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ERI 227903 R03 Former Unocal Service Station 5098,Stockton,California September 30, 1999 • Dissolved oxygenated compounds are not present in groundwater beneath and in the vicinity of the site, and separate-phase hydrocarbons are not present on the groundwater surface in the groundwater monitoring wells • With respect to the Unocal site, the dissolved hydrocarbon plume is delineated to the north, west, and east of the Unocal site by wells MW 10 and MW10A, MW 13, MW 14 and MW 14A, and MW 18 Benzene detected in well MW 15, at 1 1 ppb, does not appear to originate from the former Unocal site based upon historical groundwater flow direction and dissolved hydrocarbon concentration data collected from wells MW 10 and MW l0A • Groundwater in the upper portion of the water-bearing zone (30 to 50 feet bgs) beneath the Unocal site does not appear to be impacted by dissolved hydrocarbons (Plates 4 and 5) There is no shallow-zone impact to soil or groundwater beneath the former Unocal property • Dissolved hydrocarbons beneath the site appear to be restricted to the deep portion of the water- bearing zone (61 to 66 feet bgs) (Plates 6 and 7), and this area of hydrocarbon impacted groundwater is commingled with the Montgomery Ward plume downgradient of the site • The presence of dissolved hydrocarbons in groundwater within the deep portion of the water- bearing zone has not been evaluated beneath the Montgomery Ward site • Dissolved hydrocarbons decrease laterally downgradient of the site towards the Montgomery Wards property and the Delta irrigation well However, the vertical distribution of dissolved hydrocarbons beneath the site is inconclusive While reconnaissance groundwater samples collected from boring HP1 indicate a decrease to TPPHg and benzene concentrations with depth between 45 and 65 feet bgs, dissolved TPPHg concentrations increase with depth, based on results from the nested well MW 19S and MW 19D • The vertical distribution and extent of dissolved hydrocarbons detected in off-site nested wells MW21S and MW21D and MW22S and MW22D is inconclusive The source of dissolved hydrocarbons detected in groundwater samples collected from wells MW21S and MW21D is unclear without further investigation of the deeper zone beneath the Montgomery Ward property Dissolved hydrocarbons detected in nested wells MW22S and MW22D, located directly downgradient of the Montgomery Ward site (during operation of the Delta College well) may predominantly originate from the Montgomery Ward site • The source of dissolved hydrocarbons detected in well MW3 is inconclusive Well MW3 is screened from 47 to 67 feet bgs (across both shallow and deep zones) and may be impacted by shallow, deep, or combined shallow/deep hydrocarbons Additionally, hydrocarbon concentrations decrease from wells MW 11 to HP 1, but increase from HPI to MW3 • Dissolved hydrocarbons in groundwater in the vicinity of the site are part of a commingled plume originating from the Unocal and Montgomery Ward sites 6