Hydrochemistry and stable isotopes (¹⁸O and ²H) characteristics of groundwater in Lokoja and its environs, central Nigeria

To determine hydrochemical and stable isotope characteristics of groundwater in Lokoja and its environs, 39 samples were collected, analyzed and interpreted. The results of the analyses revealed that pH of the water samples of the area ranged from 5.9 to 8.4 indicating neutral water except few slightly acidic samples. TDS varied between 44 and 1273 mg/L but were generally < 1000 mg/L suggesting fresh water and total hardness of 1.2–360 mg/L classified the water ranging from ‘soft’ to ‘very hard water’. The concentrations of the major cations are in the order of Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ with average values of 48.5 mg/L, 29.4 mg/L, 15.3 mg/L and 13.0 mg/L, respectively; while that of anions are HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻ with average values of 143.3 mg/L, 59.5 mg/L, 35.8 mg/L and 18.3 mg/L, respectively. Groundwater is generally suitable for drinking purposes as most samples fell within the “excellent water” and “good water” categories, while the quality for irrigation ranged from suitable to unsuitable. Five hydrochemical facies were determined: CaHCO₃, mixed CaMgCl, NaCl, CaNaHCO₃ and NaHCO₃ water types. Several processes such as reverse ion exchange, weathering of silicate minerals and rock–water interactions were main mechanisms controlling ionic constituents of groundwater of the study area. In addition, the results of stable isotopes analyses showed considerable variations, ranging between − 3.9 and − 1.2% with an average of − 2.7% for δ¹⁸O and − 23.9% and 11.7% with a mean of 16.9% for δ²H. The plot of δ²H vs δ¹⁸O of groundwater system in the study area along global meteoritic water line indicated recent meteoritic recharge. However, all of the samples plotted deviated appreciable from global MWL except a sample from sandstone bedrock that was plotted along the line as defined by this equation: δ D = 8 δ ¹⁸O + 10. The deviation was a consequence of kinetic evaporative effect as well as rock–water interaction and carbondioxide exchange effects.