Fall 2000/Winter 2001 Table
Harbors and Disposing of Contaminated Sediments
Andrea Cohen, MIT Sea Grant
removing sediments in Boston Harbor. Photo: Great Lakes Dredge
& Dock Co.
are integral to the commerce of cities throughout the world. And
dredging is essential for keeping harbors navigable to increasingly
large ships. However, removing sediments is a tricky business, because
harbors bear the legacy of past industrial and domestic disposal
practices that haven't always measured up to today's standards.
Thus, while Boston Harbor has cleaned up its act remarkably since
the early 1980s, its sediments still contain organic contaminants
such as polycyclic aromatic hydrocarbons (PAHs), as well as heavy
metals. When the U.S. Army Corps of Engineers and the Massachusetts
Port Authority went about dredging the harbor, they also faced the
challenge of disposing of those contaminated sediments.
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. To minimize disturbance of the contaminated sediments,
workers used an environmental dredge-a bucket that makes a
cleaner cut and is more water-tight than a conventional clam shell
promising deeper channels for ships, the dredging also offered researchers
the chance to study the effects of CAD cells, says Eric Adams, senior
research engineer and lecturer in MIT's Department of Civil and
Environmental Engineering (CEE). In a six-year, multi-disciplinary
project funded by MIT Sea Grant, 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 (HSPH) to examine the physical, chemical, biological,
and policy aspects of using CAD cells. The work began four years
ago and project 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.
In one effort
graduate student Sanjay Pahuja is working with principal research
associate Jack Germaine and professor Ole Madsen of MIT's Dept.
of CEE, experimenting with MIT's automated fall cone device to determine
the effect of initial water content and sediment depth on the rate
of consolidation within CAD cells. Their work should help managers
determine how long to wait before the cells are strong enough to
cap. Recent graduate Chunhua Liu, now with Parsons Engineering Science,
worked with Tim Ford of HSPH studying the effects of submarine
discharge on a cap's ability to isolate toxic metals contained in
the underlying sediments. And former student Dave Shull, now with
the University of Maine, worked with Gene Gallagher of UMass-Boston
to evaluate mixing caused by benthic organisms and the role this
plays in establishing minimum cap thickness.
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.
Adams recently organized a symposium and workshops at MIT titled
"Dredged Materials Management: Options and Environmental Considerations."
The conference, sponsored by MIT Sea Grant and several other Sea
Grant Programs, drew some 200 scientists and engineers, who explored
scientific and technical issues related to nearshore disposal choices,
financial and legal issues, and policy implications. Concurrent
one-day workshops focused 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
Adams also served on a Technical Advisory Committee organized by
Massachusetts Coastal Zone Management to oversee Boston's dredging
activities. They note that the CAD cell technology used in Boston
Harbor is relatively new and, as a result, "Boston is being
watched carefully as a prototype for other locations, including
nearby Gloucester, Salem, New Bedford and Fall River."
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,
less than half of the contaminated sediments were actually 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 remaining
leaves," explains Adams.
New England District, US Army Corps of Engineers.
most important finding, says Pederson, has been that dredged materials
require longer to consolidate than had been anticipated. This 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."
As to the overall
success of the disposal and capping process, Adams says: "Its
been reasonably successful, and that success has improved with experience.
The project contractors have taken cores and made other indirect
measurements (e.g., using acoustic sub-bottom reflection) to make
sure the sediments 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. One way researchers have
been testing this is by taking measurements of contaminant concentrations
in the sediments, and by observing how benthic organisms re-colonize
thesediments. 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.