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

► Characterization of As-carriers at a microscale by in-situ techniques in a soil profile. ► Decrease of As content from the saprolite to the surface horizon during pedogenesis. ► As was initially associated to Ba-rich pharmacosiderite in the saprolite. ► As was then in goethite, hematite, ferrihydrite-type and aluminosilicates . 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 2) was located on an As-geochemical anomaly. The studied soil profile showed a significant decrease of As content from 1500 mg kg −1 in the 135–165 cm deepest soil layer to 385 mg kg −1 in the upper 0–5 cm soil layer. Directly in the stream, suspended matter and the