HYDROCHEMISTRY OF GROUNDWATER FROM THE SOUTHERN VOLTAIAN SYSTEM

The hydrochemistry of groundwater from aquifers of the southern Voltaian sedimentary basin in Ghana was modeled with the use of PHREEQC to determine the main controls on the hydrochemistry. Groundwater was further assessed on its suitability for irrigation in the area. Mineral speciation and inverse modeling indicate that groundwater from the southern Voltaian System is generally supersaturated with respect to calcite, aragonite, montmorillonite and kaolinite and undersaturated with respect to albite and anorthite at most locations. Albite and anorthite appear to weather to clay minerals along groundwater flow lines, and the incongruent weathering of these minerals appears to be responsible for the apparent supersaturation of the carbonate and clay minerals in the area. Mineral stability diagrams show montmorillonite as the most stable clay mineral phase, probably resulting from the weathering of feldspars in the study area. Stability in the montmorillonite field indicates that groundwater flow in the area is generally restricted. Further analysis of the data suggests that cation exchange activity probably plays a significant role in the hydrochemistry. The relative significance of silicate mineral weathering and cation exchange is discussed. On the basis of a sodium adsorption ratio, electrical conductivity and permeability indices calculated from the hydrochemical data, groundwater in the area appears to be of sufficient quality for irrigation purposes.