Assessment of Total Mercury Concentrations in Fish and Sediments from River Ankobra and its Major Tributaries

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A total of thirty six (36) river sediment samples and one hundred and sixty-nine (169) fish samples covering eight species were collected from River Ankobra at Heman, Prestea, and Mehame, from River Mansi at Beposo, River Bonsa at Bonsaso and River Fure at Kutukrom all of which are impacted by small-scale gold mining activities. All the samples were analysed for total mercury concentration by Cold Vapour Atomc Absorption Spectrometry (CVAAS) technique using an automatic mercury analyzer Model Hg 5000. Total mercury concentrations ranged from 36.96 to 123.91 ng/g wet weights (average 64.56 ng/g) for Alestes imberi. For Labeo coubie, Labeo chariensis and Chrysichthys nigrodigitatus the ranges were from 62.87 to 204.97 ng/g (average 122.32 ng/g), 8.20 to 409.73 ng/g (average 144.32 ng/g) and 172.00 to 1152.00 ng/g (average 457.00 ng/g wet weight) respectively. The range for Tilapia zilli and Eutropius mentalis were 46.34 — 335.09 ng/g (average 126.96 ng/g) and 132.61 — 911.46 ng/g (average 296.27 ng/g) respectively whereas Gnat honemus cyprino ides and Hydrocynus lineatus had concentrations ranging from 501.62 to 1073.00 ng/g (average 728.21 ng/g) and 865.27 to 1517.39 ng/g (average 1191.30 ng/g) respectively. Eighteen (18) fish samples comprising Chrysichthys nigrodigitatus (n = 11), Gnathonemus cyprinoids (n = 3) and Hydrocynus lineatus (n = 4) had mercury concentrations that were higher than 500 ng/g, which is the maximum acceptable limit in fish as set by the World Health Organization. The highest total Hg concentration in river sediment and fish were observed at Mehame. The mean total mercury concentration in sediment ranged from 278.20 ng/g to 682.90 ng/g. Mercury concentrations in fish showed poor correlation with fish weight and length. Only 3% and 1.2% of fish species showed positive correlation between Hg concentration and fish weight and length respectively. Positive correlation between Hg concentration in fish and sediment was observed only for Alestes imberi at Beposo and Labeo coubie at Prestea. The results in this study suggest that gold mining activities could contribute mercury to the aquatic environment as was reflected in the high fish mercury content. The poltion of the population at Mehame that eat fish frequently from river Ankobras are at risk of Hg contamination through fish consumption, and should avoid consuming large amounts of species with Hg concentration above the WHO recommended limit.
A thesis submitted to the Department of Theoretical and Applied Biology, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of Master of Science (Environmental Science, 2007.