**1. Introduction**

Pb is considered one of the most common contaminant in the environment [1] and it is considered neither essential nor beneficial for plants or animals but highly toxic. Pb is one of the less mobile heavy metals so its compounds tend to accumulate in soils and sediments, and it is slightly affected by microbial degradation [2-4]. It has a strong affinity by the sulphur, so is frequently associated to sulphides, being the galena (PbS) the most frequent ore mineral. Pb has two stable oxidation states: Pb(II) and Pb(IV), but the most dominant species in nature is the plumbous one. Geochemically, Pb2+ can replace isomorphously K+ in silicate lattices or Ca2+ in carbonates and apatites, due to the similar ionic radius with these both elements [2].

In crustal rocks, the average Pb content is approximately 16 mg kg-1 [5]. In igneous rocks trend to increase from ultrabasic (gabbro 1.9) to acid (granite 22.7), meanwhile shales and mudstones have an average Pb content of 23 mg kg-1 and sandstones 10 mg kg-1, being limestones and dolomites the rocks with higher average Pb content (71 mg kg-1). The average concentration in uncontaminated soils is estimated to be 40 mg kg-1, ranging from 2 to 200 mg kg-1 [6, 7].

On 25 April 1998, a breach in the dam of the holding pond of tailings from a pyrite mine in Aznalcóllar (S Spain) spilled around 45 x 105 m3 of acidic waters and toxic tailings containing high concentrations of Zn, Cu, Cd, As, Pb, Sb, Bi and Tl into the Agrio and Guadiamar river basins, affecting around 43 km2 [8]. The soils were polluted in two stages [9]. First, the toxic water (liquid phase) and tailings (solid phase) covered the soils and penetrated in depth through macro and micropores; the acidic water had far lower heavy-metal content (Pb concentration ranging from 2.05 to 2.60 mg dm-3) than the tailings had (Pb concentration ranging from 4352.3 to 9635.9 mg kg-1). In the second stage, the oxidation of the pyrite tailings

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causes sulphides to oxidize to sulphates, a marked fall in pH and a solubilization of heavy metals [10]. The lowest pH values were reached 25 days after the spill, and the total Pb concentration at 88 days doubled or tripled (depending the soil type) the initial value of contamination in the upper 10 cm of the soils [11].

The present study provides an evaluation of the contamination processes occurring just after the removal of the tailings and part of the upper soil, to assess the behaviour of the residual Pb concentrations in the affected area at this early stage. The aim of the present work is to analyse the mobility and availability of Pb in the initial steps of the contamination process, to obtain useful information in the planning of different strategies for future remediation activities in other affected areas based on a detailed study of the soil properties.
