Geoenvironmental characterization of unstable abandoned mine tailings combining geophysical and geochemical methods (Cartagena-La Union district, Spain)

This study presents the results of the geoenvironmental characterization of the Brunita mine pond in the Cartagena-La Unión mining district, Murcia (SE Spain). The main objective was to evaluate the present conditions of the mine tailings in order to identify related environmental hazards. Since 1956, the Brunita mine tailings were being stored in four stepped ponds at the watercourse of a NNO-SSE trending ravine. Subsequently, they were stored in a single, large pond. After the closure of the pond operations, certain work on dike stabilization was performed and debris from the Brunita mine started to accumulate. The mining works for> 50 years have led to the complete burial of the surrounding valleys and hills by mine waste material. Electrical resistivity tomography (ERT) has allowed the determining of both the general geometry of the pond base and the thickness of the mine tailings. Surveys along a total of 6 ERT profiles, 3 longitudinal and 3 transverse profiles to the largest dimension of the pond were carried out. The strong resistivity contrast between the infilling and the bedrock is high enough to clearly define the bottom pond boundary. Low resistivity values ( 200 Ω·m correspond to the Paleozoic rocks that constitute the base of the deposit. Vertical steps have been recognized defining strong variations in the thickness of the deposit. This geometry results from the interaction between the paleotopography of the previous valley and the different growth stages of the deposit. The depth of the pond's base obtained from two boreholes has confirmed the results obtained from ERT profiles. Significant amounts of pyrite and siderite and an appreciable presence of sphalerite and galena were determined by XRD of representative samples. Accordingly, figh contents of toxic metals (Cd, Cu, Pb, Sn and Zn) and As were detected in chemical analyses, which show the potential of the tailings to release substantial amounts of these pollutants into the surrounding soils and watercourses. AMD has been identified and percolation through the base of the pond could occur at its northeast sector, where a low resistivity area is recognized. The manifest instability of the tailings pond makes it necessary to include physical stabilization in remediation and monitoring in order to minimize the environmental impact of the tailings on the areas affected, which is located in one of the most popular tourist areas in SE Spain