Page 82 - Modelisation du devenir des pesticides...

Chapitre 3 Evaluation du modèle STICS version pesticide sur différents sites
expérimentaux
McCann 48 where the thermal diffusivity is assumed to be independent of soil water
conditions. The solute transfer is based on the mixing cells principle, a mixing cell
being the integration of several elementary layers. 49 The mass of solute applied
at the soil surface is first transferred by convection with water in the elementary
layers. The solute is then mixed with the solute already contained in the reservoir.
The solutes are finally disaggregated at the elementary layer scale. This method is
similar to a convection-dispersion model, if the mixing cell thickness is set to twice
the dispersivity. 45 A minimum concentration threshold can be set that prevents
leaching below that point (especially for nitrogen).

3.2.2 The new pesticide transfer module
The pesticide module was implemented within STICS to simulate the main
processes involved in the pesticide fate, i.e. linear and non-equilibrium adsorptions,
degradation, and transfer. As already stated, classical formalisms are used in this
pesticide module. Liquid transfer relies on the STICS solute transfer formalism.
Linear sorption onto soil particles is calculated with a linear isotherm according
to the equation :

(3.2.1)
Cpest ads = Kd × Cpest liq
where Cpest liq and Cpest ads are the concentrations of liquid at the equilibrium and
linearly adsorbed pesticide respectively, Kd is the soil sorption coefficient (L kg )
-1
deduced from the carbon fraction foc and the soil organic carbon sorption coeffi-
cient Koc (L kg ) as follows :
-1

Kd = Koc × foc (3.2.2)
The soil sorption coefficient Kd can thus vary with depth according to the dis-
tribution of the organic matter along the soil profile. Non-equilibrium sorption is
expressed following the Agriflux equations according to Larocque et al, 35 such
representations being based on the experimental works of Xing and Pignatello 50,51
who observed time dependent isotherm shape for sorption processes of organic
compounds. The non-equilibrium sorption in STICS is assumed to be a reversible
phenomenon including the desorption process :

h i
Mp ne ads = Mp av × 1 − e (−K ads ×foc×t) (3.2.3)

h i
Mp ne des = Mptot ne ads × 1 − e (−K des ×foc×t) (3.2.4)
Where : Mp av is the mass of pesticide available for non-equilibrium sorption,
which includes both the liquid and linearly adsorbed phases,

70