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i <br /> JFZffMA%AMAOR <br /> June 21, 1993 Working To Restore Nature <br /> i ARCO Service Station No. 2130 <br /> necessary remedial coverage, the installation of one or two additional sparge wells should <br /> be evaluated. <br /> 1 <br /> Using a 19 foot radius of influence requires the installation of two additional air sparge <br /> wells to provide sparge coverage of the soil (capillary fringe) and ground water zones that <br /> are impacted with the most hydrocarbon mass These zones are located beneath and <br /> 1 adjacent to the existing underground storage tanks (UST's) In order to remove residual <br /> hydrocarbons from historical capillary fringe soil and from other vadose zone regions <br /> beneath and beyond the UST's and to provide capture of sparge off-gas, we recommend that <br /> seven deep and two shallow vapor-extraction wells be used This will require the installation <br /> 1 <br /> of two additional deep vapor-extraction wells. In order to monitor sparge system <br /> performance,we recommend the installation of one additional sparge monitoring point The <br /> estimated limits of residual hydrocarbons in vadose and capillary fringe soil and the areas <br /> 1 of coverage for the sparge and vapor-extraction wells are shown on the attached Plates b <br /> and 7 The locations of existing and proposed sparge and vapor-extraction facilities are also <br /> shown on these plates. <br /> Data collected during the pilot study indicates that sparging is techrucally feasible within the <br /> geology and hydrogeology that comprise the sparge study zone. The existence of transitions <br /> from sand to silty sand within the saturated zone (such a transition exists between wells MS- <br /> 2 and MS-3, Plate 3) may create preferential migratory pathways for sparge air These <br /> zones of lower permeability below the ground-water surface may act as barriers to sparge <br /> air and could result in ineffective remediation at some locations Therefore, continuous <br /> sampling below the ground water surface should be performed during all future sparge well <br /> installations to evaluate sediment types and allow for program adjustments with regard to <br /> sparge well construction <br /> 30003-s 13 <br />• <br />