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Report No.

Estimation and thermodynamic analysis of natural thorium, uranium, and REE concentrations in groundwater at Horonobe URL

Sasaki, Takayuki*; Kokami, Takayuki*; Amamiya, Hiroki; Murakami, Hiroaki ; Amano, Yuki  ; Iwatsuki, Teruki ; Mizuno, Takashi ; Kobayashi, Taishi*; Kirishima, Akira*

For thermodynamic analysis of the trace metals in groundwater, concentrations of Th, U, and rare earth elements (REEs) were investigated at Horonobe Underground Research Laboratory. Groundwater was sampled in anaerobic condition directly from a packed section in the boreholes drilled to 140 m and 250 m depth in drift. The colloidal distribution was checked by an on-site batch ultrafiltration unit using 0.2 $$mu$$m or 10 kDa membranes. As the result, both Th and U concentrations could be estimated by thermodynamic calculations, assuming that the soluble solid phases are restricted to ThO$$_{2}$$(cr) and UO$$_{2}$$(cr), respectively. Meanwhile, the concentrations of REEs were also very low, and significant colloidal distribution for lighter REEs was observed. In the thermodynamic analysis where the lanthanide phosphate was assumed to be a solid phase, the experimental plots matched better with the calculated one, even though the changing trend differs from each other.



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