Polymer-resolved ecological risk of microplastics in the Vietnamese Mekong Delta: a secondary-data assessment using pollution load and polymer hazard indices

Microplastics (MPs; plastic particles smaller than 5 mm) are widely reported in the surface water and sediment of the Vietnamese Mekong Delta, but their ecological hazard has not been synthesised across the Delta. Because polymer types differ by orders of magnitude in the toxicity of their constituent monomers and additives, particle abundance alone is an incomplete measure of hazard. We compiled published MP records for the Delta and, for context, for other Vietnamese river and coastal systems (12 records from 11 studies), and evaluated them with two separate indicators: the pollution load index (PLI), which scales abundance against a reference, and the polymer hazard index (PHI), which weights the polymer assemblage by chemical hazard score. Records were analysed within sampling-method strata rather than pooled, and contamination factors were referenced to an external low-impact value from the rural upper Mekong at Kampi, Cambodia (2 items m⁻³). For the two comparable coarse-net Delta surveys, the geometric-mean PLI was about 29, well above the Tomlinson pollution threshold of 1; because it rests on a single reference point and coarse nets (300 and 250 µm), we report it as an order-of-magnitude, lower-bound estimate, ranging from about 15 to 59 as the reference is varied. The main new result concerns polymer hazard, which is strongly compartment- and land-use-dependent. River-channel assemblages (polyethylene, polypropylene, polystyrene) give a moderate PHI of 7.5 (level II), whereas Mekong Delta peatland-agricultural sediments, dominated by polyvinyl chloride (PVC, 46–61%), reach PHI in the thousands (level V); the latter is consistent with the high hazard and risk indices independently reported by Nguyen et al. (2023) for the same peatlands. Delta MP risk is therefore not uniformly low: it is load-driven and chemically mild in river channels but chemically severe where PVC accumulates. We do not combine the two indices into a single ecological-risk score, because a bulk contamination factor and an assemblage-level hazard index operate at different levels of aggregation. The principal limitation is the small, method-heterogeneous evidence base, which we document study by study.