Persistence of triclopyr, dicamba, and picloram in the environment following aerial spraying for control of dense pine invasion

Aerial application of an herbicide mixture of triclopyr, dicamba, picloram, and aminopyralid is used to control dense infestations of exotic conifers, notably lodgepole pine (Pinus contorta Douglas ex Loudon), in New Zealand. The rates of herbicide applied to control these tree weeds has the potential for off-target impacts through persistence in the forest floor, soil, and water. Persistence of three of these herbicides was investigated in cast needles, forest floor (litter, fermented humic layer [LFH]), and soil following their operational aerial application (triclopyr: 18 kg ai ha(-1); dicamba: 5 kg ai ha(-1); picloram: 2 kg ai ha(-1)) at three sites across New Zealand (KF, MD, GE) with dense invasions of P. contorta. Water was collected from a local stream at two sites (KF, MD) in the days/months after spraying. Active ingredients detected across all sites in cast needles, LFH, and mineral soil generally reflected their application rates, with total amounts comprising 81% triclopyr, 14% dicamba, and 5% picloram. Most of the active ingredients were detected in the LFH (59%), a heavy lignin-rich layer of dead needles overlaying the soil. All three herbicides persisted in this layer, at all sites, for up to 2 yr (at study termination). Only triclopyr was detected in mineral soil, where it declined to below detection levels (0.2 mg kg(-1)) within 1 yr. All three herbicides were detected in stream water on the day of spray application at KF, and during a rainfall event 1 mo later. However, amounts did not exceed New Zealand environmental and drinking water standards, an outcome attributed to a 30-m no-spray buffer zone used at this site. At MD, herbicides were detectable in water up to 4 mo after spraying, with amounts exceeding New Zealand drinking water standards on one occasion, 1 mo after spray application. No spray buffer zones were used at the MD site.