Tracking biogeochemical processes in a Subterranean Estuary (STE): Application of a multidisciplinary approach integrating isotopes, hydrogeochemistry, and dissolved organic matter (DOM) 

Subterranean Estuaries (STEs) have been recognized for their role in the transport and fate of chemical compounds that discharge to the coastal ocean. The enrichment of coastal groundwater with nutrients is affected by different sources and mechanisms. Moreover, the distribution of these substances discharging to the sea is highly affected by the reactions produced at the mixing zone between the fresh and saline groundwater. In this research, we aim to identify the nutrient sources and biogeochemical processes that are actively playing a role in the subterranean estuary located in the alluvial aquifer of Argentona, in the northeast of Barcelona, Catalonia (Spain). Coastal groundwater in the area has been continuously explored since 2014 with the development of a unique experimental site. The site is 100m long inland from the coastline and 30m wide. It is being monitored with 25 piezometers consisting of 5 nests with 4 piezometers each (with intervals at 10m, 20m, 15m, and 25m) and 4 individual piezometers, equipped with different sensors that collect data every 15 minutes. This study integrates various approaches such as the N-isotopes (δ15N-NO3-, δ18O-NO3-, δ15N-NH4+), hydrogeochemistry, dissolved organic matter (DOM), and bacteria concentration that has been measured in all piezometers during two sampling campaigns (winter and summer). The results show potential sources of ammonium and nitrate and the biogeochemical transformations that have a main role in the subterranean estuary dynamic. Acknowledgments This work was funded by the Spanish Government under the project MUCHOGUSTO (grant no. PID2022-140862OB-C21/C22) and the SENACYT – BID Scholarship by the Panamanian Government.