The influence of wetting and drying cycles on the biogeochemical dynamics of mercury in sediments

Abstract

Decommissioning hydro-dams increases sediment exposure to air, altering biogeochemical cycling of metals. Hg and MeHg mobility was studied in reservoir sediments from two Eastern Ontario lakes (Stump (SL) and Black Donald (BDL)) submitted to wet/dry cycles with artificial rainwater (pH∼4.5) every two weeks. Leachate pH, sulphate, sulphide and MeHg, and sulphate-reducing bacteria (SRB) populations were monitored over 6 months. The sediments did buffer the rainwater at the start of the experiment, but leachate pH decreased over time for both lakes. MeHg release occurred during the first draining event (2-4 ppt), and decreased thereafter, with no relationship with pH, sulphide and sulphate. SRB populations remained constant over time. Over 70% of Hg was in the non-labile refractory organic and residual phases, where Hg moved to semi-mobile phases in SL, but shifted to more immobile phases in BDL over time. Decommissioning hydro-dam reservoirs may increase acid production and alter Hg partitioning in sediments, while MeHg release will be low.