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Description of the Reinjection Trench and <br /> Discharge to the Storm Sewer <br /> The purpose of the reinjection trench is to create a hydraulic <br /> barrier such that contaminated ground water from possible offsite <br /> sources is prevented from moving onto the former Exxon site ( see <br /> Site History) . Due to the clay characteristics of the native <br /> soils, reinjection through wells was assumed to be insufficient <br /> for creating an effective hydraulic barrier. Therefore, a <br /> reinjection trench was designed ( Figure 1 , Figure 3 ) . The trench <br /> is to be constructed on the northwest portion of the site. <br /> The trench is to be approximately 75 ft ( 1 ) by 10 ft (d ) by 2 ft <br /> ( w ) . The trench is to be backfilled with pea gravel up to <br /> approximately 2 ft below grade. Drain piping ( 4" PVC or HDPE) <br /> will run on top of the gravel bed to distribute water evenly. <br /> Assuming water table depth is approximately 25 feet below grade, <br /> a maximum hydrualic head of 23 feet will be created by the <br /> reinjection trench. <br /> The elevation of the existing storm sewer, located on the <br /> northwest corner of the site, is currently unknown. If it is <br /> two feet or less below grade, a sump pump will be required to <br /> pump any excess water from the reinjection trench to the storm <br /> sewer. If the sewer access is greater than two feet below grade, <br /> excess flow will be by gravity. <br /> Flowmeters will be located on the discharge line directly <br /> following the treatment system, and also on the line leading to <br /> the storm sewer. By difference, the volume of injected water can <br /> be calculated. <br /> 7 <br />