Geochemistry of molybdenum (Mo), vanadium (V), and tungsten (W) in Swedish rivers and headwater streams

Understanding the cycling of critical oxyanion-forming trace metals is essential, considering their growing use in green technologies and their potential environmental risks. We investigated the source, transport, and fate of vanadium (V), molybdenum (Mo), and tungsten (W) across hemiboreal and boreal landscapes in Sweden, spanning 97 hemiboreal headwater streams, one boreal headwater stream, and two unregulated boreal rivers. Spatial and temporal variability was pronounced. V occurred at the highest concentrations, with hemiboreal headwaters averaging ∼78 μg/L compared to ∼8 μg/L for Mo and ∼0.2 μg/L for W. These concentrations exceeded those in boreal headwaters and rivers by factors of up to 300, reflecting enhanced weathering under milder hemiboreal conditions. Partitioning differed strongly among elements: Mo was predominantly dissolved, W occurred in both dissolved and particulate fractions, and V shifted from dominantly dissolved in hemiboreal streams to nearly equal distribution in boreal waters. Seasonal dynamics were most pronounced during spring snowmelt, when hydrological flushing and hyporheic exchange mobilized solutes from riparian soils and porewaters. Element-carrier relationships highlighted strong associations of V and W with Fe- and Al-rich phases, and Mo with organic matter, Si, and U, particularly at river mouths. Redox transitions in wetlands and hyporheic zones further mediated mobility, promoting V release and W transport while retaining Mo under reducing conditions. Our findings demonstrate that hydrology, redox dynamics, and landscape type jointly regulate the mobility and fate of V, Mo, and W from source to sea. The distinct behaviours of these elements highlight headwater streams as critical biogeochemical control points and underscore the need to integrate colloidal, redox, and hydrological processes into monitoring and modelling frameworks. These insights have implications for evaluating ecological risks and for critical raw material (CRM) exploration in northern landscapes under a changing climate.