SITE INFORMATION AND CORRESPONDENCE 1993-1995

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James L. Tjosvold, P.E. February 23, 1995 Page 3 • Limit VOC analysis to benzene, toluene, ethyl benzene, and xylene (BTEX) because these are the only VOC compounds that have been detected in any of the Zone A wells (other than a one time detection of tert-butyl-methyl ether in MW-2). • Add a one time analysis (in March, 1995) for barium and antimony to support the feasibility study. The concentration of these compounds exceeded the MCLS in a previous sampling event. Additional sampling is needed to determine if these can be attributed to upgradient concentrations. • Add a one time analysis of cyanide for MW-9 because this constituent exceeded the MCL in a previous sampling event. In summary, on-site wells MWA, MWA and MW-8 would be deleted from the sampling program. PG&E would monitor new wells MWAR and MW-18 quarterly, and existing perimeter wells MW-2, MW-3, MW-7, MW-9, MW-10, MW-11 and MW-12 annually. Water levels would be measured quarterly in all wells. The proposed revised program is presented in Table 1. Zone B: The Workplan specifies that samples from Zone B wells will be analyzed quarterly for PNAs and VOCs. The rationale was to evaluate concentrations of chemicals previously detected (PNAs) and confirm previous results (no VOCs present). PG&E recommends the following revisions to the monitoring program for Zone B wells: • Delete MW-5 from the sampling program. MW-5 is located in the center of the site and is surrounded by five Zone B wells. Concentrations in the Zone B wells are similar and this well is not necessary to characterize the nature and extent of constituents in the groundwater. • Reduce the frequency of sampling from quarterly to annually in the Zone B wells installed prior to the Phase III investigation (MW-1D and MW-6). These wells have been sampled eight and seven times respectively and PNA and VOC concentrations have been fairly consistent in these wells. Annual monitoring will be sufficient to monitor temporal changes in groundwater quality. • Limit VOC analysis to BTEX and add a one time analysis for antimony and barium for the reason stated for the Zone A wells.