Groundwater Recharge and Aquifer Transmissivity of Crystalline Basement Terrain

It is known that substantial groundwater bearing zones can develop within the weathered and fractured units of impermeable crystalline rocks if there is a good and recurrent source of recharge. On account of this, eighty-two geo-electric soundings were carried out to determine the degree and extent of weathering and bedrock fracturing; and in addition, twenty-one single –well (constant discharge) pumping tests were also conducted for the estimation of the transmissivity of the water-bearing zones across the various bedrock terrains of Ibarapa areas of southwestern Nigeria. Weathering development was found to be more pronounced in areas underlain by amphibolite and gneisses bedrocks with a regolith thickness range of 4.4 - 47.5m (av.18.41 m) as compared to the relatively shallower soil horizon range of 2.4 - 23.6 m (av. 12.03 m) that developed upon light coloured rocks of migmatite and granites. The recharge potentials as inferred from the total longitudinal conductance indicated that there is likelihood of better water infiltration in areas underlain by migmatite and granites with 0.02 – 0.60 mhos. The discharging areas underlain by amphibolite and gneisses with higher conductance of 0.01 – 1.42 mhos were characterized by thicker and finer grained overburden unit and more numerous bedrock fractures that sustain reliable groundwater yield. Only one-third of the boreholes were sustainable with transmissivities of 7.75 – 0.50 m2/day; these prolific boreholes were mostly within amphibolite and gneisses terrains. Change in the drawdowns during pumping was between 1.7 and 14m, while the groundwater yield was between 93.58 and 32.78 m3/day. Keywords: Groundwater recharge, transmissivity, sustainable yield DOI: 10.7176/JEES/9-10-13 Publication date:October 31st 201