Electrostratigraphy and hydrogeochemistry of hyporheic zone and water-bearing caches in the littoral shorefront of Akwa Ibom State University, Southern Nigeria

Ground-based electrical geophysical data calibrated with borehole information are conveniently used to delineate subsurface strata because of their inherent capability to assess the lateral and vertical variations in the pore water. In this study, joined geophysical approach of vertical electrical sounding (VES) and electrical resistivity tomography (ERT) has been steered to define the strata in the hyporheic zone and in the water bearing caches in the Akwa Ibom State University’s littoral shorefront. Four ERTs with each using Wenner array with 5 m electrode spacing were conducted along four profiles at the same locations that VES were conducted. Twelve surface VES soundings were performed with maximum current electrode separations of (AB/2 = 150 m). The integration of formation resistivity with six boreholes reveals motley topsoil/dry strata with resistivity value greater than 200 Ω - m above water table; saturated clay/saline water depository with resistivity value less than 30 Ω - m below water table; fine-grained sand/brackish water depository with resistivity range spanning between 70 and 200 Ω - m below water table; medium-grained sand/freshwater depository with resistivity ranging from 500 to 800 Ω - m below water table and gravelly sand/freshwater depository with resistivity value greater 800 Ω - m below water table were inferred from top to bottom within the maximum current electrode separations. These ranges of resistivity show lithological diversity in subsurface layer. Geochemical analysis was performed for main cations (magnesium, sodium, potassium, calcium, iron and manganese), anions (bicarbonates, sulphates, chloride, and fluoride) and other physical parameters such as, pH, electrical conductivity, total dissolved solids, dissolved oxygen, biochemical oxygen demand and chemical oxygen demand. The results of the interpretation of hydrochemical species of the groundwater samples revealed that the groundwater in most locations within the study area is fresh, slightly alkaline to acidic based on the EC, pH and TDS values. The order of abundance for anions and cations is HCO3– > Cl– > SO42– > F– and Na+ > Ca2+ > Mg2+ > K+ > Fe2+ > Mn2+ respectively. The observation of elevated BOD with lower DO even in the muddy area suggests anoxic condition (DO 5 mg/L), based on the measured DO values (00.12–2.61 mg/L). The elevated ferric iron concentrations on the surface water, which later seeps into the groundwater systems, are due to excessive accumulation of dissolved organic matter (DOM) and the consequent reduction reaction within the DOM in surface water.