Evaluation of groundwater potential of bedrock aquifers in Geological Sheet 223 Ilorin, Nigeria, using geo-electric sounding

Abstract

Electrical resistivity data acquired in one hundred and ten (110) locations using vertical electrical sounding method of Schlumberger array have been used to study the hydrogeological properties and groundwater storage potential of bedrock aquifers in an area covered by Geological Sheet 223 Ilorin, Nigeria. The aim of the study was to identify productive aquifer zones for citing boreholes for community water supply. The data acquired were processed and interpreted using auxiliary curve matching and computer automation method to delineate the different geo-electric layers, their resistivities, thicknesses, and depths. Geo-electrical layers were interpreted to their equivalent geological layers using borehole lithological logs from the study area. Then, the hydraulic conductivity, transmissivity, fracture contrast, reflection coefficient were estimated and plotted in the form of 2D maps to describe the spatial variations of these parameters in the area. The results of the study revealed the presence of three to five geo-electric layers. The geo-electric layers, from top to the bottom, corresponds to the topsoil layer, lateritic layer, weathered rock layer, fractured rock layer, and the fresh basement rock. Lateritic and/or fractured rock layers were not delineated in some places. The weathered and fractured rock layers, where present, correspond to the aquifer units. The thickness of the fracture aquifer ranges from 0.6 to 33.6 m while the thickness of the weathered aquifer ranges from 1.4 to 49.3 m. The transmissivity, $$ T $$ T , and hydraulic conductivity, $$ K $$ K , range from 3 to 1200 m2/day and 1 to 48 m/day, respectively. The reflection coefficient and fracture contrast map showed the presence of water-bearing fractures and shared some similarities with T and K maps. A mathematical model for predicting groundwater potential, $$ {\text{GW}}_{\text{P}} $$ GW P , of weathered aquifer in the basement complex terrain was proposed in this study. The consistencies between the overall groundwater potential map and aquifers parameters distributions maps suggest the appropriateness of the proposed mathematical model for predicting groundwater potential of weathered rock in the basement complex area of Nigeria. The western, northwestern, and central parts of the study area, having $$ {\text{GW}}_{\text{P}} $$ GW P greater than 0.6 (60%), were recommended for groundwater development through boreholes drilled to a depth ranging between 75 and 100 m.

Geohydraulic and vulnerability assessment of tropically weathered and fractured gneissic aquifers using combined electrical resistivity and geostatistical methods

“…The subsurface hydrogeological features of the Araromi area of Akungba-Akoko are similar to those of the surrounding towns, e.g., the main Akungba-Akoko town and Etioro-Akoko community, e.g., [3,[19][20][21], as well as some places in the crystalline basement of southwestern Nigeria, e.g., [11,12,30]. The groundwater in the study area occurs in weathered and/or fractured aquifer zones.…”

“…2b and 2c depict the elevation of the surface topography. The current electrodes AB/2 varied from 60 to 160 m, whereas the potential electrodes spread MN were varied from 0.5 to 15 m. The depth of penetration in a homogenous subsurface geologic structure is proportional to the distance between the current electrodes, whereas, varying the electrodes distance offers information regarding the subsurface lithostratigraphic units, e.g., [3,8,12]. When a remarkable resistivity of fresh bedrock, typically with values >1200 Ωm, was attained more than twice at each VES station, the survey was stopped, indicating that there was no possibility of obtaining a fracture at deeper depths even for further probing.…”

“…The sustainability of groundwater supplies also depends on the quality of the aquifers' yield, which is a function of the protective capacity of the strata overlying the aquifer zones and the depths of the aquifer zones. The occurrences of aquifer zones at shallow depths, especially within the crystalline basement terrain, give easy access for the percolation of surface runoffs and pollutants from dumpsites' leachate flows, surface and buried oil tank spillages, dissolved chemicals from mining activities, sewage from sanitation systems, etc., to degrade the stored groundwater [5,[9][10][11][12]. In addition, over-stretching of aquifers caused by over-abstraction of groundwater and silt/clay intrusion from improperly cased boreholes degrades the quality of groundwater [3, [13][14][15].…”

Review: Saltwater intrusion in fractured crystalline bedrock

“…With a high number of borehole logs, hydrological information can be achieved on a regional scale-for example, the thickness of saprolite and saprock as well as the depth of the weathering interface (e.g. Raji and Abdulkadir 2020). Several other techniques are available for the investigation and description of highly conductive structures along a bore well-for example flowing fluid electrical conductivity (FFEC; Tsang et al 1990Tsang et al , 2016 to mention only one.…”

Geohydraulic and vulnerability assessment of tropically weathered and fractured gneissic aquifers using combined electrical resistivity and geostatistical methods

“…The sustainability of groundwater supplies also depends on the quality of the aquifers' yield, which is a function of the protective capacity of the strata overlying the aquifer zones and the depths of the aquifer zones. The occurrences of aquifer zones at shallow depths, especially within the crystalline basement terrain, give easy access for the percolation of surface runoffs and pollutants from dumpsites' leachate flows, surface and buried oil tank spillages, dissolved chemicals from mining activities, sewage from sanitation systems, etc., to degrade the stored groundwater [5,[9][10][11][12].…”