The Floor of the Harbor

SEDIMENT TYPES IN BOSTON HARBOR

This map shows three types of harbor sediments: erosional, depositional, and intermediate. Erosional areas include much of the harbor’s rocky shoreline as well as the Outer Harbor islands. Large areas of the central, southern, and northwestern harbor are depositional. These muddy bottoms are home to animals like worms, clams, and crustaceans. Intermediate environments have some characteristics of erosional and depositional areas, depending on weather. MWRA’s monitoring focuses on depositional environments, which can accumulate contaminants attached to particles that settle to the bottom. Note: shoreline sediment types are estimated based on very limited data. (Map after Knebel et al 1991.)

Since the Boston Harbor Project began, some of the most surprising stories have been about the rapidity of change in the sediments at the bottom of the harbor. For example, U.S. Geological Survey studies found that levels of lead and other heavy metals in the harbor’s sediments are about half of what they were 20 years ago. There is less organic matter settling on the harbor floor, and the sediments are more oxygenated, both of which are good for the bottom-dwelling community, or benthos. The benthos is not only increasingly abundant, it is more diverse. These are truly signs of a recovering Boston Harbor.

In depositional areas, weak tidal currents or depressions in the seafloor allow solids to settle and become soft sediments. These areas are most affected by pollution because toxic materials and oxygen-consuming organic matter tend to adhere to solid particles and settle with them. Such contaminants are often swept away from erosional areas, which have strong tides and lots of water movement. Intermediate areas are sometimes depositional and sometimes erosional, depending on changing currents and waves. The map shows where different sediment types are in Boston Harbor.

The locations of the Deer Island and Nut Island outfalls in erosional areas minimized the local impacts of those discharges on the sediments, because the solids were carried to depositional areas elsewhere in the harbor or further offshore. Depositional sediments in the harbor can collect contaminants from quite distant sources. A University of Massachusetts study conducted in the late 1980s illustrates this phenomenon. The study found that contaminants in a muddy area of Dorchester Bay did not come from a nearby CSO as expected, but from sewage sludge discharged from the Nut Island Treatment Plant—more than 4 miles away.

Sediment type determines where different benthic animals and plants live, and their exposure to contamination. The benthos is particularly vulnerable to contaminants in the sediments because most benthic organisms are immobile or move very little—they cannot escape if environmental conditions deteriorate. In contrast, fish and other mobile animals will actively try to move to a higher quality habitat when conditions get worse—for example, if dissolved oxygen drops to stressful levels.

Soft-bottom benthic communities that live in potentially more contaminated, depositional areas include worms, crustaceans, clams, and other animals, mostly living below the surface of the sediments. Because these communities have the potential for more exposure to pollution, MWRA’s benthic monitoring in the harbor focuses on soft sediment habitats and investigates:

1. sediment contamination,
2. sediment metabolism, and
3. the benthic community.

Sewage sludge discharges into Boston Harbor ended in 1991. Before 1991, the solids and scum mixture removed during primary sewage treatment was broken down by bacteria (digested) to reduce its volume and organic matter. Then, the black, smelly mixture was re-combined with chlorinated discharge and sent into the harbor on the outgoing tide. The sludge-scum mixture from NITP was digested and then pumped to an outfall about 5 miles away off Long Island. Today, scum is landfilled and sludge is processed into fertilizer pellets for gardening and landscaping. The days of scum and sludge in the harbor are, fortunately, long-gone.