Environmental Engineering Reference
In-Depth Information
HYDROPOWER OR FOSSIL FUELS?
table 9.2
Concentrations of Total Mercury in Great Lakes Sediments
and Probable Effect Level (PEL) Exceedances
In countries that rely on both coal and hydropower for a sig-
nifi cant component of their energy use, the “footprint” of
the Hg can refl ect a dynamic balance between Hg emissions
associated with coal combustion, and Hg contamination
that is exacerbated by reservoir creation and operation. The
current pace of reservoir construction in the Peoples Repub-
lic of China may be particularly problematic, given the vast
land areas involved. Estimates suggest that construction of
the Three Gorges Dam alone would offset coal-based emis-
sions of carbon dioxide and sulfur dioxide by 100 metric
tons and 2 metric tons, respectively (China Three Gorges
Project Corporation, 2002). Yet because of increased energy
demands, coal use continues to increase in China, despite
the additional generating capacity of the Three Gorges
project and its upstream reservoirs. Major upstream erosion
at the Three Gorges project will sustain continuing meth-
ylation. Therefore, the construction of new reservoir facili-
ties upstream of the Three Gorges project and elsewhere
in China will certainly, because of the reservoir effect and
ongoing water-level manipulation, exacerbate the impacts
of an already Hg-contaminated landscape. The situation
described here for the People's Republic of China is occur-
ring worldwide. Yet, in areas where Hg emissions from coal
and other sources are limited to the maximum extent, and
reservoir operations are conducted in a fashion that miti-
gates methylation (e.g., appropriate drawdown regimen,
reducing upstream sediment supply), the environmental
problems of Hg within reservoirs can be controlled.
Lakewide
average (
% Exceeding PEL
(0.486
Lake
g/g)
g/g)
Michigan
0.077
0
Superior
0.088
0
Huron
0.043
0
St. Clair
0.196
0
Erie
0.187
6
Ontario
0.586
62
SOURCE
:
USEPA (2006) and Marvin et al. (2004).
All the Great Lakes (Lake Superior, Lake Huron, Lake
Michigan, Lake Erie, and Lake Ontario) are large and
physically complex and have large watersheds and long
residence times. Spatial and temporal patterns of Hg and
other contaminants refl ect sources, lake sedimentology
and bathymetry, and circulation patterns (Marvin et al.,
2004b). Lakes Superior, Michigan, and Huron generally
have lower concentrations of Hg in their sediments than
Lake Ontario and Lake Erie (Table 9.2). In all the lakes, Hg
concentrations decline from shallow near-shore coarser
sediments to deep-water depositional basin sediment of
silts and clays (Marvin et al., 2004a, 2004b). Despite lower
sediment concentrations, fi sh concentrations of MeHg are
highest in Lake Superior (greater than 0.3 ppm).
Although there is considerable information on the spa-
tial distribution of Hg in Great Lakes sediments, concen-
trations of Hg in biota are less available and not linked
to sources of MeHg in sediments. Thus, Hg fate in these
aquatic food webs is poorly understood, just as it is in large
marine systems (Chen et al., 2008). Organic contaminants
are known to bio-accumulate in Great Lakes biota and the
long food webs, which include invertebrate predators such
as
Mysis
enhance biomagnifi cation of these persistent con-
taminants (Swackhamer et al., 1998). The fi sh consump-
tion advisories for Hg and other contaminants in the Great
Lakes are established on a state-by-state (Illinois, Indiana,
Michigan, Minnesota, New York, Ohio, Pennsylvania, and
Wisconsin) or province-by-province basis (Ontario and
Quebec) and differ for each fi sh species, lake, and political
entity (USEPA http://www.great-lakes.net/humanhealth/
fi sh/advisories.html). The consumption advisories due to
Hg comprise a small percentage of the total advisories for
each of the Great Lakes (Superior 4%, Huron 9%, Erie 2%,
Ontario 7%). However, they represent a greater proportion
(93%) of the advisories in inland lakes in the Great Lakes
basin, refl ecting both the broader range of contaminants
and their respective advisories in the Great Lakes and the fact
that the highest concentrations of Hg in fi sh are from smaller,
Large Lake Ecosystems: Lake Michigan
The Great Lakes are freshwater systems that are physically
much like marine ecosystems and therefore have much to
tell us about Hg transformation, bio-accumulation, and
trophic transfer in much larger and physically complex
systems. Several characteristics predispose these lakes to
potentially higher levels of bio-accumulation in fi sh. First,
the watersheds of these lakes are vast, as are the surface
areas for receiving high Hg deposition. Second, the lakes
are relatively oligotrophic, which results in higher biocon-
centration of Hg in particulates and lower growth rates in
fi sh, which is likely to result in higher fi sh concentrations.
Moreover, the food webs in these ecosystems contain inver-
tebrate predators and numerous piscivorous species, both of
which lengthen the food chain and increase biomagnifi ca-
tion of MeHg. In the Great Lakes basin, MeHg bio-accumu-
lation in fi sh has resulted in numerous fi sh-consumption
advisories. Great Lakes sediments are the primary sink for
contaminants, including Hg, which can be subsequently
resuspended and redistributed. Persistent contaminants
in the Great Lakes have been studied since the 1960s prior
to and since mitigative binational strategies were taken to
reduce environmental impacts (Cahill, 1981; Painter et al.,
2001; Rossmann, 2002; Marvin et al., 2002, 2004a, 2004b;
Gewurtz et al., 2008).
