Importance of elemental mercury in lake sediments

Mercury (Hg) redox changes in sediments are poorly studied and understood, even though they potentially control Hg availability for methylation and can alter sediment–water Hg exchange. Elemental Hg (Hg0) concentrations in sediments of two Canadian Shield lakes were assessed by thermodesorption. Hg0 concentrations in sediments varied between 6.3 and 60.3 pg g−1 (wet weight) which represented 7.4–28.4% of total mercury (HgT) concentration. Hg0 concentrations were similar in both lakes. Hg0 was rapidly adsorbed on sediments in controlled adsorption experiments and surface sediments sampled in summer had a stronger affinity for Hg0 than deeper sediments and sediments sampled in fall. This adsorption was positively correlated to organic matter content and negatively related to particle grain size, pH and oxygen concentration in overlying water. This study demonstrates that Hg0 is a prevalent species in sediments, but not in porewater, because of the high sorptive capacity of sediments towards Hg0. Its potential availability towards Hg methylating bacteria remains to be determined.