Antecedent moisture conditions control mercury and dissolved organic carbon concentration dynamics in a boreal headwater catchment

The fate and transport of mercury (Hg) deposited on forested upland soils depends on the biogeochemical and hydrological processes occurring in the soil landscape. In this study, total Hg (THg) and dissolved organic carbon (DOC) concentrations were measured in streamwater from a 7.75 ha upland subcatchment of the METAALICUS watershed in northwestern Ontario, Canada. THg and DOC concentration-discharge relationships were examined at the seasonal-scale and event-scale to assess the role of antecedent moisture conditions on the mobilization of these solutes to receiving waters. At the seasonal-scale, subcatchment discharge poorly explained THg and DOC concentration dynamics; however, the inclusion of antecedent water storage and precipitation metrics in a multiple regression model improved the prediction of THg and DOC concentrations significantly. At the event-scale, a comparison of THg and DOC concentrations for two small summer storms with similar total discharge showed that the storm following the wet snowmelt period had a significantly lower total flux of THg and DOC than the storm following warm and dry conditions in late summer due to a distinct shift in the concentration-discharge relationship. Measurements of soil water and groundwater THg and DOC concentrations, as well as a three-component mixing analysis, suggest that there was an accumulation of potentially-mobile DOC-bound THg in the well-humified organic soil layer in the catchment during the warm and dry summer period and that as the catchment became wetter in the autumn, there was an increase in soil water THg and DOC concentrations and these solutes were subsequently flushed during the autumn storm.