Using ²²²Rn as a tracer in hydrogeological studies: a critical review of qualitative and quantitative assessments of potential sources of error

Radon-222 (²²²Rn), a radioactive noble gas produced by the decay of ²²⁶Ra in geological materials, is widely applied as a natural tracer in hydrogeological investigations. Its ubiquitous occurrence in groundwater, conservative behaviour in aqueous systems, and relatively straightforward on-site detection have made it a powerful tool for identifying groundwater flow paths, quantifying groundwater–surface water exchange and estimating water residence times. However, despite its extensive use, radon-based studies often suffer from large uncertainties and inconsistent results, frequently caused by methodological issues. This contribution presents a comprehensive critical review of the main qualitative and quantitative sources of error associated with the application of ²²²Rn as a tracer.We analyse the fundamental physical processes controlling ²²²Rn production, emanation from the mineral matrix, accumulation in groundwater under secular equilibrium conditions, and partitioning between water and gas phases and link them to practical aspects of field sampling, on-site and laboratory measurements, and data evaluation. Emphasis is given to the performance and limitations of mobile radon detectors commonly used in hydrological studies, for which we assessed how detector sensitivity, response time, air humidity, carrier gas composition, and internal air-loop configuration influence measurement accuracy and precision. We further discuss the influence of water temperature and salinity on the radon water–air partition coefficient and demonstrate how neglecting these parameters can lead to systematic biases in calculated ²²²Rn-in-water concentrations. Determining representative groundwater endmembers is identified as a key challenge with regards to natural spatial and temporal variability in aquifer properties. Finally, we discuss uncertainties arising from the stochastic nature of radioactive decay.The review identifies critical steps where avoidable errors commonly occur, including sample collection and handling, degassing during pumping and storage, diffusion losses through container materials, and inappropriate selection of water and air volumes during extraction. Practical recommendations are provided for survey design, sampling strategies, measurement protocols, and data processing, to minimise avoidable errors and improve reproducibility.By systematically addressing the physical, technical, and methodological aspects of ²²²Rn measurements, this review provides a consolidated framework for best practice in radon tracer studies, supporting more robust applications in hydrogeology and increasing confidence in both qualitative interpretations and quantitative flux estimates.