Deposition of Contaminated Sediments in Boston Harbor Studied Using Fluorescent Dye and Particle Tracers

The residence time of water and suspended particles in Fort Point Channel, a sub-region of Boston Harbor containing a major combined sewer overflow and highly contaminated sediment, was determined during three field surveys by measuring the disappearance of fluorescent tracers from the water column. Flushing by advective movement was quantified using Rhodamine WT dye, a dissolved tracer which has negligible interaction with suspended sediment. The fate of suspended particles was inferred from measured concentrations of fluorescent pigment particles which were initially well mixed with Rhodamine dye and which have a size range and settling velocity comparable to the sewage particles of interest. Dye and particle concentrations were measured by fluorescent spectroscopy of water samples obtained throughout the channel over a week following tracer introduction. Dye measurements indicate that channel water is replaced on a scale of 1–2•7 days, depending on tidal amplitude and phase during tracer release, and the magnitude of freshwater inflow. Ratios of normalized particle concentration to dye concentration suggest effective deposition velocities of 1•5–3•3 m day−1; this is an order of magnitude faster than observed in laboratory settling columns suggesting that removal of suspended tracer particles from Fort Point Channel during our surveys may have been the result of scavenging by a bottom ‘ fluff ’ layer. This finding is consistent with our previous observation of particle deposition in Salem Sound, Massachusetts, U.S.A. and in controlled laboratory studies of particle aggregation at the sediment–water interface.