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<< Click to Display Table of Contents >> Dispersion |
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In a granular layer (eg. an aquifer) or a fractured layer there can be significant localized variations in the groundwater flow. These variations will cause mechanical mixing within the layer, this process is called dispersion [Freeze and Cherry, 1979]. Although the process is very different to diffusion it can be modelled mathematically in the same manner, and the two processes can be grouped together as the “coefficient of hydrodynamic dispersion”, D, viz.:
D = De + Dmd
where,
De = effective diffusion coefficient,
Dmd = coefficient of mechanical dispersion.
In unfractured clayey soils the coefficient of hydrodynamic dispersion is often controlled by the diffusion coefficient, and the coefficient of mechanical dispersion is negligible. In sandy soils and fractured layers the opposite is generally true and dispersion dominates [Gillham and Cherry, 1982; Rowe, 1987; Rowe et al, 2004]. The mass flux for advective-dispersive transport (including diffusion) is given by:
f = n v c - n D dc/dz
where the parameters are the same as those defined previously and D is the coefficient of hydrodynamic dispersion. Dispersion is often modelled as a linear function of velocity [Bear, 1979; Freeze and Cherry, 1979; Rowe et
a;, 2004] given by:
Dmd = α v
where,
α = dispersivity,
v = groundwater (seepage) velocity.
The dispersivity tends to be scale dependent and is not a true material property [Gillham and Cherry, 1982].