## 3 Results and Discussion
### Mass Balance  
The mass balance for metribuzin is presented in Table 2. For all irrigation treatments, metribuzin concentrations were found to be below the detection limit by day 90. In both the water and surfactant-irrigated treatments, the total amount of metribuzin detectable in the system (soil + leachate), declined from an initial (applied) level of about 20 mg to less than 3 mg after 63 days and four irrigations. The relative amount of metribuzin in the two lower soil depth layers (15-45 and 45-70 cm) to that in the upper layers (0-5 and 5-15 cm) was at least 2-fold greater in the presence of surfactants from Day 5 onwards, suggesting that a greater quantity of metribuzin leached downwards in the presence (_vs._ absence) of the surfactant. This will be further addressed in the discussion of metribuzin residue concentrations in the soil which follows. _Appl. Sci._**2013**, \(3\) \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c c c c c c} \hline \hline  & \multicolumn{10}{c}{**Trrigation Solution Soil Profile Depth Range (cm/Lechate/Total)**} \\ \cline{2-13} \cline{13-13} Day & \multicolumn{1}{c}{**Tap water**} & \multicolumn{10}{c}{**Brij35 at 0.5 E L\({}^{-1}\)**} & \multicolumn{10}{c}{**Brij35 at 5 g L\({}^{-1}\)**} \\ \cline{2-13}  & **0-5** & **5-15** & **15-45** & **45-70** & **Leach** **cumul** & **Total** & **0-5** & **5-15** & **15-45** & **45-70** & **Leach** **cumul** & **Total** & **0-5** & **5-15** & **15-45** & **45-70** & **Leach** **cumul** & **Total** \\ \hline
0 & 19.25 & —Irrigation I— & 0 & 19.25 & 19.85 & —Irrigation I— & 0 & 19.85 & 20.06 & —Irrigation I— & 0 & 20.06 & —Irrigation I— & 0 & 20.06 \\
1 & 9.62 & 8.90 & — & 0.09 & 18.61 & 10.30 & 2.13 & 5.72 & — & 0.34 & 18.49 & 8.49 & 2.75 & 6.65 & — & 0.65 & 18.54 \\
5 & 5.05 & 3.08 & 0.94 & — & 0.09 & 9.16 & 4.91 & 1.79 & 2.55 & — & 0.34 & 9.59 & 2.82 & 2.26 & 3.52 & — & 0.65 & 9.25 \\
15 & 2.59 & 1.05 & 1.95 & — & 0.09 & 5.68 & 1.88 & 0.11 & 4.93 & 1.99 & 0.34 & 9.25 & 1.54 & 0.33 & 2.44 & 3.29 & 0.65 & 8.25 \\
21 & —Irrigation II— & 0.17 & 5.76 & — & —Irrigation II— & 0.80 & 9.71 & — & —Irrigation II— & 1.27 & 8.87 & — & 1.60 & 1.72 & 0.80 & 4.90 & 0.95 & — & 0.91 & 1.09 & 1.27 & 4.22 \\
22 & 1.32 & 0.21 & 1.36 & — & 0.17 & 3.06 & 0.78 & — & 1.60 & 1.72 & 0.80 & 4.90 & 0.95 & — & 0.91 & 1.09 & 1.27 & 4.22 \\
30 & 0.85 & — & — & — & 0.17 & 1.02 & 0.62 & — & 0.82 & 0.56 & 0.80 & 2.80 & 0.22 & — & 1.20 & 1.08 & 1.27 & 3.77 \\
42 & — & —Irrigation II— & 0.25 & 1.10 & — & — & \multicolumn{2}{c}{Irrigation III—} & 1.33 & 3.33 & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} \\ \cline{6-13}
60 & 0.12 & — & — & — & 0.25 & 0.37 & 0.09 & — & — & 0.93 & 1.33 & 2.35 & 0.07 & — & — & 0.17 & 1.55 & 1.79 \\
63 & — & Irrigation IV— & 0.25 & 0.37 & — & — & \multicolumn{2}{c}{Irrigation IV——} & 1.54 & 2.56 & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} & \multicolumn{3}{c}{} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Amounts of metribuzin (mg) in different soil profile depth ranges, cumulated in leachate, and overall, over a 63 day period. Soil samplings on days 0, 1, 15, 22, 30, and 60 at depths of 2, 10, 30 and 60 cm depths, as well as and irrigations followed by leachate sampling on Days 1, 21, 42, 63. (—, metribuzin was below detection levels). From 15 days onward, the total amount of measured metribuzin was higher in the treatments of surfactant compared to the tap water (Table 3). After 60 days, the total residual metribuzin was 4 and 6 fold greater in the 5 and 0.5 g L\({}^{-1}\) treatments, respectively, than in the tap water treatment. The percentage of unrecovered metribuzin (Figure 3) was similar for all three irrigation types up to 5 days; however, later values were smaller in the presence of surfactant. As a result, greater recoveries have been measured in the presence of the non-ionic surfactant Brij35 compared to tap water treatment.