## 2 Materials and Methods
### Adsorption-Desorption Study  
The adsorption isotherms of 14C-metolachlor were determined using the batch equilibrium method for the three soil types. The initial concentrations of 14C-metolachlor (0.1, 1, 5, and 10 mg L\({}^{-1}\)) were prepared in a 0.01 M aqueous CaCl\({}_{2}\) solution as recommended by the USEPA guidelines for adsorption study [14], and 2 g of air-dried soil was equilibrated with 10 mL of a 14C-metolachlor solution in 20-mL Teflon centrifuge tubes in a horizontal shaker (150 rpm) for 24 h (equilibration time based on a preliminary study) at room temperature (25 \(\pm\) 1 \({}^{\circ}\)C). Preliminary studies have shown that none of the metolachlor added was degraded during 24 h of contact with any of the soils used.  
For anaerobic treatment, a similar 2 g soil sample contained in the Teflon centrifuge tubes was flooded with the sterile deoxygenated water. The tubes were flushed with N\({}_{2}\) gas (also containing \(\sim\)5% CO\({}_{2}\)(g) and \(<\)2% H\({}_{2}\)(g)), sealed, and incubated in an anaerobic chamber (Coy Laboratory Products, Inc., Grasslake, MI, USA) at room temperature (25 \({}^{\circ}\)C) for 2 weeks to allow complete reduction. To study the adsorption, 14C-metolachlor was added using a concentrated O\({}_{2}\) free stock solution to the reduced soil (anaerobic system) in order to attain final concentrations of 0.1, 1, 5, and 10 mg L\({}^{-1}\). The sealed tubes were equilibrated on a shaker inside the anaerobic chamber for 24 h, where the oxygen was maintained at a concentration of zero.  
At the end of the equilibration period, the soil suspension was centrifuged (15 min, 12,000\(\times\)\(g\)). The aliquots were removed from each tube for a radioactivity assay using a Packard Tri-Carb (1900 TR) scintillation counter. A control (treatment without herbicide) was included for calibration and background correction purposes. The difference between the initial and final concentrations of herbicide in the solution were used to calculate the amount adsorbed. All the experiments were performed in triplicate.  
After equilibration and removal of 5 mL of the initial 10 mL of supernatant, herbicide desorption from the soil was estimated by adding an equal amount of a fresh 0.01 M CaCl\({}_{2}\) solution to the centrifuge tubes, dispersing the soil aggregates by vibration and re-shaking for 24 h. Sampling from the anaerobic soil treatments was handled inside an anaerobic glove box chamber. The soil samples were centrifuged (15 min, 12,000\(\times\)\(g\)), and an aliquot of the supernatant was removed and analyzed by the radioactivity assay. The desorption process in each tube was repeated four times. Desorption was estimated by determining the amount of herbicide in the soil solution following equilibration and calculated by subtracting the amount of herbicide remaining on the soil surface [12].  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Soil**} & \multirow{2}{*}{**pH**} & \multicolumn{3}{c}{**Texture (\%)**} & \multicolumn{1}{c}{**WHC**} & \multicolumn{1}{c}{**CEC**} & \multirow{2}{*}{**Organic**} \\ \cline{3-3} \cline{5-8}  & & **Sand** & **Silt** & **Clay** & **(\%)** & **(\(\text{Cmol}_{\text{c}}\)** & \(\text{Kg}^{-1}\))** & **Matter (\%)** \\ \hline Catlin & 7.6 & 10 & 58 & 32 & 31.4 & 13.9 & 3.3 \\ Flanagan & 6.5 & 12 & 56 & 32 & 32.0 & 17.5 & 3.7 \\ Drummer & 7.1 & 14 & 46 & 40 & 33.0 & 23.8 & 4.9 \\ \hline \hline \end{tabular}  
* WHC water holding capacity at 1/3 bar. CEC: cation exchange capacity.  
\end{table}
Table 1: Selected properties of the soils used in the experiment.