Persistence and leaching of the herbicides acetochlor and terbuthylazine in an allophanic soil: comparisons of field results with PRZM-3 predictions

Rates of degradation and adsorption of acetochlor[2-chloro-N-ethoxymethyl-6 '-ethylaceto-o- toluidide] and terbuthylazine [N-2-tert-butyl-6-chloro-N-4- ethyl-1,3,5-triazine-2,4-diamine] in a Horotiu sandy loam soil (Typic Orthi c Allophanic) were determined under controlled temperature and soil moistur e regimes. These were then combined with site-specific soil properties and climatic conditions in the Pesticide Root Zone Model (PRZM-3) to predict di ssipation and leaching of the herbicides in the field. PRZM-3 significantly under-estimated dissipation of both herbicides in the field using paramete rs derived from the laboratory incubation studies. When these parameters we re derived from the field trials, PRZM-3 adequately predicted dissipation o f both herbicides using a two-rate dissipation sub-model but under-predicte d the dissipation when a simpler single-rate sub-model was used. Earlier-th an-expected appearance of both herbicides in sub-soil layers were postulate d to result from the non-equilibrium adsorption/transport of the herbicides and preferential flow, which cannot be simulated by PRZM-3. (C) 2000 Socie ty of Chemical Industry.