ARSENIC SPECIATION IN SOIL POREWATERS FROM THE ASHANTI MINE, GHANA

At the Ashanti Au mine in southwest Ghana, arsenopyrite constitutes an important part of the hypogene ore assemblage, often hosting significant concentrations of Au. The oxidation of arsenopyrite either through lateritic weathering or during mineral processing releases As into the environment. The speciation of As in soil porewaters both from uncontaminated soils and those overlain by mine waste has been assessed. In aerobic soils overlying, weathered Au-As mineralization in the bedrock, As was present in concentrations of 189-1025 ppm in soil and porewater concentrations ranged from 86.2 to 557 mug l-1. Arsenic in these waters was found to be largely present as arsenate (approximately 78-95% of the total As) with arsenite, monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) also present. Where arsenite concentrations are greater than arsenate, no organoarsenical species were observed. In the uncontaminated soils overlying unmineralized bedrock, background As concentrations were in the range of 12.5-20.2 ppm and porewater concentrations ranged from 11.2 to 20 mug l-1. In these porewaters arsenate was the dominant species. No organoarsenical species were observed in these waters, suggesting that there is a possible threshold level of total As concentration required, above which methylation of As by soil microorganisms takes place. In the acidic soils contaminated by mine tailings, arsenite was the major species present (up to 45% of total As in the aerobic soils and 79% of total As in the anaerobic soils). Although the soils were underlain by the unmineralized bedrock, soil As concentration ranged from 40.5 to 1290 ppm and As concentration in extracted porewaters from anaerobic soils ranged from 70.2 to 110 mug l-1 and in aerobic soils from 70.8 to 599 mug l-1. In the aerobic soils MMAA, DMAA, and arsenate were also present but only arsenate was found, along with arsenite, in the anaerobic soil porewaters.