The levels of tritium in the atmosphere are nowadays almost only of natural origin and of the same range as before the era of the nuclear tests. In order to appraise the influence of tritium released from nuclear facilities to the environment, it is necessary to confirm the effect of tritium appearing overlapped on background tritium levels. Tritium concentrations and stable isotopes of oxygen and hydrogen in rain water, stream water and groundwater at the NIFS (National Institute for Fusion Science, Gifu prefecture, Japan) site were analyzed to understand behavior of the natural tritium in coupling with rain event. Conductivity, temperature and flow rate of the stream were monitored continuously. The range of tritium concentrations in rain for three year period was 0.09-0.78 Bq/l (average 0.37+0.14 Bq/l). The tritium concentrations of stream water and groundwater were almost constant, 0.34 Bq/l and 0.25 Bq/l, respectively. The isotopic ratio of oxygen and hydrogen showed a typical seasonal pattern observed in Japan. Two component separation analysis was carried out for the stream water at the time of rain using isotopic ratio, conductivity and tritium concentration.
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