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.
