URANIUM MINERAL - GROUNDWATER EQUILIBRATION AT THE PALMOTTU NATURAL ANALOGUE STUDY SITE, FINLAND

This study investigates the redox-potential, pH, chemical composition of fracture waters, and uraninite alteration associated with the Palmottu uranium mineralization in Finland. The objective is to understand the equilibrium conditions between uraninite, ferric hydroxide, and groundwater within the site's bedrock. We employed thermodynamic calculations to interpret the data collected. The results reveal that partially oxidized uraninite (UO2 33) and ferric hydroxide are in equilibrium with slightly acidic and oxidized groundwater, while primary uraninite maintains stability with deeper waters characterized by higher pH and lower Eh values. The measured Eh-pH values of groundwater cluster within a narrow range, indicating buffering through heterogeneous redox processes. Notably, we observed good consistency between measured Eh and analyzed uranium oxidation states. This research contributes to the understanding of uranium behavior in groundwater systems, especially in the context of safety assessments for planned nuclear waste repositories in crystalline bedrock. It highlights the importance of redox conditions and water chemistry in influencing the solubility and mobility of uranium in natural environments.