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.

Evaluation of factors influencing transmissivity in fractured hard-rock aquifers of the Limpopo Province

“…As a result, certain assumptions had to be made in order to determine the extent to which these lineaments influence or relate to borehole productivity in the Limpopo Province on a regional scale. According to Braathen and Gabrielsen (1998) a large lineament can enhance fracturing up to 300 m away while Clark (1985) suggested that the area of influence might be less than 150 m and Fernandes and Rudolph (2001) reduced it further to 70 m in their studies. Considering the spatial margin, all boreholes within 150 m of a lineament were considered to be targeting a lineament in this study.…”

Groundwater Potential Evaluation in a Typical Basement Complex Environment Using GRT Index—A Case Study of Ipinsa-Okeodu Area, near Akure, Nigeria

“…Fractured crystalline bedrocks remain good sources for drinkable water but sitting of highly productive wells in these rock units remains a challenging and expensive task because fractured developments on regional scale are both heterogeneous and anisotropic. Also fractured and viable aquifers wholly within the fractured bedrock are of rare occurrence because of the typically low storability of fracture systems [6]. The availability of groundwater in these aforementioned rocks is largely due to the development of secondary porosity and permeability resulting from weathering and fracturing.…”

“…In the past, groundwater potentials have been evaluated based on consideration of important parameters such as overburden thickness, weathered layer resistivity, bedrock resistivity, aquifer layer resistivity and aquifer layer thickness among others [6]- [13] and each parameter is considered in isolation. However in this study a multi-criteria GRT modelling approach was developed for evaluating groundwater potential in a typical basement complex environment.…”

Estimation of groundwater recharge in weathered basement aquifers, southern Zimbabwe; a geochemical approach

“…Crystalline basement rock in Africa is primarily of Precambrian age and granitic or gneissose in type (Clark, 1985). Basement aquifers are formed due to the effects of prolonged weathering and tectonic forces on the parent rock (Fookes, 1997).…”