RESUMEN
BACKGROUND: Since the 1970s, microplastic contamination has been discovered in various environments. These emerging pollutants threaten water quality and freshwater ecosystems. We aimed to assess microplastic contamination in the Nan River by quantifying their presence, characterising their morphology, and identifying their polymer composition. METHODS: The survey was conducted along the Nan River in northern Thailand, which is divided into three distinct zones based on land use (ie, community areas, agricultural areas, and natural areas). A wide array of samples, including surface water, sediment, and specimens from five aquatic species-Barbonymus altus, Laides hexanema, Kryptopterus cryptopterus, Pomacea canaliculata, and Pseudodon cambodjensis cambodjensis-were collected. The presence of microplastics was analysed via the wet peroxide oxidation process for sample digestion, morphological characteristics were assessed through microscopic examination, and Fourier transform infrared spectrophotometry was used to identify the polymer composition of the microplastics. FINDINGS: Natural areas had the highest mean concentration of microplastics in surface water (23·67 pcs/L, SD 14·22), whereas community areas had the highest mean concentration in soil sediment (24·67 pcs/g, 15·04). No statistically significant differences in the amounts of microplastics were observed among the various land-use zones. Microplastics were predominantly found in the gastrointestinal tracts of fish specimens, with varying quantities observed among different aquatic species; B altus had the highest concentration (11·80 pcs/g, 8·98). The primary microplastic shapes identified were filaments (53·99%), fragments (35·58%), and cylindrical forms (10·43%). We identified ten different polymer types, with polypropylene, nylons, and polymethyl methacrylate being the most prevalent among them. INTERPRETATION: Analysing microplastic pollution in this area brings attention to the issues affecting the rivers of Thailand and emphasises ecological risks, particularly microplastic presence in aquatic species. Because of the Nan River's ecological importance, we call for increased research and collaboration to address the increasing threat of microplastic pollution. FUNDING: Naresuan University.