warranted or cost- effective. The DPE/SVE system proposed by ETIC was selected in part for migration control. Cardno ERI evaluated AS/SVE as a possible alternative, and concluded that AS/SVE would be an effective remedial method for the site because it would address constituents of concern in both the vadose zone and saturated zone in a reasonable period of time and would not require dewatering of the upper 35 feet of the saturated zone to expose residual concentrations to vapor extraction. Cardno ERI proposed the installation of an AS/SVE system instead of the proposed DPE/SVE system (Cardno ERI, 2011 a). Groundwater Monitoring Activities Groundwater monitoring and sampling at the site commenced in March 2007 (Tables 1A through 1C). Dissolved -phase benzene concentrations at the site are stable to decreasing. Dissolved -phase TPHg and benzene concentrations in wells MW4, EX1, and EX2 are indicative of the presence of NAPL; however, NAPL has not been identified in these or nearby wells, indicating that the NAPL, if present, is immobile or "trapped". MTBE has not been reported in wells MW4, EX1, and EX2 with the exception of one concentration in well MW4 in February 2008; however, reporting limits for samples collected from these wells are elevated (<1,200 pg/L) due to concentrations of other target analytes. Dissolved -phase benzene and TPHg concentrations in well MW2, located downgradient of wells MW4, EX1, and EX2, have shown a decreasing trend during the monitoring and sampling program. Reported MTBE concentrations in well MW2 have steadily decreased from a maximum concentration of 1,200 pg/L in September 2007 to 390 pg/L in September 2011. WELL INSTALLATION Cardno ERI proposed the installation of an AS/SVE system at the subject site in place of the DPE/SVE system proposed by ETIC (Cardno ERI, 2011a). The Regional Board approved this change in a letter dated May 5, 2011 (Appendix A). Cardno ERI performed the fieldwork in accordance with Cardno ERI's Addendum to Corrective Action Plan and Revised Schedule of Implementation, dated April 29, 2011 (Cardno ERI, 2011 a); Cardno ERI's standard field