Groundwater Flow System in Tokyo Metropolitan Area, Japan: Focusing on Changes in the Last 60 Years

     In 1960s, the groundwater level dropped with serious land subsidence due to the excessive pumping of groundwater for economic development in Tokyo Metropolitan area, whereas the groundwater level has been recovered after 1990’s because of the strict groundwater use regulation by the government. A few studies have reported long-term changes in the groundwater including a groundwater level dropdown and the recovery in Southeast Asia, such as Ho Chi Minh City in Vietnam and Jakarta in Indonesia. However, there are not enough investigations to monitor the groundwater flow covering the water level dropdown and the recovery during more than 50 years at the megacities in Asia regions. Therefore, we investigated the change of groundwater flow system at the Tokyo Metropolitan area with a special concern on the lowland area where the impact of land subsidence was particularly large due to an excessive pumping in the 1960's.

     First, we observed a spatial distribution of hydraulic head and the chemical and stable isotopic compositions in the groundwater and the river water to understand a current groundwater flow system in the whole Tokyo area in 2019, in which the groundwater level is stable. Then, we compared those results with that monitored from 1960’s to 1990’s.

     Groundwater was sampled from May to October 2019 at multiple boreholes installed at whole of Tokyo area with the depths ranging from 5 m to 260 m, and the main inorganic dissolved ions, stable isotopes (δD, δ¹⁸O) and the dissolved gas (CFCs, SF₆) were determined on all samples. The high Cl^- concentration more than 500 mg/L is limited in the groundwater and the river water in the coastal area with the average distance of 6 km from the sea, whereas the contour line of 500 mg/L intruded inland area with the average distance of 15 km from the coastal line in 1965 and 12 km in 1971, then 8 km in 1994 (Institute of Civil Engineering of the Tokyo Metropolitan Government, 1996). The groundwater hydraulic head is -6 m (m.s.l) at the lowland area in 2019, whereas that was -58 m in 1965, -52 m in 1971, and - 14 m in 1994.

     The depleted stable isotopes and the higher solute concentrations are observed in the lowland in 2019. Also, SF₆ is not detected in the groundwater at the low land area, whereas we observe the SF₆ concentration ranging from 0.8 pptv to 78 pptv in the upland area. These suggest that the groundwater in the upland has an apparent age of approximately less than one year to 40 years, whereas the groundwater in the lowland is recharged with an age more than 80 years at the higher elevation.