THE PARTITIONING OF MERCURY IN THE SOLID COMPONENTS OF DRY AND FLOODED FOREST SOILS AND SEDIMENTS FROM A HYDROELECTRIC RESERVOIR, QUEBEC (CANADA)

Upon inundation the soils in a hydroelectric reservoir are subjected to several ye,us of physical, biological, and chemical changes as the transition from a terrestrial to an aquatic ecosystem is achieved It is suspected that changes in soil Eh and pH alter tilt metal binding capacity of organic matter, reactive iron (Fe) oxides/oxyhydroxides, and clay minerals, and may cause the mercury associated with these phases to be remobilized. Four cores were collected along a transect from an unflooded forest soil to a pre-impoundment lake bottom sediment. They were subjected to a customized sequential extraction procedure to determine the distribution of Hg between three optionally-defined solid compartments: organic carbon reactive Fe oxides/hydroxides, and the solid (clay and sulfide) residue. Results indicate that up to 80% of the Hg in the O-horizon of forest soils and flooded soils, and up to 85% of the Hg in lake sediments is bound to the NaOH-extractable organic carbon fraction. Furthermore, it was observed that the highest Hg concentrations are associated with degraded organic matter. In the B-horizon of a podzol, 40-60% of the total Hg was found associated with reactive Fe minerals. In contrast, the needed podzol contains almost no reactive Fz at any depth aid associated Hg concentrations are low. We propose that upon inundation, Fe oxides are reduced and Hg released to the pore waters where it is rapidly bound to other available substrates. Analyses of the extractions residues suggest that there is and enrichment of Hg in this fraction immediately above the B-horizon in a flooded soil.