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Site Assessment Report 1022 Frontage Road CAM B R 1 A Ripon, California December 16, 2005 the former UST cavity Groundwater samples were collected at 30, 45, and 60 ft bgs MTBE decreased with depth while diesel-range hydrocarbons increased with depth Groundwater analytical results are presented in Table 2 Fourth quarter 2005 groundwater analytical results and hydropunch analytical results from the current investigation are presented on Figure 3 " The MTBE plume is adequately defined in all directions Additional vertical and lateral assessment is Vm needed to determine the cause and extent of the deeper dissolved-phase diesel range hydrocarbons These deeper dissolved-phase petroleum hydrocarbon concentrations are possibly related to a lower historical groundwater table related to past groundwater production from the onsite truckwash supply well(DW-1)or other groundwater supply well(s)in the vicinity A City of Ripon groundwater supply well is located 750 ft west of the site Additional assessment is also needed to determine the source and extent of diesel range SPH present in well MW-6, and the extent of diesel impact related to recently identified release from the waste oil tank piping and/or the current underground diesel piping 6 0 CONCLUSIONS AND RECOMMENDATIONS Based on the results of this investigation, the extent of TPHd is not fully defined Further investigation is needed to delineate the recently identified release of TPHd related to the former waste oil tank piping and the underground diesel piping MT-86- At this time,Cambria recommends continued groundwater monitoring of the existing well network to establish concentration trends in the newly installed well MW-10 However, Cambria proposes a simplified groundwater monitoring and sampling schedule for future events We recommend that wells MW-1 through MW-3,MW-8 through MW-10,and DW-1 be sampled quarterly for TPHg and TPHd by EPA method 8015M, BTEX by EPA method 8021B, and fuel oxygenates [MTBE, di- isopropyl either(DIPE),tertiary-butyl alcohol(TBA),tertiary-amyl methyl ether(TAME),and ethyl tertiary-butyl ether(ETBE)]by EPA method 8260B,since these wells are located in or adjacent to the source area Based on the absence of any hydrocarbons or fuel oxygenates,we recommend that wells MW-4,MW-5,and MW-7 be sampled annually during the 1St quarter of the year for TPHg and TPHd by EPA method 8015M, and BTEX and MTBE by EPA method 8021B Cambria will continue to monitor groundwater Ievels in all wells on a quarterly basis Our proposed sampling schedule is provided below i 10