Metal and Organic Contaminants
Effluent
from waste water treatment plants, combined sewer overflows, rivers
and the atmosphere are all significant sources of metals entering Massachusetts
and Cape Cod Bays. Manohar-Mahanaj and Beardsley (1973) and Butman (1975)
presented data indicating that a substantial fraction of the freshwater
flowing from the Merrimack river enters Massachusetts Bay. Gardner et
al. (1986) observed an isolated low salinity surface pool of water in
the offshore section of a transect from Salem sound into Massachusetts
Bay occupied in October, 1985. The low salinity water also contained
elevated concentrations of dissolved and particulate trace metals. Gardner
et al. (1986) attributed the source of the elevated metal concentrations
to the influence of the Merrimack River plume and identified the Merrimack
as a potentially major source of metals to Massachusetts Bay, perhaps
comparable to, or exceeding in magnitude, those of atmospheric deposition
and sewage effluent inputs to the Bay.
Metal
distributions in the western Gulf of Maine, Massachusetts and Cape Cod
Bays were determined in May of 1988 primarily to test the hypothesis
that metals discharged from the Merrimack River significantly influence
concentrations in Massachusetts and Cape Cod Bays (Wallace et al., in
preparation). The results of that study support this hypothesis. Surface
concentrations of dissolved Cu and Zn are significantly correlated to
the amount of freshwater present (salinity). Extrapolation of the metal:salinity
relationships to 0 salinity indicate concentrations in the freshwater
entering the Bay from the north to be consistent with a combination
of Merrimack River (with relatively high concentrations of metals) and
northern Maine river (with relatively low concentrations of metals)
sources in rough proportion to their respective discharge volumes. Merrimack
river concentrations used in this comparison were taken from the extensive
work of
Studer
(1995) on metal dynamics in the Merrimack River and those for the northern
Maine rivers from the few reliable measurements made in the Kennebec,
Penobscott and Androscoggin Rivers by Elva Romanow (unpublished). Most
of the stations occupied in the May 1988 survey of Wallace et al. were
located in the western Gulf of Maine and in the northern section of
Massachusetts and Cape Cod Bay system. A transect was run down the center
of Cape Cod Bay and across Stellwagen Bank but the number and location
of samples were insufficient to detect any significant elevation in
trace metal concentrations resulting from the flushing and southerly
movement of water from Boston Harbor, especially if water leaving the
Harbor was primarily transported along the western part of Cape Cod
Bay (see Section II.B.). The mean and standard deviation of dissolved
metal concentrations for all samples, samples taken north of Rockport,
samples taken in northwestern Massachusetts Bay between Gloucester and
Boston and those primarily in Cape Cod Bay are given in Table 1. It
should be noted that the use of these mean concentrations ignores the
systematic relationships between salinity and concentration for some
metals but nevertheless serves as a useful descriptor of the distribution
of these metals over the study area. The lack of any strong pattern
in distribution other than that controlled by riverine discharge may
be understood if the sources of these metals are examined using the
most recent available data.
There
have been a number of efforts to assess inputs of metals (as well as
those of organic contaminants and nutrients) to the Bays from the sources
indicated above, probably the most comprehensive of which is the recent
Menzie Cura & Associates report (1991) referred to above. Unfortunately,
until recently, reliable data on the concentrations and hence source
strength of inorganic and organic contaminants in atmospheric deposition
and riverine and sewage effluent discharges have not been available.
Reliable data are still rather limited in scope despite the urgent need
to alleviate ignorance in decisions regarding management of waste disposal
in coastal waters.
Source
terms for the introduction of metals and total polycyclic aromatic hydrocarbons
(PAHs) to the Massachusetts/Cape Cod Bays system, of which the Stellwagen
Sanctuary is a part, are reevaluated here where more recent, reliable
data for their concentration and fluxes from all major sources have
been made available. Dry and wet atmospheric deposition of selected
metals and PAHs for the period between July of 1992 and January 1993
have been reported by Golomb et al. (1994). The flux of metals from
the Merrimack River has been estimated by Studer (1995) for the period
between January 1989 and September of 1991 and the flux of PAHs from
the Merrimack River estimated by Menzie-Cura Associates (1995) based
on measurements made between April 1992 and May 1993. In addition measurements
of effluent concentrations of both metals and organic contaminants in
samples collected in 1993 from June to November obtained using improved
analytical techniques have been recently reported (Hunt et al., 1995).
These data were used to estimate the relative importance of these sources
to the Bays. Flux estimates are given in Table 2 and the relative contributions
from each summarized in Table 3.
Table
1: Metal Concentrations in Various Regions of Massachusetts and Cape
Cod Bays
The data in Table 2 reflects the assumption that all of the metal and
PAH flux associated with the Merrimack River discharge reaches Massachusetts
and Cape Cod Bays. Physical and climatological factors influencing the
extent to which the Merrimack discharge reaches Massachusetts and Cape
Cod Bays are discussed in the previous section and is likely to be less
than 100%. For example Geyer et al. (1992) estimate that of the freshwater
flow along the coastal region of the western Gulf of Maine, only about
50% enters Massachusetts Bay. If one assume that the Merrimack River
discharge is completely mixed with the along-shore component of fresh
water from the north before reaching the Bay, then the same percentage
of the Merrrimack flow would enter the Bay. Thus the fluxes from the
Merrimack listed in Table 3 may be a factor of two too high at times.
Nevertheless the Merrimack flux remains an important component of the
sources of metals to Massachusetts and Cape Cod Bays and is equivalent
to or exceeds the total effluent flux even assuming a 50% transport
efficiency. In fact the argument can be made that the influence of the
freshwater from the north and its associated contaminants may be of
greater importance with respect to Sanctuary water quality than the
more local discharges to the Bays. Even if a substantial fraction of
the freshwater does not reach the inner portion of the Bays most may
still be expected to flow through the outer sections of the Sanctuary.
Table
2: Contaminant Sources to Massachusetts and Cape Cod Bays Annual Inputs
in Metric Tons
Table
3: Source strength as percentage of total iputs.
Removal of contaminants in transit by particle scavenging and settling
or by volatilization (PAHs) may be important for metals such as lead
and PAHs that have high affinities for particles and thus lower the
efficiency of transport to the bays and Sanctuary waters. On the other
hand the efficiency of transport of metals with relatively low affinities
for particles may be quite high especially in view of the short time
scale of the transport of freshwater between the Merrimack river mouth
and Massachusetts Bay and the Sanctuary boundaries (a few days). However,
because of the generally low suspended matter concentrations and rapid
transit time between the Merrimack River outlet and Massachusetts Bay,
the major fraction of even the highly particle-reactive contaminants
will reach the Bay under most conditions.
The major
sources of copper to the Bays and the Sanctuary are the Merrimack and
sewage effluent discharges while for lead, the major sources are atmospheric
deposition and the Merrimack. (It should be noted that the major fraction
of each of the metals discharged by the Merrimack can be attributed
to sewage effluent discharged to the river and its tributaries (Studer,
1995).) Atmospheric deposition and the Merrimack River are the major
sources of cadmium and nickel. The major fraction (>70%) of PAHs
entering the Bays is from sewage effluent with relatively minor and
equivalent inputs contributed by the Merrimack and atmospheric deposition.
continue
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