HYDROGEOCHEMISTRY, DRINKING AND IRRIGATION QUALITY OF GROUNDWATER IN SELECTED TOWNS OF OYO STATE

Abstract

In order to ensure groundwater sustainability, this study evaluated the hydrogeochemistry, drinking and irrigation quality of groundwater from selected towns in Oyo State. Fifty water samples from Oyo, Awe, Akinmorin, Ilora, and Ogbomosho were analyzed for major ions, pH, and Total Dissolved Solids (TDS). The pH values (6.79 ± 0.12 - 6.85 ± 0.08) were near-neutral, reflecting a well-buffered aquifer. TDS values (208.90 ± 89.62 - 295.50 ± 50.37 mg/L) indicated fresh, moderately mineralized water within the limits of the World Health Organization and the Nigerian Standard for Drinking Water Quality. The overall ionic dominance pattern was Mg²⁺>Na⁺>Ca²⁺>K⁺ and HCO₃⁻>SO₄²⁻>Cl⁻. Dietary intake assessment showed that groundwater contributes substantially to magnesium (up to 77% of RDA) but less than 30% for calcium and negligible amounts of sodium, potassium, and chloride. The Water Quality Index (83.94 - 137.93) classified samples as 44% “good” and 56% “poor.” Majority of the irrigation indices (viz. SAR, RSC, KR, PI, CBR/RI, PS, %Na, SSP, BH, MAR/MH, CR) indicated excellent suitability with no sodium and alkalinity hazards but highlighted widespread salinity and localized magnesium and corrosivity risks. Hydrogeochemical facies diagrams confirmed Ca–Mg–HCO₃ dominance with minor anthropogenic sulphate–chloride influence. Principal Component Analysis revealed that TDS (PC1: 87.45% variance) and HCO₃⁻ (PC2: 5.51% variance) were the primary factors controlling water chemistry, driven by silicate mineral weathering, rock-water interaction, and ion-exchange processes. The study concluded that Oyo State groundwater is geogenically controlled, safe after treatment, and largely suitable for irrigation, but continuous monitoring and corrosion management are recommended.