  A simple finite element formulation of the outlet gradient boundary condition
is presented in the general context of convective-diffusive transport
processes. Basically, the method is based on an upstream evaluation of the
dependent variable gradient along open boundaries. Boundary normal unit vectors
and gradient operators are evaluated using covariant bases and metric tensors,
which allow handling finite elements of mixed dimensions. Even though the
presented method has implications for many fields where diffusion processes are
involved, discussion and illustrative examples address more particularly the
framework of contaminant transport in porous media, in which the outlet
gradient concentration is classically, but wrongly assumed to be zero.
