Study On The Relationship Between Water Cycle Characteristics And Transformation In Inner Mongolia Section Of The Yellow River Basin Based On Water Chemistry And Isotopes

Water resource is the most critical basic resource element in the Yellow River Basin.Under the influence of climate change and human activities,the upper,middle and lower reaches of the Yellow River are faced with different types of water resources and environmental ecological problems.Taking the Mongolian section of the Yellow River Basin as the research area,water samples of precipitation,main stream of the Yellow River,tributaries of the Yellow River,seasonal rivers,groundwater,lake water,canal water and other water bodies were collected in July 2021（wet season）,October 2021（normal season）and April 2022（dry season）.The hydrogeochemical characteristics of different water bodies and their driving factors were analyzed by measuring the hydrochemical ion concentration andδD andδ¹⁸O values of different water bodies,using Piper three-plot,Gibbs model,ion ratio method,chlor-alkali index and other methods.The transformation relationship between precipitation,surface water and groundwater was revealed by means of precipitation line,evaporation line,d-excess model and Mix SIAR mixing model.The main research results are as follows:（1）The p H value of surface water was weakly alkaline in the period of abundance,flat and low water,and the TDS of different water bodies varied significantly with the seasons.The overall p H value of groundwater is medium alkaline,mostly low salinity water,TDS is significantly lower than that of surface water.The chemical types of surface water and groundwater are rich in diversity,mainly SO₄·HCO₃-Na type and Cl·SO₄-Na type.The average concentration of major ions in the surface water was basically the same in the period of abundant,flat and low water,showing Cl^->SO₄^2->HCO₃^->NO₃^-,Na⁺>Ca²⁺>Mg²⁺>K⁺.Order of average concentration of major ions in groundwater and surface water.（2）All the water bodies in different periods are basically located in rock weathering area and evaporation concentration area,indicating that the ions in the water bodies in the study area are mainly derived from the weathering release of minerals,and at the same time,they are located in the area with strong evaporation,which has a direct relationship with the arid and semi-arid climate in the study area.There was significant correlation between Na⁺and SO₄^2-,suggesting that there might be mirabilite leaching in groundwater in this area.Ca²⁺and Mg²⁺are significantly correlated with HCO₃^-,and HCO₃^-in groundwater is mainly derived from carbonate karst filtration,indicating that Ca²⁺and Mg²⁺are mainly derived from carbonate karst filtration.Mg²⁺is significantly correlated with Cl^-,Ca²⁺and SO₄^2-,which is similar to the correlation of surface water ions,indicating that the ions in groundwater are mainly derived from high magnesium calcite,albicite and surface water leaching.（3）Based on the measured data,the equation line of atmospheric precipitation in the study area was established:δD=6.89δ¹⁸O-2.55（R²=0.88）.The mean difference of groundwater isotope is small during abundant,flat and dry period.The slope and intercept of surface water evaporation line and underground water line are smaller than that of local precipitation line in the three periods.TheδD value of surface water is higher in wet season than in normal season and dry season.The fluctuation of surface water is the strongest in dry season and the fluctuation range in normal season is slightly larger than that in wet season.In wet season,theδD andδ¹⁸O values of groundwater concentrated in the south bank of the Yellow River,especially in the southern part of the Qianliougou-Molin River area and the middle part of the Ten Kangdui-Heidaigou area.In normal season,theδD andδ¹⁸O values showed a trend of first increasing and then decreasing from upstream to downstream,and the high values mostly appeared near the main stream of the Yellow River.In dry season,with the increase of latitude,TheδD andδ¹⁸O values of groundwater decreased first and then increased,which was slightly different from that of the two periods of abundance and drought,and the high value area appeared in the high latitude area.The height effect ofδD andδ¹⁸O values in the study area is not significant.（4）In the Mongolia section of the Yellow River Basin as a whole,the contribution rates of precipitation and surface water to groundwater recharge are 47.6%and 52.4%respectively in wet season,37.8%and 62.2%respectively in normal season,and 30.7%and 69.3%respectively in dry season.Surface water is the main source of groundwater recharge.The contribution rates of precipitation and groundwater to surface water recharge are 97.1%and 2.9%in wet season,95.0%and 5.0%in normal season,and43.4%and 56.6%in dry season,respectively.Precipitation is the main source of surface water recharge.