Fate of Pesticides in Surface Waters, Laboraty and Field Experiments

Fate of Pesticides in Surface Waters, Laboraty and Field Experiments

6 Conclusions from laboratory experiments

A large number of sorption and desorption rates for the three model pesticides with different sediments have been produced together with a large number of equilibrium partitioning constants.

Pendimethalin sorbed strongly to sediments while ioxynil and bentazone sorbed much less.

For all pesticides, the degree of sorption was depending on sediment organic matter. Thus, Koc values must be used when predicting sorption. For pendimethalin, differences in pesticide concentration influenced equilibrium partioning markedly, and sorption was higher at low concentrations.

A temperature difference of 16° C did not influence equilibrium sorption, indicating that the influence of temperature on sorption rates may be omitted in the stream and pond pesticide fate models.

Sorption was very fast in all experiments with most of the sorption happening within the first couple of hours. The sorption rate did not seem to depend on pesticide concentration level for a weakly sorbing sediment. Although the sorption was fast, it is probably too slow to be disregarded in systems with short residence times such as the small streams of Odderbæk and Lillebæk. This will be shown by model runs with the parameters generated here.

The changes in aqueous pesticide concentrations could be modelled well with the suggested sorption model.

A number of degradation rates for stream water and stream-sediment suspensions have been produced.

It was shown that for a moderately sorbing pesticide such as ioxynil, the degradation rate in water-sediment suspensions was higher than in water alone, probably due to higher degrading biomass.

For a strongly sorbing pesticide such as pendimethalin, it was shown that the presence of sediment caused a smaller degradation, probably due to reduced bioavailability caused by sorption.