Investigating the occurrence and sources of Microplastic contaminants discharged into Nakivubo Channel, Kampala City

Abstract

Microplastics (MPs) are microscopic and ubiquitous emerging organic contaminants with marked potential to bind, concentrate and act as carriers of other toxic compounds and pathogenic microorganisms. Whereas Microplastics (MPs) (i.e., plastic particles or fragments less than 5 mm in size) have been extensively studied in the marine environment, reliable data on their sources and pathways in freshwater ecosystems are limited. The aim of this study was to investigate the occurrence, characteristics and sources of MPs in Nakivubo channel that drains into L. Victoria, the largest freshwater lake in Africa. Surface water was sampled from thirteen (13) different points (S1 to S13; n = 117) along Nakivubo channel and its tributaries during the dry and wet seasons as well as on the onset of the rains. The samples were extracted and subjected to microscopic and Fourier-transform infrared spectroscopy analyses. The obtained results showed that all the samples had MPs ranging from 1568.6 ± 1473.8 particles/m3 during dry period to 2140.39 ± 3670.07 particles/m3 during the wet season. The concentration of MPs was higher in upstream reaches compared to downstream possibly due to flow dilution effects along the channel. The mean output of MPs (microplastic fluxes) from Nakivubo channel into Lake Victoria was approximately 293.96 million particles/day. The highest mean abundance of MPs (5,466.7 ± 6441.70 particles/m3) was for samples from site S3, which is characterized by poor waste management within the market area i.e., Owino market, Kisenyi bus park, car washing bay, direct discharge of wastewater and domestic water from hotels and presence of plastic collection/dumping sites. Filaments (79.7%) and fragments (17.9%) were the most common form of MPs and majorly originated from domestic effluents and surface runoff, while polyethylene (75.4%) and polyethylene-polypropylene (16.0%) were the major polymer types in the sample, explainable by its low density and wide usage in various consumer products. We found that spatial distribution of MPs was significantly different (P<0.05) for tributaries with poor waste management, within markets, bus park, car washing bay, informal settlements or waste collection and dumping sites. Our results further showed that microplastic abundances are highest during the wet season in Nakivubo channel, plausibly due to the remobilisation of plastic debris in the catchment during rain events. The results suggest, more efforts are required to improve on existing plastic waste management strategies within the hotspot areas such as markets, informal settlement areas and areas close to channels and tributaries. The waste management strategies should geared towards mitigation of MPs bioaccumulation, possible trophic transfer and biomagnification in biota and sedimentary phase of the receiving L. Victoria.