Spatiotemporal Variations Of Water Chemistry And Hydrogen And Oxygen Isotopes In Pumqu Catchments And Its Controlling Factors

Rivers are a vital component of the hydrological cycle,their important chemical characteristics can,to some extents,reflect the basic patterns of transport and transformation of the regional environmental chemistry and the overall environmental quality.The chemical composition of a river records factors such as basin lithology,vegetation cover,climate characteristics,anthropogenic activities and major soil types,can contribute to the exploration of the overall environmental quality of the basin and important background information such as rock weathering and physical geochemistry.In recent years,the impact of global warming has intensified global climate change,leading to year-round snow cover and glacial melt,which has altered the water chemistry of rivers.It is therefore important to carry out a further study of the water chemistry characteristics of watersheds in the Tibetan Plateau region.The Pumqu catchments is located in the southwestern part of the Tibetan Plateau and is one of the sources of the Kosi River,the largest river in Nepal.In this study,river water samples were collected and tested during the normal（May）,wet（August）and dry（November）seasons in2021.The study aimed to investigate the spatial and temporal characteristics of water chemistry and hydrogen and oxygen isotopes in the Pumqu basin as well as their influencing factors.The main findings of the study are as follows:（1）Overall,river water in the Pumqu basin tended to be weakly alkaline,with p H values ranging from 7.47 to 8.74.Except a few sampling sites in Yairu Tsangpo（a tributary of the Pumqu）during the wet season,the redox potential was positive during all three water seasons in the catchments,indicating an oxidation condition of the river water.The strongest oxidation state was observed during the dry season.The ranges of TDS and EC varied from 26 to 293 mg·L^-1and from 56 to 586μS·cm^-1during the normal season,from 36 to 207 mg·L^-1and from 71 to 414μS·cm^-1during the wet season,and from 47 to 277 mg·L^-1and from 94 to554μS·cm^-1during the dry season,respectively.TDS and EC displayed a consistent pattern of spatial variation,in rivers not influenced by geothermal springs,with an increasing trend from upstream to downstream,however,increased significantly in areas where geothermal hot spring water replenished the river,and then gradually decreased.（2）In the Pumqu River basin,the main anion concentrations in the river water during the normal,flood,and dry periods were HCO₃^->SO₄^2->Cl^->NO₃^-while the main cations were Ca²⁺>Na⁺>Mg²⁺>K⁺.During the normal,wet,and dry seasons,Ca²⁺and HCO₃^-accounted for66.48%,66.21%,53.89%and 76.05%,62.81%,70.17%of the total cations and anions,respectively.The chemical types of the Pumqu River water were mainly HCO₃·SO₄-Ca and HCO₃-Ca.The ion composition of the Pumqu River water was mainly affected by rock weathering.In the normal season,wet season and dry season,the contribution of carbonate rock weathering was dominated,and the contribution rate reached60.56%,63.52%and 60.46%respectively,while the contribution rate of evaporated salt rock weathering was 16.43%,21.77%and 20.27%respectively,and the contribution rate of silicate rock was 17.88%,11.01%and 14.78%respectively.The content of trace elements was low in the Pumqu River catchments and was mainly influenced by a combination of rock weathering,glacial meltwater,atmospheric deposition and geothermal water.Correlation analysis of trace elements with physico-chemical parameters revealed that the influence of each physico-chemical parameter on trace elements during the dry season was less than during the normal and wet seasons.The results show that the overall water quality in the Pumqu catchments is excellent,with the exception of the normal season when the As content of the river in the Yairu Tsangpo recharged by hot spring water exceeds the national standard for a surface water.In general,anthropogenic impact on the environmental quality of the Pumqu basin is limited.（3）The mean contents ofδD andδ¹⁸O during the normal,wet and dry seasons were-145.90‰and-18.10‰,-157.00‰and-20.20‰,-146.20‰and-17.20‰,respectively.Their spatial variation was not affected by seasonal changes,with an increasing trend of hydrogen-oxygen isotope content as altitude decreased,reaching high values at the exit of the southern slopes of the Himalayas.The hydrogen-oxygen stable isotope relationship equation isδD=8.43δ¹⁸O+6.66（R²=0.9382）during the normal season,δD=7.37δ¹⁸O-7.99（R²=0.8361）during the wet season,andδD=4.91δ¹⁸O-61.98（R²=0.80307）during the dry season.Pumqu is most recharged by atmospheric precipitation during the wet season,followed by the normal season,and least during the dry season.