Variation of radon concentration in air in observation tunnels at Toyohashi and Kikugawa, Tokai area, Japan

The observation of the concentration of 222Rn in air in tunnels have been carried out using flow-type ionization chamber in Tokai area. Observations started in April, 1977 at Toyohashi(TY), June, 1980 at Kikugawa(KI) and August, 1986 at Toyone(TN), Inabu(IB) and Asahi(AS). The 222Rn concentration observed at Toyohashi and Kikugawa shows pulse-like increase after heavy rainfall and is also affected by the atmospheric pressure change. It shows seasonal variation: it increases in summer and decreases in winter. Measurements of 222Rn concentration in groundwater flowing in the tunnel at Toyohashi had also been carried out using the method of a toluene extraction-liquid scintillation counting for 13 months from May, 1986 to May, 1987 in order to compare it with the 222Rn concentration in air in the same tunnel. The 222Rn concentration in groundwater also showed seasonal variation, though its amplitude was about a half that in air. We assumed that the 222Rn rich soil gas supplied the tunnel with 222Rn. We estimated the pulse-like disturbances of 222Rn concentration in air observed after rainfall from local strain variation caused by rainfalls. In this calculation, it was supposed that the amount of the 222Rn rich soil gas squeezed out from the basement rock, increases in proportion to the contraction rate of the basement rock. Relatively good agreement was obtained between observed and calculated values. We infer that the seasonal variations of 222Rn concentration in air and in groundwater in the tunnel are due to the seasonal variation of the 222Rn concentration in the soil gas in the basement rock. (author)