Understanding the Seasonal Dynamics of the Groundwater Hydrogeochemistry in National Capital Territory (NCT) of India Through Geochemical Modelling

Abstract

A geochemical assessment of seasonal dynamics in the groundwater chemistry of the National Capital Territory (NCT), Delhi, was attempted through geochemical modelling, mineral precipitation sequences with rainfall and water evaporation cycle. Saturation indices calculated using PHREEQC indicated that the degree of water–rock equilibrium changes significantly from pre-monsoon to post-monsoon. The schematic model of SI change with water table fluctuation showed that during monsoon, as rainwater percolates through the soil, partial pressure of CO2 becomes higher than that of the atmospheric value and led to the formation of more carbonic acid that react with the carbonate minerals to produce HCO 3 , Mg2 ? and Ca2?. The thermodynamic stability relationships of water chemistry in the Na, K, Ca and Mg silicate systems showed that for the samples with higher EC equilibrium between clay and primary minerals is not likely to be the main processes controlling variation in the groundwater chemistry. Chloro-alkaline indices (CAI) are positive when the groundwater level is high and become negative with the lowering of water level, i.e. when water level is high, reverse ion exchange is dominant. In case of pre-monsoon season, lower and negative value of CAI-1 and CAI-2 indicates dominance of ion exchange process and increases dissolved solid concentration in groundwater. The conceptual geochemical model depicted that water table fluctuation resulting from heavy pumping/withdrawal and recharge in association with the variation in DO, HCO 3 and Fe regulates the water–mineral equilibrium. The conceptual geochemical model explained the hydrogeochemical processes and their variations with water table fluctuation and, thus, highlighted the descriptive capabilities of PHREEQC. The study suggested that in the subsurface environment, complex interactions are simultaneously functioning, and hence, significant seasonal variations are likely to be very influential due to monsoonal recharge and subsequent changes in the saturation states of the water.