Mercury Subsurface Maxima in Sediments
471
2
Methods and Measurements
2.1
Study Area
The study area consisted of the lower six reservoirs (Knife Falls; Potlatch;
Scanlon; each 30-50 acres; Thomson, 339 acres; Forbay, 36 acres; and Fond du
Lac, 217 acres) of the St. Louis River, shown in Fig. 1. Specific sampling sites
within each reservoir were chosen to represent a variety of sediment depositional
zones and morphology. A Lake Superior sediment core was also analyzed to
confirm background sediment mercury levels.
2.2
Sampling Methods
2.2.1
Sediment Core Collection
Sediment core samples were collected in the summers of 1995-1996 using a 5-cm
diameter by I-m-long modified Livingstone piston corer (Wright 1967) operated
from the center of an anchored pontoon using methods described in Faegri and
Iverson (1965). Eighty cores (penetration <:::1 m) of reservoir and river sediments
were taken to provide more complete coverage of the reservoir basin. Replicated
cores were taken at selected sites within 5 m of each other in an effort to estimate
sediment mercury variability over short distances.
The Lake Superior sediment cores were obtained in 1980 using box and gravity
coring deep-water techniques (Johnson and Fields 1984; T. Johnson pers. comm.
1997).
2.2.2
Sediment Core Sectioning
Immediately after obtaining a St. Louis River core sample, the open end was
temporarily covered while the outside of the core tube was rinsed with ambient
water to remove clinging sediment. The cover was then removed and a ziploc bag
(labeled with site and sediment strata depth information) was placed against the
tube end while the core was extruded. After extruding 10 cm of core (distances
marked on the plunger), the sediment was sliced cleanly from the end of the core
Fig. 3. Sediment mercury concentrations in the three lower reservoirs of the Saint Louis River. Strata
dating at 1954-64 were determined by Cs 137 measurements by D.). Steck, Schaefer Environ.
Radiation Lab., St. John's University, Collegeville, Minnesota, pers. comm. (Glass et al. 1996; MPCA
1996)
471
2
Methods and Measurements
2.1
Study Area
The study area consisted of the lower six reservoirs (Knife Falls; Potlatch;
Scanlon; each 30-50 acres; Thomson, 339 acres; Forbay, 36 acres; and Fond du
Lac, 217 acres) of the St. Louis River, shown in Fig. 1. Specific sampling sites
within each reservoir were chosen to represent a variety of sediment depositional
zones and morphology. A Lake Superior sediment core was also analyzed to
confirm background sediment mercury levels.
2.2
Sampling Methods
2.2.1
Sediment Core Collection
Sediment core samples were collected in the summers of 1995-1996 using a 5-cm
diameter by I-m-long modified Livingstone piston corer (Wright 1967) operated
from the center of an anchored pontoon using methods described in Faegri and
Iverson (1965). Eighty cores (penetration <:::1 m) of reservoir and river sediments
were taken to provide more complete coverage of the reservoir basin. Replicated
cores were taken at selected sites within 5 m of each other in an effort to estimate
sediment mercury variability over short distances.
The Lake Superior sediment cores were obtained in 1980 using box and gravity
coring deep-water techniques (Johnson and Fields 1984; T. Johnson pers. comm.
1997).
2.2.2
Sediment Core Sectioning
Immediately after obtaining a St. Louis River core sample, the open end was
temporarily covered while the outside of the core tube was rinsed with ambient
water to remove clinging sediment. The cover was then removed and a ziploc bag
(labeled with site and sediment strata depth information) was placed against the
tube end while the core was extruded. After extruding 10 cm of core (distances
marked on the plunger), the sediment was sliced cleanly from the end of the core
Fig. 3. Sediment mercury concentrations in the three lower reservoirs of the Saint Louis River. Strata
dating at 1954-64 were determined by Cs 137 measurements by D.). Steck, Schaefer Environ.
Radiation Lab., St. John's University, Collegeville, Minnesota, pers. comm. (Glass et al. 1996; MPCA
1996)