Geochemical evolution and the processes controlling groundwater chemistry using ionic ratios, geochemical modelling and chemometric analysis in uMhlathuze catchment, KwaZulu-Natal, South Africa

The sources of chemical constituents of groundwater and its associated hydrogeochemical processes in the part of Mhlathuze catchment was identified. Groundwater of the area is classified into soft to very hard and the nature is identified as acidic to alkaline. The overall electrical conductivity is < 3000 mu S/cm except in three wells. The predominant water type is NaCl (69% of samples) and CaMgCl facies. Gibbs plots, mCa/Mg ratio, mNa/Cl ratio, Ca + Mg vs HCO3+SO4 plot, Na + K vs HCO3 plot, Ca/Na vs HCO3/Na, Chloroalkaline indices (CAI 1, CAI 2) and Ca + Mg-HCO3-SO4 vs Na + K-Cl plots confirm the impact of silicate, carbonate mineral weathering and ion exchange reaction in this aquifer. However, few wells are influenced by the evaporation process. Groundwater is highly undersaturated with sulphate, chloride minerals and saturated with carbonate minerals. CA revealed that Cl and SO4 are derived from anthropogenic sources and a significant positive correlation between HCO3 and Cl reveals that wastewater recharge has most likely simulated the mineral weathering in the vadose zone, which could have further enhanced HCO3 and Cl in the aquifer. PCA resulted in three factors. Factor 1 defines the influence of geogenic and anthropogenic processes while Factors 2 and 3 imply the mineral weathering and nitrification processes. Hierarchical cluster analysis defines that evaporation, anthropogenic input, silicate and carbonate weathering and nitrification process are the sources of chemical constituents of groundwater in this aquifer.

Automated summary

The sources of chemical constituents and hydrogeochemical processes of groundwater in the Mhlathuze catchment were identified. The groundwater in the area ranges from soft to very hard, with acidity ranging from acidic to alkaline. The dominant water type is NaCl, with CaMgCl facies. Various plots and analysis techniques confirmed the impact of silicate and carbonate mineral weathering, ion exchange reactions, and anthropogenic sources on the groundwater. Some wells were also influenced by evaporation. Groundwater is undersaturated with sulphate and chloride minerals and saturated with carbonate minerals. The study found evidence of mineral weathering and nitrification processes. Hierarchical cluster analysis indicated that evaporation, anthropogenic input, silicate and carbonate weathering, and nitrification are the sources of chemical constituents in the groundwater.