2.4. Leaching studies in the laboratory

Leaching studies were performed in triplicate, based on the EEC standard procedure (Lalah and Wandiga, 1996). The soil samples were homogenized and sieved through a 2-mm pore sieve. The sieved soil samples were packed in glass columns (I.d of 45 mm) to the height of 500 mm with a glass wool plug at the bottom. Each soil column was saturated with 0.01M calcium chloride solution by passing the solution through it by capillarity for 12 h. The columns were then allowed to drain freely overnight with the 0.01M CaCl₂ solution. The dry mass of the soil in each column was 400.5 g. The equivalent of 10 mg L⁻¹ diuron in the soil was prepared by dissolving an appropriate amount of the standard in 0.01 M CaCl₂ which was added on top of the soil column. Leaching was monitored for 8 h, draining with 1.15 L of 0.01 M CaCl₂ at a flow rate of 2.4 mL min⁻¹ which is the average field application flow rate. After 8 h, the volume of the leachate was measured before partitioning with 250 mL dichloromethane for analysis of diuron and its metabolites residues. The wet soil in the glass column was carefully removed and sub-divided into 0–7 cm, 7–14 cm, 14–21 cm, 21–28 cm, 28–35 cm and 35–42 cm portions using a ruler and extracted for residues by use of methanol and analysis as explained above. The aim of sub-dividing the soil portions to 42 cm was to observe and understand chances of underground leaching by the compound.