Hydrogeophysical Characterization of Shallow Unconsolidated Alluvial Aquifer in Yenagoa and Environs, Southern Nigeria

Groundwater of the Quaternary shallow alluvial aquifer is the only safe and reliable water resource in Yenagoa and environs, Southern Nigeria. The determination of the aquifer hydrogeophysical parameters is thus critical and constitutes the basic foundation for optimizing the short- and long-term groundwater utilization strategies. In this study, 29 Schlumberger vertical electrical soundings, borehole data, pump test, hydrochemical analysis, and measured and/or calculated hydrogeophysical parameters were integrated to reconnaissance the local aquifer conditions. Aquifer qualitative and quantitative properties in terms of electric and hydraulic conditions were also investigated. The freshwater aquifer was delineated with true resistivity and thickness ranges of 134-664 Omega m and 6.7-41.4 m respectively. Transmissivity and hydraulic conductivity values vary between 189-2609 m(2)/day and 28-118 m/day, respectively. Porosity ranges between 0.40 and 0.45 and tortuosity ranges between 1.28 and 1.31. Correlation of the results of the geochemical analysis of the groundwater samples with the layer resistivities shows that in the locations where the aquifer is overlain by clay layer (rho similar to 10 Omega m) have higher concentrations of dissolved iron (Fe > 0.1 mg/L), whereas the iron concentrations are remarkably low (Fe < 0.1 mg/l) in areas the aquifer is underneath relatively thin silty sand layer (rho similar to 349 Omega m). The near-surface lithology acts as a controlling factor for spatial distribution of groundwater iron within the very shallow depths (< 20 m). The geochemistry of the groundwater was observed to be controlled mainly by weathering of silicate group of minerals.