Geology Reference
In-Depth Information
The contamination from trace elements in the
Great Lakes is not so severe as these examples; how-
ever, the distribution and concentration of several
trace elements in Lake Erie illustrate the impact of
industrial activity on the lake.
The discovery of
mercury
in Lake St. Clair, the
Detroit River, and Lake Erie in the late 1960s led to the
banning of sport and commercial fishing in these
waterways because of the experience in Minimata Bay,
Japan, where dozens of individuals were poisoned
after consuming mercury-rich sea-food from the bay.
The bay had been contaminated by the disposal of
mercury compounds originating in the waste from a
plastics factory.
The major mercury contributors to Lake Erie
were identified as chemical plants in Wyandotte,
Michigan, and Sarnia, Ontario, Canada. These plants
manufacture chlorine gas and caustic soda by using
mercury cells in the electrolytic process. The rate of
accidental loss of mercury from these two plants was
estimated to be about 50 lbs/day from 1950 to 1970,
but greatly decreased thereafter. In the early 1970s, the
normal concentration of mercury in Lake Erie water
was less than 0.0002 ppm, but in bottom sediments it
ranged from 0.026 to more than 3.0 ppm, and the back-
ground averaged less than 0.07 ppm.
The major use of
chromium
is in the electroplating
industry. This element also appears in only minute
concentrations in water. Background chromium
concentrations in water and in the sediment on the
bottom of Lake Erie are less than 0.004 and 32 ppm,
respectively.
3. a. On Figure 14.1 draw several arrows to indicate what
you believe are the major directions of water flow from the
Detroit River to Buffalo.
b. Explain your reasoning.
4. If concentrations of both mercury and chromium are near
background levels at point A, describe the expected land use
on the north shore of the lake.
5. a. Where would you expect bottom-dwelling organisms to
contain the greatest concentration of mercury?
b. The least?
6. Should commercial fishing have been banned throughout
the lake, along the southern shore, or only in the western
part? Explain.
7. Is the chromium contamination in Lake Erie reflected in
any of the major lakeside municipal water supplies? Explain.
QUESTIONS 14, PART A
Two sets of analyses, from 1971 and 1997/1998 sediment
cores, provide the opportunity to determine change in the
Lake Erie system over time. We begin with the earlier study.
DATA FROM 1997/98 SAMPLING
In 1997/98, another set of samples was obtained to
determine trends in contamination and improvements
in environmental quality as a result of measures to con-
trol mercury and other pollutants. Ixi addition to fewer
additions of contaminants to Lake Erie, contaminants in
the upper layers of bottom sediment may have changed
because of erosion and deposition of sediment. Mercury
concentrations in sediment for 1997/98 are reported in
Table 14.2 along with the earlier (1971) values.
1.
Using the 1971 data in Table 14.2 and the maps in Figures
14.1 and 14.2, construct isolines (contours) showing the con-
centration of mercury and chromium in the bottom sediment
in Lake Erie. For mercury, use isolines representing concen-
trations of 0.25,0.5,1, and 2 ppm. For chromium, use isolines
representing concentrations of 20,60,80, and 140 ppm. Label
the isolines, showing the ppm value of each. Remember,
some isolines may terminate at the shore of the lake. Mer-
cury and chromium values for core sites 11-20 have been
entered on Figures 14.1 and 14.2, respectively and one or two
isolines drawn.
8. Table 14.2 contains mercury (Hg) concentrations for Lake
Erie sediments acquired in 1997-1998. Most of these data
have been plotted on Figure 14.3, and some of the isolines
have been drawn to show the spatial distribution of mercury.
Complete the 0.25 isoline for the Western Basin and the 0.1
isolines for the Central and Eastern Basins. Begin by plotting
values from Table 14.2 for stations in the Middle Basin (sta-
tions 47,51,61,63,99,102, and 105) in Figure 14.3.
2. From your maps of contaminated sediment, what three
areas show major contamination by both mercury and
chromium?
