Masayuki Imaizumi

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.