GEOCHEMISTRY OF MERCURY IN 2 HYDROELECTRIC RESERVOIRS IN QUEBEC, CANADA

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