Localising and quantifying groundwater inflow into lakes using high-precision 222Rn profiles

Summary Groundwater–surface water interaction is a topic gaining increasing attention. Changes in the hydrological cycle due to climatic changes as well as loading of groundwater with nutrients and pollutants motivate a precise investigation of the coupling between groundwater and surface water. Freshwater reservoirs are especially vulnerable to changes in the quantity and quality of the inflowing water. 222 Rn can be used as a sensitive tracer for the investigation of the coupling between groundwater and lakes. So far, simple one or two box models have been used to determine a general estimate of the groundwater inflow. In this study, we introduce a multi-box model for the radon distribution in a lake which accounts for vertical inhomogeneity in the groundwater inflow and transport between the boxes. Measurements of precise concentration profiles during different states of stratification enabled the depth-resolved calculation of the groundwater inflow and the validation of previous mass balance calculations. Furthermore, the multi-box model yields the possibility to detect depth levels of enhanced or suppressed groundwater input and to trace seasonal changes in the interstratification of groundwater in the lake. Additionally, the depth-dependent calculation helps to constrain several parameters in the Rn mass balance. For example, it provides an upper limit for the diffusive radon sediment flux and constrains the estimate of the gas exchange with the atmosphere. Exemplary data from the studied lake show the groundwater inflow to be restricted to a limited layer (upper 5 m) whereas the groundwater interstratification signal and depth varies with stratification pattern in the water column. The average inflow rate determined from the multi-box model (440 ± 140 m 3  d −1 ) agrees with previous one-box model calculations (390 ± 410 m 3  d −1 ), however, the new approach is more precise and yields substantially more information about the system.