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' GRANITE CONSTRUCTION FACILITY <br /> WKA No 2088 08 <br /> August 18, 1999 <br />' Page 13 <br /> • The onsite industrial supply wells are not used as a drinking water supply In <br /> addition, the nearest irrigation well is located approximately 2,600 feet southeast <br /> tof the site <br /> • MTBE has not been detected in groundwater beneath the site <br />' Based on approximately 700 cubic yards of contaminated soil removed from the former UST <br /> excavation, results of laboratory analysis of soil samples obtained from the bottom and sidewalls <br />' of the excavation, and low to nondetectable concentrations of petroleum hydrocarbons and <br /> oxygenates in groundwater, it is our opinion that subsurface soil poses no significant threat to <br />' groundwater quality onsite or offsite We base this opinion on historical groundwater <br /> concentration data obtained from the wells and mass volume calculations of petroleum <br />' hydrocarbons remaining in soil and groundwater beneath the site <br /> Recent past groundwater samples obtained from the wells indicate that petroleum hydrocarbons <br />' and oxygenates are essentially nondetectable in all wells with the exception of low <br /> concentrations ofTPH as gasoline and Tert-Butanol in MW3 only Benzene has not been <br /> detected in the water samples since January 9, 1998 <br /> The highest concentrations of TPH as gasoline has consistently been reported in well MW3 <br /> located north of the former UST excavation and does not appear to be increasing in the <br />' downgradient well (MW2) Furthermore, water samples obtained from wells MW1 and MW2 <br /> have not shown detectable concentrations of petroleum hydrocarbons and oxygenates since <br /> January 1997 Based on results of groundwater samples obtained over several past quarterly <br />' sampling events, the plume does not appear to be migrating and that as a result, some degree of <br /> natural attenuation of the contaminant plume is most likely occurring beneath the site <br /> Potential sensitive receptors in the form of offsite wells will likely not be impacted by <br />' contaminated groundwater beneath the site based on the removal of the former USTs, removal of <br /> contaminated soil, and the attenuation of dissolved-phase petroleum hydrocarbons in <br />' groundwater Based on the likelihood that sensitive receptors will not be impacted, we <br /> recommend that SJCPHD require no further action as related to the underground storage tanks at <br /> this site Furthermore, we recommend that the CVRWQCB be notified of our findings Upon <br />' concurrence for site closure by the SJCPHS and the CVRWQCB, we recommend that the eight <br /> IP <br /> DDDD°� <br /> 1 WALLACE KUHL <br /> Reryeled pope. 6 ASSOCIATES INC <br />