Archive of Spotlight Feature Articles

Dredging Harbors and Disposing of Contaminated Sediments

by Andrea Cohen,
MIT Sea Grant College Program

Bustling harbors are integral to the commerce of cities throughout the world. And dredging - or digging out material from the seafloor - is integral to keeping many harbors navigable to increasingly large ships.

Yet removing sediments is a tricky business, because along with providing transport, oceans have also historically been dumping grounds. The result is that harbors such as Boston Harbor bear the toxic legacy of the industries that spewed waste into them for decade upon decade. Boston Harbor has cleaned up its act remarkably since the early 1980s, when the city was slapped with a lawsuit because of its polluted waters. Sludge is no longer released into the harbor, secondary wastewater treatment facilities are in place, the sources of pollution have been reduced, and just this month (September 2000), the outfall pipe, which carries treated wastewater through a tunnel 9.5 miles out into Massachusetts Bay, went on line.

But two pieces of this clean-up have yet to be completed: combined sewage overflows (CSOs) still exist, and materials in the harbor still contain organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), as well as heavy metals. Thus, when the U.S. Army Corps of Engineers and the Massachusetts Port Authority went about dredging the harbor to allow for better ship navigation, they found themselves with the challenge of having to dispose of those contaminated sediments, which, while not directly harmful to humans, do present risks to marine life. The option chosen for the Boston Harbor Navigation Improvement Project was one deemed to be cost-effective and environmentally sound: digging pits for the contaminated sediments and capping them with clean sand. Specifically, the project involved what are known as confined aquatic disposal (CAD) cells, whereby in-channel pits were dug 40 to 60 feet below the seafloor, filled with dredged contaminated sediments, and capped with three feet of clean sand from the Cape Cod Canal.

It was one of the first times such a solution had been attempted in such magnitude, and along with promising deeper channels for ships, it also offered researchers the opportunity to study the effects of CAD cells, says Eric Adams, senior research engineer and lecturer in MIT's Department of Civil and Environmental Engineering. In a six-year, multi-disciplinary project funded by the MIT Sea Grant College Program, Adams teamed up with MIT Sea Grant coastal resources manager Judy Pederson and other researchers from MIT, the University of Massachusetts-Boston and The Harvard School of Public Health to examine the physical, chemical, biological, and policy aspects of using CAD cells. The scientists 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.

Indeed, CAD cells are just one alternative. Others include not dredging a harbor at all; capping sediments in situ with clean sediments; disposing of them at upland sites; using them to create wetlands; and treating them with technologies that isolate or destroy contaminants.

The choice of CAD cells for Boston Harbor was an economic one, says Adams. "Since the sediments were contaminated, and thus not suitable for open water disposal, it was the cheapest thing to do." It was also one of the first times that CAD cells had been used on such a large scale. As a result, says Adams, "it's a prototype that the Corps is looking at" to see if this would be appropriate elsewhere. "It's been fairly common to cap sediments on a level bottom-either because they were accidentally discharged there or because they were deliberately dumped there," he notes. "But besides a couple of applications in the New York Harbor area, this is the first significant activity in which cells were dug to create an unlevel bottom, with a small footprint."

To deepen areas of the harbor from 35 feet to 40 feet, workers used an environmental dredge-a bucket that makes a cleaner cut and is water tight so as not to disturb sediments-to scoop up water and sediments. This soupy mixture was loaded onto a barge and transported to one of about ten pits dug within the channel. The pits were then filled with the water and contaminated sediments, and the sediments were allowed to consolidate for one to six months before being capped.

Adams notes that he, his colleagues and students working on the MIT Sea Grant Marine Center project have learned a good bit thus far that may help in planning the disposal of contaminated sediments in other harbors. "We've learned about how much sediment released from a barge actually makes it into a cell. It doesn't all fall as a nice little cloud. There's a stem that's residual. We've learned something about what makes that stem occur." Adams notes that while this may not be significant in a shallow harbor like Boston, it would matter in a deeper harbor.

The researchers also discovered that placing the more contaminated sediments down deeper decreased the chance of their being released. They've learned that by giving the sediments a longer period to settle out-say three months as opposed to one-there's a better chance of their being contained by the blanket of clean sand. And Adams notes that "We've learned about how rapidly organisms recolonize an area when you change it."

One surprise in the project, says Pederson, was that dredged materials require longer to consolidate than had been anticipated. That finding raises "new questions about environmental impacts from open pits without caps," she states. And, she notes, "this initiated an investigation into what are the major causes of resuspension of sediments in the harbor with the finding that boat traffic causes much of the resuspension."

The study also has produced tools that should help coastal managers plan future disposal of contaminated sediments. Often, the public and managers feel their input in such matters is ignored. To help make this process more collaborative and interactive, MIT graduate student Scott FitzGerald developed a Geographic Information Systems (GIS) software for a PC that allows managers and the public to look at numerous data layers such as sediment contamination, habitats, sediment type, bathymetry, special interest areas, and physical oceanography. With an interactive GIS application, interested parties would be able to give input throughout the site selection process and get feedback on the relative importance of various data layers to proposed disposal sites. According to Pederson, who served as FitzGerald's thesis supervisor, the GIS project offers a unique way of ensuring environmentally sound decisions.

Pederson and Adams are gearing up now for a symposium and workshops to be held at MIT from December 3-6, 2000, titled "Dredged Materials Management: Options and Environmental Considerations." Sponsored by several Sea Grant Programs, the conference will address scientific and technical issues related to nearshore disposal choices, financial and legal issues, and policy implications. Concurrent one-day workshops will focus on the use of CAD cells to manage contaminated sediments in ports and harbors; sediment toxicity and risk assessment tools; and the use of dredged materials for erosion control and wetlands creation.

Meanwhile, coastal planners are keeping a close eye on the Boston Harbor Navigation Improvement Project. "Providence is looking closely at what's been done here," says Adams, as are Massachusetts towns such as New Bedford, Fall River, Salem, and Gloucester.

Those cities may also discover what Adams points out to be a larger issue in dredging a harbor: the intended scope and aim of the project. For instance, the Boston Harbor project was never intended as environmental remediation, but simply as a way to deepen the harbor. As a result, only about half of the contaminated sediments are actually being moved and capped. "In a way it's like dusting half of your bedroom floor, or raking half of your leaves. It really isn't as helpful as it could be because the wind is going to redistribute the leaves," states Adams.

As to the success of the disposal and capping process, Adams says: "It's been reasonably successful, and that success has improved with experience." They [the U.S. Army Corps and their contractors] have taken cores through the sediments to make sure they are capped and that the capping material is reasonably intact." However, testing whether or not the project has actually improved the environment is a lot harder to ascertain. One way researchers will test this is by taking measurements of contaminant concentrations in the sediments, and by observing how worms recolonize the sediments. And as deeper ships make their way into Boston Harbor, Adams, Pederson and their colleagues will be continuing their work, gathering scientific data to help coastal managers and policy makers best deal with contaminated sediments in their ports and harbors.

[9/25/00]

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