Contaminants in the Sediment
Pre-1983
sediment data from Massachusetts Bay, Cape Cod Bay and Boston Harbor
have been summarized by Manheim and Hathaway (1991; Manheim et al.,
1992). This extensive database, the Boston Harbor Data Management File
(BHDMF), contains over 7000 records, for a total of 1300 discrete sediment
samples (Figure 8). Sediment data published between 1984 and 1991 have
been compiled by Cahill and Imbalzano (1992), and are currently being
added to the BHDMF. As discussed previously, the Stellwagen Bank Sanctuary
contains areas of depositional, erosional and reworking sediments (Schlee
et al., 1973; Oldale and Bick, 1987; Pett, 1990; NOAA, 1993). Little
additional sediment data have been collected since 1991, although a
report on recent organic contaminant analyses for a number of Massachusetts
Bay stations is forthcoming (Shea, 1995, in preparation).
Depositional
areas (e.g. Stellwagen Basin) contain higher concentrations of contaminants
than do areas of high energy, such as the surface of Stellwagen Bank.
As stated above, more information concerning a contaminant present in
an area is necessary for the assessment of its toxic potential. The
bioavailability of the substance is one of these pieces of information.
Bioavailability is governed by a number of factors, including grain
size, percentage of total organic carbon (TOC), type of organic matter
present, acid volatile sulfide (AVS) concentrations, and geochemical
parameters such as the percentage of iron and manganese oxides (Di Toro
et al., 1991; U.S. EPA, 1993a, Ankley et al., 1994). High percentages
of TOC tend to bind PAHs, PCBs, and some metals, making them less available
for uptake by the biota. As a result, coarse-grained sandy sediments,
which are constantly being reworked and have low percentages of TOC,
may contain more bioavailable contaminants than fine-grained depositional
sediments (This is difficult to assess using available methodologies,
and cannot be explored further given the available data).
Figure
8: Locations of sediment contaminant and texture data archived in the
Boston Harbor Data Management File (BHDMF). (Reproduced from Manheim
et al., 1992.)
Several
methods may be used for assessing whether Stellwagen Bank sediments
are potentially causing adverse effects on the organisms that live in
contact with them. First, sediment contaminant concentrations can be
directly compared to EPA Sediment Quality Criteria (SQC; U.S. EPA 1993b,
c, d, e, f). However, SQC have only been set for five organic compounds
(dieldrin, endrin, acetnaphathene, fluoranthene, and phenanthrene) and
their application requires data on TOC as well as the individual contaminant.
Not all of the studies on Stellwagen Bank Marine Sanctuary sediments
contain TOC data.
Second,
sediment contaminant concentrations can be assessed using the National
Status and Trends (NS&T) approach (Long and Morgan, 1991; U.S. EPA
1992a). This is a correlative method in which laboratory toxicity data,
field studies and model data have been amassed and analyzed. Two criterion
points, the Effects Range-Low (ER-L) and the Effects Range Median (ER-M),
have been determined for 41 contaminants (12 metals, 18 PAHs, PCBs and
10 other organic contaminants). Sediments with chemical concentrations
below the ER-L should not exhibit adverse effects. Concentrations between
the ER-L and the ER-M may exhibit effects in sensitive organisms, whereas
those with concentrations above the ER-M will probably show biological
effects in most of the benthic organisms examined.
A third
method for assessing adverse biological effects is the Apparent Effects
Threshold (AET) method (U.S. EPA, 1992a,b). This is also a correlative
procedure which relies on the weight of evidence from a multitude of
matched chemical and biological effects data sets (laboratory toxicity
testing on field sediment samples). The AET for a particular contaminant
is defined as the sediment concentration above which an adverse biological
effect is always statistically observed. Although AET criteria should
be tailored to each geographical region, in reality differences in sedimentary
characteristics result in only a small number of data sets that are
large enough to allow region-specific AETs to be calculated. Thus, the
Puget Sound AETs will be used here to assess the potential for Stellwagen
Bank Sanctuary sediments to elicit adverse biological effects.
Gilbert
et al. (1976) conducted a comprehensive study of sediment metal and
PCB concentration throughout Massachusetts Bay (32 offshore stations
plus 4 coastal stations). While only one-third to one-half of the sites
were within the Stellwagen Bank Marine Sanctuary, comparison with the
other sites in Massachusetts Bay will give a regional perspective to
the sediment contamination distribution. A widespread, albeit slight,
elevation of metal (Cr, Hg, Ni, Pb, and Zn) concentration was observed,
particularly in the middle and northern third of Massachusetts Bay.
Stellwagen Basin, due to its depositional nature, exhibited the highest
metal concentrations of the region. Sediment concentrations of lead
exceeded the ER-L criterion at 23 offshore stations, mercury at 19 offshore
stations, zinc at 14, nickel at 12, and chromium at 11. Only rarely
were concentrations high enough in Massachusetts Bay sediments to exceed
ER-M and/or AET values (2 stations for lead and 1 for zinc). No exceedences
of ER-L, ER-M or AET were observed for cadmium, chromium, or tPCBs.
In addition
to presenting sediment contaminant concentrations, Gilbert et al. (1976)
calculated the ratio of anthropogenic to natural contributions of metals
to Stellwagen Basin sediments. Metal concentrations at a core depth
of 30cm were considered to be natural background concentrations, whereas
the surficial sediment concentrations were a mixture of natural and
anthropogenic sources. No significant contaminant enhancements to surface
sediments were observed for nickel or cadmium, and only a slight anthropogenic
enrichment was observed for chromium, copper, lead and zinc. A significant
anthropogenic enrichment was observed only for mercury (a finding that
was also observed in Buzzards Bay sediments).
These
data on metals and PCBs are complemented by a later study that examined
PAH concentrations in sediments from Boston Harbor, Massachusetts Bay,
and the Gulf of Maine (Windsor and Hites, 1979). Total PAH concentrations
ranged from 0.160 to 3.4 µg/g dry weight in Massachusetts Bay.
This range lies below ER-L and AET values.
More
recent work conducted by Boehm (1983, 1987; Boehm et al., 1984) involved
sampling of 10 stations in Massachusetts Bay, several of which were
within the Stellwagen Bank Marine Sanctuary. Total PCBs, tPAHs, individual
PAHs, and tDDT were all below ER-L and AET criteria for the August 1981
and January 1982 samplings. Fluoranthene, phenanthrene and acetnaphthene
concentrations were well below the new EPA Sediment Quality Criteria
values. One station (MB-6, at the northern edge of Stellwagen Basin)
exceeded the ER-L (but not the AET) for tPCBs during sampling conducted
during June and July 1983. In a later report, Boehm (1987) noted that
the highest concentrations of PCBs outside of Boston Harbor were found
near the Massachusetts Bay Disposal Site (MBDS) and in adjacent areas
of Stellwagen Basin.
Shea
et al. (1991) summarized most of the recent sediment data for Massachusetts
Bay, and presented a series of contour plots for PAHs, PCBs, lead, chromium
and copper, based on data from Boehm (1983, 1987), Boehm et al. (1984),
U.S. EPA (1989) and Gilbert et al (1976). Trace metals were found to
be 10 to 100 times lower in sediments of Massachusetts Bay than in those
from Boston Harbor or Salem Harbor. PAH, PCB, and metal concentrations
decreased with increasing distance from Boston Harbor, although elevated
concentrations were seen in Stellwagen Basin and near the MBDS. Shea
et al. (1991) noted a high degree of between-sample variation, which
masked any temporal trends in the data. Of the Massachusetts Bay sites,
only samples from MBDS exceeded or met ER-L (but not AET) for chromium,
lead, tPAHs and tPCBs. While the PAH values are quite low, the authors
speculate that there was a significant source of petroleum-related PAH
at the offshore sites, exhibited by a change in the Fossil Fuel Pollution
Index (FFPI).
Several
other recent studies reported sediment data for areas adjacent to the
Stellwagen Bank Marine Sanctuary. All of these studies showed a low
degree of sediment contamination when compared to levels needed to exceed
toxicity criteria. For example, surficial sediments taken from the UMB-8
site (7.5 nautical miles west of the northern tip of Stellwagen Bank)
were below ER-L criteria for cadmium, chromium, copper, nickel, lead,
and zinc, although the ER-L for mercury was exceeded (Gardner et al.,
1986). Samples taken at the Cape Cod Bay Disposal Site and its reference
site (Ocean Surveys, 1994) did not exceed EPA Sediment Quality Criteria
for phenanthrene or fluoranthene, and were below the ER-L criteria for
tPCBs, tPAHs, individual PAHs, silver, As, cadmium, chromium, copper,
mercury, lead and Sb. Lead and zinc concentrations at one station and
nickel concentrations at 11 stations exceeded the ER-L criteria (the
ER-M for Ni was exceeded at 2 stations). Pesticide concentrations, although
analyzed, could not be compared with ER-L, ER-M or AET values since
the analytical detection limits exceeded the criteria values.
As mentioned
above, elevated concentrations of contaminants have been found at the
MBDS (formerly the Foul Area) and the Industrial Waste Site, two sites
at the northern end of Stellwagen Basin. Given the function of these
areas, it is not surprising that many contaminants attain levels which
may adversely affect benthic organisms. This area has received rather
intense sampling over the last 20 years (Gilbert, 1975; Cudmore et al.,
1988; U.S. COE, 1988a,b, 1990; Pruell et al., 1989, U.S. EPA, 1989;
ADL, 1990).
Although
results can vary from one study to another due to spatial inhomogeneity,
concentrations of tPCBs, tPAHs, chromium, nickel, lead, and zinc measured
in several of these studies were high enough to exceed ER-M and/or AET
values. More generally, metals such as chromium, copper, mercury, lead,
nickel, and zinc, and tPCBs routinely exceeded ER-L values. Scientists
from Arthur D. Little (ADL, 1990) indicated that tDDT, chlordane, anthracene,
benz(a)anthracene, fluorene, phenanthrene and pyrene met or exceeded
ER-L criteria. In contrast, tPAH concentrations exceeded ER-L criteria
in some studies (Pruell et al., 1989; U.S. EPA, 1989) but not in others
(U.S. COE, 1988a; ADL, 1990).
In summary,
most of the sediments in Massachusetts Bay, including those in the Stellwagen
Bank Marine Sanctuary, have been impacted by metals, PAHs, PCBs and
pesticides, although not to such an extent that there will be appreciable
adverse effects on the benthic organisms. In general, metals such as
such as chromium, mercury, nickel, lead, and zinc can be found at concentrations
which exceed the NS&T ER-L criteria, indicating that only sensitive
species may be effected. While there have been sporadic reports of values
which exceed ER-M and AET values, it does not appear that contaminant
levels in Massachusetts Bay or Stellwagen Bank sediments are currently
eliciting significant toxicological effects. A similar conclusion was
drawn from a recent analysis of benthic community structure (Pederson
and Schull, 1995, in preparation).
While
inter-sample (inter-station) variability makes it difficult to assess
temporal changes, there are some indications that the sediments, even
those in Stellwagen Basin, should become cleaner as the years progress.
Since toxicant loadings from the MWRA' s Boston outfall have been declining
in recent years (MWRA, 1994), inputs to the sediments, even when the
new outfall is in place, should be lower than in previous years (MWRA,
1991). In addition, PAH and PCB concentrations in suspended particulates
collected from waters overlying the northern tip of Stellwagen Bank
and overlying Stellwagen Basin are currently below ER-L criteria (Menzie-Cura,
1995, in preparation). If this single sampling is representative, contaminants
in water-borne suspended particulate matter will not add to sediment
contamination, but will serve to dilute the contaminants already present.
Stellwagen
Basin (which includes the MBDS and the Industrial Waste Site) shows
clear evidence of significantly contaminated sediments which are probably
having an adverse impact on benthic species. There is a great deal of
variability from one sample to another, reflecting a markedly heterogeneous
geographical distribution of contaminants. Nevertheless, several samples
have contaminant levels of tPCBs, tPAHs, chromium, nickel, lead, and
zinc that exceed ER-M and/or AET criteria, indicating that adverse effects
are probable. There appears to be a decreasing gradient of contaminant
concentrations as samples are taken further away from the MBDS. It is
unlikely, however, that contaminants would be remobilized and impact
species on Stellwagen Bank. It is more probable that these contaminants
will remain in Stellwagen Basin and will eventually become covered with
cleaner suspended sediments. It is not entirely clear, however, what
effect the continued use of the MBDS as a site for dredge spoil discharging
will have on Stellwagen Bank's organisms.
continue
to next section
