Radon (222Rn) activation processes in shallow aquifers in fractured crystalline rocks: A model constrained by long-term monitoring and tracer tests

Abstract Sixteen years of monitoring at St. Vojtěch Springs (Giant Mountains, Czech Republic) form the basis for a calibrated model explaining variations in 222 Rn activity in crystalline aquifers. The model enables estimation of activation zone length (fractures rich in parent radioisotopes) and saturated 222 Rn activity. Eleven springs with an activation zone of primary uranium mineralization in orthogneiss at the contact with dolomite have 222 Rn activities in the range of 185–5469 Bq L −1 , which varies with flow rate. The objective of this study was to quantify the relationship between flow velocity and 222 Rn activity using tracer data and long-term observations. An exponential mathematical model was calibrated at one of the springs with an average flow velocity of 0.44 m h −1 , the saturated 222 Rn activity was 5160 Bq L −1 , and the length of the activation zone was 44 m; these parameters determine 222 Rn activity at each flow rate point. Subsequently, the model was expanded to include various lengths of activation zones and decay zones in which 222 Rn activity decreases due to missing parent isotopes. In fractured, crystalline bedrock aquifers where 222 Rn activity and flow rate relationships are established across various flow conditions, this model provides robust analytical capabilities. With sufficient monitoring data, the lengths of both the activation and the decay zone can be estimated. Without tracer tests, the model can still be applied, estimating the effective pore volumes of both zones instead of their lengths. This research advances the fundamental understanding of radon transport mechanisms in fractured crystalline bedrock aquifer systems.