Runoff of Silvicultural Herbicides Applied Using Best Management Practices

Nine small (2.2-2.9 ha) and four large (70 -135 ha) watersheds in East Texas, USA, were instru- mented to compare herbicide runoff under different silvicultural systems with best management practices (BMPs). Two treatments were evaluated: conventional, with clearcutting, aerial herbicide site preparation, and hand-applied banded herbaceous release; and intensive, in which subsoiling, aerial fertilization, and a 2nd-year aerial herbicide application were added. Herbicides were applied as operational tank mixes. The highest imazapyr concentration found in stream water was 39 gL 1 during the first storm after application (23 days after treatment (DAT)) and in-stream concentrations during runoff events dropped to 1 gL 1 in all streams by 150 DAT. The highest hexazinone concentration was 8 gL 1 for the banded application and 35 gL 1 for the broadcast application the following year and fell to 1 gL 1 in all streams by 140 DAT. The highest sulfometuron methyl concentration found during a runoff event was 4 gL 1 and fell to 1 gL 1 in all streams by 80 DAT. Approximately 1-2% of applied imazapyr and 1% of hexazinone and sulfometuron methyl were measured in storm runoff. Herbicide was found in streams during storm events only (all herbicides were 1 gL 1 in all true baseflow samples), and peak concentrations during runoff events persisted for relatively short times (24 h). These results suggest that silvicultural herbicide applications implemented with contemporary BMPs are unlikely to result in chronic exposure of aquatic biota; therefore, herbicide use under these conditions is unlikely to degrade surface waters. FOR .S CI. 59(2):197-210.