Geoelectrical characterization of coastal aquifers in AgbadoIjaye, Lagos, southwestern Nigeria: implications for groundwater resources sustainability

Water is a natural resource, its availability depends on climatic and geologic conditions, and is invariably controlled by human activities. AgbadoIjaye lies within the coastal area, where local communities have been facing incessant water shortages, especially during the dry season. This study investigated the groundwater-bearing geologic unit(s) using hydrogeophysical techniques in the coastal environment. The electrical resistivity technique, involving Vertical Electrical Sounding (VES) and two-dimensional (2D) electrical resistivity imaging via Wenner array electrode configuration, was used to characterize the geoelectric distribution. Twenty VES stations were investigated, and current electrodes (AB/2 m) spacing expanded from 1-200 m; four 2D tomographies having a length of 200 m each and inter-electrode spacing of 10 m (level 1) to 60 m (level 6) was adopted. Four geoelectric units were delineated namely: topsoil (15-251 Ωm), clayey (28-100 Ωm), clayey sand (125-190 Ωm) and sandy (205-876 Ωm) with thicknesses ranging from 0.7-1.3 m, 4.1-19.0 m, 2.6-15.6 m and undefined depth, respectively. The 2D imaging sections also detected similar geoelectric layers corroborating the VES-derived sections. The inverted sections delineated two different aquifers: the shallower low-yield aquifer comprising sandy clay/clayey sand units with a maximum depth of about 5.5 m. This layer is adjudged to be the continental plain sand of the Benin Formation. The deeper high-yield aquifer with a maximum depth of 30.4 m, is a beach sand unit that belongs to the Tertiary Alluvium of the Dahomey Basin. The study showed that hydrogeophysical investigation is vital in exploring, developing, and managing coastal groundwater resources.