3 - International Joint Commission

trations in Great Lakes sediments is scarce (Mudroch et al., 1985).
The distribution of several organochlorines in bottom sediments was
determined in 1973 (Frank et al., 1980) (Table 5). The mean values (n = 406)
for total PCBs and p,p’-DDE were 3.3 ppb and 0.71 ppb, respectively, for all
basins. Eisenreich et al. (1980) sampled Lake Superior bottom sediments in
1977 and 1978. Whereas Frank et al. (1980) analyzed the top 3 cm,
concentrations reported in Eisenreich et al. (1980) are for the top 0.5 cm
only. Eisenreich and Baker (1989) reported that the mean total PCB
concentration in 1986 (8.6 ppb) was significantly lower than that in 1978 (74
ppb). This reflects lower PCB burdens in the water column. The mean HCB
concentration of 0.2 ppb in surficial sediments in 1980 was at least an order
of magnitude lower than that in Lakes Huron, Erie and Ontario (Oliver and
Nicol, 1982). Frank et al. (1980) did not detect mirex in any of 80 samples
collected in 1973, at a minimum detection limit of 0.3 ppb. PAH concentrations
in sediments are about ten times lower than in the other Great Lakes,
reflecting both its remote location and that the atmosphere is the primary
source (Baker and Eisenreich, 1989).
Historical trends in loadings of heavy metals to Lake Superior have been
investigated by collection, sectioning, age dating and analysis of lake bottom
sediment cores. Kemp et al. (1978) calculated sediment enrichment factors for
several metals in Lake Superior. The mean values for lead, mercury, cadmium
and arsenic were 4.6, 1.8,2.9 (n = 6) and 0.7 (n = 1). respectively. The sediment
enrichment value is the ratio of the surficial sediment concentration to the
historical, pre-settlement concentration below the Ambrosia horizon. Cores
analyzed by Rossmann (1986) show that concentrations of lead have increased
in the most recent sections of sediment cores (Figure 4). Because most lead
introduced to Lake Superior comes from the atmosphere, it is likely that
concentrations in the surface sediments will now decrease as they have, for
example, in Lake Ontario (Rossmann, 1986). Anthropogenic enrichment of lead
in Lake Superior has been less than that in the other Great Lakes (Hodson et al, ,
1984). Concentrations of mercury reached peak values at a depth of 3 cm and
decreased to the surface (Figure 4).
The distribution of PCBs in sediment cores from nine sites in Lake
Superior was described by Eisenreich (1988). The top 1 cm of sediment
represented approximately 10 years. At all sites, the concentrations of PCBs
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