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Abdelwahed Facility, Ultramar Site No. 13792 May 9, 2013 <br />Stockton California NFAR Re nest and Well Destruction Work Plan <br />SG -2. These results indicate that the risk of vapor intrusion and inhalation to the human health <br />of workers in the existing site building and potential future site buildings are below levels of <br />concern. Although the analytical results for sample SG -2 indicate TPHg concentrations <br />exceed the residential ESL, the location of the sample and the intervening locations of <br />samples SG -2, SG -3 and SG -4 indicate that soil gas conditions at the SG -2 location are not <br />causing vapor intrusion conditions that exceed levels of concern near the building. The <br />petroleum hydrocarbon concentrations in soil gas detected during this investigation can be <br />assumed to have been in equilibrium and can be expected to attenuate with time. Oxygen <br />concentrations were detected at 16 to 18% indicating a well -oxygenated bio -attenuation zone. <br />REGRESSION ANALYSIS <br />Source area wells (including former well MW -I) indicate decreasing TPHg, benzene, and <br />DCA concentrations over time. Groundwater monitoring data indicates that the current <br />constituents of concern (COC) at the site are TPHg, benzene, and DCA. Historical <br />groundwater concentration data for the COCs from former well MW -1 were used to estimate <br />conservative attenuation rates for the COCs as shown on Graph I. The calculated attenuation <br />rates for the CDCs are 0.11 year' for TPHg, 0.25 year' for benzene, and 0.24 year' for DCA, <br />as shown on Graph 1. <br />Fourth quarter 2012 monitoring data continue to indicate a stable groundwater hydrocarbon <br />plume at the margins of the plume. The maximum concentrations of the COCs reported <br />during the most recent fourth quarter 2012 monitoring data were used to estimate the time to <br />reach WQOs. The maximum concentrations of dissolved CDCs were detected in wells MW - <br />13 (5,200 ppb TPHg) and MW -6 (12 ppb benzene and 24 ppb DCA). The following first <br />order attenuation equation was used to estimate the time to reach WQOs: <br />t = -in [Cgoal / Cstart] / k <br />In the above equation, t is the time (years) required for the COC to attain the WQO, Cgoal is <br />the chemical -specific WQO concentration, CST,,,z is the maximum concentration of the COC <br />detected during the fourth quarter 2012, and k is equal chemical -specific k values calculated <br />from former well MW -1. As shown in the table below, the calculated time estimates to reach <br />WQOs were: 36 years for TPHg, 10 years for benzene, and 16 years for DCA. <br />COC <br />well used <br />Cstart <br />Coal <br />k time <br />TPHg <br />MW -13 <br />5,200 ppb <br />100 ppb <br />0.11 year 36 years <br />benzene <br />MW -6 <br />12 ppb <br />1.0 ppb <br />0.25 year 10 years <br />DCA <br />MW -6 <br />24 ppb <br />0.5 ppb <br />0.24 year -116 years <br />NO FURTHER ACTION REQUIRED REQUEST <br />In an e-mail dated 25 January 2013, the EHD requested that the results of this investigation be <br />compiled, that project data be checked against the requirements of the Regional Water Quality <br />1792 NFAR rpt.doc <br />Project No. 1792.26 7 HORIZON ENVIRONMENTAL INC. <br />