Spatiotemporal distribution of microplastics and associations with river water physicochemical and hydrological parameters, including risk assessments in four tropical rivers in Sarawak, Malaysia

Abstract The microplastic pollution of freshwater systems has been a global environmental concern, yet there is a lack of knowledge on tropical rivers. This paper examined the relationships between spatiotemporal microplastic abundance and river water physicochemical and hydrological parameters in the surface water of four rivers of Sarawak in Malaysia (Baram, Miri, Sibuti, and Niah), followed by ecological risk assessment using PLI, PHI, and PERI indices. Surface waters were collected during dry and wet seasons and analyzed using stereomicroscopy and ATR-FTIR spectroscopy. The abundance of microplastics ranged from 6.33 ± 1.53 to 27.67 ± 2.31 items/L. Two-way ANOVA revealed a significant spatial difference ( p < 0.001), but no seasonal difference ( p > 0.05). The microplastics were predominantly fibers, in black, blue, and clear colors, and 100–1000 µm in size. Dominant microplastic polymers were polyethylene and polypropylene. During the dry season, microplastic abundance correlated positively with salinity ( ρ = 0.799) and temperature ( ρ = 0.708) but negatively correlated with pH ( ρ = −0.764). No significant correlations were found in the wet season. PLI values exceeded 1 in all rivers, while PHI values varied among rivers and seasons (7.16–536.46) (Category II: moderate–IV: danger). The presence of high-hazard polymers, such as rubber, elevated polymer-based risks and increased PERI values in some locations. This study provides baseline knowledge on the seasonal effects and anthropogenic pressures on microplastic distribution in tropical rivers in Sarawak, Malaysia.