Sources of Contaminants to the Water
One of
the primary sources of contaminants of environmental concern to the
Sanctuary and its surrounding environment are the direct discharge of
sewage effluents into Massachusetts and, to a lesser extent, Cape Cod
Bays. In addition, we now know that more distant sources significantly
affect contaminant loading to the Bays. Most notably, atmospheric deposition
and the discharge of contaminants to Gulf of Maine waters north of Cape
Anne, principally from the Merrimack River, also represent substantial
sources of both inorganic and organic contaminants to Massachusetts
and Cape Cod Bays (Gardner et al., 1986; Wallace et al., in preparation).
Direct discharge of sewage effluent to the coastal watersheds which
drain into the Bays also affect the water quality of the Bay. The most
recent review of existing data concerning such sources has been presented
by Menzie-Cura & Associates (1991) in a report commissioned by the
Massachusetts Bays Program.
Atmospheric
sources are a result of contaminant emissions to the atmosphere from
both local as well as more distant locations to the west (primarily
from the northeast and mid-western sections of the U.S.) and the subsequent
transport of air masses and their entrained contaminants from those
regions to the east. These airborne contaminants are removed in transit
by gravitational settling and washout of aerosol particles and their
associated contaminants and, for compounds or elements with significant
gas phase concentrations, by absorption across the air-water interface.
The Merrimack
River receives its contaminant burden primarily from wastewater treatment
plant effluents and to a lesser extent from combined sewer overflows
and non-point runoff. Because of the low suspended matter concentrations
in the river (reflecting a low supply rate of suspended matter to the
river) and seasonal periods of high runoff, there is little accumulation
and storage of sediments (and their associated contaminants) within
the main stem of the river system itself (Studer and Wallace, in preparation).
Consequently contaminants introduced into the main stem of the river
from its tributaries, direct effluent discharges, and other sources
within the drainage basin of the Merrimack are conveyed downstream to
the Merrimack River estuary. Because of the small size of the estuary
and the fact that most of the suspended matter is transported under
conditions of high flow, most of the particle-associated contaminants
are flushed from the river-estuary system and enters the Gulf of Maine.
Once
in the nearshore waters of the Gulf of Maine the discharge form the
Merrimack is entrained in the coastal current flowing to the south (see
Section II.B.) where much of the fresh water and associated contaminants
from the Merrimack may enter Massachusetts and Cape Cod Bays and Sanctuary
waters. The observational evidence for this scenario has been largely
obtained as a result of recent research supported by the Massachusetts
Bays Program, The Massachusetts Institute of Technology's Sea Grant
Program, and the cooperative efforts of the Massachusetts Water Resources
Authority and United States Geological Survey. Prior to that time the
only data available, for the most part, was produced by narrowly focused,
site specific studies that contributed little to the needed broader
understanding of the physical circulation, chemistry and biology of
the Massachusetts/Cape Cod Bays system within which assessment of anthropogenic
perturbations of the system might be reasonably analyzed.
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