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.

The fragmentation of plastics in the marine environment represents one of the most persistent and complex challenges in contemporary environmental science. Over the past decade, a limited number of laboratory experiments and numerical modeling have shown that plastics do not fragment into uniform debris but instead undergo a cascade of fragmentation processes that govern particle size distributions, transport behavior, and ecological impacts. However, real-world examples of plastic fragmentation remain rare, despite being essential for calibrating and validating generic numerical models. There is therefore a strong need for data derived from natural systems, ideally based on large and statistically representative datasets.In Normandy, France, the Dollemard coastal landfill provides a unique opportunity to address this issue through the study of a traceable anthropogenic marker: VALSTAR brand beer bottle caps. Manufactured from polyethylene, these caps were deposited in very large quantities from the 1960s to the 1990s. Due to ongoing coastal erosion, they continue to be released and are now found in significant numbers within the adjacent marine environment.The surfaces of these bottle caps exhibit advanced degradation, with numerous microplastics still attached to the remaining material. Systematic microscopic photographs of the degraded surfaces were acquired and automatically analyzed to quantify the number, size, and shape parameters of the attached microplastics. In parallel, a surface degradation index was established for the upper face of each cap, combined with a weighting method to estimate mass balance. From a total collection of 787 recovered caps, a representative subset of 107 was analyzed in detail. In addition, a single bottle cap was artificially aged in a UV chamber under controlled conditions to better isolate and characterize the role of UV radiation in the fragmentation process.The results demonstrate the coexistence of two fragmentation mechanisms: surface ablation and macro-fragmentation. Macro-fragmentation remains a secondary process, accounting for only 16% of the cases observed in the total dataset. UV-induced degradation does not appear to govern fragmentation at the scale of the smallest microplastics (µm range). Instead, it influences the particle size distribution of larger microplastics in the millimeter range. Thus, a clear bimodal size distribution is observed in the dataset, with a dominant population centered around an equivalent diameter of approximately 45 µm, and a secondary population of larger fragments, ranging from 0.8 to 0.9 mm. Comparing pristine caps, caps recovered in situ from landfills, and caps from marine settings allows a first-order estimation of fragmentation kinetics over the past 40 years. This comparison suggests an average mass loss of approximately 3.9% for the caps in the studied collection.

Coastal landfills are poorly documented reservoirs of plastic waste that may release microplastics (MPs) directly to marine environments through erosion, marine flooding, and sea-level rise. This systematic review, conducted following the PRISMA 2020 guidelines, synthesizes current knowledge on MPs associated with coastal landfills, analytical practices, and potential export pathways. Among 211 publications addressing plastics and MPs in landfills, only seven provided quantitative MP data from coastal landfill systems or associated receiving environments. Reported mean abundances were 0.0043–79 MPs L⁻¹ in water and 410–12,300 MPs kg⁻¹ d.w. in solid matrices. Comparisons among studies were strongly limited by differences in sampled matrices, pretreatment protocols, analytical techniques, particle-size thresholds, and reporting units. Using the abundance measured in marine sediment adjacent to the Dollemard coastal landfill as a local proxy, annual number-flux scenarios ranged from 3.08 × 10⁹ to 4.10 × 10¹⁰ MPs yr⁻¹. Hypothetical scaling of these scenarios to approximately 100,000 coastal landfill sites worldwide yielded 3.08 × 10¹⁴–4.10 × 10¹⁵ MPs yr⁻¹, or 1.45 × 10⁻²–7.33 × 10⁻¹ Tg yr⁻¹. These first-order scenarios are not estimates of current global emissions but highlight the need to better quantify this understudied pathway in marine MP budgets.