Irrigation return flow induced mineral weathering and ion exchange reactions in the aquifer, Luvuvhu catchment, South Africa

Abstract

Impact of irrigation return flow induced mineral weathering and ion exchange reactions in the groundwater was studied in Limpopo region, South Africa. Based on EC (>500) and HCO3 (>150 mg/l), groundwater samples are classified into two groups due to its heterogeneous nature. Water types in group 1 and 2 wells are Ca-Mg-Cl-SO4 and Ca-Mg-HCO3, respectively. Gibbs plots, bivariate plots and ionic ratios (Mg/Na vs Ca/Na; HCO3/Na vs Ca/Na; mCa + Mg vs mHCO3; mNa + K vs mHCO3; Ca + Mg-HCO3-SO4 vs Na + K-Cl; mCa + Mg/HCO3 vs Cl; Ca + Mg/SO4 +HCO3 >1; Na/Cl ratio; Cl + SO4 vs HCO3), chloro alkaline indices (CAI, CA2) and Pearson correlation analysis imply that groundwater chemistry is influenced by mineral weathering (carbonate > silicate minerals), reverse ion exchange and irrigation return flow. In group 1 wells, Cl + SO4 and HCO3 have strong positive correlation and both increases together, suggesting induced mineral weathering caused by nitrification and wastewater infiltration from the surface. Group 2 wells are generally deep with low groundwater level fluctuation and high concentration of major ions. Overall, water chemistry in the group 1 wells is highly affected by the contamination sources than group 2 wells.