• Variations of dissolved arsenic concentration according to water/rock interactions • As in river water originates from Perm-ian and Hercynian metamorphic rocks • 4.2 tons⋅year − 1 of dissolved As is transferred to the Mediterranean Sea. • The use of a mixing model supports the non-conservative nature of arsenic. A detailed study on arsenic (As) in rocks and water from the Var River watershed was undertaken aiming at identifying (i) the origin and the distribution of As in this typical Alpine/Mediterranean basin, and (ii) As input into the Mediterranean Sea. Dissolved As concentrations in the Var River range from 0.1 to 4.5 μg⋅L −1 , due to high hydrological variability and the draining through different geological formations. In the upper part of the Var drainage basin, in the Tinée and the Vésubie valleys, high levels of dissolved As concentrations occur (up to 263 μg ⋅ L −1). The two main sources of As in rocks are the Hercynian metamorphic rocks and the Permian argilites. Highly heterogeneous distribution of As in waters draining through metamorphic rocks is probably related to ore deposits containing arsenopyrite. As, U, W and Mo concentrations in water and rocks correspond to the formation of As-rich ore deposits around Argentera granite by hydrothermal fluids deposited at the end of the Hercynian chain formation, which occurred about 300 My ago. In 2009, weekly monitoring was performed on the Var River (15 km upstream of the mouth), highlighting an average dissolved As concentration (b0.45 μm) of 2.7 ± 0.9 μg⋅L −1 , which is significantly higher than the world-average baseline for river water (0.83 μg ⋅ L −1). Taking the average annual discharge (49.4 m 3 ⋅ s −1) into account and the As levels in the dissolved phase and in deposits of the Var River, dissolved As input into the Mediterranean Sea would be 4.2 ± 1.4 tons⋅year −1 which represents 59% of the total As flux. This study also reveals a probable non-conservative As behaviour, i.e., possible transfer between aqueous and solid phases, during the mixing of the Var River with a tributary.