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Geomatrix The geometric mean of TPHg concentrations in the zone with concentrations greater than 100 mg/kg is 200 mg/kg and the estimated mass of residual TPHg in that zone is about 440 kg The geometric mean of TPHg concentrations in the zone with concentrations less than 100 mg/kg is 3 mg/kg and the estimated mass of residual t TPHg in that zone is about 40 kg Based on these assumptions, the estimated mass of TPHg remaining in soil at the Site should be less than 480 kg i TPHg Mass in Groundwater The approximate extent of TPHg in groundwater is illustrated on Figures 2, 11, and 12 Assuming the same volume for affected groundwater as was estimated for affected soil and a soil porosity of 30 percent, approximately 2 6 x 106 liters of groundwater are affected The average concentration of TPHg in groundwater collected from the affected wells during 2005 was 3,780 micrograms per liter (µg/L) Based on those values, the estimated mass of dissolved-phase TPHg remaining in groundwater at the Site in 2005 was about 10 kg 16 Risk Assessment. The Site does not present potential risks to ecological receptors because it is located 1,000 feet hydraulically downgradient of the Stockton Deep Water Channel In addition, the extent of the plume is limited to less than 200 feet in length and the San Joaquin River is located approximately 4,000 feet hydraulically downgradient of the Site The residual petroleum hydrocarbons remaining in soil and groundwater at the Site could present potential risks to human health and safety The Site data are compared to nsk-based regulatory criteria for this evaluation The Site is located within the central portion of the Port within an area of heavy industrial uses The ground surface above the affected soils and groundwater is mostly covered with concrete or asphalt pavement and the Site is not easily accessible to the general public Shallow groundwater has no current or anticipated future beneficial use in the Site area Based on this information, the two potential human health risk scenarios at the Site are exposures to construction/utility trench workers and vapor intrusion to buildings The soil and groundwater data from the Site are compared with the Environmental Screening Levels (ESLs) developed by the RWQCB — San Francisco Bay Region(SFRWQCB, 2005) for this evaluation The soil ESLs for direct exposures to construction/trench workers (Table K-3, SFRWQCB, 2005) are 16 mg/kg for benzene, 400 mg/kg for ethylbenzene, ' 650 mg/kg for toluene, 420 mg/kg for xylenes, and 6,000 mg/kg for TPHg Analytical results of the soil samples collected from the Site are less than these t ESLs In addition, field screening results of soil samples during previous site investigations did not indicate the presence of petroleum hydrocarbons in soil within the top 10 feet of the subsurface and BTEX and TPHg were not detected in the soil samples collected at about 10 feet bgs from the borings Field screening included measurement of total VOCs in the headspace of the soil samples using a photoionization detector and examination of the soil samples for ` discoloration/staimng and/or petroleum odor Based on these data, the residual I 16000x16437 0001Archive16437-041 doc 5