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r 7 0 CONCLUSIONS AND RECOMMENDATIONS 71 CONCLUSIONS • The discussion of the various options tends to indicate that the action alternatives fall into three general categories in both cost and effectiveness 1) Options 6 and 7, which require extensive excavation and involve high costs, 2) Options 4 and 5 which involve bioremediation and moderate costs, and 3) Options 2 and 3, which involve a conditional closure backed by extensive groundwater monitoring and no additional corrective actions • The most effective options — 6 and 7 — are extremely costly, and are not warranted at this time without groundwater impacts • Options 4 and 5 are intermediate in both cost and effectiveness However, these alternatives will not result in reduction of TPH to background or even 100 mg/kg, while requiring site remediation activities beyond 10 years at substantial additional cost • Options 2 and 3 appear to address regulatory concerns, control cost, and meet public health protection objectives, while allowing for additional work if monitoring results indicate that impacts to groundwater are observed 7 2 RECOMMENDATIONS • Based on the site investigation data, the fact that over 47% of the original hydrocarbon mass has been removed from the site, and the feasibility analysis of remedial options, Dames & Moore recommends conditional closure for the AFPC site Under the proposed conditional closure, AFPC would continue quarterly groundwater elevation measurement and annual groundwater monitoring of five on-site wells, as described under Option 2 Annual groundwater monitoring reports would be submitted to PHSIEHD and the RWQCB Additionally, if groundwater elevations exhibit a rising trend indicating increased potential for significant groundwater impacts, the agencies would be notified within a specified period SAC12710 28 ' DAMES & MOORE