TYPES IN BOSTON HARBOR
DETERMINE THE EXTENT OF POLLUTION
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 harbors shoreline as well as the
Outer Harbor islands. They are rocky
and support seaweeds and animals like snails, blue mussels, barnacles,
and sea urchins.
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 Plantmore 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. MWRAs monitoring focuses on depositional environments,
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.