SITE INFORMATION AND CORRESPONDENCE

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y Mr. Bonham • • 2 29 September 1995 determined after analytical results were obtained. Therefore, please inform us, also, of the analytical results. • The letter states (p. 2) that piezometer P-32 was sealed in place with grout, because rig access was prevented by presence of a pipeline. The letter does not describe how this was done, or how it was determined that the sealing was complete and that no leakage pathways now exist. * From the information submitted, the new wells appear to be acceptable. Review of the July 28 Second Quarter Monitoring Report shows that the replacement well PS/WC-2s had detectable levels of benzene, toluene, ethylbenzene, and xylenes (BTEX) as gasoline and diesel. Please continue to include these wells in your regular monitoring and reporting program, as you Proposed. For convenience and clarity in review of cumulative records by future staff or investigators who may not be intimately familiar with the case history, we suggest that the new wells be given some designation that plainly shows they are replacements rather than the originals --for example, the new PS/WC-2s might be designated PS/WC-2s (n) or(1) or ('95). • Piezometer P-15 could not be located and was not decommissioned. Please inform us if future plans are made to decommission this piezometer when and if it is located. Simi]azly,please inform us of the plans for replacement or repair of piezometer PS/P-29, which, according to theSecond Quarter Monitoring Report (p.2) has been damaged and was not sampled. We note that you propose to reduce, but not eliminate, use of this piezometer in future monitoring. 2. Pilot-Scale Tessin Work lan: We have reviewed your response to our letter dated 15 May 1995 commenting on the proposed pilot testing. We concur with STTC's decision to proceed with testing simultaneously in both the southern (STA) and northern (NTA)test areas. As noted in our 15 May 1995 letter, aside from reservations about the efficacy of the proposed technologies, we have no major concerns about the proposed workplan. Our letter stated that the time schedule submitted in the workplan was acceptable. We had hoped to see work begun by this time. We encourage the STTC to proceed with the aquifer testing as proposed and to work toward developing a broad-based approach to site remediation. Please submit, therefore, a revised schedule of work for the aquifer testing and well installation. Other responses to EMCON's 26 June 1995 letter follow: Staff appreciates the additional detailed information on the rationale for the chosen test areas. While some or all of this information may have been available in older portions of our files, it greatly expedites staff review efforts (and shortens billable hours on a cost-recovery project) if reports and workplans are submitted as stand-alone documents; that is,supporting information should be either supplied or clearly referenced so that staff will know it exists and where to look for it. We agree that a test site presenting completely representative details of the vadose zone may not exist at STTC. The STA and the NTA represent different site conditions, one a less-favorable extreme and the other a more favorable extreme, and testing of both areas is reasonable. Staff also agrees that testing in the areas of high impact is logical. There was no intention to suggest