SITE INFORMATION AND CORRESPONDENCE_2004-2014

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Mr. Marshall Cloud * - 3 - • 25 February 2008 Defense Logistics Agency frequency of groundwater sampling. For consistency and Remedial Program Manager consensus, a valid scientific method or methods need to be defined, utilized, and presented in the Final Report. The method(s) should consider the rate of COC concentration changes and the magnitude of concentration variability observed. Preferred Well. The Draft Report references "preferred well"in Section 5.3.2 on page 5-3, and in Table 5.3-1 on pages 5-7 through 5-36. In Section 5.3.2, the Draft Report states the following: "If there are multiple wells that may provide similar information, a `preferred"well will be selected for sampling and the others will not be sampled." Regional Water Board staff agrees that, where nearby wells are screened in the same water- bearing zone, some of them may be sampled at a reduced frequency if it is demonstrated that they are providing redundant data. However, the Draft Report does not provide such demonstrations. The Final Report must present the sets of "multiple wells"from which each "preferred well"was selected, define the method used to select the preferred wells, and demonstrate that the wells not selected provide redundant data only. For consistency and Remedial Program Manager consensus, a valid scientific method needs to be selected, defined, and presented. The method results should be presented for each of the site's Groundwater Monitoring Program wells listed on Table 5.3-1. TREATMENT PLANT PERFORMANCE AND COMPLIANCE COMMENTS Section 5.2.2 of the Draft Report identifies three places where COC plumes may not have been captured during the year: 1) "The Central Area TCE plume in the A Zone was potentially not captured". 2) " ... it is uncertain that the southern portion of the TCE and PCE plumes in the South Balloon are being captured because of a southwesterly gradient." 3) "... in the C Zone of the Central Area west of the depot boundary, the TCE plume may not have been fully captured, ... " Continued monitoring of the downgradient edge of the plumes will be important in determining if CDCs above aquifer cleanup concentrations have the potential to migrate offsite. DESJC should evaluate the monitoring that would be required under their proposed Decision Logic at the downgradient edge of the plumes in these three places to make sure that the frequency of sample collection is protective of offsite receptors. Regional Water Board staff has no objection to the proposed extraction well pumping recommendations presented in Section 5.4.3 of the Draft Report. MONITORING AND SAMPLING COMMENTS Regional Water Board staff has no objection to the recommendations for the cone penetrometer test (CPT) investigation activities presented in Section 5.5 of the Draft Report. However, data collected from the investigation should be evaluated before a new Decision Logic for groundwater monitoring is implemented.