Anaerobic Degradation of Atrazine and Metolachlor and Metabolite Formation in Wetland Soil and Water Microcosms

The half‐lives, degradation rates, and metabolite formation patterns of atrazine (6‐chloro‐ N 2 ‐ethyl‐ N 4 ‐isopropyl‐1,3,5‐triazine‐2,4‐diamine) and metolachlor [2‐chloro‐ N ‐(2‐ethyl‐6‐methylphenyl)‐ N ‐(2‐methoxy‐1‐methylethyl) acetamide] were determined in an anaerobic wetland soil incubated at 24°C for 112 d. At 0, 7, 14, 28, 42, 56, and 112 d, the soil and water were analyzed for atrazine and metolachlor, and their major metabolites. The soil oxidation–reduction potential reached −200 mV after 14 d. Degradation reaction rates were first‐order for atrazine in anaerobic soil and for metolachlor in the aqueous phase. Zero‐order reaction rates were best fit for atrazine in the aqueous phase and metolachlor in anaerobic soil. In anaerobic soil, the half‐life was 38 d for atrazine and 62 d for metolachlor. In the aqueous phase above the soil, the half‐life was 86 d for atrazine and 40 d for metolachlor. Metabolites detected in the anaerobic soil were hydroxyatrazine and deethylatrazine for atrazine, and relatively small amounts of ethanesulfonic acid and oxanilic acid for metolachlor. Metabolites detected in the aqueous phase above the soil were hydroxyatrazine, deethylatrazine, and deisopropylatrazine for atrazine, and ethanesulfonic acid and oxanilic acid for metolachlor. Concentrations of metabolites in the aqueous phase generally peaked within the first 25 d and then declined. Results indicate that atrazine and metolachlor can degrade under strongly reducing conditions found in wetland soils. Metolachlor metabolites, ethanesulfonic acid, and oxanilic acid are not significantly formed under anaerobic conditions.