The levels of tritium in the atmosphere nowadays are those of natural origin before the nuclear test. Nuclear power stations, nuclear reprocessing plants and fusion facilities are observed as a further occurrence source. Then, in order to appraise the influence of nuclear facilities and long distance transport from the continent where tritium level is relatively high, it is necessary to investigate background levels of tritium. Tritium concentrations of 34 river waters and 6 lake waters in Japan were determined by low background liquid scintillation measurement system combined with the electrolysis using solid polymer electrolyte. Tritium concentrations of river and lake water were 0.36-2.66 Bq/l (average 1.06±0.60 Bq/l) and 0.48-1.43 Bq/l (average 0.81±0.37 Bq/l), respectively. The entire mean value was 1.03±0.57 Bq/l. This mean value equals 43% of the mean value which was measured in 1982. It was possible to calculate 11 years as an apparent half-life.
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