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The 228Ra/226Ra activity ratios obtained by the observations with 3 river waters, 2 hot-spring waters and 12 samples from sedimentary rock occurring in Okutama area, western district of Tokyo, ranged from 1.0 to 2.9, from 0.7 to 0.8 and from 1.2 to 1.7, respectively. The average values were 2.0 ± 0.03 for river waters and 1.4 ± 0.1 for sedimentary rocks, suggesting that the activity ratio of 228Ra/226Ra in river water tends to be larger than that in the associated sedimentary rocks. The 228Ra/226Ra activity ratios in hot-spring waters staying in the deeper aquifer with longer residence time may tend to be smaller than those in river waters with shorter residence time. The 228Ra/226Ra activity ratios in river waters showed that the activity ratios tended to decrease after heavier rainfalls than 20 mm/d. The decrease in the 228Ra/226Ra activity ratios in river waters suggested that heavy rainfalls forced the deeper underground water with long residence time to discharge into river water.

228Ra/226Ra Activity Ratio in River Water in Okutama Area, Western District of Tokyo