AN INVESTIGATION OF AQUIFER MORPHOLOGY IN BENIN CITY AND ENVIRONS, SOUTHERN NIGERIA

Abstract

Water-bearing formations underlying Benin City and its environs in Southern Nigeria were investigated to determine their structural characters, thickness and areal extent as well as their respective storage parameters. The aim of this study is to investigate the subsurface hydrogeological conditions and morphology of aquifers. The study area is underlain by the Coastal Plain Sand of Miocene to Pleistocene age in southern Nigeria. The methodology adopted in this study involved collection of lithologic logs from existing forty five (45) boreholes in study area and geo-referencing of the wells’ positions. The logs were plotted in space and the stratigraphic horizons were correlated in 2- & 3- dimensions using SURFER, WINLOG, WINFENCE and STRATA EXPLORER which are terrain modeling computer packages. Aquifer thickness and transmissivity (T) were calculated from profile plots. Six 2- dimensional profile plots and one 3- dimensional panel diagram were constructed for the area. The major stratigraphic units in the study area are laterite, clayey sands, clay, sands and gravelly sand. Two major aquifer horizons were found in the area and these occur as continuous beds of the sandy formations. The upper aquifer horizon is characterized by fine to medium sandy texture having average thickness of 31.72m, average transmissivity of 1820.23m2/day and mean hydraulic conductivity of 54.48m/day. It generally occurs at depths between 19.58m and 55.16m below ground surface. It has hydraulic connection with the ground surface through the overlying materials and therefore generally occurs under unconfined conditions. The lower aquifer unit is characterized by medium to coarse sandy texture having average thickness of 35.51m, it generally occurs at depth between 45.27m and 100.31m below ground surface.