Dissolved organic matter characterization and metal speciation in boreal streams

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Trace metals in the environment can provide either benefits or risks to aquatic life and humans. They may arise naturally or result partly or wholly from man-made pollution. It is no longer adequate, however, to consider only the total metal concentration in order to assess risk/benefit. Rather, it is necessary to determine the physicochemical forms (i.e. speciation) of an element because different forms of the same metal can range from essential to innocuous to strongly toxic. The modeling of metal speciation permits exploration into the processes of mobility, transport, toxicity and bioavailability of metals in water. Its application to natural aquatic ecosystems, however, is often limited due to the presence of dissolved organic matter (DOM), the main ligand for metal complexation. It is normally assumed in modeling calculations that all DOMs, regardless of their nature and origin, behave in a similar way toward complex trace metals. The diversity and variability of DOM within a specific environment depend on the potential origins of the DOM as well as on the hydrological conditions in the watershed. A better understanding of DOM cycling is required to predict how metal speciation will affect fate and bioavailability of metals in aquatic environments. The boreal shield ecozone is a particularly important and sensitive region of Ontario where most of its streams and lakes are located. In this project, a metal speciation model will be developed and tested against field observations to predict how the metal speciation will affect fate and bioavailability of water-borne metals in boreal streams. The spatial and temporal change in DOM, as well as those in the metal speciation, will be investigated in two boreal streams of south-central Ontario. Laboratory experiments will be designed to investigate the influence of DOM dynamics and composition on metal binding properties. These experimental results will be incorporated into a model that describes the importance of DOM in metal speciation. This model will allow a synthesis of the overall effects of the DOM-metal associations and of their changes as the qualitative and quantitative character of the DOM varies in the study systems.