Abstract:

Thallium (TI) is a highly toxic ultra-trace element that has received relatively little attention compared to other trace elements of similar toxicity that are usually found at much higher concentrations in the environment. Elevated TI concentrations are most often found proximal to mining and smelting centers. The Sudbury region, with nearly 150 years of mining and smelting of sulfide ores, presents itself as a possible candidate for the enrichment of TI, which may subsequently be released into the environment. Ramsey Lake, the primary source of drinking water for the southern half of the city, is located a few kilometers southeast from the Sudbury smelters, in the center of the city. The lake sediments may represent an archive of the deposition of particulate sediments within the catchment area and is thus one suitable candidate for investigation of the distribution of TI in sediment cores. Raft Lake, situated in a relatively wilderness area approximately 8 km south of Ramsey Lake, has been less affected by industrial activity and is thus used for comparison. Several sonar traverses were taken across Ramsey Lake to locate areas most suitable for collecting sediment cores, as determined by the sediment structure and bedrock topography. Three areas were identified as suitable for sample collection, and seven sediment cores were later collected from these areas: four from the deep west end, two from the main northern channel, and one from the shallow east end close to the Lake Laurentian Conservation area. After collection, the sediment cores were sub-sampled and prepared for analysis by various instrumental techniques to determine the total concentration, distribution, and speciation of TI within each core. For each sub-sample, pore water was extracted by centrifugation and subsequently analyzed by ICP-MS. The sediments were then freeze-dried, the total moisture calculated, and subjected to both an 'aqua regia' extraction and total acid dissolution followed by both ICP-MS and ICP-AES analysis. Undried sub-samples from the apparently least disturbed of the seven cores were subjected to a seven-step sequential extraction scheme of increasing aggressiveness that was designed to determine the proportion of TI in the different fractions of the sediment. TI was found to be at concentration levels of approximately 0.01 ppb in the lake water, 0.1 ppb in the pore waters, and 200 ppb in the sediments. The sequential extraction analysis revealed TI to be distributed between the organic, oxide, and residual fractions. In addition, TI in the fractions other than the residual appears to behave similarly to other base and semi-metals such as Cu, Cd, and Hg, being markedly enriched (with total concentration up to 1200 ppb) in the top 6 cm of the core that corresponds to the unconsolidated, flocculated layers of the sediment.