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Mr. Brattain October 19, 2020 Page 6 trends suggest that concentrations have been stable, high frequency data collection is not necessary. 2. Recent COC data trends: This criterion evaluates recent trends in COC concentrations to determine if high frequency data collection is beneficial for remedial performance assessment.For instance, COC concentrations in certain wells may change gradually under the effect of extraction and the concentration changes are not dynamic enough to warrant high frequency data collection. 3. ISCO Byproduct Concentrations and Proximity to the ISCO Transect: This criterion evaluates Cr(VI)and residual permanganate concentrations to assess the potential to impact the extraction wells. For instance, sentinel wells in the proximity of extraction wells provide much better definition of Cr(VI) in extracted groundwater as opposed to wells located further upgradient. Cr(VI) concentrations in wells further upgradient are slower to change and therefore do not warrant high frequency data collection. Proposed Modifications The proposed modifications to the monitoring program and well network are presented in Table 2 and visually presented in Figure 2. This section summarizes the locations where a reduction in monitoring frequency is being proposed, locations that are proposed to be excluded from current monitoring and locations that are being proposed to be destroyed. Based on a comparison with the criteria listed above, the proposed modifications can be broadly classified as follows: • Phase 1 Final Remedy treatment zone wells within the plume footprint are being proposed to be monitored annually, reduced from the current quarterly monitoring. It must be noted that some of the locations, immediately downgradient of the Phase 1 ISCO transect may have elevated Cr(VI) and/or permanganate concentrations. However, these wells are complemented by downgradient wells in closer proximity to the extraction wells that are more helpful to assess the potential for assessing Cr(VI) in extracted groundwater. Exceptions to this include MW-128A, MW-205B, MW-214B, MW-215B (Phase 1 treatment zone wells), that are being proposed for reduction from a quarterly to semi- annual frequency because while COC concentrations are somewhat elevated and decreasing at this location, concentrations do not change enough over time to warrant quarterly monitoring. engineers I scientists I innovators