Hydrochemical and isotopic interpretation of interactions between surface water and groundwater in Delingha, Northwest China

Understanding the interactions between surface water and groundwater is of considerable significance for managing water resources and protecting ecosystems in arid and semi-arid areas. This study investigated the hydrochemistry, stable isotopes (D–18O) and radioisotopes (222Rn) to identify the interactions between surface water and groundwater in Delingha, a typical arid area in Northwest China. In this area, the surface water mainly consists of the Bayin River, which flows from the north mountainous area to the southwest terminal lake areas (Hurleg and Tuosu lakes), and the groundwater also generally flows from north to southwest and is characterized by a single-layered sandy and gravelly aquifer in the alluvial–proluvial plain and multi-layered aquifers in the alluvial–lacustrine plain. The stable isotopes reveal that the water bodies in the plain are mainly recharged by precipitation in the mountainous area. The hydrochemistry and isotopes results (TDS, D–18O, and 222Rn) combined with the groundwater level indicate that the surface water and groundwater interactions exhibit spatial variability. It was found that the Bayin River generally infiltrates into aquifer in the alluvial–proluvial plain and repeatedly interchanges with groundwater in the alluvial–lacustrine plain. The flux rate of groundwater discharge in the alluvial–lacustrine plain is estimated to be 0.55–2.49 × 10−4 m3/(s·m) by the 222Rn mass balance model, showing a decreasing tendency along the flow path. In addition, the flux rate of river leakage occurring in the upper and middle areas of the alluvial–lacustrine plain is estimated to be 1.65–3.19 × 10−4 m3/(s·m). These findings provide a hydrological conceptual model and valuable flux data of water interchange for water resource management.