observed at a given sampling station observed are mostly the function of the ambient 
hydrographic regime. Sediments in Massachusetts Bay are very heterogeneous. A 1984 
survey (Metropolitan District Commission (MDC) 301(h) application) documents gravel and 
cobble deposits over much of the surveyed area. The Massachusetts Bay's extensive 
gravel deposits are glacial in origin. 
Circulation in Massachusetts Bay results from tidal forces, wind-induced 
motion, and other factors such as the Earth's rotational and atmospheric pressure 
variations. The circulation is generally counterclockwise, and winds are typically offshore 
from the west. 
Water depths in 3oston Harbor outside the navigation channels range from 10 
to 15 feet at mean low water. Depths of nearly 90 feet occur in the channel at President 
Roads. The mean tidal rise and fall of Boston Harbor is approximately 9.5 feet. 
Maximum currents have been noted at Hull Gut at 2.6 knots during ebb tide and in 
President Roads at 2 knots both during ebb and flood tides (Figure 8). 
Boston Harbor sediments have been found to contain high concentrations of 
heavy metals, particularly in the Inner Harbor and northern area of the Outer Harbor. 
Under the Massachusetts criteria for the classification of dredge or fill material, most of 
the Harbor's sediments would be classified under category two or three. These sediments 
are therefore subject to a more thorough evaluation with respect to biological impacts of 
dredging or filling than that which is required with class one material. 
Data on Boston Harbor sediment characteristics suggest that high 
concentrations of metals found in the Outer Harbor are associated with fine-grained 
sediments and organic matter. The limited data on the toxic organic compounds DDT and 
polychlorinated biphenyl (PCB) suggest that they are also associated with fine-grained 
sediments and organic deposits. 
The reported concentrations of toxic metals and synthetic organics in Harbor 
sediments are of concern due to the potential for bioaccumulation in organisms dependent 
on benthic organisms as a food source. Flounder and lobster tissues throughout the Harbor 
have been found to contain these toxic chemicals in varying concentrations. Sediments in 
the Harbor, particularly in its northern parts, are finer overall and have a higher organic 
fraction than those outside the Harbor. There also appears to be evidence of enrichment 
of metals in the most recent sediment layers in some areas of Massachusetts Bay. In 
general, coastal stations and the stations to the south and west of the foul area in 
Massachusetts Bay show elevated levels of metals in the sediments. 
In the area of water quality, even though most waters in Boston Harbor meet 
the water quality standards established by the Massachusetts Division of Water Pollution 
Control, Harbor waters have higher concentration of pollutants than are found offshore in 
Massachusetts Bay. Water quality around the Outer Harbor islands and in Hingham Bay is 
the highest in the Harbor. 
15 
