Leaching of pollutant metals (Pb, Zn) from abandoned mine tailings: A multicomponent reactive transport model of a pilot-scale experiment.

Mine tailing deposits pose a global problem, as they may contain metal contaminants in various geochemical forms and are likely to be leached from the surface into the underlying groundwater, which can result in health and/or environmental risks. Unfortunately, little is currently known regarding the water flow and mass balance related to leaching in the vadose zone as these factors are still difficult to measure at the field scale. A pilot-scale experiment was run in a 1 m instrumented column for 6 months to address this issue. This 70 cm high column was filled with highly Pb-contaminated tailings and then watered regularly. The top half remained unsaturated while the bottom half was kept saturated. Continuous water flow and water saturation measurements were recorded and the physico-chemical properties of the soil solutions were monitored weekly at different levels in the column. A 1D multicomponent reactive transport model was built to simulate the fate and transport of Pb as well as other elements. The variably saturated water flow was simulated using the Richards equation, while the van Genuchten analytic form was used to describe the unsaturated soil hydraulic properties. The main processes considered to simulate the reactive transport were advection-dispersion, thermodynamic equilibrium, and kinetically-controlled dissolution-precipitation reactions. The most reactive Pb-bearing phases accounted for in the simulation were anglesite (PbSO) and plumbojarosite PbFe(SO)(OH). The simulations accurately reproduced the water flow and mass balance as well as the drop of 2 pH units experimentally measured in the pore solution. This trend resulted from plumbojarosite dissolution followed by ferrihydrite precipitation. The increased Pb concentration in the soil solution induced by the dissolution of Pb-bearing phases was partially offset by Pb-sorption onto newly formed iron oxides and the precipitation of secondary mineral phases, e.g. anglesite. The modeling results could be used to assess potential risks of groundwater contamination by mine tailings.