Determining the settling and rising velocities of the top polluting macroplastics in rivers

The transport mechanisms of plastic pollution in rivers are currently poorly understood, hindering our ability to accurately monitor plastics, predict their fluxes, and ultimately intercept them. However, it has recently been shown that turbulent transport models designed for natural sediments, such as the Rouse model, can be used to quantify the vertical distribution of suspended plastic pollution in rivers with an uncertainty of within ±10%, despite differences in bed load or surfaced transport. These models are based on the ratio of the river’s shear velocity to the plastic’s settling (or rising) velocity, which has been shown to be represented by mono- or multi-model probability functions, due to variation in plastic shape, size, density and biofilm colonisation. In order for a Rouse-based model to be applicable as a method of monitoring plastics, and quantifying their concentration and transport in real rivers, a database of the settling velocities of the most commonly occurring macroplastic pollution in rivers, and their probability functions, is needed.

This study aims to explain the settling/rising velocities of the most commonly observed riverine plastics and to describe their full statistical functions. This includes calculating each plastic’s mono- or multi-model settling/rising velocity probability distribution. To achieve this, a state-of-the-art 2 × 2 × 2 m³ settling tank and a high-speed, synchronous multi-camera set up, with an automated plastic detection routine, will be used to describe the dynamics and calculate the settling/rising velocities and full probabilistic functions of the most prominent macroplastic items found in rivers. The plastic samples used in experiments will represent categories within the River-OSPAR litter index, which is an index used to classify plastics pollution by their type, size and material. This includes categories such as plastic cups, food wrappers, and cigarette filters. The data generated from these experiments can be used to shape Rouse-like transport models that can predict the vertical positioning and the concentration profiles of River-OPSAR plastics in the suspended layers of rivers, thus supporting the development of more accurate monitoring strategies for plastics in rivers and improving plastic quantification methods.