On the hydro-dispersive equivalence between multi-layered mineral barriers

In the context of municipal solid waste and hazardous waste disposal, the n otion of "equivalence" between different barrier designs appears in regulat ory documents from several industrialized countries. While in the past, equ ivalence has been thought of mainly in terms of contaminant travel times, i n recent years it has been defined more in terms of the magnitude of a disp osal site's potential impact on groundwater resources. This paper presents some original analytical solutions to the problem of contaminant migration through a multi-layered mineral barrier. The solutions account for the two major mechanisms of subsurface contaminant migration, namely, advection and diffusion-dispersion. An example application using the proposed solutions and a numerical model illustrates how one multi-layered mineral barrier can be considered superior to another from a strictly hydro-dispersive viewpoi nt. The influence of partial saturation of the mineral barrier is investiga ted using a numerical solution to the Richards equation for unsaturated flo w. It is emphasized that conclusions relative to the superiority of one mul ti-layered barrier, with respect to another, should not only consider hydro -dispersive aspects, but also other processes such as the mechanical and ch emical evolutions of the different barrier components. Although such phenom ena are poorly addressed by existing models, failure to take them into acco unt, at least in a qualitative fashion, may lead to unconservative conclusi ons with respect to barrier equivalence. (C) 2001 Elsevier Science B.V. All rights reserved.