Seasonal temperature oscillations drive contrasting Arsenic and Antimony mobilisation in a mining‐impacted river system

Arsenic (As) and antimony (Sb) speciation and mobility in river systems can be influenced by the redox conditions of benthic and hyporheic zone sediments. However, our understanding of how temperature fluctuations influence riverine As and Sb mobility over diel and seasonal timeframes, via moderation of sediment redox conditions, is poorly constrained. Here, we compare diel and seasonal water quality data from a river system contaminated with As and Sb with results from sediment-water incubations conducted at temperatures spanning 8 to 32^oC under low-dissolved oxygen conditions. Higher incubation temperatures caused faster microbial respiration, lower redox potential and greater As_aq concentrations, coincident with reduction of As(V), Mn(III/IV), Fe(III) and SO₄^2-. Initial rates of As³⁺_aq formation increased exponentially with temperature (Q₁₀=3.3+0.5) and were positively correlated with microbial respiration (r²=0.99, P<0.01). In contrast, Sb_aq displayed an initially negative and comparatively weak overall temperature dependence during incubations. In river waters, diel mobilisation of reduced species (As³⁺_aq, Fe²⁺_aq) and contrasting attenuation of Sb was coincident with lower redox potential during nightly respiration cycles. Distinct seasonal oscillations in river water As_aq were exponentially correlated with temperature (r²=0.68, P<0.01), whereas Sb_aq was not. These characteristics reflect the fundamental role of anaerobic metabolism (as influenced by temperature) within benthic and hyporheic zone sediments as a key driver of contrasting riverine As and Sb mobility. The findings imply that riverine As mobility may be enhanced, while Sb mobility possibly attenuated, by eutrophication or by elevated stream temperatures due to a warming climate.