Glifosato

Pest Management Science

Pest Manag Sci 64:441–456 (2008)

Review Fate of glyphosate in soil and the possibility of leaching to ground and surface waters: a review
Ole K Borggaard∗ and Anne Louise Gimsing
Department of Natural Sciences, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark

Abstract: The very wide use of glyphosate tocontrol weeds in agricultural, silvicultural and urban areas throughout the world requires that special attention be paid to its possible transport from terrestrial to aquatic environments. The aim of this review is to present and discuss the state of knowledge on sorption, degradation and leachability of glyphosate in soils. Difficulties of drawing clear and unambiguous conclusions because of strongsoil dependency and limited conclusive investigations are pointed out. Nevertheless, the risk of ground and surface water pollution by glyphosate seems limited because of sorption onto variable-charge soil minerals, e.g. aluminium and iron oxides, and because of microbial degradation. Although sorption and degradation are affected by many factors that might be expected to affect glyphosatemobility in soils, glyphosate leaching seems mainly determined by soil structure and rainfall. Limited leaching has been observed in non-structured sandy soils, while subsurface leaching to drainage systems was observed in a structured soil with preferential flow in macropores, but only when high rainfall followed glyphosate application. Glyphosate in drainage water runs into surface waters but notnecessarily to groundwater because it may be sorbed and degraded in deeper soil layers before reaching the groundwater. Although the transport of glyphosate from land to water environments seems very limited, knowledge about subsurface leaching and surface runoff of glyphosate as well as the importance of this transport as related to ground and surface water quality is scarce.  2007 Society of ChemicalIndustry

Keywords: sorption; degradation; AMPA; colloidal transport; phosphate competition

1 INTRODUCTION Glyphosate [N-(phosphonomethyl)glycine], which is the active ingredient in Roundup and other weedkilling formulations, is a broad-spectrum, postemergence, non-selective herbicide. It is the world’s biggest-selling chemical used for weed control in agricultural, silvicultural and urbanenvironments.1 – 4 This wide use can very much be ascribed to high weed-killing efficiency, low toxicity to non-target organisms and apparently a very limited risk of leaching to groundwater because glyphosate seems to be inactivated in soils by strong sorption and relatively fast degradation.1,5 – 9 Further, adding to glyphosate popularity was the introduction in 1997 of Roundup Ready crops, suchas cotton, maize and soybeans,3 which are made glyphosate resistant by incorporation of a naturally occurring, glyphosate-resistant protein. The wide use, and hence ubiquity, of glyphosate makes great demands on glyphosate safety, i.e. the absence of any harmful environmental effect except on target organisms (the undesirable weeds). In spite of many papers5,6,8,10 – 12 showing that glyphosate isrelatively safe environmentally, recent investigations
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indicate possible leaching and toxicity problems with its use.4,13 – 18 Recent studies have also shown that soil sorption and degradation of glyphosate exhibit great variation depending on soil composition and properties.9,19 – 24 Therefore, it is worthwhile reconsidering the fate of glyphosate in soils, including sorption, degradationand leachability. The objectives of this review are to point out the state of knowledge on glyphosate fate in soils, as well as to identify areas where more research is needed because of limited or inconclusive data. In order to place the outcome of this review in context, a few comments on the toxicological aspects of using glyphosate are first presented.

2 GLYPHOSATE TOXICITY The herbicidal...