Understanding biogeochemical dynamics and transformation processes in a Subterranean Estuary using Nitrogen Isotopes, hydrogeochemical parameters, and statistical analysis

Subterranean Estuaries (STEs) are very important for the discharge of nutrients and contaminants to the coastal ocean, which affects the quality of coastal environments. It is difficult to understand compounds sources and transformation processes because of the many reactions along the groundwater flow paths of the STE. The characterization of the magnitude and composition of these groundwater discharges (SGD) are strongly depended on the biogeochemical processes occurring within the STE. Most of the research have focused on using single methods to assess nutrient transformations in STE, therefore our study explores the integration of different approaches to identify the main nutrient sources and transformation dynamics applying a multidisciplinary methodology. Since 2014, an experimental site has been installed and explored to study coastal groundwater in the alluvial aquifer of Argentona, in the northeast of Barcelona, Catalonia (Spain). The area is monitored by twenty-five piezometers, distributed with five nests, each with four piezometers spaced along multiple depths and equipped with sensors. During two sampling campaigns (winter and summer), different physicochemical properties were measured in all piezometers. In this research, an approach is applied combining end-member analysis of hydrogeochemical data, N-isotopes analysis (¿15NNO3-, ¿18ONO3-, and ¿15NNH4+) and statistical analysis (Principal Component Analysis). The results from the end-member analysis shows that the aquifer zones could be defined according to redox conditions and the mixing of saline and fresh water. The N-isotopes analysis place potential sources of N-compounds such as nitrates (NO3-), ammonium (NH4+), and the dominant biogeochemical transformations of nitrates. Dissolved organic matter (DOM) contributed to identifying the biogeochemical processes that take place, and the bacteria concentration revealed different microbial communities associated with these transformation processes.