[A-HW18] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW18-P08] Spatial variation in groundwater discharge on a tidal flat in an island of western japan

*Qidi Gao¹, Shin-ichi Onodera¹, Mitsuyo Saito¹, Yusuke Tomozawa¹, Reiya Matsubara² (1.Graduate School of Advanced Science and Engineering, Hiroshima University, 2.Faculty of Integrated Arts and Sciences, Hiroshima University)

Keywords:Submarine groundwater discharge, Spatial distribution, Tidal flat, Island

To manage and conserve the seagrass bed and ecosystem in tidal flats, this study examined to confirm spatial distribution in groundwater (SGD) on tidal flats with the area of around 100 m square in a small and steep island. Various groundwater and nutrient discharges were observed at the W and E sites. 1) Based on the water budget, the groundwater flux was estimated to be 8.68 m/yr or 23.9 mm/day in E, and 17.75 m/yr or 48.6 mm/day in W, respectively. 2) Spatial distributions of groundwater contribution ratio (%) in pore water at W was around 60% in average, which was 8 times of that in E site. 3) The NO₃-N concentration in pore water at W site was about 30 times of that at E because of the low attenuation rate with high groundwater velocity. 4) The temporal variation in groundwater flux with tidal fluctuation indicated 2.5 m/day in maximum and -4.0 m/day in minimum. This huge difference between the average flux and maximum indicated also large seawater recirculation and heterogeneous distribution of SGD. This heterogeneous distribution at this tidal flat would contribute to the activity of seagrass and diversity of ecosystem.

Acknowledgment
This research was supported by JSPS Grant-in-Aid for Scientific Research (B) (No. 21H03650, PI: Mitsuyo Saito) and Fostering Joint International Research (A) (No. 20KK0262, PI: Mitsuyo Saito).