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Work Plan for Refined Plume Definition and Management ofFloating Product-7500 W 11th St., Tracy, CA. Page 46 holding them in the small interstices in the soil mass are eliminated when weather conditions are unusually dry or the groundwater table falls to unusually low elevations (United States Environmental Protection Agency 1992, 1989). False positive results for complete removal of LNAPL from beneath a site are most likely to occur when the floating product extraction system is based on skimming of free product from w. F an array of small-diameter wells. This is because, although LNAPL floating on the groundwater in the well may have been reduced to a sheen or totally eliminated, for the reasons explained in Sections 9.3.1.1, 9.3.1.3 and 9.3.1.4, the distance from the perimeter of the well from which such a system can extract free product is usually quite small, but because observations of free product thickness are only made in those same wells, conditions on the surface of the groundwater at points midway between individual wells or down- gradient or up-gradient from the extraction system remain unknown. When such systems are used, it is quite possible for LNAPL floating on the water table to bypass the extraction points and continue to migrate down the groundwater gradient. When the skimmers or pumps are turned off, floating product does not rapidly reappear in the extraction wells because, due to phenomenon related to the minimum representative volume of the subsurface that has been described in Section 9.3.1.3 and the very low rate of flow of LNAPL, particularly �-- components composed of long-chained carbon molecules such as are present in diesel fuel, there may be long delays between the time that extraction is terminated and floating product reappears in them, even though large volumes of LNAPL may remain floating on the groundwater in an area a short distance away from such a well. 9.3.5 U.S. Environmental Protection Agency Recommendation Rock-filled cut-off trenches can be used to minimize the design problems discussed above because they can be constructed at a length and depth sufficient to intersect the complete '' width of the plume of floating product that is advancing down the groundwater gradient and, because of their large dimensions, they permit floating product to be removed aggressively V over a large zone of influence, with steep flow gradients being generated in the direction of 4 r: the extraction point. In addition, their construction permits excavation and treatment or disposal of a significant volume of soil from the zone immediately above and below the water table, which is generally the most severely contaminated at any given site. That zone also contains adsorbed and absorbed LNAPL, which material is the most difficult to remove from the soil mass. For these reasons, amongst others, in its publication, How to Effectively Recover Free Product at Leaking Underground Storage Tank Sites, A Guide for Regulators, the USEPA recognizes the clear advantages of LNAPL extraction systems based on cut-off trenches over systems based on well arrays, particularly at sites such as the 7500 West Eleventh Street property, where site geography and infrastructure development permits such free product removal trenches to be utilized (United States Environmental Protection Agency 1996). sic