Fractionation Mechanism Of Hydrogen And Oxygen Stable Isotope In Unsaturated Zone Of Semi-arid Area

The unsaturated zone is the bond of water circulation from the surface water to theaquifer, dominating the pattern, intensity and dynamic features of the groundwater rechargeand discharge in arid areas. Understanding the origin, influential factors and movement law ofthe unsaturated zone flow is therefore of great significance in clarifying the mechanism andpathway of hydrological cycle. In the present study, stable isotopes of hydrogen and oxygen(D and18O) have been used to investigate the infiltration, evaporation, transpiration,penetration and so on of moisture content in aeration zone. Techniques with hydrogen andoxygen stable isotopes have subsequently become a key methodology for moisture contentmigration in unsaturated zone. It is theoretically vital as well as practical value in studyingfractionation mechanism of hydrogen and oxygen stable isotopes in aeration zone.The paper collects four samples, including eolian sand, sand from Weihe River, loess andpaleosol as unsaturated zone medium. Four methodologies are applied to conduct the study onthe adsorption of hydrogen and oxygen isotopes and its fractionation mechanism undercondition of rainfall infiltration and evaporation. They are indoor simulation experiment, fieldsite experiment, model calculation and theoretical analysis respectively. On this basis, the soilevaporation rate of the loess platform and Weihe River high overbank are estimated.The main contributions of this paper are as follows:(1) The adsorption kinetics of eolian sand for D and18O accords with dual constantequation; while the adsorption kinetics of sand sample from Weihe River, loess and paleosolconform to parabolic equation; the isothermal adsorption of D and18O accords withFreundlich equation, the rank of adsorption capacity for D is loess> paleosol> eolian sand>sand sample from Weihe River; the rank of adsorption capacity for18O is loess> paleosol>sand sample from Weihe River> eolian sand; the adsorption capacity of loess and paleosol is stronger than that of eolian sand and sand sample from Weihe River. Moreover, thefractionation of hydrogen and oxygen stable isotopes, which is an instant equilibrium reactionand the fractionation equilibrium constant equals to fractionation factor, is rather weak duringthe adsorption.(2) The results of column leaching experiment indicate that there is obvious fractionationin the four unsaturated zone medium in the process of infiltrating. The fractionation is thestrongest in the frontal surface of infiltration, and the moisture shifts towards “light” waterbecause of fractionation. The fractionation sequence from strong to weak of hydrogen andoxygen stable isotopes is eolian sand> paleosol> sand sample from Weihe River> loess.During the gravity drainage process in column leaching experiment, there are three phases,including “light” water phase,“heavy” water phase and “equilibrium” phase.(3) The meteoric water line for Guanzhong Basin is δD（‰）=7.9δ18O（‰）+4.5, in whichthe slope and intercept are lower than these of Global Meteoric Water Line. The findings forδD and δ18O in soil profile show that there is diversity among the slopes of different soilmoisture evaporation lines. To be specific, the slope of loess evaporation line is stabilizedbetween3.6-3.8. While the slope of evaporation line for sand sample from Weihe River isaround2.2. There is visible difference in hydrogen and oxygen stable isotope of loess andsample from Weihe River after precipitation, which trends to negative in loess and positive insand sample from Weihe River.(4) The soil evaporation rate in semi-arid area was estimated with hydrogen and oxygenstable isotopes. Consequently, the soil evaporation rate was7.4mm/d and3.5mm/drespectively for loess platform and Weihe River high overbank in June and July. In meanwhile,their soil evaporation rates were calculated with water balance method and flux method, andthe results were7.4mm/d and3.4mm/d by water balance method,5.8mm/d and4.6mm/d byflux method. The results by hydrogen and oxygen stable isotope coincide with water balancemethod, which indicates that hydrogen and oxygen stable isotope method can be applied toestimate soil evaporation rate in semi-arid areas.