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1 <br /> ERI 227903 R03 Former Unocal Service Station 5098,Stockton,Caltforma September 30, 1999 <br /> • It appears residual hydrocarbons that may remain beneath the site are located within the water- <br /> bearing zone at depths ranging from approximately 49 to 65 feet bgs, and at concentrations ranging <br /> from 16 ppm to 740 ppm TPPHg(Attachments C and F) <br /> Groundwater Conditions <br /> Results of laboratory analyses of reconnaissance groundwater samples collected from boring HP1 are <br /> provided in Attachment G Three discrete reconnaissance groundwater samples, collected from <br /> approximately 34, 45, and 63 feet bgs in boring HPI, were submitted for laboratory analyses The <br /> results of second quarter 1999 groundwater monitoring and sampling of existing and newly installed <br /> monitoring wells are provided in Attachment B <br /> Results of laboratory analyses of groundwater samples collected from wells MW10 and 10A, MW13, <br /> MW14 and 14A, MW16 through MW18, and MW21S did not detect dissolved gasoline hydrocarbons <br /> at or above laboratory method detection limits TPPHg and benzene were detected in groundwater <br /> samples collected from wells MW 11, MW12, MW19D, MW21D, MW22S, and MW22D ranging from <br /> 498 parts per billion (ppb) to 23,000 ppb TPPHg, and 6 5 ppb to 6,000 ppb benzene Additionally, <br /> benzene was detected in the groundwater sample collected from well MW15 at 1 1 ppb, and <br /> concentrations of toluene (1 1 ppb) and total xylenes (1 4 ppb) were detected 1n the groundwater <br /> sample collected from well MW 19S <br /> ERI evaluated dissolved TPPHg and benzene distribution in groundwater across three depth intervals <br /> within the water-bearing zone using the second quarter 1999 groundwater monitoring and sampling <br /> data from both the Unocal (GRI, July 6, 1999) and Montgomery Ward (Environmental Audit, Inc , <br /> July 14, 1999) sites The shallow interval was evaluated using dissolved TPPHg and benzene <br /> concentration data from groundwater samples collected from wells screened from approximately 30 to <br /> 50 feet bgs The iso-concentration maps for the shallow interval are provided on Plates 4 and 5 <br /> The deep interval was evaluated using dissolved TPPHg and benzene concentration data from <br /> groundwater wells screened from approximately 61 to 66 feet bgs The iso-concentration maps for the <br /> deep zone are provided on Plates 6 and 7 <br /> Plates 8 and 9 present iso-concentration maps using TPPHg and benzene concentration data from <br /> samples collected from wells screened near to, or across, some portion of both the shallow and deep <br /> intervals Please note that since the iso-concentration contours provided on Plates 8 and 9 are based <br /> upon analytical results from samples collected from wells screened near to, or across, some portion of <br /> both the shallow and deep intervals, the iso-concentration data may be reflecting data from both zones <br /> CONCLUSIONS <br /> Based upon review of the second quarter 1999 groundwater monitoring and sampling data from the <br /> existing and newly installed wells at both the Unocal and Montgomery Ward sites, reconnaissance <br /> groundwater data from boring HPI, and the iso-concentration maps, ERI's conclusions are as follows <br /> 5 <br />