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ARCADIS GERAGHTY&MILLER <br /> Work Plan Comments by SJCPHS <br /> SJCPHS <br /> Comment: In paragraph 3, your letter states, "...the depth to groundwater at <br /> the time of the release(s) is not known." <br /> Response: ARCADIS Geraghty and Miller will work with the SJCPHS, the <br /> California State Department of Water Resources (DWR), and other <br /> relevant agencies, to identify records on the elevation of, or depth <br /> to, shallow groundwater relevant to the site between the dates of <br /> the first installation of the UST's, or the date of the UNOCAL's <br /> site lease, and the present. <br /> SJCPHS <br /> Comment: In paragraph 4, your letter states, "The investigation should not be <br /> limited to current dissolved-phase hydrocarbons. The potential for <br /> future releases to the groundwater from contaminated soil should <br /> also be investigated by sampling of the soil above and below current <br /> groundwater until the vertical extent of soil contamination has been <br /> demonstrated." <br /> Response: ARCADIS Geraghty and Miller respectfully disagrees with the need <br /> to, or the appropriateness of, sampling soils beneath groundwater. <br /> It is our experience at many sites in numerous locations that <br /> delineation of the vertical extent of impact to groundwater can be <br /> accomplished using the technique described in the workplan. <br /> Further, the method indicated in the work plan will go beyond <br /> allowing the assessment of potential for future groundwater impacts <br /> from soils. It will allow a real-time assessment of the actual <br /> impacts of groundwater on an ongoing basis. <br /> Sampling of soils in the saturated zone will provide data on the <br /> total amount of constituents sorbed onto the soil particle surfaces <br /> and that which exists in the dissolved phase. <br /> A quantified assessment of the future potential for groundwater <br /> impacts based on data provided from a saturated soil sample <br /> requires an investigator to make a calculation which partitions the <br /> total amount reported in the sample analysis into the amount sorbed <br /> and the amount in the sample which exists in the pore liquids as a <br /> dissolved fraction. Next a calculation must be made which relates <br /> the constituent amount sorbed to the amount that is desorbed and <br /> the rate at which the desorption (into the dissolved phase) occurs. <br /> Next, the rate of advection in the moving groundwater must be <br /> identified. Next, the rate of migration of a constituent must be <br /> related to the rate of movement of site groundwater by a <br /> 2 <br />