Assessing nearshore submarine groundwater discharge dynamics in contrasting geological settings using amphibious electric resistivity tomography
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Submarine groundwater discharge (SGD) is a critical yet complex hydrological process influencing coastal water quality, nutrient fluxes, and ecosystem dynamics. Its quantification is challenging due to the multiple mechanisms driving groundwater flow, including hydraulic gradients, wave action, tidal forces, and the geological heterogeneity of coastal aquifers. This study employs Amphibious Electrical Resistivity Tomography (AERT) to investigate the spatial and temporal variability of SGD in the nearshore zone of two distinct coastal settings on the microtidal Mediterranean coast. AERT integrates terrestrial and marine geophysical techniques to bridge the observational gap at the land-sea transition zone, enabling high-resolution imaging of subsurface salinity variations. To validate resistivity data, we conducted complementary field measurements, including porewater sampling, manual piezometers, and seepage meters, to characterize groundwater flow and solute fluxes. Our results reveal significant differences in nearshore SGD dynamics between the two geological settings. In the karstic environment, high resistivity values indicate well-defined freshwater discharge pathways controlled by fractures and conduits, leading to rapid salinity fluctuations in marine sediments. In contrast, the detrital aquifer exhibits more diffuse discharge patterns, with slower and more gradual salinity variations. Temporal resistivity changes align with minor sea-level oscillations, suggesting that even under microtidal conditions, pressure-induced shifts can modulate groundwater discharge rates. These findings highlight the effectiveness of AERT as a tool for monitoring SGD in the nearshore zone of different coastal environments. Integrating geophysical, hydrogeological, and geochemical approaches provides insights into small-scale and short-term variations in freshwater-seawater interactions, improving the quantification of SGD, assessing its ecological and hydrological implications, and contributing to a better understanding of submarine groundwater discharge processes.