Coupled sediment and microplastic fluxes revealed by check-dam sediment records

Microplastics (MPs) are increasingly recognized as pervasive contaminants in terrestrial environments, yet their redistribution through erosion and accumulation within sediment-retention structures remains poorly quantified. This study quantified MPs in sediments retained behind check dams and examined their relationships with sediment storage and erosion-related sediment yield at the catchment scale. Twenty check dams in a semi-arid mountainous catchment in northeastern Iran were investigated. Sediment storage ranged from 531 to 9828 Mg (mean: 2683 Mg), while specific sediment yield ranged from 16.2 to 855.4 Mg km^-2 yr^-1 (mean: 247.1 Mg km^-2 yr^-1). MPs were detected in all sediment samples, with concentrations ranging from 800 to 5240 particles kg^-1 dry sediment (mean: 2702 particles kg^-1). Estimated MP inventories ranged from 9.0 × 10⁸ to 2.4 × 10¹⁰ particles per check dam. Strong positive relationships were observed between specific sediment yield and estimated MP yield (r = 0.99, p < 0.01) and between accumulated sediment mass and estimated MP inventory (r = 0.80, p < 0.01), indicating a close association between sediment dynamics and MP redistribution. Fibrous MPs dominated the assemblage (86.6%), and the 500-1000 μm size class accounted for the largest proportion of detected particles (44.7%). Raman spectroscopy confirmed several polymer types within the analyzed subset. Overall, the findings show substantial accumulation of sediment-associated MPs behind check dams and indicate that erosion-driven sediment mobilization may represent an important pathway for MP redistribution in erosion-prone catchments. However, the persistence of this retention may depend on the stability of accumulated sediments, which can potentially be remobilized during high-energy hydrological events.