Experimental uncertainty assessment of meso- and microplastic concentrations in rivers based on net sampling

Meso- and microplastics have been collected via net sampling in marine and freshwater environments, but the effect of net clogging on evaluations of their concentrations (mPC) remains uncertain. We experimentally investigated the mPC uncertainties resulting from net clogging in the Ohori and Tone-unga Rivers, typical urban rivers in Japan, throughout 16 samplings with five filtration durations in one day. The weighted mean concentration in the Ohori River was significantly lower than that in the Tone-unga River, allowing us to examine the effect of clogging in rivers with different contamination levels. The variances in both rivers consistently tended to increase with increasing filtration duration, which can be expressed by applying the integral form of the Weibull reliability function (WRF). Furthermore, application of the WRF successfully revealed the optimal filtration durations in the Ohori and Tone-unga Rivers, which depended on the plastic abundance and sample volume. Since it could be difficult to obtain the plastic contamination level in advance, our suggestion is to predict the time sustained above 85 % filtration efficiency by applying a WRF-based model. In actuality, the sustained time in the Ohori (Tone-unga) River varied between 2.6 and 6.2 min (3.2 and 7.1 min) throughout the experiment, which permitted low mPC uncertainties of 12 % and 9.5 %, respectively. If notable uncertainty exists due to a low contamination level, a net with a high open area ratio should be used to increase the filtration duration. Hence, our results emphasize the importance of considering the open area ratio of nets used for sampling in studies. Our study provides insights into the occurrence of uncertainty due to net clogging to establish a standardized methodology for meso- and microplastic monitoring in aquatic environments via net sampling and consequently contributes to improving the sampling accuracy.