The effect of solute size on diffusive-dispersive transport in porous media

The purpose of this work was to investigate the effect of solute size on diffusive-dispersive transport in porous media. Miscible displacement experiments were performed with tracers of various sizes (i.e. tritiated water (³H₂O), pentafluorobenzoate (PFBA), and 2,4-dichlorophenoxyacetic acid (2,4-D)) and a homogeneous, nonreactive sand for pore-water velocities varying by three orders of magnitude (70, 7, 0.66, and 0.06 cm h^-1). Hydrodynamic dispersion is the predominant source of dispersion for higher pore-water velocities (exceeding 1 cm h^-1), and dispersivity is, therefore, essentially independent of solute size. In this case, the practice of using a small-sized tracer, such as ³H₂O, to characterize the dispersive properties of a soil is valid. The contribution of axial diffusion becomes significant at pore-water velocities lower than 0.1 cm h^-1. At a given velocity below this value, the contribution of axial diffusion is larger for ³H₂O, with its larger coefficient of molecular diffusion, than it is for PFBA and 2,4-D. The apparent dispersivities are, therefore, a function of solute size. The use of a tracer-derived dispersivity for solutes of different sizes would not be valid in this case. For systems where diffusion is important, compounds such as PFBA are the preferred tracers for representing advective-dispersive transport of many organic contaminants of interest.