Submarine groundwater discharge and associated nutrient influx in surroundings of the estuary region at Gulf of Mannar coast, Indian Ocean

The influx of fresh groundwater and re-circulated sea water into coastal ecosystem occurs through the submarine groundwater discharge (SGD). Measurement of salinity, radium tracers (224Ra, and 226Ra isotopes) and nutrients in estuarine water, coastal surface water and groundwater during December 2019 estimated the SGD and associated nutrient fluxes near the Karameniyar estuary (Gulf of Mannar) and surroundings of the Manapad region at southern part of Tamil Nadu state in India. The presence of excessive radium tracers revealed that the SGD was contributing to Ra desorption from the sediments and enrichment in the coastal waters. We estimated SGD of approximately 0.03–0.59 m3 m−2 d−1 for the Manapad region and relatively more homogeneous but comparatively less values in the Karameniyar estuary (0.03–0.34 m3 m−2 d−1). Higher average values of dissolved inorganic nitrogen (DIN; 43.62 μmol L−1) and soluble reactive phosphate (SRP; 1.848 μmol L−1) suggested greater influence of SGD on the overall coastal water nutrient budget. This study also indicated simultaneous occurrence of fresh and saline SGD in this region. [Display omitted] •The radium isotopes-based SGD flux estimations in the Manapad region and Karamaniyar estuary (Gulf of Mannar) were 0.03–0.59 m3 m−2 d−1 and 0.03–0.34 m3 m−2 d−1, respectively.•The radon mass balance model estimated SGD flux for the Manapad region and Karamaniyar estuaries remained 0.02–0.43 m3 m−2 d−1 and 0.04–0.38 m3 m−2 d−1, respectively.•SGD influx was the primary source of dissolved inorganic nitrogen (DIN) into the marine system and the contribution of nutrient by the Karamaniyar River was lower compared to the SGD.•In the near future, a more detailed investigation of seasonal fluctuations in SGD and SGD-derived nutrient fluxes should be conducted.