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Groundwater-yielding capacity, water-rock interaction, and vulnerability assessment of typical gneissic hydrogeologic units using geoelectrohydraulic method

Akingboye Adedibu Sunny, Bery Andy Anderson, Kayode John Stephen, Ogunyele Abimbola Chris, Adeola Adedapo Oluwasanu, Omojola Olumide Olubusayo, Adesida Adeola Samuel

Acta Geophysica

2023

Vol. 71, no. 2

697--721

Geohydraulic parameters, namely hydraulic conductivity (K), transmissivity (T), effective porosity (φ), permeability (kp), anisotropy coefficient (λ), and longitudinal conductance (S), of aquifer units in Etioro-Akoko, southwestern Nigeria, were evaluated using the Schlumberger vertical electrical sounding (VES) technique. This study aimed to understand the hydrodynamics and water–rock interaction of the near-surface crustal architecture to determine the groundwater yield and vulnerability of the aquifer units in the study area. A total of 7 model curve types were generated for fifty-two geoelectrical surveyed points, with percentage distributions in the order of HA>AA>H>KH>A>HK>AK. The VES curve models constrained the subsurface layers into topsoil, weathered units, weathered/fractured bedrock units, and fresh bedrock. The weathered and fractured aquifer zones occurred at the depths of 8 m and>16 m (with depths exceeding 26.5 m for some sections). The K and T values for the aquifer units varied from 0.1901 to 0.6188 m/day and 0.7111 to 6.3525 m2/day, respectively. These parameters coupled with the aquifer φ (18.03–23.35%) and kp (0.028–0.089 µm2) classified the delineated aquifer units as low to moderate groundwater-yielding capacity aquifers, with recorded resistivity values between 85.1 Ω-m and<613.0 Ω-m. The observed positive correlations and R2 values with>32–100% prediction rates affirmed the dependence of K on T, φ, and kp for effective water–rock interactions and groundwater transmissibility. The recorded S values (0.0146–0.162 mhos) and low logarithm hydraulic resistance, Log C (0.89–1.75 years), suggested poor to weak aquifer protective capacity ratings, resulting in high aquifer vulnerability index delineated across the study area. As a result, deep-weathered/fractured aquifers should be exploited for sustainable potable groundwater supplies. However, intended wells/boreholes in the study area must be developed properly for long-term groundwater abstraction to alleviate potable groundwater deficit and optimize future operational drilling costs.

Electrical resistivity tomography for geo-engineering investigation of subsurface defects: a case study of Etioro-Akoko highway, Ondo State, southwestern Nigeria

Akingboye Adedibu Sunny, Osazuwa Isaac Babatunde, Mohammed Muraina Zaid

Studia Quaternaria

2020

Vol. 37, Nr 2

101--107

The durability of roads is dependent on the proper screening of the variations in subsurface geological characteristics and conditions through geo-engineering investigations and good construction practices. In this study, electrical resistivity tomography (ERT) technique was used to investigate the subsurface defects and potential failures along the substrate of Etioro-Akoko highway, Ondo State, southwestern Nigeria. Results of the inverse model resistivity sections generated for the two investigated traverses showed four distinct subsurface layers. The shallow clayey topsoil, weathered layer, and partially weathered/fractured bedrock have resistivity values ranging from 4–150 ohm-m, 10–325 ohm-m, and 205–800 ohm-m, with thickness values of 0–2 m, 0.5–12.5 m, and less than few meters to > 24 m, respectively. The fresh bedrock is characterised by resistivity generally in excess of 1000 ohm-m. The bedrock mirrored gently to rapidly oscillating bedrock troughs and relatively inclined deep penetrating multiple fractures: F1–F'1, F2–F'2 and F3–F'3, with floater in-between the first two fractures. These delineated subsurface characteristic features were envisaged as potential threats to the pavement of the highway. Pavement failures in the area could be attributed to the incompetent clayey sub-base/substrate materials and the imposed stresses on the low load-bearing fractured bedrock and deep weathered troughs by heavy traffics. Anticipatory construction designs that included the use of competent sub-base materials and bridges for the failed segments and fractured zones along the highway, respectively, were recommended.