HR: 0800h
AN: H11C-0641    [Abstracts]
TI: Helium Isotopic Characteristics Dissolved In Groundwater In The Area Of Heavy Soil Subsidence In The Saga Plain, Japan
AU: * Mahara, Y
EM: mahara@HL.rri.kyoto-u.ac.jp
AB: There are many rice fields at the Shiroishi area in the Saga plain, Japan. The geological setting of the Saga plain is closely located along the Beppu-Shimabara graben that has potential volcanic activities from Beppu (the active geothermal area) to Mt. Unzen (the active volcano) via Mt. Aso (the active volcano). Since this area was originally manmade by reclamation of shallow sea in a few hundred years, the plain has a poor irrigation system owing to few rivers to supply enough water to rice fields. Rice needs much water in growing in the summer, but some times water is short. Much groundwater has to be pumped from the confined shallow aquifer below a thick marine clay layer, which is impermeable and covers with a few ten meters depth. Groundwater has been recharged only at the narrow margin area between mountains and the reclaimed flat land. Pumping much water has caused heavy soil subsidence owing to shrinking the aquifer matrix. The land surface subsided by more than 120 cm depth in a long term at the center of Shiroishi area. Since the regional groundwater simply flows from mountain area to the sea, groundwater accumulates the regional crustal He component, which has a higher ratio of 3He/4He than the atmospheric He 1.4E-6, with increasing flow path. The isotopic ratio of the regional He accumulated in groundwater is characterized by a significant amount of mixing of a mantle He component with the ratio of 1.1E-5 in the subduction area. The ratio has gradually increased with increasing the dissolved He content in the entire area of the plain except the Shiroishi area and finally reached maximum 5E-6. On the other hand, the lowest ratio reached finally 8.7E-7 with increasing dissolved He content in groundwater at the center of the Shiroishi area. This suggests that the He accumulating mechanism is different between Shiroishi area and other areas in the plain. In other words, groundwater at the central Shiroishi area has selectively accumulated more the radiogenic He component, which the ratio has very low 1E-8, rather than the regional He component. It is possible to explain this contradiction if groundwater is consisted of a significant amount of pore water that has been isolated from the regional flow and squeezed from the impermeable marine clay layer. Consequently, squeezing much pore water caused the heavy soil subsidence at the central Shiroishi area.
DE: 1828 Groundwater hydraulics
DE: 1834 Human impacts
DE: 1865 Soils (0486)
SC: Hydrology [H]
MN: 2007 Fall Meeting