A Study On Contaminant Transport In Layered Media And The Performance Of Landfill Liner Systems

The high concentration and high toxicity leachate generated in landfills have become a great threat to the surrounding environment. Though the standard liner systems have been constructed for many landfills to impede leachate contaminants, the groundwater and surrounding soils do have been contaminated due to landfilling.The components and the stabilization issues of the landfill leachate from China and the foreign countries were summarized and analyzed in the present paper. The time required for the leachate to be stable was suggested for liner design. The landfill liner system should be designed in such a way that the breakthrough time of the liner systems must be larger than the leachate stabilization time plus the landfill operation time. The transport parameters for different contaminants in soils, geomembranes (GM) and geosynthetic clay liner (GCL) were summarized and the typical values for design purpose were suggested. The mechanism of contaminant diffusion through layered media was studied by models. On the basis of a presented simplified non-linear adsorption isotherm, the effect of nonlinear adsorption on contaminant diffusion through soils was also investigated. Furthermore, the leakage and diffusion transport of contaminant transport through composite liners was investigated on the basis of the concept of equivalent boundary. Using the analytical and numerical methods presented, the performance of landfill liner system was studied and the equivalency between different liner systems was also evaluated. Finally, a field investigation on contaminant transport in landfill basal soil was carried out. The main conclusion is as follows:(1) An efficient general analytical solution and an elegant computer program have been developed for the problem on one dimensional contaminant diffusion in a layered media. The average degree of diffusion of the layered system is introduced based on the analytical solution. The model accounts for the arbitrary initial conditions and the variation of contaminant in the source.(2) An analytical solution of solute diffusion through soil is presented based on a simplified non-linear adsorption isotherm. The adsorption of solute on the clay minerals can be well reflected by the present isotherm. The presented method is relatively simple to apply and can be used for evaluating experimental results, and verifying more complex numerical models.(3) An analytical solution has been obtained for the leakage and diffusion transport of contaminant through composite liner on the basis of equivalent boundary. Non-dimensional design curves were developed for preliminary performance-based design of landfill composite liners.(4) 2m CCL and GM+CCL can provide better barriers for landfill leachate. As for heavy metal ions, leakage through composite liners is not the only important mechanism of solute transport in composite liners. The diffusion coefficient and the adsorption capacity of the underlying CCL or GCL have great influences on solute transport through composite liners. GM+GCL is a good barrier for heavy metal ions in the case of low leachate level. However, the breakthrough time of GM+GCL decreased greatly when the leachate level increases. Furthermore, GM+GCL is the worst liner systems for volatile organic contaminants.(5) It is indicated that GCL is equivalent to less than 0.1m of CCL in terms of both single liner and as a part of composite liner. For media having hydraulic conductivity, diffusion coefficients, distribution coefficients, and porosity similar to those considered in this study, the additional thickness of attenuation layer (AL) underlying GCL should be about 5.0 m for equivalency with GM+CCL standard composite liner for the heavy metal ions. The additional thickness of AL should be about 2.0 m for equivalency with GM+CCL standard composite liner for volatile organic contaminants. CCL can be substituted by GCL+5 m AL when the construction quality of CCL cannot be assured.(6) The concentration of volatile organic contaminants in the leachate should be controlled since they can readily diffuse through GMs. Furthermore, it is indicated that the thickness of the AL needed to achieve the same breakthrough time as the standard liner systems can be greatly reduced when the adsorption capacity of AL can be improved. The composite liner consisting of GM, GCL and the organo-treated AL or organo-treated CCL should be adopted and popularized.(7) The filed investigation indicate the maximum depth of migration of chloride was beyond the depths at each of the boreholes, while the migration distance of COD varied between 1 m-3.5 m for the four boreholes for 13 years of landfill operation period. It is indicated that advection transport maybe the dominant mechanism rather than diffusion transport. The range of the transport parameters of the contaminant were obtained by comparing the field data with the theory of one-dimensional contaminant transport.(8) Retro-fitted measures should be taken to control the leachate level and control the leachate generation rate. Furthermore, the construction quality of geomembrane and the compacted clay liner should also be controlled.