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Fate of Pesticides in Surface Waters, Laboraty and Field Experiments

3 Introduction laboratory experiments

As stated in the introduction, the general objective of the laboratory experiments was to investigate the importance of processes and variance of process parameters between different pesticides and between different environmental conditions and to derive suitable parameters for model evaluation.

Various environmental conditions influence sorption. The pH level of the water influences the sorption of pesticides with acidic or basic properties as partitioning of charged species into organic matter, e.g. in the sediment, normally is much lower than that of neutral species. In streams originating from and flowing through calciferous soils, pH can be expected to be between 7.5 and 8.5. For pesticides with acidic properties, this could mean that virtually all of the pesticides is in the charged and more water-soluble form. In this study, however, the effect of pH was not studied as this effect is very pesticide-specific and methods already exist for the correction of pH effect on sorption.

The variation of Kom and thereby Kd with temperature is not well investigated but it seems that sorption often diminishes with increase in temperature. The size of the compound is important as it influences dissociation and the influence of temperature on sorption of larger compounds is much more pronounced than on sorption of smaller compounds (Schwarzenbach et al., 1993). The temperature of small streams in Denmark usually varies within approx. 7-25° C.

The concentration of pesticides in the water, in relation to the particle concentration, can be important for sorption rates and sorption equilibria as the sorption may be non-linear. Sorption may also influence biodegradation, due to decreasing bioavailability, and degradation rates in the water column, in water-sediment suspensions and in the sediment can be expected to be different.

Thus, the objective of the laboratory studies was to determine sediment-water partitioning equilibrium constants, sorption and desorption kinetic parameters and degradation rates for the sorption and degradation of selected pesticides in different water-sediment systems, at different temperatures and at different pesticide concentrations:

• Determination of sorption rates
• Determination of desorption rates
• Determination of sorption equilibria
• Determination of variation in sorption rates with concentration
• Determination of variation in sorption equilibria with concentration and temperature
• Determination of biotic degradation rates in surface water and in water-sediment systems

As mentioned in the introduction, the registration model was set up for two catchments, the Lillebæk catchment and the Odderbæk catchment, and thus sorption and degradation in water-sediment systems with sediments from the Lillebæk and the Odderbæk streams were investigated. Additionally, sediments from the four lakes, Vaparanta, Höytiäinen, Kuorinka and Mekrijärvi, were included in the study to broaden the span of sediment properties. As the catchments do not actually contain any suitable ponds, sorption in systems with sediment from the artificial pond at NERI and from four lakes was investigated.

The three pesticides, pendimethalin, ioxynil and bentazone, were chosen as model compounds because they are applied and found in the catchments studied and because they represent diverse physico-chemical properties.

In order to keep the number of experiments within a reasonable number, the processes were only studied with the most relevant combinations of sediment and pesticide. The combinations of pesticide and sediment used for the different studies are shown in Table 3.1.

Table 3.1
Laboratory experiments performed

	Pendimethalin	Ioxynil	Bentazone
Sorption rates	Pond Odderbæk Lillebæk Vaparanta	Odderbæk Lillebæk
Desorption rates	Pond
Sorption equilibria	Pond Odderbæk Lillebæk Vaparanta Höytiäinen Kuorinka Mekrijärvi	Pond Odderbæk Lillebæk Vaparanta Höytiäinen Kuorinka Mekrijärvi	Pond Odderbæk Lillebæk Vaparanta Höytiäinen Kuorinka Mekrijärvi
Variation in sorption rates with concentration	Pond Vaparanta	-	-
Variation in sorption equilibria with concentration	Pond Lillebæk Odderbæk Vaparanta	Lillebæk Odderbæk	Lillebæk Odderbæk
Variation in sorption equilibria with temperature	Vaparanta Höytiäinen Kuorinka Mekrijärvi	Vaparanta Höytiäinen Kuorinka Mekrijärvi	Vaparanta Höytiäinen Kuorinka Mekrijärvi
Degradation rates	No sediment Odderbæk Lillebæk	No sediment Odderbæk Lillebæk	No sediment Odderbæk Lillebæk

In order to investigate desorption phenomena, a desorption experiment was conducted in which desorption of recently sorbed pendimethalin from pond sediment was investigated. Two pesticide concentrations were studied in the desorption experiments.

An overview of the performed experiments is given in Tables 4.2 and 4.3 in Section 4.2.  

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