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Rehabilitation of waste rock pile in the Tharsis mine

The restoration of old mining areas, in particular if they are located near towns or villages, is essential to reduce their potential risks for human health and to minimize their visual impacts.

In this study, a research team of the IRNAS-CSIC, in collaboration with researchers from the University of Seville and the Environmental and Water Agency of the Regional Government of Andalusia (AMAYA), has assessed the effectiveness of the rehabilitation of an old waste rock pile (named Cabezo de los Gatos), derived from the mining activity, adjacent to the town of Tharsis (SW Spain).

The rehabilitation process, carried out twelve years ago, consisted in the reshape of the rock pile slope, followed by the application of limestone amendments and subsequent application of a topsoil layer. Finally, the revegetation of the area was implemented planting native shrubs and trees; in addition, a mixture of native grasses and shrubs was hydroseeded onto the remediated area.

In general, the applied measures were successful, covering with woody vegetation the upper part of the waste rock pile, and providing a greening visual landscape for the inhabitants of the town nearby. Moreover, the levels of trace elements contents in the aerial part of the vegetation growing in the upper of the slope were below the toxicity thresholds for the livestock potentially consuming them. In contrast, the lower part of the slope was almost devoid of vegetation. In addition, some plants, like the gum rose (Cistus ladanifer), growing at the base of the rock pile, had concentrations of Cd above the maximum tolerable level for animals, therefore representing a toxicity risk.

We interpret that partial failure in the rehabilitation process as due to the acid mine drainage, which caused downslope a decrease of soil pH and the consequent increased availability of trace elements, resulting in the accumulation of potentially toxic elements in the aerial parts of the plants. This contamination reduced the growth and establishment of plants, diminishing the vegetation cover.

To cope with the problem of acid mine drainage we propose an alternative approach to restore waste rock piles. We suggest to select extractive wastes and to place the non-acid generating ones around the periphery of the rock pile, as a protective cover. Thus, reducing infiltration rates, seepages and the negative effect of the acid mine drainage.

This study has been published in the Journal of Environmental Management:

Madejón, P., Caro-Moreno, D., Navarro-Fernández, C.M., Rossini-Oliva, S., Marañón, T. (2021) Rehabilitation of waste rock piles: Impact of acid drainage on potential toxicity by trace elements in plants and soil. Journal of Environmental Management, 280, art. no. 111848.

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