ARCHIVED REPORTS XR0009991 - Laserfiche WebLink

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i URS Mr Mike Infurna Page 4 San Joaquin County Environmental Health Department June 6, 2003 UST complex Relatively low concentrations of TPHg at MW-6 suggest that significant down-gradient migration of impacted groundwater has not occurred Benzene concentrations in groundwater are primarily located in the former UST and dispenser island areas ' An iso-concentration contour map of benzene concentrations in groundwater, as reported during the December 2002, groundwater monitoring event(Table 2) is presented as Figure 7 Figure 8 illustrates the vertical and lateral extent of the dissolved benzene concentrations at the site The maximum benzene )Lr concentrations reported in groundwater have been in vapor extraction wells VW-8 and VW-10, which are � Y located approximately 20 feet south-southeast of the dispenser islands The first groundwater sample collected from VW-10 in December 2002 had a reported benzene concentration of 2,800µg/L A block diagram of the dissolved benzene plume is presented as Figure 9 This figure shows that the highest dissolved benzene concentrations are immediately south of the western dispenser island and the lateral extent of the benzene plume has not moved off-site An MtBE iso-concentration map for the December 2002 groundwater analytical details presented as Figure . 10 Figure 11 depicts vertical and lateral extent of dissolved MtBE in groundwater at the site Dissolved concentrations of MtBE have migrated off-site at the southeastern site boundary Concentrations of MtBE up to 8 86�tg/L have been reported in off-site well MW-6,which is located downgradient from the pump islands and UST complex However,MtBE has not been detected in MW-6 since September 1999, suggesting that MW-6 currently delineates the downgradtent extent of the dissolved MtBE above laboratory reporting limits A block diagram of the dissolved MtBE plume is presented as Figure 12 This figure illustrates that the highest MtBE concentrations (20 Rg/L) are located beneath the former UST complex, indicating that the MtBE plume is stable and has not migrated significantly This is likely the result of the relatively fine-grained lithology of the site(Figure 3)and the relatively low historical hydraulic gradients across the site(Table 4) Based on the minimal amount of plume migration since groundwater monitonng began in 1995,groundwater impacts at the site do not present a threat to the two municipal supply wells located approximately 1,300 and 1,600 feet from the site CONCULSIONS Information was requested from the CSPWD to detem-une the locations of municipal production wells relative to the site Current groundwater and historical soil analytical data were complied and presented in tabular and graphical formats The following conclusions result from the above investigative work • Two active groundwater production wells are present within a 2,000-foot radius from the site These wells are screened at various depths ranging from 168 to 600 feet below ground surface Because the area of impacted groundwater is relatively stable and shallow and the two municipal wells are located K 1Wpmces.US BP ARCW21IMite Conceptual Model Rerwt12130 Site Conceptual Madel doc