Aqueous characteristics and activity concentrations of U/Th-series radionuclides were investigated in the birthplace of hokutotite－ the Peito Hot Spring Area in Taiwan. Water samples were taken in 12 locations from the hot spring pond and the Peito Creek in this study. Temperature, acidity and redox potential exhibited similar trends decreasing from the hot spring pond toward downstream Peito Creek. The aqueous environment of this area existed in an acidic and oxidative state and was found influenced by hot spring water drawing and wastewater discharge. Major ions in water were Fe³⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Cl⁻ and SO₄²⁻, which (except for Ba²⁺) generally varied with sampling locations in a tendency similar to those of temperature, acidity and redox potential. U/Th-series radionuclides were examined by α spectrometry while ²²⁶Ra were determined with ICP-QMS. Th-series was found predominant over U-series in this area. Activity concentrations of ²³⁸U (0.09 ± 0.02 to 2.69 ± 0.13 pCi/L) and ²³⁴U (0.10 ± 0.02 to 2.64 ± 0.13 pCi/L) were similar and the activity ratios for ²³⁴U/²³⁸U were greater than unity. For ²³⁰Th and ²²⁶Ra, the activity concentrations were respectively 0.07 ± 0.03 to 0.86 ± 0.06 pCi/L and 0.17 ± 0.02 to 1.49 ± 0.06 pCi/L, which led to activity ratios of ²³⁰Th/²³⁴U lower than unity while those of most²²⁶Ra/²³⁰Th greater than unity. In Th-series, the activity concentrations of ²³²Th (0.06 ±0.02 to 0.90 ±0.06 pCi/L) and ²³⁰Th were similar at the sampling area. Apparent disequilibria were discovered between ²²⁸Ra and ²³²Th resulting from abundant ²²⁸Ra (6.01 ± 0.41 to 10.5 ± 0.8 pCi/L). The variation for activity concentration of ²²⁸Th (0.43 ± 0.06 to 9.26 ± 0.18 pCi/L) with sampling locations was similar to that of ²³²Th. However, significant disequilibria were discovered between ²²⁸Th and ²³²Th. The reported data contribute to the assessment of radionuclides distribution and the investigation of hokutolite recovery in the studied area.
