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ERI 227903MI.L19 Former Unocal Service Station 5098,Stockton,Califomia September 30, 1999 <br /> Laboratory analysis of oxygenated compounds for groundwater samples collected from wells MW19S <br /> &D and boring HPI, as directed in the County's November 20, 1998 letter (Attachment B), was <br /> addressed in ERI's Addendum to the Work Plan, dated December 9, 1998. <br /> As stated in the December 9, 1998 submittal to the County, ERI contacted the County on <br /> December 7, 1998, and stated that: <br /> "Because results of groundwater samples collected near former potential sources did <br /> not detect oxygenated compounds, and given the fact that the tanks were removed in <br /> July 1985 (i.e., prior to Unocal's use of oxygenated compounds in California), no <br /> further analyses of groundwater for oxygenated compounds will be performed." <br /> The County approved the Addendum to the Work Plan in a facsimile, dated December 11, 1998 <br /> (Attachment C), accepting that groundwater samples would not be analyzed for oxygenated <br /> compounds. <br /> Regarding the County's comment referencing the deep groundwater quality in the area; the most recent <br /> investigation presents results from three deep groundwater monitoring wells (MW19D, MW21D, and <br /> MW22D) including soil analytical results. Further recommendations are included in the addendum. <br /> "Soil boring B-2 was placed north of the highest groundwater plume petroleum concentration <br /> area for the purpose of delineating the lateral and vertical extents of the contamination. Although <br /> soil data was available, PHS-EHD was unable to locate groundwater data for B-2 and is <br /> concerned that this latest phase of subsurface investigation may not have completed its task to <br /> define the extents of the groundwater plume." <br /> In the November 20, 1999 letter (Attachment B), the County requested that Unocal perform an <br /> additional soil investigation north of groundwater monitoring wells MWI l and MW12. In response to <br /> the County's request, ERI's Addendum to the Work Plan (ERI, December 9, 1998)proposed that two <br /> soil borings (B1 and B2 [Plate 1]) will be drilled north of wells MW l l and MW 12 to evaluate soil <br /> conditions in this area. ERI further indicated that laboratory analysis of soil samples collected from <br /> these borings would include analysis for oxygenated compounds. Neither the Addendum to the Work <br /> Plan (ERI, December 9, 1998) nor the County's approval facsimile (Attachment C) implied that <br /> groundwater samples would be collected from these borings. <br /> It is the opinion of ERI that the location of the ten groundwater monitoring wells (MW10 and IOA, <br /> MWll through M13, MW14 and 14A, MW18, and MW19S and 19D) located on or adjacent to, and <br /> upgradient, crossgradient, and downgradient of the boundaries of the former Unocal site (Plate 1), <br /> provide ample coverage for delineation of hydrocarbon impacted groundwater beneath the site. <br /> "Existing directives for this site include quarterly monitoring with reports due at PHS-EHD <br /> before the next sampling event, all reports to have professional conclusions and recommendations, <br /> and all seven (7) oxygenates sampled quarterly for all monitoring wells and the San Joaquin Delta <br /> College Irrigation Well." <br /> Unocal will communicate the County's request for conclusions and recommendations, as well as the <br /> submittal date requests to Gettler-Ryan, Inc. (the consultant who performs quarterly groundwater <br /> monitoring and sampling activities at this site on behalf of Unocal). Regarding the continued request <br /> 2 <br />