Low permeabiity rocks in sub-saharan Africa: an assessment of the hydrogeology of the Afram Plains, Eastern Region, Ghana

WaterAid, its partner NGOs and World Vision International are developing the limited groundwater resources of the Afram Plains area of Ghana for water supply to rural communities. Water supply borehole drilling success rates (yields > 30 l/min) in the underlying Voltaian sediments are poor, at 18-40%. The location of and data from about 370 boreholes drilled in the Afram Plains have been collated into a database to enable assessment of the hydrogeological development potential of four of the five geological units present. Near-horizontally bedded Middle Voltaian Formation sandstones and conglomerates underlie most of the Afram Plains. These are underlain by older shales and mudstones that crop out in the southern part of the area. These ancient Lower Palaeozoic age sedimentary rocks have undergone prolonged weathering, diagenesis and low-grade metamorphism; the effects of tectonic activity being limited to folding along the eastern margin of the area. The sandstones and conglomerates are generally massive and poorly jointed, and groundwater flow occurs in thin weathered zones, coarser sediment layers and a few fracture zones. Two British Geological Survey (BGS) hydrogeologists supervised the installation and testing of 4 deep exploration boreholes and six production boreholes during April-May 2001. The data gathered during this drilling programme are used with information from the database to assess the groundwater resources of the Afram Plains. The BGS hydrogeologists worked with Afram Plains Development Organisation (APDO) and Technic-Eau staff and Legon University MSc students during the drilling and testing programme. The relationship of rock out-crops to topographic features; the selection of drilling sites using geological, geomorphologic and geophysical criteria; the use of simple test pumping methods, using bailers and low capacity Whale pumps; and the collection of rock and water samples during drilling and test pumping, were demonstrated to and discussed with APDO, university and consultant staff. Borehole design was also discussed according to Community Water and Sanitation Division guidelines. APDO and Technic-Eau staff appreciate the need to collect adequate data during borehole drilling, testing and subsequent community use. The APDO are accurately locating villages and boreholes using hand held Global Positioning Systems and 1:50 000 scale base maps provided by the project. The need for improved collection of hydrogeological and related data to improve understanding of the nature of the aquifers present and the groundwater resources they contain was successfully demonstrated to stakeholders. These data allow improved selection of drilling sites and appropriate water supply technologies. Recently, a number of boreholes drilled into the feldspathic sandstone unit have failed to produce adequate quantities of water after several years of use. Due to a lack of understanding of groundwater occurrence in this formation, numerous replacement boreholes, sometimes up to three at a site, have been drilled adjacent to an original borehole. These have also failed after several years of use.