HYDRO-SEDIMENTOLOGICAL EVALUATION OF AQUIFER PROPERTIES IN PARTS OF SOUTHERN ANAMBRA BASIN

Abstract

Nigeria’s cities lack potable water supply due to failed municipal water supply systems, leading to groundwater reliance. Understanding hydrogeological settings of some basins like Anambra Basin is tough due to its complex geological heterogeneity and varying sedimentary structures. The commonly used electrical resistivity method has ambiguity issues because it often struggles to distinguish between different lithological units with similar resistivity values, leading to misinterpretation of subsurface geology. The alternative hydro-sedimentological approach estimates aquifer properties from grain size. In this study, twenty-five (25) sand samples were collected from boreholes cuttings; this is because sands are the primary aquifer-hosting materials in southern Anambra Basin. Ten empirical relationships were used to estimate hydraulic conductivities from the grain size data. Porosity, specific yield (SY), and specific retention (SR) were also empirically determined. Pumping data were available for ten (10) locations. For the remaining fifteen (15) locations, where pumping tests were not conducted, permeameter tests were performed on well cuttings to estimate hydraulic conductivity. The graphic mean of the sands ranges from 0.071 to 1.533phi, indicating that the sands are mainly medium-sized. Fractional porosity values, Specific yield and Specific retention range from 0.260 to 0.392, 0.274 to 0.039, and 0.125 to 0.322, respectively. Hydraulic conductivity ranges estimated from Hazen Simplified, Slichter, Terzaghi, Beyer, Sauerbrei, USBR, Barr, Alyamani & Sen, Chapuis, and Uma models are between 1.31-22.37m/day, 0.31-6.85m/day, 0.5-12m/day, 1.4-19m/day, 1.22-16.71m/day, 1.42-13.37m/day, 0.36-9.3m/day, 0.01-8.72m/day, 0.17-11.3m/day, and 0.02-10.64m/day respectively. Hydraulic conductivity obtained from pumping test ranges from 0.89 to 7.06m/day, while the values are 0.0864 to 25.488m/day from permeameter test, classifying the aquifers as having medium to high groundwater potential according to Terzaghi, Barr, Chapius and Uma estimates. In conclusion, this study was able to map out the groundwater potential of the area and revealed the most suitable empirical relation for estimating groundwater potentials within the area.