Sources and mobility of thallium and indium in Canadian lakes

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Chemical and biological reactions occurring in lake sediments (early diagenesis) can influence the vertical distribution of trace metals and compromise the use of sediment profiles as historical records of anthropogenic metal pollution. There is thus a need to evaluate the importance of post-depositional redistribution of metals in sediments. Large uncertainties also exist concerning sources of metal emissions. In this study, we use porewater and solid phase measurements to determine the post-depositional mobility of thallium and indium in Canadian Shield lakes and to establish atmospheric T1 and In sources and deposition chronology. Two lakes exposed to different emission sources of trace metal pollution were chosen for the study. One, L. Tantar6, is located in an ecological reserve about 40 km from Qu6bec City (about 520000 inhabitants); according to prevalent wind trajectories, this lake is under the influence of atmospheric emissions from the Midwest-Great Lakes and Eastern regions of the USA, as well as from cities located along the St. Lawrence River. The other one, L. Vose, is located 25 km downwind from an important smelter in the mining city of Rouyn-Noranda. The uninhabited drainage basins of these two lakes have not been affected by wildfires or wood harvesting for at least 100 years. Thus, atmospheric deposition has been the only input of atmospheric T1 and In to these basins. Sediment cores were obtained from the deepest part of each lake and analyzed for total T1 and In. Porewaters were collected at the coring sites by in-situ dialysis and analyzed for T1, In, as well as for major ions, sulfide, organic carbon and pH which are needed for speciation calculations. The porewater and sediment geochemical data were used, together with information on infaunal benthos, in the diagenetic reaction-transport equation which was solved numerically for net reaction rates as a function of sediment depth with the code PROFILE. This information was used to estimate the importance of diagenetic processes in shaping the solid phase T1 and In profiles, to estimate the deposition chronology of these metals and to identify the diagenetic reactions involving these metals. Comparison of T1 and In deposition chronology with those of other contaminants (e.g. PAHs, stable lead isotopes) measured at the same sites allowed identification of the main atmospheric sources of TI and In. Our results suggest that T1 and In are useful tracers for atmospheric emissions from coal combustion and metal smelters, respectively.