Transport in Nonuniform Media - Transport and Fate of Chemicals in Soils: Principles and Applications - page 311

Unsaturated ow

No adsorp. Kinetic adsorp.

1.0

0.8

Clay Sand

0.6

Water table

0.4

at χ = 100 cm

at χ = ∞

0.2

0

0 1 2 3 4

V/V o

5 6 7 8

FIGURE 9.8
Simulated effluent concentration distribution for reactive and nonreactive solutes in an unsat-
urated clay-sand profile. Open circles are based on average water content θ for each soil layer.

the reactive and nonreactive solutes when an average water content within
each layer was used. These results show that, for all unsaturated profiles
considered, the use of average water contents (open circles) provided identi-
cal concentration distributions to those obtained where the actual water con-
tent distributions were used (dashed and solid lines). Thus, when a steady
water flux is maintained through the profile, BTCs of reactive and nonreac-
tive solutes at a given location in the soil profile can be predicted with aver-
age water contents within unsaturated soil layers. Based on the above results
we can conclude that average microhydrologic characteristics for a soil layer
can be used to describe the movement of solutes leaving a multilayered soil
profile. This conclusion supports the assumption made earlier that uniform
soil water content can be used to represent each soil layer in order to simplify
the solute transport problem. However, such a simplifying approach was not
applicable for the general case of transient water-flow conditions of unsatu-
rated multilayered soils. As illustrated by Selim (1978), the transport of reac-
tive, as well as nonreactive, solutes through multilayered soils, for transient
water flow, was significantly influenced by the order in which the soil layers
were stratified.
Solute transport in a three-layered soil profile (clay over sand over loam)
is shown in Figure 9.9. In this example, we illustrate solute transport during
water infiltration in an unsaturated soil (see Selim, 1978). Here application of
a solute solution at the soil surface was assumed for an extended period of
time, that is, continuous application. The reactivity of individual soil layers to
the applied solute was assumed to follow first-order kinetics. Because of slow
kinetic adsorption, the amount of solute adsorbed S continued to increase