## 2 Materials and Methods  
### Experimental Procedure  
The methodology used to achieve the objective of this research was divided into three phases: (a) natural soil sample analysis and contamination, (b) evaluation of natural and arsenic-contaminated soils SWRC, and (c) evaluation of the permeability coefficient of the arsenic-contaminated soil samples. In the first phase, disturbed soil samples were obtained from an open pit at a site in Queretaro, Mexico. Additionally, an undisturbed cubic soil sample was obtained to determine the soil's water content and specific weight as soon as it arrived at the laboratory. Such parameters were used to fabricate the soil samples used in the suction tests. The disturbed soil samples were sieved through a No. 4 (4.75 mm) sieve. The resulting soil sample was used during the experiment, and its characterization is shown in Table 1.  
After evaluating the geotechnical properties of the natural soil reported in Table 1, the soil was air dried to avoid increasing its water content. In another case, it could happen due to contaminant addition. The soil was then divided into three parts. While one part of the soil remained uncontaminated, the other two parts were manually contaminated with an arsenic solution. According to the Mexican regulations [24] and the data shown by [PERSON] et al. [25] for the Queretaro region, it was decided to contaminate the soil with an arsenic concentration of 25 mg/kg and 50 mg/kg, respectively. To contaminate the soil at the indicated concentrations, a traceable standard arsenic solution (As\({}^{+5}\)) in the form of H\({}_{3}\)AsO\({}_{4}\) (Merck Millipore, Germany), which was traceable to the Standard Reference Material (SRM) for the National Institute for Standards and Technology (NIST), was used. Each part of the natural soil was poured into different glass containers and was sprayed with the prepared arsenic solutions to achieve final arsenic concentrations of 25 mg/kg and 50 mg/kg, respectively. Then, the contaminated soils were manually mixed and stored in hermetically sealed glass containers in the temperature-controlled room (20 \({}^{\circ}\)C \(\pm\) 1 \({}^{\circ}\)C) during a curing period of six weeks. The contamination procedure and the curing period were applied to simulate the long-term contamination effect on the soil [10]. After this curing period, the porosimetry and the specific surface area of the soil were \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Test Method** \\ \hline Unit weight (kN/m\({}^{3}\)) & 17.2 \(\pm\) 0.1 1  & ASTM D7263 \\ Water content (\%) & 27.0 \(\pm\) 0.3 & ASTM D2216 \\ Liquid Limit (\%) & 80.3 \(\pm\) 0.1 & ASTM D4318 \\ Plastic Limit (\%) & 37.6 \(\pm\) 0.1 & ASTM D4318 \\ Soil classification & MH & ASTM D2487 \\ Sand content (\%) & 92 \(\pm\) 3.3 & ASTM D6913 \\ Fines content (\%) & 8 \(\pm\) 2.1 & ASTM D6913 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Geotechnical properties of the sieved soil used for the experiments. analyzed by nitrogen adsorption-desorption isotherms to evaluate the changes promoted by arsenic contamination. The SWRC of the natural and arsenic-contaminated soils was determined in the second phase by measuring the soil suction using the filter paper method. The suction of the natural soil was measured using the calibrated curves for the Whatman Grade 42 filter paper, based on the testing procedure described in the ASTM D5298 standard [20]. Additional sodium chloride (NaCl) concentrations required to calibrate the filter paper were considered according to the procedure described in [PERSON] et al. [16]. As shown in Figure 1, each soil sample was enclosed with a filter paper into a container until moisture equilibrium was reached [17], indicating that the partial pressure of water vapor in the air gets equilibrated with the vapor pressure of the pores' water in the soil specimen. This procedure was adopted using the same method detailed by [PERSON] and [PERSON] [17] for the soil suction experimental setup.