Effect And Numerical Simulation Of Saline / Bracky Water Infiltration On Soil Permeability And Solute Transport

At present,there is a serious shortage of freshwater resources in arid and semi-arid areas.The growing industrial and agricultural production activities make the contradiction between supply and demand of water resources increasingly prominent.In order to alleviate the shortage of water resources and over-exploit groundwater resources,the unreasonable water use structure leads to the continuous decline of the groundwater level,and the water circulation system faces serious threats.Breaking through the bottleneck of the shortage of fresh water resources,finding alternative water sources to alleviate excessive consumption of groundwater sources,ensuring agricultural food security,and sustainable agricultural development have become current priorities.Salt water and brackish water as potential water sources can be used in agricultural production.The development and utilization of brackish water and brackish water resources has become the focus of attention of researchers in related fields.Agricultural irrigation using abundant saline and brackish water in the ground provides the necessary water for crop growth and alleviates the problem of insufficient freshwater resources.However,the irrigation of saline and brackish water brings a large amount of salt segregants into the soil.Changes in the physical and chemical properties of the soil cause soil compaction,soil salinization,and crop yield reduction to a certain extent.In this paper,the indoor soil column outflow experiment divides the entire soil column into four layers from top to bottom according to the position of the observation hole:Section A?4-10 cm?,Section B?10-16 cm?,Section C?16-22 cm?and section D?22-28 cm?,mainly carry out research work from three aspects:one is to explore salt water/bracky water?using a single chloride solution of different concentrations or a mixed solution of multiple chlorides?To characterize)the effect of salinity?characterization of chloride concentration?,salt cation type,sodium adsorption ratio and other factors on the spatial and temporal changes of soil permeability during infiltration;the second is to reveal the infiltration process of saline/brackish water,How the soil permeability changes caused by its salinity,ion type,sodium adsorption ratio,etc.,and how it affects the migration characteristics of salt cations;third,when using Hydrus-1D software for different salt cations and different sodium adsorption ratios The migration process of the salt cations in the soil was numerically simulated,and the following conclusions were drawn:?1?When a single salt cation,such as Na⁺,Ca²⁺and Mg²⁺solution infiltrates,as its concentration?chloride of different concentrations?increases,it will have different effects on the permeability of each layer of the soil column.When the NaCl solution is infiltrated,the permeability of each layer of the soil column gradually decreases with the increase of the chloride concentration of the infiltration solution.When the chloride concentration of the different concentration is low?3 g/L?,the soil column 10 The permeability at the depth of?16 cm changes the most,and the decrease is the most obvious.As the chloride concentration of the infiltration solution increases,the increase in Na⁺concentration causes the dispersion of particles,which leads to the decrease of soil pores and the decrease of permeability.It has the greatest influence on the permeability of surface soil?4?10 cm?.When CaCl₂and MgCl₂ solutions are infiltrated,Ca²⁺and Mg²⁺are both divalent cations,which have similar effects on soil permeability.Both have improved the permeability of the soil column to a certain extent.The degree of increase in the permeability of the soil column gradually decreases.Because Mg2+has a certain dispersion effect on soil clay particles,the effect of Mg²⁺on soil permeability is less than that of Ca²⁺.?2?When the solutions with different sodium adsorption ratios are infiltrated,the permeability of each layer of the soil column is improved to a certain extent.When the salinity is less than 5 g/L,the permeability will gradually increase with the increase of the salinity.When the salinity is greater than 5 g/L,the degree of permeability improvement gradually decreases with the increase of the salinity of the infiltration solution.When the SAR is 30,the permeability of the upper layer of the soil column is gradually increasing,and the permeability of the lower layer of the soil column fluctuates around the initial value.The solution infiltration with different salinity,ion type,and sodium adsorption ratio affects the permeability of the upper layer of the soil column to a greater extent than the lower layer.?3?The change trend of the overall soil column permeability with the infiltration water ion type,chloride concentration,sodium adsorption ratio is similar to the change trend of each layer of the soil column.The average permeability coefficient of the overall soil column is basically consistent with the layer where the minimum permeability coefficient of each layer of the soil column is located,indicating that the overall permeability of the soil column is determined by the layer with the smallest permeability of the soil column.?4?The spatial and temporal changes of soil permeability also affect the migration process of salt cations in soil.The outflow time of salt cations at different depths of the soil column and the time required for the outflow to reach the peak concentration are both affected by changes in soil permeability;at the same time,they have a certain effect on the relative peak concentration of different salt cations in the outflow.When Ca²⁺and Mg²⁺dominate the infiltrated saline/brackish water,the greater the increase in permeability,the shorter the outflow time of these salt cations and the time required for the outflow to reach the peak concentration;when the Na⁺content in the infiltration water is higher Due to the decrease of soil permeability caused by the increase of Na⁺,the migration speed of salt cations in the soil is relatively slow.The decreasing permeability and the slow cation migration speed increase the adsorption of Na+in the upper soil??5?With the help of Hydrus-1D software,a non-equilibrium single-point adsorption model is used to simulate the spatiotemporal changes of salt cations in the soil column under different influencing factors.The fitted R² is greater than 0.86,and the RMSE is less than 0.94,indicating The non-equilibrium single-point adsorption model can better characterize the migration behavior of salt cations in soil.From the simulation results,when a single salt cation is infiltrated,the adsorption capacity of soil for Mg²⁺is greater than that of Na⁺and Ca²⁺;the adsorption capacity of soil for Ca²⁺is the weakest,so the transport capacity of Ca²⁺in the soil column is the strongest,and it reaches the outflow as soon as The peak concentration;and the soil's adsorption capacity of these three cations gradually increases with the increase of infiltration depth.