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

Aerial application of a herbicide mixture of triclopyr, dicamba, picloram and aminopyralid is used to control dense infestations of exotic conifers, notably lodgepole pine (Pinus contorta Douglas), in New Zealand (NZ). 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 a.i. ha-1; dicamba: 5 kg a.i. ha-1; picloram: 2 kg a.i. ha-1) at three sites across NZ (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 rate, 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 years (at study termination). Only triclopyr was detected in mineral soil where it declined to below detection levels (0.2 mg kg-1) within one year. All three herbicides were detected in stream water on the day of spray application at KF, and during a rainfall event one month later. However, amounts did not exceed NZ 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 four months after spraying, with amounts exceeding NZ drinking water standards on one occasion, one month after spray application. No spray buffer zones were used at the MD site.