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Wendy L. Cohen -7- 1 December 1993 <br /> 23. The SAP proposes to use gamma logging to identify the lithologies in Zones B and C. <br /> Comment. The standard logging rate and the time constant for the gamma logging should be <br /> specified in the SAP since they must be coordinated to get a good log, i.e., a sufficient <br /> number of counts must be recorded over the time constant. <br /> 24. The SAP states that if elevated readings are obtained from drill cuttings using a headspace <br /> vapor monitoring technique, a sample for chemical analysis will be taken at the next depth <br /> interval. <br /> Comment. The SAP should clarify the meaning of "elevated" levels. <br /> 25. The SAP states that a pumping test will be conducted in proposed MW 17 for 24 to 48 hours. <br /> Comment: In an unconfined aquifer, the cone of depression expands more slowly, so a longer <br /> pumping period compared to a confined aquifer is required to detect boundary conditions. <br /> Groundwater and Wells recommends pumping for 72 hours. <br /> SUMMARY <br /> 1. Chevron's monitoring wells should be included in the sampling program since some of them <br /> have contained PCE. <br /> 2. In Zone C, the extent of contamination, ground water flow direction and gradient, and <br /> hydraulic connection with Zone B need to be identified. The work plan must specify the <br /> activities to achieve these objectives. <br /> 3. The work plan should propose additional investigation and extraction wells in the Village <br /> Cleaner plume. The 1989 soil gas survey report stated that it was not clear whether the <br /> source of the PCE vapor was the sewer, contaminated soil, or contaminated ground water. <br /> The uncertainty in the PCE vapor source provides justification for more investigation in the <br /> identified source areas. Regardless of the source, the PCE vapor is a threat to water quality <br /> and must be remediated. <br /> 4. The proposal to install only one upgradient well in Zone A, MW-13, is inadequate. <br /> 5. An upgradient monitoring well in Zone B is needed. <br /> 6. Upon completion of MW-IOC, there will be only two monitoring wells in Zone C. Since the <br /> LVMD wells have been abandoned, another well in this zone is needed to allow determination <br /> of ground water flow direction and gradient. <br />