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0 LFR <br /> 15 to 20 feet at the Site. This rise in groundwater elevation has caused previously unsaturated <br /> sediments to become submerged within the depth interval of 30 to 50 feet below ground surface <br /> (bgs), referred to herein as "the Resaturated Zone." Additionally, soil-vapor samples collected <br /> immediately above the Resaturated Zone in 1999 showed PCE concentrations exceeding 10,000 <br /> micrograms per liter (µg/1). The HSA is defined herein as the portion of the Resaturated Zone that <br /> underlies vadose-zone sediments that previously contained more than 10,000 ,ug/l PCE (see Figure <br /> 8 of the Revised Draft FS Report). Locations at depths of approximately 30 to 50 feet bgs within <br /> the HSA have remained at relatively elevated concentrations as this area historically became <br /> affected by volatile organic compounds (VOCs) through the downward migration of high <br /> concentrations of VOCs released from leaking sewers or other surface sources. Sediments within <br /> this interval consist of low permeability silts, clays, silty clays, and clayey silts and minor sand <br /> stringers. <br /> Because of the low hydraulic conductivity of these sediments, the VOCs that were sourced here <br /> remain within the sediments and continue to slowly diffuse VOCs into more permeable adjoining <br /> sands. Thus, the relatively low permeability sediments in this area that were once above the water <br /> table are now submerged and are acting as a continuing source of VOCs to surrounding <br /> groundwater. The Membrane Interface Probe (MIP) data recently collected at the Site show that <br /> relatively higher concentrations are bound in the shallower 30 to 50 bgs groundwater and are <br /> preferentially migrating within the sands located below 50 feet bgs (see LFR's "Proposed Phase IV <br /> IRA Detailed Plan," dated July 3, 2006) and 3-D data rendering, attached). At the same time, <br /> groundwater wells screened within sands located beneath this zone (e.g., T/C graph of MW-06, <br /> attached) have shown marked decreases since inception of groundwater and soil-vapor extraction at <br /> the Site. These data are all in agreement with the SCM. <br /> Second, the area referred to by FA as the "Eastern Hot Spot" is not a former "source" area. <br /> Concentrations of VOCs within groundwater east of the HSA migrated from the HSA primarily <br /> through advection of affected groundwater. The concept that concentrations observed in <br /> groundwater located downgradient from the HSA result from migration rather than being sourced <br /> in the area is the fundamental basis for the conceptual model developed for the Site. Further, as <br /> noted by FA, the area east of the HSA contains relatively elevated concentration of VOCs, but <br /> FA's comments fail to recognize that the results presented in FA's summary table are located <br /> primarily within the deeper and more conductive sand units. Correspondingly lower concentrations <br /> are found within the adjoining clays and silts (see cross sections presented in LFR's "Remedial <br /> Investigation Report" dated December 10, 1999). The higher concentrations observed within the <br /> area east of the HSA migrated there from the HSA primarily through more permeable sands, and <br /> can thus be effectively removed via groundwater extraction. Additionally, FA only referenced the <br /> higher concentrations detected in this area; however, review of all of the data collected from this <br /> area shows numerous locations where samples were collected from silts and clays that contained <br /> relatively lower concentrations of VOCs. In addition, with the exception of data from monitoring <br /> wells CHMW-3 and LF-108A, the results presented in FA's summary table are grab groundwater <br /> samples that are nearly 10 years old. These one-time grab groundwater samples do not accurately <br /> reflect concentration changes over time and certainly do not show trends. <br /> hr-Fart-FS-mspons -06750.da:Ifr 3 <br />