Some aspects of the hydrogeology of the crystalline basement complex rocks of South Western Nigeria
Geophysical, hydrochemical and hydrogeological investigations were undenaken to evaluate the groundwater prospects in the study area. The factors which affect/determine the spatial development of ground water in the area was also examined. The results show that the essential conditions for groundwater availability in hard rock terrains are satisfied. The average weathering depth is 34 m, rainfall is high and seasonal and the geological evolutionary sequence included tectonic phases during which suitable geological structures are expected to have been developed. Analysis of borehole drilling records revealed that each borehole encountered at least one water strike. The clay mineral stable in the prevailing physico-chemical conditions is kaolinite, an evidence that weathering sites are being flushed presumably by flowing groundwater. Available results of isotope analysis also show that the stable isotope concentration in the groundwater is similar to that of current meteoric water. It is therefore concluded that the ground water is receiving current recharge. Infact water balance calculations and results of baseflow analysis indicate that this recharge is substantial. The yields of the boreholes varied greatly spatially ranging between 0.7 and 10.9 I/s. This may be low compared with values quoted for sedimentary rock terrains, but in view of the present low level of water supply in the area, it is considered that exploitation of this resource would represent a significant improvement on the present situation. A physical catchment hydrogeological model is presented to explain the observed spatial variation in yields. In this model, a threshold elevation is considered to exist in each catchment. At elevations below this threshold, borehole yield is sustained entirely by fracture flow. At elevations above this threshold, borehole yield is sustained by storage in the weathered rocks. Pumping tests on large diameter wells show that groundwater abstration using these wells would be greatly improved by repeated pumping after every 60 - 80 minutes recovery. An equation is presented for calculating the number of times a well can be repeatedly pumped in an operating day. The drawdown of the boreholes contained a large well loss component. This well loss was incurred during the early stages of pumping when water had to be withdrawn from well storage to supplement the aquifer flow in order to meet the pumping capacity. Field results presented indicated that the drawdown can be minimised by stepping the discharge rather than imposing it all at once at the onset of pumping. Field results indicate that the use of well water levels for mapping the water table may not be valid in weathered hard rocks.