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SummaryThe retention walls in a pond containing the residues from the pyrite mine of Aznalco llar (southern Spain) broke open on 25 April 1998, spilling approximately 6  10 6 m 3 of polluted water and toxic tailings, which affected some 55 km 2 . Drying and aeration of the tailings resulted in oxidation, forming an acidic solution with high pollutant contents, the effects of which were studied in a calcareous soil. The infiltration of this solution markedly affected only the first 12 mm of the soil, where strong acidification caused the weathering of the carbonates, and where the fine mineral particles were hydrolysed. The SO 4 2± ions in the acidic solution precipitated almost entirely at this depth, forming gypsum, hydroxysulphates and complex sulphates. The Fe 3+ ions also precipitated there, mainly in amorphous or poorly crystallized forms, adsorbing to As, Sb, Tl and Pb dissolved in the acidic solution. The Al 3+ ions, though partly precipitating in the acidic layer, accumulated mostly where the soil pH exceeded 5.5 (12±14 mm in depth). They did so primarily as amorphous or poorly crystallized forms, adsorbing to Cu dissolved in the acidic solution. The Zn 2+ and Cd 2+ ions accumulated mainly at pH > 7.0 (19±21 mm in depth), being adsorbed chiefly by clay mineral. After 15 months, only the first 20 mm of the soil were acidified by the oxidation of the tailings and most of the pollutants did not penetrate deeper than 100 mm. Consequently, the speed of the cleanup of the toxic spill is not as important as a thorough removal of tailings together with the upper 10 cm of the soil.

In the carbonate soils contaminated by a toxic spill from a pyrite mine (Aznalcóllar, southern Spain), a study was made of a thin layer (thickness = 4 mm) of polluted soil located between the pyrite tailings and the underlying soil. This layer, reddish-yellow in color due to a high Fe content, formed when sulfates (from the oxidation of sulfides) infiltrated the soil, causing acidification (to pH 5.6 as opposed to 8.0 of unaffected soil) and pollution (in Zn, Cu, As, Pb, Co, Cd, Sb, Bi, Tl, and In). The less mobile elements (As, Bi, In, Pb, Sb, and Tl) concentrated in the uppermost part of the reddish-yellow layer, with concentration decreasing downward. The more mobile elements (Co, Cd, Zn, and Cu) tended to precipitate where the pH was basic, toward the bottom of the layer or in the upper part of the underlying soil. The greatest accumulations occurred within the first 6 mm in overall soil depth, and were negligible below 15 mm. In addition, the acidity of the solution from the tailings degraded the minerals of the clay fraction of the soils, both the phyllosilicates as well as the carbonates. Also, within the reddish-yellow layer, gypsum formed autigenically, together with complex salts of sulfates of Fe, Al, Zn, Ca, and Mn, jarosite, and oxihydroxides of Fe.