Facilitated transport of pesticides

Facilitated transport of pesticides

Summary and conclusions

This study combined field investigations with model work to obtain information on the dynamics and the extent of pesticide movement into subsurface drains, and the conditions and transport mechanisms governing pesticide and soil particle losses with drainage water.

The field experiments, performed on two plots (each 16 by 100 m²), were carried out in corporation with The Royal Veterinary and Agricultural University (KVL) at their test facilities at Rørrendegaard near Høje Tåstrup. Experiments with pesticides were carried out in two winter seasons; 1999-2000 and 2000-2001. The leaching dynamics of two differently sorbing pesticides, Pendimethalin (hydrophobic) and Ioxynil (hydrophilic) was observed through sampling of drain water collected below the two plots.

A model component for MIKE SHE describing the transport of colloids generated at the soil surface through the unsaturated zone and the colloid-facilitated transport of pesticide was developed. The technical documentation for the module can be found in Styczen et al. (2002). In Appendix C of this report, a sensitivity analysis for some of the parameters in the module can be found. Chapter 4 of this report describes the application of MIKE SHE with the colloid-module to the drain-flow- and concentration observations of particles and Pendimethalin at Rørrendegaard for the season 1999-2000.

Based on the observations from the two sampling seasons at the field site and the modelling of the observations, the most significant conclusions drawn from this study were:

From experiments:

Peak concentrations of soil particles and Pendimethalin occurred shortly after initiation of drain events and before the occurrence of maximum drain flow rates.
There was a significant correlation between Pendimethalin (with a Koc-value in the range 10000-18000) concentrations and turbidity (particle concentrations) in the drain flow samples.
The timing of the peak concentrations indicated that macropores were the dominant means of transport for Pendimethalin and soil particles.
Fractionation analysis (filtering through 0.7 ƒÊm filter or centrifugation) on the drain water from the field experiments showed that between 0 and 30 % of the Pendimethalin was associated with particles. Three drain water samples from two other areas (Odderbæk and Lillebæk) showed that 45-65 % of the Pendimethalin was associated with particles.
Ioxynil (with a Koc value of approx. 170) did not exhibit any sign of correlation between particle content and compound concentration.
The strong correlation between turbidity and Pendimethlin concentrations implies that turbidity may serve as a guide in determining which samples to analyse for Pendimethalin or other hydrophobic compounds.

From modelling:

Using a setup calibrated to fit the observed particle concentrations in the drains, the subsequent modelling of colloid-facilitated transport showed that with use of realistic Kd-values for Pendimethalin, colloid-facilitated transport through the macopores completely dominated (97 %) the mass transport of Pendimethalin through the unsaturated zone.
The observed low values of particle-associated Pendimethalin (0-30 %) in the drain flow samples from Rørrendegaard could be explained by the dilution of the colloidal concentrations and the subsequent release of Pendimethalin to the dissolved state.
Simulations using different Kd-values for sorption of pesticide showed that for the calibrated model setup for Rørrendegaard, particle-associated transport completely determined the mass transport of pesticides to the saturated zone in the range of Kd-values from 0.1 to 100 l/kg and was a potentially important factor for even lower Kd-values. However, the experimental evidence for Ioxynil did not support that low Kd pesticides should associate considerably with particles.
The model-approach applied in this study, where the particle-associated pesticide was obtained through sorption of dissolved pesticide could not produce concentrations of particle-associated Pendimethalin and hence total Pendimethalin concentrations that were comparable to the observed ones, since the obtainable concentrations were limited by the solubility of the Pendimethalin.
It was not possible to simulate concentrations up to the level of the observed concentrations even though Pendimethalin was considered a conservative tracer – again indicating the existence of a very considerable supplementing transport mechanism than dissolved transport.
Applying a concept, where the particle-associated concentration of the pesticide is unrelated to the solubility of the pesticide will allow for higher pesticide concentrations to be simulated.

In combination between experimental and modelling results, the following conclusions could be drawn:

The time between leaching events and sampling and between sampling and filtering of samples may be a very important factor in finding proof of particle-associated transport when taking samples from the saturated zone (and from drains), because of dilution of colloidal concentrations and subsequent release of pesticide from the particle associated state.
Ideally, for determining the importance of particle-association as transport mechanism, samples should be taken directly from the macropores and not from the saturated zone

In conclusion, the study indicates strongly that colloid-facilitated transport through macropores is a very important transport mechanism for strongly sorbing pesticides (Koc in the range of Pendimethalin) from soil surface to saturated zone, at locations where generation of colloidal particles is considerable, and that this should be taken into account when determining the potential leaching of at least strongly sorbing pesticides.

To determine the potential leaching of pesticides from different soil types at least the organic carbon content of the topsoil, the structure of macropores in the soil, and the ease of soil detachment should be considered.