Applied Geochemistry, Vol. 9, pp. 15-22, 1994 0883-2927/94 $6.00 + .00 Printed in Great Britain. Pergamon Press lad Arsenic spedation in soil porewaters from the Ashanti Mine, Ghana R. J. BOWELL Department of Geology, The University, Southampton SO9 5NH, U.K.; Department of Mineralogy, Natural History Museum, London SW7 5BD, U.K. N. H. MORLEY Department of Oceanography, The University, Southampton SO9 5NH, U.K. and V. K. DIN Department of Mineralogy, Natural History Museum, London SW7 5BD, U.K. (Received 10 September 1992; accepted in revisedform 23 December 1992) Abstract--At the Ashanti Au mine in southwest Ghana, arsenopyfite 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 #g I-1. Arsenic in these waters was found to be largely present as arsenate (approximately78-95% 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 20pg l -l . 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 1 in extracted porewaters from anaerobic soils ranged from 70.2 to 110pg l- and in aerobic soils from 70.8 1 to 599#g l- . In the aerobic soils MMAA, DMAA, and arsenate were also present but only arsenate was found, along with arsenite, in the anaerobic soil porewaters. INTRODUCTION THE CHEMICALspeciation of As in soil porewaters depends on the physicochemical characteristics of the soils (JAcoBs etal., 1970; BOYLEand JONASSON, 1973; O'NEILL, 1990). Arsenic in soils is generally present as an oxysalt or in anaerobic soils is found combined with S. Thus, in uncontaminated aerobic soils, arse- nate is the predominant species while in anaerobic soils arsenate and arsenite are the donnnant As species (JACOSS et al., 1970; THOMSON and THOR- ESSY, 1977; PORTm~ and PETERSON, 1977). Over the natural range of Eh and pH in soft, both As(III) and As(V) can be stabilized (Fig. 1). The equilibria for As in aqueous solution are given in Table 1. The activity of microbial methylation reactions is poorly under- stood, particularly in contaminated soils, partly through the inability to precisely analyse the different AS species in solution, and this has greatly restricted a comprehensive study of As speciation (THOMPSON and THO~SBY, 1977; PORTEI~ and I~TERSON, 1977; ANDRe,E, 1978; PACEYand FOIVD,1981; Ac-~Err and KADWANI, 1983). In the presence of microorganisms, methylation of the As oxyanion may occur to form monomethylarsonic acid (MMAA), dimethylarsinic acid (DMAA), trimethylarsinic acid (TMAA), and dimethylarsine (DMA) (CHALLENGER, 1945; JOHN- SON, 1972; SANDERS, 1979; A N D R E , 1978, 1985; REIMER and THOMPSON, 1988). The actual reactions which take place depend on the microorganisms and the As compounds involved. Essentially the most thermodynamically stable species over the natural range of soil pH of 4-7 (BAAs-BEcKING et al., 1960) is H3AsO ~ and in the acid mine drainage waters (pH range 2-5) 1-12AsO;. If Eh drops below 0.3V (pH 4) or -0.1V (pH 8) then H3AsO3 will be the most stable As species in the absence of complexing ions and methylating organ- isms. However, the rate of change in the oxidation state of As is not rapid, so the predicted proportions of As species (based on thermodynamic calculations) do not always correspond to actual analytical results (O'NEILL, 1990). While some organisms can fully methylate As over AS 9 : 1 4 15