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

The Mekong River delivers substantial plastic debris to the sea, and microplastics (MPs) are routinely detected in the surface water and sediment of its Vietnamese delta; their ecological implications remain unquantified. 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, 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. Contamination factors were referenced to an external low-impact value from the rural upper Mekong at Kampi, Cambodia (2 items m-3), the only mainstream point sampled with the same 300-um method. For the two method-comparable 300-um net surveys in the Delta the pooled PLI was about 29, well above the Tomlinson pollution threshold of 1; because it rests on a single reference point and the coarsest available mesh, we report it as an order-of-magnitude, lower-bound estimate rather than an exact class, and show that it ranges from roughly 15 to 59 as the reference is varied between 4 and 1 item m-3. Only one Delta survey reports polymer proportions (polyethylene 56%, polypropylene 41%, polystyrene about 3%); its PHI of 7.5 falls in the moderate band (level II), and a qualitative screen found no Lithner class IV-V polymer in any Delta assemblage. 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. A northern contrast, in which polyurethane is reported for the Red River estuary, rests on one qualitative sediment sample and is presented only as a hypothesis. The principal limitation is the small, method-heterogeneous evidence base, which we document study by study.