SITE INFORMATION AND CORRESPONDENCE

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Mr. Vaughn 9 6 • 12 May 1995 2. Rationale for Location of the Test Trench and Test Wells: The pilot-testing workplan (March 1995) does not give a rationale for the proposed STA and NTA pilot test areas, except to state (p. 3-1) that the STA is near the intermediate remedial action plan (IRAP) ground water interceptor trench that was proposed in October 1993. That trench, not yet installed, was proposed in October 1993 for placement west of Stork Road and southeast of the Santa Fe Pacific Pipeline and ST terminals. This area is downgradient of an area bounded by the western half of the ST terminal and Stork Road, an area which in 1993 had up to 2 feet of free product floating on ground water. One purpose of the trench was to evaluate, on a pilot scale, the recovery by skimming of free product in the trench. A second purpose was to place horizontal tubing in the base of the trench to facilitate enhanced biodegradation via air spargng or nutrient addition. In 1993 this area had one of the thickest accumulations of free product in the STTC terminals area. Therefore, apparently the basis for the proposed location of the interceptor trench was proximity to an area of thick accumulation of floating product nearly two years ago, and the basis of the present proposed location of the STA is proximity to the interceptor trench. In December 1994, however, the thickness of free product had decreased to approximately 0.2 feet or less in wells PS/P-I land PS/P-12, which are along Stork Road near the proposed test area. Ground water in this general area was reported as occurring approximately 8 to 9 feet bgs in the October 1993 [RAP proposal, but, the depth is quite variable and,based on recent and historical data at well PS/P-11, (the well closest to the proposed interceptor trench), the water table responds to rainfall infiltration. In October 1993, depth to water at well PS/P-I 1 was measured as 10 feet bgs; in March 1994 ground water was encountered at 3.76 feet bgs; in December 1994 it was down to 6.84 feet bgs, but by March 1995, after significant rains, it had risen to 2.32 bgs. This depth is at the shallow extreme of conditions said to be in existence at the terminals area. In addition, the site map accompanying the workplan shows that the area of the proposed tests is adjacent top (and may include?) an area described as a"subsurface depression with ponded water". There is nothing on the map to indicate whether the ponded water is a present, ephemeral, or constant phenomenon. Based on the above, it appears that STTC's consultant has offered an insufficient rationale for the location of the proposed Southern Test Area other than its proximity to the interceptor trench, which was in tum located presumably on the basis of free product thickness. That thickness has now decreased significantly, while depth of the vadose zone, a critical concern for effective use of vertical SVE wells, has also decreased significantly and shown a tendency for quick response to precipitation. If EMCON's intention was to choose a"typical" location for the tests, they have not apparently done so. If the intention was to choose a location to test the proposed remedial methods under the least favorable conditions, they may have done that. With a vadose zone now less than three feet thick and dominated by low permeability soils, conditions at the proposed STA appear to be generally even more unfavorable than at most other areas of the terminals site. 3. Sampling Frequency and other Concerns: • Although the proposal describes the general lithology of the A-zone under the entire STTC site, there is no specific information given regarding the lithology of the vadose zone in either of the proposed test areas. Examination of cross-section E-E', the nearest section to the