Method for Temporal Analysis of Exposure to the Residue
of Concern for Parent Compound and Degradates
Mohammed A. Ruhman
[email protected]
OPP/EFED
Presentation to ACS Symposium on:
“Evaluating Agrochemical Aquatic Exposure Modeling in Relation to Risk
Evaluator Needs”
Denver, CO
1
Introduction
• Overview of methods for estimating exposure EECs of the
pesticide total toxic residues:
• Total residue method (TR method)
• Residue summation method (RS method)
• Formation/Decline Method (F/D method)
• Residue Summation Method
• Present conceptual model for use of RS method with
incorporation of temporal off-set for degradate formation
• Identify issues and uncertainties with RS method with
temporal off-set
2
Required Analyses for TTR Methods
• Properties of parent and degradation products
• Environmental Fate Properties
 relative persistence
 mobility
• Mode of Action
• Relative Toxicity
• Parent and daughter formation and decline
patterns
3
Total Toxic Residue Method Comparison
Attributes
Do you have to assume similar
toxicity and/or mode of action for
chemical constituents of the
stressor?
Can you consider temporal
occurrence of transformation
products?
TR
RS
F/D
Yes
No
No
No
Yes
Yes
4
Total Toxic Residue Method Comparison
What is Needed for Modeling and Post Processing?
Attributes
TR
RS
F/D
Number of PRZM/EXAMS runs
One
Multiple
One
Application rate
Parent rate
Half-lives
Recalculated
Other parameters
F/D kinetics
Post processing of daily EECs for
the 1-in-10 year EECs
Most
conservative
None
None
Parent rate Parent rate
divided
Needed for each
chemical in the stressor
None
Yes
Needed only for chemicals
of similar toxicity and/or
mode of action
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Exposure EECs using the three TTR methods
EECs (ppb)
50
45
TR
40
35
RS
30
F/D
25
20
15
10
5
0
Peak
21-Day
60-Day
90-Day
Annual
1-in 10-Year Concentration
6
SAP Major Comments and Recommendations
• Strengths and limitations for the methods were well defined,
approach reflects current state of science, and F/D method is
more realistic
• Consider:
• toxicity and mode of action, and structural similarity analysis
• parent and degradation products as a mixture rather than individually
when assessing toxicity and biological impacts of both parent and
metabolite
• Use temporal offset of the application for the RS method
• Compare results for a range of chemicals to get a better idea of
variability/uncertainty
7
Conceptual Model for SAP Suggested
Temporal Offset
Stressor
Three Chemicals: P=
Parent; M= Metabolite
Calculate application rates for each of the chemicals
Parent
M1
1st Run @ application
1st Run @ date of its
maximum formation
2nd Run @ date of M1
maximum formation
2nd Run@ date of M2
maximum formation
M2
Run @ date of its
maximum formation
3rd Run @ date of M2
maximum formation
Daily and 1-10Years EECs from the six runs
Can Daily EECs
Be Combined for
More Than One Chemical?
No
Yes
No Need for Post Processing
Need Post Processing: Use post processing spreadsheet
Conceptual Model for SAP Suggested Temporal
Offset: Difficulties , Concerns & Uncertainties
• Assignment of appropriate application rates for various
constituents at temporal off-set requires consideration of:
• Non-toxic degradation products (CO2 and non-toxic
degradates)
• Normalizing observed concentrations
• Molecular ratios for constituents
• Post processing becomes complex for combining runs for
pesticides with more than two degradation products and more
than one application
• Difficulties in assigning 1- in-10 year averages from multiple
model times series for constituents with variable degradation
rates
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Conclusions
• More testing of the conceptual model for RS method with
temporal off-set is required for pesticides with different
degradation profiles and rates
• The TR method or the RS method without a temporal off-set
for degradation product formation have been shown to provide
reliable estimated environmental concentrations.
• The preferred method is F/D method (Gold Standard).
Additional verification is needed for routine application in
exposure modeling.
10