Spatiotemporal Variations Of Water Chemistry And Its Controlling Factors In The Nyangchu River Catchments,Tibet

As the largest tributary on the right bank of the middle reaches of Yarlung Zangbo River,Nyangchu River is located in Shigatse City,Tibet Autonomous Region.With fertile land and rich products,this basin has been a developed agricultural area in Tibet since ancient times,and is called"Tibetan granary",which plays a decisive role in the economic development of this region.Due to the unique climatic conditions,complex geological structure and active crustal activity in this basin,combined with global warming and rapidly increasing human activities in recent years,it has a certain potential impact on the hydrochemistry of Nyangchu River Basin.In addition,the Nyangchu River develops along the fault zone,and the hot springs in its basin are widely distributed,and the development and utilization of these hot springs are poor.Except for direct bathing,most of them are directly discharged into the Nyangchu River.The discharge of hot spring tail water and some hot water leaked due to the increase of water volume in the wet season have a certain impact on the water environment of adjacent rivers.At present,there are few systematic studies on the hydrochemical characteristics of river water and hot spring water in this basin.In order to reveal the hydrochemical characteristics and controlling factors of Nyangchu River Basin,and to find out the potential influence of geothermal hot spring water widely distributed in the basin and its development and utilization process on the hydrochemistry of rivers in the basin.In this study,the Nyangchu River Basin was taken as the research object,and the systematic water samples were collected from the river water in this basin in 2019 and 2020.Based on the results of data analysis in 2018,samples were collected from five geothermal hot spring waters with high development or utilization rate in2019 and 2020 respectively.The physical and chemical parameters of these waters,the temporal and spatial distribution characteristics of main ions and their controlling factors were studied.The main conclusions are as follows:（1）The river waterin Nyangchu River Basin was weakly alkaline,which had a certain buffering capacity.The conductivity,total dissolved solids,salinity and main ions of river water show the law of normal water period>dry water period>wet water period in time,and show a gradual upward trend from upstream to downstream in space.The main cations Ca²⁺,Mg²⁺,Na⁺and K⁺in river water account for 73.53%,14.34%,10.70%and 1.43%of the total cations,while the main anions HCO₃^-,SO₄^2-,Cl^-and NO₃^-account for 55.35%,41.38%and 2.13%of the total anions,respectively.（2）The hot spring water in the basin is moderately acidic and belongs to reducing water.On the whole,the conductivity,total dissolved solids and salinity of Jinga hot Spring and Kangmachado hot Spring are higher than those of other hot springs.The main cations Na⁺,Ca²⁺,K⁺and Mg²⁺in hot spring water account for 62.24%,26.01%,8.86%and 2.88%of the total cations respectively.The main anions HCO₃^-,Cl^-,SO₄^2-and NO₃^-account for 68.55%,28.04%,3.24%and0.18%of the total anions respectively.（3）On the whole,the concentration of main ions in river water is lower than that in hot spring water except SO₄^2-.Especially,the water conductivity,total dissolved solids,salinity,Na⁺,K⁺and Cl^-in the middle and lower reaches of Chongba Yongqu,Kangru Puqu and Longma River,which are widely distributed in hot springs,are higher than those in the main stream of Nyangchu River.（4）In Nyangchu River Basin,the hydrochemical types of the main stream and tributary Chongbayongqu headwaters are mainly Ca-HCO₃,the middle and lower reaches of the main stream are mainly Ca·Mg-HCO₃·SO₄,and the middle and lower reaches of the tributary Chongbayongqu,Kangru Puqu and Longma River are mainly Ca-HCO₃·SO₄.The hydrochemical types of jinga hot spring in hot spring water are mainly Na-HCO₃·Cl type,chadang micro hot spring is Ca-HCO₃ type,and kangmachaduo hot spring is Ca·Na-HCO₃·Cl type.Mengzha Hot Spring belongs to Na·Ca-HCO₃ type.Shalagang Hot Spring belongs to Na-HCO₃hydrochemical type.（5）The hydrochemical types and main ions of the river water in Nyangchu River Basin were controlled by the weathering of rocks in the basin and the interaction of carbonate rocks and silicate rocks.In addition,when the main ions in micro-hot springs are affected by carbonate weathering,the main ions in other hot spring waters may be affected by other hydrochemical processes besides evaporation and concentration and rock weathering.