Geometric and physical properties of the most persistent litter items in rivers

Plastic litter in aquatic systems is widely acknowledged as a major environmental concern, yet quantitative information on the physical characteristics of individual litter items - such as their mass, size, and shape - is still scarce. Riverine litter monitoring commonly relies on the River-OSPAR (Oslo–Paris Convention) classification scheme, which defines 109 standardized categories to support harmonized observations and policy development. While this framework offers clear advantages for categorization, it generally reports only item types (e.g. plastic bags) and lacks statistical descriptions of their physical properties. Such quantitative information is essential for multiple applications, including the design of laboratory experiments, the parameterization of numerical models describing litter transport and fate, the optimization of field sampling strategies, and the development of effective clean-up technologies such as racks or retention devices.

Here, we present a meta-analysis of 13 published studies covering 11 rivers on four continents, comprising a total of 240,571 litter items classified using the River-OSPAR index. We use detailed measurements of the longest and intermediate axes (L₁ and L₂) and mass (M) reported for 14,052 items by De Lange (2023), to derive joint probability distributions for these variables for each River-OSPAR category using copula-based statistical methods. These category-specific distributions are combined with observed category frequencies to construct a large synthetic dataset representing global riverine litter characteristics. By introducing assumptions on litter volume and density distributions, we further estimate the smallest axis (L₃), enabling a complete geometric description of individual litter items.

We identify the 25 most persistent litter categories (out of 109) in riverine environments, and we discuss full statistical distributions of L₁, L₂, and L₃, along with derived parameters commonly used in sediment and particle transport modelling, including volume, elongation (L₂/L₁), and flatness (L₃/L₂). All derived distributions are made openly available to support future experimental studies, numerical simulations, and improved monitoring and mitigation strategies for plastic pollution in rivers. This database is the basis for experimental investigation over these 25 most persistent litter categories which we developed at the moment and will be further presented and discussed.

We found that the marginal distribution of flatness peaks at 0.05, whereas the marginal distribution of elongation appears approximately uniform. When considering the joint probability distribution, nearly half of the macrolitter found in riverine environments has longest and intermediate dimensions between 1 and 10 cm and is very flat. Moreover, the ratio of the intermediate to the longest axis can take any value between 0 and 1.