2.3. Analytical methods
All soil samples were air-dried and sieved (<2 mm) before analyses, which were performed using standardized methods (MAPA, 1994). The particle size characterization was determined by the Robinson’s pipette method (USDA, 1972). The pH was measured potentiometrically in a 1:2.5 soil:water suspension and the electrical conductivity (EC) in saturated paste extract. The calcium carbonate content (CaCO3) was determined by the volumetric gas method (Barahona, 1984) and the organic carbon by the acid oxidation method (Tyurin, 1951), both parameters after fine grinding (<0.05 mm) of the sample.
The total concentration of contaminants (Pb, As, Zn and Cu) was analyzed by X-ray fluorescence (XRF), using the finely ground sample, with a NITON XL3t-980 GOLDD+ portable analyser (Niton, Billerica, USA). The precision and accuracy of the contaminating elements was made from measurement (6 replicates) of a certified reference material (CRM 052-050 RT-Corporation Limited, Salisbury, United Kingdom), obtaining the following results (certified vs. measured, mean values ​in mg kg-1 and standard error in brackets): Pb: 82.6 (4.0) vs 92.6 (4.8), As: 14.6 (2.9) vs 15.5 (3.9), Zn:89.0 (4.0) vs 94.4 (6.2), Cu: 44.2 (6.9) vs 48.5 (8.0). In all cases, the measured values were within the confidence interval of the certified value.
The concentrations of water soluble contaminants were determined in a 1:5 suspension (soil:water) according to Sposito et al. (1982), and the bioavailable forms were extracted with 0.05 M EDTA at pH 7.0 according to Quevauviller et al. (1998); in both cases, the extracted elements were measured by inductively coupled plasma mass spectrometry (ICP-MS), with a Perkin Elmer SCIEX ELAN-5000A spectrometer (Waltham, MA, USA). The precision of the method was corroborated by analysis (6 replicates) of a standard reference material (SRM 2711), obtaining the following results (certified vs. measured, mean values in mg kg-1 and standard error in parentheses): Pb: 1162.0 (31.0) vs 1138.1 (11.0), As: 105.0 (8.0) vs 102.4 (1.1), Zn: 350.4 (4.8) vs 350.3 (3.4), Cu:114.0 (2.0) vs 115.0 (1.6). For all target elements, the measured values were within the confidence interval of the certified value.