A Study On Contaminant Migration Based Performance Of Landfill Barrier Systems

The high concentration and high toxicity leachate and landfill gas generated in landfills have become a great threat to the bottom of landfill, surrounding soil and the atmosphere environment. Though the standard seepage control systems have been constructed for many landfills to impede leachate contaminants, the groundwater, surrounding soils and the atmosphere do have been contaminated due to landfilling.The research works in this thesis are funded by the National Science Foundation of China ’Evaluation of characteristics for gas migration in landfill covers and long-term service performance’and’Study on antifouling performance of landfill composite liner systems under effect of high stress and head’, International (Regional) cooperation and exchange project of NSFC’Water gas generation, migration and systematic engineering controls of municipal solid waste landfill’and973Plan project’Simulation, evaluation and sustainable control on service performance of landfills’. The migration mechanism and transport parameters for leachate and landfill gas from the landfills of China and the foreign countries were summarized and analyzed in the present thesis. Using the proposed analytical methods and numerical methods, the mechanism of leachate and landfill gas migration through saturated and unsaturated landfill liner systems and cover systems was investigated. The performance of different landfill liners and cover systems was evaluated. A field investigation on contaminant transport in Huainan landfill was carried out and analyzed using the mathematical models. The main conclusions are as follows:(1) The composition of landfill bottom liners landfill and top cover systems was studied and compared respectively. The transport parameters for leachate and landfill gas in landfill soils were summarized and suggested on the basis of the reported research on landfill contaminants from China and the foreign countries;(2) The effect of degradation is important due to the large amount of organic contaminants in solid wastes. Considering the effect of degradation as a first-order process, this thesis presented an analytical solution to organic contaminant advection and dispersion in a composite liner consisting of a geosynthetic clay liner (GCL) and an attenuation layer (AL). Results show that if the half-life of GCL is larger than1year, the degradation in GCLcan be neglected when predicting organic contaminant transport in GCL/AL;(3) An analytical solution of contaminant diffusion through unsaturated liner soil is presented based on simplified linear profile of moisture. The diffusion of contaminant through the unsaturated liner soils can be well studied by the proposed solution. The presented analytical solution is relatively simple to apply and can be used for evaluating the influence of unsaturated condition on contaminant migration in landfill bottom liners and soils;(4) The principle goal of landfill cover systems is to control the emissions of landfill gas into the atmosphere. The composition and types of landfill cover systems was discussed. A one dimensional model of gas diffusion through saturated landfill cover was developed. The model takes into account the time-dependent concentration variation boundary conditions. Based on Fick’s law and the superposition principle of linear equations, an analytical solution has been derived for one dimensional landfill gas diffusion in composite cover systems. The performance of the different landfill cover systems was then studied using the proposed analytical solutions. The obtained solutions can be useful for the preliminary design of landfill cover systems;(5) The results showed that the effectiveness of landfill cover systems for controlling the landfill gas emission is in the following order:GM+GCL>CCL> GM+CCL. For GM/GCL, when the degree of water saturation increases from0.85to1, the gas flux at the top of the cover system decreases by82.5%, and the time for the gas flux at the top of the system to reach the steady state for saturated cases is100times longer than the time for the test of unsaturated cases. For GM/CCL, the flux through the top of GM/CCL decreases by about an order of magnitude when the water saturation degree of CCL increases from0.1to0.85;(6) A one-dimensional model is developed to investigate landfill gas diffusion in the unsaturated layered cover systems. Coupled processes of gas diffusion in the unsaturated cover soils and transient water transport in the media were considered. For the1-meter-thcikness unsaturated cover soil, the relative gas concentrations for the case assuming coupled transport can be approximately8times smaller than those assuming steady state water distributions at the time range of20d to100d. The empirical formulas for determining gas diffusion coefficient were summarized and compared for cases of gas migration in unsaturated soils;(7) According to the measured atmospheric pressure on the top of cover system of landfills from foreign countries, the variation formula of atmospheric pressure was fitted. The range of the pressure fluctuations was obtained when the effect of atmospheric pressure on gas emission is significant. This study will be useful for the cases with large pressure fluctuations;(8) The field investigation of Huainan landfill indicate the maximum depth of migration of chloride was beyond the depths of the old clay layer, while the migration distance of COD varied around3m for17years of landfill operation period. It is indicated that advection transport and mechanism dispersion is important in contaminant 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. A better fit is obtained by employing an effective interface about50cm above the ground surface. The range of contaminant transport parameters can be used for the further investigation on the field studies of landfills.