  The following work compares two popular mixed finite elements used to model
subsurface flow and transport in heterogeneous porous media; the lowest order
Raviart-Thomas element and the variational multiscale stabilized element.
Comparison is made based on performance for several problems of engineering
relevance that involve highly heterogenous material properties (permeability
ratios of up to $1\times10^5$), open flow boundary conditions (pressure driven
flows), and large scale domains in two dimensions. Numerical experiments are
performed to show the degree to which mass conservation is violated when a flow
field computed using either element is used as the advection velocity in a
transport model. The results reveal that the variational multiscale element
shows considerable mass production or loss for problems that involve flow
tangential to layers of differing permeability, but marginal violation of local
mass balance for problems of less orthogonality in the permeability. The
results are useful in establishing rudimentary estimates of the error produced
by using the variational mutliscale element for several different types of
problems.
