| Abstract | Microplastic pollution in urban waterways and coastal mangrove ecosystems is a growing concern in rapidly urbanizing Southeast Asia. This study investigated microplastic transport from urban canals to coastal mangroves in Chonburi Province, Thailand, during the dry season. Water samples were collected in triplicate at eight sites along the Kapi Canal, Yi Rat Canal, and Mangrove Forest Conservation Center. Microplastics were extracted through wet sieving, hydrogen peroxide digestion, and density separation, with morphological characterization under a stereomicroscope and polymer identification by FTIR spectroscopy on approximately 4.6% of total particles.A total of 2,153 microplastic particles were identified. Site F (Bridge_2) recorded the highest mean concentration at 220.3 ± 191.3 items/L, representing 30.7% of all particles, while Site D (Bridge_1) had the lowest at 29.0 ± 8.7 items/L. Fibers were the dominant shape at 89.7%, transparent particles were the most frequent color at 62.4%, and PET was the most commonly identified polymer, consistent with textile and domestic wastewater sources. Water quality parameters showed a clear increasing gradient from the upstream canal to the mangrove, reflecting growing salinity and tidal influence. Kruskal-Wallis tests confirmed significant differences in microplastic gross mass and all water quality parameters across sites, while sequential Wilcoxon pairwise comparisons indicated a gradual rather than abrupt spatial distribution pattern along the gradient.The mangrove site (Site H) acted as a sink for fine PET fibers that were transported from upstream urban sources and deposited through hydrodynamic sorting and salinity driven particle settling. Site F (Bridge_2) was identified as the main pollution hotspot of the study, which has the highest diversity in color, shape, polymer type, and size simultaneously, reflects the mixed pollution inputs from road runoff, market waste, and direct discharge at this bridge location. These findings show that urban canals in Chonburi Province do not simply carry microplastics downstream, but also sort and filter particles before they reach the coastal mangrove ecosystem. Improved waste management and stormwater control at key urban locations such as Site F are recommended as priority interventions to reduce microplastic delivery to the mangrove. |
