Origin of major dissolved ions in groundwater within the Lower Pra Basin using groundwater geochemistry, source-rock deduction and stable isotopes of ²H and ¹⁸O

Hydrochemical and stable isotopes (¹⁸O and ²H) analyses of groundwater samples were employed to establish the origin of major dissolved ions in groundwater within the Lower Pra Basin. Results showed that, the major processes responsible for chemical evolution of groundwater include: silicate (SiO₄)⁴⁻ dissolutions, ion exchange reactions, sea aerosol spray and pyrite (FeS₂) and arsenopyrite (FeAsS) oxidations. The groundwater is strongly acidic to neutral, with pH generally range from 3.5 to 7.0 pH units and mean 5.9 (±0.5). Approximately 89 % of boreholes had pH values outside the World Heath Organization (WHO, Guidelines for drinking water quality, 2004) guideline value for drinking water due principally to natural biogeochemical processes and therefore, not suitable for potable purposes. Electrical conductivity (EC) range from 57.6 to 1,201 μS/cm with mean 279.3 (±198.8) μS/cm. Total dissolved solids (TDS) range from 32 to 661 mg/L with mean 151.7 (±106.8) mg/L, with 98.6 % of groundwater as fresh (TDS < 500 mg/L). The chemical constituents generally have low concentrations and are within the WHO (Guidelines for drinking water quality, 2004) guideline value for drinking water. The relative abundance of cations and anions is in the order: Na⁺ > Ca²⁺> Mg²⁺ > K⁺ and HCO₃ ⁻ > Cl⁻ > SO₄ ²⁻, respectively. A plot of ∂¹⁸O ‰ against ²H ‰ showed that, ground and surface waters clustered on or closely along the Global Meteoric Water Line, suggesting that, the waters emanated principally from meteoric source with evaporation playing an insignificant role on the infiltrating water.