Characterizing Abandoned Mining Dams by Geophysical (ERI) and Geochemical Methods: The Linares-La Carolina District (Southern Spain)

The mining exploitation of metallic sulphides, together with the activities associated to the mineral treatment and smelting, when maintained through centuries due to the wealth of the ores, generate important accumulations of wastes in structures of different kind of tailing dams and ponds, for instance. When no previous corrective steps are taken, as usually happens in old exploitations, this means a serious risk of environmental pollution, due to the mobilisation of heavy metals. The present study has been carried out in a mining district, actively exploited during the last two millennia, that was the first world's producer of lead during some periods (Linares-La Carolina, southern Spain). In this district, the mining activity was associated to a philonian network of metallic sulphurs and ended by the 1980s of the past century. The ancient mining operations, mostly subterranean, have generated large accumulations of residues without any prior corrective action. Therefore, this work intends to characterise these mining dams and determine the influence of these mining wastes on the quality of surface and ground waters. With this goal, three structures that store the mining refuse of different mineralogical origin have been selected. First, a geochemical characterisation of the soil was performed in the area surrounding each of the structures. In all cases, high levels of trace elements (including Pb, Zn, Cu, Cd, Mn, As, Sb and Ba) were observed. A hydrochemical study revealed the mobilisation through the aqueous medium of certain contaminants from the leachate of these ancient accumulations; these contaminants will flow to the streams that drain the area or to the aquifers of the sector. The internal characterisation of these structures was performed with geophysical techniques, specifically electrical resistivity imaging (ERI). The six generated resistivity models have allowed the identification of the morphology of the structures, variations in the vertical and horizontal distribution of the deposited material, fracture zones, water content and reload-unload zones and the contact of the mining wastes with the substrate. Thus, the ERI study confirms the lack of impermeabilisation measures for the terrain in the spill zones in all three cases, which indicates a high risk of contamination of the soil and waters. The obtained images also permit the identification of the ideal positions to conduct future borehole controls.