Development of a method to uniformly evaluate microplastic concentrations across a wide size range in rivers – High-precision estimation of unmeasured size concentrations from limited size range observations

[Research Summary and Key Points] We calculated the size spectrum of microplastics (MP) in rivers and clarified the relationship between number concentration, mass concentration, and size distribution. Using a power-law model, we demonstrated that mass concentration including unmeasured size ranges can be estimated with high accuracy from limited observed size ranges of MP. This study is expected to be an important step toward the unified organization of MP data and elucidation of the actual state of environmental pollution by MP.

[Overview of Research] Research groups including Kota Egoshi (Master's course 2nd year, 2026), Assistant Professor Mamoru Tanaka, and Professor Yasuo Nihei of the Hydraulics Laboratory, Department of Civil Engineering, Graduate School of Advanced Science and Technology, Tokyo University of Science, calculated the size spectra of number and mass concentrations of microplastics (MP) in rivers based on data collected from the Tsurumi River (*1), and demonstrated that mass concentration including unmeasured size ranges can be estimated with high accuracy from observed size ranges. MP has been detected in a wide range of environments such as oceans, rivers, atmosphere, and groundwater, and many surveys have been conducted in rivers to identify sources and understand the actual state of pollution. However, standard methods for observing and analyzing particularly small MP have not yet been established, and sufficient understanding of the relationship between MP number concentration, mass concentration, and size distribution has not been obtained. In this study, we conducted a field survey of MP in the Tsurumi River. Seven MP samples were collected using three different sampling methods according to MP size. Analysis showed that for all samples, the size spectra of MP number and mass concentrations could be statistically significantly approximated by a power-law. The estimated slopes of the power-law were -3.27 ± 0.19 for number concentration and -1.05 ± 0.20 for mass concentration, close to the theoretical assumptions of -3 and -1, respectively. Furthermore, by fitting the power-law model to measurement data from a limited size range and extrapolating to unmeasured size ranges, we demonstrated that the total mass concentration of MP can be estimated with good accuracy. This approach solves the problem of differences in MP size ranges that have hindered direct comparison between studies. This provides a foundation for cross-sectional use of accumulated MP data and is expected to contribute to a more comprehensive understanding of the actual state of MP pollution in rivers. This research was published online in the international academic journal "Environmental Pollution" on April 2, 2026.

[Background of Research] In recent years, environmental pollution by microplastics (MP), fine fragments of plastic, has attracted great attention. MP have been detected not only in natural environments such as oceans, rivers, and atmosphere but also in tap water and human blood, revealing that pollution has deeply penetrated our lives. These MP also have significant biological and physical effects on aquatic organisms, and the degree of impact strongly depends on the size distribution of MP. Therefore, it is essential to understand in detail the sources and actual state of MP pollution across a wide size range for future countermeasures. In previous studies, standardization of sampling and analysis methods for small MP (SMP) has lagged behind that for larger MP (LMP), and the size ranges investigated vary widely among studies, making unified comparison difficult. Moreover, in the analysis of size distribution, the lack of unified criteria for dividing size intervals has led to large variations in power-law exponents among studies, preventing consistent evaluation. Furthermore, few studies on size spectra have targeted rivers, and in particular, research on size spectra based on mass concentration has not been conducted in any environment. Thus, there has been no study evaluating MP in rivers from both particle number and mass perspectives across a wide size range. Our research group has previously conducted field surveys of MP in various rivers throughout Japan and achieved excellent results (*1-3). In this study, we targeted the Tsurumi River, collected and analyzed MP over a wide size range using three sampling methods, and calculated the size spectra of number and mass concentration. We also verified the applicability of the power-law model and the effectiveness of mass concentration estimation from limited size ranges.

[Details of Research Results] Microplastics were classified into three types according to size: small (SMP: 1-200 μm), medium (MMP: 200-700 μm), and large (LMP: 700-5000 μm), and sampling methods suitable for each size were used. For LMP and MMP, nets with mesh sizes of 335 μm and 100 μm were immersed in the river surface layer for 5 and 2 minutes, respectively, for collection. For SMP, surface water of 10 L or more was collected using a stainless steel bucket and recovered from it. Calculation of size spectra: Based on data collected by sampling methods suitable for each size, size spectra for SMP, MMP, and LMP were calculated. By integrating these to calculate the overall size spectrum, it was confirmed that both number and mass concentration tend to increase as size decreases. Application of power-law: It was shown that the size spectra of number and mass concentration could be statistically significantly fitted by a power-law for all samples. The slopes were -3.27 ± 0.19 for number concentration and -1.05 ± 0.20 for mass concentration, close to the theoretical assumptions of -3 and -1, respectively. Estimation of MP mass concentration: Using the power-law model to estimate total mass concentration from measurements in limited size ranges, the estimation errors were 39-56% when extrapolating from SMP side and 13-61% from LMP side. Particularly for LMP side, the error of the top three samples was about 20%, indicating that practical accuracy can be achieved by selecting an appropriate size range. This study demonstrated that mass concentration of MP can be estimated accurately by extrapolation using power-law. This bridges the gap between studies that were difficult to compare due to differences in survey size ranges, contributing to unified organization of MP data and comprehensive understanding of MP pollution in rivers.