Evaluation of groundwater occurrences in the precambrian basement complex of Ilorin metropolis, southwestern Nigeria

摘要

Surface water from the Asa and Agba dams, which hitherto supply water to Ilorin metropolis is inadequate, hence the need to supplement with water from boreholes. Forty two boreholes drilled into migma-tites, granite gneiss and quartzite in Ilorin area were evaluated. Borehole data showed varied overburden thickness (1.00 m to 36.00 m]. The static water level (SWL) contour map showed a radial groundwater flow pattern trending in the NE-SW and NW-SE directions, which is consistent with the structural trends in the area. Estimated yields of the boreholes ranged from 0.30 1/s to 2.75 1/s. Pumping/recovery test of four selected boreholes showed increase in productivity from granite gneiss (transmissivity, T = 11.42 m3/d; permeability, K = 4.94 × 10~(-1) m/d) through migma-tites (T= 13.4 m3/d; K= 6.93 x 10~(-1) m/d) to quartzites (T= 17.56 m3/d; K = 9.54 × 10~(-1) m/d). The groundwater occurrence in the area is adjudged to be moderately high. Strong correlation coefficients (+0.99) exist between Vertical Electrical Sounding (VES) and borehole log indicating that the success of borehole in the area depends strongly on production of VES report. Based on this study, minimum borehole depth of 33 m is recommended for the area.