Guidance to Calculate Representative Half-life Values and Characterizing Pesticide Degradation

The rate of transformation of pesticides in the environment is commonly described using first-order kinetics, often referred to as single first-order (SFO). The first-order 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 half-life, 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 first-order degradation. In contrast, the DT50 is the time required for the concentration to decline to half of the initial value. For non-first-order decay, the time to reach half the concentration from any other concentration point on the curve will be different.

EFED exposure models require first-order 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 half-life (t_rep)," to estimate an SFO half-life 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 first-order representation would predict. The representative half-life considers both the initial and the slower portions of the decline curve and is not necessarily numerically similar to the value of the DT50.

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