Aqueous inputs of mercury to precambrian shield lakes in ontario

The concentration of mercury in precipitation and in streams of a central Ontario lake were measured over a one year period. The concentrations in streams were generally very low (<5 ng/L), but during warm periods with low discharge, concentrations often exceeded 20 ng/L in a headwater stream draining a wetland. The concentration in precipitation varied from about 2 to nearly 30 ng/L and exhibited no seasonal pattern. By combining the concentration data with discharge and precipitation depth the inputs of mercury from runoff and direct deposition were estimated. Direct wet deposition accounted for over half of the supply of mercury to the lake. The watersheds retained most of the mercury deposited on them. The presence of wetlands appeared to decrease retention efficiency.