Aquatic Fate of Aerially Applied Hexazinone and Terbuthylazine in a New Zealand Planted Forest
Herbicides are used to control competing vegetation during tree establishment, and are often critical to the productivity and economic viability of a planted forest crop. Despite increasing public concern over herbicide use in planted forests and potential impact on the environment, there is limited information on the aquatic fate of many of these herbicides when applied in forests according to standard operational procedures. Two herbicides commonly used in New Zealand’s planted forests, terbuthylazine and hexazinone, were aerially applied during two consecutive years at 7.4 kg and 1.1 kg ha-1 respectively, to a 12.5 ha catchment of newly planted Pinus radiata (Radiata pine) in the Bay of Plenty region of New Zealand. One hundred percent of the area was sprayed during Year 1 while 85% of the area was sprayed during Year 2. The highest concentrations of both herbicides were measured in stream water, sediment and algal samples on the day of herbicide application during Year 1 (September 2012 to June 2013), mainly as a result of spray drift, and in stream water during Year 2 (September 2013 to April 2014) at the time of a rainfall event seven days after herbicide application. Terbuthylazine concentrations exceeded New Zealand and World Health Organisation drinking water standards for <24 hours on both occasions. For the remainder of the trial period, concentrations of both herbicides were below safe drinking water standards. Herbicide concentrations in stream flow declined immediately below the trial site and were close to or below detection limits at the bottom of the catchment. The concentrations of terbuthylazine and hexazinone in stream flow during this trial, posed a short-term and low risk to algae. Peak concentrations of both herbicides were several orders of magnitude lower than Lethal Concentration (LC)50 concentrations for Daphnia and rainbow trout (Oncorhynchus mykiss). These results highlight the need to include data from both field and laboratory based trials when assessing the aquatic risk of these two herbicides when applied under operational conditions in planted forests.