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SEDIMENT TYPES IN BOSTON HARBOR
DETERMINE THE EXTENT OF POLLUTION

The types of sediment in Boston Harbor correlate with pollutant impacts. This map categorizes harbor sediments into three types: erosional, depositional, and intermediate. (Note: shoreline sediment types are estimated based on very limited data. Map after Knebel et al 1991.)

Since the Boston Harbor Project began, the harbor's sediments have been changing. Levels of lead and other heavy metals are much less than what they were 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 the harbor, land contours, currents, and waves create different sedimentary environments, which can be categorized as erosional, depositional, and intermediate.

Erosional areas include much of the harbor’s shoreline as well as the Outer Harbor islands. They are rocky and support seaweeds and animals like snails, blue mussels, barnacles, and sea urchins.

The locations of the treatment plant outfalls in erosional areas minimized the local impacts of those discharges on the sediments. This is 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 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.

Depositional environments are found in large areas of the central, southern, and northwestern harbor. These muddy bottoms are home to animals like worms, clams, and crustaceans. In depositional areas, weak tidal currents or depressions in the sea floor 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 stick to solid particles and settle with them. MWRA’s monitoring focuses on depositional environments, specifically, sediment contamination,
sediment metabolism, and the health of the benthic community.

Intermediate areas alternate between depositional and erosional, depending on changing currents and waves.

Before 1991, the solids (sludge) and scum removed as part of primary sewage treatment were broken down by bacteria ("digested") after being separated out in settling tanks. Then, the mixture was discharged along with the chlorinated effluent into the harbor on the outgoing tide. This black, smelly substance, that included floating pieces of trash, represented the worst results of the old treatment plants, and the most memorable ones for boaters. Today, scum is landfilled. Sludge is processed into fertilizer pellets for gardening and landscaping. The days of scum and sludge in the harbor are, fortunately, long-gone.