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+— Work Plan for Refined Plume Definition and Management of Floating Product-7500 W 11th St., Tracy, CA. Page 53 <br /> _. system spread over a period of no more than fourteen days will permit several thousand <br /> gallons of LNAPL to be removed from the subsurface, if such a volume is, in fact, present, so <br /> that little, if any, will remain on the plume of contaminated groundwater that emanates from <br /> the 7500 West Eleventh Street property. <br /> Experience from the floating product removal operation on the south side of West Eleventh <br /> �.. Street and the 2.5 years of quarterly groundwater-quality monitoring that has been conducted <br /> at the site since the LNAPL was removed from that location indicates that additional <br /> extraction of floating product beyond that which can be accomplished by three extraction <br /> .� rounds spread over a period of two weeks will not be required. However, to permit future <br /> removal of LNAPL that may become desorbed from the affected soil mass, when the cut-off <br /> trench is backfilled, very high permeability crushed rock will be placed in it, using the <br /> «- procedure described in Section 9.4.1.5, and a 6-in, diameter slotted well casing will be <br /> installed at one end of the trench so that any floating product that might reappear on the <br /> groundwater in the trench can be efficiently removed by suctioning it into a vacuum truck. <br /> Note: <br /> �— If conditions are such that it is necessary to place rock backfill in the trench to an <br /> elevation that is above the water table to achieve adequate trench stability (see <br /> Section 9.4.1.5 below), that action will not render the cut-off trench ineffective. <br /> When rock backfill is used, the ability to pump floating product from the surface <br /> of the water table is not lost because LNAPL can be suctioned out of a local, <br /> stable depression excavated in the highly permeable rock backfill. During that <br /> process, floating product is rapidly drawn to the extraction point from the whole <br /> length and width of the cut-off trench even though it is backfilled. <br /> 9.4.1.4.1 Bulk Extraction of Contaminated Groundwater <br /> When all, or substantially all of the floating product has been removed from the cut-off <br /> trench, which will likely be the case at the time the third-round of floating product extraction- ►^'°"` `?„ 4�� <br /> is scheduled, SJC recommends that a complete volume of affected groundwater be pumped <br /> SI i <br /> from the cut-off trench. This is a simple and highly cost-effective procedure when performed <br /> b. by a vacuum truck. The maximum volume of groundwater extracted at the time would <br /> amount to some 18,000 gallons, which is a small amount compared to the total quantity that <br /> would have to be pumped through a "pump and treat" or similar remediation system to <br /> achieve the same beneficial effect. SJC's extensive experience with remediation of sites <br /> where soil and groundwater are contaminated has shown that such bulk extractions of <br /> contaminated groundwater are a very effective and economical means by which the total <br /> ,^. duration of a remediation can be significantly reduced. <br /> 9.4.1.5 Placement of Permeable Crushed Rock into Cut-off Trench <br /> To permit excavation below the water table, a technique developed by SJC for remediating <br /> sites under similar conditions to those present at the Navarra Site will be applied. This <br /> technique involves backfilling the excavation with large-sized, crushed or river-run rock. The <br /> rock is placed into the open trench by the excavator, which is also used to spread the material <br /> SJC <br />