Pesticides Research, 86 – Fate of Pyrethroids in Farmland Ponds

Fate of Pyrethroids in Farmland Ponds

16 Conclusion, part II

Suggested model formulations

It has not been possible to suggest any model for the surface micro layer. The data material is too limited and the knowledge about the 'real' layer thickness is lacking. However, some negative conclusions can be drawn. Thus, a diffusion type model including the diffusion through the micro layer, linear adsorption and 1. order volatilisation from the surface seems to predict the release of the substances from the micro layer to take place too quickly. The measurement of surface micro layer concentration may be very uncertain so the missing coincidence between model and experiment can be a result of either uncertainty in model assumptions or the uncertainty in the experimental result.

For the sediment water column system the model including sediment pore water diffusion, linear adsorption to sediment solids and degradation in the sediment seem most promising. Both experiments in the artificial ponds and in the laboratory support this type of model.

Information gained from the experimental results

For deltamethrin, permethrin and fenvalerate there was a distinct difference between pond 3 and pond 4, which were replicates. The most probable explanation is the difference in biomass due to a snail invasion in pond 3, which increased the adsorption mechanisms in pond 3, mainly due to the increased turbidity in water column caused by snail activity. Some of the substances behaved similar in the ponds 3 and 4 and the grouping is lined out in Table 16.1. Fenpropathrin seems to have higher sediments adsorption than the other substances although all substances demonstrate high sediment adsorption.

Table 16.1
The grouping of the substances in relation to similar behaviour.

Opdelingen af stofferne i forhold til ens opførsel.

Pond 3 (1996)	Pond 4 (1996)
Esfenvalerate Permethrin	deltamethrin permethrin esfenvalerate
Deltamethrin	fenpropathrin
Fenpropathrin

Laboratory experiment for fenvalerate including sediment measurements show a transport into the sediment as predicted by the pond investigation for the other pyrethroids. Furthermore, sediment degradation is observed in the laboratory experiments from sediment contamination measurements. The sediment pore diffusion coefficient in the lab scale experiment is much smaller (60 fold) than the diffusion coefficient in the pond sediment.

This indicates that the diffusion into the sediment in the pond is increased due to heterogeneity, which may be caused by bioturbation.

Generalisation of the results to other conditions

A generalised curve is constructed for fenpropathrin based on the suggested sediment/water column model. The model predicts the concentration to be proportional to the dosage and nearly independent on pond depth. This makes is possible to perform a generalised concentration time series for the water column which is independent on both dosage and depth. The tendency of independence between depth and concentration levels seems to be a result of the quick disappearance from the water column. Thus, all hydrophobic substances will tend to have this type of independence when the initial concentration is formed as a surface area specific (atmospheric) deposition.

Suggestions to improvements in relation to future experimentation design

The experimental result given in this analysis was not sufficient to make a clear distinction between the diffusion coefficient and the adsorption in the sediment. Only the product between the diffusion coefficient and the retention factor are revealed by the model calibrations. Therefore, additional adsorption tests using the sediment material will be beneficial in further experiments.

The sediment water column model can be used in experimental design. Such an optimisation is outside the scope of this work, but it should be included in future experiments. The knowledge needed is the experimental error, the cost for sampling, analysing and spraying and the constrains in form of the number of ponds and the overall time schedule. Different experimental strategies can be tested to determinate the uncertainty of the process related parameters (degradation, adsorption, diffusion). The cost and the associated uncertainty can be estimated and the optimal combination identified.

Relevance of existing models

There exist one sediment diffusion type model available in form of the Dutch model Toxswa (Adriaanse, 1996). This investigation support the use of such models where the sediment is detailed described.