Exploring temporal and spatial dynamics of fresh submarine groundwater discharge: amphibious electrical resistivity tomography in the land-ocean transition zone

Submarine groundwater discharge (SGD) is a complex hydrological process influenced by multiple mechanisms with various spatiotemporal scales. Quantifying SGD is complex because it depends on various driving forces (e.g., waves, tides, hydraulic gradient) and on the underlying geological context that modulate the occurrence and magnitude of SGD. This poses challenges to the accurate quantification of SGD, especially in heterogeneous nearshore environments. However, there is still limited understanding on SGD spatiotemporal patters hindering the accurate assessment of SGD implications for the coastal ocean.The aim of this study is to use amphibious electrical resistivity tomography (AERT), which combines terrestrial and marine instruments, to assess spatiotemporal patterns of FSGD (Fresh Submarine Groundwater Discharge) in the land-sea interface. This cost-effective method enables prospecting SGD dynamics at the land-sea transition zone based on the resistivity variations in the subsurface induced by salinity changes. The resistivity study was conducted regularly to assess hourly spatiotemporal variations in two aquifers near Barcelona, Spain, with different geological contexts, including detrital and karst formations. The resistivity data in both study sites have been validated with in situ pore water sampling for physicochemical parameters, with a good agreement between resistivity and subsurface salinity. The time-lapse results indicate that even in micro-tidal environments such as the Mediterranean Sea, SGD patterns are highly dynamic, with the karst environment exhibiting a more significant proportion of freshwater in the marine sediments and faster changes than the alluvial context. This methodology proves effective for the spatial and temporal assessment of FSGD. These findings offer a valuable approach for monitoring subsurface salinity changes in coastal aquifers and enhance our understanding of small-scale and short-term SGD variations, which is fundamental to deriving reliable SGD and nutrient flux estimates.