Residues of 3,6-DCPA in streamwater following aerial application to a radiata pine plantation in Ryan’s Creek catchment, North-eastern Victoria.  P. C. Fagg and C. J. Leitch.  December 1983.  16 pp. (unpubl.)

SUMMARY

Residues of 3, 6-dichloropicolinic acid (3,6-DCPA) in streamwater were monitored following helicopter application of the herbicide LONTREL L^† to 56 ha of radiata pine (Pinus radiata D. Don) plantation at Archerton, in North-eastern Victoria, on 21 April 1983. The plantation is located in the upper catchment of Ryan’s Creek, which provides water for the city of Benalla, and the spraying was undertaken to control the woody weed, silver wattle (Acacia dealbata Link).

Spray-sensitive cards, used to monitor spray distribution along transects from the sprayed plantation into adjacent unsprayed zones, detected only minor drift out of the sprayed area.

During and after the spraying, streamwater was regularly sampled just below the sprayed area (automatic sampling) and at a point 13 km further downstream (manual sampling) for a 19-day period, during which time there were seven substantial rainfall events totalling 143 mm. A total of 103 individual and 24 composite water samples were analysed for 3,6-DCPA residues, the highest concentration found being 0.015 mg L^-1 detected just below the sprayed area soon after the start of the first rainfall event following spraying. This level was much lower than the maximum recommended level for potable water of 1 mg L^-1. At the downstream sampling point, the highest concentration detected was 0.001 mg L^-1, and, as the domestic water supply off-take is a further 12 km downstream, this extremely low concentration would have been subject to even further dilution prior to any human usage. The pattern of results indicated that the main source of contamination was a small amount of vegetation in streamside reserves that had intercepted spray drift.

The negligible concentrations of 3,6-DCPA found in streamwater during this study, despite substantial rainfall (72 millimetres) within three days of spraying, where attributed to several factors, including: (i) the presence of unsprayed, streamside vegetation, (ii) the apparent low mobility of 3,6-DCPA in the soil, and (iii) the minimisation of spray drift through spraying in almost calm conditions, applying relatively large droplets, and using an anti-evaporant oil in the spray mix.

It is concluded that the aerial spraying with 3,6-DCPA, of areas of radiata pine plantation in water supply catchments, according to the methods described, poses no discernible hazard to water consumers.

^†Registered Trade Name. Use of this product does not in any way constitute an endorsement or recommendation of the product at the exclusion of other, possibly equally suitable products.

Also published:

Leitch, C.J. and Fagg, P.C. (1985)  Clopyralid herbicide residues in streamwater following an aerial spraying of a radiata pine plantation.  N.Z. J. For. Sci. 15(2): 195-206.