Effect of aged-sorption parameters
on calculated leaching
Jos Boesten & Sabine Beulke
Outline

Introduction

Procedure

Effect of aged-sorption parameters on DegT50

Effect of aged-sorption parameters on leaching

Effect of uncertainty in aged-sorption parameters on
leaching
Introduction

project: guidance aged sorption for leaching assessment

previous presentation: criteria based on assumption that
error of 25% in desorption rate coefficient, kd , and the factor
fNE is acceptable

aim of this part: what is effect of uncertainty of 25% in kd and
fNE on leaching concentrations ?

only a few calculations available in literature on sensitivity to
aged sorption parameters (Boesten 1990; Boesten & van der
Linden, 2001)
Procedure : scenario characteristics

FOCUS_PEARL v3.3.3 + Okehampton scenario

annual application of 1 kg/ha on 16 October in winter wheat

for five KOM – DegT50 combinations

kd range: 0 - 0.05 d-1

fNE range: 0 – 1.2
Procedure: the five KOM – DegT50 combinations
DegT50 = 20 L/kg
Contour plot of FOCUS
leaching concentration
(μg/L) not considering
aged sorption
0.1
1
10
100
KOM = 35 L/kg
Procedure: complication of change in DegT50 due to
aged sorption

Pesticide molecules sorbed at long-term sorption sites not degraded, so
definition of DegT50 changes:
conventional DegT50 is called DegT50TOT
new DegT50 is called DegT50EQ

Hypothetical example case:



Assume furthermore: at start of this day



50% of all molecules degrades in 1 day
so conventional DegT50TOT = 1 d
80% of molecules in equilibrium part
20% of molecules sorbed at long-term site
So 80% of molecules decreases to 30% of molecules in 1 day

this gives revised DegT50EQ = 0.7 d
Procedure: how to keep DegT50TOT constant when
including aged sorption ?

dossier: good description of overall degradation rate in laboratory studies
is boundary condition

all calculations based on a ‘reference’ degradation rate study in the
laboratory

Okehampton top soil at 20oC and field capacity

calculate decline from 10 mg/kg to 1 mg/kg with Single First-Order
model

fit daily SFO values to model including aged sorption (PEARLNEQ) with
DegT50EQ as the only adjustable parameter
Procedure: example of comparison between conventional
SFO model and model including aged sorption
DegT50TOT = 39 d
DegT50EQ = 30 d
So lines very similar while DegT50 values
differ significantly
Effect of aged sorption parameters on DegT50EQ
KOM = 35 L/kg
DegT50TOT = 20 d
DegT50EQ in most
extreme case almost
two times shorter than
DegT50TOT
clear need to include
this effect
Effect of aged sorption parameters on leaching
effect larger at larger kd
effect larger at larger KOM
∞
equilibrium runs for
infinite kd
Effect of aged sorption parameters on leaching
effect larger at larger kd
effect usually larger at shorter
DegT50TOT
∞
Effect of aged sorption parameters on leaching
effect larger at larger fNE
effect larger at larger KOM
Effect of aged sorption parameters on leaching
effect larger at larger fNE
effect of DegT50TOT variable
Effect of aged sorption parameters on leaching
Summary of results:

effect increases with increasing kd and fNE

effect increases with increasing KOM

no clear trend for effect of DegT50TOT

including aged sorption may decrease leaching concentrations
by one or two orders of magnitude
Effect of uncertainty in aged-sorption parameters on
leaching
effect increases both with kd and fNE so perhaps
unique relationship with product of kd and fNE ?
FOCUS leaching concentration ( g/L)
KOM = 35 L/kg
PEARL calculations
fitted exponential line
1
DegT50TOT = 20 d
reasonable fit:
c0 = 1.56 μg/L
0.1
b = 56.9 d
0.00
0.01
0.02
0.03
0.04
Product of kd and fNE (d-1)
0.05
0.06
Effect of uncertainty in aged-sorption parameters on
leaching
Contour diagram of FOCUS
leaching concentration
(μg/L) for DegT50TOT = 20 d
and KOM = 35 L/kg based
on fitted exponential
equation
previous presentation:
error of 25% in kd or fNE
is OK
Monte Carlo simulations
assuming normally
distributed kd or fNE with
CV of 12.5%
Effect of uncertainty in aged-sorption parameters on
leaching
frequency distribution of FOCUS leaching concentration
5
Frequency (L/ g)
4
deviations from
true value are
limited to about
25%
3
true value
2
1
0
0.5
0.6
0.7
0.8
0.9
1.0
FOCUS leaching concentration ( g/L)
1.1
Conclusions

significant effect of aged sorption parameters on DegT50EQ

leaching concentration decreases continuously with increasing kd and fNE

effect of kd and fNE on leaching concentration is larger at larger KOM

including aged sorption may decrease leaching concentrations by one or
two orders of magnitude; so accurate assessment of kd and fNE needed

however, exploratory calculation showed that error of less than 25% in kd
and fNE seems accurate enough for leaching assessment
Thank you for your attention !
© Wageningen UR