Spatial and temporal variation of THg concentrations in run-off water from 19 boreal catchments, 2000–2010

Total mercury concentrations are presented for 19 Swedish watercourses 2000–2010, together with an analysis of factors affecting these concentrations in space and time. Organic matter (OM) measured as absorbance at 420nm (Abs420) and total organic carbon (TOC) were the variables most strongly correlated with THg concentrations in the pooled dataset from all 19 watercourses, explaining 66% and 61% of the variance respectively. The correlation between THg and OM indicates that OM is the main controlling factor independent of geographical variation in Hg deposition, geology, or any other factor evaluated in this study. Despite an increase in TOC concentrations at most sites during the study period, THg increased in only one of the watercourses, and the THg/TOC ratio decreased significantly at six sites. The Abs420 did not increase like TOC. We suggest that OM-fractions absorbing at 420nm are more important for Hg mobilization than other OM-fractions. ► THg was measured bimonthly 2000–2010 from 19 watercourses across Sweden. ► OM quantity and quality explained most of the THg variation in space and time. ► Despite a temporal increase of TOC in most sites, THg increased in just one. ► THg was more strongly correlated with Abs420 than TOC. Despite strong spatial similarity in the THg/TOC ratio across large gradients of climate, land use and deposition, THg did not follow the temporal TOC trends in individual watercourses.