Coastal landfills as sources of plastic and microplastic pollution: a multi-scale monitoring approach

Plastic pollution monitoring remains challenging in complex and dynamic environments such as coastal landfills. These anthroposystems contain multiple plastic sources, transport pathways, and fragmentation processes that coexist and interact. Due to their proximity to the shoreline and their vulnerability to erosion, they represent a significant potential source of plastics and microplastics (MPs) into the environment. However, this environmental compartment is often overlooked and understudied in terms of its role in releasing plastics and MPs. While substantial research focuses on plastic transport and presence in marine environments, few studies consider nearshore landfills as a source of plastics and MPs. Moreover, the similarities between sediment and plastic transport processes have been little investigated. The same applies to the relationship between coastal cliff erosion and the fragmentation of plastics into MPs.This study proposes a multi-scale analytical approach combined with field-based observations. To this end, macroplastic exports were monitored using an adapted OSPAR protocol, which enabled the identification, quantification, and temporal tracking of plastic debris from coastal landfills and other sources. MP contamination in sediments was investigated using a combined approach of micro-Fourier Transform Infrared Spectroscopy (µ-FTIR) and the thermal Rock-Eval® method. The integration of these methods allows for precise polymer identification and abundance measurement via µ-FTIR, alongside mass-based quantification with the Rock-Eval® device. Those approaches were applied to two contrasting coastal landfill sites: Dollemard (Normandy, France) and Sant’Agata (Calabria, Italy).Results from macroplastic monitoring highlight spatial variations in the origins of macroplastics around both coastal landfill sites. Along transects located in the direct axis of the landfills, landfill discharge represents, on average, ~65% of the collected items, confirming these sites as active local sources of plastic pollution. In contrast, transects outside the landfill axis display highly variable compositions. At the Dollemard site, for transects downstream of the longshore drift, ~75% of plastics are attributed to beached marine litter. Additionally, across all transects, approximately 25% of the collected plastics consist of highly fragmented debris whose precise origin is difficult to determine. Furthermore, temporal variations in plastic abundance and origin were observed across all transects, reflecting the influence of storm events and short-term remobilization processes. Field observations also highlight the role of cliff erosion, gravity-driven processes, and sediment remobilization in controlling the release, transport, and fragmentation of plastics from macro- to MPs. Sediment analysis reveals high levels of plastic impregnation in both coastal landfill deposits, with MP abundances reaching up to 24,816 MPs/kg for Dollemard and 110,970 MPs/kg for Sant’Agatha. Estimations of mass-concentrations were also made using µ-FTIR and compared with Rock Eval® analysis results. These comparisons show a significant disparity in results depending on the MP abundance and nature of each sample.Consequently, this study tries to demonstrate that coastal landfills should be considered key monitoring targets for plastic pollution across the geosphere. With the multi-scale proposed approaches, long-term monitoring strategies could be implemented to better understand plastic fluxes from coastal landfills and from other sources.