Background: The use of herbicides to control competing vegetation in planted forests is an important component of forest management. The increasing public aversion to herbicide use includes the possible contamination of receiving aquatic environments. The risk of surface run-off, leaching and erosion processes transporting herbicide residues to waterways, is potentially higher in steepland forests containing soils with high leaching potential. The purpose of this study was to measure the concentrations of terbuthylazine and hexazinone, two herbicides commonly used in post-plant weed control in New Zealand, in waterways, when aerially applied in a steepland forest catchment with Recent Soils. Methods: Terbuthylazine and hexazinone were applied, at 6 and 1.5 kg ha(-1), respectively, to 30 ha distributed over a 193-ha catchment. Herbicide concentrations were measured in stream water and sediment in a 12-ha headwater sub-catchment (W1) (57 % sprayed) and in stream water at two sites further down the catchment. These sites were monitored prior to, and for 5 months after, herbicide application. Results: The highest concentrations in stream water were recorded on the day of herbicide application at W1 (terbuthylazine, 9.6 mu g L-1; hexazinone, 5.3 mu g L-1). Terbuthylazine concentrations were above drinking water standards for several hours, well below the decadal time frame required to exceed these standards. Thereafter, concentrations rapidly declined, with the highest concentrations recorded 1 month after herbicide application, 2 days after a 50-mm (24 h) rainfall event. Concentrations declined downstream and were < 5 mu g L-1 where the stream exited the forested catchment. Terbuthylazine only was detected in sediment for up to 60 days after herbicide application. Concentrations and persistence of both herbicides in stream water were well below toxic levels for aquatic organisms. Conclusions: The potential risks to aquatic environments from herbicide application in steepland conditions were partially mitigated by the 30-m 'no-spray' zone along the stream margins, along with logging slash in the stream channel and the spatial distribution of the treated areas within the catchment. Under these operational conditions, and when applied according to manufacturer's instructions, the downstream risks to human health and aquatic environments appeared to be low.
