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bioventing pilot study indicated that the bioventing was not successful in overcoming the anaerobic conditions and could not provide sufficient air flow through the fine-grained soils to successfully promote biodegradation. Hart Crowser concluded that bioventing would not be an appropriate technology to address the petroleum hydrocarbons contained in the silty soil. The results of the biosparging pilot study indicated that it was feasible to sparge air into the saturated zone; however, the radius of influence of each injection point was small. Therefore, to effectively address the hydrocarbons in the silty clay, numerous injection points would be required. This technology therefore appears infeasible for the facility because: (1) it is not able to effectively address the hydrocarbons in the silty clay soil, which contains the bulk of the hydrocarbons, without numerous, closely spaced injection points; (2) the numerous injection points will likely cause groundwater mounding beneath the Facility and the current stability of the plume would be disrupted; and (3) the technology could not effectively address the hydrocarbons beneath Tank 3302. Monitored Natural Attenuation. MNA relies on natural processes to achieve site specific remedial objectives. Natural attenuation processes include biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological transformation or destruction of constituents. Following the results of the Groundwater Characterization Results Report(Ash Creek, 2006b), an evaluation of the potential for NINA was completed (Ash Creek, 2008). As detailed in Section 2.5, results of the evaluation showed that even with residual petroleum hydrocarbons present near AST 3302, the extent of dissolved-phase + hydrocarbons has been steadily decreasing due to both aerobic and anaerobic microbial degradation. The evaluation determined that MNA would be an effective remedial option for addressing petroleum hydrocarbons and MTBE near and downgradient of the Facility. Ozone Injection. Ozone is the tri-atomic form of oxygen. Ozone has a high oxidation potential and can remediate petroleum hydrocarbons via chemical oxidation or by enhancing biological oxidation. Ozone can be introduced to the subsurface as either a gas (e.g., ozone sparging) or liquid (e.g., liquid ozone injection). The STTC conducted a pilot study of ozone injection (SECOR, 2003) and has implemented a full-scale ozone barrier wall. However, the STTC has not recommended this as a source area remedy because ozone can be corrosive to buried pipelines and other utilities and may not be safe for use within the active bulk terminals area. Groundwater Extraction and Treatment. Groundwater extraction and treatment systems are most effectively implemented to maintain hydraulic control of dissolved-phase constituents. In the Addendum to Cleanup Plan(Ash Creek, 2007) the effectiveness of a groundwater extraction and treatment to address dissolved-phase petroleum hydrocarbons in a reasonable amount of time was evaluated. As described in Section 2.7, analysis of a groundwater pump-and-treat system determined that between 180 and 4,500+ years would be required to remove sufficient hydrocarbon mass in order to achieve remediation goals supporting the conclusion that pump and treat would not accelerate the cleanup over an MNA approach. As a result, a pump-and-treat remedial alternative is not recommended Revised Cleanup Plan Page 20 NuStar Stockton Terminal-Stockton,California November 7,2012 1014-12