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

Abstract

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