The large number of rural water supply boreholes sited and drilled in northern Nigeria during the ground water decade of the eighties has provided much data on the hydrogeology of the Basement regolith and has led to the development of new techniques in locating aquifers. Regolith aquifer characteristics and their dependence on geology and environment are reviewed with particular emphasis on lithology, weathering pattern and fracturing in Bauchi, Kano and Sokoto States. Specific capacity is shown to be related to both lithology and grade of weathering; dry season water levels and frequency of occurrence of successful holes are related to lithology. Water quality data for Kano State show some high nitrate concentrations. The process of borehole siting begins with desk study (records and photointerpretation), then field reconnaissance of lithology, structure, water points, topography and soils. EM traverses and resistivity soundings follow in targeted areas. Maximum regolith thicknesses are estimated from the vertical to horizontal coil (EM) response using computed model graphs for 40 m and 20 m coil spacings. VES measurements at these sites give regolith resistivities, hence some estimate of its suitability as an aquifer. In addition EM traverses have located steeply-dipping fracture zones, dykes and pegmatites and interpretation of these in terms of strike and dip has been aided by results from a computer-controlled modelling system.

The tenfold increase in borehole siting in Northern Nigeria during the last few years has resulted in an increase in the use of electromagnetic (EM) methods and a corresponding decrease in the more traditional but less cost effective resistivity methods. The role of geophysics in exploration is optimally in combination with direct geological observation. Two types of aquifer which are amenable to geophysical investigation in the Nigerian environment are weathered, jointed crystalline rocks and alluvium. Location of joints and associated narrow zones of deep weathering in crystalline terrain is best achieved by EM techniques with resistivity backup where needed. Resistivity alone does not always reveal such features. Location of shallow aquifers in alluvium is amenable to a combination of EM and resistivity techniques. However, the complexities of alluvial and underlying Quaternary sedimentation necessitate extensive computer modelling of both sets of data in order to optimize the field techniques and interpretation methods. In particular the use of selected combinations of intercoil spacings to estimate maximum thicknesses of aquifers shows some promise.

Aquifers in the sand alluvium of the broad riverine flats (fadama) of the southern Sahel and northern Sudan climatic zones of West Africa are a major source of water for irrigation essential to crops in bad years. Annual recharge is from rivers flowing seasonally into the Sahel from the wetter south. Landforms which evolved in the mid Tertiary control the modern drainage. From the Pleistocene onwards rivers vigorously eroded the crystalline highlands of the Jos and Air massifs and carried the coarse alluvium far downstream. The resulting aquifers are often several kilometres wide and mostly over ten metres thick, sometimes much thicker, and are capped by late Pleistocene loessic soils. Aquifer properties are typical of the mostly coarse unconsolidated sands. The alluvial groundwater is reached and abstracted by methods appropriate in scale to family size farms. The system of farming and irrigating is socially beneficial and economically viable. The paper concludes with case studies of three rivers of Bauchi State in Nigeria. The alluvial aquifers are evaluated and related to irrigation potential, expressed as the percentage of land which may be irrigated using only the groundwater beneath it.