Sustained upward discharge of relict high-salinity groundwater through salt marsh tidal creeks

Salt marshes are fine-grained ecosystems widely distributed in the intertidal zone along global coastlines. Recent studies proved that submarine groundwater discharge (SGD) in salt marshes exports abundant nutrients and carbon supporting marine productivity and carbon sequestration in the ocean. However, due to our limited knowledge of groundwater discharge processes in these low-permeability systems, the accurate quantification of SGD-derived fluxes remains a great challenge. In a salt marsh system in East China, we found numerous high-salinity springs discharging from the bottom of tidal creeks. To determine their origin and trajectory, multiple field investigation methods including time-series thermal monitoring, isotope signatures and high-resolution electrical resistivity tomography were combined. Results suggest that these springs originate deep from the ancient relict marine water in the aquifer and keep discharging even during high tide. Such process represents a long-term re-distribution of the ancient marine water trapped in the unconfined aquifer. This spring-derived groundwater flow indicates a hidden SGD pathway in salt marshes, which has significant implications for studies concerning SGD-derived fluxes in similar multi-aquifer-aquitard coastal systems. These findings shed new light on the complex SGD processes in low-permeability coastal systems, while future studies are still required to further determine its driving forces and make quantitative assessments.