Metals in Lake Simcoe sediments and tributaries: Do recent trends indicate changing sources?

Metals released to the environment from various natural and anthropogenic sources accumulate in lake sediments, providing a record of metal emissions over time. Metal concentrations (Al, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Rb, Sb, Sr, V, Zn) in sediments were measured across Lake Simcoe, replicating a study conducted 25 years earlier. The highest metal concentrations were found in sediment cores from Kempenfelt Bay where concentrations peaked in the 1950s through to the 1970s. Metal concentrations generally decreased from peak levels, except for Cu and Zn whose concentrations have since returned to peak levels. In recent years, concentrations of As, Cd, Cr, Cu, Pb, Sb, and Zn were still more than double their early industrial (pre-1900s) concentrations. Across the lake, concentrations of metals in the surface sediments were generally highest in Kempenfelt Bay and the main basin, and decreased with distance away from shore and towards the lake outlet. Metal concentrations in several tributaries in the watershed were highest at locations downstream from urban areas. The temporal and spatial trends in sediment metal concentrations are consistent with a shift from point source metal pollution to diffuse sources of metals including urban runoff and atmospheric pollution.