The in-suit radon measurement system was developed for the continuous monitoring in the groundwater radon concentration of the monitoring well. This system consist of an underwater radon detector and a small-sized radon data logger. We have measured the continuous radon concentration of the groundwater during the period 18-December 1996 to 2-February 1998, by using of the new measurement system
... [Show full abstract] in the monitoring well, No.18, at the NINGYO-TOGE WORKS, Japan Nuclear Cycle Development Institute. The groundwater radon concentration was seasonally changed by the rainfall, snowfall and snowmelt. The lowest value was 0.21Bq/l on 3-September 1997, just before the falling of the temperature. The monthly average of the groundwater concentration was obtained among a year. The highest value was 64Bq/l with the lowest water temperature 7℃ in the monitoring well on February 1997. The lowest value was 0.82Bq/l with the highest water temperature 18℃ on August 1997. The groundwater radon concentration in No.18 well was suddenly increasing by the rain fall, after that was decreasing in according to 222Rn decay curve during the period 20-May 1997 to 21-July 1997. Five clear samples of the increase of radon concentration were observed at 21-May, 27-May, 6-Jun, 28-Jun and 8-July 1997 with the rainfall. The increase rate of the radon concentration Bq/l with the rainfall mm was estimated to be about 0.019(Bq/l)/mm at 28-Jun 1997.