Source activation or fluvial transport – dynamic controls on spatial patterns and temporal dynamics of plastic pollution in river corridors

Microplastic pollution has been found to be ubiquitous in freshwater ecosystems around the world, with global models predicting river network contributions to the oceans to present major and still increasing sources of marine plastic waste. While previous research has to a large degree focussed on identifying potential sources of plastic pollution to freshwater ecosystems (such as wastewater treatment plants, storm sewers, urban areas), and attributing these to observed microplastic pollution patterns in river corridors, little is known under what conditions potential pollution sources become activated and connected to surface waters, and how the fluvial transport of different micro- and nanoplastic size fractions determines spatial patterns of plastics along river networks, including long-term deposition, storage and potential resuspension.This paper integrates field-based evidence of our global river microplastic survey and several comparative large river network studies (including the rivers Ganges, Boulder Creek, Rhone, and others) with river basin to global scale plastic fate and transport models to identify major drivers of hotspots and hot moments of riverine plastic pollution. Our results highlight under what conditions prior knowledge of the source distributions of plastic pollution carries significant predictive capacity for expected river corridor microplastic concentrations and when (and where) these patters can get transformed substantially by fluvial transport (and transformation) processes. Fusing this experimental evidence with our model predictions revealed significant differences in the downstream footprint, longevity and legacy of dominant sources and transport controls of plastics in the water column and in streambed sediments, driven by gravitational settling, hyporheic exchange flow and resuspension processes.