Handbook of Solvents - George Wypych - ChemTech - Ventech!

23.1 Natural attenuation of chlorinated solvents 1575
trix results in slowing (retardation) of the contaminant relative to the average advective
ground-water flow velocity and a reduction in dissolved organic concentrations in ground
water. Sorption can also influence the relative importance of volatilization and
biodegradation. 11 Figure 23.1.2 illustrates the effects of sorption on an advancing solute
front. Sorption is a reversible reaction; at given solute concentrations, some portion of the
solute is partitioning to the aquifer matrix and some portion is also desorbing, and reentering
solution.
Sorption can be determined from bench-scale experiments. These are typically performed
by mixing water-contaminant solutions of various concentrations with aquifer materials
containing various amounts of organic carbon and clay minerals. The solutions are
then sealed with no headspace and left until equilibrium between the various phases is
reached. The amount of contaminant left in solution is then measured.
The results are commonly expressed in the form of a sorption isotherm or a plot of the
concentration of chemical sorbed (μg/g) versus the concentration remaining in solution
(μg/L). Sorption isotherms generally exhibit one of three characteristic shapes depending
on the sorption mechanism. These isotherms are referred to as the Langmuir isotherm, the
Freundlich isotherm, and the linear isotherm (a special case of the Freundlich isotherm).
The reader is referred to ref. 1 for more details on sorption isotherms.
Since sorption tends to slow the transport velocity of contaminants dissolved in
ground water, the contaminant is said to be “retarded.” The coefficient of retardation, R, is
defined as:
R v x
= [23.1.5]
v
c
where:
R coefficient of retardation
vx average linear ground-water velocity parallel to ground-water flow
vc average velocity of contaminant parallel to ground-water flow
The ratio vx/vc describes the relative velocity between the ground water and the dissolved
contaminant. The coefficient of retardation for a dissolved contaminant (for saturated
flow) assuming linear sorption is determined from the distribution coefficient using
the relationship:
K
R = 1+ N
ρ
b d
[23.1.6]
where:
R coefficient of retardation [dimensionless]
ρb bulk density of aquifer [M/L 3 ]
Kd distribution coefficient [L 3 /M]
N porosity [L 3 /L 3 ]
The bulk density, ρb, of a soil is the ratio of the soil mass to its field volume. In sandy
soils, ρb can be as high as 1.81 g/cm 3 , whereas, in aggregated loams and clayey soils, ρb can
be as low as 1.1 g/cm 3 .
The distribution coefficient is a measure of the sorption/desorption potential and characterizes
the tendency of an organic compound to be sorbed to the aquifer matrix. The