Drift Reduction Technology and Assessing Baseflow and Stormwater Runoff of Forest Chemicals

The use of herbicides as part of forest management is controversial.  Herbicides enhance reforestation and growth of crop trees but there are environmental questions, including concerns about effects on aquatic communities.  The forest and chemical communities have worked to reduce offsite delivery of aerially applied chemicals.  Historically, the highest herbicide concentrations in streams were observed where spray buffers were not used.  Without buffers, instantaneous herbicide concentrations as high as nearly 8,000 µg/L were measured in streamwater.  Decades ago, forest research demonstrated that spray buffers could dramatically reduce observed streamwater concentrations.  Aerial spray deposition models and drift reduction technologies (DRT) have further advanced our understanding and control of herbicide concentrations in streams adjacent to spray units.  DRTs include such practices as use of helicopters; use of nozzles and operational limitations to create large, fast falling drops; restrictions on acceptable weather conditions; applications at the lowest height consistent with safe operations; use of half-booms near streams; and applications utilizing on-board global position system (GPS) technology.  Tests also show that riparian vegetation can significantly reduce spray delivery to streams.  Today, the highest herbicide concentrations observed in streams are typically two to four orders of magnitude less than historic highs.  With dramatic reductions in herbicide concentrations in streams during and immediately after spray operations there are renewed concerns about stormwater runoff.  Do storm events wash herbicides into streams at durations and concentrations that can cause injury to aquatic communities?  Here we summarize monitoring results from three forest watershed studies across the United States and explain observed herbicide patterns in both baseflow and stormwater runoff.  We then assess the risk to aquatic communities associated with these exposure regimes.