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1 PAR-Eckert Cold Storage,Manteca,CA November 25,2002 Page 11 However, ozone reactions are most effective in systems with acidic pH, and the process requires closely spaced delivery points (many air sparging wells). However, the use of ozone can also provide an ' additional source of dissolved oxygen, enhancing natural attenuation. If the intent is the simple enhancement of natural attenuation by increasing dissolved oxygen, mechanisms other than ozone may be more effective. ' Air sparging is potentially suspect in the moderately heterogeneous soils of the site. The presence of silts and intermittent clay layers could significantly reduce the radial effectiveness of the air sparging and ' groundwater circulation, and could lead to short-circuiting. Pump and treat by itself could take as long as natural biodegradation due to the suspected high petroleum hydrocarbon concentrations in the soil below the groundwater table. However, soil vapor extraction should be successful in removal of the soil contamination above the groundwater. Consequently, the pump and treat method could be effective in removing and treating the contamination in the groundwater, and in creating a sufficient drawdown that soil vapor extraction could remove contaminants from the soil below the current groundwater table. The remedial option selected for the site will depend on cost, time, and contaminant concentration goals. The active remedial options, including pump and treat, soil vapor extraction and air-sparging, can potentially mitigate the site contamination very rapidly (perhaps one to two years). Their cost, however, can be substantial (perhaps as little as 100K to as much as 500K). Passive biodegradation should be ' effective but may require many years to complete (perhaps five to 25 years). The cost would be only that to conduct monitoring at the site including for intrinsic bioremediation parameters. This cost is estimated at 10 to 15K annually. Intermediate in cost and time would be enhanced in-situ bioremediation. This remedial option might involve injection of an oxygen releasing compound in the source area and then subsequent monitoring including for intrinsic bioremediation parameters. The time to complete enhanced bioremediation might be between 3 and 10 years and would involve the boring installation and injection cost of between 30 and 50K and the same cost as described above for quarterly monitoring. The cost and time for all remedial options will be highly dependent upon contaminant concentration goals negotiated with the regulatory agencies involved. ' 10.0 SUMMARY The initial work conducted at this site was performed by WHF to investigate the extent of petroleum ' hydrocarbon contamination in the subsurface caused by a former on-site UST. Soil and groundwater contamination were identified. Subsequently, as described in this report, Condor conducted an additional phase of groundwater investigation to further evaluate the vertical and lateral extent of groundwater ' contamination at the site. Based on review of the data obtained from previous phases of investigation and the current phase of investigation, it is Condor's opinion that the horizontal and vertical distribution of soil and groundwater contamination at the site has been adequately evaluated. Condor recommends that enhanced in-situ bioremediation, to include the injection of oxygen releasing compound, be conducted at the site. This recommendation is made contingent upon agreement with the ' regulatory agencies involved that the site can be closed in the future in accordance with the "low risk groundwater" criteria as described in Appendix B of the Tri-Regional Guidelines. This would involve demonstration that the groundwater plume is stable in extent, decreasing in concentration, and that potential groundwater receptors are well documented and not threatened. The reasons for proposing this approach include reasonable cost, potentially reasonable timeline, and reasonable chance for achieving site closure (given the regulatory agreement described above). The current groundwater conditions at the site could be described as generally low in concentration and fairly low in residual contaminant mass. In ' �.a CONDOR