SITE INFORMATION AND CORRESPONDENCE

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Mr. Vaughn 7 • 12 May 1995 proposed STA, shows essentially no detailed information on the vadose zone. The workplan proposes collection and characterization of soil samples every five feet in borings; clearly, if the vadose zone is only 3 to 5 feet thick, this will result in no sampling or characterization of the vadose zone in the test area. Rather, the consultant should log and sample the borings continuously, or at minimum, do a complete log of the interceptor trench when it is installed. This log should be submitted with the report on pilot test results. • The workplan proposes sequential testing at , first, the STA and later at the NTA. Given the heterogeneous and discontinuous nature of the soil strata at the STTC terminals area, it is possible that conditions at the STA and the NTA may not be sufficiently comparable within the vadose zone to allow generalized conclusions about technology feasibility to be drawn from pilot studies at only test area. In other words, if soil vapor extraction does not work in the STA because the soils are low permeability and the vadose zone is thin, can it be safely concluded that the technology also will not work anywhere else in the region, even though the vadose zone may be a foot or two thicker and the geometry of the clay and silt lenses somewhat different at another location? While it may not be cost-effective to install a complete set of wells, trench, and piping to run pilot tests simultaneously at two or more locations, it would not be prohibitively expensive to do some test borings and sampling of the vadose zone in two or more other potential test locations, while the drilling equipment and crew are already mobilized on-site. Such an effort might show immediately whether a second pilot test in the NTA would be likely to provide additional information. • An additional consideration is the appropriateness of the proposed screen intervals to the site conditions. It is proposed to screen the combination SVE/ground water wells from approximately 5 to 20 feet bgs. Under present (spring 1995) conditions in the STA, the entire screen would be within the saturated zone. Even under maximum reported depth-to-ground water conditions, more than half of the screen would be under water. This is not conducive to effective SVE as an independent process, or combined with air sparging, and possibly not, (given the low permeabilities of the soils), even accompanied by ground water pump down. • The San Joaquin County Air District permit limits the SVE pilot testing to five days. The various tests proposed by the workplan cumulatively specify only 36.5 hours of SVE and combined SVE/air sparge tests in both the wells and the trench in the STA. Based on the letter from the Air District included in Appendix E of the workplan submittal, EMCON's original plan was to test for up to seven days in each area. It is not clear from the workplan whether the consultant plans to conduct SVE testing in combination with ground water pumping in addition to the 36.5 hours of SVE and SVE/air sparging, but clearly there is room for additional testing in order to determine, among other things, whether there would be a drop-off in withdrawal rates after the initial "surge" of vapor extraction. • The workplan mentions possible future studies on enhanced biodegradation, using stimulation with increased dissolved-oxygen levels via air sparging. The existing site conditions do not really appear particularly favorable for the remedial technologies included in this pilot testing workplan. I suggest that the STTC plan to proceed, simultaneously with the other proposed pilot tests, with plans to assess feasibility of intrinsic bioremediation and enhanced in-situ or ex-situ bioremedial processes.