Guidance to Calculate Representative Halflife Values and Characterizing Pesticide Degradation
The rate of transformation of pesticides in the environment is commonly described using firstorder kinetics, often referred to as single firstorder (SFO). The firstorder representation is convenient because the rate is summarized with a single parameter (the rate constant, k), and the rate of transformation is independent of the initial concentration. The halflife, t_{1/2} =ln(2)/k, indicates the time required to reduce the concentration by 50% from any concentration point in time. It is an intuitive way to express the rate of decline of a firstorder degradation. In contrast, the DT50 is the time required for the concentration to decline to half of the initial value. For nonfirstorder decay, the time to reach half the concentration from any other concentration point on the curve will be different.
EFED exposure models require firstorder inputs for pesticide transformation processes even though pesticide transformations in soil and aquatic systems often do not follow a single exponential decline pattern. For this reason, the NAFTA guidance introduces a "representative halflife (t_{rep})," to estimate an SFO halflife for model input from a degradation curve that does not follow the SFO equation. The procedure takes into consideration the frequent observation that pesticides can degrade fast initially and then slowly as times passes, much more so than a firstorder representation would predict. The representative halflife considers both the initial and the slower portions of the decline curve and is not necessarily numerically similar to the value of the DT50.
Current Guidance and Degradation Kinetics Calculation Tools
March 23, 2015
[NAFTA] Guidance for Evaluating and Calculating Degradation Kinetics in Environmental Media
Attachments to SOP

Additional Guidance for Evaluating and Calculating Degradation Kinetics in Environmental Media

Example Runs, Analyses and Tools
Tools to Calculate Degradation Kinetics

PestDF 0.8.13 [xxDeg_0.8.13 ] (zip) Refer to SOP for instructions.

R executable file, R3.1.1win.exe, from The R Project for Statistical Computing site

Degradation Kinetics Calculator Spreadsheet (xlsx) Free Viewers
Previous (Superseded) Guidance and Degradation Kinetics Calculation Tools
Use this version only to reproduce (or verify) results initially obtained before the new guidance was issued in March 2015
March 2015

Standard Operating Procedure (Superseded) for Using the NAFTA Guidance to Calculate Representative Halflife Values and for Characterizing Pesticide Degradation Search EPA Archive

NAFTA Guidance for Evaluating and Calculating Degradation Kinetics in Environmental Media: DD Memo (December 17, 2012)

R executable file, R2.15.0win.exe, from The R Project for Statistical Computing site.
