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 detec ted in receiving waters of shallow ponds. Irradiation with simulated sunlig ht degraded hydroxychlorothalonil in distilled-deionized water, phosphate b uffer, and pond water with half-lives of approximately 30 min. Slower degra dation was observed in solutions of NaCl and NaCN, suggesting photonucleoph ilic substitution reactions may be responsible for the degradation. Hydroxy lated products resulting from the displacement of chlorine and cyano groups were detected within the first hour of irradiation but were subsequently d egraded to small aliphatic acids (succinic, oxalic, and malonic acid) withi n 96 h. Simulations of aquatic dissipation of the chemical using the U.S. E nvironmental Protection Agency standard pond scenario in the exposure analy sis modeling system indicate that the 30-min half-life measured in the labo ratory would equate to a half-life of 2.6 d in a 2-m-deep pond and as much as 3.4 din seawater. Although hydroxychlorothalonil is moderately stable an d mobile in soil, these data suggest that, in surface water, it should be r apidly degraded by sunlight to small aliphatic acids on transport into aqua tic systems.