Dissipation of [14C]acetochlor herbicide under anaerobic aquatic conditions in flooded soil microcosms

Acetochlor degradation was studied under anaerobic conditions representative of conditions in flooded soils. Soil-water microcosms were prepared with a saturated Drummer clay loam and made anaerobic by either glucose pretreatment or N-2 sparging. Sparged microcosms consisted of sulfate-amended, unamended, and gamma-irradiated microcosms. The microcosms were sampled in triplicate at predetermined time intervals during a 371 day incubation period. Volatile, aqueous, extractable, and bound (unextractable) C-14 residues were quantified with liquid scintillation counting and characterized using high-performance liquid radiochromatography (HPLRC) and soil combustion. SO42-, Fe(II), CH4, and pH were monitored. Complete anaerobic degradation of [C-14]acetochlor was observed in all viable treatments. The time observed for 50% acetochlor disappearance (DT50) was 10 days for iron-reducing and sulfate-reducing conditions (sulfate-amended), 15 days for iron-reducing conditions (unamended), and 16 days for methanogenic conditions (glucose-pretreated). Acetochlor remained after 371 days in the gamma-irradiated microcosms, and metabolites were observed. [C-14]Metabolites were detected throughout the study. Formation of one of the metabolites correlated with Fe(II) formation (r(2), 0.83). A significant portion of the C-14 activity was eventually incorporated into soil-bound residue (30-50% of applied acetochlor) in all treatments.