[A-HW22] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW22-15] Direct Measurement of High-Rate Submarine Groundwater Discharge from a Ryukyu Limestone Aquifer Using a Magnetic Flowmeter

*Jun Yasumoto¹, Ko Yasumoto², Kazumi Asai³, Chris Leong⁴, Mariko Iijima⁵, Takahiro Hosono⁷, Ryuichi Shinjo⁴, KE-HAN SONG⁶, Shinji Nakaya⁸, Akira Iguchi⁵, Bam H.N. Razafindrabe¹, Makoto Taniguchi⁴ (1.University of the Ryukyus, Faculty of Agriculture, 2.School of Marine Biosciences, Kitasato University, 3.GEO SCIENCE LABORATORY Co., Ltd., 4.Research Institute for Humanity and Nature, 5.National Institute of Advanced Industrial Science and Technology (AIST), 6.Graduate School of Engineering and Science, University of the Ryukyus, 7.International Research Organization for Advanced Science and Technology, Kumamoto University, 8.Faculty of Engineering, Shinshu University)

Keywords:High-Rate Submarine Groundwater Discharge, Direct Measurement, Ryukyu Limestone Aquife

Limestone aquifers exhibit significant heterogeneity due to the presence of numerous cavities and voids, leading to localized and rapid groundwater flow near coastal areas. Coral reef ecosystems, vulnerable to nutrient loading, rely on understanding groundwater inputs from terrestrial sources, especially in limestone regions where river development is minimal. This study investigates the Ryukyu Limestone aquifer, known for its rapid submarine groundwater discharge (SGD) that has previously not been thoroughly measured. We developed an automated seepage meter equipped with a magnetic flowmeter, tailored for continuously measuring high SGD velocities. This automated seepage meter was deployed in the coastal waters off Yoron Island to monitor SGD into an inner reef area dominated by Ryukyu limestone. Our findings indicate that the SGD velocity fluctuates with the tidal cycle, intensifying as the water depth decreases and reaching its peak at low tide. The highest SGD velocity recorded was 55.2 cm/sec, significantly surpassing previously reported velocities. Notably, no SGD was detected at depths greater than 1.64 to 1.78 m. We observed a pronounced hysteresis effect in the SGD velocities, with higher upwelling at low tide (median: 30.9 cm/sec) compared to high tide (median: 28.8 cm/sec) at equivalent depths.
The integration of automated seepage meters and magnetic flowmeters has proven to be an effective approach for accurately measuring rapid submarine groundwater upwelling velocities. This methodology offers crucial insights into land-sea interactions within coral reef ecosystems and is poised to significantly enhance our comprehension of these intricate environmental processes.