17:15 〜 18:45
[AHW22-P19] 電気抵抗トモグラフィーと数値解析を統合した黒島における淡水レンズの賦存形態と地下水流動特性の把握
キーワード：電気抵抗トモグラフィー、数値解析、淡水レンズ、地下水
Located in the Yaeyama Islands of Okinawa Prefecture, Kuroshima Island plays a significant role in the region's livestock industry. The island is underlain by Ryukyu limestone from the Quaternary period, forming an extensive underground layer that reaches depths of up to 100 meters. This geological feature creates a unique freshwater lens configuration within the groundwater system. The high permeability of the limestone is a cause for concern regarding the potential for livestock waste to contaminate the groundwater, posing risks to the surrounding marine environments. Kuroshima's proximity to the Sekisei Lagoon, situated within the Iriomote-Ishigaki National Park and home to Japan's largest coral reef, underscores the urgent need for sustainable livestock practices to protect marine biodiversity. Despite its ecological importance, there has been limited research on the structure of the freshwater lens and the dynamics of groundwater discharge from Kuroshima into the Sekisei Lagoon.
The aim of this study is twofold: to delineate the structure of the freshwater lens beneath Kuroshima using electrical resistivity survey, and to elucidate the mechanisms and characteristics of groundwater discharge into the marine environment through advanced numerical modeling. Our goal is to provide insights that can inform sustainable agricultural practices aligned with coral reef conservation efforts.
An electrical survey conducted from 14 to 17 September 2023, utilized the Super Sting R8 (AGI) equipment with the Wenner array method, chosen for its effectiveness in detecting seawater intrusion into groundwater. Measurements were taken at four locations across Kuroshima, including two inland transects (Transect 1 and 2) and two extending from the sea inland (Transect 3 and 4), with electrode spacing set at 5 meters. The lengths of the transects were 555 m, 280 m, 765 m, and 390 m, respectively. Resistivity analysis employed the Wenner method via RES2DINV version 3.50 (Geotomo Software). Numerical analysis was performed using the SEAWAT code (U.S. Geological Survey) and analyzed with FloPy (U.S. Geological Survey) to simulate density-dependent solute transport.
The survey results indicated that resistivity values below 1 Ω-m are characteristic of saline water regions, akin to seawater, while values between 10 Ω-m and 100 Ω-m suggest a mixing zone of saltwater and freshwater. The challenge from estimating groundwater level from resistivity values above 100 Ω-m due to significant variations led to an assumption of a 1 m elevation for the groundwater table. Along Transect 1, resistivity values of 100 Ω-m were distributed at elevations from -1 m to 10 m from west to east, suggesting a wedge-like shape of the fresh-salt water interface near the sea. The estimated thickness of the freshwater lens varied between 1 m and 11 m. In inland transects, the freshwater lens thickness was approximately 5 m to 9 m for Transect 2 and 3 m to 16 m for Transect 3. Along the sea transect (Transect 4), resistivity values of 100 Ω-m were observed from -2.5 m to -3.5 m below the seabed. Attempts to reproduce Kuroshima's freshwater lens configuration using SEAWAT successfully approximated the general cross-sectional shape and facilitated visualization of groundwater discharge patterns from the lens to the sea area.
This study's integration of electrical resistivity tomography and numerical modeling provides valuable insights into the hydrogeological processes influencing Kuroshima's freshwater lens and its impact on marine ecosystems, offering a foundation for developing environmentally sustainable practices.
The aim of this study is twofold: to delineate the structure of the freshwater lens beneath Kuroshima using electrical resistivity survey, and to elucidate the mechanisms and characteristics of groundwater discharge into the marine environment through advanced numerical modeling. Our goal is to provide insights that can inform sustainable agricultural practices aligned with coral reef conservation efforts.
An electrical survey conducted from 14 to 17 September 2023, utilized the Super Sting R8 (AGI) equipment with the Wenner array method, chosen for its effectiveness in detecting seawater intrusion into groundwater. Measurements were taken at four locations across Kuroshima, including two inland transects (Transect 1 and 2) and two extending from the sea inland (Transect 3 and 4), with electrode spacing set at 5 meters. The lengths of the transects were 555 m, 280 m, 765 m, and 390 m, respectively. Resistivity analysis employed the Wenner method via RES2DINV version 3.50 (Geotomo Software). Numerical analysis was performed using the SEAWAT code (U.S. Geological Survey) and analyzed with FloPy (U.S. Geological Survey) to simulate density-dependent solute transport.
The survey results indicated that resistivity values below 1 Ω-m are characteristic of saline water regions, akin to seawater, while values between 10 Ω-m and 100 Ω-m suggest a mixing zone of saltwater and freshwater. The challenge from estimating groundwater level from resistivity values above 100 Ω-m due to significant variations led to an assumption of a 1 m elevation for the groundwater table. Along Transect 1, resistivity values of 100 Ω-m were distributed at elevations from -1 m to 10 m from west to east, suggesting a wedge-like shape of the fresh-salt water interface near the sea. The estimated thickness of the freshwater lens varied between 1 m and 11 m. In inland transects, the freshwater lens thickness was approximately 5 m to 9 m for Transect 2 and 3 m to 16 m for Transect 3. Along the sea transect (Transect 4), resistivity values of 100 Ω-m were observed from -2.5 m to -3.5 m below the seabed. Attempts to reproduce Kuroshima's freshwater lens configuration using SEAWAT successfully approximated the general cross-sectional shape and facilitated visualization of groundwater discharge patterns from the lens to the sea area.
This study's integration of electrical resistivity tomography and numerical modeling provides valuable insights into the hydrogeological processes influencing Kuroshima's freshwater lens and its impact on marine ecosystems, offering a foundation for developing environmentally sustainable practices.