Occurrence of microplastics and ecological risk assessment during tidal changes in the Chao Phraya River estuary, Thailand.

River estuaries are specific transition zones that connect coastal and terrestrial environments and are recognized as primary conveyors for land-derived plastics to open oceans. The present study is the first to investigate tidal effects on microplastics (MPs) in the Chao Phraya River estuary. MPs (16-5000 μm) were collected from the water column during the changes in tidal current in order to analyze abundance, characteristics, and ecological risk. The abundance of MPs varied from 1.37 to 4.51 pieces/L and an average of 4.0 ± 3.8 pieces/L were found during the tidal cycle, which implied moderate to relatively high contamination when compared to other estuaries. Moreover, the average abundance of MPs during the low tide period was comparatively higher than that in other tidal phenomena. Morphological characteristics revealed that shape of fragments, shade of blue, size of 16-100 μm and PTFE is dominant in the MPs. The pollution load index (PLI) was 5.98, which denoted that the Chao Phraya River estuary is polluted with MPs at a low contamination level. In contrast, the risk index (RI) of MPs in the water column during the tidal cycle was 318.8, which indicated that the estuarine ecosystem of the Chao Phraya River is under considerable risk. In the present study, an ecological risk assessment was conducted for the Chao Phraya River estuary, which provides basic reference data for the management of pollution control related to MPs in the Chao Phraya River basin.