Covers installed over waste disposal sites are used to control water and gas exchanges with the surrounding environment. One example involves covers built to limit oxygen flux to sulphidic mining and milling wastes, which can be the source of acidic leachate. In this paper, the authors present an approach to evaluate oxygen flux and its controlling parameters, the effective diffusion coefficient De and reaction (consumption) rate coefficient Kr. A laboratory experimental procedure to determine these two parameters simultaneously is described, and the proposed interpretation method is presented with a few sample results. New analytical solutions are developed to calculate oxygen flux through covers with capillary barrier effects (CCBE). The proposed solutions are compared with results ensuing from a numerical treatment of Fick's laws. Specific applications of these analytical solutions are presented and discussed.Key words: unsaturated soils, covers, capillary barrier, Fick's laws, oxygen diffusion, acid mine drainage, analytical solutions, numerical solutions.
Abstract. Molecular diffusion is an important mechanism for gas transport in various natural and man-made systems. This is particularly the case with soil covers installed on acid-generating mine tailings, where oxygen availability has to be controlled. One of the most important roles of such covers is to limit gas flux, which depends on the effective diffusion coefficient D e of the cover materials. This paper presents an experimental procedure and results from oxygen diffusion tests performed on different types of materials, at various degrees of saturation. The determination of D e in the laboratory from the test data is based on analytical and numerical solutions to Fick's laws. The ensuing values of D e are compared to values calculated from available models that relate D e to basic material properties, including porosity and degree of saturation. Statistical indicators are used to evaluate the accuracy of selected models, individually and on a comparative basis. It is shown that modified versions of the Millington-Quirk (M-Q) and Millington-Shearer (M-S) models provide D e values close to the measured data. A semi-empirical expression, ensuing from these models and measurements, is proposed as a simple means of estimating D e .
Abstract. The aim of this study is the investigation of the effects of density and viscosity contrasts on miscible displacements when the regime is stable. It has been possible to quantify these effects by using an empirical model which takes into account both the physical properties of the liquids and the hydrodynamic parameters in different configurations of a miscible displacement involving distilled water and a calcium chloride solution. The introduction of a method for measuring electrical conductivities within the porous medium itself (without disturbing the flow) has enabled us to monitor the change in the mixing zone throughout the entire length of the flow path. A stability criterion based on hydrodynamic parameters has been established that depends on two dimensionless numbers.
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