Rising and settling velocities of macroplastic foils

Plastics accumulate in the environment due to inadequate waste management, and the durability of the material. A better understanding of fundamental plastic behaviour in the aquatic environment is essential to estimate transport and accumulation, which can be used for monitoring, prevention and reduction strategies. An important process for fate models is the vertical transport of particles, for which the rising and settling velocity are crucial variables. Several studies have described these for microplastics (<0.5 cm) using observations and models. For macroplastics (>0.5 cm) however, such data are scarce. In this study, the rising and settling behaviour of three polymer types (PET, PP, and PE) commonly found in the environment was investigated. The plastic particles were foils of different sizes and shapes. A new method for releasing rising plastics without interfering the flow and disturbing the column was used. Observational data were used to test the performance of four models, including one developed in this study, for estimating the settling and rising velocity based on the properties of the plastic particles. These models were validated using the data obtained in this study, as well as data from another study on plastic rising and settling rates. The newly introduced foil velocity model gave the best results (R² = 0.96 and 0.29 for both data sets, respectively). This model has the potential to estimate the rising and settling velocity of plastic foils, and should be further investigated using additional observational data. The results of our work can be used to further explore the vertical distribution of plastics in rivers, lakes and oceans, which is crucial for improving future efforts to monitor and reduce plastics.