1. Rochelle F.H. Bohaty, William P. Eckel, Katrina White, Dirk F. Young
Standard Operating Procedure for Calculating Degradation Kinetics in EPA's Office of Pesticide Programs
August 14, 2014
Rochelle F.H. Bohaty, William P. Eckel, Katrina White, Dirk F. Young

2. Purpose of the NAFTA Degradation Kinetics Project
To standardize and automate calculation of rate constants for environmental computer model input in U.S. and Canada
Find best approximation to a first-order fit
Applies to all metabolism and abiotic environmental fate studies
For single decay only (parent or total toxic residues - TTR)
Not for formation and decline of degradates

3. Guidance Documents
“Standard Operating Procedure for Using the NAFTA Guidance to Calculate Representative Half-life Values and Characterizing Pesticide Degradation” (Nov. 30, 2012)
“Guidance for Evaluating and Calculating Degradation Kinetics in Environmental Media” (Dec. 21, 2011)
Process is for Model Input, not “Descriptive” kinetics
One-year evaluation of SOP during 2013 – 2014

4. 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
All results in terms of C0 = initial concentration; k = rate constant; N = exponent; f = fraction of C0 subject to DFOP k0

5. Decision Tree yes Is SFO fit acceptable? no Use SFO yes no
Is IORE trep< DFOP
slow half life?
Use IORE trep
Use DFOP
slow half life

6. Rejecting SFO: SSFO vs. SC
If SSFO < SC , then SFO is chosen
SSFO and SC are residual sum of squares
SC based on SIORE
50% confidence bound (N vs. k) is used for comparison
Sensitivity analysis shows that choice is not sensitive to the use of 50% bound

7. 50% Confidence Region N k 50% Confidence Region 4
When N=1, First Order Model N 3 Sc 2
First Order Fit (N=1) 1 k

8. Software to Implement Guidance (Not Mandatory)
“pestDF ”, based on MS Excel & R 3.1.0
Calculates equation fits and operates Decision Tree
First, decides if SFO fits data (SSFO < Sc)
Otherwise, the “representative time,” tR is chosen from the shorter of DFOP second half-life or IORE (estimated DT90/3.32)
Implements interpretive rules from evaluation period

9. Evaluation Period Results
Mostly (>80%), pestDF worked as expected (> 50 datasets)
Some parameters were out-of-bounds (0 < f < 1) (k > 0) (N > 0)
Fits with out-of-bound parameters were difficult to interpret
Some IORE fits returned results (N < 1) less conservative than SFO
Interpretive rules required; difficult to limit parameters in R software

10. Interpretive Rules
Use SFO if IORE N < 1 (less conservative than SFO)
Use SFO if SFO k = DFOP “slow” k1 (simplest model)
Reexamine data if SFO not chosen for hydrolysis studies (SFO expected)
Examples of datasets requiring interpretation follow – using older version of pestDF

11. Example: SFO not chosen for Hydrolysis

12. Example: N < 1 and SFO = DFOP Dataset 41

13. Example: N < 1, IORE not Conservative Dataset 45

14. Example: f < 0 Dataset 35

15. Remaining Issues versus FOCUS approach (Crop Life America comments)
Data sets that do not reach DT90 are not limited to SFO
IORE “representative time” is sometimes >> empirical DT50, SFO
Statistical Fit procedures: chi-square displayed but not determinative
AIC and BIC tested but gave same results as Sc vs. SSFO
Sensitivity to N parameter in IORE

16. Example: DFOP k1 < 0, IORE tR large

17. Characterization: Comparative DT50 Values

18. Conclusions
“pestDF” implements NAFTA model-input kinetics guidance well
Some interpretive rules were needed to deal with out-of-bounds parameter estimates
Reasonable model input parameters are usually obtained
Some additional interpretation may still be needed
Open to other software implementations of guidance