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�- Work Plan for Refined Plume Definition and Management of Floating Product-75oo W IIth St., Tracy, CA, Page 41 Dual pump systems for extraction of floating product from groundwater at sites where they �... can be economically applied do have the advantage that, in addition to removal of LNAPL, considerable quantities of contaminated groundwater are also removed when the system is in ... operation, However, the element of the system that extracts the groundwater so as to induce a sufficiently steep gradient of flow in the direction of the point of extraction is, in essence, a "pump and treat" remediation system. In practice, such systems rarely perform well, ... particularly at sites where the subsurface is stratified into low-permeability silt or clay Iayers interspersed between higher permeability sand formations. Such conditions prevail at the 7500 West Eleventh Street site and over a wide area of the alluvial fan that forms the �;.. subsurface over a wide area in and around the City of Tracy. In fact, they perform so poorly that, at other sites in the Tracy area, including a site less than 1.25 miles to the west of the Navarra Site, along Eleventh Street, use of that technology was specifically disapproved by •- the SJCEHD (San Joaquin County Environmental Health Department 1995). With respect to their performance, floating product removal systems that do not include '- continuous groundwater pumping to increase flow towards an extraction point are even more ineffective than dual systems because, while exhibiting all of their limitations, they induce such a slight gradient in the direction of the extraction point that their effective zones of �- influence are extremely small and LNAPL at even short distances away bypass the system entirely and continue to advance down-gradient with the flow of groundwater. 9.3.2 Representative Elementary,Volume In addition to considerations related to the rate of flow towards a floating product extraction point as it is influenced by its gradient and the zone of influence of the extraction system relative to the area of an advancing plume from which LNAPL must be extracted, and engineering design of such a system must consider the applicable "representative elementary volume" of the subsurface soils. Representative elementary volume is a concept that incorporates an understanding that all of the physical and chemical properties of a mass of soil depend upon the scale at which they are measured and that the design of any engineered system that will influence or be influenced by those properties must consider such scale effects if it is to perform effectively. In the context of the flow of groundwater through soil, the primary parameter that must be considered at the appropriate representative elementary volume is the hydraulic conductivity, or, more correctly, the intrinsic permeability, which is independent of the properties of the fluid being transmitted through the media. It is almost universally the case that if a small volume of soil, such as that recovered from a boring, is used to measure the hydraulic conductivity of a soil, the measured value will be considerably lower than the permeability .� measured in a larger scale test such as a draw-down test or a pressure test conducted in a borehole where the test section that is isolated by packers is of significant length. If permeability testing is conducted at an even larger scale, such as is occasionally possible in tunnels or other large underground openings, the measured hydraulic conductivity will again increase, often by one or more orders of magnitude. That phenomenon occurs because as the scale of the tested mass of soil increases, the probability exists that it will include a greater number of natural conduits for hydraulic flow such as those formed by thin deposits of silt or sic