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Abstract

Gold miners (Artisanal and Small-Scale [ASGM]) operating in southwestern Ghana use exclusive mercury-gold amalgamation, cyanidation of gold-rich mercury-contaminated tailings, or a combination of both techniques during gold extraction; exposing water bodies within the catchment to mercury (Hg) and cyanide (CN) from mining discharged effluents. Additionally, sporadic CN spillage/leakage from Large and Small-scale Gold mining which employs chiefly cyanidation also impacts water bodies. Formation of Hg(CN)2 during cyanidation of Au-rich Hg-contaminated tailings, and interaction between Hg and CN in the aquatic environment increases the solubility of Hg, making it highly bioavailable for methylation. Studies on the quantitative influence of CN on Hg solubility and subsequent impact on methylation rates in the aquatic environment are rare. In Ghana, mercury contamination in the environment is mainly assessed in terms of total Hg concentrations, which lack explanation on its toxicity, mobility and bioavailability. This therefore makes an in-depth study on mercury speciation vital. Hg content in fish species depends on the age, feeding habit and location of the fish. The study investigated Hg (total) and MeHg levels in fish, water and sediment from two river systems within the Prestea-Huni Valley district, southwestern Ghana: a Hg-contaminated non-CN loaded (Ankobra River) and a Hg-contaminated CN-loaded (Aprepre River). The study also assessed Hg water-soluble fraction in sediments from both rivers, as well as identification of Hg compounds by thermal fractionation. Additionally, the depth distribution of total Hg (THg) and MeHg in soils from ASGM communities were also assessed. THg in sediments and fish was determined by Atomic Absorption Spectrophotometry (Cold Vapour technique [CVAAS]); in water by Atomic Fluorescence Spectrophotometry (Cold Vapour technique [CVAFS) after acid digestion. MeHg in sediment, fish and water was measured using aqueous-phase propylation/ethylation, preconcentration on Tenax, and gas chromatography coupled by CV-AFS. THg and MeHg levels in soils from the study sites decreased with depth, which is an indication of anthropogenic release and deposition of Hg. MeHg in sediments from the Aprepre River showed values in the range of 4.58-14.8 ng/g expressed as Hg on the dry weight (dw) basis, which represents 1.4-3.7% THg as MeHg; THg was found in the range between 241-415 ng/g, dw and 0.05–1.21 mgCN/kg, dw. For the Ankobra River, MeHg ranged 0.24-1.21 ng/g, dw (0.08-0.35% THg as MeHg) with THg in the range of 162-490 ng/g dw and CN <0.001 mg/kg. There was positive correlation (r2=0.5974; p<0.01) between MeHg and CN in the Aprepre River. The water-soluble fraction of Hg in sediment from both rivers was <1% of THg. Hg in sediments from the Aprepre River was about four times more soluble than that from the Ankobra River; indicating that Hg in sediments from the Aprepre River was more bioavailable for methylation. Accordingly, the presence of CN in Hg-dominated river sediments quantitatively influences and enhances the solubility and mobility of Hg; resulting in high rates of Hg methylation. MeHg levels in fish species exceeded the USEPA MeHg guideline of 300 ng/g by 48% and 8% from the Aprepre and Ankobra River respectively. However, species from the Ankobra River were of larger sizes having an average weight of about four times bigger than that from the Aprepre River. This is a result of the higher MeHg levels found in sediment and water samples from the cyanide loaded Aprepre River.