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.