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

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• 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 = -kCN

C0 = initial concentration

k = rate constant

N = exponent

f = fraction of C0 subject to DFOP k0

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Use SFO

Use IORE trep

Use DFOP 

slow half life

no

yes no

yes

Decision Tree

Is SFO fit 

acceptable? 

Is IORE trep< DFOP 

slow half life?

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Rejecting SFO: SSFO vs. SC

• If SSFO < SC , then SFO is chosen

• SSFO and SIORE are the residual sum of squares 

• SC is the critical value that defines the confidence 

contours based on SIORE

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

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Representative Half-life value

Equation Representative HalflifeDescription

SFO SFO half-life Standard half-life

IORE TIORE 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)

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Use SFO

Use the 

representative halflife of IORE 

TIORE<DFOP 

slow half-life?

Use slow rate halflife from DFOP

no

yes no

yes

Is SSFO<Sc?

Is IORE N<1?

Are DFOP k 

values equal 

to SFO k 

values?

Make sure that 

parameters are 

within expected 

bounds

13
SFO not chosen for Hydrolysis

14
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 (Sc vs. SSFO) 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 Sc vs. SSFO

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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

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