SITE INFORMATION AND CORRESPONDENCE 1991-1992

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Mr. Robert C. DossWE. • Page 2 June 29, 1990 archived. Provisions should be made to archive soil cores which are representative of site lithology and for reanalysis of non-volatile. contaminants. An archive of representative soil cores retained in whole would be a valuable resource if the need to re-evaluate either lithology or soil contaminants should occur. Borings SB-3B, B-2, B-3, B-4 , B-5, and B-8 contained lithologic sediments which seem to be continuous across the site. Offsite soil borings will be needed to evaluate offsite soil contaminant levels and to determine if the continuous sediments found in onsite borings extend offsite. The soil boring data collected regarding lithology, soil contaminant levels at depth, and saturated soils encountered should be compiled into stratographic cross sections. These cross sections are needed to illustrate the vertical and horizontal extent of soil contamination and indicate the saturated zones encountered during drilling activities. The site should also be contoured in terms of location, depth, and relative range (high, medium, low) of contaminants found. Several of the soil boring logs indicated the presence of "white, chalky material and residues in voids" . Any suspicious or unknown materials should be analyzed for the presence of contaminants. The areas where these questionable materials are found should be identified on a site map. Offsite soil borings should be planned in areas that represent unaffected or "background" soil conditions. Soil samples should be collected and analyzed at various depths as a means to establish soil background concentrations for metals, PAH, . arsenic, and other contaminants. Ground water Investigation The Site Characterization Report (Report) indicates that the permeability and porosity of nine soil samples were determined. The results of these analyses and a discussion of the Darcy velocity and pore velocity were briefly mentioned in the Report. This information can be used to extrapolate the extent of the ground water contamination plume, although the Report did not attempt to do so. PG&E must eventually define the vertical and horizontal extent as well as the migration velocity of the ground water contamination plume migrating from the site. An acceptable method would be to construct cluster wells screened at various depths and the performance of aquifer tests. However, aquifer testing should be postponed until the need for a ground water extraction system has been determined and prior to its design.