The impact of sulphide oxidation on dissolved metal (Cd, Zn, Cu, Cr, Co, Ni, U) inputs into the Lot-Garonne fluvial system (France)

Results are presented from observation of dissolved metal concentrations and fluxes at 5 sites in the Lot-Garonne fluvial system known for its historic metal contamination. The contamination originates from the upstream Lot River where a small tributary (Riou-Mort River) drains a smelting-waste area. Unlike non-neutralised acid mine drainage systems, the Riou-Mort waters were not acidic (6.8 < pH < 8.0) due to application of alkaline reagents for neutralisation. Relatively high dissolved U concentrations (up to 1.1 μg L−1) were attributed to these reagents. High metal concentrations (e.g., up to 23 and 1190 μg L−1, for Cd and Zn, respectively) in the Riou-Mort water resulted from the oxidation of the sulphide phases within the smelting-wastes. Pyrite oxidation rate was estimated (5530 t a−1; 35.7 t km−2 a−1) from the total amount of SO42- discharged in the river water. The related dissolved metal inputs into the Lot River were, e.g., 0.55 and 35 t a−1 for Cd and Zn, respectively. The dissolved Cd fluxes in the Lot River corresponded to 65% of those in the downstream Garonne River. The dissolved Zn fluxes were even similar to those in the Garonne River. Mass balance calculations showed that, downstream the Riou-Mort/Lot River confluence, the exchange between dissolved and particulate phases accounted for the removal of 15% of Zn and 50% of Cd from the dissolved phase. The calculated annual dissolved metal fluxes at the outlet of the Lot-Garonne River system are significant at the global scale, as they represent 0.02-0.25% of the global river budget.