Modelling aged sorption for metabolites in FOCUS-PRZM - pfmodels

Modeling Aged Sorption for
Metabolites in FOCUS-PRZM
Piyush Singh
DuPont Crop Protection

Objectives
• Propose an alternate (more realistic)
approach to simulate aged sorption for
metabolites in FOCUS PRZM GW Tool
– Current model limitation: K d factors for metabolite
are applied only after 50% parent has degraded -
not a realistic assumption considering metabolite
formation is a continuous process.

Proposed Approach
• Use a conceptual “aged sorption kinetics” model
(Model Maker ® ) to fit the experimental aged sorption
data for metabolite.
• Estimate a “rate” at which compound is moving from
dissolved phase into sorbed phase (similar to watersediment
approach in Model Manager).
• Use bi-phase degradation in FOCUS-PRZM for
metabolite:
– first phase DT 50
representing “aged sorption rate” derived
from the “aged sorption kinetics” model.
– second phase DT 50
representing normal degradation rate.

1. Rate_1 is "net" or effective transfer rate from dissolved phase
to solid phase (assuming that adsorption will be greater than
desorption at all times). By double-clicking on the arrow you
can see how this affects the fit to experimental data.
Rate_1 = k1*A {when 0 < t < Ts} Ts= Transition time
Rate_1 = F1*(Rate_1) {when t>=Ts} F1 = Fraction of Rate_1
Example: If Ts = 6 d and F1 =0.2 then the net adsorption rate
after 6 days would be 20% of the value of Rate_1
2. Rate_2 corresponds to degradation rate in solid phase
(assumed negligible, k2 = 0.00001)
Rate_2 = k2*B
2. Rate_3 corresponds to degradation rate in dissolved phase
(set equal to lab degradation rate as a starting point...then
adjust if necessary).
Rate_3 = k3*A
Note that the values of k1, k2, k3, Ts, and F1 can be adjusted
in Parameters Tab by double clicking on each parameter.

Aged sorption data for metabolite
Day Kd aging
factors
0 1.0
1 2.0
3 2.5
7 3.2
14 3.5

Parameters Derived from Aged Sorption
Kinetics Model
• Phase-1 (representing aged sorption) DT 50
for metabolite = 10 d
• Time duration for phase-1 DT 50 =15 d
• phase-2 DT 50 = 300 d (same value as used in
standard modeling scenario)

Modeling Scenarios
• Case - 1
– metabolite simulated as parent.
– no aged sorption considered.
• Case - 2
– metabolite simulated as parent.
– aged sorption factors (K d aging factors estimated
directly from the experimental data) simulated

• Case - 3
Modeling Scenarios
– metabolite simulated as parent to metabolite
pathway.
– aged sorption for metabolite simulated (model
assumption: K d factors applied after 50% parent
has degraded).
• Case - 4
– metabolite simulated as parent to metabolite
pathway
– aged sorption represented as two-phase
degradation for metabolite (proposed)

Key E-fate Properties
E-fate
Property
Parent
Metabolite
DT50 (d) 40 300
Koc (ml/g) 300 200
Molecular
Weight
300 250

E-fate/Scenario Properties
• Maximum formation level for metabolite
assumed to be 50%
• When simulating metabolite as parent
application rate for metabolite calculated as:
App rate for metab = App rate for parent * 0.5 * (MWm / MWp)
• Parent application rate = 0.5 Kg/ha
• No temperature or moisture correction used
• Crop simulated - vine (all FOCUS scenarios)

Results
• 80th percentile PECgw (µg/L) for metabolite
Scenario Case-1 Case-2 Case-3 Case-4
Chateaudun 1.63