Pollution of carbonate soils in a Mediterranean climate due to a tailings spill

Abstract

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.

Distribution of As and Zn in Soils Affected by the Spill of a Pyrite Mine and Effectiveness of the Remediation Measures

“…Therefore, population 1 can be divided into two sectors (broken line): the first one, with As and Zn concentrations below the upper limit of the geochemical background (Table 2), the presumably non-polluted soil group (≈33% in the case of As, and 23% in the case of Zn); and the second one, with As and Zn concentrations above the upper limit of the natural background, the presumably contaminated soil group prior to the spill (≈67% in the case of As, and 77% in the case of Zn). The higher concentration of Zn in the acidic waters coming from the mine (Simón et al 1999), together with the higher mobility of the Zn and deeper infiltration into the soil (Vidal et al 1999;Simón et al 2002;Galán et al 2002), explain the higher percentage below the first 30 cm of contaminated soils in Zn than in As before the spill.…”

“…Given that, prior to the removal of the tailings, the soils were not significantly affected under the first Simón et al 2002), with the exception of a few sectors in which the tailings had penetrated to depth through structural cracks in the first step of pollution (Simón et al 1999), the geochemical background and the CFD of As and Zn between 10-30 cm and 30-50 cm were used to estimate the degree of pollution before the spill and, consequently, to calculate the effect of the spill in the total pollution level of the soils. Thus, the CFD of As and Zn between 30 and 50 cm (Fig.…”

Arsenic in Natural Environments

“…Local soils were covered with wet tailings layers containing about 73 wt % pyrite and perhaps 1.4 wt % arsenopyrite (Domènech, De Pablo and Ayora, 2002), 341. The average thickness of the tailings was about seven cm (Simón et al, 2002), 322. As the tailings dried, they partially oxidized and produced salts that were rich in sulfates, arsenic, and heavy metals.…”

“…In some cases, mining wastes are catastrophically released from impoundments (Hudson-Edwards et al, 2001;Simón et al, 2002;Carbonell-Barrachina et al, 2004). On April 25, 1998, the retention walls of a tailings pond at Aznalcóllar, Spain, broke and released several metric tons of tailings and mine drainage (Simón et al, 2002;Carbonell-Barrachina et al, 2004).…”

“…On April 25, 1998, the retention walls of a tailings pond at Aznalcóllar, Spain, broke and released several metric tons of tailings and mine drainage (Simón et al, 2002;Carbonell-Barrachina et al, 2004). The break impacted an area of about 55 km 2 in the Guadiamar Valley (Simón et al, 2002;Domènech, De Pablo and Ayora, 2002). Local soils were covered with wet tailings layers containing about 73 wt % pyrite and perhaps 1.4 wt % arsenopyrite (Domènech, De Pablo and Ayora, 2002), 341.…”