PREVENTING SEDIMENT RECONTAMINATION IN AN URBAN WATERWAY, TACOMA, WASHINGTON

Under the Environmental Protection Agency's (EPA) Superfund program, contaminated bottom sediments are being remediated in Thea Foss and Wheeler-Osgood Waterways in Tacoma, WA, at a cost of approximately $88M. The waterways are located in an urban basin where the cumulative effect of ongoing sources of polycyclic aromatic hydrocarbons (PAHs) and phthalates to the waterway (conveyed via stormwater, aerial deposition, marinas, and tar seeps) could potentially recontaminate the sediments in the years following the completion of the cleanup action.To protect the sediments from stormwater recontamination, the City of Tacoma, in consultation with EPA and the Washington State Department of Ecology, has developed an approach for source-control decision-making. The primary goal of Tacoma's source control program is to ensure that stormwater discharges will not recontaminate the bottom sediments, either alone or in combination with other potential sources. To address this goal, Tacoma uses a multifaceted approach consisting of aggressive source control efforts, a comprehensive monitoring program (including stormwater, stormwater sediment, and bottom sediment quality), and computer modeling to predict the cumulative impacts of source reductions on future sediment quality conditions in the waterway. Although Tacoma's source control and stormwater monitoring programs have been in place for many years, they are being adapted to more specifically target sediment recontamination issues based on the results of the recent Superfund investigations.Although source control efforts by the City and others are believed to be sufficient to prevent recontamination of the waterway, the effectiveness of these efforts will be verified following the cleanup action. The City has developed tools to provide an advance warning of a trend toward recontamination such that adaptive measures may be implemented before recontamination actually occurs. These tools include long-term sediment monitoring in the waterway and computer-aided predictions of sediment quality using the EPA-supported WASP model. If a trend toward recontamination is evident, Tacoma will evaluate the likely cause of recontamination. To the extent recontamination is attributed to stormwater discharges, Tacoma will then evaluate additional control and/or treatment strategies as necessary.Tacoma's efforts in the Thea Foss Waterway may help other municipalities to understand how we can best protect sediment quality in our urban waterways.