Geoscience Reference
In-Depth Information
Table 12.7 Adsorption (K
d(ads)
, K
oc
)-desorption K
d(des)
coefficients for bromacil and terbuthyl-
azine, as affected by the vertical variability of the soil profile (Toiber-Yasur et al.
1999
)
Soil depth (cm)
Bromacil
Terbuthylazine
K
d(ads)
(mL/g)
0-10 56 1.7 0.7 164 4.6 2.0
10-30 65 1.5 0.8 249 3.9 2.9
30-50 61 1.2 0.5 255 2.4 1.9
50-70 96 2.6 0.6 356 2.4 2.1
70-90 77 1.1 0.5 304 2.5 2.1
90-110 111 1.9 0.7 358 3.4 2.3
110-130 108 1.7 0.7 365 3.1 2.4
Notation K
d
adsorption coefficient; K
oc
adsorption on organic carbon per unit weight of soil
K
oc
(mL/g)
K
d(des)
(mL/g)
K
d(ads)
(mL/g)
K
oc
(mL/g)
K
d(des)
(mL/g)
Table 12.8 Effect of field site spatial variability on adsorption coefficients of terbuthylazine in
soil upper layer (Toiber-Yasur et al.
1999
)
Sample location Soil organic matter (%) K
d
K
oc
2 1.19 1.59 133.27
7 1.01 1.39 137.35
9 1.04 1.14 109.19
14 1.12 1.54 136.53
17 1.18 1.54 131.06
18 1.05 1.32 125.15
19 0.89 1.06 117.92
10 1.22 2.00 163.10
Sample location was chosen randomly. Notation: K
d
adsorption coefficient; K
oc
adsorption on
organic carbon per unit weight of soil
In contrast, bromacil has a higher solubility in water and is leached downward
under the influence of the spatial variability in hydraulic conductivity.
Concentrations of terbuthylazine and bromacil in the experimental field at a
depth of 400 cm, after leaching (irrigation and rainfall) of 110 cm water, are
shown in Fig.
12.36
. The total amount of bromacil and terbuthylazine recovered
from the soil profile after this leaching period was much smaller than the amount
of chemicals applied initially. This result suggests that the reduction in concen-
tration of herbicide due to degradation is greater than that caused by leaching.
Similar results were obtained when an additional herbicide, atrazine, was applied
together with terbuthylazine on a neighboring experimental plot and leached by
irrigation and rainwater during a two-year period (Dror et al.
1999
).
The field experiments from the Bet-Dagan site were used to test different
theoretical models for field-scale chemical transport. One study expanded a simple
column model for flow and transport in partially saturated soils (Bresler and Dagan
1983
), by using a stochastic analysis (Indelman et al.
1998
). The approach ignored
