Coastal Landfills as Emerging and Poorly Quantified Sources of Marine Microplastics: Evidence Gaps, Flux Scenarios and Research Priorities

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.