[A-HW24] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW24-P14] Unconfined groundwater flow around saltwater – freshwater interface in a small granitic island

*Yusuke Tomozawa¹, Shin-ichi Onodera¹, Mitsuyo Saito², Shingo Nozaki¹, Daiki Aritomi³, Kazuyoshi Asai⁴ (1.Graduate School of Advanced Science and Engineering Hiroshima University, 2.Graduate School of Environmental and Life Science, Okayama University, 3.Yamaguchi Prefectural Government, 4.Geo-Science Laboratory Co. Ltd.)

Keywords:Coastal groundwater, recharge elevation, groundwater age, salt-freshwater boundary, bedrock groundwater

In this study, we examined to confirm unconfined groundwater flow around saltwater – freshwater interface in a small granitic island, using the hydraulic heads and natural tracers. Monitoring of water levels and electrical conductivities, water collections of groundwater rain water and spring water, and analysis of main ions concentrations, isotopic rations of water and SF6 concentration were conducted, respectively. Groundwater were monitored and collected at 10 boreholes with different depths at two coastal observation sites in Ikuchi-jima Island of western Japan, which have mountains with an altitude and area of approximately 400 m and 30 km². In addition, rainwater at 5 sites with the different altitude and spring water samples were collected, respectively. We verified the altitude effect of stable isotopes for precipitation and clarified average recharge area with the different altitudes in according to depth in the coastal groundwater. The isotopic rations of shallow groundwater up to a depth of 3 m were similar to those of recharge area with the altitude of lower than 50m , fresh groundwater from colluvial and fluvial deposits with the depth of 10m to weathered rock layer with 30m were similar to those of water recharged area at the upper part of the mountain, and blackish groundwater in the bedrock layer with the depth of 35 to 40m were lower than the water recharged at the top of the mountain, respectively. The age of the groundwater appeared to be older in the bedrock groundwater. In addition, fluctuations in the stable oxygen isotope ratio (δ¹⁸O) and chloride ion (Cl^-) concentrations were confirmed in bedrock groundwater with the weak and negative correlation. These results indicated that a deeper flow with a higher recharge altitude and lower isotopic ratios emerges with a rise in the saltwater - freshwater interface. The confirmed lower value of δ¹⁸O in bedrock groundwater suggested that recharge water of current age mixed with residual and stagnant palaeo-groundwater recharged during the glacial period.