Drivers of environmental debris in metropolitan areas: A continental scale assessment

Plastic pollution is rapidly increasing, with land-based sources being the major contributors. Understanding the factors driving waste movement from land to sea is crucial for reducing leakage to the environment and its subsequent impact. In 2023 we conducted a stratified survey of mismanaged waste in the environment across six Australian metropolitan regions, covering inland, riverine, and coastal habitats, to determine a national baseline of debris in the environment. We completed 1907 transects, and found average debris density was 0.15 items m^-2. Debris quantity was patchy and typically higher in areas with intensive land use, such as urban and agricultural zones, and socio-economically disadvantaged regions. Polystyrene (24 % of fragments) and cigarette butts (20 % of whole items) were the most common debris types. Most items could be identified by material type but not by specific use (e.g. unknown hard plastic fragments were found in 28 % of all transects). Comparing our coastal results to a survey from 10 years prior, we found a significant 39 % decrease in the national mean coastal debris density, and a 16 % increase in transects where no debris was found. Our study finds evidence to support how historical policies, practices, outreach campaigns, clean-up efforts and local custodianship have contributed to reducing debris in metropolitan coastal habitats. This national baseline study offers a benchmark to evaluate the effectiveness of new policies, practices, and awareness campaigns.