Identifying controlling factors for the evolution of shallow groundwater chemistry of Ellala catchment, northern Ethiopia

This study was designed to deal with the identification of the hydrogeochemical and anthropogenic processes controlling the evaluation of groundwater chemistry in the Ellala catchment covering about 296.5 km² areal extent. The chemical analysis revealed that the major ions in the groundwater are Ca²⁺, Mg²⁺, Na⁺, and K⁺ (cations) and HCO₃⁻, PO₄³⁻, Cl⁻, NO₃⁻ and SO₄²⁻(anions), and most of the groundwater samples (68.42%), revealed that the groundwater in the catchment is non- alkaline in nature, and the solid materials and liquid wastes discharged from different sources could be the main sources for pH and EC in the groundwater in addition to the aquifer material contribution. It is observed that the EC of the groundwater is fairly correlated with the DTS which indicates that highly mineralized water is more conductor than water with low concentration. The degree of salinity of the groundwater is increased along the groundwater flow path from east to west and is high surrounding Mekelle City due to the liquid and solid wastes discharged from the city and the industries. The groundwater facies in the catchment predominated with calcium, magnesium, and bicarbonate and the groundwater type is mainly Ca–Mg–HCO3 and Mg–Ca–HCO3. The main geochemical processes controlling the evolution of the groundwater chemistry in the catchment are rock–water interaction, particularly carbonate dissolution and reverse ion exchange due to the clay layer in the aquifer. Non-significant silicate weathering and halite dissolution also contributed to the evolution of groundwater chemistry in the catchment. The groundwater in the catchment is dominated by the meteoritic origin although it needs further groundwater chemistry study with isotope dating analysis. The groundwater is in an undersaturated state with the calcite, dolomite, and aragonite minerals where the further dissolution of these minerals is going in the groundwater. The main source of calcium and magnesium in groundwater is the dissolution of carbonate minerals (calcite and dolomite) since carbonate rocks are the dominant aquifer materials in the catchment. In addition to this, the weathering of dolerite rock is also a possible source of magnesium ions. The relatively higher concentration of sodium over chloride indicates the source for sodium ion is reverse ion exchange and/or weathering of sodium-bearing materials (such as shale and dolerite) rather than halite dissolution. The high concentration of phosphate, nitrate, and chloride in the groundwater is the main anthropogenic source that needs treatment and groundwater quality control and management in the catchment. From the Base Exchange index analysis, it is noticed that the groundwater in the catchment is dominated by the meteoritic origin, although it needs further groundwater chemistry study with isotope dating analysis.