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Work Plan for Refined Plume Definition and Management of Floating Product-7500 W l I tR St, Tracy, CA. Page 49 engineer. That person should also have sufficient education and experience in hydrogeology and the design of groundwater remediation systems that she or he would be properly qualified, according to the requirements of California law, to perform the review. 9.4.1.1 Trench Location and Dimensions The floating product cut-off trench will be excavated at the approximate location shown on Figure 11. As shown on that figure, the trench will be approximately 100 ft long so that it will extend east and west across the entire width of the plume of floating product. The actual length may be somewhat shorter or longer depending upon observations of the environmental condition of the subsurface that will be made by the California-licensed geotechnical engineer in responsible charge of the work at the time the trench is opened. Restrictions on the final geometry of the trench may also be imposed by the presence of buried utilities located in the highway shoulder on the north side of West Eleventh Street. Note: Although such denial,,-is not expected, if the SJCPWD will not permit construction of the cur-off ench in the northern shoulder of West Eleventh Street, subject to the approval of the property owner, the trench can be located immediately to the north of the curb that surrounds the parldng lot in front of the ,.. furniture retail store ay 7501 West EIeventh Street. As noted previously, the cut-off trench will be approximately 4 ft. wide. This width had been selected because it permits the floating product that will accumulate in the trench to be easily observed and the vacuum suction equipment to be efficiently operated by an experienced technician. Effective use of a vacuum lance is more difficult in narrow trenches or pits of �. more limited areal dimensions. The 4-ft. width is also equal in size to a standard excavator bucket and, more importantly, is on the same order of scale as the representative elementary volume of the soil strata. Thus, at the end walls of the trench, an adequate number of the fine silt zones and other inclusions that affect LNAPL and groundwater flow will be intersected. This will permit an extended zone of influence to develop around the ends of the trench when floating product is being extracted. (See Sections 9.3.1 and 9.3.2 for discussion of zone of influence and elementary representative volume, respectively.) r The depth of the cut-off trench has been preliminarily set at 15 ft. That depth is selected to permit a deep draw-down of the groundwater table when floating product is being removed, so that 1) a steep flow gradient in the direction of the trench can be developed and 2) the �. zone of influence will extend to a significant distance in all directions around the trench. An excavation to that depth will also permit soil severely affected by fuel hydrocarbons to be removed from the area around Monitoring Well MW 7, which removal will have a greatly beneficial affect on the quality of the groundwater in that area and, in time, the quality of the groundwater down-gradient there from. The engineer in responsible charge of the project may, by application of the observational method (Peck 1969), direct that the actual depth of the trench when excavated, while it will SJC