The quantity of groundwater available in any given area is finite and normally has two compo- nents: the accumulated reservoir over thousands of years and the seasonal replenishment, if any. Thus, when annual abstraction exceeds the annual recharge, the reservoir will be drawn down. Continuation of this excessive abstraction will eventually deplete the reserve through a process referred to as groundwater mining. In the southern parts of Nigeria, the regular seasonal replenishment reduces the chance of any large-scale depletion of the aquifers except, perhaps, in and around large urban centres, such as Lagos. The situation is different in the sub-humid northern parts where the annual rainfall and, therefore, recharge are both small and variable.
Over-exploitation has been identified in Katsina and Daura in the Tagwai-EI Fadama sub-basin where more than 40 boreholes have been sunk. Similarly, records of groundwater levels in the Kano-Hadejia area of the Chad basin indicated a 30m decline between 1973 and 1983. Even the artesian groundwater flows have declined significantly as a result of over-exploitation coupled with prolonged drought conditions and paucity of surface water. The counntry has been divided into eight hydro-geological areas or groundwater provinces. The provinces and their major hydrogeological characteristics are shown in Table 3.4.4. Area 1 to VII belong to the sedimentary formaion, while area VIII belongs to the crystalline basment complex rocks, and is scattered over four sub-area representing about 50 per cent of the country's total area. The two groups have distinct water bearing properties, the basement rocks being relatively shallow, patchy and of limited capacity as water reservoir, except where extensive jointing and fracturing occur.