High-Rate Submarine Groundwater Discharge and Its Interaction With the Freshwater-Seawater Interface in a Coastal Limestone Aquifer
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Groundwater discharge from coastal limestone and karst aquifers plays a crucial role in global freshwater pathways and nutrient transport. The porous structure of these aquifers, with voids and cavities, leads to turbulent groundwater flow, making Darcy’s law inapplicable. However, these discharge patterns remain poorly understood due to limitations in conventional measurement techniques. This study investigates submarine groundwater discharge (SGD) dynamics along the coast of Yoron Island, southern Japan, using an integrated monitoring approach. Electromagnetic flow meter, electrical resistivity surveys, and pressure monitoring were conducted to assess SGD variability in response to tidal cycles. The results highlight significant tidal influence on SGD behavior, with a pronounced hysteresis effect in efflux rates between low and high tides, suggesting complex seawater-freshwater mixing. Analysis of the hydraulic gradient between inland groundwater levels and tidal fluctuations provides additional insights into SGD mechanisms. Using observed SGD efflux rates and applying the Darcy–Weisbach equation, conduit characteristics within the limestone aquifer were estimated. The highest recorded SGD efflux rates exceeded 50 cm/s, among the highest reported in similar environments. These findings emphasize the need to refine hydrological models to account for non-Darcy flow dynamics in karst aquifers. This study demonstrates the effectiveness of electromagnetic seepage meters in capturing rapid SGD patterns in karst aquifers. The results contribute to a broader understanding of SGD pathways and their role in coastal water exchange, with implications for hydrological modeling and coastal ecosystem assessments.