Monsoon impact on groundwater chemistry and geochemical processes in the shallow hard rock aquifer

Impact of monsoon recharge on groundwater chemistry and geochemical processes was evaluated using geochemical methods in the shallow hard-rock aquifer, southern India. Groundwater samples were collected from Salem city and its outskirts during pre-monsoon (PRM) and post-monsoon (PM) seasons and analysed for major and minor ions. Results suggest that during PRM, the average concentration of TDS, major ions and electrical conductivity (EC) in groundwater are higher than the PM season except for Na+ and HCO3-. The major water Ktypes also varied with seasons (PRM: NaCl > CaMgCl > CaHCO3; PM: CaHCO3 = NaHCO3 = NaCl). Pearson correlation analysis indicates that Cl- and SO42- show a strong positive correlation with EC, TDS, TH and major cations during PRM season whereas HCO3- and NO3- have a strong positive correlation with most of the ions during PM season. Before monsoon, the water chemistry is governed by the weathering of carbonate and silicate minerals, reverse ion exchange and evaporation processes, which resulted in high EC, TDS, TH and major ions in the groundwater. After the monsoon, the water chemistry is regulated by the mineral weathering, cation exchange and dilution processes. Geochemical modelling studies indicate that groundwater is saturated and oversaturated with carbonate minerals and undersaturated with sulphate and chloride minerals. During monsoon, the recharging water flushed the weathered layer and dissolved the soil CO2 in the vadose zone, which subsequently enhanced the concentration of Na+ and HCO3- in the aquifer. Further, the high concentrations of Cl-, SO42- and NO3- imply that the aquifer is polluted by surface contamination sources. After the monsoon, NO3- implies that monsoon recharge facilitated the pollutant movement from surface to the aquifer. Hence, a proper aquifer management plan is required to protect this aquifer and to provide clean and safe drinking water to the local community.