Mobile, Immobile, and Multiflow Domain Approaches - Transport and Fate of Chemicals in Soils: Principles and Applications - page 267

α and Θ im can be estimated by plotting ln(l - C / C o ) versus application
(infiltration) time. However, the assumption of C im = C o associated with this
method is questionable and may not be correct as long as α is not equal
to 0. A slightly different approach was used by Goltz and Roberts (1988) to
estimate the fraction of mobile water as the ratio of velocity calculated from
hydraulic conductivity to the velocity measured from tracer experiment.
8.3.4 Experimental Evidence
Traditionally, solute transport through structured subsurface media is concep-
tualized as both a rapid transport process occurring within preferential flow
paths (fractures and macropores) and a diffusive mass transfer process that
occurs between the rapid flow region and a stagnant region (micropores), for
example, the mobile-immobile concept (Nkedi-Kizza et al., 1982; Parker and van
Genuchten, 1984). The disparity in solute mobility in the different pore regions
results in concentration gradients between regions, that is, physical nonequi-
libria, whereby diffusive mass transfer serves to reestablish equilibrium.
Physical nonequilibrium is often investigated by employing a miscible dis-
placement technique with the continuous injection of a nonreactive tracer
and the observation of an asymmetric breakthrough curve (Smettem, 1984;
Schulin et al., 1987; Buchter et al., 1995). Under certain conditions, nonequi-
librium cannot be readily distinguished when analyzing data obtained
with the typical continuous flow column experiment (Brusseau et al., 1989).
There are several tracer studies in the literature that illustrate deviations of
experimental breakthrough results from predictions based on the convec-
tion-dispersion equation for homogeneous porous media where physical
nonequilibrium was not considered. The tritium BTCs shown in Figures 8.4
and 8.5 illustrate lack of equilibrium conditions and is commonly considered
evidence of physical nonequilibria.
Another technique known as flow interruption or simply stop flow has been
frequently used to assess nonequilibrium processes in miscible displace-
ment experiments. This method is based on the “interruption test,” which
was developed in chemical engineering to distinguish between systems that
were controlled by intraaggregate diffusion and those controlled by film dif-
fusion. Murali and Aylmore (1980) are perhaps the earliest to implement the
flow interruption technique during the displacement of several ions through
a finely ground soil.
The flow interruption technique has been used to a much lesser extent
to assess the significance of physical nonequilibria during solute transport
(Koch and Flühler, 1993). Hu and Brusseau (1995), and Reedy et al. (1996)
among others, implemented the flow interruption method to quantify physi-
cal nonequilibria processes during displacement of a conservative mobile
dye tracer or bromide in packed columns. Their results suggested that dis-
tinct physical nonequilibria mechanisms influenced the mobility and shape
of the breakthrough results. Such examples where nonreactive or nonsorbing