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

SummaryEnvironmental 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.On the δ2H–δ18O plot, the Tertiary aquifers samples fall on an evaporation line. Their 3H values near the ephemeral rivers range from 2 to 5 TU while those away from the rivers have <1 TU and 14CDIC model ages range from 1 to 13 ka BP. The Cretaceous aquifer samples measured 3H values <1.5 TU and their 14CDIC model ages are >20 ka BP, indicating palaeo-waters. Based on 14C model ages, the groundwater velocity was estimated (Tertiary aquifers: 10−2–10−3 m d−1; Cretaceous aquifer: 10−3 m d−1). Dissolution of carbonate bearing minerals in the Tertiary aquifer and oxidation of organic matter in the Cretaceous aquifers seems to be the dominant processes and thus resulting in the corresponding enrichment and depletion in 13CDIC along the flowpaths. The values of δ34S (aq. SO42-) showed enrichment in the Cretaceous aquifer samples compared to that of the Tertiary aquifers samples. The δ34S (aq. SO42-) enrichment could be due to bacterial reduction of sulphate.From the investigation, three types of recharge processes to the aquifer system are discerned, with the overall modern recharge component being low. The Cretaceous aquifer contains fossil groundwaters and hence the resources may be finite and their exploitation is mining. Deeper coastal groundwater isotopic evidence of palaeo-climatic excursions between arid and humid phases during the Holocene and Late Pleistocene in the Southern India has been obtained for the first time.