Radon emissions from spring outlets: multi-scale assessment of local hotspots and regional atmospheric relevance.

Radon (222Rn) is a radioactive gas produced from the natural decay of uranium in soil, rock, and water, posing a significant health risk due to its association with lung cancer. While indoor radon has been extensively studied, radon degassing from groundwater sources and its contribution to outdoor air quality remain poorly characterized. This study quantifies radon emissions from spring outlets in the Cassino area (central Italy), integrating chamber-based emission measurements with year-long atmospheric monitoring. Emission rates per unit area reached up to 0.072 Bq, and atmospheric radon concentrations measured directly above spring outlets were up to 17 times higher than concurrent background levels, with peak values reaching 287 Bq m􀀀 3. Continuous background monitoring revealed persistent diurnal variability driven by boundary-layer dynamics, with morning concentrations averaging 2.3 times the afternoon minimum and a clear seasonal maximum in autumn. Despite the pronounced localized enhancement, order-of-magnitude dilution estimates indicate that the contribution of individual spring outlets to the valley-scale atmospheric radon background is negligible. These results demonstrate that groundwater-fed springs can generate outdoor radon hotspots while exerting minimal influence on regional atmospheric concentrations, clarifying their role within the broader radon budget of carbonate aquifer systems influenced by major fault structures.