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Files - 3 - 10 August The remaining comments on the work plan are based on this conceptual model of the site. NEED TO ACHIEVE FULL CHARACTERIZATION OF EXTENT OF SOIL IMPACTS: The work plan is unclear as to whether excavation will be taken to 5 feet or 8 feet below surface and whether the projected amount of soil to be excavated (300 yards) is a predictive estimate or a proposed maximum. If feasible, the entire lateral and vertical extent of contamination should be determined and impacted soil removed. Samples from the sidewalls and base of the excavation should be taken in numbers apporpriate to provide representative coverage, and should be analyzed for all COCs. In situ remedial actions may be required if significant levels of contamination are left in place. Further, clean fill materials placed in a contaminated pit may in future be considered contaminated waste materials themselves. Based on the site conceptual hydrological model, it is possible that water will be encountered, or will flow into, the pit at any depth near or below five feet. I recognize the difficulty and added expense of excavating in saturated soils. However, the references to 8 feet, a depth at or below the expected water table, suggest that Advanced GeoEnvironmental is prepared to work below the saturated zone, dewatering if necessary. Excavation should be continued vertically to the limits of soil contamination as determined by field screening methods (olfactory, visual, and/or OVA) or to limits of safety and physical or economic feasibility as determined by the field supervisor. The excavation should be continued laterally until field screening reports apparent non-detection.of COCs, or until site factors such as structures or physical barriers intervene. The field activity logs and the work report shall document the basis of the decision to cease excavation. MONITORING WELL CONSTRUCTION AND INSTALLATION: Six monitoring wells constructed of two-inch i.d. Schedule 40 PVC are proposed, with five-foot screens extending from about 10 to 15 feet below surface. This is below the peat layer and below the top of the saturated zone. The wells will be constructed through the augers, with seal tremmied in. This construction appears reasonable based on the site model, although the proposed screen placement is likely to miss any product that may be floating or be mobilized by fluctuating water levels. Also, the actual soil types present below 8 feet depth are unknown, and may be inappropriate for the proposed screen interval, slot size, or filter pack. Actual as-built construction of monitoring wells, including screen intervals and filter pack, should be determined by conditions encountered in the field, and rationale for as-built well design and any field decisions should be documented in the work report. If site conditions are determined to warrant screen coverage at shallower or deeper depths, staff recommends well clusters be considered as a possible future phase. Concerns about cross-contamination during well drilling can probably be addressed by using conductor casing. The proposed well construction is intended to minimize the potential for a pathway allowing contaminated groundwater to rise into the highly permeable peat. But the plan also proposes to backfill the soil excavation pits, which will be 5 feet or more deep, with clean crushed rock or pea gravel, an action that also potentially provides a path for movement of groundwater under saturated flow.