The Shaping of An Estuarine Superfund Site: Roles of Evolving Dynamics and Geomorphology

Abstract

Analysis of moored and shipboard hydrographic data along with suspended sediment data is used to provide the first comprehensive description of the dynamics and sediment transport processes of the Passaic River. The river has been highly depositional since dredging ceased in the 1940s and within this sedimentary record are high levels of contaminants including high levels of dioxins. Moored current meter data indicates that during low freshwater discharge both tidal period and residual flows favor a landward sediment flux. In contrast during moderate to high river flows the salt field is washed out of the system and results in a net downstream sediment flux that overcome the more persistent but weaker upstream flux during low river flow conditions. Consequently, with today’s channel morphology the Passaic exports sediment to Newark Bay that we estimate is approximately equal the annual loadings. This, together with historical bathymetric surveys, suggests that the channel is approaching geomorphological equilibrium. Bathymetric changes in the Passaic River over the past 60 years have significantly altered the river's ability to trap sediments and contaminants and are consistent with the fact that most of the dioxins released to the river in previous decades appear to be trapped within the channel, despite the fact that they were released within a tidal excursion of the mouth. The decline in trapping efficiency is also consistent with a simple scaling analysis that suggests that the salt water intrusion length, and thus the ability to trap sediment, scales with the cube of the channel depth. Consequently, as the estuary shoaled its ability to trap sediments and contaminants diminished rapidly.