A review of the groundwater characteristics of the crystalline basement rocks in the Pra River Basin, Ghana
摘要
The Pra River Basin, located in southwestern Ghana, is an essential hydrological resource in providing access to clean and reliable freshwater for agricultural, domestic, and industrial uses, but faces significant environmental degradation due to illegal mining and other anthropogenic activities. Groundwater has become a vital alternative source of water supply for the communities; however, the hydrogeology and groundwater characteristics of the basin remain under-researched. This review provides a comprehensive examination of the hydrogeology of the crystalline basement rocks in the basin, drawing from existing studies. The hydrochemical information reviewed reveal variability in water quality, with certain areas exhibiting elevated concentrations of contaminants such as Na+, Ca2+, and NO3−, especially in the south of the basin toward the ocean, and F−contamination mainly in the granitoid formation. Electrical resistivity and electromagnetic profiling reveal significant variations in resistivity values (20 to 17,574 Ω-m) across different geological units. The weathered and fractured zones revealed relatively low resistivities with thicknesses of 8–33 m, which suggests good groundwater potential, while the bedrock with higher resistivities generally had low groundwater potential. The Granitoids in the basin present the highest borehole yields, ranging from 17 to 579 m3/day, with an average of 195 m3/day, compared to borehole yields of 14–259 m3/day, with an average of 66 m3/day, for rocks of the Birimian Supergroup. Relatively high borehole yields (122–256 m3/day) occur in the central section of the basin in rocks of the Birimian and Tarkwaian formations, and this is attributable to well-developed joint structures, highly foliated systems and the presence of quartz veins. The Granitoids in the basin have relatively thin weathered zones compared to rocks of the Birimian Supergroup, which is responsible for the substantial groundwater accumulation and storage in the basin. However, due to the crystalline nature of the rocks, groundwater yield and potential were largely influenced by the nature of weathering and, in some areas, extensive fracturing of the rocks.