Unravelling SGD pathways in karst aquifer at Uvari coast of India using integrated geochemical and isotopic tracers.

Submarine groundwater discharge (SGD) is a significant pathway for the transfer of freshwater and dissolved constituents to coastal systems, particularly in karst-influenced environments. This study investigates SGD dynamics and associated nutrient fluxes along the Uvari coast, southern part of Indian Peninsula, by combining radon (222Rn) tracer with hydrochemical parameters, nutrient analyses and statistical approaches. Groundwater, porewater and seawater samples were collected during post- and pre-monsoon seasons showed spatial and temporal variabilities in hydrochemical characteristics. Groundwater exhibited low electrical conductivity (EC) and salinity but high radon and nutrients, whereas seawater shows opposite trends. Intermediate characteristics in porewater indicated active mixing within a subterranean estuary. A strong inverse relationship between radon and salinity (r ≈ -0.86), supported by ANOVA results (p < 0.05), confirms radon and EC as reliable indicators of groundwater-seawater interaction. The radon mass balance model estimates SGD fluxes ranging from 4.96- 8.60 cm day-1 (post-monsoon) and 4.80-7.30 cm day-1 (pre-monsoon) reflected clear seasonal variability. Elevated dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) and their strong positive correlation with radon (r ≈ 0.89) indicated the critical role of karst-controlled SGD as a major nutrient source for the coastal water.