Geoscience Reference
In-Depth Information
Relative concentration
C
/
C
0
0.0
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
-0.6
-0.7
-0.8
-0.9
-1.0
L
f
= 0.30
L
f
= 0.20
L
f
= 0.10
Figure 5.13
Relative salinity concentrations in the root zone for different leaching
fractions in case of a uniform root water extraction.
Early compounds, such as DDT and Dieldrin, had a very low mobility in soil and
therefore did not have the potential to reach groundwater. However, they were very
persistent and thus reached the food chain by exposure through the atmosphere or
migration in ground- and surface waters (Jury et al.,
1991
).
Modern legislation intends to prevent pesticide leaching towards the ground-
water. Implicit in this legislation is the idea that pesticides can be screened based
on their environmental fate properties at the time of their development to make
preliminary decisions about their pollution potential. As an example, Boesten and
Van der Linden (
1991
) calculated pesticide leaching to groundwater at 1 m depth
for a sandy soil continuously cropped with maize. They assumed irst-order trans-
formation, equilibrium Freundlich adsorption (
Figure 5.7
) and proportional uptake
of water and pesticides by plant roots.
Figure 5.14
shows the calculated leach-
ing amount as a function of half-life (
T
50
), adsorption (
S
d
) and application season
(spring or autumn). Pesticide leaching is very sensitive to both
T
50
and
S
d
: changing
these coeficients by a factor of 2, changes the amounts leached typically by about
a factor 10! Autumn applications result in much higher leaching of nonsorbing
pesticides with short half-lives than spring application (difference about 2 orders
of magnitude).
Question 5.14:
What are the two main reasons that nonsorbing pesticides with short
half-lives leach about 100 times more in autumn compared to spring?
5.9 Residence Time in Groundwater
To analyze solute leaching to surface water systems, we should know the solute
residence time in the groundwater system. In the vadose zone, soil water low
is predominantly vertical. Below the groundwater level, water is subject to the
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