Heavy metals in surface sediments of Lake Naivasha, Kenya: spatial distribution, source identification and ecological risk assessment

摘要

Heavy metals retained in sediments pose a threat to aquatic ecosystem and society. Knowledge of the status and sustainabilityof lake sediment in areas undergoing rapid economic development is scarce, and yet it is required in order toachieve the United Nations Sustainable Development goals. We investigated heavy metal concentration in Lake Naivashasediments using enrichment factor, geoaccumulation index, pollution load index (PLI), risk index (RI) and multivariatestatistics to determine metal accumulation, distribution and the lake’s pollution status. Sediments sampling followed agrid approach, and seven heavy metals: Co, Cr, Cu, Mn, Pb, Zn and Fe, were analyzed in total; concentrations were determinedusing portable X-ray fluorescence. The distribution maps were interpolated in a GIS platform, and the toxicity riskof sediments was determined. The concentrations of elements in the sediments are in the following decreasing order:Fe > Mn > Zn > Cu > Cr > Co > Pb. High concentration of heavy metals is found in the southern shoreline and east of thelake due to shoreline discharge and agricultural effluents drained into the lake and shoreline transgression. Pb and Znare of anthropogenic origin, while Cu and Mn originate from both natural and anthropogenic sources and Fe and Co arefrom natural sources. In addition, the mean toxicity decreases in the order: Cu > Fe > Co > Pb > Zn > Mn > Cr. The PLI valueswere above one (> 1), indicating progressive deterioration of the sediment quality. Though the overall ecological risk ofthe lake is low to moderate (RI 36.73–60.51), hotspots of pollution should be marked for intervention. This assessmentalso helps to fill the knowledge gap required to formulate the right policies in implementing sustainable developmentgoals in similar lakes.