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NA www.atcassociates.com SJCEHD approved a reduction in sampling frequency to semiannual and the reduction of certain non-reported constituents. SJCEHD identified data gaps in the SCM and recommended an additional investigation prior to submitting for site closure. ATC submitted a workplan, dated May 18, 2009, to conduct an additional soil boring investigation. SJCEHD approved the workplan in a letter dated November 17, 2009. The approved work was carried out from January 14 to January 19, 2010. ATC supervised the advancement of three direct push soil borings (MW-11, SB-4 and SB-5) and the '— installation of monitor well MW-11. Monitor well MW-11 has been sampled on a quarterly basis since its installation as requested by SJCEHD. Analytical results for well and soil boring installations are included in the Tables section of this report. 3.0 CONTROLS ON CONTAMINANT DISTRIBUTION The original source of petroleum hydrocarbons was removed in 1990 during the excavation and �.. repair of the fuel supply piping and removal of impacted soil and backfill material. Groundwater monitoring performed since 1990 has shown that the plume is stable and concentrations of dissolved petroleum hydrocarbons in the groundwater have been decreasing over time. Groundwater monitoring was conducted in targeted wells on September 2, 2010. Groundwater analytical results indicate that total petroleum hydrocarbons-gasoline range (TPHg) concentrations were reported at less than the California Department of Public Health Maximum Contaminant Level (MCL) of 50 pg/L. Benzene was not detected and MTBE was detected in `- three monitor wells: MW-1 R at 0.55 pg/L; MW-2 at 3.8 pg/L; and MW-5 at 2.3 pg/L. Historical analytical results for groundwater monitoring are included in Table 1. Historical groundwater analytical results for monitor wells MW-1 R, MW-2 and MW-5 are depicted on graphs included in Appendix B. The graphs present historical groundwater concentrations for TPHg, benzene, MTBE and tert amyl methyl ether(TAME). These ` constituents have reached asymptotic levels and/or are below laboratory reporting limits. This data and the historical analytical information demonstrate that the plume has been decreasing over time and has now reached levels near or below laboratory reporting limits due to natural attenuation. Further contaminant distribution controls would appear unnecessary for the Site. 4.0 PLUME DISPOSITION OVER TIME As stated above, the plume at the Site has been stable for many years and has been shrinking G� J due to natural attenuation. The migration rate for the plume can be expressed as less than zero feet per year for the past decade. The plume is expected to naturally attenuate until �/T/ f4 `— concentrations drop below laboratory reporting limits. The graphs presented in Appendix B `Y s include groundwater concentration versus time trend lines. Overall groundwater concentrations are decreasing to asymptotic levels and/or are below regulatory action levels. 5.0 ENVIRONMENTAL & HUMAN HEALTH SCREENING Groundwater samples were collected from monitor wells MW-1 R, MW-2, MW-4, MW-5 and MW- 11 during the September 2010 monitoring event to evaluate groundwater conditions. Quik Stop Market No. 132 July 21, 2011 ATC Project Number 75.62503.3206 Page 2 of 5