Power-law size spectra of microplastic number and mass concentration in river water

Microplastics (MPs; 1-5000 μm) have been detected in various locations, and numerous surveys have been conducted in rivers to identify their sources and characterize the extent of MP pollution. However, standardized observation and analytical methods for small MPs have not yet been established, and the relationship between MP abundance and size-distribution characteristics remains poorly understood. This study aimed to characterize the size spectra of MP number and mass concentrations over a broad size range in river water and evaluate the applicability of a power-law model to these size spectra. To achieve this, we applied three different sampling methods to surface water in the Tsurumi River, Japan, yielding seven MP samples. The results demonstrated the statistical significance of the power-law approximation for the size spectra of the number and mass concentrations of MPs across all samples. The estimated power-law slopes were -3.27 ± 0.19 and -1.05 ± 0.20 for number and mass concentrations, respectively, which are close to the assumed values of -3 and -1, respectively. Additionally, MP mass concentrations were calculated from the measured values within a limited size range using the power-law model of the MP size spectrum. The results indicated that MP mass concentrations can be accurately estimated by extrapolating across unmeasured size ranges using a power-law relationship. This approach addresses a key limitation that has hindered inter-study comparisons owing to differences in the MP size ranges considered and provides an important step toward harmonizing MP datasets and improving the quantitative understanding of MP pollution in riverine environments.