AQUIFER MORPHOLOGY IN BENIN CITY AND ENVIRONS, SOUTHERN NIGERIA
Water-bearing formations underlying Benin city and its environs in Southern Nigeria were investigated to determine their shapes, thicknesses, areal extent and water storage parameters. The area is underlain by the Coastal Plain Sand formation of Miocene to Pleistocene ages. The methodology adopted involved correlation of stratigraphic horizons from forty-five boreholes using graphic software packages. Aquifer thicknesses and transmissivities were calculated from data obtained. Panel and fence diagrams, as well as isopach maps, were constructed for the area. The major stratigraphic units identified are clayey sands, clay, sands and gravelly sands. Two major aquifer horizons were delineated. The upper horizon is characterized by sands of fine to medium textures having average thickness of 31.72m, average transmissivity of 1,820.23m2/day and mean hydraulic conductivity of 54.48m/day. It occurs under unconfined conditions at depths between 19.58m and 55.16m below ground surface. The lower aquifer unit is characterized by gravelly sands of medium to coarse textures having average thickness of 36.74m, average transmissivity of 2,197.27m2/day and mean hydraulic conductivity of 62.19m/day. It occurs at depths between 45.27m and 100.31m below ground surface. Clay unit of about 3.05 to 24.38m thick constitutes the confining stratum which separates the upper from the lower aquifer. Hydraulic pressures in the lower aquifer varies from 1.3 to 2.8 atmospheres. The pressures are not sufficient to raise the piezometric plane to the ground surface. This aquifer condition is therefore sub-artesian and it is present at Egor, Iguoshodi, Ikpoba, Ugbowo, and Iyowa parts of the study area.
|Keywords: Aquifer, Confining pressure, Hydraulic conductivity, Transmissivity, Fence diagram|
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