Representative groundwater sampling is essential for monitoring and evaluating the self-purification processes taking place in an aquifer. It entails pumping the monitoring well concerned until the pumped water matches the surrounding groundwater. The natural isotope ²²²Rn enables the hydraulic criterion of sampling to be monitored for the first time. When the radon activity concentrations of the pump water are measured, the initial concentration rises and reaches a plateau. This plateau value is a measure of the characteristic radon activity of the aquifer in the immediate vicinity of the monitoring well. Reaching the plateau indicates that the stand pipe water and the filter pore water have been completely replaced.