Evaluation of Submarine Groundwater Discharge and Chemical Fluxes along the Coastal Plains of Odisha, India.

Submarine Groundwater Discharge (SGD) and Seawater Intrusion (SWI) are two opposite components of hydrological cycle that occur across the land-sea continuum and their understanding are imperative for development and management of coastal groundwater resources. This study has attempted to identify the SGD and SWI sites along the water stressed coastal plains of Odisha through a three tier investigation system and quantify the SGD and associated chemical fluxes (nutrients and metals) though seepage metric measurements. A total 340 samples (85 each i.e., 30 porewater, 30 seawater and 25 groundwater in two post-monsoons and two pre-monsoons) were collected and their in-situ physicochemical parameters were measured along ~145 km of the coastline. Considering high groundwater EC values (> 3000 μS/cm), three probable SWI and low porewater salinities (< 32 ppt in pre-monsoons and < 25 ppt in post-monsoons), four probable SGD zones were identified. The high positive hydraulic gradient (> 10 m) near SGD site and negative gradient (< 0 m) near SWI site along with anomalous SST (due to cold/warm groundwater input) at SGD locations validated the identified locations. Lee type seepage meters were installed at SGD identified sites during the low tide period and the measured fluxes ranged from 0 to 4247.973 m³ m^-2 year^-1 in post-monsoon and 0 to 1470.46 m³ m^-2 year^-1 in pre-monsoon. The metal fluxes were found in the order of B > Sr > Li > Ba > Al > As > Fe > Ni > Cu > Co > Mo > Be > Mn with highest flux of B (2962.86 mmol. m^-2 year^-1) at Puri beach which was 7x of Sr-flux and 30x of Li-flux respectively. Among the measured nutrients, DSi fluxes were higher than NO₃^- and PO₄^3- and the highest flux of DSi (849.86 mmol. m^-2 year^-1) was observed at Puri beach. These nutrients and metal fluxes may lead to an increased chance of eutrophication/algal blooms at the SGD-identified locations and influence the sensitive coastal-marine ecosystems of Odisha; therefore, further investigations focusing on biogeochemical species transformation are needed along the fresh-saline interface.

Keywords: Submarine Groundwater Discharge, Seawater Intrusion, Nutrient flux, Metal flux, Seepage meter, Odisha Coast.