A brief review of groundwater for rural water supply in sub-Saharan Africa

Groundwater has proved the most reliable resource for meeting rural water demand in sub-Saharan Africa. There are four main hydrogeological environments in SSA. Each of these broad categories requires different methods for finding and abstracting groundwater. 1. Crystalline basement occupies 40% of the land area of SSA; 220 million people live in rural areas underlain by crystalline basement rocks. Groundwater is found where the rocks have been significantly weathered or in underlying fracture zones. Borehole and well yields are generally low, but can be sufficient for rural demand. 2. Volcanic rocks occupy 6% of the land area of SSA, and sustain a rural population of 45 million, many of whom live in the drought stricken areas of the Horn of Africa. Groundwater is found within palaeosoils and fractures between lava flows. Yields can be high, and springs are important in highland areas. 3. Consolidated sedimentary rocks occupy 32% of the land area of SSA and sustain a rural population of 110 million. Significant groundwater is found within sandstones and limestones, which can be exploited for urban as well as rural supply. Mudstones however, (which account for about 65% of all sedimentary rocks) contain little groundwater, and careful study is required to develop water for community supply. 4. Unconsolidated sediments occupy 22% of the land area of SSA and sustain a rural population of 60 million. They are probably more important than these statistics suggest since they are present in most river valleys throughout Africa. Groundwater is found within sands and gravels. Groundwater has excellent natural microbiological quality and generally adequate chemical quality for most uses. However problems can arise from the chemistry of groundwater in some circumstances, for example: high sulphate in some parts of the weathered basement and mudstones; hardness in limestone aquifers or sandstones cemented with carbonate material. Minor and trace constituents which make up about 1% of the solute content of natural waters can also sometimes lead to health problems or make the water unacceptable for human and animal consumption. For example: high fluoride in some volcanic aquifers; elevated iron and manganese where conditions are anoxic; high arsenic in some unconsolidated sediments and the lack of iodine in aquifers far from the sea. Research and experience in some of these hydrogeological environments have enabled standard techniques to be developed for finding and abstracting groundwater. Geophysical techniques in particular have proved useful in many environments for siting wells and boreholes. However, much is still not known about groundwater in Africa. Some issues that demand more attention are: • the age, recharge and sustainability of groundwater supplies in basement areas, particularly during drought; • the existence of groundwater in poorly weathered crystalline basement and mudstone areas; • sustainability of groundwater supplies from upland volcanic aquifers; • overexploitation of groundwater in sedimentary basins; • variations in natural quality and contamination of groundwater; • appropriate (technical, economic and social) choice of water technology. 4 Co-ordinated groundwater research and data collection has become more difficult in SSA due to decentralisation and demand responsive approaches to the provision of rural water supplies. Information is rarely collected from the many thousands of boreholes drilled each year, with the result that the same costly mistakes are made time and again. However techniques are available to allow local institutions to collect high value data from ongoing drilling for little additional cost. The use of these techniques could allow local institutions to assess the nature of groundwater resources in their areas and, with proper documentation and networking, increase the knowledge base of groundwater in Africa. Budgets for groundwater research in Africa could then be targeted to issues that cannot be addressed by improved data collection from ongoing drilling. Such a scenario will only occur with the dissemination of simple techniques in groundwater resource assessment to those involved in rural water supply, and when the benefits of such assessments are seen within individual water projects