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<br /> been delineated The vertical extent of the hydrocarbon impact is the underlying groundwater
<br /> aquifer Concentrations of adsorbed-phase hydrocarbons detected in the soil were compared to
<br /> I the EPA Region 9 PRGs The soil samples collected from the south and southeast walls of the
<br /> UST excavation at depths between 14 and 19 feet below ground surface (bgs) exceeded the
<br /> Industrial PRGs for benzene (1 5 mg/kg), toluene (520 mg/kg), and xylenes (210 mg/kg) The
<br /> benzene PRG was also exceeded in borings SB-2 (46 feet bgs) and SB-13D (46 5 feet bgs)
<br /> TPH-G concentrations detected in these soil samples ranged from 10 3 to 17,600 mg/kg
<br /> 3.0 GROUNDWATER ASSESSMENT ACTIVITIES
<br /> Groundwater monitoring has been conducted at the site since June 3, 1993 The latest
<br /> groundwater monitoring event (Second Quarter 2001) was conducted on May 9 - 10, 2001
<br /> There are currently seventeen groundwater monitoring wells associated with this site Monitoring
<br /> wells MW-1, 2, 3, 3A, 31), 4, 5, 6, 7, 8, 9, 10,11,12, 13, and 13A were installed in the shallow
<br />` aquifer with various screen intervals from 39— 59 feet to 64 - 90 feet bgs Monitoring well MW-
<br /> 13B appears to be installed in a deeper aquifer zone with a screen interval from 116 to 121 feet
<br /> bgs
<br /> 3.1 Summary of Groundwater Conditions
<br /> • Interpretation of the momtornng well gauging data indicates that there is a south to southwesterly
<br /> hydraulic gradient across the site (Figure 6) A review of the groundwater level data at the two
<br /> cluster well points (MW-3, MW-3A, MW-313 and MW-13, MW-13A, MW-1313) indicates that
<br /> the shallow aquifer appears to be at least partially isolated from the deeper aquifer Water level
<br /> data from monitoring well MW-13B, screened in the deep aquifer, shows a consistently lower
<br /> groundwater elevation than in wells MW-I3 and MW-13A There appears to be cychcal water
<br /> level fluctuations at the site due to weather and proximity of the site to the Stockton Diverting
<br /> Canal Historical groundwater elevations are summarized in Table 2 Graphical presentation of
<br /> the data is included in Appendix B
<br /> Groundwater at the site has been sampled and analyzed for gasoline range total petroleum
<br /> hydrocarbons (TPH-G), benzene, toluene, ethylbenzene, and xylenes, and gasoline oxygenates
<br /> Methyl t Butyl Ether (MTBE), t-Butanol (TBA), Di-isopropyl Ether (DIPE), Ethyl-t-Butyl Ether
<br /> (ETBE), t-Amyl Methyl Ether (TAME), 1,2-Dichloroethane (1,2-DCA), and Ethylene Dibronude
<br /> (EDB) A review of the groundwater analytical data indicates that the dissolved-phase
<br /> hydrocarbon constituent concentrations of benzene, toluene, ethylbenzene, xylenes, and TPH-G
<br /> have generally decreased or stabilized in all momtormg wells except MW-13 The fluctuations in
<br /> MW-13 may be the result of soul impacts in the former tank pit leaching to groundwater
<br /> it MTBE and 1,2-DCA)Two dissolved-phase oxygenate constituents ue { ) have been detected in the
<br /> groundwater MTBE concentrations were detected in monitoring wells MW-3, 3A, 313, 11, and
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