Grain-size dependence of mercury speciation in river suspended matter, sediments and soils in a mercury mining area at varying hydrological conditions

We analyzed Hg concentrations and Hg-species (Hg-thermo-desorption) in SM and different grain size fractions of soils from the Idrijca catchment to evaluate changes in solid phase Hg-species under low and high flow conditions. Concentrations of dissolved Hg did not change significantly during low and high flow (median: 21.3-28.1 ng L−1, respectively). Hg concentrations in SM in tributaries decreased with distance (∼30 km) from the mine from 32.7 to 0.47 mg kg−1 related to Hg concentrations in fine particulate soil fractions (0.45-20 μm) which ranged from 56.2 to 0.53 mg kg−1. Hg speciation in SM was dominated by NOM-bound Hg forms at low flow (median: 95.5%) which decrease during high flow conditions (median: 53.8%) attributed to increased mobilization of cinnabar from riverbanks and mine residues. Our results show that aqueous Hg transport in the Idrija mining area is dominated by heavy rain events and mobilization of NOM-bound Hg from soils indicating that the frequency of heavy rain events will control transport of bioavailable Hg to the marine environment.