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2.5.5 Monitored Natural Attenuation Evaluation Report(Ash Creek, 2008) Based on the preliminary results in 2006 supporting the presence of microbial degradation, Ash Creek evaluated the potential for MNA of dissolved-phase hydrocarbon constituents associated with the Facility. To obtain sufficient data for the evaluation, MNA parameters from select wells (ST/WC-1A, PS/P-21, ST/MW-1, PS/MW-14, PS/P-11, PS/P-13, ACA-1A, PS/MW-15, and ACA-2A ST/WC-1 B, ACA-1 B, ACA-26, and PS/WC-3M) were measured for at least four quarters. Parameters included dissolved oxygen (DO), oxidation-reduction potential (ORP), pH, nitrate, nitrite, sulfate, alkalinity, methane, ammonia nitrogen, iron, orthophosphate, and potassium as shown in Table 4. The following lines of evidence were examined to evaluate MNA as a remedial option for the Facility: (1) DO and ORP conditions; (2) petroleum concentration trends across the lateral transect of the plume; (3) MNA parameters from selected wells transecting the plumes; (4) trends in petroleum hydrocarbon concentrations over time in perimeter wells (i.e., wells on the edge of the dissolved-phase plume); and (5)the dissolved-phase extent of hydrocarbons over time. 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 a viable remedial option for addressing petroleum hydrocarbons and MTBE near and downgradient of the Facility. 2.5.6 Vertical Groundwater Investigation (Ash Creek, 2009a) In 2008,Ash Creek conducted an additional groundwater investigation to further delineate the vertical extent of facility-related hydrocarbons in groundwater downgradient of the Facility. Approximately 21 soil explorations were advanced for purposes of soil screening and depth-discrete groundwater sampling in the A/B-, C-, and D-zones. The vertical groundwater investigation defined the extent of petroleum-related constituents in the A/B-, C-, and D-zones. Figures from the investigation illustrating the extent of TPH, benzene, and MTBE in the A/B- and C-zones are contained in Appendix C for reference. Petroleum hydrocarbons in the D-zone were generally low to non-detect. As shown on the figures in Appendix C, the horizontal extent of dissolved constituents in the A/B-zone extends less than 300 feet downgradient of the Facility. Dissolved constituents in the C-zone extend less than 500 feet downgradient. Concentrations of TPHd detected in groundwater downgradient of the Facility were higher than concentrations detected at the Facility, and a groundwater flow pathway was not identified between the Facility and these downgradient maximum detections. The distribution of the TPHd suggested that the TPHd detected downgradient of the Facility was the result of other off-site sources or due to interference by non-petroleum organics(silica gel cleanup was not used prior to analysis). Revised Cleanup Plan Page 7 NuStar Stockton Terminal-Stockton,California November Z 2012 1014-12