478
G. E. Glass et al.
mean value from 128 subsamples, every 5 cm across the length of the 635 cm Lake
Superior core sample, was 25.1 ± 8.3 (SD) ng Hg/g. These low-level concentrations represent lake sediment accumulations over a period of about 8000 to
10 000 years. If there were episodic inputs of mercury from geologic sources into
the Lake Superior watershed or airshed during the period 2000-8000 B. P., then
the resulting increases in water concentrations and accumulations in algae and
diatoms and their subsequent deposition and incorporation into the sediments
should have been reflected in measured sediment levels as subsurface maxima.
No significant maxima were observed for the time spans represented by the
subsamples analyzed from the Lake Superior core, except for elevated surface
layers, as previously reported by Thomas (1974). The interpretation of changes in
global mercury cycling as reflected in Lake Superior sediments must be carefully
evaluated in light of the multiplicity of anthropogenic (Table 1) and natural
sources (Rasmussen 1994) making contributions to the total mercury deposition
to the watershed.
3.4
Sediment Mercury Subsurface Maxima
Subsurface maxima in mercury sediment concentrations were observed in most
cases where anthropogenic mercury sources were previously known to exist,
Fig. 6. The sediment mercury core profiles obtained from five sampling sites
within the Knife Falls (KF) Reservoir showed two of the five sampling sites with
subsurface maxima at 30-40 and 80-90 cm, respectively, with Hg concentrations
of 1200 and 300 ng Hg/g, respectively (cores KF-6, -5). The other three sampling
sites were all below 70 ng Hg/g (Fig. 6a). The major known sources of mercury
were industrial discharges and landfill seepage located on both sides of the river.
The industrial sources were from wood fiber products, which began in the early
1900S (MPCA/WDNR 1992), and ceased discharging to the river on or before 1978
when they became part of the Western Lake Superior Sanitary District.
The sediment mercury core profiles obtained from four sampling sites within
the Potlatch (PH) Reservoir showed subsurface maxima in all cores at different
sediment depths with Hg concentrations ranging up to 500 ng Hg/g (Fig. 6a;
Core PH-2). This reservoir received loadings from upstream sources as well as
those from other direct discharge sources.
The sediment mercury coce profiles obtained from eight sampling sites within
the Scanlon (SC) Reservoir all showed subsurface maxima in cores at different
sediment depths and Hg concentrations (Fig. 6b). One core (SC-4) showed two
extremely high concentrations of 2480 and 3560 ng Hg/g at 70-80 and 90100 cm sediment depth, respectively.
This reservoir received loadings from upstream sources as well as those from
paper manufacturing and other industries and municipal discharges up to 1978.
Mercury-containing compounds were used as slimicides in the period from
about 1950-1970S and substantial quantities were discharged with industrial
process wastewater directly to the river. The most commonly used mercury
compound was PMA (phenyl mercuric acetate). If the approximate dates are
G. E. Glass et al.
mean value from 128 subsamples, every 5 cm across the length of the 635 cm Lake
Superior core sample, was 25.1 ± 8.3 (SD) ng Hg/g. These low-level concentrations represent lake sediment accumulations over a period of about 8000 to
10 000 years. If there were episodic inputs of mercury from geologic sources into
the Lake Superior watershed or airshed during the period 2000-8000 B. P., then
the resulting increases in water concentrations and accumulations in algae and
diatoms and their subsequent deposition and incorporation into the sediments
should have been reflected in measured sediment levels as subsurface maxima.
No significant maxima were observed for the time spans represented by the
subsamples analyzed from the Lake Superior core, except for elevated surface
layers, as previously reported by Thomas (1974). The interpretation of changes in
global mercury cycling as reflected in Lake Superior sediments must be carefully
evaluated in light of the multiplicity of anthropogenic (Table 1) and natural
sources (Rasmussen 1994) making contributions to the total mercury deposition
to the watershed.
3.4
Sediment Mercury Subsurface Maxima
Subsurface maxima in mercury sediment concentrations were observed in most
cases where anthropogenic mercury sources were previously known to exist,
Fig. 6. The sediment mercury core profiles obtained from five sampling sites
within the Knife Falls (KF) Reservoir showed two of the five sampling sites with
subsurface maxima at 30-40 and 80-90 cm, respectively, with Hg concentrations
of 1200 and 300 ng Hg/g, respectively (cores KF-6, -5). The other three sampling
sites were all below 70 ng Hg/g (Fig. 6a). The major known sources of mercury
were industrial discharges and landfill seepage located on both sides of the river.
The industrial sources were from wood fiber products, which began in the early
1900S (MPCA/WDNR 1992), and ceased discharging to the river on or before 1978
when they became part of the Western Lake Superior Sanitary District.
The sediment mercury core profiles obtained from four sampling sites within
the Potlatch (PH) Reservoir showed subsurface maxima in all cores at different
sediment depths with Hg concentrations ranging up to 500 ng Hg/g (Fig. 6a;
Core PH-2). This reservoir received loadings from upstream sources as well as
those from other direct discharge sources.
The sediment mercury coce profiles obtained from eight sampling sites within
the Scanlon (SC) Reservoir all showed subsurface maxima in cores at different
sediment depths and Hg concentrations (Fig. 6b). One core (SC-4) showed two
extremely high concentrations of 2480 and 3560 ng Hg/g at 70-80 and 90100 cm sediment depth, respectively.
This reservoir received loadings from upstream sources as well as those from
paper manufacturing and other industries and municipal discharges up to 1978.
Mercury-containing compounds were used as slimicides in the period from
about 1950-1970S and substantial quantities were discharged with industrial
process wastewater directly to the river. The most commonly used mercury
compound was PMA (phenyl mercuric acetate). If the approximate dates are