Xiaohua Huang, Yuchuan Meng, Yu Chen, Peng Zhao, Li Liu, Keming Tang
Elucidating hydrodynamic-driven transport of microplastics (MPs) is essential for quantifying global riverine fluxes and assessing associated risks. Current knowledge, however, remains largely observational and fragmented across different waters. This study proceeds from MPs heterogeneous distributions and hydrodynamic-driven processes toward a mechanistic understanding from surface waters to surface-subsurface interaction zones, and proposes an open conceptual framework to better connect MPs transport across two water bodies. Results reveal that MPs behave as analogues of sediments and solutes during transport. Drawing on corresponding theories, the settling/rising velocity, vertical abundance profiles, hydro-mobilizations at water surface and riverbed, surface-water trajectories, and surface-subsurface exchanges, are studied. Key findings are threefold: (1) Mechanistic formulations from sediments and solutes are not fully applicable to diverse MPs, as current theoretical examinations rely mainly on spherical particles; (2) Theoretical developments remain nascent and poorly integrated across different research focuses, such as the well-developed settling/rising velocity formulations are not widely applied to other key behaviors; (3) Transport behaviors across surface and subsurface waters remain insufficiently connected, with boundary exchanges posing a particular challenge. To advance mechanistic understandings, an open framework is proposed based on the thoroughly literature review, which may offer prospective directions for riverine MPs transport modeling. Concurrently, research priorities for each mechanistic formulation are identified to promote practical applications and better connect MPs transport between two water bodies. This study shifts the paradigm from observation-based to process-based understanding, which offers a roadmap for future theoretical and model development to accurately assess MPs fate in river networks.