Seasonal variation of groundwater quality and potential risks in U-bearing formations revealed from hydrogeological-microbiological investigation

This study investigates the influence of hydrological and microbial conditions on uranium (U) mobility, water quality, and associated health risks in U-bearing formations near a lake in South Korea. Since 2021, natural analogue studies have been conducted to ensure the safety of geological disposal of spent nuclear fuels. The study site experiences two distinct seasons: dry and wet. Using a combination of radon, strontium isotopes, noble gases, microbial community analysis, and flow system modeling, the study examines seasonal variations in groundwater characteristics. Results show that U concentrations are low in subsurface environments under reduced conditions, which is supported by low dissolved oxygen levels and the presence of Pseudomonas and Clostridium genera. In deeper wells, radiogenic ⁴He was detected, indicating the decay of ²³⁸U. The ⁸⁷Sr/⁸⁶Sr and ²³⁴U/²³⁸U activity ratios suggest mixing between water bodies at shallow depths during the wet season, along with longer residence times indicated by radon. Flow velocity was higher during the wet season. Health risks from U exposure were generally low due to limited U mobility, but shallow groundwater was classified as undrinkable, especially during the wet season. The study demonstrates that U mobility is seasonally controlled by hydrological and microbial conditions, highlighting the importance of dual indices—groundwater quality and health risks—for effective water management. These findings are crucial for evaluating the safety of geological disposal of radionuclides under varying hydrological conditions.