Mercury Subsurface Maxima in Sediments
(c)
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Fig. 6a-e. Summary of mercury concentrations in sediment cores from various sampling sites
upstream (US, a) and within the six lower reservoirs of the Saint Louis River. The upper two reservoirs
of this study (Knife Falls: KF, a and Potlatch: PH, a) exhibited fewer contaminated sites and lower
overall mercury concentrations in sediment than those downstream (Scanlon: SC, b, Thomson: TH, c,
Forbay: FB , d), and Fond du Lac (FL, e)
correct in Fig. 3, then most of the mercury loadings to the river and river
sediments were made during the 1950S and 1960s.
The two Scanlon Reservoir cores showing the lowest concentrations with no
subsurface maxima were taken adjacent to an industrial landfill and may reflect
more recently deposited solid materials related to the solid waste disposal site.
The sediment mercury core profiles obtained from 38 sampling sites within the
Thomson (TH) Reservoir showed most sites with subsurface maxima and
secondary maxima in cores at different sediment depths and Hg concentrations
(Fig. 6c). Thirteen of the cores had maximum mercury concentrations between
1500 and 2570 ng Hg/g occurring at a variety of sediment depths.