Assessing Erosion Risk and Heavy Metal Loading in Sediment from the Inner Murchison Bay Catchment

Abstract

Lake Victoria's Inner Murchison Bay (IMB) is suffering from deteriorating water quality, with pollutants including heavy metals posing ecological and human health concerns. The information gaps on sediment from the catchment and its heavy metal content has hindered the understanding of potential ecological threats and the tailoring of mitigation and conservation measures. The aim of this research was to model erosion and quantify heavy metals embedded in sediment from the IMB catchment. Revised Universal Soil Loss Equation (RUSLE) was set up in GIS environment to model and predict potential erosion prone areas. Field walks along randomly predetermined transects within the catchment were used to validate the predicted soil erosion prone areas and identify other hotspots. Sediment yields from Nakivubo and Ggaba sub-catchments for ten individual storm events between March and May 2022 were modelled using the Modified Universal Soil Loss Equation (MUSLE). Eighteen suspended sediment samples from Nakivubo and Ggaba channels were analysed for contamination by eight heavy metals (Lead, Zinc, Copper, Iron, Cadmium, Manganese, Chromium and Nickel) following standard methods. RUSLE revealed croplands and hilly zones (LS-factor >20) as major erosion hotspots (>5 t∙ha−1∙yr−1), while the field walks revealed bare surfaces, murram roads and unlined drainage channels as other major erosion sources. MUSLE model was calibrated in Nakivubo sub-catchment and independently validated in Ggaba with R2 of 0.9, Nash-Sutcliffe efficiency (NSE) of 0.57 and a Percent Bias (PBIAS) of -15.5. For the 10 study storm events, the mean discharge and Suspended Sediment Concentration (SSC) were: 3.08 m3/s and 1088.9 mg/L; 0.495 m3/s and 831.2 mg/L for Nakivubo and Ggaba, respectively. Cadmium, lead, and zinc were the most prevalent heavy metals and Pollution Load Indices (PLI) revealed Nakivubo (PLI = 1.16) was more polluted than Ggaba (PLI = 0.43). Physical, biological and agronomic soil and water conservation measures, improved municipal effluent management, wetland and buffer restoration and enforcement of environmental regulations are recommended to ameliorate sediment and heavy metal pollution into Lake Victoria.