Cumulative Water Quality Effects Analysis of 2012-2013 Forestry Herbicide Use in the Rangitaiki River Basin, New Zealand.

The water quality effects of applying Release KT^© (43.5 mg L^-1 terbuthylazine and 6.5 mg L^-1 hexazinone) to 48 areas in a central North Island forest in the Rangitaiki River basin of New Zealand during the 2012-2013 application season was assessed using a cumulative effects approach. The herbicides were applied by helicopter for post-planting control of weeds such as gorse and broom. In one of these areas, Release KT^© was applied to a 53 ha area of a third rotation Pinus radiata stand in orthic pumice soils. Water samples were collected in streamflow during application, within and below the site, and for seven months afterwards. The monitoring provided a basis for estimating herbicide residue movement into the Rangitaiki River. A prototype spread sheet model, based on a constant flow rate was developed to estimate the effects of these two herbicides on water quality at the Murupara gaging station on the Rangitaiki River. The results of the cumulative assessment using this prototype indicated that hexazinone and terbuthylazine concentrations at this point on the Rangitaiki River were unlikely to exceed both drinking water quality standards (400 and 8 mg m³, respectively), and safety factors for aquatic species.