Distribution of mercury in water, sediment and fish from the Volta lake and its major tributaries

Abstract

Concentrations of total mercury and methyl mercury were determined in fish and sediment from the waters of the Volta Lake and its main tributaries to understand their distribution in the ecosystem. Total mercury concentrations in fish ranged from 2.11to 355.16 (mean: 75.64) ng/g wet wt. Methyl mercury concentration ranged from 1.77 to 319.48 (mean: 68.44) ng/g wet wt and accounted for, on the average 90% of the total mercury in the muscles of the fish. Methyl mercury concentrations in fish were directly proportional to total mercury concentrations with average correlation coefficient of r = 0.98. The relationship of total mercury and methyl mercury concentrations in fish to those of sediments from corresponding locations was fish-species dependent. Concentrations of total mercury in sediment ranged from 0.96 to 700.25 ng/g dry wt. which is lower than the IAEA threshold of 810ng/g. Water samples from the main tributaries of the Volta and the Volta Lake showed total mercury concentrations of 0.0027 to 0.0862 ng/L and methylmercury concentrations of 0.0004 to 0.0259 ng/L. The methylmercury concentrations accounted for 13.2 to 35.0% of total mercury in the water samples. The results of this study indicate that mean total mercury and methylmercury concentrations in fish increase with increase in trophic level of fish. All the fish samples obtained from the Volta and its major tributaries had mercury concentrations below the WHO/FAO recommended limit of 500 ng/g wet weight. The low levels of mercury in the fish analyzed in this study suggest a comparatively clean aquatic environment which has not yet been impacted by mercury contamination.