LOSS OF PESTICIDES FROM CONGAREE SANDY LOAM WITH TIME

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Abstract
Loss of 4 disparate pesticides (prometryne, carbaryl, trifluralin and metribuzin) from Congaree [South Carolina, USA] sandy loam with time is best represented by 2-stage, 1st-order curves. The relatively steep slopes of stage 1 are characterized by a relatively large, unique rate constant, k1, reflecting a relatively short-lived episode; the relatively shallow slope of stage 2 is characterized by a relatively small, less distinguishable rate constant, k2, reflecting a relatively long-lived episode. The times to 50% loss (t0.5), to onset of stage 2 (t2) and to 90% loss (t0.1) and selected ratios (k2/k1, t0.5(1)/t0.1) complete the individual characterization. Assuming that residues do not become marginally safe until 90% of the applied pesticide has disappeared, t0.1 is perhaps the most useful danger index. For the subject pesticides, t0.1 ranges from 23 days (metribuzin) to 1040 days (prometryne). The 2-stage disappearance curves observed for the disparate pesticides can serve as a model for other pesticides and that the slopes of the 2 legs of individual pesticide curves can be predicted, approximately, from limited laboratory sorption, movement and loss data.

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