Terrestrial Sources of Methylmercury in Surface Waters: The Importance of the Riparian Zone on the Svartberget Catchment

Abstract

The runoff of methylmercury (MeHg) from forested catchments to surface waters has been identified as a potentially significant input of MeHg to the aquatic ecosystem. Little, however, is known of the processes which control the transfer of MeHg from soils to surface waters. This study investigated the potential terrestrial sources of MeHg in runoff by sampling profiles of soil solution chemistry and determining the flux of water through those profiles into two tributaries on the Svartberget Catchment in northern Sweden. One study profile was from the podzol soil that covers most of the catchment area. The other profiles were taken in the riparian zone of each of the two tributaries. Soil solution was extracted from the soils by centrifugation.

High catchment soil solution concentrations of MeHg (>1 ng l^-1) occur in the surface layers of the soil, but overland flow on the catchment is rare. MeHg concentrations in the podzol profile dropped to less than 0.2 ng l^-1 in the mineral soil just 5 cm below the mor layer. In the riparian soil profiles sampled in October, MeHg concentrations were higher (ca. 0.4 ng l^-1), but in a July sampling the concentrations in the riparian profiles were comparable to those in the podzol (i.e. < 0.2 ng l^-1). Very high concentrations of MeHg were found in the streambank sphagnum mosses (>2 ng 1^-1) partially submerged within the stream.

The concentrations of MeHg observed under the podzol soil were insufficient to sustain the concentration of MeHg in runoff from the forested subcatchments where podzol profiles cover more than 70% of the surface area. The only sources of additional MeHg that lie along major runoff flow pathways are the riparian soils and mosses on the streambanks. It is therefore hypothesized that output of MeHg from thr forest areas of the catchment is controlled by the biogeochemical processes in the riparian zone.