We consider the solution of coupled fluid flow and heat or mass transport in porous media. The aim of this work is to appraise mathematical assumptions used to decrease the CPU cost of the solution of these strongly non-linear coupled equations. This purpose is reached with a reduced model for which the term q . delrho in the mass balance equation is neglected. Indeed, we show that this assumption allows an important reduction of computer time compared to the standard model. Moreover, contrarily to the Boussinesq approximation, no significant differences are found between the reduced and the standard model. Model validation is carried out with a numerical code based on mixed and discontinuous finite elements. First, the Elder and the modified Evans and Raffensperger problems are simulated to test the different assumptions. Second, a simulation of two kinds of laboratory experiment is run without any calibration. For both computations, very similar results are obtained between the complete and the reduced fluid mass balance equations. Both models give numerical results in good agreement with the laboratory layered porous medium experiments. However, these models give less satisfactory results for the salt-pool problem.