Dredged Material Plume Dispersal in Central Long Island Sound

Abstract

A simulation model based upon in situ current velocity data and records of disposal events was developed to predict the chemical exposure field resulting from dredged material disposal plumes in Central Long Island Sound (CLIS) during the spring of 1983. In the model, plumes are assumed to form as subthermocline suspended particulate clouds which are uniform vertically and Gaussian horizontally. Horizontal cloud dimensions are allowed to grow with a constant diffusion velocity as the plume is transported by tidal currents. The model permits estimation of increases or elevations in suspended solids concentration above background levels at individual sites resulting from multiple disposal operations. Model predictions of time-averaged elevations compare favorably with those estimated from tissue residues of Mytilus edulis deployed in CLIS as part of a joint Environmental Protection Agency (EPA)/Army Corps of Engineers program to investigate dredged material disposal options. Predicted elevations ranged between 0.0 and 1.6 mg/L. Based upon bulk chemistry measurements, a dredged material concentration of 1.5 mg/L translates to a water column polychlorinated biphenyl (PCB) concentration of approximately 10 ng/L, or about one third of the EPA chronic water quality criterion for that compound. The model indicates the contaminant exposure field at five biological effects field stations to be derived primarily from plume passage, rather than from postdisposal transport from a nearby disposal mound, during the period of active dredged material disposal. Both near (<1 km) and far (>1 km) field dispersal of dredged material in the form of plumes is indicated to occur within this shallow marine system.