IMPACTS OF PHYSICAL AND CHEMICAL HETEROGENEITY ON COCONTAMINANT TRANSPORT IN A SANDY POROUS-MEDIUM

A simplified numerical study of the transport of a uranyl-citric acid mixture through a nonuniform and reactive sandy porous medium is prese nted. The study seeks to identify the more important impacts of medium heterogeneity, as embodied in spatially variable physical and chemica l properties, on the migration and dilution rates of a model cocontami nant mixture, as well as on the overall partitioning among the aqueous and solid species formed from complexation and sorption reactions. So lid phase reactions are considered to occur on hydrous-ferric oxide (g oethite) coatings on the sand and are controlled by the abundance of t he oxide as a function of the specific sand surface area and larger-sc ale patterns of oxide deposition. The simulations involve calculation of fluid flow and chemical migration within highly resolved, two- and three-dimensional regions with synthetic material properties that appr oximate observed conditions in a sandy coastal aquifer. Model simulati ons in this system indicate that (1) the impact of correlation between reactive surface area and hydraulic conductivity, although evident, s eems much less significant than the overall abundance and distribution of the reactive area, such as the kind of banded goethite patterns ob served in a coastal sand body; (2) strong multicomponent interactions clearly reinforce the need to treat the mixture as a coupled system, a s opposed to a series of independently reactive compounds; (3) simplif ications can be made in extremely dilute problems that allow retardati on effects to become concentration independent; and (4) for nonlinear reaction problems, three-dimensional models will be more appropriate t han two-dimensional models to the extent that dispersion in the added dimension accelerates chemical dilution rates.