Hierarchically Nested Groundwater Flow System And Hydrological And Ecological Effects In The Northern Ordos Basin

As an important research object of groundwater flow system theory,hierarchically nested groundwater flow system has been widely studied by numerical simulation and laboratory experiments,but it is seldom successfully identified from field observation and experiment.It is difficult to provide scientific guidance for the sustainable development and utilization of groundwater resources because the formation and evolution of hierarchically nested groundwater flow system under actual conditions is relatively weak,and the understanding of groundwater circulation and hydrologic ecological effect at regional scale is not thorough.Taking the Cretaceous groundwater flow system in the northern Part of Ordos Plateau as the research object,this study uses qualitative analysis and quantitative simulation methods to quantitatively reveal the formation and evolution of the groundwater flow system.The formation and development conditions and influencing factors of nested groundwater flow systems are analyzed,the structure and characteristics of different groundwater flow systems are described,and the control effects of groundwater flow systems on hydrochemical field,isotope field and temperature field are analyzed.The contribution of different flow systems discharge to lake was analyzed,and the hydrological and ecological effects of groundwater circulation were clarified to provide scientific basis for sustainable utilization of groundwater resources.The study can provide reference for promoting and improving the theoretical research and practical application of groundwater flow system.The main contents and conclusions of the study are as follows.（1）From geological conditions of groundwater flow pattern formation at basin scale,integrated method application of lithofacies paleogeographic analysis and geostatistics,,the regional geological background of groundwater flow system development,including lithology,structure and lithofacies palaeogeographic characteristics of the cretaceous aquifer,are analysed.The controlling factors of groundwater table configuration and its effect on water flow system are discussed by analytical method.The results show that the groundwater table is controlled by topography,recharge conditions and permeability coefficient due to the climate,topography and lithology combination of the Cretaceous water flow system in Ordos Plateau which affects the development and evolution of the groundwater flow system by controlling the groundwater table configuration.At the regional scale,groundwater table configuration is mainly controlled by topography.At the local scale,groundwater table configuration in some local flow systems is controlled by topography,and others are controlled by recharge.The vertical spatial variation of the Cretaceous aquifer is characterized by obvious vertical stratification.（2）The spatial heterogeneity and anisotropy of aquifer are two important aspects of aquifer heterogeneity,which affect and control regional groundwater flow pattern.In this paper,two dimensional numerical simulation and random field simulation are used to compare and analyze the groundwater flow patterns under the condition of homogeneous anisotropy and heterogeneous anisotropy,and the influence mechanism of water-bearing layered heterogeneity on groundwater flow system is discussed.The results show that the superposition effect of layered heterogeneity at different scales of cretaceous sandstone aquifers in Ordos Basin can significantly increase the macro anisotropy of aquifers and control the circulation depth of groundwater flow system at different levels at regional scale.Three variables,horizontal correlation length,variance of permeability coefficient and anisotropy value,are used to generate random fields,which can well describe the characteristics of layered water-bearing heterogeneity and its control and influence on groundwater flow system at regional scale,and can well explain the internal reasons of unrealistic large equivalent anisotropy in large-scale groundwater flow simulation.With the increase of the aquifer depth,the porosity,water abundance and permeability decrease.The apparent resistivity variances of four boreholes indicate that the vertical correlation length of permeability coefficient of Cretaceous sandstone is 5 m.The horizontal correlation length,permeability coefficient variance and anisotropy values were used for random simulation.The results show that the effects of variance and horizontal autocorrelation length parameters under heterogeneous conditions are equivalent to the effects of anisotropy parameters on groundwater flow system under homogeneous conditions,which can both characterize the stratified heterogeneity of aquifers.（3）Taking the groundwater flow system of Ordos Plateau as a typical object,the hierarchically nested structure of groundwater flow system is described by multi-depth pumping and sampling test,multiple isotope dating and groundwater numerical simulation.Groundwater age and geochemical profiles were constructed using multi-depth isotopic data of ³H,⁸5Kr,¹⁴C and ⁸¹Kr.The inflection points on groundwater age and geochemical profile,as the key indicators to identify the boundary of groundwater flow system,can indicate the spatial structure of groundwater flow system well.In this study,the hierarchically nested structure of flow systems was successfully identified in the field for the first time by using detailed vertical multi-depth data.In particular,by combining the measured data of hydrodynamics,isotopic age,hydrochemistry and geophysical exploration,the stagnant area of the flow system is identified in the actual basin,which provides a key basis for accurately determining the interface of flow systems.By comparing the measured profile data with the results of two-dimensional numerical simulation,the spatial structure of groundwater system was delineated and the nested groundwater flow system was identified by using various evidences.（44）Guided by the theory of groundwater flow system,the parameters,water level,water quality,age and isotope of stratified aquifers were obtained by selecting typical sections by using hydrogeological drilling,packers stratified pumping test,geophysical exploration,logging and ³H,¹4C and ⁸¹Kr radioisotope dating technology.A set of key techniques and methods for field identification and fine characterization of groundwater flow system is formed through mutual verification and comprehensive judgment of various evidences and two-dimensional simulation of groundwater flow system section under various constraints.（5）The latest age profile data were obtained by using the latest isotope dating techniques.The results show that the groundwater age profile is quite different from that of previous studies,which is a new understanding of groundwater renewal capacity in this area.The method adopted in this paper can greatly reduce the uncertainty in model construction.However,the heterogeneity of permeability and soluble minerals of aquifer also produce inflection points in groundwater dating and geochemical profiles,which requires careful analysis of hydrogeology and geological profiles to identify nested groundwater flow systems.It is recommended to carry out two-dimensional or three-dimensional groundwater flow and age-mass transport-attenuation numerical simulation before drilling in field,so as to provide prior information of groundwater flow and age distribution for optimization of drilling hole location.（6）Based on the clear structure and circulation characteristics of groundwater flow system,a conceptual model of groundwater quality evolution was established by analyzing the spatial distribution and evolution law of groundwater hydrochemistry and reverse geochemical simulation.The results show that HCO₃-Ca is the main hydrochemical type in groundwater recharge area near watershed.Due to the influence of cation exchange and anionic saturation precipitation during the groundwater flow from recharge area to discharge area,the hydrochemistry type of groundwater discharge area near the watershed is transformed from HCO₃-Ca type to HCO₃-Na type and SO₄-Na type.The hydrogeochemistry of groundwater varies significantly in different parts of groundwater flow system.In the recharge area of the local water flow system,the leaching process,such as dissolution of carbonate minerals,is dominant.Cation exchange is dominant in runoff-discharge area.In the regional flow system,the interaction between water and rock is enhanced due to the long retention time of deep groundwater,and the dissolution of evaporite rocks,such as gypsum and mirabilite,is the main action.（7）The process of groundwater recharge from precipitation infiltration in the study area is complex,and the effective recharge capacity of precipitation to groundwater varies with different seasons,precipitation and precipitation patterns.By comparing the hydrogen and oxygen stable isotope characteristics of different water bodies in Ordos Plateau,it is found that the isotopic composition of groundwater is more negative than that of precipitation.Heavy precipitation with negativeδD andδ¹⁸O values has a stronger ability to recharge groundwater.At the same time,the effective infiltration capacity of winter precipitation with negativeδD andδ18O values is stronger under conditions of soil freezing and thawing.The combined action of these two factors results in negative values ofδD andδ¹⁸O.Controlled by recharge and runoff conditions of groundwater flow system,the migration time and age of groundwater at different depths in the aquifer are different,and the characteristics of stable hydrogen and oxygen isotopes of groundwater in different recharge periods are different.The isotopic composition of the deep groundwater（>150 m）is shallower than that of the shallow groundwater（<150 m）,indicating that the recharge period may be the cold period before the holocene（10ka B.P.）.When the local water age is less than 10ka（Holocene）,δ¹⁸O becomes positive gradually with decreasing age,indicating that the climate has become warming since holocene.The averageδ¹⁸O value is about 11‰when the local water age is greater than 10ka（pleistocene）,indicating that the climate was relatively cold during this period.During the last glacial maximum（LGM）of 18ka,the blank area of groundwater age samples appeared,which may be caused by the fact that the northern Ordos Plateau was covered by the glacial margin during the last glacial maximum,and the groundwater could not be recharge by precipitation infiltration.（8）According to the structural characteristics and numerical simulation results of groundwater flow system in typical sections of the basin,D.Lake and M.Lake only receive groundwater recharge from local groundwater system,that is,only from their own surface catchment area.H.Lake receives local,intermediate and regional groundwater recharge systems at the same time,and the contribution proportion of recharge is 82%,16%and 2%,respectively.The change of regional vegetation conditions has an obvious effect on groundwater and lake,which is controlled by the overall evolution of regional groundwater flow system.Under the condition of increasing vegetation coverage,lakes in Ordos area shrank,and the water level of the drainage point（lake wetland）in the upper reaches of the regional system decreased more than that in the lower reaches of the regional system,resulting in more shrinking of lakes with higher elevations than those with lower elevations.