Transport modes and pathways of the strongly sorbing pesticides glyphosate and pendimethalin through structured drained soils
Highlights
► Glyphosate and pendimethalin leached through structured soil in high concentrations. ► Transport of particle-bound pesticide accounted for only 13–31% of observed leaching. ► Particle-bound pesticide was transported solely by vertical transport in macropores. ► Dissolved pesticide was also transported laterally via discontinuities in the soil.
Introduction
With pesticides that sorb strongly to the soil, the risk of leaching from the root zone is generally considered to be low. As a consequence the use of such pesticides is not usually regulated very restrictively, with high dose application being permitted in late autumn. However, it is now generally accepted that both macropore transport (e.g. Jarvis, 2007) and particle-facilitated transport (e.g. de Jonge et al., 2004, McCarthy and McKay, 2004) and the interplay between them (e.g. Jacobsen et al., 1997, de Jonge et al., 1998, de Jonge et al., 2000, Vilholdt et al., 2000, Petersen et al., 2003) play an important role in the leaching of strongly sorbing compounds such as most pesticides.
Field studies have shown that pesticides with widely varying chemical properties are capable of rapid and deep movement in soil (Flury, 1996). Loss due to macropore flow is typically less than 1% of the applied dose but sometimes as much as 5%, which clearly raises both environmental and health concerns (Jarvis, 2007). Depending on soil type, a pesticide and/or its degradation products may end up in groundwater and, via tile drain systems, in surface waters, thereby threatening the drinking water supply and the aquatic environment.
Leaching of the strongly sorbing pesticides glyphosate (Koc 6700–29 400 mL g−1, pesticide properties database available at http://sitem.herts.ac.uk/aeru/footprint/index2.htm) and pendimethalin (Koc 884–60 000 mL g−1, pesticide properties database available at http://sitem.herts.ac.uk/aeru/footprint/index2.htm) has been reported in undisturbed soil columns (de Jonge et al., 2000), lysimeters (Gjettermann et al., 2009) and in field experiments on structured soils (Veiga et al., 2001, Petersen et al., 2003, Kjær et al., 2005, Landry et al., 2005). Glyphosate sorbs strongly to soil minerals (Gerritse et al., 1996, Borggaard and Gimsing, 2008), competing with phosphorus for sorption sites (de Jonge et al., 2001). In contrast, pendimethalin strongly sorbs to the soil organic matter fraction (Pedersen et al., 1995).
This implies that strongly sorbing substances differing in sorption affinity may potentially leach from well-structured soils. The transport processes governing the leaching of strongly sorbing compounds from structured soils subject to macropore transport are not very well understood, however. In a recent review of the fate of glyphosate, Borggaard and Gimsing (2008) concluded that although transport of glyphosate from the terrestrial to the aquatic environment seems to be very limited, little is known about subsurface leaching and surface runoff of glyphosate and the importance of this transport in relation to groundwater and surface water quality. Moreover, Jarvis (2007) concluded that while macropore transport processes are reasonably well investigated with regard to non-reactive or weakly reactive and conservative solutes, knowledge of the influence of macropore flow on the fate of the very strongly sorbing and bioreactive chemicals such as pesticides is still limited.
Likewise, little is known about the dominant transport routes for water and chemicals in tile-drained, structured soils under field conditions. It is still unclear whether leaching of strongly sorbing pesticides is dominated by particle-facilitated transport or whether a significant role is played by transport in the dissolved phase. Quantitative estimates of leaching losses via the different transport pathways are needed for cultivated soil under field conditions.
The objectives of the present study were thus to determine on drained structured soil (i) whether glyphosate, which sorbs strongly to the inorganic soil fraction, is more prone to leaching to tile drains than pendimethalin, which sorbs strongly to the soil organic fraction, (ii) the quantitative impact of particle-facilitated transport on total leaching (i.e. the sum leached in both particle-bound and dissolved form), and (iii) the pathways through which these strongly sorbing compounds enter the drainage system.
Section snippets
Chemicals
Glyphosate [N-(phosphonomethyl)glycine] (Fig. 1) – the active ingredient in Roundup – is a broad-spectrum, post-emergence, non-selective herbicide that is one of the most used herbicides worldwide. In Denmark, glyphosate is the herbicide sold in the largest quantities; in 2003, glyphosate sales for agricultural purposes accounted for 44% of all herbicide sales. By 2008, this had increased to 52% (Danish Environmental Protection Agency, 2004, Danish Environmental Protection Agency, 2009).
Leaching of glyphosate and pendimethalin
The leaching pattern of glyphosate resembled that of pendimethalin (Fig. 3), thus suggesting (i) that the leaching potential of strongly bound pesticides from structured soil is high both with pesticides that bind to soil organic matter (e.g. pendimethalin) or to the inorganic fraction (e.g. glyphosate) and (ii) that the pathways governing the transport of these two pesticides are similar.
Both glyphosate and pendimethalin leached from the root zone in average concentrations considerably
Conclusion
Pesticides leaching from the unsaturated zone may eventually pose a risk to the aquatic environment. The present 8-month study of a loamy field demonstrates that:
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Strongly bound pesticides, whether bound to the organic or inorganic soil fraction, may leach from the root zone and enter the aquatic environment in average concentrations exceeding 0.1 μg L−1.
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Particle-facilitated transport (particles >0.24 μm) accounted for only a small proportion of observed leaching (13–16% for glyphosate and 16–31%
Acknowledgements
This study was funded by the Danish Pesticide Leaching Assessment Programme. We thank the many people who have contributed to the study, including Carsten B. Nielsen, Ruth Grant, and Finn Plauborg (drainage water sampling design), Birgit Sørensen (measurement and analysis of precipitation data), Christina Rosenberg Lynge, Jørgen Munksgaard Nielsen, Rikke Guttesen and Mikko Anger (sample preparation and chemical analysis), Poul Boesen and Lasse Gudmundsson (ongoing field monitoring and data
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