Spatial profiles and sources of selected persistent organic pollutants in surface sediments of Lake Victoria, East Africa

Abstract

Lake Victoria is the second largest lake in the world and has a catchment area of 193,000km2 with only one outlet (River Nile) and numerous inlets. Several anthropogenic activities take place in the lake’s catchment area which may emit pollutants yet lake sediments act as their sink. This study determined the levels, eco-toxicological effects and sources of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) in 41 sediments from the study area. Extraction and clean-up was done using soxhlet method and fractionation column, respectively while analysis of extracts was performed by the GC-MS/MS. The mean recoveries of PCBs ranged from 65±2.83% to 112±5.66% while those of OCPs varied from 60.6±3.76% to 130±4.02%. Concentrations of 13 OCPs ranged from n.d to 376 μg kg-1, n.d to 131 μg kg-1 and n.d to 228 μg kg-1 dry weight (d.w) with p,p′-DDD as the most dominant OCP residue contributing 88.2%, 82.9% and 72.9% to the ΣDDTs for Uganda, Kenya and Tanzania respectively. The ratio of p,p′-DDE to p,p′-DDT was >1 in all sediments suggesting past DDT usage in the lake basin. α-endosulfan was the most dominant endosulfan isomer followed by β-endosulfan in Uganda, Kenya and Tanzania respectively. The predominance of α-endosulfan was attributed to its high percentage (70%) in the technical endosulfan formulation and its ease of transport in the atmosphere. The concentrations of 10 PCB congeners ranged from non-detectable (n.d) to 3.30 μg kg-1 d.w in the study sites. The most dominant PCB congener was CB 198 followed by CB 184. This was attributed to the hydrophobicity of highly chlorinated PCBs in the aquatic environment, their water partition coefficient (Kow) value and sorption to the organic matter in sediment. Principal component analysis (PCA) extracted four components which showed that OCPs were majorly due to their use in controlling pests in agriculture, eradicating tsetse flies, malaria mosquitos and black flies in public health. PCB congeners were a result of volatilization, degradation of higher CBs into lighter CBs, improper waste disposal of old transformers, hydraulic fluids, plasticizers and capacitors. Lindane, p,p′-DDD and p,p′-DDT were above the probable effect levels (PEL) and therefore, they were likely to cause an adverse effect on aquatic organisms in the lake. All PCBs were below the TEL and PEL levels in the sediments.