SUSPENDED PARTICULATE GRAIN-SIZE DYNAMICS AND THEIR IMPLICATIONS FOR TRACE-METAL SORPTION IN THE DON RIVER

We examined particle size distributions of suspended particulate matte r (SPM); physical and environmental influences on the observed distrib utions; and relationships between particle size and geochemical partit ioning of metals, over the fall and winter period in a small urban riv er (Don River, Toronto, Ontario, Canada). For this dataset, the majori ty of particles (80%) in suspension were less than 10 mu m in size. In addition, while total SPM concentrations showed a positive trend with increasing discharge (Q); the proportions of particles found within g iven size classes were independent of both SPM concentration and Q. Te mperature was the only measured environmental variable related to the particle concentrations within size classes. As water temperature incr eased, the concentration of particles in the smallest size class (1-4 mu m) decreased, while the concentration of silt and/or algae sized pa rticles (10-50 mu m) increased. Increasing water temperatures may prom ote bacterial attachment to particles and their subsequent flocculatio n into larger sized particles. Decreasing concentrations of leachable (most labile) Cd, Zn and Mn were associated with increasing concentrat ions of the largest particles (70-150 mu m) in suspension. In contrast , higher reducible (oxides) associated concentrations of Cd, Zn and Mn occurred with increasing concentrations of smaller particles (1-10 mu m) in suspension. Both of these trends are speculated to reflect the importance of particle surface area for metal sorption reactions.