Photodegradation of hydroxychlorothalonil in aqueous solutions

Hydroxychlorothalonil is the principal degradation product of the fungicide chlorothalonil in soil, and it is routinely detected in leachate from golf course greens at concentrations as high as 2 ppm. However, it is not detected in receiving waters of shallow ponds. Irradiation with simulated sunlight degraded hydroxychlorothalonil in distilled-deionized water, phosphate buffer, and pond water with half-lives of approximately 30 min. Slower degradation was observed in solutions of NaCl and NaCN, suggesting photonucleophilic substitution reactions may be responsible for the degradation. Hydroxylated products resulting from the displacement of chlorine and cyano groups were detected within the first hour of irradiation but were subsequently degraded to small aliphatic acids (succinic, oxalic, and malonic acid) within 96 h. Simulations of aquatic dissipation of the chemical using the U.S. Environmental Protection Agency standard pond scenario in the exposure analysis modeling system indicate that the 30-min half-life measured in the laboratory would equate to a half-life of 2.6 d in a 2-m-deep pond and as much as 3.4 d in seawater. Although hydroxychlorothalonil is moderately stable and mobile in soil, these data suggest that, in surface water, it should be rapidly degraded by sunlight to small aliphatic acids on transport into aquatic systems.