Fine-grained sediment and industrial waste distribution and dispersal in New Bedford Harbor and Western Buzzards Bay, Massachusetts

Abstract

New Bedford Harbor and its approaches form the estuary of the Acushnet River in southeastern Massachusetts. The estuary is weakly stratified and only partially mixed because river discharge is very small. It appears to be typical of the inlets of the coast of New England, and is a branch of a larger estuary - Buzzards Bay. Silt and clay are being transported into the estuary in suspension by landward-moving bottom currents that are driven by wave and tidal energy. These fine sediments come from Buzzards Bay, but may originate out on the continental shelf. Before the entrance to the harbor was almost completely blocked by a hurricane barrier, these sediments were accumulating in the harbor at rates of about 1-2 cm/yr in the deeps, and less than 0.5 cm/yr in the shallows. Construction of the barrier reduced the efficiency of tidal flushing, causing the rate of siltation to increase by a factor of 4-5. Outside the harbor, silt and clay accumulate in the drowned valley of the Acushnet and in related depressions at rates of 2-3 mm/yr. In the water column, silt and clay sized minerals are suspended together in organically bonded agglomerates. During sediment transport, the silt and clay become partially fractionated, probably by differential settling of the agglomerates. Because fractionation is more effective where wave and tidal energy are strongest, there is a smaller proportion of clay relative to silt in the harbor than there is seaward. Nevertheless, the net transport of clay is still landward. Fractionation due to differential settling also appears to have formed a very thin, soupy layer of clay-rich material at the sediment-water interface, that appears to carpet the study area. This layer seems to form a transition zone between the much more silty and less mobile subsurface sediments and the highly mobile suspensates of turbid near-bottom waters. Further study is needed to ascertain precisely the nature and persistence of this layer.