Mobile, Immobile, and Multiflow Domain Approaches - Transport and Fate of Chemicals in Soils: Principles and Applications - page 277

1.0

Olivier Loam
Tritium

0.8

0.6

0.4

0.2

0.0

0 1 2 3

Pore Volume

FIGURE 8.5
Breakthrough of tritium in an Olivier soil. Simulations are based on the convection-dispersion
equation where equilibrium is assumed.

similar features between these BTCs and those illustrated in the previous
figures. For f = 1, all the sites are active sites and thus there is no solute reten-
tion by the sites present within the immobile region (i.e., stagnant sites). As
the contribution of the stagnant sites increases (or f decreases), the shape of
the BTCs becomes increasingly less kinetic, with significant increase of the
tailing of the desorption side of the BTCs.
In the BTCs shown in Figures 8.2 through 8.6, the irreversible retention
mechanism for heavy metal removal (via the sink term) was ignored. The
influence of the irreversible kinetic reaction (e.g., precipitation) is a straight-
forward one and is thus not shown. This is manifested by the lowering of
solute concentration for the overall BTC for increasing values of k s . Since a
first-order reaction was assumed, the lowering of the BTC is proportional to
the solution concentration. The influence of other parameters on the behav-
ior of solute in soils with the second-order mobile-immobile model, such
as P , D , and v , have been studied elsewhere (van Genuchten and Wierenga,
1976).

8.4.3 Examples

The capability of the second-order mobile-immobile model to describe the
transport of heavy metals in soils was examined for hexavalent chromium
(Cr(VI)) by Selim and Amacher (1988). Their Cr(VI) miscible displacement
results and model predictions for three soils are shown in Figures 8.14 to 8.16.
To obtain the predictions, several assumptions were necessary for the esti-
mation of model parameters. The sorption maximum ( S max ) was estimated
from kinetic adsorption isotherms. In addition, the ratio of the mobile to