SITE INFORMATION AND CORRESPONDENCE

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Mr. Bonham 3 29 September 1995 that pilot remedial testing be done in the plume edges, an area in which natural attenuation can be expected to be proceeding. We remain concerned that the STTC site overall presents a relatively unpromising situation for SVE technology, especially for vertical wells. However, it is clearly reasonable to consider all technologies that might be applicable in a multi-technology approach. • In reviewing the pilot-scale testing proposal, staff recognized that fluctuating water tables can cause "smearing"of product in the vadose zone (as described on p. 2 of the letter), and that product might reappear as the water table drops. However, review of earlier monitoring reports as part of reviewing the pilot-testing workplan included consideration of seasonal changes in depth to water table and in floating product thickness. Although product thickness has generally been greatest in times of low water table, the data indicated that floating product thickness has shown both upward and downward trends over the past three years of seasonal cycles. In the STA, well PS/MW-15 has had no report of floating product since August of 1992. The record of ST/MW-1 shows that floating product first appeared in September 1992, (when the depth to water was greater than at any time since recorded), increased to a maximum in the period August through October 1993 (times of low water table), and has since apparently decreased proportionally even at times of low ground water. That well was not measured in June 1995. The record of PS/MW-14 also supports the observation that as water drops, thickness of floating product increases. However, re-examination of PS/MW-14 data submitted in the Second Quarter Report, with comparison of floating product thickness at times of similar vadose zone thickness, suggests that there may be an increase in floating product in this well. Well PS/MW-14 was not measured in June 1995, so its present status.is unknown. . As stated in EMCON's 26 June 1995 letter (p.4), sand units encountered in the vadose zone in the NTA and STA are 1 to 2 feet thick. For a vadose zone of only 5 feet, or a well with a screened interval of 5 feet or less, this is 20 to 40% of the effective depth over which SVE might occur. We question whether this amount of sand, if it has any lateral extent at all,will prove to be insignificant when SVE is applied. The additional lithological data included on p. 6 of the 26 June letter and in the letter's Attachment A show that there is considerable variation in lithology in the proposed test sites. Based on this,we do not agree that continuous logging of the additional borings is unnecessary, and suggest that close attention be paid to lithology when setting the screens in the SVE wells. We do not, and did not, suggest screening of vertical SVE wells at depths of less than 3 feet below surface. • We agree that monitoring of depth-to-ground water fluctuations and intermittent application of the S VE/air sparging technology is logical. Effective application of this approach will require continuous, automated monitoring of the water table and an automatic trigger for the SVE system when conditions are appropriate. • The test proposal indicates only a total of 36.5 hours of SVE and air sparge testing, out of 5 days permitted for these activities by the San Joaquin Air District. Staff expressed concern that this duration may be insufficient to determine whether there will be an initial surge of vapor extraction followed by a dropoff to a more nearly steady state rate of extraction. Staff concurs that dropoff may not be observed in a short-duration test, in fact, that was the point of the comment. Potential dropoff is a concern because the initial surge may not represent a true test