Analytical Models For Organic Contaminant Transport In Composite Liners Considering The Combined Effect Of Leakage, Diffusion And Degradation

With the development of China’s economic and accelerating urbanization, more and more cities are surrounded by the municipal solid wastes (MSWs). The leachate generated by the biodegradation of the MSWs in landfills has caused a great threat to groundwater and soil environment. The liner system which plays a’blocking’role in landfill is an important issue to domestic and foreign scholars. The organic pollutants transport mechanism in the composite liner, the service life evaluation methods and the comparisons of different composite liners are the focus and difficulty of present study. In this thesis, firstly, on the basis of previous studies, the composition and structure of different composite liner systems were summarized. The mechanisms of contaminant transport in the soil, geomembrane (GM) and the geosynthetic clay liner (GCL) were described and the related parameters involving organic pollutants advection, diffusion and adsorption in the three materials were summarized. Then, the analytical model for evaluating time lag of the composite liners including GM/GCL and GM/CCL was developed considering the leakage through geomembrane defects, the diffusion of organic pollutants in intact geomembrane. For the composite liner consisting of GM, GCL and soil liner layer (SL), an analytical model was developed for organic contaminant transport in this composite liner considering the combined effect of leakage, diffusion and degradation. On the basis of the proposed analytical models, the effect of the leachate head, the length of the connected winkles, interface transmissivity between GM and the underlying soil liner and biodegradation half-life of on the antifouling performance of the composite liner were investiged.The main conclusions are as follows:(1) Under the leachate head of0.3m, lm, and10m, the time lag of GM/GCL is quite short (about14days).The leachate head and biodegradation half-life have negligible effect on the time lag of GM/GCL. For GM/CCL, the effect of leachate head on the time lag is not as significant as degradation half-life. Under the same situation, when leachate head increased from0.3m to10m, the time lag decreased by0.1-6years. However, when the biodegradation half-life decreased from10years to1 year, the time lag was reduced by10-11years.(2) For the triple composite liner GM/GCL/SL, the leachate head has a great influence on the concentration profiles and the breakthrough curves. For example, with length of the connected winkles L=1000m, when leachate head increases from0.3m to10m, the concentration at the depth of0.6m increases by a factor of1.85for the semi-infinite bottom boundary condition. For the same length of the connected winkles when the leachate head increased from0m to10m, the30-year-bottom cumulative flux increases by a factor of17.8for the zero concentration bottom boundary. The length of connected winkles of GM (L) has a great effect on the bottom flux. For the case hw=10m, when L increases from100m to1000m, the20-year the bottom cumulative flux increases by a factor of15.3for the zero concentration bottom boundary. At the same time, L also has a great effect on the time required to reach a steady-state bottom flux. Under the semi-infinite boundary condition, when the leachate head hw=10m, the required time to reach steady state for the case with L=100m is35times greater than the case with L=1000m. The interface transmissivity between GM and GCL has negligible effect on breakthrough curves for the cases with L<100m. Under the semi-infinite boundary conditions, with L=100, when interface transmissivity0increases from5.0×10-12m2/s to1.0×10-10m2/s, the1-year-bottom concentration increase by a factor of only1.35. However, when L increases to1000m, the1-year bottom concentration increases by a factor of18.4for the same change of the interface trasmissivity.(3) The groundwater protection level achieved by GM/CCL is more effective than that by GM/GCL/SL in the earlier times (e.g.,<30years for the case with L=100m and <6years for the case with L=1000m). For example, when L=100m, the10-year-bottom concentration of GM/GCL/SL is112times greater than that of GM/CCL. However, the steady state base fluxes of GM/CCL are greater than those for GM/GCL/SL. The steady state base flux for GM/GCL/SL can be7-8times lower than that for GM/CCL.(4) Based on the proposed dimensionless design curve, illustrating examples involving the design of GM/GCL/SL liner system are presented. Under the leachte head of0.3m and3m, the dichloromethane breakthrough times of the liner systems GM+GCL+4.5m AL and GM+GCL+5m AL, respectively, can meet the requirement that the breakthrough time should be greater than30years. Similarly, under the leachte head of0.3m and3m, the toluene breakthrough times of the liner systems GM+GCL+2.5m AL and GM+GCL+3m AL, respectively, can meet the requirement that the breakthrough time should be greater than30years.(5) The time lag analytical model and analytical model of organic pollutants transport in GM/GCL/SL considering leakage, diffusion, degradation provide a relatively simple design methods for landfill design engineers, and also it can be used to verify the complex numerical model and fit the experimental data.