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1 well MW-5 Thus, the weekly hand-bailing activities appear to have successfully removed the LPH in well MW-5 9.3 Vapor Extraction System Operations On July 1, 1999, EMC initiated the operation of a SVE treatment unit The system consisted of a Baker Furnace Thermal Oxidizer connected to wells MW-2, MW-4, MW-5, TB-3, TB-4, TB-6, TB-7, TB-9 vap, TB-9 wtr, TB-11 vap, TB-1 I wtr, TB-12 vap, and TB-12 wtr Currently, only wells TB-11 wtr (50%), TB-3 (50%), and MW-5 (100%) are operating As of August 16, 2004, the system operated a total of 29,975 hours and removed 140,299 pounds of hydrocarbon 10.0 PRELIMINARY SITE CONCEPTUAL MODEL ' This PSCM 1s based on the information presented in this report As additional information is obtained from the site, the PSCM will be updated • Soil beneath the site consisted predominantly of clay, silt, clayey silt, and silty clay, interbedded with sand layers consisting of silty sand, fine- to medium-grained sand, and gravelly sand at depths of between 65 and 75 feet bgs, 90 and 100 feet bgs, and 110 and 115 feet bgs The thickest fine-grained interval within the sand layers was encountered between 75 and 95 feet bgs • Shallow groundwater beneath the site appears to occur under unconfined conditions at depths ranging from 50 56 to 52 72 feet below top of casing (August 17, 2004) In 1988, groundwater was encountered beneath the site at approximately 50 feet bgs within the 1 silty clay sediments During a subsequent site assessment investigation conducted by Thrifty in November 1992, groundwater was encountered during drilling at 80 feet bgs, indicating a significant drop had occurred in the local water table The static ' groundwater level has fluctuated from 71 08 to 48 50 feet bgs (about 22 feet) between 1992 and 2002 • Groundwater has historically flowed to the northeast at gradients ranging from 0 003 to 0 007 feet/foot Based on the hydraulic gradient of 0 0054 from data collected on August , 17, 2004, an assumed porosity of 28% for sand (Driscoll 1989), and a hydraulic conductivity value of 1 1 feet/day (HydroGeo 2002), the groundwater velocity is estimated to be approximately 0 021 feet/day or 7 7 feet/year Since there is a coarse- grained interval from approximately 65 to 75 feet bgs, sand was used for the assumed porosity to be more conservation than the 40%+porosity for silts and clays that make of the majority of subsurface soils beneath the site ' • The 50 and 70 foot screen lengths present in wells TB-7, TB-8, and MW-5 were installed due to the large fluctuation in groundwater table elevation (up to 40 feet) The screens ' penetrate primarily fine-grained soils and do not go from a thick sequence of sand through an aquitard back into sand In fact, it is just the opposite with the 70-foot screen in well MW-5 going from fine-grained soil into a 10-foot thick sand and back into fine- grained soil There is no evidence of different zones of water or of confined conditions 11