Integrated approach to investigation of occurrence and quality of groundwater in Ogbomoso North, Southwestern Nigeria

Abstract

An integrated approach involving electrical resistivity, hydrogeologic, hydrogeochemical and remote sensing has been employed to investigate the occurrence, facies, quality and hydrogeologic systems of the aquifer units of groundwater in the low latitude crystalline basement complex terrain of Ogbomoso North, Southwestern Nigeria. Vertical electrical soundings (VES) were conducted at 56 locations across the study area using Schlumberger electrode configuration with a maximum current electrode separation of 240 m along northeast–southwest azimuth. The layering models obtained from the interpretation of VES data were used to construct the 2-D geoelectric section for the study area. The static water level, depth, geographic coordinates and elevations of 130 hand-dug wells within the study area were also measured to construct the groundwater flow map. Hydrogeochemical studies involved in situ measurements of such parameters as electrical conductivity (EC), acidity (pH) and total dissolved solid (TDS) using calibrated WTW Conductivity/TDS meter (LF 90) sand HACH pH Sension meter, respectively, and laboratory analysis of 25 representative water samples for anion and cation concentration. Prominent lineaments were also mapped from side-looking airborne radar imagery of the area and used to obtain the lineaments orientations and joints systems vide Rose diagram. Interpretation of the VES data revealed a three- to four-layer earth model with the three-layer resistive–conductive–resistive geoelectrical signature as the most dominant constituting 62 % of the entire apparent resistivity curves. The fractured/weathered basement constitutes the aquifer units (confined/unconfined) while the conductive zones comprise clay/sandy clay/clayey sand. The 2-D geoelectric section revealed lenses of sand and clay, sand atoll and fresh/fracture basement at different depths. The lineament length varied from 0.72 to 1.39 km and 0.11 to 0.56 km for long and short lineaments, respectively. The long lineaments trend in the east–west direction and indicated shallow weathering while the short lineaments trend NW–SE and NE–SW directions indicating deep weathering in such directions. Two joint systems trending NW–SE and NE–SW as well as joints in the E–W direction were also revealed. The direction of groundwater flow was southward and westward with the recharge areas along topographic high trending NE–SW. The EC, pH and TDS ranged between 0.10 and 1.58 mS/cm, 5.0 and 7.3 and 60 and 920 mg/l, respectively, with corresponding mean of 0.60 ± 0.06 mS/cm, 6.8 ± 0.10, and 342 ± 34 mg/l. The anionic dominance was of the order CO₃ ²⁻ > Cl⁻ > NO₃ ⁻ > SO₄ ²⁻ > PO₄ ³⁻ > Fl⁻ and the concentrations of the cations were of the order Ca²⁺ > Na⁺ > K⁺. Furthermore, all the elemental concentrations were within the World Health Organization standard limit for drinking water save for high nitrate concentration in samples collected from very few locations. Twelve groundwater facies were identified each representing different rock types and products of weathering. The groundwater is localized within the fractured/weathered basement with moderately thick overburden and capable of sustaining industrial development, increasing urbanization as well as providing access to clean water. The corroboration between the joint system and the direction of groundwater flow indicated the groundwater flow is structural controlled.