# Characteristics of Arsenic Leached from Sediments: Agricultural Implications of Abandoned Mines
## 2 Materials and Methods
### Column-Leading Test  
Reservoir water moves slowly and is often stratified vertically by density and temperature gradients. Thus, a reservoir would not be quickly contaminated with heavy metals that leach from sediment. Leached heavy metals can react with various other environmental processes (e.g., temperature variations and redox conditions), and their concentrations and leaching rates can increase over the years. A column-leaching test was carried out using samples of water from different conditions (deionized water, water from the wet season, and water from the dry season) to examine arsenic leaching from sediment that had been on the reservoir bed for a long time and the responses of the leaching processes to hydrological changes in the reservoir. Sediment collected from the bed of the study reservoir was completely mixed and poured into a series of columns (diameter = 10 cm; height = 44 cm). The columns were fitted with an up-flow system to simulate the movement of groundwater toward the reservoir bed in the hyporheic zone. The sediment from the reservoir bed was sampled every day for 15 days. A homogenized sediment sample was prepared by mixing 30 kg of sediment for 24 h (using a V-mixer). The column-leaching test was performed under three conditions using the deionized water (control group), the wet season water (wet season group), and the dry season water (dry season group), respectively. The details of the column test are provided in Figure 2 and Table 2.  
The physicochemical properties of the soil samples used in the column-leaching test are shown in Table 3. The pH of the soil samples was slightly acidic (6.1) and the cation exchange capacity (CEC) was less than those found in other studies [6; 38; 39; 36]. The soil test showed that the soil is loamy sand; its arsenic concentration was 56.34 mg/kg. \begin{table}
\begin{tabular}{c l c} \hline \hline
**Form** & **Phase** & **Extractant** \\ \hline
1 & Nonspecifically sorbed & (NH4)SO4 \\
2 & Specifically sorbed & (NH4)H2 PO4 \\
3 & Amorphous and poorly crystalline hydrous & NH4 oxalate buffer (pH 3.25) \\  & oxides of Fe and Al phase & NH4 oxalate buffer + ascorbic acid (pH 3.25) \\
4 & Well-crystallized hydrous oxides of Fe and Al & NH4 oxalate buffer + ascorbic acid (pH 3.25) \\
5 & Residual & Residual (HNO3, H2O2) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sequential extraction procedure for the fractionation of arsenic (adapted from [36]). Figure 2: Schematic diagram of the column-leaching test (adapted from [37]). In the test, the total (accumulated) amount of arsenic leached was calculated using the following relationship:  
\[\text{Total amount leached (mg) = Arsenic concentration (mg/L) \times 0.658 L/PV \times 2 PV/day} \tag{1}\]  
where L is liters and PV denotes pore volume. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Category** & \multicolumn{4}{c}{**Details**} \\ \hline Column specifications and settings & Diameter = 10 cm, height = 44 cm; filled with mixed soil (grain size \textless{2 mm}) up to a height of 20 cm (total height = 44 cm); flow rate = 0.91 mL/min and 2 pore volumes (PVs) daily. \\ \hline Sampling & Duration of experiment = 15 days; sampling effluent rate = daily. \\ \hline Soil and water usage & Soil = 2438.4 g/column; amount of water used = 0.658 L/PV \(\times\) 2 PV/day \(\times\) 15 days = 19.74 L. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Details of the column-leaching test. Figure 3: Physicochemical properties of reservoir water in wet and dry seasons. (**a**: TP, **b**: TN, **c**: TOC, **d**: As)