Residues of clopyralid (also known as 3,6-dichloropicolinic acid or 3,6-DCPA) in streamwater were monitored after helicopter application of the herbicide LONTREL L at a rate of 2.5 kg a.i./ha to 56 ha of a Pinus radiata
D. Don plantation at Archerton in north-eastern Victoria, Australia, to control the woody weed, silver wattle (Acacia dealbata
Link). During and after the spraying, the streamwater was regularly sampled 0.5 km below the sprayed area and at a point 13 km downstream for a 19-day period, during which there were seven substantial rainfall events totalling 143 mm. The highest clopyralid concentration (0.017 mg/L) was detected just below the sprayed area soon after the start of the first rainfall event after spraying; this concentration is much lower than the maximum recommended level of 1 mg/L in potable water. At the downstream sampling point, the highest concentration detected was 0.001 mg/L. The results indicated that the main contamination was due to rainfall washing herbicide deposits from streamside vegetation that had intercepted minor amounts of spray drift.
The negligible concentrations of clopyralid found in streamwater during this study, despite substantial rainfall (72 mm) within 3 days of spraying, were attributed to several factors: (i) the small proportion of catchment (16%) that was sprayed, (ii) the presence of unsprayed streamside reserves, (iii) the use of techniques that ensured accurate spraying and minimised spray drift, and (iv) the pattern of rainfall after spraying that included low-intensity storms followed by high-intensity storms.