Résumé
The former Carnoules Pb-Zn mine (Department of Gard, France) has produced 1.5 Mt of solid waste containing pyrite and associated metals and metalloids including arsenic and thallium. The tailings are stored behind a dam. Upon oxidation they generate acid (pH≤3) water containing up to 350mg/l of arsenic, 750 to 2700mg/L of iron, sulphate (2000-7500mg/L) and a few mg/L of heavy metals (Pb, Zn, Cd). The water surges at the bottom of the dam forming the Reigous Creek. For the last ten years, the physicochemical, biological, mineralogical and hydrological characteristics of the site have been monitored by the Hydrosciences laboratory with the aim of understanding the processes responsible for the release of pollutants in the tailing stock and of reducing pollution in the mining creek. The results of this study indicate that, unlike at other sites, pyrite oxidation takes place at the bottom of the tailing stock at the entry of oxygenated underground water. The bacterial populations that are present in the tailings and may catalyse the oxidation reactions were identified. About 20 to 30% of the arsenic initially present in the spring water in the form of As(III), precipitates with Fe in less than 30 meters. This leads to the formation of ochre precipitates containing 20% of arsenic around bacterial structures. Several organisms (bacteria and euglena) that mediate Fe and As oxidation in the creek and thus contribute to accelerating arsenic retention in the solid phase were identified. On the basis of these results and pilot plant experiments, solutions for passive in-situ bioremediation are proposed.