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r+ Work Plan for Refined Plume Definition and Management of Floating Product-7500 WIN St., Tracy, CA. Page 48 9.4.1 Technolo A—Cut-off Trench and High-volume Pumping of LNAPL Application of this technology involves excavation of a cut-off trench across the whole width of the plume of floating product and evacuation of LNAPL that accumulates in the trench into a vacuum truck for off-site disposal at a permitted recycling facility. The trench, which will be approximately 4.0 ft. wide, will penetrate below the water table to sufficient depth that adequate draw-down of the groundwater table can occur, so that when floating product is pumped from the excavation, a steep groundwater gradient will be induced and the zone of influence within which the product will flow to the excavation is large. The pian location of the proposed cut-off trench is shown on Figure 11 and cross sections through it are shown on Figure 12. To provide for extraction of any floating product or i- heavily-contaminated groundwater that, over the long term, might re-accumulate in the cut- off trench, it will be backfilled with high permeability crushed rock and a 6-in. diameter well casing will be installed at one end to facilitate future extraction of LNAPL or groundwater, +� should that be necessary. This floating product remediation technology permits very large volumes of floating product .. to be removed from the subsurface very economically in a very short time. Excavation of a wide trench permits.the project engineer and equipment operators to have a clear view of the condition of the subsurface. That allows the total extent to which it is affected, including the demarcations between affected and clean soil and areas where floating product is present, to be easily and precisely evaluated. Such observations, which are essential for the practical design of an efficient and economical remediation program, represent the correct standard of ,,mow geotechnical engineering practice for the LNAPL remediation work required at the 7500 West Eleventh Street site. The ability to efficiently observe conditions in an open trench of adequate dimension and to efficiently manage floating product at that scale are greatly superior to subsurface investigations that rely on exploration by, and extraction of LNAPL from, small-diameter borings alone. Moreover, the method has been successfully applied at many sites in Northern California and elsewhere, including at 7500 West Eleventh Street itself, where, as has been discussed in Section 2.3, it was used successfully to remove a large volume of floating product from beneath the West Eleventh Street frontage of the former fueling station, greatly improving groundwater quality in that area. Note: n In its guidance letter of April 22, 2003, the SJCEHS indicated that an a_priori decision has been made to disapprove use of cut-off trench and removal of floating product from the trench by transfer to a vacuum truck as a method of remediating LNAPL at the 7500 West Eleventh Street site. However, SJC strongly believes that, due to the technical superiority and economic effectiveness of this method of removing LNAPL from the subsurface under the +- site-specific conditions, the technology should be seriously considered for application at this site. Accordingly, SJC respectfully requests that, prior to a final regulatory determination regarding selection of a floating product extraction .. technology for this site, this work plan be reviewed by a California-licensed civil engineer. Preferably, that engineer should hold title authority as a geotechnical sic