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

Abstract

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 SO 4 2 - 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.