Hydrochemical characterization of groundwater in Lokoja, North-Central Nigeria

In the face of inadequate public pipe borne water supply in Lokoja (North-Central Nigeria) and its environs, groundwater through hand dug wells has become a reliable source of water for many households. However, due to poor sitting, shallow depth of wells and poor sanitation practices, there is high risk of contamination that warrants groundwater monitoring and assessment. Hence, this study focuses on hydrochemical evaluation of groundwater in Lokoja to determine its suitability for drinking and irrigation purposes. The study involved in-situ measurements of physical parameters (temperature, pH, EC and TDS) at each sampling location using potable meter. Subsequently, 65 groundwater samples were collected in pre-cleaned polyethylene bottles and transported to the laboratory where cations (Ca²⁺, Mg²⁺, Na⁺ and K⁺) were analyzed using ICP-MS and ICP-OES, anions (Cl⁻, NO₃⁻ and HCO₃⁻) employing colorimetry method and turbimetry method for SO₄²⁻. In-situ measurements revealed that the temperature range from 25.3 to 33.7 °C (av. 30.2 °C) while pH is from 5.5 to 8.5 (av. 7.4) mostly indicating neutral and slightly acidic to alkaline water in some locations. EC and TDS vary from 30 to 860 µS/cm (av. 470.6 µS/cm) and 31–962 mg/L (av. 379.8 mg/L), respectively, an indication of low mineralized water with limited migration history. Major cations concentrations were in the order of Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ with average values of 49.91, 37.69, 14.12 and 12.92 mg/L, respectively, while that of the major anions were HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻ with av. values of 87.03, 41.59, 22.99 and 17.44 mg/L, respectively. The Water Quality Index (WQI) estimate showed that 56.9% of the samples fell in excellent (WQI = 0–25), 35.4% in good (26–50) and 7.7% in poor (51–70) water class. Evaluations based on sodium percentage (%Na) indicated that the water fell mostly within ‘good’ and ‘permissible’ categories, and Sodium Absorption Ratio (SAR) suggested mostly ‘medium’ category. Hydrochemical characterization revealed three dominant water types, which were Ca²⁺–(Mg²⁺)–HCO₃⁻, Ca²⁺–(Mg²⁺)–Cl⁻–SO₄²⁻ and Na⁺–(K⁺)–Cl⁻–SO₄²⁻ and are generally controlled by precipitation, weathering and dissolution of bedrocks. Conclusively, the groundwater in the study area is of potable quality and generally suitable for irrigation purposes due to low to moderate mineralisation and little influence of anthropogenic contamination.