The role of pumping and turnover in controlling microplastics entrapment and release in sand-bed rivers

The dynamic interaction between terrestrial and aquatic ecosystems through streams and rivers plays a crucial role in the transport and transformation of passive and reactive solutes, including nutrients and Contaminants of Emerging Concern (CECs). Among these, microplastics (MP) have received increasing attention due to their toxicity and possible impact on the transport and fate of other pollutants and CECs. While previous research has examined hyporheic exchange induced by pumping (due to near-bed head variation), the additional influence of dune migration (turnover) on MP dynamics remains poorly quantified. Here, we introduce a semi-analytical framework that couples transient hyporheic flow fields with particle transport to evaluate MP fate under the combined effects of pumping and turnover (PT). The model captures streamline pathways and solves the advection-dispersion-reaction equation with path-dependent advection, dispersion, retardation, and sorption terms. Results show that dune PT substantially alters hyporheic flow organization, shifting upwelling and downwelling fluxes, redistributing residence times, and modulating burial depth from pumping alone. PT accelerates early MP release (5th-30th percentiles) but also enhances deep retention under certain dune sizes and flow conditions, highlighting a nonlinear interplay between shallow rapid exchange and deep burial.