Hydrodynamic dispersion refers to the net effect of a variety of microscopic, macroscopic, and regional conditions that affect the spread of a solute front through an aquifer.58 Quantifying the dispersion is important to fate assessment because contaminants can move more rapidly through an aquifer by this process than would be predicted by simple plugflow (i.e., uniform movement of water through an aquifer with a vertical front). In other words, physical conditions (such as more-permeable zones, where water can move more quickly) and chemical processes (e.g., movement of dissolved species at greater velocities than the water moves by molecular diffusion) result in more rapid movement of contaminants than would be predicted by groundwater equations for physical flow, which must assume average values for permeability. Dispersion on the microscopic scale is caused by the following:
1. Velocity variations resulting from variations in pore geometry and the fact that water velocity is higher in the center of a pore space than that for water moving near the pore wall
2. Molecular diffusion along concentration gradients
3. Variations in fluid properties such as density and viscosity
Dispersion on the macroscopic scale is caused by variations in hydraulic conductivity and porosity, which create irregularities in the seepage velocity with consequent mixing of the solute. Finally, over large distances, regional variations in hydrogeologic units can affect the amount of dispersion. In hydrogeologic modeling, the hydrodynamic dispersion coefficient D is often expressed as the sum of a mechanical dispersion coefficient Dm and molecular (Fickian) diffusion D*.
In most instances, hydrodynamic dispersion is not great enough to require detailed consideration in hydrogeologic modeling for fate assessment of deep-well-injected wastes. However, regional variations (such as the presence of an USDW in the same aquifer as the injection zone, as is the case in parts of Florida) should be evaluated before a decision is made to exclude it.
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