Transport of dissolved and particulate arsenic in the Orbiel River during minor flood events downstream the historic Salsigne gold mining district, Southern France

Understanding and quantifying the transport of toxic metals and metalloids during flood events downstream of former mining sites is challenging as these events are transient, highly variable and unpredictable, resulting in a scarcity of observational data across watersheds worldwide. In this study, the concentrations and fluxes of dissolved and particulate arsenic (As) were determined to investigate the dynamic of As transfer during flood events in the Orbiel River catchment, in Southern France. This river drains the former Salsigne mining district, where gold-bearing arsenopyrite mineralization was historically exploited. In this Mediterranean catchment, characterized by long dry periods and episodic rainfall, high-frequency automatic sampling of river water was conducted at four stations during five minor to moderate flood events between 2021 and 2023. Arsenic dynamics during each flood were event-specific and varied according to the antecedent hydrological conditions of the basin and the respective contribution of mine-impacted and non-impacted tributaries, which depend on localized rainfall patterns. Dissolved As concentrations ranged from 10 to 35 μg L −1 at the outlet of the catchment, comparable to baseflow levels, while particulate As varied from 20 to 500 mg kg −1 , with enrichment factors up to 29 relative to the local geochemical background. Although these recurring, low-intensity floods did not generate strong As concentration peaks, the total As load (i.e., the sum of dissolved and particulate As) transported during the quick-flow phases of individual flood events ranged from 7.5 to 93.2 kg at the catchment outlet, reaching up to 3.9 times that measured during a 50-day low-flow summer period (24 kg). These results demonstrate that recurring minor floods, which occur several times each year, substantially contribute to As export from the Orbiel River catchment despite their moderate magnitude. These findings highlight the need to account for frequent low-intensity floods in contaminant transport assessments and management of legacy mining areas under climate change.