3 - International Joint Commission

cadmium in embayments.
The highest organic chemical residues in surflcial sediments from Lake
Huron were In Saginaw Bay. Some other nearshore localities also have
higher concentrations. In the open lake, the highest concentrations of toxic
organic chemicals are in the depositional basins. This is because of the
previously described relationship between hydrophobic organic chemicals
and fine-grained sediments. Data on toxic organic chemical concentrations
in bottom sediments are shown in Table 9. In 1969, Frank et al. (1979b)
found levels of DDT were higher in depositional ( 18.4 ppb) rather than nondepositional
zones (3.8 ppb). Of the six basins in Lake Huron, the highest
concentrations of DDT were in the Goderich Basin (31.3 ppb). Concentrations
of the parent compound (DDT) were the highest, followed by DDE and
TDE. Although the inputs were primarily diffuse, Saginaw Bay, Wasaga
Beach and the central part of eastern Georgian Bay were identified as point
sources.
In 1969, dieldrin was identified in only 5.7% of the surface sediment
samples. Concentrations were at or below 1.3 ppb (Frank et al., 1979b).
Samples containing dieldrin came from the Goderich and Mackinac Basins
and in the non-depositional zone near Saginaw Bay. Dieldrin was identified
in 30% of the sediments from Georgian Bay in 1973 and 15% of those from
the North Channel (Frank et al.,197913). Kreis and Rice (1985) reported that
mean dieldrin concentrations were high in Georgian Bay, particularly in
Lions Trough and the nearshore zone at Owen Sound. Some samples from
Saginaw Bay also had high concentrations, whereas the main lake and the
North Channel had lower concentrations. Mirex was not detected by Frank
et al. (1979b) in sediments collected in 1969 nor by Oliver and Bourbonniere
(1985) in 1980. In 1969, unlike DDT, the PCB residues (11 ppb) in the nondepositional
zones of Lake Huron were similar to the levels observed in the
depositional basins (15 ppb) (Frank et al., 1979b). Saginaw Basin contained
the highest mean residue (33 ppb) and Alpena Basin the lowest (9 ppb)
(Figure 3). The mean concentration in Georgian Bay was 13 ppb while in the
North Channel, it was 8 ppb. In Georgian Bay, substantial loadings of PCBs
probably came from the northeast shore (Frank et al., 1979b). In the review
by Mudroch et al. (1985), surface concentrations of PCBs ranged from 10-20
ppb in the open lake. In the embayments, they ranged from less than the