Abstract: Located in the Northeast Tibet Plateau,the Bayin River is the main water resource for industrial and agricultural production,residents daily living and ecological environment in Delingha City of Qinghai Province.With the rapid development of regional and social economy,this area is suffering seriously from the reduction of lake and wetland area,the increase of wasteland,saline-alkali land and sandy land,the decline of regional groundwater level.It is urgent to analyze the relationship between surface water and groundwater for the rational development and utilization of water resources,the treatment of declining groundwater level and the protection of ecological environment.A total of 23 surface water samples,13 groundwater samples and 9 spring water samples along Bayin River Basin were collected in August 2016,the major chemical ions and stable isotopes were analyzed at the laboratory.By using the technique of statistical analysis,the Piper diagram and Gibbs diagram,the paper analyzed the hydrochemical characteristics and its formation process of the Bayin River Basin,and qualitatively analyzed the interaction of surface water and groundwater by hydrogeochemical and isotopic methods based on Total Dissolved Solid(TDS),Cl^- and δ¹⁸O.At the same time,the isotope mass balance method was used to calculate quantitative relationship between surface water and groundwater by using δ¹⁸O.The results show that as the tracers,TDS,Cl^- and δ¹⁸O can effectively indicate the exchanging relationship between surface water and groundwater,and quantitatively evaluate their conversion rate.The main hydrochemical types of surface water and groundwater along Bayin River basin was HCO₃·Cl-Ca(·Mg),while the groundwater hydrochemical types was more diverse.The surface water was controlled by rock weathering,the groundwater and spring water were affected by rock weathering and evaporation.The average values of δD and δ¹⁸O for surface water were -58.00 ‰ and -8.87 ‰,respectively,they were between rain water[δD(-50.45‰), δ¹⁸O(-7.50‰)] and groundwater[δD(-59.34‰), δ¹⁸O(-9.24‰)] in the northern mountain area,indicating that precipitation and the groundwater flow were the main recharge source of surface water.Along the flow direction of the Bayin River,the surface water and groundwater have a tight hydraulic connection with each other,they have been transformed many times.In the upper reach,the groundwater was recharged by the river water leakage and its lateral runoff with the ratios being 65.33% and 34.67% respectively.Up to Heishishan reservoir upstream,the river water was recharged by the groundwater of upper reach and spring water with the ratios being 49.54% and 50.46% respectively.In the middle reach,the shallow groundwater was recharged by river water and lateral runoff coming from the mountain area in the north with the ratios being 65% and 35% respectively;in lower reach,the contribution of the shallow groundwater and spring water discharged to the river water were 53.12% and 46.88% respectively.This study could provide theoretical basis and technical support for setting up the basin hydrologic cycle,revealing the formation mechanism of water resources,sustainably developing water resources and protecting ecological environment.

Key words: hydrochemistry, stable hydrogen and oxygen isotopes, surface water and groundwater, exchanging relationship, Bayin River Basin