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 presented. The study seeks to identify the more important impacts of medium heterogeneity, as embodied in spatially variable physical and chemical properties, on the migration and dilution rates of a model cocontaminant mixture, as well as on the overall partitioning among the aqueous and solid species formed from complexation and sorption reactions. Solid phase reactions are considered to occur on hydrous-ferric oxide (goethite) coatings on the sand and are controlled by the abundance of the oxide as a function of the specific sand surface area and larger-scale 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 approximate observed conditions in a sandy coastal aquifer. Model simulations in this system indicate that (1) the impact of correlation between reactive surface area and hydraulic conductivity, although evident, seems much less significant than the overall abundance and distribution of the reactive area, such as the kind of banded goethite patterns observed in a coastal sand body; (2) strong multicomponent interactions clearly reinforce the need to treat the mixture as a coupled system, as opposed to a series of independently reactive compounds; (3) simplifications can be made in extremely dilute problems that allow retardation effects to become concentration independent; and (4) for nonlinear reaction problems, three-dimensional models will be more appropriate than two-dimensional models to the extent that dispersion in the added dimension accelerates chemical dilution rates.