Submarine groundwater discharge as sources for dissolved nutrient fluxes in Coleroon river estuary, Bay of Bengal, India

Groundwater contributed nutrients aided with increasing population threaten the global coastal ecosystems. In this study, attempt has been made using major ions and nutrients to evaluate the significance of submarine groundwater discharge (SGD) in a semi-arid estuary of south India. Surface, seepage and groundwater chemistry altered from fresh (NaK-CaMg-NO3Cl) to mixed (NaK-NO3Cl) to saline water (Na-Cl) type from upstream to outlet that connects Bay of Bengal. We predicted abundant nitrate (NO3) along upstream and towards the bay due to application of fertilizers and aquaculture activities, respectively. Elevated ammonium (NH4+) observed in the recirculated groundwater/sea water suggests contribution from sea water intrusion and higher phosphate (PO43-) noted at the outer bay suggests sources from phosphatic nodules. Decreasing Redfield ratio towards the bay suggests anoxic aquifer condition due to salinization. The SGD driven nutrient fluxes were 40.0-47.0% for NO3-, 43.0-51.0% for NH4+ and 9.0-32.0% for PO43- from the total input fluxes. The estimated nutrient fluxes showed that NO3- and PO43- discharges to the sea due to SGD and NH4+ removed from the coast due to consumption by microorganisms that creates toxic algal blooms in the study area.