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Coupled Hydro-chemical Modelling of Soluble Contaminant Transport Through Saturated Clay

Hedayati Azar, Aysa | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54214 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Sadeghi, Hamed
  7. Abstract:
  8. Leaking of leachate contamination into the clayey isolation layer at the bottom of the landfills is one of the most important environmental issues, which influences living creatures' lives by affecting groundwater quality. Coupled phenomena that play a role in contamination and fluid transportation in a saturated clayey layer are hydraulic and osmotic flow, diffusion and convection. The targeted contaminate of leachate is monovalent salts. The clayey layer is also saturated. According to the governing differential equation, membrane efficiency and hydraulic conductivity coefficients highly affect the contamination transport. Experimental results and existing models have shown that these coefficients depend on the characteristics of the clay and the solute concentration. Leachate characteristics (contaminate concentration and its volume) change as a result of fluid and contamination transport, which causes governing boundary conditions to change. However, in the previous numerical studies, membrane efficiency, hydraulic conductivity and leachate characteristics are assumed to be constant. To refine that in this study, governing models of membrane efficiency and hydraulic conductivity have been developed for contaminated clays with solute and improving existing theoretical and experimental models. An experimental model and a semi-analytical model have been proposed for membrane efficiency. Two experimental models have also been developed for hydraulic conductivity to consider the solute concentration. Changes in leachate characteristics in the boundary of clayey soil have also been modelled by mass conservation law. This study uses numerical methods in COMSOL software to stimulate coupled hydro-chemical flow phenomena and, simultaneously, considers changes in membrane efficiency, hydraulic conductivity and leachate characteristics by using developed models, which is called the "variable approach". In order to study the changes of results due to changing applied approach, numerical analysis results relating to constant values were obtained as well, which is called the "constant approach". According to the results, using the variable approach to apply membrane efficiency influences the results much more intensely in lower solute concentrations or void ratios. Furthermore, using the variable approach for applying hydraulic conductivity in the numerical analysis instead of the constant approach is more obvious in the higher concentrations or void ratios. By using the variable approach to apply boundary conditions, assuming that the solute concentration is zero at the bottom of the clayey layer, intense membrane behavior at the bottom boundary leads to increasing leachate concentration in the long term, and a steady concentration profile never reaches. Nevertheless, if the concentration gradient is zero at the bottom of the clayey layer, leachate concentration continues to drop until the steady condition happens. This study also considers leachate volume changes given the contaminate level is constant. Results have shown that by declining leachate volume, changes are more violent. Outcomes of these simulations in different conditions have shown noticeable alterations in contamination transport results. It is hoped that the design of landfills and prediction of contamination transport will improve in the future
  9. Keywords:
  10. Diffusion ; Pollutant Leakage ; Contamination ; Membrane Behavior in Soil ; Landfill Cover ; Clayey Isolation Layer ; Coupled Flow ; Hydraulic Conductivity

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