Gowanus Canal Superfund Site. IV: Delineation of Potentially Migrating NAPL Layers for ISS Treatment

Prior publications in this series presented information on the nonaqueous phase liquid (NAPL) impacts, pore fluid saturation (PFS) relationships, stabilization/solidification (S/S) treatability work, and an in-canal in situ S/S (or ISS) pilot test at the Gowanus Canal Superfund Site in Brooklyn, New York, which was impacted by three historic manufactured gas plants (MGPs). This paper reports on the decision-making strategy applied for selecting the planned areas to implement ISS to a depth of 5 ft (1.5 m) into the native sediments after dredging of the overlying soft sediments in remedial target areas (RTA-1 and -2). ISS target areas were developed primarily from in-canal Tar-specific Green Optical Screening Tool (TarGOST) analysis of sediments at a 2-in. (5 cm) layer resolution and the development of an empirical correlation relating the TarGOST percent reference emitter (%RE) response to the NAPL PFS in the cores, and the measured NAPL PFS threshold above which NAPL was considered potentially mobile. A volume accommodation model (VAM) evaluated the ability of various individual NAPL PFS exceedances and their associated layer thicknesses in the sediment to trigger a PFS exceedance above the threshold value of the entire overlying horizon, resulting in NAPL breakthrough from as deep as 10 ft (3 m) into the native sediment. The output from the VAM was integrated with ArcGIS spatial mapping and visualization tools to generate Thiessen polygons that indicated the areas having overall NAPL exceedances. In this way, and coupled with engineering judgment, mobile NAPL areas were effectively targeted for ISS, totaling 82,798 and 227,297 ft2 (7,692 and 21,117 m2) in RTA-1 and -2, respectively, or 15,227 and 49,843 cu yd (11,642 and 38,128 m3) based on the aforementioned ISS mass thickness.