Sample size requirements for riverbank macrolitter characterization

Abstract

Anthropogenic litter is omnipresent in terrestrial and freshwater systems, and can have major economic and ecological impacts. Monitoring and modeling of anthropogenic litter comes with large uncertainties due to the wide variety of litter characteristics, including size, mass, and item type. It is unclear as to what the effect of sample set size is on the reliability and representativeness of litter item statistics. Reliable item statistics are needed to (1) improve monitoring strategies, (2) parameterize litter in transport models, and (3) convert litter counts to mass for stock and flux calculations. In this paper, we quantify sample set size requirement for riverbank litter characterization, using a database of more than 14,000 macrolitter items (>0.5 cm), sampled for 1 year at eight riverbank locations along the Dutch Rhine, IJssel, and Meuse rivers. We use this database to perform a Monte Carlo based bootstrap analysis on the item statistics, to determine the relation between sample size and variability in the mean and median values. Based on this, we present sample set size requirements, corresponding to selected uncertainty and confidence levels. Optima between sampling effort and information gain is suggested (depending on the acceptable uncertainty level), which is a function of litter type heterogeneity. We found that the heterogeneity of the characteristics of litter items varies between different litter categories, and demonstrate that the minimum required sample set size depends on the heterogeneity of the litter category. This implies that more items of heterogeneous litter categories need to be sampled than of heterogeneous item categories to reach the same uncertainty level in item statistics. For example, to describe the mean mass the heterogeneous category soft fragments (>2.5 cm) with 90% confidence, 990 items were needed, while only 39 items were needed for the uniform category metal bottle caps. Finally, we use the heterogeneity within litter categories to assess the sample size requirements for each river system. All data collected for this study are freely available, and may form the basis of an open access global database which can be used by scientists, practitioners, and policymakers to improve future monitoring strategies and modeling efforts.

River plastic transport and deposition amplified by extreme flood

“…Finally, we emphasize that plastic pollution is very heterogeneous, with strongly varying characteristics (for example, size, shape and effective buoyancy) 38 . As a consequence, any effort to estimate total plastic transport will come with uncertainty as long as its diversity is not accounted for.…”

Large variation in Mekong river plastic transport between wet and dry season

“…Net sampling was done in August and September 2019 during the wet season (Haberstroh et al, 2021a). Visual counting was done in February and March 2022, during the dry season van Emmerik et al (2023). Local discharge data was only available during the wet season.…”

“…We used the data provided by van Emmerik et al (2023). During the dry season floating plastic transport was measured using the visual counting method (González-Fernández and Hanke, 2017).…”

“…Note that none of these methods require flow velocity data, which is crucial for the net sampling-based estimates. As no local data were available, we used the item-mass statistics from de Lange et al (2023).…”

River plastic transport and storage budget

“…We converted plastic item count [#] to plastic mass [kg] using a separate dataset obtained from a one-year sampling campaign conducted over the floodplain in the Netherlands (de Lange et al, 2023). This dataset was collected at eight locations along the Dutch main waterways.…”