Radioactive disequilibrium of uranium and thorium nuclide series in river waters from the Ta-Tun volcanic group area in Taiwan

Alpha spectrometry and liquid scintillation counting were used in this study to analyze the radioactivities of the naturally occurring radionuclides, U-238, U-234, Th-232, Th-230, Th-228 and Ra-226, in the river waters from the Ta-Tun volcanic group area in Northern Taiwan. The respective concentration range of U-238, U-234, Th-232, Th-230, Th-228 and Ra-226 was 0.80-48, 1.2-51, 0.05-44, 0.09-39, 0.19-342 and 3.0-22.5 mBq L-1 and the U-234/U-238 and Th-228/Th-238 activity ratios were greater than 1, indicating radioactive disequilibrium in the waters. The high Th-228/Th-232 activity ratios showed that besides the recoil effect, enrichment of Ra-228 to Th-232 in the waters was another possible factor causing an excess of Th-228. The concentrations of uranium and thorium in the river waters changed with the pH of the water. The U/Th concentration ratio was about 1; in the acidic (pH < 5) samples; whereas it was much higher than 1 in neutral (pH similar to 7) samples. (C) 1997 Elsevier Science Ltd.