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•.1 �~ • F A S S O C I A T E 5 I N C progress and to ensure that breakthrough has not occurred through the carbon canisters. The effectiveness of the vapor extraction and mobility of stripped gasoline vapor vary depending on permeability of the soil. Natural Attenuation Natural attenuation is a viable,remediation method for the ground water provided that the source of ground water contamination has .been removed and that the. contaminant plume in the ground water is stable or decreasing. Generally, eight quarters of stable to decreasing gasoline concentrations in the ground water would indicate that no further action at the site is appropriate. Enhanced Natural Attenuation Natural bioremediation can be'enhanced by the addition of oxygen to the subsurface. Several approaches have been developed to enhance the amount of oxygen availablc in the subsurface. As previously mentioned, air sparging is one approacIfto enhancing available oxygen. Another is oxygen releasing compound(ORCO). ATC would place ORC® in the existing ground water monitoring wells and inject ORC® slurry into a series of approximately 3 to S borings located on I0-foot centers within the impacted ground water plume. The ORC® increases dissolved oxygen into impacted ground water via the wells and borings and then spreads throughout the ground water underlying the site vicinity. Thee increased oxygen enhances natural biodegradation of ORCO does not negatively affect ground water quality and therefore,based on ATC's petroleum hydrocarbons' experience, is acceptable to the regulatory community. RECOMENDATIONS Based on the results of,the previous site characterization activities, the most recent groundwater monitoring data,and in accordance with the request of the SJCEHD,ATC proposes the following corrective actions: ' Soil In December 2001, ATC conducted a remedial investigation/feasibility study which indicated soil vapor extraction is a viable method;for remediating TPHg'and BTEX impacted soil at the site. Refer to ATC's Remedial Investigation/Feasibility Study dated November 19, 2002, for detailed information. Because the i concentrations of petroleum hydrocarbons detected in�soil samples collected at the site are low and the lateral 4 extent of impacted soil'is confined to the vicinity ofMWI and MW2, soil vapor extraction utilizing a high vacuum dual phase extraction mobile remediation system is recommended. ATC proposes utilizing high vacuum dual phase extraction in an effort to remediate'the vadose and capillary fringe zone soils. High vacuum dual phase extraction will be conducted using existing monitoring and remediation wells MWI, MW2, VWI, and VW3 as source and observation wells. The remedial action will be conducted as three events each of a duration of approximately five days for a total of three months. Vacuum influence and depth to water will be monitored at the observation wells to evaluate the radius of vacuum influence and groundwater draw down. During the remedial action, the soil gas oxygen ,concentration will be monitored using a GasTechTM combustible vapor analyzer. The volatile fuel.hydrocarbon (VFH) concentration will also be monitored with a { photo ionization detector. (PID). Soil gas samples will collected in TedlarTM bags or Summa canisters for laboratory analysis for volatile fuel hydrocarbons (VFH),BTEX and MTBE using EPA Test Method TO-14. Diesel Performance January 5,2004 ATC Project Number 5462577 Page 7