Dual ²²²Rn Models for Tracing Groundwater–Lake Water Exchange in a Flow‐Through Lake

Abstract Groundwater–lake water exchange in flow‐through lakes which includes both groundwater discharge into the lake and lake water seepage into the aquifer, is crucial for sustaining lake wetland ecosystems; however, these two processes are rarely addressed simultaneously by tracer methods. In this study, radon ( 222 Rn) is used as a tracer to estimate both groundwater inflow and outflow in flow‐through lakes in the Poyang Lake area, using a combination of a 222 Rn mass‐balance model and a 222 Rn production‐decay model. The results reveal that the 222 Rn flux from lake water seepage into the aquifer cannot be neglected in the 222 Rn mass‐balance model for flow‐through lakes. The velocity of groundwater discharging into the flow‐through lake was determined to be 23 ± 13 cm/d based on the 222 Rn mass‐balance model, while the velocity of the lake water seepage into the ground was estimated to be 22 ± 14 cm/d using the 222 Rn production‐decay model. A valid point dilution test was used to estimate the groundwater Darcy velocity near the flow‐through lake as 24 cm/d, which closely matches to the results obtained using the 222 Rn method. The results from 222 Rn production‐decay model enhance the accuracy of groundwater discharge estimation derived from the 222 Rn mass‐balance model. This study demonstrates both processes of groundwater–lake water exchange (groundwater discharge into the lake and lake water seepage into the ground) in a flow‐through lake can be estimated using only 222 Rn (dual 222 Rn models: mass‐balance model and production‐decay model).