Hydrogeological characteristics of crystalline rock aquifers: implication on sustainable water supply in the basement complex terrain of southwestern Nigeria

Abstract

Poor understanding of groundwater resources of the basement environment constitutes a barrier to sustainability of current potable groundwater-based supply initiatives in sub-Sahara Africa. This study assesses the influence of aquifer characteristics on groundwater yield potential and the consequent impact on water supply in Osun Drainage Basin within the basement complex area of southwestern Nigeria. Aquifer thickness, longitudinal unit conductance, hydraulic conductivity (K) and transmissivity (T) were derived from the interpretation results of 500 spatially referenced vertical electrical sounding data and subjected to descriptive and inferential statistics. Descriptive statistics show that aquifer thickness is generally low in most part of the study area with the exception of few locations where overburden thickness exceeds 50 m. Delineated aquifers have the capacity to absorb and retain water but with less ability to release the water, as indicated by low mean K (1.6 × 10−4 m/s). Only 1.4% of the aquifers have medium relative K (10−3 m/s), while the remaining 98.65% have low to very low relative K (≤ 10−4 m/s). Computed low mean T (6.2 × 10−3 m2/s) suggests low groundwater yield potential for the study basin. Currently, groundwater resources could only meet 32.7% of the household water demand of the study area and the coverage would reduce to 27.2% in the decade 2021–2030. Therefore, there is an urgent need to focus on surface water development given the fact that annual rainfall is high enough to sustain pipe-borne water supply schemes in the study basin.