Effects of land use patterns on microplastic concentration and distribution in river Rwizi, Mbarara district

Abstract

Microplastic pollution has become a pressing global environmental challenge, threatening aquatic ecosystems. In Mbarara District, growing industrial development, agricultural and urban activities, and poor waste management practices, contribute to the release of microplastics into River Rwizi. These plastic particles, typically less than 5 mm in size, largely originate from degradation of discarded plastics and can enter the food web through ingestion, biomagnification, and bioaccumulation. This research aimed to study the concentration, characteristics (colour, size, shape and polymer type), and distribution of microplastics in River Rwizi, with emphasis on the role of surrounding land use activities. Samples of water and sediments were obtained from nine purposively selected sites based on predominant land-use types, representing agricultural (upstream), urban/built up (midstream), and forested (downstream) land use types. Sample preparation involved sieving, drying, density separation, and filtration. Microplastics were then identified using stereomicroscopy and analyzed for polymer composition using FTIR spectroscopy. Land use mapping was conducted using Sentinel-2 satellite and supervised classification in ArcGIS, followed by ground truthing with a handheld GPS. Data analysis used IBM SPSS Version 29.0, while Pearson correlation and regression modeling were used to assess the influence of land use on microplastic concentrations. Results revealed that all samples collected contained microplastics. Concentrations in water samples ranged from 0.117 to 0.883 particles per liter, while sediments contained 0.012 to 0.132 particles/Kg. Built up and agricultural areas recorded higher concentrations of microplastics than forested areas. Fibers were the most common shape, most prevalent in built up land use, while blue particles were the most frequent colour. Most particles fell within 0.5–1.0 mm size range, suggesting they originated from larger plastic debris. Polyethylene and Polypropylene emerged as the dominant polymers identified across all land use categories. These findings demonstrate that land use patterns strongly influence microplastic pollution in River Rwizi. The study recommends improved waste management practices, promotion of alternatives to single-use plastic packaging materials, strengthening community awareness and policy interventions to reduce plastic pollution, and strengthening plastic recycling initiatives.