Geochemistry of Mercury in Two Hydroelectric Reservoirs in Quebec, Canada

Abstract

Carbon, nitrogen, phosphorus, and iron oxyhydroxide profiles were used to explain the retention of Hg in sediments of natural lakes and flooded soils in Quebec. In lake sediments, Hg levels increased from background concentrations of approximately 100 ng/g beneath the 10- to 16-cm depth interval to 150–490 ng/g near the surface. This increase was attributed to atmospheric inputs to the environment. In forest soils, the close association of Hg with the surficial organic horizon appears to be a potential source of contamination for the aquatic system through the transport of particulate matter by surface runoff. High organic matter content observed in the flooded soil of LG-2 Reservoir suggests that postsedimentation biodegradation is a slow process. In contrast, at Cabonga Reservoir, an 80% reduction in C, N, and Hg concentrations was observed between the flooded and the adjacent forest soil. Mechanical erosion processes (marling, waves, ice) rather than biodegradation seem responsible for the observed loss in C, N, and Hg. We propose that suspension of the humic horizon from flooded soils could constitute a significant source of Hg contamination to the overlying waters and biota until the exposed littoral zone stabilizes.