An integrated approach to environmental risk assessment of cumulatively impacted drainage basin from mining activities in southwestern Ghana

Acid rock/mine drainage and metal leaching constitute major environmental management risks in the mining industry. This paper assesses the environmental risks due to acid rock/mine drainage, and the metal leaching potential of multiple mines of gold and manganese on the Ankobra River Drainage Basin in Southwestern Ghana. The basin is a hub of mining activity in Ghana, hosting several mines. A combination of mineralogical, and static geochemical acid drainage predictive investigation of overburden of varied geological units, complimented with hydrochemical drainage quality analysis was used to assess potential environmental risks posed by acid-generating lithologies and mine spoil. Mineralogical investigations revealed sulphide-bearing lithological units with profound compositional variations due to the incorporation of potentially toxic heavy metals and metalloids, in association with carbonates and silicates. Accounting Base Accounting (ABA) and Net acid generation potential pH (NAGpH) tests delimited two tailing sites as potentially acid generating with NAGpH of 3.5 and 4.8, respectively. Five other samples, representing specific lithological units in the stratigraphic sequence, with net acid neutralization potential ratio (NNPR) less than 5.0, were classified as being potentially acid generating according to the categorization requirement of the US Forestry Service. The rest of the samples exhibited moderate to very strong buffering capabilities. The assessment also evaluated drainage quality of the network of streams and rivers constituting the basin and identified sources of drainage contaminants. Acidic waters emanate from identified acid generating sources, while high metal load regimes were identified with both low pH waters and high pH regimes, coincident with high sulphide and carbonate alteration sites, respectively. The study results show that Zn, Cu, Ni, As, Co, Sb, SO42-, pH, alkalinity and conductivity are essential and adequate parameters in routine environmental risk monitoring programmes of mines in the area. Sites characterized by low pH (<5.5) with high sulphate and metal ions are suggestive of acid mine drainage, while sites with high pH ([>7.5), metal ions and sulphate are suggestive of net acid neutralizing.