## 2 Materials and Methods
### Sample Preparation and Analysis of Metal
#### 2.4.2 Speciation Analysis  
Metal speciation was determined using the [PERSON] et al. [18] modified Sequential Extraction Procedure (SEP) (Table 2), to operationally quantify the partitioning of metals in different geochemical fractions [36]. Approximately 1 g representative dry sample was weighed; and between each successive extraction the supernatant was separated by centrifugation (Hettich, Model 320) for 15 min at 5000 rpm, to minimize loss of solid material.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Site** & **Location** & **Latitude (N)** & **Longitude (W)** & **RSG [a,b]** & **Observations** \\ \hline S1 & Asientos (Mine Out) & 22\({}^{\circ}\)13\({}^{\prime}\)48.33\({}^{\prime\prime}\) & 102\({}^{\circ}\)6\({}^{\prime}\)10.68\({}^{\prime\prime}\) & Leptosol & Agricultural \\ S2 & Asientos (Downtown) & 22\({}^{\circ}\)14\({}^{\prime}\)22.11\({}^{\prime\prime}\) & 102\({}^{\circ}\)5\({}^{\prime}\)26.94\({}^{\prime\prime}\) & Leptosol & Residential/Commercial \\ S4 & Santa Elena Reservoir & 22\({}^{\circ}\)12\({}^{\circ}\)16.15\({}^{\prime\prime}\) & 102\({}^{\circ}\)19.70\({}^{\prime\prime}\) & Durisol & Agricultural \\ S6 & El Llavero (Ranch) & 22\({}^{\circ}\)34.00\({}^{\prime\prime}\) & 102\({}^{\circ}\)5\({}^{\prime}\)57.00\({}^{\prime\prime}\) & Phaeozem & Agricultural \\ S12 & Gomez Portugal (Town) & 21\({}^{\circ}\)59\({}^{\prime}\)42.00\({}^{\prime\prime}\) & 102\({}^{\circ}\)16\({}^{\prime}\)51.00\({}^{\prime\prime}\) & Durisol & Agricultural \\ S15 & Industrial Park (PIVA) & 21\({}^{\circ}\)58\({}^{\prime}\)48.50\({}^{\prime\prime}\) & 102\({}^{\circ}\)17\({}^{\circ}\)33.30\({}^{\prime\prime}\) & Phaeozem & Industrial \\ S22 & Curtidores (City) & 21\({}^{\circ}\)53\({}^{\prime}\)20.40\({}^{\prime\prime}\) & 102\({}^{\circ}\)19\({}^{\prime}\)19.96\({}^{\prime\prime}\) & Phaeozem & Residential/Commercial \\ S24 & El Niagara Reservoir & 21\({}^{\circ}\)46\({}^{\prime}\)46.98\({}^{\prime\prime}\) & 102\({}^{\circ}\)22\({}^{\prime}\)13.81\({}^{\prime\prime}\) & Phaeozem & Agricultural \\ \hline \hline \end{tabular}  
\({}^{\text{[a]}}\) Reference Soil Group according to International Union of Soil Sciences [27], \({}^{\text{[b]}}\) Source: Instituto Nacional de Estadistic y Geografía (INEG) [28].  
\end{table}
Table 1: Coordinates of the sampling sites.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Fraction** & **Extracting Agent** & **Extraction Conditions** \\ \hline
**F1** & Exchangeable & 8 mL MgCl\({}_{2}\) 1.0 M (pH 7.0) & Continuous agitation for 1 h at room temperature \\
**F2** & Bound to Carbonate & 8 mL CH\({}_{3}\)COONa 1.0 M (pH 5.0) & Continuous agitation for 5 h at room temperature \\
**F3** & Bound to Fe/Mn oxides & 20 mL NH\({}_{2}\)OH-HCl 0.04 M in 25\% (_{\text{}}\)(_v/v_) acetic acid & Occasional agitation for 6 h at 96 \({}^{\circ}\)C \\
**F4** & Bound to Organic Carbon & 3 mL HNO\({}_{3}\) 0.02 M + 5 mL H\({}_{2}\)O\({}_{3}\) 30\% (_{\text{}}\)(_w/v_) (pH 2.0) & Occasional agitation for 2 h at 85 \({}^{\circ}\)C \\  & & 3 mL H\({}_{2}\)O\({}_{2}\) 30\% (_{\text{}}\)(_w/v_) (pH 2.0) & Occasional agitation for 3 h at 85 \({}^{\circ}\)C \\  & & 5 mL NH\({}_{4}\)OAC 3.2 M & Continuous agitation for 30 min at room temperature \\
**F5** & Residual & 25 mL aqua regia & ISO–11466:1995 protocol \\ \hline \hline \end{tabular}  
\({}^{\text{[a]}}\) 1 g substrate sample.  
\end{table}
Table 2: Sequence of extracting agent and operationally defined fractions in the SEP \({}^{\text{[a]}}\).