Facilitated transport of pesticides

Facilitated transport of pesticides

6 Conclusion

Two seasons of field experiments sampling drain flow for Pendimethalin, Ioxynil (one season) and turbidity were carried out. The following conclusions could be drawn from the experiments:

For Pendimethalin the estimated total losses to drains was on average 0.0013 % of the applied dose (2.0 kg/ha).
The maximum concentration of Pendimethalin found in the drain flow samples was 12.3 µg/l in the first season and 2.97 µg/l in the second season. In the first season, the maximum concentration occurred in the first drain flow event after application. In the second season the maximum concentration occurred in the second drain flow event after application.
The maximum concentration of Ioxynil was 0.51 µg/l, found in the second drain flow event in the second season (same event as for Pendimethalin).
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 K_oc-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 µm filter or centrifugation) on 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.
Sorption experiments showed a clear correlation between organic carbon content of soil and sorption of Pendimethalin.
Ioxynil (with a K_oc 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 the modelling of the experiments, the following conclusions could be drawn:

The model was reasonably calibrated to fit the drain flow and the particle concentrations observed in the drains.
Using this calibrated setup, the subsequent modelling of particle-facilitated transport showed that with use of realistic K_d-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 K_d-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 K_d-values from 0.1 to 100 l/kg and was a potentially important factor for even lower K_d-values. However, the experimental evidence for Ioxynil did not support that low K_d 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 (K_oc 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.