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0`- Geomatrix <br /> petroleum hydrocarbons remaining in soil at the Site are not expected to present <br />' significant risks to construction/trench workers <br /> The soil ESLs for evaluation of potential vapor intrusion concerns—commercial or <br />' industrial exposures (Table E-lb, SFRWQCB, 2005) are 0 51 mg/kg for benzene, <br /> 390 mg/kg for ethylbenzene, 310 mg/kg for toluene, and 420 mg/kg for xylenes <br /> Except for benzene in three samples, analytical results of soil samples collected <br />' from the Site are less than these ESLs The benzene concentrations in three soil <br /> samples collected in 1998 from about 16 to 20 feet bgs from borings B3-1, B3-2, <br /> and B3-3 (former UST area) ranged from 1 1 to 8 4 mg/kg and exceeded the ESL <br />' However, given the natural biodegradation of petroleum hydrocarbons, these <br /> concentrations likely do not represent current Site conditions In addition, these <br /> three soil samples were collected from the saturated zone, no BTEX or TPHg were <br />' detected in soil samples above these sample depths, and no buildings are located <br /> directly above these sampling locations As such, the residual petroleum <br /> hydrocarbons remaining in soil at the Site are not expected to present significant <br />' risks to vapor intrusion <br /> The groundwater ESLs for evaluation of potential vapor intrusion concerns — <br />' commercial or industrial land use, high-permeability vadose-zone soil type <br /> (Table E-la, SFRWQCB, 2005) are 1,800 gg/L for benzene, 170,000 gg/L for <br /> ethylbenzene, 530,000 µg/L for toluene, and 160,000 µg/L for xylenes Except for <br /> two groundwater samples collected from monitoring well MW3-6, analytical results <br /> of groundwater samples collected from the monitoring wells at the Site since 1999 <br /> are less than these ESLs The benzene concentrations in the two groundwater <br /> samples collected from well MW3-6 in 2003 and 2004 reportedly were 1,800 and <br /> 1,890 gg/L, respectively However, utilizing the 14 samples collected from this <br />' well to date, the 95% upper confidence limit of the mean benzene concentration in <br /> this well is 1,340 µg/L Based on these data, the residual petroleum hydrocarbons <br /> remaining in groundwater at the Site are not expected to present significant risks to <br /> vapor intrusion <br /> 17 Fate and Transport Modeling. Extensive soil and groundwater data have been <br />' collected from the Site for evaluation of site conditions Because the hydrogeology <br /> at the Site is well understood and sufficient data(7 years of groundwater monitoring <br /> data to date) are available to demonstrate the stability of the plume, analytical or <br />' numerical modeling has not been performed to evaluate the fate and transport of <br /> residual petroleum hydrocarbons in the subsurface of the Site <br />' 18 Conditions Remaining at the Site. Based on evaluation of the 7 years of available <br /> data, the conditions remaining at the Site will not adversely impact water quality <br /> outside of the current extent of affected groundwater(see Section 3 2 1), human <br />' health and safety (see Discussion#16 in Section 2 0), or other beneficial uses (see <br /> Section 3 2 3) <br /> 19 List of Technical Reports. The list of technical reports is included in Section 5 0 <br /> 1 16000x16437 OOO1Archivek6437 041 doc 6 <br />