SITE INFORMATION AND CORRESPONDENCE 1995-2004

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0 0 James L. Tjosvold, P.E. August 19, 1996 Page 11 based on observations made in the field, there have been varying sampling depths and types of analyses performed. To date, there have been more than 60 borings drilled in Area I alone, some as deep as 100 to 200 feet bgs. Spatially, many of the soil borings are located within 10 feet of the next closest boring, thereby indicating very detailed analytical coverage of the site. Most of the borings had samples collected for laboratory analysis at depth. PG&E acknowledges that additional data is always desirable and the need for additional data will be assessed during future meetings with the agencies. The tables referred to in Section 4 have been revised to include total BTEX instead of total VOCs and were provided in Appendix A of the Evaluation of Potential Groundwater Impacts from Residues in Soil report. Figures 2-2 through 2-5 of the above report show the spatial distribution of all subsurface soil data for Area I. Tables 4-1 through 4-6 summarize soil data for Area I. The tables and figures are attached at the end of these responses. The extent of source and nonsource areas as questioned in comments 9 through 13 is discussed below. As discussed earlier, source area determination was not only made by reviewing soil analytical data but also by reviewing the known lateral and vertical extent of former MGP structures, field observations of MGP residue or residual organic liquid, and understanding where groundwater contains COPCs. PG&E believes that source areas 1 through 3 (Sl-S3) have been sufficiently defined as discussed below. The northern and western edges of S4 have not been defined as well. Modeling of the existing data has suggested that with the present definition of S4, there will be minimal to no degradation of groundwater after treatment of S4. It should also be noted that depending on the type of soil remedy finally chosen for the site, S4 may be further refined during implementation of the remedial action. According to the National Contingency Plan, the purpose of the Remedial Investigation is to collect data necessary to adequately characterize the site for the purpose of developing and evaluating effective remedial alternatives. It is PG&E's position that this has been completed. Comment: 9. Pages 4-3 and 4-6, Section 4.2.1.1, Potential Source Area 1 (S1) The FS states that S1 is characterized by borings MW-9, MW-19, SB-3B, SB-2-95, SB-3-95, SB-4-95, and SB-23-95; boring logs show that concrete or wood barriers were encountered at depths of 15-23 feet bgs; PNAs were detected at significant levels at 24 feet bgs in MW-9 and at 19.5 feet bgs in SB-4-95; and chemical data are summarized in Table 4-1.