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Work Plan for Refined Plume Definition and Management of Floating Product-7500 W 11th St., Tracy, CA. Page 45 soils such as are present at the depth of the water table at the 7500 West Eleventh Street site, the drilling operation causes remolded clay, which has a very low permeability, to be smeared over the borehole wall. Although vigorous well development can break smeared clay away from the walls of the boring so that it does not impede well efficiency, it often occurs that smeared clay remains on the boring wall in a zone extending from a short distance below the water table upward to the ground surface because well development .. procedures tend to be less effective in those zones than they are in the deeper portions of the well. As a result, at the depth where the floating product is present, especially when the water table rises from the elevation at which it was located when the well was installed, the well is r,. highly inefficient with respect to extraction of groundwater or LNAPL from that zone. Under those circumstances, free product-skimming systems, which cause the well to be �. drawn down to only a small degree, LNAPL does not readily flow into the well and, because the well is so inefficient, its radius of influence will be very small and may not extend more than, at best, a few feet from the center of the well casing. Such well inefficiency is of little or no consequence when small-diameter wells are used for the purposes of groundwater- quality monitoring or periodic measurement of the thickness of floating product. However, if the same wells are converted to use as extraction wells or are newly installed for that purpose, their inefficiency with respect to their ability to capture floating product from distance is a major disadvantage. Because trenches, pits or other excavations that can be used to evacuate floating product from the subsurface are opened by machinery that tends to break the soil being excavated away from the in situ mass, the walls and floors of such excavations are subject to much less smearing with remolded clay. In addition, when constructed under the oversight of an experienced engineer, the excavation contractor can be directed to break away smeared claysr from the walls and floor of an excavation in those areas where the cutting action of the " excavator bucket has blocked flow through silt layers or similar conduits of groundwater flow. 9.3.4 Scale of System Required for Successful Removal of All Recoverable LNAPL As was discussed in Section 5.2.3, it is estimated that the area of the subsurface affected by floating product emanating from the 7500 West Eleventh Street site amounts to some 15,000 ftz and has dimensions of some 100 ft. from east to west and 150 ft. from north to south. Experience has shown that when attempts are made to extract large and wide-spread volumes of floating product from a plume of affected groundwater, after an extraction system has been operated for a period of time, LNAPL will often disappear from the water table in an extraction well, only to return after an extended period during which the extraction system has not been operated. This phenomenon has been observed at sites where "pump and treat" type systems have been in continuous operation for many years before they were shut down. ` This troublesome outcome frequently occurs because the free product removal system has not been sufficiently aggressive as is necessary to strip 100% of the recoverable floating product from the soil mass and due to the tendency of adsorbed LNAPL to be slowly released into the groundwater when they become detached from the soil particles and capillary tension SJC