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What are the chances that glyphosate will drift into bodies of water after being aerially applied?

Category: Environment and Wildlife

Regulatory bodies require buffer zones to be created around streams, lakes, rivers and ponds near aerial treatment sites. The use of buffers around such systems essentially negates the potential for direct overspray of these aquatic systems. Buffer zones, in combination with advanced aerial application technologies including GIS-based mapping, electronic guidance systems and low-drift nozzles also help to ensure the herbicide does not enter such bodies of water either by accidental overspray or via off-target drift.


Owing to the application of advanced aerial application techniques, including GIS-based mapping, electronic guidance systems on aircraft, low-drift nozzles and the requirement for buffers around aquatic systems, there is a very low probability of toxicologically significant concentrations of glyphosate occurring in lakes, streams, or ponds. Well-validated aerial dispersal models predict very low proportions (< 2 %) of the depositing beyond 25 m downwind of spray blocks under aerial application scenarios as typically employed in major use provinces of New Brunswick, Alberta and Ontario. Several operational or semi-operational monitoring studies provide confirmatory evidence that the probability of inputs to aquatic systems protected by buffer zones is very low, and that where measureable concentrations do occur they are well below levels known to have toxicological effects on aquatic organisms. Potential inputs into small ephemeral wetlands via direct overspray or off-target drift represent a special case of relatively higher risk to species such as amphibians that inhabit these type of aquatic systems where these may occur within or immediately adjacent to spray blocks.

Under operational scenarios in Canadian forestry, spray blocks and surrounding areas are mapped using detailed GIS-based techniques. Buffer zones designed to minimize any potential for direct input into water bodies such as streams, rivers, ponds and lakes are imposed as a protective measure. Advanced aerial application technologies including electronic guidance systems and low drift nozzles are employed and meteorological monitoring is undertaken to ensure that spray applications are made only to the target spray block and within established parameters of wind speed, temperature and humidity (Thompson et al. 2009; Thompson et al. 2012). In combination, all of these controls and mitigation measures reduce the potential for biologically significant inputs into aquatic systems. Based on validated modeling results, the amount of glyphosate depositing at distances of 25 to 65 m downwind of the spray block edge are estimated to be between 2% and 5.6% of the full application rate (Thompson et al. 2012 Payne 1993; Riley et al. 1991). Interception by vegetation within the buffer zone further reduces the potential for input. Field studies confirm both the low probability and magnitude of inputs into buffered systems under typical aerial spray operations (Thompson et al. 2004, Feng and Thompson 1990; Gluns et al. 1989; Adams et al. 2007). Similarly, Couture et al. (1995) summarizing multiple forestry studies conducted in the province of Quebec concluded that the 90th percentile of concentrations observed in water were <0.3% of the concentrations that cause high short-term mortality in aquatic organisms. Small, shallow, unmapped wetlands which may occur within spray blocks or immediately adjacent thereto are the aquatic systems most likely to receive direct chemical input via overspray or off-target drift.

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Feng JC, Thompson DG, Reynolds PE. Fate of Glyphosate in a Canadian Forest Watershed. 1. Aquatic Residues and Off-Target Deposit Assessment. Journal of Agriculture and Food Chemistry. 1990; 38:1110-8.

Feng JC, Thompson DG. Fate of Glyphosate in a Canadian Forest Watershed. 2. Persistence in Foliage and Soils. Journal of Agriculture and Food Chemistry. 1990; 38:1118-25.

Gluns DR. Herbicide Residue in Surface Water Following an Application of Roundup® in the Revelstoke Forest District. Report. Research Branch BCMoF; 1989 88001-NE.

Adams, GW, Smith T, Miller JD. 2007. The absence of glyphosate residues in wet soil and the adjacent watercourse after a forestry application in New Brunswick. Northern Journal of Applied Forestry. 24(3): 230-232.

Couture G, Legris J, Langevin R, Laberge L. Evaluation of the impacts of glyphosate as used in forests (English abstract, French text). Ministere des Ressources naturelles, Direction de l'environnement forestier, Publ No RN95-3082. 1995:187.