Sorption Kinetics - Transport and Fate of Chemicals in Soils: Principles and Applications - page 59

Solute Transport in an Unsaturated Soil
FIGURE 2.21
Solute transport experimental setup for downward flow in an unsaturated soil. A tracer, sol-
ute, and background solution are pumped sequentially at flow rate as desired.
in the effluent versus time (or volume of effluent) represent the solute break-
through curves (BTCs), which are analyzed based on various transport
equations. As illustrated by the BTCs presented in the next chapter, a BTC is
characterized by an adsorption or effluent (left-hand) side and desorption or
release (right-hand) side. It is should be noted that the rate of reaction during
release or desorption is more indicative of the fate of solute in soils than from
data based on adsorption alone.
The miscible displacement methods are often to monitor solute trans-
port during infiltration and redistribution in an initially dry soil. In other
words, column transport experiments are not limited to water-saturated
conditions where a steady water flow is maintained. This is illustrated in
Figure 2.21, where a wetting front is advancing in a dry soil, which is moni-
tored with depth and time of infiltration. When the wetting front reaches
the lower end of the column ( z = L ), outflow commences once the pressure
head reaches zero. Effluent volume is monitored with time and collected
samples analyzed for various solutes. In the example shown, a short pulse
of tritium (1.5 pore volumes) was first applied to a Commerce silt loam soil
( L = 30 cm) and was subsequently followed by a large pulse of a mixture of
three heavy metals (Cd, Cu, and Cr). Effluent solution was analyzed for the
applied solutes with time. Moreover, after the termination of the experi-
ment, the soil was sectioned and analyzed for the retained solutes versus
depth (see Figure 2.22). Analysis of results from such column transport
experiments requires solving the solute (convection-dispersion) equation
and Richard's water flow equation for partially saturated soils under tran-
sient flow conditions. Such mathematical solutions are discussed in the
next chapter.