A Comprehensive Review of MP Pollution in Global Rivers: Distribution Patterns and Fluvial Transport Dynamics

Microplastics (MPs) have a ubiquitous presence in our environment, and their abundance, particularly in fluvial systems, has been documented in most rivers worldwide over the last decade. Rivers are critical carriers of almost 70%-80% of the plastic waste that reaches the oceans from land-based sources, and MPs have become a major pollutant found in the riverine sediments. This article comprehensively reviews the available studies on the MP pollutants in riverine sediments across global rivers. A synthesis of continent-wise characteristics of MPs in river sediments, including primary sources, shape and size, polymer types, and concentration levels, based on statistical parameters, is presented. MP concentrations show a strong gradient across continents, with the highest levels detected in Africa, followed by Asia, South America, Europe, North America, and Australia. Globally, the major primary sources of MPs in river sediments include wastewater treatment plants, industrial discharges, textile and garment processing, fishing-related activities, and mismanaged municipal waste, although their relative contributions indicate substantial geographic variation. Across all continents, the occurrence of fibers and fragments is significant, and PE, PP, PS, PET, and PA emerge as the most commonly found polymers in the riverbed. Additionally, inferences are drawn from the available studies on the threshold, movement, and deposition processes of MP pollutants in river systems. Due to lower densities (particularly for PP, PE, and ABS) and decreased bed friction, MPs commence movement at lower shear stresses compared to the natural sediments of equal size. The Corey shape factor significantly influences the incipient motion thresholds, but its effect depends on particle orientation and friction. The inclusion of static friction and hydraulic roughness yields a more accurate threshold model, enabling a modified Shields framework that predicts MP incipient motion more accurately than classical sediment-based curves. Major gaps in the literature regarding the need for reliable estimates of critical bed shear stress for various MP particles and the effect of fluvial factors, such as seepage and vegetation, on MP transport are identified.