Determining Groundwater Recharge Mechanism Based On Isotope And Chloride Characteristics Of Soil Water From A Deep Soil Profile

Due to the deep vadose zone,precipitation is only water source for groundwater recharge on the Loess Plateau.Determining groundwater recharge mechanism is of great significance for the local water resource management;however,this mechanism is still controversial.Soil water isotopes(including?²H and?¹⁸O)and chloride tracing are the ideal techniques in hydrological studies,and have been applied in many researches carried out on the Loess Plateau.Unfortunately,the sampling depths of most current studies were less than 20 m,which is far less than the groundwater level.In order to understanding the groundwater recharge mechanism on Loess Plateau,we sampled an undisturbed soil core with a depth of 98 m?the groundwater table was95 m?in Changwu tableland,Shaanxi Province,and then measured soil water isotope and chloride concentrations.The classical statistics and geostatistical methods were adopted to analyze the hydrogen and oxygen stable isotopic variation and spatial correlation.The groundwater recharge was revealed by the isotopic and chloride concentration information of precipitation,deep soil water,and groundwater,and the relative contribution of piston flow and preferential flow to groundwater was calculated by the MIXSIAR Bayesian model.The main conclusions are as follows:1.Revealed the local isotope distribution characteristics of precipitation.The precipitation has obvious amount effect,and the isotopes were more depleted with the increasing precipitation amount.Further,the precipitation isotope had obvious seasonal effect,and it depleted in summer and enrichment in other seasons.Last,the isotopes variations were greater with the precipitation amount increasing.2.Revealed the isotopes vertical distribution characteristics in deep soil profile.According to the variation of the isotope,the profile was divided into four layers:the shallow layer,deep layer,transition layer,and groundwater layer.Soil water isotope obey the normal distribution in all of the four layers,and the range of isotopes decreased with increase of depth.The order of average values of?²H in four layers are:transition layer?-74.7‰??deep layer?-75.2‰??groundwater layer?-76.5‰??shallow layer?-78.6‰?.The order of average values of?¹⁸O are:deep layer?-9.9‰??transition layer?-10.2‰??groundwater layer?-10.4‰??shallow layer?-10.9‰?.The average value of?²H and?¹⁸O in deep layer were significant different from the shallow layer and groundwater layer?p<0.05?.The standard deviation and coefficient of variation of?²H and?¹⁸O decreased with the depth increased.Spherical model can describe the spatial correlation of?²H and?¹⁸O in each layer.The nugget to sill ratio from the shallow depth to the groundwater gradually decreased from 24.4%to 6.3%for?²H and from 24.0%to 13.4%for?¹⁸O.3.Revealed the groundwater recharge mechanism.The isotopes in groundwater were more depleted than that of deep soil water but more enriched than that of heavy precipitations.In addition,the chloride concentration in groundwater was smaller than that of deep soil water,but higher than that of precipitation.Therefore,we conclude that the groundwater was recharged by both piston flow and preferential flow.The MIXSIAR Bayesian model calculated the contribution of piston flow was 61.7%-90.9%,and the contribution of preferential flow was 9.1%-38.3%,judged by which the groundwater was mainly supplied by the piston flow.This research provides a data support for the deep profile soil sampling through the analysis of deep profile isotopic variation and the spatial correlation relationship.Our results show that groundwater was recharged by both the piston flow and the preferential flow,with the piston flow dominate.We also provided theoretical basis for the rational utilization and protection of groundwater in the Loess Plateau area.