Geochemical investigation of a subterranean estuary influenced by tides on the east coast of India

Submarine groundwater discharge (SGD) is an important transport pathway between land and ocean. The term SGD covers a wide range of processes, compositions and origins, including not only the direct discharge of fresh groundwater, but also diffuse and brackish fluxes through permeable sediments. The mixing zone between fresh groundwater and seawater in the coastal sediments forms a subterranean estuary (STE) where chemical elements undergo biogeochemical transformations before reaching the ocean. However, biogeochemical processes within STEs along tidally influences tropical coastlines, particularly under strong monsoonal rainfall, remain poorly constrained. This study evaluates these processes along tidally influenced section of the Odisha coast, India, which receives annual rainfall of about 1550 mm. Sampling was conducted during the pre-monsoon (May 2024) and post-monsoon (October 2024) seasons. Seawater, groundwater and sediment porewater (down to 125 cm) were collected along intertidal-zone transects parallel to the coastline. Samples were analyzed for the measurement of several parameters including nutrients, major and trace elements and carbon species. The surface seawater and pore waters along the shoreline showed a large difference in salinity values between the two seasons: during the pre-monsoon, salinities reached up to 36 PSU, while after the monsoon, the salinities decreased to a maximum of 30 PSU. During the post-monsoon season, more number and lower salinity spots were detected along the coastline, suggesting that SGD is an important phenomenon in the region causing the formation of an STE. Salinity values were positively correlated with the elevation of the beach, and a seepage line indicating the presence of diffuse SGD was found at about 2m above the sea level. The concentration of nutrients in the different systems suggests that STE plays a role in the transport of nutrients towards the sea. Further studies will continue to better understand the final subterranean element fluxes to the coastal waters of this interesting location.