Use of hydrochemical and isotopic signatures to trace hydrological connectivity in Northern Patagonian marshes.

Marshes, which are intertidal ecosystems mainly distributed in mid- and high-latitudes, exhibit complex hydrological connectivity between tidal and continental waters. While this connectivity has been more widely studied on beaches, their understanding in marshes, especially in arid and semiarid regions, is limited. On the northern coast of Argentine Patagonia, in Bahía San Blas, there are semiarid marshes adjacent to beach ridges containing freshwater lenses that are being affected by population growth. Given this context, the study aims to identify, through hydrochemical and isotopic signatures, the hydrological connectivity of marshes in Bahía San Blas area in relation to the inputs received from tidal flows and from the discharge of fresh groundwater from adjacent beach ridges. The study, conducted using multiple geochemical tools such as analysis of major ions, soluble nutrients, stable (δH y δO) and radioactive (Rn) isotopes, allowed to identify the different water inputs that support the hydrology of the marshes. This research revealed that the hydrology of the marshes is determined by the interaction between tidal inflow and the discharge of fresh groundwater from adjacent beach ridges. Although the water mixing occurring in the marsh is difficult to detect using major ions due to processes such as salt concentration from evaporation, the use of stable and radioactive isotopes enabled the identification and quantification of fresh groundwater inputs reaching up to 65 %. Groundwater discharge also supplies nutrients from beach ridges into the marsh. The results highlight the usefulness of isotopic tracers and the importance of considering groundwater discharge in arid marshes, which is often underestimated. Furthermore, the study warns that the exploitation of freshwater lenses and urban expansion could affect both the hydrodynamics and hydrochemistry of marsh groundwater, underscoring the need for integrated management and conservation strategies in light of climate change and anthropogenic pressure.