Handbook of Solvents - George Wypych - ChemTech - Ventech!

23.1 Natural attenuation of chlorinated solvents 1573
Figure 23.1.1. Breakthrough curve in one dimension showing plug flow with continuous source resulting from
advection only and from the combined processes of advection and hydrodynamic dispersion. [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. Reprinted by permission of John Wiley & Sons.]
Typical velocities range between 10 -7 and 10 3 ft/day 2 with a median national average
of 0.24 ft/day. The seepage velocity is a key parameter in natural attenuation studies since it
can be used to estimate the time of travel of the contaminant front:
x
t = [23.1.2]
v x
where:
x travel distance (ft or m)
t time
Solute transport by advection alone yields a sharp solute concentration front as shown
in Figure 23.1.1. In reality, the advancing front spreads out due to the processes of dispersion
and diffusion as shown in Figure 23.1.1, and is retarded by sorption (Figure 23.1.2) and
biodegradation.
Figure 23.1.2. Breakthrough curve in one dimension showing plug flow with continuous source resulting from
advection only; from the combined processes of advection and hydrodynamic dispersion; and from the combined
processes of advection, hydrodynamic dispersion, and sorption. [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.]
23.1.2.2 Dispersion
Hydrodynamic dispersion causes a contaminant plume to spread out from the main direction
of ground water flow. Dispersion dilutes the concentrations of the contaminant, and introduces
the contaminant into relatively pristine portions of the aquifer where it mixes with
more electron acceptors crossgradient to the direction of ground-water flow. As a result of