Water-rock Interactions Of Shallow Saline Groundwater Downward Leakage To Deep-seated Fresh Groundwater

When shallow saline groundwater migrates to deep-seated fresh groundwater,the mechanism of the migration of water and salt in the clay layer and whether the clay layer can contribute to filter salt are important issues related to water resources security.In addition to convection,dispersion,and colloids adsorption,etc.,salinity migration is also affected by the membrane effect of the clay layer.The study of the migration process and kinetics of chemical elements in the clay layer has always been a weak link in the research and is one of the key directions for the innovative research of hydrogeology.The study of the micromechanism of shallow underground salt water migrating to deep underground freshwater is not only significant in improving the theory of deepwater circulation,but also very important in eliminating or reducing the deterioration of water quality caused by saltwater intrusion.Through laboratory tests,we found the following conclusions:(1)Different types of soils in the same region have different amounts of ion release.The release amounts are:silty clay>silty sand>fine sand,indicating that the finer the particles in the aqueous medium,the greater the adsorption capacity of pore water components is;Different types of soils in the region have different levels of ion release:Silt sand>fine sand>silty clay.(2)The presence of brackish water inhibits the desorption of anions and cations in the soil.During the mixed reaction of brackish water and undisturbed soil,both cation exchange adsorption and precipitation and dissolution of calcium and magnesium salts exist.The sodium and potassium ions interact with calcium.Magnesium ions undergo cation exchange adsorption.However,it was found during the experiment that the concentration of sodium,magnesium,and calcium ions in the solution after the reaction was smaller than the original salt water,indicating that the effect of precipitation dissolution in this test was greater than in this area.The role of cation exchange adsorption,while the mineral saturation index has been greater than 0,the precipitation of calcium and magnesium in the water sample also supports this conclusion.(3)Water-rock interactions can change the hydrochemical type of groundwater and change to the same type of water chemistry.(4)In the experiment of interaction between the same type of soil and local saline water in different regions,the degree of ion exchange adsorption and mineral precipitation and dissolution are related to the composition of the original salt water and the composition of the soil,and the groundwater content in the coastal area is relatively high.Magnesium ions promote the formation of calcite and aragonite,and a large amount of calcite and aragonite in the soils of Ninghe and Jinghai are decomposed and promote the formation of gypsum.(5)For the membrane effect of cohesive soils,it is experimentally concluded that when the salt flows through the silty clay body and the silt soil,the large ion content in the water sample is smaller than that of the distilled water release experiment in the equilibrium state.The sum of the maximum value and the ion content of the original saltwater results in the fact that the clay has a certain membrane property and has a"membrane effect" effect on the retention and separation of the salt ions passing through it,and the filtering effect of the powdered clay is superior to that of the clay.Silty soil,the larger the specific surface area of clay,the better the effect of salt filtering.