Hexazinone dissipation in forest ecosystems and impacts on aquatic communities

Hexazinone (active ingredient) was aerially applied as a pellet (Velpar ULW) and as a liquid (Velpar L) to watersheds in the Piedmont of Alabama, U.S.A., at the rate of 6.72 kg·ha-1 (three times the prescribed rate for this site). An untreated watershed served as a control. We determined hexazinone half-life in days for Velpar ULW (plants, 26-59; litter, 55; bare soil, 68; soil under litter, 74) and for Velpar L (plants, 19-36; litter, 56; bare soil, 77; soil under litter, 275). Maximum stream concentrations of hexazinone (422 µg·L-1 for Velpar ULW; 473 µg·L-1 for Velpar L) were observed during application and resulted from direct overspray. Hexazinone stream concentrations peaked several times during stormflow in the first 30 days (56-70 µg·L-1 for Velpar ULW; 145-230 µg·L-1 for Velpar L) and were diluted three to five times 1.6 km downstream. Hexazinone metabolites were also monitored. Exposure of macroinvertebrates to hexazinone did not alter benthic community structure. Taxa richness, including pollution-sensitive insects, did not differ significantly between either hexazinone treatment and the control. Benthic macroinvertebrates in Piedmont streams of the southeastern United States appear insensitive to hexazinone at the exposures observed in this study.