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ARCADIS GERAGHTY&MILLER <br /> Comment In paragraph 6 your letter states "PH /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 />' areal 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 nugrated through, <br /> or are capable of aqueous nugration 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 /> 1 5 <br />