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 (²²²Rn) 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 ²²²Rn mass-balance model and a ²²²Rn production-decay model. The results reveal that the ²²²Rn flux from lake water seepage into the aquifer cannot be neglected in the ²²²Rn 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 ²²²Rn mass-balance model, while the velocity of the lake water seepage into the ground was estimated to be 22 ± 14 cm/d using the ²²²Rn 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 ²²²Rn method. The results from ²²²Rn production-decay model enhance the accuracy of groundwater discharge estimation derived from the ²²²Rn 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 ²²²Rn (dual ²²²Rn models: mass-balance model and production-decay model).