Using new mass balance methods to estimate gross surface water and groundwater exchange with naturally occurring tracer 222Rn in data poor regions: a case study in northwest China

Abstract Given that the concentration of 222 Rn in groundwater is much higher than that in surface water and that its radioactive half‐life (3.83 d) is short, 222 Rn is an effective tracer of groundwater–surface water interactions. In this study, a new mass balance method is presented, which can be used to estimate specific groundwater–surface water interactions within a river reach. Three possible situations of interaction between groundwater and surface water are considered, and equations based on the mass conservation of 222 Rn are formulated for judging specific groundwater–surface water interaction processes and for calculating water flux. A case study was conducted for the Nalenggele River, Northwest China, to demonstrate the usefulness of this method. Samples of river water and groundwater containing 222 Rn were collected from the study area to estimate the interactions between groundwater and surface water. The amount of water exchanged during these interactions was estimated and the results show that transformations between groundwater and surface water are frequent along the stream. The 222 Rn mass balance method is highly sensitive for studying such interactions, even in areas for which conventional hydrologic data are sparse. Copyright © 2014 John Wiley & Sons, Ltd.