Environmental Engineering Reference
In-Depth Information
shallower lakes than the Great Lakes themselves. In fact, the
largest fi sheries in the Great Lakes, especially Lake Erie, pro-
duce lower Hg than typical commercially available fi sh.
deposition, Hg cycling, and Hg bio-accumulation in biotic
end points in Lake Michigan have not been made.
Hg inputs to the Lake Michigan system were characterized
by measurements in atmospheric components and via trib-
utaries and the development of a deposition model. Given
its large lake surface area and low watershed:lake area ratio,
the model identifi ed atmospheric deposition as the primary
pathway for Hg input to the lake, accounting for 84% of the
estimated total annual input of 1403 kg (Landis and Keeler,
2002). However, the particulate and vapor-phase Hg at the
urban sampling site near Chicago were higher than other
more rural stations, a pattern seen in other regional studies
of Hg sources and sinks (Keeler, 1994; Mason et al., 2000).
The deposition model estimated that the Chicago/Gary
urban area contributed about 20% of the total annual input,
suggesting that Hg controls in that region could signifi -
cantly decrease Hg inputs to the lake (Landis et al., 2002).
Moreover, meteorologic analysis determined that this urban
area also impacted all other LMMBS sampling sites, mak-
ing it a major point source of deposition for the entire lake
(Landis et al., 2002). Inputs to Lake Michigan from tributar-
ies, although a lesser component of the total annual inputs,
were also highest in urban and industrialized watersheds
and lower in forested and wetland-dominated watersheds.
In contrast, MeHg concentrations were highest in tributaries
draining agricultural and forested areas and lowest in the
Chicago/Gary urban area.
Despite differences in tributary inputs of Hg, water-
column concentrations of particulate and dissolved Hg
were homogeneous across 15 sampling stations, again sug-
gesting that the primary source of Hg is atmospheric rather
than riverine. Moreover, concentrations were lower than
in the smaller and shallower lakes studied in the region
(Watras et al., 1998), which may be related to lower DOC
and particulate matter concentrations in Lake Michigan.
Unlike other studies of smaller, shallower lakes, Hg concen-
tration in the epilimnion is higher in Lake Michigan than
in the hypolimnion, which may be more characteristic of
larger, deeper lakes in which sediment resuspension is min-
imal and atmospheric inputs dominate.
LAKE MICHIGAN MASS BALANCE STUDY
Among the Great Lakes, Lake Michigan is one of the most
thoroughly studied for a whole array of contaminants
(organics and metals), chemicals (e.g., nutrients, carbon, dis-
solved oxygen), and biologic and meteorologic characteristics
through the Lake Michigan Mass Balance Study (LMMBS;
USEPA, 2004). Hg and MeHg were among the contaminants
measured in the open-lake water column, tributaries, fi sh,
lower pelagic food web, sediments, and atmosphere
(Figure 9.7). The results of the LMMBS are documented in
a series of papers and a USEPA report (Mason and Sullivan,
1997; Hurley et al., 1998a, 1998b; Sullivan and Mason, 1998;
Landis and Keeler, 2002; Rossmann, 2002; Vette et al., 2002;
Marvin et al., 2004a, 2004b; USEPA, 2004). The LMMBS
objectives for Hg were to estimate the loading rates, establish
a baseline, predict benefi ts of loading reductions, and under-
stand the processes governing Hg cycling and availability
in Lake Michigan (USEPA, 2004). Although vast amounts
of data have been collected, the links between atmospheric
CANADA
USA
Manistique River
Beaver Island
Menominee River
Sleeping Bear
Dunes
Fox River
Manitowoc
Pere Marquette
River
Sheboygan River
Muskegon River
Muskegon
Milwaukee
Milwaukee River
Chiwaukee Prairie
Grand River
Kalamazoo River
WI
SPATIAL TRENDS
South Haven
St. Joseph River
Mercury concentrations in Great Lake sediments vary greatly
(see Table 9.1) with the highest concentrations in Lake Ontario
(0.586 mg kg
1
), in which 63% of sites exceed the probable
effect level (0.486 µg g
1
) for Hg in sediments (Canadian
Council of Ministers of the Environment [CCME], 1999). As
compared with the other Great Lakes, surfi cial sediments
in the main bay of Lake Michigan are relatively uncontam-
inated with Hg (mean concentration, 0. 078 mg kg
1
). How-
ever, Green Bay, an embayment of Lake Michigan, is contam-
inated as compared with the other Great Lakes (Rossmann
and Edgington, 2000). The distribution of Hg in sediments
in the main bay of Lake Michigan is largely driven by the
lake bathymetry and physical currents, and is highest in
Benton Harbor
IIT Chicago
Chicago SWFP Intake
Grand Calumet Harbor
MI
OH
Indiana Dunes
Atmospheric station
Tributary station
Biota station
Sediment station
Water-column station
IL
IN
FIGURE 9.7
Map of sampling locations and sample types in Lake
Michigan. IIT = Illinois Institute of Technology; SWFP = South
Water Filtration Plant.
