Residues of hexazinone and four of its metabolites in streamwater after aerial spraying of a Pinus radiata plantation near Yarram, Victoria.  P. C. Fagg, J. M. James, D. W. Flinn and P. Mainey.  March 1982.  9 pp. (unpubl.)

SUMMARY

This report describes a study of the off-site movement of the herbicide hexazinone (as Velpar L) and four of its metabolites into stream water following an aerial application by the Forests Commission to part of a young Pinus radiata D.Don (radiata pine) plantation in the Yarram Forest District, south-eastern Victoria. The 28 ha study area was sprayed with 4 kg ha^-1 hexazinone in November 1980. The spraying was designed to control the woody weed Acacia dealbata Link (silver wattle), which was overtopping the P. radiata. Water draining the study area flows into the Little Albert River, which is separated from the sprayed block by a 100 m-wide strip of native vegetation.

Two sampling points were established on the Little Albert River, one immediately below the majority of the spray area and the other 4.3 km downstream, above the nearest domestic take-off point. Grab samples of stream water were taken at one to four-hourly intervals before, during and immediately following spraying at both sampling points. During the next five weeks samples were collected on the morning after any days during which rain had fallen. The stage height of the stream was measured at each sampling time.

Neither hexazinone nor four of its metabolites were detected in any of the stream water samples at detection limits of 0.05 to 0.07 ppm for hexazinone and metabolite A, and metabolites B, D and E, respectively. The National Health and Medical Research Council recommends a maximum concentration of 0.6 ppm hexazinone in drinking water. It was concluded that, under the conditions of the study, there was no detectable contamination of the Little Albert River following the aerial application of hexazinone.

Whether any stream contamination by hexazinone would have occurred following rainfall in excess of the 91.8 mm measured during the sampling period, cannot be predicted, although the likelihood of this would be minimal, due to its rapid uptake by the weed vegetation, adsorption by soil colloids, and the fact that the vegetation of the downslope buffer strip showed no signs of uptake of hexazinone up to six months after spraying.