Shinichi Kawashima

The Use of Long-term Tritium Concentration Records to Estimate Recharge to Confined Aquifers in Metropolitan Tokyo

Long-term tritium concentration records for 1969 to 1997 were used to estimate the recharge of a confined aquifer in Metropolitan Tokyo. The fluctuation patterns of tritium concentration are classified into the following four types: (1) Gently-sloping peak type (GP type), (2) Sharp peak type (SP type), (3) Decreasing type, (4) Irregularly changing type. The GP type and SP type are considered to correspond to the diffusion flow model and the piston flow model, respectively. The GP type has a broad peak of tritium concentration. The SP type has a sharp peak of tritium concentration. These peaks may correspond to the maximum tritium concentration of bomb tritium in 1963. The depth of penetration of bomb tritium into a confined aquifer was used to calculate the rate of recharge to the confined aquifer. The rate of recharge to a confined aquifer was found to range from 493 to 1,752 mm/yr assuming that the only downward groundwater flow through the flow path in an aquiclude was a result of the potential difference caused by pumping. However, the time variations of tritium concentration at the Higashi-yama and Higashi-kurume observation sites indicates that some silt beds function as an aquiclude that restricts groundwater flow. Lateral groundwater flow as well as the downward groundwater flow is dominate in the confined aquifer in Tokyo. Therefore it should be concluded that the calculated rate of recharge assuming the only downward groundwater flow is not the real rate of recharge.

Subsurface Geology and Some Soil Characters in Kitatama District, Tokyo

In order to study hydrogeological problems such as land subsidence and groundwater pollution by organic chlorine compounds in Tama District, the west part of Tokyo Metropolis, Tokyo Metropolitan Government made 18 stratigraphical drillings from 1972 to 1991 which were from 402m to 1, 003m in depth. As the result of this inverstigation, the basal Pre-Tertiary rocks were found at the deep underground in the westernmost part of Musashino Terrace, Mizuho town, Tachikawa city, Musashimurayama city, Akishima city and Hachiouji city. The Quaternary sediments covering these rocks with unconformity are divided stratigraphically into two groups: the lower is the Kazusa Group, and the upper the Tokyo Group. The Kazusa Group comprises the Kitatama Formation and Higashikurume Formation, in ascending order, the Tokyo Group comprises the Toneri Formation and the Edogawa Formation.The Kitatama Formation of the Kazusa Group, the geological oldest sediments discussed here, is developed throughout the Musashino Terrace and consists chiefly of impermeable massive silty layer. On the other hand, formations overlying the Kitatama Formation consist chiefly of permeable sand and gravel layers, and so acts as confined aquifers in this area.The geologic structure of the most western part in the Musashino Terrace is rather different from the east one. In the most western part, the structure of each formation undulates between two axes elongating parallel to the NW-SE trend. It is ascribed to the two reverse faults named Tachikawa Fault and Mizuho One.