The Experimental Study On Fluorine Migration Law In The Layered Aeration Zone Soil

The aerated zone is a complex system containing three materials: water, soil particles and air. As an important link of surface water, meteoric water and the groundwater, it plays an important role in the water cycle. It is the only way that the solute on surface pass into groundwater through aerated zone soil. With a series of physical, chemical and biological reaction between solute and soil in the process of transport, the aerated zone has a blocking effect on solute transport. Layered soil is widely distributed in the nature, the research shows that the solute transport in layered soil is much more complicated than that in the homogeneous soil. Fluorine as a typical pollutant in groundwater, has an important influence on human health. Therefore, a study on fluoride migration law in layered aeration zone soil has an important significance.This paper is based on the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection team project: The Experimental Study on Solute Migration in Unsaturated Soil(No. SKLGP2011Z001). Based on the previous study of solute transport in the aerated zone soil, the influence factors of solute transport in layered soils are analyzed. Through the soil column experiments, with different hierarchical manner, water supply approach and solute supply method, the moisture migration and fluorine migration law in layered aeration zone soil is fully studied. To compensate for the time limitations in the laboratory experiment, Hydrus-1D software is used to make numerical simulation of fluorine migration law in layered aerated zone soil under the condition of different hierarchical manners, water head heights and solute supply methods. The main research results are as follows:(1) The migration of the wetting front in layered soil is divided into two phases, at the first stage, above the hierarchical interface, infiltration rate varies nonlinearly and decreases with time increasing; at the second phase, below the hierarchical interface, infiltration rate does not change over time. Infiltration depth of wetting front changes over time, it can characterized by the power function y=axb. The moisture content in layered aeration zone soil section changes like a anti "Z", when stable, the moisture content is close to the saturated moisture content.(2) When the surface of the soil is contaminated instantaneously by fluoride with high concentration, with 15 mm / 12 h rainfall intensity, if fluorine migrates in soil of two hierarchical structures which are from fine to coarse and from coarse to fine, in each monitoring point of the soil column, its concentration increases first and then decreases over time. Due to adsorption of soil particles, peak concentration in the soil of two hierarchical structures gradually decreases with different time and depth. Comparing the two soil structures, fluorine migrates fast in soil with structure from coarse to fine. At the end of the experiment, fluorine is mainly accumulated in the upper part of the soil with a structure from fine to coarse, and fluorine concentrates at the bottom of the soil with a structure from coarse to fine. Therefore, under the condition of unsaturation, blocking effect to solute from the fine to coarse layered structure is stronger than that from the coarse to fine layered structure.(3) When the surface of the soil column is contaminated instantaneously by fluoride with high concentration, under conditions of constant head infiltration and rainfall infiltration, the migration law of fluorine in the same hierarchical soil column is basically the same, its concentration increases first and then decreases over time, however, fluorine migration rates are quite different. Fluorine migration is affected by moisture migration. With constant head infiltration, water flow in the soil is fast, therefore, convection enables a faster migration rate of fluorine, and at the end of the experiment, fluorine mainly concentrates in the bottom of the soil column. With rainfall infiltration, fluorine migration is affected by rainfall intensity. Water flow in the soil is slow, downwards migration rate of fluorine in the soil pore is also slow, so at the end of the experiment, fluorine mainly concentrates in the surface layer of the soil column.(4) The way of supplying fluorine is changed under the conditions of the same rainfall intensity and the same hierarchical manner, with continuous solute supplying, the fluorine concentration increases first and then tends to become stable over time. At the end of the experiment, fluorine was evenly distributed in the soil column, its concentration is very close to the initial concentration; If solute is supplied instantaneously, as a result of rainfall eluviation, fluoride concentration increases first and then decreases over time, and at the end of the experiment, fluorine concentrated in the column topsoil.(5) Hydrus-1D software is used to make numerical simulation of fluorine migration law in layered soil under the condition of different hierarchical manners, water head heights and solute supply methods. Fluorine migration differs in layered soil under different hierarchical manner. The nature of the surface soil has a control function on solute migration: the finer the surface soil particle is, the stronger the blocking effect of fluoride migration becomes; When the soil surface is the same, the finer the soil particle in second layer is, the stronger the retardation of the solute is. Therefore, fine, medium and coarse particle structure has the best effect on groundwater pollution control. Under different water head heights, fluorine concentration changed little with different time and depth, fluoride migration rate slightly increased with water head height increasing. If the same quality of solute is supplied at different frequencies, the faster the supplying frequency is, more peaks appear in the topsoil, and the lower the peak concentration is.(6) The soil column experiments and numerical simulation indicate that fluorine migration in layered aeration zone soil is mainly influenced by the soil moisture movement. Different hierarchical manner, water supply approach and solute supply method will have influence on fluoride migration in layered aeration zone soil to various degrees.