In spite of continuing large inputs from anthropogenic sources, average concentrations of many trace metals in the Great Lakes remain quite low: 2.8-4.5 ng L -1 for Cd, 3.2-11 ng L -1 for Pb, and 87-277 ng L -1 for Zn. These metals are rapidly scavenged by the seston (suspended particulates) and have a rapid turnover rate in the water column. Factors that affect the distribution of dissolved trace metals include water depth, seston abundance, and biological processes. Higher concentrations are generally found in nearshore stations and especially near the urban centers and polluted river mouths. In summer, there is a marked depletion of Zn (and other bioactive metals) in the epilimnion of the offshore waters. The patchiness in the concentrations of the metals is attributed to spatial differences in the biological processes. With the exception of Cr, there is no systematic increase in concentration down the drainage basin from Lake Superior to Lake Ontario. Most of the Pb, Cd, and Zn loadings into the lakes are retained in the basin, but there is a significant export of dissolved Cu, Ni, and Cr via the St. Lawrence River.

Thallium is highly toxic but has been an obscure element compared to its three popular neighbours, lead, mercury and cadmium. It is partly due to the scarcity of its analytical data, caused by its high analytical detection limit relative to the other three elements and by its generally low level in the environment. We have developed a highly sensitive instrument, a Laser-Excited Atomic Fluorescence Spectrometer, to study thallium contamination in some important Canadian ecosystems from the Arctic (containing very low thallium concentration) to coal-related industries across Canada and even to the study of thallium toxicity in an invertebrate, Hyalella azteca. Overall, our data indicate that the coal power plants and mines contain higher thallium concentrations than the other ecosystems studied, and the eastern region has the highest Tl concentrations compared to other regions. The range of thallium concentration in ng/L for the Arctic snow and ice was between not detected and 8.4, for the Great Lakes waters 0.9 to 48, for pore waters 0.1 to 213, for western coal power plants and mines 0.1 to 1326, for central coal power plants 1.2 to 175, for eastern coal power plants and mines 0.2 to 23605, and for miscellaneous sites across Canada not detected to 4390 ng/L. Some of these high concentrations and those high ones reported in industrial wastewaters exceeded the chronic toxicity endpoints for Hyalella azteca mortality, growth and reproduction, thus can cause serious distress to the environment. All data were integrated into a map of thallium distribution, the first one in Canada. Natural background level of thallium for the Arctic was estimated to be 0.02 to 0.03 pg/g.

No abstract

Metal data for Pb, Zn, Al, Cd, and Tl show seasonal variations of high levels during the winter−early spring period and low levels during the summer−fall period. In terms of Pb magnitudes and seasonal variations, our data of the late 1980s and early 1990s (unleaded gasoline era) appear compatible with those of Murozumi et al. (Murozumi, M.; Chow, T. J.; Patterson, C. C. Geochim. Cosmochim. Acta 1969, 33, 1247−1294.) of the 1950s and 1965 (leaded gasoline era). This is probably due to the close proximity and similar elevation of the two areas (Agassiz Ice Cap, Canadian Arctic and Camp Century, northwest Greenland), which are likely subjected to the same polluted air masses. Despite the implementation of unleaded gasoline since the early 1970s, our data show that the Agassiz Ice Cap still received a significant amount of lead during the late 1980s and early 1990s; this is in contrast to Summit, Greenland, which saw a very drastic decrease of the element during similar periods. This is because of the different locations and altitudes, as well as different sources, mainly Eurasian for Agassiz versus mainly U.S. for Summit. Lead was determined by laser-induced fluorescence spectrometry via the direct injection of microliter sample sizes. Its fallout flux was estimated to be 1.2 ng cm-2 year-1. The lead concentration in the surface snow appears to follow the order of Agassiz and northwest Greenland > central Greenland > Antarctica.