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 signific ant concentrations of Au. The oxidation of arsenopyrite either through lateritic weathering or during mineral processing releases As into th e environment. The speciation of As in soil porewaters both from uncon taminated soils and those overlain by mine waste has been assessed. In aerobic soils overlying, weathered Au-As mineralization in the bedroc k, As was present in concentrations of 189-1025 ppm in soil and porewa ter concentrations ranged from 86.2 to 557 mug l-1. Arsenic in these w aters was found to be largely present as arsenate (approximately 78-95 % of the total As) with arsenite, monomethylarsonic acid (MMAA) and di methylarsinic acid (DMAA) also present. Where arsenite concentrations are greater than arsenate, no organoarsenical species were observed. I n the uncontaminated soils overlying unmineralized bedrock, background As concentrations were in the range of 12.5-20.2 ppm and porewater co ncentrations ranged from 11.2 to 20 mug l-1. In these porewaters arsen ate 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 s oil microorganisms takes place. In the acidic soils contaminated by mi ne tailings, arsenite was the major species present (up to 45% of tota l 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 soil s MMAA, DMAA, and arsenate were also present but only arsenate was fou nd, along with arsenite, in the anaerobic soil porewaters.