Spatial distribution and compartmental storage of microplastic particles in a tropical lowland river system (Perak river, Malaysia)

Rivers function as critical Earth surface systems that both transfer and temporarily retain anthropogenic particles, linking terrestrial landscapes with aquatic and marine environments. Microplastics represent a rapidly increasing class of emerging pollutants, yet their behaviour within water-sediment systems remains poorly understood particularly in tropical lowland rivers. This study investigated microplastic particles in river water and riverbank surface sediments along the downstream Perak river (Malaysia). This study also aimed to quantify and characterise microplastics in both compartments, assess their spatial variability along the river continuum and examine relationships between microplastic occurrence and physicochemical parameters. In addition, microplastic pollution levels in river water and surface sediments were assessed using polymer-based ecological risk indices. Microplastics were detected in both compartments, but river surface sediment concentrations (0.7–3.8 items kg−1) were consistently higher and more spatially uniform than those in river water (0.01–2.35 items m−3). Fragments dominated in both matrices, indicating the prevalence of secondary microplastics derived from weathered consumer plastics. Polymer composition differed systematically between compartments, with polyethylene and polypropylene dominating river water, whereas sediments showed increased contributions from polystyrene and polyethylene terephthalate, reflecting density-driven settling and sedimentary trapping following biofouling and weathering. Statistical analyses demonstrated significant and consistent differences between water and sediment compartments, as well as strong associations with turbidity, electrical conductivity and fine sediment properties. Although surface waters exhibited higher short-term variability in pollution intensity and potential ecological risk, sediments displayed buffered and uniformly low risk levels, consistent with their function as time-integrated particulate sinks.

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