[A-HW25] Near Surface Investigation and Modeling for Groundwater Resources Assessment and Conservation

[AHW25-07] Shallow groundwater connectivity between upland and lowland, eastern Tokyo, Japan

*Mugiho Miura¹, Maki Tsujimura¹, Fumito Gono¹, Keita Miyoshi¹, Kei Matsuura², Teruaki Hirano² (1.University of Tsukuba, 2.Tokyo Metropolitan Research Institute for Environmental Protection)

Keywords:Upland, Lowland, Topographical boundary, Spring, Groundwater, Groundwater connectivity

This study aims to clarify groundwater connectivity between upland and lowland at topographical boundary, in urbanized area, eastern Tokyo.
In 2023 and 2024, spring, river and groundwater were corrected in the eastern part of Tokyo. Then, concentrations of inorganic dissolved ions and stable isotopic compositions of hydrogen (δ²H) and oxygen (δ¹⁸O) were analyzed.
Total anion and cation concentrations in spring were lower than that in groundwater, and the chemical characteristics in the spring and the groundwater changed from Ca-HCO₃ dominant type to Na-HCO₃ dominant type between upland and lowland. However, some spring and groundwater showed Ca-HCO₃ dominant type both upland and lowland. The δ¹⁸O in spring was more than -8.0‰ and higher than that of groundwater. And δ¹⁸O of groundwater in eastern lowland was less than -9.0‰ and lower than that in upland ranging from -8.0 to -9.0‰.
The shallow groundwater shows solution characteristics of Ca-HCO₃ dominant type and δ¹⁸O of approximately -8.0‰ in both upland and lowland areas, whereas the deep groundwater shows solution characteristics of Na-HCO₃ dominant type and δ¹⁸O ranging from -8.0 to -9.0‰ in lowland.
This suggests that the shallow groundwater seems to have connectivity between upland and lowland in a zone with a depth of approximately 10 to 50 m, whereas the deep groundwater in lowland would be sourced from the water recharged at the mountainous area with low δ¹⁸O.