Effective terms hydraulic conductivity

Present regulations assume that long-term isolation of hazardous wastes — including toxic chemical, biological, radioactive, flammable and explosive wastes — may be effected by disposal in landfills that have liners of very low hydraulic conductivity. In reality, total isolation of wastes in humid areas is not possible some migration of leachate from wastes buried in the gound will always occur. 

The bathtub effect occurs, in part, because most wastes have much higher hydraulic conductivities than the natural material into which they are placed they may also have very different unsaturated soil—moisture characteristics. The hydraulic conductivity of some wastes can be reduced by compaction. The bathtub effect also occurs because more infiltration enters the disposal excavation than would under normal undisturbed conditions. Trench covers may be constructed to achieve the desired hydraulic conductivity and to limit infiltration for the required period of containment or until compaction of the wastes occurs however, it is difficult to maintain the trench covers. The covers must withstand attack by plants, weather (freeze—thaw, wet—dry), erosion, and strains caused by consolidation within the trench. Most trench covers are not capable of meeting these demanding requirements without costly long-term maintenance programs. The cover should be designed to allow for expected consolidation and to utilize hydro-geological concepts of saturated and unsaturated flow systems present at the site. 

Permeability. Permeability is the hydraulic conductance of a medium defined with direct reference to Darcy s law. In a somewhat more general sense, the shear factor is the hydraulic resistivity of the medium. When the term permeability is used, one normally refers to linear flow systems (no inertial effects). 

This quite arbitrary method of two-dimensional transport ignores the components of vertical flow and thus underestimates the transversal dispersion thereby induced. On the other hand we tested a simple non-reacting case with the same hydraulic conditions against an analytical solution and found that the longitudinal dispersion is not influenced by numerical dispersion, whereas the numerical solution overestimates the transversal dispersion by approximately 10 %. The influence of the boundary conditions for top and bottom of the aquifer (no gradient, no flux) is more important in terms of an increased transversal dispersion for these cells. All these effects are negligible compared to the influences of inhomogeneities of hydraulic conductivities onto the modelled transversal dispersion. 

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