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‐N⁴‐ethyl‐1,3,5‐triazine‐2,4‐diamine] in a Horotiu sandy loam soil (Typic Orthic Allophanic) were determined under controlled temperature and soil moisture regimes. These were then combined with site‐specific soil properties and climatic conditions in the Pesticide Root Zone Model (PRZM‐3) to predict dissipation and leaching of the herbicides in the field. PRZM‐3 significantly under‐estimated dissipation of both herbicides in the field using parameters derived from the laboratory incubation studies. When these parameters were derived from the field trials, PRZM‐3 adequately predicted dissipation of both herbicides using a two‐rate dissipation sub‐model but under‐predicted the dissipation when a simpler single‐rate sub‐model was used. Earlier‐than‐expected appearance of both herbicides in sub‐soil layers were postulated to result from the non‐equilibrium adsorption/transport of the herbicides and preferential flow, which cannot be simulated by PRZM‐3. © 2000 Society of Chemical Industry