Environmental isotopes investigation on recharge processes and hydrodynamics of the coastal sedimentary aquifers of Tiruvadanai, Tamilnadu State, India

Environmental isotopes (2H, 18O, 3H, 13C, 14C, 34S), in conjunction with hydro-geological and hydro-chemical data, were employed to understand the groundwater recharge processes and hydrodynamics of the Tiruvadanai aquifers in the Tamilnadu State in Southern India. Hydro-chemical characterisation of the groundwater indicated that the Tertiary aquifers (depth: 50–200 m, un-confined/semi-confined/confined), contain no-dominant (Ca–Mg–HCO3–Cl) to brackish (Na–Cl) type of waters and the deeper (350–500 m) Cretaceous aquifer (confined) contains freshwater. The concentration of various chemical species along the general groundwater flow direction (northwest to east) showed a trend with decrease in Mg2+ and Ca2+ and an increase in Na+ and K+ in both Tertiary and Cretaceous aquifers which is also indicated by decreasing saturation index values for dolomite along the flow path. This could be attributed to ion-exchange processes. A higher pH value of Cretaceous aquifer samples (7.4–8.6) could also be responsible for the lowering of Mg2+ and Ca2+ concentrations by facilitating precipitation of carbonates in them.