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ARCADIS GERAGHTY&MILLER <br /> Comment: In paragraph 6, your letter states, "PHS/EHD questions the <br /> definition of a confining layer being 3 feet thick ..." <br /> Response: It was not the intent of ARCADIS Geraghty and Miller to indicate <br /> that encountering a 3 foot thick confining unit was evidence of a <br /> continuous area-wide confining layer. It is not known whether such <br /> a layer does or does not exist. Rather, the reason that drilling will <br /> be halted when encountering such a confining layer is that the layer <br /> might constitute a barrier to vertical migration of site constituents <br /> which could be compromised by drilling though. ARCADIS <br /> Geraghty and Miller will evaluate all available data to assess the <br /> area] extent of encountered potential confining layers. A soil <br /> sample will be collected within the base of any encountered 3-foot <br /> confining zone and analyzed for soil moisture and site constituents <br /> of interest, along with all other available relevant site data, to <br /> determine whether petroleum hydrocarbons have migrated through, <br /> or are capable of aqueous migration through, the confining zone at <br /> the point of sampling. <br /> SJCPHS <br /> Comment: In paragraph 7, your letter states, "Collection of groundwater <br /> samples at a minimum of 10 foot intervals from first encountered <br /> water to 50 feet is acceptable. After that depth samples should be <br /> collected at changes in lithology or a maximum of 10 foot <br /> intervals." <br /> Response: The comment is acknowledged and samples will be collected as <br /> suggested in the comment. <br /> Comment: In paragraph 9, your letter states that if MTBE is detected in any <br /> samples using EPA method 8020, they should be reanalyzed for a <br /> specified suite of compounds using EPA method 8260A. <br /> Response: The comment is acknowledged and samples will be collected and <br /> analyzed as suggested in the comment. <br /> Comment: In paragraph 10, your letter states that "the size of hollow stem <br /> auger to be used for drilling should be specified. <br /> Response: The specified size of the hollow stem auger is 8 1/4 inches outside <br /> diameter. <br /> At the direction of UNOCAL, ARCADIS Geraghty and Miller has addressed <br /> the comments in your letter cited above and submits this document as an addendum to <br /> the work plan also cited above. If you have any questions or comments regarding this <br /> 5 <br />