Environmental Impacts of PAH and Oil Release as a NAPL or as Contaminated Pore Water from the Construction of a 90-cm In Situ Isolation Cap

The placement of a sediment cap was the remedial alternative selected in the Record of Decision for the containment of PAH-contaminated sediments near the Wyckoff/Eagle Harbor Superfund site shoreline, a former log rafting area at this closed wood treatment site. Soft sediments with substantial quantities of nonaqueous phase liquid (NAPL) occurred in this area, which raised a concern that there would be environmental releases or potential cap failure in this area of the site. As part of the investigations to guide cap design, a laboratory bench study was devised to evaluate consolidation-driven NAPL and dissolved phase PAH permeation of the cap. Sediment cores collected from the site were extruded side-by-side into 20 cm diameter, 120 cm high acrylic columns to maintain sediment stratification. Synthetic seawater was added until approximately 60 cm of water covered the site sediment. The simulated cap material was added to each column in such a manner as to fall through the overlying water at a uniform rate to simulate settling velocities expected during a barge wash-off placement event. Vertical loads were applied incrementally to the cap/sediment columns until the total consolidation stress was equivalent to a 90-cm cap. Each column was extruded, inspected visually for the migration of NAPL, and sectioned into three layers with each analyzed for total petroleum hydrocarbons and PAHs. In all three test cylinders, there was no indication of impact to the top 10 cm of the cap (the biologically active zone). The results suggest that the chemicals detected at high concentrations in the native sediments would stay in place and not migrate through a overlying cap via consolidation-induced advection.