Hazard and risk assessment from geogenic radon isotopes in urban subsurface environment considering groundwater

Radon isotopes, including Rn-222 and Rn-220, are naturally occurring carcinogenic radioactive gases that pose critical health risks in urban subsurface spaces. Geogenic radon hazards are commonly evaluated using Geogenic Radon Potential (GRP), including the Predictive Geogenic Radon Potential (P-GRP) approach. This study extends the P-GRP approach by incorporating groundwater saturation-dependent transport and Rn-220 source terms for the assessment of geogenic Rn hazard and risk in urban subsurface environments. Seoul, South Korea, was selected as the study area because extensive underground development has increased the need to assess geogenic Rn hazards and risks. A hydrogeological conceptual model was constructed by vertically classifying geological media and incorporating depth-dependent parameters. Fate and transport of Rn were simulated to characterize geogenic Rn hazards, and the resulting source terms were incorporated into a conceptual site model for indoor risk assessment at subway station A. The results showed that Seoul Granite produced approximately 11-20 times higher calculated Rn concentrations than Banded Gneiss, and that Rn-220 concentrations exceeded Rn-222 concentrations across the simulated depth range in the geological setting of Seoul. Compared with the previous dry-media P-GRP model, the groundwater-coupled model produced water-table-related discontinuities and approximately order-of-magnitude enrichment in Rn concentration profiles. The indoor risk assessment suggested that Rn-220 may contribute to the calculated upper-bound risk under the well-mixed assumption, while the Rn-222-only risk also exceeded the reference risk guideline of 10⁻⁶. The extended P-GRP framework provides a physics-based screening tool for geogenic Rn hazard and risk assessment in urban underground spaces, supporting preliminary evaluation.