  We consider experimentally the instability and mass transport of a
porous-medium flow in a Hele-Shaw geometry. In an initially stable
configuration, a lighter fluid (water) is located over a heavier fluid
(propylene glycol). The fluids mix via diffusion with some regions of the
resulting mixture being heavier than either pure fluid. Density-driven
convection occurs with downward penetrating dense fingers that transport mass
much more effectively than diffusion alone. We investigate the initial
instability and the quasi steady state. The convective time and velocity
scales, finger width, wave number selection, and normalized mass transport are
determined for 6,000<Ra<90,000. The results have important implications for
determining the time scales and rates of dissolution trapping of carbon dioxide
in brine aquifers proposed as possible geologic repositories for sequestering
carbon dioxide.
