| Vadose zone is a complex system with three meterials of the rock particles, water and air, which is a hinge zone as well where atmospheric rainfall, surface water and other water bodies have contact with groundwater and exchange; at the same time, vadosezone not only has the ability to absorb moisture, retain moisture and transfer moisture, but also provides a placefor a variety of behaviors including contaminating, solute accumulation, migration and conversion in the zone. Thus, the reducing resistance ability of vadose soil is the key factor for groundwater environmental balance and there is a positive significance to research water infiltration and solute migration law under the condition of different textures and thickness in the layered vadose zone.On the basis of research results at home and abroad as well as relevant theories, this paper aims at with weak interlayer permeability soil structure characteristics of the vadose zone, prepares there kinds of the soil samplesnamed 1#, 2# and 3#, and tests the related physical and chemical properties, soil water characteristic curve and permeability characteristic curve, etc.. And through the ratio of different soil column forms, we control weak interlayer permeability structure, researching water infiltration and solute migration patterns under the condition of rainfall.First of all, combined with the obtained soil samples parameters, we theoretically guarantee that water infiltration rate is greater than the rainfall intensity, ensure no ponding on the surface layer soil column, and control the continuous rainfall about 25mm/12 h range. The tested soil column structure designed for A, B, C three kinds of types, A type soil column in homogeneous soil column, type B and C soil column containing 10 cm interlayer. In A, B, C three kinds of different soil structures, the test includes 3×2 groups of experiments, the first three groups researched the rules of water infiltration, on the basis, solute migration test were studied under the condition of the same structure. In addition, in order to further study the vadose water infiltration and influence factors of Br- solute migration, using five small scale soil column under the condition of rainfall infiltration test. Then, according to different soil column structure under the condition of the material and thickness of the weak interlayer permeability, four groups of soil column experiment was carried out respectively researching water infiltration and solute migration, controlling the continuous rainfall among 50mm/12 h, named B1, C1, B2, C2 type soil column, two groups of B1, C1 type interlayer thickness of 2cm, B2, C2 type of 5cm. Finally, on the basis of the physical test, weusing the vadose Hydrus-1D numerical simulation software for further study on the movement of water infiltration and solute migration process under the condition of the weak interlayer permeability in the soil column structure. Through a series of experiments and simulation, the conclusions is as follows:(1) With the help of water-soil characteristic curve RETC simulation software, using B-C and V-G model to fit three kinds of the samples for soil water characteristic, it is concluded that V-G model fitting is better; using indirect test method, V-G model deformation formula and the Exp type function fitting, it is concluded that the unsaturated hydraulic conductivity curve is obtained under the condition of each moisture content.(2) In contrast to homogeneous type A soil column and B type and C type of layered structure with10 cm weak interlayer permeability in the soil column experiment, the wetting front migration of type A homogeneous soil is the fastest, the first to reach the underlying soil column, and type C water infiltration is the slowest. Thus, the weak interlayer permeability of soil mass has a greater influence on the water infiltration.When the interlayer texture is the greater, the viscosity of water blocking and holding ability is stronger; About Br- solute migration test, in three kinds of soil column the trend of Br- solute concentration was of increase after decreases on shallow layer, while the middle and bottom continued to increase, the interlayer has an obvious effect of solute permeability reducing at the same time, especially in the soil column C, Br- concentration onthe interlayer was the highest, and the underlying concentration is lowest, provingthat the existence of the soil column interlining has effects on Br- solute migration, which cannot be ignored.(3) Considering the influence factors of water movement and solute transport in the vadose zone, this paper emphasizes matrix suction, air in soil column and soil temperature. When the matrix suction was at the small scope(5~20kpa), the concentration of Br- sudden droped along with the increase of matrix suction, because the soil moisture content increased, and the matrix suction decreased, while the Brsolute was accumulating and moving, especially in the clayey soil; the existence of air phase in soil column had a greater influence on the water infiltration, performingthat the gas and moisture was expelled each other in the soil, which was alway the competition process.(4) In research ofinterlayer resistance on water infiltration process of soil column, using Log-DoseResp function to fit the curves of relationship between moisture content and buried depth, the fitting effect is appropriate; Soil interlayer water reduction rate(r) showrB1<rB2<rB<rC1<rC2<rC, this demonstrates that the thicker interlayer, the greater viscosity texture, wouldbe resistance function stronger of water infiltration water, especially the terms for the mezzanine texture, had more significant effect on moisture reduction.(5) In solute migration testunder the influence of different interlayer with different buried deep, we used nonlinear Poly-Parabola function to fit the Brconcentration duration curve, it was concluded that different buried depths changed the concentration of Br- curve, the R-squarewas as high as 0.95 above. The experiments showed, the thickness and soil texture both had significant effects on Brsolute migration, as in the soil column experiment at the end of C2 type interlayerthe Br- concentration was the highest, up to 548.96 mg/L, and its concentration at the bottom was the minimum; In addition, the soil reductionrate(r) of solute migration wasrC1<rB1<rB2<rC2<rB<rC. Therefore, the greater viscosity soil texture of the permeability interlayer controledthe Br- solute migration stronger; The thicker soil interlayeris significant on the Br- solute migration, especially the thickness of the interlayer.(6) About Hydrus-1D numerical simulation, analyzingB2 and C2 simulation structure model, the simulation results both moisture movement and Br- solute migration process can reflect the general laws, where the laws of water infiltration and physical experiment is relatively close to the simulation results.And t there is a certain error between the simulation result of Br- solute migration and physical test results, which only can be used as a reference. In future research we still need to further refine this part work. |
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