Modeling microplastic dynamics in riverine systems: fate and transport analysis

The escalation of plastic production and its ubiquitous application across numerous sectors have led to increased plastic waste, contributing to the widespread occurrence of microplastics (MP) across different ecosystems. Due to their durability and resistance to breakdown, these particles persist and accumulate in the environment, threatening both ecological and human health. Rivers serve as critical conduits that link terrestrial and marine environments, thus playing a pivotal role in the distribution of MP. Consequently, one of the key research priorities is focused on understanding the primary processes that controls the transport of MP in riverine environments. Here, we propose an integrated version of a previously published model that solves the classical advection-dispersion-reaction equation (ADRE) by mimicking the removal/resuspension of MP as reactive terms. The proposed framework integrates inputs of MP from human activities and assesses transport and deposition processes-including sedimentation, burial, resuspension, and removal along riverbanks-tailored to the specific hydro-geomorphological characteristics of river segments. The model was applied in a real river network, the Tame River (UK), to evaluate four distinct MP composition scenarios: a high-fiber scenario, a high-fragment scenario, a high-pellet scenario, and an averaged composition scenario derived from literature. These scenarios represent realistic variations in particle morphology and density, allowing an assessment of how particle-specific properties influence transport and retention. The results highlight the dominant role of bank retention and sedimentation in MP transport, demonstrating that depositional processes significantly contribute to the long-term retention of MP in river networks. This tool is instrumental in enhancing our understanding of MP pollution in river systems and proposing effective mitigation strategies.