In this presentation we are concerned with the problem of developing a numerical simulator for modeling flow and contaminant transport in the vicinity of an underground nuclear waste repository for use in performance assessment. Clearly reasonably accurate and reliable simulators are needed, but as performance assessment requires many simulations, it is also important that the simulators be very efficient. The simulations under consideration necessarily involve large scale calculations both in

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... pace and in time, and yet must take into account important heterogeneities in the domain. This suggests the use of multidomain modeling: the domain is divided into more nearly homogeneous subdomains and efficient numerical solvers are used for each subdomain. The calculations on the various subdomains are related through transmission or interface conditions, and parallel computing and high level programming languages can then be used for numerical implementation. Around the waste depository the porous medium is made up of several regions of different permeability, the permeability varying over several orders of magnitude from one region to the next. In the method developed here the domain is partitioned into homogeneous subdomains and nonoverlapping domain decomposition is used for both the flow calculations and to solve the transport equation. The single phase flow equation is an elliptic equation coupling a conservation equation with Darcy's law. As the Darcy velocity is the unknown of interest, a mixed finite element method is used to solve the equation within the subdomains. In order to be able to use general hexahedral grids, Kuznetsov-Repin macroelements are used. Continuity of the pressure and continuity of the flux are enforced at the interfaces between the subdomains using a balancing domain decomposition method. This method has the advantage of being very robust with respect to large contrasts in permeability. The transport equation is of advection-diffusion type. To approximate the solution [...]