Fate and Transport in Freshwater Systems
Microplastics have been detected in rivers and streams around the world. Their measurements shed light on their occurrence in freshwater systems, but they are fragmentary in both space and time . The basis of research on microplastic pollution depends on (a) vertical and horizontal distribution in the aquatic environment and factors that affect the distribution, (b) changes of material properties due to weathering and their consequences on the fate, behavior and transport of MP and (с) MP toxicity and their dependence on different material properties .
Transport of MPs
The quantity of microplastics present in the freshwater systems is in direct co-relation with factors like human population density proximal to the water body, proximity to urban centers, proximity to major river inflows, precipitation events, movement of surface water currents, water residence time, size of the water body, the type of waste management used, amount of sewage overflow, and atmospheric depositions [50-55,34]. The population density at or near the freshwater body remains the most influential factor. Eriksen et al.  reported that Lake Erie had microplastic particle counts (466,305 particles/km2) that were 70 and 38 times greater as compared to the less populated Lakes Huron (6541 particles/km2) and Superior (12,645 particles/km2). Even within Lake Erie, sites near cities had more microplastics, including more pellets compared with rural shorelines . Also, the quantity of plastic litter which is released to the environment is directly proportional to the international differences in societal attitudes, education and investment in waste management infrastructure. In 2010, China was unable to manage its 76% of plastic waste (8.82 million metric tonnes) as opposed to 2% (0.28 million 466 metric tonnes) of the United States .
Transport of plastic particles within the river systems is largely affected by the same factors affecting sediment transport, such as hydrological characteristics and environmental conditions . Change in river depth or velocity (e.g., on a bend) may lead to deposition of particulate matter, whereas high velocity flood conditions and erosion could result in mobilization of previously sedimented particles, in addition to the introduction of particles via runoff [104,105]. Surrounding land use can also affect the dynamics of sediment and particulate transport within a river due to erosion, use of soils, irrigation and runoff . Presence of plastic in riverine systems may also be subjected to in situ degradation, either by light or mechanical fragmentation .