SUNLIGHT PHOTODEGRADATION OF METOLACHLOR IN WATER

The chemical and photochemical stability of the herbicide metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl) -N-(2-methoxy-1-methylethyl)acetamide] was determined in organic-free water and lake water containing various solutes. Metolachlor was fairly stable in lake water in the dark, with <4% loss after 100 days. Sunlight photodegradation of metolachlor was faster than purely chemical degradation but was still a relatively slow process, with estimated near-surface half-lives in lake water of 22 calendar days in summer and 205 calendar days in winter at 40 degrees N latitude. In 5 mg/L solutions of dissolved organic matter, the estimated half-lives were 2-3 times longer, depending upon the season. Four dechlorinated photoproducts were identified in lake water, accounting, after 40 days of sunlight irradiation, for 18% of the metolachlor originally present. These products resulted from dechlorination, hydroxylation, dehydrochlorination with subsequent morpholine ring formation, and N-dealkylation.