Quantification, characterization, and sources identification of macro- and mesoplastics in Rivers Sabaki and Tana in Kenya

The main objectives of this study were to characterize, quantify, identify sources, and determine the seasonal variation of plastic litter in the Rivers Sabaki and Tana in Kenya. Rivers significantly contribute land-based plastic litter to the ocean, underscoring the need to study these ecosystems. To achieve these objectives, the following aspects were investigated: i) quantification and characterization of plastic litter in the water columns of Rivers Sabaki and Tana, ii) quantification and characterization of plastic litter on the riverbanks of Rivers Sabaki and Tana, and iii) quantification and characterization of plastic litter at a potential litter accumulation site, influenced by tidal activity, the latter allowing for determining if it is a hotspot for plastic litter accumulation. In turn, this comprehensive monitoring allowed for the identification of plastic sources and seasonal variations. Cluster and factor analyses were used for source identification, and a total of five sampling campaigns were conducted over an annual cycle to capture seasonal variation. Most of the top ten litter types identified in all the study sites consisted of fragments and whole items. Plastic litter flux to the Indian Ocean through the River Sabaki was found to be 1.3 million plastic litter items per year, and the River Tana 208,000 items per year. On plastic sources, the River Sabaki water column sources are primarily driven by recreational and fishing activities, tourism, and local littering, whereas in the River Tana, illegal dumpsites, fishing, navigation, recreation, tourism, and local activities are the primary drivers. On the riverbank compartment, sources of plastic litter for the River Sabaki include recreational activities, direct littering by residents, tourism activities, agricultural activities, and upstream sources, while for the River Tana, they include illegal dumpsites, fishing activities, navigation, tourism, and local littering. Overall, this study provides valuable insights into plastic litter pollution of two major rivers in Kenya draining into the Indian Ocean, thus offering valuable insights for targeted plastic litter management and mitigation strategies.