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�a <br /> F. <br /> 4 <br /> 4; <br /> Supplementary Site Assessment Report <br /> Former Mobil Bulk Plant 04-343 <br /> April 26, 1996 <br /> ' occurring through aerobic bacterial processes. Nitrite Ievels of 28 to 40 ppm indicate that a food <br /> source for the bacteria is present within a favorable neutral range pH. Although the aquifer <br /> yields high levels of dissolved-iron, the concentrations are below toxic levels and should not <br /> inhibit biodegradation processes. <br /> The above groundwater data suggests that introduction of oxygen into Monitoring Well AW-3 <br /> would be a viable method of increasing the microbial population, and thus enhancing the natural <br /> degradation of dissolved-phase hydrocarbons and residual soil hydrocarbons. Alton has installed <br /> ' ORCTM in monitoring wells on various sites in California. Initial data indicate that oxygen <br /> content in wells containing ORCTM has increased and hydrocarbon concentrations have generally <br /> decreased. <br /> 6.0 CONCLUSIONS <br /> ' Based on the results, Alton Geoscience concludes: <br /> Residual soil hydrocarbons are characterized laterally to the west and vertically by the <br /> results from the new borings. Soil hydrocarbons do not extend to AB-3 and AB4's <br /> locations. TPH-D concentrations are present above groundwater in A13-1 and AB-2 <br /> ' (maximum concentration of 2,500 ppm in AB-2). Residual soil hydrocarbons are present <br /> below the water table in Boring AB-2, likely reflecting the presence of groundwater <br /> hydrocarbon concentrations. <br /> 1 � <br /> The background water quality parameters suggest that initiation of an enhanced <br /> bioremediation program at the site is a viable method of increasing the microbial population <br /> ' and activity, thus enhancing the natural degradation of dissolved-phase and residual soil <br /> hydrocarbons present below the water table at the site. <br /> ' 0 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 groundwater 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 feasible approach for closure of this <br /> site. Degradation of residual diesel and gasoline range hydrocarbons by natural processes <br /> (enhanced and/or unenhanced) should continue at the site. <br /> 8 <br />