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Recirculation/Bioaugmentation. As described in Section 2.6, a pilot study to evaluate the effectiveness of a recirculation bioaugmentation system was performed at the site. Because residual hydrocarbons are contained in the upper silt unit at the site, the pilot study system wells (both extraction and injection wells) were installed completely within this unit. The pilot study demonstrated that MTBE (and associated degradation products, such as TBA) are successfully degraded by biological processes. As would be expected, the most significant degradation was observed in wells that were consistently exposed to the augmented water flow—changes in groundwater gradient during the pilot study reduced the apparent effectiveness in wells farther from the injection points. Hydraulic control was difficult to achieve and maintain in the silt unit, so delivery of sufficient oxygen and nutrients for increased degradation of residual hydrocarbons in the upper silt(relative to natural processes)was therefore limited. While the system did result in an impact to the underlying sand unit (with significant petroleum degradation observed in the deep monitoring well PS-4), the pilot study results suggest that injection and/or extraction from the lower sand within the A-zone would be needed to effectively address dissolved-phase concentrations in this layer. Because the bulk of the hydrocarbons are contained in the upper silt unit, it would not be time efficient to operate the system only within the sandy layer. As stated above, operating the system within the silt layer would not achieve a significantly decreased remediation time relative to natural processes. Anaerobic In-Situ Biological Treatment. Anaerobic bioremediation technologies have been shown in field and laboratory conditions to successfully degrade MTBE, TPH, and BTEX. However, anaerobic processes are slower and are more likely to be incomplete for MTBE (which could cause a buildup of TBA concentrations)depending on specific site conditions. An anaerobic enhanced bioremediation pilot study was conducted at the adjacent SFPP parcel (LFR, 2005). Bench scale respirometric tests conducted on samples from the SFPP site confirmed that site microbes were capable of degrading petroleum hydrocarbons with the addition of nitrates. A pilot study was conducted where nitrates were applied to the ground surface and allowed to infiltrate into the subsurface. The results of the pilot study demonstrated that the nitrates were being utilized to assist in the anaerobic degradation of hydrocarbons. However, it was ultimately concluded that the technology was infeasible due to the large amounts of nutrients(e.g., nitrates)that would be required to reduce the petroleum hydrocarbon mass. The results showed that the nutrient addition did not significantly increase the biodegradation in shallow groundwater and therefore was not significantly enhancing the natural degradation occurring at the site. Bioventing and Biosparging. Bioventing involves the installation of wellpoints into the unsaturated zone and either injecting air or causing air to circulate through the subsurface. Biosparging involves installing wellpoints screened below the water table and injecting air. Hart Crowser conducted bioventing and biosparging pilot studies at the SFPP site in 1994 (Hart Crowser, 1994a and 1994b). The results of the Revised Cleanup Plan Page 19 ak NuStar Stockton Terminal-Stockton,California T_ November Z 2012 1014-12