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Work Plan for Refined Plume Definition and Management ofFloating product-7500 W 1/th St., Cy. C'4• Page 62 technology, which is no more than a simple application of a widely-used technique that has many engineering applications. However, in the past few years, a surprisingly large number of publications have appeared in the technical and government agency literature that suggests it is a novel technology and, when used in certain configurations, it has been assigned the unfortunate soubriquet of"bioslurping" (see, for example,Place, et al 2001 and Miller 1996). �. When used in semi-permanent installations to extract LNAPL from wells, this remedial technology has been claimed to have a number of advantages. These include its ability to remove floating product economically while reducing the volume of extracted groundwater to a minimum compared to techniques based on skimming of floating product or dual groundwater draw-down and skimming. In addition, the vacuum applied to extract the LNAPL may induce negative pressure conditions in the surrounding formation that increase the flow of free product toward the well and it may introduce air into the unsaturated zone of the soil above the water table, which accelerates aerobic bioremediation of hydrocarbons trapped in those zones. Some authors seem to believe that the fact that LNAPL is extracted in such a way that there is only minimum draw-down in the well (i.e., that the induced gradient c. in the extraction well is little increased compared to the natural, undisturbed groundwater table) is a significant advantage of the technology. This is clearly not the case.With very limited draw-down in the well, its radius of influence will — be correspondingly small and, as was discussed in Section 9.3.1.4, this is a distinct disadvantage when attempting to extract LNAPL from an extended plume of contaminated groundwater. Its limitations also include a significant �.. tendency for air flow in the soil mass to be bypassed to the surface within a short distance from the well casing, which further exacerbates its inability to extract product from large radial distances from the well. The limitations noted above have been demonstrated in bench-scale laboratory trials (Davila 1997), and they are also reflected in the extremely small volume of LNAPL that was extracted at the direction of the SJCEHD in three rounds of purging by SJC from Monitoring Well MW-7. As discussed in Section 9.2.4, over the period November 8 through November 22, 2003, LNAPL disappeared, at least temporarily from that well, which is located within a large plume of floating product. SJC's LNAPL purgings were effected by use of a vacuum lance, which is the same technology as it now appears fashionable to call bioslurping. �— Therefore, although it can be useful in local removal of floating product released to the subsurface from hydraulic lift equipment or similar low-volume situations, because of its limitations, which are typical of SJC's experience with LNAPL removal at numerous sites, we do not recommend use of the vacuum purging technique in small-diameter wells when the project objective is to remove large volumes of LNAPL over a wide subsurface area. As has been discussed earlier, it is expected that as the process of LNAPL extraction progresses, there will come a time when the thickness of floating product that accumulates in the extraction wells will become very small, even when extraction rounds are repeated at only very long intervals. If this occurs, LNAPL will continue to be extracted using a vacuum sic