A Study On Contaminant Transport In Layered Media

Contaminant transport in soil is a complex process, which is subject to physical, chemical and biological activities. The study on contaminant transport has important guide meaning to the actual engineering problems, such as disposal of contaminated soils, evaluation of contaminated sites. The mathematical model is a useful tool and contaminant transport through a medium is described by a partial differential equation of parabolic type. Due to the case of one-dimensional contaminant migration through layered media, the systematic study has been carried on.Considering the influence of advection and the time-dependent boundary concentration, an analytical solution of one-dimensional advection-dispersion solute transport in double layered media with the decay term is derived by using the Duhamel theorem and the separation of variables method, and several examples are given. It is found that the velocity has an impact on contaminant concentration, the effect more significant for the greater velocity. In addition, the type of boundary condition has a great influence on the bottom flux for the small velocity. As for the given boundary concentration of exponential attenuation model, the parameter Cb /Ca significantly affects the calculation results.A one-dimensional contaminant diffusion model in layered media is developed under the condition that diffusion coefficient is linear with depth. The orthogonal expansion method is used for solving the analytical solution. For the situation of single and double layered medium are analyzed. The results indicate that the relevant diffusion parameters have complex effects on the transport of contaminants. As for double-layered media, the effect of the diffusion property of the upper layer is relatively larger.Considering the time-dependent diffusion coefficient, a one-dimensional contaminant transport model with boundary conditions that are dependent upon time is presented. Analytical solutions for several different diffusion coefficient distribution models are obtained. As an illustration, the particular case of linear-asymptotic type is discussed. The results show that the parameter k of the diffusion coefficient distribution model has important influence on the transport of contaminants. Compared with boundary conditions that are constant, the distribution of contaminant concentration always increases first, and then decreases with time.Assuming that diffusion coefficient is a linear function of depth, an analytical solution for contaminant transport in layered media with considering degradation is derived. The validity and reliability of the presented solutions are verified. Based on the presented solution, the effects of the relevant parameters and the half-life of the contaminant are investigated. It is found that the concentration of contaminant can be greatly reduced when the degradation effect is considered. And the effect of degradation has a significant influence on the time required for reaching steady state.For composite liners, steady-state diffusion of the organic contaminant through the geomembrane(GM) is assumed and the effect of contaminant degradation in the soil liner beneath the GM is considered. The analytical solution for organic contaminant through composite liners with linear depth-dependent diffusion coefficient in the soil liner is derived. On the basis of the proposed model, the sensitivity of the related parameters is analyzed. It is indicated that the diffusion property of the underlying soil liner and the degradation of contaminant have a great impact on the calculation results.The presented solutions can be used for preliminary design of landfill liners, verification of complicated numerical models and evaluation of the experimental results.