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blood concentrations of Hg from insectivorous marsh birds
inhabiting the wetlands in GMNWR were generally higher
than blood from piscivorous birds (hooded mergansers and
kingfi shers) sampled in this area. It also appears that the
potential for population-level risk for birds may be great-
est at the end of the summer, when the adult birds have
accumulated dietary Hg throughout the summer. At some
sites with higher Hg loads, this may affect the ability of
some adults to successfully return to the area and breed
in the spring. This potential risk has been noted in the
South River in Virginia, where tree swallows have been
studied in relation to a point source of Hg (Brasso and
Cristol, 2008).
Thus, Hg from point sources in river ecosystems can be
transported downstream and result in the formation of
new sediment hot spots in habitats that are more favor-
able to the methylation of Hg than the original source area.
The Sudbury River is no longer a system in which Hg bio-
accumulation is highest near the source. Transport and
methylation downstream have resulted in a “moving target”
of contamination and a much more complex remediation
challenge. Nonetheless, Hg bio-accumulation and exposure
have not resulted in any discernable enduring ecologic risk
to aquatic and terrestrial organisms in the watershed.
Perch
Sediment
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14 000
120
12 000
100
10 000
80
8000
60
6000
40
4000
20
2000
0
0
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Units
FIGURE 9.9 Total Hg concentrations in sediments and yellow perch
across Reaches 2-10 in the Sudbury River, showing the elevated levels
in fi sh 30 km downstream from the Hg source. ( Source: Data from
Bart Hoskins, USEPA Region 1.)
fi sh, and invertebrate endpoints (USEPA, 2007). Across the
10 sampled reaches of the river, the peak sediment Hg con-
centrations did not coincide with peak Hg concentrations
in predator fi sh, suggesting that Hg in sediments is not spa-
tially linked with bio-accumulation in the food web (see
Figure 9.9). For each reach of the river, multiple end points
were evaluated to assess risk using a weight-of-evidence
approach. Despite the history of Hg contamination known
to exist in the Sudbury River and the high risk posed by
the contamination still existing in the sediments, most
of the biotic end points, particularly the piscivore species,
which are commonly targeted in mercury uptake stud-
ies, did not show any population-level risk in any of the
reaches of the river. Based on this assessment, it appears
that Hg contamination resulting from the Nyanza site dis-
charges is not substantially impacting ecologic resources
residing in or using the Sudbury River drainage. Note that
Conclusions
The fate of Hg and MeHg in aquatic ecosystems has been
studied in a wide range of ecosystem types, revealing that
there are multiple mechanisms that facilitate elevated
exposure of humans and wildlife to MeHg (Table 9.3). In
some cases, the sources of Hg (via high deposition over
large surface areas or point sources) are most signifi cant;
in other cases, conditions promoting Hg methylation in
table 9.3
A Summary of Sources and Strength of Each Mercury Enhancement Mechanism
for Each of the Case Study Systems Discussed in This Chapter
Transformation/
bio-availability
System
Hg sources
Bio-accumulation
Trophic transfer
Adirondacks
High atmospheric
deposition
Strong
High
High (low
productivity)
Everglades
High atmospheric
deposition
Spatially variable
Spatially variable
Variable
Reservoirs
Atmospheric, point
sources
Strong because of
reservoir formation and
fl uctuation
High after
reservoir formation
Variable
Great Lakes
Large area for
receiving atmospheric
deposition, also point
sources
Major sources poorly
known
Low
Potentially high
because of long
food chains
Sudbury River
Major point source
Spatially variable
Low
Low
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