Uranium inventory in Boeun aquifer is facing the artificial reservoir that intended for supplying water to nearby cities (40-70 m apart) where, toxic radionuclides might mobile and enter the reservoir. In order to understand U mobility in the system, groundwater and fracture filling materials (FFMs) were analyzed for microbial signatures, C, O, Fe, S and U-series isotopes. The δ18O-H2O and 14C signatures suggested groundwater was originated from upland recharges dominantly and not affected by mixing with the surface water. However, the 234U/238U activity ratios (ARs) and 230Th/234U ARs in FFMs ranged from 0.93 to 1.67 and from 0.22 to 1.97, respectively, indicating that U was mobile along the fractures. In shallow FFMs, the U accumulations (~157 mg/kg) were found with Fe enrichments (~226798 mg/kg) and anomalies of δ56Fe and δ57Fe, implied U mobility in shallow depths was associated with Fe-rich environment. Also, in the shallow depths, Fe-oxidizers, Gallionella was prevailing in groundwater while Acidovorax was abundant near U ore depth. The Fe-rich environment in shallow depths was formed by pyrite dissolution, demonstrated using δ34S-SO4 and δ18O-SO4 distribution. Conclusively, the Fe-rich aquifer was capable of immobilizing the dissolved U through biotic and abiotic processes, without significant discharge into the nearby reservoir.

The mechanical, hydraulic, thermal, and chemical properties of the subsurface can have a significant effect on the long-term performance of an underground facility. Therefore, it is important to accurately estimate the aquifer properties in order to predict the groundwater flow and solute transport and thus ensure the stability and safety of a high-level radioactive waste disposal. Using heat as a tracer has become a popular tool for the subsurface characterization. Recent studies have demonstrated that heat tracing is an effective approach to quantify both hydrogeological and thermal subsurface properties. However, most studies in natural conditions assume the local thermal equilibrium (LTE) between the solid and fluid phases, ignoring heat exchange between them. The LTE assumption has not yet been verified by experiments. This work investigates the validity of the LTE assumption by performing the laboratory tracer tests using both solute and heat in a porous medium under natural groundwater flow velocities (Reynolds number, Re < 0.37). The experimental results showed that the LTE assumption can be violated even under natural groundwater flow conditions. The violation of LTE (LTNE) had a significant impact on mechanical dispersion, whereas its effect on velocity was negligible. These results provide the first experimental evidence for LTNE effects in natural conditions. Therefore, it is necessary to consider LTNE effects especially when the mechanical dispersion is evaluated using heat tracing.