How hyporheic pumping and bedform migration redistribute microplastic burial in sand-bed rivers

The hyporheic zone plays a key role in controlling stream water quality by regulating the transport and transformation of nutrients and contaminants. Recently, growing attention has focused on hyporheic exchange as a driver of microplastic (MP) burial and resuspension, because of its ubiquity, persistence and their documented ecological impacts. Here, we investigate how MPs enter, become trapped, buried and released from sand-bed rivers with mobile dunes. We developed a semi-analytical solution of the flow field to delineate the MP trajectories considering the coupled effect of pumping (due to pressure variation at the water-sediment interface) and turnover (due to bedform migration). Along each exchange path, we then solved an advection–dispersion-reaction equation (ADRE) analytically. To represent progressive resistance/clogging effects, we incorporate spatially varying velocity, dispersion, retardation and first order removal coefficients.

Results show MP retention within the hyporheic zone is dictated by the interplay between stream hydro-morphology (dune geometry, migration speed and alluvial depth) and along-path retention processes. The transport formulation induces an exponential decay of MP concentration with both trajectory length and residence time, meaning that, beyond a certain point, longer subsurface travel does not necessarily equate to higher retention efficiency. Importantly, bedform migration does not simply increase or decrease MP burial uniformly but instead redistributes retention between near-surface and deeper sediment layers by enhancing shallow recirculation while intermittently disrupting long, deep pathways. The proposed framework shows how bedform dynamics influence the transport and persistence of microplastics with direct implications for ecosystem health and risk assessment at the watershed scale.