Predicting Microplastic Migration in Streams through Dynamic Flow Hydrologic Modeling and Direct Measurements of Accumulation Patterns, Deposition and Resuspension Rates
Résumé
Better understanding of the mechanisms that control microplastic (MP) transport in streams is vital to predict their movement in river systems. MPs are abundantly found in streambed sediments, including small and low-density particles of neutral and positive buoyancy. Hyporheic exchange, which occurs in most rivers, is the two-way movement between the overlying water and the streambed sediments. It is driven by both turbulence in the near bed region and pressure variations at the streambed surface that force water and particulate matter (e.g., microplastics, fine sediments, microbes) into and out of the sediment porewater, leading to accumulation of very small particles with very low settling rates within the hyporheic region. Recently, hyporheic exchange has been shown to substantially increase the retention of MPs in rivers. Subsequently, models need to incorporate hyporheic exchange to avoid underestimating the deposition, retention, and long-term accumulation of MPs in streambed sediments.
Additionally, MP remobilization caused by flood events through turbulence or bed sediment transport is yet to be quantified. The extent of remobilization is dependent on both the increase in magnitude of the flow and the number of accumulated MPs in streambed sediments prior to the event. Therefore, in order to accurately predict the persistence and fate of MP in streams it is crucial to not only include hyporheic exchange, but also consider both baseflow and stormflow processes within a model framework.
We present multi-scale mobile-immobile models combined with direct measurements of MP accumulation, deposition, and resuspension rates from mesocosm to reach-scale. The models account for the exchange between the surface water and streambed sediments, deposition, and resuspension during baseflow and stormflow conditions. We used this approach to relate in-stream MP transport for varying polymer shapes and types to spatial and temporal scales of retention. Linking transport mechanisms across scales advances our ability to predict MP fate in streams. These results demonstrate the complex relationship between microplastic transport and retention characteristics that lead to the heterogeneous distribution of microplastic accumulation and persistence in streambed sediments.