ENVIRONMENTAL ISOTOPE ANDHYDROCHEMICAL INVESTIGATION ONGROUNDWATER RECHARGE AND DYNAMICSOF THE COASTAL SEDIMENTARY AQUIFERS OFTIRUVADANAI, TAMILNADU STATE, INDIA

摘要

Recharge processes and dynamics of the Tiruvadanai aquifers were investigatedusing environmental isotopes and hydrochemistry, in conjuction with hydrogeologicaldata. Hydrochemical characterization of the groundwaters indicated that the shallow(<200 m) Tertiary aquifers (unconfined/semi-confined), lying below the upperQuaternary alluvial deposits, contain no-dominant (Ca–Mg–HCO3 –Cl) to saline typewaters and the deeper (350–500 m) Cretaceous aquifer (confined) is a NaCl type. Theconcentration of various chemical species along the general groundwater flow direction(northwest to east) showed a trend with a decrease in Mg2+ and Ca2+ and an increase inNa + and K + in both the aquifers. This change could be attributed to ion-exchange process.Higher pH values of Cretaceous aquifer samples (7.4–8.6) could also be responsible forthe lowering of Mg2+ and Ca2+ concentrations by facilitating precipitation of carbonates.A δ 2 H–δ18O plot shows that the Tertiary aquifer samples fall on an evaporation line.The aquifer 3 H values near the ephemeral rivers range from 2 to 5 TU while thoseaway from the rivers have <1 TU and 14 CDIC model ages range from 1 to 13 Ka BP.The Cretaceous aquifer samples had 3 H values <1.5 TU and their 14 CDIC model ages are>20 ka BP, indicating palaeo-waters. Based on 14 C model ages, the groundwater velocitywas estimated (Tertiary aquifers: 10 –2 –10 –3 m·d –1 ; Cretaceous aquifer: 10 –3 m·d –1 ). A 13 CDICenrichment along the flowpath of the Cretaceous aquifer was observed and could be