Arsenic speciation and mobility in gold mining-impacted riverine sediments from the Loddon River catchment in Victoria, Australia

The Loddon River floodplains, Victoria, southeast Australia, is contaminated with high levels of As (up to 300 mg/kg) due to re-deposited mine tailings from historical gold mining (1851-1914). Arsenic concentrations often significantly exceed Australia's sediment guidelines (ISQG-high value of 70 ppm). To determine the potential risk associated with As, this study examined the speciation of As and its mineral phase association in sediments along a 60 km stretch of the river, and investigated how As is released under redox cycling conditions. Sequential extraction procedures (SEP) revealed that As is primarily associated with crystalline Feˡˡˡoxyhydroxides (up to 40%) and residual phases (up to 70%). X-ray absorption spectroscopy (XAS) analyses showed that As was predominantly in its pentavalent state, existing mainly as Feˡˡˡoxyhydroxides-sorbed arsenate (Fe-AsO 4 ), with a small amount (up to 18%) of reduced arsenopyrite (FeAsS) present upstream reaches of the Loddon River. A column experiment simulating flooding-drying cycles was used to assess As mobility. Under reducing conditions, in the presence of a carbon source, As concentrations in the water increased dramatically, reaching up to 1,300 μg L -1 , far exceeding the WHO drinking water guideline of 10 μg L -1 . Microbially-mediated reductive dissolution of Feˡˡˡoxyhydroxides and reduction of As from As(V) to As(III), which has lower affinity for Feˡˡˡoxyhydroxides, was likely responsible for this As release. These findings highlight the enduring presence of As contamination in floodplains impacted by historical gold mining. Furthermore, this study underlines the potential environmental and health risks posed by As in floodplain sediments, particularly as wetting and drying cycles can increase its mobility. Greater efforts are needed to assess the spatial extent of historical pollution in impacted river catchments and develop comprehensive mitigation strategies to address the widespread contamination of floodplain sediments in affected river systems globally. • Arsenic concentrations in the Loddon River floodplains reach up to 300 mg/kg • Arsenic mainly bounds to Feˡˡˡoxyhydroxides and residual phases • Reducing conditions during flooding-drying cycles increase As levels in water to 1,300 μg L -1 • Arsenic contamination in floodplains poses significant environmental and health risks, requiring urgent mitigation strategies