Spatial Variation of THg and MeHg Stream Concentrations and Its Relation to TOC

Abstract: Mercury (Hg) which originates from global emissions to the atmosphere can deposit far away from the source. There is often a weak correlation between Hg deposition and concentrations in runoff, therefore catchment specific parameters might be of importance in controlling the amount of Hg ending up in watercourses and fish. Total mercury (THg) and methylmercury (MeHg) concentrations in watercourses are correlated to organic matter (OM), and in this study total organic carbon (TOC) is used as a proxy for OM. This study covers data from 125 watercourses in Sweden, and investigates the impact of e.g. Topographic Wetness Index (TWI), forest type, soil and bedrock types, deforestation and catchment size on the THg and MeHg concentrations as well as on the residuals from the regression between THg and TOC. Previous studies have found strong correlations between THg and OM as well as MeHg and OM but few previous studies have studied influences of catchment specific factors on THg and MeHg runoff concentrations with data of this large spatial and temporal scale. The catchment characteristics were extracted through ArcMap and projection to latent structures (PLS) models were created to evaluate what characteristics had the strongest influence on the variation in THg and MeHg concentrations as well as on the THg/TOC residuals. A strong correlation was found between THg and TOC, but a weaker one between MeHg and TOC. The MeHg concentrations were found to be dependent on variables which co-vary with TOC, as well as on latitude. The negative impact of latitude on MeHg concentrations could be due to methylation being temperature dependent or reflecting a spatial variation in Hg deposition from north to south of Sweden. Amount of deforestation was found to have an impact on the THg concentrations, indicating that deforestation leads to larger THg fluxes from soil to stream. The fact that variables measuring OM content in the streams were strongly influencing THg and MeHg concentrations, and the fact that few catchment characteristics were of importance, indicate the high importance of OM in explaining THg and MeHg also at this large spatial and temporal scale.