Hydrogeochemical and Mineralogical Constraints on the Origin of Elevated Uranium and Radon in a Fractured Granitic Aquifer

This study investigated the hydrogeochemical and mineralogical factors governing the occurrence of U and Rn in a fractured granitic aquifer. A 120-m-deep borehole was drilled, and depth-discrete groundwater samples were collected during wet and dry seasons for analyses of water quality and naturally occurring radioactive materials; petrographic and mineralogical analyses were conducted on the recovered drill core. U and Rn concentrations reached 488.1 μg L−1 and 1549.8 Bq L−1, respectively, exceeding Korean drinking-water criteria. Both generally increased with depth and were lower in the dry season than in the wet season. Oxic, mildly alkaline groundwater with elevated HCO3− favored U mobilization through uranyl–carbonate complexation, whereas Rn was primarily controlled by fracture connectivity and mixing between shallow and deep groundwater. Microfractures and pores in metamict U–Th-bearing minerals provided reactive surfaces and release pathways that promoted U leaching and Rn emanation. The occurrence of U and Rn therefore reflects the coupled effects of hydrogeochemical conditions, fracture development, groundwater mixing, and the susceptibility of radioactive minerals to alteration. These findings provide a mechanistic basis for interpreting and predicting naturally occurring radionuclide behavior in fractured granitic aquifers.