# Potentially Toxic Trace Elements in the Urban Soils of Santiago de Compostela (Northwestern Spain)
## 2 Materials and Methods
### Soil Sampling and Analysis  
We sampled soils at 55 points in the city, over several materials, and under different vegetations and land uses (Table 1). Composite samples (soil depth 0-20 cm) were obtained by mixing 4-5 subsamples taken with an auger at each point. Soils were taken to the laboratory, air-dried and sieved (<2 mm) before analysis. Edaphic properties of the soils, including pH, texture, OC, CEC, etc., were previously published [30].  
\begin{table}
\begin{tabular}{l l l l l l l} \hline \hline  & **Name** & **Latitude** & **Longitude** & **Altitude** & **Land Use** & **Lithology** \\ \hline
1 & Carlomagno 1 & 42°53′00” N & 8°31′23” W & 290 & Urban grassland & Amphibolite \\
2 & Carlomagno 2 & 42°52′52” N & 8°31′27” W & 285 & Urban forest & Amphibolite \\
3 & Vieiro 1 & 42°52′46” N & 8°31′38” W & 235 & Urban grassland & Amphibolite \\
4 & Fontiñas 1 & 42°53′02” N & 8°31′47” W & 260 & Urban grassland & Schist \\
5 & Fontiñas 2 & 42°53′05” N & 8°31′57” W & 265 & Urban garden & Schist \\
6 & Monte dos & 42°53′17” N & 8°31′53” W & 300 & Urban grassland & Schist \\
7 & San Caetano & 42°53′24” N & 8°32′05” W & 300 & Urban grassland & Schist \\
8 & Bouza Brey & 42°53′20” N & 8°32′21” W & 275 & Urban grassland & Schist \\
9 & Burgo Xixón & 42°53′18” N & 8°32′34” W & 250 & Urban grassland & Schist \\
10 & Auditorio & 42°53′18” N & 8°32′41” W & 235 & Urban grassland & Schist \\
11 & Vistalegre 1 & 42°53′14” N & 8°32′49” W & 240 & Urban forest & Schist \\
12 & Vistalegre 2 & 42°53′08” N & 8°32′46” W & 230 & Urban grassland & Schist \\
13 & Santa Isabel & 42°53′13” N & 8°32′59” W & 225 & Urban garden & Schist \\
14 & Espino & 42°52′59” N & 8°33′06” W & 245 & Urban forest & Schist \\
15 & Galeras 1 & 42°52′56” N & 8°33′00” W & 225 & Urban grassland & Schist \\
16 & Galeras 2 & 42°53′00” N & 8°32′58” W & 225 & Urban grassland & Schist \\
17 & Campo das & 42°52′47” N & 8°32′52” W & 235 & Urban garden & Schist \\
18 & Fonseca & 42°52′45” N & 8°32′45” W & 245 & Urban grassland & Schist \\
19 & Alameda 1 & 42°52′40” N & 8°32′58” W & 265 & Urban grassland & Schist \\
20 & Alameda 2 & 42°52′42” N & 8°32′57” W & 255 & Urban forest & Schist \\
21 & Alameda 3 & 42°52′39” N & 8°32′47” W & 250 & Urban grassland & Schist \\
22 & Parlamento & 42°52′24” N & 8°32′32” W & 230 & Urban grassland & Schist \\
23 & Granell 1 & 42°51′54” N & 8°32′48” W & 215 & Urban grassland & Schist \\ \hline \hline \end{tabular}
\end{table}
Table 1: List of sampling points.  
For the analysis of total trace elements (Cu, Pb, Zn, Ni, Cr, As), air-dry sieved soils were ground in an agate mortar down to <250 \(\upmu\)m. Ground samples were analyzed by X-ray fluorescence using an energy-dispersive Miniprobe Multielement Analyzer (EMMA-XRF) with two pieces of equipment. Copper, Ni, Zn, Pb, and As were analyzed using a Mo anode, whereas Cr was analyzed with an Ag tube and a secondary anode of pyrographite. The EMMA-XRF and its application for elementary analysis has been described elsewhere [31]. The instruments are hosted at the RIAIDT (Infrastructure Network for the Support of Research and Technological Development) facility of the University of Santiago de Compostela (Spain). The instruments were calibrated using several reference materials (NIST1515, 1547, and 1575, BCR 60 and 62, and V-1). Standard reference materials CRM143R (sewage sludge-amended soil) and 320R (channel sediment), from the European Commission Community Bureau of Reference (BCR), were used for quality control. Detection limits for the technique in soil are as follows: 3 mg kg\({}^{-1}\) for Ni and Pb, 4 mg kg\({}^{-1}\) for Cu, 10 mg kg\({}^{-1}\) for Cr, 5 mg kg\({}^{-1}\) for Zn, and As. All analyses were performed in triplicates.