Hydrochemical Characterization and Quality Evaluation of Groundwater in Parts of the Basement Complex Area of Ekiti, Southwestern Nigeria

Well water (88 samples) were collected across various bedrock units in the basement terrain of Ekiti area, Southwestern Nigeria. They were subjected to in-situ physico-chemical measurement and hydrochemical analyses using ICP-OES and ion-chromatography methods for cations and anions, respectively. To understand the water quality and utilisation aspects of groundwater, chemical indices like sodium %, sodium adsorption ratio (SAR), Wilcox diagram and salinity diagram were constructed based on the analytical results. The results show pH values ranging between 6.0-7.8 and total hardness (TH) 3.2-508.7 mg/L. Major cations concentrations were in the order of [Ca.sup.2+] > [K.sup.+] > [Na.sup.+] > [Mg.sup.2+] with average values of 28.5, 26.8, 24.2 and 7.9 mg/L, respectively while that of the anions were HC[O.sub.3.sup.-] > [Cl.sup.-] > S[O.sub.4.sup.2-] > N[O.sub.3.sup.-] with average values of 118.7, 54.2, 23.8 and 0.92 mg/L. The main hydrochemical facies being Ca-HC[O.sub.3] waters. The ionic orders of abundance varied in different rocks of the study area. These concentration trends show a low total dissolved solids (130-1544 µS/cm) indicating a low water-rock interaction due to low residence time which is an indication of CO2 dominated infiltration recharge with limited migratory history typical of the shallow basement terrain in the study area. Quality assessment revealed a potable groundwater system with chemical parameters within the acceptable limits of the WHO and SON drinking water standards with exception of Fe, Mn and Pb in a couple of locations. Also, the estimated SAR alongside TDS revealed a shallow groundwater system suitable for irrigation purposes. Keywords: basement, sodium adsorption ratio, salinity, water-rock interaction, irrigation