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

[AHW22-P08] Evaluation of the fresh and saline submarine groundwater discharge in a tidal flat area with different substrates

*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:FSGD, RSGD, tidal flat, substrate

Submarine groundwater discharge (SGD) is a hydrological process commonly occurring in coastal areas composed of freshwater-derived SGD (FSGD) and recirculated seawater-derived SGD (RSGD). This study aims to evaluate the SGD, FSGD, and RSGD with spatial variation on a tidal flat in an island with steep topography in the Seto Inland Sea during the entire spring-neap tidal cycle. The target tidal flat is about 300 m wide and substrate in the eastern part is mainly composed of sandy materials while it is mainly gravels in the western part. Temporal variations in hydraulic potential and electrical conductivity (EC) were monitored using piezometers installed on the two cross sectional lines in the eastern and the western part of the tidal flat.SGD decreased from the shallower to the deeper zone. It also indicates that SGD on the western line gradually decreases from nearshore to offshore, however, it was significantly heterogeneous on the eastern line. SGD was estimated and separated into FSGD and RSGD components based on salinity, and they were compared between the two lines during three periods (spring, neap tides, and the entire spring-neap tidal cycle). The results show that the SGD on the western line is higher than that of the eastern line in any period and major component was FSGD. In contrast, RSGD was dominant on the eastern line and it suggests tidally-driven seawater infiltration significantly occurs on the eastern line. To estimate the volume of seawater infiltration, we estimated unsaturated zone during high and low tide for the eastern line.

Acknowledgement
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).