Modeling the Fate of Acetochlor and Terbuthylazine in the Field Using the Root Zone Water Quality Model

Abstract

Data collected from a 3‐yr controlled field study in Hamilton, New Zealand were used to examine whether the Root Zone Water Quality Model is capable of predicting water movement and pesticide fate in the field based on key lab‐measured parameters and environmental variables. Acetochlor [2‐chloro‐N‐(ethoxymethyl)‐N‐(2‐ethyl‐6‐methylphenyl) acetamide; 2.5 and 5.0 kg a.i. ha−1] and terbuthylazine (C9H16ClN5; 1.5 and 3.0 kg a.i. ha−1) were applied onto nine field plots (3 by 9 m each). Soil core samples were taken to a depth of 1 m to determine soil water contents and pesticide concentrations. Dissipation of both pesticides in the field at both application rates followed first‐order kinetics (adjusted r2 > 0.91). The mean dissipation half‐life was 16 d for acetochlor and 25 d for terbuthylazine. Relatively small amounts of the pesticides leached below 5 cm and none leached below 10 cm. Predicted soil water contents in the soil profile were not significantly different from those measured in the field (p > 0.84). Predicted acetochlor and terbuthylazine masses in the soil profile based on a linear instantaneous‐equilibrium (I‐E) partitioning model matched those measured in the field (adjusted r2 > 0.93). However, predicted pesticide concentrations in the soil profile were less satisfactory, with 68 and 35% of the predicted concentrations being within a factor of 2 of the measured concentrations for 0‐ to 5‐ and 5‐ to 10‐cm depths, respectively. Calibration of each pesticide sensitive parameter individually did not significantly improve the overall predictions of pesticide mass and concentrations in the soil profile when the I‐E partitioning model was used. The predictions were improved when a two‐site, equilibrium‐kinetic (E‐K) sorption model was used.