Recharge environment and hydrogeochemical processes of groundwater in a crystalline aquifer in South India

A climatic water balance study, along with an analysis for stable isotopes (oxygen-18 and hydrogen-2), and hydrogeochemical signatures were together used to understand the groundwater recharge environment and hydrogeochemical processes in the semi-arid region of Gooty Mandal, District of Anantapur, India. The study revealed that the annual potential evapotranspiration was thrice the annual rainfall, indicating the influence of evaporation both during rainfall and after infiltration resulting in the enrichment of salts in the soil and the stable isotopes in shallow groundwater. Stable isotopic signatures suggest that the source of groundwater recharge is contributed by several components and not just direct precipitation. The argument is further supported by the relation between oxygen-18 and chloride. The most dominated water type in the dry season was found to be sodium-chloride followed by calcium-sodium-bicarbonate, while in the wet season calcium-sodium-bicarbonate was followed by sodium-chloride. Calcium-magnesium-chloride and sodium-chloride showed high temporal variability in stable isotopic concentrations, particularly in wells with shallow groundwater, suggesting that the rainwater might have undergone evaporation before recharge and/or recirculation due to agricultural activities. Hydrogeochemical signatures show that 61% of the samples sodium concentration decreased with the increase in chloride ions, revealing that the cation exchange reaction absorbs sodium in exchange for magnesium and calcium. Almost all samples from the calcium-sodium-bicarbonate, sodium-bicarbonate and magnesium-bicarbonate water types and a few samples of sodium-chloride and calcium-magnesium-chloride water types had a low chloride concentration (< 10 mmol/l), implying that the silicate weathering is also a major source through which these ions get into the groundwater.