Application of radon dynamics in analyzing karst spring flood processes:A case study of the Yaji Experimental Site,Guilin

LI Zhijie; JIANG Guanghui; GUO Fang; LIU Fan; WEI Liqiong; WANG Qigang

doi:10.11932/karst20250503

Abstract：This study utilized the naturally occurring tracer radon(222Rn)to analyze the hydrological responses of a karst spring system,aiming to elucidate variations in flow composition and recharge pathways within the karst aquifer.Three typical flood events were investigated at the Yaji Experimental Site in Guilin to understand the response mechanisms of this karst aquifer,which exhibits high heterogeneity due to its conduit-fracture system.This heterogeneity leads to rapid responses to rainfall,frequently resulting in flash floods.High-resolution monitoring of spring discharge,electrical conductivity,stable hydrogen isotope(δ2H),and 222Rn activity during these events captured the dynamic response of the system to rainfall-induced recharge,allowing for an assessment of contributions from different water sources to spring discharge.These sensitive hydrological indicators,including 222Rn and other hydrochemical parameters,revealed the complexities of aquifer recharge processes,thereby enriching our unders-tanding of karst system responses to intense precipitation. The response time and tracer variations in the karst spring exhibited significant dependence on rainfall intensity and duration,showing a notable lag effect.Peak spring discharge lagged behind rainfall peaks by 5 to 12 hours,depending on specific rainfall characteristics.A decrease in electrical conductivity after peak discharge suggested a rapid influx of surface runoff;conversely,the peak in 222Rn activity was delayed by approximately 10 hours relative to the conductivity peak,indicating multi-pathway recharge into the aquifer.While conductivity primarily reflected the rapid influx of surface runoff,the delayed 222Rn peak indicated slower percolation through the soil and epikarst zone.Variations in hydrogen isotopes further illustrated the mixing between event water and pre-event water,highlighting complex recharge dynamics under varying rainfall conditions. Under the influence of rainfall intensity and recharge pathways,the responses of various tracers to rainfall events showed significant differences.During high-intensity rainfall,rapid surface runoff infiltrated the system,significantly altering spring hydrochemistry.In contrast,recharge from the soil and epikarst zone,characterized by slow infiltration,gradually increased 222Rn activity.End-member analysis,based on tracer concentrations,differentiated pre-event water(stored water prior to rainfall)from newly introduced event water.Modeling indicated that changes in conductivity and 222Rn activity corresponded to rapid surface input and slower subsurface recharge,capturing the effects of diverse recharge sources and pathways. Post-rainfall increases in 222Rn activity suggested prolonged recharge from the soil and epikarst zone,a feature not fully captured by conductivity and δ2H data.The delayed 222Rn response highlighted the sustained recharge from the epikarst zone,thereby enhancing the understanding of recharge timing and pathways in karst systems.222Rn has been validated as an effective tracer in groundwater recharge processes,providing valuable insights for water resource management in karst regions.By distinguishing the contributions of surface runoff and groundwater flow,this method offers significant implications for flood risk prediction and groundwater recharge management,providing essential support for water resource planning and risk management in karst landscapes.