Alteration of As-bearing phases in a small watershed located on a high grade arsenic-geochemical anomaly (French Massif Central)

At a watershed scale, sediments and soil weathering exerts a control on solid and dissolved transport of trace elements in surface waters and it can be considered as a source of pollution. The studied subwatershed (1.5 km) was located on an As-geochemical anomaly. The studied soil profile showed a significant decrease of As content from 1500 mg kg in the 135– 165 cm deepest soil layer to 385 mg kg in the upper 0–5 cm soil layer. Directly in the stream, suspended matter and the <63 μm fraction of bed sediments had As concentrations greater than 400 mg kg. In all these solid fractions, the main representative As-bearing phases were determined at two different observation scales: bulk analyses using X-ray absorption structure spectroscopy (XAS) and microanalyses using scanning electron microscope (SEM) and associated electron probe microanalyses (EPMA), as well as microRaman spectroscopy and synchrotron-based micro-scanning X-ray diffraction (μSXRD) characterization. Three main As-bearing phases were identified: (i) arsenates (mostly pharmacosiderite), the most concentrated phases As in both the coherent weathered bedrock and the 135–165 cm soil layer but not observed in the river solid fraction, (ii) Feoxyhydroxides with in situ As content up to 15.4 wt.% in the deepest soil layer, and (iii) aluminosilicates, the least concentrated As carriers. The mineralogical evolution of Asbearing phases in the soil profile, coupled with the decrease of bulk As content, may be related to pedogenesis processes, suggesting an evolution of arsenates into As-rich Feoxyhydroxides. Therefore, weathering and mineralogical evolution of these As-rich phases may release As to surface waters.