Groundwater solute chemistry, hydrogeochemical processes and fluoride contamination in phreatic aquifer of Odisha, India

The work investigates the major solute chemistry of groundwater and fluoride enrichment (F^-) in the shallow phreatic aquifer of Odisha. The study also interprets the hydrogeochemical processes of solute acquisition and the genetic behavior of groundwater F^- contamination. A total of 1105 groundwater samples collected from across the state from different hydro-geomorphic settings have been analyzed for the major solutes and F^- content. Groundwater is alkaline in nature (range of pH:6.6-8.7; ave.:7.9) predominated by moderately hard to very hard types. Average cation and anion chemistry stand in the orders of Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ and HCO₃^- > Cl^- > SO₄^2- > CO₃^2- respectively. The average mineralization is low (319 mg/L). The primary water types are Ca-Mg-HCO₃ and Ca-Mg-Cl-HCO₃, followed by Na-Cl, Ca-Mg-Cl, and Na-Ca-Mg-HCO₃-Cl. Silicate-halite dissolution and reverse ion exchange are the significant processes of solute acquisition.Both the geogenic as well as the anthropogenic sources contribute to the groundwater fluoride contamination, etc. The ratio of Na⁺/Ca²⁺ > 1.0 comprises Na-HCO₃ (Cl) water types with F^- > 1.0 mg/L (range 1.0-3.5 mg/L) where the F^- bears geogenic source. Positive relations exist between F^- and pH, Na⁺, TDS, and HCO₃^-. It also reflects a perfect Na-TDS correlation (0.85). The ratio of Na⁺/Ca²⁺ < 1.0 segregates the sample population (F^- range:1.0-4.0 mg/L) with the F derived from anthropogenic sources. Such water types include Ca-Mg-HCO₃ (Cl) varieties which are recently recharged meteoritic water types. The F^- levels exhibit poor and negative correlations with the solutes in groundwater. The Na-TDS relation remains poor (0.12). In contrast, the TDS levels show strong correlations with Ca²⁺ (0.91), Mg²⁺ (0.80) and even Cl^- (0.91). The majority of the monitoring points with the anthropogenic sources of groundwater F^- are clustered in the Hirakud Canal Command area in the western parts of the state, indicating the role of irrigation return flow in the F^- contamination.