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

Groundwater age was determined by Kr-85/Kr specific activity of gases dissolved in groundwater at Kumamoto Area, in which newly developed Kr extraction system and liquid scintillation counting technique for Kr-85 were applied. Apparent mean residence times observed were 8.2 +/- A 0.7 years and 20.7 +/- A 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 Kr-85 and tritium were observed at the discharge area but the mean residence time of Kr-85 at the recharge area was younger compared to that of tritium, probably due to mixing of river water containing recent Kr-85 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.