Combining Seepage Meters and Amphibious Electric Resistivity Tomography to Investigate Pathways of Submarine Groundwater Discharge
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Submarine groundwater discharge (SGD) plays a crucial role in coastal biogeochemistry, yet it is still challenging to accurately quantify water and solute fluxes driven by this process due to its complex hydrogeological dynamics. This work aims to improve the identification and independent quantification of different SGD pathways by combining the use of seepage meters and Amphibious Electrical Resistivity Tomography (AERT) at a heterogeneous karstic system in the Mediterranean Sea. In the field, 5 AERT transects were surveyed along with 24 seepage meters, which provided a 3D view of the salinity distribution and direct discharge rate measurements, respectively. The integrated approach identified and quantified distinct SGD pathways, including beach-face recirculation, focused discharge, submarine springs, and diffusive discharge, each uniquely influencing SGD dynamics. Given that each pathway is characterized by specific geochemical signatures and discharge rates, nutrient fluxes supplied by different pathways varied significantly in magnitude. While diffusive discharge primarily facilitated the transport of fresh groundwater and ammonium, nitrate and phosphate were predominantly delivered to the coastal ocean through focused discharge, especially via submarine springs. The combined methodology proved more accurate for determining water and nutrient fluxes than straightforward extrapolations from averaged seepage meter rates, which were consistently 20 to 120% higher. This discrepancy highlights the need of combining qualitative and quantitative methods, particularly in geologically complex regions where multiple SGD pathways coexist.