As each new generation of ships is larger and deeper, dredging shipping channels is increasingly vital for keeping harbors navigable. One of the greatest challenges is disposing of the resulting contaminated sediments from urban harbors and ports that receive much land-based pollution.
At an MIT conference last week on "Dredged Material Management: Options and Environmental Considerations," some 200 scientists and engineers tackled this problem. They discussed using dredged material for wetlands creation or erosion control, evaluating risk assessment tools to test for sediment toxicity, and researching processes used for disposal options. Several Sea Grant programs, including the MIT Sea Grant College Program, sponsored the conference.
MIT researchers and colleagues described the approach for sequestering contaminated sediments that was used in recent dredging to deepen Boston Harbor. The solution was to create confined aquatic disposal (CAD) cells -- pits dug 40-60 feet below the floor of the Inner Harbor -- that were filled with the contaminated dredged sediments and capped with three feet of clean sand from the Cape Cod Canal. The underlying clean clay dug to create the cells was disposed of offshore.
The choice of CAD cells for Boston Harbor was an economic one, said conference co-organizer Eric Adams, a senior research engineer and lecturer in the Department of Civil and Environmental Engineering. "Since the sediments were contaminated and thus not suitable for open-water disposal, it was the most practical thing to do," he said. However, the technology is relativly new. As a result, "Boston is being watched carefully as a prototype for other locations, including nearby Gloucester, Salem, New Bedford and Fall River."
In related studies funded by MIT Sea Grant, the Army Corps of Engineers and other researchers from MIT, the University of Massachusetts at Boston and the Harvard School of Public Health have been examining the physical, chemical, biological and policy aspects of using CAD cells. They hope to provide a strong scientific basis for assessing the technical and environmental risks of capping in relation to other methods of disposing or isolating such sediments.
One study, presented at the conference by graduate student Sanjay Pahuja of civil and environmental engineering, highlighted laboratory work on the consolidation of "very soupy" sediments. A goal of his research is to determine how long one should wait before deposited sediments are strong enough to withstand the weight of a cap.
One surprise was that dredged materials require longer to consolidate than had been anticipated, said Judy Pederson, MIT Sea Grant coastal resources manager and conference co-organizer. That finding raises "new questions about environmental impacts from open pits without caps. This initiated an investigation into the major causes of resuspension of sediments in the harbor, with the finding that boat traffic causes much of the resuspension."
A version of this article appeared in MIT Tech Talk on December 13, 2000.