Abstract: To reveal the influence law and mechanism of rainfall on soil radon concentration, this study combined with experimental observation and numerical simulation to research the dynamic response characteristics of soil radon concentration under rainfall conditions. Real-time observation under different rainfall were carried out, and the dynamic changes of parameters such as precipitation, soil radon concentration, and soil gas permeability were continuously monitored. A theoretical assumption of soil radon concentration was proposed, which was composed of four mechanisms including pore reduce, gas attenuation, cover layer, and rainfall infiltration. A two-layer soil water-air radon transport model was constructed to achieve theoretical simulation of the radon concentration field during rainfall infiltration. The research results show that the four-coupled mechanism model constructed by the two-layer soil model can reproduce the response trend of soil radon concentration in the rainfall experiment, which first rises and then drops. About 94% of the model prediction values fall within the ±2σ range of the measured values, close to the theoretical coverage rate of 2σ confidence level (95.4%). Normalization analysis indicates that the cover layer effect is the parameter among the four mechanisms that is most sensitive to water content changes. The infiltration convective velocity decreases after reaching the peak, revealing the convective upper limit effect caused by the disappearance of the gas phase channel under high water content. As the total rainfall increased from 64 mm to 227 mm, the simulation deviation changed from the lag of radon concentration rise to systematic underestimation. The average absolute error (MAE) increased from 1.96 to 3.81 kBq/m³, reflecting that the model's expression of the surface closure and radon accumulation mechanism caused by the cover layer effect during the heavy rainfall period is not yet sufficient, and there is still needed to improve in the amplitude of the transient response of the four-coupled mechanisms. The research results can provide a theoretical basis for eliminating the interference of rainfall and improving the application of soil radon concentration in radon geological potential mapping and radon data processing in uranium geological exploration.

Key words: soil radon, rainfall, water content, two-layer soil model