Residual pollution load of soils impacted by the Aznalcóllar (Spain) mining spill after clean-up operations

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Abstract

By comparing total concentrations of potentially toxic elements in soils affected by the Aznalcóllar mining spill with those of the adjacent unaffected soils, it can be inferred that after the sludge removal, there still exists a considerable amount of residual pollution. This exceeds the suitable levels for cultivation, especially in the case of arsenic for which total concentrations are in the range of values above which eco-toxicity is considered to be possible. Elemental distribution in the soil seems to be determined by two distinctive associations (AsPbHgSb and CuZnCd) with different geochemical behaviours.

Introduction

On April 1998 probably the worst ecological disaster recorded in Spain took place as a consequence of the mine tailings accident at Aznalcóllar, near Seville (Andalusia). The failure of the tailings dam wall released out approximately 2 000 000 m3 of pyrite sludge and 4 000 000 m3 of acidic water into the Agrio-Guadiamar river system, flooding more than 4000 ha of riverbanks and adjacent fertile landfarms, and threatening ecologically sensitive areas of the Doñana National Park (Pain et al., 1998), the largest reserve of bird species in Europe. Because of the mining spill, the agricultural soils and sediments along the river course were severely impacted by potentially highly toxic elements such as Cu, Pb, Zn, Cd, As and other sulphide-related trace elements. A general overview of the accident can be found in Grimalt et al. (1999), Gallart et al. (1999), Alastuey et al. (1999) and López-Pamo et al. (1999), and preliminary data on soil pollution were provided by Simon et al. (1999), Cabrera et al. (1999) and Vidal et al. (1999), among other papers compiled in a special issue of The Science of the Total Environment (vol. 242, nos. 1–3, December 1999).

Immediately after the mining accident, the Andalusian Regional Government established an Emergency Actions Plan whose purpose was attenuation of the environmental and socio-economic effects and prevention of potential health risks (Junta de Andalucı́a, 1999). Soil remediation activities included intensive clean-up works involving removal of the pyrite slurry by bulldozing, and soil treatments by the addition of calcium carbonate and iron oxy-hydroxides to prevent metal solubilization, adsorption and dispersal of pollutants, among other emergency measures. Other long-term actions were implemented later in the framework of the Green Corridor Project, an ecological bridge connecting the Doñana Park and the Sierra Morena through the Guadiamar river basin.

This paper deals with the determination of trace element concentrations in the soils affected by the toxic spill once clean-up works and preliminary soil-reclamation activities were over, as well as an assessment of soil environmental quality, including recommendations for management.

Section snippets

Sample collection and methodology

Nine months after the accident (February 1999) when the removal of the sludge and the preliminary soil treatments had been concluded, a systematic sampling was conducted for a typical agricultural area affected by the spill around to the Quema ford, approximately 30 km downstream of the mine tailings (Fig. 1).

A collection of 149 samples (one sample per hectare) of the top 25 cm of soil profile were taken to prepare, by mixing and homogenising, 18 compounded samples for laboratory analysis.

Sludge and soil characteristics

The sludge formed a fine-grained layer several centimetres thick covering the soil, which was composed largely of pyrite (75–80 wt.%) and sub-ordinate amounts of other metallic sulphides (galena, chalcopyrite, sphalerite and arsenopyrite), quartz, and rarely chlorite and illite. Most of these mineral components ranged between 2 and 60 μm in crystal size, but it must be emphasised that a significant amount of the slurry (more than 50 wt.% of the 1QM1 and 3QM1 whole samples) was characterised by

Conclusions

Despite the immediate removal of the covering sludge, the soils affected by the Aznalcóllar mining spill still contain a significant residual contamination, especially in the vicinity of the river bed where the pollution load indices are in the range from 3 to 9. Potentially toxic elements (particularly Cu, Pb, Zn, As, Co, Cd, Hg and Sb) show high concentration factors not only in the plough layer, but also in the subsoil (up to 50-cm depth). Within soil profile, elemental distribution seems to

Acknowledgements

We thank the AFREXPORT Company for permission to take soil samples and for providing technical field assistance. Robert Readman and Yolanda Fernández-Caliani kindly helped with the translation of the manuscript.

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  • Soil-plant relationships and contamination by trace elements: A review of twenty years of experimentation and monitoring after the Aznalcóllar (SW Spain) mine accident

    2018, Science of the Total Environment
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    Cabrera et al. (2008) reported that the Hg content was 6.6 times greater in the affected soils (0–20 cm) than in unaffected soils (three years after the accident), although with a very irregular distribution (0.404 ± 0.420 mg kg− 1, mean value ± SD). In general, this level is rather similar to those reported by Galán et al. (2002) and Kemper and Sommer (2002). Mercury and other less mobile elements, such as As and Pb, reached their highest values (greatest contamination) close to the tailing dam (in the first 5–8 km away from the mine; Cabrera et al., 2008; Vanderlinden et al., 2006b).

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