Influence Mechanism Of Clay On The Hydrodynamic Characteristics Of Leakage System In Plain Area

Today,the shortage and safety of underground freshwater resources had become an important factor that restricts the development of the national economy.The deep underground freshwater production in the inland plain area was increasing year by year.Once the shallow saltwater was supplied to the deep freshwater layer,the limited deep freshwater might have the potential to be fully salted,Make the original shortage of freshwater resources more serious.Therefore,in order to developing and utilizing inland deepwater freshwater more scientifically and reasonably,it was particularly important to study the influence mechanism of cohesive soil on hydrodynamic characteristics.Taking the typical salty water area of Jiyang County in the inland plain area as an example,this study used cohesive soils and brackish water at different depths in the study area as the research object.Based on the relevant literature at Domestic and foreign,through methods of field geological surveys,drilling stratum structure analysis,the determination of physical properties of water-containing media,groundwater ion composition tests,groundwater isotope tests and methods of field pumping tests,field water level and water quality stratification monitoring described the hydrogeological characteristics and groundwater hydrodynamic characteristics of the over-flow system in the plain area.Throug field pumping tests and indoor high-pressure permeation tests investigated the hydrodynamic processes of cohesive soils in overdraft systems in the plain area.And in order to study the physical properties of cohesive soils and the effect of cohesive soil minerals on the permeability coefficient combined with electron microscopy,quantitative analysis of clay pores,ArcGIS spatial analysis,mathematical statistics and indoor adsorption isothermal tests.And then used cross-flow systems,diffusion double layer theory to study the migration characteristics of salt and major cations in cohesive soils and to reveal the role of filter salt and membrane effects of cohesive soils and the retardation mechanism of hydrodynamic characteristics.The results can provide a theoretical basis that can develop and utilize deep freshwater and improvement and utilization of saltwater for the next step.This article through the above analysis and research,draw the following conclusions:?1?The study area was located in the runoff stagnation area between paleochannels.Top-down lithology in inland plain area 500m shallow was composed of river and lacustrine cohesive soil,silty cohesive soil,silty soil with thin layer of silt.The alternating strength of shallow,middle and deep groundwater circulations was poor,and the hydraulic connection between the upper and lower aquifers was weak.The result of ¹⁴C of shallow,middle and deep groundwater ages were 80³0aB.P.,39010³6150aB.P.,and 16247¹3651a B.P.,so the groundwater renewability was poor.Combining with the dynamic characteristics of groundwater level in different aquifers and groundwater age analysis,the shallow submersible in the study area belonged to the shallow local flow system,the middle confined water belonged to the intermediate flow system,and the deep confined water belonged to the deep regional flow system.And it can reveal the geological background of the study area affects the formation of groundwater hydrodynamic characteristics.?2?Through the scanning electron microscopy analysis of cohesive soils,The porosity of cohesive soil has a significant effect on its permeability.the grain size on the surface of the soil sample after the experiment was larger than that before the experiment.The inter-particle porosity became smaller and the density becomes larger.The pore/particle decreases from1.5674 to 1.2591.The results of the infiltration test revealed that during the high pressure infiltration test,the fine particles in the soil sample migrated,flocculated,precipitated and rearranged with the seepage liquid,and the pore structure of the medium was changed,so that the porosity gradually decreased,leading to the permeability of the soil sample reduce.?3?Analysis of the chemical composition of the exudates obtained by the high-pressure permeation test showed that Na⁺replaces the original Ca²⁺cations with larger radii in the pores of the clay in the infiltrant to diffuse the particles around the clay.The layer and the hydration membrane became thicker and the porosity of the clay becomes smaller,leading to a decreasing trend in the permeability coefficient of the clay.Under the control of the adsorption of the clay,the groundwater flows through the clay to a depth of 48⁴8.2m,51⁵1.2m,The degree of decline in permeability coefficient at 71⁷1.2m and 112¹12.2m was greater than that at depths of 6⁶.2m,84⁸4.2m,97⁹7.2m,and 102¹02.2m.?4?The monitoring of groundwater quality in the field showed that the blockage of the silty clay and clay or the penetration of the block salt lead to the abrupt change of the conductivity.It was proved that the zonality of the vertical groundwater quality was controlled by the retardation of the clay.The conductivity of groundwater at the depth of 49m was abruptly changed,and the value of the groundwater reached the maximum.Concentrated polarization layer was formed on the surface of the clay,and the permeability coefficient reached the lowest value.?5?The high pressure infiltration test and the calculation of the coefficient of cohesive soil retardation indicate that the permeability change of cohesive soils is affected by the retarding effect of clay soil on the chemical composition of water.In the process of high-pressure permeation test,the Na⁺,Ca²⁺,and Mg²⁺in the tested water samples flowed through the original soil columns of depths of 42,112 m,and 97 m,respectively.The migration curve first appeared as"inverse S"or"S".It was mainly affected by diffusion and convection,and then it stabilized at a certain concentration after an upward trend.At the end of the test,the C/Ci values were all greater than 1,revealing the resistance of the clay column to Na⁺,Ca²⁺,and Mg²⁺in the tested water sample,The lag effect was significant.According to the quantitative calculation and analysis of the retardation coefficient,it was revealed that under the control of the clay soil retardation,the depth of the groundwater flowing through the clay was 48⁴8.2m,51⁵1.2m,71⁷1.2m and 112¹12.2m.The degree of decline in the coefficient of permeability at the location was greater than the depth at which the permeability coefficients at the depths of6 to 6.2 m,84 to 84.2 m,97 to 97.2 m and 102 to 102.2 m decrease.?6?The high-pressure permeability test showed that the thickness of 150mm cohesive soils at the depths of 42⁴2.2,81⁸1.2,95.2⁹5.4,118.8¹19m was affected by the water level difference of 0.75,2.25,14.25 and 18.3m respectively.The initial head values of the viscous soils with a thickness of 150 mm and depths of 95.2 to 95.4 and 118.8 to 119 m were22.2 and 24.45 m,respectively,revealing that the runoff of the groundwater was stagnant,and the groundwater flowed over under a large hydraulic gradient.?7?High-pressure permeation tests showed that under a certain pressure,the permeability of the aqueous medium as a whole first showed a downward trend,and remained relatively stable.As the pressure gradient increasing,the overall permeability of the aqueous medium increased.In the process of groundwater infiltration,it was mainly affected by cation exchange,the viscosity of seepage liquid,particle release and expansion,and the influence of groundwater temperature.There were differences in the influencing factors in different stages.In summary,the changes of hydrodynamic characteristics in inland plain areas were mainly affected by topography,meteorology,sedimentary environment,hydrogeochemical effects and the nature of aqueous media.