Mercury concentrations in Irish headwater lake catchments

An estimated 215,000 tonnes of mercury (Hg) have been emitted to the atmosphere from anthropogenic sources since the nineteenth century, igniting widespread environmental monitoring owing to its toxicity. The environmental fate of Hg is strongly determined by catchment characteristics, especially soil organic matter. In this study, concentrations and pools of Hg were determined for lakes and soils in upland peat-dominated catchments in Ireland to assess controls of aquatic Hg and soil response to changes in emissions. Headwater lakes in upland coastal regions were surveyed for water chemistry and total Hg (THg) during spring 2008. In addition, a sub-set of lakes (n = 5) were repeatedly sampled during 2009–2011, and their surface soils collected for Hg analysis, including a short (30 cm) peat core to assess temporal Hg fluxes using radiometric ²¹⁰Pb dating. Peat cores indicated a significant decrease in Hg deposition since the 1980s, in broad agreement with other ‘background’ regions. Total Hg was correlated with total organic carbon (TOC) in the survey and intensive study lakes (r = 0.70 and 0.45), indicative of the strong affinity of Hg to organic matter. At the intensive lakes, monomethylmercury (MMHg) made up 3.3 % of mean THg and exhibited a positive correlation with total SO₄ ^2? (r = 0.55). Further, both THg and MMHg were significantly correlated with conductivity (r = 0.48 and 0.54, respectively) potentially owing to marine inputs, and negatively correlated with pH (r = ?0.59 and ?0.56 respectively). Significant differences in THg (and MMHg) were observed between the five lakes, the highest concentrations (4.45 and 0.16 ng L^?1, respectively) tended to be associated with TOC in lakes and occurred at sites in the northwest, characterized by higher levels of soil organic matter (peat) and soil moisture relative to the other sites. In contrast, surface soil pools of THg ranged between 13.6 and 20.8 μg m^?2 across study sites and did not vary significantly, but were typical of global background regions. Nonetheless, the organic rich soils that dominate Ireland are a natural sink for THg, and peat harvesting for energy production may release long-term stores of Hg from deeper soil layers.