Hydrochemistry and environmental isotope signatures (δ18O and δ2H) in the crystalline basement groundwater system of Oye-Ekiti, southwestern Nigeria

Groundwater is the primary source of domestic water supply in Oye-Ekiti, a rapidly urbanizing city in southwestern Nigeria. However, the processes controlling its quality and recharge remain poorly constrained. This study presents the first city-scale, dry-season assessment integrating hydrochemistry, stable isotopes (δ 18 O and δ 2 H), multivariate statistics, and geochemical modelling. Fifty samples collected from boreholes and hand-dug wells were analysed for physicochemical parameters, major ions, and environmental isotopes. Groundwater exhibits Ca–Mg–HCO 3 and Na–K–Cl–SO 4 as the primary water types, under acidic to slightly alkaline pH conditions, while total dissolved solids are generally below 500 mg/L. Ionic relationships and multivariate analyses identify silicate weathering and cation-exchange processes as the principal controls on groundwater chemistry, with localized nitrate and phosphate enrichment linked to agricultural activities and on-site sanitation. PHREEQC modelling revealed that all mineral phases, including calcite, dolomite, gypsum, and halite, are undersaturated, confirming active rock-water interaction. Stable isotope compositions (δ 18 O: − 3.53‰ to − 2.68‰; δ 2 H: − 17.65‰ to − 11.96‰) and the δ 18 O–δ 2 H groundwater regression line plots close to the regional Local Meteoric Water Line (LMWL) [δ 2 H = 7.7 × δ 18 O + 10.2, r 2 = 0.87], indicating that the groundwater is predominantly of meteoric origin and derived from recent precipitation. The d-excess values (8.7–12.2‰), clustering around the global average of 10‰, suggest limited evaporative influence prior to recharge. Slight isotopic enrichment and higher solute variability in shallow hand-dug wells highlight their greater vulnerability to surface processes. These findings provide a new geochemical and isotopic baseline, providing critical insights for sustainable groundwater management, monitoring, and policy implementation under Sustainable Development Goals (SDG) 6 in a rapidly urbanizing environment.