  We study the mixing dynamics of a solute that is transported by advection and
dispersion in a heterogeneous Darcy scale porous medium. We quantify mixing and
dynamic uncertainty in terms of the mean squared solute concentration and the
concentration variance. The latter measures the degree of mixing of the solute
and at the same time the uncertainty around the mean concentration. Its
evolution is controlled by the creation of concentration fluctuations due to
solute spreading and its destruction by local dispersion. For moderate
heterogeneity, this interplay can be quantified by using apparent and effective
dispersion coefficients. For increasing heterogeneity we find deviations from
the predicted behavior. In order to shed light on these behaviors and separate
solute mixing and spreading, we decompose the solute plume into partial plumes,
transport Green functions, and analyze their dynamics relative to those of the
whole plume. This reveals that the variability in the dispersive scales of the
Green functions in the plume and their interactions due to the strong focusing
of preferential flow paths play an important role in highly heterogeneous
porous media.
