The partitioning of mercury in the solid components of forest soils and flooded forest soils in a hydroelectric reservoir, Québec /

Upon inundation, the soils in a hydroelectric reservoir are subjected to several years of physical, biological, and chemical changes as the transition from a terrestrial to an aquatic ecosystem is achieved. Changes in Eh, pH, and microbial activity are believed to alter the metal binding capacity of solid substrates (e.g., organic matter, reactive Fe and Mn oxides, and clay minerals) within the soil profile, leading to the remobilization of mercury associated with these phases.Four cores were collected along a transect from an unflooded forest soil to a pre-impoundment lake bottom sediment in the La Grande-2 (LG-2) Reservoir and watershed. The samples were sequentially extracted to determine the distribution of mercury between three operationally-defined solid compartments: organic carbon (1N NaOH-extractable), reactive Fe and Mn oxides/hydroxides (1N HCl-extractable), and the solid (clay and sulphide) residue.Results indicate that up to 80% of the mercury in the O-horizon in forest soils and flooded forest soils, and up to 85% of the mercury in lake sediments, is bound to the NaOH-extractable organic carbon fraction.In the B-horizon of a forest podzol where organic content is low, 40-60% of the total mercury was found to be associated with reactive Fe minerals. In contrast, the flooded soil contains very little reactive Fe at any depth and the associated mercury concentrations are low. I propose that, upon inundation, oxide minerals are reduced and Hg released to the pore waters where it is immediately bound to an available substrate. Analyses of the residues suggest that there is an enrichment of mercury in the residual fraction immediately above the B-horizon of a flooded soil while sulphide mineralization appears to play a role in sequestering mercury in lake sediments