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LFR Inc. thin zone of higher permeability sediments in the lower A-zone aquifer, between the depths of approximately 72 and 82 feet below ground surface (bgs; Figures 7 and 8). This area of PCE- and related Hazardous Substances-affected groundwater is the target for the enhanced reductive dechlorination (ERD) remedy proposed in this Phase V IRA Detailed Plan. On the basis of the review of the existing groundwater quality data and the local hydrogeology, LFR proposes to implement ERD in the A-zone aquifer. ERD employs an easily degradable carbon substrate solution, which is injected into the groundwater via injection wells. The dilute carbon substrate mixture leads to the production of excess hydrogen, which initiates a succession of biological events in the subsurface, all of which contribute to enhancing the rates of reductive dechlorination of the VOCs present in groundwater. For large plumes, ERD systems are typically configured as permeable reactive barriers (ERD barriers) to intercept and treat a contaminant plume. ERD barriers typically consist of either rows of substrate injection wells or a solid-substrate trench located perpendicular to the direction of groundwater flow. Passive biobarriers typically use slow-release, long-lasting substrates (e.g., Hydrogen Releasing Compound [HRC®], vegetable oils, or mulch) that are designed to remain in place for long periods to maintain the reaction zone. Contaminant mass is delivered to the treatment zone via natural groundwater flow. Capital and operating costs for a passive ERD barrier configuration are typically lower than for plume-wide configurations because of a limited treatment area. However, life-cycle costs could be significant if the source upgradient from the ERD barrier is not addressed. As previously discussed, Hazardous Substances in the historical source zone are being addressed by the Phase IV IRA. Groundwater data obtained from the B zone indicate that total Hazardous Substances are transforming from PCE to TCE, cis-1,2-DCE, and finally to ethane and ethene, and appear to be fully undergoing naturally anaerobic dechlorination, which is effectively remediating this zone. Review of the time-concentration graphs presented in Appendix A shows the steady decline of VOC concentrations in each of the B-zone wells, with many of them being at or below Maximum Contaminant Levels (MCLS) at this time. Given the nature of the natural transformation of the Hazardous Substances within this deeper B-zone sand channel, LFR recommends monitoring this zone, but does not propose any ERD injections. A discussion of the feasibility of natural complete degradation of Hazardous Substances in the B zone is presented in Section 3.2. The objective of the Phase V IRA is to reduce concentrations of Hazardous Substances in A-zone groundwater and to monitor the natural attenuation of Hazardous Substances in the B zone in the vicinity of the Bank of Stockton. A contingency provision for implementing ERD in the B zone if favorable geochemical conditions do not persist is presented as part of the Phase V IRA. To meet the objective, LFR proposes to perform the following activities during the Phase V IRA: wp-PhV_IRA-Sep07-Fi=1-06750.dm:1fr Page 3