Contaminants in the Water
The adverse
effects that water-borne contaminants may have on marine organisms can
be assessed in two ways: (1) comparison of contaminant concentrations
in the seawater with Environmental Protection Agency marine acute and
chronic water quality criteria (U.S. EPA, 1991; and (2) comparison of
aqueous concentrations with Maximum Acceptable Toxicant Concentrations
(MATC) (Suter and Rosen, 1986, 1988). Both methods are based on acute
and chronic laboratory toxicity testing, although the EPA marine chronic
criteria are also based on results from bioburden and aquatic vegetation
tests. EPA marine acute criteria have been established for 72 compounds,
and both acute and chronic criteria for 46 (including 3 PAHs, 6 pesticides,
tPCBs, dioxin and 9 metals). MATCs are defined as the geometric mean
of the NOEC and LOEC values derived from chronic toxicity tests (Suter
and Rosen, 1986, 1988). In most cases, MATCs are either the same or
higher than the corresponding EPA marine chronic value. However, there
are a few instances where the MATC is actually more conservative than
the EPA criteria. Additionally, there is often an MATC available where
no EPA criterion has been set.
There
are only a few studies which have provided contaminant concentrations
in the water column within or adjacent to Stellwagen Bank. Due to analytical
and sampling problems, all water column data prior to the 1970s are
considered suspect. This leaves a handful of recent studies which have
used adequate quality control/quality assurance procedures.
Gilbert
(1975) conducted metal--cadmium (Cd), copper (Cu), chromium (Cr), nickel
(Ni), lead (Pb) and zinc (Zn)--and PCB analyses on water samples collected
from six stations within and to the south of the Massachusetts Bay Disposal
Site (MBDS). Samples were collected during 4 seasons (December 1973;
April 1974; July 1974 and October 1974). Copper concentrations exceeded
or were close to the EPA marine chronic criterion during all sampling
periods. In addition, lead levels exceeded the chronic criterion during
the time when active dumping of dredge material occurred (July 1974).
PCB concentrations in one bottom sample also exceeded the chronic criterion
during this period. None of the analytes exceeded the MATC values.
The Massachusetts
Bay Disposal Site was resampled by the U.S. Army Corps of Engineers
in 1985 and 1986, the U.S. Environmental Protection Agency (1989; metal
analyses only) in 1986 and 1987, and by Battelle Ocean Sciences in 1992
. In the EPA sampling, mean mercury concentrations exceeded both the
marine chronic criterion (>0.025 µg/l) and the MATC (>1.1
µg/l). However, the measured values were close to the analytical
detection limits, and therefore could overestimate the true values.
Copper concentrations approached the water quality criterion, but did
not violate it. The September 1985 water samples, and one replicate
from January 1986 (U.S. EPA, 1989), exceeded the marine chronic water
quality criterion for tPCBs, but were below the acute value. In a more
recent study (Battelle, 1992), sixteen individual PAHs, PCBs, selected
pesticides and 8 trace metals were measured in whole water samples (unfiltered).
No exceedences of EPA criteria or MATCs were found.
A similar
trend toward cleaner waters was observed in another Massachusetts Bay
site 2.5 nautical miles west of the MBDS, and 7.5 nautical miles west
of the northernmost tip of Stellwagen Bank. When sampled in October
1985 (Gardner et al., 1986), mercury was found to exceed the EPA marine
chronic criterion (>0.025 µg/l) at this site (designated UMB-8)
and at several sites further inshore. Particulate, rather than soluble,
forms of mercury were the cause of the criterion violation. None of
the other metals sampled--cadmium (Cd), copper (Cu), chromium (Cr),
iron (Fe), nickel (Ni), lead (Pb) and zinc (Zn)--exceeded water quality
criteria. When this station was resampled in July and August 1987, none
of the analytes measured (metals, PAHs, PCBs, chlorinated pesticides)
violated the EPA water quality criteria or MATCs (Robinson and Ryan,
1988). Hg was found to be more than an order of magnitude lower in 1987
than in 1985, even though the same investigators measured it on each
occassion.
Two other
recent studies in Massachusetts Bay indicate that the waters contain
very low concentrations of contaminants. Water samples taken from the
waters over Stellwagen Basin and at a site closer to Boston Harbor (Battelle
Ocean Sciences, 1987) were free of exceedences of metal and PAH water
quality criteria. Similarly, analysis of the particulate fraction of
water samples taken from 5 sites on Stellwagen Bank in May 1992 and
from one Stellwagen Bank site sampled in October 1992 demonstrated that
PAH, PCB and chlorinated pesticide concentrations were all below EPA
water quality criteria (Menzie-Cura, 1995, in preparation). It therefore
appears that concentrations of water-borne contaminants are currently
present at very low levels. Based on both EPA water quality criteria
(U.S. EPA, 1991) and MATC values (Suter and Rosen, 1986, 1988), the
concentrations of toxicants in the water column above the Stellwagen
Bank Marine Sanctuary are below the levels known to elicit direct toxicological
effects on the biological organisms living there.
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