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, 238U, 234U, 232Th, 230Th, 228Th and 226Ra, in the river waters from the Ta-Tun volcanic group area in Northern Taiwan. The respective concentration range of 238U, 234U, 232Th, 230Th, 228Th and 226Ra 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 234U/238U and 228Th/232Th activity ratios were greater than 1, indicating radioactive disequilibrium in the waters. The high 228Th/232Th activity ratios showed that besides the recoil effect, enrichment of 228Ra to 232Th in the waters was another possible factor causing an excess of 228Th. 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 not, vert, similar 7) samples.