GROUNDWATER POTENTIAL AND VIABILITY OF THE AQUIFER SYSTEM OF CRYSTALLINE ROCKS OF IBARAPA REGION, SOUTHWESTERN NIGERIA

This research work focused on solving the problem of water deficiency in Nigeria, the most populous country in Africa, by using a combination of geomorphology, geophysics, hydrogeology as well as traditional geological mapping methods to study the underlying bedrock, the regolith units as well as the groundwater bearing zones of Ibarapa region of SW Nigeria. Ibarapa region occupies 184.29 km² and is underlain by intrusive crystalline rocks of Precambrian age; including Older granite series and varieties of amphibolites, migmatites and gneisses, that ordinarily have no potential for storing and transmitting water, except when fractured and where there is substantial regolith development.

On account of this, essential thematic maps that are viable groundwater indicators were generated from satellite imageries; and a total of 82 geo-electric soundings were carried out for characterizing the weathering pattern upon the bedrocks. In addition, single-well aquifer tests were also conducted to determine the hydraulic properties of the water bearing zones.

The geophysics results showed that the dominant VES curve is the 3-layer H type, characterized by more conductive middle layer that terminates on more resistive infinite layer. Regolith development is prominent in areas underlain by amphibolites and gneisses with a range of 4.4 – 59.0meters (av. 18.41m). The basement resistivities ranged from 105 – 29903Ω.m, and areas with high to moderate groundwater recharge potentials occurred as localised zones.

From the aquifer tests, groundwater discharge varied from the highest average yield of 3.00m³/hr in amphibolites to the lowest yield of 2.23m³/hr in gneisses. Wells on migmatites and porphyritic granites terrains have nearly similar average water discharge of 2.84m³/hr and 2.68m³/hr respectively. The transmissivities (in m²/day) of the water-bearing zones in amphibolite was 0.58 – 7.75 (av. 3.80); gneisses: 0.25 - 7.23 (av. 1.62); migmatite: 0.56 - 0.85(av. 0.72) and porphyritic granite: 0.30 – 5.75 (av. 1.90).

About one-third of the tested wells were prolific, mainly found in areas underlain by amphibolites and gneisses where there is 60% chance of having a productive well. Borehole depths have significant relationship with the prolificacy of the wells and the depths of most productive wells are within 30 – 38meters.