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BP #172 (Thrifty) - 2 - 9 March 2017 7647 Pacific Avenue Stockton, San Joaquin County TABLE 1 — December 2016 Groundwater Data Zone No. of Wells Screened Maximum TPHg Maximum Benzene Maximum MTBE Interval (ug/L) (ug/L) (ug/L) SHALLOW 7 25-45A,B 44,000 19,000 <100c (MW-14) (MW-14) (MW-14) DEEP 10 40-75A 10,000 4,300 200 (MW-22) (MW-22) (MW-3) DTW=depth to water;GWE=groundwater elevation;TPHg=total petroleum hydrocarbons as gasoline;MTBE=methyl tert butyl ether;ft bgs=feet below ground surface;ug/L=micrograms per liter A=Wells screened variably within the listed range. B=Well MW-7 is cross-screened between A and B zones from 25-56 ft bgs;concentrations in this well are below reporting limits. C=June 2016 MTBE In MW-14 was 240 ug/L. Dual Phase Extraction (DPE) remediation has been implemented at the Site between April 2001 and March 2009. During this time, approximately 210,000 pounds of vapor-phase petroleum hydrocarbons, along with 582,800 gallons of hydrocarbon impacted groundwater. In 2009, a bench-scale ozone test was performed for the Site, though full sale ozone remediation was not implemented due to the generation of hexavalent chromium and bromate during the test and their failure to attenuate in a reasonable time frame. Sensitive receptor surveys have been performed in 1992, 1993, and 2011. No public water supply wells were reported within 1,000 feet of the Site. One private water supply well was reported on the Carden School property, approximately 700 feet northwest (cross-gradient) from the Site. Five Mile Creek is located approximately 100 feet west (upgradient) of the Site. The submitted Assessment Report detailed the January 2015 installation of three (3) soil vapor extraction (SVE) wells (SVE-1 through SVE-3), and one (1) air-sparge (AS) well (AS-1) on the Site property in October 2014, and one (1) off-site deep monitoring well (MW-23) . The submitted Pilot Test Report detailed the performance of an SVE/AS pilot test in January 2015. The SVE test removed approximately 7 pounds of hydrocarbons in just under 7 hours and ATC recommended implementation of SVE and AS remediation at the Site. Central Valley Water Board staff has the following comments on the status of the assessment and cleanup of the petroleum release from the Site: 1. The Site does not meet the State Water Resource Control Board's Low Threat Underground Storage Tank Case Closure Policy(LTCP) Groundwater Specific criteria. Benzene in groundwater is greater than 3,000 ug/L, and the extent of hydrocarbon impact in groundwater remains undefined downgradient (northeast). While the newly installed deep well MW-23 appears to have defined the downgradient extent of hydrocarbon impact to groundwater in excess of Water Quality Objectives (WQOs) in the deep water zone downgradient of the Site, the extent of shallow groundwater impact remains undefined downgradient. Delineation of the extent of hydrocarbon impact to groundwater downgradient of shallow well MW-14 remains incomplete. Samples collected from well MW-14 contained the Site maximum TPHg, benzene, and MTBE concentrations, as detailed in Table 1 above.