Evaluation of groundwater discharge into surface water by using Radon-222 in the Source Area of the Yellow River, Qinghai-Tibet Plateau.

Understanding hydrological processes in the Source Area of the Yellow River (SAYR), Qinghai-Tibet Plateau, is vital for protection and management of groundwater and surface water resources in the region. In situ water measurements of exchange rates between surface water and groundwater are, however, hard to conduct because of the harsh natural conditions of the SAYR. We here present an indirect method using in situ 222 Rn measurements to estimate groundwater discharge into rivers and lakes in the SAYR. 222 Rn was measured in rivers, lakes, groundwater and springs during three sampling periods (2014-2016), and the results indicate large variability in the concentration of the isotope. The data also indicate decreasing 222 Rn trends in groundwater in the cold season (the Feb-2015 sampling period) which may be linked to frequency of capturing 222 Rn in the frozen ground caused by geocryogenic processes. In addition, permafrost spatial extent and freeze-thaw processes have strongly affected the hydrological conditions in the region.