## 3 Results and Discussion
### Evaluation of the SWRC of the Arsenic-Contaminated Soils  
The results of soil suction are presented in Figure 5. Soil suction values from 1 kPa to \(1\times 10^{6}\) kPa were obtained through the filter paper method. Water content is expressed as the normalized degree of saturation (Sr), which varies from 0 to 1. Figure 4: Calibration curves of the Whatman Grade 42 filter paper for the natural and arsenic-contaminated conditions. Figure 5: Soil–water retention curve of the natural and arsenic-contaminated soils: (**a**) total suction on wetting path, (**b**) total suction on drying path, (**c**) matric suction on wetting path, and (**d**) matric suction on drying path. In the arsenic-contaminated cases, the total and matric suction values decreased at a high saturation rate and the slope appears to be smoother than the one observed for the natural soil condition, which is evident after comparing the wetting paths because the soil behaves as a coarse-grain one when contaminated. From the wetting path of the SWRC, at a Sr = 0.90, it was observed that the soil's total suction decreased 67% (from \(\psi_{t}\) = 1055 kPa \(\pm\) 50 kPa to \(\psi_{t}\) = 348 kPa \(\pm\) 6 kPa) as the arsenic concentration in the soil increased from 0 mg/kg to 25 mg/kg. Furthermore, when the contaminant concentration increased to 50 mg/kg, the \(\psi_{t}\) decreased to 226 kPa \(\pm\) 23 kPa, which represented a 78% decrease compared with the total suction of the natural soil (\(\psi_{t}\) = 1055 kPa \(\pm\) 50 kPa) at the same saturation rate. The total and matric suction were evaluated at the saturation rate of the natural soil sample (Sr = 0.8) to compare its behavior with the arsenic-contaminated soils cases. The natural soil has a \(\psi_{t}\) = 1704 kPa \(\pm\) 41 kPa, \(\psi_{m}\) = 1681 kPa \(\pm\) 50 kPa, and \(\psi_{o}\) = 23 kPa \(\pm\) 9 kPa. According to these results, the osmotic suction contributed 1.4% to the total suction of the natural soil. This demonstrates why the \(\psi_{o}\) is frequently neglected for unsaturated soil mechanics when studying natural or non-contaminated soils. The 25 mg/kg and 50 mg/kg arsenic-contaminated soils had matric sutions of 260 kPa \(\pm\) 58 kPa and 670 kPa \(\pm\) 107 kPa, respectively. In contrast, their osmotic suction was 530 kPa \(\pm\) 168 kPa and 467 kPa \(\pm\) 30 kPa, respectively. This \(\psi_{o}\) corresponded to the 67% and 41%, respectively, of the total suction of the 25 mg/kg and 50 mg/kg arsenic-contaminated soils. Thus, the contribution of the \(\psi_{o}\) should be considered when analyzing the mechanical behavior of the contaminated unsaturated soils. Moreover, the SWRC for both the wetting and drying paths of the 25 mg/kg arsenic-contaminated soil was evaluated to verify whether the ASTM D5298 [20] calibration curve gives similar results to those obtained with the calibration curve of Section 3.2 for the filter paper Whatman Grade 42. From Figure 6, it can be seen that the ASTM D5298 calibration curve provides higher total and matric suction values for the contaminated soil. The soil suction was evaluated with the ASTM D5298 and Section 3.2 calibration curves for the 25 mg/kg arsenic-contaminated soil at Sr = 0.8. Considering the ASTM D5298 calibration curve, the total suction of the 25 mg/kg arsenic-contaminated soil in the wetting path was 7455 kPa \(\pm\) 2180 kPa, while in the drying path, the total suction was 3750 kPa \(\pm\) 2610 kPa. These values were 9.5-fold and 3.4-fold times higher than those obtained using the Section 3.2 calibration curves for the wetting and drying paths of the arsenic-contaminated condition. Figure 6: Soil–water retention curve of the 25 mg/kg arsenic-contaminated soil evaluated with the ASTM D5298 calibration curve and the one obtained in Section 3.2: (**a**) wetting path and (**b**) drying path.