Natural attenuation of an acidic plume in the aquifer underneath a uranium mill tailings
pond in Wyoming, USA was simulated using the multi-component reactive transport code
PHREEQC. A one-dimensional model was constructed for the site and the model included
advective-dispersive transport, aqueous speciation of 11 components, and precipitation-dissolution
of six minerals. Transport simulation was performed for a reclamation scenario in
which the source of acidic seepage will be terminated after 5 years and the plume
will then be flushed by uncontaminated upgradient groundwater. Simulations show that
successive pH buffer reactions with calcite, Al(OH)3(a), and Fe(OH)3(a) create distinct
geochemical zones and most reactions occur at the boundaries of geochemical zones.
The complex interplay of physical transport processes and chemical reactions produce
multiple concentration waves. For SO4(2-) transport, the concentration waves are related
to advection-dispersion, and gypsum precipitation and dissolution. Wave speeds from
numerical simulations compare well to an analytical solution for wave propagation.