Sediments
Sediments
in Massachusetts and Cape Cod Bays are extremely heterogeneous in composition
and distribution (Knebel and Circe, 1995). Sediments within the Sanctuary
boundaries range from a large area of fine-grained sediments in Stellwagen
Basin to the sandy sediments prevalent on the top of Stellwagen Bank.
Consequently the distribution of organic and inorganic contaminants
in the sedimentary environment of Massachusetts and Cape Cod Bay, including
those portions of the sedimentary environment within Sanctuary boundaries,
reflect this heterogeniety. In general both organic and inorganic contaminant
concentrations in the sediment reflect the proximity of the sediments
to sources, their organic matter content, texture (grain size) (which
usually covary), the overlying water column concentrations of the contaminant,
and the contaminant's affinity for the various solid phases present
in the sediment.
The various
data on contaminant concentrations in the sediments of Massachusetts
and Cape Cod Bays have been reviewed by Shea et al. (1991). The most
extensive data for sediments within the Sanctuary proper are those of
Boehm et al. (1984) (five samples, both organic and metal data although
metal data are unpublished but presented in Shea et al., (1991) and
Gilbert et al. (1976) (18 samples for metals in the soft-bottom area
of the Sanctuary). Unfortunately the PCB data reported by Gilbert et
al. (1976) is not comparable with the later data of Boehm et al. (1984)
due to differences in methodology (Shea et al., 1991).
Most
of the metal data reviewed by Shea et al. (1991) are based on partial
digestion of the sediment samples and are therefore operationally defined
by the particular digestion procedure used in each investigation and
do not represent the total metal present in the samples. None of the
samples taken within the Sanctuary boundaries were analyzed for their
total metal content. The lack of total metal data, as well as data on
appropriate crustal reference elements such as Al, prohibit attempts
to determine the relative contribution of inorganic detrital phases
of metals from that which might be present due to sorption by organic
matter and hydrous oxide phases of iron and manganese. Such information
is of use in discriminating between anthropogenic and natural inputs
to coastal marine sediments.
Despite
these limitations and, as Shea et al. (1991) note, that data reviewed
in their report are not directly comparable due to methodological differences,
the distribution of both organic and inorganic contaminants in Massachusetts
and Cape Cod Bays are generally consistent in that low levels are present
throughout the Bays. While the number of samples and spatial resolution
of the data reviewed by Shea et al. (1991) are not great, especially
in Cape Cod Bay, higher concentrations of both classes of contaminants
are generally found in the vicinity of the Massachusetts Bay Disposal
Site near the northwestern boundary of the Sanctuary and in patches
of fine-grained organic rich sediments in the north-western parts of
Massachusetts Bay.
The mean
and standard deviation of concentrations of total PAHs, PCBs and metals
in sediments collected within the Sanctuary boundaries are shown in
Table 4. Concentrations were generally low (relative to those in the
more nearshore areas of the Bays (Shea et al., 1991) and relatively
uniform, given potential differences in organic content and grain size
of the samples. (Only total organic carbon concentrations were reported
and only for the Boehm et al. (1984) samples.)
A more
comprehensive compilation of sediment contaminant data is being prepared
and should be available in the near future (Buchholtz ten Brink et al.,
in preparation). This compilation includes additional data from samples
taken in the eastern flanks of the Sanctuary and should allow a better
perspective of conditions over the entire area of the Sanctuary. The
more distant location of the eastern regions of the Sanctuary from point
sources as well as the physical separation afforded by the Bank's topography
serves to reduce the influence of sources within Massachusetts and Cape
Cod Bays. Thus inputs from the atmosphere and to a lesser extent the
Merrimack River are probably the principal source of contaminants to
both the water column and sediment in the eastern part of the Sanctuary.
Table
4: Mean concentrations (ug/g dry weight) of contaminants in sediments
collected within Sanctuary boundaries.
In summary both the available water column and sediment data suggest
that the extent to which the quality of these two media have been compromised
by metal and organic contaminants in the Stellwagen Sanctuary is measurable
but not large. The most probable sources having greatest influence on
the Sanctuary are riverine discharge from rivers located north of Cape
Ann, atmospheric deposition and sewage effluent. Estimates of their
relative input to Massachusetts and Cape Cod Bays made using the most
recent and reliable data available suggest that sources with the greatest
influence on the Sanctuary are the riverine and atmospheric sources.
Note however that the primary reason the riverine input is important
is directly related to the discharge of sewage effluent within the river
watersheds, primarily that of the Merrimack River.
The direct
impact of the sewage effluent discharged to Massachusetts and Cape Cod
Bays is less for several reasons. For most metals of the metals examined,
sewage effluent is a minor source (with the exception of Cu) to the
Bays. Sewage effluent is currently the dominant source of total PAHs
to Massachusetts and Cape Cod Bays, but physical circulation patterns,
removal of PAHs in the near-shore areas, and the low suspended matter
concentration in the Bays with correspondingly low removal by particle
scavenging to the sediments, greatly reduce their potential impact on
Sanctuary waters and sediments. Maintaining the reasonably high water
and sediment quality in the Stellwagen Sanctuary will depend on continued
efforts to control the introduction of contaminants of environmental
concern to the region. It must be recognized that these efforts will
need to address distant as well as local sources and that careful assessments
of the relative importance of these sources for each contaminant will
be needed in order to focus limited resources on cost-effective solutions.
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