Groundwater-surface water interactions derived by hydrochemical and isotopic (222Rn deuterium, oxygen-18) tracers in the Nomhon area, Qaidam Basin, NW China

Understanding the interaction between groundwater and surface water is of vital significance for the sustainable management of water resources in arid and semi-arid areas. In this study, multi environmental tracers (hydrochemical parameters, stable hydrogen and oxygen isotopes and radioactive Rn-222) were employed to investigate the interaction between groundwater and surface water along two rivers (Tiangeli River and Nomhon River) in the Nomhon area, southeast of the arid Qaidam Basin, northwest China. Here we observed that the Rn-222 concentration of waters were distinctly different with a decrease order of groundwater > spring > river in this area. Along the Tiangeli River, the Rn-222 concentrations of groundwater, springs and river water were about 12,120-17,407, 3013-15,819 and 809-8205 Bq/m(3), respectively. This river was characterized by the high Rn-222 concentration of the river water and was investigated to be mainly fed by springs and groundwater discharge. Along the Nomhon river, Rn-222 concentration of groundwater and river water was about 6772-60,369 and 14-519 Bq/m(3), respectively. An average background of the Rn-222 concentration in the upstream of the river was determined to be about 43 Bq/m(3). A high Rn-222 concentration zone along the river appeared in the midstream, with a peak value of 519 Bq/m(3) and a distance about 10 km, indicating that groundwater discharged to the Nomhon River in the alluvial plain. A Rn-222 mass-balance model was constructed to estimate the groundwater discharge into the river, showing a maximum groundwater discharge of 0.37 m(3)/s in the midstream of Nomhon River during the sampling period. The stable isotopic compositions (delta D and delta O-18) and hydrochemical characteristics (hydrochemical type, TDS and EC) showed corresponding changes along these two rivers, which also provided evidences for the groundwater-surface water interactions in the Nomhon area.