Evaluating the Acid Mine Drainage Potential at Abosso Goldfields Limited (AGL), Ghana* J. S. Kuma and D. N. Asamoah Abstract Occurrence of Acid Rock Drainage (ARD) affects both plants and animals that are present in a drainage system. In some base metal and coal mining environments, the presence of sulphide minerals especially pyrite and their exposure to oxygen and water may trigger Acid Mine Drainage (AMD). Evaluation of the potential for acid generation is therefore very impor- tant in any mine setting where sulphide minerals exist. Results of two Acid-Base Accounting (ABA) studies conducted at Abosso Goldfields Limited (AGL) on 120 rock and tailings samples reveal that on average, total sulphide values in rocks are low, with 83 % less than 0.20 %. More than 95 % show Neutralisation Potential Ratio (NPR) of 2 and above, indicating the presence of excess neutralising capacity in the rocks and tailings to prevent acid generation. Also, more than 93 % of the samples possess long-term acid neutralisation because their Neutralisation Potentials (NP) are more than 10 and their total sulphide are less than 0.1 %. Average up- and down- stream measurements of pH were respectively 6.55 and 6.74 while for TSS (Total Suspended Solids), the values were respectively 14.9 mg/l and 7.2 mg/l, indicating improved water quality after passing through the mine. Although these figures show very high NP’s, the presence of less than 7% samples with the potential to generate acidity necessitates that AMD monitoring should continue, especially in meta-sedimentary and siltstones to ensure acid generation does not arise during mining. * Manuscript received January 24, 2006 Revised version accepted May 20, 2007 1 Introduction ARD is an environmentally damaging phenome- non and occurs when sulphidic rocks, especially pyrite and arsenopyrite are exposed to atmospheric oxygen and water so that weathering becomes op- erational. The occurrence of ARD leads in general, to low pH, high sulphate and the mobilisation of elevated concentrations of ecotoxic metals such as cadmium, zinc, copper, lead etc, if they are present in the rocks, into the geo-environment. Pyrite oxi- dation is a multi-step process involving an oxygen- independent reaction whereby ferric iron attacks the mineral and oxygen-dependent reactions in- volving re-oxidation of ferrous iron to ferric and oxidation of reduced sulphur compounds that are produced as intermediate processes finally to sul- phate (Johnson and Hallberg, 2005). The final re- action step is written as: FeS 2(s) + 14Fe 3+ + 8H 2 O → 15Fe 2+ + 2SO 4 2+ + 16H + (1) Reaction one shows that one mole of pyrite pro- duces 16 moles of proton acidity (H + ) and is one of the most prolific reactions in nature (Younger, 1995). Additionally, the reaction rate of ARD generation is greatly accelerated by activities of iron oxidising bacteria, principally Thiobacillus ferro-oxidans. Once ARD begins, both flora and fauna in streams are negatively affected and their population may be reduced and in severe cases, die out from the point of discharge to some distance away in the stream (Jarvis and Younger, 1997). Where ARD occurs as a result of mining, the phe- nomenon is called AMD. AMD may persist sev- eral decades after a mine is decommissioned. In Ghana, many gold deposits occur with sulphides as gangue or the sulphides are present in dykes which intrude the country rocks. The presence of such sulphide ores could result in AMD when the sulphide is exposed to air and water. A require- ment of the Environmental Protection Agency (EPA) of Ghana is that prospective mines test for the possibility of their operations generating AMD (Anon, 1994). Thus, in any mining operation where sulphides occur, the possibility of the gen- eration of AMD is assessed. Since environmental issues pertaining to mining are contentious, it is necessary that transparency becomes the norm in such operations. Abosso Goldfields Limited (AGL) is a mining company in southwest Ghana with very open environmental policies. This paper presents results of investiga- tions conducted at AGL in 1994 and 2001, where 8 Kuma J. S. and Asamoah D. N. (2007), “Evaluating the Acid Mine Drainage Potential at Abosso Gold Fields Limited (AGL), Ghana”, Ghana Mining Journal, pp. 8-16 GMJ GMJ GMJ Vol. 9, December 2007