Delayed mixing of iron-laden tributaries in large boreal rivers: Implications for iron transport, water quality and monitoring

Boreal river systems globally provide specialized habitats and conduits for the transfer of nutrients, iron and trace elements (TEs) from wetlands and groundwaters to lakes and oceans. Many large boreal rivers are also the site of controversy due to their dual purpose providing ecosystem services for Indigenous peoples and supporting industrial activity; however, water quality monitoring in these systems is more complex than realized. This work demonstrates the impacts of delayed tributary mixing in a typical large boreal river on the properties and transport of Fe-bearing colloids, and water quality monitoring, in two subsequent autumns. The forms and concentrations of Fe colloids were conserved through the mixing zone for more than 100 km, despite salinity changes. Delayed mixing for ca. 60-80 and more than 100 km led to TE concentration differences of more than five-fold across the river. Using traditional samples from the middle of the river, the coincidence of mixing zones with areas of potential industrial impact obscured the potential contributions of some TEs, and caused the erroneous attribution of other TEs to industry at concentrations that were double upstream values. Adequate water quality monitoring and source attribution in large boreal rivers thus requires high-resolution spatial sampling, both across the river and upstream-downstream of potential natural and industrial sources. Since mixing behavior and water quality in large boreal rivers are highly sensitive to hydrologic changes such as seasonal fluctuations, drought/floods, and climate change, fluctuations in water quality and associated mixing behaviours render traditional monitoring approaches unsuitable.