The persistence of hexazinone and metsulfuron-methyl herbicides was investigated in an experiment using in situ enclosures deployed in a mixed-wood/boreal forest lake. Dissipation studies provided evidence that aqueous residues of hexazinone may persist (DT50 = 131-280 days) under conditions typical of northern lentic systems in the mixed-wood/boreal forest. Although dissipation rates of hexazinone differed depending upon initial concentrations (10(4) and 10(3)-mu-g L-1), such differences were of little practical significance. In contrast, concentration-dependent dissipation of metsulfuron-methyl was of practical significance with slow rates (DT50 > 84 days) observed for 10(3)-mu-g L-1 concentrations and more rapid rates (DT50 = 29.1 days) at environmentally realistic initial concentrations approximating 10-mu-g L-1. The unexpectedly slow dissipation of hexazinone observed in this study was postulated to result from the influence of low light intensity and short day length on the primary degradation pathway-photolysis. Further study of hexazinone dissipation and impact in relevant northern aquatic systems subject to photolytic inhibition is warranted.
