Transfer and Dispersion of Arsenic and Other Metal(loid)s in the Orbiel Valley (France): Spatio-Temporal Evolution and Source Tracking

With about 12 million tons of polluted tailings produced from arsenic and gold exploitation of the Salsigne mine in the last century, the Orbiel Valley is subjected to a very important contamination risk for inhabitants and ecosystems. Rehabilitation projects have been implemented for 20 years, at the end of mining activities, to safely store the waste and treat the leaching waters. However, recent studies have reported that Orbiel sediments are still contaminated by mining activities, and the 87Sr/86Sr isotopic ratio of river water is an interesting tool to highlight the anthropogenic contamination of the river. The complexity of the territory (Fig.) and these previous findings showed the importance for further studying the transport and fate of metal(loid)s during the different hydrological periods, characterizing the main sources of As contamination and distinguishing the natural geochemical baseline from anthropogenic inputs.

A total of 130 water samples were collected in the Orbiel River and its tributaries from fall 2018 to spring 2022. Physical-chemical parameters (pH, conductivity and temperature) were measured in situ. After filtration at 0.22 µm, analyses of alkalinity, DOC (dissolved organic carbon), majors and metal(loid)s concentrations, Sr isotope ratio and As speciation were performed.

Upstream the mining district, the dissolved As concentration was about 2 µg/L and increased downstream the main waste storage area (CDS, ART, Fig.) to 7 - 71 µg/L (min-max, depending on the period). The anthropic 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 (Fig.). A high proportion of As(III) with respect to total As (> 52%) is evidenced at this station, while As(V), less toxic, is the prevailing form upstream. Based on spatial variations in metal(loid) concentrations and geochemical facies of watercourses (Piper diagrams), the tributaries draining areas of mine waste storage are identified as significant carriers of contamination to the Orbiel River.

The present study will then serve as a reference point to interpret the origin, transport and fate of metal(loid)s during future extreme hydrological events, characteristics of the Mediterranean region.