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Mr. Gabriel Gonzalez, SJCEHD May 12,2015 Supplemental Information to Support Environmental Case Closure Project No.2129-1140-01 7835 Thornton Road(formerly 1140 W. Hammer Lane), Stockton,CA Page 2 aeration, and used to backfill the excavation cavity. In order to estimate the contaminant mass removed from the subsurface, Stratus has averaged concentrations of total petroleum hydrocarbons as gasoline / gasoline range organics (TPHG/GRO) and benzene reported in compliance soil samples collected in March and April 1987, and samples collected from the sidewalls and base of the excavation in January and September 1990. A copy of the analytical table containing this data is provided in Appendix A. Analytical results from 18 soil samples collected in 1987 and 1990 were used to estimate the mass of GRO and benzene removed from the subsurface. All 18 soil samples were analyzed for benzene, however only 10 of the samples were tested for GRO. Using the available analytical data, average GRO concentrations in these samples was 43.67 milligrams per kilogram (mg/Kg) and average benzene concentrations in these samples was 5.72 mg/Kg. Using these averaged concentrations of GRO and benzene, Stratus has prepared a computation sheet estimating the mass of GRO and benzene removed from the subsurface by excavation. This computation sheet is provided in Appendix B. Based on this data, Stratus estimates that approximately 353.7 pounds of GRO and 46.3 pounds of benzene were removed from the subsurface during the 1990 excavation work. WATER QUALITY PROJECTIONS The highest concentrations of petroleum hydrocarbons are currently detected in well MW-12. Therefore, Stratus is assuming that the longest period of time for groundwater quality to reach WQGs under a post-remediation natural attenuation scenario would be in the vicinity of well MW-12. Installation of well MW-12 was completed shortly before initiation of ozone injection remediation, and thus long term groundwater monitoring trends under a steady state (non- remedial) scenario are not available. In order to evaluate the time needed for groundwater contaminants to degrade to WQGs, Stratus evaluated degradation rates of contaminants at nearby well MW-11, prior to performing OI, and then assumed that future contaminant degradation rates at MW-12 would be similar to the historical MW-11 data. The following table lists annual average GRO, benzene, and 1,2-DCA concentrations in groundwater at well MW-11 between 2008 and 2012 (2008 is the first year that data is available for MW-11). KAKnowles-Hammer Lane\Reports\knowles-hammer-supplemental-closure-info.doc