Bromide Ion Migrate In The Layered Unsaturated Zone Under The Non-sufficient Water Supply

Bases on six soil column tests with different concentration profiles and the HYDRUS-1D software(developed by US Salinity Laboratory), this paper systematically and comprehensively analyzes the migration law of bromide ion under non-sufficient water in different configuration unsaturated zone. Researching the infiltration characteristic of water, the change rules of matrix suction with time, and the distribution of bromide ion in the different profile pattern, the main conclusions as follows:1.In the non-sufficient water conditions, when no perched water formed, the infiltration amount of each profile at the slope of R0, namely the intensity of water supply. After moment, the cumulative infiltration of uniform loam meets the Kostiakov model. For the Loam over sandy and loam sandy loam configuration, when wetting front reach the sand interface, the cumulative infiltration curves of both types become linear relationship.2.Before ponding occurs, the slope of the infiltration rate versus time is linear 0.219cm/h. After ponding the infiltration rate of uniform loam gradually decreases with time and becomes nonlinear.In the loam sandy loam and loam over sandy configuration, when the wetting front cross the layered interface, both soils enter into steady infiltration, but the corresponding infiltration rate is less than those in homogeneous loams.3.The advance distance of the wetting fronts in the uniform sandy and loam change over time with a non-linear relationship. For a non-homogeneous structure, when the wetting front moves in the upper soil, the advancing of wetting front behaves consistently with the uniform loam. When it the layered interface, both interfaces of sandy over loam and sand over soil, wetting front advancing weakened. There is a infiltration reduction at the interface of the layered soil.4.According to the reaching time of the wetting front at the positions of senors, the changes of water content and matric potential in different depths variation of different profile configuration of the soil can be roughly divided into 3 stages, namely the initial stage, changing stage and balanced stage. The change stage of moisture content can be also divided into 3 stages, linear increase stage, the changing stage of matrix suction, stage conforms to Logistic model.5.The shape of the breakthrough curves will change significantly when the layered loam is added into the sand loam, leading to the initial breakthrough time increases while the slope of the curve gradually decreases. The increase of the clay content prevents the bromide ion penetration rom crossing in the soil.6.The more clay content exist and the smaller of the soil pore is the more slowly of the water and solute migrate in the soil, the stronger blocking effect of soil solute relatively and solute breakthrough need more time lag, hydrodynamic dispersion coefficient relatively greater. The hydrodynamic dispersion coefficient D of different configuration profiles from small to large is uniform sandy < sandy loam sandy < sandy over loam < loam sandy loam <tloam over sandy < uniform loam. Soil solute diffusion coefficient is not only related to soil texture, soil structure is also affected.7. Below the saturated content, the bromide ion concentrations sampled at number one and two homogeneous soil increase with time, whose growth rule is in accordance with logarithmic relationship. As the concentration of samples at different profile patterns increasing with infiltration time, the changing curves of bromide ion concentration all enjoy the tendency of moving from left to right on the horizontal axis. However, when it grows to a certain time, the change of concentration becomes stable.8. After using the HYDRUS-1D software(developed by US Salinity Laboratory) to compute the geological model of sandy-soil configuration, it has been found that the simulated results and experimental results share extreme similarities. And the simulated results have visually shown that to some extent the loam interlayer has impeded the migration of water and solute, which is the same as the experimental one.