Résumé
Contamination of river sediments by metal(loid)s originating from mining activity is a worldwide problem. Because the transport of metals associated with sediments can represent more than 90% of the total flow of metals in rivers, identify their sources and evaluate their potential mobility are particularly important. The purpose of this work was to assess past and present influence of mining activity on metal(loid) enrichment in sediments of a former mining watershed now industrialized and urbanized, the Gardon River watershed (France). Samples of a sedimentary archive and current sediments of the Gardon River and its tributaries were characterized combining geochemical analyses, zinc isotopic analyses and sequential extractions. Considering metal(loid) concentrations in sediments upstream from the mining sites and in bottom sediments of the archive, natural background values were proposed. Based on these values, enrichment factors (EF) were calculated. Several tributaries were highly impacted by old mining sites (EF values up to 250 for Pb, 160 for Cd, 60 for Zn, 70 for As and 1850 for Sb) and by industrial activity. These polluted tributaries impacted metal(loid) content of the main stream sediments. EF values increased in Gardon River sediments downstream from old Pb/Zn mines about 3-fold for Pb, Cd and 2-fold for Zn, As and downstream from old Sb mines about 5-fold for Sb. Interelement relationships were used to distinguish the main contaminant sources. Although Zn isotopic signatures differed significantly for mining impacted tributaries (delta (super 66) Zn approximately 0.08ppm) and for industrial impacted tributary (delta (super 66) Zn approximately 0.31ppm), Zn isotopic composition remained homogeneous in the main stream sediments of the Gardon River watershed (delta (super 66) Zn values around 0.20ppm). Finally, the percentage of metal(loid)s present in the "mobile" pool, as estimated by sequential extraction, was: Sb (1-5%) < As (1-22%) < Zn (10-65%) = Pb (15-68%).