Microplastics journey in wetland ecosystems: From air to microlayer, to subsurface water and sediment

This study provides a short-term, dry-weather multi-compartment assessment of microplastic (MP) contamination in the Choghakhor Wetland, a vital freshwater ecosystem in western Iran. We quantified MPs in air, subsurface water, the surface water microlayer (SML), and sediments and developed a first-order mass-balance framework to clarify transport and fate. The SML showed much higher MP concentrations than the subsurface water when converted to volumetric units, while method-specific SML estimates varied among approaches (4.4–13.8 MP m⁻² using a glass tube; 196–982 MP m⁻² using a sieve; and 130–1754 MP m⁻² using filter paper). Subsurface water contained 0.083–1.5 MP L⁻¹, and the two sediment samples contained 60–400 MP kg⁻¹. Atmospheric deposition during the monitored intervals reached 2363 MP m⁻² h⁻¹. Flux analysis indicated that dry-weather influx exceeded observed outflux by more than three orders of magnitude. Using the conservative combined-outlet scenario, the wetland residence time was at least 168 days, whereas a water-only outlet scenario yielded ∼344 days. FLEXPART suggested that road dust dominated modeled source contributions, with smaller agricultural and soil-related contributions, although site-specific attribution remains model-based. These findings identify wetlands as important sinks and reservoirs of MPs, while emphasizing that the present results represent a dry-weather baseline rather than seasonal or annual conditions.