Handbook of Solvents - George Wypych - ChemTech - Ventech!

1574 Hanadi S. Rifai, Charles J. Newell, Todd H. Wiedemeier
dispersion, the solute front travels at a rate that is faster than would be predicted based solely
on the average linear velocity of the ground water. Figure 23.1.1 illustrates the effects of hydrodynamic
dispersion on an advancing solute front. Mechanical dispersion is commonly
represented by the relationship:
Mechanical Dispersion = α xv x
[23.1.3]
where:
vx average linear ground-water velocity [L/T]
αx dispersivity [L]
Dispersivity represents the spreading
of a contaminant over a given length of
flow and is characteristic of the porous medium
through which the contaminant migrates.
It is commonly accepted that
dispersivity is scale-dependent, and that at a
given scale, dispersivity may vary over
three orders of magnitude 3,4 as shown in
Figure 23.1.3.
Several approaches can be used to estimate
longitudinal dispersivity, α x ,atthe
field scale. One technique involves conducting
a tracer test but this method is time
consuming and costly. Another method
commonly used in solute transport modeling
is to start with a longitudinal
dispersivity of 0.1 times the plume
lengths. 5-7 This assumes that dispersivity
varies linearly with scale. Xu and Eckstein 8
proposed an alternative approach. They
evaluated the same data presented by
Gelhar et al. 4 Figure 23.1.3. Relationship between dispersivity and
scale. [From T.H. Wiedemeier, H. S. Rifai, C. J. Newell
and J.T. Wilson, Natural Attenuation of Fuels and
Chlorinated Solvents in the Subsurface. Copyright ©
1999 John Wiley & Sons, Inc. Reprinted by permission of
John Wiley & Sons, Inc.]
and, by using a weighted
least-squares method, developed the following relationship for estimating dispersivity:
( L )
αx =083 . log10
p
2. 414
[23.1.4]
where:
αx longitudinal dispersivity [L]
Lp plume length [L]
Both relationships are shown on Figure 23.1.3.
In addition to estimating longitudinal dispersivity, it may be necessary to estimate the
transverse and vertical dispersivities (αT and αZ, respectively) for a given site. Commonly,
αT is estimated as 0.1αx (based on data 4 ), or as 0.33αx. 9,10 Vertical dispersivity (αZ) may be
estimated as 0.05αx, 9 or as 0.025 to 0.1αx. 10
23.1.2.3 Sorption
Many organic contaminants, including chlorinated solvents, are removed from solution by
sorption onto the aquifer matrix. Sorption of dissolved contamination onto the aquifer ma-