Distribution of Total Mercury in Fish, Water and Sediments from the Densu Reservoir at Weija, Accra, Ghana

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Mercury is toxic and its ubiquitous nature makes it a global pollutant. Total mercury (THg) concentrations were determined in fish, sediments and water from the Densu Basin at Weija. Cold Vapour Atomic Absorption Spectrophotometry (CVAAS) technique using an automatic mercury analyser was used to determine total mercury after digestion of the samples. One hundred and sixty-five (165) fish samples comprising six (6) species; eighty-four (84) sediment samples and thirty (30) water samples were collected and analysed for total mercury. Mercury concentration in fish muscles ranged from 0.001 to 0.420 μg/g. Hemichromis fasciatus recorded the highest level of 0.420 μg/g whilst the lowest Hg concentration of 0.001μg/g was recorded in Chrysicthys nigrodigitatus. Mercury concentration (μg/g wet weight) in the muscle tissue of fish ranged from 0.014 to 0.420 (mean = 0.125±0.111) for Hemichromis fasciatus, from 0.022 to 0.385 (mean = 0.155±0.098) for Tilapia zilli, from 0.001 to 0.342 (mean = 0.096±0.094) for Chrysicthys nigrodigitatus, from 0.021 to 0.378 (mean = 0.181±0.115) for Tilapia mariae, from 0.010 to 0.367 (mean = 0.114±0.109) for Clarias batrachus, from 0.056 to 0.330 (mean = 0.114±0.076) for Clarias gariepinus. Mean mercury levels in sediment and water from Weija are 0.055±0.023μg/g and 0.0169±0.0077ng/L respectively. There was a significant correlation between Hg concentration in fish muscle and fresh weight of fish for Hemichromis fasciatus (r2 = 0.5739). A good correlation between Hg concentration in fish muscle and total length of fish was also observed for Hemichromis fasciatus (r2 = 0.6301). All the rest of the fish species showed poor correlation between Hg concentration in muscle and total length and fresh weight. Correlation between Hg concentration in fish and in sediment as well as fish and water was not significant. No correlation was observed between the total Hg concentration in the sediment and water. All the fish samples studied showed mercury concentrations below the World Health 6 organization (WHO) limit of 0.5μg/g wet weight. The results obtained from this study therefore showed that fish from the Densu River are unlikely to constitute a significant mercury exposure to the public through consumption. Levels of Hg in sediment and water suggest a relatively clean environment with regards to Hg.
A Thesis Submitted to the Department of Environmental Science, Kwame Nkrumah University of Science and Technology in partial fulfillment of the Requirement for the Award of the Master Of Science Degree in Environmental Science, March-2012