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Does glyphosate leach through soils and into groundwater?

Category: Environment and Wildlife

Due to the chemical structure of glyphosate, the product binds quickly to soil particles and is very unlikely to leach into the soil and ground water.

Although glyphosate is quite water soluble, the molecule also carries both positive and negative charge that explain its strong binding affinity to soil organic matter and clay particles. As a result of its strong binding properties, it is considered to have very low potential to leach down through soils and into groundwater. Unlike agricultural scenarios, in forest vegetation management glyphosate-based herbicides are applied to sites with substantial competing vegetation cover and leaf litter on the forest floor. As such, much of the depositing spray cloud is intercepted by the target competing vegetation canopy or adsorbed to leaf litter, minimizing the amount of chemical that would actually reach the soil layer. Several studies in forest sites of Canada and the northern USA demonstrate that glyphosate and AMPA are strongly sorbed and typically retained within the upper 15 cm of soil and thus unlikely to move into surface or groundwater. In cases where there is sufficient rainfall residues bound to soil particles could be transferred to surface waters, although such residues are unlikely to be biologically available again owing to the strong binding affinity to organic matter and clay constituents.

In forest soils, glyphosate is rarely detected below the upper 15 cm level (Thompson et al 2000; Roy et al 1989; Feng et al. 1990; Legris et al. 1988; Newton et al. 1984; Newton et al. 1994), indicating that glyphosate is very unlikely to percolate down through forest soils and into groundwater. Given typical forest-use scenarios, the risk for groundwater contamination by glyphosate-based herbicides is substantially lower than that in agriculture, given the small percentage of the forest land base that is ever treated, that applications are typically made only once per site in 40-80 year period, that treatment sites are typically very remote from drinking water sources and that such source areas are protected by extensive buffers. However, even under agricultural scenarios where glyphosate-based herbicides are used extensively, typically only very low-levels of glyphosate are observed and even these occur very infrequently. Vereecken (2005) reported on several studies in European agriculture that also typically showed low level residues occurring infrequently in groundwater with no detections above drinking water standards in Denmark, the UK, the Netherlands or Norway. In another example, Battaglin et al. (2014) recently reported that of the total 1,171 groundwater samples analyzed from 807 different sites, glyphosate was detected in only 5.8% and AMPA in 14.3% of all groundwater samples. The maximum reported groundwater concentrations for the two compounds were 2.03 and 4.88 ppb respectively, far below the maximum acceptable concentration of 280 ppb established by Health Canada as protective of human health assuming a lifetime (70 year consumption) of 1.5 L of drinking water per day (Health Canada 2014). Similarly, a multiyear study of pesticide residues in 4 rivers in an agricultural region of southern Quebec showed the maximal concentrations of glyphosate ranging from 3.3 to 29.0 ppb (Giroux and Pelletier 2012). The latter value was considered to be the highest concentration of glyphosate observed in surface waters that might be considered as sources of drinking water (PMRA 2015) which concluded that dietary risk were not of concern either with respect to acute or chronic toxicity to humans generally or for subpopulations such as children and women of reproductive age.

In April 2015, the PMRA released their latest review of glyphosate and declared that the weight of evidence indicates that glyphosate does not present unacceptable risk to human health. The full PMRA glyphosate review can be found here or please visit here for a summary version of the full PMRA review

Thompson DG, Pitt DG, Buscarini TM, Staznik B, Thomas DR. Comparative fate of glyphosate and triclopyr herbicides in the forest floor and mineral soil of an Acadian forest regeneration site. Canadian Journal of Forest Research. 2000; 30:1808-16.

Roy DN, Konar SK, Banerjee S, Charles DA, Thompson DG, Prasad R. Persistence, Movement, and Degradation of Glyphosate in Selected Canadian Boreal Forest Soils. Journal of Agriculture and Food Chemistry. 1989;37:437-40.

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.

Legris J, Couture G. Residues de glyphosate dans le sol forestier suite a des pulverizations terrestres en 1985 et 1986. Gouvernement du Quebec Ministere de l'Energie et des Ressources Direction de la Conservation. 1988:22p.

Newton M, Horner LM, Cowell JE, White DE, Cole EC. Dissipation of glyphosate and aminomethylphosphonic acid in North American forests. Journal of Agricultural and Food Chemistry. 1994; 42(8):1795-802.

Newton M, Howard KM, Kelpsas BR, Danhaus R, Lottman CM, Dubelman S. Fate of glyphosate in an Oregon forest. Journal of Agricultural and Food Chemistry. 1984; 32:1144-51.

Vereecken H. Mobility and leaching of glyphosate: a review. Pest Manag Sci. 2005;61(12):1139-51.

Battaglin, WA, Meyer MT, Kuivaila KM and Dietze JE. Glyphosate and its degradation product AMPA occur frequently and widely in U.S. soils, surface water, groundwater and precipitation. Journal of the American Water Resources Association; 2014 50:275-290.

Health Canada. Guidelines for Canadian drinking water quality summary table. Health Canada October 2014; http://www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/sum_guide-res_recom/index-eng.php. pp. 25.

Giroux, I, Pelletier L. Présence de pesticides dans l’eau au Québec : bilan dans quatre cours d’eau de zones en culture de maïs et de soya en 2008, 2009 et 2010, Québec, ministère du Développement durable, de l’Environnement et des Parcs, Direction du suivi de l’état de l’environnement, 2012. 46 p. et 3 annexes.

PMRA (Pest Management Regulatory Agency). Proposed re-evaluation decision – Glyphosate. PRCD2015-01. 13 April 2015. Available electronically at: pmra.publications@hc-sc.gc.ca