Solute Transport in Soil - Transport in the Atmosphere-Vegetation-Soil Continuum - page 190

100

Half-life time = 20 d

R = 1

10

R = 2

1

R = 4

0.1

0 100 200

Time (days)
Figure 5.12 Breakthrough curves at half-life T 50 = 20 d for different amounts of
retardation. The experimental data of Figure 5.2 apply.

R = 1 to R = 4, the solute concentrations at 1 m depth decrease from 90 to 0.23 μ g L -1 .
Because of the very effective immobilization of pesticides when they are adsorbed
and simultaneously decompose, screening programs of pesticides focus on high val-
ues of both R and μ .
In the case of more reactive solutes we might neglect diffusion and dispersion
and use the piston low model. The chemical moves with a velocity v = q / θ and
reaches a distance z = L in a time T res = L / v. If the chemical was added as a front of
concentration C 0 at time t = 0, according to Eq. ( 5.20 ) it will have a concentration
C when it arrives at z = L. If it is added as a pulse of mass M 0 , the

pulse mass that passes z = L will be M

e

− µ T

= C e

− µ / Lv

res

0 0

0 e − µ /

Lv

(Jury et al., 1991 ).

Question 5.11: We continue with the soil column of Figure 5.2 and the solute pulse
C 0 = C(0, t ) Δ t = 1000 (μg d cm -3 ). In the case of piston low with irst-order decay, how
much solutes will ultimately leach from the 1 m high soil column in case of no adsorp-
tion and T 50 = 50 days? And at the same half-life with adsorption R = 4?

5.7 Salinization of Root Zones
In many regions of the world, rain at agricultural ields has to be supplemented
with irrigation water in order to have viable crop production. Water from canals
and groundwater contains more salts than rain water and in the long run may sali-
nize the root zone. In this paragraph we quantify the salinity proiles in irrigated
soils. We assume (1) no solute reactions, (2) no solute dispersion, and (3) no plant
uptake of solute. These assumptions are justiied for long-term salinization processes
(10-30 years).
The steady-state soil water low equation at any depth in a root zone with uniform
root water uptake S (d -1 ), can be written as: