Groundwater contributions to metal transport in a small river affected by mining and smelting waste

The Riou Mort watershed, strongly affected by former coal mining and Zn ore treatment, has been the major source of the historical polymetallic pollution of the Lot–Garonne–Gironde fluvial–estuarine system. Two decades after the end of ore treatment, the former industrial area still contributes important amounts of metals/metalloids from various, partly unidentified, sources to the downstream river system. This study presents the high spatial variability of metal/metalloid (Cd, Zn, As, Sb, U, V) concentrations in water and suspended particulate matter (SPM) from eight observation sites during a short, intense flood event. Despite important dilution effects, the observed concentration levels at the different sites suggested additional Cd and Zn inputs, probably from polluted groundwater. This formerly unknown metal source was then localized and characterized by sampling water and SPM along two longitudinal profiles during different hydrological situations. Groundwater inputs of “truly dissolved” (<0.02 μm) Cd and Zn occurred along ∼200 m, contributing 43% and 28% to the total annual (2004) Cd and Zn fluxes in the Riou Mort River. The estimated groundwater concentrations of Cd and Zn (2500–6700 and 83,000–170,000 μg l−1, respectively) in the source zone were consistent with values measured in samples from the near aquifer (5400–13,000 and 200,000–400,000 μg l−1). The present work induced concrete remediation actions (pumping and treatment of the polluted groundwater), that are expected to strongly reduce dissolved Cd and Zn emissions into the Riou Mort River.