[A-CG39] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG39-04] Visualization of potential submarine groundwater discharge of tidal flats at the mouth of the Midori river in Kumamoto Prefecture

*Makoto Yamada¹, Ryo Sugimoto², Tomohiro Komorita³ (1.Faculty of Economics, Ryukoku University, 2.Faculty of Marine Biosciences, Fukui Prefectural University, 3.Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto)

Keywords:submarine groundwater discharge, visualization, resistivity distribution survey, tidal flat

Many submarine groundwater discharge (SGD) studies have been conducted in coastal waters, and it is widely recognized that SGD plays an important role in the supply of nutrients and other dissolved substances to the ocean. The phenomenon of SGD discharge on the seafloor is not uniform, and the area of influence of the discharged SGD is also heterogeneous. Additionally, even if freshwater or seawater mixed with freshwater exists under the seafloor, they are not necessarily discharge. Such potential SGD have the potential to discharge at any time. Although it is difficult to visually capture SGD, it is very important to understand the spatial distribution of SGD, including this potential SGD, because SGD may have an impact on the ecosystem of coastal waters.
In this study, we conducted a resistivity distribution survey at the tidal flat at the mouth of the Midori river in Kumamoto Prefecture, Japan, using a towed electric survey method, to visualize the distribution of SGD in three dimensions. We also surveyed the temperature distribution on the tidal flat surface using a drone with thermal infrared camera. In the past, towed electric sounding surveys have been conducted in this area to investigate the geological structure of shallow areas below the seafloor, suggesting the possibility that groundwater from present and old rivers may be present as SGD (Misonou et al., 2012).
The results of the resistivity distribution survey showed areas of high resistivity at several meters below the seafloor at the mouth of the estuary. This suggests the possibility of the existence of freshwater or freshwater-mixed groundwater under the seafloor (under tidal flats). In addition, the three-dimensional distribution created using multiple observation line data suggests that these groundwaters may be spatially spread out, confirming the existence of potential SGD. The presentation will detail the presence or absence of SGD discharge and the relationship between the distribution of SGD and the surrounding topography.