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1 <br /> 6 CONCLUSIONS AND RECOMMENDATIONS <br />' Based on observations made in the field during the drilling activities and on <br /> soil and ground-water sample analytical results, the soil and ground water <br /> in the vicinity of the former tank complex are impacted with petroleum <br /> hydrocarbons. The following sections present EMCON Associates' <br /> conclusions and recommendations for the site. <br /> 6.1 Soil <br />' Residual petroleum hydrocarbon concentrations remain in soil in the vicinity <br /> of the former tank excavation. The absence of detectable concentrations of <br /> petroleum hydrocarbons in the soil samples collected from borings MW-12 <br /> and 892-1 indicates that the impact to soil is relatively localized. These <br /> borings are both located within 25 feet of the former excavation (see <br /> Drawing 1). In addition, petroleum hydrocarbons were detected at moderate <br /> to high concentrations (up to 6,200 mg/kg of TPHG) in soil samples from <br /> boring MW-13 and were not detected in boring B92-1 , which is only <br /> approximately 20 feet away. <br /> Based on the results of the SVET, vapor extraction should be an effective <br /> method of removing residual petroleum hydrocarbons (the petroleum <br />' hydrocarbon impact identified in the soil surrounding well MW-13)'. In <br /> addition, vapor extraction could also reduce the concentrations of dissolved <br />' petroleum hydrocarbons in the ground water due to the relationship of <br /> aqueous phase and vapor phase equilibria. <br /> Based on the SVET results, the following conclusions can be made. <br />' V Extracting from MW-13 generates a relatively small radius <br /> of influence (12 feet) compared to the radius of influence <br />' generated while extracting from the piping gallery riser G-2 <br /> (60 feet). <br /> C The concentration of petroleum hydrocarbons in the soil <br /> vapor sample collected from well MW-13 was relatively low <br /> (0.1 percent LEL). Typically, SVE systems operate with soil <br />' vapor concentrations in excess of 25 percent of the LEL. <br /> The low flow rate from well MW-13 is most likely due to upweliing of the <br /> ground water. This upwelling limits the vacuum that can be imposed in this <br /> well and the air flow by decreasing the exposed length of screen in the <br /> pjO\0f9010f900020.1 ag 6 - 1 Rev. 0 December 15, 1992 <br />