Abiotic factors influence surface water herbicide concentrations following silvicultural aerial application in Oregon's north coast range

Nontarget impacts of routine aerial silvicultural practices on surface water quality are not well documented. Thus, uncertainty remains regarding herbicide treatment effects on ecological and human health. To investigate factors that influence silvicultural herbicide concentrations in surface water and identify any potential risks, we conducted a 2‐year study that monitored multiple streams for herbicide residues following aerial application of glyphosate, clopyralid, sulfometuron methyl (SMM), and metsulfuron methyl (MSM). The monitored streams drain recently harvested forest lands that also serve as municipal water sources for nearby communities in western Oregon's north coast range. A paired watershed design targeted predicted episodic pulses with water samples collected before, during, and after herbicide application, and during the first posttreatment storm events. We report no relic herbicide detections in control or test streams. Aerial application of glyphosate, clopyralid, SMM, and MSM resulted in no detections in control streams and only trace, episodic concentrations in test streams. Across all test streams from both study years, maximum SMM and MSM detections (≤0.030 μg/L) consistently occurred during the first storm event at sampling locations closest to the treated harvest unit. Results indicate that proximity to the treatment site, time from application, and rainfall influence herbicide presence and concentrations in surface water. Furthermore, detections of trace SMM and MSM concentrations were more than 25 000‐fold and 60 000‐fold below federal human health safety benchmarks for chronic exposure, respectively. We provide empirical context for understanding surface water herbicide presence following aerial silviculture application under modern forestry best management practices and identify potential risk to ecological and human health. Integr Environ Assess Manag 2019;00:1–14. © 2019 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC)