Seasonal variations and spatial distribution of microplastics in tributary rivers and the Mekong mainstream in Ubon Ratchathani, Thailand

Microplastic (MP) pollution has emerged as a growing environmental concern in freshwater ecosystems, yet data from northeastern Thailand, a key hydrological region within the Mekong River Basin, remain limited. In this study, we investigated the abundance, characteristics, and seasonal variations of microplastics in six tributary rivers and four Mekong mainstream sites in Ubon Ratchathani Province. Surface-water samples were collected during the dry (April 2021) and wet (October 2021) seasons using a surface-trawl microplastic sampler, and particles were isolated through density separation and oxidative digestion before identification by FTIR spectroscopy. Nine polymer types were detected across all sites and seasons, with polypropylene (PP) and polyethylene (PE) as the predominant polymers. Seasonal differences were evident, with fragment-shaped MPs more abundant in the wet season, reflecting intensified hydrodynamic fragmentation, while fibers and sheets were more prevalent during the dry season. Tributary sites exhibited consistently higher MP abundances than Mekong mainstream sites, highlighting the importance of localized land-based inputs such as municipal wastewater, stormwater runoff, and urban activities. Overall, the findings demonstrate that hydrological conditions and land-use characteristics influence microplastic distribution within the region. This study provides baseline data for northeastern Thailand and contributes to understanding microplastic distribution in the Mekong River Basin.