Spatio-temporal evolution and source tracking of arsenic in surface waters of an old mining district (Orbiel Valley, France)

Past mining activities in the Orbiel Valley pose a significant risk of As contamination to its ecosystems and inhabitants. Approximately 12 million tons of tailings from last century's As and Au mining operations remain on site. Rehabilitation works have been implemented to store mining wastes and treat leaching waters. However recent studies have revealed that contamination is still present in water and sediments (Khaska et al., 2015; Delplace et al., 2022). The complexity of the area and previous findings have shown the importance of a more in-depth study of As sources and fate in the watershed, including 1/ characterizing As contamination levels in the Orbiel River and its tributaries during different hydrological periods, 2/ identifying the main sources of As and 3/ distinguishing the natural geochemical baseline from anthropogenic inputs. Water samples (<0.22 µm) were collected in the Orbiel River and its tributaries from 2018 to 2022, representing a total of 170 samples, to analyze major element and metal(loid) concentrations, alkalinity, dissolved organic carbon, Sr isotope ratio, and As redox speciation in the dissolved fraction. Rock samples representative of the different geological formations were collected to compare the natural and anthropogenic evolution of the Sr isotope along the Orbiel Valley. Upstream the mining district, in Orbiel River, the dissolved As concentration was about 2 µg/L and increased downstream near the main waste storage area to 7 – 71 µg/L (min-max, depending on the period) with a high proportion of As(III) (> 52 %). The anthropogenic origin of this contamination was confirmed by the 87Sr/86Sr ratio, which is less radiogenic than in the upstream pristine area, in relation with lime treatment implemented in the mine waste area. However, some valley limestones exhibit a Ca-arsenate-like isotopic ratio, highlighting the need to use complementary tracers to distinguish between anthropogenic and lithological sources. Finally, the mining-impacted tributaries are identified as significant contributors of As to the Orbiel River. The present study will serve as a reference to interpret the origin, transport, and fate of metal(loid)s during future extreme flood events characteristic of this Mediterranean river.   Delplace, G., Viers, J., Schreck, E., Oliva, P., Behra, P., 2022. Pedo-geochemical background and sediment contamination of metal(loid)s in the old mining-district of Salsigne (Orbiel valley, France). Chemosphere 287 (2). Khaska, M., Le Gal La Salle, C., Verdoux, P., Boutin, R., 2015. Tracking natural and anthropogenic origins of dissolved arsenic during surface and groundwater interaction in a post-closure mining context: isotopic constraints. J. Contaminant Hydrol. 177–178, 122–135.