Tracing potential water sources of the Nagqu River using stable isotopes

Study region: The Nagqu River watershed (NRW) on the Qinghai-Tibet Plateau (QTP). Study focus: Precipitation, snowmelt, streamflow, and groundwater samples were collected in the NRW and used to analyze the contribution of different water sources to streamflow during different freeze-thaw periods. Stable isotopes of hydrogen (D) and oxygen (O-18) and water chemistry were analyzed for the period ranging from 2016 to 2019, and tritium (T) and carbon-14 (C-14) isotopes were analyzed for 2019. New hydrological insights for the region: Rain and snowmelt were found to reflect the chemical characteristics of precipitation, and the primary ions in both water types were Ca2+ and SO42- The 8D and delta O-18 isotope values of rain and snowmelt were relatively low during the entire thaw period (from June to October) and relatively high in the entire frozen period (from November to May). The results of the optimum multiparameter and mass balance analyses showed that groundwater and snowmelt accounted for approximately 39.8 % and 32.2 % to the Nagqu River water flow, respectively, whereas precipitation accounted for approximately 28.0 %. The groundwater in elastic rock fissures around the northern Cuona Lake, and permafrost and ground ice in the bedrock layer were identified as older water types with relatively low infiltration rates, and bedrock fissures in the southeastern and southwestern areas contained younger water with better recoverability.