Groundwater abstraction from Basement Complex areas of Africa

Abstract

Summary The Basement Complex of Africa is a heterogeneous mixture of crystalline rocks, predominantly of a granitic or gneissose character. Groundwater occurs within these crystalline rocks in fissures or in the superficial weathered zones. The rocks are relatively impermeable and groundwater can occur in the superficial deposits overlying the bedrock. There are no regional aquifers in Basement Complex country and each site has to be treated as unique. The occurrence of groundwater in fissure systems or weathered zones means that the resources at any one site are finite and relatively small. Well yields similarly are limited, rarely exceeding 500 m 3 /day. In this situation the groundwater reservoir is structurally controlled and, therefore, susceptible to investigation by a variety of techniques including photogeology and geophysics. Furthermore, although each borehole is unique, the data from all the boreholes in a specific area can be analysed statistically to produce well design criteria. The limited resources within the crystalline basement can lead to multiple-source solutions to supply problems where every alternative has to be examined. Shafts or cisterns, for example, may be used. The superficial deposits commonly hold significant resources of water and, in certain circumstances, can give much higher yields than the Basement Complex rocks. This superficial water can be tapped through bores, shafts or trenches, and the resource can be enhanced by water harvesting techniques. The development of a resource is instigated for a specific purpose—a village water supply or an industrial supply. It is important that the planners and engineers involved in putting forward proposals recognise the limitations of groundwater abstraction in Basement areas. The water supply must be proven and developed before the costly infra structure—the factory or distribution network—is built.