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AMB A WORK PLAN APPROVAL RESPONSE SJCEHD's September 22, 2003 work plan approval correspondence directs Shell to collect deep groundwater samples from proposed boring SB-19 SJCEHD is requesting that groundwater samples be collected from multiple depths below --90 feet below grade (fbg) in permeable zones beneath the suspected source area Shell and Cambria do not agree with this approach As explained in our work plan (August 28, 2003, Revised Site Investigation Work Plan) we have learned through past experience that assessing the vertical extent by dnilmg through shallow impacted groundwater can produce questionable data We have seen this type of problem at this site Groundwater samples collected using CPT groundwater sampling equipment in boring SB-4 contained 1,2-DCA concentrations of 1,640 parts per billion (ppb) at 72 fbg, and 141 ppb at 82 fbg (December 1999) These groundwater samples were collected in a coarse-grained unit that extents from approximately 70 fbg to 85 fbg Then we installed well 5-4-85, adjacent to the SB-4 location, and screened the same sand zone that was sampled in SB4 Quarterly groundwater sampling data from 54-85 shows 1,2-DCA concentrations ranging from 50 to 94 ppb It is likely the higher 1,2-DCA values detected in the groundwater samples from SB-4 are due to shallow impacted groundwater mixing with deeper water dunng sample collection We have not been able to find a sampling technique that can reliably seal off shallow impacted groundwater and keep it from cross contaminating groundwater samples collected at depth Until there is a method developed to do vertical assessment beneath the source area, we will not recommend vertical assessment in the source area The investigative approach proposed in our work plan was to assess deep coarse-grained zones outside of the source area and in a downgradient direction This was proposed to be protective of a water supply well located southeast of the site It is more important to assess the risk to the water supply well by assessing the downgradient extent of 1,2-DCA in deeper coarse-grained zones than to investigate the source area with methods that may produce invalid data SJCEHD suggest that by not investigating these deep zones beneath the source area while we are performing this phase of work is not cost effective We believe that by performing vertical assessment in the source area and producing invalid data is not cost effective and may result in additional investigative work(and cost)to prove or disprove the questionable data generated We ask that SJCEHD allow us to perform our investigation as proposed, without doing vertical assessment beneath the source area, which we have been directed to do We acknowledge that 0797 4