Residues of hexazinone in streamwater after aerial application to an experimental catchment planted with radiata pine

Abstract

Residues of hexazinone in streamwater were monitored over a nine-week period after helicopter application of the herbicide at a rate of 2 kg ha⁻¹ to a 46.4 ha experimental catchment. The aerial application was made in December 1981 following conversion of a steeply sloping catchment from native forest to Pinus radiata two years previously. Automatic samplers were used to sample streamwater at intervals of 0.25–2.0 h throughout the nine weeks, with the more intensive sampling occurring during and immediately following spraying and during the only substantial storm event. A total of 69 representative samples were analysed, and 4 μg L⁻¹ hexazinone only was detected in six of these samples, which was well below the maximum recommended concentration for potable water of 600 μg L⁻¹. Such low residues were attributed to several factors including the way the spraying operation was conducted (with respect to soil moisture, meteorological conditions and droplet size) and the presence of a 30 m wide vegetation reserve on each side of the stream.