MODELING THE FATE OF ACETOCHLOR AND TERBUTHYLAZINE IN THE FIELD USING THE ROOT ZONE WATER QUALITY MODEL

Authors: MA, QINGLI - ENVIRONMENTAL & TURF SERV; RAHMAN, A. - HORTRESEARCH; JAMES, T. - HORTRESEARCH; HOLLAND, P. - HORTRESEARCH; MCNAUGHTON, D. - HORTRESEARCH; ROJAS, KENNETH - USDA-USGS; Ahuja, Lajpat

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2004
Publication Date: 9/1/2004
Citation: Ma, Q.L., Rahman, A., James, T.K., Holland, P.T., McNaughton, D.E., Rojas, K.W., Ahuja, L.R. 2004. Modeling the fate of acetochlor and terbuthylazine in the field using the Root Zone Water Quality Model. Soil Science Society of America Journal.68:1491-1500.

Interpretive Summary: New Zealand has distinct weather conditions and soil properties. In particular, the Waikato region has high solar radiation and frequent rainfall all year round and volcanic soils of high organic carbon content and low bulk density that provide a unique scenario for the ARS Root Zone Water Quality Model to simulate. The two pesticides named Acetochlor and terbuthylazine were simulated with the model because they have been widely used in New Zealand and concerns have been raised regarding their potential risks to water quality and human health. Application of a time-dependent pesticide sorption sub-model that is present in RZWQM was the key to successful predictions of pesticide mass and concentration in this New Zealand soil.

Technical 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. Abbreviations: E-K, equilibrium-kinetic • FIFRA, Federal Insecticide, Fungicide and Rodenticide Act • FOCUS, Forum for the coordination of pesticide fate models and their uses • HPLC, high-pressure liquid chromatography • I-E, instantaneous equilibrium • MWHC, maximum water holding capacity • NRMSE, normalized root mean square error • OECD, Organization for Economic Co-operation and Development