Sediment and soil source apportionment using geochemical fingerprinting techniques in the Winam Gulf, Lake Victoria

• Sediment source apportionment was evaluated in the Nyando and Sondu-Miriu basins. • Soil unmixing highlights the impact of land degradation on source contributions. • Conservativeness and consensus ranking methods identified the optimum tracers. • These results benefit policymakers developing integrated catchment management plans. Accelerated soil erosion is a major cause of land degradation in East Africa’s agricultural and pastoral landscapes with severe consequences for food, water and livelihood security. In this study, we aimed to provide a tool to support the sustainable management of land and water resources in a region significantly impacted by land degradation. We employed source apportionment methods to quantify the relative contribution of sediment sources within the Nyando and Sondu-Miriu River basins and their subcatchments in the Winam Gulf, Kenya. A total of 237 riverbed sediment samples and 76 composite surface soil samples were collected from the Nyando and Sondu-Miriu River basins. The total elemental concentrations of these samples, determined using ICP-MS/MS, were utilised as geochemical tracer properties. Conservativeness index, consensus ranking and consistent tracer selection methods were then used to identify the optimum unmixing tracers before applying the frequentist unmixing model FingerPro to determine sediment provenance. Sediment source analysis revealed that the Ainamutua and Nyando-Kipchorian subcatchments, areas predominantly affected by land degradation activities such as poor crop management practices and deforestation on steep slopes, contributed 39 ± 4 % and 44 ± 4 %, respectively. In contrast, the Awach Kano and Nyaidho subcatchment, with a higher proportion of tree-cover and lower soil erosion rates, only contributed 17 ± 7 %. In the Sondu-Miriu, the Yurith and Kipsonoi subcatchments contributed 68 ± 5 % and 20 ± 6 %, respectively, due to the predominance of forest encroachment and ridges in the Yurith subcatchment. Additional fingerprinting analysis within each of the Nyando and Sondu-Miriu basins reveals the significance of land use, landform and soil types on source contributions. Quantifying sediment source contributions within large river basins provides essential information for environmental managers and policymakers developing integrated catchment management plans. The results from this study can be used to implement sustainable land use policy focused on soil restoration in the Lake Victoria drainage basin.