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 Chan nel, a sub-region of Boston Harbor containing a major combined sewer o verflow and highly contaminated sediment, was determined during three field surveys by measuring the disappearance of fluorescent tracers fr om the water column. Flushing by advective movement was quantified usi ng Rhodamine WT dye, a dissolved tracer which has negligible interacti on with suspended sediment. The fate of suspended particles was inferr ed from measured concentrations of fluorescent pigment particles which were initially well mixed with Rhodamine dye and which have a size ra nge and settling velocity comparable to the sewage particles of intere st. Dye and particle concentrations were measured by fluorescent spect roscopy of water samples obtained throughout the channel over a week f ollowing tracer introduction. Dye measurements indicate that channel w ater is replaced on a scale of 1-27 days, depending on tidal amplitude and phase during tracer release, and the magnitude of freshwater infl ow. Ratios of normalized particle concentration to dye concentration s uggest effective deposition velocities of 1.5-3.3 m day(-1); this is a n order of magnitude faster than observed in laboratory settling colum ns suggesting that removal of suspended tracer particles from Fort Poi nt Channel during our surveys may have been the result of scavenging b y 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 th e sediment-water interface. (C) 1998 Academic Press Limited.