Application of 85Kr dating to groundwater in volcanic aquifer of Kumamoto Area, Japan

Abstract

Groundwater age was determined by 85Kr/Kr specific activity of gases dissolved in groundwater at Kumamoto Area, in which newly developed Kr extraction system and liquid scintillation counting technique for 85Kr were applied. Apparent mean residence times observed were 8.2 ± 0.7 years and 20.7 ± 0.6 years for the groundwater taken from a well at the recharge area and that from an artesian borehole at the discharge area, respectively showing a consistent age trend estimated from the observed groundwater flow system of Kumamoto Area and also that determined using environmental tritium. Similar mean residence times of 85Kr and tritium were observed at the discharge area but the mean residence time of 85Kr at the recharge area was younger compared to that of tritium, probably due to mixing of river water containing recent 85Kr high in concentration into groundwater flow coming from the northern upland’s recharge area. The mean residence time of the groundwater flow coming from northern upland area and a traveling time of the groundwater between two sampling points were estimated by a simple model assuming mixing the river water with the groundwater and a piston flow between two sampling points.