Investigating submarine groundwater discharge (SGD) along the Eastern Arabian Sea coast using stable isotope tracers

Submarine groundwater discharge (SGD) plays a crucial role in coastal ecosystems by influencing nutrient cycling, water quality, and biological productivity, while also serving as a vital freshwater resource for coastal populations. Hydrogeochemical assessments, particularly isotope tracers, are pivotal in identifying water sources and understanding hydrological processes. However, the applicability of radioactive isotopes is often limited due to their decay over time. Thus, stable isotopes of oxygen (δ¹⁸O) and hydrogen (δ²H) seem promising tracers for identifying and characterizing SGD in coastal regions. Leveraging these stable isotopes, this study focuses on investigating SGD along the Eastern Arabian Sea coastline for effective water management. Groundwater discharge through the coastline was assessed by analyzing groundwater, porewater, and seawater samples for their stable isotopic compositions. In situ measurements of electrical conductivity (EC) were also conducted to differentiate between fresh, brackish, and saline SGD. Results indicate that δ¹⁸O values range from -3.23 to -2.67 ‰ in groundwater and -1.99 to -0.01 ‰ in porewater, while δ²H ranges from -20.21 to -11.36 ‰ and -16.31 to -1.12 ‰, respectively. The analysis confirms the presence of SGD at multiple sites, while few locations exhibit isotopic signatures and EC consistent with sea water (δ¹⁸O: 0.15‰, δ²H: 1.07‰, 43.80 ms/cm), likely influenced by tidal or wave-induced pumping. The SGD zones identified by hydrogeochemical analysis were further validated by sea surface temperature anomalies detected through thermal infrared data along the coastline. The findings of this study will be useful in coastal zone management, coastal urban planning, and mitigating saltwater intrusion risks in coastal aquifers.

Keywords: Stable isotopes, Submarine groundwater discharge, Eastern Arabian Sea