Fate of Pesticides in Surface Waters, Laboraty and Field Experiments
2 Model pesticidesPesticides used in experiments in this project were selected from a joint list of priority model compounds, which were identified at the start of the overall project. Five compounds were sprayed in the field experiments, three of which were also used in laboratory studies on sorption and degradation. Table 2.1 provides an overview of the structure and physico-chemical properties of the model pesticides.
Pendimethalin is a selective herbicide belonging to the chemical group of dinitroanilines. It has low water solubility and high octanol-water partition coefficient log Kow (5.18 (United States department of agriculture, 2001)). According to the Danish EPA registration database, pKa is 2.8, which, assuming that it is an acid, means that it is a rather strong acid. However, this seems to be an error as no other data indicating acidity have been found. Bentazone is a selective contact herbicide and a strong acid due to its sulfonamide group, with pKa = 2.92 (Syracuse research corporation, 2000). Bentazone has high water solubility, due to its charge at neutral pH. Kow is quite low even for the neutral form: 219. For different levels of dissociation, Kow has been measured to be 5.84 (pH 5), 0.35 (pH 7) and 0.28 (pH 9) (Tomlin, 1994). Kd values of 0.00 to 0.23 for nine Spanish, German and Italian soils have been reported with no correlation between humic acid content, clay content or pH and Kd (Fomsgaard, 1997). Ioxynil is also a selective contact herbicide and a relatively strong acid. Ioxynil is available as different salts or as octanoate. The ioxynil octanoate is practically insoluble in water (Tomlin, 1994). In this project, ioxynil salt has been used for experiments. Fenpropimorph is a systemic fungicide belonging to the morpholine group. Kow is dependent on pH. With pKa about 7, Kow varies three orders of magnitude in the pH range 5-9. Glyphosate is non-selective herbicide from the phosphonic acid group. It is ionic, water-soluble and adsorbs to soil.
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