Page 98 - Modelisation du devenir des pesticides...

Chapitre 3 Evaluation du modèle STICS version pesticide sur différents sites
expérimentaux

site year Mass Trans Sys Leach Err
app (%) (%) (%) (%)
(kg/ha)
Vredepeel 1990/91 0,63 73.97 1.02 25.08 0,07
Kerlavic 2002/03 1.25 80.80 18.80 0.15 0,15
2003/04 1.25 83.20 16.98 0,05 0.23
Thiverval- 1993 0.94 82.50 17.52 ! 0.00 0.02
Grignon 1994 0.90 98.00 2.03 ! 0.00 0.03

Table 3.7 – Mass balance of pesticide at the three study sites at 1m depth : Mass
app : Mass of pesticide applied, Trans : percentage of mass of pesti-
cide transformed (degradation) ; Sys : percentage left in the system
(liquid+adsorbed) ; Leach : percentage of liquid pesticide leached at
the bottom of the soil profile Err : numerical mass balance error

the top soil 90 DAA compared to 52 and 72 % for isoproturon and atrazine. The
lower impact of degradation is linked to the higher value of the DT50 (Table 3.4).
In addition, the low temperature during winter at Vredepeel mayhave significantly
reduce the degradation rate.
The evolution of the aeric mass of isoproturon and atrazine is first dominated
by the linear adsorption process at Kerlavic and Thiverval-Grignon sites. However
degradation becomes dominant at both sites 90 days after application with 52 and
73% of the pesticides degraded respectively. Although the DT50 of isoproturon
and atrazine are fairly similar, the dynamic of the degradation is different. This
may be due firstly, to the slow adsorption process and secondly, to the temperature
effect with warmer temperature at Thiverval-Grignon as the atrazine was applied
in late spring. The non-equilibrium process used in the model takes the creation
of bound residues into account. Consequently, this process has a larger impact
on the fate of isoproturon and atrazine on long time period. Although it is a
long-term process, the non-equilibrium adsorption shows different dynamics for
isoptruron at Kerlavic and atrazine at Thiverval-Grignon.The larger Kneq_ads for
atrazine first lead to a more larger fraction of adsorbed pesticide being adsorbed
in the Neq phase (18% compared to 7% for isoproturon 30 DAA), but this is
then compensated by a larger desorption, so that 120 DAA, the ratios are 19.5
and 15.4% respectively. Comparison of the mass balance at 30 cm and 1m depth
(Fig. 3.3.4) shows quite similar results for isoproturon and atrazine, but rather
different results for Bentazone. As isoproturon and atrazine are mostly adsorbed
and then degraded, most of the processes occur in the top soil. In contrast, the
liquid fraction dominates for bentazon and is transferred rapidly to the deeper
soil (10% 30 DAA and 70% 90 DAA). The mass balance at 1m depth is more

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