Occurrence and polymer characterization of microplastics in surface waters of tributaries draining into Winam Gulf, Lake Victoria, Kenya

Microplastics (MPs) are increasingly recognized as persistent contaminants in freshwater systems, yet data on tributary inputs to Lake Victoria, Africa's largest freshwater lake, remain scarce. This study assessed the occurrence and polymer composition of MPs in surface waters and sediments of three tributaries—Kisat, Nyando, and Sondu—draining into Winam Gulf, western Kenya. Water (5 L per station) and sediment samples were collected from upper and lower reaches of each river (n=1 per station), processed by ZnCl₂ density separation (1.6 g/cm³), and examined under a stereo microscope before polymer identification by attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy. MPs were detected in surface water at three of six stations, with concentrations ranging from 0 to 4 particles per sample (0–0.8 particles/L). Lower Kisat recorded the highest abundance (4 particles), followed by upper Kisat (3) and lower Nyando (2); no MPs were detected in river Sondu or in any sediment sample. Three polymer types were identified—polypropylene (PP, 44.4%), ethylene-propylene-diene terpolymer (EPDM, 33.3%), and low-density polyethylene (LDPE, 22.2%)—all as irregular fragments. A one-way ANOVA (F = 5.21, p = 0.041) suggested significant inter-river variation; however, this result is exploratory given the limited sample size (n=6 stations). The absence of MPs in sediment is attributed to the low density of recovered polymers relative to water, favouring surface transport. These findings provide the first polymer-resolved baseline for these tributaries and identify the urbanized Kisat catchment as a priority for targeted pollution mitigation in the Lake Victoria basin.