Yumi Yoshioka, Koji Seto, Ikuo Takeda
Received 5 January, 2026
Accepted 12 April, 2026
Published online 14 July, 2026

Yumi Yoshioka¹⁾²⁾, Koji Seto³⁾, Ikuo Takeda⁴⁾

1) Faculty of Life and Environmental Sciences, Gifu University, Japan
2) Center for Environmental and Societal Sustainability (CENSS), Gifu University, Japan
3) Estuary Research Center, Shimane University, Japan
4) Academic Assembly, Shimane University, Japan

Stable oxygen and hydrogen isotopes are key hydrological tracers for identifying the sources of groundwater recharge. Groundwater, river, lake, and precipitation isotopes were measured monthly to identify recharge sources. The study was carried out in the lowest reaches of the Hii River, located in Shimane Prefecture, Japan, and within an area surrounded by several rivers, including a ceiling river and a large brackish lake. Our sampling study was conducted at a local scale of less than 4 km² and focused on multiple groundwater recharge sources, especially those affecting evaporation (paddy fields and the lake). Even at adjacent wells, the isotopic characteristics (including the average δ values), fluctuation ranges, and the slope of the linear regression between δ²H and δ¹⁸O, differed, and they were divided into five isotopic groups. One isotopic group showed a significant influence from the Hii River, whereas the other groups showed combined influences from precipitation, rivers, and paddy water. The results suggest that stable isotope techniques can reveal the spatial and temporal distribution of groundwater beneath paddy fields from multiple recharge sources in a river-surrounded area, including a ceiling river and a brackish lake.

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