[A-HW22] 流域圏生態系における物質輸送と循環：源流から沿岸海域まで

[AHW22-15] 電磁流量計を使用した琉球石灰岩帯水層からの非常に速く局所的な海底地下水湧出の直接測定法の開発

*安元 純¹、安元 剛²、浅井 和見³、Leong Chris⁴、飯島 真理子⁵、細野 高啓⁷、新城 竜一⁴、宋 科翰⁶、中屋 眞司⁸、井口 亮⁵、Bam Razafindrabe¹、谷口 真人⁴ (1.琉球大学　農学部、2.北里大学　海洋生命科学部、3.㈱地球科学研究所、4.総合地球環境学研究所、5.産業技術総合研究所、6.琉球大学大学院　理工学研究科、7.熊本大学 大学院先端科学研究部(理)、8.信州大学　工学部)

キーワード：非常に速い海底地下水湧出、直接測定、琉球石灰岩帯水層

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.