Microplastic Pollution of Stream Water and Sediment in a Tributary of a Major Drinking Water Supplying River in Sri Lanka

Microplastic pollution in freshwater systems is a global concern. This study investigated microplastic contamination in the Rawan-Oya Tributary of the Mahaweli River, Sri Lanka’s largest river and a major drinking water source. Water and sediment samples from various areas (forested, rural, urban, and semi-urban) were collected monthly between October 2019 and December 2020. Surface water underwent sieving (5000 and 65 μm), while sediment samples were dissolved in NaCl solution, sieved, oxidized, and subjected to density separation. Microplastics were identified using microscopes and Fourier-transform infrared (FTIR) spectroscopy. A generalized linear model (GLM) analyzed microplastic density variations across sites. Microplastic levels in all tested samples (n = 64 each) ranged from 0.03–0.94 items L–1 in water and 3.1–246.9 items kg–1 in sediments. Concentrations increased along the forested-to-urban gradient (GLM, p < 0.0001). Water had higher microplastic levels in the dry season (GLM, p = 0.002), whereas sediment had more during the rainy season (GLM, p = 0.02). Fibers were the predominant microplastic type, with low-density polyethylene, polyester, and polystyrene being the most common polymers. These findings provide baseline data on microplastic contamination in a tributary of the Mahaweli River, vital for a large Sri Lankan community’s drinking water.