Abstracts Geography

Catchment-scale hydrology and methylmercury biogeochemistry in the low boreal forest zone of the Precambrian Shield

by Brian A. Branfireun

The role of catchment hydrology and biogeochemistry in the production and transport of methylmercury (MeHg) was studied in a headwater catchment in the low boreal forest zone of the Precambrian Shield. A simple, catchment-scale model found that peatlands were large sources of MeHg, and lakes were large sinks. Sensitivity analysis revealed that the volume of runoff delivered to the peatland by the upland, peatland size, and pore water MeHg concentration in the peatland are important controls on catchment MeHg yield. Contemporary atmospheric deposition of MeHg was found to be insignificant compared to the sources of MeHg within the catchment. Sulfate addition experiments were undertaken to examine the controls on MeHg production in peatlands, and it was found that the in situ addition of sulfate to peat and peat pore water resulted in an increase in pore water MeHg concentrations by a factor of 3 to 4. A supporting hydrological study found that the annual catchment hydrologic regime is strongly influenced by the volume and timing of precipitation inputs. For example, a 19% smaller than average snowpack and a dry April in 1995 resulted in the absence of a spring melt runoff event. This finding, coupled with 30% less summer rainfall in 1995 than in 1996, produced low antecedent moisture conditions in the upland soils, 68% less total runoff and reversals of hydraulic gradients. Concentrations of MeHg in pore water were variable across the catchment, with the highest found in the peatland (up to 3.02 ng/l). The delivery of sulfate, carbon quality and temperature appear to influence the production of MeHg at a variety of scales. The mass flux of MeHg within and from the catchment is dependent upon the mass flux of water and the placement of landscape units in the catchment hydrologic cascade. In the two study years, the total mass flux of MeHg over the study period was 8.65 mg and 25.9 mg in 1995 and 1996 respectively.