The aged sorption behavior of a pesticide in sandy soil was examined in two field dissipation studies. In these studies, soil samples collected from 0-5 cm horizon were first extracted by 24-hour equilibration in 0.01 M CaCl2 and then by organic solvent extraction. The 0-5 cm soil data were fit to the two-site aged sorption model to determine the parameters of equilibrium or aged sorption (if significant). The DegT50,EQL from 0-5 cm soil also accounts for leaching loss and thus does not represent degradation rate in soil. However, the other 0-5 cm soil aged parameters Kfom, Kdes, and Fne, can be used to determine the actual DegT50,EQL by fitting the 0-90 cm soil depth-specific concentrations to PEARL 4.4.4 via inverse modeling with PEST. At Step 1, the normalized (20oC and pF 2) total soil profile DegT50 values were calculated by the FOCUS kinetics guidance and then used with the 0-5 cm Kfom to examine if soil profile distribution of pesticide can be fit by equilibrium sorption. If the fit is acceptable, aged sorption is considered insignificant. If not, at Step 2, inverse modeling was further conducted with 0-5 cm soil Kfom, Kdes and Fne to optimize DegT50,EQL or only 0-5 cm soil Kfom to optimize DegT50,EQL, Kdes and Fne. The results indicate that aged sorption in German site is significant, while not in California site due to extremely low organic matter. The optimized equilibrium or aged sorption parameters provide excellent fits to soil profile distribution of pesticide in both field studies and are statistically robust for use in leaching risk assessment.

Describing aged sorption behavior of pesticide in soil field dissipation studies via inverse modeling

256th National Meeting (2018)

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