三島溶岩流内地下水の年代について

抄録

This study focused on developing a tritium and helium -3(³H-³He) dating method to survey the flow of groundwater in a large geohydrologic catchment area with groundwater basins including candidate disposal sites for wastes. The dating method is based on counting the increase of ³He produced by beta decay of tritium in groundwater. Precise detection of 3He is very difficult because the ratio of ³He to ⁴He is generally less than 10^-6 for 10^-8 ccSTP of helium dissolved in one gram of groundwater. We developed an isolating sampling method for groundwater in which atmospheric air is prevented from contacting groundwater sample and the ³He is measured by mass spectrometry. We applied this dating method to the confined aquifer in the Mishima lava flow located in the east side of Mt. Fuji. The measured age of groundwater is aligned young to old following the groundwater flow. The groundwater in the Mishima area was grouped into two different sources based on the measured groundwater ages and the stable isotope composition (δD and δ¹⁸O). The movement of groundwater in the lava flow was calculated to be an approximate 16m/day based on the difference between the groundwater ages measured at two sampling sites (Izushimada and Kakitagawa) and the geographical distance. This is in good agreement with results measured by radioisotope flow meter in a geohydrologic investigation 20 years ago and calculated by Darcy's law.