Field observations of vertical profiles of microplastic concentrations in a river

Microplastics (MPs), which are plastic pieces smaller than 5 mm, are causing widespread pollution in the aquatic environments of rivers, oceans, and lakes. Many researchers have measured MP concentrations mainly near water surfaces because the specific gravity of major plastics is lower than 1.0. In order to accurately estimate microplastic flux into oceans via rivers, it is necessary to evaluate the vertical profiles of MP concentrations in rivers. To address this, we conducted field surveys for grasping the vertical distributions of MP concentrations in a river. Two peaks in the vertical profiles of MP concentrations appeared in the surface and bottom layers of the river water. It was also noted that the specific gravity of MPs found in the bottom layer were slightly greater than those of the surface layer, showing that the vertical profiles of MP concentrations were influenced by the specific gravity of MPs. These facts indicate that the microplastics with large specific gravity (>1.0) can concentrate near the riverbed regardless of vertical mixing.