# Update to the SOP for Calculating Degradation Kinetics in EPA’s Office of Pesticide Programs April 28, 2015 EMPM Meeting Rochelle F.H. Bohaty, William P. Eckel, Mohammed Ruhman, Mah Shamim (Management Rep.), Katrina White, Dirk F. Young

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# Guidance Documents

“Guidance for Evaluating and Calculating Degradation Kinetics in Environmental Media” (Dec. 21, 2011) • NAFTA Project; harmonized approach • General approach for calculating and selecting representative half- life values “Standard Operating Procedure (SOP) for Using the NAFTA Guidance to Calculate Representative Half- life Values and Characterizing Pesticide Degradation, Version 2” (March 23, 2015) • Provides guidance on data preparation, interpretations, and precautions • Updated previous version from November 2012 Available at: http://www.epa.gov/oppefed1/ecorisk_ders/degradation_kinetics/NAFTA _Degradation_Kinetics.htm 2• Degradation kinetics do not always follow the single first - order (SFO) model • Using the SFO model can overestimate the amount of loss over time . • Aquatic exposure models use SFO equations in simulations • The guidance provides a method to estimate a “representative half- life value” for aquatic modeling, when data do not follow an SFO model. 

## Purpose of the NAFTA Degradation Kinetics Project

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# Considerations for the Guidance

• To standardize and automate the calculation of degradation kinetics and a “representative half- life value”
• Applies to all metabolism fate studies • Abiotic studies typically follow SFO • Dissipation studies and soil photolysis: use equations and tools but do not use the “representative half- life value” 

### • For single decay only (parent or total toxic residues - TTR) • Not for formation and decline of degradates

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# Equations Used in NAFTA Guidance
• Single First Order (SFO, 2 - parameter) • Indeterminate Order Rate Equation (IORE, 3 - parameter), not its mathematical equivalent FOMC (Gustafson - Holden) • Double First Order in Parallel (DFOP, 4 - parameter ) • SFO, DFOP, IORE based on same differential equation: dC/dt = - kC N C 0 = initial concentration k = rate constant N = exponent f = fraction of C 0 subject to DFOP k 0 5Use SFO
Use IORE t rep Use DFOP slow half life no yes no yes 

# Decision Tree Is SFO fit acceptable? Is IORE t rep < DFOP slow half life?

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# Rejecting SFO: S SFO 

# vs. S C • If S SFO 

## < S C 

## , then SFO is chosen • S SFO 

### and S IORE 

### are the residual sum of squares • S C 

### is the critical value that defines the confidence contours based on S IORE 

### • 50% confidence bound (N vs. k) is used for comparison

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# Representative Half- life value Equation Representative Halflife Description SFO SFO half- life Standard half- life IORE T IORE Half- life of a SFO model that passes through a hypothetical DT90 of the IORE fit DFOP Slow DFOP rate DT50 Half- life for a portion of the degradation curve

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# SOP Evaluation Period 2013 to 2014
Collected “problem”

datasets



# Phase 1 Collected “random”

datasets 

## Phase 2 Considered Comments from stakeholders 

## Phase 3 Interpretive Rules Developed Parameter Bound Considerations Tested Updates in Tool Implemented Updates

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# Example Datasets
• 56 datasets isolated for inclusion in the prepared analysis (excel file); however, 100+ datasets were examined • Data included were from the contractor, stakeholders, and FTT • SFO (20; 36%); IORE (27; 48%); DFOP (9; 16%)

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# Evaluation Summary
• PestDF worked as expected “mostly” (>80%)
• Some model fitting parameters were out - of- bounds • Expected parameter bounds 0 < f < 1; k > 0; N > 0 • Fits with out - of- bound parameters were difficult to interpret 

## • Interpretive rules required • Difficult to limit parameters in R software
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# Interpretive Rules
• Use SFO if IORE N < 1 (less conservative than SFO)
• Use SFO if SFO k = DFOP k values (simplest model)
• Reexamine data if SFO not chosen for hydrolysis studies (SFO expected)

12Use SFO

Use the representative halflife of IORE TIORE<DFOP slow half - life? Use slow rate halflife from DFOP no
yes no yes Is S SFO< S c ? Is IORE N<1?

Are DFOP k values equal to SFO k

values? 

### Make sure that parameters are within expected bounds

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# SFO not chosen for Hydrolysis

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# N < 1 and SFO k = DFOP k

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# N is negative, IORE not Conservative

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# Stakeholder Comment: Should be using SFO for more datasets
• Adjusting confidence criteria ( S c 

## vs. S SFO 

## ) didn’t change selected model (tested F ranging from 0.05 to 0.95) • AIC (Akaike Information Criterion) and BIC (Bayesian Information Criterion) tested and gave same results as S c 

## vs. S SFO

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# DT50 not reached during the study

sfo dfop iore AIC 55.07 58.3 54.06 BIC 56.99 62.13 56.61

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# Conclusions

• PestDF Tool implements NAFTA model - input kinetics guidance well • Some interpretive rules were needed to deal with out - ofbounds parameter estimates • Reasonable model input parameters are usually obtained • Some additional interpretation may still be needed • Open to other software implementations of guidance

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# Acknowledgements
NAFTA Kinetics Team EPA: William P. Eckel, Mah Shamim (Management Rep.), Dana Spatz, Dirk F. Young PMRA: Ian Kennedy, Greg Malis (Management Rep.) EPA NAFTA Kinetics Implementation Team Rochelle F.H. Bohaty, William P. Eckel, Mohammed Ruhman, Mah Shamim (Management Rep.), Katrina White, Dirk F. Young, FTT Co - chairs José L. Meléndez, Richard Shamblen FTT Science Advisory Jim Hetrick 

### Katrina White White.katrina@epa.gov 703 - 308 -4536 20