Hydrogeochemical Characteristics And Groundwater Discharge In The Buha River Basin Of Qinghai Lake

Qinghai Lake is an important barrier for ecological security in the northeast of the Qinghai-Tibet Plateau.The Buha River basin located in the northwest of Qinghai Lake,as the largest and longest river of the lake,contributing about half of the total runoff into the lake,which plays an important role in regulating the water quantity and quality for Qinghai Lake.Surface water-groundwater system is the main recharge source of Qinghai Lake,and plays a key role on the water environment.However,the studies on long time series surface water-groundwater interactions and Lacustrine Groundwater Discharge（LGD）and LGD-derived nutrient loadings in Qinghai Lake are very limited.This study focuses on the investigations of surface water-groundwater in Buha River basin of Qinghai Lake.Long time series（the time scale including every quarter,every moth and every quarter）and high resolution（the spatial scale including basin and hyporheic zone）water samples from the lake,groundwater,pore water and rivers were collected to study the spatial distribution and temporal change of hydro-geochemistry during November 2020 to October 2021.Hydrogen and oxygen isotopes and radon isotope(²²²Rn)were employed to quantify the surface water-groundwater recharge,LGD and LGD-derived nutrients and heavy metals fluxes loadings in Qinghai Lake in different seasonal.This study mainly obtained the following understandings.（1）The obvious temporal and spatial dynamic characteristics of hydrochemistry and isotopes were observed in the Buha River basin.The TDS and main ions gradually decrease along the runoff direction,while the contents of Na⁺and HCO₃^-gradually increase in the river water and groundwater.The main ions content and ²²²Rn activities of rivers and groundwater were higher in dry season than that in wet season.²²²Rn activities of river water and groundwater in the middle reaches is higher than in the upper and lower reaches,which is contrary to the distribution ofδD andδ¹⁸O.The ²²²Rn activities in lake water showed a seasonal variation pattern of high in dry season and low in normal season and wet season,and has the alternating characteristics of high days（6:00～18:00）and low nights（18:00～6:00）.The spatiotemporal distribution of hydrochemical and ²²²Rn activities in hyporheic zone become very complex,they will have different changes at different depths,different offshore distances and different seasons.（2）Water-rock interaction and evaporation are the main factors of controlling the hydrogeochemical process in the Buha River basin.The hydrochemical types of different endmember water bodies in the study area are mainly divided into two categories:Na-Cl type in lake water and pore water;Ca-HCO₃ type in river water and groundwater.The solutes of river water and groundwater in the Buha River Basin mainly come from silicate weathering and carbonate weathering.The hydrochemical of lake water and pore water is mainly controlled by the weathering dissolution of carbonate minerals and evaporation.The hydrochemical evolution characteristics of end member water show that the lake water in Qinghai Lake is mainly recharged by river water and groundwater,and the river is an important source of recharge.（3）The composition and distribution characteristics of hydrogen and oxygen isotopes in water show that river water is recharged by precipitation and groundwater,and groundwater is recharged by river water and precipitation,and lake water is recharged by river water,groundwater and atmospheric precipitation.However,the recharge and discharge relationship between groundwater and river water will change with the change of season and spatial location.Along the direction of runoff,groundwater-surface water recharge is gradually enhanced,and the discharge is greater during the abundant water period than during the dry water period in the Buha River basin.（4）The ²²²Rn activities in water is controlled by hydrochemistry,concentration of Uranium series and Thorium in sedimentary is related to the source of ²²²Rn.From the perspective of the entire watershed,the radionuclide content in the surrounding rock is the decisive factor affecting ²²²Rn activity in the water.²²²Rn activity in surface water is also affected by hydrochemical,temperature,and atmospheric escape except source.（5）The hyporheic zone is the main input path of pollutants such as nutrients and heavy metals in lake water.The hydraulic connection between surface water and groundwater gradually increased along the runoff direction in the Buha River basin of Qinghai Lake.The LGD in wet season is about 14 times of that in dry season and 1.8to 3 times of that in flat water period.LGD and LGD derived nutrients and heavy metals with a seasonal distribution pattern of wet season>normal season>dry season.The study was based on water chemistry and isotope characterization,and modeling calculations were conducted to quantify the fluxes of groundwater discharge to the lake and the fluxes of substances carried,providing a certain scientific basis and basic support for pollution management in the highland lake environment.