Forestry BMPs found to reduce the risk from forest herbicide use to aquatic organisms in streams

In managed forests, herbicides are often used to suppress competing vegetation and promote successful regeneration during stand establishment. Suppression of competing vegetation with herbicides increases wood volume yields substantially in a wide range of forest types and site conditions.

When herbicides are applied to forest land, they have the potential to impact stream water quality. The U.S. Environmental Protection Agency (USEPA) specifies on herbicide labels various practices intended to prevent contamination of water bodies from application. States also have developed forestry best management practices (BMPs) to minimize movement of silvicultural chemicals and other pollutants into bodies of water. Forestry BMPs, which rely in part on the implementation of streamside management zones (SMZs), are implemented at high rates nationally.

Recently, Dr. Vickie Tatum of NCASI and others authored a paper, which appeared in Forest Ecology and Management, that assessed the effectiveness of modern forestry BMPs for reducing the risk of forest herbicide use to aquatic organisms in streams. Other authors include C. Rhett Jackson (University of Georgia), Matthew W. McBroom (Stephen F. Austin State University), Brenda R. Ballie (Scion Research), Erik B. Schilling (NCASI), and T. Bently Wigley (NCASI). 

In the paper, the authors summarized results from recent studies of operational forestry applications of herbicides in which data on herbicide concentrations in potentially impacted streams were collected. They then used those data to conduct a screening level risk assessment for non-target aquatic organisms that might reside in those streams.

The authors’ conclusions follow.

“Operational forestry herbicide applications using modern BMPs were made at five distinctly different sites in the US [Coastal Range of Oregon (Needle Branch), East Texas (Alto), and southwest Georgia (Dry Creek)] and New Zealand (Bay of Plenty region; Baillie et al., 2015, Baillie, 2016). SMZs, ranging from 12 m to 21 m, were installed as specified in each US state’s BMP guidelines. No-spray zones equivalent to the SMZs were observed at Alto and Dry Creek. No-spray zones at Bay of Plenty Pekepeke were 10 m (Year One) or 20 m (Year Two) and those at the Eastern Bay of Plenty site were 30 m. At Needle Branch, an 18-m no-spray zone was observed, as specified in the Oregon FPA guidelines, for the fish-bearing portion of the stream. In addition, halfboom spraying, while not required, was employed along the upper nonfish- bearing portion of the stream. At most study sites, maximum herbicide concentrations in stream water were in the low ppb range and occurred as brief (< 24 h) pulses associated with storm water runoff from just the first few post-application storm events. At the Bay of Plenty Pekepeke site, peak concentrations were higher and occurred on the day of application, presumably due to direct deposition into the stream from inadvertent overspray or drift. However, the peaks were brief, lasting less than 8 h.

The herbicides used in these studies have low acute toxicity to fish, amphibians, or aquatic invertebrates (see, e.g. Tatum, 2004). At all sites, maximum stream water concentrations of herbicides were lower than the concentrations associated with acute toxicity in these organisms. Aquatic plants, on the other hand, show more sensitivity to these forestry herbicides. The lowest reported toxicity values for some species of algae and macrophytes are below the peak concentrations reported for imazapyr, hexazinone, sulfometuron methyl, and terbuthylazine at one or more sites. However, the exposure durations, especially to peak concentrations, reported in these field studies are much shorter than those used in the laboratory toxicity testing upon which the EC50 values are based and there is ample evidence that aquatic plants can tolerate much higher exposures if exposure times are short.

Overall, the low exposure levels and short exposure durations reported in the five studies described here suggest that use of herbicides in forestry while following modern BMPs poses minimal risk to nontarget aquatic animals. A greater potential of risk to non-target plants exists due to the greater sensitivity of plants to herbicides, however, this potential for risk may be mitigated by the very brief exposures to peak concentrations and the pattern of pulsed, rather than continuous, exposures.”  

Reference 

Tatum, V.L., C.R. Jackson, M.W. McBroom, B.R. Baillie, E.B. Schilling, and T.B. Wigley. 2017. Effectiveness of forestry best management practices (BMPs) for reducing the risk of forest herbicide use to aquatic organisms in streams. Forest Ecology and Management 404:258–268. https://dx.doi.org/10.1016/j.foreco.2017.08.046