Study On The Variation Law Of Hydrogen And Oxygen Stable Isotopes In The Process Of Evaporating Water Surface And Soil

Evaporation is one of the main reasons for the fractionation of stable isotopes of hydrogen and oxygen in water.Studying the hydrogen and oxygen stable isotope fractionation in the evaporation process will help to understand the hydrological cycle process.Through a series of indoor water surface evaporation and unsaturated soil column evaporation experiments with controllable temperature and humidity,the stable isotope fractionation results of the residual water ratio f of the water surface evaporation test under the influence of temperature and humidity are analyzed respectively,and the theoretical simulation results are compared.The?D and?¹⁸O changes of groundwater and profile soil water during the evaporation of unsaturated soil columns are explored,and the influencing factors of hydrogen and oxygen stable isotope fractionation during the evaporation process are analyzed from the aspects of the lithology of the unsaturated zone,the changes of profile water content and the changes of profile soil salinity.Draw the following main conclusions:(1)In the process of free surface evaporation,when the remaining water ratio is about 0.5,?D and?¹⁸O are linearly related to the remaining water ratio f.At the same humidity(temperature),the higher(lower)the temperature(humidity),the faster?D and?¹⁸O change with f.The lower the slope of the evaporation line,the smaller the residual deuterium d.Theoretical simulation results show that the dynamic fractionation simulation results under the same evaporation environment are closer to the measured results than the equilibrium simulation results,and can better reflect the degree of isotope fractionation in the actual evaporation process.(2)Regardless of the evaporation environment,regardless of the amount of evaporation,the hydrogen and oxygen stable isotopic compositions of pure soil pillars and groundwater containing 60%sand pillars are in a stable state,almost unchanged.The slope of the?D-?¹⁸O fitting line of the soil water in the soil column profile is smaller than the slope of the atmospheric precipitation line,and it becomes smaller and smaller as the evaporation progresses.(3)The profile moisture content of pure soil and 60%sandy soil pillars generally increases with depth,but the profile moisture content of pure soil soil pillars during the evaporation process is greater than the original moisture content.The water content of the 60%sandy soil column is greater than the original water content only in the deep part of the soil column.The distribution of?D,?¹⁸O,and deuterium remaining(d)on the soil column profile has a strong correlation with the distribution of moisture content.The distribution of hydrogen and oxygen stable isotopes in the process of soil evaporation is closely related to the movement of water.In the process of evaporation,isotope fractionation mainly occurs when water molecules leave the water body and enter the atmosphere,and no obvious isotope fractionation occurs at other locations.And under the same conditions,the smaller the soil pores are,the closer the isotope fractionation occurs to the ground surface.(4)The p H of the soil column profile during the evaporation process of the pure soil soil column and after the evaporation is higher than the p H of the loess before evaporation,showing strong alkalinity.The TDS of the soil leaching liquid on the surface of the soil column becomes larger and larger as the evaporation progresses,and the TDS becomes smaller and smaller as the depth increases.7 types of ions such as K⁺,Ca²⁺,Na⁺,Mg²⁺,Cl^-,NO₃^-,SO₄^2-are concentrated in the surface layer of the soil due to the movement of water during the evaporation process,and change smoothly within the soil column.CO₃^2-and HCO₃^-did not appear to be enriched on the surface,but reached the maximum at a distance of 3 to 5 cm from the surface of the soil column,and there were fluctuations inside the soil column.Correlation analysis of soil soluble salt and stable isotope D and ¹⁸O in the profile of the pure soil column shows that the migration and accumulation of salt in the soil during the evaporation process promotes the isotope fractionation in the soil column,but has little effect.