Environmental Engineering Reference
In-Depth Information
depositional basins (Rossmann, 2002). When comparing
these depositional basins throughout the lake, the mean Hg
concentrations ranged between 120 and 160 ng g
1
and did
not differ signifi cantly among depositional areas. Hg fl uxes
measured across sampling sites also did not differ from basin
to basin except for Green Bay, which was much higher
largely because of riverine sources (Rossmann and Edging-
ton, 2000). Regional atmospheric Hg deposition accounts for
about 50% of the total Hg fl ux to Lake Michigan surfi cial
sediments, with the remainder coming from tributary inputs
(Rossmann, 2002).
slight declines in the Hg concentrations of salmon from
Lake Ontario (French et al., 2006), and there have also
been declines in fi sh Hg in lakes on Isle Royale in Lake
Superior that have been attributed to decreases in sulfate
deposition (Drevnick et al., 2007). However, concentra-
tions remain high in Lake Michigan, and it is not clear
what factors are responsible for this pattern. These fi nd-
ings suggest that there is not a direct linkage between
atmospheric Hg deposition and biotic Hg contamination,
possibly due to other controls or sources of MeHg to the
food web and that perhaps legacy Hg in sediments is sup-
plying MeHg to benthic and pelagic fauna (USEPA, 2006).
In summary, the linkages between Hg sources and biotic
end points are poorly characterized in the Great Lakes,
even in Lake Michigan, for which there has been a detailed
research effort as part of the LMMBS. In Lake Michigan,
atmospheric sources are considered to be extremely impor-
tant relative to tributary inputs except for in Green Bay,
where point sources are more important. Despite the loca-
tions of river inlets, the spatial distribution of Hg in water
is homogeneous and in sediments appears highest in depo-
sitional basins associated with fi ne sediments. The source
of MeHg to the food web is possibly methylation in the
sediments; however, given the depths and stratifi cation
of the Great Lakes, it is possible that methylation in lake
sediments is not closely linked to bio-accumulation pro-
cesses in the epilimnion. Perhaps there are other zones of
Hg transformation, such as methylation at the base of the
thermocline, where an oxygen minimum exists because of
decay of particulates, as has been proposed for the open
ocean (Lamborg et al., 2002; Sunderland et al., 2009).
Given the vast sizes and depths of these lakes, the mecha-
nisms and processes controlling Hg fate and transfer in abi-
otic and biotic components are different from smaller, shal-
lower lake ecosystems in the Great Lakes basin, where Hg
methylation and bio-accumulation result in higher concen-
trations in the food web. Thus, despite the relatively long
food chains, low productivity, and large surface areas for
capturing deposition in the Great Lakes, the Hg concentra-
tions in fi sh are generally not as high as those in the nearby
smaller lakes or other smaller lake ecosystems in the region.
MERCURY BIO-ACCUMULATION
Based on the longer food chains in Lake Michigan and
the other Great Lakes, one might expect higher Hg con-
centrations in the food web; however, this is not the case.
Bio-accumulation of Hg in plankton and fi sh in Lake
Michigan have been studied, and total Hg concentrations
in phytoplankton and zooplankton averaged 35.0 ng g
1
and 54.3 ng g
1
, respectively, with a biomagnifi cation fac-
tor (BMF) of 1.55 between them. Bioaccumulation factors
for phytoplankton and zooplankton (log Bioaccumulation
factor (BAF), 5.03 and 5.22, respectively) are higher than
for other lakes in the region, but fi sh concentrations are
lower (Watras and Bloom, 1992; Monson and Brezonik,
1998; USEPA, 2004). Hg concentrations measured in adult
coho salmon and lake trout averaged 69 ng g
1
and 139
ng g
1
, values well below Food and Drug Administration
action levels of 1000 ng g
1
, but these fi sh Hg values still
warrant restrictions on fi sh consumption based on EPA
guidance for fi sh advisories. In fact, only 3% of lake trout
and 9% of coho salmon fall into the unrestricted con-
sumption category. Thus, fi sh Hg concentrations in Lake
Michigan, although lower than those in smaller lakes in
the region still pose a risk to human health.
TEMPORAL TRENDS
Sediment cores and comparisons of sediment concentra-
tions at particular sites through time provide a temporal
record of Hg deposition in the Great Lakes. Across most of
the lakes, there have been declines in Hg deposition since
peak Hg loads in the 1970s-1980s (USEPA, 2006). In Lake
Michigan, there were peak Hg concentrations between
1930 and 1950 and a consistent decrease in Hg concentra-
tion between 1969 and 1975 that continued through 1981
(Cahill, 1981; Pirrone et al., 1998; USEPA, 2004). However,
temporal trends differ in other Great Lakes. For example,
surfi cial concentrations of Hg measured in Superior and
Huron in 2001-2002 are similar to those measured in the
1960s-1970s, which is not consistent with emissions pat-
terns (Gewurtz et al., 2008). Despite the declines in emis-
sions over the past 10-20 years in the Great Lakes region,
there has not been a concurrent decline in Hg in fi sh, bald
eagles, and herring gulls. However, one analysis shows
The Nyanza Superfund Site on the Sudbury River,
Massachusetts
The earliest known hot spots of Hg contamination have
been in the receiving waters of industrial point sources of
Hg. Probably the most well known site was Minamata Bay
in Japan, where the population was exposed to elevated Hg
in a nearby fi shing village (Harada, 1995). Other sites with
point sources have often involved industrial facilities such
as chlor-alkali plants that have discharged Hg into nearby
surface or coastal waters. The Sudbury River in Massa-
chusetts has been the receiving body for many organic
and inorganic chemicals, including Hg from a number of
companies that operated from 1917 to 1978 at the Nyanza
