Dynamic variation characteristics of ²²²Rn in typical karst cave systems in south China and their influencing factors

Radon (²²²Rn) gas is a natural tracer for air exchange between cave atmospheres and external environments. Investigating the spatiotemporal variation characteristics and controlling factors of its activity is crucial for revealing seasonal cycles and short-term fluctuation mechanisms of cave air exchange. Based on three years of continuous monitoring (from January 2018 to December 2020) of atmospheric ²²²Rn in Maomaotou Big Cave, Guilin, South China, this study systematically analyzes the spatiotemporal patterns and driving mechanisms of ²²²Rn activity concentrations. The cave exhibits two distinct seasonal gas circulation patterns: a degassing circulation mode in winter and spring and a gas recharge circulation mode in summer and autumn. Seasonal variations of ²²²Rn are primarily controlled by ventilation effects driven by temperature gradients between the cave interior and exterior. When external temperatures exceed cave temperatures, atmospheric stagnation promotes ²²²Rn accumulation; conversely, ventilation-induced dilution occurs when external temperatures drop below cave temperatures. Additionally, atmospheric precipitation significantly influences ²²²Rn activity concentrations, showing an inverse relationship where higher rainfall corresponds to lower ²²²Rn activity concentrations and vice versa. This phenomenon correlates with rainfall-induced soil moisture variations that regulate radon exhalation rates and subsequent diffusion processes. Furthermore, ²²²Rn activity concentrations also depend on the distribution of its parent radionuclide ²²⁶Ra in overlying soils and soil moisture content, which influences the radon dissolution and transport to the cave by dripwater. Through multifactorial analysis, this study elucidates synergistic mechanisms involving temperature gradients, precipitation, soil properties, and air exchange in regulating ²²²Rn dynamics. The findings provide critical scientific insights into cave gas dynamics and environmental effects, enhancing the understanding of subterranean air circulation patterns while offering theoretical support for cave environmental monitoring and health risk assessment.