[A-HW24] 流域圏生態系における物質輸送と循環：源流から沿岸海域まで

[AHW24-P14] 花崗岩質諸島における塩淡水境界面付近の不圧地下水

*友澤 裕介¹、小野寺 真一¹、齋藤 光代²、野崎 真悟¹、有冨 大樹³、浅井 和由⁴ (1.国立大学法人　広島大学　大学院先進理工系科学研究科、2.岡山大学 学術研究院 環境生命科学学域、3.山口県、4.株式会社 地球科学研究所)

キーワード：沿岸地下水、涵養標高、地下水年代、塩淡水境界、岩盤地下水

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.