Mercury Subsurface Maxima in Sediments
483
ignition) are in the process of being measured in order to interpret the controlling
variables that are determinant in establishing the depositional profiles.
The sediment mercury core profiles obtained from seven of eight sampling sites
within the Forbay (FB) Reservoir showed significant subsurface maxima at a
variety of different sediment depths. Notable Hg concentration maxima ranged
from 750 ng Hg/g at 20-30 em to 2000 ng Hg/g sediment at 90-100 cm sediment
depth (Fig. 6d). This reservoir is primarily a deep channel and is used to divert the
flow from the Thomson Reservoir to hydroelectric generators at the level just above
the Fond du Lac Reservoir, the last and deepest reservoir on the St. Louis River.
Nine of the 17 cores from the sampling sites on the Fond du Lac (FL) Reservoir
had mercury levels that did not exceed 250 ng Hg/g sediment (Fig. 6e).
Subsurface maximum concentrations of 500 and 1220 ng Hg/g sediment occurred at a sediment depth of 20-30 cm for two (core FL-6R, -16) of the
remaining eight. The other six cores had maxima below 1 m of sediment depth.
For the eight sites exhibiting mercury concentrations greater than 250 ng/g, the
mercury stratigraphy was complex, especially for sediment depths >1 m, and
most likely represents sediment mercury loadings modified by sediment scouring
in upstream reservoirs during major flood events.
3.5
Surface Maxima of Sediment Mercury Levels
Nine of the 3B sediment cores from the Thomson Reservoir had mercury
concentration maxima at the surface (0-10 cm section) with values of 200, 300,
400,500,600, Boo, 1100, 1100, and 2000 ng Hg/g (Fig. 6c). All of these cores were
obtained from areas of relatively low total sediment accumulations, 20 to 40 cm,
and most from an area of the reservoir which consists of shallow wetlands. The
wetlands are subject to substantial water-level fluctuation as the reservoir is used
for hydro power peak load generation and cycles on a 24-h schedule. It has not
been determined whether or not these cores represent the total mercury loading
captured by low, limited rates of sedimentation and remain near the sedimentwater interface, or whether they represent some special circumstance such as a
new loading source, or both. These core sites may represent areas of highest
potential exposure for mercury bioaccumulation in today's aquatic food chain.
Additional studies are underway to (1) determine if possible new mercury
sources have contributed to the high surface sediment concentrations, and (2)
identify factors controlling reservoir sedimentation, mercury accumulation, and
bioturbation influences in mercury sediment deposition.
4
Summary and Conclusions
Mercury analyses of a Lake Superior sediment core revealed no subsurface
mercury maxima deposition episodes between about 2000 and Booo years B. P.,
483
ignition) are in the process of being measured in order to interpret the controlling
variables that are determinant in establishing the depositional profiles.
The sediment mercury core profiles obtained from seven of eight sampling sites
within the Forbay (FB) Reservoir showed significant subsurface maxima at a
variety of different sediment depths. Notable Hg concentration maxima ranged
from 750 ng Hg/g at 20-30 em to 2000 ng Hg/g sediment at 90-100 cm sediment
depth (Fig. 6d). This reservoir is primarily a deep channel and is used to divert the
flow from the Thomson Reservoir to hydroelectric generators at the level just above
the Fond du Lac Reservoir, the last and deepest reservoir on the St. Louis River.
Nine of the 17 cores from the sampling sites on the Fond du Lac (FL) Reservoir
had mercury levels that did not exceed 250 ng Hg/g sediment (Fig. 6e).
Subsurface maximum concentrations of 500 and 1220 ng Hg/g sediment occurred at a sediment depth of 20-30 cm for two (core FL-6R, -16) of the
remaining eight. The other six cores had maxima below 1 m of sediment depth.
For the eight sites exhibiting mercury concentrations greater than 250 ng/g, the
mercury stratigraphy was complex, especially for sediment depths >1 m, and
most likely represents sediment mercury loadings modified by sediment scouring
in upstream reservoirs during major flood events.
3.5
Surface Maxima of Sediment Mercury Levels
Nine of the 3B sediment cores from the Thomson Reservoir had mercury
concentration maxima at the surface (0-10 cm section) with values of 200, 300,
400,500,600, Boo, 1100, 1100, and 2000 ng Hg/g (Fig. 6c). All of these cores were
obtained from areas of relatively low total sediment accumulations, 20 to 40 cm,
and most from an area of the reservoir which consists of shallow wetlands. The
wetlands are subject to substantial water-level fluctuation as the reservoir is used
for hydro power peak load generation and cycles on a 24-h schedule. It has not
been determined whether or not these cores represent the total mercury loading
captured by low, limited rates of sedimentation and remain near the sedimentwater interface, or whether they represent some special circumstance such as a
new loading source, or both. These core sites may represent areas of highest
potential exposure for mercury bioaccumulation in today's aquatic food chain.
Additional studies are underway to (1) determine if possible new mercury
sources have contributed to the high surface sediment concentrations, and (2)
identify factors controlling reservoir sedimentation, mercury accumulation, and
bioturbation influences in mercury sediment deposition.
4
Summary and Conclusions
Mercury analyses of a Lake Superior sediment core revealed no subsurface
mercury maxima deposition episodes between about 2000 and Booo years B. P.,