<p>The stable isotopes of oxygen and deuterium are usually referred to as the “DNA” of water molecules because of their non-interaction with aquifer materials. These isotopes, when combined with the ionic content of water, become powerful tools to decipher the mixing between water masses of different salinities, thus enabling the tracing of water origin, determining the source of pollution and usability. Stable isotope and physicochemical data from sixty representative groundwater samples have been used to evaluate the origin, quality, usability of groundwater, and the aquifer recharge-discharge system in the basement complex of Ilorin, north-central Nigeria. Results revealed that the stable isotope values in the groundwater samples vary from −&#xa0;7.27 ± 0.35 ‰ to −&#xa0;2.45 ± 0.5 ‰, and from −&#xa0;32.0 ± 2.7 to −&#xa0;16.9 ± 3.1 ‰ for δ¹⁸O and δ²H, respectively, indicating mixing of groundwater from both isotopically enriched and depleted meteoric origins. The cross-plot of δ¹⁸O and d-values shows a distribution pattern that varies from −&#xa0;0.31 to 26.22 ‰ with a mean value of 15.55 ‰, and indicates local and regional moisture circulation and evaporative enrichment as the source of aquifer recharge in the study area. A joint interpretation of EC and δ¹⁸O of the groundwater samples for resident time suggests recent recharge, recent recharge but polluted, and recent/paleo recharge. The enriched stable isotope content was correlated with high EC and TDS, indicating an increase in salinity. The cations and anions abundance are in the order of Na⁺ &gt; Mg²⁺ &gt; Ca²⁺ &gt; K⁺ and Cl &gt; HCO₃ &gt; SO_4, respectively. The groundwater is essentially Na–Cl and Na–Mg–HCO₃ types. The information derived from the correlation of stable isotopes and hydrochemistry helped in identifying an evolutionary trend in the water chemistry. Comparisons of the measured physicochemical parameters of the groundwater with the WHO and NSDWQ confirmed that the water is suitable for human consumption and irrigation purposes.</p>