Application of Rn-222 isotope for the interaction between surface water and groundwater in the Source Area of the Yellow River

A pioneering rapid and direct measurement of dissolved Rn-222 in the field has been used here to explore interaction between surface and groundwater in the source area of the Yellow River (SAYR). The results indicate average Rn-222 activity of 2,371 Bq/m(3) in surface water and 27,835 Bq/m(3) in groundwater. The high Rn-222 activity (up to 9,133 Bq/m(3)) found in the southeast part of the SAYR suggests possible influence of permafrost on the exchange between surface water and groundwater. The remarkable contrast among the different samples of a stream in the Shuangchagou basin, a typical basin in the SAYR, clearly indicates groundwater infiltration along the north tributary and occurrence of groundwater end-member in the south tributary. Considering no Rn-222 decay and atmospheric evasion, the daily average fraction of groundwater input to the surface water through the end-member in a location (S1) is estimated at 19%. Despite the up to 40% uncertainty, this is the first estimate of a reference value for groundwater input in this basin and which can be improved in the future with more samples and measurements. Rn-222 can be a rapid and easily measured tracer of surface water-groundwater interaction for future investigation in the Qinghai-Tibet Plateau.