Hydrochemical and stable isotopic characterization of shallow groundwater system in the crystalline basement terrain of Ekiti area, southwestern Nigeria

Abstract

Assessments of hydrochemical quality and recharge source are critical components of sustainable groundwater resources evaluation and management, especially in complex weathered/fractured crystalline bedrock terrains. Assessment of the weathered/fractured basement aquifer of Ekiti area, SW-Nigeria was undertaken with emphasis on hydrochemical quality and stable isotope characterization of shallow groundwater system. The study approach involved field sampling and in situ measurements of physico-chemical parameters followed by hydrochemical and stable isotope analyses of the water samples. In situ measurements revealed EC value of 43–995 μS/cm (av 431.96 μS/cm) and total hardness of 31.3–295.1 mg/L. The pH of the water sample ranged from 7.0 to 8.9 (av 8.00). The concentrations of the major cations are in the order of Ca2+ > K+ > Na+ > Mg2+ with their respective average values of 33.35, 25.61, 24.91 and 8.7 mg/L respectively while that of the anions are in the order of HCO3− > Cl− > SO42− > NO3− with their respective average values of 165.44, 31.85, 21.27 and 5.44 mg/L respectively. However, the results of stable isotope analyses revealed δ18O value of −4.2 to −2.1 ‰ and δD of −22.3 to −10.2 ‰, suggesting meteoric water source (recent precipitation water recharging the associated shallow basement aquifer) while the estimated deuterium excess of 6.80–11.00 ‰ (av 9.6 ‰) suggested little or no imprint of kinetic evaporation. Furthermore, the overall hydrochemical profiles revealed the main hydrochemical facies as Ca(Mg)-HCO3 (dominant) and Na(K)-HCO3 (minor) water types which can be attributed to dissolution of minerals in the various rock units in the study area. Conclusively, the overall hydrochemical and stable isotope evaluations revealed a recharge predominantly derived from meteoric (rainfall) source while the corresponding low TDS is a reflection of low water–rock interaction and limited migratory history of the shallow groundwater system in the study area.