## 3 Results and Discussion
### Effect of Non-Ionic Surfactant Brij35 on Methyluzin in Leachate  
The leachate samples were analyzed to quantify the amount of metribuzin leaching to shallow ground water. Metribuzin concentrations in the leachate of both Brij35 treatments, ranging from 67.22 to 23.02 for Brij35 at 0.5 g L\({}^{-1}\), and 84.80 \(\upmu\)g L\({}^{-1}\) to below the detection limit for Brij35 at 5 g L\({}^{-1}\) (Figure 8). The levels of metribuzin in the leachate were significantly greater at the treatment of Brij35 at 5.0 g L\({}^{-1}\) than Brij35 at 0.5 g L\({}^{-1}\) after the first two irrigation events. Metribuzin concentrations reached a maximum at the third application (17.19 \(\upmu\)g L\({}^{-1}\)) in the tap water leachate samples. This observation can be explained by the fact that metribuzin adsorption in sandy soil is reversible and thus residues might escape to lower layers in the soil profile [67]. Likewise, since sandy soil possess a low CEC and large pore spaces, high leaching potential for herbicides may be observed [76]. However, in a lysimeter study, no metribuzin concentrations were detected in the leachate from a sandy soil after irrigation with either tap water or a non-ionic surfactant metabolite, NP (12 mg L\({}^{-1}\)) [25]. In the case of surfactant treatments, the amount of metribuzin moved from the surface to the leachate in the treatment of Brij35 at 5 g L\({}^{-1}\) was 84.80 \(\upmu\)g L\({}^{-1}\) after the second irrigation event. This amount exceeded the acceptable level for metribuzin in drinking water (80 \(\upmu\)g L\({}^{-1}\)), according to Canadian guidelines [77]. Generally, above the CMC, the lipophilic end (alkyl chain) of the surfactant molecule aggregate together inside the micelle, with the hydrophilic end point towards the aqueous phase on the exterior [78]. Thus, micelles enhance the solubility of low polarity organic compounds by partitioning them into the hydrophobic micelle core. Since metribuzin is a polar herbicide, it is expected to be attracted by the polar head of the surfactant by hydrogen bonding. Partitioning of pesticides with surfactants is well documented in the literature [46,79,80]. More specifically, polar compounds, such as pesticides, are solubilized in the neighborhood of stern layer of the surfactant [22]. Additionally, non-ionic alcohol ethoxylates (AE) are found to enhance the downward movement of pesticides when applied at high (_vs._ low) concentrations to soil columns [18]. Typically, sorption of herbicides onto soil surfaces occurs when the polar components in soil interact with the polar functional groups of the herbicide and since metribuzin has an amide group, interaction with polar sites may occur. However, if water is present, metribuzin tends to interact with  
Figure 8: Metribuzin concentrations in Leachate. water molecules via hydrogen bonding and form hydrophilic complexes, which in turn, facilitate the leaching process. This may explain the relationship between its solubility and leaching potential [81]. Since surfactants can interact with soil, it may compete with metribuzin for available binding sites in soil. This can lead to enhance the free downward movement of metribuzin in soil profile. Recently, [PERSON] _et al._ (2011) [82] confirmed the role of polar groups (amide) in metribuzin in the formation of water bridges via hydrogen bonding with the hydrophilic head of the non-toxic surfactant phosphatidylcholine in clay formulations. Therefore, in the presence of a non-ionic surfactant, such as Brij35 at concentrations above its CMC, metribuzin would be captured by micelles, with its polar groups oriented toward the polar head (EO) of the micelles, which would exhibit a strong hydrophilic interaction via hydrogen bonding or dipole-dipole effects. In the present study, results of the lysimeter experiment are explained by the findings of the sorption study. This conclusion confirms the role of non-ionic surfactant Brij35 in decreasing the sorption coefficient of metribuzin and hence promoting its release from soil.