Temporal and Spatial Considerations
The above
conclusions are based on average annual inputs from the three sources
and as such need to be qualified by considering the temporal and spatial
scales of these sources. The magnitude of the atmospheric fluxes of
metals to the Bays is functionally dependent on the origins of the air
masses over the Bay at any given time. When the origin of an air mass
is from areas with high emission rates (to the west and northwest) fluxes
may be much higher. When from areas with lower or little emission sources
to the atmosphere (to the south and south east) fluxes will be much
lower. For example, aerosol concentrations of metals and their estimated
deposition to the New York Bight varied by over an order of magnitude
for some metals depending on wind direction at the time of sampling
(Duce et al., 1976). Variability on the time scale of hours to days
can be expected.
River
discharge varies on the time scales of days with rainfall and snowmelt
as the primary cause of the variance. Studer (1995) observed that during
periods of low flow in the Merrimack river, metals released to the river
were apparently stored within the river system (presumably in association
with settled particulate matter in the river). Under periods of high
flow, which are highly episodic in nature, these contaminants were resuspended
and transported out of the river system. Thus the major fraction of
the estimated annual flux can be expected to occur during the few days
to weeks when flow velocities are high enough to resuspend and flush
the contaminant laden particles from the river, such as during snowmelt
during the spring (if snowmass and the rate of melting is rapid enough)
and following periods of heavy rain in the watershed.
Relative
to the temporal variabilty in the two sources discussed above, the variability
of fluxes from sewage effluents may be the lowest due to the more constant
inputs into the sewage treatment system and the significant residence
time within the system itself. However this source is by far the most
"concentrated" of the three and therefore has a disproportionately
greater influence on local spatial scales, especially in view of the
fact that almost all effluent discharges occur within semi-confined
embayments along the peripheral areas of the Bays. The confined spatial
scales and high input concentrations make the resultant impact of such
discharges much greater than those of the more diffuse riverine and
atmospheric sources affecting the Bays. Thus concentrations of metals
in Boston Harbor can be observed which may be an order of magnitude
or more than higher than their respective concentrations in the central
regions of Massachusetts and Cape Cod Bays (Wallace et al., 1983).
Higher
water column concentrations of contaminants in the vicinity of such
discharges either directly or indirectly result in higher concentrations
in sediments adjacent to the outfalls. The direct influence occurs as
a result of the release and settling of contaminant laden particles
in conjunction with the effluent. Indirect influences result from partitioning
of contaminants between sediment particles and the overlying water column
concentrations of the contaminants in contact with them. Of fundamental
importance in controlling the contaminant loading of both metals and
inorganic contaminants in sediments is the organic content of the sediments.
Thus enhancement of the organic matter content of the sediments, either
directly by the release and incorporation of particulate organic matter
discharged in the effluent, or indirectly through the release of nutrients,
biological transformation of those nutrients into particulate organic
matter, and subsequent settling of their remains to the sediments, are
critical processes largely controlling the capacity of sediments to
accumulate and retain contaminants regardless of their source.
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