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concentrations are adsorbed to clayey soils present below the water table at the base of the <br /> 1990 remedial excavation. The adsorbed-phase hydrocarbon concentrations attenuate <br /> rapidly with depth and do not appear to be migrating downward. <br /> • The lack of dissolved-phase hydrocarbon concentrations at the site suggests that adsorbed- <br /> phase hydrocarbons present from 11.5 to 16.5 fbg in the saturated zone are not readily <br /> dissolving into ground water. <br /> • Results from this phase of investigation support the conclusions of the Human Health Risk <br /> Assessment submitted to Mobil Oil and the PHS/EHD on June 29, 1993. In that report a <br /> final remediation plan involving quarterly ground water monitoring was proposed as a <br /> means of assessing potential impacts of residual petroleum hydrocarbons on human health <br /> and the environment. This proposal remains the most viable approach to closure of this <br /> site. Degradation of residual diesel and gasoline range hydrocarbons by natural processes <br /> over time will continue at the site. Further removal of remaining adsorbed-phase <br /> hydrocarbons present below the static water table is not recommended. <br /> The site assessment activities summarized in this report have been conOucted in accordance with current practice an e <br /> standard of care exercised by geologists and engineers performing similar tasks in this area. No warranty, expressed or <br /> implied, is made regarding the conclusions and recommendations presented in this report. The conclusions and <br /> recommendations are based solely upon an analysis of the observed conditions. If acti.1al_condition_q differ f om. those <br /> described in this report, our office should be notified. <br /> 5 <br />