SITE INFORMATION AND CORRESPONDENCE_2001-CURRENT

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Chevron 4 - • 1 March 2007 Stone Brothers Management ompany 5400 and 5606 Pacific Ave, Stockton Comments: 1- The Report makes a technical case for the reductions in sampling frequency and analyses for all wells. However, the MCLs and CSALs cited in the Report, while contained in the Waterboard's WQGs, are not the lowest applicable WQGs in most cases. The WQGs also include lower numeric values (Taste and Odor Threshold, Public Health Goals, etc.) that are applicable for the beneficial uses of the groundwater. After factoring in the lowest applicable WQGs, the only analytes with historic detections below WQGs are: n-Butylbenzene and sec-Butylbenzene. The ESLs are not WQGs. Therefore, the reductions in analytes (compounds) are not granted for all wells. A stronger case can be made for reduction in sampling frequency and analyses for individual wells which show a history of either: a) four consecutive quarters of non- detection of an analyte at an acceptable method detection limit, or b) four consecutive quarters of analyte detections below WQGs. However, any change to the current monitoring program will require amending or reissuing the CAO. 2- The Report states that the cost of the full scan USEPA Method 8260B, when compared to the proposed short list (BTEX and 1,2-DCA) of USEPA Method 82608, is "...excessive". However, the costs for both analyses are not provided. It has been our experience that for most labs, the difference in cost between a partial and full scan of USEPA Method 8260B is measured in tens of dollars per analysis, which does not appear to be excessive. The full scan USEPA Method 8260B data are necessary to monitor the vertical and lateral extent of the plume, to continue to show that the plume is stable and declining, and that WQGs are being or will be met in a reasonable time, which are Regional Board requirements for closure. Staff will recommend in the revised or reissued CAO that reporting all peaks in the report tables for full scan USEPA Method 8260B be continued. 3- The Report requests, in effect, the abandonment of five monitoring wells (MW-14A, MW-15, MW-21 S, MW-21 D, and MW-22S) on the western perimeter of the Site and one shallow well (MW-19S) near the core of Unocal's plume. However, MW-15 was abandoned properly with Regional Board staff approval in a letter dated 14 February 2007, due to road construction. While samples taken from these wells have had intermittent detections of pollutants, they also serve as sentinels to confirm stability and the lateral and vertical extent of the groundwater plume. Therefore, staff will recommend in the revised or reissued CAO that these wells should be sampled annually, and that MW-15 should be replaced, due to its former upgradient location. 4- Based on the Report text, listed in the summary table of requested changes above, the Report fails to address the fate of monitoring wells MW-28, MW-29, MW-30 and MW-31. Since there are no changes requested for these wells and they are downgradient and crossgradient perimeter wells, staff will recommend in the revised or reissued CAO that these wells should continue to be sampled quarterly.