LOSS OF PESTICIDES FROM CONGAREE SANDY LOAM WITH TIME

Abstract

Loss of four disparate pesticides (prometryne, carbaryl, trifluralin, and metribuzin) from Congaree sandy loam with time is best represented by two-stage, first-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 percent loss (t0.5), to onset of stage 2 (t2), and to 90 percent loss (t0.1) and selected ratios (k2/k0.5(1)/t0.1) complete the individual characterization. Assuming that residues do not become marginally safe until 90 percent 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). It is suggested that the two-stage disappearance curves observed for the disparate pesticides can serve as a model for other pesticides and that the slopes of the two legs of individual pesticide curves can be predicted, approximately, from limited laboratory sorption, movement, and loss data.