## 3 Results and Discussion
### Effect of Nonionic Surfactant Brij35 on Metribuzin Residues in Soil  
The repeated measures analysis showed the effect of treatment (surfactant concentration), depth and time, as well as their single and multiple interactions were all significant (\(p\leq 0.05\)). Mean metribuzin concentrations in soil over a 90-day period for the soil surface and at 0.1, 0.3 and 0.6 m soil depths are illustrated in Figures 4-7. As expected, the metribuzin concentration was highest at the soil surface and dropped significantly as sampling depth increased. At the soil surface, the concentration decreased as the degradation rate increased with time. The concentrations right after the first application on day 1 were 968.68, 1036.34 and 854.93 \(\mathrm{\SIUnitSymbolMicro g}\)\(\mathrm{kg}^{-1}\) in the treatments with tap water, Brij35 0.5 \(\mathrm{g}\)\(\mathrm{L}^{-1}\) and Brij35 5 \(\mathrm{g}\)\(\mathrm{L}^{-1}\), respectively. Metribuzin levels declined over time and reached 12.37 \(\mathrm{\SIUnitSymbolMicro g}\)\(\mathrm{kg}^{-1}\) in the topsoil on day 60 for the tap water treatment, whereas for Brij35 0.5 \(\mathrm{g}\)\(\mathrm{L}^{-1}\) and Brij35 5 \(\mathrm{g}\)\(\mathrm{L}^{-1}\), concentrations were lower at 9.82 and 7.29 \(\mathrm{\SIUnitSymbolMicro g}\)\(\mathrm{kg}^{-1}\), respectively. By day 90, metribuzin concentration in the topsoil was below the detection limit for all irrigation treatments. However, a considerable amount of metribuzin leached to the next depths in the treatments with surfactants. This result for the tap water treatment concurs with the findings of several studies [61; 62; 63; 64; 65]. They reported that metribuzin concentrations declined in topsoil immediately after application. A number of studies suggest that a significant portion of this decline can be attributed to the presence of soil microorganisms, which play a significant role in the breakdown of metribuzin in soil [66; 67]. For all treatments, metribuzin degradation is shown to follow the first order kinetic [68; 69]. Typically, a rapid initial dissipation is followed by a gradual decline in the degradation rate and eventually a relatively slow long term breakdown. The half-life of metribuzin under the experiment conditions is calculated. In the tap water treatment, the half- life of metribuzin was 8.89 days; however, it decreased to 8.35 and 7.96 days in the treatment of surfactant at 0.5 and 5.0 \(\mathrm{g}\)\(\mathrm{L}^{-1}\), respectively. This is consistent with the reported half -life by [PERSON] and [PERSON] (1979) [28]. At the 0.1 m depth, initial metribuzin concentrations in the tap water treatment were higher than those in either of the Brij35 treatments. For all treatments, metribuzin levels decreased over time; however, in the tap water treatment, metribuzin was detectable through day 22, whereas in the Brij35 treatments it only remained detectable through day 15. This disappearance of metribuzin could be attributed to its movement to lower depths and to the microbial degradation. Metribuzin breakdown varies according to soil layers [69] due to the differences in their microbial populations. Indeed, [PERSON] _et al._ (1982) and [PERSON] _et al._ (2002) [62; 70] observed that metribuzin degradation is decreased by depth. Likewise, [PERSON] (2010) [52] conducted a lysimeter study and observed that a considerable amount of surface-applied metribuzin had migrated through a sandy soil to a depth greater than 0.1 m within the first 7 days, and then decreased gradually afterwards. Similarly, results from a study investigating the mobility of metribuzin under free drainage conditions, revealed that a significant portion of surface-applied metribuzin was detected below a 0.1 m soil depth, six days after application [71]. Figure 4: Metribuzin concentrations at the surface. Figure 5: Metribuzin concentrations at 0.1 m depth. Figure 6: Metribuzin concentrations at 0.3 m depth.