Variation of atmospheric tritium concentrations in three different chemical forms in Fukuoka, Japan

Atmospheric tritium concentrations of tritiated water vapor (HTO), tritiate d hydrogen (HT) and tritiated hydrocarbons (primarily tritiated methane, CH 3T) have been measured in Fukuoka prefecture, Japan from 1984 to the presen t to establish a general database on the behavior of atmospheric tritium. H TO concentrations expressed in Bq/1-H2O vary within a range of 1.19 to 2.45 , giving an overall average value of 1.86+/-0.077. HTO concentrations expre ssed in mBq/m(3)-air vary within a range of 7.8 to 46.1 and have a strong c orrelation with the atmospheric humidity, being high in the summer and low in winter. In the case of HT and CH3T, no seasonal variations were observed with average monthly values of 23.1 to 61.0 mBq/m(3)-air and 8.3 to 23.9 m Bq/m(3)-air, respectively. The present HTO concentrations are already close to the tritium level before nuclear testings. However, the present HT and CH3T concentrations are still higher by a factor of about 140 and 30, respe ctively, than those before the testings. Specific activities are estimated to be 14.6-16.7 TU for HTO, 5.5.10(5)-1.0.10(6) TU for HT and 3.2.10(4)-4.1 0(4) TU for CH3T. The apparent difference in the specific activities sugges ts a very slow transformation of these species in the atmosphere or a conti nuous supply of HT and CH3T with high specific activity. Residence time for atmospheric HT was found to be 6.5 years over the period 1988-92 and 10 ye ars for 1988-95. These times are longer than 4.8 years given by Mason and O stlund in the 1970s, and thus indicate a supply to the atmosphere of HT fro m various tritium sources.