Plastic as a Sediment – A universal and objective practical solution to growing ambiguity in plastic litter classification schemes

The universal and growing challenge of inconsistency and ambiguity in plastic classification schemes restricts our ability to predict plastic routing, degradation, and accumulation in all environments worldwide. Global plastic production has risen exponentially, reaching approximately 9200 million tons between 1950 and 2017. Of this, an estimated 5300 million tons have been discarded, with a significant fraction mismanaged and entering the natural environment. Plastics are pervasive, found in nearly every terrestrial and marine environment, and their durability ensures that they can persist in the environment for thousands of years, posing escalating ecotoxicological and environmental risks. To meaningfully address plastic distribution, pathways, and the impact it has, we need a clear, universally applicable classification scheme. Whilst there have been many calls to action from the community, we do not yet have a solution offered that facilitates universal understanding through its applicability. Here we propose treating plastic as sediment, such that we may employ the well-established principles and methodologies of sedimentology within its widely applicable framework for understanding and classifying particles. By applying sedimentological techniques to plastics, we developed a classification scheme to objectively describe plastic by its fundamental sedimentological characteristics that are known to correlate with particle behavior and distribution in the environment., i.e., size, shape, density, and material properties. It centers on objective observation before classification and interpretation, recognition of spatial and temporal changes, and an adaptable and flexible framework that can adapt to the complexities of plastic characteristics and research questions. As the classification scheme isolates each physical variable seen in plastic, through using it, we will be better able to understand how plastic characteristics influence their environmental behavior. Whilst the use of this scheme will be primarily beneficial in assessing source-to-sink routing, transport processes, and accumulation tendencies of plastic objects and particles, its potential impact extends beyond this. It has the capacity to enhance environmental monitoring and management strategies through cross-disciplinary and cross-regional data comparisons and exchanges, which will benefit a broad range of stakeholders interested in understanding and managing plastic pollution.