**2. Material and methods**

After the removal of the tailings, the basin was gridded into 91 sectors of 400 x 400 m, and in the centre of each sector a plot 10 x 10 m was laid out. In every plot, soils samples were taken at the four corners and centre of this plot, at depths of 0-10, 10-30 and 30-50 cm. For each depth, 250 g of soil from the five sampling points per plot were mixed and homogenized, providing three samples per plot, for a total of 273 soil samples for the entire basin. All the samples were air dried and screened to 2 mm, and the percentages of gravels (>2mm) and fine earth (<2mm) were determined. The laboratory analyses were made with the fine-earth fraction.

Particle-size distribution was measured by the pipette method after eliminating organic matter with H2O2 and dispersion with sodium hexametaphosphate [12]. The pH was measured potentiometrically in a 1:2.5 soil:water suspension, as recommended by the International Society of Soil Science, in a CRISON Digit 501 instrument. The CaCO3 equivalent was deter‐ mined manometrically by the Barahona method [13]. Total carbon and total sulphur were measured by dry combustion with a LECO mod. SC-144DR instrument. Organic carbon was calculated as the difference between total carbon and inorganic carbon from CaCO3. The cationexchange capacity (CEC) was determined with 1N Na-acetate at pH 8.2 [14], measuring the sodium in a METEOR NAK-II flame-photometer. The total concentration of iron (Fet ) was measured by X-ray fluorescence in a Philips PW-1404 instrument, from a disc of soil and lithium tetraborate in a ratio of 0.6:5.5. Amorphous and poorly crystallized iron (Feo) and aluminium (Alo) oxides were extracted with oxalic-oxalate [15] and measured by atomicabsorption spectroscopy.

Samples of air-dried soils, ground to < 0.05 mm, were digested in open flasks by strong acids (HNO3:HF in a ratio 2:3) at 160 ºC till dry, and 100 ml HNO3 at 4% were added. In the digested samples, total Pb (Pbt ) was measured by ICP-MS with a PE SCIEX-ELAN 5000A spectrometer. A multi-element calibration standard 4 (Perkin-Elmer) was used with Rh as the internal standard. The detection limit for Pb was 0.001 µg L-1, and the accuracy of the method was corroborated by analyses (six replicates) of a standard reference material: SRM 2711 (soil with moderately elevated trace-element concentrations [16]). For Pb, the mean certified value was 1162.0 mg kg-1 with a standard deviation of 31.0; the mean experimental value was 1138.1 mg kg-1 with a standard deviation of 11.0.

Soluble Pb (Pblw) was measured in the extract from a saturated soil paste. Pb extracted by calcium chloride 0.01 M (Pbc) using the method [17], Pb extractable by acetic acid 0.43 M (Pba) following [18], and Pb extractable by oxalic-oxalate pH 3 (Pbo) as in [15]. For the EDTA 0.05 M extractable fraction (Pbe) method [19] were followed. The Pb of all the extracts was measured by ICP-MS.

For the statistical analysis, the SPSS 15.0 program was used. The areal distribution of the contamination was established by the algorithm of the least Euclidean, using the software Surfer 7.0.

The climate of this area is typically Mediterranean (hot, dry summers; cold, wet winters; temperate autumns and springs with variable rainfall). The mean annual rainfall of 613 mm, the mean temperature of 17.7 °C, and the potential evapotranspiration of 900 mm (taken from 7 weather stations near the spill, over 27 years). The affected soils were Typic Xerofluvents and Typic Xerothents [20] developed on alluvial deposits coming from the erosion of sedimentary and metamorphic materials of the Agrio and Guadiamar River basin, mainly gravels, sands, silts and clays from the Miocene and Plio-Quaternary.
