Fractionation of thallium in the Tamar estuary, south west England

Abstract

Very little information exists on the concentrations and biogeochemical behaviour of Tl in aquatic environments. In this study, the distribution and fractionation of Tl have been determined in the upper reaches of a macrotidal estuary (Tamar, south west England) whose catchment has, historically, been exploited for sulphidic metal ores. Concentrations of aqueous Tl in the estuary averaged about 6ngL−1 and were non-conservative with respect to salinity. Concentrations of Tl in intertidal sediment of around 80ngg−1 were observed throughout the tidal estuary with the exception of the most landward sample (~220ngg−1). Application of the modified BCR extraction scheme to the sediment samples resulted in reducible, oxidisable and residual Tl averaging 50%, 18% and 30% of total Tl, respectively, and exchangeable Tl that was only detected in three cases. In contrast, the fractionation of K, in many respects a geochemical analogue of Tl, exhibited a seaward increase in exchangeability from about 8% to 80%. These observations suggest that particle–water interactions of Tl in the water column are limited and that non-conservative behaviour of the metal in the aqueous phase results from external inputs to the estuary or remobilisation of reducible Tl from anoxic sediment deposits.