11:45 AM - 12:00 PM
[AHW24-09] Vertical profiles of stable isotopic composition of groundwater in Abukuma granite: comparison with radiocarbon dates
Stable isotopic composition suggests that all groundwater originated from meteoric water. The shallow around-groundwaters indicated wide isotopic range, since the monsoon climate will produce the seasonal isotopic variations of precipitation. The range of stable isotopic compositions of stream waters around Mt. Utsushiga-take showed slightly lower than that of around-groundwater. This suggested that around-groundwater is unlikely originated from recharge area in Mt. Utsushiga-take, although the local groundwater flow was not clarified. Groundwater flow from the mountain to the drilling sites might be blocked off, owing to the faults and tectonic line between Mt. Utsushiga-take and the drilling sites.
The isotopic values of borehole-groundwater beneath the depth of 100 m showed significantly lower values than those of around-groundwaters: the isotopic depletions are 5-10‰ in hydrogen isotopic ratio and 1.5-2.0‰ in oxygen isotopic ratio. These indicate that they were recharged under colder climate conditions and/or at higher altitudes. The stable isotopic compositions of stream water of the Mt. Utsushiga-take did not represent the same ranges with those of borehole-groundwater. Radiocarbon dates of groundwater beneath the depths of 100 m ranged 10-16 ka, corresponding to be the last glacial period, Oldest, Older and Younger Dryas periods. The colder climate enhances the isotopic fractionation during the air mass transportation and rainfall. Some studies reported the low values of isotopic compositions of groundwater due to recharge under the clod climate such as last glacial period.
This study is regulatory support research funded by the Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry, Japan.