Mercury Subsurface Maxima in Sediments:
a Diagnostic for Anthropogenic Origins
G.E. GLASS, J.A. SORENSEN, G.R. RAPP Jr., M. BALCER, and L. SCHWARZKOPF
Multimedia environmental surveys and monitoring studies have shown the major sources of mercury
contamination in the Lake Superior watershed to be from former industrial and municipal direct
discharges and present airborne sources. Walleye from the SI. Louis River, the largest US tributary to
Lake Superior, and siscowet trout from the lake continue to show elevated mercury tissue levels,
limiting safe human and wildlife consumption. The main routes of mercury bioaccumulation in fish
appear to be from airborne deposition and recent contaminated sediment sources and through the
benthic and planktonic food chains. No evidence for significant background from geologic sources is
indicated in upstream river sediments or from Lake Superior sediment core samples dating from
about 2000 to 8000 years. B. P.
The identification of possible mercury contamination sources from individual reservoirs and river
reaches focused on measurements of mercury concentrations in biota. The indigenous biota are an
integrator of mercury exposure, with the residue magnitudes as a function of location used to narrow
the range of possible contamination sources or source areas. Mercury concentrations in fish
populations showed a pattern consistent with localized exposure when upstream samples were
compared with those confined within a downstream series of reservoirs. The linkage between mercury
in fish and the food chain was also confirmed by increased mercury levels measured in plankton and
benthos. Sources of mercury contamination to the foodchain are indicated by the significant
correlation of mercury concentrations between surface sediments and mayfly larvae.
Documentation of sediment contamination was accomplished by extensive core sampling of river
and reservoir sites. Eighty sediment core samples from the six reservoirs on the lower St. Louis River
show some sites with mercury concentration maxima of 2 to 4 Ilgtg at 1 to 1.5 m sediment depth in
deep water, while other sites exhibit mercury concentration maxima of 0.8 to 2.0 pgtg at various
sediment depths from the surface to 1 m. The greatest surface (0-20 cm) sediment mercury
concentrations occurred in shallow water and wetland areas of a reservoir where water levels fluctuate
daily to produce hydropower for peak demands.
1
Introduction
The continuing uptake of mercury into the aquatic food chain has contaminated
fish in rivers and lakes of the Upper Midwest and elsewhere in North America.
Fish Consumption Advisories issued by the Minnesota Dept of Health (MDH
1996) establish meal limits based mostly on this contaminant. Some studies
indicate that environmental mercury levels are increasing. A 3 to 5% increase per
year for fish mercury levels was noted by Swain and Helwig (1989) when they
compared data from two time periods for a small number of Minnesota lakes. An
Environmental Science
Mercury Contaminated Sites (ed. by R. Ebinghaus et al.)
© Springer-Verlag Berlin Heidelberg 1999
a Diagnostic for Anthropogenic Origins
G.E. GLASS, J.A. SORENSEN, G.R. RAPP Jr., M. BALCER, and L. SCHWARZKOPF
Multimedia environmental surveys and monitoring studies have shown the major sources of mercury
contamination in the Lake Superior watershed to be from former industrial and municipal direct
discharges and present airborne sources. Walleye from the SI. Louis River, the largest US tributary to
Lake Superior, and siscowet trout from the lake continue to show elevated mercury tissue levels,
limiting safe human and wildlife consumption. The main routes of mercury bioaccumulation in fish
appear to be from airborne deposition and recent contaminated sediment sources and through the
benthic and planktonic food chains. No evidence for significant background from geologic sources is
indicated in upstream river sediments or from Lake Superior sediment core samples dating from
about 2000 to 8000 years. B. P.
The identification of possible mercury contamination sources from individual reservoirs and river
reaches focused on measurements of mercury concentrations in biota. The indigenous biota are an
integrator of mercury exposure, with the residue magnitudes as a function of location used to narrow
the range of possible contamination sources or source areas. Mercury concentrations in fish
populations showed a pattern consistent with localized exposure when upstream samples were
compared with those confined within a downstream series of reservoirs. The linkage between mercury
in fish and the food chain was also confirmed by increased mercury levels measured in plankton and
benthos. Sources of mercury contamination to the foodchain are indicated by the significant
correlation of mercury concentrations between surface sediments and mayfly larvae.
Documentation of sediment contamination was accomplished by extensive core sampling of river
and reservoir sites. Eighty sediment core samples from the six reservoirs on the lower St. Louis River
show some sites with mercury concentration maxima of 2 to 4 Ilgtg at 1 to 1.5 m sediment depth in
deep water, while other sites exhibit mercury concentration maxima of 0.8 to 2.0 pgtg at various
sediment depths from the surface to 1 m. The greatest surface (0-20 cm) sediment mercury
concentrations occurred in shallow water and wetland areas of a reservoir where water levels fluctuate
daily to produce hydropower for peak demands.
1
Introduction
The continuing uptake of mercury into the aquatic food chain has contaminated
fish in rivers and lakes of the Upper Midwest and elsewhere in North America.
Fish Consumption Advisories issued by the Minnesota Dept of Health (MDH
1996) establish meal limits based mostly on this contaminant. Some studies
indicate that environmental mercury levels are increasing. A 3 to 5% increase per
year for fish mercury levels was noted by Swain and Helwig (1989) when they
compared data from two time periods for a small number of Minnesota lakes. An
Environmental Science
Mercury Contaminated Sites (ed. by R. Ebinghaus et al.)
© Springer-Verlag Berlin Heidelberg 1999