Isotopic signatures to address the groundwater recharge in coastal aquifers

The dynamics of the coastal aquifers are well-expressed by geochemical and isotopic signatures. Coastal regions often exhibit complex groundwater recharge pattern due to the influence of depression in the Bay of Bengal, tidal variations on surface waters, saline water intrusion and agricultural return flows. In this research, groundwater recharge processes occurring in coastal Tamil Nadu, South India were evaluated using major ion chemistry and environmental isotopes. A total of 170 groundwater samples were collected from shallow and deep aquifers during both post-monsoon (POM) and pre-monsoon (PRM) seasons. The isotopic results showed a wide variation in the shallow groundwater, suggesting contribution from multiple recharge sources. But, the deeper groundwater recharge is mainly from precipitation. The northern part of the study area showed more depleted isotopic values, which rapidly changed towards south from -6.8 to -4.4%o. Alternatively, central and southern parts exhibited relatively enriched isotopic content with variation from -0.58 to -2.7%o. Groundwater was discerned to be brackish to saline with chloride content, 600-2060 mgL-1 and delta 18O ranging from -5.8 to -4.5%o, suggesting influence of the saline water sources. A minor influence of anthropogenic activities was also observed in the deeper groundwater during PRM, which was confirmed by tritium and Cl- trends. The old groundwater with depleted isotopic content infer recharged by distant sources while modern groundwater with enriched isotopes points to the influence of evaporated recharge.

Automated summary

The dynamics of coastal aquifers can be understood through geochemical and isotopic signatures. In this study, groundwater recharge processes in coastal Tamil Nadu, South India were evaluated using major ion chemistry and environmental isotopes. The results showed that shallow groundwater had contributions from multiple recharge sources, while deeper groundwater was mainly recharged by precipitation. The isotopic variations in different areas indicated different sources and influences. The groundwater in the study area was found to be brackish to saline, with minor influence from anthropogenic activities.