Design of Sorbent Cap for Control of Seepage and Sequestration of Coal-Tar NAPL and PAHS

The 2,400-meter (8,000-foot)-long Thea Foss Waterway in Tacoma, Washington, is undergoing a U.S. Environmental Protection Agency Superfund cleanup to address sediment contamination. Seepage of coal tar-like non-aqueous phase liquid (NAPL) and related polycyclic aromatic hydrocarbons (PAHs) near the head of the waterway presents an especially problematic condition that if not controlled will in time lead to recontamination of sediment. Batch and column treatability testing was conducted using several sorbent materials and NAPL and sediment from the waterway to confirm that the remedial objectives could be met and to develop empirical-based design parameters. The composition and dimensions of the sorbent cap were selected based on U.S. Army Corps of Engineers protocols for subaqueous capping, field and laboratory test results, and performance modeling. The Thea Foss Waterway application will be the first time this technology is used for NAPL remediation.