Page 83 - Modelisation du devenir des pesticides...

3.2 Materials and methods

Mptot ne ads (µg) is the total mass of pesticide adsorbed onto non-equilibrium
sorption sites, K ads and K des (d ) are the sorption and desorption rates to and
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from the non-equilibrium phase, t being the time in days. Moreover, the pesticides
adsorbed onto the non-equilibrium sorption sites are assumed to be compounds
not available for biodegradation.
Biodegradation is described with first-order kinetics and depends on the soil
temperature and moisture. The moisture effect is partly based on an equation
developed in the LEACHP model. 52 The biodegradation is greatest when the soil
water content is close to the field capacity and decreases linearly to the wilting
point. The soil moisture factor fθis expressed as :

fθ = θ − θ wp (3.2.5)
θ fc − θ wp
where : θ,θ fc and θ wp are the soil water content (m /m ) at time t, at field
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capacity and at the wilting point respectively. The effect of temperature is based
on the Arrhenius relationship used in the pesticide fate model PEARL : 53

! " 1 1 !#
T soil − T 0 −∆Ht
if : T 0 ≤ T soil ≤ T A,l ; ft − exp − (3.2.6)
T A,l − T 0 R T A,l T r

" −∆Ht 1 1 #
if : T A,l ≤ T soil ≤ T A,u ; ft − exp − (3.2.7)
R T soil T r

if : T soil < T 0 ; ft = 0 (3.2.8)
where : T soil is the soil temperature (K), T 0 is the freezing point (273. K), ft is
the temperature factor, T r is the reference temperature (293 K), T A,l is the lower
temperature limit for the Arrhenius range (278K), T A,u is the upper temperature
limit for the Arrhenius range (308K), ∆Ht is the molar enthalpy for transforma-
tion (J mol ) and R is the molar gas constant (J mol K ). This relation helps
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to enhance the precision of the temperature factor, which may deviate from the
Arrhenius relation in a range of temperatures above the freezing point. 54 The mass
of pesticide Mdeg transformed at each time step in the whole soil profile for liquid
and linearly sorbed pesticide is then expressed as :

X ln(2)
nb layer
Mp deg(liq,ads) = × ft i × fθ i × Mp (liq,ads)i (3.2.9)
i=1 DT50
where : nb layer is the number of elementary layers in the soil profile, Mp (liq,ads)i is
the mass of pesticide either in liquid (liq) or linearly sorbed (ads) phases in the soil

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