EspañolOn 1998 the Guadiamar Basin was scenario of one of the worst environmental disasters occurred in Europe during the last decades, the Aznalcóllar mine accident. The breakage of a tailing dam in Los Frailes mine provoked the release of 6 hm3 of trace elements-polluted waters and sludge into the rivers Agrio and Guadiamar, flooding 55 km2 towards the Doñana National Park. After the emergency cleaning-up of the contaminated land, an ambitious restoration project – the Guadiamar Green Corridor (GCC) – was implemented, representing a unique large-scale case of remediation techniques applied to contaminated soils. Since then, the Green Corridor has been the subject of many studies devoted to analyse the distribution and impact of contaminants within the different ecosystem compartments, as well as to test different techniques useful for the ecological recovering of the affected area. Most of those studies were carried out during the first years after the accident, and they were mainly restricted to the floodplain. The current re-launching of the mining activity in Aznalcóllar has fostered the interest on the environmental quality of the Guadiamar area and the effectivity of the remediation measures.
The research group SoilPlant, in the Institute of Natural Resources and Agrobiology of Seville (IRNAS-CSIC), is one of the few research teams that has maintained a continuous activity in the Guadiamar area during the 16 years since the mine accident (in April 1998). The long-term environmental monitoring carried out during those years showed that soil contaminants (mainly trace elements, such as arsenic, copper, lead, cadmium and zinc) had been progressively immobilized at the floodplain, in particular on the southern part of the basin, with soils of neutral pH and clayish texture, favouring the processes of their precipitation and retention into the soil matrix. However, the river channel and banks have been less studied, despite that the contamination levels should be much higher there, due to their difficult access for the cleaning machinery and that they received less amount of soil amendment. In a recent study by the SoilPlant group, led by Dr. María T. Domínguez and included within the European RECARE project, it has been remarked the different contamination levels at the floodplain and the channel banks. There are persistent patches of soil contamination along the river banks, especially at the northern part, from the mine to Las Doblas bridge (Sanlúcar la Mayor), where conditions are worse.
Guadiamar River banks (right side) during winter 2014, at the northern side, within the municipality of Sanlúcar la Mayor (Seville).
In the river banks of the northern part is possible to find extremely acid soils, as a consequence of sludge oxidization, where arsenic contents (indicators of mine sludge) are well above to the intervention limits and potentially causing toxicity to plants. These acidic conditions increase the solubility of elements such as copper, zinc and cadmium, favouring that those contaminants are redistributed downstream through the basin. Actually the highest concentrations of cadmium and zinc were found in the soils of the river banks on the southern basin. This pattern suggests that contaminants have been washed progressively from the acidic soils in the northern river banks and transported downstream to the south.
Detail of a soil sample with remnants of sludge (in dark colour), from the 1998 mine spill.
River banks under Mediterranean-type climate are usually exposed to high floods during the autumn favouring erosion and transport of soil particles downstream, and contaminating the water body. To avoid and mitigate the contamination with trace elements from the polluted channels and banks in the northern, upper part it would be recommended to apply new clean-up measures, to correct the acidity, and to increase the vegetation cover for physical stabilization of the soil. For the members of the SoilPlant research group, the Guadiamar Green Corridor is a benchmark in the management of a severe contamination episode, since the adopted measures contributed notably to improve the environmental quality of the area. However, monitoring the dynamics of the contaminants should be continued, in particular in the river channel and banks, and new measures to remediate soil contamination should be applied in the northern part of the basin.
The results are published in the January issue of Geoderma.
Coverage by local media:
El Guadiamar, aún contaminado (El Mundo)
Algunos márgenes del Guadiamar muestran aún alto nivel de contaminación (EuropaPress)
El Guadiamar aún mantiene restos de la contaminación de Boliden (El Correo de Andalucía)
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