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-> SO42-> CO32-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.