ARSENIC IN RURAL GROUNDWATER IN GHANA

Arsenic concentrations in groundwaters from two areas in Ghana, the Ob uasi area in the Ashanti region and the Bolgatanga area of the Upper E ast region vary from <1-64 mu g l(-1) and <1-141 mu g l(-1), respectiv ely. Sulphide minerals such as arsenopyrite and pyrite are present in the Birimian basement rocks of both areas and these form the dominant As sources. The basement aquifer is overlain by a variable thickness ( ranging from <10 m to >40 m) of weathered regolith and lateritic soil. Arsenic concentrations are low in the shallowest groundwaters, but in crease at greater depths (40-70 m below ground level in Obuasi and 20- 40 m in Bolgatanga). At depths greater than this, total As concentrati ons are relatively low. The lateral and vertical variations in dissolv ed As concentrations are controlled by ambient pH and redox conditions and by the relative influences of sulphide oxidation and sorption. In the weathered regolith and lateritic soils, oxidation has been extens ive. Shallow groundwaters are oxidizing and acidic. Under these condit ions, As is readily oxidized to As(V) and may precipitate as ferric ar senate or be sorbed onto ferric hydroxide surfaces. At greater depth, groundwaters have longer contact times with the aquifer minerals and p H values are therefore higher (>6.0 in Obuasi and >6.5 in Bolgatanga). The oxidation of sulphide minerals may proceed, but dissolved O is co nsumed during the process and mildly reducing conditions (Eh 200-300 m V) result. Sorption of As onto ferric hydroxide minerals is less favou red under such pH and redox conditions and the element is relatively m obile. At the greatest abstraction depths, groundwaters are more reduc ing (Eh less than or equal to 200 mV; dissolved O <0.1 mg l(-1)) and f ew electron accepters are available in the system to drive sulphide ox idation. Dissolved As concentrations therefore remain relatively low. The mildly reducing groundwaters in the sulphide-bearing basement aqui fer are therefore of the poorest potable quality with respect to disso lved As concentrations. Copyright (C) 1996 NERC.