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#x2212;1to a 46.4 ha experimental catchment. The aerial application was made in December 1981 following conversion of a steeply sloping catchment from native forest toPinus radiatatwo years previously. Automatic samplers were used to sample streamwater at intervals of 0.25#x2013;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 #x3BC;g L#x2212;1hexazinone only was detected in six of these samples, which was well below the maximum recommended concentration for potable water of 600 #x3BC;g L#x2212;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.
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