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5.3 Selected Approach and Rationale to Achieve Cleanup Goals The nature, extent, and trends of the COCs support that in situ biodegradation is effective at the site and vicinity, and is limiting migration of the chemicals. Based on the review of possible technologies, MNA is an appropriate remedy for the site based on the following: • Monitoring wells at the downgradient edge of the plume show stable or decreasing dissolved-phase concentration trends over time(Appendix F). • Previous assessments within the leasehold have demonstrated that both aerobic and anaerobic biodegradation is occurring and is limiting off-site migration(Ash Creek 2008). • Given the fine-grained nature of the soil containing the bulk of the petroleum mass beneath the Facility, more active remedial alternatives are not feasible or will not be likely to significantly reduce the remedial time. • There are currently no domestic or municipal uses of the groundwater in the site vicinity, and land uses are not likely to change from industrial storage of bulk fuel or other chemicals in the foreseeable future. All of these factors support that MNA will be able to achieve the cleanup goals within a reasonable timeframe relative to other potential remedies. 5.4 Scope of MNA Plan Quarterly monitoring has been conducted at the site for more than 10 years and a large dataset has been accumulated that documents the decreasing extent of dissolved constituents in groundwater. The objective j of the MNA plan will be to provide sufficient data to confirm that natural attenuation is continuing to decrease the extent of the dissolved-phase COCs and to allow comparison of dissolved-phase concentrations of I� benzene and MTBE to COC cleanup goals. To achieve these objectives,we propose semi-annual sampling of wells with concentrations that have exceeded cleanup goals in the past year. These monitoring wells have been sampled quarterly for more than 10 years, establishing a sufficient baseline that supports a 'i reduction to a semi-annual frequency. In addition, we propose sampling select downgradient wells annually to document plume stability. As detailed in the conceptual model section of this report, the up to 10 years of quarterly monitoring data from these downgradient wells have defined the dissolved extent and documented its decreasing extent such that off-Facility concentrations are generally at or near water quality objectives in most wells. Therefore, it is anticipated that annual sampling will be sufficient to continue to document that off-site concentrations are at or near water quality objectives and the extent of the plume is stable. Groundwater samples will be analyzed for TPHg, TPHd, BTEX, and MTBE. Sampling activities will include collection of groundwater elevation data to document that gradients remain consistent with historical flow directions. Groundwater monitoring reports documenting the MNA program will be submitted semi-annually, Revised Cleanup Plan Page 21 NuStar Stockton Terminal-Stockton,California November 7,2012 1014-12