Levels of Selected Water Quality Parameters, Heavy Metals and Pesticide Residues in Water, Sediments and Nile Tilapia of Lake Nakuru, Kenya

Abstract

The discharge of industrial effluents, raw sewage wastes, increased use of fertilizers and pesticides in agriculture have resulted in increased release of pollutants into the natural environment. These pollutants tend to accumulate and undergo food chain magnification thus ending up in aquatic fauna at the top of the food chain which are consumed by humans. This study was conducted to assess the levels of selected water quality parameters, heavy metals and pesticide residues in water, sediments and Nile tilapia and the potential health risk to humans who consume the fish of Lake Nakuru. Nine water, nine sediment and thirty fish samples were collected from five representative sites within Lake Nakuru between December 2020 and January 2021 and analysed. Heavy metal analysis was conducted using an Atomic Absorption Spectrophotometer (AAS) at Lake Nakuru Water Quality and Testing Laboratory whereas Organochlorine Pesticide (OCP) residue analysis was done using Gas chromatography - Mass Spectrophotometer (GC-MS) at the Directorate of Veterinary Services (DVS) laboratory, Kabete. The pH ranged from 9.52 at Mid lake point to 9.72 at Sewage discharge point. The dissolved oxygen concentrations ranged from 4.58 mg/L at Sewage discharge point to 8.62 mg/L at River Njoro mouth. Electrical conductivity was lower for such a saline system with values of 4470 - 5226 μs/cm measured during the study period. The salinity range of 2.39 - 2.81 ‰ at Sewage discharge and Mid lake point respectively was comparable to that of a freshwater system. There were no significant differences in mercury concentrations across sites whereas chromium and lead showed significant differences across sites p < 005 in water samples (One - Way ANOVA). No significant differences in mean chromium and mercury levels was recorded whereas arsenic and lead showed significant differences across sites p < 0.05 in sediment samples. For heavy metals in fish samples, there were no significant differences in mean concentrations of mercury and lead whereas arsenic and chromium showed significant differences across sites p < 0.05 (One - Way ANOVA). All the 16 OCPs targeted in this study were detected except for aldrin. The OCP residue levels ranged from BDL to 7.44 ± 0.66 μg/L, BDL to 6.39 ± 1.10 μg/kg ww and BDL to 319.74 ± 66.94 μg/kg ww in water, sediment and fish samples, respectively. The levels of lead and chromium in fish were above the WHO and EU limits while the residue level of endosulfan was above the EU standard hence not safe for human consumption. The present results show that most of organochlorine pesticides though banned in Kenya are still detected in the environment and this may have longterm hazards to humans.